6 High-Impact Polystyrene (HIPS) (Toughened Polystryenes (TPS))

438 Plastics Bused on Styrene

Very many copolymers with styrene as the principal constituent have been

prepared and a number have been marketed. In some instances there is an

appreciable increase in toughness but usually in such cases the softening point of

the copolymer is much lower than that of the homopolymer. For example

copolymers from 70 parts of styrene and 30 parts butadiene, although tough, are

leather-like. Such materials have in the past been used primarily as stiffening

resins in rubber compounds, it generally being considered that if sufficient

butadiene were present in the polymer to raise toughness sufficiently then the

softening point would be unacceptably low. One exception to this rule is the

range of materials first marketed by Phillips as BDS K-resin. These are a special

type of block copolymer dealt with later.

The materials now classified as high-impact polystyrenes, or with increasing

frequency as toughened polystyrenes, are combinations of polystyrene with

rubbery materials. The early grades that were first developed in the 1950s used

hot polymerised SBR (see Chapter 11) but for technical reasons discussed later

SBR has now been largely replaced by solution polymerised polybutadiene

rubber (also see Chapter 11). More recently, there has been interest in the use of

SBS triblock thermoplastic rubbers for this purpose. Because of the presence of

main chain double bonds both rubbers are subject to oxidation, with the

consequence that high-impact polystyrene polymers prepared using such diene

rubbers become brittle on prolonged exposure to sunlight. Materials showing

improved weathering resistance have been developed in Japan using EPDM

rubbers and similar materials have now been developed elsewhere.

The polystyrene and the rubber may be blended in a number of ways.

Originally the ingredients were compounded in a two-roll mill, in an internal

mixer or in an extruder. The impact strength of the products was, however, little

better than the unmodified polymer. Blending of SBR latex and polystyrene

latex, which is then followed by co-coagulation and drying, has also been

employed in the past but once again the improvement is only marginal.

Today the common practice is first to dissolve the rubber in the styrene

monomer and then to polymerise the styrene in the usual way. By this process the

resultant blend will contain not only rubber and polystyrene but also a graft

polymer where short styrene side chains have been attached to the rubber

molecules. This gives a marked improvement in the impact strengths that can be

obtained.

It has been demonstrated that with SBR polystyrene blends the rubber

should exist in discrete droplets, less than 5 0 p m in diameter where a good

finish is required, within the polystyrene matrix. It is believed that in such a

form the rubber can reduce crack propagation and hence fracture in various

ways." The most favoured current explanations of this were discussed in

Chapter 3. Suffice it to say here that the following features appear necessary

for a suitable blend:

(1) The rubber and the polystyrene should not be compatible. If they are there

will be molecular mixing and no improvement in toughness.

( 2 ) The rubber should not be too incompatible if good rubber-polystyrene

adhesion is to be obtained.



In effect this means that, to achieve reasonable toughness, semicompatible

rubbers should be used. Semicompatibility may be achieved (a) by selecting

mixtures of slightly different solubility parameter from the polystyrene, (b) by



High-impact Polystyrenes (HIPS) (Toughened Polystyrenes (TPS)) 439

judicious amounts of cross-linking or (c) by judicious use of selected graft

polymers. In current commercial grades it is probable that all three features are

involved.

In commerical SBR-based polymers there are three main variables to be

considered:

(1) The amount of SBR added, usually 5-20% (see Figure 16.12).An increase

in the SBR will increase the toughness but there will be an attendant

reduction in softening point.

( 2 ) The size of the rubber particles. In a typical blend these would be in the range

1-10 pm. Grades containing large particles (and which contain up to 10%

acrylonitrile residues in the matrix polymer) are used where good stress

cracking resistance is required. Small particles are used where high gloss,

toughness and stiffness are priority requirements.

(3) The gel content (toluene insoluble per cent) of the rubber and the swelling

index of the gel (the ratio of the volume of a swollen gel to its unswollen

volume). The former is a measure of the amount of cross-linked material and

the second a measure of the intensity of cross-linking. It has been found"

that a sample of medium gel content (5-20%) and a medium swelling index

(10-20) gives the best impact strength in the blend.



A high-impact polystyrene (polystyrene SBR blend) may have seven times the

impact strength of ordinary polystyrene, but about half the tensile strength, a

lower hardness and a softening point some 15°C lower. Because of the rubber

content there may be a reduction in light and heat stability and stabilisers are

normally incorporated.

