Bio-based and Biodegradable Plastics for Agricultural Applications
Creators: Rahman, Maliha
Publisher: name: University of Alberta Library
Year: 2024

<p>The intense use of petroleum-based plastics for agricultural purposes exacerbates the “plastic pollution” concern, as synthetic plastics can remain in the ground for many years. Thus, there is an increasing consumer-driven demand for plastics that can degrade into benign products at the end of their lifetime. <br>
This study aims to develop environmentally friendly biodegradable plastic films for the agricultural industry, particularly for use as bale wrap and mulch film. Polylactic acid (PLA) is a potential biopolymer with numerous attractive features; however, its inherent brittleness and photosensitivity restrict its applicability as agricultural film. To address these issues, acetyl tributyl citrate (ATBC), a bio-derived plasticizer, and kraft lignin powder, a bio-derived UV-absorbent material, were added to PLA in amounts ranging from 0 to 15 wt% for the former and 0 to 1 wt% for the latter. Biocomposite filaments were produced by melt compounding method using a twin-screw extruder. Extruded filaments were then hot pressed into dumbbell-shaped samples for tensile testing and thin films of different thicknesses for successive characterizations. <br>
The functional groups and intermolecular interactions among PLA and additives were identified by Fourier-transform infrared spectroscopy (FTIR). The mechanical properties of biocomposites were studied by tensile testing and thermal properties were analyzed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The addition of ATBC successfully plasticized PLA, resulting in a considerably higher elongation at break and lower thermal characteristic values. 12% ATBC was found to be the optimal concentration for PLA (4043D grade). 0.5% and 1% lignin had trivial effect on mechanical and thermal properties of biocomposites, but almost completely blocked the UV radiation as observed by UV-Vis spectroscopy, indicating its effectiveness as a UV absorber. <br>
Transmission electron microscopy (TEM) and optical microscopy were carried out to analyze the dispersion of lignin in biocomposite films, which revealed that lignin particles had a broad size distribution and formed agglomerates in the films. The effect of additives on PLA’s deformation behavior was investigated by scanning electron microscopy (SEM), which showed signs of substantial matrix deformation in samples containing 8-12% ATBC. However, lignin’s effect on PLA’s deformation process was not readily apparent. Water absorption test results confirmed the hydrophilic and hydrophobic characteristics of lignin and ATBC, respectively. Over a period of nine days, neat PLA films absorbed less water than PLA/lignin composites, while PLA/ATBC/lignin composites absorbed even less water than either PLA or PLA/lignin composites. Furthermore, oxygen barrier test result showed that the presence of lignin reduced oxygen permeability, whereas the presence of ATBC rendered the biocomposite films highly permeable to oxygen. <br>
To mimic the effect of agricultural films’ weathering in the field, formulated films underwent accelerated weathering. They were exposed to UV light over 300 hours while water was sprayed intermittently. After weathering, the change in molecular weight and mechanical properties of the composite films were studied. The molecular weight of the films decreased due to weathering. Consequently, the films’ strength and flexibility were reduced. The deterioration was more apparent in the films containing ATBC, which can be related to the higher accessibility of oxygen to degrade the polymer chains in these films. Despite its effectiveness as a UV-blocker, the presence of lignin showed minimal effect on weathering of the films. The hydrophilicity of lignin and the formation of agglomerates in the matrix may have reduced its effectiveness in providing weathering resistance.<br>
To investigate the degradability of the films by microbes at the end of service, enzymatic degradation tests were performed. Proteinase K from Tritirachium album was used in this study, and the degradation of the films in enzyme solution was tracked over ten days. The findings demonstrated that ATBC and lignin had opposite effects on PLA&#39;s enzymatic degradation. The decomposition of PLA by the enzyme was impeded by ATBC, while it was enhanced by lignin. A co-relationship between water absorption and enzymatic degradation rate was found. <br>
This study presents an in-depth understanding of the relationship between various physical properties of PLA and additives and the resulting behavior of the developed films. These findings suggest that further modifications of additives are needed to improve the compatibility between plastic and additives in order to achieve the required stability against weathering, while also ensuring high flexibility and biodegradability of films. </p>