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Industrial Crops and Products 50 (2013) 707714

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Industrial Crops and Products

j ou rna l h om epag e : www.e l sev i e r. com/ loca t e / i ndc rop

Cellulose nanocrystals from pineapple leaf, a new approach for thereuse of this agro-wasteRoni Marcos dos Santos a , Wilson Pires Flauzino Neto a , Hudson Alves Silvrio a ,Douglas Ferreira Martins a , Nolio Oliveira Dantas b , Daniel Pasquini a,a Instituto de Qumica, Universidade Federal de Uberlndia, Campus Santa Mnica, Av.Joo Naves de vila, 2121, 38400-902, Uberlndia,Minas Gerais,Brazilb Instituto de Fsica, Universidade Federal de Uberlndia, Campus Santa Mnica, Av.Joo Naves de vila,2121, 38400-902, Uberlndia, Minas Gerais, Brazil

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Article history:Received 7 April 2013Received in revised form 8 August 2013Accepted 16 August 2013

Keywords:Pineapple leaf Cellulose nanocrystalsAgricultural residueReuse

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Pineapple leaf (PL) isan annually renewable agricultural residue, available in abundance, which is usedvery rarely and is of limited value at present. Therefore, this agro-waste deserves to be better and/orproperly used. The aim of this study was to explore PL as a source of raw material for the production of cellulose nanocrystals (CN). The CN were extracted by acid hydrolysis at 45 C for 5, 30 or 60 min, using20mL of H2SO4 (9.17 M) for each gram of material. The resulting CN were characterized by crystallinityindex, FTIR, morphology (shape and size) and thermal stability. Among the hydrolysis conditions carriedout, the best extraction time was 30 min. At this extraction time, the CN presented a needle-shapednature, high thermal stability (225 C), high crystallinity (73%), an average length of 249.7 51.5 nm anda diameter of 4.45 1.41 nm, giving an aspect ratio (L/D) of around 60. Therefore, CN obtained from PL has great potential as reinforcement in the manufacture of nanocomposites. The production of CN fromthis underutilized agro-waste has commercial application potential that can add value to the pineapplecultivation, generate extra income for farmers and also help in agribusiness diversication. In addition,the reuse of these residues allows a signicant reduction in both the volume of waste accumulated in theenvironment and in the extraction of raw materials.

2013 Elsevier B.V. All r ights reserved.

1. Introduction

In recentyears, thequest forsustainabledevelopment hasmoti-vated efforts toward maximizing the efciency of the use of rawmaterials and minimizing the creation of waste ( Ashori, 2008 ). Inthis context, the use of biomass residues as feedstock for the pro-duction of energy and materials has been the object of intensiveacademic and industrial research ( Mishra et al., 2004; Reddy andYang, 2005; Schievano et al., 2009; Silva et al., 2009 ). The reuse of these residues allows a signicant reduction both in the volume of waste accumulated in theenvironment andin theextraction of rawmaterials. Thus, an efcient reuse of these wastes is of great impor-tance, not only for minimizing the environmental impact, but alsofor obtaining a higher prot.

Agriculture is an important sector in the Brazilian economy(Rahman, 2011 ). Diversication of the industry is crucial in encour-aging economic stability and growth. The utilization of these crop

Corresponding author. Tel.: +5534 3239 4143;fax: +55 34 3239 4208.E-mail addresses:[email protected] , [email protected]

(D. Pasquini).

residues in industrial processes for the generation of value-addedproducts, such as in the production of high performance materi-als, could be an additional source of revenue for farmers and alsohelpin agro-industry diversicationbyprovidinga non-food-basedmarket for agro-wastes ( Alemdar and Sain, 2008; Flauzino Netoet al., 2013; Rahman, 2011; Reddy and Yang, 2005; Silvrio et al.,2013 ).

