XLIV savetovanje Srpskog hemijskog društva, 6. i 7. februar 2006. HTM-P01

89

Natural Fibers in Polymer Composite Materials

G. Bogoeva-Gaceva , A. Dekanski*, V. Panić*, D. Poleti**, A. Grozdanov, A. Bužarovska, M. Avella***, G. Gentile***

Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University

Ruđer Bošković 16, 1000 Skopje, R. Macedonia * Institute of Chemistry, Technology and Metallurgy – Department of Electrochemistry

University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia and Montenegro **Faculty of Technology and Metallurgy, University of Belgrade

Karnegijeva 4, 11000 Belgrade, Serbia and Montenegro ***Institute for Chemistry and Technology of Polymers (ICTP)-CNR

Via Campi Flegrei 34, 80078 Pozzuoli, Napoli, Italy

The growing global environmental concern, high rate of depletion of petroleum resources, as well as new environmental regulations have forced the search for new fiber reinforced composite materials that are compatible with the environment. Natural fibers represent an environmentally friendly alternatives to conventional reinforcing fibers, due to the following advantages: they are abundantly available renewable resources; they are nontoxic; natural fibers used in polymer composites can lead to materials with high specific strength because of their low density. The investigations of ECO-PCCM1 research project are focused on preparation and characterization of a new class of eco-friendly and cost-effective polymer composite materials, based on kenaf and other natural fibers and biodegradable (or recyclable) polymer matrix. In particular kenaf fibers have been selected for their peculiar characteristics: stress at break 350-600 MPa, strain at break 2.5-3.5 %, modulus 40-45 GPa, density 1500 kg m−3, cellulose content 75-90 %. Natural reinforcing fibers (NF) used in polymer composite materials can be modified by chemical and physical methods with aim to reduce moisture sensitivity and biological decay and to optimize properties of the fiber-matrix interface2. Chemical treatment of NF often causes defibrillization, which also contributes to the increased reinforcing efficiency of the fibers in the composite. This work reports the effects of several chemical treatments carried out on kenaf fibers, in order to modify their properties for application in thermoplastic-based composites. Conventional modification methods such as dewaxing, alkali treatment, cyanoethylation and esterification3 were adopted so to increase the interfacial bond strength with polymer matrix. The effects obtained after the modification have been analyzed by optical microscopy, SEM, FTIR and WAXS. It was shown that during the treatment of kenaf fibers under appropriate conditions, defibrilization, changes in the surface morphology and introduction of active chemical sites, as well as changes in the crystalline structure of cellulose proceed. Surface morphology of the fibers changes with gradual removal of non-cellulosic constituents by alkali treatment. FTIR bands due to lignin and hemicellulose and their shifts due to chemical treatment are discussed. Diffraction patterns of treated fibers are typical for cellulose I, although additional maxima at about 25° 2θ are also observed in some samples. Experimental Sample preparation Kenaf fibers are chemicaly treated by: Dewaxing - 1:2 mixture ethanol/benzene for 72 h at 50 °C; Vinyl mono-mer grafting – Acrylonitrile (CAN) grafting using 0,01 M Ce4+/ 0.1M HNO3; Alkali treatment - 10% NaOH for 1 h at 30 °C and Acetylation - Acetic anhydride for 0.5 h at 20 °C, Soxhlet extraction, oven dry.

Fourier Transform Infrared Spectroscopy – FTIR FTIR measurements are carried out by using ATR technique (Perkin Elmer-2000 FTIR Spectrometer). Samples are dried for 24 hours in vacuum before analysis.

Scanning Electron Microscopy - SEM The morphology of chemically treated and dried (12 hours in vacuum) kenaf fibers was examined by JEOL scanning electron microscope, model JSM-T20 (Uw = 20 kV).

Wide-angle X-ray scattering - WAXS Data are collected on a Philips PW 1510 diffractometer (U = 38 kV, I = 20 mA), with curved graphite monochromator, using Cu Kα radiation (λ = 1.5418 Å) and step-scan mode (2θ-range: 8−30o). Scan time and step were 5 s and 0,05º2θ, respectively. Before data collection, the samples (m ≈ 0.2 g) were slightly pressed

Corresponding author: gordana@tmf.ukim.edu.mk

HTM-P01 XLIV Meeting of the Serbian Chemical Society, Belgrade, February 6 – 7, 2006

