Design of a model control and predict the silanization during mixing

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DESIGN OF A MODEL TO CONTROL AND PREDICT SILANIZATION DURING MIXING

AYUSH KHAREL

The Research described in this thesis was financially supported by Apollo Tyres Global R&D B.V

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Final Thesis to obtain the degree Professional Doctorate of Engineering

DESIGN OF A MODEL CONTROL AND PREDICT THE SILANIZATION DURING MIXING

by

Ayush KHAREL

University of Twente Faculty of Engineering Technology Elastomer Technology and Engineering April 2018

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Acknowledgement It has been an unforgettable experience working at University of Twente for the past two years. The work has been challenging and rewarding. Living and working in Netherlands has been fun and interesting thanks to all the friends and colleagues. I would like to express my gratitude to Apollo Tyres Ltd for providing me the this opportunity. I appreciate the opportunity given to me by Prof. Dr. Anke Blume to be a prat of the Elastomer Technology and Engineering group and work towards my PDEng degree. I would like to acknowledge the perseverance in guiding me through the project by Dr Wilma Dierkes I am also grateful to Dr. Ir. Louis Reuvekamp and Ing. Andries van Swaaij for all the discussion and banter and to Ceciel Ter Horst-Strootman for all her support .

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Contents 1

2

3

4

Introduction ............................................................................................................................ 1 1.1

Introduction to Tyre Technology...................................................................................... 1

1.2

Components of a Tyre ...................................................................................................... 2

1.3

Manufacturing of a Tyre ................................................................................................... 3

1.4

Motivation ........................................................................................................................ 5

1.5

Outline of the PDEng Thesis ............................................................................................. 6

1.6

Bibliography ..................................................................................................................... 6

Objective ................................................................................................................................ 8 2.1

Description of the Design Topic ....................................................................................... 8

2.2

Objective of Design Project ............................................................................................ 10

2.3

Bibliography ................................................................................................................... 10

Program of Requirements..................................................................................................... 11 3.1

Functional Requirements ............................................................................................... 11

3.2

Non-functional Requirements ........................................................................................ 11

3.2.1

Safety/Risks ............................................................................................................. 11

3.2.2

Reliability................................................................................................................. 11

3.2.3

Maintenance ........................................................................................................... 12

3.2.4

Finance/cost ............................................................................................................ 12

3.2.5

Environmental/Social impact .................................................................................. 12

Literature Review .................................................................................................................. 13 4.1

Filler reinforcement of elastomers ................................................................................ 13

4.1.1

Hydrodynamic Effect............................................................................................... 14

4.1.2

Polymer Network Interactions ................................................................................ 15

4.1.3

Filler Polymer Interaction ....................................................................................... 15

4.1.4

Filler-Filler Interactions ........................................................................................... 16

4.2

Overview of Filler Material ............................................................................................. 21

4.2.1

Silica Filler ............................................................................................................... 21

4.2.2

Physical Characteristics of Silica Fillers ................................................................... 22

4.2.3

Particle size ............................................................................................................. 23

4.2.4

Structure ................................................................................................................. 24 iv

4.2.5

Specific surface area ............................................................................................... 24

4.2.6

Surface Energy for Silica.......................................................................................... 25

4.2.7

Surface Chemistry ................................................................................................... 26

4.2.8

Silanization Reaction ............................................................................................... 27

4.3

4.3.1

Styrene-Butadiene Rubber ..................................................................................... 29

4.3.2

Polybutadiene Rubber ............................................................................................ 29

4.3.3

Processing of SBR/BR .............................................................................................. 29

4.4

Rubber Mixing Equipment ...................................................................................... 30

4.4.2

Mixing Mechanism .................................................................................................. 35

4.4.3

Mixing Steps ............................................................................................................ 37

Rubber Mixing Process Parameters ............................................................................... 40

4.5.1

Ram Pressure .......................................................................................................... 40

4.5.2

Fill Factor ................................................................................................................. 40

4.5.3

Rotor Speed ............................................................................................................ 41

4.5.4

Temperature ........................................................................................................... 41

4.5.5

Mixing Time............................................................................................................. 41

4.5.6

Order of Dosing ....................................................................................................... 42

4.5.7

Energy ..................................................................................................................... 42

4.5.8

Mixer Fingerprint and Control ................................................................................ 42

4.6

6

Rubber Mixing ................................................................................................................ 30

4.4.1

4.5

5

Overview of Polymers. ................................................................................................... 29

Bibliography ................................................................................................................... 43

Design Methodology/Design Steps....................................................................................... 47 5.1

Problem Investigation .................................................................................................... 47

5.2

Knowledge Problem ....................................................................................................... 48

5.3

Treatment Design, Validation and Refinement ............................................................. 49

5.4

Bibliography ................................................................................................................... 49

Development Phase .............................................................................................................. 50 6.1

Conceptual Design.......................................................................................................... 50

6.2

Set-up ............................................................................................................................. 54

6.3

Experiment and Results.................................................................................................. 62 v

6.3.1

Rotor Speed Trials ................................................................................................... 62

6.3.2

Device Starting Temperature Trials ........................................................................ 64

6.3.3

Temperature of the Mixing Trials ........................................................................... 70

6.4

Total Energy and Thermal Energy of Mixing .................................................................. 75

6.5

Amount of Silane ............................................................................................................ 80

6.5.1 6.6 7

Silanization Reaction ............................................................................................... 80

Bibliography ................................................................................................................... 85

Conclusion and Future Work ................................................................................................ 86

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1 Introduction Tyres are usually the most underrated components of modern vehicles, even though they provide contact of the car to the road. Moreover, tyres govern the handling ability, safety, economy and ride comfort. Complexity in their construction has increased along with the demands on the manufacturer to provide a wide array of performance targets, some of which clash with each other. In addition to the performance criteria set by the vehicle manufacturer, now there are many environmental and regulatory standards to be satisfied. The added performance criteria have forced the tyre industry to new avenues of research and exploration. There have been many advancements in compounds and manufacturing techniques to meet the requirements. New polymers have been introduced and resins are no longer just tackifier or processing aids. Similarly, traditionally used carbon black filler material has been replaced by another filler: “precipitated silica”. The primary motivating factor for this change was the better rolling resistance and less heat build-up in comparison to carbon black. However, the replacement brings with it its own unique problems in processing and performance optimizing [1]. Silica forms a similar structure of primary particles, aggregates and agglomerates compared to carbon black, but surface properties differ and problems arise in its use as a reinforcing filler. Since the surface of the silica particles is covered with silanol groups, it is polar and hydrophilic, the affinity with the polymers used in tyres is low. Moreover, there is a strong inter-particle interaction due to hydrogen bonding. Therefore it provides lower reinforcing properties when used as such. The development of a silane coupling agent by Wolff in the 1970s made it possible to use silica as reinforcing filler [2]. Later in the 1990s, the patent by Rauline introduces the use of precipitated silica in elastomers, and the adjustment of the mixing conditions to achieve higher reinforcement [3]. The increase in the filler reinforcement factor of silica brought about the emergence of the “Energy Tyre” patented by Michelin, otherwise also known as “Green Tyre”. Silica is used in these tyres as primary filler and helps to reduce the rolling resistance by 20 %, which can be translated to 5% fuel savings [4]. Silica reinforcement also improves wet handling and mechanical properties of winter and all season tyres. However, there are challenges with processing and storing of green compounds in-between manufacturing steps. Primarily the mixing time and power required is considerably larger than for carbon black reinforced compounds. Considering all the advantages of the silica reinforced rubber compounds, it is worth to ascertain and optimize the manufacturing process. It is known from literature that for any reinforced polymer, finely dispersed and distributed fillers in the polymer matrix improve the mechanical properties of the composite [5]. Moreover, properly mixed compounds can reduce waste during production; fewer rejected final products and improved product quality. The thesis presents the work done on finding and developing a system to control the dispersion and distribution of the silica-silane filler during mixing.

1.1 Introduction to Tyre Technology A tyre is placed round a wheel to protect it from wear, provide better handling and a comfortable ride by absorbing shocks. Before the discovery of rubber and pneumatic tyres, wheels were 1

circular wooden disks covered by a leather band around the circumference; providing compressive forces to hold the structure in place. Later on, with advances in metallurgy, the leather band was replaced by iron or steel following the same principle. The word ‘tyre’ is derived from this practice of tying up the wooden structure with leather or metal [6]. The discovery of vulcanized rubber by Charles Goodyear in the mid-19th century and the pneumatic [7] or air-filled tyre patented by Robert William Thomson ushered the modern era of tyres. [8, 9]. Early tyres had a rubber coated fabric tube pressurized with air and encased in a leathery outer skin. By the late 19th century, automobiles used detachable pneumatic tyres, popularized by Andre and Edouard Michelin. By 1920, the tyres were similar to what can still be seen today with single doughnut shaped inner-tubes and an outer tyre with rubber coated bias fibre plies to provide reinforcement to the outer cover, and with treads on them. Carbon black reinforced rubbers increased the tyre life from hundreds of kilometres to thousands. Synthetic rubber was invented to bridge the gap between the demand and supply of rubber during the Second World War. Tyres remained unchanged until Michelin’s steel-belted radial tyres in mid 1900s.The new design reduced the heat generated during operation which helped to reduce the rolling resistance of tyres, thus making them more fuel-efficient. The lower heat build-up also helped to increase the service life of tyres. Today, the majority of the tyres produced worldwide is of the radial type. The second half of the 20th century saw better tyres with research focusing on performance. Improvements in compounding and mould-making resulted in tyres with better wet/dry grip, wear, abrasion and puncture resistance, less rolling resistance and better steering, handling, and braking. All-season tyres have reduced the practice of buying and replacing summer and winter tyres. Recent research has focused on the impact of tyres on the environment and on reusing and recycling as much as possible. Fuel efficiency has been in the focus of the tyre manufacturer for several decades.[1, 10]

1.2

Components of a Tyre

Modern tyres are the epitome of a century of research and understanding across the globe. They are complex and have a layered construction with tens, if not hundreds, of elements making up the final tyre, as illustrated in Figure 1-1. The manufacturing industry is well established with more than one and a half billion car and light vehicle tyres produced annually.[11].

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Modern tyres are much more complex than perceived at first glance. Several layers with specialized functions are composited into a single product working in harmony to optimize performance [7]. The basic components are listed below:

Figure 1-1 Assembly of a tyre.(courtesy of Apollo Tyres Ltd)

 Inner Liner  Body Plies/ Carcass Plies  Steel Beads  Bead Reinforcement/ Bead Filler/ Apex  Steel Belt Plies  Side Walls  Treads

The components above have their specific function and requirements in the final tyre. These components all have their own constituting elements and manufacturing processes. The manufacturing process encompasses various sub-processes for producing the required components and assemble them together into a final product that can be used on a vehicle.

1.3 Manufacturing of a Tyre The basic processes in the manufacture of tyres are listed below [12] and shown in Figure 1-2:      

Mixing of raw materials and chemicals to form the rubber compound. Fiber and steel cords for beads, plies and belts are coated with rubber and calendered. Extruding treads and sidewalls. Assembly of all tyre components on tyre building drums, also known as ‘green tyre’ or unvulcanized tyre. Vulcanizing (curing) of the green tyre in a hot and pressurized mold. Inspection, quality control, labelling, storage and shipping.

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Figure 1-2 Tyre manufacturing overview.[13]

A modern tyre basically is a metal or fibre skeleton covered by different polymer blends that have various amounts of fillers and chemicals incorporated to provide an optimum functionality. The different polymers, fillers and chemicals have to be mixed to achieve a homogenous rubber compound that is easy to process in the subsequent manufacturing steps. Moreover there has to be minimum variation between different batches. Therefore, mixing can be considered in the tyre industry as the most important step [14, 15]. Rubber mixing is a complex process and requires knowledge and experience in multiple disciplines in order to understand and optimize this process. For better understanding, mixing can basically be divided into three areas:   

Equipment Design Material Science Process Engineering

The equipment designer has to design machines which are most suitable for the desired application. It should be able to perform all the tasks in a safe and efficient manner, and they also should be designed to overlook any complications that may arise. Additionally, the equipment should have proper control and monitoring systems to facilitate the operation. Material science enables the engineer to make the right selection of the raw materials according to the envisaged functionality of the compound. The process engineer is then responsible to use the recipe and mixing equipment in the most effective and efficient way to produce the best possible homogeneous compound [16]. It is essential to have an understanding of the equipment, the raw 4

materials and the processing steps of rubber compounding carried out at a typical tyre manufacturer. The main raw materials for a rubber compound are the polymer and the filler, which can be up to 90% by weight, and is compounded to achieve the desired performance. Each tyre component, depending on the performance, can have different ingredients and different ratios of ingredients. For example, passenger car tyre treads may have silica as primary filler material whereas the sidewall compounds may have carbon black as a primary filler for the same tyre. All the compounding ingredients are mixed in an internal mixer, on a two roll mill or in an extruder. Some compounds can have multiple mixing stages and multiple mixing machines may be used. Common practice is to create a “masterbatch” with the polymer, filler and most of the chemicals and mix it using an internal mixer, as it is economically more efficient to mix large volumes. Then specific ingredients that may react with other additives like zinc oxide are mixed in, as required, in subsequent steps. Therefore the quality of mixing of a masterbatch is of great interest in manufacturing of tyres. A homogeneously mixed masterbatch would reduce or remove optimization in the following steps, saving time and material in further processing. Moreover, the confidence in the consistency of the masterbatch helps to eliminate mixing as source of error and to focus more on optimizing other processing steps.

1.4 Motivation This PDEng project focuses on designing a control system to achieve a better consistency of masterbatches containing a silica-silane filler system. In masterbatch mixing, the filler is divided and distributed evenly within the rubber matrix, so as to achieve a uniform and homogeneous rubber compound. The drawback of silica as a filler is, that this filler and other chemicals may be clumped and concentrated in an area, providing less than ideal performance. Various studies have been conducted to understand the filler and polymer interactions along with the reinforcing effect of the filler material. Progress has been made towards understanding of the mechanisms involved, but they are not complete and many questions remain unanswered.[5] Nonetheless, the processing and application of silica-silane rubber compounds have been further developed. The pioneering work by A.R. Payne, A.I Medalia and G. Kraus have expanded the understanding of filler-filler and filler-polymer interactions and their contribution to the reinforcing effect [17]. The Payne effect is considered to arise from the filler-filler interactions and is characterised as a sharp decrease in storage modulus with increasing amplitude under cyclic small strain loadings [18, 19]. Unfilled compounds and compounds with fillers below a certain volume percentage don’t exhibit this strain dependant property. Above the percolation threshold, the volume of the filler is sufficient to form a filler-filler network which increases the dynamic modulus at low strains. As the strain increases, the filler-filler network is broken and the network contribution to the modulus gradually disappears. This process is reversible: when the strain is removed, the filler network reforms. This effect can be taken as the level of dispersion achieved in a compound after mixing. A strong Payne effect can be related to strong filler-filler interactions and poor dispersion and vice versa.[20]. Therefore, the measurement of the Payne effect between and after the 5

mixing steps can give information on the dispersion of the filler. However, few studies [21-23] have been conducted on the relationship between the Payne effect and the processing conditions. The main focus of this thesis is to follow an iterative process to identify the main factors that contribute to the Payne effect in a filled elastomer system and link the effects to compound mixing parameters. The project is an initiative of Apollo Global R&D, located in Enschede, Netherlands. The facility focuses on the development and testing of car and van tyres. Apollo Global R&D is part of Apollo Tyres Ltd from India. Apollo Tyres Ltd. is a multinational with offices and production locations in countries such as India, the Netherlands and Hungary. The project intends to develop a mixing process control system that can be utilized in the production plants of Apollo Tyres Ltd.

1.5 Outline of the PDEng Thesis Chapter 1 gives the introduction, background and motivation for the project. Chapter 2 and 3 are related to the project description, objective and programme of requirements. Chapter 4 is the literature review. Chapter 5 will describe the design methodology. Chapter 6 will describe the design development. Chapter 7 will be the conclusion and future work.

1.6 Bibliography 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12.

Tyre Industry Federation. Factbook: A guide to the UK tyre industry from manufacture to end of life reprocessing, (2014) [cited 2017 August 08]; Available from: http://tyreindustryfederation.co.uk/industry-fact-book/. US3873489A. Thurn F, Pochert J, Wolff S and Burmester K, Rubber compositions containing silica and an organosilane, Deutsche Gold-Und Silber-Scheideanstalt Vormals Roessler, 1975 March 25. EP05201227(B1). Rauline, R, Rubber compound and tires based on such a compound, Compagnie Generale Des Etablissements Michelin - MICHELIN & CIE, 1992-09-02. US5604286 (A). Fuchs, HBD, Dietrich G. and Steinbrecht U.D, Rubber composition, tread made therefrom and tyres containing these treads, SP REIFENWERKE GMBH, 1995 July 26. Hamed, GR. Reinforcement of rubber, Rubber Chemistry and Technology, 2000. 73(3): p. 524-533. Tire, Merriam-Webster [cited 2018 March 06]; Available from: https://www.merriamwebster.com/dictionary/tire#h3. Mark, J.E, Erman B and Roland M. The science and technology of rubber. 2013, Academic press. p. 619-661. US5104 (A). Thomson, RW, Improvement in carriage-wheels, 1847-05-08. Goodyear, C, 1844 June 15. McNeil, I. An Encyclopedia of the History of Technology. 2002, Taylor & Francis. p. 431-470. Bryan Garnier & Co. Global demand for car and light commercial vehicle tires from 2012 to 2018 (in million units), [cited 2018 March 21]; Available from: https://www.statista.com/statistics/792209/global-tire-demand/. Making of a Tyre, [cited 2017 July 31 ]; Available from: http://traditional.apollotyres.com/eneu/making-of-a-tyre.

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13. 14. 15. 16. 17. 18. 19. 20. 21.

22. 23.

Frederick, J.S. Tyre Manufacturing, [cited 2018 march 08]; Available from: http://iloencyclopaedia.org. US3075570 (A). Graver R F, Tire building machine, The B. F. Goodrich Company, 1963 Janurary 29 Stellman, J.M. Encyclopaedia of occupational health and safety. 1998: International Labour Organization. Ebell, P.C. Internal mixing of rubber: the influence of process variables on mixed material properties, (1981), Doctoral Thesis, Loughborough University. Leblanc, JL. Filled polymers: science and industrial applications. 2009: CRC Press. p. 11-335. Payne, A. The dynamic properties of carbon black-loaded natural rubber vulcanizates. Part I, Rubber Chemistry and Technology, 1963. 36(2): p. 432-443. Payne, A. The Dynamic Properties of Carbon Black Loaded Natural Rubber Vulcanizates. Part II, Rubber Chemistry and Technology, 1963. 36(2): p. 444-450. Roland, CM. Reinforcement of Elastomers, Reference Module in Materials Science and Materials Engineering, 2016. Wolff, S. Optimization of Silane-Silica OTR Compounds. Part 1: Variations of Mixing Temperature and Time during the Modification of Silica with Bis-(3-Triethoxisilylpropyl)-Tetrasulfide, Rubber Chemistry and Technology, 1982. 55(4): p. 967-989. Nakajima, N. Mechanism of mixing in internal mixer and energy‐based modelling, Polymer International, 1996. 41(1): p. 23-33. Dierkes, W. Economic mixing of silica-rubber compounds, (2005), Doctoral Thesis, University of Twente.

