Thermoset Phenolic Matrices Reinforced with Unmodified and Surface

Biomacromolecules 2005, 6, 2485-2496

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Thermoset Phenolic Matrices Reinforced with Unmodified and Surface-Grafted Furfuryl Alcohol Sugar Cane Bagasse and Curaua Fibers: Properties of Fibers and Composites W. G. Trindade,†,‡ W. Hoareau,‡ J. D Megiatto,† I. A. T. Razera,† A. Castellan,*,‡ and E. Frollini*,† Instituto de Quı´mica de Saõ Carlos, Universidade de Saõ Paulo, USP, C.P. 780, CEP 13560-970, Saõ Carlos, SP, Brazil, and Universite´ Bordeaux 1, Laboratoire de Chimie des Substances Ve´ ge´ tales EA-494, F-33405 Talence Cedex, France Received February 24, 2005; Revised Manuscript Received June 12, 2005

Composites based on phenolic matrices and unmodified and chemically modified sugar cane bagasse and curaua fibers were prepared. The fibers were oxidized by chlorine dioxide, mainly phenolic syringyl and guaiacyl units of the lignin polymer, followed by grafting furfuryl alcohol (FA), which is a chemical obtained from a renewable source. The fibers were widely characterized by chemical composition analysis, crystallinity, UV-vis diffuse reflectance spectroscopy, SEM, DSC, TG, tensile strength, and 13C CP-MAS NMR. The composites were analyzed by SEM, impact strength, and DMA. The SEM images and DMA results showed that the oxidation of sugar cane bagasse fibers followed by reaction with FA favored the fiber/matrix interaction at the interface. The same chemical modification was less effective for curaua fibers, probably due to its lower lignin content, since the reaction considered touches mainly the lignin moiety. The tensile strength results obtained showed that the fibers were partially degraded by the chemical treatment, decreasing then the impact strength of the composites reinforced with them. In the continuity of the present project, efforts has been addressed to the optimization of fiber surface modification, looking for reagents preferably obtained from renewable resources and for chemical modifications that intensify the fiber/matrix interaction without loss of mechanical properties. Introduction In the 20th century, the extraordinary growth of the application of synthetic plastics limited the application of vegetal fibers, because of the advantages of synthetic polymers concerning dimensional stability and plasticity, among other factors. The large availability of vegetal fibers all over the world and their low cost, along with the intrinsic properties of these materials, have led to the search for alternative applications for these fibers besides the traditional uses such as textile, paper production, and fuel.1,2 The present and urgent need to develop and commercialize composite materials based on constituents derived from renewable sources has had a great impact on the drive to reduce the dependence on nonrenewable materials derived from fossil sources, both from environmental and economic viewpoints.3,4 CO2 emission associated with anthropogenic activities has reached levels that, among other things, have made the interest in natural fibers, which had been considered in the first decades of the 20th century, reappear.5-9 Several enterprises have started to use composites reinforced with vegetal fibers, as, for example, those in the automotive industry. Natural fibers are very efficient in sound * Authors for correspondence. E-mail: [email protected] (A.C.); [email protected] (E.F.). † Universidade de Sa ˜ o Paulo. ‡ Universite ´ Bordeaux 1.

absorption, and in comparison to fiberglass, they are resistant to breakage into shards, have a lower cost, are lighter and biodegradable, and can be obtained using 80% less energy.4 One of the aspects that must be considered in the discussion of composites reinforced with vegetal fibers concerns fiber supply regularity, as in many countries, wood is the main source for fiber supply. Annual plants have the disadvantage of seasonal cropping over wood as fiber suppliers, which raises the need for subsequent cleaning, drying, storage, and other processes.10 In contrast, while a tree takes years to grow, annual plants have a full cycle in 12 months. The use of fibers for applications more sophisticated than the conventional ones requires process systematization, from the plantation to the storage of the fibers extracted. Highly demanding markets, such as the automotive industry, have led the required modifications. Probably, with the market growth, vegetal fibers with specific applications will be grown in areas distinct from those aimed at common applications,11 which in turn can decrease or even eliminate the disadvantage previously mentioned. With this in mind, fibers with short development cycles such as sisal,2 jute,12 sugar cane bagasse,13,14 and curaua2 have been considered as composite reinforcements. The present work deals with the two last fibers. In the mixture of two components of diverse chemical natures of any dimension or shape, the larger the contact area (interfaces) among them, the greater the possibility of

10.1021/bm058006+ CCC: $30.25 © 2005 American Chemical Society Published on Web 07/30/2005

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an interaction of physical, chemical, or physical-chemical nature between the two components.15 In all cases, the interaction between the dispersed phase and the matrix phase depends not only on the extension of the contact area, but also on the affinity between the components. The affinity can be intensified, for instance, either by physical or chemical treatments applied to the surface of the fibers.2 An important point to consider is that the reagents used in chemical modifications cannot be too expensive, and ideally, the modifications must involve a minimal number of compounds obtained from nonrenewable sources. Recently, a new selective chemical modification of the surface of lignocellulosic natural fibers has been considered. This modification is based on the selective oxidation of guaiacyl and syringyl units of lignin, generating ortho- and paraquinones able to react by Diels-Alder reaction with furfuryl alcohol,14 that is commercially prepared by reduction of furfural, which in turn is obtained from agricultural residues. To better understand this process at the molecular level, a similar chemical modification was carried out with the lignin obtained by acidolysis from sugar cane bagasse and curaua,16 which were oxidized by ClO2 and reacted with furfuryl alcohol. Similar reactions on sugar cane bagasse and curaua lignins are described in the following sections. In the field of composites reinforced with natural fibers, it can be considered that most works deal with the preparation and characterization of thermoplastic matrix composites. Comparatively few works have been made about thermoset matrices, despite their large application, and among those, even fewer deal with phenolic thermoset matrices. The use of vegetal fibers as reinforcing agents of phenolic thermosets reduces production costs, as the resin is substituted by fibers, and improves the properties of the thermoset, such as impact strength. In the present work, phenolic thermoset matrices, reinforced with unmodified and modified sugar cane bagasse and curaua fibers, were prepared and characterized. Experimental Section Fiber Characterization. Humidity content was determined according to ABNT NBR9656 (Associaç aõ Brasileira de Normas Tećnicas, the Brazilian association for technical standards), which consists of determining the percentage difference between the initial weight of the sample (1.00 g) and after 4 h drying at 105 °C. Ash content was measured by considering the percent difference between the initial weight of dried fiber of the sample and that after calcination for 4 h at 800 °C. Klason lignin content was evaluated following the TAPPI T13M-54 method, which is based on isolation of lignin after polysaccharide hydrolysis by concentrated sulfuric acid (72%). The samples (1.0 mg, previously dried) were macerated using 72% H2SO4 (15 mL) at room temperature and kept under these conditions during 24 h. After that, the material impregnated with sulfuric acid was transferred to an appropriate vessel, and 560 mL of distilled water was added. The system was then kept under reflux for 4 h. The remaining solid (insoluble lignin and ash) was filtered (sintered glass funnel, no. 4, previously weighted), dried (105

