Recycling of Plastic Materials [1 ed.] 1-895198-03-8, 9781895198034

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Recycling of Plastic Materials Francesco Paolo La Mantia Editor

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ChemTec Publishing

Copyright © 1993 by ChemTec Publishing ISBN 1-895198-03-8 All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

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ChemTec Publishing 38 Earswick Drive Toronto-Scarborough Ontario M1E 1C6 Canada

Canadian Cataloguing in Publication Data Main entry under title: Recycling of plastic materials Includes bibliographical references and index ISBN 1-895198-03-8 1. Plastics - Recycling. I. La Mantia, F. P. (Francesco Paolo) TP1122.R43 1993 668.4 C93-093134-3

W. De Winter

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Poly(ethylene terephthalate) Film Recycling W. De Winter Agfa-Gevaert N.V., Research & Development, Septestraat, B-2640 Mortsel, Belgium

INTRODUCTION The impact of man-made polymers on the environment is a problem of high priority in most industrialised countries. Mainly due to a build-up of disposed waste in landfills, and due to campaigns in the press about mistakes made in the management of waste treatment, public opinion is focusing on this problem. The fact that the corresponding percentage by volume is higher, due to the low packing density of wastes, makes the problem more visible. Although “plastics” constitute not even 10 wt% of the total amount of wastes, both residential and industrial, found in landfills (see Figure 1), public attention to them is increas1 ing. A possible explanation of such a reaction suggests that there is a lack of compatibility of plastics with the environment, despite the fact that the majority of products used in present daily life are made of materials which have also been manufactured by a chemical process. The plastic waste in landfills consists of about two-thirds polyolefines, and only ca. 15 % of styrene polymers, ca.10 % of polyvinyl chloride, and less than 10 % of all other polymers, including poly(ethylene terephthalate) (PET). The largest use of PET is in the fiber sector. PET film and PET bottles repre2 sents only about 10 % each of the total PET volume produced annually. It is also generally known that the total ECO-balance, considering energy consumption, atmospheric and water pollution, as well as solid waste content, is by a factor 2 to 5 more favorable for PET film than for its greatest competitors in the packag3 ing sector, namely glass and aluminium. 4 In addition, PET is one of the largest recycled polymers by volume, because it is suitable for practically all recycling methods.1 PET recycling by the following technological processes is discussed below:

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PET Film Recycling

• direct re-use • re-use after modification • monomer recovery • incineration • and re-use in a modified way. In addition, attention will be given to some other attempts for recycling which have not been thoroughly evaluated so far, like biodegradability and photodegradation. This paper is limited to the discussion of PET-film recycling. A global review of 2 PET-recycling in the sectors of fibres, films, and bottles was published earlier.

Figure 1. Composition of landfill-waste (domestic and industrial).

W. De Winter

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DIRECT RE-USE Over 50 % of the PET film produced in the world is used as a photographic filmbase. The manufacturers of these materials, mainly Agfa-Gevaert, Eastman Kodak, du Pont de Nemours, Fuji, Minnesota Mining & Manufacturing, and Konishiroku have long been interested in PET film recovery. An important motivation for the efforts made by these companies is the fact that photographic films are usually coated with one or more layers containing some amount of rather expensive silver derivatives, which have been recovered since the early 20th century, when cellulosics were used as a film base. Silver recovery makes 5,6 PET-base recovery more economical. In a typical way of operation, PET film recycling is coupled with the simultaneous recovery of silver, as represented in Figure 2.

Figure 2. Combined recovery of silver and PET.

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PET Film Recycling

In the first step of the process, photographic emulsion layers containing silver are washed with, for example, NaOH, and after separation, silver is recovered 2 on one side, and cleaned PET-waste on the other side. Important in this process is that the washed PET-film scrap is clean enough to be recovered by direct re-extrusion, although careful analysis remains necessary. Direct recycling of PET-waste in the molten state, before re-extrusion to PET-film, is of course the most economical process thinkable, as recovered PET-scrap can be substituted for virgin PET-granulate without requiring any additional steps. It is well-known that PET in the molten state gives rise simultaneously to polymer build-up and to polymer degradation, so that reaction conditions for this process have to be controlled very carefully in order to obtain an end-product with desired physical, chemical and mechanical properties, like color, molecular weight, and molecular weight distribution.

A large number of reaction parameters have to be kept under permanent control (temperature, environmental atmosphere, holding time in a melt state, amount of impurities, type of used catalysts and stabilizers, etc.). The order of addition of the PET flakes is very important. A typical flowsheet of a 7 batch-PET-process is represented in Figure 3. In such a process, the PET-flakes can be added after polymerization, before the melt enters the film extruder screw (Figure 3, indication 1). Such a procedure, however, has two main drawbacks: • a highly viscous melt is difficult to filter (to eliminate possible gels or microgels) • resulting low-boiling or volatile side-products cannot be discarded anymore. In order to eliminate these disadvantages, several alternative operation modes have been worked out in the past. A method to add recycled PET during

W. De Winter

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Figure 3. Batch process flow sheet. 8

the esterification step (Figure 3, indication 2) has been described by du Pont. In such a way filtration can take place in the low-viscosity phase, and volatiles can still be eliminated during the prepolymerisation phase. Although PET-recycling by direct re-use is by far the most economical process, it is only useful in practice for well characterized PET-wastes, having exactly known chemical composition (catalysts, stabilizers, impurities). Therefore, this process is the most suited for the recovery of in-production wastes, but it may not be ideal for customer-recollected PET-film. An industrial process for X-ray 9 film-recycling was worked out by the IPR-company and introduced to the market under the name REPET on the basis of a triple motivation: • availability of the waste chips on a repetitive basis • suitable purity • very competitive price.

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PET Film Recycling

RE-USE AFTER MODIFICATION Similar to the method described under direct re-use, in which PET-flakes are added during the esterification process, PET-polymer is broken down into low-molecular, low-viscous fractions. Such method could already be viewed as a method of re-use after modification. Because the intermediate products are not separated at any moment of the process, the degree of purity of PET-scrap must be high. For PET-wastes having a higher degree of contamination, other technological processes are applied, including further degradation by either glycolysis, 10 methanolysis, or hydrolysis, yielding products which can be isolated. The principles of chemical processes on which these methods are based are schematically represented in Figure 4.

Figure 4. PET degradation by glycolysis, methanolysis, and hydrolysis.

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Glycolysis can be considered as a method for direct re-use, whereas methanolysis and hydrolysis are mainly taken into consideration for monomer recovery, as discussed below. 11 The du Pont Company published many details concerning the glycolytic recycling of PET. Less costly ingredients than those required for hydrolysis or methanolysis, and more versatility than direct remelt recycling are quoted as the reasons for glycolysis choice. Goodyear has also developed the PET recycling 12 process based on glycolysis which is called REPETE. Glycolytic recycling of PET, which can be done in a continuous or in a batch process, is preferentially performed by addition of a PET waste to a boiling ethylene glycol, which leads to the formation of low-molecular weight intermediates and eventually to crystallizable diglycol terephthalate (DGT). The rate of the degradation reactions is primarily controlled by varying the holding time and temperature, which depends on a choice of suitable catalysts (e.g., titanium 12,13 derivatives), and by adjusting the PET/glycol ratio. It is also necessary to avoid side reactions which might occur, e.g., by adding “buffers” or by keeping down reaction time and temperature. The low-molecular weight depolymerizates can be introduced directly into a 14 polymerization system, preferentially after filtration. In this method, particular care has to be taken in order to avoid glycol ether formation, which may lead to PET of inferior properties. The glycolytic degradation can also be pushed to further completion, leading to DGT-recovery, rather than to direct re-use. In addition to the glycolytic recovery of PET for production of a new PET-film, granulate, or monomer (EG and DGT), alternative methods have been described for the preparation of so-called PETGs (i.e., glycol-modified PET), which can be 15,10 used for different purposes. Depending on the type of glycol (or polyol) used for depolymerization, and on the nature of dicarboxylic acid used for subsequent polycondensation, the obtained polyester may be used as a saturated polyester resin (e.g. for films, fibres or engineering plastics), unsaturated polyester resin, mixed with vinyl-type monomers, or alkyd resin, where polycondensation is performed in the presence of tri- or poly-functional organic acids. Although this method for producing unsaturated resin, e.g., for use in regular castings or in fiber-reinforced laminates, has been thoroughly studied by PET-film manufacturers, it is believed that the method is not currently used in 16 production.

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PET Film Recycling

MONOMER RECOVERY Although monomer recovery is the oldest recycling method and can be used to recover PET-waste having a high degree of impurity, it is regrettable that it is not the most economical method. The earliest methods of PET synthesizing were based preferentially on the use of dimethyl terephthalate (DMT), which could be better purified than terephthalic acid (TPA), therefore methanolysis is discussed before hydrolysis. The chemical principles of both processes are already given in Figure 4.

Methanolysis of PET-waste The waste is treated with methanol (in a ratio 1/2 to 1/10), usually under preso sure at high temperatures (160-310 C) in the presence of transesterification and 17 (or) depolymerization catalysts. Once the reaction is completed, DMT is recrystallised from the EG-methanol mother liquor, and distilled to obtain polymerization-grade DMT. Also EG and methanol are purified by distillation. Eastman Kodak has been using such a process for recycling of X-ray films for 25 18 years, and it is still improving the process, e.g., by using superheated methanol vapor, to allow the use of ever more impure PET-waste. Important factors which have to be dealt with in this process are avoiding coloration and keeping down the formation of ether-glycols.

Hydrolysis of PET-waste19 Although aromatic polyesters are rather resistant to water under atmospheric conditions, compared with other polymers, they can be completely hydrolyzed by water at higher temperatures (and) under pressure. For practical purposes, however, particularly to speed up the process, use has to be made of catalysts. Acidic as well as alkaline catalysts have been studied and worked out in practice. Figure 5 gives a flow chart of both processes. While both systems are completely realistic, their usefulness under practical production conditions remains controversial. As far as acid hydrolysis is concerned, the large acid consumption and the rigorous requirements of corrosion resistance of the equipment make profitability questionable. In addition, the simultaneous (with TPA) regeneration of ethylene glycol is difficult, ecologically undesirable (requiring the use of organic halogenated solvents), and not economical. Concerning alkaline hydrolysis, the profitability is strongly determined by the necessity of expensive filtra-

W. De Winter

Figure 5. Flow chart of acid- and base-catalyzed PET degradation.

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PET Film Recycling

tion and precipitation steps. To our knowledge, recycling of PET-waste by hydrolysis is not practiced on a production scale at present. This situation even persists in spite of the fact that the majority of newer industrial PET-synthesis 20 plants are based on the TPA-process rather than on the DMT-process.

INCINERATION Another approach which can be used to recycle plastics, particularly when they contain a large amount of impurities and other combustible solids (if such is a case, it is important to keep them away from landfills), is more recently called “quaternary recycling”, and consists of the energy recovery from the wastes by 21 burning. Research along this line has been performed, particularly in Europe and Japan, since the early 1960s. Strong emphasis has been laid on an optimization of incinerators with regard to higher temperature of their operation and reduction of the level of air pollution. PET has a calorific value of ca. 30.2 MJ/kg, which is about equivalent to that of coal. It is thus ideally suited for the incineration process. The combustion of plastics, however, requires 3 to 5 times more oxygen than for conventional incineration, produces more soot, develops more excessive heat, and incineration equipment had to be adapted in order to cope with these problems. Several processes have been worked out to overcome these technological draw22-27 backs. Examples include Leidner’s continuous rotary-kiln process, Baliko’s process for glass-reinforced PET, Crown Zellerbach Corporation’s combined system for wood fibre and PET to provide steam to power equipment, and ETH-Zurich’s fluidized bed system for pyrolysis, especially of photographic film, i.e., in combination with silver recovery. The latter system raises the additional problem of the formation of toxic halogenated compounds, stemming from the presence of silver halides. o Typical operation conditions take place at temperatures around 700 C. At lower temperatures, waxy side-products are formed, leading to clogging. At higher temperatures, in turn, the amount of the desirable fraction of mononuclear aromatics decreases. A representative sample, pyrolysed under optimized conditions, yields, in addition to water and carbon, aromatics like benzene and toluene, and a variety of carbon-hydrogen and carbon-oxygen 1 gases. Studies have been performed to avoid formation of dioxines and disposal of residual ashes containing heavy metals and other stabilizers.

W. De Winter

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At present it seems that most problems arising during incineration of PET can 25 be resolved; however, quite a few residual hurdles will have to be taken before an economically feasible and ecologically accepted industrial technical process will be available.

BIO- AND PHOTO-DEGRADATION Although there certainly has never been a great incentive for making unstable polymers, the idea of making photo- or bio-degradable polymers has long ex28,29 isted, and quite a bit of effort has gone into research along these lines. For such a process, of course, limitations with regard to the percentage of allowable impurities do not exist.

Photodegradation 30

Special photodegradable polymers were synthesized for the purpose of having them destroyed after use (e.g., in a landfill). Another approach was the incorporation of suitable groups (e.g., carbonyl) in the polymer backbone in order to make polymer photodegradable by sunlight or UV (see Figure 6). A problem arises due to the fact that light exposure conditions on a landfill cannot be regulated. The main difficulty, however, seems to be practically insurmountable: it is

Figure 6. Photodegradable monomers and polymers.31

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PET Film Recycling

hardly possible to combine rapid degradation upon exposure to light in a landfill after use with a good light-stability of the film during service. This contradictio 29 in terminis is probably the reason why this method never really caught on. Another problem is a combination of desired properties with favorable economics.

Biodegradation The main difference between biodegradation and photodegradation lies in the possibility to create in a landfill an environment completely different from that encountered under normal storage conditions; e.g., microorganisms which can destroy plastic films may be added to a landfill. In spite of the fact that substantial research time was spent on studies in this 32 field, it is claimed that surprisingly little is understood about the molecular-level interaction between polymers and microorganisms. This can be explained by a poorly defined environment (in a landfill), and by a large number of complex parameters involved in the process: methods of evaluation based solely on changes in physical properties are thus unsuitable for forming conclusions, similar to the evaluations based only on biogas production. Specifically for polyesters, however, a number of interesting data are available. Esterases (ester-hydrolyzing enzymes) and also some microorganisms are known to biodegrade 29,33 polyesters at a reaction rate depending upon the polyester structure. While many aliphatic polyesters, specifically poly(hydroxy fatty acids) - e.g., the 34-36 BIOPOL packaging material commercialized by ICI - are suited for biodegradation, the aromatic polyesters (e.g., PET) do not possess this prop32,37-39 erty.

Another approach consists of mixing small amounts of biodegradable polymers, e.g., polysaccharides, with a regular polymer (e.g., a polyolefin), in order to make the end-product destroyable as well. Examples of polysaccharides/poly38 40 ethylene have been commercialized. Mixtures of starch with other polymers,

W. De Winter

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including PET, have been studied, but no commercialization of the latter mixture is known so far. The fact, however, that the starch additive is only needed in small amounts, which hardly alters the properties of an original polymer, might show some promise for future applications. One has to realize, however, that the o thermal stability of starch derivatives above 230 C is limited, whereas the o PET-film extrusion temperature is in the range of 280 C. There also remain some controversies about the completeness of the degradation of polymer/starch mixtures. Although the development of biodegradable plastics is still in progress, it is becoming evident that the enormous market potential, forecast some years ago, re41,42 quires a real breakthrough in order to be attained. The main reason for this setback is probably the fact that organic polymers do not biodegrade fast 43,44 enough.

CONCLUSIVE REMARKS •

• • •

•

From the data presented in this overview, it seems obvious that there exists a clear hierarchy in PET-film recycling technologies. The most important criteria of classification are, first of all, the degree of “purity” of PET-scrap to be handled, and secondly, the economics of the process. For the cleanest PET grade, the most economical process, i.e., direct re-use in extrusion, is self-explanatory. For less clean PET samples, it is still possible to re-use them after the modification step (partial degradation, e.g., by glycolysis) at a reasonably low price. More contaminated PET-film waste must be degraded into the starting monomers, which can be separated and re-polymerized afterwards, of course, at a higher cost. At present, only the methanolysis process is exploited industrially, as opposed to hydrolysis processes, which are kept in reserve. Finally, the most heavily contaminated PET-shreds have to be incinerated. Here, however, economics may not be favorable enough for industrial development. As an alternative, those PET-shreds are brought to a landfill. Perhaps in future more attention will be given to modification of PET-films in such a way that they may become biodegradable, if the process can be accelerated or if a real breakthrough becomes available.

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PET Film Recycling

REFERENCES 1 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

F. P. Boettcher, ACS Polymer Preprints, 32 (2), 114 (1991). W. De Winter, Die Makromol. Chem., Macromolecular Symposia No. 57, 253 (1992). Anon., Plastics Bulletin, 174, 6 (Jan-1992). N. Basta et al., Chem. Eng., 97, 37 (Nov-1990). Brit. Pat. 1.476.539 (1977) to Barber-Colman Co. Anon., Manufacturing Chemist, 66, (Mar-1987). L. Hellemans, R. De Saedeleer, and J. Verheijen, US Pat. 4,008,048 (1977) to Agfa-Gevaert. L. Jeurissen and F. De Smedt, Brit. Pat. 1,486,409 (1977) to Agfa-Gevaert. J. Tempels, Brit. Pat. 1,432,776 (1976) to Agfa-Gevaert. W. Fisher, US Pat. 2,933,476 (1960) to du Pont. J. Burke, in Plastics Recycling as a Future Business Opportunity, Technomic Publishing Co, Pennsylvania, USA, (1986). K. Datye, H. Raje, and N. Sharma, Resources and Conservation, 11, 117 (1984). D. Gintis, Die Makromol.Chem., Macromolecular Symposia, 57, 185 (1992). R. Richard et al, ACS Polymer Preprints, 32 (2), 144 (1991). A. Petrov and E. Aizenshtein, Khim. Volokna, 21 (4), 16 (1979). US Pat. 3,884,850 to Fiber Ind. Anon., Mod. Plast. Int., 20, 6 (1990). A. M. Thayer, Chem. Eng. News, (Jan. 13, 1989). Brit. Pat. 784,248 (1957) to du Pont. Anon., Eur. Chem. News, 30 (Oct. 28, 1991). H. Ludewig, Polyester Fibers, Chemistry and Technology, Wiley Int. Publ., 1971. H. Schumann, Chemiefasern Textil, 11, 1058 (1990). U. Thiele, Kunststoffe, 79 (11), 1192 (1989). T. Randall Curlee, The Economic Feasibility of Recycling, Praeger Publishers, New York, 1986. Leidner, Polymer Plastics Techn. & Eng., 10 (2), 199 (1978). S. Baliko, Energiagazdalkodos, 28 (11), 496 (1987). D. Vaughan, M. Anastos, and H. Krause, Rpt. Battelle Columbus Lab., EPA-670/2-74-083, (Dec-1974). R. Hagenbucher et al, Kunststoffe, 80 (4), 535 (1990). K. Niemann and U. Braun, Plastverarbeiter, 43 (1), 92 (1992). W. Kaminsky et al., Chem. Ing. Techn., 57 (9), 778 (1985). Guillet, Chem. Eng. News, 48, 61 (May 11, 1970). F. Rodriguez, Chem. Techn., 409, (Jul-1971). G. Smets, Chem. Magazine, 481, (Sep-1989). Brit. Pat. 1,128,793 (1968) to E. Kodak. G. Loomis et al., ACS-Polymer Preprints, 32 (2), 127 (1991). R. Klausmeier, Soc. Chem. Ind., London, Monogr., 23, 232 (1966). Anon., Neue Verpackung, 1, 50 (1991). J. Emsley, New Scientist, 50, 1 (Oct. 19, 1991).

W. De Winter 36. 37. 38. 39. 40. 41. 42. 43. 44.

A. Steinbuchel, Nachr. Chem. Techn. Lab., 39 (10), 1112 (1991). P. Klemchuk, Mod. Plastics Int., 82, (Sep-1989). J. Evans and S. Sikdar, Chemtech, 38, (Jan-1990). K. Joris and E. Vandamme, Technivisie, 179, 5, (1992). R. Narayan, Kunststoffe, 79, 1022 (1989). N. Holy, Chemtech, 26, (Jan-1991). A. Calders, Technivisie, 156, 8 (Nov-1990). Anon., Mod. Plast., 20 (1), 72 (1990). H. Pearce, Scient. European, 14, (Dec-1990).

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E. H. Neumann

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The Importance and Practicability of Co-Injected, Recycled Poly(ethylene terephthalate)/Virgin Poly(ethylene terephthalate) Containers Eberhard H. Neumann Nissei ASB GmbH, Mündelheimer Weg 58, D-4000 Düsseldorf 30, Germany

INTRODUCTION In several European countries, packaging items, whether they are made of plastic, paper, metal, etc., are under governmental and public pressure. Well-known are: • actions to ban all plastic bags in one southern European country • the boycott of plastic packages in certain alpine villages • Denmark’s ban of metal cans for beverages • Switzerland’s removal of all PVC-packages • Germany’s set-up of a mandatory deposit on beverage bottles made of plastic and limiting sales on non-refillables. The list of restrictions on packages and their markets in Europe could be endless. Increasing environmental concerns, overloaded landfills and inadequately equipped or even not existing garbage incineration units are calling for solutions. Out of many proposals two solutions are always highlighted in public discussions: • refillable and returnable packaging articles to reduce the amount of household refuse • recycling of post-consumer packages. Regarding recycling systems for post-consumer plastic bottles, companies have already installed plants in North America which are profitable operations. However, these systems are leading to a converting technology which transforms discarded plastics into a range of second-use commodities, as well as low

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PET Co-injection Molding

cost base specialties. This paper is intended to show a technology, whereby discarded plastic bottles for beverages, food and household items - being post-consumer ware - can be used to manufacture the same range of packaging articles (bottles) for which they were originally made. Additional goal is to assure the highest level of safety offered by the original products made from virgin plastic.

BASIC TECHNOLOGY Injection molding technology involves the injection of molten plastic into one or several cavities via a hot-runner system (melt-channel distribution system) and rapid cooling of a preform to a low temperature. At this point the freshly manufactured article can be ejected from the cavity. In multilayer technology, more than one plastic resin is injected into the cavity. The different resins are molten in separate injection units, conveyed in separate hot-runner channels, under pressure and high temperature, to an injection nozzle, which is the gate area for the molten plastic, into the cavity. This injection nozzle consists of an outside and an inside tube. Both plastic streams are brought together after being released from the nozzle and they bond together because of a high pressure (up to 300 bar) and a high melting temperature (see Figure 1).

MANUFACTURING PROCESS OF MULTILAYER BOTTLES CONTAINING REGRIND For the process, a machine used had one injection unit for virgin PET and another injection unit for reground PET flakes. The individual steps of the process are described below.

Drying of PET resin and PET flakes (Figure 2) The drying of a virgin PET resin and reground PET flakes at temperature levo els of 160-180 C to below 0.005% moisture content is essential for the production of amorphous multilayer polyester bottles. Polyester is an effective desiccant. The water absorption depends on a relative humidity, residence time, temperature, and dimension of the flakes. When flakes containing moisture are heated up to the melting temperature, hydrolytic degradation occurs lowering a viscosity of the melt that results in enhanced ability of preforms/bottles to crystallize (milky appearance).

E. H. Neumann

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Figure 1. A diagram of an injection molding equipment.

The dryer and the hopper for the virgin PET chips are of standard design. The unit for the reground PET flakes is of almost the same design but it needs an internal agitator (propeller) which prevents the amorphous PET flakes from sticking together. To be exact this is not a problem inherent in the highly stretched bottle sidewalls (these flakes do not show this property). The problem is rather that ground-up flakes come from non-stretched portions of PET bottles, i.e. the neck or bottom. o Above the glass transition temperature (74 C) the flakes will stick together because of adhesional forces. It is therefore necessary to reduce a contact time between the individual flakes and keep them constantly moving in a hopper to avoid the above mentioned sticking process and bridge building in the hopper. This is carried out by the agitator installed inside the hopper. The flakes coming

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PET Co-injection Molding

Figure 2. Close-loop drying system for PET regrind

from less stretched bottle regions, i.e. the neck and bottom recrystallize during the drying process. Crystallized flakes will not stick together. The dried reground PET flakes are fed via a feeding-extruder into the throat of the satellite injection (2nd and 3rd) unit of the multilayer single stage machine. An extruder feeder is necessary because PET flakes are bulky and cause problems when they are fed by gravity into the screw of the injection unit.

Co-injection molding of virgin and reground PET flakes The virgin PET as well as the reground PET flakes are melted inside the separate injection units by means of external electrical heating of the injection barrel and the applied shear forces of the screw, driven by a hydraulic motor (see Figure 1).

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The PET melt, either coming from virgin PET or from PET flakes, is accumulated in the head-space of the barrel head and released into the hot runner (melt-channel system) under high injection pressure and at a certain melt-stream velocity. The melt is kept in a molten stage, inside the hot runner system because of the electric heating systems. The two melt-streams (virgin and reground PET) are kept, up to the injection nozzle, in separate hot runner channels. At the end of the hot-runner channels, at the gate area of the injection cavity, two nozzles are installed like a double tube or as one tube with a second smaller tube inside it. A portion of a virgin PET melt, forming the outside layers, is first injected into the preform cavity. Under controlled pressure another melt-portion coming from the reground PET is injected into the core of the virgin PET melt cake. Subsequently both injection units inject further melt (from virgin and reground PET) simultaneously during the cavity fill process. This three phase filling process results in a preform made from the following layers: inside/middle/outside (virgin PET/regrind PET/virgin PET). During the cavity filling process, the layers do not mix together because the individual melt-layers have a high melt viscosity and are not subjected to a turbulent flow. The adhesion between the individual preform layers is as good as if the layers were welded together and formed monolayer preform (made out of one melt stream).

Conditioning and stretch-blow-molding Thermal conditioning is the next step of the multilayer preform production. The purpose of thermal conditioning of a given preform is to provide the necessary temperature distribution in a preform. After leaving the injection mold and undergoing cooling process, the preform has a cross-sectional temperature distribution of an upside-down U-shape, which means that the middle of the preform wall shows higher temperatures than the two outside skin-layers. Since PET stretch properties are influenced by the temperature level above the glass transition point of PET, the preform requires more even temperature distribution, otherwise the middle layer will stretch at a different rate from the skin-layers. Thermal conditioning can be carried out by allowing the preform to equilibrate before stretch-blow-molding or by applying thermal energy from the

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PET Co-injection Molding

outside by moving the heater-pots around the preform and/or by allowing a heated core rod to plunge into the center of the preform, thus influencing the thermal profile of a preform wall from the inside. After thermal conditioning is accomplished, the preform is transferred into the blow mold of the stretch-blow-molding station. Here the preform is axially stretched by using a stretch rod and circumferentially inflated by air pressure, to match the shape of the blow mold. The final bottle is cooled down due to the contact heat losses on the metal surface of the blow mold. The stretch-blow-molding process leads to a biaxial orientation of the macromolecules resulting in better mechanical properties and lowering the gas permeation of bottles.

