Kenaf natural fiber reinforced polypropylene composites

Kenaf natural ?ber reinforced polypropylene composites:A discussion on manufacturing problems and solutions

M.Zampaloni

b,*

,F.Pourboghrat a ,S.A.Yankovich a ,B.N.Rodgers a ,J.Moore a ,

L.T.Drzal b ,A.K.Mohanty c ,M.Misra b

Department of Mechanical Engineering,2555Engineering Building,Michigan State University,East Lansing,MI 48824,USA b

Composite Materials and Structures Center,2100Engineering Building,Michigan State University,East Lansing,MI 48824,USA

School of Packaging,130Packaging Building,Michigan State University,East Lansing,MI 48824,USA

Received 26January 2006;received in revised form 20December 2006;accepted 1January 2007

Abstract

As industry attempts to lessen the dependence on petroleum based fuels and products there is an increasing need to investigate more environmentally friendly,sustainable materials to replace existing materials.This study focused on the fabrication of kenaf ?ber rein-forced polypropylene sheets that could be thermoformed for a wide variety of applications with properties that are comparable to exist-ing synthetic composites.The research done in this study has proven the ability to successfully fabricate kenaf–polypropylene natural ?ber composites into sheet form.The optimal fabrication method for these materials was determined to be a compression molding pro-cess utilizing a layered sifting of a micro?ne polypropylene powder and chopped kenaf ?bers.A ?ber content of both 30%and 40%by weight has been proven to provide adequate reinforcement to increase the strength of the polypropylene powder.The use of a coupling agent,3%Epolene enabled successful ?ber–matrix adhesion.The kenaf–PP composites compression molded in this study proved to have superior tensile and ?exural strength when compared to other compression molded natural ?ber composites such as other kenaf,sisal,and coir reinforced thermoplastics.With the elastic modulus data from testing,it was also possible to compare the economic bene?ts of using this kenaf composite over other natural ?bers and E-glass.The kenaf–maleated polypropylene composites manufactured in this study have a higher Modulus/Cost and a higher speci?c modulus than sisal,coir,and even E-glass thereby providing an opportunity for replacing existing materials with a higher strength,lower cost alternative that is environmentally friendly.ó2007Elsevier Ltd.All rights reserved.

Keywords:https://www.360docs.net/doc/a310745422.html,pression moulding;E.Thermoplastic resin;E.Forming

1.Introduction

One of the unique aspects of designing parts with ?ber reinforced composite materials is that the mechanical prop-erties of the material can be tailored to ?t a certain appli-cation.By changing the orientation or placement of the ?bers the material can be designed to exhibit properties that are isotropic or highly anisotropic depending on the

desired end result.A major drawback of this customization is the economic costs that may be associated with this pro-cessing method.While customizing individual parts may be appropriate when working with low production level parts,when the idea is extrapolated to higher production parts,the customizing process becomes highly cost prohibitive.For higher production parts the use of thermoplastic sheets that have a pre-existing ?ber orientation is a cost e?ective choice.

There has been extensive work throughout industry with forming and shaping oriented glass and carbon ?ber rein-forced https://www.360docs.net/doc/a310745422.html,mon di?culties experienced

Corresponding author.Tel.:+15174324939;fax:+15173531750.E-mail address:zampalon@https://www.360docs.net/doc/a310745422.html, (M.Zampaloni).

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Composites:Part A xxx (2007)

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show that the forming of straight,continuous?ber or woven ?ber composite sheets typically results in wrinkling of the ?bers and distortions.Randomly oriented?bers have pro-vided good formability,but without the advantages of the highly directional properties often desired in composite parts.The more formable sheets that consist of aligned,dis-continuous?bers appear to have been used with more suc-cess than continuous?bers[1].

As industry attempts to lessen the dependence on petro-leum based fuels and products there is an increasing need to investigate more environmentally friendly,sustainable materials to replace the existing glass?ber and carbon?ber reinforced materials.Therefore,attention has recently shifted to the fabrication and properties of natural?ber reinforced materials.The automotive and aerospace indus-tries have both demonstrated an interest in using more nat-ural?ber reinforced composites,for example,in order to reduce vehicle weight,automotive companies have already shifted from steel to aluminum and now are shifting from aluminum to?ber reinforced composites for some applica-tions.This has led to predictions that in the near future plastics and polymer composites will comprise approxi-mately15%of total automobile weight[2].

