Plastics Materials 7 Episode 16 ppsx

There have, however, been developments in three, quite unrelated, areas where the author has considered it more useful to review the different polymers together, namely thermoplastic ela

Shellac 867

The importance of shellac to the plastics industry has declined rapidly since

1950 Before that time it was the principal resin employed in 78 rev/min gramophone records The advent of the long playing microgroove record meant that mineral fillers could no longer be tolerated because any imperfections in the microgroove led to a high background noise on the record The record industry therefore turned towards alternative materials which required no mineral filler, and vinyl chloride-vinyl acetate copolymers eventually became pre-eminent It

is, however, still used for a number of purposes outside the normal realm of plastics

30.6.1 Occurrence and Preparation

Shellac is the refined form of lac, the secretion of the lac insect parasitic on certain trees in India, Burma, Thailand and to a minor extent in other Asian countries

The larvae of the lac insect, Luccifer lucca (Kerr), swarm around the branches and twigs of the host trees for 2-3 days before inserting their probosces into the phloem tissues to reach the sap juices There may be as many as 100-150 larvae

on each inch of twig This is followed by secretion of the lac surrounding the

cells Whereas the male insects subsequently move out of their cells the female insects become entombed for life After about eight weeks of life the male insects fertilise the females and die within a few days The fertilised females subsequently exude large quantities of lac and shed eyes and limbs The female gives birth to 200-SO0 further insects and finally dies

In commercial practice the crop is taken from the tree shortly before emergence of the new brood Some of these twigs are then tied to new trees to

provide future sources of lac but the rest, sticklac, is subjected to further

processing The average yield per tree is about 20 Ib per annum, usually one crop being allowed per tree per year

Subsequent treatment of the sticklac carried out by hand or by mechanical methods first involves removal of woody matter and washing to remove the associated lac dye to produce seedlac, containing 3 4 % of impurities This may

be further refined by various methods to produce the shellac flakes of commerce

The hand process for producing shellac has been used since ancient times and

is carried on largely as a cottage industry It has been estimated that 3-4 million people were dependent for their livelihood on this process The lac encrustation

is first separated from woody matter by pounding with a smooth stone, the latter being removed by a winnowing process The lac dye is then removed by placing the lac in a pot together with a quantity of water A villager, known as a ghasander, then stands in the pot and with bare feet treads out the dye from the resin At one time lac dye was of commercial value but is today a worthless by-

product The product, seedlac, is then dried in the sun

The next stage may best be described as a primitive hot-filtration process Two members of the village sit across the front of a simple fire resembling a Dutch oven, holding between them a bag about 30 feet long and about two inches in diameter The lac inside the bag melts and, through one of the operators twisting the end of the bag, the lac is squeezed out The lac is then removed from the outside of the bag and collected into a molten lump which is then stretched out

868 Miscellaneous Plastics Materials

by another operator using both hands and feet until a brittle sheet is produced This is then broken up to produce the shellac of commerce

In the factory processes the sticklac is first passed through crushing rollers and sieved The lac passes through the sieve but retains the bulk of the woody matter The sieved lac is then washed by a stream of water and dried by a current of hot air A second mechanical cleaning process removes small sticks which have not been removed in the earlier roller process The product, seedlac, now contains 3-8% of impurities

The seedlac may then be converted to shellac by either a heat process or by solvent processes In the heat process the resin is heated to a melt which is then forced through a filter cloth which retains woody and insoluble matter In the solvent process the lac is dissolved in a solvent, usually ethyl alcohol The solution

is filtered through a fine cloth and the solvent recovered by distillation

Variation in the details of the solvent processes will produce different grades

of shellac For example, when cold alcohol is used, lac wax which is associated with the resin remains insoluble and a shellac is obtained free from wax Thermally processed shellacs were greatly favoured for gramophone records as they were free from residual solvent and also contained a small quantity of lac wax which proved a useful plasticiser

30.6.2 Chemical Composition

The lac resin is associated with two lac dyes, lac wax and an odiferous substance, and these materials may be present to a variable extent in shellac The resin itself appears to be a polycondensate of aldehydic and hydroxy acids either as lactides

or inter-esters The resin constituents can be placed into two groups, an ether- soluble fraction (25% of the total) with an acid value of 100 and molecular weight of about 550, and an insoluble fraction with an acid value of 55 and a molecular weight of about 2000

Hydrolysis of the resins will produce aldehydic acids at mild concentration of alkali (-iN); using more concentrated alkalis (5N) hydroxy acids are produced, probably via the aldehydic acids Unfortunately most of the work done in order

to analyse the lac resin was carried out before the significance of the hydrolysis conditions was fully appreciated It does, however, appear to be agreed that one

of the major constitutents is aleuritic acid (Figure 30.9)

30.6.3 Properties

The presence of free hydroxy and carboxyl groups in lac resin makes it very reactive, in particular to esterification involving either type of group Of particular interest is the inter-esterification that occurs at elevated temperatures (>7OoC) and

Shellac 869

leads to an insoluble 'polymerised' product Whereas ordinary shellac melts at about 75"C, prolonged heating at 125-150°C will cause the material to change from a viscous liquid, via a rubbery state, to a hard horny solid One of the indications that the reaction involved is esterification is that water is evolved The reaction is reversible and if heated in the presence of water the polymerised resin will revert to the soluble form Thus shellac cannot be polymerised under pressure

in a mould since it is not possible for the water to escape 'Polymerisation' may

be retarded by basic materials, some of which are useful when the shellac is subjected to repeated heating operations These include sodium hydroxide, sodium acetate and diphenyl urea 'Polymerisation' may be completely inhibited by esterifying the resin with monobasic saturated acids A number of accelerators are

also known, such as oxalic acid and urea nitrate Unmodified lac polymerises in about 45 minutes at 150°C and 15 minutes at 175°C

Shellac is soluble in a very wide range of solvents, of which ethyl alcohol is most commonly employed Aqueous solutions may be prepared by warming shellac in a dilute caustic solution

The resin is too brittle to give a true meaning to mechanical properties The thermal properties are interesting in that there appears to be a transition point at 46°C Above this temperature, specific heat and temperature coefficient of expansion are much greater than below it The specific heat of hardened shellac

at 50°C is lower than that of unhardened material, this no doubt reflecting the disappearance, or at least the elevation, of the transition temperature

Table 30.4 Some properties of shellac

3 0 T 80°C

Some typical physical properties of shellac are given in Table 30.4

30.6.4 Applications

Until 1950 the principal application of shellac was in gramophone records The resin acted as a binder for about three times its weight of mineral filter, e.g slate

870 Miscellaneous Plastics Materials

dust The compound had a very low moulding shrinkage and was hard-wearing but not suitable for the microgroove records because of the effect of the filler on the background noise

