PROCESSING OF SHORT NATURAL FIBER REINFORCED POLYMER COMPOSITES

Abstract

The use of polymer matrix composites is increasing day by day in various engineering applications, such as, automotive, aerospace, construction etc. The industrial data indicate towards a huge market potential for composite materials in Indian industry. Synthetic fibers are dominating the polymer composite market as reinforcing agents, due to their better mechanical properties (tensile strength ≈ 2.4GPa to 4.4 GPa), low moisture absorption, good thermal resistance and chemical inactivity. But at the same time, these fibers have serious drawbacks, such as, high density, abrasive nature, cost as well as ecological concerns including CO2 emission, high energy consumption, non-renewability and non-recyclability. These concerns have forced the researchers to look for an alternate reinforcement solution, to develop sustainable composites, which is comparatively better than synthetic fibers. In order to achieve this objective, the incorporation of natural fibers into various polymer matrices has been explored. This has led to the development of composites, which were partially or fully ‘green’ in nature, based on the nature of matrix. The use of lignocellulosic (natural) fibers as reinforcement in synthetic or biopolymers has gained attention due to their specific advantages, such as, good strength, lightweight, renewability, competitive mechanical properties, corrosion resistance and low maintenance costs as compared to synthetic fibers. In spite of the several benefits offered, the practice and use of natural fibers is still limited to non-structural applications due to their thermal degradation behavior and high moisture absorption characteristics. The inherent characteristics of natural fibers affect the overall properties of composites, which necessitate their exhaustive characterization before the design stage. Further, the fiber weight fraction, selection of process and processing parameters also play a significant role in determining the overall properties of the developed composite. Subsequently, an approach of thermal post-processing of the composites can be employed to investigate its effect on mechanical behaviour of the developed composites. While developing the products, a manufacturer is highly concerned about the waste produced during processing, therefore, a recyclability analysis of the composites must be performed. The complete experimental investigation is subdivided in four broad areas focusing on characterization, process selection and optimization, thermal post-processing and recycling of the composites. The first phase of experimental investigation deals with the extraction and characterization of the lignocellulosic fibers in order to check their feasibility to be employed as reinforcement in viii polymer matrix composites. The feasibility has been investigated in terms of their thermal, chemical, mechanical and crystalline properties. The average tensile strength was observed as 280 MPa and 1.141 GPa for bagasse and munja fibers, respectively, which was attributed to the fiber structure as well as crystalline properties. Other characterization also revealed the compatibility of the selected fibers with PP and PE matrices. Pilot experimentation has been conducted to select a suitable range of process parameters for the selected processes. The second phase analyze the effect of four different processing routes, namely, Direct Injection Molding (DIM), Extrusion Injection Molding (EIM), Extrusion Compression Molding (ECM) and Loaf Compression Molding (LCM) for bagasse fiber reinforced polypropylene and polyethylene composites. EIM process has been observed as the best processing route for fabrication of short fiber reinforced polymer composites based on the mechanical properties. Subsequently, the process parameters of the injection molding process were optimized for better mechanical properties of the developed composites using Taguchi L-27 orthogonal array. The effect of each parameter on the mechanical properties of the developed composites has also been analyzed. The results substantiated that fiber weight fraction in the composite is the key parameter for deciding the mechanical properties of the developed composites, followed by injection temperature and back pressure. The third phase analyzed the effect of fiber weight fraction on the mechanical, thermal and crystalline properties of the composites developed by reinforcing bagasse and munja fibers in polypropylene and polyethylene matrix, individually. All the mechanical properties were found to be increasing, when the fibers were incorporated by 20% weight fraction in polyethylene matrix. The cooling rate of composites affects their crystalline and mechanical properties as slow cooling results in better crystallinity and mechanical properties of developed composites, but at the same time, it results in longer fabrication time as well as lower productivity. Therefore, thermal post-processing of the developed composites has been performed to investigate its effect on the mechanical behaviour of developed composites. The developed composites were exposed to an elevated temperature for different time durations (30 min to 6 hours) in a preheated hot air oven. It was observed that the thermal post processing exhibits an increase in tensile and flexural properties, yet the increase in PE based composites (≈25%) was observed higher as compared to ix PP based composites (≈17%). The increase was attributed to enhancement in interfacial adhesion, which was also confirmed with the morphological examination of fractured specimens. With the escalating demand and increased applicability of natural fiber reinforced composites, waste management is also a serious concern for the professionals working in the composite industry. Waste generation during the processing of composites as well as after the end of useful life needs to be taken care of. Hence, the fourth phase deals with the recycling behaviour of the bagasse and munja fibers reinforced polypropylene and polyethylene matrix composites. The recyclability of composites was mechanically simulated using multiple extrusion- injection cycle. The performance of the recycled composites has been assessed in terms of their mechanical properties, the aspect ratio of reinforcing fibers, their orientation, crystallinity as well as dynamic mechanical behaviour. The results revealed that the process waste of natural fiber reinforced composites can be recycled up to 6-7 time without adding any virgin material. The work being reported in the present experimental investigation will certainly help in enhancing the understanding as well as the use of natural fiber reinforced composites in various applications. Some of the application has been developed i.e. Chair parts, god sculptures, and it has been established that various other non-structural parts/ components can also be developed using the same material. Many new questions and potential research areas have been identified during the experimental investigation, which can be considered for future work in the domain. The identification of the novel research areas highlights the need for exhaustive experimental investigation in order to enhance the understanding and know-how about these materials of the future.