CHAPTER 1

INTRODUCTION

1. PLASTICIZERS

1.1. HISTORICAL OVERVIEW

The plasticizing effect of oils, waxes and balsams on brittle natural

resins and varnishes was known in antiquity and has been applied for a
long time. Success in the search for new materials was first attained
over a hundred years ago. Already at that time it was possible to nitrate
cotton, polymerize some vinyl compounds and condense phenol and
formaldehyde to form resinous materials. However, conversion of this
knowledge into large-scale production had to wait until the 20th century
when cellulose derivatives, polycondensates and, particularly, vinyl
polymers were introduced on an industrial scale giving an unexpected
impetus to plastics technology.

In the beginning, the situation with regard to plasticizers was

unsatisfactory; manufacturers of celluloid or celluloid lacquers had to
make do with natural camphor and castor oil. The discovery of triphenyl
phosphate in 1912, later used as a substitute for camphor, was a turning
point, the importance of which was not immediately recognized. This
discovery ushered in the era of ester plasticizers. The most important
resulting product was tricresyl phosphate, which is still in use today.
For a time, tributyl phosphate was highly regarded for cellulose derivatives
but lost its importance when it was replaced by less volatile products. The
same was true of glycerin acetates. From 1920 onwards, the first phthalic
acid esters, which had been known since 1908, extended the still small
range of plasticizers. Within this group di-butyl phthalate gained a
dominant position, which it held for many years and continues to hold
today for polyvinyl acetate dispersions.

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 Di (2-ethylhexyl) phthalate, known since 19301, became important de-
cades later. I ts commercial development is closely associated with the
expansion of polyvinyl chloride production. Today, more plasticizers are
consumed in processing PVC than in any other thermoplastic resin and
phthalic acid esters dominate the market both in number and production
volume.

Pressure for the development of specialty plasticizers came from

increased qua lit y requirements, the need for materials with specific char-
acteristics and compatibility problems relating to particular products. Food
legislation, industrial safety and commercial aspects played a not
inconsiderable role and. over the last 40 years, have led to the development
of the vast range of plasticizers currently available.

In itiall y only a few fatty acid esters, benzoates tartrates, and

chlorinated hydrocarbons were available but were soon joined by esters of
adipic, azelaic and sebacic acid. The latter immediately attracted interest
because they considerably improve the cold fracture temperature of
plasticized PVC. There followed numerous polyester plasticizers with high
resistance to extraction and migration. Epoxidized fatty acid esters were
especially attractive because of their synergetic thermo-stabilizing effect.
Ethyl butyric and nonionic acid esters used for the manufacture of
polyvinyl butyral films made a contribution to more stringent safety
requirements in the automobile industry because they made possible the
manufacture of laminated glass windshields. Requirements, mainly from
the cable industry, for nonvolatile plasticizers, which could be thermally
stressed over long periods of time, were first satisfied with di (isotridecyl)
phthalate and later on with the trimellitic acid esters. The lack of light
stability shown by the triaryl phosphates was countered by the introduction
of alkylated phenols as esterification components. Linear, or predominantly
linear, C6 to C11 alcohols available from new technology, have given
further impetus in recent years. Therefore, a large number of high perfor-

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mance plasticizers can be produced nowadays by systematic etherification
with aliphatic or aromatic carboxylic acids available in large-scale
production.

1.2. PLASTICIZERS AND PLASTICIZING

A plasticizer is a substance, which is added lo a material (usually a

plastic, resin or elastomer) to improve its processibility, flexibility and
stretch ability. A plasticizer can decrease melt Viscosity, glass transition
temperature and the modulus of elasticity of the product without altering
the fundamental chemical character of the plasticized material1,2. The
solvation/desolvation equilibrium and the fact that the plasticizers is bound
physically by dipole forces rather than chemically (covalently) are
characteristic of all externally plasticized polymers

There is no stoichiometric upper limit to plasticizer uptake. This

enables a processor to adjust the flexibility of the basic polymers (PVC.
rubber mixtures, cellulose derivatives) over a wide range. Thus th e product
properties are tailored to the application, e.g. for use in hot or cold climates,
for normal or elevated service temperatures, or for a rticles in contact with
oils or fuels. A disadvantage of external plasticizing is the extract-ability of
plasticizers from the plasticized material. This depends on t h e contact
medium but can never be completely avoided.

