DIRECT RESIN FILLING MATERIALS: COEFFICIENT OF THERMAL EXPANSION AND WATER SORPTION OF POLYMETHYL METHACRYLATE

¹ Philadelphia, Pa.

The data obtained indicate that fillers can be used with advantage to reduce the coefficient of thermal expansion of acrylic resins. Proper mixing of the polymer powder and the inorganic filler is of considerable importance since improper mixing might lead to poor wetting and give erratic results. The various fillers studied by us do not materially differ one from the other at low concentrations in so far as their influence on coefficient of thermal expansion is concerned; however, at high concentrations of the filler in the resin, some [see fig in the PDF file] differences are noticeable. The choice of a suitable filler should be made after taking into consideration the optical properties of the resultant resin; in this connection the index of refraction of the filler and its particle size distribution should be given adequate attention.

Studies of the increase in weight of direct filling resins in water also point to the beneficial effect of fillers. Fillers which are water-soluble in any appreciable degree are likely to cause the resin to become porous in character after a period of time; their value is doubtful. In the initial stage, most of the fillers studied reduced the water sorption of the resulting resin to about the same extent, regardless of solubility.

Addition of cross-linking materials to the acrylic resin did not significantly change its water absorption characteristics.