The swelling behavior and tensile strength of natural fiber-reinforced plastic in premium motor spirit (PMS), dual purpose kerosene (DPK) and sea water have been studied. Composite formed by reinforcing polyester resin with Okam fibers was immersed in the selected solvents for 16 weeks (4 months). Swelling characteristics of samples of this composite material were determined by monitoring- forth nightly, changes in weight of the samples within the test period and tensile test was conducted on the samples after 16 weeks using Hounsfeild (Monsanto) tensometer. The results show that the solvents reduced the strengths of the composite with PMS giving the highest reduction in strength followed by DPK and then sea water. Reduction in strength was attributed to the weakening of the interfacial bonding between the polyester matrix and reinforcing okam fibers

Njoku, RE

Obikwelu, DON

English

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SWELLING CHARACTERISTICS AND TENSILE PROPERTIES OF NATURAL FIBER REINFORCED PLASTIC IN SELECTED SOLVENTSBYR. E. Njoku and D. O. N ObikweluDepartment of Metallurgical and Materials EngineeringUniversity of Nigeria, Nsukka.E-mail: njoku_romanus@yahoo.comABSTRACTThe swelling behavior and tensile strength of natural fiber-reinforced plastic in premiummotor spirit (PMS), dual purpose kerosene (DPK) and sea water have been studied.Composite formed by reinforcing polyester resin with Okam fibers was immersed in theselected solvents for 16 weeks (4 months). Swelling characteristics of samples of thiscomposite material were determined by monitoring- forth nightly, changes in weight of thesamples within the test period and tensile test was conducted on the samples after 16 weeksusing Hounsfeild (Monsanto) tensometer. The results show that the solvents reduced thestrengths of the composite with PMS giving the highest reduction in strength followed byDPK and then sea water. Reduction in strength was attributed to the weakening of theinterfacial bonding between the polyester matrix and reinforcing okam fibers.Key words: Natural fiber; Reinforced plastic; Swelling; Tensile strength1.0 INTRODUCTIONComposite materials comprising polymermatrices reinforced with fibers usually ofglass fiber, Kevlar and carbon have gainedconsiderable applications in automotive,aerospace, marine and constructionindustries (1). Glass reinforced plastic wasoriginally developed in the UnitedKingdom during the Second World War asa replacement for moulded plywood used inaircraft Radom. Its first civilian applicationwas for building of boats where it gainedacceptance in the 19550’s. Its use hasbroadened over the years especially in theautomotive, sport equipment sectors andconstruction industries (1). According toJayamol et al (2), fibers made of Kevlar andcarbon are strong, high temperatureresistant, expensive and are nowadayslimited to high performance applicationssuch in aerospace.Since the development of reinforcedconcrete over a century ago, engineers haveconstantly studied the problem of steelreinforcement of concrete due to its poorresistance to corrosion. The new advancesin the field of materials science haveresulted in the development of fiber-reinforced plastic (FRP) materials thatcould potentially be used as bars inreinforced concrete structures. Theattractive features of this type of bar (madeof FRP) include: high strength; light weightand resistance to electrochemical corrosion(3).Philip Ritche et al (4) reported that5859SWELLING CHARACTERISTICS AND TENSILE PROPERTIES OF NATURAL FIBER REINFORCED PLASTIC ...whereas polymer resins are strong incompressive loading and relatively weak intensile strength, the glass fibers are strongin tension and weak in compression. Thusby combining the two materials, glassreinforced plastic is better material incompressive and tensile loading.Researches on natural fibers as asubstitute for glass fibers in compositematerials have gained momentum in thelast decade due to ecological concerns sinceglass fibers are non-degradable and theirdisposal after use causes environmentalproblems (5). It is for this reason thatnatural fibers based on lingo-cellulose