J O U R NA L O F M AT E R I A L S S C I E N C E L E T T E R S 1 6 ( 1 9 9 7 ) 4 6 2 – 4 6 4

Effect of water absorption on dimensional stability and impact energy

of jute fibre reinforced polypropylene

A. C. KARMAKERInstitute of Materials Science, University of Connecticut, Storrs, CT 06269, USA

Low cost and high specific stiffness have made jutefibres a potential alternative to man-made fibres, likeglass and aramid [1, 2]. Recent studies have renewedthe long-standing interest in jute as a reinforcingfibre for thermoplastic materials [3, 4]. Beinghydrophilic, jute fibre does not adhere well tohydrophobic matrix polymers such as polyethyleneand polypropylene. When a single jute fibre isembedded in the polypropylene matrix, the thermalshrinkage of polypropylene creates a gap surround-ing the fibre. The maximum width of such a gap wasfound to be 700 nm [5]. On the other hand, theaverage increase in diameter of jute fibres caused byswelling in water amounts to 3.6 µm [6], whichleads to an increase of interfacial shear strengthbetween fibre and matrix [7]. The purpose of thestudy reported here was to investigate whether suchan increase in interfacial shear strength can alsoimprove the mechanical properties of bulk compo-sites from jute fibre and polypropylene.

Short jute fibres (average length 2 mm) were hand-mixed with commercially available polypropylene(Fortilene 1602, Solvay Polymers, Deer Park, TX) atfive different volume fractions of fibre (7, 14, 21, 28and 35 vol %) with or without coupling agent. Thecoupling agent was a maleic anhydride graftedpolypropylene (MAPP; G-3002, Eastman Kodak,Kingsport, TN), which was added by 3 wt % of thefibre content. The hand-mixed materials were com-pounded by a high intensity thermokinetic mixerknown as K-mixer. American Society for Testing andMaterials (ASTM) standard test specimens wereprepared with a 33 t reciprocating screw injectionmoulder at a flat temperature profile of 188 8C. Adetailed description of sample preparation has beenreported elsewhere [8]. Unnotched Izod impactenergy was determined according to ASTM D256-90b [9], by using an impact tester from BaldwinLocomotive Works (Washington, DC). Water absorp-tion tests were carried out according to ASTMD570-81 [10] on the samples used for impact tests,after 1 and 14 days of water immersion. Toinvestigate the effect of water absorption on impactenergy, the samples were immersed in water for 14days at room temperature. After the immersion time,the samples were removed from water, and excesswater was removed from the sample surface withtissue paper. In all cases an average value was takenfrom five specimens.

Water absorption of jute=polypropylene compositeincreased with the increase in fibre volume fraction

(Fig. 1). When the samples were immersed in waterfor 1 day, water absorption increased linearly withthe increasing fibre content, and was not influencedby the coupling agent. On the other hand, when thesamples were exposed to water for a longer time(14 days), the coupling agent reduced water absorp-tion, especially at higher fibre contents. Thicknessswelling increased linearly with the fibre content,when the samples were immersed in water for 1 day(Fig. 2). After 14 days of immersion, however, thethickness swelling was reduced by the couplingagent, especially at higher fibre content. The resultsindicate that both the water absorption and thethickness swelling of the jute=polypropylene com-posite are mainly dependent on the fibre content.

The impact energy decreased nonlinearly withincrease in fibre volume fraction (Fig. 3). In all casesthe slope of the curves decreased gradually with theincrease in fibre content. When coupling agent was

0261-8028 # 1997 Chapman & Hall

7 14 21 28 35Fibre content (vol%)

Thi

ckne

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Figure 2 Thickness swelling of jute fibre=polypropylene compositecaused by water treatment at different fibre volume fractions, with andwithout coupling agent. (n), 1 day without MAPP; (u), 1 day withMAPP; (s), 14 days without MAPP; (3), 14 days with MAPP.

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Figure 1 Water absorption of jute fibre=polypropylene compositeprepared at different fibre volume fractions, with and without couplingagent. (n), 1 day without MAPP; (u), 1 day with MAPP; (s), 14 dayswithout MAPP; (3), 14 days with MAPP.

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added, the impact energy against fibre content curveslevelled off at the fibre volume fraction of 21%,indicating that further increases in fibre content didnot have much influence on impact energy. It couldalso be observed that the impact energy wasimproved by water absorption, independent ofwhether coupling agent was added.

