Comparison Study of the Bio-degradation Property of Polylactic Acid (PLA) Green Composites Reinforced by Kenaf Fibers

Degradation of polylactic acid (PLA) was studied to investigate the best method of degradation to assist in reducing environmental pollution. Two condition of testing were conducted, which are natural weathering and landfill burial test. The rate of degradation was determined by weight loss, which was calculated once a month for six months. Natural weathering was achieved by exposing the samples to the natural environment, and this method was adopted according to ASTM D1435. Landfill burial testing was conducted by adopting ASTM G160-12 with a few modifications. The result shows that degradation was faster in a landfill burial condition. This was determined by the calculated weight loss, which was 2–4% of the total weight loss of PLA and its composites after six months’ exposure to a natural weathering environment. In contrast, the landfill burial condition showed 4–17% weight loss after six months. The addition of natural fiber, whether bast or core fiber, assisted in the degradation of the composites. As well, the addition of natural fiber led to a 0.3–1.3% higher weight loss among the composites compared to neat PLA following natural weathering, while an 11–13% weight loss was recorded for composites exposed to the landfill burial condition. Natural weathering and landfill burial testing provide a time frame for the degradation of composite products. This is a good information for commercial composting facilities, providing data on the time frame required for material biodegradation.

Biodegradation; Kenaf fiber; Landfill burial; Natural weathering; Polylactic acid

In recent decades, the effects of globalization have created an extremely competitive atmosphere in all aspects of society. However, this flourishing competition must consider the harmony and balance between human needs and the environment to create a sustainable future (Suwartha et al., 2017). Researchers have played a role in identifying potential biomaterials to replace fossil fuel-based products. Polylactic acid (PLA) is a degradable polyester that has been widely used in various applications. PLA is completely mineralized to CO₂, water, and a small amount of biomass after four to six weeks’ exposure to a composting condition with a temperature of approximately 60°C (Lunt, 1998; Drumright et al., 2000; Farrel et al., 2001; Itavaara et al., 2002). The degradation of PLA was studied to investigate the best method of degradation to assist in reducing environmental pollution. Although it is degradable, the process of PLA degradation could take longer than expected. This is due to the thousands of polymer chains that must be broken before continuing the process. Zhang et al. (2008), Li and McCarthy (1999), Tokiva and Jarerat (2004), and Auras et al. (2004) studied and proved that PLA degradation was completed in aerobic (composting) or anaerobic (biomethanation) environments over a period of six months to five years.

PLA degradation was affected by various factors, including moisture, heat, lights, microorganisms, etc. Otherwise, the sizes of the samples were also an important factor, where a smaller size experienced easier degradation than larger samples due to a higher surface area exposure to microorganism attacks (Kunioka et al., 2006; Kale et al., 2007). In addition, types of filler also reflect the degradation rate, where natural fiber reinforcement and nanoclay could increase and fasten the rate of composite degradability due to the hydrophilic nature of the material.

Various research studies have been conducted to investigate the degradability of PLA composites. Rudnik and Briassoulis (2011a) studied the degradability of PLA film at various thicknesses, as determined through real composting in a Mediterranean condition and laboratory simulation setup. They found that both conditions assisted in the degradation of PLA film; however, the laboratory simulation setup has a higher rate of degradability due to the control temperature and pH, which are favorable for microorganism reproduction. Cadar et al. (2012) studied the degradation of commercially available PLA, and they synthesized PLA under a controlled composting condition. They found that PLA degradation was dependent on the lactic acid content. Larger amounts of lactic acid content speed up the degradation rate of the composites. Sikorska et al. (2012) studied the effect of recycled PLA on degradation properties. PLA degradation was achieved in a composting pile and in water at 70°C. They reported that the multi-processing of PLA did not affect the rate of degradation. The results obtained indicate that hydrolytic degradation occurs preferentially via random ester bond scission. Degradation within a composting pile was completed after 42 days of incubation, while degradation in water decreased continuously starting from the beginning of the process.

In this paper, the degradation of neat PLA and its composites was studied using natural weathering and the landfill burial method. Although PLA is one of the most degradable polymers, its degradation behavior could vary when exposed to different conditions. This study was conducted for six months, where the earliest testing period was the rainy season and the dry season occurred toward the end of this study. The weight loss and chemical changes of the composites were determined in terms of degradation properties. Morphological studies were conducted to observe the effect of weathering and landfill burial on the samples.

PLA-kenaf green composite degradation was determined using two different tests, which are natural weathering and the landfill burial test. Both conditions show degradation of the samples occurred, but PLA-kenaf green composite degradation was faster in a landfill burial condition. This was determined by the calculated weight loss, which is approximately 2–4% of the total weight loss of PLA and its composites after six months’ exposure to a natural weathering environment. In contrast, the landfill burial condition shows 4–17% weight loss after six months buried in a landfill. The addition of natural fiber, whether bast or core fiber, assisted in the degradation of the composites. As well, the addition of natural fiber led to a 0.3–1.3% higher weight loss among composites, compared to neat PLA in a natural weathering condition. Meanwhile, an 11–13% higher weight loss was recorded for composites exposed to the landfill burial condition than for neat PLA. The addition of natural fibers acts to attract microorganisms and increase water absorption, which assist in the hydrolysis of polymer and enhance the degradation rate of the composites.

Thank you to Universiti Teknologi MARA, Shah Alam, UPM INTROP, and Quasi UKM for their assistance in completing this project.

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