Enhancing Mechanical Properties of Polymer Composites by Modification of Surface Acidity or Basicity of Fillers

Abstract

AbstractThe surface of a powdered glass has been treated to optimize its performance as a filler in polymer composites cast from solution. The aim has been to enhance acid-base interactions between the functional sites of polymers and the surface sites of the glass particles so as to insure complete coverage of the glass particles by adsorbed polymer molecules, and to thereby enhance both the adhesion and dispersion of the filler. The glass chosen for this study is a magnesium aluminum silicate of very low alkali metal content, and its surface sites have negligible basicity and only moderate acidity, as evidenced by weak adsorption of polymethylmethacrylate and negligible adsorption of chlorinated polyvinylchloride from solution in methylene chloride. Because of the negligible surface basicity of the filler, acidic polymers such as polyvinylbutyral and polyvinyl chloride are not adsorbed sufficiently to promote adhesion or dispersibility, and brittle or weak composites result.Surface treatments to enhance surface acidity or basicity of the glass particles were evaluated by electrophoresis in organic solvents, in which acidic particles had negative zeta-potentials and basic particles had positive zeta-potentials; untreated glass particles had zero zetapotentials. The most effective treatments were done with silane coupling agents; amino-silanes provided strong negative zeta-potentials and methacryloxy-silanes provided strong positive zeta-potentials. Angleresolved ESCA spectra showed that the adsorbed amino-silanes averaged about a monolayer in thickness and that half of the nitrogens bonded to the surface as ammonium groups and half were in the outer surface as basic amine groups; the carbonyl oxygens of the methacryloxy-silanes appeared to be bonded to the surface.Heavily-loaded polymer composites were made with the silane-treated and with untreated glass powders, using two acidic binders and a basic binder. The acidic binders were a polyvinylbutyral (PVB) binder cast from solution in a methanol-MIBK mixture, and a post-chlorinated polyvinylchloride (CPVC) binder cast from solution in a mixture of cyclohexane and tetrahydrofuran (THF): the glass transition temperature (Tg) for CPVC is about 140 C, but for PVB it is only about 35 C. The basic polymer (PEHM, polyethyl-hexylmethacrylate) has a Tg well below room temperature. Thus the extension to break in tensile tests was about 1% for CPVC, 10% for PVB, and 20 % for PEHM. About ten stress-strain Instron tests were made on each composite, and the composites with appreciable acid-base interactions between the matrix polymer and the surface sites of the treated glass fillers were in all cases much tougher than those without such acid-base interactions. The brittleness of the polyvinylbutyral composites filled with untreated glass powders was not observed at all on silane-treated glass powders.