Pineapple is one of the most popular tropical fruits in the worldand their crop occupies a prominent position in the Brazilianagricultural sector. Brazil is one of the main producers of thiscrop, accounting for approximately 10.9% of the world production(http://www.cnpmf.embrapa.br/planilhas/Abacaxi Mundo 201 0.pdf ). Currently, the main focus of the pineapple industry in thiscountry is the fruits and related foodstuffs and consequently theother plant parts (stems, roots and especially leaves) are consid-ered agricultural residues of pineapple cultivation ( dos Santoset al., 2001; Fagundes and Fagundes, 2010 ). The post-harvestresidue comprises mainly pineapple leaves, which are mostlyburned to eliminate fungi and other parasites, composted or justcrammed to rot ( de Aquino, 2006; Maniruzzaman et al., 2011 ). Thisis due to the lack of adequate technology for this purpose, as wellas the ignorance of the farmers about the existence of commercial

0926-6690/$ see front matter 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.indcrop.2013.08.049
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uses for leaves that can generate extra income for these farmers(de Aquino, 2006; Mishra et al., 2004 ).

The practices of decomposing and burning the pineapple leaf (PL)insitudonot contributeto theimprovementof plantation yield,as reported in previous literature ( Ahmed et al., 2002; Mohamedet al., 2009 ).

PL is an annually renewable agricultural residue, that isbiodegradable, available in abundance (inexpensive), used veryrarely and of limited value at present. In addition, after harvest-ing, PL waste remains, causing various problems forfarmers to dealwith. There is a great demand to nd other end uses for these agri-cultural cellulosic wastes ( Cherian et al., 2010, 2011; KengkhetkitandAmornsakchai, 2012; Maniruzzamanet al., 2011 ). Hence,with-outanyadditional cost input,PL berscanbe obtained forindustrialpurposes.

Several processes and products have been reported that uti-lize PL as a raw material. These include the extraction of celluloseber and nanober, the production of paper, textiles and com-posites ( Banik et al., 2011; Cherian et al., 2010; Chollakup et al.,2011; Kengkhetkit and Amornsakchai, 2012; Mishra et al., 2004;Threepopnatkul et al., 2009 ). However, there is not yet any pub-lished work on the extraction of cellulose nanocrystals (CN) fromPL.

CN have attracted immense interest as a novel nanostruc-tured material during recent years. CN are very high crystallinitynanoparticles derived from cellulosics bers. CN are a very high-value material, since they can transform the performance of existing products as well as helping to create new, unique andimproved products. The unique combination of amazing physico-chemical properties andenvironmental benetsallows that theCNoffer a wide range of potential applications. At present, the mainapplication of CN is as a reinforcing agent in the nanocompositeeld. Others elds of potential applications are packaging, paints,coatings, special papers, cosmetics, pharmaceuticals, biomedicalmaterials, textiles, the automotive industry, aerospace, buildingmaterials, the electronic and electrical industry, and many others(Moon et al., 2011; Peng et al., 2011; Podsiadlo et al., 2005; Silvrioet al., 2013 ). One specic example of the application of CN is thesolidication of liquid crystals for optical applications, e.g. securitypaper ( Beck et al., 2011; Revol et al., 1998 ).

It is known that the morphology and properties of the CNdepend mainlyon the sourceof theoriginal cellulose, of theextrac-tion process and their parameters ( Beck-Candanedo et al., 2005;Elazzouzi-Hafraoui et al.,2008 ). Therefore, the isolationand furtheranalysis of the characteristics of CN from many kinds of cellulosicresourcesis necessaryand relevant forthe efcient comparison andexploitation of these resources ( Chen et al., 2011; Flauzino Netoet al., 2013; Silvrio et al., 2013 ).

CN have been isolated from different vegetable sources, suchas cotton and wood pulp ( Beck-Candanedo et al., 2005; Teixeiraet al., 2010 ), and from animal sources such as tunicates ( Berg et al.,

2007 ). In addition, there are only a few papers which describe theisolation of whiskers fromagriculturalbyproducts,such as soyhulls(Flauzino Neto et al., 2013 ), corncob ( Silvrio et al., 2013 ), rice husk(Rosa et al., 2012 ) and sesame husk ( Purkait et al., 2011 ).

