The Effects of Weathering on Mechanical Properties of Kenaf Unsaturated Polyester Composites (KFUPC)

Transcription

1 The Effects of Weathering on Mechanical Properties of Kenaf Unsaturated Polyester Composites (KFUPC) A.A.A. Rashdi 1, S.M. Sapuan 1 *, M.M.H.M. Ahmad 2 and A. Khalina 3 1 Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia 2 Department of Mechanical Engineering, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, Kuala Lumpur, Malaysia 3 Department of Biological and Agricultural Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia Received: 26 November 2009, Accepted: 24 February 2010 SUMMARY Exposure to natural weathering changes the mechanical properties of unsaturated polyester composites. These changes can negatively affect the mechanical properties of natural fibre composites, through UV radiation and relative humidity. The idea of this study was to determine the durability of kenaf unsaturated polyester composites (KFUPC) in natural conditions, and in addition, to determine the effects of weathering on the performance of these composites and the biodegradability possessed by the composite when exposed to natural weather. KFUPC composites were exposed to the atmosphere for four months. Humidity up-take tests were performed on composite specimens containing (10%, 20% and 30%) weight percentage of fibre content. The tensile properties of exposed specimens were evaluated, and compared alongside dry composite specimens. A decrease in tensile properties of the weather exposed composites was demonstrated, compared with the dry samples. The percentage of moisture uptake increased as the weight percentage of fibre increased, due to the high cellulose content. 1. INTRODUCTION Much recent research in the area of composite materials focuses on the use of natural fibres as reinforcement for polymer composites. Natural fibres have attracted research and engineering interest due to its important benefits, and various desirable properties including lower cost and density, high specific strength, high specific stiffness, ease of preparation, biodegradability, and wider availability than traditional reinforcing fibres. However, they have some disadvantages, including high absorption of moisture, which negatively affects the mechanical properties 1,2. *Corresponding author. Smithers Rapra Technology, 2010 On the other hand, kenaf, sisal, coir, banana, jute flax, pulp, wood flour, and cellulose fibres are the main natural fibres used as reinforcement 3. Kenaf fibres provide high stiffness and strength values. They also have high aspect ratios, making them particularly suitable to be used as reinforcement of polymer composites 4. Kenaf (Hibiscus cannabinus, L. family Malvaceae) is an herbaceous annual plant. Kenaf has a bast fibre which contains 75% cellulose and 15% lignin and offers the advantages of being biodegradable and environmentally safe 5. Natural plant fibre reinforced unsaturated polyester composites also have some disadvantages such as the incompatibility between the hydrophilic fibres and hydrophobic thermoplastic and thermoset matrices requiring appropriate treatments to enhance the adhesion between the fibre and the matrix 6,7. Natural weathering is generally done to determine the durability of the material in natural conditions 8. The idea was to manipulate to determine the extent of biodegradability possessed by the composite when exposed to natural weather. Sunlight, rain and dew are common aspects of natural weathering. Ultraviolet (UV) rays from sunlight are reportedly the most common cause of polymer article failure during natural weathering 9. Chalking, yellowing, cracking, brittleness and loss of transparency are common observation faced with conventional polymers exposed to natural weather 9. Unsaturated polyester has been a 337