Today the most important applications are in surface coatings, including some use as French polish, as adhesives and cements, including valve capping and optical cements, for playing card finishes and for floor polishes The material also continues to be used for hat stiffening and in the manufacture of sealing wax Although development work on shellac in blends with other synthetic resins has been carried out over a period of time, the only current use in the plastics industry is in the manufacture of electrical insulators At one time electrical insulators and like equipment were fabricated from mica but with increase in both the size and quantity of such equipment shellac was introduced as a binder for mica flake For commutator work the amount of shellac used is only 3-5%

of the mica but in hot moulding Micanite for V-rings, transformer rings etc., more than 10% may be used The structures after assembly are pressed and cured, typically for two hours at 150-160°C under pressure

In recent years the dominance of shellac in mica-based laminates has met an increasing challenge from the silicone resins

30.7 AMBER

In addition to shellac a number of other natural resins find use in modern industry They include rosins, copals, kauri gum and pontianak Such materials are either gums or very brittle solids and, although suitable as ingredients in surface coating formulations and a miscellany of other uses, are of no value in the massive form, i.e as plastics in the most common sense of the word

One resin, however, can be considered as an exception to this Although rarely recognised as a plastics material it can be fabricated into pipe mouthpieces, cigarette holders and various forms of jewellery It may also be compression moulded and extruded It is the fossil resin amber

Amber is of both historical and etymological interest as its property of attracting dust was known over 2000 years ago From the Greek word for amber,

elektron, has come the word electricity Pliny in his works makes an interesting and informative dissertation on the occurrence and properties of amber Amber is a fossil resin produced in the Oligocene age by exudation from a now extinct species of pine It occurs principally in the region known before World War I1 as East Prussia It may be obtained by mining and also by collecting along the seashore Small amounts of amber may also be found off the coasts of England, Sweden, Holland and Denmark Similar resins are found in Burma, Rumania and Sicily but only the Baltic variety, known also as succinite, is considered a true amber At one time a Royal Amber Works existed in Konigsberg (now Kaliningrad) and in 1900 annual production was approximately

500 tons

30.7.1 Composition and Properties

The chemical nature of amber is complex and not fully elucidated It is believed

not to be a high polymer, the resinous state being accounted for by the

complexity of materials present The empirical formula is C10H160 and true amber yields on distillation 3-8% of succinic acid

Bituminous Plastics 871 The resin is fairly soluble in alcohol, ether and chloroform and is decomposed

by nitric acid It becomes thermoplastic at temperatures above 150°C and decomposes at a temperature rather below 300"C, yielding an oil of amber and leaving a residue known as amber colophony or amber pitch

X-ray evidence shows the material to be completely amorphous as might be expected from such a complex mixture The specific gravity ranges from 1.05 to 1.10 It is slightly harder than gypsum and therefore just not possible to scratch with a fingernail Yellow in colour, it is less brittle than other hard natural resins and may therefore be carved or machined with little difficulty The refractive index is 1.54

Amber has been a much prized gem material for many millennia and has been found at Stonehenge, in Mycenaen tombs and in ancient European lake dwellings In modern times it is used for beads and other ornaments, cigarette holders and pipe mouthpieces

At one time the small fragments of amber produced during the fabrication and machining operations were used to produce varnishes In 1880 they were first

used in the production of Ambroid This is made by pressing the fragments in a hydraulic press at temperatures somewhat above 160°C The moulded product has a close resemblance to amber A form of extrusion has also been used to produce amber rods for subsequent conversion into pipe and cigarette-holder mouthpieces

30.8 BITUMINOUS PLASTICS

Although generally ignored in plastics literature the bituminous plastics are still

of interest for specific applications The moulding compositions consist of fibrous and mineral fillers held together by a bituminous binder together with a number of minor ingredients

A number of types of bituminous material exist and terminology is still

somewhat confusing The term bitumens in its widest sense includes liquid and

solid hydrocarbons but its popular meaning is restricted to the solid and semisolid materials The bitumens occur widely in nature and may be considered to be derived from petroleum either by evaporation of the lighter fraction under atmospheric conditions or by a deeper seated metamorphism The purer native

bitumens are generally known as asphaltites and include Gilsonite, extensively

used for moulding, which occurs in Utah

Where the bitumens are associated with mineral matter the mixture is

referred to as native asphalt These are widely distributed in nature, the best

known deposit being the asphalt lake in Trinidad which covers an area of about

100 acres (40 hectares) The terms asphalt or asphaltic bitumen are applied to

petroleum distillation residues and these today form the bulk of commercial bituminous matter Related chemically and in application but not in origin are

the pitches These are the industrial distillation residues They inc!ude wood

tar, stearin pitch, palm oil pitch and coal tar pitch The last varies from soft semisolid to hard brittle products Of these materials those most useful in moulding compositions are coal tar pitches with a softening range of llS-130"C and natural bitumens such as Gilsonite and Rafaelite with softening points in the range 130-160°C

The bitumens are complex mixtures of paraffinic, aromatic and naphthenic hydrocarbons A small amount of unsaturation is usually present which accounts

872 Miscellaneous Plastics Materials

for the slow oxidation which occurs on exposure to ultra violet light and the ability to bring about a form of vulcanisation on heating with sulphur

The bulk of bituminous materials are used for road making and building applications which are outside the scope of this book Only a very small percentage is used in moulding compositions and few data have been made publicly available concerning the properties of these compositions

The bitumens have a good order of chemical corrosion resistance, have reasonably good electrical insulation properties and are very cheap Their main disadvantages are their black colour and their somewhat brittle nature

Moulding compositions contain a number of ingredients These may include:

(1) Bituminous binder

(2) Fibrous filler

(3) Inert filler

(4) Softener

(5) Drying oil and drier

Of the fibrous fillers which greatly reduce the brittleness, blue asbestos fibre

is normally used for battery boxes, the principal outlet Other materials that may

be used include cotton fibres, ground wood, slag wool and ground cork Mineral fibres are incorporated to reduce cost and to raise the softening point China clay, natural silicas, talc and slate dust are frequently used

To facilitate moulding a softener is incorporated These may include soft industrial pitches or heavy tars, coumarone-indene resins or waxes

In the United States softer stocks have been employed using a drying oil which

is incorporated with a drier such as cobalt naphthenate to harden the oil The compositions are mixed in heated trough mixers, the mixing tem- perature being in the range of 150-200°C Skill is required in order to achieve good dispersion of the fibrous filler without charring the butuminous matter Moulding is carried in compression moulds using prewarmed doughs For battery boxes the mould temperature on charging the composition is about 1OO"C, which is reduced to at least 50°C before extraction of the moulding Some simple mouldings can be carried out using prewarmed mixes but cold moulds

