Morphology and tensile properties of bast fibers extracted from cotton stalks

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 grows from the epidermis of the seeds of cotton plants of the genus Gossypium. The seed fiber contains the most pure crystalline cellulose found in nature. The cotton plant is a shrub or tree native in tropical to semi-arid regions around the world. Today cotton is grown in more than 100 countries on about 2.5% of the world’s arable land,¹ making it one of the most significant crops in terms of land use after food grains and soybeans. The area planted to cotton has remained quite stable for the last 50 years.¹ In excess of 24 million tonnes of cotton were produced worldwide in 2010/2011.² Cotton growth provides livelihood for many people in developing countries.

Cotton stalk is a necessary by-product of cotton growth. As a rough approximate, each hectare of cotton produces about three tonnes of dry stalk^3,4 and the total biomass produced worldwide amounts to about 100 million tonnes annually. Traditionally, most cotton stalks are incorporated into the soil³ or burnt in the field⁴ after harvesting. Both of these traditional operations have problems.^3–5 Utilization of cotton stalk for production of pulp and paper had been considered by several researchers.^6–8 However, the relative low density of cotton stalks for transport and mechanical difficulties in removing the outer bark from the thin stalk creates some problems for pulping and papermaking.⁶ Cotton stalk has also been considered to replace wood as regenerated cellulose for making rayon fibers⁹ and as feedstock for particle board using the so-called CIRCOT process.⁴ Undecorticated cotton stalks (i.e., the whole stalk containing both the bast fibers and the woody core) have also been used in polymer and cement composites.^10–13

From a botanical perspective the cotton stalk or plant stem serves to hold the chief photosynthetic organs of the plant (i.e., the leaves) to the light and to conduct photo-assimilates, nutrients and water between parts of the plant. The vascular system contained within the stalk comprises two major cellular components, xylem, through which water and nutrients passes upwards through the plant, and phloem, through which photo-assimilates manufactured in the leaves and other photosynthetic tissue are transported around the plant. In woody plants like cotton, thick-walled phloem cells form a supporting tissue around the plant stem and these can be extracted from the stem as fibers. The phloem cells in cotton are similar in substance and structure to jute, flax, hemp and kenaf fibers, known as bast fibers that are widely used for traditional textiles and more recently as reinforcement for 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 traditionally processed into fibers for textile or composite applications and as such has not been widely studied. Recently, Reddy and Yang¹⁵ used chemical methods to extract bast fibers from cotton stalks (cultivar DP 555 BG/RR) and determined their chemical constitution, crystallinity and mechanical strength.

Despite their abundance, bast fibers from cotton stalks are an unutilized resource and their properties have not been widely studied by the scientific community. In this paper, we report a study of mechanically decorticated bast fibers from cotton stalks. The study focuses on the morphology and the tensile strength variability of the fibers and a preliminary investigation of the suitability of the fibers as reinforcement in polymer composites.

Experimental details

Cotton stalk and fiber extraction

The fiber used in this study was extracted from the stems of a currently typical Upland (Gossypium hirsutum sp.) cultivar bred by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and grown outside in experimental, furrow irrigated plots at the Australian Cotton Research Institute (ACRI) near Narrabri, New South Wales, Australia. The seeds were sown in October 2010 and cotton and stems were harvested in early April 2011. The stems were cut at their base two weeks after a standard harvest that included defoliation and mechanical harvesting. They were then stored in a dry and dark environment for two months before examination.

Specimens of the bast fibers for structural study and tensile testing were obtained by manually removing (stripping) the outer bark layer of the stem and then cutting through the cuticle and outer cortex into the phloem tissue to initiate stripping of bast fiber bundles by hand. Each bast fiber bundle consists of many single phloem cell fibers held together by binding substances in the plant, that is, hemi-celluloses and lignin. Single phloem cells or elementary fibers are the smallest morphological units; these fibers are also known as ultimate fibers or “ultimates”.

Fiber bundles for the manufacture of composites were extracted from the stem using the mechanical decortication method developed in this study. All fiber samples were taken from the main stem of the cotton plant stem, that is, branches were not used. Although the stem fibers were refined to produce material largely containing bast fiber bundles, other fibrous plant tissue resulting from the decortication method, for example, fibrous and splinter lengths of the stem cortex and bark, were also included in the composite manufacture.

