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 DOI: 10.1177/0040517513495949

2014 84: 303 originally published online 15 July 2013Textile Research JournalAlban Yzombard, Stuart G Gordon and Menghe Miao

Morphology and tensile properties of bast fibers extracted from cotton stalks  

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Original article

Morphology and tensile properties of bastfibers extracted from cotton stalks

Alban Yzombard, Stuart G Gordon and Menghe Miao

Abstract

Bast fiber contained in cotton stalk, a residue from the growth of cotton fiber, is available in very large quantities,

estimated at more than 15 million tonnes annually. The stalk is currently burnt or buried into soil. In this study, bast fibers

were extracted from cotton stalk using a mechanical decortication method. The morphology of single bast fibers, also

known as ultimates or ultimate fibers, were characterized by an effective diameter and a cell wall thickening factor

(maturity) derived from a concentric circle model reconstructed using an image analysis technique. Fiber cells within the

same plant are quite consistent in diameter but can vary considerably in maturity depending on their position in the plant.

Eighty percent of the bast fibers were contained in the lower half of the stalk where the fiber maturity was high. Cotton

bast fibers are as strong as other bast fibers, such as jute and hemp, and can be used as reinforcement for polymer

composite materials.

Keywords

bast cellulose fibers, morphology, mechanical testing, polymer matrix composites

Cotton refers to the soft, fluffy staple fiber that growsfrom the epidermis of the seeds of cotton plants of thegenus Gossypium. The seed fiber contains the most purecrystalline cellulose found in nature. The cotton plant isa shrub or tree native in tropical to semi-arid regionsaround the world. Today cotton is grown in more than100 countries on about 2.5% of the world’s arableland,1 making it one of the most significant crops interms of land use after food grains and soybeans. Thearea planted to cotton has remained quite stable for thelast 50 years.1 In excess of 24 million tonnes of cottonwere produced worldwide in 2010/2011.2 Cottongrowth provides livelihood for many people in develop-ing countries.

Cotton stalk is a necessary by-product of cottongrowth. As a rough approximate, each hectare ofcotton produces about three tonnes of dry stalk3,4

and the total biomass produced worldwide amountsto about 100 million tonnes annually. Traditionally,most cotton stalks are incorporated into the soil3 orburnt in the field4 after harvesting. Both of these trad-itional operations have problems.3–5 Utilization ofcotton stalk for production of pulp and paper hadbeen considered by several researchers.6–8 However,the relative low density of cotton stalks for transport

and mechanical difficulties in removing the outer barkfrom the thin stalk creates some problems for pulpingand papermaking.6 Cotton stalk has also been con-sidered to replace wood as regenerated cellulose formaking rayon fibers9 and as feedstock for particleboard using the so-called CIRCOT process.4

Undecorticated cotton stalks (i.e., the whole stalk con-taining both the bast fibers and the woody core) havealso been used in polymer and cement composites.10–13

From a botanical perspective the cotton stalk orplant stem serves to hold the chief photosyntheticorgans of the plant (i.e., the leaves) to the light andto conduct photo-assimilates, nutrients and waterbetween parts of the plant. The vascular system con-tained within the stalk comprises two major cellularcomponents, xylem, through which water and nutrients

Materials Science and Engineering, Commonwealth Scientific and

Industrial Research Organisation (CSIRO), Australia

Corresponding author:

Dr M Miao, Commonwealth Scientific and Industrial Research

Organisation, Henry Street, Belmont 3216, Australia.

Email: menghe.miao@csiro.au

Textile Research Journal

2014, Vol 84(3) 303–311

! The Author(s) 2013

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passes upwards through the plant, and phloem,through which photo-assimilates manufactured in theleaves and other photosynthetic tissue are transportedaround the plant. In woody plants like cotton, thick-walled phloem cells form a supporting tissue around theplant stem and these can be extracted from the stem asfibers. The phloem cells in cotton are similar in sub-stance and structure to jute, flax, hemp and kenaffibers, known as bast fibers that are widely used fortraditional textiles and more recently as reinforcementfor polymeric composites.