90

(5 MPa) in pellets of 20 mm diameter. Crystallinity index (Cr.I.) was calculated according L. Segal4. Integral crystallinity was determined as described by Majdanac et al 5 after a full WAXS profile fitting. The patterns were modelled as a Gaussian-like amorphous halo plus a set of pseudo-Voight peaks.The approximate average crystallite size was calculated using Scherrer formula6. Full widths at half maximum (FWHM) obtained after profile fitting were corrected for instrumental broadening and Kα1-α2 doublet. Results Fourier Transform Infrared Spectroscopy – FTIR The chemical modification applied on natural fibers in order to improve compatibility with polymer matrix can also increase or decrease the strength of the fibers, inducing different structural changes. The removal of surface impurities is advantageous for fiber-matrix adhesion as it facilitates both mechanical interlocking and the bonding reaction at the interface. FTIR analysis of the fiber surface has provided additional information on the chemical changes caused by different treatments (Fig 1).

Fig. 1. FTIR spectra of treated kenaf fibres

The most significant changes are induced by acetylation and alkalization. The removal of hydroxyl groups by acetylation (as evidenced by disappearance of the strong absorption between 3200-3600 cm−1 caused by the OH groups of the fiber constituents as a result of esterification) will drastically change the polarity of the surface and consequently their compatibility (reactivity) with polymer matrix (polar or non-polar). The band in the spectrum of untreated fibers near 1740 cm-1 which is also present in the spectra of treated fibers, is assigned primarily to the C=O stretching vibration of the carboxyl and acetyl groups in the ‘xylan’ component of hemicelluloses and also to chemical groups of lignin. Further analysis is needed for assignation of the bands in the region 1000-1300 cm−1 of acetylated fiber. Disappearance of the band at 1740 cm−1 in the spectrum of alkali treated fibers is clearly seen, indicating changes in the fiber surface chemical composition. Scanning Electron Microscopy - SEM SEM images of kenaf fibres are shown in Fig. 2. Surface morphology of the fibers changes with gradual removal of non-cellulosic constituents (like lignin and hemicellulose). Alkalization of kenaf fibers has changed the surface topography of the fiber and their crystallographic structure (see Table 2). The untreated fibers appeared in separated bundles with relatively smooth surface (Fig. 2 A) while alkalized fibers have a rough surface with more separation of individual fibers Fig. 2 E). The most significant changes and deterioration of the structure are observed on acetylated fibers (Fig. 2 C) modified under non-optimized conditions, confirmed by WAX analysis as well.

XLIV savetovanje Srpskog hemijskog društva, 6. i 7. februar 2006. HTM-P01

91

A

B C

D

E

Fig. 2 SEM images of kenaf samples:

A – As received fiber B – ACN grafted fiber C – Acetylated fiber D – Dewaxed fiber E –Alkali treated fiber

Wide-angle X-ray scattering - WAXS WAXS patterns of examined Kenaf samples are shown in Fig. 3, and their structural characteristic are listed in Table 1. Diffraction pattern of as received kenaf sample is typical for cellulose I and it is very similar to the literature data7. The major structural characteristics of other samples also well correspond to the cellulose I. However, more or less pronounced additional maxima observed at about 25º 2θ usually are not visible in cellulose I samples. In acetylated sample the broad envelope centered at about 15.5º 2θ is shifted toward higher 2θ values and contains at least three, instead of usual two peaks. The most significant changes in crystalline structure are observed after alkali treatment of kenaf fiber which caused increase of crystallinity as well as in crystallite size.

Table 1. Structural characteristics of Kenaf samples

Crystallite size (Å) No. Sample Crystallinity index(%)

Integral Crystallinity(%) D(101) D(10-1) D(002)

1 As received 69 70 28 45 37 2 Dewaxed 73 75 30 49 39 3 ACN-grafted 72 75 28 47 40 4 Alkali treated 79 77 34 40 45 5 Acetylated 61 67 − − 41

10 15 20 25 30

5

4

3

2

1

Inte

nsity

(arb

. uni

ts)

2θ (o)

Fig. 3. WAXS patterns of different Kenaf samples.

