Plastics Materials 7 Episode 7 docx

Compounding Ingredients
327
of the samples are then bound on to cards giving data on the processing and
heating conditions. These are then available for visual comparison of colour
forming. For technical reports it is common to give the colour a numerical rating.
Thus water-white samples can be given a number
0,
a pale yellow
1,
an orange
2,
a brown
3
and a black sample
4
and these numbers can be tabulated in the
report. It is, however, common experience that such a technique is far less
effective in imparting information than the use of well-displayed samples.
In some laboratories samples are heated for prolonged periods in a press at a
suitably elevated temperature. Such results may frequently fail to correlate with
oven-heated samples since oxygen is largely excluded from the samples.
As already indicated, the measurement of dehydrochlorination rates is not a
practical way of assessing the effect of a stabiliser. Thus the congo red test
sometimes specified in standards, in which a piece of congo red paper is held in
a test tube above a quantity of heated PVC and the time taken for the paper to
turn blue due to the evolution
of
a certain amount of hydrogen chloride, cannot
be considered as being of much value.
Many stabilisers are also useful in improving the resistance of PVC to
weathering, particularly against degradation by ultraviolent radiation. This is an

important consideration in building applications and other uses which involve
outdoor exposure. The efficiency
of
stabilisers in improving resistance of PVC
compounds to degradation is best measured by lengthy outdoor weathering tests.
Accelerated weathering tests using xenon lamps or carbon arcs have not proved
to be reliable even for purposes of comparison.
The most important class of stabilisers are the lead compounds which form
lead chloride on reaction with hydrogen chloride evolved during decomposition.
As
a class the lead compounds give rise to products of varying opacity, are toxic
and turn black in the presence of certain sulphur-containing compounds but are
good heat stabilisers.
Of these materials
basic lead carbonate (white lead)
has been, and probably
still is, the most important stabiliser for PVC. It may be considered as typical of
the lead compounds and has a low weight cost. It is appreciated that weight cost
is not, however, the best criterion to be considered in assessing the economics of
a stabiliser. Far more relevant is the cost required to stabilise the material to an
acceptable level for the processing and service conditions involved. One
additional disadvantage of the lead carbonate is that it may decompose with the
evolution of carbon dioxide at the higher range of processing conditions, thus
leading to a porous product.
For this reason
tribasic lead sulphate,
a good heat stabiliser which gives
polymer compounds with better electrical insulation properties than lead
carbonate, has increased in popularity in recent years at the expense of white
lead. Its weight cost is somewhat higher than that of lead carbonate but less than

most other stabilisers. This material is used widely in rigid compounds, in
electrical insulation compounds and in general purpose formulations.
Other lead stabilisers are of much specific applications.
Dibasic lead phosphite
gives compounds of good light stability but because of its higher cost compared
with the sulphate and the carbonate its use is now restricted. In spite of its even
greater weight cost
dibasic lead phthalate
finds a variety of specialised
applications. Because it is an excellent heat stabiliser it is used in heat-resistant
insulation compounds (for example in 105°C wire). It is also used in high-fidelity
gramophone records, in PVC coatings for steel which contain polymerisable
plasticisers and in expanded PVC formulations which use azodicarbonamide as
328
Vinyl Chloride Polymers
a blowing agent.
In
this latter application the phthalate stabiliser also acts as a
‘kicker’ to accelerate the decomposition of the blowing agent.
Normal
and
dibasic lead stearate
have a stabilising effect but their main uses are
as lubricants (see section 12.5.4).
Lead silicate
is sometimes used in leathercloth
formulations but is today of little importance. Other lead compounds now
of
negligible importance are coprecipitated
lead orthosilicates

and
lead salicylate.
The use
of
lead compounds as stabilisers has been subject to regulation and
legislation arising from their toxicity. Whilst legislation varies from country to
country, lead stabilisers are not generally allowed in food packaging compounds
but in most countries are allowed for use, under certain conditions, in pipes for
conveying drinking water. Where lead stabilisers continue to be used there has
commonly been a reduction in the level of use in a particular compound. The
resulting lower level
of
protection may cause problems in the use
of
scrap and in
general polymer re-work operations.
Whilst lead compounds have been, and still are, the most important class of
stabiliser for PVC the metallic soaps or salts have steadily increased in their
importance and they are now widely used. At one time a wide range of
metal
stearates, ricinoleates, palmitates
and
octoates
were offered as possible stabilisers
and the efficiency of many of them has been examined. Today only the compounds
of
cadmium, barium, calcium
and
zinc
are prominent as PVC stabilisers.