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2 Objective 2.1 Description of the Design Topic In manufacturing of tyres, the rubber compounds used for tread, sidewall and apex are shaped by extrusion. Extrusion allows to create objects of a fixed cross-sectional area by forcing it through a die of the desired cross-sectional area and shape. The rubber compound, being a viscoelastic material, experiences swelling when it exits the die due to the relaxation of the polymer chains. This swelling changes the dimension of the cross-section and has to be taken into account during extrusion. The degree of swelling is specific for a given material, temperature, die and extrusion speed. Therefore, once optimized, a constant dimension of the extrudate can be obtained. Extrusion is a continuous process and can run as long the extrudate material is fed into the extruder. Rubber compounds have to be mixed before they can be fed into an extruder and shaped to a desired cross-section area. Compound mixing is done in batches and the batch weight depends on the capacity of the mixing device being used. Continuous mixing of rubber compounds is possible, but is economically not attractive due to the wide variety of compounds with different compounding ingredients used in manufacturing of tyres [1]. Extrusion is a continuous process that needs a continuous feed of the rubber compound to be extruded. Any variation in the material being fed to the extruder changes the degree of swelling when the material exits the die. Accordingly, any variation of the material properties between batches leads to variations in the degree of swelling of the extrudate. It is known that the die swell and the viscosity of the mixed rubber compound reach a plateau when the dispersion of

Figure 2-1 Relationship between dispersion, viscosity and die swell.[2]

the filler is completed as shown in Figure 2-1. Therefore it is apparent that the fluctuation in dispersion of filler particles between the batches of mixed rubber is one of the key factors for fluctuations in die swell. Practically, rubber compounds are not mixed until complete dispersion 8

is reached due to limitations on resources (time/energy). It is also possible that compound properties deteriorate at higher dispersion levels due to polymer degradation[3]. Therefore, during mixing of rubber compounds, proper control methods need to be employed to maintain a constant level of dispersion amongst batches. Traditionally, rubber compounds used in tyres had carbon black as filler material. Carbon black and polymer have the same non-polar chemical nature and are compatible. The processing of carbon black filled rubber compounds only requires the breaking of carbon black agglomerates incorporated into the rubber matrix. This is achieved, during mixing, by generating the required shear forces in the rubber matrix. As the shear forces generated during mixing depend on the viscosity of the material being mixed, parameters influencing viscosity like temperature were monitored to control the dispersion level [4]. However, with the introduction of silica as filler material, the process of mixing had to be revised. The inherent nature of silica requires additional processing steps during mixing in conjunction to the shear forces generated in the rubber matrix during mixing. Silica is incompatible with rubber due to its chemical nature, and mixing of silica in rubber is like mixing oil and water. Silica is polar, but rubber is non-polar, therefore, silica particles in rubber tend to attract each other and form domains with high concentration of silica.[5-7]. To overcome this compatibility issue, a coupling agent is used, which forms a nonpolar shielding layer around the filler particles and a bridge between the polar silica and nonpolar rubber. During mixing, the coupling agent has to chemically react with the silanol moieties on the silica surface, resulting in a decrease in the polarity of the filler particles and an increase in the mobility within the rubber matrix. This chemical reaction has to occur in a controlled way, in addition to breaking of the filler clusters by shear forces generated during mixing. A higher degree of dispersion control during mixing is not only beneficial for reducing the variation in die swell during extrusion, but also to achieve a higher efficiency during mixing by reducing the need for remixing of compounds. [8, 9]. The coupling agent has sulfur moieties which can also react with the rubber chains and form crosslinks, thus care must be taken to avoid this reaction from taking place during mixing. Consequently, a higher degree of precision in process control is required for mixing of silica filled rubber compound.[6, 7] The masterbatch mixing at the production facilities of Apollo Tyres Ltd. is performed in internal mixers. The rubber compounding ingredients have to be mixed to a homogeneous compound, and this is rather a challenge. Furthermore, every batch needs to have the same level of homogeneity or having uniform shape and structure of fillers and this translates to controlling the level of dispersion of the filler incorporated into the rubber matrix. The divergence in the consistency level is reduced by remixing or in the following mixing steps. At present, the ingredients are mixed together, removed from the mixer and visual methods of dispersion measurements using optical microscopes are employed. However, optical measurement methods are limited by the resolution of the optical microscope and can only give information about macro dispersion. It provides information on the level of physical breakdown 9

of the filler agglomerates by measuring the percentage of undispersed particles above a certain dimensional threshold. Another challenge is to prepare samples with a clean and smooth surface to be viewed under the microscope. As the ‘green’ unvulcanised rubber compound flows under shear, the surface roughness of the samples is high. The small focal length at higher magnification makes it difficult to get the whole image on focus. In addition, there is no information on the level of the chemical reaction that the filler surface has undergone with the coupling agent. The project intends to use the Payne effect as the measure of dispersion and to draw correlations between mixing conditions and the Payne effect. The relation can then be used to design a predictive mixing system that can adjust the mixing conditions based on the perceived level of dispersion and the expected level of dispersion. The Payne effect can be an effective tool in characterizing the various influences on the filler dispersion. It can not only be used to evaluate the influence of the mechanical breakdown of the filler, but also as an indicator of the degree of silanization.[10]

2.2 Objective of Design Project The project objectives can be summarized by the following targets:     2.3 1. 2. 3. 4. 5. 6. 7.

8.

9.

10.

Identify factors influencing the Payne effect in a rubber compound. Draw correlations between the Payne effect and mixing parameters. Design an algorithm to calculate the Payne effect based on the mixing parameters. Validate the algorithm. Bibliography Wood, P.R. Report 90: Rubber Mixing, 8(6),1996, Rapra Technology Limited, p.3-95, Tokita, N and Pliskin, I. The dependence of processability on molecular weight distribution of elastomers, Rubber Chemistry and Technology, 1973. 46(5): p. 1166-1187. Payne, A. Effect of dispersion on dynamic properties of filler-loaded rubbers, Rubber Chemistry and Technology, 1966. 39(2): p. 365-374. Freakley, P.K. Rubber processing and production organization. 2012: Springer Science & Business Media. p. 43-67. US3873489A. Thurn F, Pochert J, Wolff S and Burmester K, Rubber compositions containing silica and an organosilane, Deutsche Gold-Und Silber-Scheideanstalt Vormals Roessler, 1975 March 25. Wolff, S. Silanes in Tire Compounding After Ten Years — A Review, Tire Science and Technology, 1987. 15(4): p. 276-294. Wolff, S. Optimization of Silane-Silica OTR Compounds. Part 1: Variations of Mixing Temperature and Time during the Modification of Silica with Bis-(3-Triethoxisilylpropyl)-Tetrasulfide, Rubber Chemistry and Technology, 1982. 55(4): p. 967-989. Mihara, S. Reactive Processing of Silica-reinforced Tire Rubber: New Insight Into the Time- and Temperature-dependence of Silica Rubber Interaction, (2009), Doctoral Thesis, University of Twente. Cichomski, E.M. Silica-silane reinforced passenger car tire treads: effect of silica morphology, silica-polymer interface structure and rubber matrix network on tire-performance indicators, (2015), Doctoral Thesis, University of Twente. Luginsland, H.D, Frohlich, J and Wehmeier, A. Influence of different silanes on the reinforcement of silica-filled rubber compounds, Rubber chemistry and technology, 2002. 75(4): p. 563-579.

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3 Program of Requirements 3.1 Functional Requirements The functional requirements are as follows:    

The algorithm has to define the dispersion of the mixed rubber compound. The algorithm has to link the dispersion of the rubber compound to the mixing parameters. The algorithm has to predict the dispersion of the fillers based on the mixing parameters. The algorithm has to be able to adjust the mixing parameters to change the predicted dispersion level in the mixed rubber compound.

3.2 Non-functional Requirements The project focuses on designing an algorithm that can be followed to control the dispersion of the mixed compound. The algorithm has to be able to satisfy the following requirements: 

The algorithm has to be compatible with the existing machine being used during production.  The algorithm has to be conform to the existing mixing methods used during production.  The algorithm has to be applicable to the materials generally used in the production.  The algorithm has to include the motoring and feedback control system and be adaptable to manual user control or an automated system. In addition to these requirements, there may also be other non-functional requirements that could be encountered during the implementation of the algorithm in the control system. 3.2.1 Safety/Risks Risk is the probability of gain or loss from performing or not performing an action. It is calculated based on the probability of an event happening and the consequences of the event. The risks associated with modelling a mixing process to predict the material properties is related to the accuracy of the algorithm. Risk here may be calculated based on the probability or degree of deviation of the prediction from actual values and the consequences of such a deviation. As predictions are based on finding relations and fundamentally understanding them, the assumptions made can also be a possible source of risk. The risk assessment can be based on the algorithm studied, input parameters and boundary conditions applied. The algorithm by itself has no safety requirements but it has to comply with all the safety regulations that are in place for the use of mixing devices or the safety procedures at the production site. 3.2.2 Reliability Reliability is the ability of a product to perform a function in a given environment for a given time. It can be measured by the rate of failure or probability of success. The reliability of the system being developed would be calculated based on the accuracy of prediction. It can be synonymous with the percentage of match between the measured value and predicted value. As the project 11

focuses on identifying the algorithm to base the control system, the reliability requirements are difficult to estimate. 3.2.3 Maintenance Maintenance are actions performed to keep or return to a level of functionality. Maintenance of the system used to predict the desired property depends on the material being processed. Maintenance here means to keep the algorithm in line with changes in materials: Materials from different suppliers can bring unforeseen changes to the material, making adjustments of the algorithm necessary. Therefore, the algorithm should be periodically validated. 3.2.4 Finance/cost The costs of such a system is high at the initial stages and comprises primarily the costs of the research and development activities. Once the algorithm is successfully developed, the costs associated are low, as it is only necessary to periodically validate the algorithm. However, algorithms are case-specific and expansion may require a similar investment as in the beginning of this project. 3.2.5 Environmental/Social impact The predictive algorithm does not have a direct impact on the environment. But there could be an indirect influence as it helps to reduce inefficiency and to save time and material. Similar to environmental impact, the social impact would also be indirectly felt by the user as it helps to reduce the resources used.

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4 Literature Review 4.1 Filler reinforcement of elastomers Reinforcement of elastomers by fillers like carbon black and silica is a common practice in the industry and gives the elastomers improved mechanical properties. Filler reinforcement is mainly governed by two factors: filler-filler network and filler-polymer network. They arise due to the physical and chemical nature of the filler and their surfaces [1]. The main requirement for reinforcement of elastomers by filler material is an interaction between the filler and polymers. The interaction can be physical like in case of carbon black or chemical like with silica fillers in combination with silanes. Besides the interaction between the filler and the elastomer there may be interaction between the filler particles. The primary particle size is the most important factor influencing the amount of interaction between filler and elastomers. Particles having sizes larger than 1 µm do not have a reinforcing capacity due to less interactive surface and they only increase the reinforcement by the simple hydrodynamic effect. Reinforcement can be realized with a filler size smaller than 100 nm [2]. The amount of filler material incorporated into the elastomer can have a direct effect on the amount of inter filler interaction and the filler polymer interaction. Beside these there is also the hydrodynamic effect generated due to the viscoelastic properties of the elastomer and the interactions between the elastomer network that contributes to the reinforcement of elastomers.

Figure 4-1 Contribution of different reinforcement effects on the complex shear modulus.

13

The reinforcing effect of silica and carbon black is shown in the Figure 4-1. The figure shows the effect on the complex modulus of the filled elastomer against the log of strain. Also the contribution by the different interactions are shown in this figure. It can be seen that the hydrodynamic effect and the polymer network contribution are constant at all values of strain and similar for carbon black and silica fillers. Silica fillers have a higher reinforcing effect than carbon black fillers but the effect is more due to the strong inter particle interaction. Carbon black shows higher filler-polymer interactions than silica as it is more compatible with the polymer network [3]. The Payne effect is generally considered as the measure of the filler network interaction[4, 5]. For this reason, Payne effect can be used as a measure of the dispersion of the filler material in the elastomer matrix. The influence of dispersion on the reduction of the modulus of the filled elastomer has been well documented [6]. Higher wear resistance and lower rolling resistance have been observed due to the improvement of silica dispersion [7]. The reinforcement by different mechanisms is briefly described in the following sections. 4.1.1 Hydrodynamic Effect The addition of fillers to polymers leads to an increase in mechanical properties and viscosity. The increase in viscosity is explained as the forces exerted by the particles on the continuous matrix due to its rigid structure that resists shear forces. The theory for the filler influence on viscosity was first studied by Einstein for colloidal suspension in a liquid medium [8]. The model is used to describe the change in viscosity 𝜂 in relation to the amount of filler volume added [9]. 𝜂 = 𝜂0(1 + 2.5𝛷) Where η0 = viscosity of continuous phase and

(1)

Φ=filler volume fraction. However, this equation is only applicable to lower filler volumes and spherical monodisperse filler particles with no interaction between them. Smallwood proposed that the viscosity could be replaced with the Young’s modulus for elastic material.[10] 𝐸 = 𝐸0(1 + 2.5𝛷) Where E0 = elastic modulus of un filled rubber

(2)

E = elastic modulus after adding fillers This equation for elastic material was later modified by Guth and Gold and a second degree term was included to account for higher filler concentration in the medium. (3) 𝐸 = 𝐸0(1 + 2.5𝛷 + 14.1 𝛷2 ) The equation could be used for filler volume fractions of up to 0.3 but the shape factor of the filler particles were not taken into account and Guth proposed a modification to the equation [11]. 𝐸 = 𝐸0(1 + 2.5𝑓𝛷 + 14.1𝑓 2 𝛷2 )

(4) 14

Where f is the ratio of length to width of the aggregates. Guth also proposed that the Young’s modulus could be substituted by the shear modulus G [9]. 𝐺 = 𝐺0(1 + 2.5𝑓𝛷 + 14.1(𝑓 2 )(𝛷 2 )) Where G0 = Shear modulus of unfilled rubber

(5)

The models are not applicable for higher loadings because the inter-particle interaction makes the formula invalid [12]. The formula does take into account the shape of the particles (f in equation 4 and 5) but not the structure of the fillers. Nonetheless the model can be used to estimate the modulus of the filled polymer by the hydrodynamic effect [13]. 4.1.2 Polymer Network Interactions The polymer network interaction arises due to the crosslinks formed between the chains of the polymer. The chemical and physical properties of the elastomer are determined by the length and density of crosslink, the chemical structure of the polymer, the molecular weight and the molecular weight distribution . Crosslinking can be achieved through chemical reactions or vulcanization and are stable and don’t break easily. The crosslinking of long chain rubber molecules increases the hardness and the glass transition point but decreases flexibility and elongation at break. The shear modulus G can be described by the classical theory of rubber elasticity at small deformations using the following formula [14]. < 𝑟2 > (6) ) 2 < 𝑟0 > Where k is the Boltzmann constant in Joules/Kelvin, v is the number of elastically active chains per unit volume, T is the temperature in Kelvin and / is the root mean square ratio of the end-to-end length of the undeformed polymer chain and the end-to end length of polymer chain under swelling of elastomer. The equation gives a theoretical estimation of the contribution of the crosslinking to the shear modulus. The crosslinking is one of the strain independent contribution to the modulus due to polymer network interaction and is proportional to the crosslink density of the networks [15]. 𝐺 = 𝑣𝑘𝑇 (

4.1.3 Filler Polymer Interaction The filler polymer interaction is generated by the interaction of polymer network with the structure and the surface characteristics of the filler material. The interaction can be physical adhesion or adsorption, chemisorption and mechanical interaction of the polymer to the surface of the filler due to wetting of the surface by the polymer material. The polymers can be adsorbed at the porous or the rough surface of the fillers aggregates and forms a monolayer around it like a shell. This rubber on the surface of the filler material is termed as bound rubber. The bound rubber is not only affected by the filler and the surface characteristic but also depends upon the polymer type. The chemically bound rubber is the dominant mechanism for the generation of bound rubber in a silica filled polymer system. The chemical structure of the polymer and their configuration, molecular weight and distribution can influence the amount of bound rubber. The bound rubber content can also depend on the processing condition and the storage time of the 15

mixture. This idea of bound rubber was first proposed by Twiss in 1925 and extensive research was followed by Boiry and J.H. Fielding developing the bound rubber test [15, 16]. Medalia [17] determined that the morphology of the filler aggregates also contributes to the interaction between the filler and polymer. The voids present in the aggregates and the agglomerates of the filler materials can be filled by the polymer and is termed as the occluded rubber. In addition to that, similar to bound rubber in aggregates, there is a layer of rubber that encapsulates the agglomerates as well named the shell rubber. The trapped rubber is shielded by the filler structure and immobilized. Occluded rubber is the amount of rubber that behaves like a filler material as it cannot contribute to the elastic behaviour of the polymer. The occluded rubber increases the effective volume of the filler material and it can be estimated that half the volume of the occluded rubber behaves as filler [17]. The filler particle size, morphology, surface area and surface activity determine the amount of bound rubber and thus reinforcement. 4.1.4 Filler-Filler Interactions The filler-filler interaction was extensively studied by A.R. Payne and has been named after him as the Payne effect. This effect is a unique stress-strain behaviour of the filled polymer system. Payne studied the effect of cyclic loading at small strain amplitudes on the storage and loss modulus of the filled system and observed that above the 0.1% strain the storage modulus decreased rapidly until about 20% strain whereas on the same range the loss modulus is rising [18]. The effect is not noticed with unfilled polymer system but can be observed in the polymers with very low filler volumes as well. The effect has been attributed to the breakdown of the fillerfiller networks. Due to the increase in the distance between them at higher strains the attractive London or van-der-Waals forces become weaker and network breakage occurs. The Payne effect is strain-dependent and temperature dependent. It is mostly reversible and independent of the type of polymer used but depends on the type of filler used. Silica reinforced polymers show higher Payne effect than carbon black filled polymers (see Figure 4-1) [19].The temperature effect on Payne effect is most prominent in the range of 20 to 90°C with approximately 50% decrease in the storage modulus at low strains occurring in this temperature range [15]. Kraus proposed a model based on the van-der-Waals forces of attraction between the filler particles that contribute to the agglomeration and de-agglomeration of the filler particles. Depending on the distance between and the number of particles he estimated that the filler would coalesce or break depending on the cyclic strain applied [20, 21]. Based on the formation and deformation of the filler network Kraus derived the equation for Payne effect [22]. Kraus assumed that the network breakdown rate (Rb) is proportional to the number of existing network (N), where N0 is the initial number of contacts, and a function of strain amplitude (f b). Then the network reformation rate (Rm) is proportional to the number of broken contacts (No-N) and a function of the strain amplitude (fm). 𝑅𝑏 = 𝑘𝑏 𝑁𝑓𝑏

(7)

16

𝑅𝑚 = 𝑘𝑚 (𝑁0 − 𝑁)𝑓𝑚

(8)

Where, kb and km= Proportionality constant At equilibrium (undisturbed network) the network formation is equal to the network breakdown Rm=Rb: 𝑁=

𝑁0 𝑘 𝑓 1+( 𝑏 𝑏) 𝑘𝑚 𝑓𝑚

(9)

The contribution to the storage modulus is taken as the difference of the modulus at infinite strain (G’∞) and at given strain G’(γ) which is proportional to the number of existing network. The total number of contacts between particles is proportional to the difference of modulus at infinitely low strain (G’0) and modulus at infinitely high strain (G’∞). The strain amplitude function for an existing network (fb) is replaced by γm in Equation 7 and the strain amplitude function for network breakdown (fm) by γ-m in Equation 8 where γ is ½ peak to peak test strain amplitude and m is a constant that gives the shear strain sensitivity. Replacing the values in equation 9 and the constant (km/kb)1/2m by yc the following equation can be obtained. ′ 𝐺 ′ (𝛾) − 𝐺∞ = ′ 𝐺0′ − 𝐺∞

1 𝛾 2𝑚 1 + (𝛾 ) 𝑐

( 10 )

Where: G’(γ) = Storage modulus at strain γ (Pa) γ = Shear strain rate (1/s) γc = Shear strain where half of the filler network is broken m = shear strain sensitivity G’0 = Modulus at zero strain (Pa) G’∞ = Modulus at infinite strain (Pa)

The viscous modulus due to the energy dissipation of breaking and forming network is the difference of G’’(у) and G’’∞ and is proportional to the rate of network breakdown. From this the following equation is obtained : ′′ 𝐺 ′′ (𝛾) − 𝐺∞ = 𝐶1 𝐾𝑏 𝑁𝑓𝑏

( 11 ) 17

Where C1 is a constant. If integrated with fb = γm we get the expression of G’’m, the maximum modulus. ′′ (𝛾)

′′ 𝐺 − 𝐺∞ 𝐺′′𝑀 − 𝐺′′∞

=

𝛾 𝑚 2 (𝛾 )

( 12 )

𝑐

𝛾 2𝑚 1 + (𝛾 ) 𝑐

Where: G’’(γ) = Loss modulus at strain γ (Pa) γ = Shear strain rate (1/s) γc = Shear strain where half of the filler network is broken m = shear strain sensitivity G’’M = Maximum Loss modulus (Pa) G’’∞ = Loss modulus at infinite strain (Pa)

This Kraus model does not take into account the influence of temperature on the Payne effect, but he does note that G’0 decreases with a rise in temperature [20]. Huber and Vilgis considered the effect of the fractal nature of fillers and derived a relation for Payne effect. Their approach is similar to the one of Kraus but considers additionally that the filler network breaks into smaller networks when strain is applied. They take into account the fractal dimension of filler aggregates and also the shortest path in the aggregate structure. The relation [22, 23] derived by Huber and Vilgis is expressed below: ′ 𝐺 ′ (𝛾) − 𝐺∞ 1 = ′ ′ 𝐺0 − 𝐺∞ 1 + 𝑘 2 𝛾 2𝑚 ′′ 𝐺 ′′ (𝛾) − 𝐺∞ 2𝛼𝑘𝛾 𝑚 = 𝐺′′0 − 𝐺′′∞ 1 + 𝑘 2 𝛾 2𝑚

( 13 )

( 14 )

Where: G’(γ) = Storage modulus at strain γ (Pa) γ = Shear strain rate (1/s) G’0 = Modulus at zero strain (Pa) 18

G’∞ = Modulus at infinite strain (Pa) m = shear strain sensitivity 1 𝑚

k = (𝛾 ) 𝑐

′

′

′

𝐺′′𝑀 − 𝐺 ′ ∞ = 𝛼(𝐺 ′ 0 − 𝐺 ′ ∞ ) α = Constant G’’(γ) = Loss modulus at strain γ (Pa) γc = Shear strain where half of the filler network is broken G’’M = Maximum Loss modulus (Pa) G’’∞ = Loss modulus at infinite strain (Pa)The Huber-Vilgis model is similar to the Kraus model but modified to take into account the temperature effect and the fractal dimension of fillers. The k value takes into account the temperature effect and the m parameter is related to the fractal dimension and connectivity of the filler network. The alternative Maier and Göritz model describes the Payne effect based on the molecular interaction of the fillers and polymer [24]. During mixing the polymers are adsorbed on the filler surface. The first polymer chain is strongly adsorbed on the surface and facilitates the adsorption of other polymer chains. The first polymer chain is termed as a “stable bond” and the subsequent chains have weaker adhesion to the surface, eventually leading to the last chain which forms “unstable bond”. These unstable bonds desorb from the surface when the temperature increases or a mechanical stress is applied. This effect gives rise to the Payne effect observed in filled polymers. The contribution to the elastics modulus of the filled rubber is assumed to be by the polymer chain interaction and by the filler-polymer interaction. 𝐺′ = 𝑁𝑘𝐵 𝑇

( 15 )

Where: G’ = Storage modulus (Pa) kB = Boltzmann Constant (J/K) T = Temperature (K) N = Crosslink density (mol/cm3). Further look in to the crosslink density gives the components contributing to the crosslink density, which is as follows: 𝑁 = 𝑁𝑐 + 𝑁𝑠𝑡 + 𝑁𝑖

( 16 ) 19

Where: Nc = Chemical crosslink density (mol/cm3). Nst = Stable crosslink from filler polymer bonds (mol/cm3). Ni = Unstable crosslink from filler polymer bonds.(mol/cm3).

