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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
81
cock floats) and for shoe heels. Light-stabilized polymer is used for light
fittings but because of the tendency of polystyrene to yellow, poly(methyl
methacrylate) is preferred. Polystyrene monofilament finds limited use for
brushes and for handicraft work. Both in North America and in Western
Europe about two-thirds of the expanded polystyrene produced is used for
thermal insulation. Most of this is used in building construction. It is also
used to some extent in refrigeration insulation. In this field it meets
intensive competition from polyurethane foams. The expanded polystyrene
has a low density, a low weight cost, is less brittle and can be made fire
retarding. On the other hand polyurethane foams produced by systems
using auxiliary blowing agents have a lower thermal conductivity and can
be formed in situ. This latter property makes the polyurethane foam sel fsealing, thus aiding the overall insulation characteristics of the whole
construction rather than just that of the foam.
Nearly all the expanded polystyrene that is not used for thermal
insulation is used for packaging. Uses range from individually designed
box interiors for packing delicate equipment such as cameras and electronic
equipment, thermoformed egg-boxes to individual beads (which may be up
to 5 cm long and about 1 cm in diameter) for use as a loose fill material.
There is also some use of thin-wall containers for short-term packaging and
conveying of hot food from take-away service areas. A small amount is
used for buoyancy applications and as decorative flower pots.
Expanded polystyrene accounts for over 20% of the weight
consumption of polystyrene and high-impact polystyrene. The volume of
expanded material produced annually exceeds even the volume production
of the aliphatic polyolefins.
2.2.2 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 mor e than a few
specialized 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 strengt h
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(methyl
82
methacrylate), i.e. about ten times that of polystyrene but about one-tenth
that of cellulose acetate.
The market for styrene-acrylonitrile plastics, referred to in many
countries as SAN has remained small relative to that for polystyrene and
ABS 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(methyl
methacrylate) for a number of applications. In general the copolymer is
cheaper than poly(methyl methacrylate) and cellulose acetate, tougher than
poly(methyl 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(methyl methacrylate). As a result of these considerations the styreneacrylonitrile 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 31% 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.
2.2.3 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
83
are commercially available. For example, the opaci ty 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 copolymerization 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
polymerization 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.
Besides the MBS materials, related terpolymers have been
prepared. These include materials prepared by terpolymerizing methyl
methacrylate, acrylonitrile and styrene in the presence of polybutadiene
(Toyolac, Hamano 500); methyl methacrylate, acrylonitrile and styrene in
the presence of a butadiene-methyl methacrylate copolymer (XT Resin),
and methylacrylate, styrene and acrylonitrile on to a butadiene-styrene
copolymer.
Because the polybutadiene component is liable to oxidation, ABS
materials are embrittled on prolonged exposure to sunlight. By replacing
polybutadiene rubber with other elastomers that contain no main chain
double bonds it has been possible to produce blends generally similar to
ABS but with improved weathering resistance. Three particular types that
have achieved commercial status are:
(1) ASA polymers which utilize an acrylic ester rubber.
(2) AES polymers which use an ethylene-propylene termonomer
rubber.
(3) ACS polymers based on elastomeric chlorinated polyethylene.
The ASA materials were introduced by BASF about 1970 as Luran
S. Similar to ABS, they show improved light resistance and heat resistance
84
(both during processing and in service). Because of their generally very
good weatherability these materials have become best known for
automotive grilles and mirror housings and have also been successfully
used in garden equipment including pumps, marine equipment and satellite
dishes. Other applications reported include chain covers and guards for
agricultural machines, moped guards, housings for street lighting, road
signs and mileage indicators. Where great er toughness is required alloys of
ASA and polycarbonate resins are available from BASF (Luran SC). The
extension of ABS-type materials into such exterior applications means that
these products have to be considered alongside other plastics that show
good weathering behaviour such as poly(methyl methacrylate), cellulose
acetate-butyrate and several fluorine-containing polymers.
Whilst the ASA materials are of European origin, the AES
polymers have been developed in Japan and the US. The rubber used is an
ethylene-propylene terpolymer rubber of the EPDM type which has a small
amount of a diene monomer in the polymerization recipe. The residual
double bonds that exist in the polymer are important in enabling grafting
with styrene and acrylonitrile. The blends are claimed to exhibit very good
weathering resistance but to be otherwise similar to ABS.
