S2Biocomposite05!06!07 08th

2Plant Fibers as Reinforcement for Green Composites

Alexander Bismarck, Supriya Mishra, and Thomas Lampke

CONTENTS2.1 Introduction 2.2 Plant Fiber Composition and Structure 2.3 Fiber Production Chain 2.4 Agricultural Fiber Crop Cultivation

2.4.1 Harvesting Bast Fiber Crops 2.4.2 Fiber Extraction, Separation and Processing

2.4.2.1 Dew or Field Retting 2.4.2.2 Stand-Retting 2.4.2.3 Cold-Water Retting 2.4.2.4 Warm-Water Retting 2.4.2.5 Mechanical or Green Retting 2.4.2.6 Wet-Retting Processes

2.4.2.6.1 Ultrasound Retting 2.4.2.6.2 Steam Explosion Method 2.4.2.6.3 The Duralin Process 2.4.2.6.4 Enzyme Retting 2.4.2.6.5 Chemical and Surfactant Retting

2.5 Impact on Fiber Properties: Different Fiber Separation/Retting Procedures

2.6 Fiber Treatment and Modification 2.7 Bast Fibers

2.7.1 Flax (Linum usitatissimum L., Linaceae) Fibers 2.7.2 Hemp (Cannabis sativa L., Cannabaceae) Fibers 2.7.3 Jute (Corchorus capsularis, Tiliaceae) Fibers 2.7.4 Kenaf (Hibiscus cannabinus L., Malvaceae) Fibers 2.7.5 Ramie (Boehmeria nivea L. and Boehmeria viridis,

Urticaceae) Fibers 2.7.6 (Stinging) Nettle (Urtica dioica L., Urticaceae) Fibers Copyright 2005 by Taylor & Francis

2.8 Leaf Fibers 2.8.1 Sisal (Agave sisalana, Liliaceae) Fibers 2.8.2 Henequen (Agave fourcroydes, Liliaceae) Fibers 2.8.3 Pineapple (Anannus comosus, Bromeliaceae)

Leaf Fibers (PALF) 2.8.4 Abaca (Musa textilis Nee, Musaceae) Fibers 2.8.5 Oil Palm (Elaeis guineensis, Palmacea) Fibers

2.9 Seed Fibers 2.9.1 Cotton (Gossypium spp., Malvaceae)

2.10 Fruit Fibers 2.10.1 Coconut Husk or Coir (Cocos nucifera, Palmae) Fibers

2.11 Stalk Fibers: (Cereal) Straw Fibers 2.12 Conclusion 2.13 Outlook Acknowledgments References

ABSTRACT Plant fiber crops belong to the earliest known cultivatedplants. They were cultivated for fiber production and were extensivelydeveloped through breeding and selection according to the human needsand values. These fibers used to possess great agricultural importance forthe production of textiles until the late 19th century. However, the produc-tion of cheap synthetic textile fibers nearly terminated the production of tra-ditional fiber crops, especially in Western Europe and North America.

The increasing environmental awareness, growing global waste problems,the continuously rising high crude oil prices motivated governments toincrease the legislative pressure; see, for instance, the European Union End-of-Life Vehicles* as well as Waste Electrical and Electronic Equipment (WEEE)Directive. This in turn prompted researchers, industry and farmers to developconcepts of environmental sustainability and reconsider renewable resources.

As a result of new legislation, the composite and polymer manufacturers,the processing industry and end-users but also the local communities willneed to move away from traditional materials. New strategies will have tobe developed for environmentally and economically viable materials manu-facturing and processing, but also reuse and recycling. Composites withmoderate strength will perform for many noncritical structural applicationsin the automotive and electronic, but also for packaging, housing and build-ing industry. Green composites made entirely from renewable agricultural

* Directive 2000/53/EC of the European Parliament and of the Council of 18 September 2000 onend-of life vehicles, link in: http://europa.eu.int/eur-lex/pri/en/oj/dat/2000/l_269/l_26920001021en00340042.pdf accessed on: 01.09.2004. Directive 2002/96/EC of the European Parliament and of the Council of 27 January 2003 onwaste electrical and electronic equipment (WEEE), link in: http://europa.eu.int/eur-lex/pri/en/oj/dat/2003/l_037/l_03720030213en00240038.pdf accessed on: 27.05.2004.

Copyright 2005 by Taylor & Francis

resources could offer a unique alternative for these applications. Plant fiberswill be used as the reinforcing phase in such composites. They offer a realalternative to the commonly used synthetic reinforcing fibers, such as car-bon, glass or aramid, because of their low density, good mechanical proper-ties, abundant availability and problem-free disposal.

Farmers might benefit from fiber crops because of their quick turnaroundtime. Fiber crops often produce very long fibers. The energy consumptionfor fiber crop cultivation, harvesting and fiber separation is much lower thanthe energy needed to manufacture synthetic fibers.

However, there are also some drawbacks related to the use of plant fibersas reinforcement for polymers. Restrictions for the successful exploitationare their high moisture absorption, low microbial resistance and low thermalstability.

Further research is necessary to gain a better understanding to developsustainable economically viable materials processes for composites and todesign innovative products of common interests.