Trindade et al.

°C, 24 h), and then weighed. The acid-insoluble lignin content was calculated by considering the weight difference between the initial sample (fibers) and that of the filtered solid and subtracting the ash content from the weight of this last sample. The Klason-soluble lignin was determined by ultraviolet spectroscopy, considering both the solution obtained after filtering off the insoluble lignin and a reference consisting of a sulfuric acid solution with the same acid final concentration of the soluble lignin solution (TAPPI T13M54). Absorbance values were measured at 280 (A280) and 215 (A215) nm. The concentration (C, g/L) of Klason-soluble lignin was determined using the following equation, based on the Lambert Beer law: C (g/L) )

4.53 × A215 - A260 300

The holocellulose content was determined according to TAPPI T19m-54, which consists of a selective degradation of the lignin polymer. Sodium hypochlorite (2.5 g) and glacial acetic acid (1 mL) were added to an aqueous suspension (120 mL put in an Erlenmeyer flask) of previously dried and milled vegetal fibers (3 g). The system was covered with an inverted Erlenmeyer flask and kept at 70 °C, under magnetic stirring for 1 h. This sequence was repeated twice, that is, sodium hypochlorite (2 × 2.5 g) and glacial acetic acid (2 × 1 mL) were again added and the system kept under magnetic stirring for 1 h (2 × 1 h). Then, after 3 h (3 × 1 h), the system was cooled to nearly 5 °C, and the hollocellulose was filtered and washed exhaustively with water and methanol and then dried at 40 °C, in a vacuum oven, until constant weight. For R-cellulose content determination, sodium hydroxide solution (10 mL, 17.5%) was added to cellulose (1.0 g) at room temperature. Then, the mixture was triturated for 8 min, and sodium hydroxide (10 mL, 17.5%) was added to the mixture, which remained at rest for 20 min. Then, water (40 mL) was added, and the solid residue was filtered and washed exhaustively with aqueous acetic acid and water. This remaining solid, considered R-cellulose, was dried at 40 °C, in a vacuum oven, until constant weight.17 If the alkaline solution is kept at room temperature longer than 20 min, as usually described for wood, cellulose can also degrade, leading to erroneous results for hemicellulose content. An average on three samples was done for all mentioned analyses. The hemicellulose content was obtained by subtracting the R-cellulose part from the holocellulose content. The crystallinity index, Ic, was determined by X-ray diffraction using a RIGAKU Rotaflex model RU-200B diffractometer operating at 40 kV, 20 mA, and λ (Cu KR) ) 1540 Å. The crystallinity index was calculated using the Buschle-Diller and Zeronian equation: Ic ) 1 - I1/I2,18 where I1 is the intensity at the minimum of the crystalline peak (18° < 2θ < 19°) and I2 is the intensity at its maximum (22° < 2θ < 23°).18 Scanning electron microscopy (SEM) was carried out in a Zeiss-Leica apparatus model 440, electron acceleration 20 kV. The samples analyzed were covered with a thin layer of gold in a sputter-coat system.

Composites of Phenolic Resin and Sugar Cane or Curaua

Tensile strength was analyzed by considering fiber bundles 15 mm long and nearly 0.5 mm diameter. A dynamic mechanical analyzer (DMA) model 2980, from TA Instruments, was used under the following conditions: 25 °C, 1 N/min to 18 N. A minimum of twenty samples was tested for each material, and average values are reported in the next section. Differential scanning calorimetry (DSC) analyses were carried out using a Shimadzu DSC model 50. Samples of approximately 6.5-7.6 mg were placed in appropriate sealed pans and heated from 20 to 600 °C, at 20 °C/min, under N2 atmosphere (10 mL/min). Thermogravimetric (TG) analyses were carried out using a Shimadzu model TGA-50TA. Samples of approximately 7.5 mg were placed in appropriate pans and heated from 20 to 800 °C at 10 °C/min, under N2 atmosphere (10 mL/min). The UV-vis diffuse reflectance spectra were recorded on a Perkin-Elmer Lambda 18 spectrometer equipped with an integrating sphere (Labsphere, 150 mm). Solid-state 13C CP-MAS (cross polymerization-magic angle spinning) NMR spectra of unmodified and modified sugar cane bagasse were performed at room temperature on a Bruker DPX-400 NMR spectrometer (Bruker), using MAS rates of 4 and 8 kHz, at a frequency of 100.61 MHz. Samples were packed in MAS 4 mm diameter zircon rotors. Chemical shifts were relative to tetramethylsilane (TMS) used as an external standard. The acquisition time for all spectra was set at 16 h (30 000 scans). Oxidation of Bagasse and Curaua Fibers. 1. Oxidation of Bagasse Fibers. The sugar cane fibers (kindly provided by Mr. Bernard Siegmund, CIRAD, Reunion Island, France) and curaua (kindly given by Prof. Alcides Leaõ, Depto Ciencias Ambientais, UNESP, Botucatu, SP, Brazil) were extracted (Soxhlet) with cyclohexane (Labsynth, Brazil, 99%) //ethanol (Labsynth, Brazil, 99%), 1:1 v/v, for 48 h and then with water for 24 h. The fibers were dried in an air-circulated oven (60 °C) until constant weight. The fiber extraction with cyclohexane/ethanol allows removal of waxes from the fiber surface, preventing weakening of the interactions at the interface fiber matrix of these compounds. Oxidation of fibers (2 g) was performed with a ClO2-water solution (18 mL, 1.88 mmol), prepared according to a procedure described by Mark et al.,19 and acetic acid (0.5 mL) at 55 °C. After reaction, the fibers, which turned yellow-red, were washed with water until neutrality. 2. Reaction of Oxidized Bagasse and Curaua with Furfuryl Alcohol. The oxidized fibers (2 g), impregnated with furfuryl alcohol (Sigma-Aldrich, PA) (FA, 11.35 g), were heated at 100 °C for 4 h. The excess of FA was removed by Soxhlet extraction using ethanol for 16 h. Then, the fibers were dried 24 h at 45 °C, and weight gains due to reaction were determined on the basis of original and final oven-dried fiber weights. They are reported as weight percent gain (WPG). Prepolymer Synthesis. Phenolic prepolymer was synthesized by mixing phenol (Labsynth, Brazil, 99%), formaldehyde (Labsynth, Brazil, 37%), and potassium hydroxide (Labsynth, Brazil, 85%), 1.38:1.00:0.06%, respectively, under mechanical stirring, at 70 °C, for 1 h. Then, the solution