The use of stabilisers (antioxidants) may, however, have adverse effects in

that they inhibit cross-linking of the rubber. The influence of phenolic

antioxidants on polystyrene-SBR alloys blended in an internal mixer at 180°C

has been studied. It was found that alloys containing 1% of certain phenolic

antioxidants were gel-deficient in the rubber phase. l2 The gel-deficient blends

were blotchy in appearance, and had lower flow rates compared with the

normal materials, and mouldings were somewhat brittle. Substantial improvements in the impact properties were achieved when the antioxidant was added

later in the mixing cycle after the rubber had reached a moderate degree of

cross-linking.



5



0



SBR CONTENT IN BLEND IN%



Figure 16.12. Effect of adding SBR on the impact strength of polystyrene



440 Plastics Based on Styrene

SBR has now been superseded by high cis-1,4-polybutadine because of the

greater effectiveness of the latter.l 3 , I 4 Reasons given for this greater effectiveness

include a better balance of the compatibility-incompatibility factors, the lower

glass transition temperature of the rubber (-100°C instead of -55°C for SBR),

greater resilience than SBR and the higher reactivity towards grafting. In addition

the polybutadiene is not contaminated with the large amounts of soap associated

with emulsion polymerised SBR and so leads to better gloss and lower moisture

pick-up. For the best results a typical poly-1,4-butadiene would have a high cis

content (for minimum T g ) , low gel (<0.05% for maximum gloss) and low

molecular weight (Mooney Viscosity of approx. 38) to facilitate dissolution and

would be water-white and free of residual traces of catalyst fragments such as

titanium which may cause severe discolouration. High-impact materials with a

degree of transparency have been prepared from 1:2-polybutadiene. In this case

the double bond is present in a pendant vinyl group rather than in the main chain.

It is thus more reactive and unless the grafting process is camed out at

temperatures below 100°C and under carefully controlled conditions excess

cross-linking may occur.

Transparent toughened polystyrene polymers are produced by blending

polystyrene with SBS block copolymers (see Section 11.8). During the 1970s and

1980s most development was with block copolymers with a radial (or star) shape.

Two types were developed: block copolymers with a central butadiene block, and

block copolymers with a central polystyrene block.

More recently Fina Chemicals have introduced linear SBS materials

(Finaclear) in which the butadiene is present both in block form and in a mixed

butadiene-styrene block. Thus comparing typical materials with a total styrene

content of about 75% by weight, the amount of rubbery segment in the total

molecule is somewhat higher. As a result it is claimed that when blended with

polystyrene the linear block copolymers give polymers with a higher impact

strength but without loss of clarity.

Tough transparent sheet may be produced by blending standard polystyrene

with block copolymer in an extruder in the ratios 80:20 to 20:80, depending on

the application of the products subsequently thermoformed from the sheet. For

example, sheet for thermoforming an egg tray will not require the same level of

impact strength as that required for jam jars.

Optical properties of the blends are somewhat dependent on the molecular

weight of the polystyrene, presence of additives such as lubricant in the

polystyrene, ratio of polystyrene to SBS, processing conditions and mixing

effectiveness of the extruder. It is stated that the optical properties of the sheets

are similar whether linear or radial type stereoblock polymers are used.

A number of polyi~oprenes'~

have also been investigated as potential

toughening agents. By careful control of grafting conditions high-impact

blends have been made from 3:4-polyisoprenes, Neither natural rubber

(essentially cis-1,4-polyisoprene) nor the other synthetic polyisoprenes give a

significant reinforcing effect. Ethylene-propylene rubbers which have been

peroxidised by bubbling oxygen through a solution of the polymer whilst

irradiating with ultralviolet light may also toughen polystyrene. The peroxidised rubber is dissolved in styrene monomer in the concentration range of

5 2 0 % and the solution reacted at above 70°C. The peroxide or hydroperoxide groups present then decompose with the formation of free radicals

which initiate growth of a polystyrene branch on the chain of the rubber

molecule.



ABS Plastics 441

16.7 STYRENE-ACRYLONITRILE COPOLYMERS

Styrene-acrylonitrile copolymers (-20-30% acrylonitrile content) have been

commercially available for a number of years. Initially, however, the price of

these materials was too high for them to find more than a few specialised outlets.

