Impact of hydrolysis pretreatment on the compostability of biodegradable poly(caprolactone) and poly(lactic acid) films†

Abstract

The biodegradation performance of non-pretreated and pretreated commercial polyesters was evaluated under simulated composting conditions to understand how abiotic pretreatment accelerates biotic degradation. Polylactic acid (PLA) and polycaprolactone (PCL) were subjected to hydrolysis pretreatment and assessed under simulated composting conditions for 120 days. In addition to tracking CO₂ evolution, polymer-intrinsic factors such as chain scission, measured by reductions in intrinsic viscosity molecular weight (M_η), and changes in crystallinity (X_c) were also evaluated for both non-pretreated and pretreated samples during the biodegradation process. Hydrolysis pretreatment resulted in a reduction of initial M_η and an increase of initial X_c for all polymer samples. The initial decrease in M_η was particularly marked for PLA, which showed about 30% decrease, while PCL exhibited a reduction of just around 7%. Regarding initial X_c, the most significant increase was also seen in PLA, which jumped from approximately 0% to c. 30%. Hydrolysis of semi-crystalline polymers primarily affects the amorphous region, where elevated temperatures allow water to break polymer chains easily. However, for PLA, the disruption of the crystalline structure leads to a less stable type of crystal, probably due to an increase in the rigid amorphous region that enhances the overall biodegradation process. The effect of pretreatment on the biotic phase showed minimal differences for PCL but a noticeable overall increase in biodegradation for the pretreated PLA.