The most important of these are
cadmium-barium
systems. These first became
significant when it was discovered that stabilisers often behaved synergistically.
Of the many stabilising systems investigated, cadmium-barium systems gave
considerable promise. The first of these systems to be used successfully were
based
on
cadmium octoate in conjunction with barium ricinoleate. Alone the
cadmium salt gave good initial colour but tumed black after a relatively short
heating period. The use of the barium soap in conjunction with the cadmium salt
extended this period. The addition of antioxidants such as
trisnonyl phenyl
phosphite
was found to greatly increase the heat stability whilst the further
addition of
epoxidised oils
gave even better results.
It
was, however, found that
on
exposure to light an interaction took place between the ricinoleate and the
epoxidised oil, with the formation of products incompatible with the PVC. These
products exuded and caused tackiness
of
the compound and problems in
calendering. Replacement of the octoate and ricinoleate with
laurates
avoided
the undesirable interaction but instead led to plate-out, difficulties in heat sealing

and printing and compounds yellowish in colour and lacking in clarity. However,
the laurates continue to find some limited use in PVC compounding.
Somewhat better results have been obtained with
octoates
and
benzoates
but
these still lead to some plate-out. The use of liquid
cadmium-barium phenates
has today largely resolved the problem of plate-out whilst the addition of a trace
of a zinc salt helps to improve the colour. Greater clarity may often be obtained
by the addition of a trace of stearic acid or stearyl alcohol. Thus a modem
so-
called cadmium-barium stabilising system may contain a large number of
components.
A
typical ‘packaged’ stabiliser could have the following
composition:
Cadmium-barium phenate 2-3 parts
Epoxidised oils 3-5 parts
Stearic acid
0.5-1
part
Trisnonyl phenyl phosphite
1
part
Zinc octoate
0.5
parts
Compounding Ingredients

329
It appears that the zinc salt functions by preferentially reacting with sulphur to
form
white zinc sulphide rather than coloured cadmium sulphide and thus helps
to reduce colour in the compound.
The use of cadmium stabilisers, as with the lead compounds, gives rise to some
concern because of possible toxicity and environmental problems. This has led to
large efforts to produce non-toxic systems. Mixtures
of
calcium and zinc soaps,
sometimes in conjunction with epoxidised oils, have been used for many years
but these soap-based materials are both less powerful than the Cd-Ba complexes
and also fail to give glass-clear products. Calcium/zinc non-soap liquid
compounds, and to some extent, strontium/zinc compounds have found
increasing use in recent years in efforts to cope with this problem. For flooring
compositions, magnesium-barium, calcium-barium and copper-barium com-
pounds are sometimes used in conjunction with pentaerythritol. The latter
material has the function of chelating iron present in the asbestos and thus
reducing colour formation.
Another group of stabilisers are the
organo-tin compounds.
These materials
found early applications because of their resistance to sulphur and because they
can yield crystal-clear compounds. The older organo-tin compounds such as
dibutyltin dilaurate,
however, give only limited heat stability and problems may
arise with high processing temperatures.
Dibutyltin maleate
imparts somewhat
greater heat resistance. The availability of a number of sulphur-containing

organo-tin compounds, such as
dibutyltin
di-iso-octylthioglycollate,
which
impart excellent heat resistance and clarity, has to some extent increased the
scope of organo-tin compounds. They
are,
however, more expensive in terms of
weight cost. It should be noted that the sulphur-containing organo-tin compounds
should not be used where lead derivatives are present in the PVC compound
since cross-staining will occur to form black lead sulphide. Such lead compounds
could be present as added stabiliser or even because the polymer on manufacture
was washed with water fed through lead pipes.
The butyltins generally show a level of toxicity that prevents them being used
in application in contact with foodstuffs. On the other hand the octyltin materials
such as dioctyltin dilaurate and dioctyltin octylthioglycollate are much better in
this respect and many of them meet stringent requirements for use in contact with
foodstuffs. The low toxicity, excellent stabilising performance and improving
relative price situation has led to considerable growth in the organo-tin market
during the
1970s
and their status has changed from special purpose to that of
general purpose stabilisers. Furthermore, additions to this class of material have
been made, including the ester tins, characterised by low odour, volatility and
toxicity, and the methyl tins which, with their high metal content, may be used
in lower amounts than the more common organo-tins to achieve comparable
efficiency.
Mention has already been made of
epoxide
stabilisers. They are of two classes

and are rarely used alone. The first class are the epoxidised oils, which are
commonly employed in conjunction with the cadmium-barium systems. The
second class are the conventional bis-phenol A epoxide resins (see Chapter
22).
Although rarely employed alone, used in conjunction with a trace of zinc octoate
(2
parts resin,
0.1
part octoate) compounds may be produced with very good heat
stability.
There has been a revival of interest in recent years in antimony mercaptides as
alternatives to the organo-tin stabilisers. This stems from the low level
of
toxicity
and the strong synergism with calcium stearate. However, compared to the
330 Vinyl Chloride Polymers
organo-tins they have lower resistance to sulphur staining and to ultraviolet
radiation, particularly with transparent sheet.
A further class of stabilisers are the amines, such as diphenylurea and
2-phenylindole. These materials are effective with certain emulsion polymers but
rather ineffective with many other polymers.
There is somewhat more interest in the aminocrotonates of the general
formula
CH,
,
C=CH-CO-
OR
/
NH,
Figure

