Elaborating Mechanisms behind the Durability of Tough Polylactide Monofilaments under Elevated Temperature and Humidity Conditions

Abstract

Melt-spun poly­(l-lactic acid) (PLA) monofilaments with excellent toughness have been subjected to environmental weathering under varying temperatures in the range of 50–70 °C, that is, below and close to the glass transition temperature T g, and at 20–95% relative humidity in order to follow the evolution of their mechanical performance. Environmentally triggered structural changes and the hydrolytic degradation of the monofilaments have been evaluated by analysis of their thermal and mechanical properties as well as their long-term relaxation behavior using a self-developed model. Despite structural changes, the fibers demonstrate a long-term preservation of the toughness under environmental conditions of physical aging. The mechanisms behind the observed durability of the PLA material are attributed to the relaxation of the confined amorphous phase presumably as a result of the local chain scission. Conditions when hydrolytic degradation leads to mechanical failure correspond to an absolute humidity of 90 g/m3 independent of the temperature of the aging. Presented results reveal a persistence of the mechanical performance of PLA fibers upon aging under moderate conditions, thus offering possible design strategies toward tough and durable PLA materials for sustainable technologies.

Evaluating the Potential of Polylactide Nonwovens as Bio-Based Media for Air Filtration

“…Degradation of tough PLA monofilaments under elevated temperature and humidity conditions has been reported earlier [29]. In that study, a combination of physical and chemical methods, including DSC measurements, molar mass determination, tensile tests and relaxation measurements were employed to assess initial stages of mechanical failure of the fibers as a result of hydrolytic degradation (Figure 1).…”

“…We note that, while thermal properties and molecular weight distribution indicate measurable changes in the samples, which show advanced degradation behaviors, the relaxation measurement, evaluated via a lifetime prediction model, has been proven to be the most sensitive in revealing changes, which leads to a slight improvement of the mechanical performance. This knowledge allows one to use defined environmental conditions in order to self-reinforce the highly crystalline and oriented structure allotted by the fabrication [29].…”

“…As can be seen in first DSC heating run (Figure 3a), all samples are semi-crystalline with a missing cold crystallization peak. The degree of crystallinity, calculated as described in [29], using an ideal melting enthalpy of ∆H m 0 = 93.7 J/g, is~72% for pellets,~75% for nonwovens and slightly higher (~78%) for the monofilaments. The crystallinity has a high impact on the resulting mechanical properties of the fibers, i.e., on the elasticity and tensile strength.…”

“…Numerous studies have established that PLA has specific physical properties, such as relatively low glass transition and melting temperatures [15], brittleness [16], slow crystallization behavior [17][18][19] as well as high hydrolytic and thermal instability [20][21][22][23], which make the resulting mechanical performance of the generated fibers sensitive to downstream processing steps as well as to the environmental conditions [24][25][26][27][28][29].…”

“…Recent findings suggest that forcing the orientation of PLA nanocrystallites results in meso-morphic phases [37][38][39] that have enough long-term stability to enable the industrial use of PLA nonwovens in air filters at mild environmental conditions [29].…”