Impact strength of polymers: 1. The effect of thermal treatment and residual stress

Abstract

The effect of thermal annealing and quenching on the notched Izod impact strength of several polymers has been studied. Primary emphasis was placed on polycarbonate, but ABS, PVC, polysulfone, and polymethylmethacrylate were also studied. It was determined that residual stresses created by thermal quenching from above the glass transition temperature can have a great effect on impact strength for the polycarbonate, PVC, and polysulfone polymers studied. In fact, it is shown that the thickness transition observed in impact strength for polycarbonates is governed by the residual stresses and not by thickness. In polycarbonates, quenched sheets up to 3/8 in. in thickness have shown impact strengths of 18 ft‐lb/in. whereas sheets 1/8 in. in thickness can be embrittled by annealing, showing an impact strength of 2 ft‐lb/in. However, it has been shown that this embrittlement results from the absence of residual stress. Residual stresses having maximum values up to 3000 psi (in Compression) have been determined at the polycarbonate sheet surface using birefringence measurement techniques. The existence of these compressive stresses is postulated to restrict the extent of craze growth at the notch tip, and the impact specimen can yield rather than fail in a brittle manner if the stress state is sufficient.

Physical aging of linear and network epoxy resins

“…Should this be the case, the initial relaxation behavior might be expected to be different as implied from the work of Struik (18) and Broutman (19). Our results shown in Figure 6 suggest that within scatter of the relaxation data, no significant difference exists in either the initial stress relaxation rate or within the rate of change with sub-T annealing.…”

An overview of the nonequilibrium behavior of polymer glasses

“…Referring again to Figure 7, it is seen that the aging range of these polymers occurs between T and Tg That polymters only age in a cer~tain temperature range 9' has thus been determined experimentally and explained by the free volume theory [3J During the quenching process from the melt to the glassy state, as well as during the cure of thermosets, temperature gradients develop within the polymer materials due to their rather low thermal conductivity. 16 These temperature gradients, in turn, may result in the formation of stress gradients through the material [25,26,27]. While we will not discuss the consequences of thermal stress gradients and their time dependence on the behavior of bulk glasses, an awareness of this quenching or rapid cooling effect must be recognized.…”

Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: Influence of fibre loading, fibre length, fibre orientation and voids

“…When a critical specimen thickness is exceeded, the impact behaviour can change from ductile to brittle and the toughness is reduced signicantly [15,16] (IST, AG, Vilters, CH). A detailed description of this method can be found in [17].…”