Biomacromolecules, Vol. 6, No. 5, 2005 2487 Table 1. Properties of Sugar Cane Bagasse and Curaua Fibers component (%)

sugar cane bagasse

curaua

humidity ash holocellulose cellulose hemicelluloses klason lignin crystallinity

9.5 1.1 72.1 55.2 16.8 25.3 47

7.9 0.9 83.5 73.6 9.9 7.5 67

was cooled to room temperature, water was eliminated under reduced pressure, and the mixture was neutralized with concentrated hydrochloric acid (Labsynth, Brazil, 36.5%). Cure Reaction and Composite Preparation. Thermoset materials were obtained by mixing the prepolymer with resorcinol (Labsynth, Brazil), the curing accelerator (10:1, w/w) through mechanical stirring at 50 °C for 30 min. The compression molding was carried out in a mold (220 × 99.5 × 5 mm3) under pressure. The molding cure cycle (75 °C/1 h/2.5 ton, 85 °C/2 h/5.0 ton, 95 °C/30 min/7.5 ton, 105 °C/ 30 min/7.5 ton, 115 °C/1 h/10.0 ton, 125 °C/1.5 h/10.0 ton) was previously determined by DSC measurements.20 Composites reinforced with bagasse or curaua fibers (chemically modified or unmodified) were obtained by adding the fibers (18 g) to the prepolymers (102 g), the mixture being submitted to mechanical stirring (30 min, 50 °C) in order to get an optimum impregnation of the lignocellulosic materials in the prepolymer. The curing procedure was analogous to the one described for thermoset preparation (vide infra). Composites were prepared with randomly oriented fibers (near 15 mm length). Composite Characterization. For the Izod impact test, 10 unnotched samples were cut from each plate and shaped according to ASTM D256 (63.5 × 12.7 × 4.0 mm3). Impact strength was assessed using an Izod impact testor (CEAST Resil 25). Impact tests were carried out at room temperature with an impact speed of 4 m/s and incident energy of 2.75 J. In each experiment, at least 6 measurements were used for average calculation. SEM, using the same conditions described for fibers, was used to characterize the fractured samples. Dynamic mechanical thermoanalysis (DMA or DMTA) was carried out using a thermal analyzer, TA Instruments model 2980, operating in the cantilevered horizontal measuring system. It was used in flexural mode to evaluate the specimens. The experimental conditions used to analyze all samples were as follows: oscillation amplitude of 10 µm, 1 Hz frequency, heating rate of 3 °C/min, and temperature range 30-225 °C. Before starting every experiment, the equipment was stabilized at 30 °C for 5 min. Results and Discussion Table 1 shows fiber composition data, which are in agreement with other data for these lignocellulosic materials.2,13 To further understand the chemical modification at the molecular level of the lignins present in the inner part of

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Figure 1. UV-vis diffuse reflectance spectra of fibers expressed as log(1/R). (-) Unmodified fibers; (-) ClO2 oxidized fibers; (- - -) ClO2 oxidized fibers and treated with FA modified fibers. Left: sugar cane bagasse fibers. Right: curaua fibers. Table 2. Quantification of Several Hydroxyl Groups (mmol/g lignin) in Curaua (CLig) and Bagasse (BLig) Isolated Lignins from 31P NMR Analysis of Their Phosphitylated Derivatives (see ref 16) CLig CLigoxa CLigox-FAb BLig BLigoxa BLigox-FAa aliphatic S-OH G-OH H-OH 5-condensed total phenol acids a

1.47 0.38 0.18 0.18 0.08 0.82 0.13

0.99 0.09 0.07 0.05 0.02 0.23 0.19

1.33 0.04 0.07 0.07 0.01 0.20 0.01

2.35 0.06 0.32 0.48 0.02 0.88 0.06

1.18 0.05 0.13 0.06 0.02 0.26 0.16

0.57 0.02 0.07 0.04 0.01 0.14 0.07

Lignin oxidized. b Lignin oxidized and modified with FA.

