The Manufacturing Technology of Continuous Glass Fibres [3 ed.] 0444893466, 9780444893468

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Glass Science and Technology 6

The Manufacturing Technology of Continuous Glass Fibres Third, completely revised edition

K.L. LOEWENSTEIN, B.Sc., Ph.D., F.S.G.T. Director, K.L. Loewenstein Ltd., CamberJey, Surrey, U.K.

ELSEVIER Amsterdam - London - New York - Tokyo 1993

GLASS SCIENCE AND TECHNOLOGY

Volume

J. Stanek, Electric Melting at Glass

Volume 7. Volume 8. Volume 9. Volume 10. Volume 11. Volume 12.

C.R. Bamford, Colour Generation and Control in Glass H. Rawson, Properties and Applications of Glass J. Hlavac, The Technology of Glass and Ceramics: An Introduction I. Fanderlik, Optical Properties of Glass K.L. Loewenstein, The Manufacturing Technology ot Continuous Glass Fibres M.B. Volt, Chemical Approach to Glass Z. Strnad, Glass-Ceramic Materials M.B. Volf, Mathematical Approach to Glass M.B. Volt, Technical Approach to Glass I. Fanderlik, Silica Glass and its Application J. Menbk, Strength and Fracture of Glass and Ceramics

1. Volume 2. Volume 3. Volume 4. Volume 5. Volume 6.

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 p.a. Box 211, 1000 AE Amsterdam, The Netherlands

Library of Congress Cataloging-in·Publication Data Loewenstein, K.L. (Klaus Leopold), 1923The manufacturing technology of continuous glass fibres / K.L. Loewenstein. - 3rd. completely rev. ed. p. cm. - (Glass science and technology: 6) Includes bibliographical references and index. ISBN 0-444-89346-6 1. Glass fibers. I. Title. 11. Series. TP860.5.L6 1993 666' .157-dc20 92-39909 CIP

ISBN: 0 444 89346 6

© 1993, Elsevier Science Publishers B.V. All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted. in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without Hle prior written permission of the Publisher, Elsevier Science Publishers B.V., Copyright & Permissions Department, p.a. Box 521, 1000 AM Amsterdam. The Netherlands. Special regulations for readers in the USA: This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from ttle CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, includi ng photocopying outside the USA, should be referred to the Publisher, un less otherwise specified. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods. products, instructions or ideas contained in the material herein. This book is printed on acid-free paper Printed in The Netherlands

._-_ ... __. _ - -

-----._-~--

----._------

Preface to the Third Edition This book clcab with glass fibres \\-hieh, to distinguish thelll from glass \voo!. arc drawn as CCl1ltilJUOUS straight fihres. They IW1~' clld up ns chopped fihre:-;. may 1)(' chopped ('\TOll during the ntt('ll1wt ion process itself hut.. pla.f:Jtic and doe.':> not set hard until it i8 hentecL These arc prc-illlpregnatcd materials: in the form of sheet, they are referred to as sheet moulding COlllpounds (sJVrC): ill the form of a. bulk material, as dough moulding compollnds (DNIC). The precombinatioll of resin and glass fibre i:-; llsually clone by a resin manufacturer or the moulder. These lnat.crials an: llsc~d widely ill ))1"C'S5 moulding.

18

The products and the manufacturing processes

Fig. 3.5. Typical ravings of different sizes and weights, some on cardboard cores for external unwinding. The roving in the top right hand corner is the standard, about 250 mm in diamet.er and 280 mm high. (Courtesy of Fibreglass Ltd., V.K.)

Another material falling into this category is nodular moulding compound (NDC). It is made from short glass fibres and resin into nodules about 1.5-2 m111 in diameter which are free-flowing and can therefore be used in automatic injection or direct transfer moulding machines for the manufacture of small parts, e.g. electric components. 3.1.3. Roving cloth or woven roving fabrics

These are fabrics made by interweaving ravings. Unit weights vary from 2001000 g 111- 2 and fabric thickness from 0.2-0.9 mm. Various fabric constructions are possible, from almost uni-directional when the warp consists of high-tex and the weft of a few low-tex ravings, to square-weave cloths (fig. 3.6), both in plain weave and twill constructions. Roving cloth is used both for open-lay-up and press mouldings. Over a wide range of applications roving cloth and chopped strand mat constitute alternatives. For laminates making use of both materials, the mat is placed in the centre sandwiched between inner and outer layers of roving cloth. This is the preferred orientation

The products

19

Fig. 3.6. Selection of woven roving fabrics of different weights. (Courtesy of Fibreglass Ltd., U.K.)

since a higher glass content is achieved with roving cloth and thus the outer layers, which carry most of the stress, also contain the extra reinforcement. At one time, the weaving and selling of roving cloth was carried out by companies specialising in weaving. Today the tendency is for the primary glass fibre producer to manufacture and sell roving cloth as part of his range of reinforcing materials, especially since he is, in any case, supplying the market that makes use of it.

3.1.4. Cornbination mats A not insignificant market has been created for the use of combination mats, that is mats or fabrics consisting of combinations of chopped strand mat and woven rovings, or needled mat and woven ravings which, when taking into account the various densities and constructions of each 'which can be combined, yield speciality products for specific applications. Examples of the use of these mats are used in press moulding, pipe manufacture and boat hull and deck construction. 3.1.5. Yarns Yarns of glass fibre (fig. 3.7) are analogous to other textile fibres in that they are twisted and doubled for subsequent weaving into glass fabrics. They have also been

20

The products a.nd the manufacturing processes

Fig. 3.7. Examples of glass fibre yarns wound on milk bottle bobbins. (Courtesy of Silcnki'l 8.\1., The Netherlands.)

ul-led in the manufacture of heavy cords for reinforcing elastomers. The weaving of cloth is a specialised art and is usually carried out by specialist companies; the weaving of fabrics from yarns will therefore not be considered in this book. Glass yarns as such are used for the electrical insulation of wires. The major uses of glass fabrics is in the production of laminates for printed circuit boards, high qualit.y composites for more critical applications, e.g. parts of aeroplane structures, and for fireproof curtains and wallpaper. 3.1. a. Chopped stmnds

Chopped strands (fig. 3.8) are exactly what they say they are, namely strands which arc chopped by the gIns.") fibre manufacturer into lengths specified by the cllstomer. Thcy arc supplied ill lcngths varying from 1.5--50 mm. There arc three major application areas. The reinforcement of thermoplastics IIlndc by injection moulding tcchniques is achieved by incorporating; short chopped

--~

The pmdncts

2]

Fig. 3.8. Chopped strands. (Courtesy of Fibreglass Lt.d., U.K.)

strands of up to a few mm in length, well-consolidated, and free-flowing; the nse of chopped strands for this application has been an arca of great expanf'iion in the last decade, most of the reinforced thermoplastics being llsed in the automotive industry. The second is for the manufacture of roofing mat (or tissne) when directl.ychopped strands of 50 mm length are required. The third application is for the ll1amlf8cture of dough Illonlding componnds (DJVIC) \\Then chopped strand of 25-50 mill length are llsed.

