Silicone Rubber Properties During Consolidation/Curing of Advanced Composites Using Specialized Elastomeric Tooling

Abstract

Specialized elastomeric tooling (SET) is a patented process that can replace autoclaving for consolidating and curing advanced thermoset and thermoplastic composite parts. The process mimics autoclave conditions of uniform pressure and temperature by clamping an uncured laminate or sandwich structure with known force between a temperature-controlled lower tool and an engineered, rubber-faced upper tool. Several published studies involving small- to medium-sized parts have shown that SET provides equal or better quality, but at fraction of the energy, waste, and capital and consumable costs. The elastomer of choice for the upper tool is a castable, platinum-catalyzed silicone rubber because of its high working temperature, high tear strength, and negligible shrinkage. To date, there is limited understanding about the properties of silicone rubber subjected to high temperature and compression for long periods of time over multiple cycles. This paper discusses recent work that characterizes silicone rubber under these conditions for design and simulation purposes. Compression testing performed per ASTM-D575 exhibited linear behavior at 125 °C (typical processing temperature for epoxy resins), whereas tensile testing (per ASTM-D412) at the same temperature exhibited strain softening. To show the repeated effect of compression on rubber properties (i.e., mimic multiple loading cycles on rubber mask) at typical process temperature (125 °C) and pressure, a fatigue testing apparatus was custom designed and fabricated. Over repeated cycles between 0 and 1.35 MPa (typical consolidation pressure for advanced composites), silicone rubber exhibited slight hysteresis and a minor stiffening effect that appears to plateau at a particular modulus. Static and kinetic frictional coefficients, also used in modeling, between silicone rubber and several materials commonly used in SET ranged from 0.5 to 2.4 (per ASTM-D1984). Finally, pressure injection and in-line mixing of uncured rubber resulted in significantly less entrained air bubbles (and resulting surface defects in contact with composite part) than the current standard practice of hand mixing. Results are applicable to both SET and any advanced composite forming or curing/consolidation processes involving rubber-faced tools.