Morphological study of elementary fibers

Cross-sectional view

To obtain cross-sectional images of single bast fibers (phloem cells) herein referred to as ultimates, segments of the stem were cut from dry cotton stalk and treated in 70% ethanol at room temperature for 24 hours before being sectioned using a sledge microtome set to 30 µm. The sections were stained with 0.1% Toluidine Blue O followed by washing in water and then mounted on glass slides. A Leitz Dialux 22 optical microscope fitted with a phototube mounted with a Firewire Leica DFC290 HD camera was used for capturing cross-section images of the elementary fibers in the stained sections.

Longitudinal view

Bundles of ultimate fibers are held together by gummy materials in the plant. In order to assess the length of single fibers they were separated from the matrix using an alkali process known in the textile industry as degumming.¹⁶ In this study 20-mm-long fiber bundle segments cut from the stem were boiled in a 2.0 N sodium hydroxide solution for two hours. The bundles were then thoroughly washed in warm water, filtered and then rinsed in dilute acetic acid (3% w/w) to neutralize any remaining alkali on the fiber bundles. The bundles were then macerated in an equal proportion of 10% (w/w) nitric acid and 10% (w/w) chromic acid solution for 24 hours at room temperature (∼20℃). This was followed by rinsing in water and centrifuging in 70% ethanol to obtain single ultimate fibers, as shown in Figure 1. The separated ultimate fibers were examined for length using the above-mentioned Leitz Dialux 22 optical microscope equipped with a graticule.

Figure 1.

Optical microphotograph of separated single ultimate (phloem) fibers after the alkali and acid treatment. These fibers were used for fiber length measurement.

Tensile testing

Bast fibers are commonly extracted from plant stem, further processed and used in bundle form (also known as technical fibers, in contrast to single cell fibers or ultimates). The tensile properties of these technical fibers are of greater interest for manufacturers than that of the ultimates. Fiber bundle strips approximately 2–3 mm wide were extracted from cotton stalks using tweezers. Care was taken to ensure homogeneity of fibers selected, that is, free from any obvious defects along the fiber length. The outer bark (epidermis) of the specimens could be lifted off the specimen easily by finger nails or tweezers. All prepared fiber bundle strips were cut to a standard length of 35 mm to facilitate measurement of linear density. Prior to mechanical testing, all fiber bundles were dried in an oven at 110℃ for 2.5 hours and weighted individually, from which the linear density of the specimen in tex (1 tex = 1 mg/m) was calculated. To avoid inconsistency in strength measurements from using overly large or small specimens, only fiber bundles with weights between 1.8 and 6.0 mg were used. Fiber bundle specimens were glued to stiff paper handling frames using a commercial adhesive (Selleys Araldite®). The handling frames were cut with a slot that exposes the fiber specimen to a predetermined testing gauge length. The fibers were then tested at different gauge lengths, namely, 4, 6, 10 and 16 mm. Fibers were conditioned at 21℃ and 65% relative humidity for 24 hours before tensile testing.

Tensile testing of the fiber bundles was conducted according to ASTM D3822-07. An Instron (55 R 4501) tensile testing machine set to a crosshead speed of 1 mm/min was used to measure the tensile behaviors of the fiber bundles. A pretension of 0.5 cN was applied to remove any slackness in the fiber placed in the clamps. Specimens fractured at the edges of the paper frame (i.e., jaw breakage) were excluded. The tensile load was divided by the fiber linear density to obtain a specific stress (N/tex, or cN/tex), which is known as tenacity in the textile industry. The specific tensile strength in N/tex can be converted into stress unit (GPa) by multiplying the fiber tenacity using the fiber density estimated in this instance to be 1.5 g/cm³.

