Effect of surface modification of bamboo cellulose fibers on

Composites: Part B 51 (2013) 28–34

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Composites: Part B

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Effect of surface modification of bamboo cellulose fibers on mechanicalproperties of cellulose/epoxy composites

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.02.031

⇑ Corresponding author.E-mail address: [email protected] (Z. Zhou).

Tingju Lu a, Man Jiang a, Zhongguo Jiang b, David Hui c, Zeyong Wang a, Zuowan Zhou a,⇑a Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University,Chengdu 610031, PR Chinab School of Light and Chemical Engineering, Xichang College, Xichang, Sichuan 610500, PR Chinac Department of Mechanical Engineering, University of New Orleans, LA 70148, USA

a r t i c l e i n f o

Article history:Received 14 December 2012Accepted 25 February 2013Available online 7 March 2013

Keywords:A. FibersA. Thermosetting resinB. Mechanical propertiesB. Interface

a b s t r a c t

Bamboo cellulose fibers were treated with NaOH aqueous solution and silane coupling agent, respec-tively, before they were applied into epoxy composites. The effect of surface modification on mechanicalproperties was evaluated by tensile and impact tests under controlled conditions. Compared with theuntreated cellulose filled epoxy composites, the NaOH solution treatment increased the tensile strengthby 34% and elongation at break by 31%. While silane coupling agent treatment produced 71% enhance-ment in tensile strength and 53% increase in elongation at break. The scanning electron microscopy(SEM) was used to observe the surface feature of the cellulose fibers and the tensile fractures as wellas cryo-fractures of the composites. The Fourier transform infrared (FTIR) was employed to analyze thechemical structure of the cellulose fibers before and after modifications. The results indicated differentmechanisms for the two modifications of cellulose. The NaOH solution partly dissolved the lignin andamorphous cellulose, which resulting in splitting the fibers into smaller size. This led to easier permeat-ing into the gaps of the fibers for epoxy resin (EP) oligmer and forming effective interfacial adhesion.Based on the emergence of Si–O–C and Si–O–Si on the cellulose surface, it was concluded that theenhancement of mechanical properties after coupling agent modification could be ascribed to the forma-tion of chemical bonds between the cellulose and the epoxy coupled with the coupling agent.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The growing environmental awareness and increasing scarcityof natural resources have provoked a growing demand for environ-ment-friendly materials, with the desire to lower the cost of tradi-tional synthetic fiber reinforced composites [1,2]. Natural fiberreinforced green composites, virginly initiated in the automotiveindustry, have regained great research interest in recent years be-cause of its cheap price and low energy consumption [1,3]. Naturalfibers exhibit many advantages over their synthetic counterparts(e.g. carbon, glass and aramid), such as light weight, low cost, easyprocessing, excellent specific strength and high specific modulus,and especially, their renewability and recyclability [1,3]. Thus nat-ural fibers are long regarded as being promising candidates forreplacing conventional synthetic reinforcing fibers in compositesfor semi-structural and structural applications [4].

Cellulose, one of the most abundant natural fibers on the earth,has been studied for years in varied fields. Till now, numerousresearchers have reported advances in cellulose fibers/polymer

composites. The cellulose can act as reinforcement of both thermo-plastic and thermosetting polymers, as well as template for func-tional composites [5–18]. On the basis of earlier reports, bamboois a very good resource of cellulose fibers because of its high con-tent of cellulose and relatively small microfibrillar angle [19].The bamboo is also very abundant and cheap in China, especiallyin Sichuan province. Epoxy resin (EP), with excellent comprehen-sive performance and a wide range of applications, has been themost researched thermosetting matrix of cellulose reinforced com-posites. The cellulose fibers were proved to have good effect onimproving certain properties of the EP matrix. Fracture toughnessand impact behavior were reported to have significant increaseafter adding the cellulose fibers, while flexural strength andYoung’s Module hardly increased [20–23]. The strength improve-ment was unsatisfied because of the poor compatibility betweenthe cellulose and the matrix, which limited application of the cel-lulose/EP composites.

