A Review on Water Absorption Behavior and its Effect on Mechanical Properties of Natural Fibre Reinforced Composites

Transcription

1 A Review on Water Absorption Behavior and its Effect on Mechanical Properties of Natural Fibre Reinforced Composites K.S.Meenalochani 1, Vijayasimha Reddy.B.G. 2 1 Asst.Professor, 2 Principal, Department of Mechanical Engineering, Vemana Institute of Technology, Bangalore Abstract Composites are superior to conventional materials due to their high strength to weight ratio. Of late, because of environmental concerns, natural fibres are finding their places in composites. However, their hygroscopic nature affects the mechanical properties of the composites adversely. Applications of composites for decking, flooring and outdoor facilities have made it necessary to evaluate the water uptake characteristics of natural fibre composites. The objective of this paper is to review the water absorption behavior, its effect on mechanical properties and the efforts to reduce the absorption of various natural fibre reinforced composites. Keywords Natural fibres, Water absorption, Hydrophilic, Mechanical properties I. INTRODUCTION Natural fibres are available abundantly and are easy to handle and process. However, the application of natural fibres in composites was almost redundant due to high performance of conventional reinforcement such as glass and carbon fibres. With the recent increase in environmental concerns due to high usage of traditional fibres, natural fibres have been receiving attention from researchers. Fig 1: Effect of water on fibre- matrix interface [1] The advantages of natural fibre based composites include light weight, posing minimum health hazard, abundance, renewability, low cost and comparable specific strength and stiffness. Low energy consumption during extraction, wide variety of natural fibres available locally are added benefit to manufacturers. Natural fibres are divided into straw fibres, bast, leaf, seed or fruit and grass fibres. The fibres most widely used in the industry are Flax, Jute, Hemp, Kenaf, Sisal and Coir. However, natural fibre composites (NFC) also suffer from few drawbacks such as high moisture absorption, poor wettability and weak interfacial bonding, the major problem being the interfacial bonding between hydrophilic fibres and hydrophobic polymer matrices. IJIRAE , All Rights Reserved Page -143