The largest outlet for the bituminous plastics has been for automobile battery boxes Bituminous battery boxes do, however, have a susceptibility to electrical breakdown between the cells and in Europe their use has been mainly confined

to the cheaper batteries installed initially in new cars Bituminous compositions have also been used for toilet cisterns and to some extent for cheap containers They are no longer important

Bibliography

Casein

COLLINS, J H , Casein Plastics, Plastics Institute Monograph No C5, 2nd Edn, London (1952)

PINNER, s H., Brit Plastics, 18, 313 (1946)

SUTERMEISTER, F., and BROWNE, F L., Casein and its industrial Applications, American Chemical

Society, Monograph No 30, New York (1939)

Rubber, Derivatives of Rubber and Similar Polymers

BLOW, c M., and HEPBURN, c (Eds.), Rubber Technology and Manufacture (2nd Edition),

Butterworths, London (1982)

Bibliography 873

BRYDSON, I A., Rubber Chemistry, Applied Science, London (1978)

BRYDSON, J A,, Rubbery Materials and their Compounds, Applied Science, London (1988)

DAVIES, B L., and GLAZER, J., Plastics derived from Natural Rubber, Plastics Institute Monograph No

NAUNTON, w J s (Ed.), The Applied Science of Rubber, Arnold, London (1961)

SCOTT, J R., Ebonite, MacLaren, London (1958)

Shellac

CIDVANI, B s Shellac and Other Natural Resins, Plastics Institute Monograph No S1, 2nd Edn

Shellac, Angelo Brothers Ltd., Calcutta (1956)

Amber

HERBERT SMITH, G F., Gemstones, Methuen, London (1952)

LEY, WILLY, Dragons in Amber, Sidgwick and Jackson, London (1951)

Pliny, Book 37, Chapter 3

C8, London (1955)

London ( 1954)

31

Selected Functional Polymers

3 I 1 INTRODUCTION

Chapters 10 to 29 consisted of reviews of plastics materials available according

to a chemical classification, whilst Chapter 30 rather more loosely looked at plastics derived from natural sources It will have been obvious to the reader that for a given application plastics materials from quite different chemical classes may be in competition and attempts have been made to show this in the text There have, however, been developments in three, quite unrelated, areas where the author has considered it more useful to review the different polymers together, namely thermoplastic elastomers, biodegradable plastics and elec- trically conductive polymers

All three types of material have now been available for some years and it is probably also true that none have yet realised their early promise In the case of the thermoplastic elastomers most of the commercial materials have received brief mention in earlier chapters, and when preparing earlier editions of this book the author was of the opinion that such materials were more correctly the subject

of a book on rubbery materials However, not only are these materials processed

on more or less standard thermoplastics processing equipment, but they have also become established in applications more in competition with conventional thermoplastics rather than with rubbers

The concept of degradable polymers arose largely from concern about the large quantities of plastics materials used for packaging and which, having fulfilled their function, were then discarded and unwanted Interest has, however, now moved on to include medical and related applications

Electrically conductive polymers are just one of a number of esoteric possible uses for synthetic polymers These materials are now being considered for a variety of applications

It was pointed out in Chapter 3 that conventional vulcanised rubbers were composed of highly flexible long chain molecules with light cross-linking

874

Thermoplastic Elastomers 875 (covalently) which enabled the chains to coil and uncoil but prevented them slipping past each other This gives a highly extensible network structure Once cross-linking has taken place, such materials cannot be reprocessed (at least without severe degradation) In addition, the ‘setting’ operation involves a chemical reaction rather than the, apparently, simpler setting brought about by cooling as used with thermoplastics materials

For this reason, many attempts have been made over the years to produce a rubbery material which has a network structure over a useful temperature range but which, if heated further, loses this structure In many cases this involves a

form of cross-linking that is said to be heatfugitive In Section 3.4 four types of

heat-fugitive cross-link were identified, namely:

of showing rubbery behaviour The explanation lies in the fact that the morphology is more complex, and it is better to consider the hard phase as a reticulated structure which allows large-scale deformations to occur (rather like that in a human skeleton), with the rubbery phase facilitating the recovery from deformation

The main commercial types of thermoplastic elastomers are:

( I ) Styrene-butadiene-styrene triblocks and the related S-I-S and SEBS

(2) Polyester-based thermoplastic polyurethane elastomers (Section 27.4)

( 3 ) Polyether-based thermoplastic polyurethane elastomers (Section 27.4) (4) Thermoplastic polyester elastomers (Section 25.10)

(5) Thermoplastic polyamide elastomers (Section 18.15)

(6) Thermoplastic polyolefin rubbers (Section 11.9)

materials (Section 11.8)

It may also be argued that plasticised PVC may be considered as a thermoplastic elastomer, with the polymer being fugitively cross-linked by hydrogen bonding via the plasticiser molecules These materials were, however, dealt with

extensively in Chapter 12 and will not be considered further here The ionomers

are also sometimes considered as thermoplastic elastomers but the commercial materials are considered in this book as thermoplastics It should, however, be kept in mind that ionic cross-linking can, and has, been used to fugitively cross- link elastomeric materials

This section is intended to summarise some basic principles of the chemistry

of such materials and to compare the various types

876 Selected Functional Polymers

Although there will be specific requirements for specific applications, the principal properties of importance with the thermoplastic elastomers are: (1) The minimum temperature at which the material will be a serviceable (2) The maximum service temperature

(3) Oil resistance and chemical resistance

(4) The range of hardness possible

(5) Recovery from deformation and general high-elasticity properties

(6) Density

(7) Cost, not simply raw material cost, but the cost of making, installing and rubber

servicing the product

The minimum service temperature is determined primarily by the Tg of the soft phase component Thus the SBS materials can be used down towards the T g of the polybutadiene phase, approaching -100°C Where polyethers have been used

as the soft phase in polyurethane, polyamide or polyester, the soft phase Tg is about -6O"C, whilst the polyester polyurethanes will typically be limited to a minimum temperature of about 4 0 ° C The thermoplastic polyolefin rubbers, using ethylene-propylene materials for the soft phase, have similar minimum temperatures to the polyether-based polymers Such minimum temperatures can also be affected by the presence of plasticisers, including mineral oils, and by resins if these become incorporated into the 'soft' phase It should, perhaps, be added that if the polymer component of the soft phase was crystallisable, then the

higher T , would also affect the minimum service temperature, this depending on

the level of crystallinity

It should also be pointed out that the Tg of the soft blocks, which consist of fairly short polymer chains, will be somewhat lower than for a corresponding homopolymer of high molecular weight, for the reasons given in Section 4.2 This effect may, however, be more than compensated by the loss of molecular freedom due to the presence of and interaction with the hard phase polymer present

