Effects of Moisture on the Properties of High-Performance Structural Resins and Composites

Abstract

Graphite, boron, S-glass, and DuPont's PRD-49-III fiber reinforced composites, as well as castings of current epoxy resin systems, were evaluated to determine the effects of moisture and high humidity on their physical properties and their room- and elevated-temperature mechanical properties. All of the neat resin castings were found to absorb moisture and swell. Associated with moisture absorption is a loss in elevated-temperature tensile strength. All of the composite systems showed weight gains and thickness increases when subjected to a high-humidity environment. However, the effect of absorbed moisture on the elevated-temperature mechanical properties of composites is determined principally by fiber orientation and test method employed. Unidirectional composites may show a significant reduction of 350°F (177°C) flexural strength due to absorbed moisture, whereas a multidirectional lay-up may show only a minor loss of 350°F tensile strength after equivalent moisture absorption. Fiber-controlled composite properties are relatively unaffected by absorbed moisture whereas matrix-controlled properties are adversely affected.