Study of Structural Morphology of Hemp Fiber

Study of Structural Morphology of Hemp Fiberfrom the Micro to the Nanoscale

Bei Wang & Mohini Sain & Kristiina Oksman

Received: 9 October 2006 /Accepted: 27 December 2006 /Published online: 30 January 2007# Springer Science + Business Media B.V. 2007

Abstract The focus of this work has been to study how high pressure defibrillation andchemical purification affect the hemp fiber morphology from micro to nanoscale.Microscopy techniques, chemical analysis and X-ray diffraction were used to study thestructure and properties of the prepared micro and nanofibers. Microscopy studies showedthat the used individualization processes lead to a unique morphology of interconnectedweb-like structure of hemp fibers. The nanofibers are bundles of cellulose fibers of widthsranging between 30 and 100 nm and estimated lengths of several micrometers. The chemicalanalysis showed that selective chemical treatments increased the α-cellulose content ofhemp nanofibers from 75 to 94%. Fourier transform infrared spectroscopy (FTIR) studyshowed that the pectins were partially removed during the individualization treatments.X-ray analysis showed that the relative crystallinity of the studied fibers increased aftereach stage of chemical and mechanical treatments. It was also observed that the hempnanofibers had an increased crystallinity of 71 from 57% of untreated hemp fibers.

Key words cellulose nanofibers . hemp . microfibrils . nanostructures . characterization

1 Introduction

Lately, there has been considerable interests in the isolation and study of novelnanomaterials manufactured from renewable resources. An important class of nanomaterialshas been nanofibers and fibrils from different cellulose sources and cellulose crystals

Appl Compos Mater (2007) 14:89–103DOI 10.1007/s10443-006-9032-9

DO9032; No of Pages

B. Wang :M. Sain (*) : K. OksmanCentre for Biocomposites and Biomaterials Processing, Faculty of Forestry,University of Toronto Earth and Science Centre, 33 Willcocks Street, Toronto, ON M5S 3B3, Canadae-mail: [email protected]

B. Wange-mail: [email protected]

K. OksmanManufacturing and Design of Wood and Bionanocomposites, Luleå University of Technology,SE-93187 Skellefteå, Sweden

(whiskers) [1–4]. These novel nanofibers, fibrils and crystals have been shown to result inunique properties when incorporated in different polymers [1, 5–8]. The sources of thesenanomaterials have been wheat straw, bacterial cellulose, kraft pulp, sugar beet pulp, potato,and swede root [2, 5, 7–10]. Since 1998, Canada has grown industrial hemp for seed andfor fiber. Interest in hemp arises from the plant’s amazing versatility. The seeds are used toproduce healthy food, nutraceuticals, and bodycare products and the stalk is starting to beprocessed into high performance fiber products such as paper, textiles, biocomposites andbuilding materials [11].

The chemical constituents of the hemp plant’s cell wall consist not only of cellulose, butalso of hemicellulose, pectin and lignin. The properties of each constituent contribute to theoverall properties of the fiber [12]. The smallest building element of the cellulose skeletonis considered by some to be an elementary fibril. The fibril can be about 5–10 nm indiameter and its length varies from 100 nm to several micrometers depending of the sourceof cellulose [13]. The cellulose molecules are always biosynthesized in the form ofnanosized fibrils; up to 100 glucan chains aggregate together to form cellulose nano-sizedmicrofibrils or nanofibers [14–17]. The mechanical performance of cellulose nanofibers interms of the tensile strength and Young’s modulus is comparable to other engineeringmaterials such as glass fiber, carbon fiber, etc. Therefore, the cellulose nanofibers can beconsidered to be an important structural element of natural cellulose in a number ofapplications such as plastic reinforcement, gel forming and thickening agents [3, 18, 19].Furthermore, a cellulose nanofiber has more than 200 times the surface area of isolatedsoftwood cellulose [20] and possesses higher water holding capacity, higher crystallinity,higher tensile strength, and a finer web-like network. In combination with a suitable matrixpolymer, cellulose nanofiber networks show considerable potential as an effectivereinforcement for high quality specialty application of bio-based composites. Another typeof nanoreinforcement that can be obtained from cellulose fibers are nanowhiskers. Theelementary fibril is made up of amorphous and crystalline parts. The crystalline parts can beisolated by various treatments producing the cellulose nanowhiskers [4, 21].