Providing the polymer is thermally stable in the range under consideration, the maximum service temperature is largely determined by the Tg of the hard phase (or the T , if the hard phase is crystallisable) As a general rule, thermoplastic elastomers with a crystallisable hard phase will be usable to higher temperatures, although some amorphous thermoplastic polyamide elastomers with a high hard phase Tg can have good maximum service temperatures As pointed out in the

previous paragraph, the effective Tg (and for that matter T,) of the short polymer

blocks may differ somewhat from the transition points of a high molecular weight homopolymer

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase If the soft phase is non-polar but the hard phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR) If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer

will be lost As with all plastics and rubbers, the chemical resistant will depend

on the chemical groups present, as discussed in Section 5.4

Most of the thermoplastic elastomers can be produced in a wide hardness range without resort to additives If it is practical to use soft and hard phases in any proportions, then the hardness range will be from that of the soft phase

Thermoplastic Elastomers 817

polymer to that of the hard phase polymer This may not always be possible, since, for example, a minimum level of hard phase component may be necessary for the elastomer to have acceptable recoverable elasticity Unfortunately, there

is no word in the English language describing a condition between ‘hard’ and

‘soft’ and one can only talk about ‘degrees or levels of hardness (or softness)’ The polymer technologist (and more specifically this writer) has a problem in classifying a number of the harder grades of the so-called thermoplastic elastomers which are more like leather than traditional rubber or thermoplastic

Properties such as low permanent set, low creep and low hysteresis are really measures of the efficiency of the ‘heat fugitive’ network system This is a

complex function of the morphology As a very general statement, the problem

would seem to be less important with the harder grades of thermoplastic elastomer

The density of the polymer will clearly depend on the density of the soft phase (usually low), and the density of the hard phase (generally higher with crystallisable polar blocks) and the ratio of the soft and hard phases present It will also clearly depend on the additives present and to some extent on the processing conditions, which may affect the crystalline morphology

Table 31.1 Comparative properties of commercial thermoplastic elastomers

Tg (“C) T, or T, (“C) resistance range gravity

60-90A 30-40A 65-75A 70A-70D 40-90A 75A-65D 60A-75D 35-75D

0.94 0.92 0.91 1.18-1.24 1.1 1.15-1.45

1 O-1.15 0.9-1.1

Approximatelcost Relative to SBS

Special features

~ ~~~ ~~~~

S-B-S or (S-B),x

S-EB-S 200 Improved aging and weathering

Polyester-urethane 200 Abrasion and oil resistance

pol yether-urethane 200 Better hydrolytic stability and resistance to fungal Polyester 250 Similar to PU but can be harder

100 Low density and cost Good chemical resistance

growth than above but less good abrasion resistance

Better at low temperatures Low compression set and creep and less variation of modulus with temperature

Polyamide 250-450 Similar to PU but can he softer

Very good at low temperatures Wide range of products and wide range of properties possible

Notes:

( I ) Data are given for unfilled polymers

(2) Transition data are very dependent on the method of measurement

(3) Hardness values are for Shore A and Shore D

878 Selected Functional Polymers

The basic cost of thermoplastic elastomers is often higher than that of more conventional and comparable rubbers and plastics However, there can be considerable savings in their use It is clearly a positive feature that the materials will not need complex vulcanising processes taking time and energy and also that they may be processed on conventional thermoplastics material, with scrap being capable of being recycled There have also been examples of industrial and auto uses where a part had previously been made by impregnating cloth with rubber (frictioning), assembling layers of the impregnated cloth and then compression moulding the assembly The cloth had been used to give a level of stiffness of which the rubber compound was on its own incapable Replacement of the traditional rubber with a hard grade of a thermoplastic elastomer can enable the reinforcing fabric to be dispensed with and production to become a compar- atively simple one-stage operation (It should be stressed that the use of cloth or other laminating material may still be required if the product is required to have different levels of stiffness in different directions.)

31.2.1 Applications of Thermoplastic Elastomers

In general, the thermoplastic elastomers have yet to achieve the aim of replacing general purpose vulcanised rubbers They have replaced rubbers in some specialised oil-resistant applications but their greatest growth has been in developing materials of consistency somewhat between conventional rubbers and hard thermoplastics A number of uses have also been developed outside the field

of conventional rubber and plastics technology

A manufacturer considering using a thermoplastic elastomer would probably

first consider one of the thermoplastic polyolefn rubbers or TPOs, since these tend to have the lowest raw polymer price These are mainly based on blends of polypropylene and an ethylene-propylene rubber (either EPM or EPDM) although some of the polypropylene may be replaced by polyethylene A wide range of blends are possible which may also contain some filler, oil and flame retardant in addition to the polymers The blends are usually subject to dynamic vulcanisation as described in Section 11.9.1

TPOs have found a number of mechanical rubber goods applications, particularly in the automotive industry Examples are convoluted bellows, flexible diaphragms, protective sleeving, steering gear boots, extruded profiles, torque couplings and tubing There has also been a considerable market for these materials for car bumpers, rather more in Europe than America Other major applications include wire insulation and weather stripping TPOs are of interest where low manufacturing cost and good weathering properties are of more importance than a high level of oil resistance and heat (deformation) resistance TPOs are one of the two most widely used thermoplastic elastomers

The other ‘most widely used’ thermoplastic elastomers are the S-B-S and

related materials These are usually blended with large amounts of a stiffening resin (polystyrene), mineral oils, fillers and even other polymers so that the amount of S-B-S in the finished compound is usually less than 50% In the area

of replacements for commercial vulcanised rubber, the S-B-S compounds are

largely used for shoe soling, tubing, sound deadening and flexible automotive parts The related hydrogenated polymers (S-EB-S) are of particular interest for wire insulation and other applications where enhanced aging and weathering properties are required

Thermoplastic Elastomers 879

S-B-S, S-I-S and S-EB-S polymers are widely used in adhesive, sealing and coating formulations as well as being important additives to many asphalt formulations

Thermoplastic polyurethane elastomers have now been available for many years (and were described in the first edition of this book) The adipate polyester- based materials have outstanding abrasion and tear resistance as well as very good resistance to oils and oxidative degradation The polyether-based materials are more noted for their resistance to hydrolysis and fungal attack Rather specialised polymers based on polycaprolactone (Section 2.5.1 1 ) may be considered as premium grade materials with good all round properties