arec o n s i d e r e d  a s  i n t e r e s t i n g  a n denvironmentally safe alternative to the useof glass fibers as reinforcement inengineering polymeric materials. Accordingto Jarrod Schemenauer (6), plant fibers likeflax, hemp, kenaf, jute and sisal have anumber of techno-economic and ecologicaladvantages over glass fibers and are beingconsidered as potential candidates toreplace them (glass fibers). Moreover, plantfibers are abundant, renewable (sourcedfrom annual plant), cheap, recyclable,exhibit low density and the extent of itsenvironmental pollution is less compared tosynthetic fibers (7).Generally, natural fibers arecategorized according to their origin:plants; animals and minerals (3). Plantfibers, however, can be grouped into: Bast(jute, banana, flax, hemp, kenaf, mesta);Leaf (pineapple, sisal, screw pine) and Seedor Fruit (coir, cotton, palm oil). Accordingto Youngquist, J. A et al (8), cellulose is themain component of plant fibers and it(cellulose) consists of other naturalsubstances such as lignin and waxes.Cheremisinoff (9) reported that themechanical and physical properties of plantfibers are influenced by the followingstructure parameters :  degree ofpolymerization; void structure and size;crystal structure; degree of crystallinity;specific interface and fiber diameter. Theultimate mechanical properties ofcomposites are influenced mainly by theadhesion between matrix and fibers (2). Astrong adhesion at the interface is neededfor effective transfer of stress and loaddistribution throughout the interface.The primary drawback of agro-basedfibers is associated with their inherent polarand hydrophilic nature and the non-polarcharacteristics of most thermoplasticswhich results  in difficulties incompounding of the fiber and the matrix.Also plant fibers have poor resistance tomoisture absorption and this causesswelling and quality variation in plantfiber-based composites (10). Possiblesolution to this problem is by treating thefibers with appropriate chemicals. Variouschemical agents have been employed toenhance the compatibility between theconstituent materials (11).Despite these drawbacks, therenewability and biodegradability of naturalfibers make them very attractive asreinforcements of engineering polymersystems (5, 6). As fiber reinforced plasticsare gradually replacing steel in pipelineapplications, this research aims at studyingthe swelling characteristics of “Okam”fiber-reinforced-polyester compositeimmersed in petroleum product media andsea water and the effects of its exposure tothese environment on the tensile strength ofthis material under uniaxial tension.NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 27 NO.2, SEPTEMBER 2008 60                R. E. NJOKU & D D. O. N OBIKWELU2.0 MATERIALS AND METHODThe composite was made from polyesterresin which served as the matrix and okamfiber-serving as the reinforcing material.Polyester resin is a thermosetting polymericmaterial consisting of linear polyester in8 8styrene (C H ) monomer. The structuralformula of polyester is shown below.        The polyesters are formed by thepolycondensation of decarboxylic acid withdihydric alcohols. In the composite, thefibers provide stiffness and strength whilethe resin provides a matrix to transfer loadto the fibers, give stability to the fibers andprovide relatively impermeable andchemical resistant surface. Okam is afibrous plant grown in abundant quantitiesin some parts of Nigeria. The ones used inthis work are sourced from Kogi state.Other additives in the polyester-okam fibercomposite include a catalyst (methylethylketone peroxide) and an accelerator(cobalt nephthenate). The catalyst is across-linking agent which when added insmall quantities to the unsaturated polyesterresin initiates the reaction of the styrenemonomers contained in the resin with otheringredients to produce a molecular cross-linked polymer. The accelerator activatesthe catalyst at lower temperatures andallows polymerization to take place atlower temperatures.2.1 Fiber PreparationStems of okam plants were soaked inflowing stream and dried in ambientcondition. The okam fiber was, thereafter,soaked in chemical  solution ofcomposition: vinyl triethoxy saline,methanol and water for 3 hours and dried.The fibers were later soaked in sodiumhydroxide (NaOH) solution for 2 hours andfinally washed with water and dried. Theabove surface treatments were performed tomodify the physico-chemical properties ofthe okam fibers and make them chemicallycompatible with the polyester resin.                                                                       