Page 359 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.39

5.5.6.7 Dispersants. Dispersants are used to separate pigment particles from each other,
distribute them uniformly in the liquid polymer system, and stabilize the dispersed state.
5.5.6.8 Bonding Agents. Phenolic resins are added to plastisols for adhesion to metals.
Isocyanates are added to plastisols for adhesion to fabrics.
5.5.6.9 Nucleating Agents. Crystallization of polymers from the melt occurs in two
successive stages: nucleation (formation of tiny incipient crystal lattices), and growth (ad-
dition of layers of polymer molecules to the surface of the nucleus or growing crystal. Low
temperature (near T ) promotes the formation of nuclei, but high temperature (near T ) fa-
g
m
vors growth. To solve this dichotomy and promote formation of small uniform crystallites,
compounders add nucleating agents, which then permit processors to crystallize finely,
uniformly, and rapidly at higher temperatures.
Nucleating agents give faster crystallization, finer crystal size, and higher percent crys-
tallinity, hardness, modulus, strength, elongation, impact strength, heat deflection temper-
ature, and transparency.
Two types of materials nucleate crystallization: tiny filler particles with sharp edges
and corners, and high-MP polymers that crystallize first and then nucleate crystallization
of the lower-MP major polymer.
5.5.6.9.1 Polyethylene. Crystallization is fast but may be coarse. One percent of K
stearate reduces spherulite size.

5.5.6.9.2 Polypropylene. Coarse crystallinity makes it opaque. To reduce crystal size
and increase transparency, 0.1 to 0.3 percent of substituted sorbitols are added as “clarifi-
ers.” Dibenzylidene sorbitol, and especially its methyl derivatives, are the best. One per-
cent of Na benzoate is used for injection molded food and drug packaging. Other metal
benzoates, t-butyl benzoate and naphthoate, are also mentioned.

5.5.6.9.3 Polyethylene Terephthalate. Crystallization is difficult and slow. Particu-
larly for injection molding, 0.5 percent of 3-µm metal oxide, metal salt, pigment, or other
minerals, and ionomer are mentioned. For thermoformed food trays, 1 to 3 percent of low-
MW polyolefin.
5.5.6.9.4 Nylon. Mentioned most often is 0.1 percent of silica. Others mentioned in-
clude Na benzoate, minerals, MoS , FeS, TiO , talc, Na phenyl phosphinate, and higher-
2
2
MP polymers.
5.5.7 Effects of Lubricants on Other Final Properties
Fogging is caused by condensation of moisture on clear film, forming droplets that scatter
light. It is a problem in food packaging for refrigerator and freezer display and in agricul-
ture for greenhouses. It is worst on hydrophobic films of low surface energy, particularly
polyolefins. Some lubricants also serve as antifogging agents. Glycerol monoesters, sorbi-
tol esters, and polyethoxylates of fatty acids and alcohols are designed for semimiscibility,
to exude to the surface of the film. Once there, they provide a hydrophilic surface of higher
surface energy. Moisture that condenses on the surface is able to wet and spread across the
surface in a uniform film, which remains transparent.
As mentioned earlier, when lubricants smooth the surface of extruded products, this
can increase gloss.
One negative effect is often encountered in the use of silicones. While they are very ab-
hesive and prevent polymer from sticking to steel process equipment, they also prevent

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