2 Natural fibres have high hydroxyl content of cellulose which makes it susceptible to absorb water and thus affects the mechanical properties. The water absorption will increase with increase in fibre content and temperature. The evaporation of moisture absorbed may also influence the porosity in the matrix. II. DIFFUSION MECHANISMS Water absorption can influence mechanical and electrical properties. Factors such as the type of material, additives, temperature, and length of exposure can affect the amount of water absorbed.moisture diffusion in composites may degrade mechanical properties by three different mechanisms [1, 2].The first mechanism involves the diffusion of water molecules inside the micro gaps between polymer chains. The second mechanism involves capillary transport into the gaps and flaws at interfaces between fibre and matrix. The third mechanism involves swelling effects which propagates micro cracks in the matrix. In a wet environment, water molecules penetrate in natural fibre reinforced composites through micro-cracks and reduce interfacial adhesion of fibre with matrix. This causes swelling of the fibre and eventually lead to debonding between fibre and matrix. A schematic illustration of this process is shown in Fig 1. Diffusion in polymer composites is by and large reported to take place as per the Fickian rule [3,4] and mass of absorbed penetrant as a function of time t, compared with M [1,2,5,6] is = 4 The coefficient of diffusion D = π Where M is the water content at time t,m is the equilibrium water content, h is the sample thickness, k is the slope of linear portion of the plot M and t. The diffusion coefficient shows the ability of solvent molecules to move across the polymer segment. III. EFFECT OF WATER ABSORPTION ON MECHANICAL PROPERTIES The moisture absorption test is done either as per ASTM D570 or as per BS EN ISO 62: Before immersion in water, the specimens are dried in an oven at 50 C [2], cooled in desiccators and immediately weighed. This process is repeated until the mass of the specimen reaches a constant value. Water absorption is expressed as increase in weight percent. Percent Water Absorption = [(Wet weight - Dry weight)/ Dry weight] x 100 Water absorption leads to swelling of the fibre, formation of voids and micro cracks at the fibre matrix interface region. All these result in a reduction of the mechanical properties and dimensional stability of composites. The effect of water absorption of cotton fabric (CF) reinforced geopolymer composites was investigated by [1] and it was seen that the flexural strength of composites decreased markedly from 15.8, 19.7 and 28.1 MPa to 9.3, 13.4 and 21.4 MPa for 4.5, 6.2 and 8.3wt% of CF respectively. It was also found that the hardness of the composites decreased in all samples in wet condition as shown in Fig 2. Fig 2: Hardness of CF reinforced geopolymer composites in dry and wet conditions [1] In this case, when water uptake reaches saturation level, the bound water and the free water remains in the composite as a reservoir. This leads to softening of the fibres and weakening of fibre matrix adhesion resulting in reduced mechanical properties. A reported work on JF/CF-HDPE composites (Jute fibre (JF) reinforced chemically functionalized high density polyethylene) [7] showed that the tensile modulus of the wet 10/90, 20/80 and 30/70 (% fibre / % matrix) decreases by 5%, 8.5% and 22% respectively and the tensile strength decreases by 5%, 6% and 9% respectively, as compared to that of the dry sample of the same composite composition. The water absorption behaviour of kenaf fibres in reinforced polyester resins at two different temperatures of 25ºC and 50 C was studied [8]. The flexural strength and modulus of dry composites of 20%wt of fibre were 69.6 MPa and 4.28 GPa respectively. On exposure to water immersion, the flexural strength for the same was reduced to MPa at 25ºC and to MPa at 50 C. The flexural modulus also followed the same trend with 1.67 GPa and 1.18 GPa at 25 C and 50ºC respectively. This is because moisture uptake increases with rising environmental temperature due to increase of water motion inside the composites. Another study,[2] on hemp reinforced unsaturated polyester composites showed that weight gain percentage at moisture saturation point at boiling temperature is approximately 23% higher than at room temperature for a five layer hemp (0.26 volume fraction of fibre) reinforced composites. IJIRAE , All Rights Reserved Page -144