Thus the poor compatibility between the cellulose fiber and thematrix is a problem we must overcome to get composites with highproperties. The strong polarity of cellulose and high dense hydrogenbonds between molecules and intra-molecules in its structure leadto poor accessibility of matrix. Therefore, the interfacial tension

T. Lu et al. / Composites: Part B 51 (2013) 28–34 29

between the cellulose fiber and the matrix is quite high, which leadsto the peeling off the fibers, and increasing of the porosity in thecomposites [24,25]. To improve the compatibility, physical, chemi-cal or other modifications of the cellulose are very necessary[26,27]. In the blending system of natural fiber and polymer matrix,alkali treatment, acetylation, cyanoethylation and coupling agenttreatment have been included in the modification methods. Bledzkiet al. reported that alkali and coupling agent treatments of cellulosefiber had relatively better effects on the mechanical properties ofcellulose/PP composites [28], but no mechanisms or explanationswere mentioned. Based on the earlier reports, the APS and GPS (alsonamed KH560 in China) could be the silanes used for the natural fi-ber/polymer composites [29]. We chose KH560 to modify the bam-boo cellulose because the glycidoxy organofunctionalities in KH560make the reaction between the fiber and the silane milder and lowerenergy consumption.

In this work, the bamboo cellulose fibers were pretreated withsilane coupling agent and NaOH aqueous solution respectively.The effects of the modification of cellulose fibers on mechanicalproperties of cellulose/epoxy composites were comparatively stud-ied, and the distinct mechanisms of the two modifications wereput forward. The materials were very cheap and the experimentswere simple and low energy consumption, but the results werepositive.

Table 1Different composites in the experiments.

Cellulose content (wt%)

10% 20% 30%

Untreated BCF10/EP BCF20/EP BCF30/EPNaOH treated N-BCF10/EP N-BCF20/EP N-BCF30/EPKH560 treated K-BCF10/EP K-BCF20/EP K-BCF30/EP

2. Experimental part

2.1. Materials

The bamboo cellulose fibers (BCFs) used in the experimentswere smashed bamboo fibers pulp procured from Sichuan AnxianPaper Co., Ltd. The bamboo fiber pulp was dried for 4 h at 105 �Cin the electric blast oven (Model 101-1A) after cut into smallblocks, then crushed the cellulose blocks for 3 min with a high-speed universal grinder (FW-400A, 26,000 r/min), kept the cellu-lose powder under sealing after drying again.

The epoxy-E44, acetone, and the curing agent-T31 were com-mercial products that were procured from Chengdu ShunmeiComposite Materials Co., Ltd. The silane coupling agent (commercialcode: KH560), NaOH and ethanol were of chemically purity and pro-vided by Chengdu Kelong Chemical Reagent Factory. Deionizedwater was laboratory-made.

2.2. The treatment of the bamboo cellulose

The alkali treatment of BCF was carried out in a conical flaskwith 2 wt% NaOH aqueous solution and 10 g of cellulose powder.The container was placed in a magnetic stirrer for 4 h at 60 �C untilthe reaction was completed. The NaOH treated BCFs were filtratedand washed several times to neutral before dried at 80 �C.

The silane coupling agent treatment of cellulose was treatingBCF for 3 h at room temperature with 3% of KH560 solution. TheKH560 solution was mixed by 2.5 ml KH560, 77.5 ml deionizedwater and 120 ml anhydrous ethanol, stirred continuously for 1 hto fully hydrolyze. Next, the BCFs were soaked in the solution for3 h, and then washed and dried at 100 �C in an electric oven.

2.3. Preparation of test samples of epoxy and composites

The EP was prepared by casting method at room temperature.The initial EP was heated at 60 �C for 5 min to a better fluidity, fol-lowed by adding the curing agent, then stirred and poured into themold and cured for 24 h at room temperature. The BCF/EP compos-ites were prepared by adding BCFs to the diluted EP oligmer, andcured after adding curing agent and stirring. Composites with

different modifications and gradient loadings of BCFs were accord-ingly prepared. The details for the composites’ preparation in theexperiments are shown in Table 1.

2.4. Mechanical properties

In order to evaluate the mechanical properties of composites, atensile testing machine (AGS-J, Suzhou, China) was employed tostudy the tensile strength and elongation-at-break of the compos-ites; a pendulum impact tester (XC, Chengdu, China) was employedto test the impact strength of composites. The samples for tensiletesting were standard dumbbell-shaped with a 112 mm gaugelength, the tensile speed was 10 mm/min. The size of the samplesfor impact test was 65 � 10 � 4 mm, the maximum impact energywas set to be 1 J. All the tests were carried out at 25 �C. Values ofthe mechanical properties of each sample were decided by themeans of five tests. The percentage of error as determined by per-centage error (Standard Deviation/Mean) ⁄ 100 was less than 10%[30].