In general, even small quantities of plasticizer have a noticeable

effect and most polymers are plasticized with only a small amount of
plasticizer. However, PVC possesses the ab ilit y to take up large quantities of
plasticizer, so that, as more is added and the brittle point is passed, its
mechanical properties gradually change from rigid to soft/gel-like
(viscoelastic) 4 .

The principle of "internal" plasticizing involves monomers leading to

homopolymers with high glass transition temperature being selectively
copolymerized with monomers whose homopolymers have a substantially
lower glass transition temperature.
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 The advantage of internally plasticized products lies in the strong
chemical combination of "hard" with "soft" segments, which cannot be
separated from each other by extraction. However, the technique is limited;
every copolymer is only suited to certain flexibility requirements. The
mechanical properties of internally plasticized materials show marked
temperature dependence and the materials have insufficient dimensional
stability at elevated temperature. Of course, in t e r n a l l y plasticized
polymers can be plasticized externally as well.

The classification of organic substances into primary plasticizers,

secondary plasticizers and extenders is useful as a guide. Primary
plasticizers must gel polymers in the usual processing temperature range
quickly and satisfactorily, be usable alone and not exude from the
plasticized material. On the other hand, secondary plasticizers have lower
gelling power and limited compatibility with the polymer so that
combination with a primary plasticizer is necessary. Least useful alone,
extenders gel PVC very poorly, tend to exude strongly if used singly and are
only suitable for application in restricted amounts as diluents for primary
plasticizers.

1.3. ANTI-PLASTICIZERS AND ANTI-PLASTICIZING

On incorporation of small amounts of a plasticizer into certain

plastics there is no increase in elongation and no decrease in tensile strength.
On the contrary, the polymers become harder and more brittle. This is
termed "anti-plasticizing4-6 . In addition to PVC, this effect is found with
polycarbonate, polysulfone7 , polyphenylene ether 8, polyesters and even with
polyamide 669 . The brittleness which is brought about in PVC by small
quantities of plasticizer should not be confused with the occurrence of stress
cracking in molded plastics parts.

Although small amounts (less than 10%) of a plasticizer facilitate

the thermoplastic processing of PVC, the end product is extremely brittle
and is liable to fatigue failure.
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 The reasons for this behavior are to be found in the degree of order as
a consequence of increased mobility of the PVC molecules the degree of
crystallinity rises. At 4 to 15% plasticizer content, the degree of order is at
its highest and the mechanical parameters (e.g. tensile strength at break)
show maximum or minimum values. Increasing amounts of plasticizer
reduce and finally eliminate crystallinity and plasticizing is achieved.
Plasticizers with high polarity have especially pronounced anti-plasticizing
effects. With these materials, steric hindrance of the forces of attraction
between PVC chains, caused by the plasticizer, can also be a contributing
factor.

All plasticizers can cause brittleness in PVC; the maximum effect

depends on the plasticizer structure (aliphatic/aromatic acids,
linear/branched alcohols) and generally occurs at plasticizer content of up
to 15%. Emulsion PVC shows a reduced effect compared to suspension
PVC.

1.5. THEORIES OF PLASTICIZING AND PLASTICIZER EFFECT

1.5.1. LUBRICATION THEORY

In the lubrication theory the plasticizer is viewed as a lubricant,

which exhibits no bonding forces with the polymer. Lubricants only lower
inter-molecular forces and therefore only cause partial plasticizing. In the
early days of plastics technology, relatively ineffective plasticizers have
been used in small quantities as processing aids. They decreased melt
viscosity thereby facilitating processing while generally affecting the
properties of a polymer only insignificantly. In contrast, effective
plasticizers lower the glass transition temperatures of polymers and can
no longer be described as processing aids.