the sugar cane and curaua fibers, lignin was extracted from the two vegetal sources considered in this work and submitted to the same modifications afterward. The lignins were reacted with chlorine dioxide to oxidize the phenolic units of this macromolecule and then treated with furfuryl alcohol. Weight percent gains of 14% and 10% were obtained for sugar cane bagasse and curaua, respectively.16 1H NMR of oxidized lignins (figures not shown) indicated the diminution of the aromatic and methoxy regions after ClO2 oxidation, due to the partial degradation of the macromolecule.16 Probably, this kind of degradation of lignin was one of the factors that reduced some mechanical properties of fibers, when they were submitted to the same reaction, and thus of composites reinforced with the chemically modified lignocellulosic fibers, as will be discussed later. To quantify the different hydroxyl groups present in the lignin samples, curaua and bagasse, a 31P NMR study of the phosphitylated polymers was carried out.16 Quantification of the different hydroxyl groups is presented in Table 2. The total phenol and aliphatic hydroxyl groups for bagasse and curaua lignins after oxidation reaction are lower. ClO2 is known to react by one-electron oxidation with phenol to give phenoxy radicals, followed by chlorine dioxide addition to the latter.21 This process conduces to quinones and muconic derivatives, mainly esters, in accordance with the lower content of phenol and acid groups observed (Table 2). The low content of aliphatic hydroxyl groups might be due to degradation of the propane chain, after chain displacement from carbon 1 of the aromatic ring in the radical process. After modification with furfuryl alcohol, the values are also lower. This is indicative of the involvement of the hydroxyl group of furfuryl alcohol in the reaction. The

decrease of hydroxyl groups in the lignins after chemical reactions (oxidation and grafting furfuryl alcohol) is tentatively confirmed by acetylation of the polymers. After the acetylation procedure, oxidized and chemically modified lignins were found not soluble in tetrahydrofuran, because of a lack of acetoxy groups necessary to render the polymers soluble in this specific solvent.16 Once the reaction with ClO2 and FA was studied for sugar cane and curaua lignins, the related fibers were submitted to the same treatment. The WPG of sugar cane bagasse was 18%, and the WPG of curaua was 7%. Sugar cane bagasse has a higher amount of lignin in its composition compared with curaua, and this reflects on the WPG, because the chemical modification of lignocellulosic fiber considered in the present work mainly involves this macromolecule. Observation of the red color displayed by the fibers of sugar cane bagasse and curaua fibers after oxidation with ClO2 is in accordance with the expected formation of orthoquinones. Although UV-vis diffuse reflectance spectra of the fibers measured before and after oxidation (Figure 1) are not quantitative, they show a maximum near 455 nm; this value and the shape of the curve are indicative of the formation of guaiacyl and syringyl ortho-quinones.14 Also, it is likely that p-quinones, absorbing in the near-UV region, are produced from the p-hydroxybenzyl alcohol structural elements of lignin. The UV-vis diffuse reflectance spectra of sugar cane fibers (Figure 1, left) and curaua (Figure 1, right), ClO2 oxidized and treated with FA, show intense absorption in the visible region. This is in accordance with the formation of complex quinonoid structures, as expected from Diels-Alder reactions between ortho-quinones and conjugated dienes.22 The content of lignin in sugar cane bagasse fiber changed from 25.3% (untreated fiber, Table 1) to 40.9% for the fiber oxidized and then treated with FA. Concerning curaua fibers, the content of lignins changed from 7.5% (untreated fiber, Table 1) to 11.7% after FA treatment. Probably for both fibers, besides lignin, the FA polymer formed at the surface also remained insoluble during the analysis applied to quantify lignin, with the weight of FA polymer then added to that of lignin. The crystallinity of sugar cane bagasse changed from 47% to 52% after oxidation, probably because of the partial extraction of lignin, present in the amorphous region. The

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Table 3. Resonance Assignments of 13C CPMAS Spectrum of Sugar Cane Bagasse Fibers (refs 23-25) peak chemical shift number (ppm) 1 2

Figure 2. Schematic representation of the structures of lignin, cellulose, and poly(furfuryl alcohol).

3 4 5 6 7 8 9 10 11 12 13 14

173 168 153 150-142 140-122 120-100 105 88 83 75 73 64 56 22

assigmentsa hemicelluloses: -COOsR and CH3COOlignin: Cγ in ArsCHdCHCOOR and LigOsArCHdCHCOOH lignin: S3(e), S5(e) lignin: S3(ne), S5(ne), G3, G4 lignin: S4, S1, G1, H2, H6 lignin: S2, S6, G2, G6 carbohydrates: C1 carbohydrates: C4 lignin: Cβ. carbohydrates: C4 lignin: CR. carbohydrates: C2, C3, C5 carbohydrates: C2, C3, C5 carbohydrates: C6 lignin: OCH3 hemicelluloses: CH3COO-

a Abbreviations: S, syringyl; G, guaiacyl; H, p-hydroxyphenyl; ne, nonetherified; e, etherified.

Figure 3. 100-MHz CP-MAS 13 solid-state NMR spectra of bagasse fibers (A), chemically modified bagasse fibers (oxidized by ClO2 and treated with FA) (B), difference spectra (C) ) (B) - (A).

reaction of oxidized fiber with FA decreased the crystallinity to 48%, which can indicate that the cellulose domain was affected in the considered reaction conditions. A similar behavior was observed for curaua fibers, although with changes occurring in a smaller scale than those in sugar cane: The crystallinity changed from 67% (untreated fiber, Table 1) to 70% (oxidized fiber) and 64% (oxidized and then reacted with FA). To confirm the occurrence of FA polymerization on the surface of fibers, sugar cane bagasse was analyzed by solidstate 13C CP-MAS NMR. Only sugar cane was considered for analysis because of its higher content of FA polymer on the surface, in order to obtain spectra with adequate intensity peaks. Part of the structures of lignin, cellulose, and poly(furfuryl alcohol), with the numbers and symbols assigned to the atoms, are showed in Figure 2. The spectrum presented in Figure 3(A) shows characteristic signals of lignocellulosic fibers23 and more specifically of sugar cane bagasse fibers.24,25 Main resonance assignments are given in Table 3. The spectrum displays a signal at 22 ppm, assigned to the methyl carbon of the acetyl group in hemicelluloses. The region between 60 and 110 ppm is dominated by strong signals, which are assigned mostly to the different carbons of cellulose, namely C1 (105 ppm), C4 crystalline (88 ppm), C4 amorphous (83 ppm), C2, 3, and 5 (75-73 ppm), and C6 (64 ppm). Hemicelluloses also give signals in