S.1.7. Milled ,fibTeo'3 l\'Iillecl fibres (fig. 3.9) are made by subjecting strand to milling in a ball 1nil1. Filamentisation is extensive. The problem is to obta,in milled fibre in a free-flowing 1'01'111. Lengths of fibres arc difficult to control but lie below 1.5 111111. l\Iillec! fibre are

22

The products and the manufacturing processes

Fig. 3.9. Milled fibres. (Courtesy of TBA Industrial Products Ltd., u.K.)

incorporated into phenolic and cresolic mouldings used in electrical applications.

3.2. An outline of the manufacturing processes

The manufacture of glass fibre products is best considered in three stages (see fig. 3.10): (1) Glass manufacture. (2) Fibre forming, i.e. the conversion of glass into glass fibres. (3) The conversion of glass fibres made under (2) above into saleable glass fibre products. Stage 1 consists of glass manufacture, i.e. the fusion of selected, weighed and mixed raw materials such as sand, limestone, boric acid, etc. in a glass-making furnace. This stage ends either with liquid glass flowing directly into the fibre forming furnaces, caIled 'bushings', i.e. as in the direct-melt process, or the glass being made into an intermediate product in the form of marbles or other suitable shapes, annealed and cooled to room temperature, and stored under clean conditions ready for use in the remelt process at some future date.

An outline of the m.anufacturing p7'Ocesses

Stage 1: Glassmaking

23

Glassmaking raw materials

I Molten glass ;n furnace (Direct melt)

(Rellelt)

\

I Glass marbles •

•

•

•

•

•

•

•

•

•

•

•

•

•

•

••

••••••••••••

i

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Stage 2: Fibre forming

•

•

•

•

•

•

•

•

•

•

•

•

~

•

•

•

i

•

•

•

•

Glass rrbles I

I

Fibre cakes •••••••••••••••

Stage 3: Conversion into Products

I.'

••••••••••••

.;

• • • • • .,

•••••••••••.••••••••••

Fibre cakes

.-----.---'----r---,..-----.------, Chopped strand mats

Cont. strand mats

Ravings

I

Chopped fibre

Mi 11 ed fibre

I

Roving cloth

I

Cont. strand mats

Directdrawn ravings

I Roving cloth

I

Yarns

(Fabrics)

I

I

Surfacing mat

Overlay mat

Di reetl ychopped fibre

I Roofing mat

Fig. 3.10. Flowchart of glass fibre products manufacture. Some prodnct.s appeal' in more than one sequence; this is clue t.o t.he fact. t.hat they are made both by the remeJt and t.he direct-melt processes.

24

The products and the manufacturing processes

Stage 2 consists of fibre forming. Continuous glass fibres are made by the rapid mechanical attenuation of molten drops of glass exuding from nozzles located on the underside of a heated furnace, called a bushing. A bushing is provided with a large numbers of nozzles, usually 200 or a multiple thereof, and is supplied either with cold glass in the form of marbles or other suitably-shaped pieces which first have to be melted before the liquid glass can pass through the nozzles for attenuation, or with hot liquid glass directly from the glass melting furnace. The fibre forming process is shown diagrammatically in fig. 3.11. Molten glass exudes from each nozzle where, during fibre drawing, the glass forms a meniscus as a result of the attenuation. The whole fan of individual fibres, called filaments, passes through a light water spray and then over an applicator which transfers a protecting and lubricating size onto the filaments before they are gathered on a suitably-shaped shoe into a cohesive bundle of filaments called a strand or, if desired, into a number of smaller strands. From there the strand passes to the machine responsible for the attenuation, in this example, a winder. A winder consists of a slightly expandable rotating cylinder, called a collet, covered during the winding process by a removable paper or plastic tube onto which the strand is wound, and a device, called a traverse, which lays successive lengths of strands onto the tube at small angles to one another to facilitate subsequent unwinding. The thickness of fibre in the form of strand being wound onto a collet is allowed to reach 25-30 mm before the collet is stopped and the package of strand, now called a cake, is removed; the winding operation is then recommenced. Other machinery for attenuation will be discussed in Chapter 5. Each group of equipment as shown in fig. 3.11, consisting of a bushing and its electric supply, fibre size applicator, gathering shoe, water sprays and winder, constitutes a production unit. In a fibre forming department, many of these, depending on the types of products and tonnages to be manufactured, are placed together, usually in lines, each bushing at a distance of between 600 and 1200 mm from the next. The cake, having been coated with a fibre size, is wet and requires drying before further processing. After drying and conditioning the fibre cakes are ready to be converted into saleable products. This, stage 3 of the manufacturing process, will be considered in some detail in Chapter 7. At one time, the above three stages always constituted separate manufacturing operations, even if they were located in the same plant. In the 1950's and 1960 18 stages 1 and 2 were integrated into one continuous process for products which were required in sufficiently large quantities; it was called the 'direct-melt' process as the molten glass made at stage (1) is converted directly into fibre. For technical reasons, surfacing tissue and overlay mat were always made directly from the molten glass but, because of production volumes involved not by the direct-melt process but from bushings in which marbles were used as feed stock. Over the last 20 years, competitive pressures have worked towards a. full integration all three production stages whenever this was technically possible and economically desirable. \Vhile continuous strand mat can be made from cakes, it is in fact made either from

-

An outline of the manufacturing processes

25

Table 3.1 The more commonly produced nominal filament diameters in inches and micrometers and their designations [1]. Inches

x104 1.5 1.9 2.1 2.5 2.9 3.2 3.6 4.3 4.8 5.3 5.6 6.4

6.8 7.2 8.0 8.4

8.8 9.2 9.6

Filament designation B C

Micrometers 3.5

D

4.5 5.0

DE

6.0

E F

7.0

G

9.0 11.0

H

J K L M N

o p

Q R

T U

8.0

12.0 13.0 14.0 16.0 17.0 18.0

20.0 21.0 22.0 23.0 24.0

marbles using the remelt techniques or by the direct-melt method. V\Tith the development of bushings of 2000-4000 nozzles and the acceptance in the marketplace of filament diameters of over 17 l.i·m and over, it became possible to make all ravings except those for chopping by the direct-roving winding technique; in this process the traditional '\Tinder is replaced by a roving machine adapted for this purpose. Similarly, chopped strand for manufacture of roofing mat is now made directly from the molten glass by replacing the winder by a chopping machine which can cut wet glass fibre strands. These will be discllssed in more detail in Section 5.6. Glass fibre strands are defined in terms of the diameter of inelividnaJ filaments and the weight per unit length (or its inverse). These, by implication, also involve the number of filaments per strand. Since the industry developed first in the D.S.A., designations were introduced based on American units; there, a strand is defined bv a letter which denotes the average diameter of the filamcnt in the strand i and a .J number which denotes the 'count'; the COllnt is defined by the l1\lmber of hundred yards per pound (454 g). In the metric system the filament is stated by its diameter in micrometers (nearest \\'hole number) and its 'tex', defined as grammes per kilometer. Count and tcx arc related by the equation: count x tex

= 4961

-----

---)

26

The products and the manufacturing processes

Bushing

Light ::::-~'" water spray -

Fibre size

applicator Gathering shoe

Traverse

Collet

Fig. 3.11. Diagrammatic representation of the manufacturing process of continuous glass fibres. The above is the most typicaL but there are variations.