1.5.2. SOLVATION THEORY

The solvation theory is based on concepts of colloid chemistry. The

polymer/plasticizer system is regarded as a lyophilic colloid in which the

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plasticizer forms solvation shells around the polymer particles (disperse
phase). There is, from the physical point of view, no difference in
principle between materials termed solvents and those termed plasticizers11 .
In both cases there is a non-chemical interaction between the components.
Two substances are miscible when the Gibbs free energy of mixing is
negative12. Mixing energies can be determined in various ways, e.g. vapor
pressure measurements on plasticized PVC, or by DSC 13 . According to such
studies, plasticizers are weak solvents possessing poor to moderate solvation
power for polymers. They render a strongly dipolar polymer flexible at room
temperature by gel formation with an equilibrium between solvation and
desolvation.

The solvating or swelling power of a plasticizer depends on its

molecular weight and on its functional groups 14 . Whether or not a
plasticizer is effective as a solvent depends on three intermolecular forces:
plasticizer/plasticizer, plasticizer/polymer and polymer/polymer. Plasticizers
should have small molecules and possess appropriate forces attractive to
the polymer, which must, however, be less than those between the polymer
chains. The effectiveness of a plasticizer is improved the lower the
plasticizer/plasticizer forces compared with the polymer/polymer forces.
This is summarized in the leihlich rule that states that the viscosity of a
plasticizer should be as low as possible and have a low temperature
coefficient. Judged by these criteria the plasticizers, which are used in
practice, seem to be relatively inadequate. This is a demonstration of the
fact that commercial products often involve compromises between
incompatible demands.

To summarize: A plasticizer must have molecules, which are not

too small otherwise it would be too volatile. It should also not be too poor a
solvent as it could then only be used in small quantities and the danger of
separation and exudation would be too great.

1.5.3. THERMODYNAMIC THEORY

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 The thermodynamic theory uses solution and swelling as
explanations of gelling but views plasticizing as the reduction of brittleness
of polymers (alteration of the glass transition temperature). As
temperature is increased, polymers undergo successive changes from hard to
viscoelastic, viscoliquid, limited elastic and plastic forms. The
thermodynamic theory attempts to interpret the intermolecular forces in the
plasticizer/polymer system by a model based on the resistance to deformation
of a three-dimensional gel. Bonding forces, which are effective along the
polymer chains, forms the gel. As an example, in a stiff, brittle polymer
the intermolecular separations are small compared with an elastomer and
every deformation causes internal stresses, which the molecules cannot
accommodate; consequently, elasticity of t h is polymer is very low (hard
rubber). The macro Brownian motion of the chain molecules and the micro
Brownian motion of molecular segments are hindered in PVC by strong
dipole forces, while a thermoplastic elastomer with only a few
intermolecular bonds (coarsely networked) is flexible even without
incorporation of a plasticizer (weakly cross-linked rubber).

The effect of a plasticizer is now treated as that of reducing the

intermolecular forces (dipole and dispersion forces and hydrogen
bonding) as much as possible and loosening the bonding of polymer
molecules with each other, i.e. shielding the force centers which hold
together the polymer chains. This causes the macromolecules to have fewer
points of attachment and the polymer becomes elastic and flexible, although,
after cooling, linkages between polymer segments or crystallites can be
formed again. The network structure of PVC is explained in this way15 .

Plasticizers are only absorbed into the amorphous region of polymers;

they do not remain permanently firmly bound (secondary bond formation). It
is more likely that t h e r e is a continuous exchange of plasticizer molecules.
At salvation/desolvation equilibrium, when a certain proportion of the force
centers of the polymer chain remain shielded, the gel is stable.