Many studies have been done on extracting cellulose microfibrils from various sources andon using them as reinforcement in composite manufacturing [1, 2, 6, 7, 22–24]. Thesemicrofibrils can be extracted from the cell walls by three types of isolation processes: simplemechanical methods, a combination of chemical and mechanical methods, or an enzymaticapproach. A purely mechanical process can produce refined, fine fibrils several micrometerslong and between 20 to 90 nm in diameter [25]; however, this nano-scalar web-like structureof fibrils causes a reduction of strength [5]. In contrast, chemi-mechanical treatments canextract cellulose nanofibers from the primary and secondary cell walls without degrading thecellulose. A chemi-mechanical process can also achieve finer fibrils of cellulose, rangingbetween 5 and 60 nm diameter [1, 7]. Depending upon the raw materials and defibrillationtechniques, the degree of polymerization, morphology and aspect ratio of the nanofibers willdiffer. The separation of nanoreinforcement from natural materials and the processingtechniques have been limited to laboratory scale [13]. Therefore, it is important to developnew processing techniques which will be at use in large scale production.

Removal of lignin left after chemical treatment of fibers is the goal of the bleachingprocess. Chlorine-based processes still dominate, but more environmentally friendly non-chlorine processes are becoming more prevalent. A bleaching treatment using a sodiumchlorite solution was performed to remove phenolic compounds or molecules havingchromophore groups, in order to whiten the fibers [1]. This is a popular technique at thelaboratory scale to remove lignin from plants. Lignin is rapidly oxidized by chlorine andchlorites. Lignin oxidizing leads to lignin degradation and to dissolution in an alkaline

90 Appl Compos Mater (2007) 14:89–103

medium [1]. A further treatment is often required to fully bleach the suspension. Removalof a part of the flax fibers’ noncellulosic compounds by sodium chlorite was reflected in themechanical and physical characteristics of the surface state [26].

Key issues are the size and the dispersion of the nano-sized reinforcements and the effectof this fine structure on fiber properties. Transmission electron microscopy (TEM) andatomic force microscopy (AFM) aid the interpretation of structures from the nm to the μmsize scale. Typical information obtained from conventional TEM is length, aspect ratio,shape and the aggregated or isolated state of fibers [27]. Thin evaporative carbon coatingswere used for TEM sample preparation in this study. Recently, AFM has also been used toexamine plant cell walls at a similar resolution to that of the TEM [28–30]. This type ofmicroscopy has the important advantage of reducing the risks of introducing artefactsresulting from the preparative techniques.

In order to investigate the potential of cellulose nanofiber as a reinforcement in polymercomposites, this study was focused on the development of a new isolation technique toextract cellulose nanofibers from hemp by chemi-mechanical treatments. This research aimsto clarify how the various levels of high pressure defibrillation affect the morphology fromlong hemp fiber towards nano-scale fibrillated cellulose and to compare the morphology ofbleached and unbleached fibers at the different stage of the individualization of thenanofibers. The structural details were studied with SEM, TEM and AFM. The crystallinitywas determined before and after different stages of the chemi-mechanical treatments ofhemp fibers. The changes of the chemical composition of fibers after different treatmentswere studied. Infrared measurements were performed to identify the removal of the pectins.Further research work is required for incorporating these nanofibers into a polymer matrixto evaluate the mechanical properties of nanocomposites.

2 Experimental

2.1 Materials and Methods

The raw material used in this study was hemp fibers (Cannabis sativa L.) from southwesternOntario, Canada (Hempline, Ontario). These fibers have diameters of approximately 22–25 μmand lengths of 15–25 mm.

Reagent grade chemicals were used for fiber surface modifications and bleaching,namely, sodium hydroxide, hydrochloric acid, sodium chlorite, chlorine dioxide, peroxideand sulfuric acid.

2.2 Individualization Process

The individualization process of nanofibers is a multi-step process, shown in Fig. 1.Chemical and mechanical treatments together are applied onto the hemp fibers toindividualize nanofibers. The chemical treatments include pre-treatment, acid hydrolysis,alkaline treatment and bleaching. The mechanical treatments include cryo-crushing byliquid nitrogen and high-pressure defibrillation [1].

