The Structure and Moisture Stability of the Matrix Phase in Glass-Reinforced Epoxy Composites

The utilization of reinforced plastic composites is generally limited by their sensitivity to long-term environmental exposure. Glass-reinforced thermosetting polymers in particular are detrimentally affected, reversibly and/or irreversibly, by exposure to water vapor or liquid water. A reinforced composite consists of three mutually interacting regions: fiber, matrix, and the fiber/ matrix interface. Each region may, for purposes of convenience, be further subdivided according to its location or properties. Most common is the listing of regions comprising the “interface”: glass/ coupling agent interface, the coupling agent polymer itself, and the coupling agent/matrix interface. Even the amorphous cross-linked matrix nearby the filler is believed to possess properties dissimilar to those of the bulk. Each region and subregion is altered by moisture exposure to some extent; at this time studies on composite stability still focus on determining which regions are most affected by the environment and on the mechanisms of those effects, as well as on devising systems with minimal environmental sensitivity. Moisture effects on glass-reinforced epoxy composites, the most common of the reinforced plastic systems, will be emphasized in this paper. Multifunctional epoxy resins are typically cross-linked by acid anhydrides, forming 3-D polyesters, and by amine functional compounds, yielding 3-D amine-ether polymers. There are many structural varieties; the same hydrothermal degradation mechanisms apply universally, and hereafter the term “epoxy” matrix will be employed. The stability of the glass reinforcement itself and of the interface will first be briefly reviewed as a prelude to a more detailed consideration of the reversible and irreversible moisture effects on the cross-linked epoxy matrix.