The more commonly produced filament diameters are given in table 3.1. For example, GI50 means a fibre strand of.200filaments of average cliarneter 3.75 x 10- 4 inches and 15000 yards per pound. The metric equivalent is a filament diameter of 9.53 pIll and a tex of 33. Since productivity and efficiency increase with increasing filament diameter, there is always economic pressure to move towards the manufacture of coarser filaments, or if the specification does not permit this, to manufacture at the upper end of the filament diameter and tex ranges. The typical range of commercially important fibre strands given in table 3.2 must therefore always be regarded as temporary as each manufacturer and customer will decide between themselves the specification and price of the materials required . The following are useful formulae interrelating filament diameter (d), number of filaments per strand (N), count (Y), and tex (T):

d = 0.06220 (Y N)1/2

inches

T ) d= po; . 0.8 ( 0,4961N

1/2

pm

References

27

Table 3.2 Examples of commercially important fibre strands. Strand designation

U.S.A.a D 1800 D 900 D 450 D~E 150 E 300 E 450 E 225 150 G G 130 75 G H 110 H 55 J 90 J 45 23 J K 31 K 16 K 10 K 8 NI 6 M N N T T

5 4 2 2 1

metric b EC52.75 EC55.5 EC511 EC63.3 EC616.5 EC711 EC722 EC933 EC938 EC966 EC11 45 ECll 90 EC1255 EC12110 EC12220 EC14160 EC14320 EC14480 EC14640 EC16800 EC161000 EC171200 ECl7 2400 EC242200 EC244400

Number of filaments per strand 50 100 200 400 200 100 200 200 200 400 200 400 200 400 800 400 800 1200 1600 1600 2000 2000 4000 2000 4000

Average filament diameter pm 5.33 5.33 5.33 6.35 6.35 7.32 7.32 9.14 9.65 9.14 10.67 10.67 11.75 ] 1.75 11.75 14.19 14.19 14.19 14.19 15.86 15.86 17.38

] 7.38 23.52 23.52

a

Filament designation and count. E = E glass, C = continuous, then nominal filament diameter and tex. b

d2

X

O.4961N 15.8 2

T=-----:---

J'l

15.8 2 X T ([2 X 0.4961

= ----=-----

References [1] Owens Coming Fiberglas Corp. Publication No. 5-1NT-15396 (.January 1989).

4.

Glass manufacture

4.1. General It is impossible to overstress the importance of glass quality when considering the manufacture of glass for conversion into continuous glass fibres. Filaments of continuous glass fibres range from 3 to 25 pm; solid inclusion of even submicron dimensions act as stress concentrations and can cause filament breakage during attenuation causing loss of production and wastage of glass. Such inclusions could be residues of solid materials such as unmelted raw materials, specks of refractory from the side walls of the melter, dislodged, possibly, due a to small change in glass level, or a small piece of devitrified glass which had grown in a cold corner ofthe forehearth, or metal dust rubbed off feeding tubes into a marble bushing. They all cause breakage of a single filament in the first instance, the tail of which can break others; this often leads to an unintentional but catastrophic interruption of the fibre forming process. Inhomogeneity, such as could arise from inadequately mixed raw materials, 01' the inadequate dissolution and dispersal of striae of alumina or zirconiaarising from incomplete solution of refractories in the glass, call create a slllall volulllc of the glass outside the range of viscosity required for stable fibre formation; this will suddenly vary the flow of glass through a bushing nozzle and lead to filament breakage. Contrary to expectation, gas bubbles in the molten glass, unless very big, do not cause filament breakage but cause a ca.pillary about 5 m in length to form in the filament. While for many reinforced plastics prodllcts the presence of such capillaries does not cause a problem, their presence is undesirable in fibres destined for manufacture into certain products such as printed circuit boards, where they can become the channel for electrkal failures, or in thelll3nufadure of translucent sheeting as the presence of capillaries makes these fibres more visible in the composite. Slight fluctuations of the glass composition from day to clay \vould not interfere with the production process as such but would cause variations in the physical properties of the fibre and the production rates of giveu fibre-forming furnaces (bushings). Therefore, for consistency of products being manufactmed, all factors which contribute to this objective must be controlled. Such factors blclude the glass composition, the temperature of the furnace, the melting rate, and the temperature/time schedule to which the glass will be sllbjcct~cl between melting and fibre forming. Although optical glasses are usua.lly thought. to be the apex of glass quality 29

30

Glass manufacture

and, for reasons of purity, are often manufactured in furnaces lined with platinum alloys, the quality requirements of glass for the drawing of continuous fibres are significantly higher since 'sorting' of good glass from bad is inherently impossible in glass fibre manufacture. The problem of achieving the necessary quality standard is linked to a proper choice of raw materials for glassmaking, the proper selection and placing of refractories in the glass melting furnace and the proper control of the glassmaking and subsequent conditioning processes. The growth of the glass fibre industry since 1945 and the comparative price stability of glass fibre products since the 1970's are linked directly to improvements in glass quality over that period brought about by the development of refractories of better performance, higher melting temperatures, longer furnace life, better understanding and control over parameters affecting productivity and quality, improved control instrumentation and computerisation of processes, as well as the larger scale of operations resulting from greater confidence and experience. Thus, in the 1950's the first direct-melt furnace had a production capability of 3 V.S. tons per day, and the furnace life was 9 months. In 1966 the biggest direct-melt furnace had a capacity of 10 D.S. tons per day. By 1970 furnace life had extended to 4 years; single furnaces of less than 15 D.S. tons per clay were becoming uneconomic and furnaces of 40 D.S. tons per day, and over, were being successfully operated. The largest furnaces currently in operation (1992) are believed to have a. rated output of ahout 200 D.S. tons (180 metric tons) per day.