A method for liberating fibers from cotton stalks by‘‘digestive softening’’ was patented as early as 1954.14

However, the bast fiber from cotton stalk is not trad-itionally processed into fibers for textile or compositeapplications and as such has not been widely studied.Recently, Reddy and Yang15 used chemical methods toextract bast fibers from cotton stalks (cultivar DP 555BG/RR) and determined their chemical constitution,crystallinity and mechanical strength.

Despite their abundance, bast fibers from cottonstalks are an unutilized resource and their propertieshave not been widely studied by the scientific commu-nity. In this paper, we report a study of mechanicallydecorticated bast fibers from cotton stalks. The studyfocuses on the morphology and the tensile strengthvariability of the fibers and a preliminary investigationof the suitability of the fibers as reinforcement in poly-mer composites.

Experimental details

Cotton stalk and fiber extraction

The fiber used in this study was extracted from thestems of a currently typical Upland (Gossypium hirsu-tum sp.) cultivar bred by the Commonwealth Scientificand Industrial Research Organisation (CSIRO) andgrown outside in experimental, furrow irrigated plotsat the Australian Cotton Research Institute (ACRI)near Narrabri, New South Wales, Australia. Theseeds were sown in October 2010 and cotton andstems were harvested in early April 2011. The stemswere cut at their base two weeks after a standard har-vest that included defoliation and mechanical harvest-ing. They were then stored in a dry and darkenvironment for two months before examination.

Specimens of the bast fibers for structural study andtensile testing were obtained by manually removing(stripping) the outer bark layer of the stem and thencutting through the cuticle and outer cortex into thephloem tissue to initiate stripping of bast fiber bundlesby hand. Each bast fiber bundle consists of many singlephloem cell fibers held together by binding substancesin the plant, that is, hemi-celluloses and lignin.

Single phloem cells or elementary fibers are the smallestmorphological units; these fibers are also known asultimate fibers or ‘‘ultimates’’.

Fiber bundles for the manufacture of compositeswere extracted from the stem using the mechanicaldecortication method developed in this study. Allfiber samples were taken from the main stem of thecotton plant stem, that is, branches were not used.Although the stem fibers were refined to producematerial largely containing bast fiber bundles, otherfibrous plant tissue resulting from the decorticationmethod, for example, fibrous and splinter lengths ofthe stem cortex and bark, were also included in thecomposite manufacture.

Morphological study of elementary fibers

Cross-sectional view. To obtain cross-sectional images ofsingle bast fibers (phloem cells) herein referred to asultimates, segments of the stem were cut from drycotton stalk and treated in 70% ethanol at room tem-perature for 24 hours before being sectioned using asledge microtome set to 30 mm. The sections werestained with 0.1% Toluidine Blue O followed by wash-ing in water and then mounted on glass slides. A LeitzDialux 22 optical microscope fitted with a phototubemounted with a Firewire Leica DFC290 HD camerawas used for capturing cross-section images of theelementary fibers in the stained sections.

Longitudinal view. Bundles of ultimate fibers are heldtogether by gummy materials in the plant. In order toassess the length of single fibers they were separatedfrom the matrix using an alkali process known inthe textile industry as degumming.16 In this study20-mm-long fiber bundle segments cut from the stemwere boiled in a 2.0N sodium hydroxide solution fortwo hours. The bundles were then thoroughly washedin warm water, filtered and then rinsed in dilute aceticacid (3% w/w) to neutralize any remaining alkali on thefiber bundles. The bundles were then macerated in anequal proportion of 10% (w/w) nitric acid and 10%(w/w) chromic acid solution for 24 hours at room tem-perature (�20�C). This was followed by rinsing in waterand centrifuging in 70% ethanol to obtain single ultim-ate fibers, as shown in Figure 1. The separated ultimatefibers were examined for length using the above-mentioned Leitz Dialux 22 optical microscopeequipped with a graticule.