3 The moisture uptake results in this study show Fickian behaviour at room temperature and non Fickian at boiling temperature. This is due to the moist, high temperature environment and the micro cracks developed on the surface as well as inside the materials. IV. EFFORTS TO REDUCE THE WATER ABSORPTION Research [6] has shown that alkali treated fibres enhance the mechanical properties of the hybrid polymer composites. For alkali treatment, both jute and sisal fibres were initially immersed in 5% NaOH solution for 30 min. Results revealed that J50S50 (Jute=50%,Sisal =50%wt) had 50% lower water absorption as compared to jute composite J100S0 when the fibres were not alkali treated. But, hybrid composite (J50S50) showed 72% lower water absorption as compared to J100S0 after treatment of fibres. It also had the lowest value of diffusion and permeability coefficient. Moisture absorption is reduced [9] through the removal of the OH groups of fibres seen in equation below: Fibre-OH + NaOH Fibre-O Na + +H 2 O Other chemical treatments that have been reported are acetylation, etherification, benzoylation, permanganate, acrylation and sodium chlorite. Experimental results of [10] showed addition of glass fibre as a skin protection drastically reduced the water absorption of jute-hemp epoxy composite from 21% to 6%. Data from [11] shows that the water uptake of unfilled composite of sisal fibre (SF) in epoxy was more than clay filled composites. Equilibrium water content continuously decreased as nano clay content increased and maximum reduction was observed for 5% wt. nano clay filled composite. It was also explained that nano clay layer creates an impermeable medium to arrest the water flow and cause the water to take an indirect path and hence more time is needed for water uptake. This can result in decrease in equilibrium water content in nanoclay filled composite system. All of the equilibrium water content value for each specimen is illustrated in Table 1. TABLE 1: WATER UPTAKE CHARACTERISTIC OF UNFILLED AND CLAY FILLED COMPOSITE SERIES [11] MATERIAL EQUILIBRIUM WATER CONTENT (%) EPOXY+SF 62.1 EPOXY+SF+1% NANO CLAY 24.3 EPOXY+SF+3% NANOCLAY 15.8 EPOXY+SF+5% NANO CLAY 12.5 A work by [12] was to find a solution to deal with water absorption for natural fibre reinforced epoxy. In the research, nano silica carbide (n-sic) was added to observe the effect on water absorption of n-sic reinforced RCF/epoxy composites. The author verified that the addition of n-sic will eventually decrease the water uptake and enhance the mechanical properties of the polymer. Fig 3: Water absorption curves of n-sic filled RCF/epoxy nanocomposites [12] From Fig.3 it can be observed that the high content of nano particle decreases water uptake of the specimen and exhibit low value of weight gain. In the case of PP-g-MA (polypropylene-g-maleic anhydride)and PE-g-MA(polyethylene-g-maleic anhydride) /flax fibre composites, the hydrophilicity of the fibre can be reduced using MAPP and MAPE treatment due to the esterification reaction between flax fibre hydroxyl groups and anhydride part of MAPP and MAPE [13]. It was observed that the treated flax/pp composite exhibited a decreasing rate of water absorption compared to untreated fibre composites. The compatibilizer (coupling agent) builds up chemical bonds and hydrogen bonds which reduce the moisture-caused fibre/matrix debonding. It was found that all the modifications reduce the hydrophilic nature of the flax fibre thereby favouring a strong interfacial adhesion between the fibre and matrix. The effect of compatibilizer on the tensile and flexural strength of the composites after immersion in water at room temperature for 504 hrs is presented in Figs 4 (a) and (b) respectively. It is found that the compatibilized composites showed higher strength than uncompatibilized composites for all fibre content. The PP-g- MA compatibilizer composites showed the best tensile and flexural strength properties at 30% fibre content both before and after water absorption. Some of the other coupling agents that have been used in natural fibre composites are silane [14], titanates, amides, sodium silicates etc. [15] IJIRAE , All Rights Reserved Page -145

4 Fig 4(a): The tensile strengths of PP/flax fibre composites with various compatibilizers before and after water immersion [13] Fig 4(b): The flexural strengths of PP/flax fibre composites with various compatibilizers before and after water immersion [13] CONCLUSION Exposure of natural fibre reinforced composites to moisture results in significant drop in mechanical properties due to degradation of the fibre-matrix interface. Chemical treatment of fibres, adding skin protection, using nano fillers and using coupling agents have been proved to reduce the water absorption, thus enhancing the mechanical properties. ACKNOWLEDGEMENT The authors are thankful to Dr.Yella Reddy for his critical comments and helpful suggestions on the manuscript. REFERENCES [1]. T. Alomayri, H. Assaedi, F.U.A Shaikh, I.M. Low, Effect of water absorption on the mechanical properties of cotton fabric reinforced geopolymer composites, Journal of Asian Ceramic Societies 2, , [2]. H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites, Composite Science and Technology, Science Direct,Vol.67, Issue 7-8, , [3]. Shen, Chi-Hung and Springer George S. Moisture absorption and desorption of composite materials, Journal of Composite materials, Vol 10, 2-20, [4]. Springer George S, Moisture content of composites under Transient conditions, Journal of Composite Materials, Vol 11, , [5]. E. Munoz & J.A. Garcia-Manrique, Water absorption behavior and its effect on the mechanical properties of flax fibre reinforced bio-epoxy composites, International Journal of Polymer Science, Article ID , [6]. M.K. Gupta & R.K. Srivastava, Mechanical, thermal and water absorption properties of hybrid sisal/jute fibre reinforced polymer composite, Indian Journal of Engineering & Material Sciences, Vol 23, , 2016 [7]. Anshu Anjali Singh & Sanjay Palsule, Effect of water absorption on interface and tensile properties of jute fibre reinforced modified polyethylene composites developed by Palsule process, Applied Polymer Composites, Vol 1, No2, , 2013 IJIRAE , All Rights Reserved Page -146

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