this region. These signals overlap those due to different aliphatic carbons of lignin. In this region, the only signal that can be assigned to lignin is resonance peak 13 at 56 ppm due to methoxy groups of aromatic moieties (Table 3). The region between 110 and 160 ppm is specific to the aromatic carbon of lignin. At 173 ppm appears a signal due to carbonyls of acetoxy and other ester groups in hemicelluloses. The peak at 168 ppm originates from esterified p-coumaric acid or from the Cγ acidic carbon in etherified ferulic acids. The p-hydroxyphenyl (H) residues show a resonance signal at 128 ppm due to C2 and C6 carbons (Figure 3A and Table 3) The chemical modification of bagasse fibers with chlorine dioxide and FA affected mainly the lignin polymer component, as is show in spectrum (B) (Figure 3). The carbohydrate signals remain unaffected, whereas signal 13 at 55.8 ppm, assigned to lignin methoxy groups, disappeared because of the formation of quinones after the oxidation reaction with ClO2. Also, the lignin region between 110 and 160 ppm is changed to a large extent. The difference spectra, (C) ) (B) - (A), Figure 3, shows new resonance peaks at 153, 108, and 30 ppm due to the grafting of FA on the lignin polymer. The chemical shifts of these peaks are in accordance with those found in linear poly(furfuryl alcohol) and might be assigned as follows: 153 ppm (C2), 108 ppm (C4 and C5), and 30 ppm (CR).26-28 These observations are in accordance with covalent grafting of the furfuryl molecular framework on the lignin polymer, via the oxidation of the phenolic lignin groups into quinones and/or muconic acids. The tensile strength and elongation of lignocellulosic fibers were evaluated, and the results are shown in Table 4. The maximum difference between each measurement and the average tensile strength and elongation (Table 4) was nearly 25%, corresponding to the value of sugar cane bagasse oxidized with ClO2 fiber. The data of Table 4 show that the oxidation of fibers with ClO2 strongly reduces the tensile strength of both curaua and sugar cane bagasse. On the other hand, the reaction with

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Figure 4. SEM images of (a) unmodified and (b) modified curaua and (c) unmodified and (d) modified sugar cane bagasse fibers. Table 4. Tensile Strength and Elongation (%) for Unmodified and Modified Sugar Cane and Curaua Fibers

fiber unmodified sugar cane bagasse oxidized sugar cane bagasse oxidized sugar cane bagasse reacted with FA unmodified curaua oxidized curaua oxidized curaua reacted with FA

tensile strength elongation (mpa) (%) 222 126 238 636 218 565

1.1 0.7 0.9 0.8 0.5 1.2

FA seems to protect the fiber, probably because of the polymeric layer introduced on the surface of fiber, as can be inferred by the increase in tensile strength when the oxidized fibers react with FA (Table 4). Figure 4 shows SEM images of unmodified and modified fibers By comparing the SEM images shown in Figure 4, it is possible to see that the chemical modification introduced a coating at the surface of the lignocellulosic fiber, which is more evident for sugar cane bagasse because of the higher content of FA polymer introduced, as has already been mentioned. The unmodified and modified fibers were characterized in relation also to thermal stability (TG and DSC analysis, Figures 5 and 6). Studies show that the thermal decomposition of lignocellulosic fibers is not necessarily an additive function resulting from the contribution of each fraction of its components, that is, cellulose, hemicellulose, and lignin, because of the interactions between these fractions.29,30 In general, the thermolysis reaction of polysaccharides (cellulose and hemicellulose) occurs by the cleavage of glycoside bonds, C-H, C-O, and C-C bonds, as well as by dehydration, decarboxylation, and decarbonylation. Con-

sidering the mixture arising from the degradation of cellulose, levoglucosan, produced by transglycosylation intramolecular reactions,31 is the most abundant product.32 Around 600 °C, there can be carbonization of levoglucosan with the release of water.30,31 One of the degradation mechanisms of lignin to be considered occurs through dehydration, yielding derivatives with lateral unsaturated chains and the release of water. Yet, carbon monoxide, carbon dioxide, and methane are also formed.32 The decomposition of aromatic rings occurs above 400 °C.33 Continued burning leads to the saturation of the aromatic rings, the rupture of C-C bonds present in lignin, the release of water, CO2, and CO, and structural rearrangements.29 Considering the three main components of the lignocellulosic material, lignin is the component that presents thermal degradation at a larger temperature range.30,34 The DSC and first-derivative TG curves (Figures 5 and 6) obtained show peaks related to the decomposition processes mentioned already; however, the TG curves show in a more clear way the decomposition of polysaccharides (near 300 °C) and lignin (near 480 °C), the main components of sugar cane bagasse fibers. As the content of polysaccharides is considerably higher than that of lignin, the firstderivative TG peak related to the decomposition of polysaccharides is more intense than that of lignin. It must be pointed out that when end use of some material is considered, the temperature of the beginning of the degradation is the limiting factor concerning thermal stability. In the case of the fibers used in the present work, that temperature is near 280 °C (Figure 5). The profiles of the TG curves are similar for samples oxidized only and for those oxidized and then reacted with FA. The DSC curve of this last one probably incorporates several events in the large exothermic peak that appears from



Figure 5. (a) TG curves of sugar cane bagasse, oxidized with ClO2 and modified with furfuryl alcohol. (b) TG curves of sugar cane bagasse oxidized with ClO2, N2 atmosphere, 10 mL/min, 20 °C/min.

Figure 6. (a) DSC curve of sugar cane bagasse oxidized with ClO2 and modified with furfuryl alcohol, (b) DSC curve of sugar cane bagasse oxidized with ClO2, (c) DSC curve of curaua oxidized with ClO2 and modified with furfuryl alcohol. N2 atmosphere, 10 mL/min, 20 °C/min.