Natural?bers that have been evaluated as replacements for glass and other non-recyclable?bers include?ax,hemp, kenaf,and sisal.These?bers are abundant,cheap,renew-able,and easily recycled.Other advantages include low density,high toughness,comparable speci?c strength prop-erties,reduction in tool wear,ease of separation,decreased energy of fabrication,and CO2neutrality[3].These natural ?bers can be split into two categories,bast and leaf.The bast?ber composites include kenaf,hemp and?ax,while sisal may be considered a leaf?ber.The bast?bers exhibit a superior?exural strength and modulus of elasticity (MOE),but the leaf?bers show superior impact properties. Compared to glass?bers,the bast?bers tend to show approximately the same?exural strength and a higher MOE[3].The main drawback in using these natural?bers is the hydrophilic nature of the natural?bers,which may lead to problems of adhesion with the hydrophobic poly-mer matrix.High temperatures must also be avoided due to the possibility of?ber degradation.In addition,since they are grown naturally,the properties of the?bers can vary immensely from plant to plant.

2.Manufacturing of natural?ber reinforced thermoplastic sheets

Mckenzi and Yuritta[4]compared di?erent types of wood?ber–reinforced polymers to determine if wood?ber has advantages as a reinforcing material over other?brous https://www.360docs.net/doc/a310745422.html,parisons were made with nylon,rayon, glass,and Kevlar.The short length of the wood?ber led to the conclusion that the bonding of the matrix with the ?ber was crucial since the full strength of the?ber would only be utilized if a strong bond were formed.It was also found that a wood?ber composite would have to be67%thicker than a glass?ber composite in order to have the same strength.This increased polymer matrix requirement reduces the cost bene?t of wood?ber over glass,but wood ?ber provides advantages by consuming less energy during ?ber manufacture and the potential for lower mass structures.

Michell[5]studied di?erent types of composites contain-ing wood pulp?bers.Michell has shown that wood pulp ?bers are cheaper than other organic polymers and also lead to improvements of both the strength and tensile mod-ulus of composites.The study concluded that although the wood pulp?bers were already being used in thermoset applications,there exists an opportunity for use as rein-forcements in thermoplastic composites.

Wambua et al.[6]evaluated several di?erent natural ?ber–polypropylene composites to determine if they had the ability to replace glass?ber–reinforced materials.Poly-propylene with a very high melt?ow index was used to aid in?ber matrix adhesion and to ensure proper wetting of the?bers.Samples were made with40%?ber content of kenaf,coir,sisal,hemp,and jute.After the samples were fabricated,tensile and impact tests were run to compare the properties of these composites to those made with glass ?ber.The tensile strengths all compared well with glass, except for the coir,but the only sample with the same?ex-ural strength was hemp.It was shown with kenaf?bers that increasing?ber weight fraction increased ultimate strength, tensile modulus,and impact strength.However,the com-posites tested showed low impact strengths compared to glass mat composites.This study demonstrated that natu-ral?ber composites have a potential to replace glass in many applications that do not require very high load bear-ing capabilities.

Mohanty et al.[3]compiled an overview on the di?erent bio?bers,biopolymers,and biocomposites comparing advantages and disadvantages of these materials.Di?erent ?ber–polymer composites were fabricated and tested in this work.The speci?c properties of natural?bers such as hemp,kenaf,jute,?ax,and sisal were compared with tradi-tional composite reinforcements showing that the density of these natural?bers were much lower than the traditional materials thereby leading to strength/weight ratios compa-rable with those of such widely used materials as E-glass and Aramid.Testing of the biocomposites demonstrated an optimal?ber content of approximately30%.

Mohanty et al.[7]evaluated biocomposites formed using chopped hemp?ber and cellulose ester biodegradable plastic.The e?ect of two di?erent processing approaches was studied.For the?rst process,the chopped?ber(30% by weight)was mechanically mixed in a kitchen mixer for 30min followed by compression molding using a picture-frame mold.The second process involved two steps,?rst an extrusion process yielded pellets of cellulose acetate plastic(CAP),secondly,the pellets were fed into a twin-screw extruder while chopped hemp?bers were fed into the last zone of the extruder.This process yielded thin strands of the composite,which were pelletized for

2M.Zampaloni et al./Composites:Part A xxx(2007)xxx–xxx

injection molding into tensile test coupons.One of the main advantages of melt mixing is the superior mixing of?ber and matrix,but at the cost of high shear forces,which may lead to?ber damage.The mechanical mixing and compression molding process,allows for these forces to be avoided.In addition,preforms are produced in one step. However,the lack of adequate mixing may adversely a?ect the overall performance.The composites formed using the second process,extrusion and injection molding,showed superior overall strength due to the adhesion and distribu-tion of the?bers.

Nishino et al.[8]used kenaf?ber sheets in an attempt to fabricate composites with better dispersion and adhesion. The kenaf?ber was bought in sheets,were dried,and then soaked in a dioxane solution under vacuum.These samples were easily fabricated and exhibited good performance with a?ber content of approximately70%by volume.This study concluded that the?ber orientation plays an impor-tant role in the?nal properties of the composite,both anisotropic and quasi-isotropic composites were fabricated and evaluated.

