(2gm) Aquatic Biodegradation of Fibers/Bio-Based Polymers

As of 2019, about 20 % of plastic wastes had been mismanaged, and a significant amount of the plastic wastes had leaked into aquatic environments, which are the main driver of plastics discharged into aquatic environments. Natural fibers and bio-based polymers have been considered possible candidates for replacing the non-biodegradable synthetic polymers, and their biodegradation has been extensively studied. However, the biodegradation research has been mainly focused on soil biodegradation and composting.

My Ph.D. research has focused on the aquatic biodegradability of renewable materials, including natural-based fibers or bio-based polymers, with an interest in sustainable solutions for plastic issues. During my Ph.D. study with Dr. Richard Venditti and Dr. Joel Pawlak, we investigated several natural and bio-based polymers, including wood-based fibers, polyester-typed, and polysaccharide-typed biopolymers, and polymer blending, regarding the key factors affecting biodegradation in aquatic conditions.

Test methods:

Our project investigated aquatic biodegradation by tracking the oxygen consumption by microorganisms’ biodegradation activities, following an ISO method (ISO 14851). Activated sludge from the wastewater treatment plant was used as inoculum and added to the test flask with test materials and nutrients. The % biodegradation extent was calculated based on the theoretical oxygen demand of each test material. The biodegradation results were fitted to the Gompertz model to investigate the biodegradation kinetics of each material.

Key results:

Several factors essential to understanding the biodegradation of the polymers were suggested based on the study. Critical factors affecting the biodegradation were different depending on the type of polymers.

1. Wood fibers:

Wood-based fibers were mainly composed of cellulose, hemicellulose, and lignin, and their components tightly coalesced in the fiber structure. The biodegradability of the components was as follows: hemicellulose > cellulose >> lignin. The hemicellulose and cellulose showed over 75% biodegradation after 27 days, but lignin was essentially not biodegradable in the present aquatic condition. Therefore, the biodegradation of wood-based fibers was closely related to the chemical composition. Lignin content was negatively correlated with the final biodegradation extent after 27 days of biodegradation. In addition, the impurities from the recycling process also had an effect on the biodegradation of wood fibers.

2. Bio-based polymers:

Twelve biopolymers, including bio-based polymers and purported biodegradable polymers, have been investigated. Six kinds of polysaccharides were tested. Except for cellulose acetates, they are readily biodegradable, showing over 80% biodegradation due to the abundant microorganism population that can degrade the polysaccharide materials. Cellulose acetates were not biodegradable in the present conditions, possibly due to the high degree of substitution. The biodegradation of polyesters was related to the crystallinity and the hydrophilicity of the polymers. The crystallinity was negatively correlated to the ultimate biodegradation extent and the initial biodegradation rate. The hydrophilicity of the polyesters seemed to affect the beginning of biodegradation process, decreasing the lag phase of biodegradation.

3. Polymer blends:

Further investigation on biopolymers was done with polymer blending to investigate the effect of polymer blending with non-biodegradable polymers and miscibility on the biodegradation of biodegradable polymers. Two types of polymer blends, such as polypropylene (PP)/ polyhydroxy butyrate (PHB) and polylactic acid (PLA)/PHB blends, were spun as fibers, and their biodegradation was investigated. The biodegradation of PHB was affected by the blending with PP and PLA. However, the effect of non-biodegradable polymers on the PHB biodegradation differed depending on the miscibility.

In PP/PHB blends, since PP can cover the surface of the PHB surface, the biodegradation of PHB was hindered significantly when the PP concentration was over 50%. However, when the PP concentration was less than 50%, the hindrance of PP to the biodegradation of PHB was insignificant, and the biodegradation of PHB was about the same as the pure PHB condition showing about 60% of final biodegradation. After the biodegradation, the fiber residuals were collected and analyzed with a scanning electron microscope. The residuals images showed the nano- and micro-sized fibrils after the biodegradation of PP/PHB blended fibers, confirming that PP and PHB were phase-separated, and PP parts can remain as residuals after the PHB biodegradation. Furthermore, a compatibilizer, Maleic anhydride (MA)-grafted PP (MAPP) was added to the PP/PHB blends to investigate the effect of the improved miscibility on the biodegradation of PHB in the PP/PHB blends. PP and PHB were still phase-separated but less distinctly when MAPP was added. The biodegradation of PHB was significantly hindered by the introduction of MAPP, suggesting that the improved miscibility may not have a positive effect on the biodegradation of biodegradable polymers, even though it may improve some performance of polymer blends, such as mechanical properties.

In PLA/PHB blends, the blending with PLA inhibited the biodegradation of PHB more significantly compared to the PP with PHB. Even in high PHB ratio (75%) conditions, only 18% of PHB biodegraded. The control non-blending condition of PLA with PHB and PLA with microcrystalline cellulose (MCC) showed that the presence of PLA also significantly inhibited the biodegradation of PHB and MCC. This result suggests that the blending with PLA may inhibit the biodegradation of biodegradable polymers by blocking the surface of the biodegradable polymers and inhibiting the biodegradation activities of microorganisms simultaneously. Similar to PP/PHB blends, when MA was used as a compatibilizer, the biodegradation of PLA/PHB blends was inhibited significantly with the increased miscibility.

Overall, it has been shown that various factors, including chemistry, crystallinity, hydrophobicity, and miscibility, can affect the biodegradation of polymeric materials. These results can be basic information for further research on more sustainable and environmentally materials.

Based on my Ph.D. study, my interest in renewable and sustainable materials has been growing. Also, it was shown that depending on the process or treatment of the polymeric materials, biodegradability can be changed. Even small amounts of additives can change the biodegradability of the base materials significantly. This result inspired me to study more about the so-called environmentally friendly treatment and its effects on the sustainability of subject materials. Even though a single material is biodegradable, the mixture of materials or composite materials can show the difference in the biodegradation or effect on the environment. For the practical avenue for the environmental issues, it is necessary to systematically investigate the effects of the treatments or blending on the sustainability of subject materials, which can be suggested for the subsequent studies.