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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
111
The TMC polycarbonate homopolymer has a glass transition
temperature of 238°C, nearly 100°C above that of the bis-phenol A
polycarbonate. Therefore, copolymers will have intermediate glass
transitions depending on the relative proportions of TMC and bis-phenol A.
Commercial grades (marketed by Bayer as Apec HT) have Vicat softening
points from 158 to 205°C. Other properties are broadly similar to those of
conventional bis-phenol A polycarbonates. For example, the polymer is
clear, with a light transmittance of about 90% at 1 mm thickness. The
polymer is also tough, although the notched impact strength is somewhat
lower. Being copolymers, the materials are amorphous. These materials are
of particular interest where the material is subject to extensive thermal
stressing, such as in car headlights, lamps, household appliances and
medical applications (where the copolymers are suitable for superheated
steam sterilization at 134°C). There is at present no other plastics material
which provides the particular combination of high heat resistance, high
transparency, high impact strength and good flow properties at such a level.
6) Polycarbonates with superior notched impact strength, made by
reacting bisphenol A, bis-phenol S and phosgene, were introduced in 1980
(Merlon T). These copolymers have a better impact strength at low
temperatures than conventional polycarbonate, with little or no sacrifice in
transparency. These co-carbonate polymers are also less notch sensitive
and, unlike for the standard bis-phenol A polymer, the notched impact
strength is almost independent of specimen thickness. Impact resistance
increases with increase in the bis-phenol S component in the polymer feed.
Whilst tensile and flexural properties are similar to those of the bis-phenol
A polycarbonate, the polyco-carbonates have a slightly lower deflection
temperature under load of about 126°C at 1.81 MPa loading.
The addition of carbon fibre to polycarbonate can lead to
composites with flexural strength three times and flexural modulus seven
times that of unfilled resin. Notched Izod impact values are amongst the
highest for any fibre-filled thermoplastics material. Flexural creep after
2000 hours loading at 68.97 MPa is also minimal. Carbon-fibre-reinforced
grades also exhibit enhanced deflection temperatures (149°C for 30% fibre
loading under 1.8 MPa loading), low volume and surface resistivities,
facilitating dissipation of static charge, lower coefficient of friction and
increased wear resistance.
Incorporation of PTFE, silicone resins and glass or carbon fibres
can lead to important internally lubricated composites.
Flame retardant grades may not only use additives such as sodium
112
2,4,5-trichlorobenzene sulphonate but also an anti-dripping agent which
can cause cross-linking as the polymer burns, thus reducing the tendency to
drip.
General properties
Although somewhat more expensive than the general purpose
thermoplastics, polycarbonates have established themselves in a number of
applications. The desirable features of the polymer may be listed as
follows:
(1) Rigidity up to 140°C.
(2) Toughness up to 140°C.
(3) Transparency.
(4) Very good electrical insulation characteristics.
(5) Virtually self-extinguishing.
(6) Physiological inertne ss .
The principal disadvantages may be listed as:
(1) More expensive than polyethylene, polystyrene and PVC.
(2) Special care required in processing.
(3) Pale yellow colour (now commonly masked with dyes).
(4) Limited resistance to chemicals and ultraviolet light.
(5) Notch sensitivity and susceptibility to crazing under strain.
The resistance of polycarbonate resins to “creep” or deformation
under load is markedly superior to that of acetal and polyamide
thermoplastics. A sample loaded at a rate of one ton per square inch for a
thousand hours at 100°C deformed only 0.013 cm/cm. Because of the good
impact strength and creep resistance it was felt at one time that the
polycarbonates would become important engineering materials. Such hopes
have been frustrated by the observations that where resins are subjected to
tensile strains of 0.75% or more cracking or crazing of the specimen will
occur. This figure applies to static loading in air. When there are frozen-in
stresses due to moulding, or at elevated temperatures, or in many chemical
environments and under dynamic conditions crazing may occur at much
lower strain levels. Aging of the specimen may also lead to similar effects.
As a result moulded and extruded parts should be subjected only to very
light loadings, a typical maximum value for static loading in air being
14MPa.