ACS polymers, developed primarily in Japan, are grafts of
acrylonitrile and styrene onto elastomeric chlorinated polyethylene.
Although the polymer has good weathering properties it is somewhat
susceptible to thermal degradation during processing and to date these
polymers have been of limited interest.
Blends have also been produced containing neither acrylonitrile
and styrene in the glassy phase nor polybutadiene in the rubbery phase.
One such system involved grafting 70 parts of methyl methacrylate
on to 30 parts of an 81-19 2-ethylhexyl acrylate – styrene copolymer. Such
a grafted material was claimed to have very good weathering properties as
well as exhibiting high optical transmission.
Perhaps the greatest resistance to development with these materials
is the strong competition offered by the clear impact-modified grades of
unplasticized PVC which are generally less expensive.
2.2.4 Styrene-maleic Anhydride Copolymers
There has been some interest in random copolymers of styrene with
small amounts of maleic anhydride. Manufacturers included Monsanto
(Cadon), Dow (Resin XP5272) and Dainippon (Ryurex X-15). However,
the only current manufacturer of high molecular weight materials appears
85
to be Arco, which markets its products under the trade name Dylarc. The
abbreviation SMA is commonly used for these mater ia ls.
The unmodified copolymers are transparent and have a Tg and
deflection temperature under load in excess of 125°C. Toughened grades
may be obtained by incorporating a graftable rubber during the
polymerization stage. Glass-fibre reinforcement of the copolymer is also
common. Long glass-fibre grades have recently become available in
addition to the more common grades obtained by melt blending of
polymers with glass fibre.
The processing of SMA materials is largely predictable from a
consideration of the structure. The polymer is easy flowing but setting
temperatures are somewhat higher than for polystyrene and thus facilitate
short cycle times. The low shrinkage, typical of an amorphous polymer,
does, however, require that excessive pressures and pressure holding times
during injection moulding should not occur since this could hinder mould
release.
Styrene-maleic anhydride copolymers have achieved a good market
penetration in the USA for auto instrument panels, where factors such as
good heat resistance, rigidity, predictable impact properties and
dimensional stability are important. Commercial blends of SMA with
polycarbonate resin have been marketed. Such blends have deflection
temperatures about 15°C above those for straight SMA copolymers and are
also attractive for their ductility, toughness and ease of mouldability. A
composite material consisting of an SMA foamed core sandwiched between
an elastomer-modified SMA compound has been of interest as a car roof
lining. This interest arose from the ability to expose components to the
elevated temperatures that occur in hot paint drying equipment and in
metallising baths. Other applications include car heating and ventilating
systems and transparent microwave packaging material.
In addition to the above SMA materials, low molecular weight
(1660-2500) copolymers with 25-50% ma leic anhydride c ontent have be en
made available (SMA Resins – Elf Atochem). These find use in such
diverse applications as level ling agents in floor polishes, embrittling/antiresoil agents in rug shampoos, and pigme nt dispersants in inks, paints and
plastics. They are also used in paper sizing and metal coating. The suppliers
of these materials lay emphasis on the reactivity of such materials. For
example, the maleic anhydride groups may be esterified with alcohols,
enabling a wide spectrum of chemical structures to be grafted onto the
chain, neutralized with ammonia, or imidised by reaction with an amine. As
86
with all styrene polymers, the benzene ring may also be subject to a number
of chemical reactions such as sulphonation.
Production of SMA materials is of the order of 25000 t.p.a. and
recent reports refer to an annual growth rate of the order of 10-15%.
2.2.5 Butadiene-styrene Block Copoly mer s
Random copolymers of butadiene and styrene have been known for
over half a century and such polymers containing about 25% of styrene
units are well known as SBR. Styrene-butadiene-styrene triblock
copolymers have also been known since 1965 as commerci al thermoplastic
elastomers.
Closely related to these but thermoplastic rather than rubber-like in
character are the K-resins developed by Phillips. These resins comprise
star-shaped butadiene-styrene block copolymers containing about 75%
styrene and, like SBS thermoplastic elastomers, are produced by sequential
anionic polymerization.