2.2.1 Chemical Treatments

The main objective of the chemical treatments was to remove the starch, hemi-cellulose,lignin/pectins surrounding cellulose. Generally, the first step for all of the fiber surface

Appl Compos Mater (2007) 14:89–103 91

treatments is mercerization (pretreatment process) which will change the crystal structure ofcellulose. The essence of mercerizing hemp is that in the swelling of cellulose fibers due toexposure to alkalis, the natural crystalline structure of the cellulose relaxes and under anappropriate tension, the dimensions can be set by the conditions [31].

Hemp fibers were soaked in a sodium hydroxide solution of 12% w/w at roomtemperature for 2 h, enabling chemical molecules to penetrate through the crystallineregion of the cellulose. Acid hydrolysis with 1 M hydrochloric acid followed by alkalinetreatment with 2% w/w sodium hydroxide was applied to remove the undesired com-ponents.

After the successive chemical treatments, lignin was still remained within the fibers andremoved by multi-stage bleaching. The bleaching was done in four different stages:

1) Chlorine dioxide stage (D), where the fiber consistency was adjusted to 3.5%. Sodiumchlorite solution was applied based on the Kappa number of the fibers. Therefore theKappa number was determined and then chlorine dioxide was calculated based onlignin content in the sample. The retention time was 1 h.

2) Extraction stage (E), where the consistency was adjusted to 10% using boiling water.Sodium hydroxide and peroxide was added to the fiber stock based on 1% OD fiberunder mechanical stirring.

3) Acid stage (A), where the consistency was adjusted to 4% and sulfuric acid was addedto fiber mixing well for 1 h.

4) Peroxide stage (P), where the consistency was adjusted to 10% and peroxide wasadded to fiber. After filtration, the fiber was washed and air dried.

Raw Material (hemp)

Pretreatment (12% w/w NaOH, 2h)

Acid Hydrolysis (1M HCl, 80°C, 1.5h)

Alkaline Treatment (2 % w/w NaOH, 2h, 80°C)

Successive Bleaching

Chlorine Dioxide Stage

Extraction Stage

Acid Stage

Peroxide Stage

Cryo-crushing in Liquid Nitrogen

High Pressure Defibrillation

Fig. 1 Isolation of nanofibers


Process for chemical pulping was obtained from literature [32]. However, at thispurification level, microfibrils are not individualized and further steps of mechanicaltreatments are needed.

2.2.2 Mechanical Treatments

The hemp fibers were first cryo-crushed with liquid nitrogen to reduce the length and size. Theobjective of the cryo-crushing is to form ice crystals within the fiber cell wall. When highimpact is applied on the frozen fiber, ice crystals exert pressure on the cell wall, causing arupture and thereby liberating the microfibrils [33]. The chemically treated and cryo-crushedfibers were then diluted in water and dispersed evenly in a disintegrator (Cramer) for 10 min.The disintegrator is used to disperse the fibers uniformly in the water suspension before thehigh pressure defibrillation process. The water suspension with higher concentration of fibers(1–2%) was subsequently passed through the defibrillator (purchased from GEA-modified bythe Centre for Biocomposites and Biomaterials Processing Laboratory, University of Toronto,Ontario). The pressure was above 500 bar and several passes were needed to crush the cellwall and fully release the nanofibers. The detailed method for mechanical treatment toproduce the nanofibers is described in recent patents [33, 34].

2.3 Microscopy Characterization

Scanning electron microscope (JEOL JSM-840, Tokyo, Japan) was used as a routine formicrostructural analysis of the fibers after various stages of chemical and mechanicaltreatments. All images were taken at an accelerating voltage of 15 kV. The sample surfaceswere coated with a thin layer of gold on the surface using an Edwards S150B sputter coater(BOC Edwards, Wilmington, MA) to provide electrical conductivity.

Transmission electron microscopy (TEM) observations were achieved with a PhilipsCM201 (Philips, Eindhoven, The Netherlands) operated at 80 kV. A drop of a dilutecellulose nanofiber suspension was deposited on carbon-coated grids and allowed to dry.

