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Polyvinylchloride (PVC)

Polyvinylchloride (PVC), widely known as vinyl, is one of the largest volume synthetic
thermoplastic used globally. Polyvinylchloride is a very unique polymer compared to other widely
used thermoplastics, due to its superior fire performance. Usually, rigid polyvinylchloride products
pass fire standard testing such as UL-94 V-0. Due to the excellent performance in combustion of
polyvinylchloride, it's widely used in wide variety of applications including electric cables,
upholstery, mattresses, wall linings, and floor coverings etc.
The basic structure of the vinyl chloride monomer
is as follows.
Polyvinylchloride structure

Polyvinylchloride is known as vinyl polymer, due to its
vinylic repeating unit. The basic structure of polyvinylchloride
is represented below
-[CH2-CH]-n ; where n can be from 600 -2700 for a PVC
polymer with reasonable properties
Vinyl Chloride monomer structure
Polyvinylchloride structure
Bulk properties of a polymer mainly depend on three
major factors:
- Number of repeating units in the polymer chain -
(molecular weight and molecular weigh distribution)
- Imperfections - (structural defects)
- Spatial arrangement of the polymer chain - (tacticity)

The molecular architecture of
Polyvinylchloride

The molecular architecture of Polyvinylchloride is
influenced by several factors including
polymerization temperature, the choice of chain
transfer agents, type of initiators, polymerization
type (suspension, mass or micosuspension
polymerization) etc. Further, the process conditions
including the type /size/shape of vessel, the position
of the blade, oxygen contamination and many other
factors could influence the final properties of the
polymer.
Vinyl chloride monomer
Molecular weight of Polyvinylchloride

The molecular weight of Polyvinylchloride is usually
characterized by one-point solution viscosity, which
is expressed as inherent viscosity (IV) or K-value.
The selection of Polyvinyl chloride grade that is
suitable for various applications including extrusion,
molding, calendaring, sheet, film or other
applications is usually done using the K value of the
polymer. For example, Polyvinylchloride with K
value of 56 may used for injection molding pipe
fittings, films or sheet applications, where as the
material with a K value of 70 may be used in
medical tubing or automotive molding applications.

Structural Defects of Polyvinylchloride

Ideally, Polyvinyl chloride has the repeating unit of -CH2CHCl-
. During the polymerization, the repeating units could be
added in normal head to tail arrangement yielding a
-CH2CHClCH2CHCl- structural unit. However, if it is added in
head to head fashion, structural defect could occur yielding a
-CH2CH2CHClCHCl- (defected) structural unit. Another defect
that could occur during the polymerization is forming an
unsaturated end group. Further, formation of chloromethyl
group or the presence of double bonds along the polymer
chain backbone could also occur.
The presence of oxygen during the polymerization could lead
to the production of alternating copolymer of oxygen and
vinylchloirde which is Poly(vinylchloride peroxide). This is a
very dangerous polymeric material which could detonate upon
impact. Thus, PVC manufacturers takes extra precaution to
not to get the reaction vessels contaminated with air.


Presence of all these defects affects the thermal stability of
Polyvinylchloride, resulting the need of the addition of a heat
stabilizer for effective processing. If Polyvinylchloride is
heated above 100 C without a heat stabilizer, it eliminates
hydrogen chloride resulting a color change of the material
(yellow, red and finally black), due to the formation of
conjugated double bonds along the Polyvinylchloride
backbone. Most polymers upon heating tend to
depolymerize yielding the monomer and other byproducts.
However, Polyvinylchloride doesn't depolymerize to form
vinyl chloride monomer, but degrades via HCL elimination,
which is better, as vinyl chloride is considered as a
carcinogen.
Tacticity of Polyvinylchloride

The presence of chlorine unit in the Polyvinylchloride backbone causes configurational and conformational isomers
of the polymer. The chlorine atom can disposed in one side with respect to the backbone (isotactic), either sides
of the backbone in a regular pattern (syndiotactic), or can be randomly distributed in both sides (heterotactic).

The configurational and conformational isomerism of PVC has a direct impact on the crystallinity of the polymer.
Polyvinylchloride (PVC) synthesis:

Polyvinyl chloride is synthesized by the free-radical polymerization of vinyl chloride monomer (VCM). The vinyl
chloride monomer is synthesized with natural gas, oil and salt as the main raw materials. The salt is coming from
the sea water, thus the supply of chlorine to make PVC is limitless.
Polyvinylchloride is considered as chlorine containing carbon-carbon polymer, as it has a chlorine group attached to
the chain, and as there is no heteroatom in its backbone respectively.
There are three major PVC manufacturing processes used industrially.
- Suspension polymerization
- Mass polymerization
- Emulsion and microsuspension polymerization

Depending on the production parameters, all these methods could yield Polyvinylchloride with distinctive properties.