𝑁 = 𝑁𝑐 + 𝑁𝑠𝑡 + 𝑁𝑖

( 17 )

The storage modulus at strain is given by the equation: 𝐺𝑖′ 𝐺′(𝛾) = 𝐺 𝑠𝑡 + 1 + 𝑐𝛾 ′

( 18 )

Where: G’st = (Ng+Nst)kbT G’I = Ni0kbT Ni0 = Initial unstable crosslink from filler polymer bonds.(mol/cm3). Ng = number of elastically active rubber chains per unit volume. G’st is the modulus value for high deformation and G’i the Payne effect amplitude. The parameter c gives the position of the curve on the strain axis. The loss modulus is defined by: 𝐺′′(𝛾) = 𝐺 ′′ 𝑠𝑡 + 𝐺𝑖′′

𝑐𝛾 (1 + 𝑐𝛾)2

( 19 )

Where: G’’st is the very low or high strain modulus, G’’i gives the amplitude variation, γ is the shear strain and c gives the maximum of G’’ on strain axis. Wang et al. proposed a more detailed model that took into account the compatibility of the filler and polymers. They postulated that a highly compatible filler and polymer would form a layer of bound rubber whereas an incompatible filler would tend to stick together and form a much rigid filler network. Heinrich and Klüppel demonstrated a connection between the filler morphology and the viscoelastic properties of rubber materials. The mechanisms of the Payne effect are dependent on the filler morphology and bound rubber. The viscoelastic nature of the bound rubber suggests the role of the filler elastomer structure in which filler–filler bonds are created via an adsorbed layer of polymer [25].

20

These different interactions contribute all to the final reinforcement of the polymer matrix by the fillers. A different degree of reinforcement is attributed to different interactions. Generally, the hydrodynamic effect, filler-polymer interaction and the crosslinking contribution to the shear modulus are considered as strain independent but the filler-filler interaction is strain dependent (see Figure 4-1). Better understanding of the strain independent part of the storage and loss modulus could help to optimize the processing conditions. However, the understanding of the strain dependent part is of significance in understanding the performance aspects of the filled polymer system.

4.2 Overview of Filler Material The term “fillers” are widely used for particulate material added to a binder material to mix a composite. In the past fillers were primarily used to reduce costs as binder materials were expensive and fillers helped to increase the volume at low costs. Presently, fillers are extensively used to modify or improve the physical and mechanical properties of material. The addition of fillers may alter thermal conductivity, electrical resistivity, friction and wear resistance. Fillers can be classified according to their functionality. Conductive fillers increase the electrical and thermal conductivity, extender fillers can increase the volume of materials and reinforcement fillers can enhance properties. The worldwide annual consumption of filler material exceeds 53 million tons and is a EUR 16 billion industry [26]. Fillers are used as raw materials in production of pharmaceuticals, paper, plastics, rubber, paints, coatings, adhesives, sealants and more. Clay, aluminium, carbon fibre, calcium carbonate, silica are some of the widely used filler materials. Filler material selections are made depending on the use and filler specifications. A polymer combined with filler material is referred to as filled polymer [27]. A filled polymer system is used to produce many everyday rubber and thermoplastic products. The tyre industry is among the biggest with 3.3 billion tyres produced worldwide annually [28]. The most commonly used filler materials in the tyre industry are carbon black, silica, calcium carbonate and clay [2]. Reinforcement fillers used in the production of tyres are silica and carbon black. In the beginning of the 20th century carbon black was used as the primary filler material. Carbon black provided the soft natural rubber higher stiffness, strength and elongation at break. Carbon black is relatively cheap, interacts physically with polymers used in a tyre and the manufacturing process is simple and quick. However, with the new focus on reducing emission and achieving better fuel economy the tyre industry has had to rethink its compositions. Recent years have seen an increase in silica as filler material because of the superior rolling resistance and wear resistance. Silica is polar and hydrophilic, therefore does not disperse readily in the hydrophobic polymer matrix. 4.2.1 Silica Filler Silica is a compound made up of one part of silicone and two parts of oxygen and found abundantly on the earth’s surface. It is generally found in crystalline form and has been used by humans for millennia primarily in glass making and metallurgical activities (see Figure 4-2 for different types of silica). There are numerous industrial applications with numerous types and 21

grades available in the market. The tyre industry uses the precipitated amorphous form of silica either in pellets or powder form as reinforcement fillers [27]. Silica has been in use as fillers for tyres since as far back as 1940 where it was used in combination with carbon black to enhance the adhesion between the steel cords and rubber. The earlier applications as reinforcing fillers gave unsatisfactory results because of the poor mixing tendency of silica with carbon compounds. Silica, being polar and hydrophilic, and rubber comparatively nonpolar and hydrophobic, when used without additives tends to bond with each other more than with the rubber matrix. For better bonding and better reinforcement, bridging compounds that react with both the silica surface and the rubber molecule, also called silane coupling agents, are often used [29]. Silica

Natural

Crystaline

Quartz Tridymite

Synthetic

Amorphous

Opals Kieselguhr

Wet Process

Thermal/Pyro genic

Silica Gel

Arc Silica

Precipitated Silica

Plasma

Viterous Silica

Flame Hydrolysis

Figure 4-2 Different types of silica

Tyres with silica filled compounds in the tread have a few distinct advantages over carbon black filled tyre tread compounds. Reinforcement of elastomers by fillers is a common practice in the tyre industry and gives the elastomers improved mechanical properties. A homogenous dispersion of silica is an important factor that greatly influences the reinforcing effect of fillers. The dispersibility of silica is a complex phenomenon depending on many other factors along with coupling agent [16, 30]. It is of general knowledge among people in the tyre industry that a better dispersion provides better properties in a tyre. But mixing of rubber compounds is a batch process and the inconsistencies between the batches of mixed rubber compounds have to be minimized. A brief discussion on the various parameters of silica that can affect the dispersibility of silica is included in the following sections. Silica is manufactured by many different methods depending on the application. In the tyre industry, precipitated silica is generally used extensively and will be the focus of this study. 4.2.2 Physical Characteristics of Silica Fillers The flocculation is the process by which the individual primary particles of the silica come together in solution and bond to form a collection of primary particles called aggregates. 22

Although the ultimate particles that constitute silica gels, pyrogenic (fumed) precipitated, and coacervated silica have lost their mobility by aggregation. They are also in the colloidal range of particle size and are therefore known as colloidal silica [31]. The stability of a colloidal system is inherently dependent on the formation of aggregates due to flocculation of the particles. The formation or separation of the particles is dependent on the local forces acting between the particles. The individual particles tend to cluster together due to chemical reactivity at the surface. After the formation of the particles the surface energy reduction to reach a minimum energy is one of the main reasons for aggregate formation. These aggregates form agglomerates of complex branched 3D structures and networks of agglomerates. The aggregates and agglomerate are present when the silica is mixed with rubber compound. The formation of aggregates and an agglomerate network during and after the mixing process has a substantial effect on the dispersibility of silica aggregates as well as the properties of the filled polymer system. Some properties that can be used to characterize silica are discussed in the following sections [31]. 4.2.3 Particle size Synthetic precipitated amorphous silica used in tyre industry is made up of mostly condensed and dense spherical particles, also known as primary particles that cannot be broken further. The primary particle is the basic building material and influences all physical, chemical and mechanical properties. In the manufacturing process of silica, the silica nucleates to form primary particles, which in turn randomly combine to form aggregates. The aggregates are the most stable form made up of primary particles. They combine to form agglomerates of size reaching up to 100 µm [31, 32]. Agglomerates are held together mainly by Van-der-Waals forces or hydrogen bonds. They can be broken down reversibly to aggregates by application of force and stabilized under right conditions. The dispersion mechanism follows the rupture of agglomerates and this rupture is dependent on the shear rate and the agglomerate size. The larger agglomerates rupture easily, however, a critical shear stress is required to disintegrate them completely. Agglomerates above 100μm radius do not disintegrate in one step [33].

Figure 4-3. Different spatial organization of silica filler particles. [34]

The capacity of silica to form various levels of spatial organization is shown in Figure 4-3. The hierarchical arrangement of silica makes it difficult to analyse its structure as it can exist in aggregated and agglomerated forms. The varying primary particle size and distribution also 23

causes the aggregate sizes to vary. In addition, the size distribution of aggregates and agglomerates is wide. From the point of view of dispersion and homogeneity it may be prudent to select fillers with primary particles of low particle size range and distribution as this can limit the range and distribution of the aggregates and agglomerate sizes. The dispersion of aggregates can increases with decreasing particle size as interactions with the polymer matrix is higher [35]. The smaller particle sizes have more exposed surface area and thus interact easier with the polymer network. The Payne effect is also shown to be influenced with the change in particle size, i.e. fillers with smaller primary particles show higher Payne effect than for fillers with larger primary particles [36]. 4.2.4 Structure The primary particles coalesce to form aggregates and commonly these aggregates are the most dominant sub-form with the primary particles tied up in them and many aggregates linking with each other to form an aggregate network or agglomerate. The size, shape and the spread of this agglomerate network influences the physical, rheological, mechanical and electrical properties. Aggregates can form a dense, solid structure with few voids or an open lattice-like configuration and can occur as tight groups of particles or as chain-like with long dimensions. The higher structure has more surface area exposed to interact with the polymer but the lower structure has many overlaps and surfaces shielded by other particles. From this description of structure it can be expected the higher structure to be easily dispersed as each particle have more possibility to interact with the rubber matrix. [17, 37]. The aggregates form agglomerates which have a complex 3D structure. The density of the aggregate will have a direct effect upon the agglomerate and polymer matrix, thus influencing the dispersion of the filler in the rubber matrix. 4.2.5 Specific surface area The specific surface area (SSA) is the ratio of total surface to mass or volume of a material with unit of m²/g or m-1. The SSA can be used as a tool to determine the effective reactive surface area of the filler materials. In general, the specific surface area can be taken as the measure of the average size of the primary particles. Higher specific surface area can be correlated to smaller particle size [38] . But the specific surface area can be used to qualitatively estimate the size of primary particle and provides no information on the actual structure. The relationship between the primary particle size, structure and surface area is closely related. Carbon black and silica fillers form a branched porous structure. The branches and porosity gives them a higher surface area. Silica manufactured by pyrogenic methods has a smaller primary particle size and thus larger surface area in comparison to precipitated silica. James Leblanc noted that the specific surface area of the filler had a direct effect on the dynamic mechanical properties. Higher surface area can also lead to a higher Payne effect and to more difficulties to get it well dispersed [33, 36].

24

4.2.6 Surface Energy for Silica The surface energy can be defined as the sum of all intermolecular forces that are on the surface of a material. In other words, the degree of attraction or repulsion forces of a material surface exerts on another material [39]. The surface energy of a substrate determines the wetting of the surface and the adhesion of the particles and the surface tension of the elastomer determines the wettability to the substrate. When a particle has a high surface energy it tends to be easy to adhere to and if the polymer has a low surface tension a good wetting of the particle by the polymer is produced with strong adhesion. The chemical and physical nature of the surface determines the surface energy and it can be split into two components: Ys=Ysd+ Yssp

( 20 )

Where, Ys= Surface Energy (J/m2) Ysd = Dispersive energy or Vander Walls forces (J/m2) Yssp = Total polar/hydrogen bond between fillers (J/m2) The total surface energy of silica is the sum of the polar component and the dispersive component of the surface Ysd and Ysp. The dispersive component is greater in the case of a carbon black but the polar component is higher for silica. Therefore, from the surface energy point of view the silica primary particles easily aggregate and agglomerate in comparison to carbon black. Also due to the bigger polar component of silica the filler-filler interaction is dominant and it is difficult to incorporate silica into the non-polar polymers. Therefore, the components of the total surface energy can be used as a tool to estimate the dispersibility of silica in polymers. The surface chemistry of silica is more active and distinct for silica particles than for carbon black. The silica surface has silanol groups and siloxane groups that contribute to the polarity of the silica particle. Primarily, increasing number of silanol groups present on the surface leads to a certain polarity. These silanol groups can form hydrogen bonds with each other and other polar molecules. This polarity also effects the particle-particle interaction of silica giving rise to stronger particle interaction due to hydrogen bonds. Silica primary particles form string of pearls or complex 3D clusters through chemical bonds. Thus, in Equation 20 it can be seen that Y ssp can be considerably higher than the dispersive component in the total energy. It follows logically that for an easy dispersion the energy required should exceed the total energy of the hydrogen bonds. The same effect can also be expected to be observed in the Payne effect with higher surface activity having less Payne effect due to better filler-polymer interaction [36]. The main focus in dispersion is to break down agglomerate and have stable optimally sized particles that do not re-agglomerate. This process generally involves three distinct stages: wetting, de-agglomeration, and stabilization. However, in the tyre filled with silica the fillers are polar and hydrophilic and polymers are hydrophobic. The wetting of the surface of silica by the polymer is very difficult. Less particle-polymer interactions take place which results in poor dispersion. Therefore, a good rubber–filler interaction has to be created with the help of bridging 25

agents that help to link the fillers to the rubber matrix. Considerable research has been performed on the bi-functional chemicals that reacts with silanol groups on silica and also form a covalent bonding with the rubber matrix while vulcanizing. Thus, silica forms bonds with the rubber matrix by a chemically linking whereas for carbon black this interaction is a physical adsorption. Therefore, the mechanism of reinforcement and dispersion parameters for carbon black and silica vary considerably. 4.2.7 Surface Chemistry Silica is a chemical compound of silicon and oxygen. Silica has a tetrahedron structure with four oxygen atoms at the corners and oxygen atom occupies the central cavity. In an amorphous silica the structure is a random packaging of [SiO44-] units. This random packing causes the amorphous silica to have less density than crystalline structure. The surface of silica is much more active than carbon black. The surface of silica primarily has two functional groups, silanol groups (Si-OH) and siloxane (Si-O-Si) groups. These two groups have a significant effect on the surface properties and the characteristic of silica. However, impurities may give an additional negative charge to the surface. The silanol group gives the silica its hydrophilic character and can easily be wetted by water. The siloxane groups are mostly inert but give the silica a slightly hydrophobic nature. However, the silanol group is more predominant and silica is hydrophilic. The surface modification of silica is done by the reaction of silane coupling agent with the silanol groups. The reaction hinders the effect of silanol group on the silica surface making it more hydrophobic. The surface of silica may be saturated by the silanol groups in an aqueous medium but the configuration may be different. Some of the configuration that is presently considered to exits is given below.  A single hydroxyl group or an isolated silanol group at one silicon atom.  Two hydroxyl groups or geminal silanols at one silicon atom.  Two hydroxyl groups or vicinal silanol group on two adjacent silicon atoms. Other than the silanol groups there are some stable siloxane bridges and chemically and physically bound water in the structure. Moreover, the siloxane groups can undergo hydration to form silanol in the presence of water [40]. Heating the silica can be used to remove the additional silanol groups by dehydration. The drying process reduces the number of silanol groups at the surface of the silica. The average agglomerate size increases with the increasing number of silanol group per unit surface area of the silica particle and similar results can be seen when mixed in rubber matrix. Moreover silica with surface silanol group density of one or less per unit surface area did not aggregate and also no trace of bound rubber was found when composited with polymer [41]. The lack of bound rubber in the absence of a silanol functional group suggests that the link between silica and polymer is more of chemical bonding than physical. The implication is that the dispersive surface energy component of silica, which helps bonding of silica and polymer at the interface, is not the dominant factor for filler interaction with the polymer network.

26

4.2.8 Silanization Reaction The traditionally used filler, carbon black, is slowly but surely being replaced by precipitated silica as the primary filler for tyres. The polar and hydrophilic silica is not compatible with elastomers and do not provide the high reinforcing effect of carbon black compounds. Therefore, bifunctional organic silica compounds (organosilanes) have been developed to be used as a coupling agent to link the silica and elastomer. Therefore, in contrast to carbon black that reinforces by physical interaction, the silica-silane system leads to a chemical reinforcing. Due to the chemical reinforcing nature of the silica filled polymer system the processing parameters have to be adjusted to include a chemical reaction step. This step happens during the mixing cycle inside the internal mixer during the dispersion and distribution cycles. In general, chemical reactions are dependent on concentration, time, temperature and catalysts used. Therefore, during the mixing cycle these parameters also have to be taken into consideration to achieve a high efficiency of the reaction. A thorough understanding of the reaction kinematics and reaction steps can help to optimize the process of chemical reinforcing elastomers by silica using organosilanes. The coupling agents link silica fillers to the elastomer by using two reactive groups in the organosilane molecule. The alkoxy group reacts with the silanol group at the silica surface during compound mixing and the sulphur group reacts with the elastomer during the vulcanization [42]. To achieve the maximum reinforcement effect both reactions need to occur. However, the reaction needs to happen at the right stage. The reaction with fillers should occur during the mixing and the reaction with the elastomer during the vulcanization. Therefore, the control of these reactions is of paramount importance. There is a noticeable difference in the Payne effect in silica filled elastomers that use a silane coupling agent and that do not. The coupling agent interacts with the hydroxyl group at the surface of the silica and makes it less hydrophilic, thus more compatible with the elastomer. The surface modification of the silica by a coupling agent can work in two ways: reacting with and reducing the number of –OH group and shielding the – OH group from further reactions [30].

Figure 4-4 Primary silanization reaction.[40]

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The reaction of silanes with the hydroxyl groups on the silica surface follows a primary and secondary reaction sequence (see Figure 4-4 and 4-5). The primary reaction in the silanization reaction is that of silane with a silanol group of the silica. During mixing the coupling agent can react with silica either by a direct condensation or by hydrolysis then followed by a condensation reaction. The reaction follows a first order reaction. This is followed by the secondary reaction between adjacent silane molecules. The primary reaction is much faster and the dominating reaction and the secondary reaction does not have any significant effect on the properties [15]. Most of the bi-functional silanes have sulphur functional groups in their chain that is capable of reacting with the elastomer matrix. The sulphur in the silanes react with the elastomer at high temperatures (above 150°C) and pre-scorch or premature vulcanization may occur. If pre-scorch happens during the rubber compounding stage, the reaction of the sulphur with the rubber increases the viscosity of the compound, making it more difficult to process. Therefore, to avoid pre-scorch during mixing the temperature control is very important [15, 30].

Figure 4-5 Secondary silanization reaction.[40]

Bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT) is one of the most common bi-functional coupling agents used in silica filled tyre compounds and will be used for subsequent experiments. The reaction conditions, such as the presence of other compounding ingredients, reaction temperature, energy input and ethanol removal, influence the silanization reaction. The competing reaction of other additives such as zinc oxide, stearic acid, alcohols and amines influence the kinetics of the silanization reaction as they may block active sites at the filler surface. The rates of the primary and the secondary reaction are increasing with an increase in temperature. The reaction also depends on the concentration of the silanol groups at the silica surface and their accessibility. The reaction is catalysed by moisture and the reaction rate also decreases with a lower moisture content in the silica and a lower pH value of the silica filler. At a temperature interval between 140°C and 160°C, the silanization reaction takes place at a significant rate [40].

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4.3 Overview of Polymers. Rubber or elastomer is a type of polymer with unique properties. The main property of an elastomer is its ability to stretch. In other words, it is a viscoelastic material with low yield strength but very high failure strain. The viscoelastic properties of the elastomer arises from the ability of the long chains of monomers to reconfigure themselves when stress is applied. The weak van-der-Waals force ensures that the polymers return to their original configuration when the stress is removed. There are several different types of elastomer used ubiquitously at present like polybutadiene, chloroprene, silicone rubber, ethylene propylene diene rubber and many more. Rubbery materials are ideal for tyre compounds because they are flexible and tear resistant. Most common elastomers used in tyres are natural rubber, polybutadiene and styrenebutadiene copolymers. The tread compounds used in the passenger car tyres are generally made out of polybutadiene and styrene-butadiene copolymer blends and will be the focus in this section. 4.3.1 Styrene-Butadiene Rubber Styrene-butadiene rubber or SBR is a synthetic rubber made from copolymer of styrene and butadiene. It can be polymerised in solution or an emulsion medium, has good abrasion and aging resistance and is a good substitute for natural rubber. The Tg and other properties of SBR are highly dependent on the ratio of the styrene and butadiene in the monomers. Higher ratio of styrene makes the polymer more saturated and decreases the number of monomers. The number of monomers is crucial to the molecular weight and distribution and processing of SBR. The Tg increases with the increase in styrene content and the tensile strength is maximum at 50:50 styrene and butadiene blend. Higher styrene contents also increase the modulus of SBR and results in a more plastic-like nature. SBR is available in various grades and molecular distribution like oil extended emulsion SBR [43]. 4.3.2 Polybutadiene Rubber Polybutadiene rubber or BR is a synthetic polymer made of butadiene monomers and is available in cis, trans and vinyl configuration. BR is used in blends with SBR and is used in tyre manufacturing to provide improved fatigue life, abrasion resistance, lower rolling resistance and to promote better heat transfer. The processing of BR is difficult and handling on open mills and extrudability is also poor. However, the low temperature performance and the abrasion resistance are very good as the Tg is very low compared to other rubbers. BR has a very good low temperature flexibility and higher elasticity at normal temperatures. Therefore, it is blended with SBR to improve its physical properties while having better processing properties [43]. 4.3.3 Processing of SBR/BR The properties of the polymer are highly influenced by the monomer that constitute the polymer or the repeating unit. The degree of polymerization or the number of monomer is also of equal importance as it determines the chain length of the polymer. In addition, the molecular weight and the molecular weight weigh distribution is also of equal importance. The molecular weight and distribution determine the physical and processing properties of the polymer. The molecular 29

weight distribution directly affects the elasticity, tensile strength, melt viscosity and other properties of the elastomer. SBR and BR are the main synthetic polymers used by the tyre industry and because of their structure and narrow range of molecular weight distribution they provide better elasticity, less abrasion and less heat build-up. The mixing and processing of SBR is relatively easier as is not as temperature sensitive as natural rubber and oil and plasticizers can be used to adjust the viscosity.. Therefore, during mixing it is possible to use oils to change the viscosity of SBR or increase the temperatures to change the viscosity. Similarly, BR compounds are also able to absorb high amounts of filler and oil and their rate of change of viscosity with respect to temperature is low. This slow change in viscosity makes the processing difficult but, blended with SBR, this problem is less imposing. BR does have a tendency to cold flow and can be compacted when stored improperly, making it difficult to incorporate filler materials. It also has a sticky surface and can attract dirt and other pollutants [43, 44].