4.2. Glass compositions Over 99% of all continuous glass fibre produced is of a composition referred to as 'E glass'. Although E glass was originally developed for electrical applications, and there is a growing market for E glass in electrical applications, the use of E glass has spread into many other applications where the properties of the fibre as such were found to meet the requirements. E glass therefore dominates the 'world market for reinforcing fibre; in cases where glasses of different compositions are used, there is a specific reason for this, not the other way round. The composition of E glass is best given in terms of ranges of its constituents (table 4.1). Up to 20 years ago, fair quantities of fibre were also being made from soda-lirnesilica glass of sheet composition, called 'A glass' in the industry. This was attractive for producers during the period when patents covering E glass prevented them from using it, and/or when a local source of cheap sheet glass scrap was available; it was never, to the best of the author>s knowledge, set up as a direct-melt process but was always produced by the remelt process. As a proportion of the total world fibre production it is now insignificant, although for many general purpose composites A glass fibre reinforcement has been shown to be perfectly adequate and, on all equivalent scale, should be cheaper to produce than E glass. E glass has one particular disadvantage: it is easily dissolved in dilute mineral

.. _ - - - - -

~-------

Glass compositions

31

Table 4.1 The composition of E glass, the most important composition for the formation of glass fibres [11. Constituent Si02 AI 2 0;j B2 0 3 Ti02 MgO CaO Na20+K20 Fe20;, F2

Weight %

52-56 12-16 5~1O

0-1.5 0-5 16~25

0-2 0-0.8 0-1

acids. For this reason a chemically resistant glass, called C glass, is used in the form of fibre for composites which will be in contact with, or will be containers for, acidic materials, e.g. tanks in the electroplating industry. C glass is, in theory, also cheaper to produce than E glass. As a result, where production volumes warrant it. C glass is also used as an alternative to E glass in the manufacture of roofing mat for the reinforcement of bitumen. In practice, since the volume of E glass manufactured for this purpose exceeds that of C glass by far it competes sllccessfully with it due to the scale of manufacturing operations and the high degree of mechanisation which has been achieved. Two special glasses are used in applications \vhere composites of highest mechanical performance are required. They are S glass 1 (ma.l1ufacturecl by Owens Coming Fibreglass Corporation) and R glass (manufactured by Vetrotex). These glasses are si1nilar in composition (see tahle 4.2), are clif£cult and costly to make and their use is therefore limited to sophisticated applications in the fields of aircraft and engine construction, missiles and specialised sports equipment. Typical compositions of E glass [1] and of the other glasses mentioned together with some of their basic properties arc given in table 4.2. Cemfil [2] and AR [3] glasses are two glass compositions used for the reinforcement of cement; the amounts manufactured for this purpose probably do not exceed 20000 tons/year. They are very small when compared to E glass; however) this type of fibre reinforces 20-30 times its own weight of cement. With the growth of the computer and associated industries, glasses of dielectric constant lower thelll that of E glass have been developed in order to provide glasses of lower dielectric constant and faster response time of composite printed circuit boards. Examples from .Japan [4], the United States and Europe are given in table 4.3. 'S glass' is a registered trade name.

Glass man1lfactllTe

:32

Table 4.2 Composition in weight % of glasses used in fibre manufacture and some of their properties in fibre form. Constituent or property Si0 2 Ab03 B203 Zr02 MgO CaO Na20 K20 Li20 Fe20::l F2 Liquidus temp. Dca Fiberising temp.o cb Tensile strength of single fibre at 25°C, kgjmm 2 Density, g/cm 3 Refractive index no Coefficient of linear thermal expansion per °Cx 10 6 Volume resistivity in cm Dielectric constant at 25°C and 10 10 Hz Loss tangent at 25° C and 10 10 Hz X 10 3

E glass 55.2

14.8 7.3

C glass 65

4

A glass 71.8 1.0

glass 65.0 25.0

R glass 60 25

Cemfil glass[2]

71

AR glass[3] 60.7

1

5

3.3

3

3.8

18.7

14

8.8

0.3 0.2

8.5

13.6 0.6

0.3 0.3

0.3

0.5

1140 1200

370

S

10.0

tl'.

1010 1280

337

2.53 1.550

2.49

5.0

7.1

310 2.46 1.541

9

1201 1470

468

449

2.48 1.523

2.55

2.85

4.10

1172 1290

292

n

Na20 loss/h/g in water at 100°C, /lg

6.11

5.21

3.9

6.8

112

274

The liquidlls temperature is the highest temperature at which a glass, if held t.here sufficiently long, will develop crystals. The greater the difference between this and fiberising temperature, the more stable the fibre forming' process. b Indicates temperature at which the viscosity of the glass is 1O:~ poises. C Measured at lOG l:Iz. (1

E glatis dOJninates the continuous glass fibre market. As is evident from table 1, it docti not have a defined composition, but is a glass of defined electrical properties. Since these are governed by the alkali content of the glass, E glass is usually defined in national and international specifications in terms of the alkali content of the glass, i.e. that the glass should not have an alkali content when calculated as Na20 exceeding 1% by mass. (Japanese standards limit the alkali content to 0.8% as Na20.)

Gla.ss compositions

33

Table 4.3 D glass compositions in weight % ('D' stands for 'dielectric'). Composition/properties Si02 Ah 0 3 B 2 03 CaO MgO CaO+MgO+ZnO L i2O+ Na20+K 20 Miscellaneous Dielectric constant at 10 10 Hz Loss tangent at 1010Hzx 10 3 Q

Japan[4] 45-65 9~20

13-30

USA 75.5 0.5 20.0 0.5 0.5

Europe Q 72-75 20~23

4-10 0~5

3.0

max.l max.4

4.3-4.9

3.8

3.85 0.5

Manufactured by Vetrotex.

Basically, E glass is a calcium alumino-borosilicate glass containing less than 1% of alkali oxide when calculated as Na20. The actual alkali oxide content, as well as the presence of other trace elements, are usually governed by the choice of raw materials which, for cost reasons, should be natural materials whenever possible. Most glasses contain small but important quantities of fluoride added to assist the dissolution of raw materials and lower the liquidus temperature of the gla.ss. Some glasses also contain deliberate substitutions of IvlgO for CaO. It might be thought that the composition of E glass is: after over 50 years of its existence, now an established composition varying only as a result of variations in locally-available materials; but this is not so. Several factors encourage and/or force manufacturers to re-examine the composition of E glass, namely the need to reduce atmospheric pollution due to gases and dust discharged from E glass furnaces, production problems and cost of raw materials, especially of B 2 0 3 -containing materials. table 4.4 shows a range of E glass compositions including fluoride-free and ftuoride-and-boric-oxide-free versions. In this group the glass in column 6 is of particular interest since its development involved a careful study of the benefits or otherwise of the presence of :rvlgO; it concluded that: at about 1.8% I\!IgO, the liquidus temperature 2 was at a minimum (1083°C) and the fibre forming temperature "vas also lower than with traditional E glass (1212°C). Of the minor constituents, it is worth noting that the presence of small quantities of fluoride assist melting of the glass, contribute to a reduction in the liquidus tem2 The liquidus temperature is the highest temperat.ure at which a glass, if held there sufficiently long, will develop crystals. For reasons already stat.ed, the presence of even SUbmicroscopic crystals is disastrous for fibre manufacture. The lower the liquidus temperature, and the greater the difference between liquiclus and fiberising t.emperature, the 1110re stable t.he composition as a glass.