Atomic force microscope (AFM) study was obtained using a digital instrumentsdimension 3100 AFM (Veeco Metrology Group, Santa Barbara, CA) with a nanoscope IIIacontroller. The system was operated in tapping mode at room temperature with DI tappingmode tips having a resonant frequency of 280 kHz. A droplet of the aqueous nanofibersuspension was allowed to dry on a cleaved mica surface.

2.4 Chemical Characterization of Fibers

Over the different stages of nanofiber development, untreated hemp fibers, acid/alkalitreated fibers, bleached fibers and nanofibers were chemically analyzed for hemicellulose,lignin and cellulose contents. The procedure used here for cellulose determination wasgiven by Zobel et al. [35]. Lignin content was determined based on Tappi T 222 om-02.2002 (acid-insoluble lignin in wood and pulp) and Tappi useful method UM250 (rawmaterial and pulp-determination of acid-soluble lignin).

2.5 Spectroscopy

Fourier transform infrared spectroscopy (FTIR), Tensor™ 27, Bruker Optics, Billerica,MA) was used to identify the removal of pectins at different purification levels bymeasuring the transmitted radiation of various infrared light wavelengths of pectin


functional groups in the sample. The Tensor 27 standard FTIR spectroscopy was used toobtain spectra for the fibers after each chemical treatment. Fibers were ground and mixedwith KBr (sample/KBr ratio, 1/99) to prepare pastilles. FTIR spectra were recorded in aspectral range of 4,000–400 cm−1 with a resolution of 4 cm−1.

2.6 X-Ray Analysis

The crystallinity determination was made using a powder X-ray diffraction method(PXRD). X-ray crystallography was carried out to investigate the relative crystallinity aftervarious stages of the chemi-mechanical treatment and of nanofibers obtained by air dryingof the nanofiber suspension. A D8 advance Bruker AXS diffractometer (Bruker AXS,Madison, WI), Cu point focus source, graphite monochromator, and 2D-area detectorGADDS system were used. Samples were analyzed in transmission mode.

3 Results and Discussion

3.1 Individualization of Hemp Nanofibers

The individualization step of nano-sized fibers from the plant cell walls requires chemicaland mechanical treatments. The properties of the nanomaterials cannot be physicallymeasured without separating them from the plant cell wall. These treatments may result insignificant chemical or mechanical damage on the fibers.

Figure 2 shows how the fiber morphology is changed from the micro to the nanoscaleduring the individualization process. Figure 2a shows an untreated hemp fiber bundle wherethe individual fibers are bound together by lignin. The size of the bundle is around 100–200 μm. In Fig. 2b, it is clearly visible that the chemical treatments are reducing the bundlesize and the surface roughness compared to the fibers in Fig. 2a. Figure 2c shows how themorphology is affected by the cryo-crushing. This process imparted sufficient energy tobreak the bundles into single fibers which are around 20 μm in width. In Fig. 2d, the singlefibers are defibrillated showing a web-like structure. No individual hemp fibers are visibleafter the defibrillation step and the size is reduced to the nanometer level. The fibrillarstructure of individual fibers was revealed from the Fig. 2e after the fiber bleaching andmay be due to the leaching out of waxes and pectic substances. In Fig. 2f, it was observedthat the diameter of cryo-crushed fibril with bleaching is much smaller compared to cryo-crushed fibril without bleaching. High pressure defibrillation provided high turbulence andshear that created an efficient mechanism of reduction in size. Figure 2g shows the structureafter the high pressure defibrillation, showing nanoscale fibrils and microfibril bundlescontributed a unique morphology of the interconnected web-like structure of fibrils. Thiscombination of forces promoted a high degree of microfibrillation of cellulose fibers,resulting in cellulose nanofibers.

Figure 3 demonstrates how the number of passes through a defibrillator is affecting theindividualization of hemp nanofibers. Figure 3a shows the morphology after five passeswhich did not result in nanofiber structure. The fibers were still entangled with each otherand the size was in the range of microns. In Fig. 3b, after 10 passes, the fibers were splitapart into smaller bundles. Figure 3c and d show a large extent of defibrillation after 15 and20 passes, these small bundles were additionally separated into thinner fibril bundlesincreasing the exposed surface area of the cellulose (Fig. 3d). High pressure and highenergy were needed to defibrillate hemp fibers and achieve acceptable dispersion level. The


separated fiber bundles were shown to create small entanglements that were fibrillated intosmaller entities as the number of passes through the defibrillator was increased [5].