Whilst approximately twice the raw material cost of TPO- and S-B-S-type polymers, thermoplastic polyurethane elastomers find applications where abrasion resistance and toughness are particular requirements Uses include gears, timing and drive belts, footwear (including ski boots) and tyre chains Polyether-based materials have also achieved a number of significant medical applications There is also some minor use as hot melt adhesives, particularly for the footwear industry

Thermoplastic polyester elastomers such as the Du Pont product Hytrel were developed later than the polyurethane materials, being first introduced in 1972 They have similar characteristics to the polyurethanes but there is an upward shift

in the hardness range (i.e the softest grades are not so soft, but the hardest grades are harder than the corresponding extreme grades in the polyurethanes)

In spite of their cost the thermoplastic polyester rubbers have found use as replacement for oil-resisting rubbers such as the polychloroprenes, not simply because of their improved processability but also because of such superior physical properties as tear and tensile strength up to 150°C They tend to be somewhat easier to process than the thermoplastic polyurethane elastomers Applications include flexible couplings and diaphragms, convoluted bellows, ski boots, quiet-running gear wheels, high-pressure hose lines, outer coverings for wire and optical fibre cables, seals and segmented tracks for snow vehicles Thermoplastic polyamide elastomers first became available in 1978 and have many features similar to those of the polyurethane and polyester thermoplastic elastomers They are block copolymers with an oil-resistant hard phase polymer There is, however, a considerable flexibility in formulation Either polyesters or polyethers may be used for the soft phase and a wide range of polyamides for the hard block These can include crystallisable polymers such as nylons 6, 66, 610,

11 and 12, or amorphous aromatic materials which rely on a high Tg to give

polymers of good heat resistance Block copolymers with polyester soft blocks have excellent resistance to thermal and oxidative degradation whilst those with polyether soft segments are more resistant to hydrolysis and remain rubbery down to lower temperatures Whilst moisture resistance is usually quite good, special hydrophilic products are available which can absorb up to 120% of their weight in water

The thermoplastic polyamide elastomers may be considered as premium grade materials available in a wide range of hardness values with, in some instances, very good heat resistance Particular properties of interest are the flexibility and impact resistance at low temperatures and the good dynamic properties and related resilience, hysteresis and alternating flexural properties

In addition to the above well-known groups of thermoplastic elastomer, other materials continue to be introduced One such material is Alcryn, introduced by

Du Pont in 1985 but of undisclosed composition This material exhibits the good

880 Selected Functional Polymers

weathering, ozone and heat resistance of TPOs but with enhanced oil resistance Other potentially important blends have been made by mixing nitrile rubber and polypropylene (and then subjecting the rubber to dynamic vulcanisation) and by reacting PVC and ionically cross-linked butadiene-acrylonitrile rubber It is perhaps worth restating at this point that the all-important plasticised PVC materials could reasonably be classified as thermoplastic elastomers

There remains considerable scope for producing novel block copolymers but these tend to involve sophisticated polymer science and any commercial products are likely to be more expensive than the easier-to-make blends They would thus need to possess some quite outstanding properties for them to be a commerical proposition

31.2.2 The Future for Thermoplastic Elastomers

Thermoplastic elastomers have now been available for over 30 years and the writer recalls organising a conference on these materials in 1969 In spite of considerable publicity since that time these materials still only comprise about 5-10% of the rubber market (equivalent to about 1-2% of total plastics consumption) It is important to appreciate that simply being a thermoplastic material (and hence being processed and reprocessed like a thermoplastic plastics material) is not enough to ensure widespread application Crucially the material must have acceptable properties for a potential end-use and at a finished product price advantageous over other materials

The styrenic and the TPO type elastomers share, very roughly equally, about 85% of the total market but in the case of the former over half the output goes into what might be considered ‘non-rubber’ applications such as bitumen and thermoplastic modifiers and for adhesives and coatings, with the bulk of the remainder being used in footwear On the other hand TPOs have achieved broader penetration particularly into the automotive market It is to be expected that growth in this area will exceed general economic growth for a number of reasons These include ‘recyclability regulations’ now existing in many countries, high polymer stiffness allowing thinner section products sometimes eliminating the need for fabric reinforcement (with the number of additional operations that this involves) and the ability to use processes such as blow moulding that are not appropriate to conventional rubbers The more expensive polyester, polyamide, polyurethane and other materials will continue to find niche markets where their properties justify their cost

In recent years there has rightly been a marked increase in concern for the environment The continual global population explosion together with the increase in purchasing power has led to a vast increase in the amount of pollution and rubbish Because of their visibility and inability to degrade at a reasonable rate, plastics materials have been particularly criticised

For more than 20 years, polymer scientists and plastics technologists have been working to develop plastics materials that would be more acceptable environmentally, and in the third edition of this book, published in 1975, the author devoted a section to photo- and biodegradation of polymers In spite of such effort, an article in 1992 stated that ‘Degradable plastics are still in the early

Degradable Plastics 881 stages of their technological and usage evolution’, and more recently in 1998 a spokesman for a leading polymer supplier could state that ‘the market for biodegradables is still in its infancy’ It has been subsequently forecast that the biodegradable polymer market should reach 70 000 t.p.a by 2001

Whereas cellulose films are biodegradable, that is they are readily attacked by bacteria, films and packaging from synthetic polymers are normally attacked at

a very low rate This has led to methods of degrading polymers to a sufficiently low molecular mass (typically about 10000) which are then accessible to biodegradation

Several approaches are used, either individually or collectively, to degrade polymers in this way Of these the most important are:

It is important that any photodegradation should be controlled It is essential that plastics should not degrade prematurely Possible approaches are to use photodegradants that are only activated by light waves shorter than those transmitted by ordinary window glass This will help to ensure that samples kept indoors will not deteriorate on storage Dyestuffs which change colour shortly before the onset of degradation could also be used to warn of impending breakdown

One example of the use of photodegradable polyethylene is for beverage can ring holders in North America

The author is unaware of any commerical polymers that are specifically designed to degrade oxidatively, although oxidation may be involved in association with hydrolytic and biological degradation It may be of interest to note that before World War I1 products known as rubbones were produced by degrading natural rubber with cobalt linoleate in the presence of cellulosic materials to produce low molecular weight, fluid oxidised natural rubber (Section 30.4)

An example of the hydrolysis approach is that used by Du Pont with their Biomax polymer This is a copolyester based on poly(ethy1ene terephthalate) technology, can therefore be produced on conventional PET polymerisation plant and is therefore only marginally more expensive than PET Film from this copolymer is claimed to have general physical properties similar to PET (see Chapter 25) other than a lower melting point of about 200°C and may be processed on standard equipment designed for use with PET Up to three different comonomers may be used according to the end-use and these are designed to provide weak points in the polymer chain making them susceptible

