Characterization of the Mechanical, Thermal, Rheological, Biodegradation and Morphological Properties of Sisal Fiber Reinforced Polylactic Acid Composites for Automotive Industry

In recent years, the use of polymeric composite materials has gained popularity in various applications due to their advantageous properties such as strength, stiffness, lightweight, and corrosion resistance. However, most products manufactured by companies utilize non-biodegradable fibers like glass and carbon, posing challenges for disposal after their end-use. This has led to increased interest in exploring natural, sustainable, biodegradable, and renewable resources. Natural reinforcing fibers such as sisal, hemp, jute, flax, and bamboo have been widely accepted in thermoplastic composites across different application areas. The objective of this study is to develop and characterize polylactic acid (PLA) bio-composites using sisal fiber (SF) as a reinforcement and incorporating a plasticizer. Before developing the bio-composites, the single fiber characterization was performed, comparing retted and unretted sisal fibers in terms of diameter, tensile strength, strain at failure, and elastic modulus. The results indicated that retting primarily influences the strain at failure of sisal fibers. This is expected since water acts as an active plasticizer and increase fiber strain. The influence of retting on fiber strain was statistically significant considering the standard deviation. The injection molding process was utilized to prepare the bio-composites, and the mechanical properties were analyzed. The results indicate that the incorporation of short sisal fiber into the PLA matrix lead to an improved tensile and flexural moduli. Whereas, the tensile strength of the composites reinforced with short sisal fibers was lower compared to the neat PLA material. In addition, flexural strength decreased at low fiber content, although it recovered when 10 wt% of SF was added. The impact strength increased with the addition of SF. The mechanical properties were adversely affected by the inclusion of ATBC plasticizer, except for the impact strength which improved. Furthermore, as the immersion time increased, the water absorption of the composite specimens also increased. This led to a decrease in the tensile and flexural strengths of the material, but an increase in its impact strength. Additionally, the bio-composite exhibited higher water absorption, lower strength, and lower modulus with high fiber content after water absorption. Morphological studies revealed relatively good compatibility between sisal fiber and PLA matrix composites, but the fiber-matrix interfacial strength deteriorated after water absorption. The study also investigated the influence of injection molding process parameters on the mechanical properties of the composites. The results revealed that mold cooling water temperature did not significantly affect the strength of the composite, while melt temperature had an impact. Numerical simulations further confirmed that variations in melt temperatures significantly influenced the strength of the molded composite parts. Thermal and rheological behaviors of the composites were also investigated. The results revealed that thermal conductivity increased with the incorporation of SFs and decreased with increasing temperatures. In addition, differential scanning calorimetry (DSC) analysis revealed that PLA crystallinity increased with SF and ATBC plasticizer. Specific heat capacity increased with ATBC v plasticizer but decreased with increasing fiber content. Furthermore, dynamic mechanical property test analysis (DMA) showed that storage moduli increased with higher SF content and abruptly dropped around the glass transition temperatures. Hence, the developed bio-composites are suitable for low-temperature applications. Rheological analysis indicated effective fiber-fiber and fiber-matrix interactions and a good level of fiber dispersion within the matrix, resulting in uniform shear viscosity for different capillary dimensions. The shear viscosity of the bio-composite material increased with fiber content but decreased with plasticizer content.