Glass mamtfacture

34

Table 4.4 E glass compositions 1940-1990 (weight %). 1 original E glass

[6] SiOz AlzO:J BZ03 TiO z MgO CaO ZnO

RzOu FCZ03

F?

60 9

4 27

2 'improved' E glass [7] 54.0 14.0 10.0 4.5 17.5

3 621 glass [8] 54.0 14.0 10.0

22.0

1.0 t.race

1.0 trace

0.5 e

o.se

4 MgO-free glass 54..3 15.1 7.4

5 816 glass [9] 58.0 11.0

0.1

2.4 2.6

22.1

22.5

0.4 0.2 0.6

2.6 1.0

0.1 0.01

6

F -free glass [10J 55.3 13.9 6.8 0.2 1.8 21.4 0.4 0.2

7 B&F-free glass

[I1J 59 12.1 1.5 3.4 22.6 0.9 0.2

8 low nD glass [12] 55.8 ]4.8 5.2

21.0

1.4" n.d. 0.5

Tot.al of alkali oxides. Note alkali cont.ent (reported as NazO) above 1.0%. C Estimated from statement in patents that 'fluorspar lJIay be substituted in amount.s of 1.-3% for part of the boron oxide ... '. a

b

perature a.nd ea.se fibre formation. The presence of iron oxide also has a significant influence on the stability of the fibre forming operation due to the fact that its presence increases the rate of infra-red emission, i.e. the rate at which heat is lost from the glass as it leaves the nozzles of a bushing. In the last 20 years, increased consideration of environmental effects has led to legally enforceable restrictions on permitted pollution levels resulting from industrial activities. This is now of such importance that a more detailed discussion all the current situation is called for (see Section 4.5.9). The causes of atmospheric pollution originating in the glass composition and melting operation for a particular E glass composition are: - fluoride vapours, probably in the form of fiuorosilicic acid, I-hSiFc, hydrofiuoric acid, HF, and f1uoborates; - sulphur oxides, from sulphur in the oil used for combustion plus a trace of sulphate in the raw materials; - nitrogen oxides, resulting from the oxidation of nitrogen in the air; - batch dust carried over from the melting chamber and into the atmosphere; and - boric oxide, as vapour from the furnace which condenses to a smoke on cooling. The manufacturer, when faced with this problem, has certain technical choices which can minimise pollution. He has a limited choice of composition and a choice of furnace (electric heating within the glass eliminates pollution by sulphur and nitrogen oxides and reduces losses of B 2 0 3 , fluoride, a.nd dust - see Section 4.5.8.1). He may also have a choice of fuel which can minimise the formation of sulphur

Glass compositions

35

oxides, and he can now deal with nitrogen oxide pollution by installing a firing system which avoids them being formed. If he cannot control pollution by these means he has to rely on treating the polluted waste gases to reduce all pollutants to acceptable levels before discharge to the atmosphere. A more detailed discussion of pollution control is given in Section 4.5.9. In the glass industry in general, dust has been significantly reduced, melting rates increased and the quality of the glass improved by pelletising the batch [5]. So far, at least, the glass fibre industry does not appear to have followed this example. To consider a change in glass composition is a very serious undertaking since it involves changes to a large number of parameters all of which have been, up to then, considered as an established part of an integrated production process. Many problems are likely to surface downstream in the production line and all unexplained faults will tend to make the suitability of the new glass composition suspect. Despite these problems, economic pressures and pollution regulations, the high cost of some materials and a better understanding of the factors interlinking glass composition with process engineering and the properties of glass fibres has given some manufacturers sufficient courage to experiment with and undertake changes in the composition of E glass. Consideration of these compositions might as well include a look at how E glass developed between 1940 and 1990. Table 4.4 gives seven examples. Composition 1 is that of the original E glass which resulted from very careful work to find a substantially alkali-free glass that could be manufa.ctured at the then practical founding temperatures and using available refractories [6]. This was improved by Composition 2; in it, the addition of fluorspar as a partial replacement for B 20 3 was suggested to help melting and reduce the liquidus temperature [7]. This glass suffered from the fact that, although the melting temperature ,vas, by present standards, very low, the then available refractories, namely pressed zircon, dissolved in the glass very rapidly. This not only gave very short furnace campaigns, but the dissolution of zircon in the glass raised its liquidus temperature, thus increasing the risk of the glass devitri(ying before or during fibre forming. This problem was attacked in Composition 3, known as '621' glass [8]. The major change was the elimination of lVlgO and corresponding increase in CaO content. The liquidus temperature of 621 glass was lower than that of the 'improved' E glass, and was thus able to accommodate the addition of Zr02 from refractories without such a high risk of devitrification. Composition 4 is a variant 621 glass in which the B 2 0 3 content has been reduced. With improved and new refractories, especially the development of isostaticallypressed chrome and zircon refractories, the rate of corrosion of refractories decreased to such an extent that melting temperatures could be increased to nearly 1600°C and furnace campaigns extended. Nevertheless, the two types of E glass composition, i.e. with and without 1vIgO as a significant constituent have remained in use. With increasing founding t'€mperatures, B 2 0 3 could be reduced. Both types contain fluorides at about 0.4% F 2· As laws governing pollution came into force some manllfacturers tackled the

36

Glass manufacture

problem of fluoride emissions by developing glasses which were free of deliberate additions of fluoride, i.e. the only fluoride still found was due to trace amounts present in, for example, the clay (used as source of alumina). It was also found at this stage that the presence of even very small amounts of fluoride, i.e. 0.01% F 2 , significantly assisted fibre forming; the effect is as if a trace of fluoride increased the surface tension between glass and the platinum alloy of the nozzles of the bushing for fibre forming, thus helping to stabilise the fibre forming process (see also Section 5.2.1). Having started on the elimination of fluoride pollution by means of composition changes, attention was also drawn to the smoke of condensed B 2 0 3 which is discharged from furnace chimneys. The losses are very considerable since, in a normal gas or oil-fired tank furnace, about 10-20% of the boric oxide added in one form or another is lost into the atmosphere. Since B 2 0 3 is the most expensive raw material being used - it accounts for about one half of the raw materials costs - and has, on occasions, been in short supply, the concept of eliminating all problems by avoiding the raw materials which caused them was inviting. This led to Composition 5, known as '816' glass [9]. It is worth noting, however, that raw material costs were not reduced and the process details had to be adjusted to accommodate the higher liquidus temperature of this glass. Composition 6 is a fluoride-free B 20 3 -containing E glass of a type used in Japan for many years, although this particular composition is a recent improvement of this type [lOJ. Composition 7 is an American E glass free of both fluoride and boric oxide, differing only slightly in composition from 816 glass [11]. It is no[; clear to what extenf1hese two glasses are in large-scale use. Most manufacturers have preferred to use established E glass formulations and to install pollution control equipment while continuing their efforts at minimising the evolution of contaminants; in this connection the contribution of electric melting is significant (see Sections 4.5.8.1 and 2). Also: some of the pollution equipment itself permits all or part of the pollutants to be recycled back into the glassmaking process Csee Section 4.5.9). Composition 8 is (almost) an E glass specially formulated to possess a low refractive index for its use in translucent sheeting [12]. Although its alkali content of 1.4% as Na20 takes it outside the limits for E glass proper (1.0% Na20 max.), there is no reason why it should not be Ilsed for non-electrical applicat.ions.