Tensile testing

Bast fibers are commonly extracted from plant stem,further processed and used in bundle form (alsoknown as technical fibers, in contrast to single cell

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fibers or ultimates). The tensile properties of these tech-nical fibers are of greater interest for manufacturersthan that of the ultimates. Fiber bundle strips approxi-mately 2–3mm wide were extracted from cotton stalksusing tweezers. Care was taken to ensure homogeneityof fibers selected, that is, free from any obvious defectsalong the fiber length. The outer bark (epidermis) of thespecimens could be lifted off the specimen easily byfinger nails or tweezers. All prepared fiber bundlestrips were cut to a standard length of 35mm to facili-tate measurement of linear density. Prior to mechanicaltesting, all fiber bundles were dried in an oven at 110�Cfor 2.5 hours and weighted individually, from which thelinear density of the specimen in tex (1 tex¼ 1mg/m)was calculated. To avoid inconsistency in strengthmeasurements from using overly large or small speci-mens, only fiber bundles with weights between 1.8 and6.0mg were used. Fiber bundle specimens were glued tostiff paper handling frames using a commercial adhesive(Selleys Araldite�). The handling frames were cut witha slot that exposes the fiber specimen to a

predetermined testing gauge length. The fibers werethen tested at different gauge lengths, namely, 4, 6, 10and 16mm. Fibers were conditioned at 21�C and 65%relative humidity for 24 hours before tensile testing.

Tensile testing of the fiber bundles was conductedaccording to ASTM D3822-07. An Instron (55R4501) tensile testing machine set to a crosshead speedof 1mm/min was used to measure the tensile behaviorsof the fiber bundles. A pretension of 0.5 cN was appliedto remove any slackness in the fiber placed in theclamps. Specimens fractured at the edges of the paperframe (i.e., jaw breakage) were excluded. The tensileload was divided by the fiber linear density to obtaina specific stress (N/tex, or cN/tex), which is known astenacity in the textile industry. The specific tensilestrength in N/tex can be converted into stress unit(GPa) by multiplying the fiber tenacity using the fiberdensity estimated in this instance to be 1.5 g/cm3.

Decortication

To extract enough cotton bast fibers for an evaluationof fiber yield and to produce composite samples, weused a mechanical decortication method that involvedbeating the dry cotton stalk with a hammer causing thestalk to split longitudinally and the bast fibers to sep-arate from the woody core of the plant, as shown inFigure 2(a). This proved to be a very efficient methodfor extracting the bast fibers. The method could bemechanized easily (for example by using a pair ofcrushing rollers) for large-scale decortication of drycotton stalks. The decorticated bast fibers were thenrefined using a Shirley Analyzer, which is essentially aminiature carding machine that acts in this situation toseparate finer bast fibers from coarser and heavierbundle segments. The effects of this process areshown in Figure 2(b) and (c). The refined fibers havethe potential to be further processed into non-wovenmatting, for example by using an air-laying method.17

Figure 2. Cotton bast fiber decortication and refining: decortication tools (a); first carding pass (b); and second carding pass (c).

Figure 1. Optical microphotograph of separated single ultimate

(phloem) fibers after the alkali and acid treatment. These fibers

were used for fiber length measurement.

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To provide a yield comparison based on the extrac-tion method, we also extracted bast fibers from cottonstalk using a simple tank water-retting method. Thecotton stalk was immersed in a cylindrical containerfilled with untreated water taken from a nearby river.The immersion time was 37 days during the SouthernHemisphere winter (from 2 August to 7 September).After retting, long strips of bast fibers were removedfrom the stalk by hand while the stems were wet.