Mase et al.[9]worked with recycled paperboard and began to develop a heat/pressure formable wood?ber ther-moplastic composite.This new material was composed of a relatively?ne polypropylene powder(20microns)and paperboard waste.Samples were tested using20%and 30%polypropylene with six layers of the preform pressed together.Results have shown that the tensile modulus of this combination was increased up to twice the value of polypropylene alone.Additionally,the modulus of elastic-ity and ultimate tensile strength were increased,and the strain at the ultimate tensile strength was decreased.From this study it has was concluded that while the material exhibits satisfactory properties,there was not enough polypropylene added to make a formable material.There was limited resin?ow during the compression forming and further processing studies were to be completed in the future.

Bhattacharyya et al.[10]showed that wood?ber–poly-propylene composites are indeed formable.The sheets they manufactured used pinus radiate?bers along with polypro-pylene powder,for a total through thickness of1.3mm. Two types of composites were made,layered and homoge-neous with polypropylene and wood?bers mixed during formation.Results showed a tensile modulus increase of up to250%with a25–30%?ber mass fraction.Several formability tests were studied,dome forming with matched die and cup drawing being most relevant.For both forming techniques the material was heated to approximately 190°C(374°F)before beginning formation.This study demonstrated the ability to form these composites into not only two-dimensional shapes,but also simple three-dimensional shapes.

All of the previously mentioned studies have shown that natural?ber composites may be fabricated in a variety of ways to gain a variety of di?erent properties.There is one common conclusion from each study;the most perti-nent problem in manufacturing natural?ber reinforced composites is the assurance of?ber–matrix adhesion. George et al.(2001)[11]attempted to solve the problem of?ber–matrix adhesion when manufacturing biocompos-ites by applying an alkaline solution to the?bers.Natural ?bers are mainly composed of cellulose,whose elementary unit,anhydro d-glucose,contains three hydroxyl(–OH) groups.These hydroxyl groups form intramolecular and intermolecular bonds,causing all vegetable?bers to be hydrophilic.The alkaline solution regenerated the lost cel-lulose and dissolved unwanted microscopic pits or cracks on the?bers resulting in better?ber matrix adhesion. Another method of improving?ber matrix adhesion dis-cussed in this work was through the use of coupling agents. The coupling agents had two functions:to react with–OH groups of the cellulose and to react with the functional groups of the matrix with the goal of facilitating stress transfer between the?bers and the matrix.There have been numerous studies conducted on the use of coupling agents for improving the adhesion between natural?bers and polypropylene,with the most common approach being the use of a coupling agent containing maleic anhydride, creating maleated polypropylene(MAPP),which has been shown to immensely improve the properties of these mate-rials[12,14,15].

2.1.Fabrication of the kenaf–polypropylene composites

Kenaf is an herbaceous annual plant that is grown com-mercially in the United States in a variety of weather con-ditions,and it has been previously used for rope and canvas.Kenaf has been deemed extremely environmentally friendly for two main reasons;(a)kenaf accumulates car-bon dioxide at a signi?cantly high rate and(b)kenaf absorbs nitrogen and phosphorous from the soil[5].Addi-tionally,kenaf,like most other natural?bers,demonstrates low density,high speci?c mechanical properties,and is eas-ily recycled[3].Because it is commercially grown in the United States,kenaf maintains a competitive price of approximately$0.44–$0.55/kg(compared with E-glass at $2.00–$3.25/kg[3]).

One of the main obstacles that need to be addressed in the fabrication of kenaf?ber–reinforced composites regards uneven?ber distribution.The kenaf?bers are dif-?cult to manually separate and visually disperse evenly during manufacturing.Two di?erent composites were man-ufactured,one with long kenaf?bers,approximately 130mm(5.1in.)and a second using shorter,chopped kenaf ?bers with an approximate length of20mm(0.79in.).Both samples were fabricated in the same manner.First,poly-propylene sheets were made by compression molding a polypropylene(PP)powder(Profax6501;material proper-ties supplied by the manufacturer can be found in Table1) on a Carver Laboratory Press.A layer of10g of PP pow-der was heated to190°C(374°F)under minimal pressure for3min.The pressure was increased to69MPa(10kpsi) for a period of10min,and then increased again to

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165.5MPa(24kpsi)for another5min.The melted PP was allowed to cool to100°C(212°F)still under pressure before being released and removed immediately.This pro-cess yielded the two sheets of polypropylene that were used to make the kenaf–polypropylene composites.The thick-ness of the sheets was maintained at1mm(0.04in.)by using a steel picture frame.After the polypropylene sheets had been created,the kenaf?bers were added by sandwich-ing them between the two layers of PP.This layered system was then placed inside a1.5mm(0.06in.)thick frame and placed into the same press and the parts were fabricated using the same method outlined above.As shown in Fig.1a and b,both samples showed extremely poor?ber distribution.

