Moisture absorption and hydrothermal aging of phenylethynyl-terminated pyromellitic dianhydride-type asymmetric polyimide and composites

Abstract

The effects of moisture on a polymerized monomeric reactant (PMR)-type polyimide (TriA X) and associated composites were investigated. Water uptake tests were performed on the polyimide at various temperatures and relative humidity levels to investigate moisture absorption behavior. Two-stage moisture absorption was observed, in which the first stage was diffusion controlled, whereas the second stage was moisture plasticization controlled. As exposure temperature increased, the equilibrium moisture content of the polyimide decreased, indicating an exothermic absorption process. The Arrhenius temperature dependence and moisture saturation as functions of temperature and humidity in the neat polymer were determined using curve fitting based on the published mathematical models. Long-term hydrothermal aging at 95°C was conducted on the neat polyimide and associated carbon fiber composites. Reversible hydrolytic reactions and a trace of irreversible hydrolysis were observed in the long-term exposure. The tensile ductility of the neat polyimide and the short-beam shear strength of the composites decreased with increasing aging time, while the tensile strength and modulus and thermal properties of the polyimide exhibited little change after 2000-h aging, demonstrating hydrothermal stability. The decrease in the ductility of the neat polymer after long-term moisture exposure was attributed to the network structure change, driven by hydrolysis and moisture plasticization.

Effects of hygrothermal and thermal-oxidative ageing on the open-hole properties of T800/high-temperature epoxy resin composites with different hole shapes

“…In the initial stage of moisture absorption, moisture rapidly diffused in the composites because the moisture first entered into the void space inside the material or into defects, such as bubbles and cracks, resulting in a rapid increase in mass. 23 As the hygrothermal ageing time increased, the mass change rate increased slowly and then tended to stabilize; finally, the mass change rate entered the relative saturation stage, wherein the equilibrium moisture absorption rate was only approximately 0.72%. In the initial stage of thermal-oxidative ageing, the mass loss rate increased rapidly, which was mainly due to the evaporation of moisture inside the samples and the volatilization of residual low molecular substances.…”

“…The interfacial damage occurred because the carbon fibres do not absorb a substantial amount of moisture, and the resin expands after moisture absorption, which destroys the interfacial bond between the fibre and the resin matrix, thereby causing interfacial damage that degrades the mechanical properties of the composites. 23 After thermal-oxidative ageing, the fibres and the matrix obviously debonded, voids appeared between the fibres and fibres, no resin filled the voids, and the matrix cracking was extensive. These morphological changes occurred because thermal-oxidative ageing causes the volume of the matrix to shrink, and the thermal expansion coefficient of the fibre and the matrix do not match, resulting in shrinkage stress and interfacial damage.…”

“…However, the absorption peaks of other characteristics did not change significantly, and there was no increase or decrease in the peaks, indicating that no new substances were formed during the hygrothermal ageing process, and no chemical reaction occurred. 23…”

Polyimide fabric-reinforced polyimide matrix composites with excellent thermal, mechanical, and dielectric properties

“…An emerging type of high performance fiber material, that is, a polyimide (PI) fiber, possesses excellent mechanical properties of high strength and high modulus, remarkable thermal and dielectric properties, and outstanding chemical and irradiation resistance. [1][2][3][4][5] Owing to a satisfactory combination of the aforementioned properties, PI fibers have attracted significant attention in the field of advanced composite materials for aviation and aerospace applications. In particular, because PI fibers can be used within a wide range of temperatures, they match well with the ultrahigh temperature and pressure applied in the processing conditions of high performance resins and achieve long-term service stability under extreme environments.…”

Effects of thermal–oxidative aging on the mechanical properties of open-hole T800 carbon fiber/high-temperature epoxy composites

“…Among them, thermal–oxidative aging, hygrothermal aging, and hot water aging have recently become major concerns in the aerospace industry. 13 –17 At present, extensive tests have only studied the opening factors or environmental factors on the properties of composites but not combinations of the two.…”