300 °C. It must be pointed out that the chemical modification occurs mainly at the surface, and in the thermal analyses, the material as a whole is analyzed. The content of FA grafted on the surface is low, and besides, its thermal decomposition probably occurred in the same temperature range of the components of fibers, because exothermic peaks near 325 °C are found for poly(furfuryl alcohol).28 Curaua fibers presented similar TG and DSC curves (figures not shown). Table 5 gives the impact strength results obtained for composites reinforced with sugar cane bagasse and curaua fibers, both unmodified and chemically modified. Cellulose content and fiber crystallinity are important factors for the mechanical properties of fibers. The larger cellulose content and the larger proportion of crystalline regions of the curaua fiber in relation to those of the sugar cane bagasse fiber (Table 1) have an effect on the impact strength of composites reinforced with curaua fibers, as their

Table 5. Izod (unnotched) Impact Strength of Phenolic Composites Reinforced with Unmodified and Chemically Modified Sugar Cane Bagasse and Curaua Fibers and Their Respective Standard Deviations (%) fiber

composite impact strength (J/m) Unmodified 28 ( 7 88 ( 19

sugarcane curaua Oxidized ClO2 sugarcane curaua

15 ( 2 71 ( 7

sugarcane curaua

After Reaction with FA 17 ( 2 39 ( 1

phenolic composites are considerably more resistant than those reinforced with sugar cane bagasse (Table 5). The data given in Table 5 confirm the data on tensile strength; that is, in the conditions of the present work, ClO2

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Figure 7. SEM images of the impact fracture surface of sugar cane and curaua reinforced phenolic composites. (a) Bagasse unmodified (×5000); (b) bagasse oxidized with ClO2 (×5000); (c) bagasse oxidized with ClO2 and modified with FA (×5000); (d) bagasse oxidized with ClO2 and modified with FA (×300); (e) curaua oxidized with ClO2 and modified with FA (×500; pull-out mechanism occurrence indicated by arrows).

oxidation led to fiber degradation, which in the case of influence on impact strength of curaua composites, was intensified by the later reaction with furfuryl alcohol, as the properties of curaua fibers decreased noticeably after these modifications. In contrast, electronic scanning (SEM) images of the composites reinforced with sugar cane bagasse (see Figure 7 and compare a with b and d) show that adhesion was significantly intensified in the fiber/matrix interface by these treatments. With relation to curaua, the SEM images

of composites reinforced with modified fibers also showed regions with good adhesion at the interface, although it is also possible to detect regions in which the pullout mechanism occurred, as a consequence of poor adhesion (Figure 7e). Probably, the chemical modification the surface suffered facilitated the interaction with the phenolic prepolymer by increasing its interdiffusion in the lignocellulosic network, leading to a more intense fiber/matrix interaction during the



Figure 8. Storage modulus (E′) for phenolic thermoset (PT) and composites (PC) reinforced with (a) sugar cane bagasse and (b) curaua fibers; (c) PT submitted to post cure (175 °C, 1 h).

fiber cure step, and consequently also facilitating the establishment of physical interactions and/or possibly of fiber/matrix chemical bonds. Although the SEM images showed, at least for sugar cane bagasse fibers, that the fiber/matrix interactions were improved because of the modifications of fiber surfaces, the impact strengths of the composites reinforced with both modified curaua and sugar cane bagasse are lower than those of composites reinforced with unmodified fibers (Table 5). These results indicate that the intensification of interactions at the interface was overshadowed by the worsening of the mechanical properties of the fibers caused by the chemical treatment applied on their surfaces. The composites were also characterized by DMA, a technique not extensively used for vegetal fiber-reinforced composites, in comparison to synthetic-fiber reinforced composites.35 However, studies on the dynamical mechanical properties of these vegetal fiber-reinforced composites are of great importance, considering that these materials can suffer dynamical stressing during service.36 Considering the three parameters obtained from this analysis, that is, the storage and loss moduli (E′ and E′′, respectively) and tan δ, it is possible to get information on molecular mobility.37 When the polymer under consideration corresponds to a thermoset, the mobility is related mainly to the segments located among the cross-linkages. Besides, when the thermoset is part of a composite material, other

features must be considered. The dynamic mechanical properties of a composite are determined by the properties of its components, the system morphology and nature of the interface between the two components. The layer of the matrix that circles the fibers, that is, the layer immediately posterior to the interface, can have different properties from those of the remaining material. Figure 8 shows that the storage modulus (E′) of sugar cane bagasse reinforced composites is lower than that of the thermoset and decreases when the fibers are modified, in the whole temperature interval considered. On the contrary, composites reinforced with curaua have higher (modified fibers) or nearly the same (unmodified fiber) E′ as thermoset (Figure 8) Relating to the lower E′ values of composites reinforced with sugar cane bagasse when compared with phenolic thermoset (Figure 8a), this is probably mainly caused by the poor mechanical properties of fiber, which lead to a less rigid material compared to the thermoset. Besides, differences between the coefficients of thermal expansion of the fiber and of the polymer can be another factor that can explain this decrease in E′. Because of this difference, stress in the matrix can appear, and as a consequence, the modulus becomes lower than the value obtained in the absence of stresses. The same behavior was observed in a previous work for composites reinforced with unmodified sugar cane bagasse.13

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Figure 9. Loss modulus (E′′) and tan δ for phenolic thermoset (PT) and sugar cane bagasse reinforced phenolic composites (PC).

Figure 10. Loss modulus (E′′) and tan δ for phenolic thermoset (PT) and curaua fiber-reinforced phenolic composites (PC).

Figure 11. Loss modulus for sugar cane-reinforced composite: (a) unmodified, (b) oxidized and modified with FA.

Curaua is stiffer than sugar cane fiber, mainly because of both its higher cellulose content and crystallinity than bagasse (Table 1). As a consequence, the storage modulus of curauareinforced composites is higher than that of thermoset, because of the incorporation of a high-modulus fiber into the polymer, which contributes to the overall properties. (Figure 8b). The DMA study for both composites revealed still other interesting features. The tan δ and E′′ values of the composites reinforced with modified sugar cane fibers were lower than those of the composites reinforced with unmodified fibers (Figure 9).