Because of the polar nature of the acrylonitrile molecule these copolymers have

better resistance to hydrocarbons, oils and greases than polystyrene. They also

have a higher softening point, a much better resistance to stress cracking and

crazing and an enhanced impact strength and yet retain the transparency of the

homopolymer. The higher the acrylonitrile content the greater the toughness and

chemical resistance but the greater the difficulty in moulding and the greater the

yellowness of the resin. Typical resins have a water absorption about that of

poly(methy1 methacrylate), Le. about ten times that of polystyrene but about onetenth that of cellulose acetate.

Some typical properties of styrene-acrylonitrile plastics, referred to in many

countries as SAN, are compared with those of other styrene-based plastics in

Table 16.7.

The market for SAN has remained small relative to that for polystyrene and

ABS (discussed in the next section) and is probably only about 5% that of the

latter. Major producers are BASF, Dow, Monsanto and Montedison.

The important features of rigidity and transparency make the material

competitive with polystyrene, cellulose acetate and poly(methy1 methacrylate)

for a number of applications. In general the copolymer is cheaper than

poly(methy1 methacrylate) and cellulose acetate, tougher than poly(methy1

methacrylate) and polystyrene and superior in chemical and most physical

properties to polystyrene and cellulose acetate. It does not have such a high

transparency or such food weathering properties as poly(methy1 methacrylate).

As a result of these considerations the styrene-acrylonitrile copolymers have

found applications for dials, knobs and covers for domestic appliances, electrical

equipment and car equipment, for picnicware and housewares, and a number of

other industrial and domestic applications with requirements somewhat more

stringent than can be met by polystyrene.

SAN is also used for pharmaceutical and cosmetic packaging. Usage

breakdown for Western Europe in the early 1990s has been estimated at 28% for

household products, 21 % for domestic electrical applications, 8% for battery

casings, 12% for pharmaceutical/cosmetic packaging and a large figure of 3 1%

for other applications.

Glass-reinforced grades of SAN exhibit a modulus several times that of the

unfilled polymer and, as with other glass-filled polymers, a reduced coefficient

of thermal expansion and lower moulding shrinkage. The materials are thus of

interest on account of their high stiffness and dimensional stability.

One unusual but nevertheless important application of SAN has been in the

manufacture of polymer polyols used in the manufacture of flexible polyurethane

foams. Proportions of up to 40% of the polyol may be used to increase stiffness

as foam bulk densities are lowered (see Section 27.5.4).

16.8 ABS PLASTICS

Although tough enough for many uses, styrene-acrylonitrile copolymers are

inadequate in this respect for other purposes. As a consequence, a range of



442 Plastics Based on Styrene

materials popularly referred to as ABS polymers first became available in the early

1950s. Since that time the ABS polymers have become well established, with

production now of the order of 3 X 10' tonnes per annum and thus in tonnage terms

only surpassed by the 'big four'; polyethylene, polypropylene, PVC and

polystyrene. In particular, the material has become of considerable importance for

quality equipment housings. The reasons for its widespread acceptance are:



(1) High impact resistance.

( 2 ) Good stiffness.

( 3 ) Excellent surface quality.

(4) High dimensional stability at elevated temperatures.

(5) Good chemical resistance.

( 6 ) Good stress cracking resistance.

Its main disadvantages are:



(1) Lack of transparency.

( 2 ) Poor weathering resistance.

(3) Poor flame resistance.



16.8.1 Production of ABS materials

The term ABS was originally used as a general term to describe various blends and

copolymers containing acrylonitrile, butadiene and styrene. Prominent among the

earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN)

(which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery).

Such materials are now obsolete but are referred to briefly below, as Type 1

materials, since they do illustrate some basic principles. Today the term ABS

usually refers to a product consisting of discrete cross-linked polybutadiene rubber

particles that are grafted with SAN and embedded in a SAN matrix.

The Type 1 materials may be produced by blending on a two-roll mill or in an

internal mixer or blending the lattices followed by coagulation or spray drying.

In these circumstances the two materials are compatible and there is little

improvement in the impact strength. If, however, the rubber is lightly crosslinked by the use of small quantities of peroxide the resultant reduction in

compatibility leads to considerable improvements in impact strength (see Figures

16.14 and 16.15).A wide range of polymers may be made according to the nature

of each copolymer and the proportion of each employed." A typical blend would

consist of

7 0 parts (70:30 styrene-acrylonitrile copolymer)

40 parts (63:35 butadiene-acrylonitrile rubber).