Because of a small dipole polarization effect the dielectric
113
constant is somewhat higher than that for PTFE and the polyolefins but
lower than those of polar polymers such as the phenolic resins. The
dielectric constant is almost unaffected by temperature over the normal
range of operations and little affected by frequency changes up to 10
6
Hz.
Above this frequency the dielectric constant starts to fall.
In common with other dielectrics the power factor is dependent on
the presence of polar groups. At low frequencies and in the normal working
temperature range (20-100°C) the power factor is almost surprisingly low
for a polar polymer (-0.0009). As the frequency increases, the power loss
increases and the power factor reaches a maximum value of 0.012 at 10
7
Hz. The polycarbonates have a high volume resistivity and because of the
low water absorption these values obtained are little affected by humidity.
They do, however, have a poor resistance to tracking.
Although the general electrical properties of the polycarbonates are
less impressive than those observed with polyethylene they are more than
adequate for many purposes. These properties, coupled with the heat and
flame resistance, transparency and toughness, have led to the extensive use
of these resins in electrical applicati ons.
Early grades tended to be yellow in colour due to impurities in the
bis-phenol A. Some darkening also occurred during processing and service.
Later grades masked the yellowness with the use of a small amount of blue
dye whilst modern grades are of much higher purity and virtually waterwhite. The polymer has a refractive index of 1.586 at 25°C. As may be
expected of a polar polymer the dielectric constant (3.17 at 60 Hz) is
greater than the square of the refractive index (2.51) but does tend towards
this value at very high frequencies.
Peilstöcker has studied in some detail the dependence of the
properties of bis-phenol A polycarbonate on temperature. He found that
if the resin is heated to just below the glass transition temperature some
stiffening of the sample takes place owing to some ordering of the
molecules. The degree of molecular ordering did not affect the form of
the X-ray diagram. The annealing effect takes place quite rapidly and is
complete within 80 minutes at 135°C. This effect may be partially
reversed by heating at about the transition temperature, viz. (140-160°C),
and completely reversed by raising the temperature of the sample to its
optical melting point. The rubbery range extends from the glass
transition temperature to the optical melting point. Samples maintained
at this temperature, i.e. the Tg, will slowly crystallize. The maximum rate
of crystallization occurs at about 190°C, spherulitic structures being
formed at this temperature within eight days.
114
The chemical resistance of polyester materials is well recognized
to be limited because of the comparative ease of hydrolysis of the ester
groups. Whereas this ease of hydrolysis was also observed in aliphatic
polycarbonates produced by Carothers and Natta in 1930, the bis-phenol
A polycarbonates are somewhat more resistant. This may be ascribed to
the protective influence of the hydrophobic benzene rings on each side of
the carbonate group. The resin thus shows a degree of resistance to dilute
(25%) mineral acids and dilute alkaline solutions other than caustic soda
and caustic potash. Where the resin comes into contact with organophilic
hydrolysing agents such as ammonia and the amines the benzene rings
give little protection and reaction is quite rapid.
The absence of both secondary and tertiary С–H bonds leads to a
high measure of oxidative stability. Oxidation does take place when thin
films are heated in air to temperatures above 300°C and causes crosslinking but this is of little practical significance. The absence of double
bonds gives a very good but not absolute resistance to ozone.
Although moulded polycarbonate parts are substantially
amorphous, crystallization will develop in environments which enable the
molecules to move into an ordered pattern. Thus a liquid that is capable of
dissolving amorphous polymer may provide a solution from which
polymer may precipitate out in a crystalline form because of the
favourable free energy conditions.
For solvation to take place it is first of all necessary for the
solvent to have a solubility parameter within about 1.4 units of the
solubility parameter of the polycarbonate (19.4-19.8 MPa
1/2
). A number
of solvents meet the requirement but some are nevertheless poor solvents.
The reason for this is that although they may tend to dissolve the
amorphous polymer they do not interact with the polycarbonate molecule,
which for thermodynamic reasons will prefer to crystallize out. If,
however, some specific interaction between the resin and the solvent can
be achieved then the two species will not separate and solution will be
maintained. This can be effected by using a solvent which has a proton-
donating ability (e.g. symtetrachlorethane δ = 19.2MPa
dichloride, δ = 19.8 MPa
1/2
), as a weak bond can be formed with the
1/2
or methylene
proton-accepting carbonate group, thus preventing crystallization. Other
good solvents are cis-1,2-dichloroethylene, chloroform and 1,1,2trichl oroethan e. Thio phene, dioxane and tetrahydrofuran are rated as fair
solvents.
