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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
101
intermediate in their degree of interaction, are the best all-round materials.
Phthalates prepared from alcohols with about eight carbon atoms
are by far the most important class and probably constitute about 75% of
plasticizers used. There are a number of materials which are very similar in
their effect on PVC compounds but for economic reasons di-iso-octyl
phthalate (DIOP), di-2-ethylhexyl phthalate (DEHP or DOP) and the
phthalate ester of the C7-C9 oxo-alcohol, often known unofficially as
dialphanyl phthalate (DAP), are most commonly used. (The term
dialphanyl arises from the ICI trade name for the C7-C9 alcohols-Alphanol
79.) These materials give the best all-round plasticizing properties.
In the 1950s phosphate plasticizers had an importance comparable
with the phthalates. However, during the 1960s the development of the
petrochemicals industry resulted in the phthalate plasticizers becoming
available at much lower prices than obtained for the phosphates such as
tritolyl phosphate (TTP) for which the cresols were obtained from coal tar.
During this period trixylyl phosphate (TXP) tended to replace TTP because
of its lower price structure. Because of their high price phosphates tend to
be limited to products where good flame resistance is required, such as in
insulation and mine belting. Other advantages of these materials are their
high compatibility with PVC and good solvent resistance. On the debit side
they are toxic and give products with a high cold flex temperature.
The development of natural gas as a fuel source in the UK has led
to reductions in tar acid supplies and this has prompt ed the petrochemicals
industry to make available synthetic alkylated phenols such as the isopropyl
phenols. Tri-isopropylphenol phosphates are more price stable than the
older phosphates, but have otherwise similar properties. Some problems
are, however, said to arise with PVC pastes based on these materials
because of their high pseudoplasticity and thixotropy leading to draining
difficulties in dipping operations.
For some applications it is important to have a compound with
good low-temperature resistance, i.e. with a low cold flex temperature. For
these purposes aliphatic esters are of great value. They have a lower
interaction with PVC and thus are incorporated with greater difficulty and
extracted with greater facility. For many years the sebacates such as dibutyl
sebacate (DBS) and dioctyl sebacate (DOS) were used where good lowtemperature properties are required. Today they have been largely replaced
by cheaper esters of similar effect in PVC derived from mixed acids
produced by the petrochemical industry. The most important of these mixed
acids are the AGS acids (a mixture of adipic, glutaric and succinic acids).
102
These are esterified with octyl, nonyl and decyl alcohols to give plasticizers
now generally referred to as nylonates but occasionally as sugludates. The
sebacate, adipate and sugludate-type plasticizers can also be used to give
compounds of high resilience.
Esters based on trimellitic anhydride, the trimellitates, have
become very popular primary plasticizers for use at high temperatures or
where a high level of resistance to aqueous extraction is required. Because
of their frequency of use at elevated temperatures, they are usually supplied
commercially containing an antioxidant.
A number of other special purpose plasticizers are also available.
Lubricants
In plasticized PVC the main function of a lubricant is to prevent
sticking of the compound to processing equipment. This is brought about
by selecting a material of limited compatibility which will thus sweat out
during processing to form a film between the bulk of the compound and the
metal surfaces of the processing equipment. When used for such a purpose
the additives are known as external lubricants.
In Britain calcium stearate has been most commonly used with
non-transparent products and stearic acid with transparent compounds. In
the United States normal lead stearate, which melts during processing and
lubricates like wax, is commonly employed. Dibasic lead stearate, which
does not melt, lubricates like graphite and improves flow properties, is also
used.
The quantity of an external lubricant to be used has to be chosen
with care. If too little is used sticking problems occur; if too much the
compound may develop haze and greasiness, printing and heat sealing may
be difficult and gelation and fusion of the compound may be slowed down.
