Influence of Matrix Chemistry on the Short Term, Hydrothermal Aging of Vinyl Ester Matrix and Composites under Both Isothermal and Thermal Spiking Conditions

Abstract

Moisture absorption in vinyl ester (Derakane 441-400) resin and vinyl ester/glass fiber (EXTREN®) composites has been investigated. Moisture uptake for the neat polymer under short term isothermal aging conditions in water was found to obey Fick's second law. Information on moisture uptake in the polymer at different water bath temperatures namely, room temperature, 45°C, 66°C and 84°C was used to compute the diffusion coefficients and the Arrhenius activation energy for the diffusion process. In this study, glass transition (Tg) of the polymer aged to different extents in a water bath at 66°C was monitored as a function of moisture content using a dynamic mechanical analyzer (DMA) run in fixed frequency mode. Chemical structure changes were also monitored for both unaged and fully saturated thin film specimens aged under similar conditions using Fourier transform infrared spectroscopy (FTIR). Specific interactions in the form of hydrogen bonding with water were found to exist. In addition to isothermal aging, the effects of short term thermal spiking on the equilibrium moisture content was studied both for polymer and composite. Samples were aged in water under a 66°C-room temperature-66°C temperature profile, wherein changes in exposure temperature were made only upon saturation at the previous temperature. The reverse thermal effect (RTE) was found to exist. The activation energy for the process was experimentally computed and compared with that of typical hydrogen bonding to find excellent correlation confirming that hydrogen bonding was occurring. Moisture uptake at 66°C was also monitored on a model resin system (i.e., CH3-GMA) similar in chemical structure to vinyl ester except for the absence of hydroxyl (-OH) groups on the backbone and FTIR tests were performed on this new resin. The absence of the "-OH" peak in the unaged specimen and the resulting shift upon aging motivated the above mentioned thermal spiking experiment to be performed on the system. The RTE was not observed for this resin, and it was concluded that along with the previous work that the RTE related phenomenon could be attributed to hydrogen bonding. In addition to the neat polymer, quasi-static tensile properties were found to deteriorate as a function of equilibrium moisture content for E-glass/vinyl ester composites aged under RTE conditions.