Physical aging toughness

Thermal annealing at 140 °C in an inert dark atmosphere resulted in decreases in strength, ductility, and toughness, as seen in Fig. 3 and Table 1. These changes are attributed to physical aging processes occuring in the glassy polymer. As an additional... 

In order to study the effect of physical aging on the carbon-fiber reinforced epoxy, the freshly quenched materials were then sub-Tg annealed at 140 °C. After annealing for only 10 minutes at that temperature, the toughness of the composite was restored to a level comparable to that of the postcured material (see Fig. 7). It is likely that residual thermal stresses resulted from the quenching were annealed away during this 10 minutes thermal aging at 140 °C. 

Physical aging in the epoxy matrix definitely affects the long-term mechanical properties of the composite. As the material was further sub-Tg annealed at 140 °C for 102, 103, 104, and up to 105 minutes, the toughness and other ultimate properties... 

The inplane shear stress-strain tests reported here have been well demonstrated to be a reliable test for matrix-dominated properties in composites 141). For the selected mechanical properties that were monitored, their sensitivity to the thermal history was well demonstrated. In particular, the embrittlement process during the sub-Tg annealing or physical aging has been clearly observed. This decrease in molecular mobility, which gives rise to an increase in relaxation time and hence a decrease in toughness, can be rationalized as a decrease in free volume in an approach towards the equilibrium glassy state. 

Polycarbonate has been found to lose its toughness when aged at temperatures below its T and impact modifiers are often added to counteract this effect of physical aging. Blends of PC with... 

The influence of physical aging on the comparative and intrinsic toughness of PEEK and PPS carbon-fiber composites was determined by Ma et al. [1992]. Both types exhibited a loss of toughness with increase in aging time and temperature. The PEEK composites tended to retain more impact toughness but after aging were less ductile than the PPS composites. 

Crystallization and morphology [156-158] have been considered. With materials processed at high temperatures, the type of size used on the carbon fiber is important [159] and the effect of physical aging on the toughness of PES composites [160] has been determined. 

Ma CCM, Lee CL, Chang MJ, Tai NH, Effect of physical aging on the toughness of carbon fiber-reinforced poly(etheretherketone) and poly(phenylene sulfide) composites. Part 1, Polym Composites, 13(6), 441-447, 1992. 

Polysulfone has a second, low-temperature glass transition at — 101°C similar to other tough, rigid thermoplastic polymers. These polymers also resist aqueous acid and alkaline environments. Properties such as physical aging and solvent crazing can be improved by annealing the parts. This also reduces molded-in stresses. 

By contrast, and analogous to physical aging processes, molecule structure is not attacked by physical media. Instead, interactions between the diffused-in medium and the molecular chains takes place. The increased toughness in polyamide due to water absorption is an example of physical media effects [93]. 

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