In addition, too much slip of compound against machinery walls will
prevent t he development of the shear forces which are required to ensure a
smooth and even flow. In an extruder the extent of lubricant may be used to
control the gelation of powder blends. If this occurs too early then undue
working of the polymer, causing some degradation, may occur before the
material emerges from the die. On the other hand there should be sufficient
time for proper gelation and homogenisation to take place before the die is
reached. A further problem that may arise with lubricants (and those
stabilizers which can also act as lubricants) is that because of their low
compatibility they may deposit on to calender and mill rolls and on to
extruder dies, carrying with them particles of pigments, fillers and other
103
additives. Such a phenomenon, which is often more severe at high shear
rates, is commonly known as plate-out.
In unplasticized PVC it is common practice to incorporate at least
one other lubricant. Such materials are primarily intended to improve the
flow of the melt, i.e. they lower the apparent melt viscosity. Known as
internal lubricants, such materials are reasonably compatible with the
polymer and are rather like plasticizers in their behaviour at processing
temperatures, although at room temperature this effect is negligible. Such
materials do not retard gelation, cause haze or cause greasiness. It should
be pointed out that whereas the above classification of lubricants into
internal and external types appears clear cut, in reality the situation is more
complex and some materials seem to have a more or less dual function.
Amongst materials which are usually classified as internal lubricants are
wax derivatives, particularly from montan wax, glyceryl esters, particularly
glyceryl monostearate, and long chain esters such as cetyl palmitate.
It has been pointed out that for rigid PVC extrusion compositions
best results are obtained with a lubricant, or mixture of lubricants, whi ch
melt in the range 100-120°C, since these generally give a lubricating film
of the correct viscosity at the processing temperature of about 165°C. For
calendering operations it is suggested that lubricants should be chosen with
higher melting points, i.e. in the range 140-160°C. Aluminium and
magnesium stearate fall within this melting point range.
Fillers
Fillers are commonly employed in opaque PVC compounds in
order to reduce cost. They may also be employed for technical reasons such
as to increase the hardness of a flooring compound, to reduce tackiness of
highly plasticized compounds, to improve electrical insulation properties
and to improve the hot deformation resistance of cables.
In evaluating the economics of filler it is important to consider the
volume of filler that can be added bringing the processing and service
properties below that which can be tolerated. Thus in some cases it may be
more economical to use a filler with a higher volume cost because more can
be incorporated. To judge the economics of filler simply on its price per
unit weight is of little merit.
For electrical insulation china clay is commonly e mp loyed whilst
various calcium carbonates (whiting, ground limestone, precipitated
calcium carbonate, and coated calcium carbonate) are used for general
purpose work. Also occasionally employed are talc, light magnesium
104
carbonate, barytes (barium sulphate) and the silicas and silicates. For
flooring applications asbestos has been an important filler.
Pigments
A large number of pigments are now commercially available which
are recommended for use with PVC. Before selecting a pigment the
following questions should be asked:
(1) Will the pigment withstand processing conditions anticipated,
i.e. will it decompose, fade or plate-out?
(2) Will the pigment adversely affect the functioning of stabilizer
and lubricant?
(3) Will the pigment be stable to conditions of service, i.e. will it
fade, be leached out or will it bleed?
(4) Will the pigment adversely affect properties that are relevant to
the end-usage?
When there remains a choice of pigments which fulfil the above
requirements then economic factors have to be taken into account. The cost
function relevant is again not the weight cost or volume cost but the cost of
adding the amount of pigment required to give the right colour in the
compound. Thus a pigment with a high covering power may be more
economic to use than a pigment of lower cost per pound but with a lower
covering power.
Polymeric impact modifiers and processing aids
Unplasticized PVC has a high melt viscosity leading to some
difficulties in processing. The finished product is also too brittle for some
applications. In order to overcome these problems it has become common
practice to add certain polymeric additives to the PVC. The impact
modifiers generally are semi-compatible and often somewhat rubbery in
nature. In practice it seems that the greatest improvement in impact strength
occurs with polymer additives having a solubility parameter about 0.4-0.8
l/2
MPa
different from that of PVC (δ=19.4 MPa
1/2
).