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1.5.4. POLARITY THEORY

According to the polarity theory, the intermolecular forces between

the plasticizer molecules, the polymer molecules and the polymer/plasticizer
molecules must be well balanced to ensure that t h e gel is stable. Therefore,
plasticizers contain one or more of both polar and nonpolar groups, which
must match the polarity of the particular polymer. The polarity of a
plasticizer molecule depends on the presence of groups containing oxygen,
phosphorus and sulfur. Proven plasticizers contain polar ester groups,
polarizable phenyl groups and nonpolar alkyl groups as shielding groups. In
terms of chemical structure a plasticizer is:

o A polar aromatic compound comprised of polar and nonpolar

portions. Examples: di (2-ethyhexyl) adipate, tri (2-ethylhexyl)
phosphate;

o a polar aromatic/aliphatic compound comprised of polar, polarizable

and nonpolar portions. Examples: benzyl butyl phthalate, diphenyl 2-
ethylhexyl phosphate.

The polar, directional and orientating groups of a plasticizer

interact with the dipoles of the polymer. The purpose of the aromatic,
polarizable parts of the molecule is to conduct further the attractive forces
of the dipoles and thereby to promote the polar character of the plasticizer
molecule. The aliphatic, nonpolar portions shield the dipoles of the
polymer from one another. Substances, which consist of polar groups
only, associate and exude.

Pictures taken by electron microscope and small-angle X-ray

scattering investigations clearly show changes in the morphology of a
polymer after being plasticized. Incorporation of plasticizers as ball-shaped
agglomerates in PVC is conceivable.

1.6. CHOICE OF PLASTICIZERS

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 Processing and properties of plasticized polymers are markedly
affected by the type and quantity of plasticizer. The following points must
be particularly considered:

I. Volume cost analysis,

II. Compatibility,

III. Processing characteristics,

IV. Thermal, electrical and mechanical properties,

V. Resistance to water, chemicals, weathering, dirt and micro organisms,

VI. Toxicity,

VII. Effect on rheological properties.

The volume cost analysis is based on the fact that the sale of most end
products is by meters, square meters etc., that is by "volume". The volume
cost analysis gives advantages to plasticizers with low density and low
plasticizing activity (e.g. di-isononyl phthalate plasticizers PVC less than
di-isoctyl phthalate). This is, of course, true only as long as the volume
cost of PVC is not lower than that of the plasticizer.

The choice of a plasticizer demands experience and the exact

definition of the desired properties of the end product. Important
parameters for PVC are:

A. The processing method starting either from dry-blends, agglomerates,

or granules: extrusion, calendering

B. , injection molding, compression molding; processing as plastisol by

coating, dipping, casting or spraying,

C. Special demands on the end product, such as high or low temperature

application or resistance to petrol, require specific plasticizers. e.g.
low volatility plasticizers, low-temperature plasticizers or polyester
plasticizers.

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 In general, an optimized plasticizer mixture rather than a single
plasticizer is used. Furthermore, the cost of a mixture can be reduced by
the use of extenders or fillers such as chloroparaffins or calcium carbonate.
Plasti cize rs with low effectiveness, used in large amounts, can also reduce
t h e volume cost of a mixture.

1.7. COMPATIBILITY

The compatibility of a plasticizer w i th a given polymer is its most

important property. High compatibility means that a stable, homogeneous
mixture of plasticizer and polymer is produced. Plasticizers may be com-
patible at the processing temperature but exude from the material on
cooling: the reverse can also occur.

Primary plasticizers are highly compatible with PVC in all

proportions, and neither exudes as drops or as a surface film, nor do they
bloom (form a firm deposit). Substances, which show a certain amount of
exudation or blooming, particularly at high concentrations, are termed
secondary plasticizers, e.g. the aliphatic dicarboxylic acid esters. The
boundary between primary and secondary plasticizers is not clear-cut and
depends on concentration and ambient conditions. In PVC, phthalates such
as dioctyl phthalate and butyl benzyl phthalate show satisfactory compati-
bility under almost all service conditions.