Double-layer preforms The injection molding technique using double layer technology is an alternative method. By this method the first preform is made of a thin layer of virgin PET. In the next step, melt from reground PET flakes is injected into the exterior of the preform layer made of virgin PET. Such technology requires two injection preform molds and subsequently a five station machine, compared with a standard machine which has four.

TRIALS OF CO-INJECTING VIRGIN PET AND REGROUND PET FLAKES The following trials were carried out at NISSEI ASB headquarters in Komoro, Japan.

Quality of the raw materials Virgin PET had an intrinsic viscosity of 0.78 dl/g and was crystallized and solid state-polycondensed (upgraded) by the chemical manufacturer. PET reground flakes came from an unknown PET source. The flakes consisted of 100% PET before the reground bottle flakes went through a separation and clean-up process. The flakes were not regranulated since this step involves separate machinery and would add to the cost of the reground polymer material. The intrinsic viscosity was 0.03 to 0.05 dl/g lower compared to that of virgin PET. Such difference is negligible. Only extreme variation in viscosities of melt-layers can lead to pro2 cessing problems. The size of flakes was in general between 9 and 49 mm .

E. H. Neumann

Figure 3. Bottle shape and the thickness of layers.

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PET Co-injection Molding

The trial processing For the trial approach, a NISSEI ASB-250 T-Series machine (multilayer type) was used. The processing parameters, as chosen, were within standard set-ups. The bottle shape for these trials and its layer distributionare shown in Figure 3.

Trial results The manufactured bottles were of a high quality. When supplied with a clean regrind, the transparency was similar to that of comparable monolayer bottles. Physical and mechanical properties of the multilayer bottles were the same as the monolayer types (creep under CO2 pressure, top-load, impact strength). Standard treatment like drop testing and bottle squeezing did not result in delamination of the layer structure. In conclusion, all trial results indicated that multilayer bottles had comparable properties to those having monolayer structures.

COST SAVINGS The cost saving in PET bottle production by co-injection technology, using an inexpensive polymer layer, is about 20% as calculated in Table 1. The calculation includes a higher investment cost of machinery for the co-injection technology as compared to the monolayer bottle production which employs less expensive monolayer machinery.

CONTAMINATION ASPECTS Several aspects regarding potential contamination of the middle layer made from the post-consumer PET bottle flakes, are discussed below.

Bacteriological contamination The majority of micro-organisms found on PET bottle surfaces is washed away during the cleaning process of the post-consumer reground PET flakes. The remaining minor amounts do not survive the drying and processing temperature o in the injection molding unit, which reaches 180 and 270 C, respectively.

Contamination by foreign substances There is a possibility that PET bottles are used for storage of substances which can cause danger to human health. Analysis was conducted, using health-endangering substances, to evaluate the effect of PET surfaces exposure. The migration and leaching of these substances were examined. It was found that, even

E. H. Neumann

25

Table 1 Manufacturing cost comparison of PET/PET-regrind bottles Container: round bottle, free-standing

Material: PET/regrind

Content: 1000 ml

Equipment/operation

Output

Resin price (DM/kg)

Bottle weight (g)

Price per bottle (DM)

NISSEI ASB-650 N PET 8 cavities (monolayer)

1,371

Virgin-injection-stretch-blow PET 3.20

35

0.1767

NISSEI ASB-650 NT 8 cavities (multilayer)

1,371

Regrind PET (bottle flakes) 1.30

35

0.1435-0.1501

Machinery type

NISSEI ASB

Unit DM/h

650 N (monolayer M/C)

650 NT (multilayer

650 NT (multilayer

M/C)

M/C)

1. Equipment cost amortization

48.57

2. Interest 8% p.a.

3.89

3. Energy consumption 0.15 DM/kWh (main equipment)

11.55

12.30

4. Energy consumption 0.15 DM/kWh (auxiliary equipment)

3.75

3.75

5. Rent for space 8 DM/m2 per month: 500 h

0.28

0.35

6. Labour (1/3 operator) 33 DM/h

11.00

11.00

7. Maintenance 20% of machine price

9.71

12.43

8. Entire cost per hour

88.75

106.97

106.97

9. Cost and output per hour (production cost) = DM/PC

0.0647

0.0780

0.0780

10. Total resin cost including scrap = DM/PC

0.112

0.0721

0.0655

100% virgin PET

62.17

60%

4.97 PET-regrind

70% PET-regrind 30% virgin PET

40% virgin PET

with strong toxic substances, like pesticides, the migration into PET surfaces is extremely low and furthermore, the re-migration rate (possible leaching of substances) is so low (a small fraction of the migration rate) that the values were well below the average daily intake specified by FDA.

26

PET Co-injection Molding

Furthermore, the PET flakes are exposed to a melting temperature in the injection barrel which would degrade most organic substances in the middle layer. In a new multilayer PET bottle, this middle layer is shielded by layers made of virgin PET. The contents of the bottle (food) are therefore separated by a layer of virgin PET from the middle layer. In the case of inorganic toxic substances which might have migrated into the PET surfaces in the ppb-range and furthermore survived the processing temperatures, these extremely small fractions will be diluted with the melt cake in the injection barrel and hot runner system.

CONCLUSIONS • • •

•

Pressure from environmentalists and legislations require creative solutions for re-using of packages. Discarded plastic packages can be reprocessed into articles similar to the originals. Co-injection technology, allowing for the injection of a layer of melt of reground PET flakes into the center of the preform wall, offers a technology of using low cost post-consumer regrind PET bottle resin. The layer of the melt of reground PET flakes is completely insulated in the middle of the bottle and therefore has no contact with the filled-in product such as food, nor with an outside environment. Contamination of filled-in products is therefore excluded. Cost savings by this method are about 20%.

F.P. La Mantia and D. Curto

27

Recycling of Post-consumer Greenhouse Polyethylene Films: Blends with Polyamide 6 F. P. La Mantia and D. Curto Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universitá di Palermo, Viale delle Scienze, 90128 Palermo, Italy

INTRODUCTION Problems connected with recycling of polymeric materials arise mainly from the heterogeneous nature of plastics waste and from degradation of polymers occuring either during their processing or their lifetime. As for the first point, important only when mixtures of plastics have to be recycled, it is well established that a strong incompatibility is typical for polymers usually found in plastics wastes (PE, PET, PVC, PS, PP). This incompatibility gives rise to materials which are very difficult to process and/or have inferior mechanical properties. Regarding the degradation experienced by polymeric materials, the formation of lower molecular weight fractions, possible presence of crosslinking, and oxygenated groups can further worsen properties of the recycled materials. Polyethylenes are the most popular polymers and as such are the most frequently recycled. In particular, films from greenhouses, although highly degraded by UV radiation, are recycled by means of well-established processes, leading to materials frequently used to manufacture films and molded products with low mechanical properties. Even more important problems in the recycling of greenhouse films arise from the presence of products of photooxidation, which modify both structure and 1-5 morphology of polyethylene, strongly affecting its mechanical properties. Especially, the presence of low molecular weight chains and of oxygenated groups affects the properties of a recycled material. 6,7 In previous works, a possibility of use of photooxidized polyethylene in blends with nylon 6, to improve blend compatibility, was demonstrated.

28

Greenhouse PE Film Recycling by Blending with PA

Table 1 Physico-chemical properties of polyethylene samples Sample code

Photooxidation time*

MFI

Gel

C=O

(h)

(g/10 min)

(%)

(mol⋅L )

PEPh0

0

0.08

0

0

PEPh24

24

-

11

0.07

PEPh48

48

-

19

0.11

PEPh72

72

-

25

0.28

-1

* photooxidation was carried out using eight UVB fluorescent lamps for 24, 48, and 72 h. The cycle adopted was: 8 h UV radiation at T = 60oC and 4 h condnesation at T = 50oC

In this paper, after a short review of the method, the properties and morphology of blends and coextruded films made out nylon 6 and recycled photooxidized polyethylene will be presented.

State of Art Blends of polyamides and polyolefines are potentially very interesting, but, because of the strong incompatibility of both polymers, the product of blending usually has poor properties. Many efforts have been devoted to compatibilize 8-12 these blends. Most of the methods consider functionalized polyolefines which can react with the amino groups of polyamides, giving rise to copolymers, stabilizing the blend. Such functionalization, in general a long and expensive step, is mostly performed by chemical modification of the polyolefine structure. 6 In a previous study, we have demonstrated that photooxidized polyethylene offer similar results. A low density polyethylene sample with different degree of photooxidation (see Table 1) was blended with nylon 6 (LDPE 25 wt%). The results of the Molau test are shown in Figure 1. The solution is clear in the case of blend PEPh0/Ny. The solution consists of nylon 6, which is soluble in formic acid. The upper part of the same test tube is a suspension of polyethylene particles. The tests regarding blends containing photooxidized polyethylenes show, on the contrary, an increasing and persistent turbidity which represents a suspension of colloidal particles.

F.P. La Mantia and D. Curto

29 11-13

Figure 1. Molau test. From left to right: PEPh0/Ny; PEPh24/Ny; PEPh48/Ny and PEPh72/Ny.

Following Molau and other authors, the presence of colloidal suspension is attributable to the existence of LDPE/Ny 6 graft copolymers, acting as interfacial agents. In our case, we believe that a chemical reaction between carbonyl groups (formed during photooxidation of polyethylene) and amino groups of nylon 6 occurred during melt blending. The suspension turbidity increases along with an increase in the photooxidation time of polyethylene, and it is also parallel to the concentration of carbonyl groups. The SEM micrographs of samples of blends containing virgin and the most extensively photooxidized polyethylene (Figures 2 and 3) confirm such an interpretation. In the blend containing virgin polyethylene, the polyamide forms the continuous phase and the polyethylene the discrete phase. The polyethylene particles have average dimensions ranging from 5 to 10 µm. The

Figure 2. SEM micrograph: PEPh0/Ny.

Figure 3. SEM micrograph: PEPh72/Ny.

30

Greenhouse PE Film Recycling by Blending with PA

Figure 4. Mechanical properties of blends. PEPh0/Ny, PEPh24/Ny, PEPh48/Ny, and PEPh72/Ny.

micrographs of blends containing photooxidized polyethylenes show an almost homogeneous phase and are hardly distinguishable as a result of good dispersibility. Elastic modulus, tensile strength and elongation at break are reported in Figure 4. Tensile strength and elastic modulus increase with an increase in the photooxidation time, whereas the elongation at break increases as the photooxidation time increases until it begins to decrease again, which occurs after 50 h of photooxidation. The elongation decrease, as will be explained in the following, can be attributed to a very low value of the elongation at break of a more extensively photooxidized polyethylene. The SEM micrographs and the results of the mechanical properties are consistent with the proposed chemical reactions between carbonyl groups and amino end-groups forming a graft copolymer. This copolymer acts as interfacial agent between continuous and dispersed phases. The presence of such an interfacial agent causes smaller dimensions of the dispersed particles and a good adhesion between discrete and continuous phases. These results suggest the possibility of using degraded (photooxidized) polyethylene, from recycling of films for greenhouses, as a functionalized polymer to obtain compatibilized nylon/polyethylene blends.

F.P. La Mantia and D. Curto

31

Table 2 Raw materials Sample

Supplier

Mw

Mw/Mn

Gel %

V

Enichem Polimeri

250,000

7.2

0

R1

-

-

-

40

R2

-

-

-

56

Ny

Snia

37,000

2.1

-

EXPERIMENTAL Materials The materials used in this work were: virgin polyethylene (V), nylon 6 (Ny), and two samples of recycled polyethylene (R1 and R2). These latter materials were obtained from films for greenhouses which were photooxidized to a great extent. The main physico-chemical properties of the raw materials are reported in Table 2. The blends were prepared by melt extrusion in a Brabender laboratory single-screw extruder (D = 19 mm, L/D = 25) at 100 rpm with a die temperature of o 260 C. The R2/Ny blend was also prepared by melt mixing of homopolymers in the same Brabender Plasticorder equipped with a mixer head model W 50 EH at o 260 C and 100 rpm for 15 min. The mixing time was sufficient to attain a practically constant value of torque. Homopolymers were subjected to the same treatment. All blends were prepared with 80 wt% of nylon 6. For comparison, a blend with 20% of nylon 6 was also prepared. Coextruded films were manufactured by using two layers coextruder Toyplast (Crespi, Italy) with a blowing unit. The coextruder was operated at a flow rate of about 20 kg/h. The thickness of the nylon layer was adjusted by the extruder rate, maintaining a layer ratio of about 4. o The die temperature was 215 C for all the runs. Runs were carried out with an axial draw ratio of 50 and a blow up ratio of 2 for all the blends.

32

Greenhouse PE Film Recycling by Blending with PA

Structural studies The gel content of two samples of recycled polyethylene was determined by means of a Soxhlet extractor. Approximately 0.3 g of any photooxidized polyethylene sample was extracted by refluxing p-xylene close to its boiling point for 48 h. Samples of all blends, fractured under liquid nitrogen, were observed, using a scanning electron microscope, Philips Model 505. The surface of the specimens was coated with gold. 11 Molau tests were carried out by dissolving 200 mg of sample in 10 mL of 80 % formic acid.

Mechanical properties Tensile properties and peel adhesion were determined, using an Instron machine Model 1122 at room temperature. A crosshead speed of 5 cm/min and a gauge length of 3 cm were used to obtain the stress-strain curves. The specimens used for tensile tests were cut out of sheets obtained by como o pression molding at T = 240 C (180 C for the pure polyethylene). All the reported results are an average of at least 10 measurements. Impact strength was determined at room temperature on notched samples using a Fractoscope (CEAST) in an Izod mode. Ball drop measurements were carried out using a Ceast apparatus. Before testing, the specimens were o equilibrated in ambient conditions (T = 20 C and 60% R.H.) for at least 3 days.

RESULTS AND DISCUSSION Blends Figure 5 shows the results of the Molau tests of the following blends: V/Ny, R1/Ny, and R2/Ny prepared by extrusion. The solution is clear for the blend V/Ny, consisting of nylon 6 dissolved in formic acid. The upper part is a suspension of polyethylene particles. The tests of blends containing recycled photooxidized polyethylene show a turbidity which is due to a suspension of colloidal particles. As already pointed out, this colloidal suspension is caused by the presence of polyethylene/nylon graft copolymers formed during melt extrusion in chemical reaction between carbonyl groups of the recycled photooxidized polyethylene and amino groups of nylon 6.

F.P. La Mantia and D. Curto

Figure 5. Molau test. From left to right: V/Ny, R1/Ny, R2/Ny prepared by melt extrusion.

33

Figure 6. Molau test. From left to right: R2/Ny by melt extrusion, R2/Ny (Ny = 20%), and R2/Ny by melt mixing.

As the degradation degree of the polyethylene rises, the suspension turbidity also increases. The R2 sample has a larger amount of C=O groups and thus its blend shows a more intense turbidity. The results of Molau test of R2/Ny blend, prepared by extrusion and melt mixing, are reported in Figure 6. The blend prepared by melt mixing shows a more intense turbidity. The influence of a composition was tested for blend, having 80 wt% polyethylene. The Molau test (Figure 6) shows that at the low content of nylon, no turbidity is observed.

34

Greenhouse PE Film Recycling by Blending with PA

Figure 7. SEM micrographs: (a) V/Ny, (b) R1/Ny, (c) R2/Ny, prepared by extrusion, (d) R2/Ny prepared by melt mixing.

The SEM micrographs of the same samples are given in Figure 7a-d. The blend containing virgin PE (Figure 7a) has polyethylene particles of average dimension ranging from 5 to 30 µm. Furthermore, a negligible adhesion between the two phases is observed. The micrographs of the samples with recycled PE show that the dimensions of the discrete phase decreases with an increase in the degradation degree of the polyolefines (Figures 7b and c) and with severity of the mixing process (mixing vs. extrusion) (Figures 7c and d). Moreover, the adhesion improves with the

F.P. La Mantia and D. Curto

35

Table 3 Mechanical properties of polyethylene/nylon blends Sample

E (MPa)

TS (MPa)

EB (%)

IS (J/m)

V/Ny

700

30

140

110

R1/Ny (extr)

800

25

30

120

R2/Ny (extr)

1000

30

10

125

R2/Ny (mix)

1200

46

25

135

Table 4 Elongation at break of polyethylene samples Sample

EB (%)

V

490

R1

180

R2

110

same trend. In particular, the blend with R2 prepared by melt mixing (Figure 7d) shows an almost homogeneous phase. All the above results indicate that the formation of graft copolymers increases with: • the photooxidation degree of the polyethylene • the content of nylon • the intensity of the mixing. The presence of graft copolymers, acting as reinforcing interphase between the two polymers, improves some mechanical properties of these blends, compared with blends which were not compatibilized. Modulus, E, tensile strength, TS, elongation at break, EB, and impact strength, IS, for all blends are reported in Table 3. Modulus and impact strength can be increased by the use of recycled degraded polyethylene and, for blends with the same components, by increasing the mixing intensity (mixer vs. extruder).

36

Greenhouse PE Film Recycling by Blending with PA

Table 5 Mechanical properties of compatibilized PE/Ny blends Sample

E (MPa)

TS (MPa)

EB (%)

IS (J/m)

FPE/Ny*

79

52

40

70

R2/Ny (mix)

120

46

25

135

*FPE - functionalized polyethylene (taken from Ref.13)

Table 6 Mechanical properties of coextruded films TS (MPa)

EB (%)

BD (cN)

Peeling (cN/m)

450

105

80

440

165

900

draw

transv

draw

transv

V/Ny

31

28

240

R2/Ny

21

23

500

Tensile strength is only significantly improved when the blending process is performed in the mixer. The elongation at break is drastically worsened when recycled polyethylene is used. In this case, however, it should be noted that the elongation at break of the polyethylene dramatically decreases with an increase in the photooxidation degree (see Table 4). The elongation at break of blends R1/Ny and R2/Ny drastically decreases, due to a low elongation at break of the recycled polyethylene. Nevertheless, the blend with R2 prepared by melt mixing shows a value higher than that of the same blend obtained by extrusion. This is probably due to the larger amount of compatibilizer formed during this severe processing. The mechanical properties of R2/Ny blend are similar to those reported for blends of nylon with a functionalized polyethylene (FPE) (these latter data are taken from Ref. 13) (see Table 5). Even if the results relative to this blend have been obtained for dry samples, both modulus and impact strength of the blend with more photooxidized PE are better than those of the blend with the functionalized PE, whereas TS and EB are only slightly lower.

F.P. La Mantia and D. Curto

37

Coextruded films In Table 6, the mechanical properties of two coextruded films are reported for samples cut out in the draw direction and in the transverse direction. The tensile properties in the draw direction of the V/Ny coextruded film are significantly larger than those of the R2/Ny film, whereas in the transverse direction they are similar. This unusual behavior is probably due to the different rheological properties of two PE samples. Indeed, the film blowing operation has been carried out using the same processing parameters, which give rise, in this case, to more oriented film in the case of virgin polyethylene. This hypothesis is confirmed by the results of the ball drop, which are better for the more balanced R2/Ny film. The more interesting result is, however, related to the improvement of a peeling value for the recycled material. This means that two phases at the exit of the die adhere much better in presence of photooxidized polyethylene because of interaction between the carbonyl groups of the R2 sample and the NH2 groups of the nylon.

CONCLUSIONS The above results show that use of the recycled polyethylene in blends with nylon, giving rise to PE/Ny graft copolymers during processing, improves the mechanical properties of the resultant material. The graft copolymers act as compatibilizing agents and the properties of nylon-rich blends are very similar to those of blends compatibilized by functionalized polyethylene. A good adhesion between the two layers of coextruded films helps to avoid a need for the addition of a third layer binding two incompatible phases.

ACKNOWLEDGMENT This work has been financially supported by MURST 60% and by Plastionica SpA.

REFERENCES 1. 2. 3. 4. 5.

J. F. Heacock, F. B. Mallory, and F. B. Gay, J. Polym. Sci., 6, 2921 (1968). J. M. Adams, J. Polym.Sci., 8, 1279 (1970). M. U. Amin, G. Scott, and L. M. K. Tlillekeratne, Eur. Polym. J., 11, 85 (1975). F. P. La Mantia, Radiat. Phys. Chem., 23, 699 (1984). F. P. La Mantia, Eur. Polym. J., 20, 10 (1984).

38 6. 7. 8. 9. 10. 11. 12. 13.

Greenhouse PE Film Recycling by Blending with PA D. Curto, A. Valenza, and F. P. La Mantia, J. Appl. Polym. Sci., 39, 865 (1990). F. P. La Mantia and D. Curto, Polym. Deg. Stab., 36, 131 (1992). F. Ide and A. Hasegawa, J. Appl. Polym. Sci., 18, 963 (1974). S. Cimmino, L. D’Orazio, R. Greco, G. Maglio, M. Malinconico, C. Mancarella, E. Martuscelli, R. Palumbo, and G. Ragosta, Polym. Eng. Sci., 24, 48 (1984). S. Cimmino, F. Coppola, L. D’Orazio, R. Greco, G. Maglio, M. Malinconico, C. Mancarella, E. Martuscelli, and G. Ragosta, Polymer, 27, 1874 (1986). G. E. Molau, J. Polym. Sci., A3, 1267 (1965); Kolloid Z.Z. Polym, 238, 493 (1970). G. Illing in Polymer Blends: Processing Morphology and Properties, Eds., E. Martuscelli, R. Palumbo and Kryszewski, Eds., New York, 167 (1980). H. K. Chuang and C. D. Han, J. Appl. Polym. Sci., 30, 165 (1985).

E. Gattiglia et al.

39

Recycling of Plastics from Urban Solid Wastes: Comparison Between Blends from Virgin and Recovered from Wastes Polymers E. Gattiglia, A. Turturro, A. Serra, S. Delfino*, and A. Tinnirello* Centro di Studi Chimico-Fisici di Macromolecole Sintetiche e Naturali e Istituto di Chimica Industriale, Corso Europa 30, 16132 Genova, Italy * Enichem Polimeri, Centro Tecnologico, Via Iannozzi 1, 20192 S. Donato Milanese (Mi), Italy

INTRODUCTION The recycling of plastic wastes is not a recent problem for plastic users and producers. Since long, industrial scraps are recycled within the production cycle itself or recovered as lower grade materials and refabricated into new products. On the contrary, plastics recovered from urban solid wastes are being considered only since recent times. A general problem of waste management is becoming more and more relevant. Traditional disposal policies such as landfill and incineration are facing increasing opposition due to the ecological drawbacks and rising costs. Recycling, i.e. reprocessing and reshaping of plastic wastes into a new object seems the logical solution “to close the plastic cycle” which will be fully exploited in the future. The major problem still facing the viable recycle of plastic wastes is their chemical multiplicity: several different polymers are present in plastic wastes and many of them are mutually incompatible, making the reprocessing of them, as a whole, practically impossible. On the other hand, the separation of the individual plastics is extremely expensive and must consequently be simplified as much as possible. Therefore, many separation methods currently used, such as for instance flotation, cannot provide the individual components but only fractions which may contain several different polymers. Such heterogeneous mixtures are often scarcely compatible, giving rise to processability and quality problems.

40

Recycling of Plastics from Urban Solid Wastes

By using the flotation method, generally two fractions are obtained from the plastic wastes: a light fraction, floating on water, and a heavy fraction (density 3 >1 g/cm ). The first fraction is essentially made of low and high density poly(ethylene) (LPDE and HDPE), polypropylene (PP) and high impact polystyrene (PS); the heavy fraction is formed by polyvinylchloride, cross-linked resins, high melting thermoplastics. As well known, polymers of such fractions are incompatible, so the mechanical properties of their blends are expected to be poor. Nevertheless, the light fraction is at present used to produce different kinds of products. In order to obtain products with expected mechanical properties, people often overdesign them or limit their use to “safe” applications, therefore reducing market potentiality. From the fundamental point of view, a multitude of studies on polyolefin blends have been reported in literature: binary blends of LDPE/HDPE and 1-11 LDPE or HDPE mixed with PP, polyvinylchloride, PS, etc. as well as ternary 1,12 mixtures of LDPE/HDPE/PP have been examined. Several aspects have been investigated, such as crystallization and melting, rheology, mechanical properties and morphology. Since polyolefins represent the large majority of polymers in the plastic wastes, some of the above works were specifically focussed on the 1,3-5 wastes. In this study we will report results concerning a wide analysis of: • • •

quaternary blends of virgin LDPE/HDPE/PP/PS, in which LDPE is the major component, as in the plastic wastes a light fraction from the urban solid wastes 13,14 a blend of HDPE with the heavy fraction recovered from wastes as well.

The aims of our research are the following: • • • •

to understand why the incompatible polymers of the light fraction give rise, without compatibilizing agents, to a material with useful final properties to establish the lowest content of the major component (LDPE) in the blend required to obtain always a product with good properties to estimate the influence of possible impurities on the mechanical performance of the light fraction from the municipal solid wastes to test a possible use of the heavy fraction as filler for virgin HDPE.

E. Gattiglia et al.

41

EXPERIMENTAL Materials The virgin polymers used in this study are described in Table 1 as well as the composition of their mixtures. Note that LDPE is a blend of two LDPE’s having different molecular weights and PS is a high impact PS, containing some elastomers selected to improve similarities with real wastes components. Table 1 Some physical characteristics of homopolymers and compositions of the studied mixtures Polymer

Trade name

MFI

Density 3

Mixture

(g/10 min)

(g/cm )

1

2

3

4

LDPE

Riblene CF 2200

2

0.917

70

63

49

42

LDPE

Riblene EF 2200

0.7

0.917

20

18

14

12

HDPE

Eraclene H ZB 5515

0.3

0.963

10

9

7

6

PP

Moplen X 305

7.5

0.895

-

8

24

32

PS

BASF Polystirol 454C

1.53

1.003

-

2

6

8

Table 2 Blends of the plastic fractions from urban solid wastes Polymer

Composition wt% Mix 5*

Mix 6

LDPE + HDPE

91

-

PP

3

-

PS

6

-

HDPE Eraclene HZB 5515

-

75

Heavy fraction

-

25

Light fraction

*composition estimated by thermal analysis

42

Recycling of Plastics from Urban Solid Wastes

By the flotation method, a light and a heavy fractions were separated from urban plastic wastes in an industrial plant. In Table 2, Mix 5 corresponds to the 15 light fraction, whose composition was estimated by thermal analysis; Mix 6 is a blend of virgin HDPE with 25 wt% of the heavy fraction.

Blend Preparation 2

Before blending, the light fraction films were ground to pieces of a few mm and the heavy fraction to particles of about 100 µm size. The blends were prepared in o a Krauss-Maffei double screw extruder at T = 180 C and a rotation speed of 20 rpm. The extrudates were injection molded with a Negri and Bossi nb 55 mao chine, at T = 220 C in a cold mold and specimens for mechanical tests were produced.

Rheological measurements The shear viscosity was measured according to ASTM D3835 in a gas pressure o viscometer at T = 190 C. Die of diameter 0.5 mm and L/D ratio of 10 was used. The melting flow index, MFI, was determined according to ASTM D 1238/c in a Karl Frank model 73694.

