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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
71
increase in the ease of flow. It is not, however, good practice to change the
flow properties in this way as the monomer will volatilize in the processing
machine and the bubbles formed will be distorted to produce such faults as
“silver streaks” and “mica marks” in the finished product.
The thermal properties are of interest to both the user of the endproduct and to the processor. From the user's point of view the principal
features are the very low thermal conductivity (approx. 0.13 W/mK) and
the comparatively low softening point. Standard tests give softening points
of about 90°C, that is below the boiling point of water. In addition many
properties are affected by temperature.
In common with other thermoplastic melts polystyrene exhibits
pseudoplastic behaviour.
The specific heat of polystyrene is dependent on temperature and at
200°C the value is approximately double that at room temperature.
Stereoregular polystyrene
Polystyrene produced by free-radical polymerization techniques is
part syndiotactic and part atactic in structure and therefore amorphous. In
1955 Natta and his co-workers reported the preparation of substantially
isotactic polystyrene using aluminium alkyl-titanium halide catalyst
complexes. Similar systems were also patented by Ziegler at about the
same time. The use of n-butyl-lithium as a catalyst has been described.
Whereas at room temperature atactic polymers are produced,
polymerization at -30°C leads to isotactic polymer, with a narrow
molecular weight distribution.
In the crystalline region isotactic polystyrene molecules take a
helical form with three monomer residues per turn and an identity period of
6.65 Å. One hundred percent crystalline polymer has a density of 1.12
compared with 1.05 for amorphous polymer and is also translucent. The
melting point of the polymer is as high as 230°C. Below the glass transition
temperature of 97°C the polymer is rather brittle.
Because of the high melting point and high molecular weight it is
difficult to process isotactic polystyrenes. Various techniques have been
suggested for injection moulding in the literature but whatever method is
employed it is necessary that the moulding be heat ed to about 180°C, either
within or outside of the mould, to allow the material to develop a stable
degree of crystallinity.
The brittleness of isotactic polystyrenes has hindered their
commercial development. Quoted Izod impact strengths are only 20% that
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of conventional amorphous polymer. Impact strength double that of the
amorphous material has, however, been claimed when isotactic polymer is
blended with a synthetic rubber or a polyolefin.
Syndiotactic polystyrene
The first production of syndiotactic polystyrene has been credited
to research workers at Idemitsu Kosan in 1985 who used cyclopentadienyl
titanium compounds with methyl aluminoxane as catalyst.
Whereas the isotactic polymer has not been commercialized Dow
were scheduled to bring on stream plant with a nameplate capacity of
37000 t.p.a. in 1999 to produce a syndiotactic polystyrene under the trade
name Questra. The particular features of this material are:
(a) Tg of about 100°C (similar to that of amorphous polystyrene)
and Tm of 270°C.
(b) Low density with crystalline and amorphous zones both having
densities of about 1.05 g/cm3. An advantage of the matching densities of
the two zones or phases is that there is little warping and generally good
dimensional stability.
(c) While the unfilled polymer is somewhat brittle, impact strength
is substantially increased by the use of glass fibres and/or impact modifiers.
(d) While the heat deflection temperatures of unfilled materials are
similar to Tg, that of glass-filled grades approaches Tm.
(e) Electrical, chemical and thermal properties and dimensional
stability are similar to those of general purpose (“atactic”) polystyrene and
thus has some advantages over more polar crystalline, so-called,
engineering plastics such as the polyamides and linear polyesters.
Potential applications for glass-filled grades include
electronic/electrical connectors, coil bobbins, relays; automotive lighting
and cooling system components and pump housings and impellers. Unfilled
grades are of interest as capacitor film with a heat resistance that can
withstand infra-red reflow soldering combined with excellent electrical
insulation properties little affected by temperature and frequency. Nonwoven fabrics with good heat, moisture and chemical resistance are of
interest for filter media.
There has also been some interest in melt blending with polyamides
to increase the toughness but at some sacrifice to dimensional stability and
moisture resistance.