Secondary plasticizers, which are incompatible when used alone, are

also called extenders. Alkylated aromatic hydrocarbons and chlorinated hy-
drocarbons have adequate dispersion forces and sufficiently high solubility
parameters but are too weak in polarity and in their hydrogen bonding
ability to be usable as primary plasticizers.

The compatibility of a plasticizer is also influenced by the presence

of other additives, and by pressure, temperature, moisture and light.
Plasticized polymers are in dynamic equilibrium at a particular

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temperature. As soon as the temperature changes, the effectiveness of the
forces alters. With more energy added (at elevated temperature) it
becomes easier to break the bond between polymer and plasticizer
(favoring desorption) and the opposite is true at lower temperature. Under
normal service conditions diffusion always ensures that there is a certain
amount of plasticizer on the surface. This does not mean, however, that the
plasticizer is exuding. Exudation means that the surface feels oil y and fatty
because of separated plasticizer. In this case the dynamic adsorption
desorption equilibrium between plasticizer and polymer molecules is
disturbed.

The exuded material generally contains all of the liquid components,

which were used in th e original mixture. Solid deposits are not usually
caused by incompatibility of plasticizers.

Table 1.1: Effect of temperature on Shore hardness A and D) of

plasticized PVC with a plasticizer content of 30%.

Plasticizer Shore A/Shore D hardness

-10 0 +10 +20 +30 +40 +50 +60 +70

DPB 98/75 97/69 94/58 85/- 78/- 71/- 66/- 62/- 60/-

DOP 98/76 97/71 94/65 90/47 85/- 79/- 72/- 66/- 62/-

DINP 98/82 98/76 97/70 90/54 90/45 82/- 73/- 97/- 63/-

DOA 97/62 95/56 91/48 86/- 82/- 77/- 70/- 64/- 59/-

1.12. TESTING OF PLASTICIZERS

For characterization and control of consistency and of purity a series

of characteristic values are determined for plasticizers and plasticized PVC.
There are specific standards for molding compounds, individual plasticizers,
plasticizer groups and plasticized PVC (DIN 7749 part 1, ISO 2898-1-86).
Table 3 lists the purpose for determining a number of characteristic values

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and test methods for obtaining them (DIN 53400).

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 Table 1.3: Test on plasticizers

Property Purpose, application, assessment DIN

Plasticizer Symbols 7723
Density Calculation of mass and volume 1306
Density/temperature Consistency and purity 51757
dependence
Refractive index n D 20 Consistency, purity and optical 53491
properties
Flash point Flammability of vapors 51758
Ignition temperature Assignment to hazard classification 51794
Viscosity Consistency, purity; plasticizing 51550
effectiveness and structure (degree of 51561
branching) for plasticizers based on
the same acid but different alcohols 51675
53015
Viscosity/temperature Storage, transport See above
dependence and 51 563
Acid number (mg Consistency, degree of purity 53402
KOH/g)
Saponification Consistency, degree of purity 53401
number (mg/KOH g)
Hydroxyl number Consistency, degree of purity 53240
Pour point Thickening at low temperature 51597
Hazen color 53409
Ash content 51757
Bromine content 51774
Nitrogen content 51772
Sulfur content Consistency, degree of purity 51768
Water content 51777
Iodine number 55934
Aniline and mixed 51775
aniline points 51787

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1.13. TYPES OF PLASTICIZER

In the literature, plasticizers are divided according to different view

points into:

I. Internal (copolymers), vulcanizable or cross-linkable plasticizers (in

which the plasticizer reacts with the polymer or with itself).

II. Physical mixtures,

III. Primary plasticizers,

IV. Secondary plasticizers, thinners or extenders (plasticizer diluents).

Use of any of these proposed methods for the classification of

plasticizers is inadequate because there is always a remainder, which
cannot be assigned to any class. The clearest classification is a chemical
one:

a) Phthalates,

b) Aliphatic/aromatic monocarboxylic acid esters,

c) Aliphatic dicarboxylic acid esters,

d) Phosphates,

e) Polyester or polymeric plasticizers

f) Special plasticizers,

g) Extenders (hydrocarbons, chlorinated hydrocarbons).