The tan δ value can be taken as an indication of the intensity of interactions at the fiber/matrix interface, because the interactions are stronger and the mobility of polymer segments is lower at the interface, and then, the damping is reduced. Yet, an improved composite interface leads to more efficient transference of stress between the fiber and the matrix and lower energy dissipation and thus E′′.38 In this way, both values, tan δ and E′′, seem to indicate that the chemical modification of sugar cane bagasse increased the interface interaction. On the other hand, E′ is lower for the composite reinforced with modified fiber than that for the composite reinforced with unmodified sugar cane bagasse,


as was already mentioned. The storage modulus (E′) reflects mainly the stiffness of the material, and as the treatment weakened the fiber, the material as a whole became less rigid. Figure 10 shows that tan δ and E′′ values of composites reinforced with chemically modified curaua fibers are higher than those of composites reinforced with unmodified fibers. Considering the previous discussion for sugar cane bagasse composite, it seems that the treatment of curaua fiber leads to a less intense interaction with the matrix, thus increasing tan δ and E′′ values. As was already mentioned, the sites of the fiber that reacted with ClO2 and FA are mainly located in the lignin portion, and bagasse has a higher content of lignin than curaua (Table 1). Besides, curaua has a morphology different from that of bagasse (see Figure 7), and the chemical modification can change the surface in a way that decreases the interactions with the matrix at the interface. The E′′ curves for both composites (Figures 9 and 10) show two peaks, near 125 and 200 °C, the first one probably related to the movement of the segments located among the cross linkages, that is related to the glass transition (Tg). The interactions at the interface seem to be different for each fiber (curaua and bagasse) and even for unmodified and modified fibers, as was mentioned, and at first, this can lead to a displacement in Tg peaks for higher temperatures as the interactions increase. However, in the present work, the shifts in Tg peaks were not significant (Figures 9 and 10). The peak at 200 °C is probably related to a residual cure that occurs during the scanning, because in some storage modulus curves (Figure 8a,b), the E′ value increases between 150 and 200 °C, which is an indications of postcure, which in turn leads to a more rigid material and then to a second transition as is observed in loss modulus curves near 200 °C. Figure 8c shows both the E′curves for phenolic thermoset before and after postcure. It can be observed that the increase in E′ values between 150 and 200 °C practically disappeared after the sample was subjected to 175 °C for 1 h, indicating that the main process leading to the observed increase in storage modulus was the residual cure. Figure 8c shows also that the postcure leads to a stiffer material, with higher storage modulus than the previous one, in all the temperature intervals considered. It can be pointed out that not necessarily a fully cured matrix must be obtained, because it can lead to a more brittle material, as was already observed in previous works.2,20 As the postcure occurs at higher temperatures than those normally related to the end use of this kind of material, this process does not limit the application of them. The peak around 50 °C that appears in some curves probably is related to the mobility of some more free segments. The E′′ peaks near 125 and 200 °C are enlarged as a consequence of fiber treatment, as can be observed in Figure 10 for composites reinforced with curaua and in Figure 11, where the curves for composites reinforced with sugar cane both unmodified and oxidized and modified with FA appear in a scale more suitable than in Figure 9 to observe the differences mentioned. The treatment introduces different sites at the fiber surface and then diversifies the kind of interactions that can occur at the fiber/matrix interface. The enlargement of E′′ curves then reflects the increase in heterogeneity of segments, as they are involved in fiber


interactions with still more varied intensity compared with the unmodified fibers. Conclusions A new process was set for chemically modifying sugar cane bagasse and curaua fibers. A quite specific oxidation by chlorine dioxide of syringyl and guaiacyl phenols of the lignin polymer creating quinones was obtained. The latter were reacted with furfuryl alcohol, creating a coating around the fiber more compatible to phenolic resins, to prepare composites. This modification favored the fiber/matrix interaction at the interface for sugar cane and curaua fiberreinforced composites, but caused some fiber degradation that affected their mechanical properties and decreased the mechanical resistance of the composites reinforced with them. It must be pointed out that the chemical modification process of the fibers was conducted in water with matter coming from natural resources such as nonwood fibers and furfuryl alcohol. On a molecular basis, UV-vis diffuse reflectance spectroscopy has pointed out the formation of ortho-quinones, whereas CP-MAS 13C NMR have shown disappearance of methoxy groups in lignin phenyl units with strong chemical modification of the aromatic ring. Concomitantly, peaks of furfuryl alcohol polymer were observed. As this is an ongoing project, it has been sought to optimize fiber surface modification in order to intensify fiber/matrix interaction without the loss of fiber mechanical properties. Curaua and sugar cane bagasse fibers presented different behaviors in relation to similar treatment and worked with different intensities as reinforcing agents of the same matrix. These results indicated that is important to find optimal conditions for each lignocellulosic fiber individually, with respect to its action as a reinforcing agent. Although the composites reinforced with curaua fibers showed higher impact strength than those reinforced with sugar cane bagasse, at first both of them can be used in some nonstructural applications, as, for instance, inner trim parts in automotive applications. Acknowledgment. The authors are grateful to CAPES for research fellowship for W.G.T. and I.A.T.R. and to CAPES/COFECUB (project 422/03) for traveling missions between Brazil and France. E.F. is grateful to CNPq (National Research Council, Brazil) for research productivity fellowships and to FAPESP (The State of Saõ Paulo Research Foundation, Brazil) for a research fellowship for J.D.M. and for financial support. W.H. is very grateful to Conseil Re´gional de la Reúnion (France) and to Fond Social Europeén (FSE) for a doctoral grant and A.C. is very indebted to B. Siegmund (CIRAD) for his participation in the sugar cane bagasse project. References and Notes (1) Young, R. A. Utilization of Natural Fibers: Characterization, Modification and Applications. In Lignocellulosic-Plastics Composites; Leaõ, A. L, Frollini, E., Carvalho, F. X., Eds.; UNESP-USP, Saõ Carlos, SP, Brazil, 1997; p 1. (2) Frollini, E.; Paiva, J. M. F.; Trindade, W. G.; Razera, I. A. T.; Tita, S. P. Lignophenolic and Phenolic Resins as matrix in vegetal fibers reinforced composites. In Natural Fibers, Polymers and Composites - Recent Advances; Wallenberger, F., Weston, N., Eds.; Kluwer Academic Publishers: New York, 2004; p 193.