882 Selected Functional Polymers

A further approach is used by Bayer with their polyesteramide BAK resins A film grade, with mechanical and thermal properties similar to those of polyethylene is marketed as BAK1095 Based on caprolactam, adipic acid and butane diol it may be considered as a nylon 6-co-polyester An injection moulding grade, BAK 2195, with a higher melting point and faster crystallisation

is referred to as a nylon 66-co-polyester and thus presumably based on hexamethylene diamine, adipic acid and butane diol

Some typical properties of these two materials are given in Table 31.2

Mention should also be made here of the extensive use of poly(viny1 alcohol)

in potentially biodegradable applications At appropriate hydroxyl contents these polymers will dissolve in water (see Chapter 14) and can apparently be conveniently washed away after use as a water-soluble packaging Biodegrada- tion does, however, appear to be slow and first requires an oxidative step involving enzymatic attack to a ketone such as polyenolketone, which then biodegrades more rapidly

There has been considerable use of polyethylene film containing about 5-15%

of cornstarch for making dustbin bags The starch biodegrades rapidly, when for example it is composted under aerobic conditions In the late 1980s production

of over 50000 t.p.a was being reported More recently plastics with starch contents of 60-70% have been introduced using a synthetic polymer binder which is itself degradable

In 199 1 Rhone-Poulenc offered biodegradable cellulose acetate compounds in which an additive acts both as plasticiser and biodegrading agent (see Section 22.2.2.1)

There has been recent interest in lactic acid polymers and copolymers These materials are environmentally attractive in that renewal and cheap source materials such as potato waste and cheese whey may be used Such materials have been used for some time in degradable and resorbable surgical sutures but recent efforts have been directed at food packaging applications There is

Degradable Plastics 883 particular interest in the use of the materials as a biodegradable plastic coating for paper and board

Polycaprolactones (see also Section 25.1 I), although available since 1969, have only recently been marketed for biodegradable purposes Applications include degradable film, tree planting containers and slow-release matrices for pharmaceuticals, pesticides, herbicides and fertilisers Its rate of biodegradability

is said to be less than that of the polylactides

After many years of research initiated in the 197Os, IC1 introduced polyhydroxybutyrate-valerate copolymers in 1990 Because of their particularly interesting manufacturing technology, these materials are dealt with in depth separately in the next section

31.3.1 Polyhydroxybutyrate-valerate Copolymers (PHBV)

These copolymers are very unusual for a commercial plastics material in that they are produced by biochemical methods A fermentation process is used in which a bacterium or micro-organism Alcaligenes eutrophus is fed with a carbohydrate (usually glucose) This causes the bacteria to produce and accumulate 'bacterial fat' or energy-storing polyesters in their bodies At the end

of fermentation 80% of the weight of the bacteria is composed of the polyester, which is then 'harvested' by breaking open the cells and extracting and purifying the polymer

This is a copolymer consisting of hydroxybutyrate and hydroxyvalerate units incorporated randomly along the chain The hydroxyvalerate content may be varied by adding controlled amounts of a simple organic acid

hydroxybutyrate hydroxy valerate

(2) The presence or otherwise of a plasticiser

The hydroxybutyrate homopolymer has a T,, in the range 173-180°C and a Tg of about 5°C

The hydroxyvalerate (HV) content is usually in the range of about 5% to 12%

As might be expected from considerations of the relation of structure to properties given in Chapter 4 and 5, the ethyl side chain of the valerate unit will reduce chain packing and lower crystalline melting point, modulus and tensile strength and at the same time increase flexibility, impact strength and ductility This is shown in Figures 31.1, 31.2 and 31.3

884 Selected Functional Polymers

Figure 31.3 Effect of HV content on tensile strength

Degradable Plastics 885

It would seem that the two units are reasonably isomorphous in that the copolymers are crystalline although the higher hydroxyvalerate materials exhibit lower crystallisation rates All commercial grades contained a nucleating agent

to facilitate crystallisation and shorten processing cycles during moulding operations

Also, as might be expected, the use of plasticiser has a similar effect to that of increasing the hydroxyvalerate content It also has a more marked effect on depressing the glass transition temperature and therefore improves properties such as impact strength and ductility at lower temperatures

As with most polyesters, the polymers have quite good resistance to oils, particularly hydrocarbons, but are hydrolysed by acids and bases

Some typical properties are shown in Table 31.3

Table 31.3 Some selected properties of polyhydroxybutyrate-valerate copolymers

(Biopol-Zeneca)

high HV

MFI (ASTM 1238-906) 190OC (2.16kg load) 8 12

Izod impact strength (ft Ib/in) notch 1.12 3.15 6.74

The Biopol copolymers are fully degradable in a range of microbially active environments Because of the enhanced crystallinity due to the presence of nucleating agents, biodegradation commences at the surface of the polymer Enzymes degrade the resin into molecular fragments of hydroxybutyrate and hydroxyvalerate Under aerobic conditions these fragments break down into carbon dioxide and water, and under anaerobic conditions into carbon dioxide and methane Where polymer is immersed in an activated sewage sludge, according to the procedures of ASTM DS209 it is found that carbon dioxide evolution reaches 85% of its theoretical limit after 40 days In a composting test 90% biodegradation (as measured by carbon dioxide evolution) was achieved in 7-8 months Experiments have also been carried out with moulded bottles to study the effect of composting in industrial composting plant at 70°C Up to 80% weight losses were observed after just 15 weeks Bottles have also been tested under simulated managed landfill conditions A weight loss of about 50% after

40 weeks has been reported

There are two main points to bear in mind when processing PHBV plastics:

( 1 ) The limited thermal stability of the polymer, the polymer degrading rapidly above 195°C

( 2 ) The need to optimise conditions to allow a maximum rate of crystallisation and thus reduce cycle times The maximum rate of crystallisation is reported

to be at about SS-6O0C, which, interestingly, is significantly closer to the Tg

than the T , (see Section 3.3)

886 Selected Functional Polymers

Processing temperatures should not exceed 1 8O"C, and the duration of time that the material is in the melt state should be kept to a minimum At the end of a run the processing equipment should be purged with polyethylene When blow moulding, the blow pin and mould should be at about 60°C to optimise crystallisation rates Similarly, injection moulds are recommended to be held at 6Ok5"C

The low-HV unplasticised grades are the most critical to process, requiring the higher processing temperatures Conditions are slightly less critical with the higher HV and plasticised grades