4.3. Selection of raw materials for E glass manufacture The selection of raw lnaterials must be based on composition, reliability of quality and supply, and cost. Natural materials usually contain sometimes major, sometimes minor proportions of oxides which are not the main constituents for which the raw material in question is being used. The presence of minor constituents mus£ be checked carefully, e.g. of alkali and iron oxides, so that certain permissible maxima in the glass composition are not exceeded.

--------------------====~ -~-

S eleeti.on of raw materials for E glass manufacture

.... -----"------=---------

37

Table 4.5 Comparision of particle sizes in micrometers compared to U.S.A. and British Standard Sieve equivalents. U.S.A. standard sieve 275

micrometers

213 141 99

75 100

53

150 210 250

70

60

British standard sieve

300 240 156 100 72

61

A major difference between lUsual raw materiab and raw materials for E glass is that the raw materials must be finely powdered; if they are not, silica and other materials of low solubility will separate out on top of the melt as a scum. With the exception of raw materials for introducing boric oxide, most other raw materials should be available within reasonably short distances. \\Then considering transport costs to the plant, bulk shipments usually reduce the costs considerably. 4.3.1. Raw material for int1'Oducing sUica (Si0 2 )

Glassmaking sand finely powdered but not of the lowest iron content used in the glass industry is suitable. A typical analysis is: 8i0 2 Ab 0 3 Na20 K 20 Fe2 0 3 H2 0

98.05%

mm.

0.85

0.1 0.4 0.1 0.1

A typical particle size distribution is: coarser than 150 pm between 150 and 75 pm between 75 and 45 lun finer than 50 pm

0.1% 0.8%

6.0% rema.inder

For a comparison of micrometer and mesh sizes, see table 4.5.

(1) The preferrcclmaterials is a china clay of low alkali amI iron contents. A typical specification would be:

Glass manufacture

38

Si0 2 Al 2 03 CaO Na20 Fe203 Water

44%

37 0.6 2.0 1.0 1.0

max. max, max.

The clay should be supplied as a very fine powder, nominally 50 pm with the following particle size distribution: coarser than 150 /.Lm between 150 and 75 /-lm between 75 and 50 pm

1% 1% 99%

max. max.

(2) In the event that a suitable clay is not available, synthetic alumins composition (see table 4.4). (2) The alternative material available in North America is burnt dolomite (MgO·CaO). It does not decrepitate. A typical composition is: 8iO z A1 2 0 3 CaO MgO Fe 2 03 80 3 H2 0

0.2% 0.2 56.8 41.0 0.11 0.78 0.11

In the absence of the above forms of dolomite, other magnesia-containing minerals can be llsed provided their contribution to the total batch composition fits into the overall composition of the E glass targeted; such minerals could be olivine, or magnesite (magnesium carbonate).

4·3.5. Raw material for introducing calcium o.Tide (Ca 0) Universally available limestone or calcite is used. A typical specification would be: CaO

Al 2 0 3 P2 0

[j

~Vln02

FeZ03 H2O

S

55.4% 0.2 0.1 0.1 0.05 0.4 0.1

min. max. max. max. max. 111clX.

max.

The particle size distribution, nominally finer than 150 Inll, should be:

' - - ----~--~- --~-

Selection of raw materials for E glass mamtfacture

coarser than 150 t-tm between 150 and 75 t-tm between 75 and 50 /-1,111

2.0% 28.0% 70.0%

41

max. max. mm.

4.3.6. Raw materials for introducing fluoride (F2 ) The most common raw material used is fluorspar, a natural calcium fluoride: F2 CaO Si0 2 A1 2 0 3 Fe203 PbO Water

47.3 70.2 1.0 1.0 0.25 0.2 0.4

max. max. max. Inax. lllax.

An acceptable particle size distribution could be: coarser than 400 /-I.m between 400 and 150 t-tlYl between 150 and 50 ILlll finer than 50 11.111

nil 4% 41% remainder

max.

In calculating the quantity of fluorspar required, it is necessary to allow for the fluoride losses. In fuel-fired tank furnaces these \vill be about one half of the F 2 added; in cold-top electric furnaces about one quarter. It is evolved probably as H 2 SiF 6 , and the 8i0 2 thus lost must be allmved for in the calculation by replacing it. Note that no CaO is lost: it ends up as CaO in the glass. 4.3.7. Use of 8odi'u.m su.lphate For melting in a fuel-fired tank furnace it is normal practice to add 3-4 kg of sodium sulphate for a mix designed to give 1000 kg of glass. The purpose is fm it to act as fining agent and to assist in dissolving any residual particles of silica which may have floated to the glass surface. The specification for sodium sulphate is: N a 2 S0 4 NaCl A1 2 03 Fe203 I-hO H 2S04

95%

3.0 0.03 0.16 0.4 2.0

max. max. max. max. max.

It is suitable in the form of granular powder of a. particle size of about 150 I'm. Sodium sulphate is not used in E glass batches melted in the Pochet furnace or in electrically-boosted unit melters, as sulphates attack the molybdennm electrodes.

"'----'---

-'---'~--~~"=-=-

42

Glass manufacture

E glass usually contains about 0.3-0.4% or iron oxide. Because of the infra-red emission and absorption characteristics that iron oxides impart to a glass, its presence assists the fibre forming process. Usually, enough iron oxide is introduced as impurities in the raw materials; should this not provide enough Fe203 in the glass, extra should be added in the form pulverised iron oxide. It is important that, once a level of iron oxide has been established which is judged satisfactory, then it should be held constant since changes in iron oxide content will affect the fibre forming process.