Due to the uneven?ber distribution in the sandwiched composites,a new method was tested.Instead of making layers and using polypropylene sheets,the chopped kenaf ?bers were mechanically mixed with powder polypropylene in a kitchen mixer.The PP was slowly added while mixing the?bers.This proved to be a di?cult task due to the?ne nature of the kenaf?bers.The mixer balled up the kenaf, and the denser PP fell through to the bottom of the mixer. Using gloved hands,the powder was spread more evenly though the?bers.These composites were then spread evenly into a1.5mm(0.06in.)thick frame and compres-sion molded using the same method described previously.

As shown in Fig.1c,the?bers were not distributed evenly through the sample.There was a tendency for the ?bers to create a swirling pattern causing clumping and voids,thereby making it di?cult to achieve an even distri-bution of?bers.While neither of these methods provided a desirable sheet,it was concluded that using a method to chop and sift the individual?bers in a random orientation might create the fabrication of the kenaf?ber–reinforced thermoplastics with an even?ber distribution.

Table1

Typical physical properties of polypropylene(Profax6501)

Property

Melt?ow rate4g/10min

Density at23°C(73.4°F)0.9g/cm3(56.18lb/ft3)

Tensile stress at yield34MPa(4.93kpsi)

Tensile elongation(%)12

Flexural modulus 1.4GPa(203.1kpsi)

Notched izod impact strength

at23°C(73.4°F)

39N(8.77

lbf)

Fig.1.(a)composite fabricated by sandwiching layers with long?bers,(b)composite fabricated by sandwiching layers with chopped?bers,(c)kenaf–polypropylene composites fabricated by dry mixing,(d)kenaf–polypropylene composite fabricated by multiple layering of powder and?bers,(e)and(f) kenaf–polypropylene composite fabricated by multiple layering of powder and?bers,and(g)and(h)?nal kenaf–polypropylene biocomposite with30%?ber by weight.

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The new sifting process involved sprinkling the polypro-pylene powder as a base layer,adding individually sifted ?bers,sprinkling more polypropylene,etc.until all the desired materials were used.Problems were found trying to sift the PP powder so a new?ner PP powder was acquired that was easier to spread and sift(Micro?ne Poly-ole?n Powder–Polypropylene,Equistar Chemicals;aver-age particle size of20microns,compared with the original PP used that averaged400microns).This method was originally tested on a small,177.8mm by139.7mm(7 in by5.5in.)sample,using20g of polypropylene powder and8.5g of chopped kenaf?ber(approximately30%by weight)with the same original process.From Fig.1d,it is evident that this process enabled better?ber distribution. Based on these results this fabrication method was used to create304.8mm(12in.)square samples that could be used for material characterization.When fabricating these larger samples some problems were encountered with?ber wet-ting and the surfaces of these samples were rough with some exposed?bers.Additionally,as shown in Fig.1e and f,not all of the polypropylene was melted in the process.

Further analysis of the volume fractions led to the con-clusion that,in order to utilize the optimized process for the larger sample,six times the amount of each of the mate-rials must be used.In order to ensure even heating,a Tetrahedron Smart Press was used to fabricate these larger samples.This press is similar to the Carver Laboratory Press with the main di?erence being that the Tetrahedron has a larger compression bed thereby allowing the

samples Fig.2.Flow chart of optimal fabrication process for kenaf?ber reinforced thermoplastic using polypropylene matrix.

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to heat uniformly all the way to the edges.By incorporat-ing these changes,kenaf–polypropylene samples were fab-ricated as illustrated in Fig.1g and h.

In order to further improve the composite samples,both the kenaf?bers and the polypropylene powder were altered.In order to remove any excess moisture,the kenaf ?bers were baked in a vacuum oven at80°C(176°F)for at least4h prior to fabricating.The decrease in moisture con-tent decreased the weight of the?ber;therefore,more?ber reinforcement was added to the composite to maintain a ?ber/matrix ratio of either30or40%.In addition,with-drawing the moisture allowed the?bers to be chopped and spread more easily.The polypropylene was modi?ed by the addition of a coupling agent to increase?ber to matrix adhesion.The chosen coupling agent is Epolene Wax G3015(MAPP)in powder form from Eastman Chem-ical Company.This coupling agent has been previously used in a study of maleated polyole?n coupling agents for agro?ber composites and has been proven to increase ?exural and tensile strengths by more than60%[14]. According to this and other studies[12,15],the optimal amount of coupling agent is3%for a range of di?erent ?ber amounts.In order to accommodate the coupling agent,3%of the polypropylene powder was replaced with the Epolene Wax.