Density The homopolymer and blend densities were measured using a gradient column filled with water and isopropyl alcohol in such a proportion as to obtain a density 3 range from 1.000 to 0.870 g/cm , at a room temperature.

Mechanical properties The tensile properties were determined with an Instron Dynamometer model 1122 at a crosshead speed of 5 mm/min according to ASTM D638. The flexural modulus was measured in an Arquati Dynamometer AG7E at a speed of 100 mm/min and a distance of 50.8 mm according to ASTM D790. All results are average of at least 10 measurements. The IZOD impact tests were performed aco o cording to ASTM D256 on a CEAST pendulum model 6545/000 at T = 30 C, 0 C o and 23 C. The last results are average of 15-20 measurements.

Morphology The samples were cryogenically fractured and the surface fractures were gold coated and observed in a Scanning Electron Microscope, model Cambridge Stereoscan MK 250.

E. Gattiglia et al.

43

Figure 1. Viscosity of homopolymers vs. shear rate at 190oC.

Thermal analysis The melting and crystallization behaviors were studied in a Mettler TA 3000 DSC. The non-isothermal crystallization was performed as follows: heating at o o 20 C/min up to 200 C, 3 min. of dwelling time, cooling down at cooling rates vario able from 30 to 1 C/min.

RESULTS AND DISCUSSION Rheology The rheological behavior of polyolefin blends has been widely studied by many authors. It was found that the shear viscosity exhibits maxima and minima when plotted as a function of composition. In general, the viscosity vs. shear stress trends for the binary blends, which present negative (PS/LDPE, PS/PP, 16-21 22 PS/HDPE) and positive (PP/HDPE) deviations, cannot be super-imposed into a master curve by a simple horizontal shift. In other words, the shape of dependence changes with composition.

44

Recycling of Plastics from Urban Solid Wastes

Figure 2. Viscosity of blends vs. shear rate at 190oC.

Table 3 Density and melt flow index measured at 190 oC and calculated by the additivity law Mixture

MFI (g/10 min)

Density 3

experimental

calculated

(g/cm )

Mix 1

1.19

1.57

0.9234

Mix 2

1.31

2.04

0.9261

Mix 3

1.81

2.01

0.9261

LDPE CF/LDPE EF = 77.8/22.2

1.71

0.92

-

In our case, also considering the rather complicate composition of the mixtures, we do not intend to fully study the rheological characteristics of these materials, but simply to check whether such blends may present melt properties which can prevent processability with usual machinery. Figures 1 and 2 show the viscosity, η, vs. shear rate, γ&, of homopolymers and their blends, respectively. The addition of PP and PS reduces the viscosity of the blends in the range of

E. Gattiglia et al.

45

Figure 3. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 81/9/8/2 blend.

shear rates under investigation and then increases the value of the melt flow index. In Table 3 experimental MFI values are compared with calculated values assuming the additivity law. The experimental values are lower than calculated suggesting that weak interactions among components exist, due to a similar chemical structure of blend ingredients. Therefore, it seems that the processability of blends does not present particular problem.

Density In Table 3 the density values of the Mix 1, 2, and 3 are presented. These values are about 0.5% higher than those calculated according to the (additivity rule) weighted average of contributions of several components, assuming that the degree of crystallinity of crystallizable polymers is not affected by the blending. It can be thus deduced that the blends have a compact structure, without holes or voids.

Morphology The morphologies of the fracture surfaces of the Mix 2, Mix 3, and Mix 4 are shown in Figures 3, 4, and 5. The morphology of Mix 1, not reported here, looks

46

Recycling of Plastics from Urban Solid Wastes

Figure 4. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 63/7/24/6 blend.

Figure 5. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 54/6/32/8 blend.

E. Gattiglia et al.

47

Figure 6. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 180oC. Parallel Nicols. Magnification 200×.

like a single phase material, meaning that the two components are indistinguishable in such composition. As it will be discussed later, HDPE certainly gives rise to homodomains inside the LDPE matrix because it crystallizes before LDPE. However, from the point of view of the morphology LDPE/HDPE 90/10 mixture will be considered homogeneous. Mix 2 presents a ductile fractured PE matrix with inclusions of PP and PS not easily recognizable. The same morphological characteristics were observed in the case of Mix 5 from plastic wastes. The morphology of Mix 3 (Figure 4) presents clearly visible PS and PP domains. PS domains break with a typical rough, globular surface while PP domains, whose size distribution is broad, on fracture, expose a circular cross-section with smooth surface. On increasing the amount of PP (Figure 5) to 32% (Mix 4) this component almost tends to form a co-continuous matrix with PE’s; PS is still visible as separated spherical particles. It is important to notice that in all blends PS and PP domains are well locked into the matrix. The interfacial adhesion is good and no voids are observed at the phase boundary. Of course the structures of the blends are not in thermodynamic equilibrium, as can be seen by melting and molding the samples in different processing conditions. However, no morphological modifications or shrinkage phenomena were observed by annealing of the mixtures at about o 100 C for more than two months.

48

Recycling of Plastics from Urban Solid Wastes

Figure 7. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 140oC. Crossed Nicols. Magnification 200×.

Phase segregation is present also in the melt, as can be observed by the optical o microscope. Figure 6 clearly shows segregated PS droplets at 180 C, even at a few percent of PS as in Mix 3. On the contrary, PP domains in the molten state are visible only when its content is higher than 10 wt%. However, cooling down slowly one can follow the crystallization of PP as shown in Figure 7 for Mix 3 at o 140 C.

Crystallization behavior Since the crystallization process is very important in controlling the morphology and thus the mechanical properties, we will discuss in more detail the melt behavior during cooling. In Figures 8 and 9 the effect of cooling rate on the crystallization temperature, Tc, of the LDPE, HDPE, and PP pure and in mixture, is shown. o The point of interest here is the fact that for cooling rate higher than 1 C/min. the Tc of HDPE is higher than PP and both are, as known, well above that of the LDPE. Only at very low cooling rates, PP crystallizes before HDPE and probably o at very fast cooling rate (> 40 C/min), they crystallize at the same time. In Figure 8, the crystallization peaks of PP and HDPE components of Mix 2 merge

E. Gattiglia et al.

Figure 8. Crystallization temperatures of pure homopolymers and LDPE/HDPE/PP/PS = 81/9/8/2 blend as a function of cooling rate.

Figure 9. Crystallization temperatures of pure homopolymers and LDPE/HDPE/PP/PS = 54/6/32/8 blend as a function of cooling rate.

49

50

Recycling of Plastics from Urban Solid Wastes

into one and only one Tc is detected in the range of cooling rate examined. This simultaneous crystallization of HDPE and PP is only due to particular conditions of cooling and blend compositions. In fact, increasing the PP content (Mix 4) two o peaks occur, when the cooling rate is less than 10 C/min (Figure 9). A comparison between the crystallization behaviors of the Mix 5, prepared from the light fraction of plastic wastes, and Mix 2, from virgin polymers only puts into evidence that LDPE of Mix 5 crystallizes earlier than LDPE of Mix 2. This agrees well with the smaller crystalline grains observed in optical microscope and may be attributed to some nucleating power of the present impurities. The Tc‘s of PP and HDPE in the blends are lower than those of the pure homopolymers. On the contrary the Tc of LDPE in the blend is a few degrees higher than that of the single component and the difference increases on increasing the cooling rate. This behavior is understandable considering the favorable effect of the already crystallized HDPE and PP on the nucleation process of the LDPE. Therefore, the LDPE matrix crystallizes always when HDPE and PP are solid and PS is below its glass transition. During this crystallization a volume reduction occurs and the matrix shrinks over the domains of the dispersed phases clinging them together very solidly. This is the main reason of a good contact between the matrix and the different polymer domains, observed in the electron microscope analysis.

Mechanical properties Tensile behavior Values of tensile modulus, E, yielding stress, σy, tensile strength, σb, and elongation at break, ε b, of the homopolymers and mixtures are presented in Table 4. Modulus values of the homopolymers scatter by about ±5% whereas for blends by about ±8%. The scatter of σb and σy data ranges from ±12% for homopolymers to about ±18% in the case of blends values; ε b results have wider scatter ranging from ±20 to ±27%. Reducing the amount of LDPE in a blend, the modulus and the yield stress increase, whereas σb does not practically change and ε b seems to reach a maximum for the Mix 3 having 70% PE. The Mix 5, prepared with the light fraction of plastic wastes, shows practically the same tensile modulus and strength as pure homopolymers (Mix 2); however, samples break before reaching the high deformation of Mix 2, probably due to defects created by impurities not completely removed during the flotation process. The increase of the E modulus, reducing the percentage of LDPE, is well below the additivity rule prediction, as shown in

E. Gattiglia et al.

51

Table 4 Tensile characteristics of injection molded specimens Sample

E (MPa)

σy (MPa)

σb (MPa)

εb (%)

LDPE CF

119

-

12

613

LDPE EF

143

-

13.8

548

HDPE

640

-

30.4

960

PP

704

-

25.2

720

PS

850

-

15.1

60

Mix 1

151

10.6

13.4

353

Mix 2

196

11.5

13.5

377

Mix 3

264

16.1

14.4

440

Mix 4

330

17.9

13.9

147

Mix 5

190

-

12.0

80

Mix 6

1026

-

22.2

-

Figure 10, even if the morphology does not reveal any kind of holes or voids between the dispersed phase domains and the matrix. If we consider the mixtures as a matrix-filler composite in which the matrix is the LDPE/HDPE blend and the fillers are PP and PS taken together, it should be possible to compare our mechanical data with the model developed for polymer-filler systems by Nielsen. We recognize that this approximation is quite simplistic, because the difference between the moduli of LDPE and PP-PS components is not as high as in the case of a polymer matrix and an inorganic filler. Nevertheless, we think that this approach can be attempted making the following assumptions: • the matrix is a mixture of LDPE and HDPE in the constant weight ratio 9/1 for all blends, and its modulus is that experimentally measured for the Mix 1 (≈ 151 MPa) • the filler consists PP and PS in a constant ratio 4/1 and its modulus is taken as the weighted average (≈ 739 MPa) of the moduli of two components.

52

Recycling of Plastics from Urban Solid Wastes

Figure 10. Comparison between experimental and calculated tensile moduli of the mixtures. 23,24

For a polymer-filler system, the Nielsen model equation: E b = E m (1+ ABφ f ) / (1 − Bφ f ψ )

is described by the following [1]

where: A = K Ef − 1

[1a]

B= (E f / E m − 1) / (E f / E m + A)

[1b]

ψ = 1+ (1 − Φ max ) / (Φ 2max ⋅φ f )

[1c]

E. Gattiglia et al.

53

and: Eb, Em, Ef are the blend, matrix, and filler modulus, respectively; KE is the Einstein’s constant, depending on the geometry and size of the filler particles, as observed from the SEM morphology; f is the correction factor related to the Poisson’s ratio, ν, of the matrix; φf is the volume fraction of the filler; Φ max is the maximum packing fraction of the filler. The SEM pictures offer evidence that the geometry of the dispersed phase is complex, due to the presence of ellipsoid and sphere shaped domains. We have applied the Eq [1] taking into account spheres and ellipsoids with aspect ratio 24 r = 4. According to Nielsen, KE = 2.5 and φmax = 0.60 for random loose packed spheres and KE = 3.08, φ = 0.6 for random packed rods or ellipsoids were used. Moreover, f = 0.9 was assumed on the base of the matrix ν= 0.4. Figure 10 shows the experimental data of Eb and the calculated values as a function of the volume fraction, φf, of the filler (PP + PS). The trends are similar and suggest that PP and PS have a reinforcing effect on the PE matrix and the stress transfer at low strain is good. However, the experimental values are higher than the calculated values, and a difference between them increases with increasing φf. This must be attributed to the geometrical shape of the dispersed PP domains, which tend to become more and more elongated.

Flexural modulus The flexural modulus and the impact strength are very important properties in the field of applications of recovered plastics. Table 5 presents data on the mechanical properties for single components and blends. The increase of flexural modulus by addition of more rigid polymers is evident, confirming that the stress transfer between the phases is good also in the flexural mode.The flexural modulus of Mix 5 is practically equal to that of Mix 2 if one considers the experimental uncertainty.

Impact resistance The impact strength, reported in Table 5, shows a very strong dependence on phase heterogeneity and on the presence of rigid inclusions in LDPE. Although the impact properties of the blends are generally decreased, when LDPE content decreases, the effect is dramatic at low temperature which requires special attention if such application is required. Nevertheless the absolute values are still acceptable for most applications. Data of mixtures containing less than 50% LDPE (Mix 3 and Mix 4) indicate very poor impact properties suggesting that

54

Recycling of Plastics from Urban Solid Wastes

Table 5 Flexural modulus and impact strength of injection molded specimens Sample

Flexural modulus

IZOD (J/m) o

(MPa)

-23 C

0C

o

30 C

o

LDPE CF

182

n.b.

n.b.

n.b.

LDPE EF

216

n.b.

n.b.

n.b.

HDPE

1035

196*

212*

268*

PP

1143

4

24

33

PS

1709

44

53

72

Mix 1

223

58

n.b.

n.b.

Mix 2

273

31

78*

n.b.

Mix 3

511

19

29*

136*

Mix 4

607

20

24

200

Mix 5

256

-

-

220

Mix 6

3381

-

-

56

n.b. = no break *partially broken

50% LDPE is the lower limit for acceptable properties balance. Once again, the detrimental influence of the impurities is the cause of a low impact resistance at o 30 C (220 J/m) of Mix 5 compared to Mix 2, which does not break. This suggests that more precise separation and washing stages are needed before processing the blend.

HDPE/heavy fraction blend The HDPE/heavy fraction blend, 75/25 was examined to evaluate a possibility of reusing of a heavy fraction as a filler of HDPE to obtain extrudates with appropriate rigidity. As indicated before, the heavy fraction was first ground to

E. Gattiglia et al.

55

particles of about 100 µm and then blended with HDPE pellets in a double-screw o extruder at 180 C. At this temperature, all the components (PVC, PET, nylons, crosslinked polymers, etc.) are still solid in the molten HDPE. Morphological observation by SEM shows that the particles distribution is not homogenous. This is probably the main reason for poor performance of a blend at high deformations. In fact, both the tensile and the flexural moduli of HDPE improve significantly, as shown in Tables 4 and 5, while the strength at break and the impact resistance of the blend are much lower compared to pure HDPE. Perhaps, a change of some parameters of the blending process and optimization of a filler content or reduction of a percentage of the heavy fraction offer a possibility to prepare HDPE-based materials, having technological properties suitable for extrudates, like pipes used for general purposes.

CONCLUSIONS Although LDPE, HDPE, PP, and PS are incompatible polymers and their blends show phase separation, the mixtures containing more than 60% LDPE maintain good mechanical properties. In general, the introduction of HDPE, PP, PS in LDPE causes an increase in stiffness, flexural modulus and strength at break and an obvious reduction in the ultimate elongation and impact resistance. The latter seems to be the most critical characteristic to be evaluated for applications. The reason for this behavior is explained by a very good adhesion between the LDPE matrix and the dispersed phases, obtained during a melt solidification process of the matrix. The presence of a LDPE matrix as a binding agent is indispensable for a transfer of the mechanical stress. As a consequence, with LDPE content decreasing below 50 %, the mechanical properties of the material become affected. The materials can be grossly schematized as composites in which PE is the matrix while PP and PS are the fillers, with a good adhesion at the phases interface.Within this phase frame, the tensile modulus fits reasonably well to the theoretical prediction. Compared to blends made from virgin materials, the real waste recovered plastics present obvious problems due to the presence of impurities but the overall properties are still acceptable.

56

Recycling of Plastics from Urban Solid Wastes

The properties can be modulated by varying the LDPE content in the mixture which allows one to reprocess and reuse the light fraction from plastic wastes from various sources whose original composition is outside the proposed optimized range. The possible alternative approach for a reuse of the heavy fraction, as filler of the HDPE, can be considered. Approach is promising but needs further work to optimize the processing steps, with special attention to the optimization of a filler size and blending conditions.

ACKNOWLEDGMENTS Special thanks to Dr. S. Astengo for a part of the experimental work and to Mr. G. Dondero for precious assistance with the Scanning Electron Microscope.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

R. E. Robertson and D. R. Paul, J. Appl. Polym. Sci., 17, 2579 (1973). D. R. Paul, C. E. Vonson, and E. C. Locke, Polym. Eng. Sci., 12, 157 (1972). O. Laguna, O. Castellanos, and E. P. Collar, Resources, Conservation and Recycling, 2, 37 (1988). O. Laguna, E. P. Collar, and J. Taranco, J. Polymer Eng., 7, 169 (1987). O. Laguna, E. P. Collar, and J. Taranco, J. Appl. Polym. Sci., 38, 667 (1989). A. P. Plochocki, Polymer Blends, vol. 2, Eds., D. R. Paul and S. Newmann, Academic Press, New York, (1978). A. K. Gupta and S. N. Purwar, J. Polym. Sci., 30, 1799 (1985). T. Kyn and P. Vadhar, J. Appl. Polym. Sci., 32, 5575 (1986). K. Min, J. L. White, and J. F. Fellers, J. Appl. Polym. Sci., 29, 2117 (1984). D. W. Clegg, A. A. Collyer, and K. Morton, Polymer Comm., 24, 10 (1983). R. Wycisk, W. M. Trochimczuk, and J. Matlys, Eur. Polym. J., 26, 5 (1990). L. Bohn, Rubber Chem. Technol., 41, 495 (1968). S. Astengo, Thesis, University of Genoa, (1989). A. Serra, Thesis, University of Genoa, (1991). C. Perrone, Poliplasti (Milan), 5, 72 (1987). Y. Shimomura, J. E. Spruiell, and J. L. White, Polym. Eng. Rev., 2, 417 (1983). C. D. Han and J. E. Kim, Trans. Soc. Rheology, 19, 254 (1975). L. A. Utracki and M. R. Kamal, Polym. Eng. Sci., 22 (2), 96 (1982). C. D. Han and Y. W. Kim, J. Appl. Polym. Sci., 19, 2831 (1975). B. L. Lee and J. L. White, Trans. Soc. Rheology, 19, 481 (1975). N. Alle and J. Lyngaae - Jørgensen, Rheolog. Acta, 19, 94 (1980).

E. Gattiglia et al. 22. M. Kasajima, A. Suganuma, D. Kunii, and K. Ito, Proceed. Intl. Conf. Polym. Process., Cambridge, Mass. (1977). 23. T. B. Lewis and L. E. Nielsen, J. Appl. Polym. Sci., 14, 1449 (1970). 24. L. E. Nielsen, Mechanical Properties of Polymers and Composites, Vol. 2, Marcel Dekker, New York, (1974).

57

O. Laguna Castellanos et al.

59

Management of Plastic Wastes: Technical and Economic Approach O. Laguna Castellanos, E. Pérez Collar, and J. Taranco González Instituto de Ciencia y Tecnología de Polímeros, U.E.I., Tecnología de Plásticos. Grupo de Ingeniería de Polímeros, c/Juan de la Cierva, 3, 28006 Madrid, Spain

INTRODUCTION It seems that a general agreement has been reached on the real recycling possibilities of plastic wastes, considering the place and the manner in which wastes are generated and restriction to thermoplastic polymers. The basic principles of recycling are included in studies conducted during the 1980s which considered the technical validity of recycling of various plastic wastes. Most of these studies have been carried out by corporate research and they are covered by patents or 1-12 sold in the form of utilities. Plastic wastes or scraps are generated from two main sources: industrial wastes and post-consumer wastes. The problem of the industrial wastes was addressed from the beginning by the companies which generated them in order to improve the economics of the process. Present public interest in the environmental impact has affected further strategies of big companies. Their knowledge on the recyclability of their industrial wastes was applied to solve the early 13-15 steps of the recyclability of the products manufactured by these companies. It is important to mention that economic aspects play a secondary role under the expectations of regulations and laws which control recycling of plastic wastes. The second and the biggest source of plastic wastes generation, which creates the real problem, is termed “post-consumer wastes”. Three kinds of wastes are generated: municipal, agricultural, and uncontrolled plastic wastes. The last group of plastic wastes must not be considered in the sense of a technical problem because they can only be avoided due to common consent of users of public

60

Management of Plastic Wastes

facilities. The agricultural plastic wastes can be collected and well classified in the place of their generation. After their proper characterization, their recyclability can be determined. Municipal plastic wastes are the most visible. A social cost is inherent in the disposal of all kinds of wastes and so it is generally recognized to the extent that wastes can be reduced and treated for reuse or recycling in specific ways considered attractive by consumers. On the other hand, many advantages are derived from the application of polymers in the packaging industry, even considering waste disposal: • In economic terms, the costs of the packaging industries would be increased by approximately four hundred percent by the weight if non-plastic materials were used. • An increase by two hundred and fifty percent by volume of wastes would become real if plastic packaging materials were not used. • Approximately two hundred percent increase in energy consumption and costs of materials is feasible for packaging without plastics. As a consequence, plastics offer many advantages in packaging materials. • The total amount of plastics present in municipal solid wastes is estimated to be currently about 7% of total waste. The present paper shows the scheme followed in the development of a strategy of study and determination of recycling feasibility of the plastic waste fraction.

Recycling of urban plastic wastes 1

The initial step, i.e., source identification, was previously discussed. Once the source is identified, the first question regards the composition of the source: • if plastics are mixed with other materials (glass, paper, organic), a separation is needed • if plastics are dirty (clays or similar contaminants), wastes must be cleaned •

if the plastic waste consists of a polymer blend, the situation is much more complex. The problem was studied by comparison between the polymer blend of the plastic waste fraction and polymer blend, having similar composition, but obtained from virgin polymers. The technology was applied to the film plastic waste fraction present in urban solid wastes from Madrid (Spain) and was developed during the 1983-1986 period.

O. Laguna Castellanos et al.

61

Figure 1. Flow diagram of treatment and separation plant for municipal solid wastes.

The first step of separation of plastic wastes from the overall waste stream was 4 successfully achieved in an industrial plant having a capacity of 7×10 tons/year which was designed by ENADIMSA (Spain) and operated in Madrid according to the layout given in Figure 1. The economic evaluation, once the recyclability

62

Management of Plastic Wastes

was determined, was made taking into account the actual sale price of the wastes based on the amortization costs and profitability of the plant during the 1983-86 period. A preliminary economic study was conducted using a classic method based on Engines and Machinery. The results obtained were then taken as starting data for more realistic evaluation, based on the state of the Spanish polymers market in the 1985-1988 period. Only the main results of these works will be presented here, with the goal of showing that the plastics recycling business is also very attractive from an economical point of view.

EXPERIMENTAL Materials The film plastic wastes were supplied by ENADIMSA from the Municipal Treatment Plant of Urban Solid Wastes in Valdemingómez (Madrid). The identification of polymers present in plastic wastes was carried out by IR and DSC methods. The results are compiled in Table 1. Table 1 Average composition of the plastic film wastes of urban origin (1985) Polymer

%

PVC

4

HDPE

20

LDPE

68

Solid waste

8

Remarks half is removed by flotation in water

insoluble solids

A flotation method was applied in order to separate ninety percent of the polyolefins present in the film plastic wastes fraction from rejectable materials. The physical properties of virgin polymers supplied by Repsol Química S.A. (wastes, and of polymers chosen as interfacial modifiers) are collected in Table 2.

Procedures and Utilities The study of the rheological behavior of wastes and HDPE/LDPE system was 16 carried out using a torque-rheometer.

O. Laguna Castellanos et al.

63

Table 2 Physical properties of polymers and wastes used in this work Material

Molecular weight

[η]

Melt index o

Density -3

Mw

Mn

dl/g

T ( C)

P (kg)

(g/10 min)

kg⋅m

HDPE

141,000

19,000

0.87

190

2.160

6.7

916

LDPE

114,700

22,000

1.84

190

5.000

2.3

945

Wastes

0.76

190

2.160

1.1

941

LMWPE

0.74

125

0.325

1.1

924

190

2.160

3.0

1,000

EVA (24% av) ClPE (36% Cl)

1,160

Blends, having 15/85, 50/50, and 85/15 of HDPE/LDPE ratio, were prepared according to an experimental design method by Box-Wilson. The experimental design consists of thirteen experimental combinations distributed as follows: 2 • four combinations corresponding to the factorial 2 • four combinations to obtain the central design rotability (star combinations) • five experimental points in the center to calculate the experimental error. The amounts of HDPE in a blend and the shear rate were chosen as independent variables in this study. Experiments were carried out in the melt state at o 17 190 C. A Goodrich method was used for determination of effective instrument 18 dimensions, which allows one to employ the Daane et al. procedure in order to relate torque-rheometer data to more fundamental rheological values. The same HDPE/LDPE ratios were used in the study of mechanical behavior of HDPE/LDPE system of injected specimens. As said previously, DSC thermograms were used to determine weight concentration of two crystalline components both in HDPE/LDPE blends and wastes. Tensile and impact tests were carried out according to the UNE standards and in agreement with the ISO standards. The preparation of the specimens was

64

Management of Plastic Wastes

Figure 2. IR spectra of HDPE and LDPE homopolymers and film plastic wastes.

done by conventional processing methods: blending in a two-roll mill, followed by the injection molding of the materials. Thin sections, around 20 µm in thickness, from injected parts were cut by a microtome. Samples were taken from the nearest to, and farthest from, the gate. The study of a microstructure of different systems was made by interference and phase contrast microscopy. SEM was also employed for observations of a fracture surface of specimens from the impact tests. An etching of samples was 19 carried out according to Olley et al.

O. Laguna Castellanos et al.

65

RESULTS AND DISCUSSION Identification of polymers present in the film plastic wastes and the rheological behavior of the HDPE/LDPE system

Figure 3. DSC thermograms of HDPE and LDPE and their blends obtained from waste materials.

In Figures 2 and 3 IR plots and DSC curves of HDPE, LDPE, wastes, and some blends are reported. Two remarks can be made concerning these data: • the film plastic wastes from Madrid constituted, in 1985, mainly of HDPE and LDPE, having 20/70 HDPE/LDPE ratio determined by the peak area 20 method. • no co-crystallization was found 21 in HDPE/LDPE blends. The rheological study of HDPE/LDPE system and wastes was conducted to confirm the processability of wastes and their thermomecha- nical stability during the processing operations. The results obtained demonstrated a good processability of wastes. Torque, shear stress, and shear rate of pure components, blends, and wastes are reported in Figure 4. No thermal-oxidative degradation of the wastes was detected during and at the end of the processing. The flow curve of the HDPE sample is higher than that of LDPE and the flow curves of blends and wastes fall in between. The curve for wastes is between the curves for blends containing 85 and 50% of

66

Management of Plastic Wastes

Figure 4. Rheological properties of HDPE/LDPE blend and wastes.

HDPE. This is because of a presence of ten percent of inorganic solid particles in the wastes which increases their rheological response in terms of viscosity.