73
Processing of polystyrene
Polystyrene and closely related thermoplastics such as the ABS
polymers may be processed by such techniques as injection moulding,
extrusion and blow moulding. Of less importance is the processing in latex
and solution form and the process of polymerization casting. The main
factors to be borne in mind when considering polystyrene processing are:
(1) Th e negligible wat er absorption a voids the need for predrying
granules.
(2) The low specific heat (compared with polyethylene) enables the
polymer to be rapidly heated in injection cylinders, which therefore have a
higher plasticizing capacity with polystyrene than with polyethylene. The
setting-up rates in the injection moulds are also faster than with the
polyolefins so that faster cycles are also possible.
(3) The strong dependence of apparent viscosity on shear rate. This
necessitates particular care in the design of complex extrusion dies.
(4) The absence of crystallization gives polymers with low mould
shrinkage.
(5) Molecular orientation.
Although it is not difficult to make injection mouldings from
polystyrene which appear to be satisfactory on visual examination it is
another matter to produce mouldings free from internal stresses. This
problem is common to injection mouldings of all polymers but is
particularly serious with such rigid amorphous thermoplastics as
polystyrene.
The main reason for extruding polystyrene is to prepare highimpact polystyrene sheet. Such sheet can be formed without difficulty by
vacuum forming techniques. In principle the process consists of clamping
the sheet above the mould, heating it so that it softens and becomes rubbery
and then applying a vacuum to draw out the air between the mould and the
sheet so that the sheet takes up the contours of the mould.
Expanded polystyrene
Polystyrene is now available in certain forms in which the
properties of the product are distinctly different from those of the parent
polymer. Of these by far the most important is expanded polystyrene, an
extremely valuable insulating material now available in densities as low as
16 kg/m3. A number of processes have been described in the literature for
the manufacture of the cellular product of which four are of particular
interest in the manufacture of large slabs.
74
(1) Polymerization in bulk of styrene with azodi-isobutyronitrile as
initiator. This initiator evolves nitrogen as it decomposes so that expansion
and polymerization occur simultaneously. This method was amongst the
earliest suggested but has not been of commercial importance. There has,
however, been recent resurgence of interest in this process.
(2) The Dow “Log” Process. Polystyrene is blended with a low
boiling chlorinated hydrocarbon and extruded. The solvent volatilizes as
the blend emerges from t he die and the mass expands. This process is still
used to some extent.
(3) The BASF Process. Styrene is blended with a low boiling
hydrocarbon and then polymerized. The product is chipped. The chips are
then converted into expanded polymer as in method (4).
(4) Bead Processes. These processes have generally replaced the
above techniques. The styrene is polymerized by bead (suspension)
polymerization techniques. The blowing agent, typically 6% of low boiling
petroleum ether fraction such as n-pentane, may be incorporated before
polymerization or used to impregnate the bead under heat and pressure in a
post-polymerization operation.
The impregnated beads may then be processed by two basically
different techniques: (a) the steam moulding process, the most important
industrially and (b) direct injection moulding or extrusion. In the steam
moulding process the beads are first “prefoamed” by heating them in a
steam bath. This causes the beads to expand to about 40 times their
previous size. At this stage the beads should not fuse or stick together in
any way. It has been shown that expansion is due not only to volatilization
of the low boiling liquid (sometimes known as a pneumatogen) but also to
an osmotic-type effect in which steam diffuses into the cells with the bead
as they are formed by the expanding pneumatogen. The entry of steam into
the cells causes a further increase in the internal pressure and causes further
expansion. It has been estimated that about half of the expansion is due to
the effect of steam, which can diffuse into the cells at a much greater rate
than the pneumatogen can diffuse out. The expansion of the beads is
critically dependent on both temperature and time of heating. At low
steaming pressures the temperature obtained is about that of the softening
point of polystyrene and it is important to balance the influences of
polymer modulus, volatilization rates and diffusion rates of steam and
pneumatogen. In practice prefoaming temperatures of about 100°C are
used. Initially the amount of bead expansion increases with the time of
prefoaming. If, however, the beads are heated for too long the pneumatogen
75
diffuses out of the cells and the residual gas cannot withstand the natural
tendency of the bead to collapse.