4.4 Rubber Mixing Mixing is the process to form a mixture by combining two or more substances to obtain a homogenous material by forcing a mass transfer between different substances and phases in the mixture. A mixture may be of different state materials or of the same state materials like gas-gas blends or solid-liquid blends. Mixing of miscible and same state material like water and milk requires a low amount of energy. In contrast, different state mixing like salt dissolved in water requires more energy. Immiscible substances may be mixed with the help of substances that aid in the dispersion of one of the constituting mass in the system matrix. The complexity of the mixing process increases with the number of constituents, multi-phase materials and degree of miscibility. For industrial mixing, batch and continuous mixing are two modes of mixing currently used to process compounds and an appropriate system must be adopted according to the application. In the batch process, ingredients are added to the mixer all at once or in a predefined sequence and mixed to a level of homogeneity and discharged. In contrast to this, in the continuous mixing the ingredients are continuously fed into the mixture and mixed en route to a discharge nozzle. Continuous methods are generally used for a single mixture type required in high volume. For the tyre industry batch mixing is employed due to the big variety of compounds to be mixed. Every compound used in tyres is a mixture and its properties controlled by the constituting elements and the process of mixing. Due to the viscoelastic nature of rubber, a power intensive sturdy machinery like mills or internal mixers is necessary to achieve the mixing of additives into the polymer [45, 46]. 4.4.1 Rubber Mixing Equipment The majority of the rubber mixing for tyres is conducted on a two roll mill or internal mixers. The master batch is almost exclusively produced in the internal mixer and incorporation of cure packages maybe done using the two roll mill. Master batch contain most of the additives optimally dispersed in a carrier polymer and later maybe diluted with other polymers as required. Both machines are of equal importance to a tyre manufacturer but due to the focus on the master 30

batch, the internal mixer and especially the tangential rotor type mixer will be the focus in this work. The different types of rotors and different parts of the internal mixer is discussed later in this section. Two roll mill Two roll mills have been used in the mixing of rubber compounds in the tyre industry from the very beginning. Edwin M. Chaffee introduced the two-roll mill in 1863 to soften the natural rubber obtained from plantations and to facilitate the mixing of pigments [47]. A two roll mill is a machine that uses shear forces between two rolls to mix, disperse and homogenize the rubber compound passed through the rolls (Figure 4-6). Mills use horizontally placed metallic cylinder or rollers offset at a set distance from one other, rotating in opposite directions and at varying speeds to produce the required shear forces. The distance between the rolls or nip can range from 2-20mm and determines the shear forces generated along with the speed difference between the rollers. The gaps between the rolls can be adjusted to achieve the desired shear forces. The speed of the two rolls maybe same or different with one of the rolls rotating faster than the other. The difference in speed creates a friction on the material and shears the material. The rollers may have internal tubes or nozzles for heating or cooling systems. The high surface area of the rollers keeps the temperature change small and gradual. The material pass between the rolls and due to the narrow space between the rolls the amount of material that makes it through the rolls experiences very high shear force due to the different rotation speeds of the two rolls. This cycle is repeated several times until a homogeneous mixture is obtained. The main disadvantage of a two roll mill is that it is a time consuming, labour intensive process and the quality of the final product is highly dependent on the operator’s skills; therefore, the batch consistency is difficult to maintain. The mills are large and occupy much space and fine powder ingredients tend to escape the mill to create dust. Taking into account all these facts, modern tyre manufacturers mix most of their master batches in internal mixtures and use mills for specific operations only [48].

Figure 4-6 Two-roll mill.[49]

Internal Mixers Internal mixers used for mixing rubber compounds consist of an enclosed chamber with two rotors. It is made for heavy duty performance with high production capacity. The first of the 31

internal mixers were conceptualized and realized by Thomas Hancock in 1820 at London to facilitate the manufacturing of novelty rubber items [50]. These machines were of simple

Figure 4-7 Internal Mixer. [54]

construction, usually were hand or horse driven and with very little capacity. Hancock noted the beneficial effects of mastication, heat generation, different rotor speeds and different degree of chamber fillings [50]. It was not until the early 19th century did the engineering practices advance for the development of better internal mixtures. At present there are many mixture manufacturers and equally diverse industrial applications and modifications. The internal mixtures used in the manufacturing of tyres generally have a feeding hopper with a pressurized ram, an internal chamber with rotors where actual mixing takes place and a discharge door to remove the final mixture [51-53]. The sub-assemblies will be discussed in detail to get a better understanding of the whole mixture and the mixing process.

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Hopper and Ram assembly The feeding hopper is assembled on top of the machine and consists of a very large cylinder or ram that moves up and down in order to apply pressure on the batch. The feeding hopper has an opening on the side for solid or liquid ingredients to be fed into the mixture. In new mixers the fluids maybe directly injected to the mixing chamber. The ram maybe pneumatically actuated but the modern machines use a hydraulic system to actuate both the ram and the feeding doors as it provides a better control and efficiency. The main door at the front (see Figure 4-7) is generally used to feed bulk materials and multiple side doors may be fitted to feed different materials and to make the cleaning and maintenance easier. The inside of the ram body may have cooling tubes to control temperatures and prevent material sticking to the ram head. The feeding hopper may also have vents and openings around the ram to channel dust or gases generated while mixing. The top surface of the ram is all angled to prevent dust settling and may also be equipped with air nozzles to remove dusts. The main function of the ram hopper assembly is to facilitate the loading of the material into the mixing chamber, apply pressure with the ram so that all materials are incorporated into the compound and to make sure that the materials being mixed take part in the mixing process. The process of loading materials in the mixing chamber is done by a system of conveyers and automatic weighing system. The materials are delivered in form of a bulk from the suppliers and stored inside huge containers on site. The materials are stored in day storage bins and weighed as per the requirements of the batch. There is a conveyers and piping system that transports materials from the storage to the day bins and further to the hopper for feeding to the mixers. The modern weighing and the conveying system is generally automated and centrally controlled and monitored. Mixing Chamber The mixing chamber is the heart of the mixing machine and the place where all the material comes together and are actually mixed together. The main parts of the chamber are the two end frames, two rotors and the chamber walls. The top of the mixing chamber has the ram and hopper assembly and the bottom has the discharge door. The end frames form the limits of the chamber wall on one side and two other are the side walls. The end frames have circular openings for the shaft to be seated and can be split in two halves to easily remove the shaft for maintenance. The gap between the rotor and the end frames are fitted with sliding ring dust seals. All the parts inside the chamber, the rotors and the ends of the ram and discharge door that come in contact with the elastomer during mixing are hard coated to resist wear and to increase the service life of the machine. They also have holes inside them for a cooling or heating fluid to flow through to control the temperature during mixing. The rotor bodies are made of strong cast steel shrink fitted onto forged steel shafts. The rotors are the main driving parts of the mixer and come in two different types, intermeshing and Tangential rotors, divided according to their geometry and the way they perform [48].

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Intermeshing Rotors In intermeshing rotors, the paths followed by the two rotor blades overlap and thus are at a fixed distance from each other and rotate at the same speed (Figure 4-8). Due to the overlapping of the rotor paths, the rotors occupy a larger surface area resulting in a fill factor of only about 50%. Yet due to the larger surface area they provide better heat transfer and thus better cooling. Also due to the overlapping and narrow clearance between the rotors the forces generated are higher at lower energy inputs [55].

Intermeshing Rotor

Tangential Rotor Figure 4-8 Rotor design.[56]

Tangential Rotors The tangential rotors do not overlap each other during operation and a gap exists between them. Modern tangential rotors can have an adjustable gap and variable individual rotor speeds(Figure 4-8). The shape of the rotor is of critical importance for tangential rotors as the protrusion or wings define the flow characteristic of the rotor. There are various designs with changes to the number, shape, length, angle and position of wings and each design has different mixing efficiency. The wide gap between the rotors make it easy to load compounds and has a maximum fill factor of 80%. Each rotor revolves at different speed in the opposite directions and circulates the material around the chamber. The rotors pull material down into the mixing chamber, in the gap between the rotors, and forces the material into the gap between the chamber and the rotor to generate forces needed for mixing. The narrow gap between the rotors and chamber walls generate very high shearing forces. Besides the rotors the mixing chamber can also have an injection system for oils and other liquids to be introduced into the machine and tubing to transport cooling fluids within all the parts in contact with the compound. Along with that there are temperature sensors fixed on the chamber walls to measure the temperature during mixing [48].

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Drop door or Base Plate The base plate of the mixer is the part that bears the whole weight of the compound during mixing. It is usually made very sturdy. Similar to the inside of the mixing chamber the inner surface of the base plate is hard coated and has voids inside the base plate for fluids to flow through them and control the temperature. The base plate is fitted by a latch and a hinge that can be used to open and close the base plate to remove the mixed compound. The latch and the swing system may be automated and operated by a hydraulic ram system. After the doors are opened the mixed compound fall down by action of gravity and the rotors keep moving at few RPMs to remove all materials. The mixed compounds are moved to cooling and storage or directly dropped to machines for other operations [15, 48, 57, 58]. 4.4.2 Mixing Mechanism Rubber compounds have to be reinforced and curative agents, protective agents and other process aids have to be added. The polymer itself or blends of different polymers and the ingredients can be in the form of a liquid or a solid. All these ingredients have to be homogeneously mixed into a coherent mass inside an internal mixer to form a master batch. The mixing of compounds is primarily done to achieve the best possible homogeneity and is driven by the mass transport between the different materials. In an internal mixer the rotors introduce the energy into the compound and are the cause of mass transfer between the two hemispheres of the mixing chamber. Three zones on a rotor can be identified, as shown in Figure 4-9, that gives rise to three different forms of transport and force mechanism that promote the mixing; rolling bank (excess material on the entry side of the rotors), gap between rotors and gap between rotors and chamber walls [48, 59]. The first and the main contributor is along the rolling bank, by the wings, with rotation of the shaft towards the gap between the rotors, similar to a two roll mill (shown with red arrows in Figure 4-9). This area is where most of the material is stretched, folded and incorporated. The materials that cannot be carried along the rolling bank either escape towards the gaps in between wings of the rotor and the gaps between the wing tips and the side walls. The materials that slide from the gaps in the shaft do not experience shear forces and only move around for distributive mixing. The gap between the wing tip and the side walls is narrow and the volume of material here is low but experiences very high amount of shear forces [48, 57, 58]. The amount of material flowing along the rotor depends mainly on the material properties, mixer characteristics, rotor features and processing parameters. If the above criteria are not optimized, the amount of material transported onto the rolling bank is reduced or more materials slip between the

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Figure 4-9 Mixing mechanism.[57]

gaps; the mixing efficiency decreases. The ingredients undergo several mixing steps which are described in brief in the following paragraphs. The mixing process can be sub-categorized into three modes [48, 57]: Simple Mixing or Distribution: The distributive mixing phase or homogenizing is characterized by the even circulation of all the materials inside the compound. In an ideal distributive mixing the position of the particles changes and are spread evenly through the mixture. There is an increase in the configurational entropy of the system and the randomness increases with the increase in the mixing time. The main characteristic of this stage is the change in location and not the change in the size of the particles. The material transport capacity of the mixture is the main driving force of the distribution phase. It can be characterized by the rotor speed and time in a mixing device. Laminar Mixing: During this phase the rubber is stretched and the interphase or the boundary between the rubber and the filler are increased as much as possible by the deformation of the rubber matrix. It is achieved by shear forces generated by the rotor which are transferred to the rubber. The elongation causes the thickness to decrease while the area increases. During this mixing the rubber matrix may be stretched by more than twice. The laminar mixing is the dominant form of mixing in elastomer blends as the viscosity is high and the critical Reynold’s number for turbulent mixing is not achieved. It can be characterized by the ram pressure, fill factor and total number of revolutions in a mixing device. Dispersive Mixing: Dispersive mixing is the dominant mixing for solid and rigid substances that don’t deform like the rubber matrix. Dispersive mixing is characterized by the agglomerate broken down to aggregates and aggregates break down to smaller aggregates. The breakdown of larger particles is achieved by application of shear or strain forces. It is known from practice 36

that straining is much more effective than shearing as it requires 3 time more energy to shear the rubber than to stretch it [60]. The dispersive mixing is time dependent. A higher degree of dispersive mixing can be achieved with longer time but after a certain time a stable point is reached. Since reinforcement by fillers depends on the particle size, particle size distribution and surface area, adequate forces must be applied to achieve the optimum dispersion. This makes the dispersive mixing an extensively energy intensive process. It can be characterized by the power and torque in a mixing device. The viscosity of the compound is one of the main parameters of the material that has to be taken into consideration as it influences behaviour of the material during mixing. Nonetheless, the geometry and the features of the machine have an influence on the mixing properties. As it can be seen from the flow of the material induced by the rotors, this is highly reliant on the type, number and arrangement of the wings. In addition, it is also influenced by the distance between the rotors and the chamber wall. Similarly, other mixer features like hydraulic or pneumatic ram system and pressure, the gear and the differential system to transmit power from the motor to the rotor, ways of dosing liquid materials and temperature control system used, influence the compounding process and properties. Nevertheless, in the context of this work the mixer used in the laboratory scale is always the same and as such the influence can be considered constant through all the experiments. However, if the process is scaled to industrial applications or for a different mixer a better understanding of these parameters is essential to obtain comparable results. 4.4.3 Mixing Steps The processing of rubber material begins with the polymer used in the process. The basic principle for the mixing is that polymers are viscoelastic materials and can be easily sheared at higher temperatures for the incorporation of fillers and chemicals. The first step in rubber processing is the mastication or the breakdown of polymers. This step is carried out to break the polymer chains, orient the polymer chains in a certain direction, to blend various

Figure 4-10 Change in viscosity of polymer with time during mixing operations.[61]

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polymers and to increase the interface between the polymer, filler and chemical so at to facilitate incorporation. Mastication can be achieved by mechanical shearing induced by the rotors or by high temperatures degrading the polymer chains. The change in viscosity while mixing is shown in Figure 4-10. The figure also shows the contribution of the different factors that contribute to the change in viscosity. The mechanical process is preferred as it can be done at lower temperatures and without affecting too much the properties of the polymer. The process can be facilitated by adding processing aids like oils, wax or resins to decrease the viscosity. The process of introducing additives to increase the free volume is also known as plasticization and increases the flexibility of the polymer chains due to the addition of lower molecular weight materials [62]. Plasticization helps increase the flexibility and mobility of the polymers and helps ease the pressure on the processing equipment, the amount of energy used and enables a better flow of the materials through the processing machines. The various mixing stages are summarised in Figure 4-11. When the fillers and other chemicals are added, the incorporation process begins and if they are in bulk some sub-division or rupturing of the materials can also occur. When the viscosity of the blend begins to decrease then the filler is added to incorporate into the polymer. The rubber wets the surface of the filler and the voids between the agglomerates in the filler are also filled by the polymer; which is also known as the bound rubber and occluded rubber, respectively. The mastication and the plasticization process continue throughout the mixing cycle no matter the mixing process employed.

Figure 4-11 Mixing process for polymer and fillers.[63]

The incorporation stage is followed by the dispersion stage (Figure 4-11). The dispersion stage is typically considered as the stage where the majority of the agglomerates in the filler material is broken. The fillers are broken down to aggregate size mainly by action of shear stress. The bound and occluded rubber increases the effective radius of the filler thus increasing the effective shear stress acting on them and helping the dispersion. The shear forces should exceed the cohesive 38

forces and the surface energy for the dispersion to take place. Due to the breaking of filler materials the surface area of interaction between the filler and the polymer increases and the occluded rubber is freed from the void spaces. The viscosity of the compound continues to decrease until a minimum dispersion particle size is reached, the viscosity levels out. The dispersive mixing process is highly dependent on shear rate and on the viscosity of the material. It is considered that after this stage there is no further reduction in the filler particle size and there is a decrease in dispersion efficiency. Better dispersion provides the best reinforcing properties but for some properties larger particle sizes are more favourable. Therefore, the control of dispersion is essential to achieve the desired effect of filler materials [59, 64-66]. The distribution stage is defined by the mass transfer to reach a homogenous mix. In this stage already dispersed filler is randomly spread through the mixture. The filler particles are spread throughout the filler matrix without any physical change in filler materials. The phenomenon is dependent on the flow path of the material. The flow is aided by shearing and folding of the material by the rotor action and the concentration of fillers at a given location in the compound is reduced. The flow of the material causes a change in the viscosity; slowly changing from domains of high viscosity to have uniform viscosity throughout the material. The division of the mixing steps is primarily done to facilitate the study of the mixing process. The division in this form helps to isolate properties and study them. However, during the mixing

Figure 4-12 Shaded area under the curve gives the theoretical estimation about the amount of power each stage consumes with respect to time.

process some or all the steps mentioned above may happen at the same time with one being dominate. The mastication and the plasticization of the polymer is done in order to simplify the incorporation of fillers and other additives into the polymers. The dispersion and the distribution stages are the stages that influence the properties of the mixed compounds most. Figure 4-12 shows a generalized and simplified view of the amount of power consumed by each stage during a mixing cycle for carbon black. As it can be seen from this the distribution and mastication 39

process continue through the mixing cycle. In contrast to that, the power consumption of the incorporation stage and the dispersive stage are high at the beginning but decreases almost exponentially with increasing time.