Mechanical behavior of HDPE/LDPE blends The mechanical properties of blends of virgin HDPE and LDPE and plastic 22-29 wastes are below those expected on the basis of an additive rule. In Figure 5

O. Laguna Castellanos et al.

67

Figure 5. Polar representation of mechanical behavior of HDPE/LDPE system containing either virgin polymers or composition from urban wastes.

a polar representation was plotted showing the mechanical properties of HDPE/LDPE system inside the contour lines assigned for homopolymers. Also the contour lines for the wastes were plotted. As can be seen, the values are inside the range for homopolymers, indicating a mechanical behavior typical for material which does not degrade. Thus the recyclability of these plastic wastes can be attained without a danger of degradation. The stress-strain curves of semicrystalline polymers show, at low strain rate, a sharp drop in stress after the yield point. After the neck formation, and during a certain time period, the stress does not change appreciably with further strain. Finally, there is a slight increase in stress and then the specimen breaks. Such behavior is typical for HDPE but not for LDPE. The drop in stress after the neck

68

Management of Plastic Wastes

formation in LDPE is very small. HDPE-rich blends show a stress-strain behavior similar to that of pure HDPE, whereas the stress-strain curves of LDPE-rich blends resemble that of the pure LDPE. Figure 6 reports the tensile strength values at yield and at break for the HDPE/LDPE system and for wastes. The values relative to a blend from wastes are very similar to those of a blend containing 15% of virgin HDPE. As known, the crystalline polymers are composed of lamellae containing folded chains. The lamellae are held together by the tie molecules which extend from one crystalline layer to another. Molecular imperfections (e.g., branches in polyethylene) tend to reside in the amorphous portion between crystallites, which suggests that a different behavior of HDPE and LDPE is related to their crystalline behavior. According to these observations, three interfacial modifiers of HDPE/LDPE system were chosen based on their chemical similarity to ethylene units but havFigure 6. Tensile strength: at yield (—) and at ing different crystallization capabilibreak (—) for HDPE/LDPE system containing ties. An ethylene-vinyl-acetate interfacial modifiers. amorphous copolymer (EVA), chlorinated polyethylene (ClPE) having 10-15% of residual crystallinity, and a low molecular weight polyethylene (LMWPE) practically without branches but having a very high crystallinity were used. These additives were also incorporated into the wastes.

O. Laguna Castellanos et al.

69

Table 3 Mechanical properties of HDPE/LDPE and HDPE/waste blends obtained by injection molding with and without interfacial agents (IA) Concentration HDPE

100%

85%

50%

14%

0%

wastes

E (MPa)

%

Tensile strength (MPa)

Impact strength 2 (KJm )

Def. 3 (m⋅10 )

Fmax (MPa)

none

993

820

17

10.4

4.2

3.8

1% EVA

387

700

18

11.0

3.9

4.0

1% ClPE

346

750

16

15.4

3.2

6.1

1% IMV

245

750

18

21.3

3.6

5.8

none

375

610

17

9.2

3.8

3.9

1% EVA

408

700

23

9.6

3.8

4.0

1% ClPE

316

700

14

9.5

2.6

5.5

1% IMW

246

730

24

13.5

2.7

5.2

none

375

522

18

6.3

3.2

3.2

1% EVA

346

610

18

7.3

3.7

3.1

1% ClPE

280

604

19

16.0

4.4

5.3

1% IMW

198

690

17

22.0

5.1

5.9

none

184

340

12

11.1

4.8

3.2

1% EVA

173

340

12

14.0

5.2

2.9

1% ClPE

158

370

12

22.1

7.6

4.0

1% IMW

163

500

12

26.0

6.8

4.9

none

151

212

10

∞

∞

∞

1% EVA

155

150

9

∞

∞

∞

1% ClPE

100

155

9.5

23

5.4

5.0

1% IMW

164

280

11

40

6.2

5.0

none

240

230

12

4.0

2.6

2.3

1% EVA

300

340

11

3.5

2.5

2.4

1% ClPE

251

300

11

6.4

2.1

4.4

1% IMW

228

330

12

5.2

2.0

3.5

IA

70

Management of Plastic Wastes

Figure 7. Elongation at break of HDPE/LDPE system in a presence of the interfacial agents.

Table 3 contains data on mechanical properties of HDPE/LDPE system and wastes, both containing 1% of each interfacial modifier. Figures 7 and 8 give the relative values of elongation at break and impact strength, respectively, for modified systems. The elongation at break (Figure 7) is especially improved by incorporation of a low molecular weight polyethylene, which gives the most significant improvement when the matrix is composed of LDPE. The elongation at break of the film plastic wastes is improved by about 30-50% due to the action of additives. A surface improvement of the injected specimens can also be achieved by adding the additives. The blend with a low molecular weight polyethylene as an interfacial agent shows better properties. The impact strength of the HDPE/LDPE system and wastes is also improved by the presence of additives (Figure 8). Similarly, the best results are obtained with a low molecular weight polyethylene. On the contrary, these additives give rise to a lower elastic moduli compared with data for the unmodified system. It is, however, evident that it is possible to

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Figure 8. The impact strength of HDPE/LDPE system at -30oC in a presence of interfacial agents.

markedly change, and in general to improve, the mechanical behavior and the surface quality of plastic waste fractions by adding an interfacial modifier.

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Figure 9. Micrographs of HDPE/LDPE system. Magnification 500×.

O. Laguna Castellanos et al.

Figure 10. Micrographs of the etched samples of the same composition as in Figure 9.

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Management of Plastic Wastes

Figure 11. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% EVA copolymer.

Figure 12. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% ClPE.

Microstructural aspects of HDPE/LDPE blends Figures 9 to 10 show micrographs of the microtomed sections from injection molded bars. The different microstructure of HDPE/LDPE blends, having different compositions, are shown (Figure 9). In fact, these thin sections taken in the parallel flow direction inside the cavity of a mold and near the gate, show differences in

O. Laguna Castellanos et al.

Figure 13. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% LMWPE.

Figure 14. SEM micrographs showing fracture surface of HDPE (left) and LDPE (right). Magnification 30x.

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Management of Plastic Wastes

Figure 15. SEM micrographs showing the fracture surface of the film plastic wastes. Magnification: (left) 30×, (right) 50×.

response of the material under polarized light. When HDPE is the matrix, smooth surface and high birefringence are found, as in the case of pure HDPE, with ringed and impinged macroaggregates of crystals also visible. These features indicate that the microstructure of these materials is basically due to HDPE. In Figure 10 the above observations are fully confirmed for samples which have undergone a chemical attack directed to the amorphous zones of blends. Figures 11-13 show micrographs of HDPE/LDPE blends containing 1% of EVA copolymer, chlorinated polyethylene, and a low molecular weight polyethylene, respectively. The changes on the microstructural level of these specimens, compared to the unmodified HDPE/LDPE system, previously discussed, are evidently in agreement with changes observed in the mechanical properties and the surface appearance of the injected bars. The changes in the size of the

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Figure 16. SEM micrographs of HDPE/LDPE system showing differences in fracture mechanism due to different matrix.

Figure 17. SEM micrographs showing brittle fracture (HDPE) and ductile fracture (LDPE).

macroaggregates of crystals and the presence of the interfacial additives enhance the stretchability of the compatibilized blends.

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Figure 18. SEM micrographs of wastes showing their brittle fracture.

Because of the presence in the wastes of about 10% of inks, pigments, fillers and so on, their microstructure cannot be studied by polarized optical microscopy. Thus, a microstructural study by scanning electronic microscopy, using the fracture surface of impact tested specimens, was carried out in order to confirm the previous data. Figures 14 and 15 give SEM micrographs of the fracture surface of HDPE, LDPE, and wastes at a very low magnification level. A similarity between the fracture surfaces from impact tests of the wastes and LDPE in liquid nitrogen is found. A smooth surface is observed at a low magnification in the front of a semi-brittle fracture observed in the hollowed region of HDPE specimen. Also, a lower amount of the solid particles (pigments) in the micrographs of the wastes is seen. At higher magnification (Fig. 16), it is possible to observe a significant difference between the fracture surface of blends with HDPE as a matrix and with LDPE as a matrix. Indeed, rounded features in the 85% HDPE blend fracture surface suggest a semi-brittle fracture around macroaggregates of crystals. At a

O. Laguna Castellanos et al.

Figure 19. Mass balance of the recycling plant designed for treatment of film plastic wastes from municipal origin.

Figure 20. Air view of the plant from Figure 19.

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Management of Plastic Wastes

Figure 21. Results of the ROI for economic study of viability of the utilities in figures 19 and 20.

higher level of magnification this feature is clearly evident and can be compared with the micrographs in Figure 17, where the homopolymers show striking differences in their fracture mechanism due to their different microstructure in macroaggregates of crystals. Furthermore, we must keep in mind that the o LDPE does not break at -30 C and its fracture surface was obtained in liquid nitrogen. In Figure 18, the fracture surface of the waste specimens also has numerous rounded features at the higher level of magnification. Probably solid particles from inks act in the wastes as nucleation agents. Also, a very small size of the rounded forms is observed. In any case, the higher level of stress concentrators present in the wastes makes them more brittle and explains their low impact strength value. The failure mode of the wastes is similar to that of the matrix.

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The economical approach Based on preliminary results, the design of a plant for treatment of the film plastic wastes was completed in 1985. The facilities were designed to produce pellets from the film plastic wastes ready to run a production with substitution of the required percentage of LDPE virgin resin. The mass balance and implementation of the plant design can be seen in Figures 19 and 20, respectively. The preliminary economic study was carried out according to Machinery and 30 31,32 Engines Charge. Satisfactory results were obtained and these were chosen as a starting point for two economic projects with immediate release in the 33,34 35 1986-89 period, and as a Final Master Project. A summary of one project is given in Figure 21 based on the results of the ROI (Rent Over Investment). It is very remarkable that in the 1986-1988 period, 43 % of ROI was achieved.

CONCLUSIONS The main conclusion of this paper is a real and practical possibility of recycling of thermoplastic materials using an integrated management scheme in which plastic wastes are subjected to sequential recycling from initial high performance and high cost applications to subsequently lower performance and lower cost applications. Thus, based on the technical conclusions regarding recyclability of plastic wastes, it is necessary to create laws regarding generation, collection, and location of sites having economic aspects in mind. The classic commodity - thermoplastics can be successfully recycled into a sequential scheme from the top applications to the lower performance articles. Additives can be found to improve performance, processing, mechanical properties, surface properties, etc. In future, and probably according to international standards and regulations, recyclability will become the most economical way to manage the Earth’s resources, in good agreement with the expected performance of the recycled plastic materials and their fundamental properties.

REFERENCES 1. 2. 3. 4. 5. 6.

M. J. Curry, Secondary Reclamation of Plastic Wastes, Vol. I and II, Technomic Pub., (1987). Proceedings of the Melting, Recycling Plas I, Technomic Pub., (1986). Ibid, Recycling Plas II, Technomic Pub., (1987). Ibid, Recycling Plas III, Technomic Pub., (1988). Ibid, Recycling Plas IV, Technomic Pub., (1989). Ibid, Recycling Plas V, Technomic Pub., (1990).

82 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

34. 35.

Management of Plastic Wastes R. Leaversuch, Modern Plast., 4 (1990). Proceeding of the Meeting, Plastics Recycling-88, SPE Scandinavian, Copenhagen (1988). Ibid, Plastics Recycling-91, SPE Scandinavian, Copenhagen (1991). Proceeding of the Meeting, Recycle-87, Davos, Switzerland (1987). Ibid, Recycle-89, Davos, Switzerland (1989). Ibid, Recycle-91, Davos, Switzerland (1991). Technical Bulletins, General Electric Plastics, Belgium (1991). Technical Press Bulletins, Neste Chemicals, Belgium (1992). Technical Bulletins, Opel, Germany (1992). G. E. P. Box, W. G. Hunter, and J. S. Hunter, Statistics for Experimenters, John Wiley, New York (1978). J. E. Goodrich and R. S. Porter, Polym. Eng. Sci., 7, 45 (1967). L. L. Blyler and J. H. Daane, Polym. Eng. Sci., 7, 178 (1967). S. Y. Hobbs, J. Macromol. Sci., Rev. Macromol. Chem., C19, 221 (1980). B. B. Stafford, J. Appl. Polym. Sci., 9, 729 (1965). E. P. Collar, J. Taranco, and O. Laguna, J. Appl. Polym. Sci., 38, 667 (1989). C. D. Han, Rheology in Polymer Processing, Academic Press, New York (1976). L. A. Utracki, Polym. Eng. Sci., 23, 602 (1983). C. D. Han and Y. W. Kim, Trans. Sco. Rheol., 19, 245 (1975). D. D. Patterson, Polym. Eng. Sci., 22, 64 (1982). C. D. Han, J. J. Kim, H. Chuang, and T. H. Kwack, J. Appl. Polym. Sci., 28, 3435 (1983). O. Laguna, E. P. Collar, J. Taranco, and J. P. Vigo, J. Polym. Mat., 4, 195 (1987). D. R. Paul and S. Newman, Polymer Blends, Vols. I and II, Academic Press, New York (1978). E. Martuscelli, Polym. Eng. Sci., 24, 563 (1984). A. Vian, El Pronóstico Económico en Química Industrial, Alhambra, Madrid (1975). E. P. Collar, Doctorate Project, U. Complutense, Madrid (1985). O. Laguna, J. Tijero, E. P. Collar, J. A. Serrano, V. E. Ibañez, D. Blanco, and J. Taranco, Rev. Plást. Mod., 50, 451 (1985). B. Cantabrana, M. J. San Pablo, V. Evangelio, T. G. Iglesias, and J. A. Serrano, End’s Project, Enterprises Direction Master, Escuela de Organizacion Industrial, E.O.I., Madrid (1986). S. Niddam and A. Sanz, End’s Project, Enterprises Management Master, Instituto de Empresa, Madrid (1989). T. G. Iglesias, J. A. Serrano, O. Laguna, E. P. Collar, and J. Taranco, Rev. Plást. Mod., 52, 209 (1986).

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Blends of Polyethylenes and Plastics Waste. Processing and Characterization F. P. La Mantia, C. Perrone*, and E. Bellio** Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy *I.P.I. SpA, S. S. Passo del Giogo, 50038 Scarperia (FI), Italy **Centro Ricerche Riciclo Materie Plastiche, Enichem Polimeri, Ragusa, Italy

INTRODUCTION Recycling of mixed plastics waste is not new. The first industrial applications 1 in Japan date from 1973. It is well known that it is possible to manufacture rods, stakes, bars, boards, plates, etc. from mixed plastics waste but the mechanical properties of these products are inferior. The recycled products cannot compete with virgin plastic materials, and therefore they have to find their market in areas dominated by cheap materials like wood and concrete. Moreover, the market of park benches, playgrounds, fences, etc. cannot absorb, in the long run, the massive amounts of plastic wastes that are produced every year. A possible route to recycle mixed plastics wastes to obtain secondary materials with acceptable mechanical properties could be to blend them with virgin poly2 mers or, at least, with recycled homopolymers. In a previous paper, processing and properties of blends of virgin low density polyethylene (LDPE) and mixed plastics waste (MPW), have been presented. The experimental results have shown that all the mechanical properties, with an exception of elongation at break, are very similar to those of the virgin material if the MPW content does not exceed 50%.

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Considering these first experimental results, we have investigated the possibility to use the mixed plastics waste as “filler” for three types of polymeric matrices, namely, low density polyethylene, high density polyethylene (HDPE), and recycled polyethylene (RPE). Such approach may offer two important advantages: • improvement of utilization of huge amounts of a mixed plastics waste which are generated by municipalities and industries • savings in non-renewable raw materials and energy, both associated with the manufacturing of the fraction of the virgin materials that can be replaced by plastics waste. Even if the percentage of plastics waste used as a filler cannot be high, its common use may absorb sizable amounts of wastes. As an example, in Italy, a blending of 10% of mixed plastics waste with non-food thermoplastics, excluding those for film manufacturing, could utilize 172,000 t/years, that is about 75% of all the plastic bottles manufactured every year in our country. It should be thus possible to fully satisfy the recycling objectives of the Italian law that requires a 40% recycling in 1992. Aim of this work is to investigate the processing conditions and the mechanical properties of blends made using different homopolymeric matrices and different amounts of heterogeneous plastics waste.

EXPERIMENTAL The characteristics of materials used as a matrix in this work are reported in Table 1. The recycled polyethylene (RPE) was obtained from films for agricultural use and is, therefore, mostly composed of a low density polyethylene. The average composition of the mixed plastics waste was: PEs 33% (mainly HDPE from blow molded detergent bottles) PVC 39% (plain mineral water bottles) PET 28% (soft drink and carbonated mineral water bottles). Such composition is representative of the average composition of the plastic fraction obtained by separate collection of bottles, as required by the Italian law. The plastics waste was reduced to small flakes by a rotating knives mill. Polyethylenes and MPW were mixed by hand and fed to a laboratory single-screw extruder (D = 19 mm, and L/D = 25) fitted with a venting port. For all o the extrusion runs, the die temperature was 270 C, and the screw speed 60 rpm.

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Table 1 Polyethylene used in blends Sample

Supplier

Commercial name

MFI

Density (g/cm )

LDPE

Enichem Polimeri

ZF 2200

0.30*

0.922

HDPE

Enichem Polimeri

AF 5015

0.24**

0.953

RPE

-

-

-

-

3

*ASTM D-1238 cond B **ASTM D-1238 cond E

The extruded material was cooled in a water bath, granulated and extruded again in order to reach a good homogenization. The unblended polymers were also treated in the same way. Three compositions were investigated for each blend, namely, 10, 25, and 50% MPW. Blends with a higher content of plastics waste were prepared only with LDPE. Blends of the same composition were also prepared by adding 10 and 20% of calcium carbonate. Small amounts of commercial antioxidants and lubricants were added to each blend. The samples for the stress-strain tests were prepared by compression molding of the granules of the extruded materials in a Carver laboratory press. The bars for the impact tests were obtained by the injection molding in a laboratory molder (Mini Max molder CS 183, Custom Scientific, USA). In both cases o the temperature was 270 C. Stress-strain curves were obtained with an Instron model 1122. Impact tests were carried out in Izod mode, using a CEAST Fractoscope. The results were averages of, at least, seven measurements.

RESULTS AND DISCUSSION Processing When mixed plastics waste is processed, one of the main problems is to find the best compromise between homogenization and degradation. In the waste material, there are polymers with different melting points and different thermal stability. Especially, PVC and PET are very difficult to process together, because o the melting point of PET is over 250 C, and at this temperature PVC is easily decomposed forming HCl and chary residues. Therefore, the optimal processing

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Figure 1. The modulus and impact strength of LDPE/MPW blend (MPW = 25%) vs. the number of extrusions.

conditions must ensure a good dispersion of the materials with high melting point in a continuous phase of molten polymers, avoiding gas bubbles, low molecular weight compounds, and crosslinked residues that are formed by thermal degradation. 2 In the previous work, it has been demonstrated that all mechanical properties reach a maximum or a plateau after two extrusions. As an example, Figure 1 gives the data on the elastic modulus and the impact strength of a blend with 75% of LDPE. The data indicate that a double extrusion gives the best balance between homogenization (which improves with a number of passages through the extruder), and the degradation which is increased by the severity of the thermomechanical treatment. Based on these results, two extrusion steps were used for the preparation of all samples. During processing, no significant evolution of HCl from the venting port of the extruder was found, only at the die lips some acid vapors were observed. This is a rather surprising fact, because the HCl evolution from PVC is a very common occurrence in the industrial operation of plastics recycling. One possible explanation could be the residence time in the screw, which is much longer for the industrial extruders than used in laboratory extruder. Additionally, it should also be considered that some antioxidants and lubricants were added.

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Figure 2. Dimesionless modulus vs MPW content.

The stability of PVC, under the processing conditions of experiment, was confirmed by X-ray analysis with an energy-dispersive Philips apparatus. Significant amounts of chlorine are present in all extruded blends. The only processing difficulty, found during extrusion, was due to the presence of non-polymeric materials (paper, aluminum, etc.) in the MPW which required frequent changes of filter at the end of the screw.

Mechanical Properties The mechanical properties of all the investigated materials are reported in Figures 2-5 as a function of the mixed plastics waste concentration. The experimental results are reported in dimensionless form, i.e., the value relative to each blend is divided by the corresponding value of the unblended matrix. These latter values are reported in Table 2 for three matrices. As expected, all the mechanical properties, except for the elastic modulus, decay on increasing the MPW content. The extent of this deterioration, however, is strongly dependent on the investigated property and on the nature of the matrix.

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Figure 3. Dimensionless tensile strength vs. MPW content.

Figure 4. Dimensionless elongation at break vs. MPW content.

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Table 2 Mechanical properties of the three matrices Sample

E

TS

EB

IS

(MPa)

(MPa)

(%)

(J/m)

LDPE

180

9.6

450

470

HDPE

580

23.2

660

750

RPE

500

10.1

350

450

Figure 5. Dimensionless impact strength vs. MPW content.

As already mentioned, the elastic modulus (see Figure 2) increases with the mixed plastics waste content. Moreover, the improvement is larger for the blends containing LDPE and recycled polyethylene as a matrix. This behavior can be attributed to the high modulus of the polymers forming the MPW mixture. The remarkable increase of the modulus found for blends containing LDPE and RPE can be explained by considering their relatively low moduli compared

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Figure 6. Stress-strain curves for LDPE and its blends with 10 and 50% MPW.

with polymers of the MPW phase which have considerably higher moduli). On the contrary this improvement is less pronounced for the blends containing HDPE as a matrix, because of its higher elastic modulus, similar to that of the PE component in the MPW phase. 2 As demonstrated in the previous work, the values of the modulus are lower than expected on the basis of an additive rule. It is due to the incompatibility among the different polymeric phases present in the blends. Tensile strength (Figure 3) is only slightly influenced (especially for LDPE and RPE) by adding mixed plastics waste but an elongation at break decreases rapidly even at low MPW content (Figure 4). The stress-strain curves of some blends are reported in Figure 6. The blends with MPW fail, in general, at a deformation in proximity of elongation at yield of the matrix when the stress reaches a plateau, extending almost up to the break point. It is probably due to this reason that the tensile strength slightly depends on the MPW content. The elongation at break (Figure 4) is low for all blends due to the incompatibility between the various phases formed in the blends. In Figure 7, the SEM micrograph of a sample of a blend of LDPE and MPW (75%) highlights the

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Figure 7. SEM micrograph of LDPE/MPW blend (MPW = 75%).

multiphase morphology of these materials and the poor adhesion among the different components. The lack of adhesion gives rise to microdefects in the structure of these material inducing a significant fragility. The blends containing HDPE have also poor properties. Impact strength (Figure 5) is also significantly influenced by the presence of a heterophasic dispersed plastics waste, but the matrix seems to play a more important role. The impact strength of LDPE slightly decreases on increasing the MPW content within investigated composition range. Blends with HDPE and RPE show a different behavior. The samples prepared with the recycled polyethylene as a matrix do not show any change in the impact strength when MPW content is lower than 30%, which rapidly decreases at the MPW concentration of 50%. The blends containing HDPE show a remarkable decrease of the impact strength also at the low MPW concentrations. These latter results are difficult to explain on the basis of the nature of the components of these blends. Only a better adhesion between the LDPE continuous matrix and the different phases of the MPW can be considered. This fact is, however, quite unexpected considering that the PE phase in the plastics waste is mostly HDPE.

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Figure 8. Modulus of blends containing 50% MPW vs. CaCO3 content.

Figure 9. Tensile strength of blends containing 50% MPW vs. CaCO3 content.

F.P. La Mantia, C. Perrone and E. Bellio

Figure 10. Elongation at break of blends containing 50% MPW vs. CaCO3 content.

Figure 11. Impact strength of blends containing 50% MPW vs. CaCO3 content.

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Table 3 Mechanical properties of blends with CaCO3 CaCO3 = 10% MPW = 10%

CaCO3 = 20%

LDPE

RPE

HDPE

LDPE

RPE

HDPE

E (MPa)

270

260

620

280

300

620

TS (MPa)

9

10

22

9

9.5

20

EB (%)

150

110

50

25

20

20

IS (J/m)

500

480

200

300

290

70

E (MPa)

370

340

640

400

400

640

TS (MPa)

8.5

10

21

8

8.5

19

EB (%)

50

25

15

7

5

5

IS (J/m)

400

300

100

140

140

40

MPW = 25%

Blends containing calcium carbonate Calcium carbonate is widely used as an inexpensive filler for many polymers. For this reason (a significant reduction of the cost of these recycled materials), PE/MPW blends with different amounts (10 and 20%) were prepared and characterized. The mechanical properties of the blends with 50% of MPW are reported in Figures 8-11 as a function of the calcium carbonate content. The blends with 10 and 25% of MPW and with the same amounts of CaCO3 were also prepared and tested. The results are reported in Table 3. The elastic modulus of blends containing LDPE and RPE remarkably improves on increasing the CaCO3 content, while the modulus of the blends containing HDPE as a matrix is only slightly improved by adding calcium carbonate. Tensile strength, elongation at break, and impact strength show the same features for all blends. Tensile strength does not change significantly by adding these amounts of calcium carbonate. The explanation of these results

F.P. La Mantia, C. Perrone and E. Bellio

Figure 12. Modulus vs. PE content for blends having varying content of LDPE and CaCO3.

Figure 13. Tensile strength vs. PE content for blends having varying content of LDPE and CaCO3.

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Figure 14. Elongation at break vs. PE content for blends having varying content of LDPE and CaCO3.

Figure 15. Impact strength vs. PE content for blends having varying content of LDPE and CaCO3.

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could be the same as the above proposed to interpret the small sensitivity of these properties to the MPW concentration. Elongation at break and impact strength of blends with LDPE and RPE decrease slightly only by adding 10% CaCO3. A 20% addition of calcium carbonate significantly decreases elongation and impact strength of blends. The elongation at break and impact strength drastically deteriorate on addition of 10% CaCO3 to HDPE blends.

Blends containing LDPE On the basis of the above results, blends made from LDPE with larger amounts of MPW were prepared. It is worth noticing that all blends previously characterized were composed of polyethylene matrix (PE > 50%) with dispersed PET and PVC particles. Blends with larger amounts of MPW show a PE content lower than 50%. The mechanical properties of LDPE/MPW blends with and without calcium carbonate are reported in Figures 12-15 as a function of the polyethylene content. The total amount of polyethylene was evaluated considering the polyethylene in the plastics waste (about 33%). The modulus increases with the MPW and is only slightly influenced by the CaCO3 content. As already discussed, the tensile strength does not significantly change with decreasing the PE and CaCO3 content. The elongation at break rapidly decreases even at a low content of plastics waste (for PE content lower than 70% the influence of CaCO3 is negligible). At high PE contents, CaCO3 seems improving the elongation at break. It is however worth noticing that the comparison is not made for the same MPW content. The data for PE = 80% (graph for 20% CaCO3) refers to a blend without plastics waste showing that the effect of the heterogeneous plastics waste is more deleterious for the elongation at break of the polyethylene than that of the CaCO3 addition. The impact strength, on the contrary, strongly depends on the presence of CaCO3. With increasing the CaCO3 concentration, the amount of MPW at which the impact strength declines decreases. Whereas at CaCO3 = 0%, impact strength diminishes at a PE content lower than 40%, at a concentration of CaCO3 = 20%, impact strength is considerably reduced at a PE content lower than 70%.