In this work, CN were extracted from PL under different condi-tions of sulfuric acid hydrolysis in order to obtain a material witha high crystallinity index, thermal stability, aspect ratio and yield.Different techniques were employed to characterize the PL at dif-ferent stages of treatment. The characteristics investigated werethe chemical composition, crystallinity index, thermal stability,surface charge and morphology (shape and size). The aim in thisstudy was to investigate the viability of this agricultural residue asa simple and low-cost source of CN and the possibility of addingvalue to the pineapple cultivation through a new approach to their

utilization.

2. Experimental

2.1. Materials and methods

The PL used in this study was obtained from residues after har-vesting pineapple in the So Mateus farm (Comendador Gomes,Minas Gerais, Brazil). The plant species used was Ananus coso-mus belonging to the Bromeliaceae family. The other reagentsemployed in this study were: sulfuric acid (95.098.0 wt.%, Vetec,P.A.), sodium hydroxide (Vetec), potassium hydroxide (Vetec),sodium chlorite (NaClO 2 , technical grade, 80%, SigmaAldrich),glacial acetic acid (Synth), and cellulose membrane (D9402,SigmaAldrich).

2.2. Preparation of cellulose nanocrystals

2.2.1. PuricationThe raw PL was ground in a mill. After that, the PL was treated

with a sodium hydroxide aqueous solution of 2% (w/w) for 4h at100 C under mechanical stirring, washed several times with dis-tilled water until the alkali was completely removed, and nallydried at 50 C for 12h in an air-circulating oven. After this treat-ment, the material was bleached with a solution made up of equalparts (v:v) of acetate buffer (27g NaOH and 75ml glacial aceticacid, diluted to 1 L of distilled water) and aqueous sodium chlorite(1.7wt% NaClO2 in water). Thisbleaching treatment was performedat 80 C for 4 h. The bleached material was washed repeatedly indistilled water until the pH of the material became neutral andsubsequently dried at 50 C for 12h in an air-circulating oven. Thematerial content throughout these chemical treatments was about46% (w/w). The material which resulted after the purication wasthe treated pineapple leaf (TPL). The bleaching process utilized hasbeen used/adapted by other authors ( de Rodriguez et al., 2006;Siqueira et al., 2010a; Flauzino Neto et al., 2013; Silvrio et al.,2013 ).

2.2.2. Isolation of cellulose nanocrystalsAfter thechemical treatmentwas completed,the TPLwas milled

with a blender and then used for the extraction of CN by acidhydrolysis.The hydrolysiswas performedat 45 Cfor5min,30minor 60min under vigorous and constant stirring. For each gram of TPL we used 20mL of H2SO4 64% (w/w) (9.17M). Immediately fol-lowing the hydrolysis, the suspension was diluted 10-fold withcold water to stop the hydrolysis reaction, and centrifuged twicefor 10min at 7000 rpm to remove the excess acid. The precipitatewas then dialyzed with tap water to remove non-reactive sulfategroups, salts andsoluble sugars, until the neutral pH (

4 days) was

reached. Subsequently, the resulting suspension of dialysis processwas ultrasonicated for 10min and stored in a refrigerator at 4 C.Some dropsof chloroform were added inthe CNsuspension toavoidany bacterial growth. The cellulose nanocrystals from pineappleleaf were labeled CNPL 5 or CNPL 30 or CNPL 60 , depending on thetime of extraction.

2.3. Characterizations and measurements

2.3.1. Birefringence analysisAliquots of CN suspensions at same concentration

(7.10 3 g mL 1 ) were placed in a glass bottle, and then thesebottles were placed in front of a source of polarized lightand photographed by a camera equipped with a polarizedlight lter while being agitated with the aid of a magnetic

stirrer.


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R.M.d. Santos et al. / Industrial Crops and Products50 (2013) 707714 709

Fig. 1. Photographs of (a) the pineapple cultivation, (b) untreated pineapple leaves, (c) ground pineapple leaves, and (d) treated pineapple leaves.

2.4. Gravimetric analysisThe hydrolysis yields were calculated by drying aliquots of the

CN suspensions with a known volume at 105 C for 12h in an air-circulating oven.