2.2  Sample PreparationLaminates of polyester-okam fibercomposite were produced by compressionmoulding of polyester resin with theadditions of methyl ethylketone peroxideand cobalt nephthenate catalyst andaccelerator respectively. Chopped strandmats of the okam fibers were embeddedinside the “resin mix”. A resin-fiber ratio of60:40 was used. The whole arrangementwas allowed to cure in wooden mould andwas subjected to mild pressure by placingof loads on the upper mould. Test couponswere cut from the laminate, initiallyweighed and immersed in petrol, keroseneand sea water for 16 weeks. The sampleswere re-weighed after every 2 weeks.Standard testpieces (prepared according toASTM standard D 638) were cut for tensiletest. They were immersed in separate bathof petrol, kerosene and sea water for 16weeks and, thereafter, tested for ultimatestrength.2.3 Tensile TestNIGERIAN JOURNAL OF TECHNOLOGY, VOL. 27 NO.2, SEPTEMBER 2008 61SWELLING CHARACTERISTICS AND TENSILE PROPERTIES OF NATURAL FIBER REINFORCED PLASTIC ...Sample specimens measuring 300mm ×16.85mm × 5.75mm were cut from thepolyester-okam fiber composite and testedfor tensile strength using the Hounsfield(Monsanto) tensometer. The test wasperformed on the samples which were notimmersed in the selected solvent and onthose immersed. The test pieces weresecurely clamped on he holding devices ofthe tensometer and subjected to a graduallyincreasing uniaxial load until theyfractured. The force applied to thespecimens and the corresponding extensionon the samples were monitored andrecorded. The tensile strength of thecomposite was estimated using the formula:(1.0)Whereas strain was calculated using therelationship given below:(2.0)3.0 RESULTS AND DISCUSSIONSFigures 1.0 shows the Results of PercentageSwelling Index of composite material in thesolvents. Swelling Index (S.I) wascalculated using the formula:(3.0)Where Wo and W1 are initial and finalweights of samples.3.1 Swelling of Polyester-okam FiberComposite in Selected  SolventFigures 1 is the graph of swelling index ofthe polyester-okam fiber composite inpetrol, kerosene and sea water after every 2weeks for 16 weeks. Results show that theswelling index of the samples increased asthe time of immersion was increased.Swelling of the samples in sea water couldbe attributed to the interaction between thehydrophilic cellulose molecules of theokam fibers and water molecules.According to Jayamol et al (2), cellulosewhich is the main constituent of plant fiberscontain three hydroxyl (-OH), groups.These hydroxyl groups form hydrogenbonds between the cellulose molecules andexposing these bonds to humidity causesthem to be broken. The hydroxyl groupsthen form new hydrogen bonds with watermolecules which induce swelling. In the case of the petroleum products,Jiagun li et al (12), suggested that theorganic liquids penetrate and swell thepolymeric composite through diffusion andabsorption phenomena. Hence allowingcrazing (formation of micro voids) to takeplace at lower stresses and once a craze hasformed; the active environment is able todiffuse along the craze more easily thanthrough the bulk material.Calculation of percentage SolventAbsorption and Swelling index of thecomposite material shows that theseparameters are highest for samplesimmersed in premium motor spirit and leastfor those immersed in sea water.NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 27 NO.2, SEPTEMBER 2008 62                R. E. NJOKU & D D. O. N OBIKWELU3.2 Tensile Strength of Okam Fiber-reinforced Composite Immersed inSelected SolventFigure 2.0 shows the tensile stress Vs straincurves of the samples before and after theirimmersion in the solvents for 16 weeks. Itcan be deduced