Cotton stem fiber-reinforced composite

Cotton bast technical fibers extracted by the mechan-ical decortication method described above were usedwithout any further refinement for the fabrication ofcomposite samples. The fibers were pre-dried at 80�Cfor two hours and then manually laid in approximateparallelization in a mold that was open at its two ends.Escon unsaturated polyester (UP) resin type F61347supplied by Fibre Glass International was used tomanufacture cotton bast fiber-reinforced compositesamples. The UP resin was brushed onto the fibersevenly layer by layer at approximately a 1:1 fiber:resinweight ratio. The impregnated material was cured atroom temperature while being held in a press underapproximately 5MPa constant pressure. The samplewas then post cured at 70�C in an oven for 35minutes.This procedure was the same as that used to producealigned flax fiber (sliver) composites based on the sameresin system.18

The resulting composite sample was 4.25mm inthickness. The composite sample was cut into speci-mens for flexural testing using a three-point bendingtest. Flexural testing was carried out in accordancewith ASTM D 790-0312 using an Instron 5500R tensiletesting machine. Six specimens from the compositepanel were tested.

Results and discussion

Fiber yield

Approximately three kilograms of cotton stalk wasused in the yield comparison of extractable materialby the mechanical decortication and water-rettingmethods described above. Extracted fibers were driedin a ventilated oven at 100�C for two hours before theirdry weights were determined. The fiber yield (dryweight of bast fiber to dry weight of stem) was 16.6%for the water-retting method and 18.0% for the drymechanical decortication method. These values areindicative because there was no repeated experiment.The lower yield of water-retted fibers might be causedby leaching of water dissolvable materials in the fiberduring retting.

Fiber morphology

Cross-sectional view. Figure 3 shows sections of the cottonplant stem at three positions along the length of thestem, that is, top, middle and bottom. Ultimate fiberssituated close to the outer layer of the stem in each sec-tion are highlighted. The figure shows the ultimatesorganized in a hierarchical structure that changesaccording to the position along the length of the stem.At the first level, single cells form closely packed bundlesin roughly rectangular shapes. The rectangular bundlesare stacked in pyramid-shaped piles, with the apexpointing away from the plant center. Each pile may con-sist of as many as 10 or more layers of fiber bundles atthe bottom of the plant and reduce to only a single layerat the top of the plant.

The fiber bundles are typically 50–100mm in thick-ness and up to 1mm in width. Cross-sections of theultimate fibers have a simple convex polygonal shapeand prominent cell protoplasm (lumen). The number ofultimates in a bundle also changes according to its pos-ition along the plant height.

The quantity of ultimates in the cross-section of thestem changes dramatically according to the height ofthe plant. We counted the number of ultimate fiberscontained in one pyramid from optical images andthe number of pyramids in a stem cross-section, fromwhich we estimated the total number of ultimates in thestem cross-section according to its height position in theplant. The number of ultimates per cross-section esti-mated in this way is plotted according to height in theplant in Figure 4. Clearly, the great majority of fibersoriginate from the lower part of the plant. We foundfiber recovery yields of 80% from fibers in the lowerhalf of the stalk.

Cell diameter and wall thickness. The cross-sectional shapeof an ultimate fiber is largely round, albeit polygonaland irregular in shape (Figure 5). The shape of ultimatefibers has similarities with cotton seed fibers before bollopening and dehydration. The cross-section of cottonseed fibers is usually described by two parameters: theouter perimeter of the cross-section otherwise known as‘‘biological fineness’’, and the relative thickness of thefiber cell wall, which is known as the ‘‘maturity’’ of thefiber. The cross-sectional perimeters of ultimate fibersextracted from the cotton stalk may be described in thesame way as for the cotton seed fibers.