2.5. Fourier transform infrared (FTIR) of cellulose

The virgin bamboo cellulose fiber (BCF), NaOH treated bamboocellulose fiber (N-BCF) and KH560 treated bamboo cellulose fiber(K-BCF) were submitted to FTIR analysis to find out the transfor-mation in their chemical structures. The powders of the three dif-ferent BCF were completely dried at 105 �C in a drying oven beforeprepared as KBr pellet and being tested by FTIR (Nicolet 5700,America) on the sample holder. All spectra were recorded in thewave number range of 400–4000 cm�1 with resolution of 4 cm�1.

2.6. Scanning electron microscopy (SEM)

The tensile fractures and the cryo-fracture surfaces of the com-posites as well as the pure EP were studied by SEM (QUANPA200,Netherlands) operating in the high vacuum mode at acceleratingvoltage of 20 kV. The morphologies of those three kinds of BCFswere also characterized by SEM observations. Prior to examination,the samples were sputtered with gold using JEOL Fine Coater JFC-1200 for 50 s at argon pressure of 8 Pa and current of 30 mA.

3. Results and discussion

3.1. Pretreatment of cellulose fiber

Before preparation of the composites, SEM observations of BCF,N-BCF and K-BCF were applied to learn about the size and surfacefeatures of the cellulose fibers. From Fig. 1, one could see that thevirgin cellulose displayed a fibril shape, having an average diame-ter of 8 lm, and the modifications led to obvious differences inmorphology. As shown in Fig. 1a, the surface of the untreated cel-lulose was found to be considerably covered with impurities andamorphous cellulose [31,32]. Comparatively, the cellulosic microf-ibers treated with NaOH displayed a neat and ordered surface(Fig. 1b), which demonstrated dislodging of impurities and amor-phous cellulose, agreeing to the observation of Peng et al. [33].

Fig. 1. SEM micrographs of (a) virgin BCF, (b) N-BCF and (c) K-BCF.

30 T. Lu et al. / Composites: Part B 51 (2013) 28–34

Particularly, the NaOH treatment of cellulose resulted in the forma-tion of nano-sized cellulose fibrillations (also indicated in Fig. 8b).As reported, the natural cellulose fibers were composited by crys-talline cellulose in nanosize and para-crystalline, surrounded byamorphous cellulose [34]. Gaps between the nanofibers wereformed after the removal of amorphous cellulose, resulting in split-ting the fibers into smaller sizes. From Fig. 1c, one can see that a

H2C CHCH2OCH2CH2Si (OCH3)3+ H2O

O

H2C CHCH2OCH2CH2Si(OH )3

O

H 2C CH CH 2OCH 2CH 2Si(O H)3+Rcell

O

OH H2C CHCH2O CH2CH2Si__O __Rcell

O

tinny thin film appears on the cellulose surface after KH560modification, which was ascribed to the coupling agent.

The chemical structures of the virgin and treated cellulose wereinvestigated by FTIR, as shown in Fig. 2. The FTIR spectrum of thevirgin BCF (Fig. 2a) was associated with the typical spectrum ofcellulose [23]. Weak characteristic peaks of lignin were found, thepeak at 1633 cm�1 indicated an absorption peak of carbonyl groupin lignin. Comparing Fig. 2b with a, no new functional groups wereintroduced to the cellulose structure by NaOH treatment, but someabsorption peaks in N-BCF shifted from the corresponding peaks inthe virgin BCF as marked in curve-b. The –OH stretching vibrationabsorption peak at 3407 cm�1 had a left shift by 13 cm�1, whilethe –CH2 bending vibration absorption peak shifted from1436 cm�1 to 1422 cm�1 after the NaOH modification. Associatedwith the SEM observations in Fig. 1, it can be concluded that thestructure of the BCF might have been changed because of theremotion of amorphous cellulose and other impurities after NaOHtreatment. Two new peaks appeared in curve-c at 1103 cm�1 and

802 cm�1 after KH560 modification, wherein 1103 cm�1 corre-sponded with the stretching vibration of Si–O–C, and 802 cm�1

related to Si–O–Si stretching vibration. These results demonstratedthat chemical bonds have been formed after the cellulose beingmodified by the coupling agent of KH560, and CH2CH(O)CH2-

O(CH2)3SiO– group had been grafted onto the cellulose molecules.The process can be described as follows:

3.2. Mechanical properties of the composites

The mechanical properties of the pure EP and the compositeswith different loadings of cellulose fibers are shown in Figs. 3–5.The composites filled with unmodified BCF increased the impactstrength and elongation at break, while decreased the tensilestrength. This might be caused by the poor compatibility betweenBCF and EP matrix, which could be reinforced by the SEM observa-tions as shown in Fig. 6. From Fig. 6b–d, relating to BCF10/EP,BCF20/EP and BCF30/EP respectively, cellulose fibers were foundto be dispersed in the matrix, and the surface of the BCFs weresmooth and clean, not coated by matrix EP. In general, clear pullout, detachment or debonding of some of the BCFs could be ob-served (as indicated with arrows), which indicated the poor inter-facial adhesion. Failure firstly occurs at the interfaces, thus virginBCF produced defects in the composites rather than reinforcedthe matrix, leading to reduction in the tensile strength. At the sametime, the tensile fracture of pure epoxy (Fig. 6a) was very flat,

0 10 20 30

3

4

5

6

Elon

gatio

n Pe

rcen

tage

(%)

Cellulose Content (wt%)

KH560 treatedNaOH treated Unmodified

Fig. 4. Elongation at break of BCF/EP composites with different loadings ofcellulosic fibers.

0 10 20 302

4

6Im

pact

Stre

ngth

(kJ/

m2 )

Cellulose Content (wt%)

Unmodified KH560 treated NaOH treated

Fig. 5. Impact strength of BCF/EP composites with different loadings of cellulosicfibers.

4000 3500 3000 2500 2000 1500 1000 500 0

1633

1268

1103

1422

1436

1253

802

3420

3407

Inte

nsity

Wave number (cm-1)

(a)

(b)

(c)

Fig. 2. FTIR spectra of the celluloses: (a) virgin BCF, (b) N-BCF and (c) K-BCF.

T. Lu et al. / Composites: Part B 51 (2013) 28–34 31

showing a brittle fracture. The fractures of the composites wererougher than pure EP, indicating a higher degree of toughness thatresulted in enhancing the impact strengths of the BCF filled com-posites. This could be ascribed to the loose structure of cellulose fi-bers that can absorb more impact energy.

After NaOH solution treatment, the tensile strength and theelongation at break increased by different degrees with varyingloadings of the fibers (N-BCF). KH560 modification caused signifi-cant increases both in tensile strength and elongation at break.Both modifications decreased the impact strength slightly com-pared with BCF/EP composites. The N-BCF or K-BCF reinforced EPcomposites still had much higher impact strength than those ofpure EP. For all the three kinds of composites, increasing thecontent of the filler, the magnitudes of tensile strength, impactstrength and elongation at break increased at first. When thecontents of cellulose fibers were 20 wt%, the composites had theultimate mechanical properties. More loading of BCFs, such as30 wt%, resulting in the reduction of mechanical properties. Wespeculated this phenomenon on the higher filling that caused poordispersion of the fibers, leading to more and/or larger defects, asshown in Fig. 7b and d.

The tensile strength of BCF20/EP increased by 34% and elonga-tion at break increased by 31%, while impact strength just de-creased by 8%, still 1.97 times higher that of the pure EP after

0 10 20 30

12

18

24

Tens

ile S

treng

th (M

Pa)

Cellulose Content (wt%)

KH560 Treated NaOH Treated Unmodified

Fig. 3. Tensile strength of BCF/EP composites with different loadings of cellulosicfibers.

NaOH solution treatment. The increments of tensile strength andelongation at break caused by KH560 treatment were 71% and53% respectively, while decrement in the impact strength was only3% for the composites with 20 wt% cellulose content. Lívia et al.reported considerable decreases in interfacial adhesion betweencellulose and PP after benzylation treatment [4]. Nearly noenhancement in mechanical properties of cellulose compositesby surface modification of cellulose fibers was reported by bothAbdelmouleh et al. [35] and Lu et al. [34]. Compared with thesestudies, the results in our work were quite positive.