Phthalates are the most common, especially di (2-ethylhexyl) and

diisooctyl phthalate and, recently diisononyl and diisodecyl phthalate.
Aliphatic dicarboxylic acid esters (adipates, sebacates and azelainates) are
used to impart good low-temperature flexibility. Epoxidized plasticizers
made from natural oils can be used as primary plasticizers. In general they
improve heat and light stability as well as resistance to extraction.
Extenders (secondary plasticizers) are used in combination with primary
plasticizers as a way of reducing cost. In general they lower heat and light

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stability and show some incompatibility, so that they can be used only in
quantities limited to about 20% of that of the primary plasticizer.

Polyester (polymeric) plasticizers are primary plasticizers with very

good resistance to extraction by aliphatic hydrocarbons but they reduce low-
temperature flexibility of articles containing them. Elastomers, such as
nitrile rubber, ethylene/ vinyl acetate copolymers, chlorinated polyethylene,
thermoplastic polyurethane etc., can be used for special articles where
high demands are made with respect to low volatility as well as to
migration and chemical resistance of the plasticizer. They are used in
powder form, which can be easily incorporated.

As an example, table 4 shows the distribution of plasticizer

production in the USA according to these groupings. The percentages given
are probably as a rough approximation applicable worldwide.

Since 80% of plasticizers are used in the manufacture of plasticized

PVC, developments required for PVC and plasticized PVC products are
decisive in determining the market importance of plasticizers; although use
is stagnating in some branches, in others there is actually showing a
tendency to increase.

Out of the more than 20000 substances with plasticizing properties

described so far, about 200 arcs used in practice with about 40 products
having achieved noticeable market significance. About 12 products account
for half of the plasticizer production in the Western world 41 . The most
important starting materials are given in table 4.

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 Table 4: Breakdown of plasticizer production in the USA (1987,
source: US International Trade Commission).

Product Group Share

Phthalates 61

Trimellitates 1

Phosphates 6

Adipates 3

Sebacates 1

Polyesters 2

Fatty other esters (oleates, stearates etc) 2

Fatty acid esters, epoxidized 5

All other cyclic plasticizers 13

All other acyclic plasticizers 6

At present the worldwide production capacity for plasticizers is

estimated at around 4 mio-tons per annum and the annual consumption at
approx. 3.1 mio-tons. The fourfold increase in production in the years 1960
to 1970 was not maintained between 1970 and 1980 and a lower figure is
also likely for the 1980 to 1990 period.

2. INTRODUCTION OF NITRILE RUBBER

The Introduction of acrylonitrile (ACN) into the polymer backbone

imparts oil resistance and affects many other properties. Grades that vary in
acrylonitrile content from 18 to 50 % are commercial available, the percentages
of acrylonitrile present forming the basis of the following grade descriptions:
Low 18-24 % ACN
Medium low 26-28 % ACN
Medium 34 % ACN
Medium 38-40 % ACN

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High 50 % ACN

Many properties are influenced by the acrylonitrile content:

Properties Acrylonitrile content
18 % --------
-----50 %
Oil resistance improvement
Fuel resistance improvement
Tensile strength improvement
Hardness increase
Abrasion resistance improvement
Gas impermeability improvement
Heat resistance improvement
Low temperature flexibility improvement
Resilience improvement
Plasticizer compatibility

A typical Tg of an 18% CAN co-polymer is -38 0C, and that of a 50% ACN co-
polymer -20C. Carboxylated nitriles, hydrogenated nitrile, liquid nitriles, and
blends with polyvinyl chloride (PVC) are also commercially available.