In summary,the process that was developed for the fab-rication of strong,useful kenaf?ber–reinforced polypro-pylene composites is illustrated in Fig.2.Additionally, the amounts of each material per unit volume may be found in Table2.These amounts have been tested in the fabrication of picture frame volumes between22,580mm3 and278,700mm3(1.38in.3and17in.3).For this range of volumes,the process has proven to be very e?ective and provided samples that could be compared against the mechanical properties of other natural?ber and glass?ber reinforced thermoplastics taken from existing literature.

3.Characterization of kenaf–polypropylene–epolene thermoplastic composites

In order to properly characterize the material behavior during deformation,there exists the need to perform a ser-ies of di?erent material tests including compression(or squeeze?ow)tests and multi-axis uniaxial tension tests. Characterization methods will be based on the techniques used by Zampaloni[16]to characterize glass mat?ber rein-forced thermoplastics.The goal of the characterization is to compare the mechanical performance of the kenaf?ber reinforced structures versus more traditional materials such as other natural?bers and glass?ber reinforced materials.

3.1.Squeeze?ow testing

Thermoplastics reinforced with a random?ber orienta-tion are not designed to provide directional strength and sti?ness but to ensure that the load is transferred to the ?bers instead of the matrix while the part is being utilized. Theoretically,a thermoplastic that has a truly random?ber orientation should provide isotropic material properties. Since these materials are manufactured by hand it is antic-ipated that some directionality will be imparted to the material.This implies that the material will not behave in an isotropic manner but rather as a material with a number of preferred?ber orientations.The goal is to determine these preferred directions and to characterize the material based on this inherent directionality.

A squeeze?ow test or a compression test,as illustrated in Fig.3,was utilized in order to determine the preferred ?ber orientations for kenaf?ber reinforced materials.Typ-ically for randomly oriented materials an orientation distri-bution function is determined from these types of tests and that function becomes a factor in the constitutive model-ing.One of the goals of this work was to determine whether there are a minimum number of directions that can be used to accurately capture the behavior of the material before resorting to the determination of the speci?c orientation distribution function for each material type.Details of the testing procedure can be found in Zampaloni[16].

As shown by Fig.4the kenaf?ber reinforced thermo-plastic deformed anisotropically.An isotropic part would have resulted in a circular sample at the completion of the compression process instead of the elliptical behavior found with these samples.This con?rms the assumption that there is some directionality associated with the?ber arrangements within the matrix and demonstrates that the?ow velocity and strain rate are not constant within the material and vary with respect to an angle f.Assigning a polar coordinate system based on f,lines were drawn on the deformed parts corresponding to5°increments,start-ing at the initial0°direction line that was marked on the

Table2

Optimal quantities of composite components

Material Amount by weight(g)a

Kenaf37.6

Polypropylene powder(PP)82.2

Epolene wax G-3015P(MAPP) 3.8

a Scale these quantities for every100,000mm3(6.11in.3)

volume.

Fig.3.Schematic of squeeze?ow test experimental setup[17]. 6M.Zampaloni et al./Composites:Part A xxx(2007)xxx–xxx

material samples prior to compression,and proceeding around to the full360°.The lengths of these lines,mea-sured from the original center point of the material to the edge of the compressed sample were recorded and plotted as shown in Fig.5.

For the kenaf?ber reinforced thermoplastics24samples were evaluated and averaged to determine the preferred ?ber orientations associated with these materials,Fig.5. To choose preferred?ber orientations,the goal is to evalu-ate all the data,and determine where there are noticeable peaks or valleys in the plot(Fig.5).The initial four angles chosen for the evaluation were40,170,250,and350°.In order to make sure these results were as accurate as possi-ble;a few of the outlying samples were removed due to a visual observation of?ber clumping during testing.These four preferred?ber orientations were used to initially char-acterize the material properties and will be used to popu-late the numerical model in future stages of this work.

3.2.Tensile and?exural testing

Tensile and?exural testing are static tests that were per-formed on the kenaf?ber reinforced composites,as well as on base polypropylene and Epolene G3015(PP/MAPP) sheets at room temperature.As determined in the previous section,the samples used for these tests have been cut out of the fabricated sheets at two of the speci?ed preferred?ber angles of40°and170°.All tests were performed on a UTS Machine,Model SFM20from United Calibration Corporation.Originally,sampling coupons were cut to a length of228.6mm(9in.)and a uniform width of 25.4mm(1in.)according to the guidelines of ASTM Stan-dard D3039.Trials with these samples proved to break near the grips,giving inconclusive results.It was then determined that the dog-bone tensile coupon shapes detailed in ASTM Standard D638would be needed in order to obtain accurate results.The test conditions for the tensile and?exural tests are illustrated in Table3while the results for each material type are summarized in Table4

Fig.4.Squeeze?ow test sample before(left)and after(right)compression.