An interesting feature of these polymers is that they have a
tetramodal molecular mass distribution which has been deliberately built in
and which is claimed to improve processability. This is achieved by the
following procedure:
(1) Initiating polymerization of styrene with sec-butyl-lithium.
(2) When the styrene has been consumed, to give living polymers
of narrow molecular mass distribution, more styrene and more catalyst are
added. The styrene adds to the existing chains and also forms new polymer
molecules initiated by the additional sec-butyl-lithium.
(3) When the replenishing styrene had also been consumed
butadiene is added to give a living diblock and when the monomer has been
consumed the diblocks will have two modal molecular weights.
(4) The linear diblocks are then coupled by a polyfunctional
coupling agent such as epoxidised linseed oil to give a star-shaped polymer.
Commercial materials of this type have a tetramodal distribution.
Polymers of this sort possess an interesting combination of
properties. They are clear and tough (although notch sensitive) and exhibit
a level of flexibility somewhat higher than that of polypropylene. Typical
properties are given in Table 4.
The block copolymers are easy to process but in order to obtain
maximum clarity and toughness attention has to be paid to melt and mould
temperatures during injection moulding.
87
Table 4 – Some typical properties of styrene-butadiene block
Property
Value
Specific gravity
Transparency
1.04
transparent
copolymer thermoplastics (Philips K-Resins)
Tensile strength, MPa
Tensile modulus, MPa
Hardness (Rockwell R)
Heat deflection temperature (at 1.81 MPa stress ), ºC
Vicat softening point, ºC
Water absorption (24 hours), %
are of interest for medical applications and a wide variety of miscellaneous
industrial uses.
commercially available polymers should also be, in terms of tonnage
consumption at least, one of the two most important plastics materials
available today. Yet this is the unusual position held by poly(vinyl
chloride) (PVC), a material whose commercial success has been to a large
extent due to the discovery of suitable stabilizers and other additives whi ch
has enabled useful thermoplastic compounds to be produced.
1835 although it may have been prepared earlier by Liebig. The method
used was to treat ethylene dichloride with an alcoholic solution of
potassium hydroxide. Vinyl bromide was also obtained by a similar method
using ethylene dibromide. It is reported that in 1872 vinyl bromide was also
prepared by reacting acetylene and hydrogen bromide. The analogous
reaction with hydrogen chloride, discovered by F. Klatte in 1912,
subsequently became one of the two major routes for vinyl chloride
production.
bromide, when exposed to sunlight, was reported in 1872 by Baumann.
Further work on these polymerizations was carried out by Ostromislensky
in Moscow and this was duly reported in 1912.
27-30
1400
72
71
93
0.09
Polymers of this type find application in toys and housewares and
2.3 Polymers of Halogenated Unsaturated Hydrocarbons
2.3.1 Poly(vinyl chloride)
It is an interesting paradox that one of the least stable of
The preparation of the monomer was first reported by Regnault in
The polymerization in sealed tubes of vinyl chloride and vinyl
88
Commercial interest in poly(vinyl chloride) was revealed in a
number of patents independently filed in 1928 by the Carbide and Carbon
Chemical Corporaration, Du Pont and IG Farben. In each case the patents
dealt with vinyl chloride-vinyl acetate copolymers. This was because the
homopolymer could only be processed in the melt state at temperatures
where high decomposition rates occurred. In comparison the copolymers,
which could be processed at much lower temperatures, were less affected
by processing operations.
During the next few years PVC was steadily developed in the
United States and in Germany. Both countries were producing the material
commercially before World War II. In Great Britain, ICI in 1942 and the
Distillers Company in 1943 also commenced pilot-plant production of
PVC, a material then in demand as a rubber substitute for cable insulation.
Paste-forming grades suitable for the production of leathercloth also
became available soon afterwards.
After the war developments in Britain and the United States were
concerned largely with plasticized PVC, handled mainly by extrusion,
calendering and paste techniques.
By the early 1970s PVC was being manufactured in a large number
of countries and was contending with polyethylene for the title of the
world's number one plastics material in terms of tonnage consumption. In
1971, however, some evidence was presented which showed that the
vascular disturbance known as acro-osteolysis, Reynauds syndrome and
scleroderma were all associated with exposure to vinyl chloride monomer.
