Page 348 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.28 CHAPTER 5

TABLE 5.27 Markets for Reinforced Plastics
Market Percent
Transportation 25
Construction 21
Corrosion resistance 16
Marine 13
Electrical/electronic 9

Consumer products 7
Appliances and business machines 5
Aerospace 2
Other 2

5.3 COUPLING AGENTS

5.3.1 Polymer/Filler Interface
When inorganic fillers and fibers are dispersed in an organic polymer matrix, the interface
is weakened by sharp differences in modulus, thermal expansion, polarity, chemical attrac-
tion, and chemical reactivity. This gives rise to many practical problems as listed below:
• Dispersion of solid fillers in the liquid matrix is difficult, slow, and incomplete.
• Strength is limited by premature failure at the weak interface.
• Impact strength is critically lowered by the weakness of the interface.
• Thermal cycling produces mismatch at the interface, resulting in premature failure.
• Pigmentation with colored fillers is inefficient and expensive.
• Humidity attacks the interface preferentially, causing hydrolysis and premature failure.
These problems are often solved by using coupling agents to strengthen the interface.

5.3.2 Commercial Coupling Agents
Some commercial coupling agents are illustrated in Fig. 5.8.
5.3.2.1 Dispersants. Dispersants for blending pigments into liquid systems include a
wide range of anionic, nonionic, and cationic surfactants that help to wet the pigment par-
ticles and disperse them in the liquid system.
5.3.2.2 Stearic Acid. This is frequently used to coat filler particles. In calcium carbon-
ate, it is the preferred coupling agent. Presumably the –CO H group orients toward the
2
filler particle, and probably reacts with it, while the –C H chain penetrates into the
17 35
polymer matrix.

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