Suspension Polymerization of Polyvinylchloride

Suspension polymerization route is the most widely used method in industry to synthesis Polyvinylchloride. The
whole process of producing PVC by this method involves polymerization, removing the residual vinyl chloride
monomer (by stripping), removing water (through centrifuging), drying, screening to remove larger particles, and
bagging or loading to bulk tankers. The polymerization process involves the suspension of vinyl chloride monomer
in continues water phase through vigorous agitation in the presence of a dispersant. A typical suspension
polymerization reactor could have a size between 20 m3 to 200 m3. The formulation for suspension polymerization
may include water as the phase, vinyl chloride monomer, a pH regualator, dispersan(s), and initiator(s).
Polymerization of vinyl chloride is highly exothermic, thus, designing a reactor that is stable in an acidic
environment, hold high pressure and easily remove the heat buildup is critical. Usually the inner lining of reactors is
made from stainless steel, and get thicker when the reactor get larger.

The final properties of Polyvinylchloride can be affected by several process variables including, polymerization
temperature, conversion, dispersant system, agitation, water:monomer ratio, presence of oxygen etc. One of the
main drawbacks in suspension resins is the presence of thin skin called pericellular membrane formed at the
water-vinyl chloride monomer interface, which could lead to processing issues. Suspension polymerization
produces porous resin particles with average particle size of about 100 -200 microns.

Mass Polymerization of Polyvinylchloride

The mass polymerization involves no solvents or second dispersing phase. Typically mass polymerization of
Polyvinylchloride is done in a two different vessels called pre-polymerizer and post-polymerizer. The polymerization
is carried out up to about 10 - 12% in the pre-polymerizer, where the agitation speed and temperature is controlled
to adjust the grain size and porosity of the final resin. Then the free flowing mass called seed particles are
transformed to the post-polymerizer which holds additional monomer and initiator. Mass polymerization at
post-polymerizer is usually done at 50-60 C, and driven by high pressure rather than temperature. Controlling the
final molecular weight is done at the post-polymerizer.
The advantages of using mass polymerization technique includes, no involment with water, no pericelluar
membrane in resin particles which in turn makes it easier for absorbing other additives including plasticizers,
energy conservation as the drying step doesn't involve applying large amounts of heat etc.
The major drawbacks are the presence of large amounts of fine particles which creates dusting issues and process
issues for some industries, need to apply high heat at stripping to get down the vinyl chloride monomer content to
the required 1 ppm level etc.

Emulsion and Microsuspension Polymerization of Polyvinylchloride

Emulsion polymerization of vinyl chloride had being the most predominant method for producing Polyvinylchloride
until the suspension process was developed. In the emulsion polymerization, emulsifies (soap) are used to disperse
the vinyl chloride monomer in water, where as suspending agents are used in suspension polymerization. Two main
routes of emulsion polymerization of vinyl chloride includes microsuspension and conventional emulsion
polymerization. Both methods produce nonporous very fine grains in the range of 1 micron, where as the mass and
suspension processes produce much larger particles. The ability to make fine grains has the advantage of making
films and products without the graininess-surface.

Conventional emulsion polymerization of vinyl chloride involves using a surfactants, usually anionic surfactants such
as sodium lauryl sulfate, to increase the surface tension and conductivity until it reaches the critical micelle
concentration (c.m.c). At this stage, the emulsifies molecules are arranged in way that their hydrophilic groups are
at the outer space contacting with water, and hydrophobic groups are gathered inside the micelle. Most of the vinyl
chloride monomer droplets get dispersed in the micelle. For the conventional emulsion polymerization, water
soluble free radical initiators are used, as opposed to monomer-soluble initiators used in suspension
polymerization. When a free radical generated in water enters the micelle containing monomer droplets,
polymerization get initiated. Usually the particle size of the resulting resin would be less than 0.5 microns. In order
to get larger particles a seeding technique is usually used, where the polymerized batch gets transferred to another
vessel where more monomer is added and the polymerization continues.

During the microsuspension polymerization, monomer soluble initiators are used as in the suspension technique.
Although the microsuspension process uses a high load of the emulsifier, it doesn't require the formation of
micelles, and rely on the use of a homogenizer -- colloid mill, high speed pump or an ultrasonic device--, to make
submicron size monomer droplets.
The particle properties of the PVC made by emulsion and microsuspension methods depends on process variables
including emulsifier system, the type of the electrolytes used, homogenization process, seeding technique as well
as the spray drying process used. Although it is very costly to make the resins by these techniques, the ability to
make fine particle makes it useful to be used them in plastisol applications. Usually a combinations of PVC resins
with various particle size ranges are blended together to produce the grade suitable for plastisol applications.

PVC Synthetic Scheme