01-07-2013, 04:24 PM
STUDY ON MECHANICAL BEHAVIOUR OF WOOD DUST FILLED POLYMER COMPOSITES
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ABSTRACT
The use of wood dust filled polymer composites has been considerably studied both from a scientific and a commercial point of view over the last decades, as these materials are particularly attractive for their reduced environmental impact and the globally pleasant aesthetical properties. Wood dusts are attractive fillers for thermoplastic polymers, mainly because of their low cost, low density and high-specific properties. They are biodegradable and non-abrasive during processing etc. Although there are several reports in the literature which discuss the mechanical behavior of wood/polymer composites, however, very limited work has been done on effect of wood dust types on mechanical behaviour polymer composites. Against this background, the present research work has been undertaken, with an objective to explore the potential of wood dust types as a reinforcing material in polymer composites and to investigate its effect on the mechanical behaviour of the resulting composites. The present work thus aims to develop this new class of natural fibre based polymer composites with different wood types and to analyse their mechanical behaviour by experimentation. Finally the morphology of fractured surfaces is examined by using scanning electron microscopy (SEM).
Introduction to composite materials
In recent years, the interest in composite materials is increasing due to its advantages as compared to monolithic metal alloys. Composites materials can be defined as engineered materials which exist as a combination of two or more materials that result in better properties than when the individual components are used alone. Composites consist of a discontinuous phase known as reinforcement and a continuous phase known as matrix. In practice, most composites consist of a bulk material (the „matrix‟), and a reinforcement of some kind, added primarily to increase the strength and stiffness of the matrix.
Fibrous composites:
Fibers, because of their small cross- sectional dimensions, are not directly usable in engineering applications. They are, therefore, embedded in matrix materials to form fibrous composites. The matrix serves to bind the fibers together, transfer loads to the fibers, and protect them against environmental attack and damage due to handling. In discontinuous fibre reinforced composites, the load transfer function of the matrix is more critical than in continuous fibre composites. An example of particle reinforced composites is an automobile tyre, which has carbon black particles in a matrix of poly-isobutylene elastomeric polymer
Natural Fiber Reinforced Polymer Composites
Over the past two decades, natural plant fibers have been receiving considerable attention as the substitute for synthetic fiber reinforcement such as glass in plastics [1,2]. The advantages of plant fibers are low cost, low density,
acceptable specific strength, good thermal insulation properties, reduced tool wear, reduced dermal and respiratory irritation, renewable resource and recycling possible without affecting the environmental damage, and together with biodegradable ability [3–7]. In the literature, many works devoted to the properties of natural fibres from micro to nano scales are available. In these, the effects of reinforcement of matrix (thermoplastic starch) by using cellulose whiskers, commercial regenerated cellulose fibres are also proposed.
The past decade has seen fast and steady growth of wood plastics industry. Among many reasons for the commercial success, the low cost and reinforcing capacity of the wood fillers provide new opportunities to manufacture composite materials. Although the use of wood-based fillers is not as popular as the use of mineral or inorganic fillers, wood-derived fillers have several advantages over traditional fillers and reinforcing materials: low density, flexibility, during the processing with no harm to the equipment, acceptable specific strength properties and low cost per volume basis. The main application areas of wood flour filled composites are the automotive and building industries in which they are used in structural applications as fencing, decking, outdoor furniture, window parts, roofline products, door panels, etc. [8,9]. There are environmental and economical reasons for replacing part of the plastics with wood but the wood could also work as reinforcement of the plastics. The elastic modulus of wood fibres is approximately 40 times higher than that of polyethylene and the strength about 20 times higher [10].
Mechanical Testing
After fabrication the test specimens were subjected to various mechanical tests as per ASTM standards. The tensile test and three-point flexural tests of composites were carried out using Instron 1195. The tensile test is generally performed on flat specimens. A uniaxial load is applied through both the ends. The ASTM standard test method for tensile properties of fiber resin composites has the designation D 3039-76. Micro-hardness measurement is done using a Vicker‟s micro-hardness tester. A diamond indenter, in the form of a right pyramid with a square base and an angle 1360 between opposite faces, is forced into the material under a load F. The two diagonals X and Y of the indentation left on the surface of the material after removal of the load are measured and their arithmetic mean L is calculated. In the present study, the load considered F = 1Kgf. Low velocity instrumented impact tests are carried out on composite specimens. The tests are done as per ASTM D 256 using an impact tester. The charpy impact testing machine has been used for measuring impact strength. Figure 3. 4. (a-c) shows the tested specimens for flexural, tensile and hardness test respectively. Figure 3.5a and b. show the experimental set up and loading arrangement for the specimens for tensile and three point bend tests respectively.
Scanning electron microscopy (SEM)
The scanning electron microscope (SEM) JEOL JSM-6480LV (Figure 3. 6) was
used to identify the tensile fracture morphology of the composite samples. The
surfaces of the composite specimens are examined directly by scanning electron
microscope JEOL JSM-6480LV. The samples are washed, cleaned thoroughly, air-dried and are coated with 100 Å thick platinum in JEOL sputter ion coater and observed SEM at 20 kV. Similarly the composite samples are mounted on stubs with silver paste. To enhance the conductivity of the samples, a thin film of platinum is vacuum-evaporated onto them before the photomicrographs are taken.
Effect of wood types on Micro-hardness
The measured hardness values of all the three composites are presented in Figure 4.1. It can be seen that the hardness value of teak wood dust filled epoxy composites is more as compared to rubber wood and sal wood dust filled epoxy composites. Among three types of wood dust filler, rubber wood dust filled epoxy composites showing less hardness value.
Effect of wood types on Tensile Properties
The test results for tensile strengths and moduli are shown in Figures 4.2 and 4.3, respectively. It can be seen that the tensile strength of teak wood dust filled epoxy composites is more as compared to rubber wood and sal wood dust filled epoxy composites. This may be due to the good compatibility of teak wood dust and epoxy resin. Among three types of wood dust filler, rubber wood dust filled epoxy composites showing less tensile strength value. From Figure 4.3 it is clear that the similar trend is observed for tensile modulus of different wood types as observed for tensile strength.
Effect of wood types on Flexural Strength
Figure 4.4 shows the comparison of flexural strengths of the composites obtained experimentally from the bend tests. It is interesting to note that teak wood dust filled epoxy composite much more superior as compared to other two types of wood dust filled composites. This may be again due to the good dispersion of teak wood dust filler in epoxy resin. However rubber wood dust filled epoxy composite is showing less flexural strength.
Effect of wood types on Impact Strength
Effect of wood types on impact energy values of different composites is shown in Figure. High strain rates or impact loads may be expected in many engineering applications of composite materials. The suitability of a composite for such applications should therefore be determined not only by usual design parameters, but by its impact or energy absorbing properties. From the figure it is observed that resistance to impact loading of teak wood dust filled epoxy composites is more as compared to sal and rubber wood dust filled epoxy composites.