Page 351 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.31

5.3.5.5 Strength. The strength of filled and reinforced plastics, resistance to thermal cy-
cling, and efficiency of flame retardants may be improved by coupling agents, but com-
mercial secrecy and salesmanship have obscured any objective benefits.
5.3.5.6 Wet Aging Resistance. This may be the most demonstrable benefit in reinforced
plastics. When glass-fiber-reinforced thermoset polyesters are immersed in water, particu-
larly boiling water, they lose strength rapidly. When they are properly prepared with orga-
nosilane coupling agents, their strength retention is markedly improved. The coupling
agent strengthens the interfacial bond between glass fiber and polyester matrix, and it pro-
vides hydrophobicity to repel moisture and keep it from intruding into the interface.

5.4 PLASTICIZERS

Plasticizers are most commonly liquid esters of low volatility, which are blended into rigid
thermoplastic polymers to make them soft and flexible. Most are esters of phthalic, phos-
phoric, and adipic acids. Major use is in polyvinyl chloride (PVC) elastoplastics. Another
major use, rarely mentioned in the literature, is the addition of hydrocarbon oils to rubber
to improve processability. Plasticizers are also used to improve melt processability and
toughness of rigid plastics such as cellulose esters and ethers, and they are used in a vari-
ety of specialized applications. In some cases, they perform dual functions such as thermal
stabilization or flame retardance. This gives the individual processor the ability to tailor
properties for each product.

5.4.1 Compatibility
The first requirement of a plasticizer is that it should be compatible with the polymer; that
is, it should be completely miscible and remain permanently in the polymer. In general,
this requires that polymer and plasticizer should have solubility parameters within one to
two units of each other. Strong mutual hydrogen-bonding is a second factor favoring com-
patibility. And low molecular weight also favors miscibility.
When a plasticizer meets these requirements, it is actually a solvent for the polymer
and can speed melt processing. When solubility parameters are a little farther apart, the
plasticizer must be heated to dissolve the polymer; on cooling to room temperature, it
forms a gel, which may favor optimum balance of flexibility and strength. When solubility
parameters are still farther apart, it is incompatible unless used in combination with a com-
patible “primary” plasticizer and is referred to as a “secondary” plasticizer.

5.4.2 Efficiency
There are 600 commercial plasticizers. Some are very efficient in softening the polymer.
Others are much less efficient and are used for other reasons. Efficiency is measured by
plotting modulus versus plasticizer concentration and comparing the plots for different
plasticizers. It is reported either (1) as the amount of plasticizer required to reach a stan-
dard modulus or (2) as the modulus produced by a standard amount of plasticizer.
Three factors determine plasticizer efficiency. (1) A flexible plasticizer molecule, con-
taining long (CH ) chains, is more efficient in flexibilizing the polymer; rigid units such
2 n
as benzene rings are much less efficient. (2) Low polarity and low hydrogen-bonding pro-
vide less attraction between polymer and plasticizer (borderline compatibility), permitting

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.