Figure 4, compares the structure of unbleached and bleached nanofibers. Figure 4ashows that the unbleached nanofibers were either individual or bundles and coarsercompared to the bleached nanofibers, shown in Fig. 4b. The Fig. 4b shows that bleachednanofibers were thinner and shorter than unbleached nanofibers in diameter and length. It islikely that harsh chemicals used for bleaching will reduce the chain length of the cellulosewhich can result in cutting the length and weakening cellulose nanofibers. Aspect ratio(fibril length to diameter ratio) is one of the most important parameters in determiningreinforcing capability of the nanofibers. Aspect ratios of the extracted cellulose nanofiberswere estimated from transmission electron micrographs. In some cases total fibril lengthwas not visible, therefore only the visible portion was considered for the calculation inTEM graphs (provide statistical significance of this assumption). The aspect ratio of thesebleached nanofibers (82) is comparable to unbleached nanofibers (88). We can expect ahigh reinforcing capability from both nanofibers. There is a direct relation between degreeof polymerization and length of the nanofibers, as cellulose synthesizes in extended chainconformation. Degree of polymerization (DP) of nanofibers was calculated using intrinsicviscosity method (ASTM D1795–96). Unbleached nanofiber has a similar valve of DP(1,155) compared with that of bleached nanofibers (1,138).

The nanofiber suspension obtained after the high-pressure defibrillation was alsoanalyzed to determine the width using AFM, shown in Fig. 5. It is seen that the fibers areindeed nano-sized and the width is within the range of 30–100 nm. The length is estimatedto be at micrometer level. The network of nanofibers can also be seen in force mode image

Fig. 2 Scanning electron micrographs of: a untreated fiber, b after acid and alkaline treatment, c after cryo-crushing, d after defibrillation, e after bleaching, f bleaching followed by cryo-crushing, g bleachingfollowed by cryo-crushing and defibrillation


Fig. 5a by interwoven microfibrils overlapping each other. As evident from the TEM andAFM images, high-pressure defibrillation leads to individualization of the cellulosenanofibers from the cell wall without degrading them. Figure 6 shows the widthdistribution of the nanofibers obtained through chemi-mechanical treatments. In Fig. 6a,it was observed that the widths of unbleached nanofibers were estimated between 50–100 nm and most of them had a diameter range of 70 to 100 nm. Bleached nanofibers

Fig. 4 Transmission electron micrographs of hemp nanofibers (a) unbleached, (b) bleached under the samemagnification (15,000×)

Fig. 3 Scanning electron micrographs of: bleached hemp nanofibers after 5 (a), 10 (b), 15 (c) and 20 (d)passes during the defibrillation


Fig. 5 Atomic force micrographs of unbleached hemp nanofibers a force mode image, b height mode image

(a)

0

0.1

0.2

0.3

0.4

Width Range (nm)

Fre

quen

cyof

Wid

th

<30 30-50 50-70 70-100 >100

(b)

0

0.1

0.2

0.3

0.4

0.5

Width Range (nm)

Fre

quen

cyof

Wid

th

<30 30-50 50-70 70-100 >100

Fig. 6 Size distribution of hempnanofibers a unbleached,b bleached


produced smaller widths (30–100 nm) compared with that of unbleached nanofibers asshown in Fig. 6b. Most of bleached nanofibers had a diameter range of 30–50 nm. By thefiber image statistical analysis, the average width of unbleached nanofiber was 87.5 nmand of bleached nanofiber was 54 nm based on averaging 50 individual fibers. It isexpected that better reinforcing effect will be obtained by using bleached nanofibers. It isclearly shown from The TEM picture (Fig. 4) and the fiber size distribution chart (Fig. 6)that the size is less in bleached nanofibers, but more surface areas will be contributed fromthese fibrils.