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CONCLUSIONS • • • • •

Blends of polyethylene with mixed plastics waste, up to 50-75%, can be extruded without significant difficulties and accelerated degradation of the PVC component. The mechanical properties depend on the structure of the polyethylene matrix, but, in general, are very similar to those of the matrix up to a MPW content of about 25-50%. The elongation at break rapidly decreases even at low MPW content; however, in many applications this characteristic is not very important and the values recorded for blends under the study can be considered adequate. Considering polyethylenes tested in these blends, LDPE and polyethylene recycled from greenhouse films gave the best results. The addition of CaCO3, up to 10% content, does not significantly change the processability and the mechanical properties of blends but the cost of the blends is reduced because of replacing a corresponding amount of polyethylene.

ACKNOWLEDGMENT This work was financially supported by MURST and Enichem Polimeri.

REFERENCES 1. 2.

Plastic Waste: Resource Recovery and Recycling in Japan. Plastic Waste Management Institute, Tokyo, Japan 1985. F. P. La Mantia, Polym. Deg. Stab., 37, 145 (1992).

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Techniques for Selection and Recycle of Post-Consumer Bottles E. Sereni Tecoplast and Govoni, 44047 Casumaro, Ferrara, Italy

INTRODUCTION Techniques for selection and recycling of post-consumer plastics are closely related to the characteristics of plastic containers consumption, which are extremely diversified according to the geographical areas and the relevant law regulations governing activities in this sector. In each area, socio-economic and legislative features as a whole determine the first stage of the recycling process, that is collection. This, in turn, influences the layout of the recycling plant which aims at the re-use of plastic materials reclaimed in the most economical way. Hence the enactment of laws which bind municipalities to recycle containers in general, as is the case in Germany, or in a more restrictive sense, in Italy, where recycle is limited to containers for liquids only. On the other hand, consumption features play a major role in the choice of the materials to be recycled. So, in the United States materials chiefly recycled are PET bottles and PE containers; in France, on the contrary, they began to recycle PVC on account of the large quantity of such material used in the packaging of drinks. In Australia recycling includes PET and PE, whereas in Japan PET alone. In Italy plastics recycled are PET, PVC and PE. In short, the material to be recycled and the enforced legislation are determinants for the choice of the collection system. In the United States, Australia, France, Austria and Switzerland, different fractions of post-consumer plastics are collected in the most homogeneous way possible. In other countries, on the contrary, plastics are collected more heterogeneously, that is to say, different types of plastics out of different types of manufactured articles, such as foil, containers, bottles, are collected together.

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At any rate, collection must necessarily be a “differentiated” one, although the degree of differentiation may vary extremely. In Italy, collection, according to Law 475, is differentiated as far as recycle of plastic containers is concerned, whereas it is non-differentiated with regards to type of plastics, since liquids containers may be made of either PVC, or PET, or PE, or else. The outcome of the collection system, therefore, constitutes the raw material for the recycling process. Consequently, as we are taught by experience in this field, the more selective the collection, the higher the degree of purity from foreign bodies in the final product. In Italy, for instance, collection of liquid containers by means of plastic boxes, shows 15% to 20% contamination resulting from foreign bodies. Collection of drink containers carried out with the AZURE-Bottle-eater - an automatic machine installed inside supermarkets presents pollutants solely composed of other types of containers, and not exceeding 1%. Aim of this paper is to present the more recent technologies of automatic separation of plastic items. A brief paragraph on a typical recycling plant of homogeneous plastics is also presented.

GENERAL CONSIDERATIONS The collection always yields a polluted product, and this fact poses the need for the first operation of the recycling process, namely the cleaning of foreign bodies. The machinery required at this stage may be of either manual or automatic type. Manual cleaning is simpler from the installation standpoint, nevertheless it equally requires a number of operating steps. Normally, the first operation is carried out by a machine called de-baler. The collected material, indeed, for transport cost reasons, is reduced into bales. There exist very efficient de-balers and the best brands have been equipped with specific devices designed in accordance to the composition of the bales to be loosed. Such factors as the proportion of PVC, PET or PE in the bales, the collection features, the container typology, and the share of foil plastics, determine the type of de-baler construction technique. After the first step, the following operations are determined by the type of recycling process to which the material is to be subjected. Basically there are two main recycling processes: • recycle of heterogeneous plastics • recycle of selected polymers.

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The former technique consists of the manufacture of extruded or injected products, whose main component is either PE or PVC, whereas other plastics act as fillers. The latter consists of separation of the mix of collected plastics into homogeneous fractions, subjected to a recycling process which brings their characteristics and purity as near as possible to those of original polymers. The two techniques are not contrasting, in fact they can be regarded as complementary to each other. The polymers recycling process is the technique an industry is the most concerned with. Attention is called to the fact that the utmost degree of purity of the finished product should be aimed at, because reclaiming process requires various plant operations determinant for the final cost of the material. The difference in outcome - namely a product similar to the original one or a polluted product - does not entail such a difference in installation and operating costs as there may be between the price of an optimum product and that of a lower-quality product. If this is true for every type of plastics, it is the more so for the PET, which, if it is not recycled to the utmost level of quality, has virtually no market. The cleaning of foreign bodies to obtain recycled plastics as polymers is followed by the selection by fractions which can be carried out as well with manual or automatic techniques. Cleaning and selection operations, in general, have the purpose to obtain a selected material to be “refined” by processing it through a recycling plant. The process of cleaning and selection has therefore the function to remove, during this stage, all materials that cannot be selected during the recycling process. It is therefore necessary to eliminate in advance metals and foreign bodies and sort out plastics presenting such physical affinity with the type of plastic to be recycled as to make it impossible to separate them by the methods employed in the recycling process. Typical is the case of PET and PVC, which contaminate each other mutually and have very similar specific weights. As a consequence, they cannot be separated through such ordinary physical processes as water flotation and, therefore, must be separated prior to recycling. The simplest method to perform the cleaning and selection operation consists of a selection platform where a number of trained sorters separate the different types of plastics on the basis of visual assessment. This is a hard and unpleasant job. Therefore, planning of works station according to ergonometric criteria is paramount in order to attain manpower’s maximum efficiency, the more so if the cost of labor is considered.To the advantage of manual selection is the fact that

102

Techniques for Selection and Recycle of Bottles

sorters operate to such a degree of intelligence as the automatic equipment cannot reach. On the other hand, manual selection is comprehensibly always liable to human error. For this reason, in Italy, selection platforms, which are being installed, are equipped with detectors to check the quality of the selected material. The detectors employed in Italy are the clue to the automatic selection. These are electronic appliances capable of recognizing PET in a flux of PVC and vice versa. The checking equipment is further completed by detectors able to identify traces of metal overlooked during manual sorting, such as aluminum from caps and rings. The material manually selected and then electronically checked is therefore of best quality and can be sold at the maximum market price. The most serious inconvenience with manual platforms lies in the high cost of labor and in the complexity that the management of a large number of workers poses when considerable quantities of material are to be selected. Such drawbacks may be avoided by resorting to automatic platforms. Automation is introduced at the stage of de-baling. In order to obtain a product suitable for the recycling process, operations to remove undesired impurities must be carried out. The machines required are manifold and the necessity to employ them is related to the quality of the collected material. Essential machines are: • • • •

Rotary screen, by which parts of the desired dimension are sorted out, separating them from smaller and larger ones. Light-parts separation equipment. In this machine lighter parts, such as for example films, are separated from the plastic material to be recycled. The method of separation is by air blowing. Heavy-parts separation equipment. Also this operation is carried out by means of air which shifts the material selectively, so that heavy particles are separated. Aluminum rejection equipment. It normally consists of an electromagnetic drum placed in suitable location on the train of operations.

All such machines are preliminary to the stage of selection into homogeneous plastics fractions.

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Selection can be carried out by different methods which fall mainly into three groups: • Molecular separation • Microseparation • Macroseparation.

MOLECULAR SEPARATION This type of selection is still in the stage of study and there is no evidence of any industrial plant in operation so far. The technique is based on the dissolution of the various plastics in selective solvents with reclaim of the dissolved substances. The system appears to be very promising, but no more detailed information is currently available.

MICROSEPARATION Microseparation is a method by which a suspension medium is used to separate plastics with density higher or lower than the suspension medium. For example, water can be used as medium to separate PE from PVC or PET. In this case special tanks are used in which various types of plastic flakes are mixed with water and then given a sufficient time to position themselves in the most suitable way according to their density. Materials are subsequently extracted separately from the top or bottom. This method is not suitable for separating PVC from PET. 1 Researchers at the Rutgers University studied a method of PVC microseparation from PET, by which PVC is subjected to a process of selective bulking which causes it to float. Such method may perhaps be applied in the future to separate small quantities of PVC from large quantities of PET, as normally is the case in United States. In Italy, for example, PVC is a major component of the containers mix and, therefore, it is necessary not only to separate PET, but also to recycle PVC for re-use. For this reason the above method does not appear suitable. 2 There are known experiments on separation in super critical fluids. Beckman reported separation of PVC and PET, using carbon dioxide and sulphur esofluoride. The conditions under which experiment was conducted are objectively difficult and, at present, an application of this method is not practical. The electrostatic separation is presently arising great expectations. By exploiting the characteristics of the triboelectrical charge, two mixed polymers obtain opposite charges. The particles, which are allowed to fall in electromagnetic

104

Techniques for Selection and Recycle of Bottles

field, are attracted by to the opposed pole. The equipment recently shown by 3 4 Kali and Saltz and Carpco let us reasonably predict encouraging results. However, prior to the industrial introduction of the method described above, it will be necessary to make an accurate assessment of the final cost of treatment and to evaluate whether the conditions under which the system is efficient are those actually encountered in the treatment of post-consumer plastics. One should also mention some other studies and experiments conducted by various institutes regarding techniques applicable to PVC and PET containers, previously reduced to flakes, namely: flotation with surface active agents and separation by thermal treatment, which enables to retain PVC by its softening on a belt having a suitable temperature. Decontamination is already applied in practice, the most frequently by micronization which takes advantage of a higher brittleness of PVC as compared to PET. Such grinding produces PVC particles of smaller size than those of PET. Consequently PVC can be separated by screening with a satisfactory degree of efficiency. The method is employed in Micronil, France and Cabot in Belgium. Low-temperature micronization techniques, used in Australia by Cryogrind, do not appear useful in Europe because of a high energy cost involved.

MACROSEPARATION Separation of plastic fractions when waste materials are still in initial form continues to appear the most conveniently applicable system, considering the increasing possibilities of automation it offers. It is of little interest as long as separation is manual, but it becomes worth discussing when conducted by an automatic separation process which includes various systems employing detectors currently available in the market. The first detector was developed by Tecoplast in Casumaro, Ferrara-Italy, to separate PVC from PET. The application of this system resulted in the introduction of an automatic plant processing drink plastic bottles using the AZZURRA machine. A peculiar composition of the collected material made it possible to fully automate the operations of selection, thus totally excluding manual handling of bottles. The Tecoplast’s detector consists of an X-ray source and a receiver which measures the bottle absorption while passing between the source and receiver. PVC has a higher absorption compared to the other plastics due to the presence of a

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chlorine atom. The value of PVC absorption, electronically processed through algorithms, makes it possible to detect its presence and consequently to eject bottle. Another detector is employed to line up bottles using a suction robot. The aligning process enables bottles to be arranged in the most suitable fashion for identification of transparency and color. Taking advantage of this technology, Tecoplast developed the first optical detector capable of establishing the quality of PET, thus determining the separation between clear PET and colored PET, besides the mentioned separation from PVC. At present Govoni’s technology employs detectors performing the above described operations without alignment. 1 Other detectors have been developed in the USA. ASOMA built a detector which identifies PVC by fluorescence induced to non-aligned bottles. These fall through a chute at the end of which PVC detected bottles are extracted by a com4 pressed-air jet. The NRT detector is based on the same principle and operates, using bottles descending along a tilted plane. An optical detector has been devel5 oped by the American MSS. The machine is a part of a system consisting of PVC detector to obtain separation into homogeneous material fractions, including PVC, clear PET, colored PET, multicolored HDPE, translucent HDPE. A near-infrared (NIR) spectrophotometry detector was been recently devel6 oped by AIC, Automation Industrial Control for identification of the resin type. The equipment is connected with another detector for color determination. The resulting data are processed by computer with highly sophisticated software. The equipment enables separation of a container mix into various components with a high degree of selectivity in regard to typology and color. This type of detector requires material singularization and lining-up. All detection systems are based on techniques which in principle should be highly accurate, almost infallible. In practice, however, this is not the case. As known, for instance, PET with maximum admissible impurities of 100 ppm, should have a PVC detector with 99,99% efficiency. In reality to reach this percentage is virtually impossible, because detection must be unfailingly followed by rejection of the detected bottle, and this is not always the case either. Therefore detectors with a more likely efficiency of 99,5%, but also 99% only, if installed in series and in number of at least two units, can bring the level of impurities within the limits required. Returning to the previous example: with a mix in the proportion of 90% of PET and 10% of PVC, two detectors with 99,5%

106

Techniques for Selection and Recycle of Bottles

Figure 1. Automatic platform for plastics selection.

efficiency placed in series enable to obtain a level of impurities of 2,5 ppm, whereas with efficiency of 99% the value of residue is 10 ppm. A further point of consideration is the efficiency of separation which could nullify all the above calculations. Experience has shown that the inefficiency of separation of the detected product is exceeding by far the detector inefficiency. For instance, if the efficiency of PVC separation is 99%, in the former case 22,5 ppm will be obtained, in the latter 400 ppm. Therefore, a check on the operations of selection of the detected product is of primary importance.

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There is an all-Italian project, meeting all the primary prerequisites so far illustrated, which is a system developed by Govoni based on Tecoplast’s experience. It consists of a platform for selection according to plastics typology, where all the identification steps are performed twice and where the efficiency of selection following detection is always subjected to check (Figure 1). The notion of “error reduction” in the execution of an operation by a way of repetition of the same step is adopted also in the phases of pre-cleaning of the parts to be passed onto electronic selection. In conclusion, we believe that the described system allows one to achieve the aim of selection purity using an automatic equipment, having a sufficient degree of reliability.

RECYCLE INSTALLATIONS A special importance is generally devoted to the techniques of electronic selection of homogeneous fractions, while disregarding the phase of regeneration of selected plastic flakes. This minor interest would be justified if bottles were manufactured following criteria of perfect recyclability. However, such criteria are not yet universally accepted and complied with; therefore the recycling plant designer must give an accurate response to the problems posed by various elements which normally compose the item to be recycled. If we refer to liquid containers in general, these, beside the body made out of plastic, consist of additional components which are to be regarded as foreign bodies to be removed. Such elements are: • caps made of: PE, PE with PVC gaskets, aluminum • labels of tacky paper with different types of glue, composed of PE or PVC • base cups of PEAD • residues and dirt which may have been added during waste collection phase. Various operations, to be carried out in a specific sequence because of the problems posed by the type of material, are the following:

Grinding It is the first step following selection. It requires attention and accuracy in design to ensure optimum homogeneity of the ground product.

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Techniques for Selection and Recycle of Bottles

Air flotation Specially designed machines combining the effects of vibration and air flotation ensure separation of flakes with different specific weight. Such machines are very useful for removal of parts of labels which were freed by grounding. The purpose of this operation is the removal of a part - important with regard to volume and weight in the dry phase. Process allows to avoid problems of dissolution in water and relevant contamination. An extremely interesting and certainly unique application of this method is the separation of PVC labels from PET bottle flakes.

Washing equipment Residues are normally washed out of material flakes by means of equipment whose construction details, guaranteeing of their performance, are kept secret by manufacturers as much as possible. This class of equipment includes: • • • • • • •

centrifugal cleaners washing tanks autoclaves for flakes-detergents contact settling tanks combined-action machines scraping machines (mechanical friction) centrifugal machines, normally used for water separation.

The installation of the above machinery according to a specific sequence is part of a know-how of various manufacturers. In conclusion, in view of complex problems posed by post-consumer plastics installations, we deem it necessary to suggest that the design of a plastic recycling plant be inspired by one major principle: it is not possible to expect miraculous results from key-processes carried out in a single passage therefore the efficiency is maximized by repetition of the same operation in more than one phase.

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REFERENCES 1. 2. 3. 4. 5. 6.

H. Frankel, Proceedings of Recycle 92, Davos (Suisse), April 7-10, 1992. E. Beckman, M. S. Super, and R. M. Enick, Proceedings of Recycle 92, Davos (Suisse), April 7-10, 1992. U. Kleine-Kleffmann, Proceedings of Recycle 92, Davos (Suisse), April 7-10, 1992. R. T. Gottesman, Proceedings of Recycle 92, Davos (Suisse), April 7-10, 1992. G. Kenny and R. S. Bruner, Proceedings of Recycle 92, Davos (Suisse), April 7-10, 1992. AIC, Baltimore, USA, Technical Bulletin.

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111

Hydrolytic Treatment of Plastics Waste Containing Paper C. Klason, J. Kubát,* and H. R. Skov** Polymer Center, University of Lund, Box 118, S-22100 Lund, Sweden *Department of Polymeric Materials, Chalmers University of Technology, S-41296 Gothenburg, Sweden **Chemistry Consultants Co., Copenhagen, Denmark

INTRODUCTION Recycling of pure plastics is not difficult from a technical point of view. However, the majority of municipal solid wastes consists, however, of plastics waste which is often contaminated with significant amounts of paper. This is not only the case with the plastics fraction of municipal solid waste (PFMW), but also with such industrial waste as used packaging materials, laminates, trimmings, etc. The reprocessing of plastics waste contaminated with more than 5% paper is difficult using conventional plastics processing machinery, and becomes almost impossible at paper levels exceeding 15%. Normally, the aim, at a municipal sorting plant, is to remove the paper component from the light plastics fraction to a level well below 1%, and this results in a plastics fraction mainly consisting of polyolefines (polyethylene and polypropylene). Up till now municipal sorting plants have not been very successful in this respect; the material-handling side has been difficult and the costs far have exceeded the low price of abundant vir1 gin material. A simpler solution to the problem of paper contamination is to allow for a paper component in the plastics fraction and to use a processing method which can disintegrate the cellulose fibers into small fragments such that they act as particulate fillers in the plastics. Such a method has been developed at Chalmers University of Technology (the CUT-method) making it possible to reprocess both the plastics fraction of municipal solid waste (PFMW) and a number of different 2-4 industrial plastic waste materials contaminated with paper.

112

Hydrolytic Treatment of Plastic Waste Containing Paper

The aim of this study is to demonstrate that the CUT-method, consisting of a pre-hydrolytic treatment of the paper component, is an industrially applicable method of reprocessing paper-contaminated plastics waste of various origins. The main advantage with the CUT-method is that the plastics fraction and the paper component do not need to be separated during the pre-hydrolytic treatment. The hydrolysis does not degrade the plastics fraction but reduces the molecular weight, i.e. the chain length of the cellulose component to a level at which the cellulose fibre becomes extremely brittle and the shear field acting in normal plastics processing machinery (compounding extruders and molding machines) can easily disintegrate the paper parts into small fibre fragments. The results of a previous investigation show that the disintegration of the embrittled paper component into an almost pulverized substance is the key to the success of the method, resulting in greatly enhanced melt flow properties, better homogeneity, 4 and, thus an improvement in the mechanical properties of the material. The main work in this investigation has been to adapt the hydrolytic method to the conditions of industrial practice. Therefore, much attention has been paid to the method of hydrolysis, and a gas-phase method was developed.

EXPERIMENTAL The samples for the experiments were obtained from the following sources: Sample A: source-separated plastics waste from households, collected in Gothenburg (collection test), Sweden, representing post-consumer mixed plastics waste containing paper. Sample B: source-separated plastics waste from households consisting of HDPE 65%, LDPE 20%, and PP 15% (Kolding, Denmark). The addition of edge trimmings from the packaging industry (15% LDPE, 85% cellulose) comprised the paper component, representing industrial plastic waste with paper. Sample C: plastics fraction from mechanically-sorted household waste, mainly post-consumer LDPE-film waste with 5% paper (4S Plant, Skive, Denmark). Paper was added (outdated telephone directories). Sample D: post-consumer mixed plastics waste, HDPE 70%, LDPE 20% and various plastics 10% (supplied by Cadauta, Turin, Italy), with addition of a paper component (telephone directories). It should be noted that all samples represented post-consumer plastics waste. Such waste will become a dominating problem in municipal waste management in the near future. One component in sample B, the edge trimmings, consisted of

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113

Table 1 Approximate composition of samples A-D Material

Sample A (%)

Sample B (%)

Sample C (%)

Sample D (%)

LDPE

27

20

70

90

HDPE

5

65

350

Table 7 UV resistance of HIFax CA12A (UV exposure in Xenotest 1200 according to ISO-4892) UV package

∆E after aging 500 kJ

1750 kJ

2750 kJ

1

0.19

0.34

1.63

2

0.45

0.50

0.69

structure. The consequence of a rapid change in modulus is an unfavorable condition for expansion that could be obtained only in a narrow temperature range. Hence the problem is to improve dramatically the strength-temperature properties near the melting point. The rate at which polymer crystallizes from melted state is the other important physical property in the foaming process. For instance, polypropylene crystallizes much slower than polyethylene. This means an increasing of the time required to build up sufficient modulus to avoid cell-wall rupture and cell-coalescence during the expansion which is a serious process limitation to overcome. The cushioning and soft-feeling properties required for some car interior applications were achieved by matrix modification as well as by the structural pa2 rameters affecting the compression-deflection of the foamed material (i.e. density, open cell content, cell size).

134

Processing of Mixed Plastic Waste

Figure 6. Dashboard sketch.

Figure 7 Sketch of floor covering.

Table 8 Fogging test for HIFax CA12A skin (transmittance measured according to DIN 75201-C) Transmittance (%) Stabilization package

1

2

HIFax CA12A

black

98

97

dark grey

99

99

light gray

99

99

Plastified PVC

65-80

Plastified PVC/ABS

80-95

Technologies In order to obtain all TPO recyclable applications described below, different assembling techniques have been specifically studied to obtain the basic composite 3,4 structures. The most interesting technique is one that allows to obtain simultaneously thermoforming, embossing, and coupling in one stage operation yielding a foamed-synthetic leather bilayer on a rigid support (all TPO based)

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135

Table 9 Properties of PP recycled from dashboard Method

Units

Recycled material

Standard PP

Tensile strength

ASTM D638

MPa

18

22

Elongation

ASTM D638

%

520

580

Flexural modulus

ASTM D790

MPa

1130

2080

Charpy impact

ASTM D256

MPa

3.13

3.68

without adhesives. This process can eliminate problems of embossing distortion present in thermoforming of an embossed synthetic leather.

AUTOMOTIVE APPLICATIONS Dashboard Figure 6 shows a sketch of the dashboard designed for recycling and obtained with a process based on the previously described assembling technique. The system has three basic components, that, in the past were made of different materials; in a new design this system has been molded from TPO based polymers to provide rigidity, comfort, safety, and aesthetics. Some prototypes have been recycled following the traditional procedures for the scrap recovery of thermoplastic materials. The mechanical characteristics of the obtained materials are reported in Table 9.

Floor covering The structure proposed does include a lower high density TPO based sheet, an intermediate polypropylene felt layer, and a polypropylene fiber carpet ( loop pile or cut pile ) on the top. A sketch of this component is shown in Figure 7. The recovery possibilities of the scraps from the sandwich production (about 20%) and material coming from the whole structure are the following: after passing through a mechanical mill, the material can be calendered and then processed by injection molding as well as mixed with virgin polypropylene and afterwards

136

Processing of Mixed Plastic Waste

Table 10 Properties of PP recycled from floor covering Property

Method

Units

Recycled material

Tensile strength

ASTM D638

MPa

8.4

Elongation

ASTM D638

%

141

Hardness Shore D

6 mm/3 sec

points

54

Charpy impact

ASTM D256

MPa

1.84

Figure 8. Door panel prototype.

extruded. Materials, obtained from sandwich samples and processed as discussed above, have been usefully utilized to produce the lower layer of the floor covering and have met the required specifications. The characteristics of the recycled materials are reported in Table 10.

Other components All the other interior components are now being designed following this approach procedure: Door-panels - Polypropylene materials allow the production of integral structures of aesthetic and functional elements produced within the same family of

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137

materials. The frame and the arm-rest are obtained by injection molding of glass filled PP. Trim parts are produced in lightweight panels of thermoformed sheets which are laminated with fabrics, calendered film materials and/or carpeting. PP air ducts are also included. Figure 8 shows a prototype of a door-panel. Pillar Trim & Rear Shelf - Blow molding has been used for the pillar trim to provide the double functions of air ducting and aesthetic trimming. Finishing is provided during the molding process with an application of fabric or textured film. The support structure may be filled or unfilled types, depending on the mechanical properties required. For the rear shelf, the design presented is a vacuum formed laminate of extruded PP and woven fabric.

CONCLUSIONS • • • •

The general solution to the problem of plastic recycling at the life-end cannot be unique and some suitable approaches have been studied, depending on application sectors. Although mixed plastics can be processed and recycled through some technologies, the use of homogeneous or compatible plastics seems to be the most suitable way to allow a direct and more profitable recycling. Recent technological advances have made available thermoplastic families of materials that can be tailored in order to meet different requirements of each application sector. It is possible to design automotive structures, for instance, made of the same chemical material and provide an important aid to the recycling of plastic.

REFERENCES 1. 2. 3. 4. 5.

J. C. Haylock, A. Addeo, and A. J. Hogan, Thermoplastic Olefins for Automotive Soft Interior Trim, SAE International Congress and Exposition, Detroit, Michigan, February 26 - March 2, 1990. A. Addeo, Mechanical Energy Absorption by Plastic Foam, Sitev Forum, Geneve 15-18 May 1990. A. Addeo, Novel TPE for Car Interior Trim : a PVC Replacement, TPE 90 Deaborn, Michigan, March 28-29,1990. A. Addeo, New Materials for Automotive Interiors, 22nd ISATA Florence, Italy 14-18 May, 1990. F. Forcucci, D. Tompkins, and D. Romanini, Automotive Interiors Design for Recyclability, 22nd ISATA Florence, Italy 14-18 May, 1990.

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The Use of Recyclable Plastics in Motor Vehicles Michael E. Henstock and Klaus Seidl* The University of Nottingham, Nottingham NG7 2RD, England *BMW, Section EG-554, München, Germany INTRODUCTION In market economies recovery operations are judged on financial criteria. Thus, whether or not a discarded waste is exploited for the materials it contains depends, among other things, on the technical ease of recovery of a saleable secondary product. Since industry must also consider the environmentally-sound 1 disposal of its residuals the ease of residuals disposal is financially important.