The second stage of the process is to condition the beads, necessary
because on cooling after prefoaming pneumatogen and steam within the
cells condense and cause a partial vacuum within the cell. By allowing the
beads to stand in air for at least 24 hours air can diffuse into the cells in
order that at room temperature the pressure within the cell equilibrates with
that outside.
The third stage of the process is the steam moulding operation
itself. Here the prefoamed beads are charged into a chest or mould with
perforated top, bottom and sides through which steam can be blown. Steam
is blown through the preform to sweep air away and the pressure then
allowed to increase 0.11 MPa. The beads sof ten, air in the cells expands on
heating, pneumatogen volatilizes and steam once again permeates into the
cells. In consequence the beads expand and, being enclosed in the fixed
volume of the mould, consolidate into a solid block, the density of which is
largely decided by the amount of expansion in the initial prefoaming
process. Heating and cooling cycles are selected to give the best balance of
economic operation, homogeneity in density through the block, good
granule consolidation, good block external appearance and freedom from
warping. This process may be used to give slabs which may be
subsequently sliced to the appropriate size or alternatively to produce
directly such objects as containers and flower pots. The steam moulding
process, although lengthy, has the advantages of being a ble to make very
large low-density blocks and being very economic in the use of polymer.
Whilst it is possible to purchase standard equipment for the steam
moulding process, attempts continue to be made to make sweeping
modifications to the process. These include the use of dielectric and
microwave heating and the development of semicontinuous and continuous
processes.
The outstanding features of steam moulded polystrene foam are its
low density and low thermal conductivity. These are compared with other
important insulating materials in Table 3.
One alternative approach to the two-stage steam moulding process
is that in which impregnated beads are fed directly to an i njection moulding
machine or extruder so that expansion and consolidation occur
simultaneously. This approach has been used to produce expanded
polystyrene sheet and paper by a tubular process reminiscent of that used
with polyethylene. Bubble nucleating agents such as sodium bicarbonate
76
and citric acid which evolve carbon dioxide during processing are often
Parameter
Density, g/cm
3
Thermal conductivity,
Expanded polystyrene
Polyurethane foam (with
Expanded PVC
0.016
0.04
0.031
0.031
incorporated to prevent the formation of a coarse pore structure. Typical
film has a density of about 0.05 g/cm3. Injection moulding of impregnated
beads gives an expanded product with densities of about 0.22-0.24 g/cm3).
This cannot compare economically with steam moulding and the product is
best considered as a low-cost polystyrene (in terms of volume) in which air
and pneumatogen act as a filler. Such products generally have an inferior
appearance to normal polystyrene mouldings. Nevertheless, there has been
considerable interest recently in higher density cellular polymers
(sometimes known as structural foams). In some processes it is possible to
produce mouldings with a non-cellular skin. The dependence of the
properties of such cellular polymers on structure has been studied.
Table 3 – Density and thermal conductivity of some polymers
W/mK
chloro-fluorocarbon gas)
Expanded ebonite
Cork (expanded)
Wood
Glass wool
It is important to note that the thermal conductivity is dependent on
the mean temperature involved in the test: the higher the mean temperature
the higher the thermal conductivity.
Structural foams
The term structural foam was originally coined by Union Carbide
to describe an injection moulded thermoplastic cellular material with a core
of relatively low density and a high-density skin. The term has also been
used to describe rigid “foams” that are load bearing. Today it is commonly
taken to imply both of the above requirements, i.e. it should be load bearing
and with a core of lower density than the skin. In this section the broader
load-bearing definition will be used. Whilst structural foams are frequently
made from polymers other than polystyrene, this polymer is strongly
associated with such products.