4.5 Rubber Mixing Process Parameters The mixing process of rubber compounds can be a challenging process as the ingredients can have different phases, structures, viscosities and affinities towards each other. Additionally, the high number of ingredients and their interaction make it even more complex. The vast number of formulations, multiple ingredients in a single formulation and variance in individual ingredients with multiple mixing parameters make it practically impossible to do any standardized mixing. Therefore, it is on the one hand common practice to control the processing parameters during mixing in hopes of controlling the desired properties. This makes rubber processing more of an art than a science. On the other hand, it is essential to know the relationship of process parameters to the in-rubber properties to be able to develop effective control mechanisms. In a tangential internal mixer the following parameters are controllable during a mixing cycle [15, 48, 57, 66, 67]: 4.5.1 Ram Pressure The ram pressure is the amount of force applied to the batch during mixing by the ram. The ram can be opened to input the material into the chamber or to exhaust the ethanol formed during mixing by the silanization reaction and maybe opened and closed several times during mixing. The increase in ram pressure during the mixing cycle helps the material to stay inside the mixer and are properly transported by the rotors. Ram pressure helps to reduce the mixing time and incorporation. They have a very small effect on the dispersion of fillers as they do not directly provide the shearing forces needed for dispersion. However the action of ram does pressurise the mixing chamber to certain extent increasing the effective fill factor of the chamber and providing added shear forces. 4.5.2 Fill Factor The fill factor is one of the most influential parameters for the mixing efficiency. The fill factor is the amount of free volume of the mixing chamber occupied by the compound. If all the volume of the mixing chamber is completely filled by the mixture the fill factor would be 1. If the mixer is under filled, the material only moves around the mixing chamber without being sheared and no mixing takes place where as if its overfilled the mass transport becomes more difficult and the homogeneity is not achieved. Therefore, the optimization of the fill factor can enhance the dispersion of fillers in the compound. The fill factor depends on the viscosity of the mixture, lower viscous material permits higher fill factors. In general, the fill factor ranges from 0.5 to 0.92 and the batch weight can be calculated by: Total Batch Weight = Empty mixture volume x density of mixture x fill ( 21 ) factor. For fill factors of 0.7 and above the gap between the rotor and the chamber wall is always filled. [68]. The fill factor increases with increasing ram pressure and decreases with increasing rotor 40

speed. For silica compounds the fill factor is usually lower than this optimum because the incorporation of the filler causes a sharp increase in viscosity and decreases only when finally the silanization reaction takes place. 4.5.3 Rotor Speed The rotor speed is the number of revolutions of the rotor in a minute. The two rotors may operate at the same speed or at different speeds and they can generate friction in the material due to the difference in the relative angular motions. An increase in rotor speed generates higher torque and helps to increase the mixing efficiency but also causes an increase in temperature. The rise in temperature could be controlled by a decrease in the fill factor or using a better temperature control system. Speeds at the beginning of the mixing cycle build heat quickly that helps in the plasticization of the mixture and causes a decrease in the viscosity of the material. This also facilitates the incorporation of various oils, solid softeners and processing aids for an easier compound formation. The rotor speed can be increased or decreased to regulate the temperature gradient in combination with the liquid cooling system integrated into the mixer. The rotor speed could also be controlled to maintain the mixture temperature at a constant. 4.5.4 Temperature The temperature gradient of the mixing process is an important factor in the mixing process. Most of the work performed by the internal mixer is converted to heat that increases the temperature of the mixture. The energy used to plastically deformation of polymer is transformed into heat. The temperature changes the flow behaviour and then the processing conditions [69]. The rate of temperature change of the compound increases with the rotor speed and higher fill factor. The temperature affects the viscosity of the mixture and consecutively the effective rotor speed and the required mixing time. Typically for silica mixing including the silanization reaction, is to choose higher temperatures to increase the reaction kinematics. According to the Arrhenius power law 10°C rise in temperature can reduce the reaction time by 50%. However, processing at higher temperature has two disadvantages: It does not only cause premature scorch of the material but also degrades the polymer. Mixing at lower temperature generates more shear on the rubber which leads to better dispersion. But mixing at low temperatures also can cause problems with higher power demand, unnecessary stress on the machine and poor and uneven mastication and plasticization. Therefore, in order to maintain the temperature gradient, water is circulated inside the mixer. The temperature can be controlled by knowing the heat generated during mixing and the amount removed by the cooling system. 4.5.5 Mixing Time The mixing time of a machine is directly proportional to the rotor speed. In order to achieve a good mixing, a minimum number of revolutions of the rotor is desirable. In other words, it means the amount of time it takes for the dispersive and distributive mixing to be completed. The mixing time can be longer depending on the need for mastication and blending of polymers and 41

plasticizers. Furthermore, chemicals and additives in a small quantity require longer mixing times to guarantee even dispersion and distribution. 4.5.6 Order of Dosing The sequence of addition of ingredients in an internal mixer is critical and can be divided into a conventional and an upside-down method. There are specific temperatures for the incorporation of each ingredient but fillers are added as soon as possible to achieve already a good dispersion while the elastomer is relatively cool and higher shear forces can be applied. The fillers may be added in one portion or in multiple steps and the oils and plasticizers are usually added immediately for better processability. If the addition of the oil and plasticizers is too late there can be very high temperature gradients that could scorch the rubber. The timing of the addition of the coupling agent is also of great importance for silica filled compounds. If the silane is added too late to the compound it may not be able to completely react with the silica and early addition can cause it to react with other ingredients. The time of adding further ingredients can also be crucial depending on their effect on viscosity and on the resulting temperature change. Furthermore, the sequence of feeding the mixer is even more of concern when compounding with silica fillers as they can have undesired reactions with other materials like zinc oxide [66]. 4.5.7 Energy Rubber compounds are currently mixed for a fixed amount of time or the mixing is stopped after reaching a specific temperature. But the temperature and time of mixing is highly reliant on the mixer features, the environmental conditions, the cooling system and many other factors. Therefore, mixing with respect to time and temperature creates inconsistency between batches. The energy used for mixing is independent of the type and size of the mixer [69]. Mixing according to the amount of energy introduced to the machine can improve the mixing consistency as this has be found to be independent of mixer features and characteristics. The viscosity of the compound inside an internal mixture decreases with a higher energy input. The energy estimation method can be used to control the quality of the compound by relating the amount of energy input into the machine and dispersion or viscosity [64]. The compound should be dropped when a fixed energy has been introduced. With the optimization of this energy balance method the dump temperature and the time of mixing can be reduced to increase the efficiency of mixing. This is related to the fact that the compounds mixed according to a fixed energy input have a shorter mixing cycle because the compound can be dumped at the upper limit [70]. 4.5.8 Mixer Fingerprint and Control The fingerprint is a visual representation of the signals received from various sensors for rotor speed, temperature, pressure, ram position and other parameters. It is measured and then displayed graphically in real-time, with time on the x-axis and other parameters on the y-axis.

42

Figure 4-13 Fingerprint generated during mixing of rubber compounds.[15]

Figure 4-13 shows a fingerprint generated during the masterbatch mixing of a rubber compound. The graph gives information on the temperature, the rotor speed, current, power, ram force, and ram position. The temperature is the main parameters that is monitored during mixing. The temperature curve is used to determine mixing is terminated. It is also monitored to control the silanization reaction as it is temperature dependent. The torque curve can be used to analyse the progression of the different stages of mixing.

4.6 Bibliography 1. 2.

3.

4. 5. 6. 7. 8.

Hamed, G.R. Reinforcement of rubber, Rubber Chemistry and Technology, 2000. 73(3): p. 524-533. Mujtaba, A. Viscoelasticity of filled elastomers: determination of surface-immobilized components and their role in the reinforcement of SBR-Silica nanocomposites, (2014), Doctoral Thesis, Universitäts-und Landesbibliothek Sachsen-Anhalt, p.5-25. Gurovich, D, Macosko, C. W and Tirrell, M. The influence of filler-filler and filler-polymer interactions on the physical properties of silica-filled liquid polyisoprene, Rubber Chemistry and Technology, 2004. 77(1): p. 1-12. Payne, A. The dynamic properties of carbon black-loaded natural rubber vulcanizates. Part I, Rubber Chemistry and Technology, 1963. 36(2): p. 432-443. Payne, A. The Dynamic Properties of Carbon Black Loaded Natural Rubber Vulcanizates. Part II, Rubber Chemistry and Technology, 1963. 36(2): p. 444-450. Payne, A. Effect of dispersion on dynamic properties of filler-loaded rubbers, Rubber Chemistry and Technology, 1966. 39(2): p. 365-374. Kim, K, et al. Styrene-butadiene-glycidyl methacrylate terpolymer/silica composites: dispersion of silica particles and dynamic mechanical properties, Composite Interfaces, 2014. 21(8): p. 685-702. Einstein, A. Berichtigung zu meiner Arbeit:"Eine neue Bestimmung der Moleküldimensionen”, Annalen der Physik, 1911. 339(3): p. 591-592.

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17.

18. 19. 20. 21. 22.

23. 24. 25. 26. 27. 28. 29. 30.

31.

Guth, E and Gold, O. On the hydrodynamical theory of the viscosity of suspensions, Physical Review, 1938. 53(322): p. 2-15. Smallwood, H.M. Limiting law of the reinforcement of rubber, Rubber Chemistry and Technology, 1945. 18(2): p. 292-305. Guth, E. Theory of filler reinforcement, Journal of applied physics, 1945. 16(1): p. 20-25. Wolff, S and Donnet, J.B. Characterization of Fillers in Vulcanizates According to the Einstein-GuthGold Equation, Rubber Chemistry and Technology, 1990. 63(1): p. 32-45. Othman. A and Gregory. M.J. Stiffening Effect of Carbon Black: Its Interpretation by a Modified Guth-Gold Equation, Journal of rubber research, 1988. 3(1): p. 7-20. Rehage, G. Elastic Properties of Crosslinked Polymers, Pure and Applied Chemistry, 1974. 39(1-2): p. 161-178. Dierkes, W. Economic mixing of silica-rubber compounds, (2005), Doctoral Thesis, University of Twente. Kaewsakul, W. Silica-reinforced natural rubber for low rolling resistance, energy-saving tires: aspects of mixing, formulation and compatibilization, (2013), Doctoral Thesis, University of Twente. Medalia, A.I. Morphology of aggregates: VI. Effective volume of aggregates of carbon black from electron microscopy; Application to vehicle absorption and to die swell of filled rubber, Journal of Colloid and Interface Science, 1970. 32(1): p. 115-131. Chazeau. L, Brown. J.D, Yanyo. L.C and Sternstein. S. S. Modulus recovery kinetics end other insights into the Payne effect for filled elastomers, Polymer Composites, 2000. 21(2): p. 202-222. Payne, A and Whittaker, R. Low strain dynamic properties of filled rubbers, Rubber Chemistry and Technology, 1971. 44(2): p. 440-478. Kraus, G. Reinforcement of elastomers. 1965: New York, Interscience Publishers p. 2-150. Kraus, G. Swelling of filler‐reinforced vulcanizates, Journal of Applied Polymer Science, 1963. 7(3): p. 861-871. Clement, F, Bokobza, L and Monnerie, L. Investigation of the Payne effect and its temperature dependence on silica-filled polydimethylsiloxane networks. Part II: Test of quantitative models, Rubber Chemistry and Technology, 2005. 78(2): p. 232-244. Huber, G, Vilgis, T.A and Heinrich, G. Universal properties in the dynamical deformation of filled rubbers, Journal of Physics-Condensed Matter, 1996. 8(29): p. 409-412. Maier, P and Goritz, D. Molecular interpretation of the Payne effect, Kautschuk Gummi Kunststoffe, 1996. 49(1): p. 18-21. Gauthier, C, Reynaud, E, Vassoille, R and Ladouce-Stelandre, L. Analysis of the non-linear viscoelastic behaviour of silica filled styrene butadiene rubber, Polymer, 2004. 45(8): p. 2761-2771. Cassagnau, P and Melis, F. Non-linear viscoelastic behaviour and modulus recovery in silica filled polymers, Polymer, 2003. 44(21): p. 6607-6615. What is Silica?, (2010) [cited 2016- 6-07]; Available from: http://www.eurosil.eu/what-silica The Freedonia Group. World Tires: Industry Study with Forecasts for 2015 & 2020, (2012). Study #2860. Sims. E and Galli. C. Understanding the Role of Fumed Silica in Adhesives and Sealants Formulations, Adhesive & Sealant Industry Magazine 2014. Cichomski, EM. Silica-silane reinforced passenger car tire treads: effect of silica morphology, silicapolymer interface structure and rubber matrix network on tire-performance indicators, (2015), Doctoral Thesis, University of Twente. Flörke, O.W, et al. Silica, Ullmann's Encyclopedia of Industrial Chemistry, 2008.

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32.

33.

34. 35. 36.

37. 38. 39. 40. 41. 42. 43. 44. 45.

46. 47. 48. 49. 50. 51. 52. 53. 54.

JRC DIRECTORATE-GENERAL. Integrated Pollution Prevention and Control Reference Document on Best Available Techniques for the Surface Treatment of Metals and Plastics, European Commission, Editor. 2006. Collin. V, Roux. C, Boudimbou. I and Peuvrel-Disdier. E. Characterization of dispersion mechanisms of agglomerated fillers in an elastomer matrix under shear by in-situ observations, in 9th Fall Rubber Colloquium. 2010: Hannover, Germany. Liu, M. Coating technology of nuclear fuel kernels: a multiscale view, in Modern Surface Engineering Treatments. 2013, InTech. Walter, D. Primary particles–agglomerates–aggregates, in Nanomaterials. 2013. p. 9-24. Frohlich, J, Niedermeier, W and Luginsland, H.D. The effect of filler-filler and filler-elastomer interaction on rubber reinforcement, Composites Part a-Applied Science and Manufacturing, 2005. 36(4): p. 449-460. Medalia, A. Elastic modulus of vulcanizates as related to carbon black structure, Rubber Chemistry and Technology, 1973. 46(4): p. 877-896. Hewitt, N and Ciullo, P. Compounding Precipitated Silica in Elastomers: Theory and Practice. 2007: Elsevier Science. p. 1-21. What is the surface energy and surface tension?, [cited 2016 7-2]; Available from: http://www.adhesiveandglue.com/surface-energy.html. Goerl, U, Hunsche, A, Mueller, A and Koban, H. Investigations into the silica/silane reaction system, Rubber chemistry and technology, 1997. 70(4): p. 608-623. Yatsuyanagi, F, Suzuki, N, Ito, M and Kaidou, H. Effects of secondary structure of fillers on the mechanical properties of silica filled rubber systems, Polymer, 2001. 42(23): p. 9523-9529. Liang, J.Y, et al. Effect of Polysulfane Silanized Silica on the Morphology and Mechanical Properties of Brombutyl Rubber Vulcanizates, Rubber Chemistry and Technology, 2013. 86(4): p. 558-571. Kobayashi, S and Müllen, K. Encyclopedia of Polymeric Nanomaterials. 2015: Springer Berlin Heidelberg. Arayapranee, W. Rubber abrasion resistance, in Abrasion Resistance of Materials. 2012. p. 147166. Mixing and Mix Design Advances in Mixing Technology, Rubber Machinery World 2015; Available from: https://rubbermachineryworld.com/2015/08/10/editors-pick-mixing-and-mix-designadvances-in-mixing-technology-part-1/ Mixing And Mix Design – Advances In Mixing Technology (Part 2), Rubber Machinery World 2016; Available from: https://rubbermachineryworld.com/category/mixing-machinery/. US16(A). Chaffee, E.M. Limper, A. Mixing of Rubber Compounds. 2012: Hanser Publishers. Ontsuka, H and Toh, M. Mill Behaviour of Rubber on Two Roll Mill with Temperature, Nippon Gomu Kyokaishi, 2015. 88(4): p. 130-135. Hancock, T. Personal narrative of the origin and progress of the caoutchouc or India-Rubber manufacture in England. 1857: London: Longman, Brown, Green, Longmans, & Roberts. US4455091 (A). Bamberger. Wolfgang and Wiedmann. Werner, Process for regulating the mixing process of rubber mixtures in an internal mixer, WERNER & PFLEIDERER, 1984-06-19. US4234259 (A). Wiedmann. W and Schmid. HM, Mixing apparatus for kneading of plastic substances, WERNER & PFLEIDERER, 1980-11-18. US6908221 (B2). Proni. A, Balasso. D and Bottomley. A, Closed mixer working process with strokecontrol ram, PIRELLI, 2005-06-21. Kumar. V. Banbury Mixing, [cited 2018 03-23]; Available from: http://banburymixing.blogspot.nl/.

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55. 56. 58. 57. 59. 60. 61. 62. 63. 64. 65. 66.

67. 68. 69.

70.

Moribe, T. Advanced intermeshing mixers for energy saving and reduction of environmental impact, Mitsubishi Heavy Industries Technical Review, 2012. 49(4): p. 201. Sapkota, J. Influence of clay modification on curing kinetics of natural rubber nanocomposites, (2011), Masters Thesis, Tempere University, p.18. Wood, P.R and Limited, RT. Rubber Mixing. 1996: Rapra Technology Limited. Grossman, RF. The Mixing of Rubber. 2012: Springer Netherlands. Nakajima. N and Harrell. ER. Contribution of elastomer behaviour to mechanisms of carbon-black dispersion. in Elastomerics. 1983. Gent A.N. Chapter 1 – Rubber Elasticity: Basic concept and behaviour. 1978, Science & Technology of Rubber. p. 1-22. Flohrer. J, Jentzsch. J and Michael. H. Zusammenhang zwischen temperaturentwicklung und leistungsaufnahme beim mischen im innenmischer, Plaste und Kautschuk, 1983. 30(4): p. 216. Ceresa, R and Watson, W. Mastication of rubber. VII. Mechanical degradation of polymers during mastication, Journal of Applied Polymer Science, 1959. 1(1): p. 101-106. Palmgren, H. Processing conditions in the batch-operated internal mixer, Rubber chemistry and technology, 1975. 48(3): p. 462-494. Nakajima, N. Mechanism of mixing in internal mixer and energy‐based modelling, Polymer International, 1996. 41(1): p. 23-33. Nakajima, N. An approach to the modeling of mixing of elastomers, Rubber Chemistry and Technology, 1981. 54(2): p. 266-276. Reuvekamp, L.P.A.E.M. Reactive Mixing of Silica and Rubber for Tyres and Engine Mounts: Influence of Dispersion Morphology on Dynamic Mechanical Properties, (2003), Doctoral Thesis, University of Twente. Ebell, P.C. Internal mixing of rubber: the influence of process variables on mixed material properties, (1981), Doctoral Thesis, Loughborough University. Freakley, P.K and Wanidris, W.Y. Visualization of Flow during the Processing of Rubber in an Internal Mixer, Rubber Chemistry and Technology, 1979. 52(1): p. 134-145. Wijayarathna, B, Chang, W and Salovey, R. Effects of Processing Variables on the Mechanical Properties of Carbon Black Filled Rubber, Rubber Chemistry and Technology, 1978. 51(5): p. 10061022. Myers, F.S and Newell, S.W. Use of Power Integrator and Dynamic Stress Relaxometer to Shorten Mixing Cycles and Establish Scale-up Criteria for Internal Mixers, Rubber Chemistry and Technology, 1978. 51(2): p. 180-193.

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5 Design Methodology/Design Steps 5.1 Problem Investigation The goal of this project is to design a control algorithm based on a correlation which links the mixing parameters to the Payne effect of a batch, thus the degree of dispersion. An in-depth study of the rheological behaviour of silica rubber compounds and its dependency on the mixing process has to be performed. Finally, an algorithm to predict the dispersion of the silica during mixing has to be designed. The final aim is to design an improved control method for mixing of silica compounds, and thus to reduce the variation between different batches at Apollo Tyres Ltd. The artefact being designed is a control loop in the context of mixing silica-filled rubber compounds. The stakeholder is Apollo Tyres Ltd., who defines the goals and provides the budget. Stakeholders within Apollo Tyres Ltd. are the process engineers, operators and operation managers. There is a lack of knowledge within Apollo Tyres Ltd. in terms of alternative methods to implement a control mechanism. Within the design, the plan is to answer the questions, gaining knowledge and design a viable algorithm.

Figure 5-1 The design and engineering cycle.[1]

The design of the artefact requires some questions concerning available knowledge to be answered, and to look at the beginning if there are already viable methods. The project focuses more on the design and knowledge cycle than on the engineering cycle, which can be addressed after validation of a working artefact. As shown in Figure 5-1, the three steps of the design cycle will be followed to structure the design process. The design methodology or steps to be followed is elaborated in detail in the following paragraphs. In the design process we analyse the problem and the requirements of the stakeholder. This will help to understand the problem and its context, but also what the costumer hopes to achieve. Furthermore, it is necessary to know the ingredients being used to make the rubber compounds in question. In addition, it is necessary to understand the equipment used to mix the rubber compound and the process and mechanism of mixing. The information required is collected from available literature and also from experts at Apollo Tyres Ltd. Most of this information is summarized in the objective and the literature review chapter.

47

The design goal is to identify a criterion to relate the dispersion of fillers in the mixed rubber compound to the mixing process. One requirement is, that the dispersion has to be controlled during the masterbatch mixing stage. Besides, the process must have the possibility of automation as well as being able to be manually operated. The method must be integrable into the existing devices and methods with least disturbance. Using literature research, patents and gathering the relevant information on the problem from experts at Apollo Tyres Ltd., a method to measure dispersion and to monitor mixing parameters for correlation were identified. A preliminary screening of several parameters related to mixing was performed before a method for deriving correlations was developed. Understanding these correlations was also a part of this study and provided a framework for the control algorithm. Besides, a strategy to test the validity of the method was designed. The latter will complete the knowledge problem cycle and information can be obtained on the various mixing parameters and how they influence the independent and dependent variables that can be used to develop a control algorithm.

5.2 Knowledge Problem The design cycle described in Figure 5-2 needs to be supported by the knowledge cycle that helps to answer questions and to develop a feasible theory to base the design on. This project primarily focuses on solving the knowledge problem and then developing a corresponding system for the control mechanism. In the knowledge cycle, the first question is to identify the correct method to measure dispersion. The second question is to identify mixing parameters related to dispersion. A single-case mechanism experiment is used to test the change in dispersion with variation of the mixing parameters. The dispersion of the silica filler in the polymer matrix will be the dependent variable and the mixing parameters the independent variables.

Figure 5-2 Design and Knowledge cycle for the design problem.[1]

48

5.3 Treatment Design, Validation and Refinement At this stage, the required dispersion level will be defined, the mixing parameters will be chosen, the initial concept will be developed and a setup to test the validity of the algorithm control method will be designed. After the validity of the proposed control method has been confirmed, a set-up can be selected to test the various mixing parameters and if they are conform to the algorithm. The correlations between the mixing parameters can be elucidated by selecting the same model compound used in the knowledge cycle and performing mixing on a lab scale mixer. This is performed in conjunction to the knowledge problem cycle iteration, to further understand the knowledge context, to develop a control algorithm and to use the information in framing the control algorithm. The algorithm can be tested with different ingredient ratios in the rubber compound to further test the validity. It is also necessary to test the algorithm with different mixing sequences and new ingredients. Within the design cycle, the limits of the control algorithm need to be identified and the boundary conditions need to be tested. After the validation, information on the effectiveness and sensitivity of the control method will be gained. Further alterations and refinement of the design artefact can then be made, and the iterative process of the design cycle can continue. When a satisfactory level of effectiveness and sensitivity is reached, the engineering cycle of implementation and evaluation can begin. However, the engineering cycle is beyond the scope of this project.

5.4 Bibliography 1.

Wieringa R.J. PDeng Design Science Research [Powerpoint Presentation]. 2017.

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6 Development Phase 6.1 Conceptual Design

Control system

Compounding Ingredients

Mixing Device

Mixing sequence

Rubber compound with X dispersion

Figure 6-1 Architecture of rubber compound mixing.