2.1 Introduction

Fiber crops have accompanied human society since the start of our time. Inearly history, humans collected the raw materials for ropes and textiles fromthe wild. Later societies learned to cultivate such crops. Plant fiber crops areamong the earliest known cultivated plants and humans continued thedomestication of these crops over millennia.1 Fiber crop varieties have beenextensively developed through breeding and selection, according to the soci-eties needs and values. For instance, hemp and linen fragments were foundin Neolithic sites in Syria, Turkey, Mesopotamia (present-day Iraq), and Persia(present-day Iran), and have been carbon dated back to 80006000 B.C.24 Theancient Egyptians wrapped their corpses in linen cloth for thousands of years.Tomb paintings and hieroglyphs show and describe the production of flax, ret-ting, spinning, and weaving as well as the treatment and dyeing of linencloths. In Central Europe, the Swiss lake dwellers started flax cultivation andthe production of linen more than 4000 years ago. Fiber crops have been bredfocusing on fiber quality, climatic adaptability, and yield factors. Ingeniousfiber crops, such as flax, hemp, and nettle, possessed great agricultural impor-tance for the production of textile fibers until the late 19th century. However,the mechanization of cotton harvest, processing, and development, and thegrowing demand for and production of cheap synthetic textile fibersdestroyed the production of traditional fiber crops. Gradually, they becameless significant and almost vanished in Western Europe and North America.

More recently, increasing environmental awareness, concern for environ-mental sustainability, and the growing global waste problem, initiation ofecological regulations and legislation such as the end-of-life vehicles


regulation,5,6 the depletion of fossil fuels, and the increasingly higher price ofcrude oil have together created a groundswell of interest in renewableresources. Legislative pressures for greener technologies as well as customersdemands for more environmentally friendly consumer goods are forcing mate-rials suppliers and manufacturers to consider the environmental impact oftheir products at all stages of their life cycle, including materials selection pro-cessing, recycling, and final disposal. This and the worldwide availability ofplant fibers7 and other abundantly accessible agrowaste is responsible for thisnew research interest 810 in the field of polymer science, engineering, and isresponsible for a new interest in research in sustainable technology. Researchhas as its objective the development, processing and manufacturing, recycling,and disposal of green plastics, adhesives, polymer composites, blends, andmany other industrial products from renewable resources. Consequently, acradle-to-grave approach is emerging (Figure 2.1).

Renewable resources11 from agricultural or forestry products form a basisfor new industrial products or alternative energy sources. Plant-based fibersare already used in a wide range of products. Plant fibers find applicationsas textiles and geotextiles, twines and ropes, special pulps, insulating andpadding materials, fleece, felts and nonwoven materials, and increasingly asreinforcement for polymers.

The Sun

Photosynthesis

Intermediate productfiber mats, fleece etc.

naturalCO2 & H2Odecomposition

Production

Raw fibers

Compostbio-waste

Fiber extraction& separation

Processing

Final product eco-composite

Renewable resourcesfiber crops

FIGURE 2.1Truly green ecocomposites in natures circle of life. (Adapted from Fakten & Trends 2002 ZurSituation der Landwitschaft, Eggenfelden, 2002, p. 193.)


The mechanical properties of plant fibers are much lower when comparedto those of the most widely used competing reinforcing glass fibers (Table2.1). However, because of their low density, the specific properties (property-to-density ratio), strength, and stiffness of plant fibers are comparable to the values of glass fibers.1214 The public generally regards products of renew-able raw materials as environmentally friendly. A comparison of the ecobal-ances (see also Figure 2.2) for glass and plant fibers starting with the seedproduction and/or acquiring the raw material to the finished fleece showedthat the energy input is up to 83% lower for mats manufactured from plantfibers.15,16 *

Other ecologically relevant parameters provide some distinct advantages tothe use of plant fiber crops. These are the lower ecotoxicity, due to the reduced

* This issue is still under debate.10 If plant fibers are to replace glass fibers in composite appli-cations, then it should be made clear that overall energy balance, i.e., the energy involved in har-vesting, separating, treating, and processing the fibers as well as their incorporation incomposites plus their disposal is less compared to glass and glass fiber composite processing.

TABLE 2.1

Characteristic Values for the Density, Diameter, and Mechanical Properties ofVegetable and Synthetic Fibers

Density Diameter Tensile Youngs Elongation

Fiber (g cm23) (m) Strength (MPa) Modulus (GPa) at Break (%)

Flax 1.5 40600 3451500 27.6 2.73.2Hemp 1.47 25500 690 70 1.6Jute 1.31.49 25200 393800 1326.5 1.161.5Kenaf 930 53 1.6Ramie 1.55 400938 61.4128 1.23.8Nettle 650 38 1.7Sisal 1.45 50200 468700 9.422 37HenequenPALF 2080 4131627 34.582.5 1.6Abaca 430760Oil palm EFB 0.71.55 150500 248 3.2 25Oil palm mesocarp 80 0.5 17Cotton 1.51.6 1238 287800 5.512.6 78Coir 1.151.46 100460 131220 46 1540E-glass 2.55 ,17 3400 73 2.5Kevlar 1.44 3000 60 2.53.7Carbon 1.78 57 3400a4800b 240b425a 1.41.8

a Ultra high modulus carbon fibers.b Ultra high tenacity carbon fibers.

Source: Adapted from Mohanthy, A.K. et al., Macromol. Mater. Engin., 276277, 1, 2000. Further

developed with Kennwerte von Fasern nach KOHLER & WEDLER 1996 and Kennwerte von

Faserwerkstoffen nach BOBETH. Link in: http: //www.inaro.de/Deutsch/ ROHSTOFF/indus-

trie/FASER/mechkenn.htm accessed on July 25, 2003, and Sreekala, M.S. et al., J. Appl. Polym.