The carboxylated types (XNBR) contain one or more acrylic type of acid as a
terpolymer, the resultant chain being similar to nitrile except for the presence
of carboxyl groups, which occur about every 100-200 carbon atoms. This
modification gives the polymer vastly improved abrasion resistance, higher
hardness, higher tensile and tear strength, better low temperature brittleness,
and better retention of physical properties after hot oil and airing ageing when
compared to ordinary nitrile rubber.
Low molecular weight liquid nitrile grades are available, and these can be used
as compatible plasticizers in the compounding of nitrile rubber, Such

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plasticizer can be partially cross-linked to the main chain during cure; hence
they exhibit low extractability.
PVC / NBR poly blends can be produced as colloidal or mechanical blends, the
former generally giving superior properties. Commercially available poly
blends have PVC contents ranging from 30 to 55 %. The poly blends have
reduced elasticity, which gives improved extrudability, but they also exhibit
superior ozone resistance, improved oil swell resistance, and tensile and tear-
strength; these properties, however, are achieved at the expense of low
temperature flexibility and compression set. The ozone resistance of such
polyblends is improved only if the PVC is adequately distributed and fluxed.
Failure due to ozone attack can occur if proper distribution and fluxing are not
achieved, but this is harder to do in mechanical blends.
Another method by which the ozone resistance of nitrile rubber can be
improved is the removal of the double bonds in the main chain of the co-
polymer by hydrogenation. Hydrogenated nitrile rubbers also exhibit much
greater resistance to oxidation and extend the useful service temperature range
of nitriles up to 150 0C commercially available grades offer different degrees of
hydrogenation, with residual double bonds ranging from 0.8 to 6 %.
Nitriles have good resistance to oil, aliphatic and aromatic hydrocarbons, and
vegetable oils, but polar solvents such as ketones swell them. The unsaturated
main chain means that protection against oxygen, ozone, and UV light is
required.
In compounding, choice of the correct grade is required if the required balance
of oil resistance and low temperature flexibility is to be achieved.
Sulphur, sulphur donor systems and peroxides can cure nitrile rubber.
However, the solubility of sulphur in nitrile rubber is much lower than in NR,
an a magnesium carbonate coated grade (sulphur MC) is normally used this is
adds as early in the mixing cycle as possible. Nitrile rubber requires less
sulphur and more accelerator than is commonly used for curing natural rubber.
A cadmium oxide/magnesium oxide cure system gives improved heat

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resistance, but the use of cadmium. A heavy metal will increasingly be
restricted.
The hydrogenated nitrile grades that contain the lowest level of residual
double bonds can be cross-linked only by the use of peroxides and radiation.
While those with a level of residual double bonds greater than about 3.5% can
be cured by sulphur.
In addition to the normal sulphur cure systems, metal oxides can be used
to cure the carboxylated nitriles. The low temperature properties of nitriles can
be improved by the use of suitable plasticizer (e.g. ester plasticizers).
Nitrile rubber, because of its oil resistance, is widely used in sealing
applications, hose liners, roll coverings, conveyor belts, shoe soles, and plant
lining. Nitrile rubber is also available as latex.

1. INTRODUCTION OF PVC

Polyvinyl chloride (PVC) is the world’s most versatile thermoplastics

with a wide rage of applications than any other plastics. Virtually all areas of
human activities whether domestic or industrial, leisure or commercial, PVC’s
presence is dominating. It plays an important role in every field such as
agriculture, electrical, irrigation, house roofs, shoe soles and many other fields.
Vinyl chloride was first synthesized and reported by Reghawlt in 1835.
Hoffmann published the first report of polyvinyl chloride in 1860. The full-
scale commercial production of PVC resin began in 1931in Germany. In India
the manufacturing of PVC began in 1961when calico started its plant in
Mumbai.
- Presence of chlorine atom causes an increase in inters chain attraction
and increase hardness and stiffness of polymer. Hence PVC is ‘horny
material’.
- PVC is polar in nature due to presence of C-Cl dipole, thus having high
dielectric constant and power factor.