Measurements were made using an image analysissoftware Fiji (http://fiji.sc/wiki/index.php/Fiji). Theexterior and interior profiles in optical microscopeimages were digitized, so the internal perimeter (Cint)and external perimeter (Cext) could be measured andthe cell wall area integrated. We then constructed twoconcentric circles using the measured exterior and inter-ior perimeters, as shown in Figure 5. The interior and

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exterior diameters of the reconstructed fiber cross-section are therefore

Dint ¼ Cint=� ð1Þ

Dext ¼ Cext=� ð2Þ

The wall thickness of the reconstructed cell fiber isthus

w ¼Dext �Dint

2ð3Þ

The ratio of the area of the cell wall to the total areainside the exterior circle is referred to as fiber maturityin the study of cotton seed fiber.19 If we borrow thisterm, the ‘‘maturity’’ of an ultimate fiber can be calcu-lated from

� ¼D2

ext �D2int

D2ext

ð4Þ

We analyzed more than 1000 cell cross-sectionsextracted from two plants from the same test plot(>500 cross-sections per plant). Table 1 gives the sum-marized fiber maturity and diameter averages. Therewere no significant differences between the two plants.Cotton stem ultimate fibers (with an average diameterof 12 mm) are much finer than jute ultimate fibers (aver-age 20 mm),20 and are similar to that of cotton seedfibers (12–17 mm).21

Figure 3. Hierarchical organization of single cell (or ultimate) bast fiber bundles in the cross-section of cotton stem.

Figure 4. Estimated total number of single cell bast fibers

(ultimates) in stem cross-section plotted against plant stem

height (position 1¼ bottom of stem, position 10¼ top of stem,

distance between segments¼ 100 mm).

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We then examined the variability of these morpho-logical parameters in relation to their positions in theplant. Cell fibers from the bottom and middle sectionsof the two cotton stalks were analyzed. The exteriordiameter of the cell fibers did not change significantlyin relation to their radial and height positions in theplant (Figure 6).

Ultimate fibers from the bottom part of the plantshowed very consistent maturity, as shown inFigure 7(a), that is, fibers grown close to the woodycore had approximately the same maturity as thosenext to the epidermis. However, the maturity of fibersfrom the middle section of the plants showed a trend ofincreasing maturity towards the epidermis, as shown inFigure 7(b). The inner layer fibers (close to the woodycore) had a maturity ratio of slightly lower than 0.6,while the outer layer fibers (close to the epidermis) hadmaturity ratios of about 0.8, similar to the value fromthe bottom part of the plant. This result perhaps reflectsthe growth sequence of bast fiber cells in plant stems.

Length of single cell fibers

The length of ultimate fibers was measured from speci-mens separated using the alkali and acid macerating

treatment described earlier using an optical microscopeequipped with a graticule, as described earlier. Theaverage length of clearly separated ultimates was1.8mm, with a standard deviation of 1.4mm. This isvery similar to the average of length of jute ultimatefibers (2mm),20 but much shorter than cotton seedfibers, which are typically about 25mm for Uplandcotton.19

Tensile properties of fiber bundles

We assigned numbers 1–10 for the samples according tothe height positions of the segments on the cotton stalkfrom the bottom to the top. Each segment had

Figure 5. Single cell fiber cross-sectional parameters.

Figure 6. Fiber exterior diameter distribution. Position in plant

cross-section refers to the sequence of fiber bundles in the

pyramids as shown in Figure 3.

Table 1. Cross-sectional parameters of cotton stem ultimate

fibers.20

Diameters (mm) Wall

thickness

(mm) MaturityExterior Interior

Plant A 12.2� 3.6 5.4� 2.4 3.4� 1.3 0.79� 0.1

Plant B 11.8� 3.2 5.6� 2.4 3.1� 0.9 0.77� 0.1

Average of A&B 12.0 5.5 3.3 0.78

Cotton seed fiber 12–17 – 2–5 0.7–1.0

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approximately the same length. Twenty specimens wereprepared and tested from each of the 10 segments,giving a total of 200 specimens for tensile testing.Figure 8(a) shows a typical tensile curve. The averageand standard deviations of fiber-specific strength for the10 segments are plotted in Figure 8(b). Despite the dif-ference in cell cross-sectional morphology, the tenacity

(specific strength) of fiber bundles from different heightpositions along the stalk, with the exception of segment1, did not change significantly. Segment 1 showed some-what lower average strength than the other nine seg-ments. The overall average specific strength was0.34N/tex with a standard deviation of 0.07N/tex;again very similar to values for cotton seed fiber.