3.3. The effect of fiber modifications

The fracture morphology of the composites with 20 wt% loadingof cellulose fibers were observed by SEM. From Fig. 8a, related toBCF/EP composite, small pieces of polymeric matrix was foundon the fibers, which further demonstrated that there was poorcombination between the fibers and the matrix. In contrast, a goodlink between the filled fibers and the epoxy could be seen from thetensile fracture of the N-BCF/EP composite. From Fig. 8b, we couldsee that it displayed a lot of burrs on the surfaces of the cellulosefibers. In addition, it seems that some of the fibers were brokenby tensile loading (as shown by arrow in Fig. 8b). These results

(a)

Pulled out Fiber

(b)

(c) (d)

Fig. 6. Tensile fractures of (a) Pure EP, (b) BCF10/EP, (c) BCF20/EP and (d) BCF30/EP.

Cellulose Reunion

Separated with matrix

(a) (b)

(c) (d)

Fig. 7. Cryo-fracture of (a) K-BCF20/EP, (b) K-BCF30/EP, (c) N-BCF20/EP and (d) N-BCF30/EP.

32 T. Lu et al. / Composites: Part B 51 (2013) 28–34

Broken Fibers

Cellulose Fibrillation

(a) (b)

(c)

Fig. 8. Tensile fractures of (a) BCF20/EP, (b) N-BCF20/EP, and (c) K-BCF20/EP.

T. Lu et al. / Composites: Part B 51 (2013) 28–34 33

suggest that the interfacial adhesion between BCF and EP matrixbecome much more favorable and stronger upon the treatmentof the cellulose with NaOH solution. The removal of impuritiesand amorphous cellulose in BCF by alkaline treatment has led tosmaller sized fibers and increasing the surface area available forcontact with the matrix. What’s more important, after the cellulosefiber being treated by NaOH solution, the epoxy resin will perme-ate into the gaps of cellulose fibrillation and then strongly joint thefibers together. As after the removal of the cementing materials,originally a whole fiber become cellulose fibrillation, partly withnanometer sizes (as shown by arrow in Fig. 8b). The NaOH solutionmodification broke the loose structure of cellulose fibers and theepoxy filled the gaps between the fibrillations, so the impact en-ergy absorbed by them was lower.

Fig. 8c showed that the K-BCF well trapped by the EP matrixafter KH560 treatment. The fibers were covered with a layer of ma-trix, demonstrating a fine linking between the fiber and EP. Com-pared with the BCF/EP (Fig. 8a), the coupling agent caused asignificantly wetting of the BCF through the matrix (Fig. 8c). Thiswas because of the chemically interaction between BCF and matrixcoupled by KH560. The action mechanisms for KH560 modificationcan be suggested as: the epoxy groups in KH560 that have graftedonto the cellulose can react with the matrix molecules and formchemical bonds, as it has been illustrated in FTIR spectra (Fig. 2),which will significantly improve the adhesive force between theinterfaces of the components. Secondly, the reaction between the–OH group in cellulose molecules and the Si–OH in KH560 willweaken the hydrogen bonds in celluloses, resulting in improvingthe compatibility between the cellulose and EP. While the chemicalbonds would generate more crosslinking points in the compositesand embrittle the material, the impact strength has slightly de-creased after KH560 modification.

4. Conclusions

The epoxy composites reinforced by unmodified bamboo cellu-lose fibers, NaOH treated and KH560 treated cellulose fiber wererespectively prepared. Both coupling agent modification and alkalitreatment apparently improved the tensile strength and the valuesof elongation at break. The K-BCF/EP composites exhibited bettermechanical properties than those of N-BCF/EP composites havingthe same cellulose content. For all these three kinds of composites,the ultimate mechanical properties were obtained with 20 wt%loading of cellulose fibers. The result indicated optimum cellulosecontent and selected modifications were positive to the mechani-cal properties of the cellulose composites.

Two different modifying mechanisms were suggested in thisstudy. After being treated with NaOH aqueous solution, cellulose fiberbecame cellulose fibrillation with much smaller diameters, thus eas-ier permeating for EP as well as increasing effective surface area avail-able for contact with the matrix. After the reaction of KH560 withboth the cellulose and EP happened during KH560 modification, thesilane molecules linked the EP and the cellulose together by chemicalbonds, and significantly improved the interfacial interactions.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Nos. 51173148, 90305003), theNational Key Technology R&D Program of China (No.2011BAE11B01), the Fundamental Research Funds for the CentralUniversities in China (Nos. SWJTU11ZT10, SWJTU11CX052), theScience and Technology Planning Project of Sichuan Province

34 T. Lu et al. / Composites: Part B 51 (2013) 28–34

(Nos. 2010GZ0251, 2011GZX0052) and the Applied Basic ResearchPrograms of Sichuan Province (No. 20095Y0064).

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