Page 113 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.53

2.2.28 Vinyl-based Resins
2.2.28.1 Polyvinyl Chloride (PVC). Polyvinyl chloride polymers (PVC), generally re-
ferred to as vinyl resins, are prepared by the polymerization of vinyl chloride in a free rad-
ical addition polymerization reaction. Vinyl chloride monomer is prepared by reacting
ethylene with chlorine to form 1,2-dichloroethane. 379 The 1,2 dichloroethane is then
cracked to give vinyl chloride. The polymerization reaction is depicted in Fig. 2.41.

FIGURE 2.41 Synthesis of polyvinyl chloride.

The polymer can be made by suspension, emulsion, solution, or bulk polymerization
methods. Most of the PVC used in calendering, extrusion, and molding is prepared by
suspension polymerization. Emulsion polymerized vinyl resins are used in plastisols and
organisols. 380 Only a small amount of commercial PVC is prepared by solution polymer-
ization. The microstructure of PVC is mostly atactic, but a sufficient quantity of syndio-
tactic portions of the chain allow for a low fraction of crystallinity (about 5 percent). The
polymers are essentially linear, but a low number of short-chain branches may exist. 381
The monomers are predominantly arranged head to tail along the backbone of the chain.
Due to the presence of the chlorine group, PVC polymers are more polar than polyethyl-
ene. The molecular weights of commercial polymers are M = 100,000 to 200,000;
w
M = 45,000 to 64,000. 382 M /M = 2 for these polymers.
w
n
n
The polymeric PVC is insoluble in the monomer; therefore, bulk polymerization of PVC
383
is a heterogeneous process. Suspension PVC is synthesized by suspension polymeriza-
tion. These are suspended droplets approximately 10 to 100 nm in diameter of vinyl chlo-
ride monomer in water. Suspension polymerizations allow control of particle size, shape,
and size distribution by varying the dispersing agents and stirring rate. Emulsion polymer-
ization results in much smaller particle sizes than suspension polymerized PVC, but soaps
used in the emulsion polymerization process can affect the electrical and optical properties.
The glass transition temperature of PVC varies with the polymerization method but
falls within the range of 60 to 80°C. 384 PVC is a self-extinguishing polymer and therefore
has application in the field of wire and cable. PVC’s good flame resistance results from re-
moval of HCl from the chain, releasing HCl gas. 385 Air is restricted from reaching the
flame, because HCl gas is more dense than air. Because PVC is thermally sensitive, the
thermal history of the polymer must be carefully controlled to avoid decomposition. At
temperatures above 70°C, degradation of PVC by loss of HCl can occur, resulting in the
generation of unsaturation in the backbone of the chain. This is indicated by a change in
the color of the polymer. As degradation proceeds, the polymer changes color from yellow
to brown to black, visually indicating that degradation has occurred. The loss of HCl ac-
celerates the further degradation and is called autocatalytic decomposition. The degrada-
tion can be significant at processing temperatures if the material has not been heat
stabilized, so thermal stabilizers are often added at additional cost to PVC to reduce this
tendency. UV stabilizers are also added to protect the material from ultraviolet light,
which may also cause the loss of HCl.
There are two basic forms of PVC: rigid and plasticized. Rigid PVC, as its name sug-
386
gests, is an unmodified polymer and exhibits high rigidity. Unmodified PVC is stronger
and stiffer than PE and PP. Plasticized PVC is modified by the addition of a low-molecu-
387
lar-weight species (plasticizer) to flexibilize the polymer. Plasticized PVC can be for-
mulated to give products with rubbery behavior.

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