2.5. Chemical composition

The chemical composition of PL and TPL was investigated asfollows: the lignin content was measured according to a standardmethod of the Technical Associat ion of Pulp and Paper Indus-tryTAPPI T13M-54; the holocellulose ( -cellulose+ hemicellulose)content was estimated by the acid chlorite method ( Browning,1967 ); the -cellulose content was determined treating the holo-

cellulose with potassium hydroxide solutions of 5 and 24% (w/w),according Browning ( Browning, 1967 ); the hemicellulose contentwas found by subtracting the -cellulose part from the holocellu-lose content; and the ashcontent wasmeasured by considering thepercentage difference between the initial weight of the dried berof the sample and that after calcination for 4 h at 800 C (Trindadeet al., 2005 ). An average of three replicates was calculated for eachsample.

2.5.1. Fourier transform infrared spectroscopy (FTIR)A Shimadzu IRPrestige-21 Infrared spectrophotometerwas used

toobtain spectra forthe PL,TPL, CNPL 5 ,CNPL 30 and CNPL 60 .TheKBrdisk (ultra-thin pellets) method was used in taking the IR spectra.Samples wereground andmixedwith KBr(sample/KBrratio,1/100)

to prepare pastilles. The experiments were carried out in the rangeof5004000 cm 1 witharesolutionof4cm 1 andatotalof32scansfor each sample.

2.6. X-ray diffraction (XRD)

The X-ray diffractograms of PL, TPL, CNPL 5 , CNPL 30 and CNPL 60were obtained at room temperature withina 2 range from5 to40 anda scanrateof 2 min 1 . The equipment used was a diffractome-ter Shimadzu LabX XRD-6000,operating at a power of 40kV with acurrent of30 mA andCuK radiation(1.5406 A).BeforeperformingtheXRD,allsamples were dried at50 C for12 h inanair-circulatingoven.

The diffractograms were deconvoluted into peaks and halos

referring to the crystalline and amorphous regions, respectively.

Thesedeconvolutionswereobtained usingthe Pseudo-Voigt 2 peakfunction from Origin 7.0, which is shown in Eq. (1) , and thesedeconvolutions were evaluated according to the two-phase model(Cerqueira et al., 2006 ).

y = y0 + A mu2 wL

4( x xc)2+w2L

+(1 mu ) 4 ln 2

W Ge(4 ln 2 /w

2G)( x xc )

2 (1)

where wL and wG are the width at half maximum for Lorentz andGausscomponents of the aboveequation, respectively, Aisthe areaand mu is the prole shape factor.

Considering this model, the crystallinity indexes (CrI) of thesamples were calculated using Eq. (2) :

CrI = Ac

Ac + Aa 100 (2)

where Ac and Aa are the areas under the crystalline peaks and theamorphous halos, respectively determined by the deconvolutions.

2.6.1. Atomic force microscopy (AFM)AFM measurements were performed withShimadzu SPM-9600

equipment for evaluating the morphology of CNPL 5 , CNPL 30 andCNPL 60 . A drop of a diluted nanocrystals aqueous suspension (toabout 5.10 5 g mL 1 ) was deposited onto a freshly cleaved micasurface and air-dried. AFM images were obtained at room temper-ature in the dynamic mode with a scan rate of 1Hz and using Si

tips with a curvature radius of less than 10nm and a spring con-stantof42Nm 1 . The dimensionsof nanocrystals weredeterminedusing VectorScan software (software for Shimadzus SPM-9600).To eliminate the effect of tip radius on width measurements, wemeasured the heights of the nanocrystals, which are not subjectto peak broadening artifacts, and assumed the nanocrystals to becylindrical in shape ( Beck-Candanedo et al., 2005 ). Seventy-venanocrystals were randomly selected to determine the averagelength, width and aspect ratio. For each nanocrystal, one measure-ment of the length and two measurements of the diameter wereperformed and the aspect ratio was calculated.