Sci., 66, 821, 1997.


need for pesticides and herbicides, and, therefore, lower CO2 emissions fromdiesel fuel during farming. However, overall energy and ecological balancedepends strongly on the final product, its production process, and the circum-stances of the journey of life of the product.17 For example, the contribution ofhemp long-fiber textiles to emissions of CO2 (greenhouse effect) stronglydepends on the efficiency of the fiber processing, the spinning equipment, andthe design life of the final product. This dependency prevents simplified con-clusions on the ecological superiority of plant fibers.18 Yet, flax and/or hempfibers are up to 40% cheaper compared to glass fibers.15 Moreover, plant fibersare nonabrasive to mixing and molding equipment, which can contribute tosignificant cost reductions.13 These are the primary advantages of using suchfibers as reinforcements for polymer matrix composites.

Additionally, plant fibers are of less concern to occupational health and safetyduring handling than other fibrous materials. The favorable aspect ratios andhigh specific properties at low cost make them an ecologically-friendly alterna-tive to conventional reinforcing fibers in composite materials.19 The ecologicalcharacter, the biodegradability, and the price of plant fibers are very importantfor their acceptance in large volume engineering markets such as the automo-tive and construction industry. Innovative plant fiber-reinforced polymers arerapidly finding more and more applications in secondary structural applica-tions20 especially for various applications in the automotive industry.2125 Plantfibers are currently used in considerable quantities in various applications in theautomotive industry only in the interior of passenger cars and truck cabins. Forexample, fiber crops, such as flax and hemp, currently grown in the EuropeanUnion (EU) (and in North America) provide an alternative to the overproduc-tion of food crops and land division. The most important end user of natural

Resources

Raw materialproduction

Processing &production Application

Waste usageor removal

Transport

Storage

Transport

Storage

[waste]Transport

Storage

Recycling

[harmful]Emissions

[harmful]Emissions

[harmful]Emissions

[harmful]Emissions

Transport

Storage

Transport

Storage

Environment

Environment

FIGURE 2.2Ecological product life cycle. (Adapted from Bennauer, U. and Dyckhoff, H., Katalyse-Institut(Hrsg.), Hanf & Co. Die Renaissance der heimischen Faserpflanzen, Verlag Die Werkstatt,Gttingen, 1995, p. 75.)


fiber products in Europe is the automobile industry (1999: 14,000 t(Germany/Austria), 2000: 17,000 t natural fibers). Plant fibers are used as trimparts in door panels or cabin linings. Coir fibers bonded with natural latex areused as seat cushions. Plant fibers are increasingly used for thermoacousticinsulation purposes. These insulating materials are mainly based on recycledtextiles and have high fiber content exceeding 80 wt%.26 At the present time,there are very few exterior parts made from plant fiber composites.

Sustainable biobased ecoproducts are products with commercial viabilityand environmental acceptability that are derived from renewable resourcesand that have recycling capabilities and triggered biodegradability.27 Byincorporating plant fibers into biopolymer matrices,28 such as derivatives ofcellulose, starch, shellac, lactic acid, cashew nut shell liquid, polyhydroxy-alkanoates (bacterial polyesters), and soy-based plastics, new truly greenbiodegradable ecocomposites (also called biocomposites) are created. Thisclass of materials is currently under development2932 and heavilyresearched.3342

An increased demand for renewable resources, including plant fibers, aswell as sources for energy or raw materials (reactants) for paints, lacquer,varnishes, adhesives, and polymers43 can have an important socioeconomicimpact. It will generate a nonfood crop source for the economic developmentof farming and rural- and agricultural-based areas of the world.44 Theincreased demand for fiber crops offers alternatives for employment andincome.17 Conversely, the recently adopted EU end-of-life vehicles regula-tion, applying to all passenger cars and light commercial motor vehicles,threatens the most important market for fiber crops. Starting from 2015 theregulation allows only an incineration quota of 5% for disused cars. This par-ticularly concerns composite materials, since a recycling and reutilization ofthe material is technically and economically impossible.45 Aggravating thissituation is the regulation forbidding composite waste landfill by the end of2004 in most EU member states. The incineration of composite waste willhave imposed limits that depend on the level of the energy content of thewaste. Consequently, the composite suppliers and users have to agree toaccept responsibility for the recycling of the end-of-life waste by providing agreen FRP (fiber-reinforced plastics) recycling label on their products.46

Fiber crops produced in the EU (and North America) cannot and will notcompete on the global market without EU and governmental subsidies inthe near future. This is due to the relatively high processing costs as com-pared to the synthetic reinforcing fiber materials they are supposed to sub-stitute.47,48 The amount of subsidies in the EU is decreasing in the past coupleof years (for hemp 1999/2000 721/ha, 2000/2001 646/ha,49,50 down to392/ha in 2002 /200351), which makes the cultivation of fiber crops lessinteresting for farmers.52 Therefore, a profitable production of fiber crops inWestern Europe (and probably North America) can only be realized if the pro-duction costs can be recovered without subsidies in the long term.53 One wayto compete on the world market is to produce and supply high-quality fibersfor high value-added products for niche applications in the textile or compos-ite sector.5456 Up to now the greatest barrier against increased industrial use of