0.032
0.06
0.10
0.40
0.064
0.022
0.03
0.038
0.094
0.036
77
Cellular thermoplastics can be made by feeding a blend of polymer
Tensile strength
Water absorption
0.11-0.14 MPa
2 g / 100 cm3 (max)
and chemical blowing agent to an injection moulding machine. The agent
decomposes in the heated barrel but because of the high pressures in the
melt in the barrel gases do not form until the material is injected into the
mould. In order for the process to work satisfactorily the machine should
have a cylinder shut-off nozzle to prevent egress of material during the
plasticating stage, a non-return valve on the screw tip, a capability of
operating at high injection speeds and good control over screw back
pressure. As an alternative to chemical blowing agents, volatile blowing
agents or, more commonly, nitrogen may be introduced into the polymer
melt shortly before mould filling.
Typical properties for a 0.016g/cm3 expanded polystyrene material
are:
Flexural strength
Compression strength
0.14-0.21 MPa
0.07-0.11 MPa
Moulding systems are usually divided into low-pressure and highpressure systems.
In the low-pressure systems a shot of material is injected into the
mould which, if it did not expand, would give a short shot. However, the
expanding gas causes the polymer to fill the mould cavity. One important
form of the low-pressure process is the Union Carbide process in whi ch the
polymer is fed to and melted in an extruder. It is blended with nitrogen
which is fed directly into the extruder. The extruder then feeds the polymer
melt into an accumulator which holds it under pressure (14-35 MPa) to
prevent premature expansion until a predetermined shot builds up. When
this has been obtained a valve opens and the accumulator plunger rams the
melt into the mould. At this point the mould is only partially filled but the
pressurized gas within the melt allows it to expand.
Although such products do not have a high-quality finish they do
exhibit two typical characteristics of structural foams:
(1) The internal pressures can prevent the formation of sink marks,
particularly on faces opposite to reinforcing ribs.
(2) Thick mouldings may be produced, again without distortion
such as sink marks.
Perhaps, however, the greatest virtue of structural foams is the
ability to increase the ratio of part rigidity / weight. A foam of half the
density of a solid material only requires a 25% increase in wall thickness to
maintain the rigidity.
78
High-pressure processes generally involve partial mould opening
after mould filling. In several cases these processes may also be described
as counter-pressure processes. The principle involved in such processes is
to fill the mould cavi ty with a gas such as air or nitrogen under pressure
before injection of the polymer / blowing agent melt. This pressure
prevents bubbles at or near the surface of the advancing front from
breaking through the surface and subsequently marring the appearance of
the moulding.
One such process is the TAF process, the basic patent being held
by Dow. It was developed in Japan by Asahi in conjuction with Toshiba.
Foam expansion after mould filling is made possible by use of retractable
mould cores. Because of the difficulty of allowing expansion in more than
one direction this process has been largely limited to the production of flat
products. Efficient gas sealing systems are also vital and the process needs
close control. For this reason it has not been widely used in either Europe
or North America.
A counter-pressure process was also used by Buhler-Miag, details
of which were only disclosed to licensees. It has been stated that expansion
does not involve mould movement or egression back through the sprue but
that the key to success is in the venting. This suggests that egress of melt
through mould vents allows the expansion. This process has been used in
England for furniture, computer housings and sailing boat rudders.
A high-pressure process not involving counter-pressure is the
sandwich moulding process developed by ICI in the United Kingdom a nd
by Billion in France. The principle of the process is to inject two polymer
formulations from separate injection units one after the other into a mould
through the same sprue. If a foamed core is desired the mould is partially
opened just after filling to allow the foamable polymer in the core to
expand. To seal off the core the injection stage is completed by a brief
injection through the sprue of the first (skin) material injected. A
modification of the sandwich process involves co-injection simultaneously
through two concentric nozzles, a process generally credited to Siemag and
developed by Battenfield.
Oriented polystyrene
Deliberately oriented polystyrene is available in two forms:
filament (mono-axially oriented) and film (biaxially oriented). In both cases
the increase in tensile strength in the direction of stretching is offset by a
79
reduction in softening point because of the inherent instability o f oriented
molecules.
Filament is prepared by extrusion followed by hot stretching. It
may be used for brush bristles or for decorative purposes such as in the
manufacture of “woven” lampshades.