2.6.2. Thermal characterization (TG)Thermal stabilities of PL, TPL, CNPL 5 , CNPL 30 and CNPL 60 were

evaluated using Shimadzu DTG-60H equipment. The analysis


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Fig. 2. FTIR spectra of PL, TPL, CNPL 5 , CNPL 30 and CNPL 60 .

conditions were: a nitrogen atmosphere with ow 30mLmin 1 ,heating rate of 10 Cmin 1 , temperature range from 25 to 600 C,sample mass between 5 and 7 mg and aluminum pans.

3. Results and discussion

3.1. Purication, chemical composition and FTIR

A diluted alkali treatment was performed to remove the lignin,hemicelluloses, waxes, pectins, proteins, soluble mineral salts, sil-ica and ash, while bleaching was applied to remove the ligninresidues ( Dufresne et al., 1997; Flauzino Netoet al., 2013; Sheltamiet al., 2012 ). The yield of purication was 40% (by dry mass). Fig. 1shows the physical aspect of the PL before and after purication(TPL). Visually, the white color of the material after puricationindicates that a great part of the initial non-cellulosic componentswas removed. The contents of the cellulose were 36.3 3.8% and74.5 4.2%, of the hemicelluloses were 22.9 2.0% and 20.4 2.6%,of the lignin were 27.53 1.94% and 8.72 1.63%, and ash were2.85 0.24% and 2.28 0.11%, for PL and TPL respectively (the val-ues are based on dry basis).

It was veried that the percentage of mass consumption of themain components of thePL dueto thepuricationprocesswas18.0,64.4 and 87.4% for -cellulose, hemicelluloses and lignin, respec-tively. The mass loss of cellulose during the purication processcan be related to two factors: (i) the breakdown of cellulose chainsduring bleaching and (ii) the loss of material inherent in variousltering processes and mass transfer ( Flauzino Neto et al., 2013 ).

Taking into account that purication was performed to remove

non-cellulose components and mainly lignin, which acts as a natu-ral agglutinative hindering acid hydrolysis, the purication processhas reached its goal because the composition of TPL is suitable forthe extraction of cellulose nanocrystals (low content of lignin andhigh content of cellulose).

Fig. 2 shows the FTIR spectra of PL, TPL, CNPL 5 , CNPL 30 andCNPL 60 . The prominent peak at 1742cm 1 in the spectrum of PL is attributed to the acetyl and uronic ester groups of the hemi-celluloses or the ester linkage of carboxylic group of ferulic andp-coumaricacids of lignin and/orhemicelluloses( Kejunetal.,2011;Sun et al., 2005 ). In the same spectrum, the peak at 1514cm 1 isassociatedwiththe C C aromaticskeletal vibration oflignin( Kejunet al., 2011; Sun et al., 2005; Vargas et al., 2011; Xiao et al., 2001 ),and the band near 1254cm 1 corresponds to the axial asymmetric

strain of C O C, which is commonly observed when C O e.g.

Fig. 3. The resultingcolloidal suspensions for CNPL 5 , CNPL 30 andCNPL 60 aftera fewhours of rest.

in ether, ester, and phenol groups are present ( Siqueira et al.,2010b ). Comparing the data shown in the FTIR spectra for PL andTPL,it could benotedthatthelack ofpeaksat 1742 cm 1 ,1514cm 1and 1254cm 1 in the spectrum of TPL is due to the signicantremoval of hemicelluloses, and mainly lignin, by the puricationprocess (alkali and bleaching treatments).

The peakat 1061 cm 1 isassigned totheC O stretchingandtheC H rock vibrations of the cellulose ( Alemdar and Sain, 2008 ). Thesmall increase in this peak forTPL in relation to PL indicatesthat theTPL have higher cellulose content. Similar behavior was observedwhen comparingthe spectraof TPLwith CNPL 5 , CNPL 30 andCNPL 60 .This peak appeared in all of the spectra and the differences pre-sented suggest that the CNPL 5 , CNPL 30 and CNPL 60 samples has avery high content of cellulose.

3.2. Isolation of Cellulose nanocrystals, gravimetric andbirefringence analysis

Theyields of theCNPL,with respect to theinitial amountof driedTPL bers, for CNPL 5 , CNPL 30 and CNPL 60 were 77, 65 and 55wt%respectively; these values are consistent with the literature data(Silvrio et al., 2013; Teixeira et al., 2011, 2010 ).