By altering theses variables, blends may be produced to give products varying

in processability, toughness, low-temperature toughness and heat resistance.

Although the nitrile rubbers employed normally contain about 35% acrylonitrile the inclusion of nitrile rubber with a higher butadiene content will increase

the toughness at low temperatures. For example, whereas the typical blend cited

above has an impact strength of only 0.9 ft Ibf in-' notch at O'F, a blend of 70

parts styrene-acrylonitrile, 30 parts of nitrile rubber (35% acrylonitrile) and 10

parts nitrile rubber (26% acrylonitrile) will have an impact value of 4.5 ftlbf in-'

notch at that temperature. I '

From Figure 16.13 it will be seen that a minimum of about 20% cross-linked

nitrile rubber is required in order to obtain tough products. For high-impact



ABS Plastics



443



NITRILE RUBB


Figure 16.13. Variation in impact strength against concentration of rubber in ABS Type 1 blends"



material the acrylonitrile-styrene copolymers should have a high molecular

weight. The commercial copolymers containing 20-30% acrylonitrile are

suitable for the preparation of Type 1 ABS polymers. The amount of gelled

rubber also has a profound effect on the strength, as shown in Figure 16.14. The

samples used were prepared by milling the rubber and peroxide for 15 minutes,

the styrene-acrylonitrile resin then being added and blended for a further 15

minutes. The physical nature of the blend is more complex than with the SBRmodified polystyrene, as it appears that rubber molecule networks may exist in

the resin phase.



0'



Figure



I

I

1.0

20

T-BUTYL HYOROPEROWIDE CONTENT IN



%



0



Effect of cross-linking agent on impact strength of 75/25 styrene-acrylonitrile/ nitrile

rubber blend.



To produce the Type 2 polymers, styrene and acrylonitrile are added to

polybutadiene latex and the mixture warmed to about 50°C to allow absorption

of the monomers. A water-soluble initiator such as potassium persulphate is

then added to polymerise the styrene and acrylonitrile. The resultant materials

will be a mixture of polybutadiene, polybutadiene grafted with acrylonitrile

and styrene, and styrene-acrylonitrile copolymer. The presence of graft

polymer is essential since straightforward mixtures of polybutadiene and

styrene-acrylonitrile copolymers are weak. In addition to emulsion processes

such as those described above, mass and mass/suspension processes are also

of importance.



444 Plastics Based on Styrene

The resulting polymers may vary in the following respects:

(a)

(b)

(c)

(d)

(e)



SAN-rubber ratio;

the styrene to acrylonitrile ratio in the SAN component;

the amount of grafted SAN;

rubber particle size and particle size distribution;

cross-link density of the rubber;

(f) use of modified styrenes such as a-methyl styrene to increase heat deflection

temperatures;

(g) use of saturated rubbers instead of polybutadiene to improve

weatherability.



It is obvious that the range of possible ABS-type polymers is very large. Not

only may the ratios of the three monomers be varied but the way in which they

can be assembled into the final polymer can also be the subject of considerable

modifications. Neither is it necessary to be restricted to the use of acrylontirile,

butadiene, and styrene. Because of the wide range of products available and

because the chemical nature of these materials is rarely divulged it is not possible

to give detailed properties of these materials unless one resorts to lists of

properties of named proprietary materials. In general, however, these materials

have a high impact strength, have softening points as high as, and sometimes

higher than, general purpose polystyrene, and moulded specimens generally have

a very good surface appearance. Some typical properties of ABS polymers

compared with other styrene-containing polymers are given in Table 16.5.

In recent years there has been an increased demand for a variety of special

ABS grades, for example products with improved flame retardancy.

Improvements in flame retardancy have been met in two ways:



(1) By the use of fire-retardant additives.

( 2 ) By blending ABS with PVC.

