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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
201
Thermoplastic chlorinated polyethylene is seldom used on their
own but primarily in blends with other polymers, particularly PVC. If
chlorination is taken to a level at which the polymer is only semicompatible with the PVC, a blend with high impact strength may be
obtained. In these circumstances the material is classified as an impact
modifier.
There have been some attempts to develop chlorinated
polyethylene elastomers. The rubbers possess such attractive properties as
very good oil, heat, flame, ozone, and weathering resistance and are also
available in a convenient powder form. In spite of being marketed at
competitive prices, the chlorinated polyethylene rubbers took some time to
become established, probably largely as a result of difficulties in curing the
materials. Today, however, they are of importance for cable sheathing and
are now also being investigated more generally in technical goods such as
hydraulic and radiator hoses, gaskets, seal caps, membranes and spark plug
caps. The major competition for these materials is in the chemically closely
related chlorosulphonated polyethylene (Hypalon - Du Pont).
2.2.3 Chlorinated PVC
The process of post-chlorinating PVC was carried out during
World War II in order to obtain polymers soluble in low-cost solvents and
which could therefore be used for fibres and lacquers. The derivate was
generally prepared by passing chlorine through a solution of PVC in
tetrachloroethane at between 50°C and 100°C. Solvents for the product
included methylene dichloride, butyl acetate and acetone. These mat erials
were of limited value because of their poor colour, poor light stability,
shock brittleness and comparatively low softening point.
In the 1960s materials became available which are said to have
been obtained by chlorination at lower temperatures. In one process the
reaction is carried out photochemically in aqueous dispersion in the
presence of a swelling agent such as chloroform. At low temperatures and
in the presence of excess chlorine the halogen adds to the carbon atom that
does not already have an attached chlorine. The product is therefore
effectively identical with a hypothetical copolymer of vinyl chloride and
symmetrical dichloroethylene. An increase in the amount of postchlorination increases the melt viscosity and the transition temperature.
Typical commercial materials have a chlorine content of about 66-67%
(c.f. 56.8% for PVC) with a Tg of about 110% (c.f. approx. 80°C for PVC).
The main applications for CPVC arise from the fact that the
202
material has a softening point of about 100% and very good chemical
resistance. Particular interest has been shown in waste and soil systems
which may pass hot water effluents. Calendered sheet may be vacuum
formed for uses where hot filling techniques are employed, for example in
jam packing.
2.2.4 High-impact Polystyrene (HIPS) (Toug hened Polystyrene (TPS))
For many applications polystyrene might be considered to be too
brittle a pol ymer. Because of this, polystyrene manufacturers have made a
number of attempts to modify their products.
The methods of approaching this problem include:
(1) Use of higher molecular weight polymers.
(2) Use of plasticizers.
(3) Incorporation of fillers such as glass fibre, wood flour, etc.
(4) Deliberate orientation of the polymer molecules.
(5) Copolymerization.
(6) The use of rubbery additives.
Of these methods the first gives only marginal improvements
whilst the second approach has far too severe an effect on the softening
point to be of any commercial value. The use of fillers has been practiced to
some extent in the United States but is not of importance in Europe.
Deliberate or ientation is limited to filament and sheet.
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.
The materials now classified as high-impact polystyrene, or with
increasing frequency as toughened polystyrene, are combinations of
polystyrene with rubbery materials. The early grades that were first
developed in the 1950s used hot polymerized SBR have now been largely
replaced by solution polymerized polybutadiene rubber. 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
203
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.
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 polymerize 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.
The following features are 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 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 involv ed.
In commerical SBR-based polymers there are three main variables
to be considered:
(1) The amount of SBR added, usually 5-20%. An incre ase 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 µm. 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
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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.
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 stabilizers are normally incorporated.
The use of stabilizers (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. 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.
SBR has now been superseded by high cis-l,4-polybutadine
because of the greater effectiveness of the latter. Reasons given for this
greater effectiveness include a better balance of the compatibilityincompatibility 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
polymerized SBR and so leads to better gloss and lower moisture pick-up.
For the best results a typical poly-l,4-butadiene would have a high cis
content (for minimum Tg), low gel (<0.05% for maximum gloss) and low
molecular weight to facilitate dissolution and would be water-white and
free of residual traces of catalyst fragments such as titanium which may
cause severe discolouration .
Transparent toughened polystyrene polymers are produced by
blending polystyrene with SBS block copolymers. 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 butadi ene is prese nt both in blo ck form
205
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.
2.2.5 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 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 3·106
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.
206
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, sin ce
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 cross-linked by the use of small quantities of peroxide the
resultant reduction in compatibility leads to considerable improvements in
impact strength. 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:
70 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 tempera tur es.
For high-impact 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.
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 polymerize the styrene and
acrylonitrile. The resultant materials will be a mixture of polybutadiene,
polybutadiene grafted with acrylonitrile and styrene, and styreneacrylonitrile copolymer. The presence of graft polymer is essential since
straightforward mixtures of polybutadiene and styrene-acrylonitrile
207
copolymers are weak. In addition to emulsion processes such as those
described above, mass and mass/suspension processes are also of
importance.
The resulting polymers may vary in the following respects:
(a) SAN-rubber ratio;
(b) the styrene to acrylonitrile ratio in the SAN component;
(c) the amount of grafted SAN;
(d) rubber particle size and particle size distribution;
(e) cross-link density of the rubber;
(f) use of modified styrenes such as α-methyl styrene to increase
heat deflection temperature s;
(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 Table16.
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-retard ant addi tiv es.
(2) By blending ABS with PVC.
Bromine compounds are often used as flame retardant additives but
15-20 pts phr may be required. This is not only expensive but such large
levels lead to a serious loss of toughness. Of the bromine compounds,
octabromo-diphenyl 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
208
some by systems based on ABS or ASA with polycarbonates. Better levels
Very
Styre
Specific
gravity
1.05
1.05
1.02
1.06
1.04-
1.07
1.01-
1.04
1.07-
1.10
Tensile
MPa
Tensile
MPa
2000-
1500-
1700-
Vicat
point, °C
of toughness may be achieved by the use of ABS/PVC blends but the
presence of the PVC lowers the processing stability.
Table 16 – Typical mechanical properties of compression-moulded
polystyrene plastics as measured by appropriate ASTM tests
Medium
-impact
PS-SBR
blend
35
3250
94
high
impact
PS-SBR
blend
14
1750 3700
94
acrylo
nitrile
40-70 38-48 24-40 40-55
ne
Medium
impact
-
106 100-108 104 96-99
ABS
3000
High
impact
ABS
2000
MBS
2500
Property
strength,
modulus,
softening
G.P.
poly-
styrene
52
3900
100
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
(µ) of the rubber particle, graft and matrix phases and eliminating any
additives or polymerization ingredients that may cause opacity. A variety
of approaches have been used including the use of SBR or carboxyterminated 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.
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 Tg intermediate to
209
the two homopolymers. Fortunately this is usually the case.
(2) The addition of third monomers that enhance Tg.
(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 α-methylstyrene, 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 α-methylstyrene.
The second approach is typified by maleic anhydride. This material
does not homopolymerize but will polymerize 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 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 Tg. One blend
has been offered (Arylon T by USS Chemicals) which has a higher
softening point than the ABS-polycarbonates.
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 ABS/PVC blends are used to produce fireretarding ABS-type materials, as already mentioned, while 10:90 blends are
considered as impact-modified forms of unplasticized PVC. ABS materials
have also been blended with plasticized PVC to give a crashpad sheet
material.
Blending of ABS with an acrylic material such as poly(methyl
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 unplasticized PVC.
210
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) T he 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.
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.