Achieving Dimensional Stability Using Functional Fillers

Abstract

Several materials exhibiting negative coefficients of thermal expansion have recently been identified. Such materials incorporated as filler in a polymer matrix should result in a composite with reduced, if not zero, expansion when exposed to elevated temperatures. One such filler material, ZrW2O8, has a highly negative coefficient of thermal expansion and its cubic structure leads to isotropic thermal behavior. Consequently, Zr2WO8 was investigated as functional filler for polymer matrices, with the goal being improved dimensional stability of the filled system. The choice of polymeric matrix material is also crucial when seeking dimensional stability, and fabrication of a useful end product. In this regard, several aerospace grade epoxies along with a novel cyanate ester resin were investigated. The dynamic mechanical properties along with thermal behavior of the filled systems were compared. A cyanate ester/ZrW2O8 system exhibited the lowest coefficient of thermal expansion (CTE), and consequently was investigated in most detail. Cylindrical samples of the cyanate ester/ZrW2O8 system were molded and cured per schedule. The ZrW2O8 particles were allowed to settle at their natural rate. Thus, the final samples contained various weight percentages and sizes of ZrW2O8. The sectioned samples allowed investigation of filler percent and how it impacts overall CTE. Decreases in CTE of ~80% are reported for samples containing ~82 wt% ZrW2O8. The lowest value observed for composite CTE was 2.8 ppm/K at 32 °C for a sample having 83% zirconium tungstate, which had been subject to one temperature cycle..