On the Rheological Memory and Cumulative Damage of Thermoplastic Starch Biodegradable Films Reinforced with Nanoclay

Although the strain hardening phenomenon has been studied in different types of materials, there are only a few such reports regarding flexible food packaging. To address this issue, nanoclay-reinforced and control starch-based films were subjected to sequential and weekly tension and the rheological index, defined as the ratio of the tensile strength observed under weekly to that under consecutive elongation, was measured. The results showed that the values of the rheological index were >1, implying a strain hardening effect that was more notable when nanoclay was added and when the stress duration was increased. Additionally, a cumulative damage test was conducted, involving the gradual increase of two factors in each step: the percentage of the elongation level and the duration of each step. The data were fitted to a linear model, describing the correlation between the ln failure time (μ) and the tensile strength (X), μ = 6.021 − 0.478 X. This model enabled the prediction of the failure probability and the hazard rate of the films that were studied. In addition, from the survival of the units in the initial steps of the cumulative damage experiment, it can be hypothesized that the elongation of the units under low stress levels, for prolonged periods of time, exhibits rheological memory properties, which leads to an increase in their mechanical strength.