RECOVERABLE MATERIALS IN THE MOTOR VEHICLE Discarded vehicles yield ferrous metals, aluminium, copper, lead, and zinc. However, as currently processed by shredding, they also generate a valueless fraction, containing the non-metallic detritus of seats, carpets, tyres, and other components. The historical position of steel in the automotive scrap industry emerges from data for the composite car described in the pioneering work of 2 Dean and Sterner. The recovered steel, even when contaminated with the residual copper in the car, would have generated 43% of gross revenue in 1969, 3 60% in 1976, and 56% in 1986 at prevailing metal prices for those years.

PRESENT RECOVERY PRACTICE A discarded vehicle may first be stripped of resaleable parts by a salvager (wrecker) or dismantler, or it may go directly to a scrap processor, who may or may not strip such parts before processing the hulk. Until some twenty years ago processing usually took the form of compression into a cuboid or rectangular bale, known in the USA as a No. 2 bundle, and in the United Kingdom as a No. 5 bale. Sometimes, where air pollution regulations permitted, the hulk was incinerated to remove non-metallic items, but since this step involved cost it was often omitted. The bale, containing all the non-ferrous and non-metallic materials

140

The Use of Recyclable Plastics in Motor Vehicles

that it had not been worthwhile to remove, was then charged to a steelmaking furnace. Because it is impossible to inspect the interior of bales their purity is unknown and their desirability limited. Hence, this procedure is now much less common. Early in the 1960s the market was considerably disturbed by the introduction of the scrap shredder, which comminutes feed, including whole vehicles, to fragments some 5-20 cm in diameter. For physical and chemical reasons steel mills usually prefer shredded scrap to bales. Hence, shredded steel scrap enjoyed a rapid rise in popularity and market. The shredder generates, however, a non-metallic residue, currently worthless, whose disposal involves cost. In 1991 there is, however, increasing realization that shredding, though offering financial advantages over baling so far as materials recovery is concerned, is nevertheless a noisy, energy-intensive, and self-destructive process. There is, at last, some interest in designing road vehicles to make them easier to dismantle. Ironically, far from being pleased by this prospect, some sectors of the scrap industry view with disquiet the setting up, by certain vehicle manufacturers, of 4 in-house recycling plants.

CHANGES IN THE MATERIALS USED IN VEHICLES Changes in vehicle materials are made for a variety of reasons, including cost, absolute performance, lightness, (to improve fuel economy), and longevity. The magnitude of fuel savings over an estimated 174,000 km vehicle road life has 5,6 been computed as 15-25 l/kg of weight saved. Though improved engineering design may achieve weight reductions, a point is reached where further improvements may be made only through the use of light materials. Considerable effort has therefore been devoted to substitution of steel and cast iron by lighter materials, such as aluminium and polymers. Polymers are used in manufacturing for financial and technical reasons. In 7 transport they can provide reductions in the first cost. Such cost reductions are not necessarily permanent, since technological improvements and shifts in the raw materials prices continually give one material a cost advantage relative to another. Such a case is the substitution of plated ABS for zinc-based die castings in trim. However, plastics also offer other advantages: their use can achieve significant weight savings relative to metals and, whether for this reason or for others, they may offer substantial lifetime energy savings. For example, it has been

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141

calculated that although manufacture of a given automotive fuel tank in HDPE typically consumes 7% more energy (taking into account both the energy content of the material and the energy required for fabrication) than does a corresponding tank in steel, (0.738 GJ/tank in plastic compared with 0.685 GJ in steel) 68% of the total energy consumed in the production of the latter is associated with the conversion of sheet steel into tanks and so cannot be reclaimed through recycling processes. By contrast, most of the total energy of manufacture of a plastic fuel tank is inherent in the material and only relatively little (12%) with fabrication into the finished article. Thus, plastics have a low requirement for energy once the material has been manufactured. In addition, weight savings associated with the plastic fuel tank represent a saving of 23.3% in the fuel required to transport the tank during an estimated vehicle lifetime of 150,000 km. If the energy required for recycling is also included in the calculations it may be seen that, relative to the recycled steel fuel tank, the recycled plastic tank can effect a 8 28.8% energy saving during manufacture, lifetime use, and recycling. There is, therefore, some advantage in reclaiming the plastic as material rather than burning it for energy recovery. So far as recyclability in its widest sense is concerned the most important change has been the reduction in the amount of steel used. Of increasing significance now, however, is the disposal cost for the non-metallic residuals of vehicle and appliance dismantling and shredding operations; their low density makes them expensive to transport and landfill. This problem is likely to intensify. The 2 1960 model composite car described by Dean and Sterner contained a mere 0.9 wt% polymers; in the model years 1972 and 1973 U.S.-built cars typically contained almost 5% polymers, largely in safety and comfort applications. By 1985 the plastics contents of automobiles produced in various countries were 8% (Japan), 9% (USA), and 10.5% (Federal Republic of Germany). It is estimated that these figures will, by 1995, have risen to 13%, 14.5%, and 15%, respectively, with a corresponding increase in the quantities of non-metallic waste during scrap9 ping.

THE EFFECTS OF MATERIALS SUBSTITUTION ON VEHICLE RECYCLING The energy crisis of 1973 changed the purchasing patterns for cars. The real price of oil has fallen significantly since that time, and large cars are again in demand, especially in the United States. However, some of the move towards

142

The Use of Recyclable Plastics in Motor Vehicles

smaller and lighter vehicles is undoubtedly permanent, and the future shape of the vehicle recycling industry may be analysed in terms of possible changes in size and weight of cars, and also in their composition and mode of manufacture. 10 A detailed analysis by Roig et al. divided the car by size into three market categories for the U.S.A. It presented data on 1975 model year vehicles and predicted materials use in 1980 and 1990 model year cars. Significant conclusions regarding changes in recoverable values are difficult to make since no data are available which are precisely comparable with those of 2 Dean and Sterner for the 1960 composite car. However, the nominal composi10 tions derived by Roig may be used, with appropriate scrap metal values, to determine the relative value of the recoverable materials contained in 1960, 1975, 1980, and 1990 model year vehicles. These changes are given in Table 1, which shows scrap value changes at metal prices obtaining in April 1976 and December 1986. At those prices there is no clear trend in the values of materials recoverable from cars between the model years 1960 and 1990. Scrap values recalculated at December 1976 prices show that probable revenues for the 1980 and 1990 model years exhibit a drop of around 10% compared with the 1960 model year. What is clear is that, at any reasonable set of metal prices, revenues have not kept pace with inflation. Since metal prices can change significantly, particularly in the short term, there may be correspondingly large changes in profitability. The increase in the value of secondary aluminium between 1976 ($353/tonne) and 1986 ($554/tonne) reversed the decline in hulk dollar values attributable to reduced ferrous metal content. This change in the value of the recoverable scrap aluminium and the reduction in labour costs brought about by introduction of the shredder have been the dominant factors in maintaining the profitability of the recovery operation. A fall in the unit value of secondary aluminium could be equally significant in 11 making it unprofitable. Figure 1 shows that a notional composite car would yield metals which would, in 1976, have commanded $106.85; at 1986 metal prices they would command only $108.51, an increase in ten years of 1.6% of the 1976 value. The cost of the necessary labour for hand-dismantling would, however, have risen from $31 to $54, a rise of 74% on the 1976 costs in the same period. No realistic data are available for the changes in the costs of disposal of the non-metallic residues, but they are known to be rising rapidly.

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143

Table 1 Changes in scrap values for the 1975 composite car and the 1960 composite car (1976 and 1986 metal prices) 1975

1980

1990

a,b

c,d

a,b

c,d

a,b

c,d

change

change

change

change

change

change

(wt)

(value)

(wt)

(value)

(wt)

(value)

kg

$

kg

$

kg

$

-233

-12.11

-460

-23.92

-565

-29.38

58

5.22

-19

-1.71

-102

-9.18

22

11.97

58

31.55

113

61.47

0.5

0.33

-3

-1.95

-8

-5.2

Zinc

-11

-3.01

-19

-5.21

-21

-5.76

Lead

-

-

-

-

-1

-0.06

Material

Steel Cast iron aluminium Red metals

e

Net change relative to 1960*

2.4

-1.24

11.89

Net change relative to 1960**

-0.86

-11.19

-9.55

a b c d e * **

data for 1975 vehicle and projections for 1980 and 1990 from Ref.10 reference points are the data from Ref.2 other metals (alloying elements, etc.) and non-metallics are assigned zero value metallic values are from Table 2 for 1986, and from a similar exercise for 1976 red metals: copper and its alloys calculated at December 1986 prices calculated at April 1976 prices

Clearly, by suitable choice of date, the analysis could be made to yield very different answers. However, it is clear that, to remain profitable in the face of any future increases in the costs of utilities and labour, the vehicle recycling industry must seek further improvements in its efficiency.

144

The Use of Recyclable Plastics in Motor Vehicles

Figure 1. Changes in recoverable metal values and labour costs for vehicles of 1960 - 1990 model year at April 1976 and December 1986 metal prices.

DISPOSAL OF RESIDUALS An important financial element in recycling is residue disposal, whose costs are part of the overall economics of the recovery operation. Four general methods have been advanced for commercial utilization of the light fraction: landfill, polymer-enriched fractions, structural materials, and fuels. It is unlikely that any revenue could be generated by landfill, since the non-metallic residue is voluminous, compressible, and lacks the properties desirable in hard-core. Current technology cannot profitably separate individual plastics from shredder residues to provide fractions which can be used in applications similar to the original use. It has been estimated that, in the period 1985-1990, only about 25% of the total quantity of plastics waste is likely to be diverted from the municipal waste stream and recycled. Of this, some 27%, i.e. approximately 7% of the total is likely to come from the transport sector in the United States.

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A step lower in the hierarchical use of polymers, attempts have been made to separate polyurethane foams from shredder residues by flotation and to hydrolyse the recovered foam to reusable mixtures of polyether glycol monomers and toluene diamine. However, no commercial process seems to have gone into oper12 ation, presumably because of unattractive economics. Mixed plastics scrap has been converted to a variety of useful products including fencing, pallets, piling, and moulded items; however, this has usually been (clean) in-plant scrap rather than that recovered from vehicles and appliances. Post-consumer plastics scrap present a more intractable problem. There has been much effort world-wide to utilise heterogeneous wastes from plastics and fibrous materials, from metal-plastic composites, from laminates, and from residues such as waste tyres, as raw materials of various kinds. Although such scraps can often be fabricated into useful structural shapes it is desirable to improve their properties through bonding agents, by blending with a scrap material having an increased content of thermoplastics, or by blending with certain inexpensive resins, such as PVC. Various proprietary binders have been evaluated in the preparation of experimental building panels. Non-metallic shredder residues have an estimated heat value of 16.6 MJ/kg, 13 compared with about 30 MJ/kg for anthracite. Some studies suggest an even higher heat value, of 24 MJ/kg. This could, in principle, be used to generate in-plant steam, or even electricity and so improve the profitability of the scrapyard. Thermal degradation of plastics by any route should, though, be the method of last resort. They are too valuable to be burned. They should, if possible, be retained in their most useful state, in this case as structural materials. If, however, they are too badly contaminated for recovery and recycling in structures, thermal degradation for heat recovery and volume reduction is arguably better than landfill. Shredder residues cannot currently be exploited economically because the material is not readily combustible, has a very high (50%) ash content, and gener14 ates slag which, when solidified, is difficult to extract. It also contains certain components, of which PVC has given particular cause for concern, which release corrosive and polluting fumes. Some 80-90% of these can be removed by suitable techniques, but the need for scrubbers may so increase the cost as to minimise any financial gain from incineration.

146

The Use of Recyclable Plastics in Motor Vehicles

RECYCLABLE PLASTICS COMPONENTS An alternative approach to the recovery of automotive plastics is to use them as large, easily removable components which offer potential for reclamation as well-characterised individual polymers. Some particularly complex components, such as vehicle front- and rear-end systems, exhibit special suitability for manufacture in plastics instead of metals, because of their ease of production and assembly. Plastic fuel tanks are now in common use, and an investigation has been made of some possibilities for the recycling of the materials used in them. The project comprises of four stages: • Initially, the recyclability of virgin high-density polyethylene (HDPE) was assessed, on a labouratory-scale, by processing, regrinding, and reprocessing for 15 cycles, with property assessment after each cycle. • Since fluorine is used to minimise the quantity of fuel which migrates through the HDPE material, the next step was to recycle, again on a labouratory-scale, unused fluorinated and unfluorinated fuel tanks, and hence to determine the effect, if any, of fluorine on the properties of the recycled material. • The results of Stage 2 were then compared with those obtained by recycling of used fuel tanks, again on a labouratory-scale, to assess the effects of ageing and exposure to and absorption of fuel; the tanks were some ten years old and had been taken from cars which had covered some 150,000 km. • Stage 4 will involve the recycling of used plastic fuel tanks on an industrial scale.

PRELIMINARY RESULTS Comparison of virgin and recycled HDPE The plastic fuel tanks investigated are produced from Lupolen 4261A, supplied by BASF AG, Ludwigshafen. In Stage 1 of the trials the virgin HDPE was formed into injection-moulded test pieces using an `Engel CC 80’ injection moulding machine, and tested in a variety of mechanical tests. The tested samples were then successively reground and remoulded. The following mechanical

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147

properties were assessed, and the change in each property reported after 15 cycles: Modulus of elasticity Yield stress Elongation at yield stress Charpy impact test Unnotched Notched MVI (melt volume index)

-8.2% -3.0% -20.7% Not measurable +12.0% -60.5%

Comparison of fluorinated and unfluorinated HDPE The HDPE of fuel tanks contains small amounts of fluorine, determined as 0.3-1 g per 10 kg tank, incorporated as a sealant and present as polytetraethylene (PTFE). In Stage 2, to determine the effect of this fluorine, samples of fluorinated and unfluorinated tanks were sawn into small pieces and reground before being formed into injection-moulded test specimens for determination of torsion modulus, and of tensile, impact and ballistic properties. The thermal properties investigated were the degree of crystallinity, melting temperature, and flow characteristics. The measured properties were compared with those provided for in the relevant quality order for new material.

Torsion test Torsion properties satisfied the quality order, with no significant difference between fluorinated and unfluorinated materials.

Tensile test The tensile properties of the recycled material lay within the 10% tolerance band of the values of new material, and hence satisfied the provisions of the quality order.

Charpy impact test The quality order requires that the test samples must not break. Some of the test specimens fractured and, for those, the results are therefore invalid in the context of the quality order.

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The Use of Recyclable Plastics in Motor Vehicles

Ballistic test All test samples showed a ductile fracture. It is, however, difficult to compare the values determined for injection-moulded test specimens with those exhibited by the original tank material and those specified by the quality order since the test specimens were only 3 mm thick, compared with the average thickness of a tank, which is between 5 and 8 mm.

Degree of crystallinity The degree of crystallinity of both the fluorinated and unfluorinated materials, both after one regrinding and one remoulding, lay within the provisions of the quality order.

Melting temperature The remelting temperature of fluorinated and unfluorinated materials, both after regrinding and after remoulding, lay below the temperature range stipulated by the quality order.

Flow index No difference could be detected between fluorinated and unfluorinated samples evaluated by a spiral flow mould test.

The recycling of material from used fuel tanks Parallel investigations were carried out to establish how much fuel the tank material absorbs, and the time involved for the fuel to migrate from the material when no longer exposed to fuel. At a room temperature, samples punched from the original (fluorinated) tank achieved constant weight, with weight increases in the range 6.6-7.3%. Since material recycling cannot take place without elimination of the fuel, drying tests were carried out under a range of conditions. It was established that temperao tures exceeding 100 C were necessary to eliminate all fuel. The material used for the investigation was obtained from Volkswagen Golf and Passat vehicles, which have employed plastic fuel tanks for some 10 years. The time required for tank removal, cleaning, and elimination of tank components, made either of metal or a plastic other than HDPE, averaged some 5 min 30 sec. At a labour cost of DM 45/h this implies some DM 4.2 for recovery of a fuel tank yielding 3.8 kg of HDPE with, at a price of about DM 1.8/kg for secondary HDPE, a nominal value of DM 6.9.

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Before the tank was reground and reprocessed the fuel was extracted for 24 h o at 120 C, to avoid the risk that reprocessing might reach the fuel ignition temo perature of 230-260 C. Subsequent material reprocessing caused no problems other than the unpleasant odour, which was absent from samples made from virgin material. Since all finished fuel tanks are in fact fluorinated, the mechanical, rheological and thermal properties of the recovered material were compared only with the fluorinated samples.

Torsion test The recovered material met the requirements of the quality order in respect of torsional properties.

Tensile test The moduli of elasticity of samples made from recovered material were much higher than those made from unused tanks. Though perhaps partially attributable to cross-linking, it is more likely to reflect a change of specification of the 2 Lupolen 4261A, whose modulus of elasticity was in 1978 given as 1200 N/mm 2 and in 1989 as 850 N/mm . The higher value of the modulus of elasticity does not necessarily constitute a poorer quality of material so far as the requirements of plastic fuel tanks are concerned.

Charpy impact test As before, none of the test specimens broke, so satisfying the criteria of the quality order. It was noted that the impact properties did not deteriorate on recycling, as usually occurs.

Ballistic testing Since all specimens exhibited ductile fracture it was possible to compare used and unused polymers. It was noted that, although the materials had been in service for ten years before recycling, the low-temperature impact properties had not deteriorated. As before, it is difficult to compare the values determined for injection-moulded test specimens with those exhibited by the original tank material and those specified by the quality order since the test specimens were only 3 mm thick, compared with the average thickness of a tank, which is between 5 and 8 mm.

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The Use of Recyclable Plastics in Motor Vehicles

Degree of crystallinity The results fell within the ±10% tolerance band of the quality order.

Melting temperature The melting temperature, evaluated by differential scanning calorimetry, was lower than that specified by the quality order.

SUMMARY AND CONCLUSIONS The monetary value of recoverable materials in road vehicles has fallen as plastics have replaced steel. Increases have occurred in the cost of the labour for scrapyard dismantling and in the amount of unsaleable residue and, therefore, in the associated disposal costs. Improvements in scrapyard economics may possibly be achieved by the prior removal from vehicles of large polymeric components and their recycling as well-characterised plastics fractions. Some trials with material recovered from used plastic fuel tanks show promising results for the manufacture of new tanks.

REFERENCES 1. 2. 3. 4. 5. 6.

7. 8. 9.

M. E. Henstock, Design for Recyclability, London, The Institute of Metals, pp. 3-6 (1988). K. C. Dean and J. W. Sterner, Dismantling a Typical Junk Automobile to Produce Quality Scrap, United States Bureau of Mines, RI 7350, Washington, (1969). M. E. Henstock, Conservation and Recycling, 2 (1), 69 (1988). R. Franklin, Recycling Cars, Materials Reclamation Weekly, 8 December, 21 (1990). K. D. Marshall, The Economics of Automotive Weight Reduction, Soc. of Automot. Engrs, Paper No. 700 174, Automot. Engng. Congr., Detroit, 12 January, 1970. M. C. Flemings, K. B. Higbie, and D. J. McPherson, Report of Conf.: Energy Conservation and Recycling in the Aluminum Industry, Massachusetts Institute of Technology, (co-sponsored by the Center for Materials Science, M.I.T. and the U.S. Bureau of Mines, with the cooperation of the aluminium Ass.), 18-20 June, 1974. Anon, The Energy Content of Plastics Articles, Association of Plastics Manufacturers in Europe, Distributed in the United Kingdom by the British Plastics Federation, Publication No.309/1, (April, 1986). K. Seidl, Development of a Total Energy Balance for the Manufacture of Fuel Tanks in Steel and Plastics, Unpublished calculations (1991). K. Muller, The Increasing Use of Plastics and Its Impacts on the Recyclability of Automobiles and on Waste Disposal in West Germany, The United States and Japan, Vortrag zur Recyclingplas II, Washington, D.C., 18-19 June, 1987.

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10. R. W. Roig, M. Narkus-Kramer, and A. L. Watson, Impacts of Materials Substitution in Automobile Manufacture on Resource Recovery, Symp., The Technology of Automobile Recycling, Univ. of Wisconsin, 16 October, 1975. 11. The calculations are intended to illustrate only the absolute changes in quantities of recoverable metal with change in model year and to facilitate comparison by application of metal prices at arbitrary times, i.e. April, 1976 and December, 1986. No attempt has been made to determine the revenues obtainable by scrapping each model year after a predetermined period and then deflating prices to 1988 levels. 12. M. E. Henstock, Design for Recyclability, The Institute of Metals, London, p. 74 (1988). 13. K. E. Boeger and N. R. Braton, Resources and Conservation, 14, 133 (1987). 14. G. R. Daborn and M. Webb, Treatment of Fragmentizer Waste by Starved Air Incineration - a Brief Feasibility Study, Department of Industry, Warren Spring Laboratory, LR 465 (MR) M, October, 1983.

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Ground Rubber Tire-Polymer Composites K. Oliphant, P. Rajalingam, and W. E. Baker Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6

INTRODUCTION Discarded tires represent a significant component of the overall plastics recycling challenge. They are an easily segregated, large volume part of the waste stream and present their own, somewhat unique, waste utilization problems. The whole issue is complicated by many alternative proposals, varying government legislation and preferences, incomplete technical information, and eco1 nomic uncertainties. A number of reviews have already discussed this overall disposal problem and 1-5 examined many of the proposed approaches. The major problem lies in finding approaches that are both economically and environmentally sound. Some of the methods of utilizing scrap tires that have been investigated are: burning, pyrolysis, use in cleaning up oil spills, road surfaces, roofing materials, and playground surfaces (for details see above mentioned reviews). While some of these approaches have been put into practice, the scrap tire disposal problem is still clearly a case where supply far outstrips available uses, and new methods of utilization (or technological advances to extend existing ones) are clearly needed. One area that has the potential to utilize large volumes of discarded tires is their use as a filler in polymer composites. It is the problems associated with this approach, and the technological advances made in overcoming these problems, that are the focus of this review. Unfortunately, much of the work in this area has been undertaken by industry and is not available in the literature. The literature which is available, however, is presented, along with an in-depth look at the work carried out in our laboratories.

154

Ground Rubber Tire-Polymer Composites

Figure 1. Ground rubber tire particles. (left) cryogenically ground, (right) ambiently ground.

GROUND RUBBER TIRE COMPOSITE BEHAVIOR Although the use of ground rubber tire (GRT) as a filler in polymer blends is a potentially attractive approach, it is fraught with a number of difficulties. Generally, when the large GRT particles are added to either thermoplastic or thermoset matrices there is a large drop in mechanical properties, even at rela6,7 tively low filler loadings. Given that the approach here is to use the GRT as a low cost additive, and that there are a number of other materials competing in this regard, overcoming this large drop in properties has to be accomplished with little added cost (both in terms of additives and additional processing). This has proven to be quite a challenging task. In the following sections the major factors influencing thermoplastic GRT composite properties are discussed along with approaches to improving these properties.

Tire Grinding In order to be used as a filler in polymer composites, tires are first ground into a fine powder on the order of 100-400 µm. This is accomplished typically through either cryogenic or ambient grinding. General reviews of the size-reduction pro8-10 4 cess have been published. A typical process generally involves tire splitters to cut the tire initially, followed by a two-roll grooved-rubber mill or hammer

K. Oliphant, P. Rajalingam, and W. E. Baker

155

mill. The bead wire is removed by hand or with magnets and fiber is removed at intermediate operations with hammer mills, reel beaters, and air tables that blow a steady stream of air across the rubber, separating the fiber. Between ambient and cryogenic grinding there is a noticeable difference in the nature of the ground rubber tire (GRT) particles. As shown in Figure 1, the surface of the cryogenically ground rubber is smooth and regular (because the particles are cooled below their glass transition temperatures before fracture) compared to the rough irregular surfaces of ambiently ground material. There has been no complete study (to our knowledge) on the advantages (or disadvantages) of cryogenically versus ambiently ground GRT particles in polymer composites. Such a study is complicated because of the influences of particle size, particle size distribution and contaminant levels on GRT-polymer composites, and the fact that these all vary from supplier to supplier. It has been found, however, that significant differences in composite properties are found for GRT of similar mesh (particle) size sourced from different suppliers and further study of particle characteristics is currently underway in our laboratories.

Characteristics of Tire Particles It is the complex nature of the GRT particle that complicates its use as a filler in polymer composites. The most pertinent feature of GRT particles is that they are still highly cross-linked. This has two major consequences: • there is little breakdown of the particles under normal melt compounding conditions (see section on particle size) • there is a sharp interface resulting in poor adhesion between the GRT particles and the matrix (see section on adhesion). GRT particles are also compositionally quite complex. Tires contain a number of different rubbers (SBR, butyl rubber, natural rubber, polybutadiene rubber etc.), carbon black filler, antioxidants, and additional additives, the exact composition depending on the type of tire and the part of the tire (e.g. tread vs. sidewall, vs. liner). Elementally, a typical tire is comprised of carbon 83%, hydrogen 1 7%, ash 6%, oxygen 2.5%, sulfur 1.2%, and nitrogen 0.3%. There is approximately 45-55% rubber hydrocarbon, 10-15% acetone extractables, 20-30% car11 bon black, and 6% ash.