Table3

Tensile and?exural test parameters

Tensile test to break Tensile test for g Flexural test

ASTM D638D638D790

Temperature30°C(86°F)30°C(86°F)30°C(86°F)

Strain recorder Laser extensometer Biaxial strain gage Strain gage

Load cell capacity 4.45kN(1000lb) 4.45kN(1000lb) 4.45kN(1000lb) Test speed 5.08mm(0.2in.)/min 5.08mm(0.2in.)/min 1.52mm(0.06in.)/min

Specimen dimensions Gage length–50.8mm(2in.),

width–12.7mm(0.5in.),

thickness–3.05mm(0.12in.)Gage length–50.8mm(2in.),

width–12.7mm(0.5in.),

thickness–3.05mm(0.12in.)

Span–50.8mm(2in.),

width–12.7mm(0.5in.),

thickness–3.05mm(0.12in.)

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In order to evaluate the e?ectiveness of the kenaf–PP composites,they have been compared to other natural?ber composites including hemp/PP,coir/PP,sisal/PP and?ax/ PP.Figs.6and7show a comparison of the tensile and?ex-ural strengths of both the30%and40%by weight kenaf–polypropylene composites fabricated for this study to other 40%by weight compression molded natural?ber–polypro-pylene composites.Data for the other material types was taken from existing literature where these composites were fabricated using polypropylene?lms,with natural?ber lay-ers randomly spread between them[6,13,15and18].From these?gures the bene?t of the kenaf?ber reinforced ther-moplastics is readily apparent.Through repeated testing both the30%and the40%kenaf polypropylene samples demonstrated equivalent tensile strengths.When compared against the other natural?ber reinforced polypropylene

Table4

Mechanical properties derived from tensile and?exural tests

Elastic modulus Poisson’s ratio Flexural modulus

PP/MAPP1,797MPa(260.67kpsi)––

Kenaf–PP/MAPP40°4,153MPa(602.41kpsi)0.353,632MPa(526.83kpsi) Kenaf–PP/MAPP170°4,835MPa(701.33kpsi)0.424,035MPa(585.29

kpsi)

8M.Zampaloni et al./Composites:Part A xxx(2007)xxx–xxx

systems both the30%and the40%kenaf polypropylene systems provide a tensile strength that is very similar to the?ax and hemp polypropylene systems while providing tensile strengths that are greater than either coir or sisal polypropylene systems.

When comparing the?exural strengths of the materials, Fig.7,the40%kenaf polypropylene samples performed signi?cantly better than the30%kenaf/PP samples.The 40%kenaf/PP was equivalent to the?ax/PP,larger than the hemp/PP and almost double the coir/PP and sisal/PP systems.The30%kenaf/PP showed results that were equiv-alent to the40%hemp/PP systems while also outperform-ing the coir/PP and sisal/PP systems.

With the elastic modulus data calculated from testing,it is also possible to compare the bene?ts of using this kenaf ?ber reinforced composite over other natural?bers as well as E-glass.Figs.8and9illustrate that these kenaf/PP com-posites have a higher Modulus/Cost and a higher

speci?c

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modulus than sisal,coir,hemp,?ax and E-glass.Sources of the density and cost data can be found in Table 6.These results demonstrate that the 40%and 30%by weight kenaf polypropylene composites that were compression molded compare favorably with the more commonly used hemp and ?ax ?ber polypropylene composites that are 40%?ber by weight.This reinforces the idea that compression molded kenaf polypropylene thermoplastics can provide an alternative that has less ?ber but similar performance,at a lower weight and potentially even a lower cost.3.3.Thermoforming of kenaf ?ber reinforced thermoplastics After fabrication and characterization,the kenaf–poly-propylene with maleated polypropylene (PP/MAPP)com-posites were tested for formability by using the stamp thermo-hydroforming process developed by Zampaloni et al.[17](please see [17]for details of the process).Fig.10illustrates a schematic of the stamp thermoforming process.In order to form usable parts out of the kenaf–polypropylene sheets,they must be heated to the forming

temperature of the polypropylene.At room temperature,thermoplastic materials are extremely brittle and will sim-ply fail when stamped.The forming temperature chosen for this work can be de?ned as the temperature at which the composite becomes malleable and easily shaped.For the kenaf polypropylene the forming temperature used was 190°C (374°F).