12.19
many of which are approved for food packaging applications in Western Europe.
They are used mainly in UPVC compounds for packaging film and blow
moulded containers.
In
addition to stabilisers, antioxidants and ultra-violent absorbers may also be
added to PVC compounds. Amongst antioxidants, trisnonyl phenyl phosphite,
mentioned previously, is interesting in that it appears to have additional functions
such as a solubiliser or chelator for PVC insoluble metal chlorides formed by
reaction of PVC degradation products with metal stabilisers. Since oxidation is
both a degradation reaction in its own right and may also accelerate the rate
of
dehydrochlorination, the use of antioxidants can be beneficial.
In
addition to the
phenyl phosphites, hindered phenols such as octadecyl 3-(3,5-di-tert-butyl-
4-hydroxypheny1)propionate
and 2,4,6-tris
(2,5-di-tert-butyl-4-hydroxybenzyl)-
1,3,5-trimethylbenzene may be used.
Low levels of ultraviolet absorbers (typically 0.2-0.8 pphr [parts per hundred
resin]) can also be useful in preventing initiation of degradation mechanisms.
Modified benzophenones and benzotriazoles are in widest commercial use.
12.5.2
Plasticisers
The tonnage of plasticisers consumed each year exceeds the annual tonnage
consumption of most plastics materials. Only PVC, the polyolefins, the styrene
polymers, the aminoplastics and, possibly, the phenolics are used in large
quantity.
As explained in Chapter

5,
these materials are essentially non-volatile solvents
for PVC. Because of their molecular size they have a very low rate of diffusion
into PVC at room temperature but at temperatures of about 150°C molecular
mixing can occur in a short period to give products of flexibility varying
according to the type and amount of plasticiser added.
All PVC plasticisers have a solubility parameter similar to that of PVC. It
appears that differences between liquids in their plasticising behaviour is due to
differences in the degree of interaction between polymer and plasticiser. Thus
such phosphates as tritolyl phosphate, which have a high degree of interaction,
gel rapidly with polymer, are more difficult to extract with solvents and give
compounds with the highest brittle point. Liquids such as dioctyl adipate, with
the lowest interaction with polymer, have the converse effect whilst the
phthalates, which are intermediate in their degree
of
interaction, are the best all-
round materials.
Compounding Ingredients
33
1
Phthalates prepared from alcohols with about eight carbon atoms are by far the
most important class and probably constitute about 75% of plasticisers 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 IC1 trade name for the
C7-C9 alcohols-Alphanol 79.) As mentioned in the previous paragraph these
materials give the best all-round plasticising properties. DIOP has somewhat less
odour whilst DAP has the greatest heat stability. Because of its slightly lower
plasticising efficiency, an economically desirable feature when the volume cost
of
a plasticiser is less than that of polymer,
dinonyl phthalate
(DNP) may also be
an economic proposition. Its gelation rate with PVC is marginally less than with
DIOP, DAP and DEHP.
In spite of their high volatility and water extractability,
dibutyl phthalate
and
di-isobutyl phthalate
continue to be used in PVC. They are efficient plasticisers
and their limitations are of greatest significance in thin sheet.
Until comparatively recently the bulk of general purpose phthalate plasticisers
have been based on the branched alcohols because
of
low cost of such raw
material. Suitable linear alcohols at comparative prices have become available
from petroleum refineries and good all-round plasticisers are produced with the
additional advantage of conferring good low-temperature flexibility and high
room temperature resistance to plasticised PVC compounds. A typical material
(Pliabrac
810)
is prepared from a blend of straight chain octyl and decyl

alcohols.
Certain higher phthalates are also available. For example,
ditridecyl phthalate
and
di-isodecyl phthalate
are used in high-temperature cable insulation, the
former having the better high-temperature properties. Because of its greater
hydrocarbon nature than DIOP, di-isodecyl phthalate has lower water extract-
ability and
is
used, for example, with epoxidised oils in baby-pants.
Developments in the
USA
have led to the availability of terephthalate
plasticisers, for example dioctyl terephthalate (DOTP). Whilst these materials are
very similar to the corresponding o-phthalate esters they are generally less
volatile and are best compared with o-phthalates with one or more carbon atom
in the alkyl chain. As with the linear dialkyl phthalates the terephthalates show
good fogging resistance. This is a phenomenon in which new cars on storage
fields awaiting delivery develop misting on the windows due, apparently, to the
volatility of additives in PVC compounds used with the car.
In
the 1950s phosphate plasticisers had an importance comparable with the
phthalates. However, during the
1960s
the development of the petrochemicals
industry resulted in the phthalate plasticisers 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 prompted the petrochemicals industry to make
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Compounding Ingredients
333
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 low-temperature 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). These are esterified with octyl, nonyl and decyl alcohols to
give plasticisers now generally referred to as nylonates but occasionally as
sugludates. The sebacate, adipate and sugludate-type plasticisers can also be used
to give compounds of high resilience.

