Surface treatments for particulate fillers in plastics

From surface energy theory, the work of cohesion in a liquid is equal to twice the surface energy. For a polymer this would be approximately 70mjm . It has been shown that for cases when either or both the filler and polymer are non-polar, Wpp, the reversible work necessary to separate unit area of filler-polymer interface is given by  

Many of the available coatings react with the filler particles, but do not react with the polymer matrix. Most are monofunctional. A typical reaction for an organic add and magnesium hydroxide is shown here  

Other difunctional coatings, which are strictly coupUng agents, react with both filler and matrix polymer. Organosilanes are the most common type in this category. 

Stearic acid reacts to form stearate as shown above, and such coatings can also be applied as stearate salts. The use of salts has been shown to have favourable effects on compound properties. For example, when magnesium stearate is used as an alternative to stearic acid for coating magnesium hydroxide, tensile and impact properties (measured using an instrumented drop weight impact tester) are both improved as shown in Table 1. 

Stearic acid coated calciuirt carbonates are commercially available, and are widely used. Stearic acid coating of magnesium hydroxide has been studied in some detail, and fatty acids could be used for coating other hydroxide and carbonate containing fillers. 

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The reader is referred to the entry in this book, entitled Surface treatments for particulate fillers in plastics, for further information. 

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