156

Ground Rubber Tire-Polymer Composites

Polymer Matrix As this paper focuses on the use of GRT as a filler in thermoplastic systems the literature pertaining to thermoset systems will only be briefly reviewed. In general, addition of GRT to rubber vulcanates reduces all physical properties, the extent of deterioration depending upon the amount and particle size of the GRT 12-14 15 added. The use of coupling agents has been reported to improve the properties of these systems. GRT particles are also found to decrease the tensile, flexural and storage shear modulus in composites with an unsaturated polyester 16 15 resin. Very poor properties are again reported for a GRT-phenolic compound. Similar poor behavior for GRT-thermoplastic composites is often reported. 17 Deanin and Hashemielya report on GRT composites made with six different polymers (HIPS, PP, HDPE, LLDPE, LDPE, and ABS) and five different elastomers (SBS, SEBS, SIS, butyl, and EDPM). The addition of the GRT reduced the tensile strength noticeably, and is not reported to provide for any increase in impact strength. Poorer properties are reported for more brittle matrices. The GRT in this study was, however, broken down to some extent in a prior mastication step, reportedly enabling it to form into a “thin thermoplastic sheet”. Its exact nature, compared to the still highly cross-linked large GRT particles typically 15 employed in GRT composites was not reported however. Tuchman and Rosen examined composites of GRT and PP, ABS, PS, LDPE, and HDPE. Addition of GRT was reported to reduce all the mechanical properties of ABS, LDPE, and HDPE. The Izod impact strengths of PP and PS, however, are reportedly increased (by up to three times for PP at 40 wt% GRT) upon the addition of GRT. 18 Phadke and De, however, report that there is a decrease in the impact strength of PP when GRT is added. 6 Duhaime and Baker report on the properties of LLDPE-GRT composites. The impact strength is seen to drop by 35% even at 10 wt% filler loading. The impact strength continues to drop until at 30 wt% filler it is 50% lower than that of the pure LLDPE. It remains approximately constant, however, from 30-60 wt% GRT. The impact behavior was characterized using a Rheometrics drop-weight instrumented impact tester, in which a piezoelectric load cell is tip-mounted to a high velocity dart permitting the recording of the load-displacement curve for the entire impact event. For pure LLDPE, impact failure is seen to be a ductile 19 yielding process in which the dart draws the material out as it passes through. For the GRT-LLDPE composites the failure is similar except that ‘yielding‘ occurs at much lower forces and elongations, and the material is not drawn out to

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Table 1 Influence of melt flow index on the impact energy of LLDPE/GRT composites MFI

Impact energy (J)

Drop (%)

Pure PE

40 wt% GRT

0.3*

15.8

7.6

52

1.0

21.0

13.9

34

5.0

14.9

7.9

47

12.0

12.9

7.4

43

20.0

11.4

6.8

40

*LDPE

as high elongations because of premature failure caused by the large, poorly bonded rubber particles. In tensile tests, the tensile strength and ultimate elongation are seen to decrease steadily with increasing GRT content. At 40 wt% GRT the tensile strength has decreased by 48% and the tensile elongation by 80%. 7 Oliphant and Baker report on blends of GRT with LLDPE and HDPE. The addition of GRT to LLDPE results in composites with properties similar to those 6 described by Duhaime. The deleterious effects of the GRT particles on HDPE is, however, more pronounced than for blends of the GRT with LLDPE (70% decrease in impact strength for HDPE compared to 50% for LLDPE at 40 wt% GRT), although the observed trends are similar. In contrast to the failure of LLDPE described previously, the failure of pure HDPE, although it involves some plastic deformation, is observed to occur through catastrophic propagation of a crack through the impact zone. This type of failure is also observed in the GRT-HDPE composites (compared to the ductile tearing process observed for GRT-LLDPE composites). It is this difference in impact failure which is suggested to be responsible for the poorer properties of GRT-HDPE composites. It is postulated that in HDPE-GRT composites the failure remains semi-brittle because the particles are too large to induce a brittle to ductile transition (or a shift in the brittle to ductile transition temperature to below the test temperature). Failure then occurs largely through crack propagation and the large particles

158

Ground Rubber Tire-Polymer Composites

act as serious flaws, providing an easy path for the crack to follow. The addition of GRT to a semi-brittle matrix is therefore believed to require much higher levels of adhesion (to retard crack growth at the particle/matrix interface), or much lower particle sizes (to lower the brittle-ductile transition temperature). This is borne out to some extent experimentally (see reference 7 and section on adhesion). 20 Rajalingam and Baker have studied GRT composites with a number of different LLDPE’s and a LDPE. The impact properties of the pure matrices and their corresponding 40 wt% GRT composites are given in Table 1. Although there is some variation in the percent drop in impact strength with MFI, the addition of GRT is shown in all cases to have a similar influence on mechanical properties. There are two exceptions of note: the higher molecular weight 1 MFI LLDPE appears to produce composites with lower material property drop, and the reduction in impact strength for the LDPE is slightly higher than for the LLDPE 17 composites. Deanin and Hashemiolya have also reported that LDPE produced poorer GRT composites than LLDPE. It is also interesting to note that the LLDPE of MFI = 1.0 produces a 40 wt% GRT composite with slightly higher impact strength than for pure LLDPE’s with MFIs of 12 and 20 dg/min. Thus, if the higher viscosities of the composite can be tolerated in processing, these materials may prove to be useful composites. In general then, it is seen that simple addition of GRT to most polymers by melt blending results in a significant deterioration in mechanical properties.

Particle Size The large rubber particle size used in GRT composites is reported to be one of the two major factors (the other being adhesion) contributing to the poor mechanical properties generally observed for GRT-polymer composites. The importance of particle size (and particle size distribution and shape) on mechanical 21 properties of composites in general is well known in the literature. For rubber toughening applications it is generally reported that there is an optimum parti21 cle size (typically in the 0.1-5 µm range) for toughening brittle polymers, and a minimum particle size (or inter-particle distance) for toughening semi-ductile 22 polymers (typically less than 1 µm). For hard particulate fillers, impact strength is generally observed to increase with a decrease in particle size. In general, for optimum composite properties, a low particle size is desired. In GRT-polymer composites, however, the particle size is (relatively) quite large.

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The problem is that there is a little (or no) breakdown of the particles under nor7 mal melt blending conditions, due to the highly cross-linked nature of GRT. The particle size is, therefore, controlled largely by the grinding process, which in turn, is influenced by process choice and economics. In order for GRT to be used as an economical filler the particle size then has to be kept as large as possible to minimize grinding costs. The lower limit on particle size necessary to produce economical composites is generally regarded to lie in the 40-80 mesh range (≅ 400-100 µm), which is well above the particle sizes commonly used in rubber toughened and hard particulate filled systems. There may be some advantage, however, in going to smaller particle sizes if significant gains in mechanical properties are realized. It has been reported that the detrimental effects of add12-14 ing GRT to cured rubbers decreases as the particle size is decreased. Similar 23 behavior is observed for GRT recycled back into tires, and with the use of 12 ultrafine rubber (20 µm) the detrimental effects are almost eliminated. 7 Oliphant and Baker found that decreasing the GRT particle size from ≅ 350 µm to ≅ 100 µm resulted in improvements in impact strength of 20%, and in tensile elongation of 40-50%. The particle size was seen, however, to have no effect on the yield point. Accompanying the mechanical property increase was a decrease in the melt flow index (MFI). This work was extended by Rajalingam 20 and Baker to cover particle sizes ranging from 28 mesh (600 µm) to 200 mesh (74 µm). These materials were obtained by sieving off different mesh sizes from a sample originally ground to 40 mesh (300 µm). Four wt% of a reactive coupling agent was added to the blends (see section on adhesion). Figure 2 shows the influence of particle size on impact energy and the MFI for blends of 40 wt% GRT in LLDPE. On going from 600 to 74 µm particles there is only a minor increase in impact energy (15%), while the MFI is seen to drop from 2.5 to 0.75 g/10 minutes. It should be pointed out that the impact values for these particular rubber samples are lower than those usually obtained at these filler loadings. It is well known that the addition of solid particles to a polymer increases the melt viscos24 ity and that this effect is more pronounced for smaller particle size. The MFI data are then as expected. At higher shear rates, however, the effect of particle 24 size is often negligible. The observed MFI decrease may therefore not translate into any problems in processing (which occurs at higher shear rates than in a melt index tester). Nonetheless, according to these results there is little advantage to be gained through reducing particle size over the range studied, especially given the increased costs this would entail.

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Ground Rubber Tire-Polymer Composites

Figure 2. Influence of ground rubber tire particle size on MFI and impact energy of 40 wt% GRT-LLDPE composites.

Adhesion 25

The importance of adhesion in the rubber toughening of polymers and in particulate and fiber composites is well documented. Weak interfaces, resulting in poor stress transfer, generally lead to poor overall composite performance. Poor adhesion is generally believed to be the major factor (in addition to the large particle size) leading to the large mechanical property decreases observed upon incorporation of GRT into most polymers.

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This poor adhesion is, at least in part, due to a high degree of crosslinking in the GRT particles. The highly crosslinked nature of the particles inhibits molecular diffusion across the interface so that there is little or no interpenetration of the phases, resulting in a sharp interface. In studying the role of molecular dif26 fusion in the self adhesion of elastomers, Ellul and Gent have shown that even in systems with complete thermodynamic compatibility, relaxation times are so large in highly cross-linked systems that interpenetration of the phases to form 7 a strong interface is not possible. Oliphant and Baker have shown in 180° peel tests of LLDPE melt bonded to a tire compound which simulates the tire tread composition that there is negligible adhesion. That this was largely an artifact of the cross-link density was shown in the significantly higher adhesion of LLDPE to a tire compound having a much lower concentration of curing agents and hence a lower degree of cross-linking. In this sense then GRT particles are akin to hard particulate fillers where specific surface interactions (e.g. covalent and/or H-bonding) are necessary to improve adhesion. Given the influence of particle size on GRT composites (and on the properties of particulate composites in general) there is some question as to what properties are ultimately obtainable with the large GRT particles even if good adhesion can be obtained. In other words, is there any justification for focusing on improving adhesion while leaving particle size fixed? To this end, blends of ground tire bladder (GTB) with 7 LLDPE and HDPE were studied by Oliphant and Baker. The GTB was a carbon black filled butyl rubber vulcanizate ground to 40 mesh (= 300 µm). Unlike the typical rubber components in a tire (natural rubber, polybutadiene, SBR, etc.), which have reactive sites on every monomer unit, the unsaturation in 28 butyl rubber is widely spaced along a saturated, flexible hydrocarbon chain. GTB therefore has a lower degree of cross-linking than GRT and consequently, 7 as shown by peel tests, a much higher level of adhesion to LLDPE and HDPE. In blends with LLDPE at 40 wt% the GTB actually results in an increase in the impact strength over pure LLDPE, compared to a 50% drop for GRT composites of similar particle size. Lower cross-link density and differences in the carbon black filler loads may result in a lower modulus for the GTB particles than for the GRT particles, which could also lead to the mechanical property improvements. However, no difference in modulus of the particles is suggested on comparing the relative stiffness of blends with GTB and GRT. Scanning electron

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micrographs (SEM) of the composites revealed little or no breakdown of the particles on blending. This suggests that if reasonable adhesion can be obtained quite worthwhile composites can be developed, and provides some justification in focusing more on developing methods for improving adhesion despite the large particle size. Little is reported in the open literature on the nature of the GRT particle surface or on its potential for chemical interactions that would increase adhesion. Infra-red spectroscopy has been reported to show the presence 28 of unreacted double bonds but does not mention the presence (or absence) of other functional groups. Electronic Spectra for Chemical Analysis (ESCA) of 20 GRT tire surfaces reveals an oxygen surface content of 5-15%, which may indicate the presence of -OH or -COOH functionalities. There have been a number of reports of processes that claim to improve properties of GRT-polymer composites through enhancing adhesion. Many of these are from industrial sources and include little specific information. For thermoset systems, coating the surface of the GRT particles with an unsaturated liquid and curable or cross-linking polymer has been reported to extend the level of 29 GRT that vulcanizates can tolerate. 15 Tuchman and Rosen report the use of an aqueous slurry process using a water-soluble initiator system to graft styrene to GRT. The styrene grafted GRT particles were found to give composites with properties superior to straight mechanical blends. 6 Duhaime and Baker investigated the use of co-reactive compatibilizers. In these systems the matrix phase (LLDPE) was doped with ethylene acrylic acid (EAA) copolymer while attempts were made to introduce an amine terminated butadiene nitrile (ATBN) reactive liquid rubber onto the GRT surface. Simple mechanical blends of all components, without any pre-mixing, resulted in GRT composites with little improvement over non-reactive systems. Pre-mixing, through melt blending, of the reactive additives with their respective phases resulted in composites with much better mechanical properties. The lower modulus of these composites, compared to those of straight blends of all components, might suggest that the ATBN is to some extent plasticizing the GRT particles. Much higher mechanical property improvements are obtained (up to 58% increase in impact energy over unreactive systems - or an impact strength 78% of that for pure LLDPE - at 40 wt% GRT) when a peroxide initiator is added during the pre-mixing of the GRT and the ATBN in an attempt to graft the ATBN to the 30 GRT surface. In subsequent work, this was shown to be, at least partially, due

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to cross-linking of the PE phase by residual initiator. In fact, spray coating of the GRT surface with low concentrations of a peroxide initiator (= 0.1 wt%) before blending with LLDPE resulted in composites with impact strengths up to 80% of those of the pure LLDPE. Although straight addition of the peroxide resulted in similar mechanical property improvements the precoating step was found to be essential in controlling the subsequent melt rheology and, thus, processability. Although the use of reactive additives in both phases has been shown to provide for reasonable mechanical property improvements, it is not the most attractive approach from the standpoint of economics. A search was therefore made for compatibilizing agents that would be compatible with the matrix phase and pro7 vide for some interaction directly with the GRT surface. It was found that pre-coating of GRT particles with an EAA copolymer overcame most of the deleterious effects of adding GRT to LLDPE (while still retaining composite processability). A blend of 40 wt% EAA coated GRT particles (4 wt% EAA) with LLDPE was shown to have impact and tensile strengths 90% of those for pure LLDPE, representing increases of 60% and 20% respectively, over blends with uncoated particles. It was suggested that an interaction between the EAA copolymer and functional groups on the GRT surface, resulting in increased adhesion, were responsible for the mechanical property improvements. These interactions may involve H-bonding of the acid groups in the EAA copolymer with oxygen groups shown to be present on the GRT surface. The precoating step was found to be necessary because addition of EAA to pure LLDPE resulted in a decrease in mechanical properties at the level of additive used. This prompted an investigation of coupling agents which might be more compatible with the PE matrix. The use of maleic anhydride grafted PE (PE-g-MA) resulted in increases in the impact energy of LLDPE-GRT compos31 ites of as much as 43%, without the need for a precoating step. Since the epoxy group readily reacts with a wide range of functional groups such as -OH, -COOH, -SH, -NH2 , the use of ethylene-co-glycidyl methacrylate 31 (EGM) as a coupling agent was investigated. Figure 3 shows the load displacement curves for the impact tests for (a) pure LLDPE, (b) 40 wt% GRT-LLDPE composite, and (c) 40 wt% GRT-LLDPE composite with 4 wt% EGM added (obtained using an instrumented impact tester as described previously). The impact energy (area under the force-displacement curve) of the reactively coupled composite is 58% higher than that of the straight GRT-LLDPE blend (without the necessity of a pre-coating step). Blends of EGM and LLDPE show virtually

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Figure 3. Effect of reactive coupling on GRT-LLDPE composites: (a) pure LLDPE, (b) 40 wt % GRT-LLDPE composite, (c) 40 wt% GRT-LLDPE composite with 4 wt% EGM.

identical impact behavior to pure LLDPE, and the increase in the impact behavior of the composite is, therefore, attributed to an interaction between the epoxy groups of the EGM and functional groups on the GRT surface. It is seen, therefore, that through use of a single compatibilizing additive it is possible to produce GRT composites with properties at least as good as those utilizing two reactive additives, without any necessity of a pre-coating step. This provides for significant savings in both material and processing costs. A range of one compo31 nent compatibilizing copolymers has been evaluated in our lab. In blends of

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GRT with HDPE, the addition of 4 wt% EGM results in a 70% increase in impact energy for 40 wt% filler loadings.The properties still lie well below the pure HDPE (= 50% of pure HDPE). This is attributed to the belief that less ductile matrices (or matrices where there is some crazing or crack formation on failure) 7 are less tolerant of the large GRT particles and, as described previously, consequently require either much higher levels of interaction (adhesion) or much smaller particle sizes.

Matrix Modification An alternative approach to improving upon GRT-thermoplastic composite properties is to add materials that will enhance the properties of the matrix, and thus the overall composite. Ideally the added modifier would result in a synergistic effect; the improvement in composite properties being greater than those of the matrix alone. A potentially even more attractive route is the use of an additive that not only modifies the matrix properties but also increases interaction (adhesion) between the matrix and the GRT particles. 32 In this regard, Rajalingam and Baker have studied the influence of reactive and nonreactive thermoplastic elastomers on GRT composites. Two styrene-ethylene-butadiene-styrene (SEBS) block copolymers of similar composition, one with maleic anhydride (MA) grafts and one without, were each added to both LLDPE and HDPE. For pure LLDPE the two thermoplastic elastomers result in similar increases in impact strength (15% for non-reactive SEBS and 17% for MA modified SEBS at 6.7 wt% additive). For the GRT composites, however, at 40 wt% GRT the non-reactive SEBS results in a 34% increase in impact strength while the MA-modified SEBS results in a 51% increase over GRT-LLDPE composites with no additives. The greater increase on addition of the MA-modified SEBS is attributed to interaction between functional groups on the GRT surface and MA groups on the SEBS. In pure HDPE, addition of both thermoplastic elastomers results in little increase in the impact energy (6.4% and 3.2% for the SEBS and MA-modified SEBS, respectively), although ductility is enhanced. In the 40 wt% GRT composites, however, the increases in impact energy are 58% and 62%, for the SEBS and MA-modified SEBS, respectively. These larger increases compared to those

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Figure 4. Properties of compatibilized GRT-PCPEPP composites: (a) unmodified RCPEPP, (b) 3:2 RCPEPP:GRT composite, (c) 14:10:1 RCPEPP:GRT:SEBS composite.

for LLDPE composites are believed to be a result of : • the larger percentage drop in properties on addition of GRT to HDPE (70% decrease at 40 wt%) compared to LLDPE (50% decrease at 40 wt%) • the increased ductility afforded the HDPE composites on addition of the thermoplastic elastomers. As discussed in the section on the influence of the polymer matrix, it is the semi-brittle failure of HDPE which is believed to be responsible for its greater intolerance to the large, poorly bonded GRT particles. The increase in the ductility of the HDPE matrix on addition of the thermoplastic elastomers, then, results in significant impact improvement through crack suppression, thus rendering the matrix more tolerant of the large GRT particles. The small increase on going from non-reactive to reactive SEBS may be an indication that

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Table 2 Impact properties of RCPEPP/GRT composites Blend composition

Impact energy (J)

RCPEPP

4.7

RCPEPP:GRT (3:2)

4.4

RCPEPP:GRT:SEBS (14:10:1)

10.5

RCPEPP:GRT:SEBS (13.8:22.2:1)

11.2

the adhesion, although increased, is still below the level necessary to achieve additional improvements in the properties.

GROUND RUBBER TIRE AND RECYCLED PLASTICS In order for GRT composites to find high volume applications they have to be a lower cost replacement for virgin, commodity polymers. To achieve this, each aspect of the composite fabrication process, from tire collection and grinding to compounding and final product manufacture, has to be optimized on a cost/performance basis. One very attractive approach to minimizing overall composite costs is to utilize recycled polymers as the matrix phase in place of virgin materials. There are also a number of advantages in addition to cost that this method may provide. As GRT composites are already black (although they can be col29 ored to some extent ), the use of colored post-industrial or post-consumer waste is easily tolerated. Low levels of dirt or impurities, which may render scrap unsuitable for some applications, will probably be of secondary concern compared to the large GRT particles. comingled waste, where material properties are usually very poor and require the addition of compatibilizing agents anyway, may provide for particularly interesting opportunities. As part of a larger study on 33 compatibilizing a comingled waste stream of PE and PP, Rajalingam and Baker have investigated composites of scrap PE/PP with GRT. The initial PE/PP mixed waste (RCPEPP) was found to have an impact energy of 4.7 J (See Table 2). Addition of a number of thermoplastic elastomers was found to provide significant increases in impact strength. For a particular SEBS copolymer, the addition of 6.7 wt% to the PE/PP blend increases the impact energy from 4.7 J to

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18.6 J, clearly showing a major compatibilizing and toughening effect for this potential matrix material. A blend of RCPEPP with GRT in a 3:2 ratio has an impact energy similar to pure RCPEPP, about 5 J. The addition of the SEBS copolymer to produce a composite of 14:10:1 (RCPEPP-GRT-SEBS) increases the impact energy to 10.5 J. A composite of even higher GRT content (13.8:22.2:1) has an impact energy of 11.2 J. Impact curves for select composites are shown in Figure 4. These results were obtained with a Rheometrics drop-weight instrumented impact tester (described previously). Curve A represents the impact failure of the unmodified RCPEPP blend, showing the highly brittle failure. Curve B shows the impact failure of the RCPEPP-GRT (3:2) composite, showing how the ductility is increased although the impact energy (integrated area under the curve) remains approximately constant. The large increase in ductility for the composite modified with SEBS (14:10:1) is shown in curve C. Clearly, the use of comingled waste as the matrix phase for GRT composites provides for interesting opportunities given that suitable compatibilizers can be found.

CONCLUSIONS In general it is seen that simple addition of GRT to most polymers results in significant decreases in mechanical properties due to the large particle size and poor adhesion. Although some of these materials may find limited application in low level usages, it is clearly necessary to improve on the properties of GRT-polymer composites for them to become a large volume material. Since lowering particle size results in only small improvements in material properties (and increased grinding costs) strategies for overcoming the deleterious effects of adding GRT to polymers have focused on methods of improving adhesion. In this area it is seen that judicious selection of a compatibilizing agent can lead to composites with quite reasonable mechanical properties at significant levels of GRT (as high as 50-60 wt%). As added compatibilizer levels are low (4-7 wt%) and no specialized processing steps are necessary, these higher value composites can be produced at little additional cost over simple GRT-polymer blends. Perhaps even more attractive is the use of recycled polymers as the matrix phase for GRT-polymer composites. Although only one example is presented in this review, this is an area that would seem to have great potential for increasing the recycling of both GRT and polymer wastes.

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The studies of ductile LLDPE and semi-brittle HDPE indicate that mechanical properties of GRT filled thermoplastics can be better retained when GRT is added to a ductile matrix in which failure occurs with little or no crazing or crack formation. One issue that may seem neglected in this review is that of applications for GRT-polymer composites. It is the opinion of the authors that if GRT composites can be developed where 1) the mechanical properties are close to virgin commodity polymer properties, 2) rheological properties are retained, and 3) costly processing steps are not required, the applications will follow. This question, as to whether cost effective composites can be produced, depends, as with any recycling effort, on a number of factors, and is not easily answered. The potential, however, is evident.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

J. A. Beckman, G. Crane, E. L. Kay, and J. R. Laman, Rubber Chem. Technol., 47, 597 (1974). J. D. Snyder and R. F. Hickox, Rubber World, 161 (3), 49 (1962). J. Winker, Chem. Eng. News, 49 (20), 29 (1971). J. Paul, Encyclopedia of Polymer Science and Engineering, Vol. 14, 2nd ed., Ed. H. Mark, John Wiley & Sons, New York, 787 (1986). R. J. Sperber and S. L. Rosen, Polym. Plast. Technol. Eng., 3 (2), 215 (1974). J. R. M. Duhaime and W. E. Baker, Plast. Rubber Comp. Process. Appl., 15, 87 (1991). K. Oliphant and W. E. Baker, Polym. Eng. Sci., in press A. Ratcliffe, Chem. Eng., 79 (7), 62 (1972). A. A. Hershaft, Environ. Sci. Technol., 6 (5), 412 (1972). Cryogenic Size-Reduction Technology Provides Economical Recycling Method, Elastomerics, 109 (12), 39 (1977). A. A. Phadke, Plast. Rubber Process. Appl., 6 (3), 273 (1986). R. A. Swor, L. W. Jenson, and M. Budzol, Rubber Chem. Technol., 53, 1215 (1980). G. Cheater, Eur. Rubber J., 161, 11 (1979). D. Dempster, Eur. Rubber J.,159, 87 (1977). D. Tuchman and S. L. Rosen, J. Elast. Plast, 10, 115 (1978). E. L. Rodriguez, Polym. Eng. Sci., 28, 1455 (1988). R. D. Deanin and S. M. Hashemielya, Polym. Mater. Sci. Eng., 57, 212 (1987). A. A. Phadke and S. K. De, Polym. Eng. Sci., 26, 1079 (1986). T. Liu and W. E. Baker, Polym. Eng. Sci., 31, 753 (1991). P. Rajalingam and W. E. Baker, Report being prepared for the Ontario Ministry of the Environment. S. Wu, Polymer Interface and Adhesion, M. Decker, New York, (1982). S. Wu, Polymer, 26, 1855 (1985). F. G. Smith and W. B. Klingensmity, Paper Presented at a Meeting of the Rubber Division, American Chemical Society, Washington, 9-12 October, 1990.

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24. D. H. Chang, Multiphase Flow in Polymer Processing, Academic Press, New York (1981). 25. N. C. Liu and W. E. Baker, Polymer Eng. Sci., in press. 26. M. D. Ellul and A. N. Gent, J. Polym. Sci., Polym. Phy. Edn., 22, 1953 (1984). 27. H. C. Wang, R. H. Schatz, and E. N. Kresge, Encyclopedia of Polymer Science and Engineering, Vol. 8, 2nd ed., Ed. H. Mark, John Wiley & Sons, New York, 423 (1986). 28. G. Koski, Annual Technical Conference of the Soc. of Plastics Engineers, 1799 (1988). 29. F. J. Stark Jr. and A. Leigton, Rubber World, 12, 36 (1983). 30. K. Oliphant and W. E. Baker, unpublished results. 31. P. Rajalingam and W. E. Baker, Rubber Chem. Technol., submitted. 32. P. Rajalingam, K. Oliphant, and W. E. Baker, Paper presented at the IUPAC International Symposium on Recycling of Polymers - Science and Technology, Marbella, Spain, 18-20 September, 1991. 33. P. Rajalingam and W. E. Baker, Annual Technical Conference of the Society of Plastics Engineers, Detroit, May, 1992.

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Quality Assurance in Plastics Recycling by the Example of Polypropylene Report on the experience gathered with a scrap battery recycling plant

K. Heil and R. Pfaff Metallgesellschaft AG, Zentral-Laboratorium, Reuterweg 14, 6000 Frankfurt/Main 1, Germany

RESOURCE RECYCLING Originally, industrial production relied on a linear material flow. Raw materials were mined and upgraded, premixes produced for the manufacture of components and equipment which were then utilized and eventually dumped into landfills after their useful life had elapsed. As the proportion of synthetic products in the total production volume increased, landfilling became ever more of a problem. The consequences of this disposal practice became unforeseeable. The price of landfilling rose dramatically. The resulting bottlenecks called for a fundamental change in our attitude towards disposal. Both industrial producers and consumers have since been looking for new concepts and solutions. As compared to the linear material flow, a cyclical concept offers the advantage of avoiding waste or at least postponing landfilling to a later date. Recycling of materials and products into the production processes soon became a commonly used phrase. Recycling can start in a production process itself or during or after the product’s life. Thus, the maintenance of goods can be viewed as product recycling. Therefore the return of defective or partly renewed products should have the highest rank among the recycling priorities. Here, the product undergoes no changes in its form. If, on the other hand, the product is disassembled or decomposed into its basic components or substances, this is referred to as a resource recycling. Ideally, these materials or substances should be sorted and returned to the production route. A flow diagram of this cyclical material flow concept is shown in Figure 1.

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Figure 1. Flow diagram of material recycling.