4·3.9. The recycling of waste glass fibre It is common practice throughout the glass industry to return waste clean glass

back into the manufacturing process by charging it into the glass melting furnace together with batch. As broken glass it is generally referred to as 'cullet'. 'With increasing levels of collection of glass containers of all types the proportion of cuHet for recycling into the glassmaking process is steadily increasing and has reached levels of over 80% in some cases; the glass quality does not seem to be any the worse for it. The quantity of waste fibre produced in glass fibre manufacturing operations can vary widelYl depending on the products being made and the degree of mechanisation used in the plant; it can be assumed to lie somev.rhere between 10 and 25% of the glass which was originally melted. This glass, as E glass, contains between 5 and 8% B 2 0 3 and is therefore potentially valuable as a raw material. Furthermore, it must be expected, in line with experience in the glass industry in general, that the use of 'E glass cullet' would bring significant economic advantages to the mclting of E glass, i.e. more rapid melting, lower melting temperatures, and a saving in energy requirements, quite a.part from the saving in raw materials. In addition, in the case of waste fibre, there would be Cl. considerable saving in the cost of disposing of this very bulky material. Much work has had to be done to investigate the use of waste glass fibre as a. raw materia.l for making E glass. This arises from the fact that waste glass fibre from E glass operations is usually coated with organic material, i.e. the fibre size with possibly, mat binder. Although it is recognised that the organic materials would burn off before any reactions between batch constituents took place, i.e. before the batch reached 650-700°C, they could, in so doing, create local reducing conditions during the glass melting operation. This could, in turn, afTect the redox equilibrium of the glass, in particular the equilibrium between Fe203 and FeO in the glass and, thereby, the infra-red emission of the glass, a factor of considerable importance in fibre forming. The picture at the moment is not a little confusing, but economic pressure to make use of waste fibre by recycling is such that it must be assumed it is only a question

._---_.------_.

Selection of raw materials for E glass manufaetU1'e

43

of time and, perhaps, care in devising technically sound recycling treatments, before the vast bulk of waste glass fibre will be recycled and/or used in some form. For well over a decade some companies in Japan have simply cut their waste fibre (other than from chopped strand mat) into lengths preferably between 20 and 30 mIll, dried it, the fed it separately into the batch charger just upstream from its entry into the furnace. In this way, waste fibre was mixed with batch [13]. In another case, also in Japan, the waste fibre was broken down wet in a grinder, excess water removed in a centrifugal separator until, with a water content of between 2 and 3%, the material was used as a batch raw material [14]. Bearing in mind the extremely high quality standards required of glass for fibre forming, the following approaches seem to take care to clean and purify waste fibre before using it as a gIassmaking raw material. In one example, from the United States, the waste fibre is chopped wet, then drained, passed through an oven to burn of[ all organic matter, ground to give a free-flowing pmvder, and transferred to a silo in the batch mixing department; from that point onwards, the waste fibre is handled exactly like all other raw materials, i.e. a controlled proportion of waste fibre is included in each batch mix [15]. A similar process seems to have been used in Japan [16]. If batch is pelletised, ground \vaste fibre can be included in it [17]. An alternative approach proposed (perhaps used) in the United States consists of burning ofl the organic matter on the fibre, then melting the fibre and passing the molten glass into the glassmaking furnace [18]. It is difficult to obtain a clear picture of the present situation and technology. There are several companies which include recycled v.'aste fibre in the batch up to a maximum of 15%; it is claimed that, above 15%, the glass quality, as determined by fibre forming efficiency, suffers. Sometimes, the quality suffers at levels even below 15% and manufacturers have felt forced to reduce the waste fibre content to lower figures. By preference, waste fibre from the fibre forming department is used as the amount of organic matter deposited on the fibre is low at this stage, and the majority of this is washed off by the various water sprays which cause water to flow through the waste continuously before its removal. The waste fibre treatment procedure, in general terms, includes the following steps: _ collection of waste fibre from the fibre forming department; this may involve raising the waste fibre out of a cellar; _ crudely cutting the waste into short length of 10-20 mm for easier handling and lower bulk; _ passing the waste fibre through a metal detector and ejector; _ burning off all organic matter at about 650-750°C in a furnace which provides easy access for air; this also changes the characteristics of the fibre in that it becomes very brittle; _ grinding of the clean fibres in a ball mill to give a free-flowing powder passing a 200 mesh; and

44

Glass manufacture

- storage or transfer to silos from where the ground waste fibre is used either as a batch constituent or fed at predetermined rates into the batch charger(s) of the melter. Even with these precautions, some companies, and this includes some large manufacturers, have ceased to recycle waste fibre as they found it lowered the quality of the glass for fibre forming, thus increasing rather than decreasing product costs. The reason for the deterioration of glass quality is believed to be contamination by organic matters rather then material picked up during the grinding process. One must conclude that waste fibre recycling will become established, even if some technical problems remain unresolved at this date. It could well be that, for critical products, such as fibre for fine yarns having filament diameters of 7 fJ,ll1 or less, glass made only from batch should be llsed; however, for less critical products such as are made into direct chopped fibres (1724 1',111) or ravings for weaving or winding (14-17 Inn), or even chopped strand lllat and continuous strancl mats (11-13 /"m), recycling ohvastefi bre could well become standard practice.

4.4. Handling, weighing, and mixing of raw materials into batch

Batch is the mixture of weighed raw materials carefully blended to give homogeneous glass of the intended composition. For obvious reasons batch preparation is a critical operation in glassmaking and the following conditions must be met: (1) The raw materials (and this includes recycled waste fibre, if used) must be received, stored and handled in such a "lay that their compositions are known when they are used, and that segregation according to particle size is avoided. In cases where the composition can vary from one delivery to the next, arrangements must be made for each delivery to be stored separately. In any case, each delivery must be analysed or, better, the supplier should be under an obligation to provide a certified chemical and particle size analysis with each shipment. (2) The weighing of each material must be carried out to a degree of accuracy which ensures that the properties of the glass, and that means its composition remains within specified limits. (.3) The handling of raw materials must not release dust into the atmosphere. This is an objective which can only be met in a fully automated batch plant in which all materials are handled in totally enclosed systems. Depending on the location of the plant this is not always possible; at least some of the materials will be received in bags which have to handled, even if by fork lift truck. However, all ba.tch plants should be designed \vith the objective to be c1ust-free, and constant vigilance must be applied to ensure cleanliness and good housekeeping: if not, conditions will soon arise which will undermine control over the glassmaking

Handling, weighing, and mixing of mw materials into batch

45

process, quite apart from the very unsatisfactory working conditions that this creates. (4) The batch once made must be conveyed and stored and then passed to the batch charger of the furnace in such a way that segregation of raw materials is avoided.

