This article addresses the effect of thermal aging on unreinforced and glassreinforced recycled polyamide 66. As an accelerated test, injection-molded test bars were aged at 110, 140, and 170ЊC for up to 4000 h in air to simulate service life. FTIR spectroscopy demonstrated that the oxidative degradation primarily occurred between the surface and a depth of 0.5 mm. Furthermore, the degradation in the surface region was more pronounced with recycled as well as unstabilized materials. Reprocessing resulted in a faster increase of carbonyl groups, a decrease in melting peak temperature, and elongation at break during subsequent aging. Because of process-induced fiber shortening, however, the elongation at break of recycled reinforced samples was always at least as high as that of virgin samples for up to 4000 h of aging at 140ЊC. The decrease in melting peak temperature as determined by differential scanning calorimetry (DSC) indicated that the surface or boundary regions of the crystallites in the material are affected by aging. The loss in elongation at break for the reinforced material was shown to correlate with the reduction in melting peak temperature of material taken from the surface region of aged samples. The contribution of the degraded surface region to the properties was studied by removal of surface layers prior to testing. The degradation in the surface region was the sole cause, even of glass fiber-reinforced polyamide, for the embrittlement of aged samples. Furthermore, aging-induced changes in tensile strength and modulus were independent of the removal of the surface region, indicating that these properties are controlled by changes occurring in the bulk of the material.

An experimental study of the mechanical performance of in‐plant recycled fiberglass reinforced polyamide 66 is reported. The fiber length distributions were used to investigate and to predict the influence of process induced fiber shortening on the short term performance of recycled samples compared to that of virgin samples. The results indicate that fiber shortening has a strong influence on strength. Applying a modified Kelly‐Tyson model to the fiber length distribution gave excellent agreement with measured strength. There was no need to vary interface or matrix properties in the theoretical analysis. The effect of reprocessing on these factors does not appear to influence strength within the bounds of the model. The decrease in strength during a continuous in‐plant recycling process is small at a 30 wt% regrind level. Indeed, below 50 wt% regrind, the strength remains within design limits. The impact strength of dry unnotched samples indicated that the resistance is related to the reciprocal fiber length.

A study of the mechanical properties in an accelerated service-related environment of recycled glass-fiber-reinforced polyamide 66 is reported. Material reinforced with 30 wt % of short fibers was reground and remolded up to seven times. Thermal aging in air at 140ЊC for up to 3000 h and coolant aging at 110ЊC for up to 1000 h showed no significant differences in behavior pattern. In addition to mechanical testing, the fiber length measured directly and the matrix stability measured by differential scanning calorimetry (DSC) were used to determine the influence of process-induced degradation on the durability of recycled samples compared with that of virgin samples. The results indicate that fiber length controls the initial properties. The differences in tensile strength and modulus between recycled and virgin samples were similar within the examined times of aging and could be explained by process-induced fiber shortening. The onset of embrittlement during both aging conditions is revealed first in a decrease in tensile elongation at break. Because of a lower degree of fiber reinforcement, the elongation at break of recycled samples was always as good as that of virgin reference samples. However, increasing the number of molding operations up to four to five times resulted in a faster deterioration rate in elongation at break of recycled samples. Further processing had less effect on the deterioration rate. The oxidative stability of the matrix as determined by DSC decreased as a result of repeated processing. The results suggest that matrix stability is related to changes occurring in elongation at break during accelerated aging of samples remolded up to about four times.

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The fiber length distribution was found to control the overall short term performance of reprocessed heat‐stabilized short fiberglass reinforced polyamide 66. Length changes, and matrix and interface degradation were studied. Fiber shortening dominates during compounding and during the first injection molding cycle. Further regrinding and remolding has a lesser effect. The short term mechanical strength decreased for reprocessed samples. Using a modified Kelly‐Tyson model, the lower tensile strength of reprocessed samples, compared with virgin samples, can be explained by fiber shortening. Reprocessing had a negligible effect on the strength for both the fiber matrix interface and the matrix of this system. Studies on unreinforced samples confirmed that thermal degradation of the matrix during reprocessing had a negligible effect on short term mechanical performance.