The components in the mixing cycle of rubber compounds are the compounding ingredients, the mixing device and the mixing process as shown in Figure 6-1. A specific mixing sequence is used to load the mixing device with the compounding ingredients. The mixing device is controlled by a monitoring system which can be used to control the various mixing parameters, but it does not give any information on the degree of dispersion. Effective mixing of rubber compounds requires the incorporated filler granules and agglomerates to break and form filler aggregates (as seen in Figure 4-3 in Section 4.3.1), which need to be dispersed throughout the rubber matrix. After the filler clusters are broken down to the required unit size, the silanol groups on the silica surface need to react with the coupling agent so that they do not re-agglomerate and have a better compatibility with the rubber matrix. The silanization is the crucial step of this mixing process, which has to happen simultaneously with the dispersion and distribution of the filler. It is a chemical reaction during which ethanol is generated, which influences the efficiency of the mixing process. The detailed reaction mechanism of the silanization reaction is given in Section 4.3.6. The equipment used for mixing of silica compounds is designed for physical mixing, thus not optimal for this physio-chemical process and therefore less efficient. The completion of these steps will result in a rubber compound with a certain dispersion. The incorporated filler agglomerates are broken down by shearing the rubber compound, including the filler clusters. As the agglomerates are broken down to smaller size, the compound 50

reaches a critical viscosity level, in which the shearing forces can no longer break the filler agglomerates due to the low viscosity of the compound. In carbon black filled rubber, the filler agglomerates are physically broken and dispersed in the rubber matrix, as they are non-polar like rubber and therefore can physically interact with the rubber matrix which prevents reagglomerating. Therefore the mixing of carbon black filled rubber compounds can be stopped when the critical viscosity is reached. The viscosity of the compound is dependent on volume fraction of the filler according to the hydrodynamic effect described by the Guth and Gold equation [1]. In the case of equal volume fractions of filler material with different particle sizes, the filler with a larger particle size results in a higher viscosity. Thus as the particle is sheared, a decrease in viscosity due to change in particle size is occurring. In addition to the hydrodynamic effect, the viscosity also changes with temperature and shear rate. The effect of shear on viscosity decreases as the temperature is increased. At constant shear rate, the viscosity is higher at lower temperature and decreases with temperature. As the viscosity is highly temperature dependent, we can control the mixing based on the temperature and shear rate that corresponds to the critical viscosity. When the compound viscosity passed the critical value, the shearing of the rubber does not translate to shearing of the filler agglomerates any more as the rubber flows more easily. The same mixing control is applied for silica filled systems, with the compound mixed until the critical viscosity is reached, then the temperature is increased to correspond to the optimum reaction temperature for the coupling agent. The temperature is kept constant for the required time of reaction between the silica and silane coupling agent. This mechanism, where mixing is done by maintaining the optimum reaction temperature during mixing, doesn’t appear to be suitable for silica filled systems, because wetting of the silica surface by the polymer is improved with the degree of silanization and the available silica surface depends on the shearing of the filler aggregates. The improved surface wetting supports the shear transfer by improving the interaction between the polymer and the filler. Thus the change in viscosity is no longer only temperature, shear rate and dispersion-dependent, but also reaction dependent. The mixing process can be terminated and the compounds can be dumped based on different criteria. There can be one of several criteria for terminating and dumping the compound. 

Mixing time is one of the criteria. The compound is mixed for a predetermined time period or for a certain number of rotations of the rotor.  Another criteria is temperature; the compound can be mixed until a predetermined temperature is reached.  The heat history of the compound can be used to terminate the mixing process.  Similarly, energy input during mixing can also be measured as an indicator to dump the compound. There is no consensus on the correct method or the efficiency and effectiveness of the variables. In addition to these parameters, there are observations of the ram position and machine vibrations that are used to judge the incorporation stage. The compound properties depend highly on the dump criteria used, e.g. compounds mixed based on a predetermined time can show variability due to differences in the ambient temperature. 51

The dispersion during mixing of a rubber compound is an energy-intensive process. It requires energy to shear and strain the rubber and also to break the filler agglomerates. This implies that ‘total energy consumed’ or ‘unit energy consumed’ concepts can be used for quantification within the control system. This idea is not new and much literature can be found on this control system for rubber mixing [2-5]. The unit work concept is independent of the size of the mixer or the speed of mixing. The total energy can be taken as the electrical power consumed by the motor. The energy consumed by the motor (ET) is the sum of energy used to mix the compound (EM) and the mechanical losses (EL) of the device. The energy for mixing is converted into energy needed for dispersive mixing (ED) and energy converted to heat (EH): 𝑬 𝑻 = 𝑬𝑳 + 𝑬𝑴

( 22 )

𝑬 𝑴 = 𝑬𝑫 + 𝑬𝑯

( 23 )

𝑬𝑻 = 𝑬𝑫 + 𝑬𝑯 + 𝑬𝑳

( 24 )

The energy input can also be calculated based on the torque and rotor revolutions (RPM), which can be obtained from the fingerprint (see Figure 6-2) given by the mixing device. The torque is measured on the rotors of the device and is zeroed to torque at zero load. The energy calculated this way is the power being applied to the rubber and is less than the power drawn by the motor. The difference of the two can give us the mechanical losses during transmission from the motor to the rotor. The following relation can be used to get the energy calculated (EC) based on the fingerprint : 𝑷 = 𝝉. 𝝎

𝑷 = 𝝉. 𝟐. 𝝅.

𝑹𝑷𝑴 𝟔𝟎

( 25 )

( 26 )

𝑬𝑪 𝒕

( 27 )

𝑬𝑪 = 𝑬𝑴

( 28 )

𝑷=

Where, P = Power in Watt τ = Torque in Newton meter 52

ω = Angular velocity in radians per second E= Energy in Joules t = Time in seconds

400 300 200 100 0 0:00:00 -100

0:01:26

0:02:53

0:04:19

0:05:46

0:07:12

Time (min)

Torque [Nm]

Temp.(Stock) [°C]

Speed [rpm]

Figure 6-2 Mixing fingerprint of the reference compound mixed according to the recipe, mixing sequence and mixing parameter in Table 2.

Dispersion relates to the size of the filler aggregates in relation to the rubber matrix, as well as the spatial distribution of the latter. It can be measured by optical methods and by the Payne effect. The agglomerates with dimensions in the micron range can be measured by using a simple microscope. The material is considered well dispersed when the particles can’t be identified by the microscope i.e. when all the particles are below the micron size. However, this simplified Control system

Calculate current energy

Calculate necessary energy input range Change Parameter Measure energy energy Energy-Payne Compounding Mixing Device effect Relationship Ingredients

Mixing sequence Rubber compound with X dispersion Figure 6-3 Architecture of rubber compound mixing with control loop.

53

definition is inadequate as there is always a certain distribution of aggregates and agglomerates with different dimensions. As the rubber compound is mixed for a longer period, the dispersion shifts to a smaller unit size, and this process also happens at lower than micron scale range. Silica fillers tend to re-agglomerate, and even if they are mixed for a longer time it is possible for them to recombine and form larger clusters. As the microscope measurement focuses on a small area and gives only local information, measurements at multiple locations are used in this study to get an average dispersion level. For this project, the focus will be on the Payne effect as measurement of the dispersion. The Payne effect is the measure of the force required to break the filler network as proposed by A.R Payne and modelled by G. Kraus [6-8]. In this study, the Payne effect was measured using a RPA 2000 based on the ASTM 8059 standard for measuring rubber properties. The Payne effect is defined as the difference in shear storage modulus at 0.56% strain and 100% strain. This method provides a broader range of measurement than a microscope measurement. Moreover it is fast, simple and convenient.

6.2 Set-up The mixing process depends strongly on the materials to be mixed, their physical state, the order of loading the mixer, the compounding principles, the system layout, the machine design and other aspects of the mixing process. This project is focused on the mixing conditions and how they can be tailored to achieve a homogenously mixed and silanized compound for every batch (Figure 6-3). Considering this, a model compound was used to reduce the influencing parameters, to easier understand the process and to study the various influences of the mixing process on the final compound properties. The silica-silane filler system is mainly used in tread compounds, therefore this was the first choice. However, there are different types of tread recipes depending on the type of tyres they are used in. The model compound selected for this project is based on the first patented silica tread compound used for tyre tread manufacturing [9]. Similarly, to reduce the influence of the mixing device and of the sequence of loading the ingredients into the mixer, all compounds were mixed in the same mixer using the same mixing sequence. The information about the mixing device, compound recipe and the mixing sequence can be found in Table 6-1 and Table 6-2. After having identified the various mixing parameters and developed and understood the control method, the technique can be tested for different compounds and also for different mixing devices. Finally, scale up criteria can be deducted to transfer the control system to production mixers. The various factors are summarised in Figure 6-4.

54

Mixing sequence

Control Parameters: RPM Temperature Fill factor

Control system

Calculate current energy

Mixing Device

Calculate necessary energy input range

Compounding Ingredients

Rubber compound with X dispersion

Energy-Payne Effect Relationship

Figure 6-4 Architecture of rubber compound mixing with control loop.

Brabender Internal Mixer 350S Volume

390 cm3

Heating/Cooling

Liquid

Torque Max

400 Nm

Max. operating temperature

250°C

Speed

1-350 RPM

Rotor

2 Wing tangential rotor

Table 6-1 Specification for the mixing device.

55

The mixing device used for the experiments is a Brabender lab mixer, see Table 6-1. It has tangential type rotors with two wings in “Z” configuration, as shown in Figure 6-5. The total volume of the mixing chamber is 390 cm3. The mixing device is heated/cooled by passing oil through the inner tubing in the chamber walls. The oil temperature is controlled by an external unit that can be set to the desired temperature. The mixing device is connected to a driver which is computer controlled. Graphical as well as numerical values of the rotor speed, torque, temperature and mixing time are given by the “winmix” software, which is also used to control the driver. The software is used to set the parameters for mixing. Ingredient SBR/BR

Amount [phr] 75/25

Silica/Silane 90/7.2 Oil

32.5

Zinc oxide 2.5 (ZnO) Stearic acid 1

Figure 6-5 Tangential Rotors with two wings in "Z" configuration.

Time (seconds) 00

Mixing sequence

20

Close Ram

80 110

Open ram; add: ½ silica, ½ silane, ½ oil, ZnO and stearic acid Close ram

170

Open ram; add: ½ silica, ½ silane, ½ oil

210

Close ram

270

Open ram; Sweep

Open ram; add rubber

Wax

1.5

285

Close ram

6PPD

2

405

Dump compound

Table 6-2 Reference Compound recipe, mixing sequence and mixing parameters.

Parameter Rotor speed [rpm] Mixer temperature [°C] Fill factor [%] Mixing time [s]

Value 90 50 75 405

Table 6-3 Optimized mixing parameters used for mixing the model compound.

The focus of the experimentation is on the masterbatch, without cure package. The mixing time and sequence of mixing was taken from the PhD thesis of Louis Reuvekamp [10]. As can be seen in Table 6-2, silica, silane and oil are dosed into the mixer in two stages. This is because the chamber volume of the mixing device is too small to fit all the ingredients in in a single step.

After the selection of the recipe and the mixing device, the mixing parameters needed to be selected such that acceptable Payne effect values are achieved. After several trials, it was 56

determined that mixing with 90 RPM rotor speed, set temperature of the cooling oil as well as the device temperature at 50°C, and a fill factor of the mixing chamber of 75% will give a suitable Payne effect without having to readjust the mixing parameters during the 405 seconds of mixing. The mixing parameters given in Table 6-3 were taken as the reference parameter settings for the rest of the project. After selecting the model compound and determining the mixing sequence and parameters, the reference compound was mixed three times to determine the level of variation in Payne effect values between batches. The Payne effect measurement was performed after allowing the compound to relax for 16 hours. In addition to measuring the variation between batches, a square sheet was taken from a single batch and nine samples were taken as shown in Figure 6-6. The square sample was used to determine the in-batch variation in the Payne Figure 6-6 In-batch Payne effect measurement sample. effect measurement. Figure 6-7 shows the curves of the storage modulus versus the strain, which is commonly used to determine the Payne effect, of the three batches. Figure 6-8 shows the Payne curves for the in-batch measurements. Table 6-4 and 6-5 summarize the results for the batch-tobatch and in-batch variation measurements along with the average and standard deviation for the measurement.

Payne effect curves for different batches 2500

G' (Pa)

2000 1500 Ref_Batch_1 1000

Ref_Batch_2 Ref_Batch_3

500 0 0.1

1

10

100

Log Strain (%)

Figure 6-7 Payne effect curves for three batches of the reference compounds.

57

Batch-to-batch variation Batch 1 Batch 2 Batch 3 Average Standard Deviation

Payne effect [G’0.56%-G’100%](Pa) 1845 1808 1970 1874 84

Table 6-4 Payne effect measurement, average and standard deviation for three batches.

In-batch variation of the Payne effect masurements 2500 R1

G' (Pa)

2000

R2 R3

1500

R4 1000

R5 R6

500

R7 0

R8 0.1

1

10

100

Log Strain (%)

R9

Figure 6-8 Payne effect curves for in-batch variation measurements.

In-Batch variation Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Sample 9 Average Standard Deviation

Payne effect [G’0.56%-G’100%](Pa) 2134 1970 2024 1993 2010 2034 1987 2060 2141 2141 61

Table 6-5 Payne effect measurement, average and standard deviation for in-batch variation.

58

The reference Payne effect value was taken to be the average of all the samples measured for batch-to-batch variation and the in-batch variation along with the standard deviation as an acceptable range. After determining the variation and identifying the reference Payne effect value, the various parameters that influence the energy of mixing were defined. The parameter range was set and a series of experiments was defined to identify the parameters with significant influence on the Payne effect. The parameters and the experimental design are shown in Table 6-6. The model compound recipe given in Table 6-2 was used and the compound was mixed with the reference parameters for 6 minutes and 45 seconds. Rotor speed [RPM](1/min) 1 110 2 110 3 110 4 110 5 70 6 70 7 70 8 70 REFERENCE 90

Mixer Fill Factor [FF] Device starting temperature (%) (°C) 85 25 65 75 65 25 85 75 85 75 85 25 65 75 65 25 75 50

Table 6-6 Series of experiments with control parameters.

59

25°C and 70 RPM

75°C and 110 RPM 1500

85 FF

65FF 500

PE (Pa)

PE (Pa)

4000 1000

0

65 FF

85 FF

2000

0 363

685

316

Energy (kJ)

456

Energy (kJ)

a

b

85 FF and 110 RPM

65 FF and 70 RPM

1000

4000

25°C

50°C

500

PE (Pa)

PE (Pa)

1500

0

25°C 2000

50°C

0 666

685

316

Energy (kJ)

Energy (kJ)

c

d

75°C and 85 FF

25°C and 65 FF 6000

70 RPM

1000

110 RPM

500 0

PE (Pa)

PE (Pa)

1500

321

4000

70 RPM

110 RPM

2000 0

375

685

Energy (kJ)

e

316

446

Energy (kJ)

f

Figure 6-9 Payne effect values for model compounds mixed with different mixing parameters.

60

From the results of the initial experiments it became clear that the rotor speed and device starting temperature were of significant influence in comparison to the fill factor. Graph a and b in Figure 6-9 show the compounds mixed at 75°C and 110 RPM and 25°C and 70 RPM with different fill factors: the Payne effect values are not significantly different for these compounds mixed with different fill factors. In contrast to this, there is a significant difference when other conditions change. This can be seen in graphs c to f in Figure 6-9, where rotor speed and temperature are varied. However, the small influence of the fill factor is contrary to what was expected, as the amount of energy spent during the mixing process should be proportional to the amount of material. It is possible, given that the volume of the mixing chamber is relatively small, that the changes in mass (see Table 6-1 and Table 6-2) are not significant enough to influence the weight/energy relationship. Besides, the mixing efficiency is generally high due to the small volume of the mixer. Considering this, the fill factor was removed from the further analysis. Compound Rotor speed Device starting temperature Mixing time (s) [RPM] (°C) 1 90 50 240 2 90 50 270 3 90 50 300 4 90 50 330 5 90 50 360 6 90 50 390 7 90 50 420 8 90 50 450 Table 6-7 Mixing parameters for the series varying mixing time.

After performing measurements with fixed time settings and identifying the influencing parameters, trials varying the mixing time were conducted to check the evolution of the Payne effect during a mixing cycle. The model compound was mixed without varying any other parameters for 240 seconds, dumped and the Payne effect was measured. Then a new batch was mixed for 270 seconds, followed by the Payne effect measurement. This process was continued with 30 seconds intervals being added to every subsequent batch until the final compound was mixed for 450 seconds, making a total of eight compounds (see Table 6-7). The energy is calculated based on Equation 5 and 6. The individual Payne effect values were plotted against energy input, and the result is shown in Figure 6-10. The figure shows the Payne effect values on the Y-axis and the energy input for mixing is given on the X-axis. When a fitting line is drawn, it appears to show a linear relationship between Payne effect and energy input during mixing.

61

Energy input Vs Payne effect 8.00 7.00

PE (kPa)

6.00 5.00 4.00 3.00 2.00 1.00

y = -0.0198x + 10.354 R² = 0.9829

0.00 150

200

250

300 350 Energy (kJ)

400

450

500

Figure 6-10 Payne effect measured after mixing for certain periods, plotted against energy input during these mixing periods.

The results indicate that variation in energy can be linearly related to the change in energy consumed during mixing of the rubber compound. This set-up can now be used to further elaborate and understand the relationship when varying the rotor speed and temperature.

6.3 Experiment and Results 6.3.1 Rotor Speed Trials After determining the correlation of the energy of mixing to the Payne effect values as demonstrated in Section 6.2, the influence of rotor speed and temperature on the energy of mixing and in turn, its influence on the Payne effect values should to be evaluated. The same model compound, mixing device and mixing sequence were used in the temperature and rotor speed trials, with the same dump times as the tests explained in Section 6.2. The first compound was mixed for 4 minutes and the following compounds mixed for 30 seconds longer, with a maximum of 7 minutes and 30 seconds, giving a total of 8 mixed compounds. In the rotor speed trials, the compounds were mixed at 110 RPM and 70 RPM respectively, while the other settings were the same as the reference compound selected in Section 6.2. The Payne effect was measured following the same procedure as mentioned in Section 6.2 and plotted against the energy of the mixing calculated using Equation 5 and 6 given in Section 6.2. The Payne effect (PE) against energy input graph for the compound mixed with different rotor speeds is shown in Figure 6-11.

62

Rotor speed 12000.0 10000.0

PE (Pa)

8000.0 Energy (REF: 90RPM, 50°C)

6000.0

Energy (70RPM,50C)

4000.0

Energy (110RPM, 50°C) 2000.0 0.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ)

Figure 6-11 Different rotor speeds used for mixing the model compound with all other parameters except mixing time constant.

The red dots represent the compound mixed at lower rotor speed (70 RPM), the orange dots for the high rotor speed (110 RPM) and blue is for the reference mixed at 90 RPM and 50°C. In Figure 6-11, the graph shows that changing the rotor speed moves the data points along the curve traced by the reference mix. At a lower rotor speed, when less energy is used, the Payne effect values are higher, and as the rotor speed is increased and more energy is added to the compound, the Payne effect values decrease. Besides being an effect of the higher energy input, this is also caused by a more efficient silanization reaction. The lower Payne effect indicates an improved filler dispersion. It can also be seen from the graph that at around 400 kJ of energy input, the Payne effect reaches a plateau and does not show a significant change anymore, even when more energy is added into the compound. At this stage, the degree of the silanization as well as the dispersion are rather high, and no measurable improvement is achieved anymore. A steep decrease exists at the beginning of the mixing cycle and can be explained with a minimum energy which is required to incorporate the filler into the rubber, resulting in a strong decrease in the Payne effect value. If all the data points are plotted together, a master curve can be obtained for the energy input and Payne effect as shown in Figure 6-12. This relation can be used to get a Payne effect value for a certain amount of energy used for mixing. It is also clear that the Payne effect values change exponentially with increase in the energy input. Changing the rotor speed changes the amount of energy input into the compound for a given time.

63

Rotor speed 14000.0 12000.0

PE (Pa)

10000.0 8000.0 6000.0 4000.0 R² = 0.9445

2000.0 0.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ)

Figure 6-12 Different rotor speeds used for mixing the model compound with all other parameters except mixing time constant, plotted all together and an exponential curve fitted.

6.3.2 Device Starting Temperature Trials The temperature trials were conducted on the same model recipe and with the same mixing sequence. All the mixing parameters were kept constant except the device temperature at the beginning of the mixing. The speed of the rotors was constant at 90 RPM and two different starting temperatures, 25°C and 75°C, were selected. In total, 8 compounds were mixed, with the first compound mixed for 4 minutes and every following compound was mixed for 30 seconds longer till it reached 7 minutes and 30 seconds, similar to trials in Section 6.2.

Temperature 12000.0

PE (Pa)

10000.0 8000.0

90RPM, 75°C

6000.0

90RPM, 25°C

4000.0

REF: 90RPM, 50°C

2000.0 0.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

Energy (kJ)

Figure 6-13 Different temperatures used for mixing the model compound with all other parameters constant.

64

The Payne effect was measured for all compounds as mentioned also in Section 6.2, and the energy of mixing was calculated using Equation 5 and 6. The Payne effect values were plotted against the energy of the compound and can be seen in Figure 6-13. The grey data points are the compounds mixed at 25°C, the orange data points are mixed at 75°C and the blue data points are for the reference mixed at 50°C. It can be seen from the figure, that the curve as a whole shifts due to the change in the temperature. The same amount of energy spent during mixing at higher temperature gives lower Payne effect values than a compound mixed at lower temperature. This phenomenon can be explained due to the degree of silanization that takes place during mixing: At lower temperature, the amount of reacted silane is less and the degree of silanization is poor. At higher temperature, more silane reacts. However, this temperature dependence is not linear: the change is less, the higher the temperature. It may be possible to add a shift factor for various temperatures and determine the energy required for mixing of the compound at different device starting temperatures. The Payne effect values globally change by a factor of two with threefold change in temperature, between 25°C to 75°C.

