2. Hydrogen Peroxide Treatment
2.6. Processing Techniques
Fiber-reinforced plastic composite manufacturing is similar with thermoplastic processing. Most thermoplastic processing operations involve heating, forming into the desired shape, and then cooling. These include compression molding, extrusion, injection molding, rotational molding, etc. Selection of a suitable processing method for the generation of fibre reinforced polymer composites is an important task in order to enhance the mechanical, physical and thermal properties of the final product. Temperature, pressure and speed of processing are the major factors which determining properties of the composites. Since the degradation resistance of fiber is low, it has been limited for using it with the thermoplastic matrices with higher melting points.
2.6.1. Compression Moulding
Compression moulding is a frequently used method for the processing of polymer composites, also known as hot press method. Previous studies showed that compression moulding gives composite materials with superior mechanical properties when compared to other processing methods. Polymer fibre mixture is placed between preheated mould cavities and then the mold is
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closed, heat and pressure are applied to obtain a homogeneously shaped composite. Applied pressure and heat depend on the thermal and rheological properties of the polymer. A preheating time is needed to reduce holding time. Slow cooling or rapid cooling (quenching) can be applied at the end of holding time.The temperature and applied pressure of the mould cavities are fixed according to the types of matrix and filler materials used and thickness of composite samples[32].
2.6.2. Injection Moulding
Injection molding is commonly used industrial process for manufacturing of thermoplastics, in which high temperature and shear rate are associated with. In injection molding process, the compounded samples are preheated in cylindrical chamber to a temperature at which it can flow and then it is forced into a cold, closed mold cavity by means of quite high pressure, which is applied hydraulically through the ram or screw type plunger. The screw rotates to pick up the polymer and melt it, mix the melt and deliver it to the closed mold. The screw is then moved forward to force a fixed volume of the molten polymer into the closed mold. After melting, polymer is solidified in the cool mold; the screw rotates and moves backward to charge the polymer for the next cycle. Many studies have been conducted on the potential of using natural fibers as reinforcement for polymers to make a composite through injection molding. It is also used with other pressing techniques like extrusion. During the injection molding process for composites, a complex molten polymer flow field is generated and fibers are therefore oriented in the direction of shearing and stretching[33]. A good distribution of fibers is always achieved in this process. However the processing of natural fiber composites is limited via injection molding because of the low thermal stability of the fibers.
2.6.3. Extrusion
Extruder is a versatile machine, which forms thermoplastic items with a uniform cross-section such as pipe, hose, wire and cable. Melting, compression and metering sections are basic sections of an extrusion screw. In melting part, the solid pellets are conveyed from the hopper and converted into molten polymer. In compression section, the molten polymer is compacted and mixed with the additive (if required). The metering section is needed to produce the desired product cross-section. Twin-screw extruder and single screw extruder are basic types of extruders. Twin screw systems have been shown to give better dispersion of fibers and better
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mechanical performance than single screw extruders[34]. The processing temperature PE can be in the range of 190–230°C. The mixtures are fed into the hopper of the extruder, compounded, cooled and granulated. The compounded samples are prepared as test specimens by injection molding machine or hot press molding machine.
2.6.4. Rotational Moulding
Rotational molding or rotomolding is one of the most important polymer processing methods for producing stress free, hollow products. Rotational molding involves powder mixing, melting, sintering and melt solidification. In this process, polymer mix is filled up in a half of mold then closed and subjected to biaxial rotation in an oven at a temperature of 200–400°C. Once the polymer has melted, the mold is moved out of the oven with biaxial rotation. For cooling the mold, water or air fan can be used. Rotational molding has particular advantages in terms of relatively low levels of residual stresses and inexpensive molds. Polyethylene can be used in rotational molding because of its low melting point, low cost and good thermal stability.
Reinforcements can be incorporated into the rotationally molded components to increase their mechanical properties. Rotational molding is the less explored process for natural fiber thermoplastic composites. The low-shear characteristics of rotomolding limit the amount of fiber that can be added to produce good composite materials as most researchers used fiber contents much less than 30%[35] . Wang et al. prepared linear low-density polyethylene (LLDPE) with treated flax fibers by benzoylation, and their maximum fiber amount was 10%[36]. They found that tensile strength increased from 15.2 to 16.1 MPa, while impact strength also increased from 190 to 220 kJ/m2. Lopez-Banuelos et al. prepared LMDPE composites with 5, 10, and 15% of agave fibers[37]. The impact strength decreased with fiber content. Nevertheless, a maximum tensile modulus was achieved at 10% of fiber which was 70% higher than the neat matrix.
Raymond and Rodrigue produced LLDPE-wood composites by rotational molding with fiber contents up to 25%. They found a maximum tensile modulus is 17% higher than LLDPE at 20%
of wood. But the tensile strength decreased with fiber content from 16 MPa for neat LLDPE to 9.2 MPa at 25% of wood.
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Figure 2.11 (a) External and (b) internal surfaces of the rotomolded composite[38]
Bubbles are one of the most common defects in rotomolded parts[38]. The composites produced had a small amount of bubbles, suggesting that good sintering occurred under these processing conditions as well as an acceptably good dispersion of the fiber without a specific orientation (Figure 2.11). The external surface was smoother than the internal surface, where many fibers stuck out, especially when higher fiber content and larger fibers were used.
Table 2.2: Summary of physical modifications on natural fiber
Composite Modificati
mechanical property es than gamma treated composites.The UV treated composite showed an increase of 18% TS and 20% bending strength respectively.
SEM images of the fracture sides of the composites were supported that gamma treated jute fabrics/PP composites had poor fiber matrix adhesion than that of the UV treated jute fabrics/PP composites.