As the use of fiber reinforced plastics increases in such industries as aerospace, wind energy, and sporting goods, factors effecting long-term durability, such as environmental exposure, are of increasing interest. The primary objectives of this study were to examine the effects of extensive environmental exposure (specifically UV radiation and moisture) on carbon/epoxy composite laminate structures and to determine the relative effectiveness of polymer-based coatings at mitigating degradation incurred due to such exposure. Carbon/epoxy composite specimens, both coated and uncoated, were subjected to accelerated weathering in which prolonged outdoor exposure was simulated by controlling the radiation wavelength (in the UV region), temperature, and humidity. Mechanical test data obtained for the uncoated specimens indicated a reduction in strength of approximately 6% after an environmental exposure duration of 750 hours. Test data revealed that no further degradation occurred with increased exposure duration. This reduction resulted from the erosion of the epoxy matrix in additional to the formation of matrix microcracks. The protective coatings evaluated were all epoxy based and included two different surfacing films applied during initial cure of the carbon/epoxy composite laminate and a chromate containing epoxy based paint primer applied after the cure was complete. Although the chromate primer performed well initially, degradation of the underlying substrate was detected with extended exposure durations. In contrast, the surfacing films provided superior protection against environmentally induced degradation. Although similar degradation attributes were identified in the surfacing film as observed in the uncoated composite, it is likely that this degradation was either confined within the surfacing film layer or only penetrated the very near surface of the carbon/epoxy substrate, as it did not result in a substantial reduction in mechanical strength.

Experimental and theoretical characterization techniques are developed to illuminate relationships between molecular architecture, processing strategies, and physical properties of several model epoxy-amine systems. Just beyond the gel point partially cured networks are internally antiplasticized by unreacted epoxy and amine which leads to enhanced local chain packing and strain localization during deformation processes. Additional curing causes the antiplasticization to be removed, resulting in lower modulus, density, yield stress, and less strain localization. Physical and mechanical probes of network formation are discussed with respect to several different partially cured model epoxy-amine chemistries. The non-linear fracture energy release rate and the molecular architecture of virgin and healed epoxy networks are related through an effective crack length model. The inelastic component of the fracture energy release rate is attributed to the failure of network strands in a cohesive zone at the crack tip. Data from fracture and healing experiments are in good agreement with the model over more than three orders of magnitude. Changes in the shape of the process zone and deviation from planar crack growth cause deviations from the model for the toughest networks tested. Double network epoxies are created from stoichiometric blends of an epoxy resin cured sequentially with aliphatic and aromatic amine curing agents. Unreacted epoxide and aromatic amine functionality antiplasticize the partially cured materials. The thermal and mechanical properties of the fully cured networks vary according to composition. No evidence of phase separation is observed across the entire composition and conversion range. However, the breadth of the glass transition in the double networks increases due to the difference in the molecular stiffness of the two curing agents. Techniques are developed to monitor the evolution of residual stresses and strength in complex multicomponent epoxy-amine based coatings. The evolution of properties is attributed to loss of volatile small molecules from the coatings. The stresses that develop in biaxially constrained membranes are monitored through mechanical excitation. The strength of the membranes is determined by monitoring the size and shape of center cracks. This fracture analysis technique allows the evolution of stresses and toughness of the materials to be monitored simultaneously.

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