Decortication

To extract enough cotton bast fibers for an evaluation of fiber yield and to produce composite samples, we used a mechanical decortication method that involved beating the dry cotton stalk with a hammer causing the stalk to split longitudinally and the bast fibers to separate from the woody core of the plant, as shown in Figure 2(a). This proved to be a very efficient method for extracting the bast fibers. The method could be mechanized easily (for example by using a pair of crushing rollers) for large-scale decortication of dry cotton stalks. The decorticated bast fibers were then refined using a Shirley Analyzer, which is essentially a miniature carding machine that acts in this situation to separate finer bast fibers from coarser and heavier bundle segments. The effects of this process are shown in Figure 2(b) and (c). The refined fibers have the potential to be further processed into non-woven matting, 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).

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

Cotton stem fiber-reinforced composite

Cotton bast technical fibers extracted by the mechanical decortication method described above were used without any further refinement for the fabrication of composite samples. The fibers were pre-dried at 80℃ for two hours and then manually laid in approximate parallelization in a mold that was open at its two ends. Escon unsaturated polyester (UP) resin type F61347 supplied by Fibre Glass International was used to manufacture cotton bast fiber-reinforced composite samples. The UP resin was brushed onto the fibers evenly layer by layer at approximately a 1:1 fiber:resin weight ratio. The impregnated material was cured at room temperature while being held in a press under approximately 5 MPa constant pressure. The sample was then post cured at 70℃ in an oven for 35 minutes. This procedure was the same as that used to produce aligned flax fiber (sliver) composites based on the same resin system.¹⁸

The resulting composite sample was 4.25 mm in thickness. The composite sample was cut into specimens for flexural testing using a three-point bending test. Flexural testing was carried out in accordance with ASTM D 790-03¹² using an Instron 5500R tensile testing machine. Six specimens from the composite panel were tested.

Results and discussion

Fiber yield

Approximately three kilograms of cotton stalk was used in the yield comparison of extractable material by the mechanical decortication and water-retting methods described above. Extracted fibers were dried in a ventilated oven at 100℃ for two hours before their dry weights were determined. The fiber yield (dry weight of bast fiber to dry weight of stem) was 16.6% for the water-retting method and 18.0% for the dry mechanical decortication method. These values are indicative because there was no repeated experiment. The lower yield of water-retted fibers might be caused by leaching of water dissolvable materials in the fiber during retting.

Fiber morphology

Cross-sectional view

Figure 3 shows sections of the cotton plant stem at three positions along the length of the stem, that is, top, middle and bottom. Ultimate fibers situated close to the outer layer of the stem in each section are highlighted. The figure shows the ultimates organized in a hierarchical structure that changes according to the position along the length of the stem. At the first level, single cells form closely packed bundles in roughly rectangular shapes. The rectangular bundles are stacked in pyramid-shaped piles, with the apex pointing away from the plant center. Each pile may consist of as many as 10 or more layers of fiber bundles at the bottom of the plant and reduce to only a single layer at the top of the plant.

Figure 3.

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

The fiber bundles are typically 50–100 µm in thickness and up to 1 mm in width. Cross-sections of the ultimate fibers have a simple convex polygonal shape and prominent cell protoplasm (lumen). The number of ultimates in a bundle also changes according to its position along the plant height.

The quantity of ultimates in the cross-section of the stem changes dramatically according to the height of the plant. We counted the number of ultimate fibers contained in one pyramid from optical images and the number of pyramids in a stem cross-section, from which we estimated the total number of ultimates in the stem cross-section according to its height position in the plant. The number of ultimates per cross-section estimated in this way is plotted according to height in the plant in Figure 4. Clearly, the great majority of fibers originate from the lower part of the plant. We found fiber recovery yields of 80% from fibers in the lower half of the stalk.

Figure 4.

Estimated total number of single cell bast fibers (ultimates) in stem cross-section plotted against plant stem height (position 1 = bottom of stem, position 10 = top of stem, distance between segments = 100 mm).

Cell diameter and wall thickness

The cross-sectional shape of an ultimate fiber is largely round, albeit polygonal and irregular in shape (Figure 5). The shape of ultimate fibers has similarities with cotton seed fibers before boll opening and dehydration. The cross-section of cotton seed fibers is usually described by two parameters: the outer perimeter of the cross-section otherwise known as “biological fineness”, and the relative thickness of the fiber cell wall, which is known as the “maturity” of the fiber. The cross-sectional perimeters of ultimate fibers extracted from the cotton stalk may be described in the same way as for the cotton seed fibers.