3.2 Chemical Characterization of Individualized Nanofibers

The fibers obtained after the chemical treatment contained mainly alpha-cellulose withsome hemi-cellulose and lignin. As shown in Table 1, the α-cellulose content in thechemically treated fibers was 94% as compared to the original 75% and the hemicellulosecontent was reduced to 1.6%. Chemical analysis of these fibers after each stage of thepurification showed a drastic increase in cellulose content and a decline in hemicelluloseand lignin content. Lignocellulosic fibers contain a bit amount of hemicellulose, which is ahetero polysaccharide consisting mainly of pentoses and hexoses. The treatment ofcellulosic, starch, or hemicellulosic materials using acid solution to break down thepolysaccharides to simple sugars allows the solubilization of both pectins and hemi-celluloses. Dilute sodium hydroxide treatment of lignocellulosic fibers causes separation ofstructural linkages between lignin and carbohydrate and disruption of lignin structure [32].Fibers samples from hemp appeared brownish in color even after carrying out the acid andalkali treatments. Chemically treated hemp fibers were bleached before proceeding to themechanical treatment to ensure that most of the lignin was removed from the fibers. Afterbleaching treatment, it was found that nanofibers contain both soluble and insoluble lignin.The lignin content of hemp significantly decreased from 6.6 to 3.18%. According to theresult of successive bleaching extractions, the nanofibers lost most of their non-cellulosicconstituents. Cellulose can be also partially degraded during these bleaching.

Chemical treatments lead to almost pure cellulose fibers, which ensure the high stiffnessand strength. The mechanical behavior of nanohemp reinforced composites changes as afunction of hemp cellulose microfibrils purity level [5]. Although cellulose possessesexcellent strength and good stability, it can be partially degraded due to the harsh chemicaland mechanical treatment. The alkali extraction is expected to hydrolyze pectin by a β-elimination process and solubilize it [1, 7]. Figure 7 shows the transmission electronmicrographs of hemp nanofibers under two controlled concentrations of NaOH solution forthe fiber pretreatment. 17.5% of sodium hydroxide induced undesirable reactions to cutdown the cellulose chains, therefore reducing the aspect ratio of nanofibers. Too harsh

Table 1 Chemical analysis of hemp fibers after selective chemical treatments

Holocellulose(%)

α-cellulose(%)

Hemicellulose(%)

Insolublelignin (%)

Solublelignin (%)

Total lignin(%)

Untreated fibers 86.22 75.56 10.66 4.89 1.72 6.61Acid treated 91.42 85.66 5.76 4.49 0.66 5.15Acid and alkalinetreated

92.82 89.78 3.04 4.40 0.53 4.93

Bleached 95.72 93.87 1.85 2.83 0.35 3.18Nanohemp 96.12 94.53 1.59 2.53 0.18 2.71


treatments led to the loss of the microfibrillar morphology as seen in Fig. 7a. In Fig. 7b, itwas shown that the hemp fibers released their nanofibers either individually or in bundles ata lower alkali content leading to the formation of a strong network of microfibrils.

Pectin is a heterogeneous grouping of acidic structural polysaccharides. The untreated,chemically treated and bleached hemp fibers were characterized by FTIR, shown in Fig. 8.By this technique, it was possible to follow the removal of pectins due to the vanishing of

400 600 800 1000 1200 1400 1600 1800 2000 2200

Wavenumber cm-1

Tra

nsm

ittan

ce (

%)

Untreated Chemically treated Bleached

1739

1631

1259895

Fig. 8 FTIR spectra of hemp fibers a untreated, b after acid and alkaline treatment and c after bleaching

Fig. 7 Transmission electron micrographs of hemp nanofibers a under 17.5% alkali extraction, b under 12%alkali extraction (15,000×)


the characteristic bands for the carboxylate groups at 1,740 cm−1, acetyl and methyl estergroup at 1,590 and 1,240 cm−1 [7]. In the transmittance spectra, the absorption bandassigned to the pectin carboxylic groups was observed at 1,739 cm−1 in untreated fibers, butdisappeared upon chemical treatments and the successive bleaching. This is because thecarboxylic groups were partially removed by alkali treatment through a process called de-esterification. During de-esterification, the ester groups on the pectin can be removed aswell. The alkali treatment allows the ionization of pectin carboxylic groups (–COOH) andthe formation of the corresponding sodium carboxylate (COONa), which decreases theability of hydrogen-type intermolecular bonds to form [7] and the solubility of the pectins.Reduction in the peak intensity found at around 1,631–1,633 cm−1 in chemically treatedand bleached fibers indicates the partial reaction of the C=O bonds of hemicelluloses. Theintensity of the 1,259 cm−1 peak is sharply weakened after the bleaching treatment, due tothe removal of hemicellulose materials. The peak observed at 895 cm−1 in both untreatedand chemically treated fibers indicates the presence of the glycosidic linkages between themonosaccharides and disappears in the spectrum for bleached fibers.