Among the several methods for preparing CN, acid hydroly-sis is the most well-known and widely used ( Peng et al., 2011 ).This process breaks down the disordered and amorphous parts of the cellulose, releasing single and well-dened crystals. Thus, thisprocess is based on the quicker hydrolysis kinetics presented byamorphous regions, as compared to crystalline ones ( Habibi et al.,2010; Peng et al., 2011; Teixeira et al., 2011 ).

During the hydrolysis process with sulfuric acid, sulfate groupsare introduced on the surface of the CN by esterication of thehydroxyl groups from cellulose.This allows an anionic stabilizationby repulsive forces, leading to the achievement of stable aque-ous dispersions of CN ( Beck-Candanedo et al., 2005; Lima andBorsali, 2004; Silva and DAlmeida, 2009 ). Also, it is known thatthe increased extraction time results in higher sulfate content inthe CN ( Flauzino Neto et al., 2013; Roman and Winter, 2004 ). Onlythe hydrolysis times of 30 and60 minled to stable aqueous suspen-sions being obtained (as shown in Fig. 3). The suspension obtainedwith 5 min of hydrolysis tended to aggregate after a few hours (asseen in Fig. 3), probably this particle agglomeration is related to thelarge size and small amount of surface charges of these particles.

Birefringence was used to conrm the presence of isolated

nanowhiskersin a suspension andis considered by some authors as


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Fig. 4. Photograph of aqueous suspension of CNPL 5 , CNPL 30 and CNPL 60 observedbetween cross nicols showing the formation of birefringent domains.

a good dispersibility criterion in suspension ( Silva and DAlmeida,2009; Silvrio et al., 2013 ). This birefringence results from two ori-gins: (1) a structural anisotropy form of cellulose and (2) a owanisotropy resulting from the alignment of the nanocrystals underow, generally operated before observation. Fig. 4 shows a bire-fringence of different cellulose suspensions in water, indicatingthat CN were isolated in all three isolation conditions. The birefrin-gence was somewhat weaker for CNPL 5 suspension compared tothe CNPL 30 and CNPL 60 suspensions. It is possible that the hydroly-sis time was not sufcient to separate single CN from TPL bersas effectively as the others hydrolysis times and, therefore, thebirefringence is not as strong ( Oksman et al., 2011 ).

3.3. X-ray diffraction

The XRD patterns of PL, TPL, CNPL 5 , CNPL 30 and CNPL 60 areshown in Fig.5 . These patterns are typical of semi-crystallinemate-rials with an amorphous broad hump and crystalline peaks. In all

Fig. 5. X-ray diffractograms of PL,TPL, CNPL 5 , CNPL 30 and CNPL 60 .

of the diffractograms proles ( Fig. 5), there is a predominance of type I cellulose, veried by the presence of peaks at 2 = 15 (plane10 1), 17 (plane 10 1) , 21 (plane 02 1), 23 (plane 00 2) and 34 (plane 00 4) ( Borysiak and Garbarczyk, 2003; Flauzino Neto et al.,2013 ). The peak at 2 =26.5 present in all the spectra is related tothe sample holder.

The CrI was found to be about 49, 64, 69, 73 and 68% for thePL, TPL, CNPL 5 , CNPL 30 and CNPL 60 , respectively. The higher CrIvalue of TPL compared to PL can be well understood by the reduc-tion and removal of amorphousnon-cellulosic compounds inducedby the alkali and bleaching treatments performed in the puri-cation process. The increase in the CrI value of CNPL 5 in relationto TPL was also observed, due to the partial removal of the para-crystalline domains during the acid hydrolysis. A similar behaviorwas observed when comparing the diffraction patterns of CNPL 5and CNPL 30 samples.

The CNPL 60 sample presented a decrease in crystallinity withrespect to CNPL 30 . A similar effect of hydrolysis time in excess wasobserved by some authors for cellulose nanocrystals from sugar-cane bagasse bers and pea hull bers ( Chen et al., 2009; Teixeiraet al., 2011 ), although the rod-like structures were maintained,supported by AFM images.

