Page 114 - Handbook of Plastics Technologies

THERMOPLASTICS

2.54 CHAPTER 2

PVC is often compounded with additives to improve the properties. A wide variety of
applications for PVC exist, because one can tailor the properties by proper selection of ad-
ditives. As mentioned above, among the principal additives are stabilizers. Lead com-
pounds are often added for this purpose, reacting with the HCl released during
388
degradation. Among the lead compounds commonly used are basic lead carbonate or
white lead and tribasic lead sulphate. Other stabilizers include metal stearates, ricino-
leates, palmitates, and octoates. Of particular importance are the cadmium-barium systems
with synergistic behavior. Organo-tin compounds are also used as stabilizers to give clear
compounds. In addition to stabilizers, other additives such as fillers, lubricants, pigments,
and plasticizers are used. Fillers are often added to reduce cost and include talc, calcium
389
carbonate, and clay. These fillers may also impart additional stiffness to the compound.
The addition of plasticizers lowers the T of rigid PVC, making it more flexible. A
g
wide range of products can be manufactured by using different amounts of plasticizer. As
the plasticizer content increases, there is usually an increase in toughness and a decrease in
the modulus and tensile strength. 390 Many different compounds can be used to plasticize
PVC, but the solvent must be miscible with the polymer. A compatible plasticizer is con-
sidered a nonvolatile solvent for the polymer. The absorption of solvent may occur auto-
matically at room temperature or may require the addition of slight heat and mixing. PVC
plasticizers are divided into three groups, depending on their compatibility with the poly-
mer: primary plasticizers, secondary plasticizers, and extenders. Primary plasticizers are
compatible (have similar solubility parameters) with the polymer and should not exude. If
the plasticizer and polymer have differences in their solubility parameters, they tend to be
incompatible or have limited compatibility and are called secondary plasticizers. Second-
ary plasticizers are added along with the primary plasticizer to meet a secondary perfor-
mance requirement (cost, low-temperature properties, permanence). The plasticizer can
still be used in mixtures with a primary plasticizer, provided the mixture has a solubility
parameter within the desired range. Extenders are used to lower the cost and are generally
not compatible when used alone. Common plasticizers for PVC include dioctyl phthalate,
di-iso-octyl phthalate, and dibutyl phthalate, among others. 391
The plasticizer is normally added to the PVC before processing. Since the plasticizers
are considered solvents for PVC, they will normally be absorbed the polymer with only a
slight rise in temperature. 392 This reduces the time the PVC is exposed to high tempera-
tures and potential degradation. In addition, the plasticizer reduces the T and T , there-
g m
fore lowering the processing temperatures and thermal exposure. Plasticized PVC can be
processed by methods such as extrusion and calendering into a variety of products.
Rigid PVC can be processed using most conventional processing equipment. Because
HCl can be given off in small amounts during processing, corrosion of metal parts is a
concern. Metal molds, tooling, and screws should be inspected regularly. Corrosion-resis-
tant metals and coatings are available but add to the cost of manufacturing.
Rigid PVC products include house siding, extruded pipe, thermoformed, and injection-
molded parts. Rigid PVC is calendered into credit cards. Plasticized PVC is used in appli-
cations such as flexible tubing, floor mats, garden hose, shrink wrap, and bottles.
PVC joints can be solvent welded rather than heated so as to fuse the two part together.
This can be an advantage when heating the part is not feasible.
2.2.28.2 Chlorinated PVC. Post-chlorination of PVC was practiced during World
393
War II. Chlorinated PVC (CPVC) can be prepared by passing chlorine through a solu-
tion of PVC. The chlorine adds to the carbon that does not already have a chlorine atom
present. Commercial materials have chlorine contents around 66 to 67 percent. The materi-
als have a higher softening point and higher viscosity than PVC. They are known for good
chemical resistance. Compared to PVC, chlorinated PVC has higher modulus and tensile
strength. Compounding processes are similar to those for PVC but are more difficult.

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