Impact of chemical treatments on the mechanical and water absorption properties of coconut fibre (Cocos nucifera) reinforced polypropylene composites
Isiaka Oluwole OLADELE, Okikiola Ganiu AGBABIAKA* and Paul Tolu OLORUNLEYE
Dept. Metallurgical and Materials
E-mails: firstname.lastname@example.org; email@example.com
*Corresponding author, phone: +2348107769172
In this work, chemically treated coconut fibres were used to reinforce Homopolymer Polypropylene in order to ascertain the effect of the treatments on the mechanical and water absorption properties of the composites produced. Coconut fibre was first extracted from its husk by soaking it in water and was dried before it was cut into 10 mm lengths. It was then chemically treated in alkali solution of sodium hydroxide (NaOH) and potassium hydroxide (KOH) in a shaker water bath. The treated coconut fibres were used as reinforcements in polypropylene matrix to produce composites of varied fibre weight contents; 2, 4, 6, 8 and 10 wt.%. Tensile and flexural properties were investigated using universal testing machine while water absorption test was carried out on the samples for 7 days. It was observed from the results that, NaOH treated samples gave the best tensile properties while KOH treated samples gave the best flexural and water repellent properties.
Coconut fibre; Polypropylene; Chemical treatment; Mechanical properties
Natural fibres are now considered as serious alternative to synthetic fibres for use in various applications in human endeavour [1-2]. The use of natural fibres as reinforcing materials in both thermoplastic and thermoset matrix composites provide positive environmental benefits with respect to ultimate disposal and best utilization of raw materials . Due to the relatively high cost of synthetic fibres like glass, plastic, carbon and Kevlar that are been used in fibre reinforced composites, and the health hazards of asbestos fibres, it has become necessary to explore natural fibres as source of alternative reinforcement materials for composites development [4-5]. Natural fibres produced from renewable resources, are biodegradable and relatively inexpensive compared to the traditionally used synthetic fibres. Fibres of these types, for instance, hemp and flax have been successfully used as packaging material, interior decorations of automobiles and building components among others [6-7]. Other advantages of natural fibres over traditional reinforcing materials like glass and carbon fibres are their specific strength properties, easy availability, light weight, ease of separation, enhanced energy recovery, high toughness, non-corrosive nature, low density, low cost, good thermal properties, reduced tool wear, reduced dermal and respiratory irritation, less abrasion to processing equipment, renewability and bio-degradability . In light of this, researchers have focused their attention on natural fibre composites for various engineering applications [9-10]. Accordingly, manufacturing of high-performance engineering materials from renewable resources have been pursued by researchers across the globe because renewable raw materials are environmentally sound and do not cause health problem [9-11].
However, high moisture sorption, low strength and poor compatibility of natural fibres in matrix have restricted their use in engineering applications . These problems can be solved by carrying out chemical treatments on fibres before being used as reinforcements for composites development. Commonly used chemical treatments for fibre modification are alkalization, silane, acrylation, maleic acid, acetylation and benzoylation . This present work aimed to contribute to knowledge by investigating the effect of alkali treatments of selected natural fibre on the mechanical and water absorption properties of the developed composites.
In this work, Homopolymer Polypropylene, a thermoplastic polymer was used as the matrix while coconut (Cocos nucifera) fibre was used as reinforcement to develop the composites using compression moulding process. The effect of alkali treatment of coconut fibre using sodium hydroxide (NaOH) and potassium hydroxide (KOH) was investigated on the flexural, tensile and water absorption properties of the developed composites.
Materials and method
major materials for this work are: coconut fibre
gotten from the farm plantation in Akure while Polypropylene, distilled water,
sodium hydroxide (NaOH) and potassium hydroxide (KOH) were procured from
chemical and Allied stores in
Fibre preparation and treatment
Coconut fibres were locally sourced and were cut into 10 mm with scissors before they were subjected to alkali treatments. The selected fibres were divided into 2 parts where one part was treated with NaOH solution while the other part was treated with KOH solution. The process involved immersing the fibres into 1M NaOH and KOH each in a shaker water bath operated at 50°C for 4 hours, respectively. After the treatments, the fibres were washed thoroughly with tap water and then with distilled water until a pH of 7 were achieved. Then, the fibres were sun dried for 4 days to remove moisture.
The purpose of chemical treatment is to remove the moisture content of fibre, increase the tensile strength and roughened the surface of the fibre to enhance its interfacial bonding with the matrix as well as reduces the susceptibility of the fibres to fungi attack .
