Page 248 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Free Radical Chain Polymerization 211

bags. Further, baby and children toys should be constructed from materials that do not contain these
plasticizers. Another group that is at particular risk is the critically ill or injured patients. Again,
special care should be exercised when dealing with special groups of people. As we are dealing
with toxic materials, we need to remember that all materials can be harmful if present in the wrong
concentrations and/or locations. Even so, where we can we might minimize exposure to known
potentially toxic materials.
Poly(vinyl chloride) materials are often defined to contain 50% or more by weight vinyl chloride

units. PVC is generally a mixture of a number of additives and often other units such as ethylene,
propylene, vinylidene chloride, and vinyl acetate. Structurally similar products, but with differing
properties, are made from the chlorination of PE but almost all PVC is made from the polymeriza-
tion of vinyl chloride. Typical homopolymers are about 400–1,000 units long.
Poly(vinyl chloride) is commercially produced by a number of techniques but mainly suspension,
emulsion, bulk, and solution polymerization. Typically, product properties and form can be tailored
through the use of a particular synthetic process and conditions. Particulate architecture is then con-

trolled to achieve materials with specific sizes and distributions for specific uses and applications.
Because of the tendency of PVC to split off hydrogen chloride, forming materials with high char
at relatively low-general processing temperatures, special care is taken with respect to temperature
control and particulate architecture that allows ready processing of PVC by most of the common
processing techniques.
Tacticity of the PVC varies according to the particular reaction conditions, but generally manu-
factures favor a syndiotactic form with many PVC materials being about 50% sPVC. The reported
amount of crystallinity is in the range of 5%–10%. This allows for a material with some strength,

but one with sufficient amorphous regions to retain good fl exibility.
Poly(vinyl chloride), in comparison to many other polymers, employs an especially wide variety
of additives. For instance, a sample recipe or formulation for common stiff PVC pipe, such as that
used in housing and irrigation applications, may contain in addition to the PVC resin, tin stabilizer,

acrylic processing aid, acrylic lubricant-processing aid, acrylic impact modifier, calcium carbon-
ate, titanium dioxide, calcium sterate, and paraffin wax. Such formulations vary according to the

intended processing and end use. In such nonflexible PVC materials, the weight amount of additive
is on the order of 5%–10%.
Plasticizers weaken the intermolecular forces in the PVC reducing crystallinity. A relatively

stable suspension, called a plastisol, of finely divided PVC in a liquid plasticizer can be poured
o
into a mod and heated to about 175 C, producing a solid fl exible plastic as a result of fusion of the
plasticizer in the PVC.
As noted before, there is a tendency for PVC to undergo elimination of hydrogen chloride
when heated. The most labile chlorine atoms are those at tertiary or terminal sites. Once the
initial chloride is eliminated, continued unzipping occurs with the formation of unsaturated
backbone sites and the evolution of hydrogen chloride. The purpose of the stabilizer is to cap
unzipping sites by substitution of more stable groups for the evolved chloride as depicted in
Equation 6.59.

-----C–Cl + Sn–R → -----C–R + Sn–Cl (6.59)

Some of the tin stabilizers are based on oligomeric materials first made by Carraher and cowork-
ers. These oligomeric materials are essentially “non-migratable.”
A sample formula for a flexible upholstery fabric covering might contain PVC resin, medium-

molecular weight polymeric plasticizer, stearic acid lubricant, calcium carbonate, pigment, anti-
mony oxide, linear phthalate ester, epoxidized soy bean oil, and linear phthalate ester. Here the
weight amount of additive is in the range of 40%–70% by weight with the plasticizer often being on
the order of about 60%.

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