From the point of view of quality assurance, recycling of used materials into production poses its own special problems. The used material originates from different material streams varying in their impurity content and age. They bear the traces of their former service life. Their original properties have suffered, changed or disappeared altogether. Another important factor to be considered is the material damage resulting from the stresses to which the materials were exposed during their former service life. The impact of such damage is not known and cannot be predicted without comprehensive testing. Processing of recycled materials involves unknown risks. This applies to post-use of metal, plastics just as well as to minerals and textiles. When it comes to supply, recyclers face a problem of finding reliable sources of used materials, and organizing their collection and transportation. Supply may be a subject of considerable seasonal fluctuations as it is for example the case of used batteries. Continuous operation of the recycling process

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therefore requires that a large volume of used materials are held in stock. Collection often requires the sophisticated organization and transportation, frequently not possible without the use of special means of transport. Thus logistics alone may jeopardize the profitability of a recycling process. This cost factor increases with decreasing value of the post-used materials to be collected and transported. From the point of view of quality assurance, pure post-use material streams rank highest on the value ladder while mixed or contaminated materials are 1 grouped into an inferior category. Metallgesellschaft AG as a process and raw material-oriented technology and services group has a long and successful track record in recycling of post-use metals. Ever since recycling of post-consumer goods and durables gained a foothold in industry, polymers recycling has been attracting growing attention. The first commercial-scale plastics recycling plants are currently in the planning phase. Several pilot plants are already in operation. BSB RECYCLING GmbH in Braubach, a subsidiary of Metallgesellschaft AG operates a secondary lead 7 smelter for lead recovery from post-use lead-acid batteries. They process some 60,000 tons of batteries per annum which accounts for about half of the used battery volume to be disposed in the western states of the Federal Republic of Germany. BSB started to segregate the polypropylene from the battery casings and route it to a separate recycling process as far back as in 1984. For the recycling process, a quality assurance system geared to the specific requirements of the 3 application has been developed and implemented.

USED BATTERY RECYCLING Lead-acid batteries from automotive applications normally have a shorter service life than the car itself. After their service life has elapsed, they are no longer suitable for use. Because of their high lead content, lead-acid batteries have always been eagerly snapped up by secondary lead smelters. Via the car dealer and garage network, lead-acid batteries are collected in large quantities and then transported to secondary lead smelters. The logistics system is geared to lead recycling. The first battery reprocessing step yields not only lead but also PP in a form of the casing fragments. Accordingly, the polymer is available without additional cost. As the casing makes up a substantial part of the total battery, the quantities of polypropylene obtained are sufficient to warrant the operation of a plastics recycling plant (Figure 2). The secondary lead smelter

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Figure 2. Parts of an original 12V 44Ah lead-acid battery with a casing made of polypropylene.

processes up to 60,000 tpa of used batteries corresponding to about 3000 tpa of polypropylene. For this material stream, a recycling plant was developed, built and put into operation in 1986.

Crushing and separation In the first step, the batteries are processed through a crushing and separation ® system operating on the TONOLLI principle (Figure 3) which has been successfully employed in various battery recycling plants in Europe and North America. Next, the heavy fractions (lead, lattice metal) and Ebonite are separated from the light fractions (polypropylene and impurities). At this stage, the polypropylene has a purity of 97 %, which is still insufficient for its further processing. It is therefore routed to an upgrading stage, where it is further reduced in size in a wet-type rotary grinder and subsequently separated from water by sedimentation. After having passed through two series-connected driers and a cyclone separator, the polypropylene is available as so-called regrind with a purity of 99.5 %. The regrind consists of various types of polypropylene differing in their formulation, molecular composition and stabilizer content, having a broad spectrum of characteristics. Suitable mixing yields an intermediate product with a narrowed range of statistically uniform product characteristics.

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Figure 3. Process steps in preparation of polypropylene regrind.

Further processing to polypropylene granulate As a next step, the regrind is routed to compounding (Figure 4). By controlled addition of additives, polymers and fillers, the feed mix can be adjusted to suit the specific customer requirements. This feed mix is then gravimetrically metered into a special twin-screw kneader where it is molten under the dual action of an external heater and internal shear forces to obtain a homogeneous compound. Volatile matter is extracted and impurities resulting from unmolten components are filtered out. Subsequently, the melt is pelletized in a melt granulator. The resulting granulate is quenched in a water bath, centrifuged and finally processed through a hammer mill to break up lumps. The end product is a packagable granulate suitable for injection molding. This granulate is a secondary raw material which not only meets customer specifications but is also manufactured under a quality assurance system.

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Figure 4. Flow diagram (according to DIN 28004) of the compounding plant.

QUALITY ASSURANCE TO EN 29,000 PP The EN 29,000 pp standards provide a selection of quality assurance elements 2 and a guideline for establishing quality assurance systems. The selection of an optimum quality assurance system is governed by many diverse factors such as the application-specific objectives, the specific products, special processes, and the size of the plant. Corresponding models are given in EN 29,001 to 29,003. However, the key elements are always the same, irrespective of the quality assurance system selected. The quality cycle, illustrated in Figure 5, shows the interaction of the individual QA elements.

QA element - raw materials The raw material element is a key component of the quality assurance system. Unlike the quality-controlled trade with finished products, raw material and

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Figure 5. Quality loop with quality elements.

used material trading uses a completely different acceptance procedure generally known by the term “telquel”. From the quality assurance point of view, this means that a buyer accepts a processing risk without having an option of rejection. If the recycling process is robust and flexible, this kind of used material application is of a low risk alternative. If not, processing of the used materials is liable to lead to considerable problems. In recycling of battery casing plastics, the battery crushing and separation process as well as the subsequent mechanical polypropylene upgrading process have a decisive influence on the quality of the regrind produced because they determine the residual impurity content, product purity level, and particle size distribution. Mechanical separation breaks up molecular bonds with a resulting reduction in the mechanical properties of polypropylene. Only by gentle size reduction processes tailored to the specific application, it is possible to limit losses of mechanical properties and ensure a high quality of the end-product. Accordingly, feedstock qualification testing only makes sense after materials have undergone the first processing steps. Extensive tests and analyses conducted over many years were necessary to characterize the material - “polypropylene from used batteries”. For this pur-

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Figure 6. Impurities in regrind.

Figure 7. Improvement of product characteristic by homogenizing (shown by mold flow index).

pose, trace elements, mechanical and process parameters were determined repeatedly. The examples of wood impurities (Figure 6) and flow properties of the melt (Figure 7) demonstrate that the characteristics are scattered at random. Although, the used material stems from different sources and is delivered in batches, the parameter distribution is homogeneous. Major variations in the parameters only occur when used batteries of a new design enter the recycling

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Figure 8. Flow diagram of process control system.

market in large quantities. However, even in such a case, controlled process conditions in terms of quality assurance can still be expected. After crushing and upgrading, the regrind exhibits a purity of over 99.5 %, is color-mixed, and has a characteristic particle size distribution of 2 to 4 mm. In this condition, it meets all the requirements of the subsequent compounding step.

QA element - process control Here, quality assurance is faced with new demands. The secondary polypropylene granulate must compete directly with virgin granulate on the market (Figure 1). This means that quality assurance has to consider both user requirements and the technological and economic requirements of the compounding process. It is at this point where the process control element of quality

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assurance is brought to bear. The 5 M influencing factors (man, method, machine, milieu, material) have to be planned, organized and implemented to achieve a desired result (Figure 8). In plastics recycling, process control gains added significance since compounding constitutes a special technology whose performance cannot be fully verified by subsequent testing of the product. Therefore, supervision of the process and maintaining the quality-relevant process parameters constant are imperative. For this purpose, we have developed and implemented a process control concept which has for many years been employed with great success in a day-to-day operation even for plant startup, after process modifications and after the introduction of new formulations. Out of various process parameters, six were classified as quality-relevant, twelve as indirectly quality-relevant and the others as neutral. The actual values of these process parameters are recorded and compared with given target values. The target values were determined by previous trial runs. A trial run is defined as a production phase in which a TO-SPEC product is produced under realistic conditions over a prolonged period. Trial runs are performed to verify the process capability of production of a product, having a defined range of characteristics. The data, especially the process and product data established during the trial run are investigated by statistical methods to obtain target values for the individual process parameters. These target values relate to the level and dispersion of the parameters, and are incorporated into the process formulation. It should, however, be borne in mind that the process control element of quality assurance aims at the compliance of the end product with defined quality criteria rather than at defining target values for process parameters. It means that correlations between process parameters and product parameters must be determined to enable conclusions to be drawn on suitable process conditions. However, this is normally not possible. Influences are so diverse as to make it virtually impossible to develop a closed model. Also, to our knowledge, no such model exist as yet for the compounding process. A certainty, that the defined process conditions lead to the desired product quality, is solely based on experience from previous production phases such as for example the trial run. Accordingly, the process changes, whether technical, organizational or staff-related, always create uncertainty as to their effects on the end-product properties. These drawbacks notwithstanding, the deductive approach of determining the process parameters after verification of the product parameters can define the

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Figure 9. Graded actions to process and product.

conditions under which the desired result is reliably met. A point yet to be proven - but only by theory - is whether there are no other process conditions yielding the same result, however, as long as no other such conditions are known, this question is of no relevance to practice. The approach here described is known as indirect process control and applied by us in polypropylene recycling. The scattered target data and the actual data from production are compared by a computerized data acquisition system with the target data for different formulations being administered by a corresponding software program. This permits a rapid comparison. As both the process and the plant are designed for processing of many diverse formulations, computer administration of the formulation data makes for a great simplicity of handling. The target-versus-actual comparison of the process parameters identifies deviations from the process target condition. From this information, staged responses to the process are derived with the aim of reestablishing and maintaining the target conditions.

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The staged responses after detection of a deviation are depicted in Figure 9. In this connection, a distinction is made between tolerable and intolerable deviations. In addition, time-staged measures are initiated. Immediate local control interventions are the fastest and the most direct method to bring the plant and process parameters back to target conditions. This method always has first priority irrespective of the deviating process parameter. If the deviating process parameters is of direct relevance to the product quality, product flow to the product bins is immediately interrupted. Only after the target condition has been re-established, is the product stream once again directed to the product bin. Repetitive parameter deviations, even if the parameter is only of indirect relevance to product quality, are additionally processed through a statistical monitoring programme. The latter uses a CUSUM control card with a memory to compare 5 such deviations with customer-specific acceptance criteria. If the cumulated deviations at the end of a batch run are below the acceptance limit, the batch is rejected. In such a case, further processing of the batch will not be allowable unless expressly authorized by the customer. This quality assurance approach to process control is new in the field of raw material production. It provides maximum possible reliability by disclosing all the process parameters and is thus especially suited to the production of secondary raw materials as additional influences on the product, due to process deviations, are reliably ruled out. The process runs more smoothly.

Quality assurance in after-sales service The fact that a product like polypropylene granulate has been produced under controlled conditions and meets all the quality specifications does not yet guar6 antee its success on the market. Although quality is an indispensable prerequisite for the success of the product, it is far from being the only criterion. This holds true for secondary raw materials which frequently have to overcome considerable reservations on the buyers side. The chief objection raised to secondary raw materials is that their properties vary greatly both within one batch and from one batch to another. This constitutes a major impediment to the recycling market. Suppliers could provide much greater volumes of secondary raw materials than the buyers side is prepared to take. Currently, plastics recycling is receiving some impetus from legislation. In future, there are good chances that the

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Figure 10. Manufacturer’s test certificate.

use of defined proportions of recycled materials will be prescribed for the manu4 facture of certain products. However, as yet the recycling market is still operating with two product classes. On one hand, the industry offers recycles which have been produced without specific pre-sorting process. There is no denying to 1 the fact that the properties of such products vary greatly. The products from such recycling processes are definitely inferior. On the other hand, there are

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recyclers who put great efforts into producing high-purity secondary raw materials. As described before, this normally requires comprehensive sorting and upgrading processes. This approach normally yields a product which can be adjusted within a broad range to suit the specific customer requirements. For such products, suppliers can assume contractual obligations in the form of specifications. This requires an after-sales service which provides the customer with extensive technical assistance. Such an after-sales service has to include activities such as providing written information in the form of data sheets, test certificates (Figure 10) and product information just as well as application-specific advisory services, processing instructions, and solving of customer problems. To answer to these needs, we have established a “Technical Marketing Service” which coordinates and provides all these activities.

OUTLOOK Materials used today for the production of consumer goods and durables will appear on the recycling market tomorrow, provided they are routed for reuse. Polymers constitute a major source material for the production of commodities. Currently, concepts are being developed and first steps taken to facilitate and improve the separation of polymers from the product after its useful life has elapsed. This can be achieved by designing products for maximum ease of disassembly and identifying them clearly. The practice of combining a great variety of materials in a single appliance or assembly is doomed to find an end as it stands on the way of comprehensive product recycling. Accordingly, greater amounts of pure polymers will enter the recycling market in the future. In parallel, new processes will be developed and implemented to complement the existing recycling and processing capacity. It is also well conceivable that the advent of new materials will create new customer requirements or open up new markets. Comprehensive quality assurance applied at the right point and at the right time and taking into account the specific circumstances of the recycling application and the customer requirements will boost the image of secondary polymers and facilitate their market introduction.

K. Heil and R. Pfaff

185

REFERENCES 1. 2. 3.

4.

5. 6. 7.

H. Dominghaus, Stand und Entwicklungstendenzen beim Kunststoff-Rycycling, Tagungsberichte DIF, Kempen, November, 1991. EN2900ff, Qualitätsmanagement - und Qualitätssicherungsnormen, Qualitätssicherungssysteme, Mai, 1990. J. Wortberg and J. Haussler, Moderne Konzepte der kontinuierlichen Prozessuberwachung. Qualität und Zuverlässigkeit 37 Jahrg.,2/72, Hanser, München. Bundesminister F. Umwelt, Naturschutz und Reaktorsicherheit WA UU 3 530 111-2/6 Entwurf 15, August 1990, Zielfestlegung der Bundesregierung zur Vermeidung, Verringerung oder Verwertung von Abfallen aus der Kraftfahrzeugentsorgung. H. Rinne and J. J. Mittag, Statische Methoden der Qualitatssicherung, Hanser, München, Wien, 1989. N. Ehlert, Anforderungen an die Rohstoffhersteller aus der Sicht eines Spritzgiessverarbeiters Fachtagung, Würzburg, Dezember, 1990. R. Pfaff, Material Recycling of Polypropylene from Automotive Batteries - Process and Equipment, II Int. Symp., Ed. van Linden, The Mineral, Metals & Materials Society 1990.

index

187

Index A

D

C adhesion 91, 160, 161 air flotation 108 aluminium 1, 102, 113 amorphous phase 74 antioxidants 86 automation 104

B bacteriological contamination 24 ballistic test 148 battery recycling 173 battery scrap 171 biodegradation 11, 12 blend compatibility 27 bottles 2,18, 23, 99, 105 branches 68 butyl rubber 155

calorific value 10 car industry 129, 130, 139 car interiors 131 carbon black 155 catalysts 8 cellulose 111, 113 chlorinated polyethylene 63, 68 cleaning 101 co-injection molding 17, 18, 19, 21 coextrusion 37 collection system 100 combustion 10 compatibility 40, 90 compatibilizer 40, 62, 70, 71, 76 compatibilizing 166 composites 7, 153 compression molding 32 containers 17 crushing 174 crystallinity 68, 76, 148 crystallization 48, 49, 50

dashboard 134, 135 density 45, 63 detectors 102, 104, 105 direct re-use 2, 3, 4 disposal 144 door panel 136, 137 drying system 20 DSC 65

E elastic modulus 30, 35, 36, 51, 89, 95, 125 electrostatic separation 103 elongation 30, 35, 36, 50, 68, 70, 88, 89, 90, 93, 94, 96, 119, 121, 125, 126 ESCA 162 esterification 4 EVA 63, 68 extrusion

188

index

36, 54, 71, 86, 89, 91, 93, 94, 96, 125 impurities 178 incineration 2, 10 industrial scrap 39 injection molding 18, 69, 116, 124 interfacial adhesion 47 IR 64

31, 32, 33, 35, 42, 84, 124, 127

F fiber 1, 2, 114 fillers 55, 76, 93, 94, 95, 96, 97, 125, 153 film 1, 2, 3, 10, 12, 27, 28, 30, 36, 37, 60, 62 filmbase 3 flexural modulus 53, 54 floor coverings 134, 135, 136 flotation 42, 62, 104 flow index 148, 157 foam sheets 132 fogging 133 fracture 78 fuel tanks 146 functionalization 28

L lamellae 68 landfills 1, 2, 10, 11, 12, 17 lead 173, 174 lubricants 86

M macroseparation 104 melt index 63 methanolysis 6, 8 microgels 4 microseparation 103 Molau test 29, 32, 33 molecular separation 103 molecular weight 63 monomer recovery 2, 7, 8 monomers 11 morphology 42, 45, 91

G gel content 32 glass 1 glass fiber 126 glycolysis 6, 7 greenhouse 27 grinding 107, 125, 154, 155, 175

H homodomains 47 hydrolysis 6, 8, 9, 113, 114, 115, 117, 118, 120

I

N Nielsen model 52 NIR 105

O impact energy 157 impact resistance 53 impact strength 32, 35,

oil spills 153

P packaging 60 paper 111, 112, 114, 115, 120 parking area 128 particle size 29, 114, 155, 158, 159 peel adhesion 3 2, 36, 37 phase segregation 48 photodegradation 11 photooxidation 27, 28, 29, 30, 32, 35, 36 pigments 77 pilar trim 137 playground surfaces 153 Poisson’s ratio 53 polyamide 27, 28, 29, 31, 32, 35, 37, 113 polyethylene 27, 28, 30, 31, 32, 33, 34, 35, 36, 37, 40, 41, 43, 44, 47, 48, 50, 54, 62, 63, 64, 65, 67, 68, 70, 74, 76, 83, 84, 85, 86, 89, 90, 94, 95, 96, 97, 99, 100, 101, 103, 111, 113, 124, 126, 146, 156 polyethyleneterephthalate 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 17, 18, 20, 22, 21, 26, 27, 84, 99, 100, 103, 104, 105, 113, 124 polymer degradation 4, 27, 65, 85, 86, 125, 145 polypropylene 27, 40, 41, 43, 47, 48, 54, 111, 113, 120, 124, 131, 171, 174 polystyrene 1, 27, 40, 41, 43, 47, 54, 113, 116, 124 polyvinylchloride 1, 17, 27, 40, 84, 87, 99, 100,

index

101, 103, 104, 105, 113, 116, 125, 128, 145 process control 179

selection 101 SEM 29, 30, 32, 33, 34, 42, 45, 46, 53, 55, 64, 72, 73, 74, 75, 76, 77, 78, 91, 162 separation 61, 100, 102, 106, 174 shear rate 44 shrinkage 48 silver 3, 4 stretch-blow-molding 21, 22 synthetic leather 132

Q quality assurance 176, 182 quality loop 177

R re-use after modification 2, 6 reactive coupling 164 rear shelf 137 rheology 42, 43, 66 rigidit 55 road surfaces 153 roofing materials 153 rotary screen 102

S SBR 155 SBS 126 scrap value 142, 143, 144

189

U urban wastes 39, 40, 60, 62 UV resistance 133

V vacuum filtration 114 viscosity 19, 44

W T

washing equipment 108 water absorption 18, 19 talc 125, 126 tensile modulus 50, 52 tensile strength 30, 32, 35, 36, 42, 50, 67, 68, 88, 89, 90, 92, 94, 95, 126 test certificate 183 tiles 127 tires 153 triboelectrical charge 103 turbidity 29

X X-ray 5

Y yielding stress 50

7

Preface Recycling of plastic materials is now an important field in the plastics industry, not just an activity born under environmental pressure. The recycling processes include industrial operations in which secondary materials are reprocessed and/or monomers recovered for further polymerization; such processes are termed secondary and tertiary recycling. Although the plastics industry considered recycling for many years, attention was mainly focused on the recycling of industrial scraps and homogeneous post-consumer plastics which are easy to collect and reprocess. More recently, the plastics industry accepted the challenge of recycling of heterogeneous plastic waste based on new technologies of separation and reprocessing. Scientific research, scarcely visible only a few years ago, is now a very active, fast-growing discipline, contributing numerous papers which appear in the scientific literature. Several congresses and scientific symposia are attended by specialists every year and new books on this subject demonstrate the great scientific and industrial interest in the recycling of plastic materials. This book is intended to focus on the state of the art in recycling, the most recent technologies of recycling, and some recent scientific research in the field. Polyolefines and poly(ethylene terephthalate) (PET) are the most frequently recycled polymers, and as such they are given significant attention in the research and technology which this book reflects. Two reviews characterize the state of the art in PET recycling. De Winter presents a review on recycling of PET film and Neumann on a co-injection technology which allows one to use recycled PET as an intermediate layer in bottles. Both processes are common in industrial practice and are thus able to offer an overview of experience in plastic recycling which is of interest in other areas of recycling as well. Other references to PET recycling are presented by Sereni and La Mantia, Perrone, and Bellio. Polyethylene (PE) and other polyolefines are discussed from various angles. La Mantia and Curto propose methods of recycling of photooxidized polyethylene in blend with Nylon 6. It is shown that the recycled PE behaves like a functionalized PE, having compatibilizing attributes due to which blends exhibit improved mechanical properties. Recycling of urban wastes is discussed by Gattiglia et al. and by Laguna et al. Generation source, separation possibilities, and cleaning technology are discussed in relation to blend properties, such as rheology, morphology, and mechanical properties. Comparison is also made with blends having similar

8

composition but made from virgin polymers. The major problems in recycling of mixed plastic waste are due to their inferior processability, which results in materials having poor mechanical properties. La Mantia et al. and Vezzoli et al. present experimental results which disclose the possibility of obtaining recycled materials with acceptable properties from mixed plastic waste. Plastic wastes are often contaminated with paper. Klason et al. present an industrial method of reprocessing paper-contaminated plastic waste which does not require a difficult and costly separation process. Instead, cellulose from paper is converted to a filler. The method and equipment suggested allow for excellent dispersion of in situ formed filler. Recycling of plastic component from car scrap is a very important challenge for the plastics industry and car manufacturers, since the plastic content in cars is systematically increasing. Henstock and Seidl show results on the recycling of plastic fuel tanks, Oliphant et al. describe the methods of application of ground discarded tires as a filler in polymer composites; Vezzoli et al. present new strategies of design of easily recyclable car interiors; while Heil and Pfaff show how battery recycling can utilize all initial components, offering quality assurance for recycled polypropylene. An alternative method of recycling of mixed plastic waste is based on a separation of different components into homogeneous fractions. Sereni describes opportunities in this area and interesting industrial equipment required for effective separation of PET and PVC. The above short summary shows that this book combines lessons from the past experiences of an industrial practice with evaluation of modern trends and current research in the field of plastic recycling.

F. P. La Mantia Palermo, September, 1992

i

Table of Contents Poly(ethylene terephthalate) Film Recycling Introduction Direct Re-use Re-use After Modification Monomer Recovery Methanolysis of PET-waste Hydrolysis of PET-waste Incineration Bio- and Photo-degradation Photodegradation Biodegradation Conclusive Remarks References

1 1 3 6 8 8 8 10 11 11 12 13 14

The Importance and Practicability of Co-injected, Recycled Poly(ethylene terephthalate)/Virgin Poly(ethylene terephthalate) Containers Introduction Basic Technology Manufacturing Process of Multilayer Bottles Containing Regrind Drying of PET Resin and PET Flakes Co-injection Molding of Virgin and Reground PET Flakes Conditioning and Stretch-blow-molding Double-layer Preforms Trials of Co-injecting Virgin PET and Reground PET Flakes Quality of the Raw Materials The Trial Processing Trial Results Cost Savings Contamination Aspects Bacteriological Contamination Contamination by Foreign Substances Conclusions

17 17 18 18 18 21 21 22 22 22 24 24 24 24 24 25 26

ii

Recycling of Post-consumer Greenhouse Polyethylene Films: Blends with Polyamide 6 Introduction State of Art Experimental Materials Structural Studies Mechanical Properties Results and Discussion Blends Coextruded films Conclusions Acknowledgment References

27 27 28 31 31 32 32 32 32 37 37 37 37

Recycling of Plastics from Urban Solid Wastes: Comparison Between Blends from Virgin and Recovered from Wastes Polymers Introduction Experimental Materials Blend Preparation Rheological Measurements Density Mechanical Properties Morphology Thermal Analysis Results and Discussion Rheology Density Morphology Crystallization Behavior Mechanical Properties Tensile Behavior Flexural Modulus

39 39 41 41 42 42 42 42 42 43 43 43 45 45 48 50 50 53

iii

Impact Resistance HDPE/Heavy Fraction Blend Conclusions Acknowledgements References Management of Plastic Wastes: Technical and Economic Approach Introduction Recycling of Urban Plastic Wastes Experimental Materials Procedures and Utilities Results and Discussion Identification of Polymers Present in the Film Plastic Wastes and the Rheological Behavior of the HDPE/LDPE System Mechanical Behavior of HDPE/LDPE Blends Microstructural Aspects of HDPE/LDPE Blends The Economical Approach Conclusions References Blends of Polyethylenes and Plastics Waste. Processing and Characterization Introduction Experimental Results and Discussion Processing Mechanical Properties Blends Containing Calcium Carbonate Blends Containing LDPE Conclusions Acknowledgment References Techniques for Selection and Recycle of Post-Consumer Bottles Introduction

53 54 55 56 56

59 59 60 62 62 62 64 64 65 70 80 81 81 83 83 84 85 85 87 94 97 98 98 98 99 99

iv

General Considerations Molecular Separation Microseparation Macroseparation Recycle Installations Grinding Air Flotation Washing Equipment References

100 103 103 104 107 107 108 108 108

Hydrolytic Treatment of Plastics Waste Containing Paper Introduction Experimental Hydrolysis Processing Results Conclusions Acknowledgement References

111 111 112 113 115 116 121 121 121

Processing of Mixed Plastic Waste Introduction Mixed Plastics from Household Waste Plastics from Industrial Sectors Concepts for Car Interiors TPO Based Materials Synthetic Leather Foam Sheets Technologies Automotive Applications Dashboard Floor Covering Other Components Conclusions References

123 123 123 129 131 132 132 132 134 135 135 135 136 137 137

v

The Use of Recyclable Plastics in Motor Vehicles Recoverable Materials in the Motor Vehicle Present Recovery Practice Changes in the Materials Used in Vehicles The Effects of Materials Substitution on Vehicle Recycling Disposal of Residuals Recyclable Plastics Components Preliminary Results Comparison of Virgin and Recycled HDPE Comparison of Fluorinated and Unfluorinated HDPE Torsion Test Tensile Test Charpy Impact Test Ballistic Test Degree of Crystallinity Melting Temperature Flow Index The Recycling of Material from Used Fuel Tanks Torsion Test Tensile Test Charpy Impact Test Ballistic Testing Degree of Crystallinity Melting Temperature Summary and Conclusions References

139 139 139 140 141 144 146 146 146 147 147 147 147 148 148 148 148 148 149 149 149 149 150 150 150 150

Ground Rubber Tire-Polymer Composites Introduction Ground Rubber Tire Composite Behavior Tire Grinding Characteristics of Tire Particles Polymer Matrix Particle Size Adhesion

153 153 154 154 155 156 158 160

vi

Matrix Modification Ground Rubber Tire and Recycled Plastics Conclusions References Quality Assurance in Plastics Recycling by the Example of Polypropylene Resource Recycling Used Battery Recycling Crushing and Separation Further Processing to Polypropylene Granulate Quality Assurance To EN 29,000 PP QA Element - Raw Materials QA Element - Process Control Quality Assurance in After-sales Service Outlook References Index

165 167 168 169

171 171 173 174 175 176 176 179 182 184 185 187