These conditions nlUst be applied regardless of the scale of the plant. Fortunately, most industrial countries have had experience with these problems and specialist engineering companies exist which can meet these objectives. Each complete batch plant must be designed for the specific task bearing in mind possible future expansion of the plant, and the availability and cost of labour, land and buildings. In industrial countries with plants targeted for 50 to 200 tons of E glass per clay, fully automated batch plants are used \vhich receive raw materials in bulk in large trucks or rail cars, and transfer them to their respective storage silos. The silos are designed for easy discharge without segregation which is achieved by providing a very steep cone at the silo outlet, the included angle of the cone being 700 or less. The necessary quantities arc ,vithdrawn from the silos, weighed~ and collected in a batch mixer, then mixed and tra,nsferred to a batch silo above the batch charger. This is equipped with level indicators which, when the signaJ is given, will cause another batch to be mixed and transferred to the silo. The whole operation is computer-controlled so that changes in composition of raw materials can be entered into the programme which then makes the necessary changes to the batch formulation. In less industrialised countries, at least some of the rav,T materials can only be obtained in bags. vVhile there is no reason why some stocks or inventory cannot be carried in bags, carefully stored on pallets, it is best to transfer these materials before use into a silo by means of a bagslitter coupled to a mechanical or pneumatic elevator. Thereafter the weighing and mixing process can proceed as described above, Various types of mixers can be used provided they are sealed to prevent leakage of dust. Horizontal cylindrical mixers fitted with plows rotating on a central horizontal shaft and pan mixers with plows rotating horizontally around a central vertical shaft are used successfully. More recently, air blenders have come into use; they are, in principle, a blow umk which receives the unmixed raw materials. Compressed air is injected to mix the materials, which is effected very quickly; the mixed batch is then transferred from the blender (acting as a blow tank) pneumatically to the batch silo near the furnace. Apa,rt from the effective mixing tha,t this tank provides, it also has the advantage that there are no mechanical parts that can wear and the whole mixing and transporting system is sealed (see fig. 4.1). Pneumatic transfer and storage have to be designed to avoid batch segregation. For this reason, transfer into silos is done by a dense phase pneumatic conveying system in which small amounts of air move large amounts of powdered materia.l in closely associated plugs through a conveyor line.

-nlo glass fib1'C

this, the fibre made must be kept separate throughout the production processes as, otherwise, the wrong types of fibre are incorporated into a product. In most cases, one fibre size looks very much like another, so do the cake.'3. Therefore the fibre size, cakes and, possibly, intermediate products need to be coded. One solution is to colour code the fibre size, marking mixing tanks, supply lines, fibre forming position, cake tubes, cake carriers, etc. all with the same colour. In this approach, the risk of using a tube of the wrong colour code is the most critical and calls for effective management and operator appreciatioll of the critical importance of this point. The quantity of fibre size deposited can be checked simply by taking a sample of strand (say, 100 metres), drying it at HO°C to constant weight, then burning off the fibre size at 620-640°C; the change in \V'eight gives the fi bre size content. It is best to take a sample from a cake immediately after it has been made and reject the outer 200 meters; in this manner the effect of any fibre size migration can be avoided. As far as the fibre size itself is concerned, it is not easy to check that it has been prepared correctly. Hence the fibre size preparation department is the first to be blamed for any mistakes. The solids content of fibre sizes can be checked easily by taking a small sample and evaporating the water at 110°C; in some cases, hydrometers can also be used to ascertain the specific gravity. Assurance that the mix itself consists of the right raw materials, correct.ly weighed a.nd mixed., relies heavily on the conscientiousness of the operator. Independent weight checks of every component for each fibre size mix go a long way to ensure reliability in this area.

5.10.2. Water content The water content of a dried cake is not easy to determine. In the first place, water will tend to remain in the interior of the cake. To take a sample from there is not satisfactory, as the act of unwinding will evaporate a large part of the water. In practice, it is more satisfactory to control the cake weight and the drying process. Generally speaking, the maximum cake weight should be sHch that, for direct reinforcing fibres, the thickness does not exceed 25 mm in most cases. For yarn fibres, which are conditioned rather than dried, the control of weight is related to the weight of yarn per bobbin to be made and the fact that joints in yarns are generally not allowed. The weight is therefore set to give fibre for one or more bobbins plus an allowance for starting\vaste. Control of cake weight is simple if automatic winders are used. By trial and error the correct number of revolutions per cake is preset so that, except for a few cakes where fibre forming, for some reason, was terminated prematurely, all cakes will have the same weight and thickness. With manual winders, a time can be set so that, after it has passed, a light indicates that the forming of a cake is complete and that a new one has to be started. Since winding should not be stopped until the operator can effect the change-over, this, too, relies on the operators to react promptly to the signal.

Control of the fibre forming process

225

The drying process depends on the type of fibre; consistency between batches can be ensured by controlling time, temperature and relative humidity. The optimum conditions are determined by trial and error for each type. The final water content is more critical for some type of products than others. Generally, for direct reinforcing fibre, a residual water content must be limited as water acts as a temporary plasticiser, thus changing the handling characteristics of the strand, e.g. a strand which should be springy, turns out to be limp; a maximum of 0.05% is usually acceptable. For yarn fibre, the water content can lie anywhere between 0.5 and 2.0%; it is also possible to provide final drying on the yarn machines while unwinding the cake (see Section 7.7). The problems of residual water content can all be resolved or brought under control by the use of di,electric drying (see Section 5.9.1 above).

5.10.3. Splitting efficiency

This factor is important in the manufacture of chopped strand mats and certain types of ravings. Subdivision of strands is necessary whenever chopped fibres are distributed randomly whether in the manufacture of mat, or by simultaneously chopping and spraying the chopped strands with resins in open lay-up moulding. By splitting the strands a much more uniform layer of chopped strands can be obtained even by manual techniques. Split strands (see also Section 5.5.2) are made when a group of filaments, instead of being gathered into a single strand by passing over a guide, are passed through a number of grooves, thus giving a multiplicity of strands, and these are wound up substantially in parallel on the collet of a winder or the mandrel of a direct-roving winder. It is possible to achieve 100% splitting efficiency if the separate substrands are never allowed to meet during cake winding; in practice this means using a winder with a non-reciprocating traverse and a reciprocating collet (see Section 5.6.1.2). Control of efficiency of splitting is necessary whenever the process is such that 100% splitting is not inherent in the process, and this means, in most cases. The problem is that, as the split strands pass over the traverse and are thrown to and fro, there is a tendency for one or more substrands to meet at the moment of reversal of direction and stick together for some distance; this reduces the number of substrands that are created after chopping. The efficiency of splitting is governed by the geometry of the fibre forming set-up, the type of traverse being used, the tension of the fibre during winding, and the type of winder (stationary or reciprocating collet) being used. Generally speaking, the lower the tension, and the nearer to the traverse it is possible to place a guide to keep the substrands apart, the better the splitting efficiency. The splitting efficiency can be defined as the ratio of the number of sllbstrands actually formed compared to the number that should have been formed. The determination involves counting the number of substrands in a sample of known weight.

226

The conversion of glass into glass fibre

Thus if tex of whole strand: sample weight: strand is intended to be split: chopping length: number of substrands found in sample: then the number of substrands should be: and the splitting efficiency is:

T

g/km

lV 5 L N

0' b

times mm

HFFV 5 LT NLT Cl S !O.

]()4Hi

5.10·4. Control of tex (or' count)

The control of tex is carried out by a. simple mcasurcnwnL ll