Esters based on trimellitic anhydride, the trimellitates, have become very
popular primary plasticisers 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 plasticisers are also available. The
epoxidised oils and related materials are good plasticisers and very good light
stabilisers and are often used in small quantities in PVC compounds. Polymeric
plasticisers such as polypropylene adipate, polypropylene sebacate and similar
products capped with lauric acid end groups are used where non-volatility and
good hydrocarbon resistance is important. They are, however, rather expensive
and are rather difficult to flux with PVC. Solid ethylene-vinyl acetate modified
polymers have also recently been offered as polymeric plasticisers (e.g. Elvaloy
by Du Pont). These are claimed to be true plasticisers and are non-volatile, non-
migratory and, unlike most PVC plasticisers, have a high resistance to
biodegradation. Certain esters of citric acid, such as acetyl tributyl citrate, find
an outlet where minimum toxicity is of importance.
The development of PVC as a metal-finishing material has led to the need for
a good PVC-metal adhesive. For some purposes it is found convenient to
incorporate the adhesive component into the PVC compound. Esters based on
allyl alcohol, such as diallyl phthalate and various polyunsaturated acrylates,
have provided useful in improving the adhesion and may be considered as
polymerisable plasticisers. In PVC pastes they can be made to cross-link by the
action
of
peroxides or perbenzoates simultaneously with the fluxing of the PVC.
When the paste is spread on to metal the ‘cured’ coating can have a high degree
of
adhesion. The high adhesion of these rather complex compounds

has
led
to
their development as metal-to-metal adhesives used, for example, in car
manufacture. Metal coatings may also be produced from plasticised powders
containing polymerisable plasticisers by means of fluidised bed or electrostatic
spraying techniques.
Many other liquids have been found to be effective plasticisers for PVC but are
of
limited commercial value, at least in Britain. The effect of plasticisers on the
properties of PVC is illustrated in Figure
12.20
(a-e).
334
PLASTICISER CONTENT IN
%
(a)
Figure
12.20.
See
page
336
for
key
335
0
FILLER
CONTENT IN
'/a
BY

WEIGHT
(d
1
Figure
12.20.
See
page
336
for
key
336
Vinyl Chloride Polymers
PLZSTICISER
CONTENT
IN
'/a
Figure
12.20.
Effect of change of plasticiser on the properties of polyvinyl chloride compounds.'!
(a) Tensile strength.
(b)
Cold flex temperature. (c)
BS
softness number. (d) Elongation at
break
(e)
100%
modulus. (The Distillers Company Ltd.)
12.5.3
Extenders

A
number of materials exist which are not in themselves plasticisers for PVC
because
of
their very limited compatibility with the polymer, but in conjunction
with a true plasticiser a mixture is achieved which has a reasonable compatibility.
Commercial
extenders,
as these materials are called, are cheaper than plasticisers
and can often be used to replace up to a third of the plasticiser without serious
adverse effects
on
the properties of the compound.
Three commonly employed types
of
extender are:
(1)
Chlorinated paraffin waxes.
(2)
Chlorinated liquid paraffinic fractions.
(3)
Oil extracts.
The solubility parameters
of
these extenders are generally somewhat lower
than
that of PVC. They are thus tolerated in only small amounts when conventional
plasticisers
of
low solubility parameter, e.g. the sebacates, are used but in greater

amounts when phosphates such as tritolyl phosphate are employed.
12.5.4
Lubricants
In
plasticised 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
Compounding Ingredients
337
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 the 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 stabilisers 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 additives. Such
a phenomenon, which is often more severe at high shear rates,
is
commonly
known as
plate-out.
In unplasticised 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
plasticisers 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'6 that for rigid PVC extrusion compositions best results
are obtained with a lubricant, or mixture of lubricants, which 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
140-1 60°C.
Aluminium
and magnesium stearate
fall within this melting
point range.
12.5.5
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 plasticised
338
Vinyl Chloride Polymers
compounds, to improve electrical insulation properties and to improve the hot
deformation resistance of cables.

In
evaluating the economics of a 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 a filler simply on its price per unit weight is of little merit.
For electrical insulation
china clay
is
commonly employed 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 carbonate, barytes
(barium
sulphate) and the
silicas
and
silicates.
For flooring applications
asbestos
has been
an important filler. The effect of fillers on some properties of plasticised PVC are
shown in
Figure
12.21
(a-d).