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(3) Joshi, S. V.; Drzal, L. T.; Mohanty, A. K.; Arora, S. Composites, Part A 2004, 35, 371. (4) Mohanty, A. K.; Wibowo, A.; Misra, M.; Drzal, L. T. Composites, Part A 2004, 35, 363. (5) Wambua, P.; Ivens, J.; Verpoest, I. Compos. Sci. Technol. 2003, 63, 1259. (6) Pervaiz, M.; Sain, M. M. Resour. ConserV. Recycl. 2003, 39, 325. (7) Keener, T. J.; Stuart, R. K.; Brown, T. K. Composites, Part A 2004, 35, 357. (8) Maffezzoli, A.; Calo`, E.; Zurlo, S.; Mele, G.; Tarzia, A.; Stifani, C. Compos. Sci. Technol. 2004, 64, 839. (9) Thielemans, W.; Wool, R. P. Composites, Part A 2004, 35, 327. (10) Rowell, R. M.; Sanadi, A. R.; Caulfield, D. F.; Jacobson, R. E. Utilization of Natural Fibers in Plastic Composites: Problems and Opportunities. In Lignocellulosics-Plastics Composites; Leaõ, A. L., Carvalho, F. X., Frollini, E., Eds.; UNESP-USP, Saõ Carlos, SP, Brazil, 1997; p 23. (11) Benatti, R., Jr. Natural fibers in Brazil-Utilization, Perspective and Processing for Sisal, Ramie, Jute, Kenaf and Allied Fibers for Composites Production. In Lignocellulosics-Plastics Composites; Leaõ, A. L., Carvalho, F. X., Frollini, E., Eds.; UNESP-USP, Saõ Carlos, SP, Brazil, 1997; p 343. (12) Razera, I. A. T.; Frollini, E. J. Appl. Polym. Sci. 2004, 91, 1077. (13) Paiva, J. M. F.; Frollini, E. J. Appl. Polym. Sci. 2002, 83, 880. (14) Trindade, W. G.; Hoareau, W.; Razera, I. A. T.; Ruggiero, R.; Frollini, E.; Castellan, A. Macromol. Mater. Eng. 2004, 289, 728. (15) Yosomiya, R. In Adhesion and Bonding in Composites; Marcel Dekker, Inc.: New York, 1990; p 1. (16) Hoareau, W.; Trindade, W. G.; Siegmund, B.; Castellan, A.; Frollini, E. Polym. Degrad. Stab. 2004, 86, 567. (17) Browning, B. L. In Methods of Wood Chemistry; Wiley: New York, 1967; Vol. 2, p 499. (18) Buschle-Diller, G.; Zeronian, S. H. J. Appl. Polym Sci. 1992, 45, 967. (19) Mark, H. F.; Othmer, D. F.; Overberger, C. G.; Seaborg, G. T. Castor oil to chlorosulfuric acid. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley and Sons: New York, 1993; Vol. 5, p 981.

Trindade et al. (20) Paiva, J. M. F.; Trindade, W. G.; Frollini, E.; Pardini, L. C. In Natural Polymers and Composites IV; Frollini, E., Leaõ, A. L., Mattoso, L. H. C., Eds.; USP-UNESP-EMBRAPA, Saõ Carlos, Brazil, 2002; p 528. (21) Ni, Y.; Van Heiningen, A. R. P. J. Wood Chem. Technol. 1994, 14, 243. (22) Hergert, H. L. In Lignins, Occurrence, Formation, Structure, and Reactions; Sarkanen, K. V., Ludvig, C. H., Eds.; Wiley-Interscience: New York, 1971; p 165. (23) Bardet, M.; Foray, M. F.; Tran, Q. K. Anal. Chem. 2002, 74, 4386. (24) Sun, J. X.; Sun, X. F.; Sun, R. C.; Fowler, P.; Baird, M. S. J. Agric. Food Chem. 2003, 51, 6719. (25) Sun, X. F.; Sun, R. C.; Sun, J. X. Bioresour. Technol. 2004, 95, 343. (26) Choura, M.; Belgacem, N. M.; Gandini, A. Macromolecules 1996, 29, 2839. (27) Principe, M.; Ortiz, P.; Martinez, R. Polym. Int. 1999, 48, 637. (28) Gonzales, R.; Figueroa, J. M.; Gonzales, H. Eur. Polym. J. 2002, 38, 287. (29) Pe`rez, M. G.; Chaala, A.; Yang, J.; Roy, C. Fuel 2001, 80, 1245. (30) Paiva, J. M. F.; Trindade, W. G.; Frollini, E.; Pardini, L. C. Polym. Plast. Technol. Eng. 2004, 43, 1179. (31) Scheirs, J.; Camino, G.; Tumiatti, W. Eur. Polym. J. 2001, 37, 933. (32) Meier, D.; Faix, O. Bioresour. Technol. 1999, 68: 71. (33) Rohella, R. S.; Sahoo, N.; Paul, S. C.; Choudhury, S.; Chakravortty, V. Thermochim. Acta 1996, 287, 131. (34) Orfao, J. J. M.; Antunes, F. J. A.; Figueiredo, J. L. Fuel 1999, 78, 349. (35) Ray, D.; Sarkar, B. K.; Das, S.; Rana, A. K. Compos. Sci. Technol, 2002, 62, 2002, 911. (36) Joseph, P. V.; Mathew, G.; Joseph, K.; Groeninckx, G.; Thomas, S. Composites, Part A 2003, 34, 275. (37) Pothan, L. A.; Thomas, S. Compos. Sci. Technol. 2003, 63, 2003, 1231. (38) Keusch, S.; Haessler, R. Composites, Part A 1999, 30, 997.

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