Figure 5.

Single cell fiber cross-sectional parameters.

Measurements were made using an image analysis software Fiji (http://fiji.sc/wiki/index.php/Fiji). The exterior and interior profiles in optical microscope images were digitized, so the internal perimeter (C_i_nt) and external perimeter (C_e_xt) could be measured and the cell wall area integrated. We then constructed two concentric circles using the measured exterior and interior perimeters, as shown in Figure 5. The interior and exterior diameters of the reconstructed fiber cross-section are therefore

D_{int} = C_{int} / π

(1)

D_{ext} = C_{ext} / π

(2)

The wall thickness of the reconstructed cell fiber is thus

w = \frac{D_{ext} - D_{int}}{2}

(3)

The ratio of the area of the cell wall to the total area inside the exterior circle is referred to as fiber maturity in the study of cotton seed fiber.¹⁹ If we borrow this term, the “maturity” of an ultimate fiber can be calculated from

β = \frac{D_{ext}^{2} - D_{int}^{2}}{D_{ext}^{2}}

(4)

We analyzed more than 1000 cell cross-sections extracted from two plants from the same test plot (>500 cross-sections per plant). Table 1 gives the summarized fiber maturity and diameter averages. There were no significant differences between the two plants. Cotton stem ultimate fibers (with an average diameter of 12 µm) are much finer than jute ultimate fibers (average 20 µm),²⁰ and are similar to that of cotton seed fibers (12–17 µm).²¹

Table 1.

Cross-sectional parameters of cotton stem ultimate fibers.²⁰

	Diameters (µm)		Wall thickness (µm)	Maturity
	Exterior	Interior	Wall thickness (µm)	Maturity
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

We then examined the variability of these morphological parameters in relation to their positions in the plant. Cell fibers from the bottom and middle sections of the two cotton stalks were analyzed. The exterior diameter of the cell fibers did not change significantly in relation to their radial and height positions in the plant (Figure 6).

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.

Ultimate fibers from the bottom part of the plant showed very consistent maturity, as shown in Figure 7(a), that is, fibers grown close to the woody core had approximately the same maturity as those next to the epidermis. However, the maturity of fibers from the middle section of the plants showed a trend of increasing maturity towards the epidermis, as shown in Figure 7(b). The inner layer fibers (close to the woody core) had a maturity ratio of slightly lower than 0.6, while the outer layer fibers (close to the epidermis) had maturity ratios of about 0.8, similar to the value from the bottom part of the plant. This result perhaps reflects the growth sequence of bast fiber cells in plant stems.

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.

Length of single cell fibers

The length of ultimate fibers was measured from specimens separated using the alkali and acid macerating treatment described earlier using an optical microscope equipped with a graticule, as described earlier. The average length of clearly separated ultimates was 1.8 mm, with a standard deviation of 1.4 mm. This is very similar to the average of length of jute ultimate fibers (2 mm),²⁰ but much shorter than cotton seed fibers, which are typically about 25 mm for Upland cotton.¹⁹

Tensile properties of fiber bundles

We assigned numbers 1–10 for the samples according to the height positions of the segments on the cotton stalk from the bottom to the top. Each segment had approximately the same length. Twenty specimens were prepared 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 average and standard deviations of fiber-specific strength for the 10 segments are plotted in Figure 8(b). Despite the difference in cell cross-sectional morphology, the tenacity (specific strength) of fiber bundles from different height positions along the stalk, with the exception of segment 1, did not change significantly. Segment 1 showed somewhat lower average strength than the other nine segments. The overall average specific strength was 0.34 N/tex with a standard deviation of 0.07 N/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 1 = bottom of stem, position 10 = top of stem, distance between segments = 100 mm.