European-grown plant fibers is the mismatch between production costs ofthe fibers and their value added.47,57 Obviously, the profitable production inthe long run is not sustainable unless a technological breakthrough in fiberprocessing can be realized. The price competition between European-grownfibers and fibers cultivated and harvested in developing countries, such asjute from India and Bangladesh, will be severe.47

The use of plant fibers as reinforcement for polymer matrix compositeshas also some drawbacks. One restriction to the successful exploitation ofplant fibers for durable composite applications is their high moistureabsorption and their low microbial resistance and susceptibility to rotting.These properties need to be considered, particularly during shipment andlong-term storage, as well as during composite processing. The hydrophiliccharacter of the fibers and, therefore, the high moisture uptake affects notonly the fiber surface and bulk properties but also the properties of theircomposites.5861 The high moisture absorption leads to fiber swelling, whichin turn results in changing mechanical and physical properties and reducesthe dimensional stability of the composite.62 The predominantly hydrophiliccharacter of plant fibers is also the reason for incompatibility, and poorimpregnation behavior of the fibers with some (e.g., polyolefins63) hydropho-bic thermoplastic matrices. Another limiting factor for the extended use oflignocellulose fibers in composites is their low thermal stability. In order toavoid degradation of the fibers during processing (Figure 2.3), the tempera-tures are limited to ca. 200C (taking into account that the processing time

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FIGURE 2.3Effect of thermal stress on the mechanical properties of retted flax fibers. The fibers wereexposed to air at varying temperatures. (Reprinted from Wielage, B. et al., Thermochim. Acta,337, 169177, 1999. With permission from Elsevier.)


should be as short as possible),64 which further restricts the choice of thepolymer matrix materials.

A major drawback of plant fibers is their nonuniformity and the vari-ability of their dimensions and of their mechanical properties (evenbetween individual plants in the same cultivation) as compared tosynthetic reinforcing fibers. A precondition for increased use of plant fibersin technically challenging applications is the availability of reproduciblefiber mechanical and morphological properties. The major task to besolved, in order to boost the acceptance of plant fibers as a quality alterna-tive to conventional reinforcing fibers is to develop fiber quality assuranceprotocols.6567

Nevertheless, plant fibers as fillers and reinforcements for thermoplasticsand thermosets are the fastest-growing type of polymer additives.Significant markets are emerging in building products, transportation(automotive, railroad, trucking), furniture, and marine. It has been pre-dicted that the demand of the North American market for both wood andagricultural fiber used as plastic additives will grow from 15% to 20% peryear in automotive applications, to 50% or more per year in selected build-ing products.68

2.2 Plant Fiber Composition and Structure

Natural fibers are subdivided based on their origins, whether they arederived from plants, animals, or minerals (Figure 2.4). Plant fibers includebast (or stem or soft or sclerenchyma) fibers, leaf or hard fibers, seed, fruit,wood, cereal straw, and other grass fibers.

Natural fibers

Mineral fibers

Seed Bast(or stem)

Fruit Leaf(or hard)

Wool/hair Silk

CottonKapokMilkweed

Coir FlaxHempJuteRamieKenaf

Pineapple (PALF)Abaca (Manila-hemp)HenequenSisal

Lamb's woolGoat hairAngora woolCashmereYakHorsehairetc.

Tussah silkMulberry silk

AsbestosFibrous bruciteWollastonite

Wood Stalk

WheatMaizeBarleyRyeOatRice

Cane, grass & reed fibers

BambooBagasseEspartoSabeiPhragmitesCommunis

Vegetable(cellulose or lignocellulose)

Animal(protein)

FIGURE 2.4Classification of natural fibers.


The chemical composition (Table 2.2) as well as the structure of plantfibers is fairly complicated. Plant fibers are a composite material designed by nature. The fibers are basically a rigid, crystalline cellulose microfibril-reinforced amorphous lignin and/or hemicelluloses matrix. Most plantfibers, except for cotton, are composed of cellulose, hemicelluloses, lignin,waxes, and some water-soluble compounds, where cellulose, hemicellu-loses, and lignin are the major constituents.69

The major component of most plant fibers is cellulose (-cellulose). Celluloseis a linear macromolecule consisting of D-anhydroglucose (C6H11O5) repeatingunits joined by -1,4-glycosidic linkages (Figure 2.5) with a degree of polymer-ization (DP) of around 10,000. Each repeating unit contains three hydroxylgroups. These hydroxyl groups and their ability to hydrogen bond play a majorrole in directing the crystalline packing and also govern the physical propertiesof cellulose materials.

Solid cellulose has a semicrystalline structure, i.e., consists of highly crystalline and amorphous regions. Cellulose forms slender rodlike crystalline

TABLE 2.2

Chemical Composition, Moisture Content, and Microfibrillar Angle of VegetableFibers

Moisture Microfibrillar

Cellulose Hemicelluloses Lignin Pectin Content Waxes Angle

Fiber (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (deg)

Flax 71 18.620.6 2.2 2.3 812 1.7 510Hemp 7074 17.922.4 3.75.7 0.9 6.212 0.8 26.2Jute 6171.5 13.620.4 1213 0.2 12.513.7 0.5 8Kenaf 4557 21.5 813 35Ramie 68.676.2 13.116.7 0.60.7 1.9 7.517 0.3 7.5Nettle 86 1117Sisal 6678 1014 1014 10 1022 2 1022Henequen 77.6 48 13.1PALF 7082 512.7 11.8 14Banana 6364 10 5 1012Abaca 5663 1213 1 510Oil palm EFB 65 19 42Oil palm

mesocarp 60 11 46Cotton 8590 5.7 01 7.858.5 0.6 Coir 3243 0.150.25 4045 34 8 3049Cereal straw 3845 1531 1220 8