Originally,the sheets were heated to the forming tem-perature in a conventional oven for 20min,and then formed using a cool die.During these tests the ambient environment cooled the sheets below the forming tempera-ture before stamping could be executed thereby resulting in material shear and premature fracture during forming,as illustrated in Fig.11.In order to help maintain the elevated temperature needed for forming,the die was preheated to 165°C (329°F)prior to placing the heated sheet in the die.A variety of sheets were tested at a multitude of di?erent forming conditions using the above-mentioned techniques.Optimum forming parameters for the kenaf–polypropylene composites are listed in Table 5.Experiments were con-ducting using a hemispherical punch as illustrated in Fig.10,with results shown in Fig.12.The kenaf polypro-pylene sheets showed consistent formability even though each sheet was fabricated by hand thereby validating the fabrication process.Sheets were drawn to a depth of 50.8mm (2in.)without rupture or ?ber/matrix separation.All parts exhibited wrinkling behavior around the ?ange area as expected.An optimization of the process parameters would eliminate this behavior but is not necessary at this stage of the project.The goal of this stage of this on going project was to evaluate the e?ectiveness of the kenaf poly-propylene composite fabrication methods and to evaluate these materials as potential core materials for the fabrica-tion of sandwich structures.The results from

subsequent

Fig.11.Kenaf–PP/MAPP composite formed in ambient conditions.

Table 5

Thermoforming parameters Preform diameter Preform thickness Preform heating time Die heat temperature Draw depth Gap between dies 165.1mm (6.5in.)

3mm (0.12in.)

15min

165°C (329°F)

50.8mm (2in.)

19mm (0.75in.)

Table 6

Density of the glass ?bers and natural/bio-?bers Fiber Density (g/cm 3)Cost (kg à1)Ref.Flax 1.4–1.5$$0.40–$0.55[19–21]Hemp 1.48$$0.40–$0.55[19,21]Jute 1.3–1.45$$0.40–$0.55[19–21]Sisal 1.45$$0.40–$0.55[20,21]Ramie

1.50$$0.44–$0.55[21,22]Pineapple leaf 1.53$$0.40–$0.55[20]Cotton 1.5–1.6$$0.44–$0.55[22]Coir 1.15$$0.40–$0.55[23]Kenaf 1.4$$0.40–$0.55[23]Softwood 1.4$$0.44–$0.60[19]Hardwood 1.4$$0.44–$0.60[19]E-glass

2.5$US $2[20,24]S-glass

2.5

$US $2

[20,24]

Fig.10.Schematic of stamp thermoforming process [17].

M.Zampaloni et al./Composites:Part A xxx (2007)xxx–xxx

stages of the work,including the fabrication,characteriza-tion and stamping of the sandwich structures,the numerical modeling of the structures and the fabrication and charac-terization of the 40%by weight kenaf polypropylene ther-moplastics will be published in the near future.4.Conclusions

The research done in this study has proven the ability to successfully fabricate kenaf–polypropylene natural ?ber composites.The optimal fabrication method for the com-pression molding process has proven to be the layered sift-ing of a micro?ne polypropylene powder and chopped kenaf ?bers.A ?ber content of both 30%and 40%by weight has been proven to provide adequate reinforcement to increase the strength of the polypropylene powder.The use of the coupling agent,3%Epolene G3015,has enabled successful ?ber–matrix adhesion.

The kenaf–PP composites compression molded in this study provide both tensile and ?exural strength that is very similar to the 40%by weight ?ax and hemp polypropylene systems.In contrast the tensile strength is higher and the ?exural strength is almost doubled when compared against the coir and sisal systems.

Also shown through this work was that kenaf–PP com-posites have a higher Modulus/Cost and a higher speci?c modulus than sisal,coir,hemp,?ax and E-glass.This implies that the 30%by weight kenaf polypropylene com-posite that was compression molded compares favorably with the more commonly used hemp and ?ax ?ber polypro-

pylene composites that are 40%?ber by weight.In addi-tion,the kenaf–PP–MAPP composites fabricated in this study have a higher Modulus/Cost and a higher speci?c modulus than sisal,coir,and even E-glass.

Experiments with the in-house stamp thermoforming equipment have proven successful.It was found that the temperature of both the die and the preform must be ele-vated in order to prevent cooling and tearing during form-ing.The kenaf polypropylene sheets showed consistent formability even though each sheet was fabricated by hand,thereby validating the fabrication process.Acknowledgements

The ?nancial support from NSF Award DMI-0400296‘‘PREMISE-II:Design and engineering of ‘green’com-posites from bio?bers and bioplastics’’,is gratefully acknowledged.The authors also wish to express their appreciation to Flaxcraft Inc.,Cresskill,NJ,to Eastman Chemical Company,Kingsport,TN,and to Basell Poly-ole?ns,Elkton,MD,USA,for supplying the kenaf ?ber,Epolene G-3015wax,and polypropylene,respectively.References

[1]Hsiao SW,Kikuchi N.Numerical analysis and optimal design of

composite thermoforming https://www.360docs.net/doc/a310745422.html,put Meth Appl Mechan Eng 1999;177:1–34.

