Page 115 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.55

394
Chlorinated PVC can be extruded, calendered, or injection molded. Extrusion
screws should be chrome plated or stainless steel. Dies should be streamlined. Injection
molds should be chrome or nickel plated or stainless steel. CPVC is used for water distri-
bution piping, industrial chemical liquid piping, outdoor skylight frames, automotive inte-
rior parts, and a variety of other applications.

2.2.28.3 Copolymers. Vinyl chloride can be copolymerized, with vinyl acetate giving a
polymer with a lower softening point and better stability than pure PVC. 395 The composi-
tions can vary from 5 to 40 percent vinyl acetate content. This material has application in
areas where PVC is too rigid and the use of plasticized PVC is unacceptable. Flooring is
one application for these copolymers. Copolymers with about 10 percent vinylidene chlo-
ride and copolymers with 10 to 20 percent diethyl fumarate or diethyl maleate are also
available.

2.2.28.4 Dispersion PVC. If a sufficient quantity of solvent is added to PVC, it can be-
396
come suspended in the solvent, giving a fluid that can be used in coating applications.
This form of PVC is called a plastisol or oganisol. PVC in the fluid form can be processed
by methods such as spread coating, rotational casting, dipping, and spraying. The parts are
then dried with heat to remove any solvent and fuse the polymer. Parts such as handles for
tools and vinyl gloves are produced by this method.
The plastisol or organisols are prepared from PVC produced through emulsion poly-
merization. 397 The latex is then spray dried to form particles from 0.1 to 1 µm. These par-
ticles are then mixed with plasticizers to make plastisols or with plasticizers and other
volatile organic liquids to make organisols. Less plasticizer is required with the organisols
so that harder coatings can be produced. The polymer particles are not dissolved in the liq-
uid but remain dispersed until the material is heated and fused. Other additives such as sta-
bilizers and fillers may be compounded into the dispersion.
As plasticizer is added, the mixture goes through different stages as the voids between
398
the polymer particles are filled. Once all the voids between particles have been filled,
the material is considered a paste. In these materials, the size of the particle is an important
variable. If the particles are too large, they may settle out, so small particles are preferred.
Very small particles have the disadvantage that the particles will absorb the plasticizer
with time, giving a continuous increase in viscosity of the mixture. Paste polymers have
particle sizes in the range of 0.2 to 1.5 µm. Particle size distribution will also affect the
paste. It is usually better to have a wide particle size distribution so that particles can pack
efficiently. This reduces the void space that must be filled by the plasticizer, and any addi-
tional plasticizer will act as lubricant. For a fixed particle-to-plasticizer ratio, a wide distri-
bution will generally have lower viscosity than for a constant particle size. In some cases,
very large particles are added to the paste, as they will take up volume, again reducing the
amount of plasticizer required. These particles are made by suspension polymerization.
With the mixture of particle sizes, these larger particles will not settle out as they would if
used alone. Plastisols and organisols require the addition of heat to fuse. Temperatures in
the range of 300 to 410°F are used to form the polymer.
2.2.28.5 Polyvinylidene Chloride (PVDC). Polyvinylidene chloride (PVDC) is similar
to PVC except that two chlorine atoms are present on one of the carbon groups. 399 Like
PVC, PVDC is also polymerized by addition polymerization methods. Both emulsion and
suspension polymerization methods are used. The reaction is shown below in Fig. 2.42.
The emulsion polymers are either used directly as a latex or dried for use in coatings or
melt processing.
This material has excellent barrier properties and is frequently used in food packaging
applications. Films made from PVDC have good cling properties, which is an advantage

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