Source: Adapted from Mohanthy, A.K. et al., Macromol. Mater. Engin., 276277, 1, 2000. Further

developed with Sreekala, M.S. et al., J. Appl. Polym. Sci., 66, 821, 1997, and Olesen, P.O. and

Plackett, D.V., Natural Fibers Performance Forum, Copenhagen, 1999. Link in: http:// www.

ienica.net/fibresseminar/olesen.pdf accessed on May 30, 2003. Lilholt, H. and Lawther, J.M.,

Vol. 1, Fiber Reinforcements and General Theory of Composites, Chou T.-W., ed., Comprehensive

Composite Materials, Kelly, A. and Zweben C., eds., Elsevier Science, Amsterdam, 2000, pp.

303325.


microfibrils. The crystal structure (monoclinic sphenodic) of naturally occurringcellulose is known as cellulose I. Cellulose is resistant to strong alkali (17.5 wt%)but is easily hydrolyzed by acids to water-soluble sugars. Cellulose is relativelyresistant to oxidizing agents.

Hemicelluloses are polysaccharides composed of a combination of 5- and6-ring carbon ring sugars. The polymer chains are much shorter (DP around50 to 300) and branched, containing pendant side groups giving rise to itsnoncrystalline nature. Hemicelluloses form the supportive matrix for cellu-lose microfibrils. Hemicellulose is very hydrophilic and soluble in alkali andeasily hydrolyzed in acids.

Lignin is the compound that gives rigidity to the plants. It is thought to bea complex, three-dimensional copolymer of aliphatic and aromatic con-stituents with very high molecular weight. Its chemistry has not yet beenprecisely established, but most of its functional groups and building units ofthe macromolecule have been identified. It is characterized by high carbonbut low hydrogen content. Hydroxyl, methoxyl, and carbonyl groups havebeen identified. Lignin has been found to contain five hydroxyl and fivemethoxyl groups per building unit. It is believed that the structural units ofa lignin molecule are derivatives of 4-hydroxy-3-methoxy phenylpropane.70

Lignin is amorphous and hydrophobic in nature. It is a thermoplastic poly-mer, exhibiting a glass transition temperature of around 90C and meltingtemperature at which the polymer starts to flow of around 170C.71 It is nothydrolyzed by acids, but soluble in hot alkali, readily oxidized, and easilycondensable with phenol.

Plant fibers are bundles of elongated thick-walled dead plant cells. A sin-gle or elementary plant fiber is a single cell typically of a length from 1 to50 mm and a diameter of around 1050 m. Plant fibers are like micro-scopic tubes (Figure 2.6), i.e., cell walls surrounding the center lumen. Thelumen contributes to the water uptake behavior of plant fibers.72 The fiberscomprise different hierarchical microstructures (Figure 2.6).73 The cell wallin a fiber is not a homogeneous membrane. It is build up of several layers:the primary cell wall that is the first layer deposited during cell growth, andthe secondary cell wall (S), which again consists of three layers (S1, S2, andS3). The cell walls are formed from oriented reinforcing semicrystalline

1

11

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HO

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H O H

H

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CHH

H

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OH OH

OH

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HH

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HH

FIGURE 2.5Cellulose.


cellulose microfibrils embedded in a hemicelluloses/lignin matrix of vary-ing composition. Such microfibrils have typically a diameter of about 1030nm, are made up of 30 to 100 cellulose molecules in extended chain confor-mation, and provide mechanical strength to the fiber. The amorphousmatrix phase in a cell wall is very complex and consists of hemicellulose,lignin, and in some cases pectin. The hemicellulose molecules are hydrogenbonded to cellulose and act as a cementing matrix between the cellulosemicrofibrils, forming the cellulose/hemicellulose network, which isthought to be the main structural component of the fiber cell. Thehydrophobic lignin network affects the properties of the other network in away that it acts as a coupling agent and increases the stiffness of the cellu-lose/hemicellulose composite. The cell walls differ in their composition, theratio between cellulose and lignin/hemicellulose, and in the orientation ofthe cellulose microfibrils. In most plant fibers the cellulose microfibrils areoriented at an angle to the normal fiber axis called the microfibrillar angle(Figure 2.7). The characteristic value for this structural parameter variesfrom one plant fiber to another. The outer cell wall is porous and containsalmost all of the noncellulose compounds, except proteins, inorganic salts,and coloring matter, which have been found in the fiber lumen,74 and it isthis section that creates the problemspoor absorbency, poor wettability,and other undesirable textile properties.75 In most of todays plant fiberapplications, fiber bundles or strands are used rather than individual fibers.Technical bast fibers isolated from the plants, consist of several elemen-tary fibers or cells, and are, therefore, fiber bundles with an average lengthup to a meter and diameters that are typically between 50 to 100 m.7678Within a fiber bundle, the fiber cells overlap and are bonded together bypectin that gives strength to the bundle as a whole (Figure 2.8). However,the strength of the bundle structure is significantly lower than that of theindividual fiber cell.

Middle lamella

Primary wall (fibrils of cellulosein a lignin/hemicellulose matrix)

S3

S2 S1

Lumen

S : secondary walls1,2,3

(a) (b)

FIGURE 2.6Structure of a vegetable (flax) fiber cell. (a) Schematic and (b) a microscopic cross section.(From Bismarck, A. et al., Polym. Composites, 23, 872, 2002. With permission of the Society ofPlastics Engineers.)