Rotor speed and Temperature 14000.0 12000.0

PE (Pa)

10000.0 8000.0

90RPM, 75°C

6000.0

90RPM, 25°C

4000.0

REF: 90RPM, 50°C RPM

2000.0 0.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ) Figure 6-14 Different temperatures used for mixing the model compound with all other parameters except mixing time constant, plotted along with the results for different rotor speed results.

The rotor speed and temperature trials indicate that these two parameters have different influences on the mixing energy versus Payne relationship. Mixing at higher rotor speed provides more energy input per unit time, which correlates with an exponential decrease in the Payne effect values, as seen in Figure 6-12. In contrast, when mixing at a higher temperature, there is a shift to lower Payne effect values for the compound mixed at 25°C compared to the compound mixed at the reference temperature (50°C). The shift of the Payne effect values for the compounds mixed at 25°C and 75°C is slightly smaller. Two different mechanisms, breaking of the particles and reaction with the coupling agent, are involved in mixing of silica as filler in rubber compounds, and they have a different influence on the energy and Payne effect value

65

relationship. A higher rotor speed generates more shear, which results in a faster breaking of the particles. The temperature is related to the amount of silane that reacted during the mixing cycle. Figure 6-14 shows all the data points from the rotor speed and temperature measurements. In all cases, the Payne effect values follow an exponential decrease with the increase in energy input into the compound. The blue curve represents the master curve generated from the combination of all the various rotor speeds used plotted against energy. It appears that the blue and orange curves converge together at higher Payne effect values. The change in slope can be explained by the effect caused by different device starting temperature. To better see the convergence and the change in slope, the Y-axis was converted to natural logarithmic scale. Figure 6-15 shows the same temperature and rotor speed data, but with the Y-axis in natural logarithmic scale. It can be seen that the curves at higher temperatures, 75°C and the REF at 50°C, do converge, whereas the curve for the compound mixed at lower temperature, 25°C, crosses both other curves. Therefore we can assume that they all have the same Payne effect at the beginning. This is likely determined by the types of silica, polymer and coupling agent used in the compound, and for better understanding further investigation is required. The slope of the curve changes with the change in the temperature. From this change, it is assumed that this possibly corresponds to the amount of silane reacting at the different temepratures. Another point to be noted is that for the grey curve, the change in Payne effect at the beginning or at low energy inputs is smaller than for the blue curve. The Payne effect value changes with bigger steps in the beginning for the orange curve which is mixed at higher temperature. Therefore it can be assumed that the temperature plays an important role in the rate of change of Payne effect.

Rotor speed and Temperature

Ln. PE (Pa)

12500.0

2500.0

90RPM, 75°C 90RPM, 25°C

500.0

REF: 90RPM, 50°C RPM

100.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ)

Figure 6-15 Different temperatures used for mixing the model compound with all other parameters except mixing time constant, plotted along with the results for different rotor speed results.

66

To further understand the effect of temperature and rotor speed, the model compound was mixed changing both, the rotor speed and the temperature, at the same time. The compound was mixed at 70 RPM and 75°C device starting temperature. These settings were selected, as the combined effect of temperature and rotor speed will probably have a different effect than the two separate effects added up: material temperature will influence the viscosity, and this in turn will influence the input of mechanical energy by the rotor. It was not possible to select higher rotor speeds and higher temperatures, as the temperature of the material during mixing exceeded 200°C before the mixing time was reached. Such a high temperature will lead not only to pre-scorch by creating crosslinks with the released sulphur from the silane, but also causes the polymer to degrade. The Payne effect was measured, the energy of mixing calculated and plotted against the Payne effect values. Figure 6-16 shows the same graph as Figure 6-14, but

RPM and Temperature 14150.0 12150.0

PE (Pa)

10150.0

90RPM 75°C

8150.0

90RPM 25°C

6150.0

REF: 90RPM, 50°C

4150.0

RPM

2150.0

70RPM, 75°C

150.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ) Figure 6-16 Graph of all the various parameters used for mixing the model compound.

with the addition of the energy and Payne effect values for the model compound mixed at 70 RPM and 75°C represented by the blue curve. It can be seen from the blue curve that the Payne effect value starts at the same position as the compound mixed at 90 RPM and 25°C, but towards the end it moves closer to the curve of the compound mixed at 90 RPM 75°C. clearly there is an intermediate effect on the Payne effect value when both, the rotor speed and the temperature, are changed. To better see the change in Payne effect value, the graph of Figure 6-16 was plotted with the natural logarithmic Y-axis as seen in Figure 6-17. In Figure 6-17, the slope of the curve showing the relationship between the Payne effect and energy input during mixing for the model compound mixed at 70 RPM and 75°C, is the same as for the compound mixed at 90 RPM and 75°C. If the slopes are the same, the rate of change of the Payne effect is the same for these compounds mixed at the same temperature but at different rotor speeds. From the slope of the curves in Figure 6-17 it can also be concluded, that the temperature effect is more dominant on the rate of change of the Payne effect value than the rotor speed in this range.

67

RPM and Temperature

7500.0

Ln. PE (Pa)

90RPM 75°C 90RPM 25°C REF: 90RPM, 50°C RPM 70RPM, 75°C 150.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ) Figure 6-17 Natural logarithmic Y-scale for the compounds mixed at different parameter settings.

The device starting temperature directly influences the rate of the silanization reaction, and the rate of change of the Payne effect in turn depends on the rate of silanization reaction taking place. This would also explain why starting with a different rotor speed during mixing without changing the device starting temperature results in a Payne effect master curve with the points for different rotor speed lying on the same master curve with the same slope as the rate of change of Payne effect would be the same. However, the curve for the compound mixed at 70 RPM and 75°C does not coincide completely with the curve for the compound mixed at 90 RPM and 75°C. This might be caused by the fact that the amount of filler broken down and distributed throughout the rubber matrix depends on the rotor speed. In order to understand this better, further experiments were conducted with variable rotor speeds. The model compound was mixed for 4 minutes at 90 RPM and then the rotor speed was increased or decreased within the mixing process of this compound to 110 or 70 RPM respectively with the device starting temperature at 50°C. In total, 8 compounds were mixed, with the first compound mixed for 4 minutes and every following compound was mixed for 30 seconds longer till it reached 7 minutes and 30 seconds. The first data point on the graph represents the compound mixed for the same period of time using the same sequence and settings. The Payne effect was measured and the energy of mixing was calculated. Figure 6-18 shows the graph of energy input against the Payne effect values for the compounds mixed at variable rotor speeds. The change in rotor speed during mixing does not appear to change the rate of decrease of the Payne effect. Similar to variable starting rotor speed tests, the compounds mixed by changing the rotor speeds during mixing also move along a master curve. At lower rotor speeds, less energy is supplied to the compound and there is a smaller decrease in Payne effect for each time interval and vice versa. The explanation for this behaviour may again be related to the reaction rate of the silanization and the amount of silane being reacted, as well as to the degree of filler being broken down which creates new surfaces for the ethanol to evaporate. As the silanization reaction is an 68

equilibrium reaction, removal of ethanol will speed up the reaction. The rate of reaction of silanization reaction does not change but the amount of silane reacting is larger in the given time interval.

Variable rotor speed

12000.0 10000.0

PE (Pa)

8000.0 90/70RPM, 50°C

6000.0

90/110RPM, 50°C 4000.0

REF: 90RPM, 50°C

2000.0 0.0 0

100

200

300

400

500

600

700

Energy (kJ)

Figure 6-18 Graph of compounds mixed while changing the rotor speed during mixing.

From the rotor speed and device starting temperature experiments it can be seen that the rate of change of the Payne effect is highly dependent on the starting temperature of the mixing device. For a given starting temperature, the speed of change of the Payne effect is dependent on change of rotor speed. This series of experiments with the model compound points to the fact that the change in the Payne effect values, or in other words the change in dispersion of the particles in the compounds, depends on the temperature as well as the speed of the rotor acting on the ingredients. However, as stated above, the degree to which they influence the Payne effect is very different. This can lead to some assumptions about how the model compound will perform with different compounding ingredients and types of mixing device. A mixing device with a better temperature control would provide a better property control [1]. Homogeneity of the temperature within the mixing device will also influence the dispersion of the filler in the rubber compound. In tangential mixers, highest shear forces are exerted on the rubber compound between the rotor tips and the chamber wall [2]. This region would also see higher temperatures than the rest of the compound. Rubber is a poor thermal conductor [3] and controlling the temperature of the compound will introduce new challenges. Rotors with better distributive mixing quality can homogenize the compound better, providing a smaller temperature gradient in the compound being mixed [4]. In the experiments conducted at fixed times with varying the fill factor, trials described in Section 6.2, the fill factor was not highly influential as the volume is rather small in this mixing device, and changes in fill factor only leads to a change of a couple of 10s of grams. However, in larger mixers this factor will have to be taken into account as changing the fill factor changes the mixing 69

efficiency considerably [1]. In order to get a better understanding of the temperature and the rotor speed on the relationship between the Payne effect value and the energy input into the material, further experiments were conducted by adjusting the rotor speed to achieve the desired temperature for an efficient silanization reaction. These experiments will be discussed in Section 6.3.3. 6.3.3 Temperature of the Mixing Trials The temperature during the mixing of the ingredients changes while the compound is being mixed. The temperature changes depend on many of factors. The starting temperature of the device, temperature of the cooling fluid for the mixer and rotor speed are the few controllable factors that influence the material temperature. In addition, there are other factors like the heat capacity of the ingredients, losses of thermal energy from various parts of the mixing device, the rate of heat exchange between the device, cooling fluid and the environment, and many of the factors cannot be controlled which makes it difficult to adjust the temperature precisely. It is possible to control the temperature to a certain degree, but that requires heavy modifications which cannot be achieved in the time frame of this project. Thermally isolating the device, adding more thermocouples at different locations for better temperature measurement, measure the heating and cooling rate of the temperature control unit, decreasing the temperature of the compound during mixing are some of the possible modifications of the mixing device to better understand the thermal behaviour of mixing. RPM Compound 1 2 3

90 90 90

Starting Temperature (°C) 50 50 50

Time at low temperature (min) 4 5 5.5

Time 140°C (min) 3.5 2.5 2

at Total mixing time (min) 7.5 7.5 7.5

Table 6-8 Mixer setting and mixing time for the experiment.

To understand the influence of the temperature during mixing on the dispersion of the filler in the compound, a set of experiments was conducted on the model compound using the same mixing device and mixing sequence. The model compound (compound 1 in Table 6-8) was mixed at 90 RPM and 50°C device starting temperature for 4 minutes, and then the rotor speed was adjusted within 15 seconds to increase the temperature to 140°C and kept at that temperature. Finally, the compound was dumped at 7 minutes and 30 seconds. The next compound (compound 2 in Table 6-8) was mixed with the same settings and mixed for 5 minutes and then the rotor speed was adjusted within 15 seconds to increase the temperature to 140°C. The compound was again dumped at 7 minutes and 30 seconds. The next compound (compound 3 in Table 6-8) was mixed for 5 minutes and 30 seconds before adjusting the rotor speed to increase the temperature to 140°C and dumped at 7 minutes and 30 seconds as shown in Table 6-8.

70

The Payne effect values against energy input during mixing are given in Figure 6-19 along with the reference compound. All the compounds, which are mixed for 7 minutes 30 seconds with the temperature increased to 140°C for the silanization step, but with different mixing times before increasing the temperature to 140°C by adjusting the rotor speed, show the same level of Payne effect even though the energy input during mixing is different: they were mixed for longer periods before the temperature was increased by adjusting the rotor speed. This indicates that a compound is perhaps over-mixed and that the limit of the measurement by the Payne effect value was already reached for the compound with the shortest time at low temperature. To check this, a compound was mixed for 14 minutes at 120°C with the device starting temperature at 50°C and 90 RPM. The Payne effect was measured, the energy input was calculated and this point was added to the ones given in Figure 6-19. The results are shown in Figure 6-20. 8000.0 7000.0 6000.0

PE (Pa)

5000.0 4000.0

Same amount of time

3000.0

REF: 90RPM, 50°C

2000.0 1000.0 0.0 0

100

200

300

400

500

600

Energy (KJ)

Figure 6-19 Graph of compounds mixed for the same amount of time but variable time at 140°C along with the reference.

The Payne effect level reaches the plateau and does not change with increasing amount of energy anymore: the Payne effect was not further reduced by the longer period of mixing at low temperature and shorter silanization time, as the dispersion reached its best value within the earlier specified mixing sequence.

71

8000.0 7000.0 6000.0

PE (Pa)

5000.0 4000.0

Same amount of time

3000.0

REF: 90RPM, 50°C 90RPM, 120°C

2000.0 1000.0 0.0 0

100

200

300

400

500

600

700

Energy (KJ)

Figure 6-20 Graph of compounds mixed for the same amount of time, but variable time below and above 140°C, along with the reference and the compound mixed for an extended period of time at very low temperature (120°C).

In the next set of experiments, the model compound (compound 1 in Table 6-9) was mixed at 90 RPM and 50°C for 4 minutes, then the rotor speed was adjusted to instantaneously increase the temperature to 140°C and held there for 2 minutes. The next compound (compound 2 in Table 6-9) was mixed for 4 minutes 30 second before the temperature was instantaneously increased to 140°C and held for 2 minutes. This was repeated with the time at low temperature increased by 30 seconds to get a total of 5 compounds as shown in Table 6-9. All the compounds in this series of experiments (see Table 6-9) are mixed at 140°C for 2 minutes to complete the silanization. This variation is expected to lead to a change in the Payne effect values. It can be seen from Figure 6-21, that the increase in the amount of time spent at low temperature for Compound

RPM

1 2 3 4 5

90 90 90 90 90

Starting Temperature (°C) 50 50 50 50 50

Time at low temperature (min) 4 4.5 5 5.5 6

Time at Total mixing 140°C (min) time (min) 2 2 2 2 2

6 6.5 7 7.5 8

Table 6-9 Mixer settings and mixing time for the experiments.

shearing the rubber and breaking the filler has an influence on the reaction part of the mixing: Higher shearing leads to smaller particles, what can then influence the kinetics of the reaction. The shearing of the filler particles decreases the agglomerate size, increases the surface area of the filler which can react with the silane, and thus increases the reaction rate. But still the energy input is the determining factor for the Payne effect. 72

8000.0 7000.0 6000.0

PE (Pa)

5000.0 4000.0

90RPM, 50°C+2min

3000.0

REF: 90RPM, 50°C

2000.0 1000.0 0.0 0

100

200

300

400

500

600

Energy (KJ)

Figure 6-21 Graph of compounds mixed at 90RPM and 50°C for variable length of time at low temperature, followed by 2 minutes of silanization period.

In the next series of experiments, the rotor speed and the temperature of the mixing process were varied. The compounds were mixed until four minutes and then the rotor speed was adjusted to increase the temperature to 140°C as shown in Table 6-10. The experiment was conducted at 70 RPM and two different temperature settings: 50°C (compound 1, 2 and 3 in Table 6-10) and 25°C (compound 4, 5, 6 and 7 in Table 6-10). Figure 6-22 shows the reference compound along with the same compound mixed at 70 RPM and 50°C as well as a compound mixed at 70 RPM and 25°C. The time interval between these compounds is 1 minute. That means each compound is mixed for one extra minute before the temperature is increased to 140°C and held for 2 minutes. The last compounds for a certain temperature setting was mixed for 30 seconds extra (compound 3 and 7 in Table 6-10). It can be seen that the compounds mixed at the same rotor speed but at lower device starting temperature (the first yellow data points in Figure 6-22) reach lower Payne effect values faster than the compounds mixed at higher device starting temperature (first grey data point in Figure 6-22). However, the compound mixed at higher device starting temperature reaches a lower end Payne effect value at a lower energy input of less than 500 kJ (third grey data point in Figure 6-22) than the compound mixed at lower device starting temperature (last yellow data point in Figure 6-22) . It is known from literature [4, 5], that for mixing of silica with rubber, the filler particles have to be broken down as well as have to react with the silane coupling agent. From the results obtained from the above experiments, it can be concluded that a compound experiencing higher shear forces due to a lower material temperature shows a sharper drop in the Payne value. The compounds mixed at higher device starting temperature shows a higher silanization reaction yield than the compound mixed at lower device starting temperature due to the higher thermal energy present in the mix, and thus a more efficient silanization reaction. 73

Compound

RPM

1 2 3 4 5 6 7

70 70 70 70 70 70 70

Starting Temperature (°C) 50 50 50 25 25 25 25

Time at low temperature (min) 4 5 5.5 4 5 6 6.5

Time at Total mixing 140°C (min) time (min) 2 2 2 2 2 2 2

6 7 7.5 6 7 8 8.5

Table 6-10 Mixer setting and mixing time for the experiment. 8000.0 7000.0

del PE (Pa)

6000.0 5000.0 4000.0

70RPM, 50°C+2min

3000.0

70RPM, 25°C+2min REF: 90RPM, 50°C

2000.0 1000.0 0.0 0

100

200

300

400

500

600

700

Energy (KJ)

Figure 6-22 Graph of compounds mixed at 70RPM/50°C and 70 RPM/25°C for variable lengths of time mixed at low temperatures.

From these sets of experiments it can be concluded that both, temperature and rotor speed, have a complex influence on the Payne effect. It is necessary to keep track of the amount of particles broken down and the amount of particles reacting with the silane coupling agent to better control the dispersion of the silica filler in the rubber compound. It is evident that the energy input approach primarily characterizes the influence of the energy used to break down the filler particle, but the reaction cannot be completely modeled with only this parameter as it is difficult to separate the influence of the temperature and shearing forces. The shearing and distributive mixing continue throughout the mixing cycle and the silanization reaction can occur at lower temperatures albeit at a slower rate. Silanization helps to improve the affinity of the silica particles to the rubber matrix making it easier to mix the filler into the rubber [6, 7]. The higher shearing helps to expose more surface of the silica particle that can then take part in the reaction process. However, the silanization reaction is more efficient at higher temperatures, 74

whereas the shearing is more efficient at lower temperatures. In Section 6.4 and 6.5, the thermal energy and the silanization reaction and their influence on the Payne effect will be analyzed.

6.4 Total Energy and Thermal Energy of Mixing In order to get a better understanding of the energy input during mixing, the total energy input (t.energy) for different mixing time intervals was plotted against time. The energy input during mixing of the model compounds with various mixing parameters as described in Section 6.3 was used to generate the graph.

Total energy input against time 700.0 600.0

Energy (kJ)

500.0 400.0 t. energy REF: 90RPM, 50°C 300.0

t. energy 70RPM, 50°C

200.0

t. energy 110RPM, 50°C

100.0 0.0 0

100

200

300

400

500

Time (s)

Figure 6-23 Total energy spent on mixing the compounds for different rotor speeds against time.

Figure 6-23 shows the energy for the model compound against the time of mixing along with the model compounds mixed at 110 RPM and 70 RPM. As can be seen from Figure 6-23, increasing the time interval for the compounds mixed with same mixing parameter changes the total energy input linearly over time. The compound mixed at 110 RPM diverges from the linear trend towards the end, after 390 seconds of the mixing. This is probably due to the sulphur in the silane forming crosslinks within the rubber network (scorch), that increases the energy required for shearing the rubber. When the rotor speed is increased, the energy input during mixing for every time interval increases and vice versa. The change in rotor speed causes the Payne effect value to move along a master curve as seen in Figure 6-12 in Section 6.3.1. This effect could probably be explained by the overall increase in energy input for every time interval which makes it possible to reach the desired dispersion level faster. Figure 6-24 shows the energy input of the model compounds (same as Section 6.3) mixed at 25°C and 75°C in addition to the compound mixed at 70 RPM, 90 RPM and the reference compound. As can be seen, the compounds mixed at different temperature have the same energy input as the reference compound, but this is different from the compounds mixed at higher and lower rotor speed. The Payne effect value of the compounds are different in relation to the energy input (see Figure 6-13 section 6.3.2). To better understand

75

the influence of the temperature, further study of the influence of the temperature and the thermal energy of mixing is required.

Total energy input against time 700.0 600.0

Energy (kJ)

500.0 t. energy REF: 90RPM, 50°C

400.0

t. energy 70RPM, 50°C 300.0

t. energy 90RPM, 75°C

200.0

t. energy 90RPM, 25°C

100.0

t. energy 110RPM, 50°C

0.0 0

100

200

300

400

500

Time (s)

Figure 6-24 Total energy spent on mixing of the compounds for different rotor speeds and temperatures against time.