Butadiene-acrylonitrile copolymers (nitrile rubbers) were the most
important impact modifiers. Today they have been largely replaced by
acrylonitrile-butadiene-styrene (ABS) graft terpolymers, methacrylate-butadiene-styrene (MBS) terpolymers, chlorinated polyethylene, EVA-PVC
graft polymers and some polyacrylates.
ABS materials are widely used as impact modifiers but cause
opacity and have only moderate aging characteristics. Many grades show
105
severe stress-whitening, generally a disadvantage, but a phenomenon
positively employed in labelling tapes such as Dymotape.
There are a number of applications such as bottle and film where
tough materials of high clarity are desired. The advent of MBS material has
been a significant advance to meet the requirements. It has been found
possible here to produce an additive with sufficiently different solubility
parameters from the PVC for it to exist in the disperse phase but with a
very similar refractive index to the PVC so that light scattering at the
interface between the two phases is at a minimum. However, owing to
differences in the formulation of PVC compounds, a particular MBS
modifier may not have exactly the same refractive index as the PVC
compound.
When the disperse phase has a slightly higher refractive index the
compound tends to be blue; when it is lower than that of the PVC the
compound tends to be yellow and hazy. In order to overcome this a
carefully determined quantity of a second MBS additive, with an
appropriate refractive index and which is compatible with the PVC
compound and hence forms a continuous phase with it, may be added to
match the refractive indices. Such a matching operation should be
evaluated at the proposed service temperature range of the product since the
temperature coefficients of the two phases are usually different and a film
which is blue at processing temperature may become yellow at 20°C.
MBS materials vary considerably in their tendency to cause stresswhitening in PVC and in their effect on impact strength. They are generally
considered to lead to better aging than ABS additives but are marginally
more expensive.
Chlorinated polyethylene has also been widely used as an impact
modifier, particularly where good aging properties are required. Such good
aging behaviour arises from the absence of butadiene and hence double
bonds in such materials. Such materials tend, however, to give lower
softening points and higher processing die swell to the PVC compounds.
In addition to acting as impact modifiers a number of polymeric
additives may be considered as processing aids. These have similar
chemical constitutions to the impact modifiers and include ABS, MBS,
chlorinated polyethylene, acrylate-methacrylate copolymers and EVA-PVC
grafts. Such materials are more compatible with the PVC and are primarily
included to ensure more uniform flow and hence improve surface finish.
They may also increase gelation rates. In the case of the compatible MBS
polymers they have the special function already mentioned of balancing the
106
refractive indices of the continuous and disperse phases of impact-modified
compound.
Miscellaneous additives
A number of ingredients may be used from time to time in PVC
formulations. For example, blowing agents such as azodicarbonamide and
azodi-iso-butyronitrile are frequently used in the manufacture of cellular
PVC.
Antimony oxide is useful in improving the fire retardance of PVC
compounds. This is sometimes necessary since, although PVC itself has
good flame retardance, phthalate plasticizers will burn.
For some applications it is necessary that static charge should not
accumulate on the product. This is important in such diverse applications as
mine belting and gramophone records. The use of antistatic agents such as
quaternary ammonium compounds has been of some limited value in
solving this problem.
The viscosity of PVC pastes may be reduced in many instances by
the presence of certain polyethylene glycol derivatives and related
materials. Because of their tendency to exude, the use of these viscosity
depressants should be restricted to levels of less than 1% of the total mix.
Formulations
It is obvious that the range of possible formulations based on
poly(vinyl chloride) and related copolymers is very wide indeed. For each
end-use the requirements must be carefully considered and a formulation
devised that will give a compound of adequate properties at the lowest cost.
In assessing cost it is not only important to consider the cost of the
compound but also comparative processing costs, the possible cost of
storing additional materials and many other cost factors.
Properties of PVC compounds
Because of the wide range of possible formulations it is difficult to
make generalisations about the properties of PVC compounds. This
problem is illustrated in Table 5, which shows some differences between
three distinct types of compound.