3.3 X-Ray Diffraction

X-ray crystallography was used to investigate the crystallinity of the sample after differenttreatments. X-ray powder diffraction photographs from untreated, acid and alkali treatedfibers and nanofibers are shown in Fig. 9. The percentage crystallinity of these samples wascalculated based on X-ray analysis by Eq. 1 and they are given in Fig. 9 as well. The maindiffraction intensity was at about 2θ=21° for each sample. The peak observed close to 2θ=22.4° is from cellulose. Untreated fiber exhibited very low crystallinity (57.4%) and a

10 15 20 25 30 35

Diffraction angle 2θ (degree)

Inte

nsity

(a.u

.)

Untreated (57.4%) Acid treated (61.9%)Acid & Alkaline treated (69.7%) Nanofiber (71.2%)

Fig. 9 X-ray diffractometry and crystallinity estimation after each stage of chemo-mechanical treatment forunbleached hemp fiber


single peak at about 2θ=22.75° and a broad hump showing amorphous nature. It can beseen that acid treated and acid/alkali treated fibers show peaks at 2θ=22.8° and 2θ=21.4°respectively. In the case of acid treated fibers, an additional peak was seen at 2θ=20.4° witha small hump. It is possible that acid treated fiber contains some residual lignin andhemicellulose that contribute to the slightly lower crystallinity (61.9%). Further the humphas disappeared showing that amorphous chains have been rearranged to crystallineregions. Acid/alkali treated fiber shows only one prominent peak and exhibits morecrystalline nature (69.7%) than acid treated fiber. Nanofibers showed a high crystallinefeature with a narrow strong peak at 2θ=21.6°. This peak can be attributed to cellulosecrystals. It was observed that the hemp nanofibers had an increased crystallinity from57.4% of untreated hemp fibers to 71.2% of nanofibers. As shown in Fig. 9, the relativecrystallinity of the samples increased after each stage of chemical treatments. Consecutivechemical treatments and processing of cellulose fibers give different X-ray patterns.According to X-ray testing on cellulose, cellulose is not made up of single perfect crystals.Disordered cellulose molecules as well as hemicelluloses and lignin are located in thespaces between the microfibrils. The hemicelluloses are considered to be amorphousalthough they apparently are oriented in the same direction as the cellulose microfibrils.Lignin is both amorphous and isotropic. It is believed that these crystallites are connected toeach other by disoriented amorphous zones [36]. The crystalline nature of the cellulosenanofibers is not only influenced by the chain conformation but also by the packing ofadjacent chains. Nanofibers are pure cellulose chains having different arrangements of theglucose chains as in the native cellulose of untreated fibers. The hemp nanofiber, aftersuccessive chemical treatments, only possessed 4.3% lignin and hemicellulose combined.The X-ray powder diffraction pattern showed a narrow peak which was more prominentand sharp for nanofiber, indicating the crystalline nature of this reinforcement and higherrelative crystallinity.

C;% ¼ IcrystallineIcrystalline þ Iamorphous

� 100 ð1Þ

4 Conclusions

This study has been concerned how the degree of individualization affects the hemp fibermorphology from the micro to the nanoscale. The chemi-mechanical process resulted inhemp nanofibers having a width in the range of 30–100 nm. The used chemical treatmentsresulted in the individualized hemp microfibers and further mechanical treatment formed anetwork structure of hemp nanofibers. The high pressure defibrillation contributed a uniquemorphology of the interconnected web-like structure of nanofibers.

Chemical analysis of the cellulose fiber after each stage of purification showed anincrease in cellulose content and a decrease in lignin and hemicellulose content. Successivebleaching helped with the cellulose purification. FTIR graph indicated the partial removalof the pectins during the fiber extraction. It was also seen that the relative crystallinity of thehemp fibers increased after each stage of chemical treatments.

Acknowledgment We gratefully acknowledge financial support of this study given by NSERC (NaturalSciences and Engineering Research Council of Canada).


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