The energy for the mixing is generated by the torque of the rotor acting on the rubber compound. The energy is spent on straining and shearing the rubber compound and breaking the fillers. The elastic deformation is stored and released by the rubber, but the plastic deformation generates thermal energy which causes the rise in temperature of the rubber compound during mixing. Increasing temperature helps wetting of the silica surface by the rubber, in turn helping incorporation. However, the increase in temperature reduces the effective dispersion, as the decrease in viscosity decreases the effective shear forces being applied to the rubber. However, for a silica compound the increase in temperature promotes the silanization reaction and makes silica more compatible with rubber. The silanization reaction follows a first order kinetic, and the reaction rate increases exponentially with temperature. This makes it necessary to keep track of the thermal energy and separate it from the mechanical energy. The thermal energy can be calculated by using the heat equation. 𝑸 = 𝒎 × 𝜟𝑻 × 𝑪𝒑

( 29 )

Where, Q = Heat Transfer (Joules) m = Mass (grams) ΔT = Temperature change (°C) Cp = Specific heat capacity (Joules/gram °C) 76

Equation 8 can be used to calculate the specific heat capacity of the rubber compound. The heat transfer in the mixed rubber compound for different temperatures was measured using a Differential Scanning Calorimeter (DSC). The sample was placed in the DSC cup and heated at a rate of 10 Kelvin per minute from 20°C to 180°C, then cooled from 180°C to 20°C at 20 Kelvin per minute. From the energy needed to heat the samples, the specific heat capacity at constant pressure, Cp, of the model compound was calculated using Equation 8. Figure 6-25 shows the specific heat capacity of the model rubber compound plotted against temperature. The heat capacity increases slightly with temperature, as expected. As the range of the heat capacity of the model compound within this temperature range was not too high, an average number for the specific heat capacity was calculated. The average is taken as 0.02312 J/g°C with a standard deviation of 0.0018786. The fill factor was removed as a parameter, as the influence on the Payne effect was minimal as described in Section 6.2. Therefore, the mass of the compounds is always 354 grams. The Cp value along with the mass and the temperature during mixing is then used to calculate the thermal energy required for the temperature change during mixing using Equation 8.

Specific Heat for model compound

Cp (Joules/g.°C)

0.030

0.025

0.020

0.015

0.010 0

20

40

60

80

100

120

140

160

180

Temperature (°C)

Figure 6-25 Calculated Specific heat capacity for the model compound from the measurement done using a DSC.

Figure 6-26 shows the thermal energy, calculated using Equation 8 and the average Cp obtained

77

from Figure 6-25, plotted against the mixing time. The thermal energy required to achieve the change in temperature during mixing is in the range of 500 - 2000 Joules. The overall energy input during mixing is in the range of 100 – 600 kilojoules in the same time intervals (see Figure 6-24). It is clear, that the energy required to raise the temperature while mixing is a small fraction of the total energy used to mix. The progression of the change in thermal energy follows the same trend for all the compounds. This could be one of the reasons why the change in rotor speed does not affect the rate of change of the Payne effect values. The mechanical energy input is only contributing to breaking of the filler particles and the filler-filler interaction is primarily influenced by the degree of silanization reaction. The energy generated by the shearing action of the rotors is spent to heat the mixing device walls, lost to the atmosphere and removed by the heating/cooling fluid running through the mixing device. Thermal energy is spent on changing the viscosity of the rubber and in phase changes of different ingredients like melting of the wax present in the compound. This makes it difficult to accurately predict the amount of thermal energy spent effectively on mixing. However, Figure 626 shows that the rotor speed has the most significant influence on the energy spent to raise the temperature. The influence of the device starting temperature is also present, but to a lesser extent than the rotor speed. It is clear from the graph that the model compound mixed at 70 RPM has the lowest thermal energy level despite the different device starting temperature, while the compound mixed at 110 RPM has the highest. The compound mixed at a device starting temperature of 25°C has a higher level of thermal energy than the compound mixed at 75°C device starting temperature, as the temperature difference between them is greater and more energy is required to change the temperature. The thermal energy in the compound could not give a clear understanding of the influence it has on the Payne effect values. It can be seen from Figure 6-27, that the compounds mixed at 70 RPM at different temperatures and the compound mixed at 90RPM and 75°C show the same trend and require the same amount of thermal energy. In contrast to this, the energy requirement for the compound mixed at 110 RPM and 25° show similar trend of change in Payne effect value but require more thermal energy than the compounds mixed at other settings. Nonetheless, temperature as such is an important factor in determining the level of the Payne effect in a mixing cycle as can be seen in Section 6.3.2.

78

Thermal energy input during mixing 2500 2000

Energy (J)

REF: 90RPM, 50°C 1500

70RPM, 50°C 90RPM, 75°C

1000

90RPM, 25°C 500

110RPM, 50°C 70RPM, 75°C

0 0

100

200

300

400

500

Time (s)

Figure 6-26 Thermal energy in the compound during mixing for different rotor speeds and temperatures against time.

Payne effect against the thermal energy of mixing 12000 10000 REF 90RPM 50°C

PE (Pa)

8000

70RPM 50°C

6000

90RPM 75°C

4000

90RPM 25°C

2000

110RPM 50°C 70RPM 75°C

0 0

500

1000

1500

2000

2500

Thermal Energy (J)

Figure 6-27 Payne effect values plotted against the thermal energy required to change the temperature.

It was necessary to look at the temperature in relation to how the mixing stages are progressing throughout the mixing cycle. Mixing of this rubber compound requires not only braking of the fillers, but also reacting with the coupling agent as known from experiments in Section 6.3.3. As this is a chemical reaction, the reaction kinetics will be influenced by the temperature. This avenue of reasoning was perused and the following section will focus on the reaction kinetics of the silanization reaction. 79

6.5 Amount of Silane To better understand the influence of the silane and the silanization reaction, experiments were conducted with various amount of silane in the recipe. The model compound (recipe and mixing sequence in Section 6.2) was modified to have half the original amount of silane, and another compound without any silane along with the model compound was mixed using the same mixing device and sequence. The compounds were mixed under the same mixing conditions as the reference compound: 90 RPM and 50°C. The Payne effect was measured and the energy of mixing was calculated. The plotted graph of energy against Payne effect values is given in Figure 6-28: It is clear from this graph that the same amount of energy of mixing gives a higher level of Payne effect when the silane amount is reduced. The highest level is exhibited for the compound mixed without any silane. The change in Payne effect in this case is linearly linked to the energy input of mixing. However, the data points do not show if the Payne effect values level off at a higher energy input into the compound. The curve for the compound mixed with half of the silane is inbetween the reference compound and the compound mixed without any silane. The Payne effect change in relation to the energy required for mixing also appears to have changed from a linear to an exponential change. The exponential change of the Payne effect is probably due to the silanization reaction: this chemical reaction will slow down the further it proceeds, and it finally ends when the silane is consumed. At this stage, the filler-filler interaction will not further be reduced by the formation of the silane-shielding around the silica particles.

Payne effect against energy for different silane concentration 16000.0 14000.0

PE (Pa)

12000.0 10000.0 8000.0

Energy (REF2)

6000.0

Energy (1/2silane)

4000.0

Energy (no silane)

2000.0 0.0 0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

Energy (kJ)

Figure 6-28 Model compounds mixed with different amounts of silane with the same mixing parameters.

6.5.1 Silanization Reaction The previous set of experiments shows that shearing generated by the rotor and the temperature change influence the dispersion of the filler in the model compound. From Section 6.4, it can be seen that the change in thermal energy follows the same trend for the change in mixing time intervals for all compounds mixed with different parameters, but the amount of the thermal 80

energy change is different. It is necessary to investigate the change in temperature and its influence on the silanization reaction and how that influences the Payne effect value. To understand the reaction kinetics of the silanization reaction, the Arrhenius relation was used to estimate the amount of silane consumed during the reaction. The Arrhenius relation is as follows: 𝒌 = 𝑨. 𝒆−𝑬/(𝑹𝑻) 𝒍𝒏(𝒌) = 𝒍𝒏(𝑨) −

( 30 )

𝑬 𝑹𝑻

( 31 )

Where, k = rate constant T = temperature in Kelvin A = pre-exponential factor E = activation energy in Joules R = 8.3144598 J/ K. mol (universal gas constant) The rate constant for the reaction between silica and silane was taken from the work performed by Göerl [8] and is summarised the Table 6-11: Temperature °C k /min

30 0.0008

40 0.0015

50 0.0027

60 0.0043

Table 6-11 Reaction constants for silanization reaction.

Temp. °C

Temp. (Kelvin)

30

1/Temp.

k\min

ln(k)

E/R

E

ln(A)

303.15

0.00329 0.0008

-7.11

5696

47359.16

11.67

40

313.15

0.00319 0.0015

-6.51

5696

47359.16

11.67

50

323.15

0.00309 0.0027

-5.94

5696

47359.16

11.67

60

333.15

0.00300 0.0043

-5.42

5696

47359.16

11.67

Using the rate constants and temperature in Table 6-11, the 1/T and ln(k) values in Table 6-12 were calculated. Then the graph of ln (k) versus 1/T was plotted as shown in Figure 6-29. A fitting line was drawn through the points and the equation of the line was obtained. The equation for the fit line follows the linear correlation of the general form y=mx+c, which corresponds to

81

Equation 10. The y-intercept or c gives the ln(A) value and the slope or m of the line gives the E/R value. The calculated values can be replaced in Equation 10 to get Equation 11. Table 6-12 Activation energy and pre-factor calculation table.

Activation energy 0 0.00295 -1

0.003

0.00305

0.0031

0.00315

0.0032

0.00325

0.0033

0.00335

-2

ln k

-3 -4

y = -5696x + 11.677 R² = 0.9984

-5 -6 -7 -8

1/T(K)

Figure 6-29 Relationship between the logarithm of the reaction constant and 1/T.

After obtaining the values for the activation energy E and the pre-exponential factor A, Equation 11 allows to calculate the rate constant k for any temperature during mixing. 𝒍𝒏(𝒌) = 𝟏𝟏. 𝟔𝟕𝟕 −

𝟓𝟔𝟗𝟔 𝑻

( 32 )

After obtaining k or the reaction rate for a certain temperature, it is possible calculate the decrease in concentration of the of silane using the following relation for a first order kinetic reaction:

[𝑺] = [𝑺𝟎 ]. 𝒆−𝒌𝒕

( 33 )

Where, [S] = concentration of silane at time t [S0] = initial concentration of silane t = time k = rate constant 82

The temperature recorded at time t during mixing was used to first calculate the reaction rate k at a time t. Then the reaction rate was used in addition to the initial concentration of the silane to calculate the amount of silane consumed within the time the compound was mixed. The amount of silane consumed with time is shown in Figure 6-30 for the model compounds mixed with different rotor speed, temperature and the reference compound.

Change in silane concentration Silane concentration (%)

120 100 80

REF: 90RPM, 50°C 70RPM, 50°C

60

110RPM, 75°C

40

90RPM, 25°C 20

110RPM, 50°C

0 0

100

200

300

400

500

Time (s)

Figure 6-30 Amount of silane in the compound plotted against mixing time.

At higher temperatures and rotor speeds, a higher amount of silane reacted in comparison to the reference and the compounds mixed at low temperature and rotor speeds. At higher temperatures, the reaction conditions are favourable and at higher speeds, the temperature rise is faster, therefore more of the silane reacts. In addition, at higher rotor speeds more fresh surface is exposed allowing the ethanol to evaporate. As it is an equilibrium reaction, more ethanol evaporates and leaves the system, allowing the silanization reaction to move forwards faster as well. Figure 6-31 shows the Payne effect values plotted against the amount of silane consumed during the reaction for compounds mixed at different rotor speeds, temperature and the reference compound. As it can be seen from Figure 6-31, all the compounds mixed for various rotor speed and temperature coincide on the same curve. Besides, the compounds mixed at lower rotor speed and lower temperatures show higher Payne effect values in comparison to the compounds mixed at higher temperature and rotor speeds. These results indicate, that the Payne effect values and thus the dispersion of the silica filler in the rubber matrix is directly correlated to the silanization reaction. At a lower level of consumption of the silane, the variation in the Payne effect values is probably due to the variability in the temperature gradient within the material in the mixer, creating zones of higher temperature where the silanization reaction takes place at a higher rate and thus decreasing the Payne effect at a higher rate. If this assumption is correct, homogeneity of the temperature in the compound during mixing will play a crucial role in maintaining batch homogeneity. However, it is interesting to note that the change in Payne 83

effect is directly correlated to the amount of silane consumed for all the mixing parameters. It is highly likely that the silanization reaction is the primary driver for the dispersive mixing stage, and dispersion modelling of mixing of tyre tread compounds with silica fillers can be made based on the reaction kinetics irrespective of the various mixing parameters. However, changing the mixing parameters influences the reaction kinetics and in turn influences the dispersion of the fillers in rubber.

Payne effect against silane consumed Payne effect value (kPa)

12.00 10.00 8.00

REF: 90RPM, 50°C 70RPM, 50°C

6.00

110RPM, 75°C

4.00

90RPM, 25°C 2.00

110RPM, 50°C

0.00 0.0

10.0

20.0

30.0

40.0

50.0

60.0

Silane consumed (%)

Figure 6-31 Payne effect values plotted against silane consumed during mixing for model compounds mixed at different rotor speeds and temperature.

Evaluating the thermal energy and the amount of silane reacted based on the temperature in addition to the energy spent on breaking the fillers can be a suitable method to control the dispersion during mixing of the silica filled rubber compounds. The thermal energy can be an important parameter as it can influence the temperature change of the compound being mixed. It is also probable that during mixing, the amount of thermal energy generated is higher or lower than the heat capacity of the material and in turn influences the temperature change. However, the methods evaluated in the various sections of Chapter 6 need to be further evaluated to accurately gauge the effectiveness of the method in combination with different recipes, mixing devices and mixing sequences. This technique can give an indication of the change in Payne effect with respect to the mixing parameters. No other technique is able to give the complete picture of the process. But this procedure is complicated by the fact, that the silanization reaction depends on several parameters which are difficult to control in the bulk of the rubber being mixed and inside the mixing device. However, this technique could be further developed to achieve a higher degree of predictability and may be possible to include different types of silica and coupling agents.

84

6.6 Bibliography 1. 2. 3.

4. 5.

6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17.

18.

Guth, E and Gold, O. On the hydrodynamical theory of the viscosity of suspensions, Phys. Rev, 1938. 53(322): p. 2-15. Nakajima, N. Mechanism of mixing in internal mixer and energy‐based modelling, Polymer International, 1996. 41(1): p. 23-33. Van Buskirk, P, Turetzky, S and Gumberg, P. Quantitative evaluation of emulsion styrene-butadiene rubbers and compounding variables controlled by mixing, Rubber Chememistry and Technology, 1975. 48: p. 557-568. Dizon, E. Processing in an internal mixer as affected by carbon black properties, Rubber Chemistry and Technology, 1976. 49(1): p. 12-27. Myers, F and Newell, S. Use of Power Integrator and Dynamic Stress Relaxometer to Shorten Mixing Cycles and Establish Scale-Up Criteria for Internal Mixers, Rubber Chemistry and Technology, 1978. 51(2): p. 180-193. Payne, A. The dynamic properties of carbon black-loaded natural rubber vulcanizates. Part I, Rubber Chemistry and Technology, 1963. 36(2): p. 432-443. Payne, A. Effect of dispersion on dynamic properties of filler-loaded rubbers, Rubber Chemistry and Technology, 1966. 39(2): p. 365-374. Kraus, G. Reinforcement of elastomers by carbon black, in Fortschritte der HochpolymerenForschung. 1971, Springer. p. 155-237. EP05201227(B1). Rauline, R, Rubber compound and tyres based on such a compound, Compagnie Generale Des Etablissements Michelin - MICHELIN & CIE, 1992-09-02. Reuvekamp, L.P.A.E.M. Reactive Mixing of Silica and Rubber for Tyres and Engine Mounts: Influence of Dispersion Morphology on Dynamic Mechanical Properties, (2003), Doctoral Thesis, University of Twente. Dierkes, W. Economic mixing of silica-rubber compounds, Print Partners Ipskamp, Netherlands, 2005. Clarke, J and Petera, J. Modeling dispersive mixing of rubber compounds, Rubber chemistry and technology, 1999. 72(5): p. 807-828. Carwile L.C.K and Hoge H.J. Technical Report 66-49-PR: Thermal conductivity of soft vulcanized natural rubber, 1966, p.3-39, Limper, A. Mixing of Rubber Compounds. 2012: Hanser Publishers. Leblanc, J.L. Filled polymers: science and industrial applications. 2009: CRC Press. p. 11-335. Wolff, S. Silanes in Tyre Compounding After Ten Years — A Review, Tyre Science and Technology, 1987. 15(4): p. 276-294. Wolff, S. Optimization of Silane-Silica OTR Compounds. Part 1: Variations of Mixing Temperature and Time during the Modification of Silica with Bis-(3-Triethoxisilylpropyl)-Tetrasulfide, Rubber Chemistry and Technology, 1982. 55(4): p. 967-989. Goerl, U, Hunsche, A, Mueller, A and Koban, H. Investigations into the silica/silane reaction system, Rubber chemistry and technology, 1997. 70(4): p. 608-623.

85

7 Conclusion and Future Work The project objective was to design a control algorithm based on a correlation which links the mixing parameters to the Payne effect of a batch, thus the degree to which the silanization reaction took place. For this study, a model compound was chosen from literature. This compound was mixed using a predefined mixing sequence in a laboratory Brabender mixer, and the dispersion of the compound was determined by the Payne effect. The measured Payne effect was compared to the calculated energy input during mixing. In a first screening study, the different mixing parameters rotor speed, fill factor, mixing time and starting temperature of the mixing chamber were varied in order to determine, which of these factors was the most determining one for the dispersion of the silica in the rubber matrix. However, the outcome was that not one of these single parameters is determining the dispersion, but a combination thereof. This mutual influence of different parameters can be summarized in the energy input: for a given device starting temperature, the dispersion of the model compound changes exponentially with the energy input for mixing. The mechanical energy input into the compound can be changed by varying the rotor speed during mixing of the compound. The same level of dispersion can be obtained at a shorter mixing interval at an increased rotor speed. The correlation between energy input during mixing and Payne effect not only depends on the mechanical energy input, but also on the device starting temperature. The level of the Payne effect for a given thermal energy input changes with the change in device starting temperature. At higher device starting temperature, a lower Payne effect can be obtained for the same level of total mechanical energy input in the device and vice versa. However, the change of the level of the Payne effect with change in device starting temperature is not linear: the Payne effect changes more while moving from a lower device starting temperature to higher temperatures. The analysis of the thermal energy exchange during the mixing process revealed that a high rotor speed provides a maximum thermal energy exchange in the material being mixed. Increasing the device starting temperature while maintaining the rotor speed had negative influence on the thermal energy exchange during mixing (see Figure 6-26). These results made clear that the influence of temperature of the compound is more dominant than the thermal energy exchange that influences the viscosity of the material being mixed and mechanical shear transferred through the rubber matrix to shear the filler particles. The results also showed that for higher device starting temperatures but lower rotor speeds, the change in thermal energy was not so significant in comparison to the compound mixed only by varying the device starting temperature. At the higher device starting temperature, the mechanical energy converted to thermal energy due to shearing of the compound while mixing is not substantial enough to provide an extra temperature increase as the viscosity of the compound is low or the energy converted by shearing is low. Then the silanization reaction is the primary factor determining the level of Payne effect during a mixing cycle. When the amount of silane reacting during mixing was correlated to the Payne effect, a master curve was generated with the Payne effect as a function of the amount of silane consumed for 86

the silanization reaction during mixing. Besides, it was observed that at lower temperatures the relation between the amount of silane consumed and the Payne effect value has more deviations than at higher temperatures. It is highly likely that at lower temperatures, at which the silanization reaction is inefficient, the influence of a mechanical breaking of particles is higher. As the silanization reaction is the dominant factor in determining the level of the Payne effect, it can be concluded that the factors influencing the reaction kinetics during mixing have to be monitored and controlled to control the dispersion properties of the mixed compound. Besides, the shearing forces during mixing will mechanically break filler aggregates at lower temperatures, thus creating fresh surface of the filler that influences the reaction kinetics. At higher temperatures, the shearing forces exerted onto the compound expose fresh surface of the material from which ethanol can be evaporated . This as a consequence will enhances the reaction rate of the silanization reaction. As two different effects, the shearing forces exerted onto the fillers at low temperature and the silanization reaction at higher temperatures determine the Payne effect of the mixed compound, it is necessary to incorporate both factors into the control system. Initial silane concentration

Reaction Kinematics by Arrhenius

Temperature during mixing

Amount of silane consumed

Mixing Parameters: Rotor Speed Device Starting Temperature Time of Mixing

Thermal Energy Energy input

Evolution of Payne effect

Level of Payne effect Payne Effect

Figure 7-1 Architecture of the proposed control algorithm.

The formulated algorithm would follow the architecture shown in Figure 7-1. The device starting temperature as well as the progression of the temperature during mixing of the rubber compound in combination with the reaction kinematics determined by the Arrhenius equation would determine the initial and the final level of the Payne effect. The progression of the Payne effect with respect to mixing time is determined by the energy input for mixing the compound. The energy input for mixing in combination with the thermal exchange occurring during the mixing would enable the prediction of the temperature progression during the mixing.

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Further investigation is required with regard to how the amount of silane consumed and the energy input during mixing influence the Payne effect with compounds other than the model compound. The various influences of different polymers, blends of fillers, varied types of coupling agents as well as other compound ingredients need to be studied. As the result from this work points to the silanization reaction being one of the primary factors determining the level of Payne effect, it is essential to understand how other silanes and silica types react during mixing and how this influences the filler-filler interaction of the mixed compound. It is also necessary to look at how the relations change when a mixing device with different parameters are considered. The influences of different rotor types, mixer volumes and efficiency of the heating/cooling unit need to be further investigated to develop a complete control system. This project was able to identify the basic algorithms that can be utilized to predict the dispersion of the compound based on the Payne effect and the energy input in during mixing, the thermal energy generated and the amount of silane consumed in the silanization reaction during mixing. However, this method still needs to be further evaluated in larger production mixers for industrial applications.

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