To convert the specific strength to commonly used, area-based strength expressed in MPa, we need to estimate the fiber density. The density of crystalline cellulose 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 of 1.5 g/cm³ (i.e., similar to jute), the overall average specific strength of 0.33 N/tex is equivalent to an area-based engineering strength of 510 MPa, which is within the range of strength commonly reported for other bast fibers, such as jute, kenaf, flax and hemp.^26–29

The averages and standard deviations of fiber-specific moduli for the 10 segments are plotted in Figure 8(c). These values follow a distribution pattern similar to that of the specific strength in Figure 8(b). The overall average of the specific modulus is 8.80 N/tex, which is equivalent to an initial modulus of 13.2 GPa based on the fiber density 1.5 g/cm³. The corresponding specific moduli for jute, flax and hemp fibers reported in literature are in the range from 11.8 to 21.7 N/tex.^19,29

Cotton stem fiber-reinforced polyester composite

The composite showed a flexural strength of 78.4 MPa (standard deviation 13.9 MPa) and a modulus of 11.4 GPa (standard deviation 3.5 GPa). In Table 2, these values are compared with values of composites fabricated from aligned flax fiber (sliver) and UP in our lab using the same fabrication procedure¹⁸ and composites made from aligned long hemp fiber (sliver) and polyester resin by Aziz and Ansell³⁰ and thermoplastic composites made from carded mat of cotton stalk bast fibers and polypropylene (pp) fibers by Reddy and Yang.¹⁵ Our cotton stalk bast fiber/UP composite showed much higher mechanical properties than the cotton stalk bast fiber/PP composite by Reddy and Yang.¹⁵ In comparison, the cotton stalk bast fiber/UP composite showed significantly lower flexural strength than our aligned flax composites reported previously, but similar modulus. It also showed similar strength and significantly higher modulus than the composites made from aligned long hemp fiber (untreated) reported by Aziz and Ansell.³⁰ Therefore, the bast fiber mechanically extracted from cotton stalk demonstrated good reinforcement for the polymer composite.

Table 2.

Mechanical properties of cotton bast fiber/unsaturated polyester composite in comparison with other previously reported natural fiber/unsaturated polyester composites

Fiber/laying method	Strength (MPa)	Modulus (GPa)	Reference
Cotton bast fiber, hand laid	78.4	11.4	This paper
Flax sliver, hand laid (same resin, fabricated in same lab using same method)	123–157	8.3–12.1	Miao et al.¹⁸
Long hemp sliver, hand laid	∼80 (untreated), ∼100 (treated)	∼7.5 (untreated), ∼9.5 (treated)	Aziz & Ansell³⁰
Cotton bast fiber, random	13.5–15.7	0.8–0.98	Reddy & Yang¹⁵

Conclusion

In this study we investigated the morphology of cotton bast (phloem) cell fibers (ultimates) and the strength distribution of technical fibers along the height of a cotton plant stem from the base to the tip. A simple mechanical decortication method was used to extract bast fibers from cotton stalk without retting. The morphology of single cell fibers or ultimates was characterized using an effective diameter and a cell wall thickness ratio (i.e., maturity) derived from a concentric circle model using data measured from optical microscope images. The morphological study shows that the maturity of ultimates is higher in the lower part of the stalk than that in the upper part and that 80% of the bast fibers are contained in the lower half of the stalk. Cotton bast fibers were found to be as strong as commonly used bast fibers, such as hemp, flax, jute and kenaf. The mechanically decorticated cotton stalk bast fibers demonstrated good reinforcement effect in unsaturated polyester composites.

Footnotes

Acknowledgments

The authors gratefully acknowledge the kind support of their CSIRO Plant Industry colleagues, Dr Mike Bange and Ms Jane Caton, for supplying the cotton stalks used in this work and Dr Colleen MacMillan for advice on staining and sectioning the cotton stems.

Funding

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

References

FAO and ICAC. A summary of the world apparel fibre consumption survey 2005–2008. 2011.

ICAC. Summary of the outlook for cotton. Cotton: Review of the world situation 2012; vol. 66: 3–4.

Gomes

Wilson

Coates

. Cotton (Gossypium) plant residue for industrial fuel: an economic assessment. Ind Crops Prod 1997; 7: 1–8.

Sreenivasan S. Utilisation of cotton plant stalks for value-added products, http://www.expresstextile.com/20041230/infocus01.shtml (2004, accessed 7 March 2012).