2 A.A.A. Rashdi, S.M. Sapuan, M.M.H.M. Ahmad and A. Khalina popular thermoset used as polymer matrix in composites 10,11. For the current work, unsaturated polyester resin trade name Reservol P9509, was convenient for hand lay-up application and easy air release. All polymer composites absorb moisture in humid atmosphere and when immersed in water. The effect of absorption of moisture leads to the degradation of fibre-matrix interface region, creating poor stress transfer efficiencies and resulting in a deterioration of mechanical properties 12. One of the main concerns for the use of natural fibre reinforced composite materials is their susceptibility to moisture absorption and the effect on physical and mechanical properties 13. Therefore it is important that this problem is investigated in order that natural fibre may be considered as a viable reinforcement in composite materials. 2. EXPERIMENTAL 2.1 Materials Kenaf Bast Fibres Long form kenaf bast fibres obtained from Department of Biological and Agricultural Engineering, Universiti Putra Malaysia were used. Kenaf fibres were dried for 3 hours at 100 o C using a vacuum oven to remove storage moisture before use Resin The matrix material used in this study was based on the unsaturated polyester resin, trade name Reservol P9509 supplied by Revertex (Malaysia) Sdn. Bhd. Company. This type of resin is a rigid, low reactivity, thixotropic general purpose orthophthalic one; the matrix was mixed with curing catalyst, methyl ethyl ketone peroxide (MEKP) at a concentration of 0.01 w/w (weight ratio) of the matrix for curing. Unsaturated polyesters have many advantages compared to other thermosetting resins including room temperature, low pressure moulding capabilities which make it particularly valuable for large component manufacturing at relatively low cost Methods Samples with 10%wt, 20%wt and 30%wt kenaf fibre content were manufactured using a combination of hand lay-up and compression moulding methods. A measured quantity of unsaturated polyester resin mixed with 0.01 w/w catalyst (MEKP) for rapid curing was poured on the kenaf fibre, which placed in a mould. The mould was closed and the composite was left to cure under pressure at a temperature of 50 C for about 1 h. The mould was opened and composite was removed from the press. The schematic of a mould press was used to consolidate composite panel is shown in Figure Natural Weathering Test Natural weathering test was carried out as described in ASTM D1435. Sample Figure 1. Schematic of the composite consolidation Figure 2. Schematic of the natural weather test samples (W) dimensions were 30x30x1 mm 3, encoded as W1-4 (Figure 2). The samples were exposed to atmosphere in an open area at the Strength of Materials Laboratory of the Faculty of Engineering, Universiti Putra Malaysia for four months. Then samples were collected every 10 days and dried to a constant weight at 40 C. The percent moisture content or percentage weight gain was calculated. Weight-gain or moisture up-take due to humidity was plotted versus the square root of time for all the samples in order to investigate the absorption behaviour of the samples. 2.3 Tensile Testing The tensile composite samples according to ASTM D 3039 (250x15x2 mm 3, encoded as N) as shown in Figure 3 were cut for tensile testing and natural weathering evaluation. Fifteen samples were used with weight percentages of fibre 10%wt, 20%wt and 30%wt. The tensile strength and modulus of kenaf 338

3 fibre reinforced unsaturated polyester composite after water absorption were measured using an Instron 5569 Universal Electromechanical Testing System with 50 kn loading capacity, with a cross-head speed of 2 mm/ min. The clamping procedure was performed carefully to ensure that the specimen was aligned with the loading axis. 3. RESULTS AND DISCUSSION 3.1 Natural Weathering Natural weathering is generally done to examine durability of materials in natural conditions 8 and to determine the extent of biodegradability possessed by composites when exposed to natural weather. The tests showed that most the KFUPC samples turned brittle at the end of exposure (see Figure 4). Changes of composite colour are mainly due to bleaching of wood components due to UV radiation 8. Samples did not lose their dimensional stability, which means that no warping was remarked. The effect of UV with moisture on the surface of samples is a complex process of photochemical degradation of polymers due to atmospheric moisture and oxygen 15. Absorption of UV light by the polymer creates free radicals near the surface of the samples. These radicals react with oxygen and attack the polymer molecules in the samples 16. Unsaturated polymer may undergo certain types of degradation processes when exposed to natural environment. 3.2 Humidity Absorption from Natural Weathering Composites Figure 5 shows the weight gain due to moisture uptake of natural weathering samples W after four months. It is observed that the amount of moisture absorbed was quite low (less than 1%), due to the fibre void volume, which as we have seen in the absorption tests, plays a role in the rate of uptake and the overall uptake at the saturation point. Initially the samples had sharp linear increases in moisture absorption and they reached their saturated state with a maximum moisture content of 0.125% after 240 hours. Figure 6 discusses the behaviour of the 30%wt KFUPC samples exposed to natural weathering. For the process of weight gain due to moisture uptake, the samples again showed sharp linear increases in moisture absorption and reached saturation with a maximum moisture content of 0.146% after 960 hours with linear initial gain. Similarly, as shown in Figure 7, the 20%wt KFUPC samples reached their saturated state after a linear increasing region at the same time of 960 hours, but with a lower moisture content at equilibrium 0.11%. The 10%wt KFUPC samples (see Figure 8) show an initial linear region and about 960 hours after natural weathering exposure these samples reached their saturation state with a maximum moisture content of 0.075%. Figure 3. Schematic of the tensile composite samples (ASTM D 3039) Figure 4. Photographs of KFUPC composites before and after natural weathering test 339

