Increasing the Biodegradation Rate of Poly(Lactic Acid) in Composting Conditions

摘要

Poly(lactic acid) (PLA), a well-known compostable and bio-based aliphatic polyester, has found applications in the medical, textile, plasticulture, and packaging industries. PLA has been blended with several polymers and compounded with different micro and nanoparticles to fulfill desirable properties and to extend its range of applications. The growing interest in PLA-based materials and other biodegradable polymers has required the development of methodologies to evaluate their biodegradability and understand the different factors affecting their biodegradation mechanisms and rate. One of the current limitations of biodegradable polymers, like PLA, is that they do not biodegrade as fast as other organic wastes during composting, affecting their general acceptance in industrial composting facilities. In this work, the results of two different approaches to accelerate the biodegradation rate of PLA are presented: 1) the addition of layered silicate nanoparticles to the PLA matrix, and 2) the addition of selective PLA-degrading microbial strains to the media, i.e., bioaugmentation.;For structural changes, three different nanoclays were used as model systems due to their different surface characteristics but similar chemistry: organo-modified montmorillonite (OMMT), Halloysite nanotubes (HNT), and Laponite RTM RD (LRD). Additionally, the organo-modifier of OMMT (Cloisite RTM 30B), methyl, tallow, bis-2- hydroxyethyl, quaternary ammonium (QAC) was used to investigate its effects on the biodegradation of the polymer. PLA and PLA bio-nanocomposite films (BNCs) were produced and fully characterized. Films were tested for biodegradation in simulated composting conditions by analysis of evolved CO2 with an in-house built direct measurement respirometer. The molecular weight of the films was monitored during the biodegradation tests and correlated with the degradation kinetics. Additionally, a biofilm formation essay and scanning electron microscopy were used to evaluate microbial attachment on the surface of PLA and BNCs. The biodegradation test results showed a higher mineralization and microbial attachment of the films containing nanoclay in comparison to the pristine PLA. However, the effect of the nanoclays on the initial molecular weight and thickness played a crucial role in the evolution of CO2.;For bioaugmentation, microorganisms present in the compost and capable of degrading PLA were isolated through an enrichment technique with PLA as the sole carbon source at 58°C. The isolates were identified as Geobacillus using 16S rRNA gene sequencing and further used to study the effect of bioaugmentation on the biodegradation rate of PLA and BNCs in solid environments. The results showed that bioaugmentation with Geobacillus increased the evolution of CO2 and accelerated the biodegradation phase of PLA and BNCs when tested in compost and vermiculite inoculated with a compost-derived mixed culture.;This work provides the insights gained during the performance of different biodegradation tests and unique understanding about the biodegradation mechanism of PLA. Increasing the biodegradation rate of PLA-based materials will greatly benefit their general use and their acceptance in industrial composting facilities at their end of life.