The first applications for PHBV materials were for shampoo bottles in 1990 and since then they have been increasingly used for packaging of cosmetics and toiletries There is also interest as a packaging material for pharmaceuticals and for 'medical disposables' such as urine bags and surgical trays Of greater potential is the use of the material as a laminating coating for paper products such

as paper cups and packages for household detergents, so that the whole material will be biodegradable Other potential applications include disposable razors, plant pots and fishing nets In some of these applications it may be expected that PHBV will be in competition with other biodegradable materials such as the biodegradable cellulose acetate discussed in Section 22.2.2.1 and the polylactides and polycaprolactones referred to above

One noteworthy application announced in the Autumn of 1998 was the use of PHBV for biodegradable credit cards issued by at least one bank and one major conservation organisation

Although first marketed by Zeneca, a company split off from IC1 in 1993, under the trade name of Biopol, marketing was transferred to Monsanto in May

1996 In 1993 production capacity was 600t.p.a but prior to the Monsanto takeover had been expected to rise to 5000-1000Ot.p.a by the late 1990s However, in November 1998 Monsanto announced that it was discontinuing the Biopol programme

31.3.2 The Future for Degradable Plastics

In the past development and widespread acceptance of degradable plastics has been very restricted because of the rather poor all-round performance of the materials and the high premium cost inevitable with small-quantity production Furthermore, to an extent contrary and contradictory demands are made on these materials To be acceptable as a degradable material it should fit in with normal composting cycles, completely degrade within three months and be compostable

At the same time the material must be stable during storage, processing and the service life of the product To date these materials have penetrated little more than niche markets but there is now some evidence that the more recent polylactic acid polymers, the Du Pont Biomax copolymers and related copolymers and the polyesteramides will find uses because of their general properties and not simply on their degradability If this happens then the degradable plastics could become of significance

3 1.4 INTRINSICALLY ELECTRICALLY CONDUCTING POLYMERS (ICPS)

For many years it has been common practice to improve the electrical conductivity of plastics and rubbers by incorporating certain additives, such as special grades of carbon black Such compounds have been important, for

Intrinsically Electrically Conducting Polymers (ICPs) 881

example, in hospital operating theatres, where it was essential that static charges did not build up, leading to explosions involving anaesthetics

During the past 30 years considerable research has been undertaken that has led to electrically conducting polymers that do not rely on the use of fillers, the

so-called intrinsically conductive polymers Such polymers depend on the

presence of particles which can transport or carry an electric charge Two types may be distinguished:

(1) Ionically conductive polymers

(2) Intrinsically electronically conductive polymers

An example of an ionically conductive polymer is polyethylene oxide containing LiC104, which is used as a solid phase electrolyte in batteries

The intrinsically electronically conductive polymers are a result of the presence of an extensive system of conjugated double bonds with n-electrons, to which electrons may be either added or removed so that they exhibit an electric charge The conjugated backbones have low ionisation potentials and high electron affinities and they can be oxidised and reduced more easily than conventional polymers They may in fact conceptually be considered as polymeric salts The process of creating an excess charge involves treatment of the polymer with oxidising or reducing agents and, by analogy with semi- conductor technology, is referred to as doping The excess charge is in the range

of one per two to ten monomer units

The polymers which have stimulated the greatest interest are the polymers of acetylene, thiophene, pyrrole and aniline, poly-p-phenylene, polyphenylvinylene and poly-I ,6-heptadiyne Of these materials polypyrrole has been available from BASF under the trade name Lutamer PI60 since 1988

acetylene thiophene PYnoIe aniline

A variety of methods have been used to produce these polymers but the use

of chemical or electrochemical oxidative polymerisation has been particularly important Whilst the doping operation may follow the polymerisation stage,

in the case of polypyrrole an excess charge may be formed during the oxidative polymerisation stage Where doping is carried out subsequent to polymerisation, oxidising agents such as AsF5, SbF5, NOPF, and FeC1, have been used successfully

888 Selected Functional Polymers

The properties and applications of intrinsically conductive polymers have been reviewed (Frommer and Chance, 1986; Sauerer, 1991) The important poly- pyrolles have been separately reviewed (Jasne, 1988)

Whilst the conductivity of these polymers is generally somewhat inferior to that

of metals (for example, the electrical conductivity of polyacetylenes has reached more than 400 000 S/cm compared to values for copper of about 600 000 S/cm), when comparisons are made on the basis of equal mass the situation may be reversed Unfortunately, most of the polymers also display other disadvantages such as improcessability, poor mechanical strength, poor stability under exposure

to common environmental conditions, particularly at elevated temperatures, poor storage stability leading to a loss in conductivity and poor stability in the presence

of electrolytes In spite of the involvement of a number of important companies (e.g Allied, BASF, IBM and Rohm and Haas) commercial development has been slow; however, some uses have begun to emerge It is therefore instructive to review briefly the potential for these materials

Clearly, ICPs will not find application solely on their conductivity They do, however, offer an interesting range of properties in addition to conductivity, such as:

(1) Ability to store a charge

( 2 ) Ability to ion exchange

(3) Absorption of visible light to give coloured products

(4) Transparency to X-rays

Sauerer (1991) has listed possible applications for these polymers (Table 31.4)

Successful application will, however, depend on achieving the following features:

(1) Advantages in processing

(2) Better product properties

Table 31.4 Possible applications for ICPs

Electrodes Rechargeable batteries (accumulators) fuel cells, photoelectrochemical

cells, analytical sensors (pH, 0 2 , NO,, SOz, NH3, glucose), electrocardiography (ECG)

on ICPs (electron beam lithography), novel photoluminescent diodes

(LED), data storage (e.g spatially resolved electrochromics) Optic\ Electrochromic displays, optical filters (windows with adjustable

transparency), materials with non-linear optical properties

Reviews 889

(3) Lower price

(4) Greater product stability

(5) Demonstration of ecological advantages

(6) More efficient recycling

At the present time, doped ICPs are not normally capable of being processed like normal thermoplastics Processes usually involve high-pressure moulding of finely powdered polymers under vacuum or an inert gas However, modification

of some ICPs with, for example, alkyl or alkoxy side groups may produce soluble, and thus more tractable, polymers

The poor stability on exposure to air and water, particularly at elevated temperatures, which results in a reduction in conductivity, also poses problems

In the case of polypyrrole it has been found that conductivity can, however, be maintained either by the drastic measure of storing under the protective layer of the inert gas argon or embedding polypyrrole film in a matrix of an epoxide resin-glass-fibre composite

Somewhat more practical is the use of an ICP as a conductive filler instead of carbon black The addition of as little as 10% of carbon black will severely reduce the tensile strength, elongation at break and impact strength of polypropylene mouldings In contrast, up to 40% of polypyrrole will have little effect on tensile strength and also give a much higher impact strength than obtained with a carbon black-filled compound at only 10% black loading Such compounds are of interest in electromagnetic shielding, as are also laminated structures in which a polypyrrole film is sandwiched between protective non- conductive polymers