These diseases were characterized by symptoms such as increased
sensitivity to cold, changes in skin colour and thickness, changes in bone
formation and the shape of the digits. Further studies then indicated that
animals exposed to monomer levels in the atmosphere as low as 250 ppm
had a higher than normal incidence to the rare cancer form, angiosarcoma
of the liver, and in 1974 it was found that this cancer had occurred at
above-normal incidence among workers in some PVC polymerization
plants and by 1976 a total of 55 deaths due to angiosarcoma of the liver had
been recorded amongst those who had worked with vinyl chloride.
These findings led to rapid and important changes in the details of
PVC manufacture in the mid-1970s. As a result monomer concentration
levels around polymerization plants were reduced from 300-400 ppm i n the
late 1960s to about 2-5 ppm by late 1976. Not surprisingly the effort
expended in this direction as well as the adverse public image retarded the
expected growth patterns for PVC and also led to its withdrawal from
89
certain applications. However, with the containment of the hazards
associated with the monomer, the natural growth patterns for PVC were
resumed within a few years. This growth has occurred in spite of
continuing questions on health and safety aspects such as the toxicity of
plasticizers such as phthalates and of stabilizers, and the nature of products
produced during processing, thermal decomposition and composting of a
halogen-containing polymer.
Polymerization
In commercial practice vinyl chloride is polymerized by free
radical mechanisms in bulk, in emulsion and in suspension, the latter
process dominating with about 85% of the market. Today emulsion
polymers are mainly used for paste applications and the bulk polymerized
material for UPVC applications. The general kinetics are influenced by a
significant chain transfer to monomer reaction, which increases more
rapidly with temperature than the chain propagation reaction. In
consequence the molecular weight of the resultant PVC is determined by
the polymerization temperature and little affected by the initiator
concentration. In practice, polymerizations are usually carried out in the
temperature range 50-75 °C although this range may be extended upwards
by the use of chain transfer agents.
Bulk polymerization processes have been known for many years
but until the mid-1960s the only commercial process was one operated by
Pechiney-St Gobain in France. This process was a one-stage process and
according to one patent example vinyl chloride was polymerized with 0.8%
of its own weight of benzoyl peroxide in a rotating cylinder containing steel
balls for 17 hours at 58°C.
Bulk polymerization is heterogeneous since the polymer is
insoluble in the monomer. The reaction is autocatalysed by the presence of
solid polymer whilst the concentration of initiator has little effect on the
molecular weight. This is believed to be due to the overriding effect of
monomer transfer reactions on the chain length. As in all vinyl chloride
polymerization oxygen has a profound inhibiting effect.
One function of the steel balls was to facilitate the removal of heat
but difficulties in control, particularly of particle size of the polymer which
was ground by the tumbling of the balls, limited the attraction of the
process. This solution changed quite considerably with the development by
Pechiney-St Gobain (now Rhone-Poulenc) of a two-stage process. The first
stage is carried out as a liquid with up to about 15% conversion whilst the
90
second stage carried out as a powder takes the conversion to 80-85%. The
Monomer
Modifier
Vinyl chloride
Trichlorethylene
30-50 parts
0.1 parts
presence of two stages allows considerable flexibility for the process,
particle characteristics usually being determined by the operation of the
first stage and average molecular weight by the second stage. By early
1970s at least 20 companies throughout the world had taken out licences to
operate this process.
there is little, if any, loss in clarity or electrical insulation properties.
Particle shape, size and size distribution may be controlled by varying the
dispersing systems and the rate of stirring. A typical polymerization vessel
for suspension polymerization is shown in Figure 12. It should, however,
be noted that many more modern reactors are fitted with shorter bottomentry agitators. Autoclave sizes of 40-80m
although autoclaves as large as 200m
Dispersing agent
Figure 12 – Typical polymerization vessel suitable for suspension
or emulsion polymerization of vinyl chloride
Suspension polymerizations are generally easier to control and
3
are used by most manufacturers
3
are in commercial use.
A typical charge would be:
Gelatine
0.001 parts