Koeser

HJK

Siemers

Stueven

. Technical and economic assessment of charcoal and densified fuel from water hyacinths and cotton stalks. Fuel Process Technol 1983; 7: 23–42.

Ali M, Byrd M and Jameel H. Chemimechanical pulping of cotton stalks. In: TAPPI pulping conference, 2002.

Fahmy

Ibrahim

. Cotton stalks as a fibrous source for fine paper and rayon: Part I: Bleached pulp for fine paper. Cellul Chem Technol 1976; 10: 723–735.

Pandey

Shaikh

. Utilisation of cotton plant stalk for production of pulp and paper. Biol Waste 1987; 21: 63–70.

Fahmy

Ibrahim

. Cotton stalks as a fibrous source for fine paper and rayon: Part II: Dissolving pulp for viscose rayon. Cellul Chem Technol 1979; 13: 385–390.

10.

Habibi Y, El-Zawawy WK, Ibrahim MM, et al. Processing and characterization of reinforced polyethylene composites made with lignocellulosic fibers from Egyptian agro-industrial residues. Compos Sci Technol 2008; 68: 1877–1885.

11.

Hassan

Nada

A-AMA

. Utilization of lignocellulosic fibers in molded polyester composites. J Appl Polym Sci 2003; 87: 653–660.

12.

Zhao

. Properties study of cotton stalk fiber/gypsum composite. Cem Concr Res 2003; 33: 43–46.

13.

. Research on adaptability between crop-stalk fibers and cement. Cem Concr Res 2004; 34: 1081–1085.

14.

Spencer AM, Jacobson A and Sixt K. Method of fibre liberation in cotton stalks and the pulp. US1954.

15.

Reddy

Yang

. Properties and potential applications of natural cellulose fibers from the bark of cotton stalks. Bioresour Technol 2009; 100: 3563–3569.

16.

Bhattacharya

Das

. Alkali degumming of decorticated ramie. Color Technol 2001; 117: 342–345.

17.

Miao

Finn

. Conversion of natural fibres into structural composites. J Textil Eng 2008; 54: 165–177.

18.

Miao M, Milanovic N and Shen SZ. Effect of moisture exposure during fabrication on flax/polyester composites. In: ICCM 18, Jeju, Korea, 2011.

19.

Morton

Hearle

JWS

. Physical properties of textile fibres, 3rd ed. Manchester: The Textile Institute, 1993.

20.

Pickering

. Properties and performance of natural-fibre compoistes, Cambridge: Woodhead Publishing Ltd, 2008.

21.

Hertel

Craven

. Cotton fineness and immaturity as measured by the arealometer. Textil Res J 1951; 21: 765–774.

22.

Madsen

Hoffmeyer

Thomsen

. Hemp yarn reinforced composites – I. Yarn characteristics. Composites Part A 2007; 38: 2194–2203.

23.

Baley

. Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase. Composites Part A 2002; 33: 939–948.

24.

Wang

W-m

Cai

Z-s

J-y

. Changes in composition, structure, and properties of jute fibers after chemical treatments. Fibers Polym 2009; 10: 776–780.

25.

Cook

. Handbook of textile fibres, Watford: Mwrrow, 1968.

26.

Xia

Jyy

Cheng

. Study on the breaking strength of jute fibres using modified Weibull distribution. Composites Part A. 2009; 40: 54–59.

27.

Islam

Church

Miao

. Effect of removing polypropylene fibre surface finishes on mechanical performance of kenaf/polypropylene composites. Composites Part A 2011; 42: 1687–1693.

28.

Velde

KVd

Baetens

. Thermal and mechanical properties of flax fibres as potential composite reinforcement. Macromol Mater Eng 2001; 286: 342–349.

29.

Zhang

Miao

. Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites. Compos Sci Technol 2010; 70: 130–135.

30.

Aziz

Ansell

. The effect of alkalization and fibre alignment on the mechanical and thermal properties of kenaf and hemp bast fibre composites: Part 1 – polyester resin matrix. Compos Sci Technol 2004; 64: 1219–1230.