Bromine compounds are often used as flame retardant additives but

15-20pts phr may be required. This is not only expensive but such large levels

lead to a serious loss of toughness. Of the bromine compounds, octabromodiphenyl ether has been particularly widely used. However, recent concern about

the possibility of toxic decomposition products and the difficulty of finding

alternative flame retarders for ABS has led to the loss of ABS in some markets

where fire retardance is important. Some of this market has been taken up by

ABS/PVC and ASA/PVC blends and some by systems based on ABS or ASA

(see Section 16.9) with polycarbonates. Better levels of toughness may be

achieved by the use of ABSPVC blends but the presence of the PVC lowers the

processing stability.

There is also a demand for glass-clear forms of ABS since standard forms are

opaque. This can be achieved by matching the refractive indices (p,) of the rubber

particle, graft and matrix phases and eliminating any additives or polymerisation

ingredients that may cause opacity. A variety of approaches have been used

including the use of SBR or carboxy-terminated NBR to modify the refractive

index of the rubber phase and to use terpolymers of methyl methacrylate, styrene

and acrylontrile instead of the normal SAN. In such applications it is important

that the refractive index match should be over the operating temperature

range.



s?



vi



446 Plastics Bused on Styrene

There has also been a demand in recent years for an ABS-type material with

an enhanced heat distortion temperature. In ABS polymers this is largely

controlled by the Tg of the resin or glassy component. Consequently three

approaches to raising the distortion temperature have been developed. They

are:

(1) Replacement, in full or in part, of the styrene by a monomer whose

homopolymer has a higher Tg than polystyrene. In the case of partial

replacement it is also important that the copolymers have Tgs intermediate to

the two homopolymers. Fortunately this is usually the case.

(2) The addition of third monomers that enhance T g .

(3) Blending the glassy phase polymer with another polymer of higher Tg such

as a polycarbonate.

The first approach has been important commercially. The monomer most

commonly used is a-methylstyrene (see Section 16.11), whose polymer has a Tg

of about 120°C. The heat distortion temperature of the resultant-ABS type

polymer will depend on the level of replacement of styrene by the a-methylstyrene. (It may be noted in passing that a-methylstyrene-acrylonitrile binary

copolymers have been available as alternatives to styrene-acrylonitrile materials

but have not achieved commercial significance.)

The second approach is typified by maleic anhydride. This material does not

homopolymerise but will polymerise with styrene or styrene and acrylonitirle, in

the latter case to give terpolymers with Tg above 122°C.

Blends of ABS with polycarbonates have been available for several years (e.g.

Bayblend by Bayer and Cycoloy by Borg-Warner). In many respects these

polymers have properties intermediate to the parent plastics materials with heat

distortion temperatures up to 130°C. They also show good impact strength,

particularly at low temperatures. Self-extinguishing and flame retarding grades

have been made available. The materials thus provide possible alternatives to

modified poly(pheny1ene oxides) of the Noryl type described in Chapter 21. (See

also sections 16.16 and 20.8.)

The process of blending with another glassy polymer to raise the heat

distortion temperature is not restricted to polycarbonate, and the polysulphones

are obvious candidates because of their higher T g .One blend has been offered

(Arylon T by USS Chemicals) which has a higher softening point than the ABSpolycarbonates.

Blending of ABS with other polymers is not restricted to the aim of raising the

distortion temperature. Blends with PVC are made for various purposes. For

example, 80:20 ABSPVC blends are used to produce fire-retarding ABS-type

materials, as already mentioned, while 10:90 blends are considered as impactmodified forms of unplasticised PVC. ABS materials have also been blended

with plasticised PVC to give a crashpad sheet material.

Blending of ABS with an acrylic material such as poly(methy1 methacrylate)

can in some cases allow a matching of the refractive indices of the rubbery

and glassy phases and providing that there is a low level of contaminating

material such as soap and an absence of insoluble additives a reasonable

transparent ABS-type polymer may be obtained. More sophisticated are the

complex terpolymers and blends of the MBS type considered below. Seldom

used on their own, they are primarily of use as impact modifiers for

unplasticised PVC.



ABS Plastics



447



16.8.2 Processing of ABS Materials

The processing behaviour of ABS plastics is largely predictable from their

chemical nature, in particular their amorphous nature and the somewhat

unpleasant degradation products. The main points to bear in mind are:



(1) ABS is more hygroscopic than polystyrene. (It will absorb up to 0.3%

moisture in 24 hours.) It must therefore be dried carefully before moulding

or extrusion.