Biaxially stretched film has proved of value as a packaging
material. Specific uses include blister packaging, snap-on lids,
overwrapping, “envelope windows” and de luxe packaging.
It may be produced by extrusion either by a tubular process or by a
flat film extrusion. The latter process appears to be preferred commercially
as it allows greater flexibility of operation. The polystyrene is first extruded
through a slit die at about 190°C and cooled to about 120°C by passing
between rolls. The moving sheet then passes above a heater and is
rewarmed to 130°C, the optimum stretching temperature. The sheet is then
stretched laterally by means of driven edge rollers and longitudinally by
using a haul-off rate greater than the extrusion rate. Lateral and longitudinal
stretching is thus independently variable. In commercial processes stretch
ratios of 3:1-4:1 in both directions are commonly employed.
Commercial oriented film has a tensile strength of 70-83 MPa ( c.f.
41-55 MPa for unstretched material) and an elongation of break of 10-20%
(c.f. 2-5%). The impact strength of bars laminated from biaxially stretched
film has impact strengths of the order of 15 times greater than the basic
polymer. The heat distortion temperature is negligibly affected. Whereas
toughness and clarity are the principal desirable features of oriented
polystyrene film the main disadvantages are the high moisture vapour
transmission rate compared with polyethylene and the somewhat poor
abrasion resistance.
Although it is possible to vacuum form these films the material has
such a high modulus at its shaping temperatures that an exceptionally good
vacuum is required for shaping. As a consequence of this the pressure
forming technique has been developed. In this process the sheet is clamped
between the mould and a heated plate. Air is blown through the mould,
pressing the sheet against the hot plate. After a very short heating period
the air supply is switched so that compressed air passes through holes in the
heater plate and blows the sheet into the mould.
Applications
Unmodified polystyrene first found application where rigidity and
low cost were important prerequisites. Other useful properties were the
80
transparency and high refractive index, freedom from taste, odour and
toxicity, good electrical insulation characteristics, low water absorption and
comparatively easy processability. Carefully designed and well-made
articles from polystyrene were often found to be perfectly suitable for the
end-use intended. On the other hand t he extensive use of the polymers in
badly designed and badly made products which broke only too easily
caused a reaction away from the homopolymer. This resulted, first of all, in
the development of the high-impact polystyrene and today this is more
important than the unmodified polymer (60% of Western European
market).
In recent years general purpose polystyrene and high-impact
polystyrene have had to face intensive competition from other materials,
particularly polypropylene, which has been available in recent years at what
may best be described as an abnormally low price. Whilst polystyrene has
lost some of it markets it has generally enjoyed increasing consumption and
the more pessimistic predictions of a decline have as yet failed to
materialize. Today about 75% of these materials are injection moulded
whilst the rest is extruded and / or thermoformed.
The largest outlet for polystyrene is in packaging applications.
Specific uses include bottle caps, small jars and other injection moulded
containers, blown containers (a somewhat recent development but which
has found rapid acceptance for talcum powder), vacuum formed toughened
polystyrene as liners for boxed goods and oriented polystyrene film for
foodstuffs such as creamed cheese. Vacuum formed cigarette packets were
introduced in the United States in the early 1960s and were claimed to be as
economical to produce as those from cardboard.
A second important outlet is in refrigeration equipme nt, where the
low thermal conductivity and improved impact properties of polystyrene at
low temperatures is an asset. Specific uses in this area include door liners
and inner liners made from toughened polystyrene sheet, mouldings for flip
lids, trays and other refrigerator “furnishings” and expanded polystyrene
for insulation. Although in the past most liners have been fabricated from
sheet there is a current interest in injection moulding these parts since these
will give greater design flexibility. It is also claimed that with sufficiently
high production rates the injection process will be cheaper.
Polystyrene and high-impact polystyrene mouldings are widely
used for housewares, for example storage containers, for toys, games and
sports equipment, radio and electrical equipment (largely as housings,
knobs and switches), for bathroom and toilet fittings (such as cistern ball-