Figure 8. A typical tensile curve (a); tenacity (specific strength) distribution of bundle fibers along plant height (b); and specific

modulus distribution (c). Note that position 1¼ bottom of stem, position 10¼ top of stem, distance between segments¼ 100 mm.

Figure 7. Average maturity of cell fibers in bottom section of plant (a) and in middle section of plant (b). Position in plant cross-

section refers to the sequence of fiber bundles in the pyramids as shown in Figure 3.

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To convert the specific strength to commonly used,area-based strength expressed in MPa, we need to esti-mate the fiber density. The density of crystalline cellu-lose is 1.64 g/cm3.22 Bast fibers (jute, flax and hemp)typically have densities in the range of 1.50–1.58 g/cm3

with jute at the lower end.22–25 Based on a density of1.5 g/cm3 (i.e., similar to jute), the overall average spe-cific strength of 0.33N/tex is equivalent to an area-basedengineering strength of 510MPa, which is within therange of strength commonly reported for other bastfibers, such as jute, kenaf, flax and hemp.26–29

The averages and standard deviations of fiber-specific moduli for the 10 segments are plotted inFigure 8(c). These values follow a distribution patternsimilar to that of the specific strength in Figure 8(b).The overall average of the specific modulus is8.80N/tex, which is equivalent to an initial modulusof 13.2GPa based on the fiber density 1.5 g/cm3. Thecorresponding specific moduli for jute, flax and hempfibers reported in literature are in the range from 11.8 to21.7N/tex.19,29

Cotton stem fiber-reinforced polyester composite

The composite showed a flexural strength of 78.4MPa(standard deviation 13.9MPa) and a modulus of11.4GPa (standard deviation 3.5GPa). In Table 2,these values are compared with values of compositesfabricated from aligned flax fiber (sliver) and UP inour lab using the same fabrication procedure18 andcomposites made from aligned long hemp fiber(sliver) and polyester resin by Aziz and Ansell30 andthermoplastic composites made from carded mat ofcotton stalk bast fibers and polypropylene (pp) fibersby Reddy and Yang.15 Our cotton stalk bast fiber/UPcomposite showed much higher mechanical propertiesthan the cotton stalk bast fiber/PP composite by Reddyand Yang.15 In comparison, the cotton stalk bast fiber/UP composite showed significantly lower flexuralstrength than our aligned flax composites reported pre-viously, but similar modulus. It also showed similarstrength and significantly higher modulus than thecomposites made from aligned long hemp fiber

(untreated) reported by Aziz and Ansell.30 Therefore,the bast fiber mechanically extracted from cotton stalkdemonstrated good reinforcement for the polymercomposite.

Conclusion

In this study we investigated the morphology of cottonbast (phloem) cell fibers (ultimates) and the strengthdistribution of technical fibers along the height of acotton plant stem from the base to the tip. A simplemechanical decortication method was used to extractbast fibers from cotton stalk without retting. Themorphology of single cell fibers or ultimates was char-acterized using an effective diameter and a cell wallthickness ratio (i.e., maturity) derived from a concentriccircle model using data measured from optical micro-scope images. The morphological study shows that thematurity of ultimates is higher in the lower part of thestalk than that in the upper part and that 80% ofthe bast fibers are contained in the lower half of thestalk. Cotton bast fibers were found to be as strong ascommonly used bast fibers, such as hemp, flax, jute andkenaf. The mechanically decorticated cotton stalk bastfibers demonstrated good reinforcement effect in unsa-turated polyester composites.

Funding

This research received no specific grant from any fundingagency in the public, commercial or not-for-profit sectors.

Acknowledgments

The authors gratefully acknowledge the kind support of theirCSIRO Plant Industry colleagues, Dr Mike Bange and Ms

Jane Caton, for supplying the cotton stalks used in this workand Dr Colleen MacMillan for advice on staining and sec-tioning the cotton stems.

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�100 (treated)

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