4 A.A.A. Rashdi, S.M. Sapuan, M.M.H.M. Ahmad and A. Khalina Figure 5. Weight gain of KFUPC specimens (W) versus the square root of exposure time Figure 6. Weight-gain of 30%wt specimens versus the square root of exposure time Figure 7. Weight-gain of 20%wt specimens versus the square root of exposure time We have seen from the absorption tests that KFUPC samples with high fibre content and KFUPC samples with lower fibre content differ in their ability to absorb humidity as shown in Figure 9. It was observed that the KFUPC samples with 30%wt gained more moisture and reached their maximum moisture content of 0.146% after 960 hours (40 days). A high fibre content in the sample leads to more water penetration into the interface through the micro cracks induced by swelling of fibres creating swelling stresses leading to composite failure 17. Moisture gain for W samples was faster, and it reached the maximum of 0.125% after 240 hours (10 days). In general, the overall moisture uptake was not very great, as it did not exceed 1% of specimen s weight. The moisture uptake followed a Fickian diffusion process Effects of Moisture Absorption on Mechanical Properties of Composites Mechanical tests were performed on all samples before and after natural weathering test. Figures present these results. Obviously, the tensile strength of KFUPC composite drops slightly after natural weathering exposure. The moisture uptakes, due to the humidity change the structure and properties of the fibres, matrix and the interface between them. For the 30%wt KFUPC samples the strength slightly dropped by 13.8 MPa, whereas that of the 20%wt and 10%wt KFUPC dropped by 20 MPa, and 27 MPa respectively. This might suggest that the kenaf fibre volume fraction did not reach the minimum value which ensures fibre-controlled composite failure giving way to matrix dominated failure 18,19. The presence of voids might also be a contributing factor for the drop in tensile strength at lower kenaf fibre volume 20,21. The 30%wt KFUPC samples reached that maximum values of Young s modulus ( MPa) and ultimate tensile 340

5 stress of 134 MPa whereas for the 20%wt KFUPC samples the Young s modulus was MPa and the ultimate tensile stress was MPa. However, it is noticeable that for 10%wt KFUPC reinforced samples, the ultimate tensile stress is low (89.6 MPa) compared to other samples. Strain at maximum load decreased after the natural weathering test for all KFUPC composites. However, the maximum strain was reduced with higher fibre content. Water acts as a plasticizer in the composite material, which should lead to an increase of the maximum strain for the composite after water absorption CONCLUSIONS The effect of natural weathering on mechanical properties and durability of kenaf fibre reinforced unsaturated polyester composite (KFUPC) was studied. The weight percentage of fibre content plays a role in the rate of moisture uptake and the overall uptake at saturation. Thus, KFUPC composites samples differed in their ability to absorb ambient humidity. Among all the KFUPC composites tested, the moisture uptake from the humid environment increased with the increase in percentage of fibre volume content. The amount of moisture uptake was lower than 1 wt.%. The water absorption pattern of these composites was found to follow Fickian behaviour. Moisture uptake had a significant impact on the mechanical properties of composites. The tensile strength of KFUPC dropped and the strength was slightly reduced by the introduction of moisture. Increasing the weight percentage of fibre content led to an increase in the mechanical properties of the KFUPC composites for dry specimen, so that water absorption from relative humidity dramatically affected the mechanical properties. Figure 8. Weight-gain of 10%wt specimens versus the square root of exposure time Figure 9. Comparison of average weight gain between the naturally-weathered samples W and tensile test samples (N) Figure 10. Effect of the natural weathering humidity on the stress for KFUPC ACKNOWLEDGEMENTS The authors wish to thank University Putra Malaysia for the financial support 341

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