Page 353 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.33

dation of oily droplets on the surface is generally evidence of poor compatibility.
Migration of plasticizer into an adjacent unplasticized surface can ruin the surface. Hu-
midity can reduce compatibility in borderline cases.
Whenever a plasticizer is too fugitive for the application, the problem is easily solved
by using a higher-molecular-weight plasticizer. Thus, changing from dioctyl to didecyl ph-
thalate is enough to prevent windshield fogging or to get a higher temperature rating from
Underwriters Laboratories. Going to ditridecyl phthalate achieves an even higher tempera-
ture rating. Going from dialkyl phthalate to trialkyl trimellitate is even better. In serious
problems, the best solution is to use polymeric plasticizers—linear polyesters of MW
1000 to 3000. Ultimately, high-molecular-weight polar elastomers such as ethylene/vinyl
acetate, chlorinated polyethylene, butadiene/acrylonitrile rubber, and polyurethane can
provide absolute permanence.

5.4.4.2 Aging. Plasticizers have less age resistance than polymers such as PVC. Ther-
mal oxidation occurs in electrical wire and cable insulation and is commonly retarded by
addition of antioxidants. Microbiological attack on plasticizers is commonly retarded by
addition of biostabilizers (Sec. 5.1.5).

5.4.5 Special Effects

5.4.5.1 Air Molecules. Air molecules have lower molecular weight and higher mobility
than any liquid plasticizer. Consequently, foaming a polymer (Sec. 5.6) can produce much
greater softness than conventional plasticization.
5.4.5.2 Water Absorption. Water absorption in polar hydrogen-bonding polymers can
provide powerful but unreliable plasticization, depending on humidity and immersion. The
effect is most often noticed in nylon. It also occurs in cellulosic plastics.
5.4.5.3 Antiplasticization. This is an unexpected stiffening effect observed when small
amounts of plasticizer are added to PVC, acrylic, and polycarbonate plastics. Probably,
these stiff polymer molecules do not pack neatly on cooling from the melt, and the resid-
ual free volume leaves some molecular mobility. Adding a little plasticizer may fill the free
volume or may give the polymer molecules enough mobility to pack more neatly, elimi-
nating free volume and thus increasing their rigidity.
5.4.5.4 Internal Plasticization. This is produced by building the plasticizing structure
right into the polymer molecule. In homopolymers such as polyolefins and vinyl and
acrylic esters, lengthening the side-chains up to 6 to 12 CH groups pushes the polymer
2
main-chains farther apart (“tent-pole effect”), creating free volume that increases the mo-
bility of the polymer molecules, changing rigid plastics into flexible, rubbery, and even
tacky polymers.
In some polymer families, copolymerization with more flexible comonomer units is
very effective in producing the amount of flexibility desired. Major commercial examples
are ethylene/propylene rubber, styrene/butadiene plastics and latex paint, vinyl chloride/
vinyl acetate plastics, vinyl acetate/acrylic ester latex paints, and methyl methacrylate/
acrylic ester plastics and latex paints.
In all these cases, the resulting plasticization is absolutely permanent. However, there
are two drawbacks. (1) A plasticizing structure bound into the polymer molecule is less ef-
ficient than an independent monomeric structure. (2) The product can be made by the
polymer manufacturer only on a large scale, not by the independent processor on a daily
basis for individual customer needs.

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