HTM-P01 XLIV Meeting of the Serbian Chemical Society, Belgrade, February 6 – 7, 2006

92

Conclusions Experiments on surface chemical fiber modification have shown that commonly used treatment applied on cellulosic NF is efficient for kenaf as well. During the treatment of kenaf fibers under appropriate conditions, defibrilization, changes in the surface morphology and introduction of active chemical sites, as well as changes in the crystalline structure of cellulose proceed. Surface morphology of the fibers changes with gradual removal of non-cellulosic constituents by alkali treatment. Diffraction patterns of treated fibers are typical for cellulose I, although additional maxima at about 25° 2θ are also observed in some samples. Accknowledgement: This research is part of the European Commision FP-6 Project: ECO-PCCM, “Eco-houses based on Eco-friendly Polymer Composite Construction Materials”, FP6-2002-INCO-WBC-1, INCO-CT-2004-509185, http://elchem.ihtm.bg.ac.yu/ECO-PCCM References: 1. ECO-PCCM, “Eco-houses based on Eco-friendly Polymer Composite Construction Materials” FP6-2002-INCO-WBC-

1, INCO-CT-2004-509185 2. D. Feng, D.F. Caulfield, A.R. Sanadi, Polymer Composites, 22(4) (2001) 506 3. V. Tserki, N.E. Zaferopoulos, F. Simon, C. Panayiotou, Composites P.A, 36 (2005) 1110 4. L. Segal, Decrystallized Cotton, Watford Herst, Merrow Publishing, 1971 5. Lj. Majdanac, M. Teodorović, D. Poleti, Acta Polymerica, 42 (1991) 351-356 6. H. P. Klug, L.E. Alexander, X-ray diffraction procedures, 2nd ed., Wiley, New York, 1974, p. 687 7. P. M. Bonatti, C. Ferrari, B. Focher, C. Grippo, G. Torri & C. Cosentino, Euphytica, 140 (2004) 55–64

Prirodna vlakna u polimernim kompozitnim materijalima

Globalni trend zaštite životne sredine, uključujući i pozitivne zakonske propise, ali i velika brzina trošenja naftnih rezervi u svetu, pokrenuli su veliki broj istraživanja u cilju pronalaženja novih kompozitnih materijala ojačanih vlaknima, koji će istovremeno biti ekološki pogodni. Prirodna vlakna predstavljaju veoma prihvatljivu alternativu konvencionalnim vlaknima, jer su netoksična, predstavljaju obnovljiv i široko dostupan materijal, a zbog svoje male gustine, u polimernim kompozitima mogu dati materijale velike specifične jačine. Istraživanja u okviru ECO-PCCM projekta1 usmerena su na izradu i karakterizaciju nove vrste ekološko prihvatljivih i jeftinih polimernih kompozitnih materijala, zasnovanih na kenafu i drugim prirodnim vlaknima i biodegradabilnim (ili ponovo upotrebljivim) polimernim matricama. Posebna pažnja je posvećena kenafu zbog njegovih osobina: jačina istezanja 350-600 MPa, prekidno izduženje 2,5-3,5 %, modul elastičnosti 40-45 GPa, gustina 1500 kgm-3 i sadržaj celuloze 75-90 %. Prirodna vlakna koja se koriste u polimernim kompozitnim materijalima mogu se hemijski i fizički modifikovati s ciljem da se smanji njihova osetljivost na vlagu i biološke uticaje (truljenje), odnosno da se optimizuju osobine međufaze vlakno-matrica2. Hemijski tretman prirodnih vlakana dovodi do njihove defibrilacije, što opet doprinosi armirajućim osobinama vlakana u kompozitu. U ovom radu prikazani su rezultati različitih hemijskih tretmana vlakana kenafa s ciljem da se modifikuju njihove osobine za primenu u kompozitima sa termoplastičnom matricom. Primenjeni klasični postupci modifikacije, kao što su odmašćivanje, alkalni tretman, esterifikacija i cijanoetiliranje3 imali su za cilj da povećaju jačinu veze na međupovršini vlakno-polimerna matrica. Efekti dobijeni različitim tretmanima su analizirani optičkom mikroskopijom, skenirajućom elektronskom mikroskopijom – SEM, infracrvenom spektroskopijom sa analizom signala Fourier transformacijom – FTIR i širokougaonom difrakcijom X-zraka – WAXS. Pokazano je da ovakvim tretmanima dolazi do defibrilacije vlakana kenafa, promena u površinskoj morfologiji vlakana, uticaja na hemijski aktivna mesta na površini, kao i do promena u kristalnoj strukturi celuloze. U radu su diskutovane ligninske i hemicelulozne trake na FTIR spektrima, kao i njihova pomeranja zbog hemijskog tretmana. Difrakcioni spektri tretiranih vlakana su tipični za celulozu I, ali su registrovani i dodatni maksimumi na oko 25° 2θ na nekim uzorcima.