A jute fibre is a bundle of short cellulose fibres,which are bonded with each other by substancessuch as lignin, pectin etc. Each cellulose fibre ishollow and contains a lumen at its centre (Fig. 4).Consequently, there are three main regions where theabsorbed water in the jute fibre=polypropylenecomposite sample can reside: the lumen (Fig. 4),the gap between fibre and polypropylene (Fig. 5),and the cell wall. In the system without couplingagent, water can access all three regions. When thecoupling agent (MAPP) was added the gap surround-ing the fibre, caused by thermal shrinkage of poly-propylene, disappeared because of good fibre=matrix interfacial adhesion (Fig. 6). The goodadhesion was supposed to be promoted by theesterification between the anhydride groups ofMAPP and the hydroxyl groups of jute fibres[3, 11]. While the coupling agent diminished theinterfacial gap between fibre and matrix, the waterabsorption was controlled mainly by the lumen and the cell wall. The lumens were not filled by the

matrix polymer, because the high viscosity poly-propylene melt could not penetrate into the jutefibres.

Fig. 7 shows the ratio of impact energy after(impact-wet) and before (impact-dry) water treat-ment. In the case of coupling agent, the ratio ofimpact-wet=impact-dry increased slightly with theincrease in fibre content, and there was almost alinear relation between the ratio and the fibrecontent. When there was no coupling agent, theratio was found to be almost constant up to the fibrecontent of 21 vol %. By further increase of fibre con-tent, the ratio of impact-wet=impact-dry increasednonlinearly. The results indicate that although thecoupling agent improves the overall impact energy ofthe jute/polypropylene composite system, the effectof water treatment on the impact energy is higherwhen there is no coupling agent.

7 14 21 28 35Fibre content (vol%)

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Figure 3 Effect of water absorption (after 14 days) on unnotchedimpact energy of jute fibre=polypropylene composite prepared atvarious fibre volume fractions, with and without coupling agent. (u),without MAPP=dry; (n), without MAPP=wet; (s), with MAPP=dry;(3), with MAPP=wet.

Figure 4 Scanning electron microscopy micrograph of the fracturesurface of a jute fibre (after tensile test), showing the lumen in theindividual cellulose fibre.

Figure 5 Scanning electron microscopy micrograph of the fracturesurface of the jute fibre=polypropylene composite (after tensile test),showing the gap between the fibre and the polypropylene. Withoutcoupling agent.

Figure 6 Scanning electron microscopy micrograph of the fracturesurface of the jute fibre=polypropylene composite (after tensile test),showing good wetting between the fibre and the polypropylene. Withcoupling agent.

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It can be concluded that water absorption resultedin dimensional changes in jute=polypropylene com-posites. The Izod impact energy was improved bythe coupling agent. When the jute=polypropylenecomposite samples were immersed in water, the Izodimpact energy was increased independent of additionof coupling agent. However, the effect of water onIzod impact energy was higher when no couplingagent was added.

AcknowledgementsThe experimental work was performed during mystay at the Forest Products Laboratory (FPL) in

Madison, WI. I wish to thank John Youngquist andhis group for providing me with laboratory facilities.The valuable comments of Craig Clemons during thepreparation of this manuscript are also highlyacknowledged.

References1. A . C . K A R M A K E R and G . H I N R I C H S E N, Polym. Plast.

Technol. Engng 30 (1991) 609.2. J. G . K I S H O R E and R . M . V. G . K . R AO, J. Reinf. Plast.

Compos. 5 (1986) 141.3. A . C . K A R M A K E R and J. P. S C H N E I D E R , J. Mater. Sci.

Lett. 15 (1996) 201.4. J. P. S C H N E I D E R and A . C . K A R M A K E R, in Proceed-

ings SPE=ANTEC, Boston, MA, May (1995) p. 2086.5. A . H O F F M A N N, MS thesis, Technical University of Berlin

(1993).6. A . C . K A R M A K E R and G . H I N R I C H S E N, J. Text. Inst.

85 (1994) 288.7. A . C . K A R M A K E R , A . H O F F M A N N and G . H I N -

R I C H S E N, J. Appl. Polym. Sci. 54 (1994) 1803.8. A . C . K A R M A K E R and J. YO U N G Q U I S T, ibid., 62

(1996) 1147.9. A M E R I C A N S O C I E T Y F O R T E S T I N G A N D M AT E R I -

A L S , ‘‘Annual book of ASTM standards’’, ASTM D256-90,Vol. 8.01 (American Society for Testing and Materials,Philadelphia, Pennsylvania, 1991).

10. Idem, ‘‘Annual book of ASTM standards’’, ASTM D570-81,Vol. 8.01 (American Society for Testing and Materials,Philadelphia, Pennsylvania, 1988).

11. G . - S . H A N, H . I C H I N O S E , S . TA K A S E and N. S H I -R A I S H I , Mokuzai Gakkaishi. 35 (1989) 1100.

Received 6 Augustand accepted 26 November 1996

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Figure 7 Ratio of impact energy of jute fibre=polypropylene compo-site after (impact-wet) and before (impact-dry) water treatment againstfibre volume fraction. Samples were immersed for 14 days in water.(n), without MAPP; (s), with MAPP.

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