Unplasticized PVC is a rigid material whilst the plasticized
material is flexible and even rubbery at high plasticizer loadings. It is of
interest to note that the incorporation of small amounts of plasticizer, i.e.
less than 20%, does not give compounds of impact strength higher than that
107
of unplasticized grades, in fact the impact strength appears to go through a
Parameter
Unplasticized
Vinyl chloride-
copolymer
PVC + 50
Specific gravity
Vicat softening point,
1.4
1.35
1.31
temperature
minimum at about 10% plasticizer concentration. As a result of this
behaviour, lightly plasticized grades are used only when ease of processing
is more important than in achieving a compound with a good impact
strength.
Table 5 – Properties of three types of PVC compound
Tensile strength, MPa
Elongation at break, %
ºC
some plasticizers, particularly the less polar ones such as dibutyl sebacate,
are extracted by mat erials such as iso-octane. The polymer is also resistant
to most aqueous solutions, including those of alkalis and dilute mineral
acids. Below the second order transition temperature, poly(vinyl chloride)
compounds are reasonably good electrical insulators over a wide range of
frequencies but above the second order transition temperature their value as
an insulator is limited to low-frequency applications. The more plasticizer
present, the lower the volume resistivity.
points than the homopolymers and compounds and may be processed at
lower temperatures than those used for analogous homopolymer
compounds. The copolymers have better vacuum-forming characteristics,
are soluble in ketones, esters and certain chlorinated hydrocarbons but have
generally an inferior long-term heat stability.
polymers is most conveniently made under the following divisions:
PVC
58
5
80
vinyl acetate
48
5
70
p.h.r. DIOP
19
300
flexible at
room
Poly(vinyl chloride) has a good resistance to hydrocarbons but
Vinyl chloride-vinyl acetate copolymers have lower softening
Processing
Consideration of the methods of processing vinyl chloride
(1) Melt processing of plasticized PVC.
(2) Melt processing of unplasticized PVC.
(3) Processing of pastes.
(4) Processing of latices.
(5) Copolymers.
108
Plasticized PVC
The melt processing of plasticized PVC normally involves the
following stages:
(1) Pre-mixing polymer and other ingredients.
(2) Fluxing the ingredients.
(3) Converting the fluxed product into a suitable shape for further
processing, e.g. granulating for injection moulding or extrusion.
(4) Heating the product to such an extent that it can be formed by
such processes as calendering, etc. and cooling the formed product before
removal from the shaping zone.
Over the years dry blending techniques have become more popular.
In these processes the mixture of ingredients is either subjected to vigorous
stirring, gentle heating or both. As a result of such treatment the plasticizer
is absorbed into the polymer particles to give dry free-flowing powders.
This process is most easily worked with easy-processing polymers.
A large amount of plasticised PVC is fabricated by calendering
techniques using calenders of either the inverted-L or, preferably, of the
inclined-Z type.
Fluidised-bed techniques, pioneered with low-density polyethylene,
have been applied to PVC powders. These powders can be produced by
grinding of conventional granules, either at ambient or sub-zero
temperatures or by the use of dry blends (plasticised powders). The
fluidised bed process is somewhat competitive with some well-established
paste techniques, and has the advantage of a considerable flexibility in
compound design.
Unplasticized PVC
The processing of unplasticised PVC (UPVC) is more critical than
that of the plasticised material since UPVC only becomes processable in
the temperature range at whi ch decomposition occurs at a measurable rate.
Any unnecessary heating, either through a needless processing stage, by
undue frictional working of the viscous melts, by too high temperature
settings on the equipment or by poor flow lines which cause hold-up of the
polymer in processing machines, should be avoided. At the same time the
polymer compound should be designed so that it has a low melt viscosity
and of a high degree of thermal stability such as is provided by the newer
organo-tin stabilisers.
Most UPVC compounds are prepared by dry blending of powders.
