Effects of prior aging at 288°C in air and in argon environments on creep response of PMR‐15 neat resin

Abstract

The creep behavior of PMR-15 neat resin, a polyimide thermoset polymer, aged in air and in argon environments at 2888C for up to 1000 h was evaluated. Creep tests were performed at 2888C at creep stress levels of 10 and 20 MPa. Creep periods of at least 25-h in duration were followed by 50-h periods of recovery at zero stress. Prior isothermal aging increased the elastic modulus and significantly decreased the polymer's capacity to accumulate creep strain. The aging environment had little influence on creep and recovery behaviors. However, aging in air dramatically degraded the tensile strength of the material. Dynamic mechanical analysis revealed an increase in the glass transition temperature from 3308C to 3368C after 1000 h in argon or in air at 2888C. The rise in the glass transition temperature with aging time is attributed to an increase in the crosslink density of the PMR-15 polyimide. Increase in the crosslink density due to aging in both air and argon environments is likely behind the changes in the elastic modulus and the decreased capacity for inelastic straining. A visibly damaged surface layer of 0.16 mm thickness was observed in specimens aged in air for 1000 h. Results indicate that the unoxidized core material governs the overall mechanical response, whereas the oxidized surface layer causes a decrease in tensile strength by acting as a crack initiation site and promoting early failures.

A model for the thermo-oxidative degradation of polyimides

“…We also note that we chose the functions for the material moduli (μ p ,μ G ,η) such that they increase as α goes from 0 to 1. This is consistent with the experiments (see figure 5 in Ruggles-Wrenn and Broeckert (2009)) where it seen that the elastic modulus increases with aging. We further note that such a choice of functions for the material moduli is different from what Rajagopal et al (2007) have chosen in their work.…”

Effects of prior aging at 288°C in argon environment on time‐dependent deformation behavior of a thermoset polymer at elevated temperature, part 1: Experiments

“…Figure 2 presents the tensile stress–strain curves obtained at strain rates of 10 −6 and 10 −4 s −1 for the unaged material and for specimens aged for 100, 250, and 1000 h. It is seen that the elastic modulus increases with prior aging time. The effect of prior aging on the elastic modulus, which is further illustrated in Figure 3, is consistent with the observations reported earlier 19. Furthermore, stress–strain curves in Figure 2 show that for a given strain rate the departure from quasi‐linear behavior occurs at higher stress levels for specimens subjected to longer prior aging times.…”

“…The PMR‐15 neat resin panels were manufactured and postcured by HyComp Inc. (Cleveland, OH). The details of the PMR‐15 chemistry and processing, as well as the schematic of the free standing postcure cycle are given elsewhere 19, 20. Dogbone shaped test specimens of 150‐mm total length with a 7.6‐mm wide gage section were machined from the 3.18‐mm thick panels using diamond‐grinding.…”

“…Several research efforts evaluated the effects of aging on the tensile and compressive properties of PMR‐15 based composites 4, 5, 12, 18. Recently Ruggles‐Wrenn and Broeckert19 investigated the effects of thermal aging at a temperature near the T g on creep and recovery behavior of the PMR‐15 neat resin at elevated temperature. Creep tests at stress levels of 10 and 20 MPa were conducted at 288°C on specimens subjected to prior thermal aging at 288°C in air or in argon environment.…”

BPDA-PDA Polyimide: Synthesis, Characterizations, Aging and Semiconductor Device Passivation

“…The effect of long time aging of polyimide at high temperature (>200 °C) and in oxidative environment on the mechanical properties [39], weight loss [40,41], and chemical properties [42,43], was widely investigated for thick polyimide matrix composites (1 mm thick) used in high temperature aerospace applications. It was found that while thermal degradation occurred throughout the material, the oxidative degradation occurs mainly within a thin surface layer where oxygen diffuses into the material.…”