The composites were produced by compression moulding process. Random fibre orientation was used with varied fibre content of 2, 4, 6, 8 and 10 wt.% . The treated fibres were mixed with polypropylene in the moulds before taking them to compression moulding machine to produce the composites. The mixtures were heated up to melting temperature of about 160°C and maintained at this temperature for about 10 minutes which then caused them to flow in the moulds. After, cooling and solidification, the composites were separated from the moulds and were allowed to cool for 27 days before carrying out tests on them.
Tensile and flexural tests were carried out on each samples using universal testing machine. It was done after the samples were produced from compression moulding machine, cleaned and chamfered. Tensile tests were performed on INSTRON 1195 at a fixed Crosshead speed of 10 mm/min-1 . Samples were prepared according to ASTM D412 standard test methods for rubber properties in tension. Flexural tests were carried out in accordance to ASTM D 790 Standard test method for polymer matrix composite materials . The test was performed at a speed of 100 mm/min. Three samples were tested for each representative samples from where the average values for the test samples were used as the illustrative values.
Water absorption test
The water absorption property of the composite samples was determined by weighing those samples in air before and after immersion in water. This test was done for 24 hours for the various samples of the composite span for 7 days. This test was used to determine the extent at which the composites can absorb water.
Results and discussion
Figure 1 shows variation of flexural strength at peak against composite samples.
Figure 1. Variation of flexural strength at peak against samples
Flexural strength at peak is the strength at a particular cross section and not the load carrying capability of the overall beam. From the chart, it was observed that sample denoted as C8 from KOH treatment which connotes 8 wt.% of KOH treated coconut fibres +92 wt.% polypropylene possess the highest value which is about 46.33 N/mm2 followed by sample denoted as C2 which means 2 wt.% of NaOH treated coconut fibres +98 wt.% polypropylene with a value of about 38.65 N/mm2.
Figure 2 shows the variation of flexural modulus with the samples.
Figure 2. Variation of flexural modulus against samples
From the plots, it was observed that sample denoted as C8 from 8 wt.% KOH treated coconut fibre +92 wt.% polypropylene possess the highest value which is about 2186 N/mm2 while sample denoted as C2 from 2 wt.% KOH treated coconut fibre +98 wt.% polypropylene follows with a value of about 1635 N/mm2.
Figure 3 revealed the results of the tensile strength at peak for the various samples.
Figure 3. Variation of tensile strength at peak against samples
Tensile strength at peak is the maximum tensile stress the material can withstand before it will fracture. From the chart, it was observed that the neat sample possess the best result with a value of 27.89 N/mm2 followed by sample denoted as C6 which contains 6 wt.% NaOH treated coconut fibre +94 wt.% polypropylene that a value of about 17.93 N/mm2 and sample denoted by C2 from 2 wt.% KOH treated coconut fibre +98 wt.% polypropylene with a value of about 13.90 N/mm2. The plots of the variation of Young’s modulus for the various samples were shown in Figure 4.
Figure 4. Plots of young modulus for the samples
Young’s modulus is also known as tensile modulus or elastic modulus which is a measure of the stiffness of an elastic material and it is defined as the ratio of stress along an axis over the strain along the same axis in which Hooke's law holds. From the chart, it was observed that NaOH treated coconut fibre reinforced polypropylene composites gave the best results from the various reinforcement contents used compared with KOH treated coconut fibre reinforced polypropylene composites. From the results, it was observed that sample denoted as C4 which contains 4 wt.% NaOH treated coconut fibre +96 wt.% polypropylene has the highest value which is about 1308 N/mm2 followed by C6 which connotes 6 wt.% NaOH treated coconut fibre +94 wt.% polypropylene with a value of about 1157 N/mm2.
Water absorption test
Figure 5 shows the water absorption rate for the samples.
Figure 5. Plots of water absorption for the samples
This test was used to determine the probability of future degradation of the composite materials if subjected to environment with constant water supply.
The test was carried out on the samples to determine the extent at which the composites can absorb water over a given period of time. From the graph, it was observed that sample denoted as C6 containing 6 wt.% KOH treated coconut fibre +94 wt.% polypropylene has the best water repellent property followed by C2 from 2 wt.% KOH treated coconut fibre +98 wt.% polypropylene. It was noticed from the results that, KOH treated samples gave better water repellent results compare with NaOH treated samples, though NaOH treated samples gave moderate results in terms of water repellent property.
The impact of chemical treatments on the tensile, flexural and water absorption properties of coconut fibre reinforced polypropylene composites have been investigated. From the results, it was apparent that KOH treatment was suitable for the enhancement of flexural and water repellent properties due to the performance of the chemically treated fibre reinforced composites samples by this treatment. In addition, it was evident that NaOH treatment for coconut fibre aids the enhancement of tensile properties. Alkaline treatment of coconut fibre is therefore seen as the potential means of enhancing the properties of their corresponding composite materials.
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