115
A number of materials exist which neither attack the polymer
molecule chemically nor dissolve it but which cannot be used because
they cause cracking of fabricated parts. It is likely that the reason for this
is that such media have sufficient solvent action to soften the surface of
the part to such a degree that the frozen-in stresses tend to be released but
with consequent cracking of the surface.
The very low water absorption of bis-phenol A polycarbonates
contributes to a high order of dimensional stability.
When fabricated polycarbonate parts are exposed to ultraviolet
light, either in laboratory equipment or by outdoor exposure, a progressive
dulling is observed on the exposed surface. The dullness is due to
microscopic cracks on the surface of the resin. If the surface resin is
analyzed it is observed that it has a significantly lower molecular weight
than the parent polymer.
Such degradation of the surface causes little effect on either
flexural strength or flexural modulus of elasticity but the influence on the
impact properties is more profound. In such instances the minute cracks
form centres for crack initiation and samples struck on the face of samples
opposite to the exposed surface show brittle behaviour.
Because polycarbonates are good light absorbers, ultraviolet
degradation does not occur beyond a depth of 0.075-0.125 cm. Whilst this
is often not serious with moulded and extruded parts, film may become
extremely brittle. Improvements in the resistance of cast film may be made
by addition of an ultraviolet absorber but common absorbers cannot be
used in moulding compositions because they do not withstand the high
processing temperatures.
Heat aging effects are somewhat complex. Heating a t 125°C will
cause reduction in elongation at break to 5-15% and a slight increase a
tensile strength in less than four days. Further aging has little effect on
these properties but will cause progressive darkening. Heat aging in the
presence of water will lead to more severe adverse effects.
Unmodified polycarbonates are usually rated as slow burning, with
an oxygen index of 26. Flame-retarding grades are available with an
oxygen index as high as 35. Some of these grades also have limited smoke
and toxic gas emission on burning.
Processing characteristics
Satisfactory production of polycarbonate parts may be achieved
only if consideration is given to certain characteristics of the polymer.
116
In the first place, although the moisture pick-up of the resin is
small it is sufficient to cause problems in processing. In the extruder or
injection moulding machine it will volatilize into steam and frothy
products will emerge from die and nozzle. It is therefore necessary to keep
all materials scrupulously dry. Commercial materials are supplied in tins
that have been vacuum sealed at elevated temperatures. These tins should
be opened only after heating for several hours in an oven at 110°C and the
granules should be used immediately. The use of heated hoppers is
advocated.
The melt viscosity of the resin is very high and processing
equipment should be rugged. The use of in-line screw plasticizers is to be
particularly recommended.
Processing temperatures are high and fall between the melting
point (~230°C) and 300°C, at which temperature degradation occurs quite
rapidly.
Polycarbonate melts adhere strongly to metals and if allowed to
cool in an injection cylinder or extrusion barrel may, on shrinkage, pull
pieces of metal away from the wall. It is therefore necessary to purge all
equipment free of the resin, with a polymer such as polyethylene, after
processing.
There is little crystallization on cooling and after-crystallization
has not been observed. Mould shrinkage is consequently of the order of
0.006-0.008 cm/cm and is the same both along and across the flow.
In the case of glass-filled polymers, moulding shrinkage is
somewhat lower (0.003-0.005 cm/cm).
The rigidity of the molecule means that molecules may not have
time to relax before the temperat ure drops below the glass transition point.
Frozen-in strain may be gauged by noting how well the sample will
withstand immersion in carbon tetrachloride. In general, moulding strain
will be reduced by using high melt temperatures, preplasticizing machines,
high injection rates, and hot moulds (~100°C); where used, inserts should
be hot. Annealing at 125°C for up to 24 hours will be of some value.
Provided due care is taken with respect to predrying and to crazing
tendencies, polycarbonates may also be thermoformed, used for fluidised
bed coating and machined and cemented. Like metals, but unlike most
thermoplastics, polycarbonates may be cold formed by punching and cold
rolling. Cold rolling can in fact improve the impact resistance of the resin.