12.5.6
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 stabiliser 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? (NB many pigments will reduce the volume resistivity of a
compound.)
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.
12.5.7
Polymeric Impact Modifiers and Processing Aids
Unplasticised 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. Whilst the mechanism of
toughening is not fully understood it appears to be
on
the lines suggested
in
Chapter
3.
In practice it seems that the greatest improvement in impact strength
occurs with polymer additives having a solubility parameter about
0.4-0.8
MPa1I2
different from that of PVC
(6
=
19.4MPa’’2). Data by Bramfitt
and Heaps” are largely in accord with this supposition
(Table
12.3).
339
FILLER CONTENT IN

'/o
BY
WEIGHT
(a)
"
20
40
FILLER
CONTENT
IN
'/o
BY
WEIGHT
6)
Figure
12.21.
Effect
of
filler content on the properties
of
plasticised
PVC
compounds." (a) Tensile
strength.
(b)
BS
softness
number.
340
4.001

0
I
I
40
60
FILLER CONTENT
IN
*/e
81
WElCHl
(c)
CAR0ON 0LACK
TUFKNIT
CS
PRECIPITITED
WHITING
FILLER
CONTENT IN
*/e
BY
WEIGH1
(d
Figure
12.21.
(c)
Elongation at break. (d) Macklow-Smith flow index.
(The
Distillers Company
Ltd.)
Compounding Ingredients

341
Table
12.3
Effect
of
solubility parameter of rubber on its effect on the impact strength
of
a
PVC-rubber blend
(5
parts rubber
per 100
parts
PVC)”
Solubility parameter
(MPa’”)
Rubber
Izod
impact strength
(ft Ibf/in
of
notch)
Butadiene-2-vinylpridine
(30:70)
Chlorinated polyethylene (44% chlorine)
Butadiene-styrene-acrylonitrile (67: 17: 16)
Butadiene-2-vinylpyridine
(40:60)
Butadiene-methyl methacrylate (35:65)
Butadiene-methyl acrylate

Butadiene-methyl isopropenyl ketone
Butadiene-diethyl fumarate
I
19.3
19.2
18.7
18.7
18.2
17.9
17.4
17.2
3
4.4
17.4
10.0
2.8
3.3
15.9
15.0
The anomalous effect of the last two rubbers in the table with their low
solubility parameters is possibly explained by specific interaction of PVC with
carbonyl and carboxyl groups present respectively in the ketone- and fumarate-
containing rubbers to give a more than expected measure
of
Compatibility. It is
important to note that variation of the monomer ratios
in
the copolymers and
terpolymers by causing changes in the solubility parameter and compatibility will
result in variation in their effect

on
impact strength.
At one time 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-buta-
diene-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 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.
342
Vinyl
Chloride
Polymers
MBS materials vary considerably in their tendency to cause stress-whitening
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 refractive indices of the continuous and disperse
phases of impact-modified compound.
12.5.8
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’8 is useful in improving the fire retardance of PVC
compounds. This is sometimes necessary since, although PVC itself has good
flame retardance, phthalate plasticisers will bum.
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.
12.5.9
Formulations
It
is
obvious that the range of possible formulations based
on
poly(viny1 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.
The few formulations given below are intended as a general guide. They
should not be taken as recommendations for a specific application where many
factors, not considered in the brief discussion here, would need to be taken into
account. Formula
1
gives a typical general purpose insulation compound.
Compounding Ingredients
343
Formula
1
Suspension polymer (IS0
No.
125) 100
DIOP 40

Trixylyl phosphate 20
China clay 20
Tribasic lead sulphate
7
Pigment 2
Stearic acid
0.5
Suspension polymer is chosen because its relative freedom from emulsifier
and other surface active material gives polymers of better electrical insulation
characteristics than emulsion polymers. Di-isoctyl phthalate is a low-cost good
all-round plasticiser whilst some trixylyl phosphate is added to improve the fire-
retarding properties. China clay is a cheap filler with good insulation properties,
whilst lead sulphate gives compounds of high heat stability, long-term aging
stability and good insulation characteristics. Formula 2 is a transparent
calendering compound
Formula
2
Suspension polymer
(IS0
No.
125) 100
DIOP 40
Ba-Cd phenate
3
Trisnonyl phenyl phosphite 1
Epoxidised oil
5
Stearic acid 0.4
Each of the ingredients is chosen with a view to obtaining high clarity at a
moderate cost. In Formula 3 the stabilising system has been replaced by a less

powerful, but also less toxic, stabiliser.
Formula
3
Suspension polymer (IS0
No.
125) 100
DIOP 40
Ca-Zn stabiliser 2.5
Epoxidised oils
5
The requirements for garden hose are somewhat less critical and both filler and
extender may be incorporated to reduce cost (Formula 4).
Formula
4
Suspension polymer (IS0
No.
125)
100
DAP
50
Extender
25
Whiting
30
Tribasic lead sulphate
6
Calcium stearate
1
Pigment
3