The structure, microfibrillar angle, cell dimensions and defects, and thechemical composition of the plant fibers are the most important variablesthat determine the overall properties of the fibers.79,80 Figure 2.9 shows theaveraged fiber tensile strength as a function of the cellulose content and themicrofibrillar angle. In general, the tensile strength and Youngs modulus ofplant fibers increase with increasing cellulose content of the fibers. The ori-entation of the cellulose microfibrils with respect to the fiber axis determinestiffness of the fibers. Plant fibers are more ductile if the microfibrils have aspiral orientation to the fiber axis. Fibers are inflexible, rigid, and have a hightensile strength if the microfibrils are oriented parallel to the fiber axis.81

LumenSecondary wall S3

Helicallyarrangedcrystallinemicrofibrilsof cellulose

Amorphousregion mainlyconsisting of ligninand hemicellulose

Disorderly arrangedcrystalline cellulosemicrofibrils networks

Primary wall

Secondary wall S1

Secondary wall S2

Spiral angle

FIGURE 2.7Structure of an elementary plant fiber (cell). The secondary cell wall, S2, makes up about 80% of the total thickness. (Reprinted from Rong, M.Z. et al., Composites Sci. Technol., 61, 14371447, 2001. With permission from Elsevier.)

FIGURE 2.8Overlapping flax fiber cellscemented together by pectin (optical micrograph).


Satyanarayana et al.82 established a semiempirical relationship to correlatethe fiber elongation and the microfibrillar angle :

52.7817.283102217.731023 2

and also the tensile strength and microfibrillar angle with the cellulosecontent W.

5334.00522.830 112.22W

2.3 Fiber Production Chain

The production chain for fiber crops can be divided into three major steps:(1) agricultural production, (2) fiber processing, and (3) fiber utilization.55

The agricultural production includes breeding, growth, and harvesting ofthe fiber crops as well as storing the raw material. Afterward, the raw mate-rial is processed. First the fibers have to be extracted and separated from theplant. Once the raw fibers are extracted, they need to be cleaned, refined, and

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FIGURE 2.9Influence of the cellulose content and the microfibrillar angle on the average tensile strengthof vegetable fibers. (Adapted and modified from Flemming, M. et al., Faserverbundbauweisen.Fasern und Matrices. Springer-Verlag, Berlin, 1995, p. 159.)


processed into the required fiber product by spinning or weaving. Differentapplications require different fiber qualities. When the design life of the fiberproduct is finally over, it needs to be disposed of. It could be recycled andreused or incinerated to partially regain energy or it could be compostedunder certain circumstances.

During the various steps of fiber crop and plant fiber production (Figure 2.10), a large number of factors influence the final fiber quality.55,83

The fiber crop variety (genotype) and the plants biology, climate, location ofthe origin region (i.e., soil conditions and so on), even the farmer himself, theripeness at the harvesting time, and the harvesting machinery and fiberextraction and separation process affect the fiber quality.12,84,85 Furthermore,each stage and additional workstep in the production chain reduces thevalue added of the final product. Many extensive, labor-intensive steps arenecessary to manufacture high-quality fibers, which makes the price of someplant fibers, as in case of European-grown flax, noncompetitive as comparedto standard E-glass fibers.86 In the end, the final fiber quality will determineits end application and the production and processing costs will determineits economics.

The traditional fiber crop production and the fiber processing are com-posed of many labor-intensive worksteps (Figure 2.11) utilizing productiontechnology that was established shortly after World War II and, is therefore,quite unproductive.87

Plant production

SpeciesCrop productionLocationClimate

Degree of ripenessHollow fiberOptimum strengthLignification

Retting or other fiberextraction methods

Decortication

Fiber yield Fiber degradation

Fiber processingDry/mechanical

Steam explosionWet Ultrasound

FIGURE 2.10Factors that influence fiber quality. (Adapted from Katalyse-Institut (Hrsg.), Hanf & Co. Die Renaissance der heimischen Faserpflanzen, Verlag Die Werkstatt, Gttingen, 1995, p. 72.)


2.4 Agricultural Fiber Crop Cultivation

The farmers interest in fiber crops stems primarily from the potential ascommercial fiber, the wooden core, and seed oil source. Fiber crops also offerproduction alternativesthey can be grown as shelter crop and they are effec-tive pollen insulators, since they form impermeable hedges, which minimizeoutside pollination.88 Fiber crops have excellent crop rotation properties.52

Furthermore, only a small amount of crop residue returns back to the soil afterharvesting because the entire top growth is harvested.89 In agricultural pro-duction the aim is to improve crop yields in order to improve the farmersincome. Particularly in the field of fiber crop production, additional emphasishas been invested in agronomic aspects, such as breeding of new varieties, disease and lodging resistance, cultivation, and crop management.55,88,90,91

Fiber crop production efficiency has been and still is improving by the mech-anization of soil tillage and harvesting, an optimized use of fertilizers, and the

SowingSwathing, mowing, or pulling

DryingThreshing

Dew-rettingBaling & storing

Breaking & scutchingTransportation

Degumming & cottonizationdrafting

Wet spinning

HacklingCombing

ModificationScutched tow

Hackled tow

Dry spinning

Coarse yarnsFine yarnsFleeces, mats, etc.