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 - Solubility parameter of PVC is 19.4 MPa. Hence polymer is resistant to
non- polar solvents, which are having less solubility parameter. Suitable
solvents for PVC are Cyclohexanone, Tetrahydrofuran and Xylene.
- Presence of Chlorine atom makes the material ‘flame retardant and self
extinguishing’ and also presence of plasticizer reduces rate of burning.
- These polymers are amorphous in mature. Properties of Poly Vinyl
Chloride are given in table

3.1. PROPERTIES OF PVC

Table 3.9: Propertied of Poly Vinyl Chloride
Properties Unit Value
Specific gravity - 1.18- 1.70
Tensile Strength MPa 5.5- 26.2
Tensile modulus MPa 4.8- 12.4
Flexural modulus MPa 30
Elongation at break % 150- 450
0
Glass transition temperature C 80- 85
Dielectric Strength KV/mm 9.9- 15.8

3.2. Compounds of PVC

A PVC compound may contain the following ingredients
3.2.1. STABILIZERS
Heating of PVC at temperature above 70 0C has a number of adverse
effects on the properties of the polymers. Sufficient degradation may take place
during standard processing operations (150- 200 0C) to make the product
useless.
Therefore to avoid degradation ‘stabilizers’ are found useful, so that useful
moldings can be obtained.
The most important stabilizer for PVC is ‘basic lead carbonate” (white
lead). Tri basic lead sulphate is a good heat stabilizer but due to its heavy cost
(when compare to sulphate & carbonate), its use is restricted. ‘Dibasic lead

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phthalate’ is also used in PVC. Today, only the compounds of Cd, Ba, Ca and
Zn are prominent as PVC stabilizers.

3.2.2. PLASTICIZERS
These are found useful,
- For reducing processing temperature of polymer below the
decomposition temperature.
- To modify the properties of finished products such as flexibility or
extensibility etc.
- To modify processing properties.
All PVC plasticizers have a solubility parameter to that of PVC.
Di-isooctyl phthalate (DIOP) and di-ethyl hexylphthalate (DEHP) are most
important plasticizers used in PVC.

3.2.3. EXTENDERS
Sometimes plasticizers are not found useful in PVC because of their
limited compatibility with the polymer. But when mixed with ‘true plasticizer’
(commercially called extenders), a reasonable compatibility is acquired.
Therefore ‘Extenders’ can often replace plasticizers without any adverse effects
on the properties of compound.
Commonly used extenders in PC are
- Chlorinated paraffin waxes
- Chlorinated liquid paraffin fraction
- Oil extracts
The solubility parameters of these extenders are generally lowers that of PVC.

3.2.4. LUBRICANTS
In plasticized PVC the main function of a lubricant is to prevent sticking
of the compound to the processing equipment. This is brought about by
selecting a material of limited compatibility with PVC which will bleed out
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from melt during processing to from a film between the bulk of the compound
and the metal surface of the processing equipment. The additives used for this
purpose are known as ‘external lubricants’. Calcium stearate, normal lead
stearate, dibasic lead stearate, graphite are employed to improve flow
properties.
In unplasticized PVC the internal lubricants are incorporated to improve
the flow of the melt and lower the apparent melt viscosity. These materials are
reasonably compatible with the polymer. Internal lubricants are wax derivatives
and glyceryl esters (glyceryl mono stearate)

3.2.5. FILLERS
Fillers are commonly employed in opaque PVC compounds in order to
reduce cost. They may also be incorporated 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.
For electrical insulation, china clay is commonly employed while
various carbonates (ground limestone, Precipitated CaCO3 and coated CaCO3)
are used for general-purpose work. Occasionally employed fillers are talc, light
magnesium carbonate, barium sulphate (barites) and the silica’s and silicates.
For flooring applications ‘asbestos’ has been important filler.

3.2.6. PIGMENTS
A large number of pigments are now commercially available which are
recommended fro use with PVC.
To add a pigment, firstly the following facts are to be considered
- Will it decompose, fade or plate out?
- Will the pigment adversely affect the functioning of stabilizer and
lubricant?
- Will it fade, bleached out or will it bleed?

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 - Will the pigment adversely affect properties that are relevant to his end
usage (because many pigments will reduce the volume resistivity of a
compound)?

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