3.4. Atomic force microscopy (AFM)

An accurate morphological examination of the CN is essentialnot only for the promoters of the manufacturing process of the CN,but also for developers of functional applications. In this way, AFMtopography measurements were performed in order to have a pre-cisecharacterizationof the dimensionsof the individualcrystallites(Flauzino Neto et al., 2013 ).

Determining the exact dimensions of CN is complicated by thespecic limitations of the different analytical methods used. In thecase of AFM, tip/sample broadening representsthe main limitation,resulting in an overestimation of CN dimensions. Since the CN areassumed to be cylindrical in shape, the height of the CN was takentobe equivalent to thediameter,to compensate forimagewideningdue to the convolution of thetip and the particle ( Beck-Candanedoet al., 2005;Flauzino Neto et al., 2013;Kvien et al., 2005 ). However,the tip broadening effects causes an error in the length measure-ments, but this is unavoidable ( Beck-Candanedo et al., 2005 ).

Fig. 6 shows AFM images of CNPL 5 , CNPL 30 and CNPL 60 . For thesamples CNPL 30 and CNPL 60 , the AFM images presented needle-like nanoparticles throughout, conrming that the extraction of CN from PL was successful. However, the AFM images of sampleCNPL 5 showed micro-sized bers and some needle-like nanoparti-cles. Therefore, it is clear that the hydrolysis conditions employedforthis sample(CNPL 5 ) were notsufcient to completely isolate CNfrom TPL bers. This is in agreement with the weak birefringenceand particle agglomeration observed for this suspension sample(CNPL 5 ).

Fig. 7 shows the length ( L), width ( D) and aspect ratio ( L/D) his-tograms of CNPL 30 and CNPL 60 obtained by several AFM images, asdescribed in the Experimental Section. The statistics of the length,width and aspect ratio are shown in Table 1 .

The increase in extraction time resulted in a slightly shorterlength forCNPL 60 when compared with CNPL 30 . This wasexpected,since the long extraction time (60min) partially destroyed areas of the crystalline domains, as seen by XRD analysis.

Table 1Length, width,aspect ratio, forCNPL 30 and CNPL 60 obtained by AFM pictures.

Length (nm) Diameter (nm) Aspect ratio (nm)

CNPL 30 249.7 51.5 4.45 1.41 60.1 19.5CNPL 60 190.2

36.5 4.18

1.44 50.4

20.7


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Fig. 6. AFMimages of CNPL 5 , CNPL 30 and CNPL 60 .

By studying histograms it is clear that the increase in hydrolysistime resulted in a reduction in particle size, but no signicant dif-ference in width among the CNPL 30 and CNPL 60 could be detected,when the standard deviation of each value was taken into account.

As a consequence of the whiskers preparation conditions,a decrease in the aspect ratio ( L/D) of CNPL 60 compared withCNPL 30 was observed. This suggests that CNPL 30 can give a bet-ter reinforcing effect than CNPL 60 at the same ller loading levels(Eichhorn et al., 2010 ). The average aspect ratio for the CNPL foundin this work is close to the largest values ever reported in the lit-erature; therefore, these particles have great potential to be usedas reinforcing agents in nanocomposites ( Kalia et al., 2011; Silvrioet al., 2013 ).

The results of the morphological investigation by AFM areconsistent with other reports in the literature, where CN wereextracted from different sources ( Bai et al., 2009; Beck-Candanedoet al., 2005; de Rodriguez et al., 2006; Elazzouzi-Hafraoui et al.,2008; Kvien et al., 2005; Rosa et al., 2010; Siqueira et al., 2010a;Teixeira et al., 2011 ).

