UV Light Degradation of Polylactic Acid Kickstarts Enzymatic Hydrolysis

Abstract

Polylactic acid (PLA) and bioplastics alike have a designed degradability to avoid the environmental buildup that petroplastics have created. Yet, this designed biotic-degradation has typically been characterized in ideal conditions. This study seeks to relate the abiotic to the biotic degradation of PLA to accurately represent the degradation pathways bioplastics will encounter, supposing their improper disposal in the environment. Enzymatic hydrolysis was used to study the biodegradation of PLA with varying stages of photoaging. Utilizing a fluorescent tag to follow enzyme hydrolysis, it was determined that increasing the amount of irradiation yielded greater amounts of total enzymatic hydrolysis by proteinase K after 8 h of enzyme incubation. While photoaging of the polymers causes minimal changes in chemistry and increasing amounts of crystallinity, the trends in biotic degradation appear to primarily be driven by photoinduced reduction in molecular weight. The relationship between photoaging and enzyme hydrolysis appears to be independent of enzyme type, though commercial product degradation may be impacted by the presence of additives. Overall, this work reveals the importance of characterizing biodegradation with relevant samples that ultimately can inform optimization of production and disposal.

Figure 1

Percentage of PLLA polyester bonds broken over time of pristine (purple), 1 (blue), 2 (green), 3 (gold), and 4 h (red) irradiated PLLA. Markers represent the average of triplicate values with error bars as the standard deviation.

Figure 2

(A) Carbonyl and hydroxyl indices of the irradiated PLLA samples. (B) Percent bulk crystallinity (%) of irradiated PLLA samples. (C) Peak temperature, °C of irradiated PLLA samples from DSC thermograms. Markers represent the average of triplicate values with error bars as the standard deviation.

Figure 3

SEM imaging of pristine and aged (8 h UV) PLLA. These are representative images of n = 1 sample per condition with >20 images taken per sample.

Figure 4

Total percentage of PLLA polyester bonds broken after 8 h of proteinase K exposure in 2, 50, 90, and 300 kDa PLLA molecular weight standards. Values represent the average of 3 replicate samples and error bars as standard deviations.

Figure 5

Percentage of PLLA polyester bonds broken over time of pristine (purple), 1 (blue), 2 (green), 3 (gold), and 4 h (red) irradiated PLLA utilizing lipase. Markers represent the average of triplicate values with error bars as the standard deviation.

Figure 6

(A) Percentage of PLLA polyester bonds broken over time of pristine PLLA doped with 3% w/w bis (2,2,6,6-tetramethyl-4-piperidyl) (UV Stabilizer), 4% w/w titanium dioxide (TiO₂), and 11% w/w Talc compared to a neat (additive free) PLLA. (B) Percentage of PLLA polyester bonds broken over time of pristine PLLA doped with 2.75, 5.5, and 11% w/w Talc compared to a neat (additive free) PLLA. Markers represent the average of triplicate values with error bars as the standard deviation.