from the graph that thesamples degraded in strength as a result oftheir interaction with these media. According to Ko (13), the strength andother mechanical properties of a compositematerial depend on a number ofmicrostructural parameters which include:volume fraction of the constituents; shapeof second phase (particulate, platelet orfiber), dimensions of second phaseconstituents (diameter, length and aspectratio), orientation of fibers with respect tothe loading direction and the properties ofthe interface between the constituents. For auniaxially loaded composite, the ultimatestrength can be calculated from the rule ofmixture approach given by:(4.0)fWhere σ  is the tensile strength of themfibers, σ  is the tensile strength carried bythe matrix when the fibers are strained tof their ultimate strength and V is the volumefraction of the fiber. The critical fiberfcvolume (V ) which must be exceeded forfiber strengthening to occur is:(5.0)uWhere σ  is the ultimate strength of thematrix. For a composite to develop its fullstrength, a critical length   (6.0)Must be exceeded. Where τo isshear yield stress of the matrix andd is the fiber diameter. Figure 2.0again revealed that the greatestreduction in strength was noticedin samples immersed in petrolfollowed by the ones immersed inkerosene. Sea water produced theleast effect on the strength of thecomposite material. Reduction instrength of the composite materialcould be attributed to theweakening of interfacial bondbetween the matrix and fiber and the factthat the presence of the solvents lowers thesurface energy of the composite materialand makes deformation easier at lowerstresses (13).4.0 CONCLUSIONS1. The solvents investigated reduced thetensile strength polyester- okam fibercomposite. 2. Swelling of the composite materialwas greatest in petrol, followed bykerosene and was least in sea water.REFERENCES1. Fiber-reinforced Plastic. Wikipedia, thefree encyclopedia. (Accessed on July 20,2007 from http://en.wikipedia.orgNIGERIAN JOURNAL OF TECHNOLOGY, VOL. 27 NO.2, SEPTEMBER 2008 63SWELLING CHARACTERISTICS AND TENSILE PROPERTIES OF NATURAL FIBER REINFORCED PLASTIC .../wiki/fiber-reinforced plastic)2. Jayamol, G.S., A Review of InterfaceModification and Characterization ofNatural Fiber-Reinforced PlasticComposites, Polymer Engineering andScience, Vol. 28, Number 11, 2001. PP. 9623. Fares, E. T., Civil Engineering Database,Vanasse Hengen Brustlin Inc, 19991, PP.302-3204. Philip, A. R, David, A. T and Guy, M. C.,Structural Journal, Vol. 88, Number 4,1991, PP. 490-5005. Jayamol, G. S., Effects of Environment onthe Properties of Low DensityPolyethylene Composites Reinforced withPineapple-leaf Fiber, Composite Scienceand Technology, Vol. 13, Number 8,1991. PP. 2466. Jarrod, J. S, Melt Rheological Propertieso f  N a t u r a l  F i b e r - R e i n f o r c e dPolypropylene, Polymer Engineering andScience, Vol. 86, Number 24, 2000. PP.482.7. Wretfors, C and Svennerstedt, B., Biofiber Technology used for MillitaryApplications- An Overview, JBT Rapport,Number 142, 2006. PP. 1-408. Youngquist, J. A and Krzysik, A. M.,Lignocellulosic Composites, CellulosePolymer, Vol. 10, Number 12, 1994.PP115-130.9. Cheremisinoff, N. P., Handbook ofEngineering Polymeric Materials, Vol.25, Number 14, 1999. PP. 506.10. Nair, K. C. M and Thomas, S., Journal ofApplied Polymer Science, Vol. 60, 19996.PP 1483.11. Wang, B and Panigrahi, S., Agriculturaland Biological Engineers AnnualMeeting, Paper Number 046083, 2004.PP.25612. Jiajun, L and Arnold, J. C.,Environmental Stress Cracking behaviorof  Urethane Methacrylate based Resin,Journal of Materials Science, Vol. 29,1994.PP. 3095-3101.13. Ko, F., Preform Fiber Architecture forCeramic Matrix Composites. AmericanCeramic Society, Vol. 68, 1989. PP. 40-47.  NIGERIAN JOURNAL OF TECHNOLOGY, VOL. 27 NO.2, SEPTEMBER 2008 

Swelling Characteristics and Tensile Properties of Natural Fiber Reinforced Plastic in Selected Solvents

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Swelling Characteristics and Tensile Properties of Natural Fiber Reinforced Plastic in Selected Solvents

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