PROCESSING TECHNIQUES AND DEGRADATION BEHAVIOR OF SUSTAINABLE POLYMERIC COMPOSITES

Abstract

The use of composite materials has multiplied manifold owing to their superior combination of properties, usually difficult to achieve using traditional/conventional engineering materials. However, the non-biodegradability, non-recyclability, and nonrenewability of most of the currently used conventional composite materials is a major limitation and is motivating the researchers and scientists to explore the feasibility of conceptualizing, designing and developing composite materials based on sustainable resources. The research community has identified the natural fibers and biopolymers as an alternative to overcome the challenges associated with biodegradability, recyclability, and sustainability. The commercial viability of the biocomposites depends on the selection of manufacturing method ensuring less processing time, simplicity in operation with exceptional dimensional properties and repeatability. The literature reveals that one of the most commonly used processing technique for the fabrication of short fiber/thermoplastic based composites is injection molding. It is a quick process and offers flexibility in product design variation. However, the lack of information regarding the processing of biocomposites, the poor interfacial bonding between the natural fibers and polymeric matrices, the tendency of natural fibers to absorb moisture, and the degradation characteristics of the composites during the service period are limiting the application spectrum and commercial viability of these composites. Hence, the present experimental study focusses on the development of partially and fully biodegradable composites based on polypropylene and poly-lactic acid incorporating natural fibers (banana and pineapple fibers). In order to improve the interfacial interaction between the fibers and polymeric matrices, an eco-friendly route of chemical treatment has also been investigated. In order to investigate the recycling behavior, the recyclability assessment of the developed composites has been conducted. To investigate, in-service performance and the degradation behavior at the end of service life, the environmental aging and degradation study under different environmental conditions have also been explored. The present research endeavor is divided into four broad areas focusing on the development of partially and fully biodegradable polymer composites, recyclability assessment, environmental aging, and the degradation behavior of the polymeric composites. In the first study, polymer composites based on two types of natural fibers (banana and pineapple fibers) and two types of matrices (PLA and PP) were fabricated using three viii different processing techniques, namely direct injection molding (DIM), extrusion injection molding (EIM) and extrusion compression molding (ECM). The thermal and mechanical characterization, as well as dynamic mechanical analysis, have been performed to understand and compare the performance of the developed composites. Moreover, the behavior of the composites has been analyzed in context of extracted fiber morphology as well as the distribution and orientation of fibers within the developed composites. The study also focuses on the feasibility of using sodium bicarbonate and borax for the treatment of banana and pineapple fibers, whereas the potassium permanganate treatment was conducted for the reference. The effect of chemical treatment on the behavior of the fibers as well as the composites based on treated fibers has also been investigated. The results reveal that as compared to DIM and ECM, the crystallinity, as well as the tensile and flexural properties of composites fabricated by EIM, were found to be superior. The storage and loss modulus of composites fabricated by EIM was also found to be maximum. The second study explores the recyclability assessment of banana and pineapple fiber reinforced PP and PLA based composites. The performance of the recycled composites has been investigated in terms of fiber aspect ratio, mechanical, thermal, and dynamic mechanical behavior as well as the crystallinity. The investigation revealed that even after five recycling stages, the strength (tensile) of the recycled composite is still higher than the strength of the neat polymer which indicates the excellent recycling characteristic of the developed composites. In the third study, the environmental aging behavior of the developed composites has been investigated under exposure to various environments (tap water, 5% NaOH solution, 5% H2SO4 solution, 5% NaCl solution, and outdoor weather). The composite specimens were exposed to these environments for six months. The effect of environmental aging on the behavior of the developed composites has been assessed in terms of variation in weight, mechanical properties and crystallinity as well as the change in morphology. The study reveals that degradation of the PLA based composites was maximum in basic solution (BS). The exposed specimens dissolve completely after six months in the basic solution (i.e. 5% NaOH solution) due to hydrolysis of the PLA in alkaline medium. Other than BS, the percentage reduction in mechanical properties (TS, FS and IS) was found to be maximum in acidic solution (AS), followed by outdoor weather (OW), water (W) and salt solution (SS), respectively. ix The fourth study focusses on the degradation behavior of the neat polymers and developed composites when exposed to different mediums (cow manure, organic compost, and farmland soil) for six months. The degradation behavior of the composites has been investigated in terms of variation in weight, mechanical properties, crystallinity as well as the change in morphology. Investigation reveals that as compared to organic compost (OC), the reduction in mechanical properties of PLA based composites was higher in cow manure (CM) and farmland soil (SB). In the case of PP based composite specimens, the maximum reduction in mechanical properties was observed in farmland soil, followed by cow manure and organic compost, respectively. However, the maximum reduction in TS and FS was found to be less than 4% and 10%, respectively. The current experimental research endeavor gives an insight into the development, recycling, and degradation behavior of the partially and fully biodegradable composites for the industrial and research fraternity working in the broad area of sustainable biocomposites.