DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The composite filler (A) constituting the composition of the present invention is obtained by non-catalytically polymerizing a carboxylic monomer with one or more other polymerizable vinyl monomers in a polymerization system having an inorganic compound dispersed therein.
No particular limitation is placed on the method for preparing the composite filler (A).
According to one preferred method, the composite filler (A) is prepared by carrying out an aqueous heterogeneous polymerization reaction under temperature conditions which do not cause thermal polymerization to occur.
More specifically, a vinyl monomer and an inorganic compound are suspended and dispersed in an aqueous medium, and a carboxylic monomer is added to this suspension.
Then, the resulting reaction mixture is stirred under the aforesaid temperature conditions for a predetermined period of time.
The specific carboxylic monomer of the above general formula [I] or [II], which is used for the preparation of the composite filler (A), should indispensably have a carboxyl group as an active site for imparting polymerizability thereto, and a double bond as an active site for establishing a firm bond between the formed polymer and the inorganic compound.
There may be used any of the compounds having a chemical structure containing those functional groups.
Specific examples of useful carboxylic monomers include acrylic acid, methacrylic acid, crotonic acid, tiglic acid, cinnamic acid, maleic anhydride and citraconic anhydride.
Among these, acrylic acid, methacrylic acid, crotonic acid and maleic anhydride are preferred because of their high polymerizability.
The inorganic compound used for the preparation of the composite filler (A) is selected from among metals of groups I, II, III and IV of the periodic table; transition metals; oxides, hydroxides, chlorides, sulfates, sulfites, carbonates, phosphates and silicates of such metals; and mixtures and complexes of such compounds.
Among these, barium sulfate, barium fluoride, silicon dioxide, aluminum oxide, titanium oxide, quartz powder, glass powder, glass beads, glass fibers, glass fillers containing a barium, lead or strontium salt, silica gel, zirconium oxides and tin oxides are preferred because they are highly effective in activating vinyl monomers and forming a firm bond with polymers
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