Page 328 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.8 CHAPTER 5

TABLE 5.6 World Consumption of
Antioxidants
Type Percent

BHT 14
Higher phenols 42
Phosphites 31
Sulfides 9
Other 4
Thousand metric tons 207

keeps ozone from reaching the rubber. Paraffin waxes bloom rapidly but are too brittle.
Microcrystalline waxes bloom more slowly but are less brittle. Mixture of the two types
gives broader protection. These are adequate for static performance but are too brittle for
dynamic stretching and flexing.
A saturated rubber can be coated on the surface to provide a barrier against ozone. Eth-
ylene/propylene, plasticized PVC, and polyurethane are typical coatings. However, these
involve problems of adhesion, elasticity, and cost, so they are not commonly used.
5.1.2.2 Chemical Antiozonants. These are mostly secondary alkyl aryl amines R-NH-
Ar and related compounds. They give excellent protection. Most of them discolor badly,
but several are recommended for nonstaining applications.
Most compounders use a combination of physical and chemical antiozonants and
achieve excellent protection in this way. For more severe ozone-resistance problems, there
are, of course, a number of specialty elastomers that are saturated and therefore com-
pletely ozone-resistant: ethylene/propylene rubber, chlorinated and chlorosulfonated poly-
ethylene, ethylene/vinyl acetate, ethylene/acrylic esters, butyl rubber, SEBS, plasticized
PVC, butyl acrylate copolymers, polyepichlorohydrin and copolymers, polyetherester
block copolymer, polyurethane, and silicone.

5.1.3 PVC Heat Stabilizers
PVC is very heat sensitive. When it is heated during processing, or even during use, it
loses HCl, which is toxic and corrosive; forms C=C bonds which cause discoloration; and
cross-links, causing clogging of process equipment and embrittlement of products
(Fig. 5.2). The problem is caused by an occasional unstable Cl atom that is destabilized by
being adjacent to a branch point, a C=C group, a C=O group, or an oxygen atom. It re-
quires strong and precise stabilization for practical use. There are three major classes of
heat stabilizers for PVC, as described below.

5.1.3.1 Lead Compounds. These were the earliest in commercial practice. “Normal”
lead salts included sulfate, silicate, carbonate, phosphite, stearate, maleate, and phthalate.
“Basic” lead salts combined these with lead oxide, giving greater stability. They were low-
cost, efficient, and gave excellent electrical resistance. Disadvantages were opacity, sulfur-
staining, and toxicity. Due to worries about toxicity, their use has been restricted to electri-
cal wire and cable insulation.

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