End User

Farmer

Furtherprocessing

FIGURE 2.11Processing steps of bast fiber production: from the farmer to the end user. (Adapted fromFlster, T., Textilveredelung, 30, 2, 1995 and Flemming, M. et al., Faserverbundbauweisen. Fasernund Matrices. Springer-Verlag, Berlin, 1995, p. 162.)


efficient use of herbicides and pesticides.55 It can be expected that new bastfiber crop varieties will be generated in the future. Such new plant varietieswill contain high-quality fibers that will be easier to extract.91

However, plant fiber crops certainly have merit as an alternative crop, butthey are not automatically sustainable. The modern, yield-oriented agricul-tural techniques for the production of fiber crops (especially cotton) requirehuge amounts of water, pesticides, fungicides, and herbicides. If fiber cropsare grown like any other row crops, as monocrops (see also ref. 92) and evenworse without crop rotation, with tillage, commercial fertilizers, herbicides,and pesticides, they are contributing to soil degradation just as other cropsare89 (see also ref. 93). These factors tend to disrupt ecological balances (seealso Figure 2.2). However, fiber crops as a source for plant fibers and asrenewable resources could be of major importance for the worlds environ-ment and economy (e.g., farmers income); however, this outcome will bedecided through the global market.

2.4.1 Harvesting Bast Fiber Crops

The timing of harvesting bast fiber crops depends on whether the plants aregrown for high-quality fibers or for their seeds, or both.94 The technical matu-rity (defined by the ratio of the cross sections of the walls to that of the wholefiber95) of hemp is reached at around the time the plant has completed pro-ducing pollen. The optimal time for harvesting, however, is at the beginningof seed maturity, about three to four weeks after the anthesis (flowering).

Figure 2.12 shows increasing formation of bast fibers from the time the flaxplants are in full flower (a) to the optimal time for harvesting (b). At this timethe secondary fiber cell wall occupies more than 90% of the plant stalk.76

The mechanical strength of the bast fibers increases with increasing fiberripeness. However, competing with the development of bast fibers is the

(a) (b)

FIGURE 2.12Optical micrographs (200:1) of cross sections through flax stalks at the time (a) of full anthesisand (b) of the harvest. (From Lampke, T., Technische Universitt Chemnitz, Lehrstuhl frVerbundwerkstoffe, 2001, p. 64. With permission.)


ever-increasing lignifications of the stalk, i.e., the fibers get coarser.Therefore, the fibers adhere much more strongly to each other and to thewood core.95 The decortication and separation of the fibers gets much moredifficult if the plants are left growing well beyond optimal ripeness.95

However, it was also reported that a relatively late harvesting did not affectthe tensile strength of the bark and is advantageous for mechanical decorti-cation of green (nonretted and dried) hemp stalks.96 Normally, farmers aimto achieve maximum stem yield at best fiber quality. In the case where plantsare grown for both their seeds and fibers, the actual harvesting process startsabout six weeks after anthesis at a time when all seeds are properlyripened.97 The fibers obtained from plants grown for seed production arestiffer, coarser, and more brittle. Traditionally, all hemp and flax crops areharvested by pulling the plants with their roots to preserve the full fiberlength (in some cases by cutting them near the ground). The plant strawremains for a certain time (to dry or to rett and dry) in swaths on the field.To enhance the efficiency of the harvesting process new harvesting machin-ery is currently being developed.98100

2.4.2 Fiber Extraction, Separation and Processing

After mowing or pulling bast fiber crops, the fibers must be separated andextracted from the woody tissue of the fiber crops. The process that causesthe separation of the technical fiber bundles from the central stem, whichloosens the fibers from the woody tissue, is called retting. Most availablemethods of retting rely on the biological activity of microorganisms, bacte-ria, and fungi from the environment to degrade the pectic polysaccharidesfrom the nonfiber tissue and, thereby, separate the fiber bundles. The rettingor rotting of the straw is caused with time by exposure to moisture and,sometimes, by the help of a mechanical decorticator. The fiber separationand extraction process has a major impact on fiber yield and final fiber qual-ity.101 It influences the structure, chemical composition,69,102 and properties ofthe fibers.103 Retting procedures can be divided into biological, mechanical,chemical, and physical fiber separation processes (Figure 2.13).

Retting

Physical ChemicalBiological Mechanical

Dew (or field) rettingCold-water retting

STEXultrasound retting

Green retting Surfactant rettingNaOH retting

Hot water or canalretting

Natural Artificial

FIGURE 2.13Classification of retting processes.


2.4.2.1 Dew or Field Retting

Dew or field retting83,104,105 is the most commonly applied retting process inregions that have appropriate moisture and temperature ranges. After beingmown, the crops should remain on the field until microorganisms have sep-arated the fibers from the cortex and xylem. The degradation of the cortexprimarily occurs due to the action of indigenous fungi. Mycelium grows onthe carbohydrate-rich tissue, utilizes the easy to access pectin, and degradesthe pectin in the phloem with excreted enzymes. In order to guarantee ahomogeneous retting of the plant straw, the plant has to be turned over atleast once. After the retting the stalk is dried and baled. The retting processhas to be stopped at the right time to prevent overretting, i.e., degradation ofthe cellulose of the bast fibers by the fungi, thereby lowering their tensilestrength. However, underretting results in fibers that are difficult to separateand to further process. Therefore, farmers have to monitor the rettingprocess to ensure the quality of the fibers. The duration of the dew-rettingprocess is between 3 and 6 weeks and is highly dependent on uncontrollableweather conditions. Rainfall and humidity, sun hours and temperature, andthe way the fiber crops are spread on the ground affect the retting processand, therefore, the quality of the fiber. The unpredictability of the dew-ret-ting process leads to varying and inconsistent fiber quality and results everyso often (statistically every 6 to 7 years) in complete loss of the fiber cropharvest.106

