INDERDEEP SINGH and SAURABH CHIATANYA

ABSTRACT

With increasing awareness about environmental concerns, Natural fiber composites (NFCs) have emerged as a potential replacement to traditional polymer composites, which are derived from nonrenewable resources and are nonbiodegradable. NFCs can be classified into green and partially green composites based on the polymer matrix used. NFCs have several advantages over traditional polymer composites leading to worldwide increase in their demand. In order to meet this demand, prima­ry processing techniques should be developed exclusively for fabrication of NFCs. Injection molding process is the most extensively used process in the industry for the production of polymer composites. In this chapter, injection molding process parameters and the various issues and challenges during fabrication of NFCs using injection-molding process has been discussed.

8.1 INTRODUCTION

The continuous demand of materials having high strength and stiffness, light weight and low cost has led to the development of fiber reinforced composites to replace metals in structural applications. The polymer matrix composites (PMCs) based on petroleum derived polymers and synthetic fibers have been developed and used as structural material for engineering applications for decades, owing to their high strength and stiffness and light weight.’PMCs have been used for numerous appli­cations ranging from aerospace, automobiles, electrical, sports goods, construction and household items. Due to superior mechanical properties and wide applications, the use of PMCs has increased in the last few decades. Traditionally used PMCs consists of synthetic fibers like glass, aramid and carbon fibers, resulting in com­posites that can be easily used for structural applications. These PMCs are made up of nonrenewable and nonbiodegradable materials and the resulting composites are also nonrecyclable. These drawbacks have led to the disturbance in the ecological balance. Limited petroleum resources, rapidly depleting land fill space and strict en­

vironmental rules and regulations have forced the researchers and plastic industries worldwide to look for alternate matrix and reinforcement materials to overcome the drawbacks as well as to meet the performance of traditional PMC.

The possibility of reinforcing natural fibers into the polymer matrix has been explored by many researchers and industries worldwide. Natural fiber composites (NFCs) have been seen as a potential replacement to the traditional PMCs in many structural and nonstructural applications. Natural fibers are the largest and fastest growing renewable resource of fibers available that can serve as a potential replace­ment to synthetic fibers. NFCs consisting of natural fibers reinforced polymer ma­trix are being developed and studied as a potential material for the replacement of traditional PMCs. Natural fibers have distinct advantages over traditionally used glass fibers as natural fibers are renewable, biodegradable and have a low density (1.2-1.6 g/cm3) as compared to glass fibers (2.5 g/cm3) which result in the fabrica­tion of lighter composites. Natural fiber composites can also be recycled and cause no abrasion to tools used for their processing. Natural fibers can be broadly classi­fied into three types as plant fibers, animal fibers and mineral fibers (see Fig. 8.1).

The incorporation of natural fibers in the petroleum derived polymer matrix re­sults in PMCs called Partially Green Composites (PGCs). PGCs have been studied by many researchers and it has been found that they have a distinct advantage of be­ing recyclable and have comparable properties to traditional PMCs. However, PGCs still remain nonbiodegradable. To overcome the drawbacks of traditional PMCs a material called biocomposite was innovatively developed by reinforcing natural fi­bers into a biopolymer matrix (fully biodegradable polymers) by the DLR Institute of Structural Mechanics, in 1989.2 Biocomposites are still being developed and have attracted the attention of researchers worldwide in the last decade. Biocomposites consists of a renewable and biodegradable matrix (cellulose, starch, lactic acid, etc. derived) like polylactic acid (PLA), poly hydroxyl alkanoates (PHA), polyhydroxy butyrate covalerate (PHBV), etc. and natural fibers (plant, animal and mineral based) like sisal, hemp, flax, etc. The classification of polymers is shown in Fig. 8.2.

Biocomposites derived from renewable natural resources are termed as Green Composites. The cost and availability of biopolymers is restricting their wide spread use. The incorporation of natural fibers into the biopolymer not only reduces its cost (cheaper comparable to glass fibers) and weight but at the same time provides more strength and stiffness compared to base bio polymer.

Due to the increasing awareness among society about environmental concerns, the demand for NFCs has drastically increased during the last decade. According to a market forecasting report by BCC research (leading market forecasting agency), the global market for applications of wood-plastic composites, cellulosic plastics, plastic lumber and NFCs during the 5 year period (2011-2016) is estimated to grow at a compound annual growth rate (CAGR) of 13.8%. The global market for build­ing products and automotive application is expected to experience a growth at a CAGR of 12.4% and 17.1%, respectively.3Also the global use of bioplastics is ex­pected to increase up to 3.7 million metric tons by 2016, at a CAGR of 34.3%.4 With the increase in demand, the need to identify and develop primary processing tech­niques especially for the fabrication of NFCs arises. As other composites, NFCs can also be tailor made according to the specific properties required for specific applica­tions by careful selection of the polymer matrix, natural fiber and a suitable manu­facturing process. NFCs can be manufactured in a similar way as traditional PMCs by compression molding, resin transfer molding, hot pressing, direct extrusion and injection molding. The NFCs are being manufactured using the processes designed for the manufacturing of traditional PMCs for a controllable output. However, the properties of NFCs are highly variable compared to synthetic fibers. The properties of the natural fibers vary in terms of mechanical, thermal and structural properties. Also, there are several problems in fabrication of NFCs such as distribution of fibers in the matrix, fiber attrition during mechanical mixing, interfacial bonding between hydrophobic matrix and hydrophilic fibers and thermal degradation of the fibers during processing. Hence, there is a need for identification and development of pro­cessing technologies for NFCs.

Although compression molded parts exhibit better mechanical properties than other processes, but the compression molding process can only be used for small to medium parts with simple geometries. Short fiber reinforced polymers are used ex­tensively as structural material as they provide superior mechanical properties and can be easily processed by the rapid, low-cost injection molding process.5Injection molding process is used for the fabrication of small to medium parts with complex geometries. The parts which require precision, dimensional accuracy and excellent surface finish can be easily processed by injection molding. Also the polymers and the fibers are exposed to higher temperatures for a very short period preventing their degradation due to exposure to higher temperature for a long time. Apart from these advantages, there are several issues and challenges related to the fabrication of NFCs by injection molding process. This chapter focuses on the identification and selection of the processing parameters to successfully overcome the issues and chal­lenges faced by the industry in fabrication of NFC by injection molding.