The main requirements for a flooring composition are that it should be hard,
durable and competitive in price with other materials. This calls for highly filled
344
Vinyl
Chloride Polymers
materials which are consequently harder to process than unfilled materials. The
problem is alleviated by use of copolymers with their easier processing
characteristics. Formula
5
is an example of a flooring recipe.
Formula
5
Vinyl chloride-vinyl acetate copolymer
(IS0
No.
55-80)
DAP
Extender
Whiting
Asbestos
Ba-Zn
complex
Calcium stearate
Pigment
100
30
15
150
150
3

1
as required
Copolymers were also used in gramophone record formulations (Formula
6).
No
filler can be tolerated and stabilisers and lubricants are chosen that give
records of minimum surface noise. Antistatic agents may also be incorporated
into the compound.
Formula
6
Vinyl chloride-acetate copolymer
(IS0
No.
60)
Diabasic lead stearate
0.75
Dibasic lead phthalate
0.75
100
Lamp black
2
Formula
7
is a leathercloth formulation for use in spreading techniques. There
are many possible formulations and that given is for a product with a soft dry
feel.
Formula
7
Paste-making polymer (IS0
No.

125-140)
100
DIOP
50
Extender
20
Ba-Cd system
2
Epoxidised oil
3
Pigment as required
In
the case
of
unplasticised polymer the main concern is with control of
gelation and ensuring processability and adequate stabilisation. The formulation
selected thus depends almost as much
on
the actual processing equipment used
as
on
the end-product. Formula
8
gives a typical rigid opaque formulation
suitable for pipes and Formula
9
a transparent calendering compound.
Formula
8
Suspension polymer (IS0

No.
85-100) 100
Tribasic lead sulphate
6
Lead stearate
1
Glyceryl monostearate 0.4
Acrylic process aid
2
Properties
of
PVC Compounds
345
Formula
9
Mass polymer
(IS0
No.
80)
Octyltin stabiliser
1.5
Acrylic process aid
3
.O
Montan wax 2.0
Glyceryl monostearate
0.5
100
MBS impact modifier 10
12.6 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
12.4,
which shows some differences between three distinct
types of compound.
Table
12.4
Properties of three types of PVC compound
Specific gravity
Tensile strength Ibf/in2(MPa)
Elongation at break
%
BS
softness
No.
Vicat softening point
(“C)
Unplasticised
PVC
1.4
8500(
5
8)
5
80

-
Vinyl chloride-
vinyl acetate
copolymer (sheet)
1.35
7000(48)
5
70
-
PVC
-+
50
p.hx
DIOP
1.31
2700(19)
300
35
flexible at room temperature
Mechanical properties are considerably affected by the type and amount of
plasticiser. This was clearly shown in
Figure
12.20.
To a lesser extent fillers will
affect the physical properties, as indicated in
Figure
12.21.
Unplasticised PVC is a rigid material whilst the plasticised material is flexible
and even rubbery at high plasticiser loadings. It is of interest to note that the
incorporation of small amounts of plasticiser, i.e. less than 20%, does not give

compounds of impact strength higher than that of unplasticised grades, in fact the
impact strength appears to
go
through a minimum at about
10%
plasticiser
concentration.
As
a result of this behaviour, lightly plasticised grades are used
only when ease of processing is more important than in achieving a compound
with a good impact strength.
Poly(viny1 chloride) has a good resistance to hydrocarbons but some
plasticisers, particularly the less polar ones such as dibutyl sebacate, are extracted
by materials 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(viny1 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 plasticiser present, the lower the volume
resistivity.
Vinyl chloride-vinyl acetate copolymers have lower softening points than the
homopolymers and compounds and may be processed at lower temperatures than
346
Vinyl Chloride Polymers
IO
20
so
40

0
COMONOMER CONTENT
IN
'/a
BY
WEIGHT
COMONOMER CONTENT
IN
Yo
BY
WEIGHT
COMONOMER CONTENT
IN
'/a
BY
WEIGHT
a
COMOIICMER CONTENT IN
'/o
BY
WEIGHT
Figure
12.22.
Effect of copolymerisation of vinyl chloride with other monomers
on
the properties
of
unplasticised compounds. (After Weldon'')
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.
The effect of percentage comonomer
on
the properties of a copolymer are
illustrated in
Figure
I2.22l9.
12.7 PROCESSING
Consideration of the methods of processing vinyl chloride polymers is most
conveniently made under the following divisions:
(1)
Melt processing of plasticised PVC.
(2)
Melt processing
of
unplasticised PVC.
(3)
Processing
of
pastes.
(4)
Processing
of
latices.
(5)
Copolymers.
Processing
341
12.7.1 Plasticised
PVC