Film casting is comparatively straightforward but when film is
produced above a critical thickness it tends to become cloudy. This is
117
presumably because with such a thickness the solvent remains in the film
Polycarbonate
Polycarbonate/ABS alloy
Western
Europe
USA
Western
Europe
USA
Consumption, tones
Other
506000
4.7
358000
3.9
156000
4.4
78000
-
longer, giving the molecules freedom for a longer period to move into a
crystalline state. Since we have already found that the higher the molecular
weight of polyethylene and of polypropylene the more difficult does
crystallisation become, it is not surprising to find that the critical thickness
with polycarbonate film increases with an increase in molecular weight.
For polymers with molecular weights (Mn) in the range 75000-100000 the
critical thickness can be as high as 275 μm.
One recent development is rotational moulding. This process has
enabled large mouldings of polycarbonate to be made using reasonably
simple and inexpensive equipment.
Applications of bis-phenol A polycarbonates
In spite of their rather complicated chemical structure, which
consequently involves rather expensive production costs, the bis-phenol A
polycarbonates have achieved an important place amongst the speciality
plastics materials.
About 15% of bisphenol A polycarbonates are used in alloys with
other thermoplastics such as ABS. Western Europe has about 45% of the
market with the United States 30% and Japan 25%. There are differences in
the pattern of consumption in the two areas (Table 6).
Table 6 – U sage pat ter ns for polycarbonates and polycarbonate/ABS alloys
in Western Europe and the USA in 1997
Application
breakdown, %:
Glazing
Electric/electronic
Optical media
Lighting
Appliances
Transportation
Packaging
Recreation
Medical
28.7
21.8
17.8
8.7
5.2
4.3
3.6
2.6
2.6
23.5
7.5
16.2
2.5
8.4
17.0
4.2
7.3
9.5
-
34.0
-
1.3
7.1
52.0
-
1.3
-
-
25.6
-
-
10.3
57.7
-
5.1
1.3
118
Such success in the use of polycarbonates arises from the
advantages of toughness, rigidity, transparency, self-extinguishing
characteristics, good electrical insulation characteristics and heat
resistance. The main factors retarding growth are the cost, the special care
needed in processing, limitations in chemical and ultraviolet light
resistance, moderate electrical tracking resistance and notch sensitivity.
Other polymers are as rigid, others are as transparent, others are
even both more rigid and as transparent, but the bis-phenol A
polycarbonate is the only material that can provide such a combination of
properties, at least at such a reasonable cost. The application of
polycarbonates therefore largely arise where at least two and usually three
or more of the advantageous properties are required and where there is no
cheaper alternative.
The largest single field of application for moulded polycarbonates
is in electronics and electrical engineering. Covers for time switches,
batteries and relays, for example; utilize the good electrical insulation
characteristics in conjunction with transparency, flame resistance and
durability. The polymer is widely used in making coil formers. In this case
the ability to wind the wire tightly without deformation of the former, the
heat stability, the oxidation resistance and the good electrical insulation
characteristics have proved invaluable. Polycarbonate mouldings have also
been made for computers, calculating machines and magnetic disc pack
housing, terminals, contact strips, starter enclosures for fluorescent lamps,
switch plates and a host of other miscellaneous electrical and electronic
applications. Polycarbonate films of high molecular weight are used in the
manufacture of capacitors.
The polymers are extensively used in telecommunications
equipment, a major use being in telephone switching mechanisms.
Polycarbonates now dominate the compact disc market, where material of
very high purity is required. Fibre-filled lubricated grades have become of
interes t in business machine applications such as ribbon cartridges, paper
tractors and printed circuit boards.
Polycarbonates have proved attractive in domestic appliances.
Examples include food processor bowls, coffeemaker cold water
reservoirs, vacuum cleaner housings, food mixer housings, power tool
housings, hair drier and electric razor housings, and microwave cookware.
In the photographic field polycarbonates now complete with ABS
for projector housings, whilst in cameras polycarbonates are now used in
the shutter assembly, film drive, flash-cube sockets and lens holders. One
119
popular low-cost camera recently introduced into the UK market had at
least eight parts moulded from polycarbonate. Polycarbonate film is also
used for photographic purposes, e.g. for quality colour fine engravings.