(2) The heat resistance in the melt is not so good as that of polystyrene and

unpleasant fumes may occur if the melt is overheated. This can occur at the

higher end of the processing range (250-260°C) and when high screw speeds

and high back pressures are used when injection moulding. Volatile

decomposition products can also lead to bubbles, mica marks (splay marks),

and other moulding defects. The problem is often worse with flame-retarding

grades. It is usual to purge the material at the end of a run.

(3) The flow properties vary considerably between grades but some grades are

not free flowing. Flow path ratios in the range 80 to 150:1 are usually quoted,

generally being lower with the heat-resistant grades.

(4) Being amorphous, the materials have a low moulding shrinkage

(0.044-0.008 cm/cm).

One particular feature of the material is the facility with which it may be

electroplated. In order to obtain a good bond the ABS polymer is first treated by

an acid etching process which dissolves out some of the rubber particles at or

near the polymer surface. After sensitisation and activation electroless metal

deposition processes are carried out. Much of the strength between the ABS and

the plating depends on a mechanical press-stud type of effect. It is commonly

observed that low peel strength usually arises not through failure at the interface

but in the moulding just below the surface. It would seem that the greater the

molecular orientation in such regions the lower the interlayer forces and hence

the lower the peel strength.



16.8.3 Properties and Applications of ABS Plastics

Because of the range of ABS polymers that may be produced, a wide range of

properties is exhibited by these materials. Properties of particular importance are

toughness and impact resistance, dimensional stability, good heat distortion

resistance (relative to the major tonnage thermoplastics), good low-temperature

properties and their capability of being electroplated without great difficulty.

Several classes of ABS which show the above general characteristics but with

specific attributes are recognised. One supplier for example classifies ABS

materials into the following categories:

general purpose grades

fire retardant grades

improved heat resistance grades

enhanced chemically resistant grades

static dissipation grades

extrusion grades

fire retardant extrusion grades



448 Plastics Based on Styrene

transparent grades

electroplating grades

blow moulding grades.

Over the years there has been some difference in the balance of use between

UPVC and ABS in the United States compared with Western Europe. This was

due largely to the earlier development in Western Europe of UPVC and in the

United States of ABS. Thus, for example, whilst ABS consolidated its use for

pipes and fittings in the United States, UPVC was finding similar uses in Europe.

Whilst some of these traditional differences remain, ABS is now well established

in both Europe and the United States.

As well as unplasticised PVC, ABS also finds competition from polypropylene. In recent years polypropylene has been the cheaper material on a tonnage

basis and even more economic on the more relevant volume basis. On the

other hand the properties listed above, in particular the extreme toughness and

superior heat distortion resistance, lead to ABS being preferred in many

instances. Because ABS, typically, has a higher flexural modulus than

polypropylene, mouldings of the latter will have to a wall thickness some

15-25% greater in order to show an equal stiffness. It is also interesting to

note that because of its higher specific heat as well as possessing a latent heat

of fusion, polypropylene requires longer cooling times when processing (see

Section 8.2.3). Applications of ABS are considered in more detail in Section

16.16.



16.9 MISCELLANEOUS RUBBER-MODIFIED

STYRENE-ACRYLONITRILE AND RELATED COPOLYMERS

The commercial success of ABS polymers has led to the investigation of many

other polyblend materials. In some cases properties are exhibited which are

superior to those of ABS and some of the materials are commercially available.

For example, the opacity of ABS has led to the development of blends in

which the glassy phase is modified to give transparent polymers whilst the

limited light aging has been countered by the use of rubbers other than

polybutadiene.

Notable among the alternative materials are the MBS polymers, in which

methyl methacrylate and styrene are grafted on to the polybutadiene backbone.

This has resulted in two clear-cut advantages over ABS. The polymers could

be made with high clarity and they had better resistance to discolouration in

the presence of ultraviolet light. Disadvantages of MBS systems are that they

have lower tensile strength and heat deflection temperature under load.

The MBS polymers are two-phase materials, with the components being

only partially compatible. It is, however, possible to match the refractive

indices providing the copolymerisation is homogeneous, i.e. copolymers

produced at the beginning of the reaction have the same composition as

copolymers produced at the end. Such homogeneity of polymerisation appears

to be achieved without great difficulty. The poor aging of ABS appears to be

due largely to oxidative attack at the double bonds in the polybutadiene

backbone. Methyl methacrylate appears to inhibit or at least retard this process

whereas acrylonitrile does not.