The melt processing
of
plasticised 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.
The many possible variations and modifications to this sequence have been
admirably summarised by Matthews2'
(see
Figure
12.23).
In
most of these routes, premixing is carried out in a trough mixer at room
temperature to give a damp powdery mass or 'mush'. This may then be fluxed
on
a two-roll mill, in an internal mixer, or in a continuous compounder such as the
Werner and Pfleiderer Plastificator. For many operations the compounded mass
r
POLYMER
STAEILISER
LUBRICANT

t
It1
I
el
LAMINATING
HACHINE
XTRUDER
FINISHED
PRODUCT
.i
I
PRESS
I
EXTRUDER
INJECTION MOULDly
I
EXTRUDER
INJIC'TION
t
T
HOULDING
Figure
f2.23.
Routes from raw materials to finished products illustrating different compounding
techniques with
PVC
compounds"
348
Vinyl Chloride Polymers
is then granulated or pelleted. This can be carried out as part of the continuous

compounding process, whereas a mass mixed in an internal mixer may be either
fed to an extruder-pelletiser, which extrudes strands which are then cut up into
pellets, to an extruder-slabber, which produces a sheet subsequently fed to a
dicing machine, or to a two-roll mill, which also provides a sheet for subsequent
dicing. Sheet may also be fed directly to a calender whilst still hot and it may also
be used for pressing into sheet.
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 plasticiser is absorbed
into the polymer particles to give dry free-flowing powders. This process is most
easily worked with easy-processing polymers. Although the intensity
of
mixing
is not
so
great as in
an
internal mixer, mixing is taken to a stage where it can be
subsequently completed during the plasticising stage of an extrusion operation.
The important advantage of this process is that it frees the polymer from
subjection to one high-temperature process and thus reduces risk of decomposi-
tion and of deterioration in electrical insulation properties. For successful use of
dry blending processes it is important to ensure an adequate degree of mixing in
the product. This involves not only care in the development of the mixing process
but also care in the choice of extrusion machinery and conditions.
Extrusion operations are involved in making cables, garden hose-pipes and
sections. It is necessary to ensure the following conditions:
(1)
That the compound is dry. (This is not normally a problem and special
predrying operations are rarely necessary if the material has been properly

stored.)
(2)
That the compound is not allowed to stagnate in heated zones of the extruder
and thus decompose. The ‘life’ of many compounds at processing
temperatures is little more than the normal residence time of the material in
the extruder.
(3)
That there is a good means of temperature control.
It is not possible to give detailed recommendations of processing conditions as
these will depend on the nature of the product, the formulation of the compound
used and the equipment available. However, the temperature is generally
of
the
order of 130°C at the rear of the barrel and this increases to about 170-180°C at
the die. Screw speeds are of the order of
10-70
rev/min. A typical screw would
have a length/diameter ratio of about 15:l and a compression ratio of
2:l.
It is,
however, possible to extrude plasticised PVC on extruders with lower length/
diameter ratios and compression ratios, particularly with extruders
of
screw
diameters in excess of 5cm. In some cases it may, however, be necessary to
improve the degree
of
homogenisation by use of stainless steel mesh screens
behind the breaker plate and by the use of screw cooling water. Higher length-
diameter screws are often preferred for dry blends in order to ensure adequate

mixing.
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. A
major problem is the control of gauge (thickness). Transverse variations due to
bowl bending may be reduced or partially compensated for by the use
of
bowls
of greater diameter/width ratio, by cambering the bowls, by bowl cross-
alignment or by deliberate bowl bending. Longitudinal variation may be largely
Processing
349
eliminated by preloading the bowls to prevent the journals floating in their
bearings. For technical reasons it is more convenient to preload
on
Z-type
calenders than
on
L-and inverted L-type calenders. Bowl temperatures are in the
range 140-200°C. Large-scale leathercloth manufacture is today commonly
carried out using calendering techniques.
Plasticised PVC was not easily injection moulded in ram-operated machines.
This is because it is difficult to bring compound furthest from the cylinder wall
to a temperature which gives the compound adequate flow without decomposing
the material nearest to the cylinder wall. The introduction of preplasticising
machines, which amongst other advantageous characteristics do not rely
on
heat
transfer solely by conduction, has largely overcome this problem. A number of

special low-pressure machines have been especially developed for use in the
manufacture of PVC shoes and shoe soles. For these purposes, compounds based
on lower molecular weight homopolymers (K-value
55-60)
or copolymers are
frequently employed.
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.
12.7.2
Unplasticised
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 which 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 (by the use of polymers with low
IS0
number
and the incorporation of internal lubricants and process aids) and of a high degree
of thermal stability such as is provided by the newer organo-tin stabilisers.