The toughness and transparency of polycarbonates has also led to a
number of other industrial applications. In Great Britain one of the first
established uses was for compressed air lubricator bowls. In the first five
years of commercial production it was estimated that over 100000
breeding cages for rats were produced. Transparent milking pail lids have
also been moulded.
Polycarbonates have also found applications in domestic
mouldings. Cups, saucers and tumblers are adequately tough and are not
stained by the usual domestic beverages and fruit juices. They are thus
competitive with melamine-formaldehyde mouldings, the latter having
superior resistance to scratching. Tough transparent babies' bottles may be
blow moulded at very high rates because of the high setting-up
temperatures. Medical uses of polycarbonates include transparent filter
bowls used in transferring blood and intravenous fluids. Because of the
option of disposibility or sterilization, they have also replaced some
stainless steel surgical equi pm en t.
In 1973 polycarbonate structural foams, i.e. expanded or cellular
polycarbonates, became available. Densities as low as 0.6 g/cm3 are
possible whilst the rigidity of the stress-free mouldings is such that the
flexural strength to weight ratio is twice that of most metals. Furthermore
the products may be nailed and screwed like wood. Initial applications
were largely in business machine housings but glass-reinforced grades
have extended the range of use. For example they are used in water ski
shoes because of the high rigidity and resistance to fatigue.
1.4.5 Alloys Based on Bis-phenol A Polycarbonates
Alloys of bisphenol A polycarbonates with ABS and MBS resins
have been known for many years. Subsequently many other alloys
containing polycarbonates have been introduced so that by the mid-1990s
they comprised at least 15% of the polycarbonate market.
The styrene-based terpolymers were originally used to the extent
of some 2-9% in order to reduce the notch sensitivity of the polycarbonate
and to improve the environmental stress cracking resistance. More recently
emphasis has been on alloys with 10-50% of SAN or ABS.
Vicat softening points are usually in the range 110-135°C,
depending on the level of ABS or MBS (decreasing with increasing ABS
120
or MBS content). The Bayer ABS-PC alloy (Bayblend) retains its high
impact strength and notched impact strength down to -50°C. The alloys are
also claimed to be “non-splintering”. The hardness of the alloys is
comparable to that of polycarbonate.
Because of the above properties, together with other features such
as the ability to mould to close dimensional tolerances, low warpage, low
shrinkage, low moisture absorption and good surface finish, polycarbonateABS alloys have become widely used in the automotive industry, for
electrical applications and for housings of domestic and business
equipment.
Examples of applications in the automotive industry include
instrument panels, air vents and ventilation systems, cowl panels, wheel
covers, rear light chassis, headlamp housings, central electrical control
boxes, electroplated insignia, loudspeaker grilles, double rear spoilers and
window trim. Electrical applications include fuse switch housings, plug
connectors, safety sockets, fuse switch housings, power distribution
fittings, control switch housings, thermostat housings, switches, appliance
connectors and telephone dials. The alloys are also used for housings of
typewriters, small radio receivers and hair dryers, steam iron handles and
vacuum cleaner motor bear ings.
In addition to standard grades varying in the ABS/PC ratio, fireretarded, glass-fibre-reinforced and glass-fibre-reinforced fire-retarded
grades are available. Typical properties of three grades of ABS-PC alloys
are given in Table 20.9.
Polycarbonate-polybutylene terephthalate alloys (Macroblend–
Bayer; Xenoy–General Electric) were introduced in the 1980s. The blends
are particularly notable for their high levels of toughness (down to -40°C)
and resistance to petrols and oils. Initial interest was for car bumpers and
front ends but the alloys have found intensive competition from
polypropylene-based materials and more recently emphasis has been placed
on the suitability of these materials for demanding uses such as lawn
mower and chain saw housings. Typical properties of a general purpose
grade are given in Table 7.
Blends with styrene-based polymers were introduced in 1980, and
compared with PC/ABS blends, are claimed to have improved hydrolytic
resistance, lower density and higher heat deflection temperatures.
Suggested applications are as dishes for microwave ovens and car
headlamp reflectors.