3.5. Thermogravimetric analysis (TGA)

At present, the main application of CN is as a reinforcing agentin the nanocomposite eld, and typical processing temperaturesfor thermoplastics rise above 200 C, thus the thermal stability of

thesecrystalsis a key factor in order for them tobe used aseffectivereinforcing materials ( Roman andWinter,2004 ). The thermogravi-metric (TG) results of the PL, TPL, CNPL 5 , CNPL 30 and CNPL 60 areshown in Fig. 8. In all cases, a small weight loss was found in therange of 25150 C, due to the evaporation of the water of thematerials or low molecular weight compounds (about 7%). The ini-tial degradation temperatures were found to be around 216, 244,245, 225 and 220 C for the PL, TPL, CNPL 5 , CNPL 30 and CNPL 60 ,respectively.

The initial degradation temperature of the TPL (244 C) wasappreciably increased compared to that of the original PL (216 C).Due to the low initial decomposition temperatures of hemicellu-loses, lignin and pectin, the higher initial temperature of thermaldecomposition of the TPL is related to the partial removal of hemi-celluloses, lignin, and pectins by purication processes (alkali andbleaching treatments) ( Alemdar and Sain, 2008; Chen et al., 2011;Flauzino Neto et al., 2013 ).

As reported in previous studies, treatment with sulfuric acidleads to a remarkable decrease in thermal stability of CN. Thisoccurs because the incorporation of sulfate groups on the surfaceof the cellulose after hydrolysis hasa catalytic effect in its reactionsof thermal degradation ( Roman and Winter, 2004 ).

Taking into account that acid hydrolysis leads to a remarkabledecrease in thermal stability of CN, it was expected that the initialdegradation temperature of CNPL 5 were smaller than for TPL, but

Fig. 7. Length( L), width ( D) and aspect ratio ( L/D) histograms of CNPL 30 and CNPL 60 obtained by several AFM images.


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Fig. 8. Thermogravimetric (TG) curvesof thePL, TPL, CNPL 5 , CNPL 30 and CNPL 60 .

it was actually almost the same (245 vs.244 C). This can be clearlyexplained by the higher cellulose content of CNPL 5 compared to

TPL and the small hydrolysis time (small amount of sulfate groups)of sample CNPL 5 .The CNPL 30 sample presented a decrease in initial degradation

temperature with respect to CNPL 5 , as expected since the CNPL 30sample had a higher sulfate content than CNPL 5 . Similar behaviorwas observed when comparing the initial degradation tempera-ture of samples CNPL 30 and CNPL 60 . These results are consistentwith results obtained from the Chemical Composition, XRD andFTIR measurements.

4. Conclusions

The present work shows that PL is an interesting source of raw material for the production of CN, due to the characteristicsof the obtained nanocrystals. Chemical treatment performed withsodium chlorite and alkali removed the non-cellulosic componentsresulting in bers with a low content of lignin and a high con-tent of cellulose, which were therefore suitable for extracting CN.Through theAFM images it was observed that there was an incom-plete isolation of CN after 5min of hydrolysis by the employedconditions (sample CNPL 5 ). However, above 30min of hydroly-sis it was possible to obtain stable aqueous suspensions of CNPL which are negatively charged, due the presence of sulfate groups.The yields of acid hydrolysis, with respect to the initial amount of dried TPL bers, for CNPL 5 , CNPL 30 and CNPL 60 were 77, 65 and55 wt%, respectively. The increase in the hydrolysis time resultedin a decrease in the dimensions and also in the aspect ratio ( L/D) of the CNPL.

For an extraction time of 30min, the CN presented a needle-shaped nature, high thermal stability (225 C), high crystallinity(73%), an average length of 249.7 51.5nm and a diameter of 4.45 1.41nm, givingan aspect ratio of around60 (which is amongthe largest values reported in the literature). It can be concludedfrom these results that the CN obtained from PL has great poten-tial to be used as reinforcement agents for the manufacture of nanocomposites and also for diversied applications.

The production of CN from this underutilized agro-waste hascommercial application potential that can add value to the pineap-ple cultivation, generate extra income for farmers and also helpin agribusiness diversication. In addition, the reuse of theseresidues allows a signicant reduction both in the volume of waste accumulated in the environment, as in the extraction of raw

materials.

Acknowledgements

The authors thank CAPES/PROAP, CNPq and FAPEMIG for nan-cial support.

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