[2]Mohanty AK,Drzal LT,Misra M.Novel hybrid coupling agent as

an adhesion promoter in natural ?ber reinforced powder polypro-pylene composites.J Mater Sci Lett

2002;21:1885–8.

Fig.12.Thermoformed kenaf–MAPP composite sheets.

M.Zampaloni et al./Composites:Part A xxx (2007)xxx–xxx

[3]Mohanty AK,Misra M,Hinrichsen G.Bio?bres,biodegradable

polymers and biocomposites:An overview.Macromol Mater Eng 2000;276/277:1–24.

[4]McKenzie AW,Yuritta JP.Wood?bre reinforced polymers.Appita J

1979;32(6):460–5.

[5]Michell https://www.360docs.net/doc/a310745422.html,posites containing wood pulp?bres.Appita J

1986;39(3):223–9.

[6]Wambua P,Ivens J,Verpoest I.Natural?bres:can they replace glass

?bre reinforced plastics?Compos Sci Technol2003;63:1259–64. [7]Mohanty AK,Wibowo A,Misra M,Drzal LT.E?ect of process

engineering on the performance of natural?ber reinforced cellulose acetate https://www.360docs.net/doc/a310745422.html,pos:Part A2004;35:363–70.

[8]Nishino T,Hirao K,Kotera M,Nakamae K,Inagaki H.Kenaf

reinforced biodegradable https://www.360docs.net/doc/a310745422.html,pos Sci Technol 2003;63:1281–6.

[9]Mase T,Drzal LT,Jurek B.Development of a heat/pressure

formable wood?ber thermoplastic composite from recycled paper-board.In:ANTEC’02,proceedings of the annual technical confer-ence of the society of plastics engineers.2002.

[10]Bhattacharyya D,Bowis M,Jayaraman K.Thermoforming wood-

?bre-polypropylene composite https://www.360docs.net/doc/a310745422.html,pos Sci Technol2003;63: 353–65.

[11]George J,Sreekala MS,Thomas S.A review on interface modi?ca-

tion and characterization of natural?ber reinforced plastic compos-ites.Polym Eng Sci2001;41(9):1471–85.

[12]Feng D,Caul?eld DF,Sanadi AR.E?ect of compatibilizer on the

structure-property relationships of kenaf-?ber/polypropylene com-posites.Polym Compos2001;22(4):506–17.

[13]Mutje P,Girones J,Lopez A,Llop MF,Vilaseca F.Hemp strands:

PP composites by injection molding:e?ect of low cost physico-chemical treatments.J Reinf Plast Compos2006;25(3):313–27.[14]Keener TJ,Stuart RK,Brown TK.Maleated coupling agents for

natural?bre https://www.360docs.net/doc/a310745422.html,pos:Part A2004;35:357–67.

[15]Mohanty AK,Drzal LT,Misra M.Engineered natural?ber

reinforced polypropylene composites:in?uence of surface modi?ca-tions and novel powder impregnation processing.J Adhes Sci Technol2002;16(8):999–1015.

[16]Zampaloni M.A multi-preferred?ber orientation constitutive model

for?ber mat reinforced thermoplastics with a random orientation applied to the stamp thermo-hydroforming process.PhD Disserta-tion,Michigan State University,2003.

[17]Zampaloni M,Pourboghrat F,Yu WR.Stamp thermo-hydroform-

ing:a new method for processing?ber reinforced thermoplastic composite sheets.J Thermoplast Compos Mater2004;17(1):31–50.

[18]Van Den Oever MJA,Bos HL,Van Kemenade MJJM.In?uence of

the physical structure of?ax?bres on the mechanical properties of ?ax?bre reinforced polypropylene composites.Appl Compos Mater 2000;7(5–6):387–402.

[19]Lilholt H,Lawther JM.Natural organic?bers.In:Chou T-W,editor.

Comprehensive composite materials.Fiber reinforcements and gen-eral theory of composites,vol. 1.NewYork:Elsevier;2000.p.

303–25(Chapter1.10).

[20]Nabi Saheb D,Jog JP.Natural?ber polymer composites:a review.

Adv Polym Technol1999;18(4):351–63.

[21]Ugbolue SCO.Text Inst1990;20(4):1.

[22]Bledzki AK,Gassan J.Prog Polym Sci1999;24:221.

[23]Bledzki AK,Reihmane S,Gassan J.J Appl Polym Sci1996;59:1329.

[24]Chamis https://www.360docs.net/doc/a310745422.html,minated and reinforced metals.In:Grayson M,

editor.Encyclopedia of composite materials and components.New York,NY:John Wiley and Sons;1983.p.613.

12M.Zampaloni et al./Composites:Part A xxx(2007)xxx–xxx