Not only does this avoid unnecessary heating in internal mixers, mills and
109
granulating equipment but it also leads to substantial economies
Twin-screw extruders now dominate the extrusion field,
particularly because of their positive pumping action which is so important
with PVC in powder form.
Because PVC evolves corrosive hydrochl oric acid on heating, care
should be taken in the choice of metals for machine construction and the
use of plating and/or special steels is widely practised.
Extrusion blow moulding of bottles has been successfully
accomplished in recent years.
Injection moulding of unplasticised PVC was only really made
possible by the advent of the in-line screw preplasticizing machines. As
with extrusion the main points to bear in mind are the high melt viscosity,
the need to avoid overheating and steel corrosion by hydrochloric acid
evolved during processing. In practice this demands good control of
operating conditions, short runners, reasonably generous gates and mould
cavities which, preferably are either chrome or gold plated.
Copolymers
Vinyl c hl ori de -vinyl acetate copolymers may be processed at lower
temperatures than those used for the homopolymer. Their main applications
were in gramophone records and flooring. Gramophone record
compositions are unfilled and contain only stabilizer, lubricant, pigment,
and, optionally, an antistatic agent. Preheated compound is normally
moulded in compression presses at about 130-140°C. Flooring
compositions contain about 30-40 parts plasticizer per 100 parts copolymer
and about 400 parts filler (usually a mi xture of asbestos and chalk). Internal
mixer discharge temperatures are typically about 130°C whilst calender roll
temperatures are usually some 10-20°C below this.
Applications
PVC first became well known during World War II as a substitute
for natural rubber for wire insulation and for waterproof sheeting. After the
war it retained some of these applications and found additional uses in the
areas of flexible sheeting, hose piping and small mouldings. In many
instances poor formulation gave the material a bad reputation which took
some years to eradicate. Over the years many of the additives came under
scrutiny concerning their toxicity but the discovery that the monomer had a
number of undesirable toxic characteristics caused considerable alarm and
revision of manufacturing procedures in the 1970s. More recently there
110
have been worries concerning the use of plasticizers in applications
requiring contact with food. In addition there has been concern about the
nature of the decomposition products of fire and of composting. Finally, as
a long-established material it was first used in a more utilitarian age so that
many of its early applications may be considered to be downmarket or,
indeed, obsolescent. Not surprisingly, it has been subject to increasing
substitution by newer polymers.
In spite of these problems it was estimated in late 1997 that
consumption at the end of the century would be about 24·106 t.p.a., second
only to polyethylene (46·106 t.p.a.) and just ahead of polypropylene (20·10
6
t.p.a.).
Over 80% of the market is accounted for by suspension
homopolymer, 13% by emulsion and paste-forming homopol ymer, and the
rest is mainly bulk homopolymer and vinyl chloride-vinyl acetate
copolymer. In Western Europe about two-thirds of consumption is in the
field of unplasticized PVC.
One reason for the success of PVC is its formulation versatility.
Products range from rigid piping and window frames to soft flexible foams,
with such diverse materials as vinyl leathercloth, flexible sheeting and
playballs somewhere in between.
The principal advantages of UPVC are:
(1) Low cost.
(2) Good resistance to burning, which may be further enhanced by
appropriate selection of additives.
(3) Excellent weathering behaviour.
(4) Good clarity.
(5) Very good chemical resistance – par ticularly to hydrocarbons.
(6) Rigidity and toughness.
The largest single use area for UPVC is for pipes and fitting s.
The second largest market is that of profiles, particularly for the
building industry. UPVC has become well established for guttering, waste
piping and conduits, where economies arise not just in basic product costs
but also in transportation and installation costs. Unlike with cast iron
products, c orrosion and maintenance is less of a problem, although UPVC
products are more liable to damage. UPVC is widely used for soil pipes and
for drainage, with pipes up to 60 cm in diameter being not uncommon.
After many years of steady development and careful formulation, UPVC
has become widely accepted for window frames. UPVC profiles have also