The ecological advantages of ICPs have made them of particular interest in the field of rechargeable batteries, since they do not involve heavy metals and do not appear to have any serious toxicological problems

As indicated in Table 31.4, the potential of ICPs is in somewhat esoteric applications In some instances the potential has reached commercial realisation For example, coating the walls of boreholes in circuit boards before electroplat- ing with copper involves fewer stages than with older established processes and

is claimed to be cost effective, faster and simpler ICPs are also now being marketed in Japan for use in solid electrolyte conductors

MOORE, J w., Modern Plastics, December, p 58 (1992)

Intrinsically conducting polymers

FROMMER, I E and CHANCE, R R., Encyclopedia of Polymer Science and Technology, Vol 5 , p 462,

Wiley, New York, (1986)

IASNE, s., Encyclopedia of Polymer Science and Technology, Vol 13, p 42, Wiley New York (1988)

w., Kunstofle, 81, 8 (1991)

by the wide range of plastics materials made from PVC It is convenient to consider materials based on a general chemical structure as a generic group, e.g polycarbonates It is also useful to consider distinctive variants of such a generic group as sub-generic groups In the case of polycarbonates one well-known software package identifies nine sub-generic groups for polycarbonates, including standard grades, structural foam, high flow, glass or carbon reinforced, PTFE lubricated, ultraviolet stabilised and fire retardant Even this is a considerable simplification, since in 1993 one manufacturer alone was offering

88 grades of polycarbonate (not including the important polycarbonate alloys with ABS)

Indeed in recent years there has been increased use of blends of polymers

as a less expensive way of extending the spectrum of properties available rather than that of developing highly specialised polymers Clearly the selection of material for a particular application is not a simple exercise Before discussing material selection it is important to appreciate the extent to which processing conditions can affect polymer properties Table 32.1 '

illustrates the range of values of selected properties that can be obtained with

a specific grade of material in a given injection mould simply by varying processing parameters such as melt temperature, injection time and injection pressure Such a variation may well be greater than the difference in properties shown between materials under consideration Intelligent use of the moulding process may well be more effective and more economical than the use of a more expensive material

890

Economic Factors Affecting Material Choice 89 1

Table 32.1 Range of values of physical properties of injection mouldings obtained by alteration of

moulding conditions (after Allen and Van Putte, 1974)2

Tensile load at failure (N)

Flexural strength (MPa)

Izod impact strength (ft I b h notch)*

Ball-drop impact strength (J)

Shrinkage across flow ( m m h m )

Shrinkage with flow ( m m h m )

Stiffness modulus (MPa)

578-898 698-1 120

0.113-6.666 5.197-13.558

0-0.005 0.007-0.016 0-0.006 0.0 10-0.014

~ ~

32.2 ESTABLISHING OPERATIONAL REQUIREMENTS

The first requirement must be to specify carefully the operational requirements of the item to be produced and of the material(s) to be used in its construction This

is seldom easy and is usually the most difficult part of the selection process The most common factors to be considered are:

( 1 ) Regulations and specifications There may be international (e.g EU), national or local regulations concerning the use of materials These may be with respect to factors such as flammability, contact with foodstuffs, environmental considerations and medical criteria

(2) Mechanical properties Properties commonly of importance here are rigidity, creep resistance, strength and toughness For some applications resistance to repeated flexing or abrasion may be critical

(3) Environmental operating conditions This will include such factors as operating temperatures, including duration of use at elevated temperatures, presence of water, solvents, oils and chemicals which may be reactive with the plastics materials (not just the polymer but also with the additives) and expected humidity levels

(4) Other requirements These could include electrical insulation properties, including resistance to tracking and arcing, transparency, frictional proper- ties, surface finish, scuff resistance and specific gravity

(5) Particular fabrication requirements, including the need to assemble or plate parts

Once such criteria have been established and a shortlist of possible materials drawn up, it will be important to consider:

(1) Processing problems associated with the shortlist materials

(2) Economics

32.3 ECONOMIC FACTORS AFFECTING MATERIAL CHOICE

In most instances there will be more than one material that will meet a technical specification, and choice will then be largely a matter of economics Since parts

892 Material Selection

are usually made by volume rather than by weight, a simple comparison of the price per unit weight, e.g &/kg or $/tonne, is quite misleading Figures for comparative volume cost are an important first requirement

It may also be the case that, of two materials, that with a higher volume cost may prove to be more economical One reason for this may be that the more expensive material may be stiffer and can thus be used in thinner section mouldings, so allowing material savings Simpler processing operations and conditions, including shorter downtimes and more economical purging require- ments, may also tilt the economic balance In the previous chapter mention was made of the replacement of fabric-reinforced vulcanised rubbers with easily moulded intrinsically stiff thermoplastic rubbers for diaphragms and other automotive parts Implications of possible differences in scrap rates should also

be considered

Sources for material data may be classified into three groups:

(1) Journals and textbooks (such as Plastics Materials!)

(2) Trade literature issued by raw material suppliers

(3) Computer-based information sources

The writer would suggest that the use of all three in combination would be synergistic

No textbook can provide data on all the materials available, nor can it ever be completely up to date It can, however, provide a useful background, helping the user to understand material behaviour It can guide the reader between different classes of materials and it can point out deficiencies in materials

Trade literature can provide a wealth of information Users should, however, bear in mind that suppliers will naturally wish to emphasise data in the best possible light For example, if the Izod impact strength increases sharply with decrease in sample thickness, then results may be quoted for thinner section test pieces Whilst the facts may be stated, the underlying significance may not be fully appreciated by the casual reader

Much trade literature is of a high standard, particularly that of suppliers of the so-called ‘engineering polymers’ In many cases these manufacturers supply a range of such polymer types and they provide much useful comparative material This may be in the form of descriptive material and tables of numerical data Suitable choice of graphs and other diagrams can often give the reader a more immediately absorbed visual comparison

One such graphical device about which this writer is equivocal is the polar diagram In this case a number of properties, e.g 6, are selected and the value of the property is indicated on a radial line The points are then joined up In some

cases, as in Figure 32.1, maximum and minimum values of the properties (which

vary between grades) is given Diagrams for three such materials are given in

Figure 32.1

Whilst the data presented in such diagrams is useful, particularly where many such diagrams may be inspected simultaneously, I do have certain reservations

The purpose of a graph or diagram is to provide an instant visual impact My

personal experience when I look at these diagrams is that the instant impact is of

Figure 32.1 Polar diagrams for three thermoplastic materials, CYCOLOY (a PC/ABS blend),

ULTEM (polyetherimide) and NORYL (a styrenic PPO) The shaded area indicates the range

available with different grades