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Composite materials have attracted considerable attention in the past decades, and many current important industrial products are made with them. A composite is the result of the combination of two or more distinct components,' which often belong to different classes of materials. Many composites are natural (e.g., bone), but there is a large number of synthetic composites. Most often, composites are made of a continuous phase, or matrix, interspersed with particles, fibers, tissue, or other form of a second phase.' The interfacial regions are very important in composite materials, and they have a decisive role in the ultimate mechanical properties.
Composite (or encapsulated) particles are special and important reinforcing materials. They are interesting because of the properties that may be obtained, both as particulate materials or as precursors of bulky solids. There are many interesting examples of encapsulated particles from the recent literature. Hasegawa et al.3-5 studied the encapsulation of BaSO,, CaC03, and TiOz with poly(methy1 methacrylate), obtained by emulsion polymerization. In thi's case, monomer adsorption on the particles is SYNOPSIS Stable, thick poly(dimethylsi1oxane) (PDMS) coatings are formed on iron(II1) and aluminum oxide and calcium carbonate particles by impregnating the particles with dimethylsilicone oil and heating at 250-280OC. These coatings are strongly resistant to solvent extraction and to exposure in a water-saturated atmosphere. Coated particles are strongly hydrophobic, as evidenced by their greater stability in apolar solvents. Silicone coating formation on oxide particles is interpreted as a result of two reactions: siloxane chain opening at higher temperatures, followed by the reaction of active chain end-groups with hydroxo groups at the metal oxide or carbonate surfaces and silicone cross-linking by methylene or siloxane bridges. The procedures described in this paper differ from usual silanization or siliconization procedures because it uses stable PDMS and yields thicker coatings. 0 1996 John Wiley &