Due consideration of these principles has made it possible to process
unplasticised PVC by all the standard melt processes, including injection
moulding and bottle blowing, a state of affairs hardly conceivable in the
1950s.
Most UPVC compounds are prepared by dry blending of powders. Not only
does this avoid unnecessary heating in internal mixers, mills and granulating
equipment but it also leads to substantial economies. The fine powders do,
however, cause problems of dust and contamination of hydraulic systems of
processing plant unless particular care is taken. High-speed mixers are preferred,
frictional heating causing a temperature rise bringing the PVC above its
Tg
and
thus facilitating rapid absorption of liquid and semi-solid additives.
Too
early
addition of lubricants may retard the heat build-up and the point of their addition
can be critical. The blends continue to increase in temperature as mixing
proceeds, causing agglomeration and an increase in bulk density. This leads to
increasing output in, for example, extruders but also reduces the thermal stability
of the compound.
Twin-screw extruders now dominate the extrusion field, particularly because
of their positive pumping action which
is
so
important with PVC in powder form.
350
Vinyl
Chloride
Polymers
(This is probably because efficient pumping of the granules or powder in a

single-screw extruder depends
on
the adhesion of the polymer to a hot barrel
being greater than that of the polymer to a cooler screw. With PVC there is a
much lower temperature coefficient of adhesion than with other polymers.)
In
some cases the desired compression ratio (of about
2:l)
is achieved by using
tapered barrels. Dies must be designed
so
that there is
no
chance of polymer
stagnation and all flow path cross-sections should only change slowly, with
narrow lead-in angles to the die parallels. Accurate temperature control is also
clearly important. As previously mentioned the selection of lubricant is critical in
that it controls the point of gelation in the extruder barrel; too early a gelation
leads to unnecessary working and frictional heat, too late a gelation leads to
imperfect extrudates.
Because PVC evolves corrosive hydrochloric 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 by attention to the points mentioned above. It is to be noted here that
UPVC has a much lower average specific heat between the processing
temperature and room temperature than polyethylene and, being essentially
amorphous,

no
latent heat
of
fusion. This leads to much less heat needing to be
removed
on
cooling of mouldings and very short cycle times are possible.
Injection moulding of unplasticised PVC was only really made possible by the
advent of the in-line screw preplasticising 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. Although it is possible to extrude rigid PVC sheet, it is commonly made
by compression moulding techniques, either by laminating hide from a sheeting
or mixing mill or by moulding granules. Such sheet may be welded using hot gas
welding guns to produce chemical plant and other industrial equipment. The
sheet may be shaped by heating and subjecting it to mechnical or air pressure.
The methods used are similar to those originally developed to deal with
poly(methy1 methacrylate).
UPVC film or sheet may also be made by the Luvitherm process using a
technique used only with PVC. High molecular weight PVC
(IS0
No.
145-165)
is compounded and partly agglomerated in an extruder-mixer. The heated mix is
then fed to an L-type calender (a vertical three-roll calender with a fourth roll
horizontally aligned with the bottom roll of the three-roll stack). The hot calender

rolls simply partly consolidate the granules
so
that the resulting film or sheet is
strong enough to be drawn over a train of heated drums which are well above the
fluxing temperature of the compound. The PVC is therefore subjected to only a
very short but intense heating process. The resulting films with the high
mechanical properties consequent on the use of high molecular weight polymers
are used for magnetic tapes and for packaging applications.
12.7.3
Pastes
As explained in Section
12.4.1
a
paste is obtained when the voids between
particles are completely filled with a plasticiser
so
that the polymer particles are
suspended in it. It has also been pointed out that to ensure a stable paste there
is
Processing
35
1
an upper and a lower limit to the order of particle size. Finally it has been stressed
that both the flow and fluxing characteristics of a paste are, to
no
small extent,
dependent on the particle shape, size and size distribution.
A
number of basic paste types may be distinguished. The most important
classes are the plastisols, the organosols, plastisols incorporating filler polymers

(including the rigisols), plastigels, hot melt compounds, and compounds for
producing cellular products.
The first four types are most conveniently distinguished by reference to
formulations
A
to
D
in
Table
12.5.
Formulation
A
is a conventional
plastisol.
The
viscosity of the paste is largely controlled by the choice of type and amount of
polymer and plasticiser.
In
order to achieve a sufficiently low viscosity for
processing, large quantities of plasticiser must
be
added, thereby giving a product
of lower hardness, modulus, tensile strength and other mechanical properties than
would be the case if less plasticiser could be used. In many applications this is
not a serious problem and plastisols are of some considerable importance
commercially.
Table
12.5
A
100

80
10
4
-
-
-
Paste-making polymer
Filler polymer
Plasticiser (e.g. DOP)
Filler (e.g. china clay)
Stabiliser
(e.g.
white lead)
Naphtha
Aluminium stearate
B
C
100
55
45
30
80
10
10
4
4
50
-
-
-

-
~
D
100
80
10
4
4
-
-
The influence of plasticiser content on viscosity is shown in
Figure
12.24.
It
is also to be noted that because of plasticiser absorption the viscosities of pastes
do invariably increase on storage. The rate of increase is a function of the
plasticiser used
(Figure
12.25).
Figure
12.24.
Effect
of
plasticiser (diaphanyl phthalate)
on
initial paste
commercial paste polymers
viscosity with
two