2.4.2.2 Stand-Retting

A modified field-retting procedure is the thermally induced stand-rettingprocedure.107 Open gas flames are used to terminate the plant growth of flaxcrops. The plant bases are heated up to ~100C. The plants afterward dryhomogeneously within 1 to 2 days, thus allowing replacement of theweather-susceptible dew-retting procedure on the ground. Even thoughweather dependence and therefore risk of fiber damage decreases, costs forretting increase because additional special machinery is needed. Themechanical properties of the fibers are not affected by thermal treatment.Furthermore, such a retting procedure allows for obtaining desired fiberquality by adjusting the retting parameters.

2.4.2.3 Cold-Water Retting

Traditional cold-water retting is mainly used by fiber producers in EasternEurope. The process utilizes anaerobic bacteria that break down the pectin ofplant straw bundles submerged in huge water tanks,108 ponds, hamlets orditches, rivers, and vats. The process takes between 7 and 14 days anddepends on the water type, the temperature of the retting water, and anybacterial inoculum.109 Even though the process produces high-quality fibers,environmental pollution is high due to unacceptable organic fermentationwastewaters.110 Because of that and the malodor of fermentation gases, theprocedure was abolished in 1918 in Germany.83


2.4.2.4 Warm-Water Retting

Warm-water retting is a form of accelerated retting and produces homo-geneous and clean fibers of high quality in only 3 to 5 days. Plant bundlesare soaked in warm (28 to 40C) water tanks. The process as such is pro-hibited in Europe because of the extreme impact on the environment.83

After sufficient retting the bast fibers have to be separated from the woodyparts. The shieves or hurds are loosened and extracted from the raw fibers ina breaking and swingle or scutching process. The coarse processed fibers arepassed through fluted rolls to break the woody portion into small particles.The scutching is done by mechanically beating the material with metalblades. Afterward the short fibers are separated from the long fibers by hack-ling or combing. All the consecutive processing stages (retting, scutching, andhackling) affect fiber properties. The physical form of the fibers (Figure 2.14)obtained after different separation steps can range from bundles to elemen-tary fibers or even further opened, smaller fibers.111

Most of the emerging technical and industrial applications of plant fibersrequire sufficient quantities of constant quality with good mechanical properties at competitive prices. However, this can only be achieved if the varying raw fiber quality can be refined and adjusted using improvedand flexible processes.112 In order to supply fibers that fulfill these require-ments, several new fiber retting and separation techniques have beendeveloped.

Hacking

Breakingscutching

Flax stem23 mm

Bast fiberbundle

Technical fiber50100 m Elementary fiber

2020 m

Meso fibril0.5 m

Micro fibril410 nm

FIGURE 2.14Schematic of flax stalk and fiber structure. (From Bos, H.L. et al., J. Mater. Sci., 37, 1683, 2002.With permission.)


2.4.2.5 Mechanical or Green Retting

Mechanical or green retting is a much simpler and more cost-effective alter-native to separate the bast fibers from the plant straw. The raw material forthis procedure is either field dried but only slightly retted (2 to 3 days, but 10days maximum) plant straw113 or technically dried straw. The bast fibers areseparated from the woody part by mechanical means. Weather-dependentvariations of fiber quality are eliminated. However, the produced green fibersare much coarser and less fine as compared to dew- or water-retted fibers.114

The properties of the resulting green fibers are much less sensitive to variableraw material input.115

2.4.2.6 Wet-Retting Processes

Homogeneously fine and clean fibers can only be obtained from wet-rettingprocesses. Wet-retting processes offer a wider range of parameters that allowfor modifying the fibers with respect to given applications by adjusting theprocess to take into account varying properties of the raw material.95

2.4.2.6.1 Ultrasound Retting

ECCO Gleittechnik GmbH (Seeshaupt, Germany) has developed an ultrasound-retting process to separate the bast fibers from the rough plantstraw which are neither subjected to dew nor to water retting.106,116 After har-vest, the stems are broken and washed. The slightly crushed stems areimmersed in a hot water bath (~70C) that contains small amounts of alkaliand surfactants, and then exposed to high-intensity (1 kW) ultrasound(40 kHz). This continuous process separates the hurds from the fiber. Theprocess separates the fibers from the woody components to a degree suffi-cient for technical and nontextile applications. The retting solution contain-ing the hurds can be used to produce fuel or can be used for many otherapplications. One advantage of the process as compared to traditional ret-ting methods is that green (unretted) raw material is used, thereby avoidingthe unreliable dew-retting process.

2.4.2.6.2 Steam Explosion Method

The steam explosion method (STEX)87,95,117,118 represents another suitablealternative to the traditional field-retting procedure.119 Under pressure andincreased temperature, steam and additives penetrate the fiber interspacesof the bast fiber bundles. The center lamella is thus removed at optimumconditions of reaction. The subsequent sudden relaxation of the steam leadsto an effective breaking up of the bast fiber composite which results in anextensive decomposition into fine fibers. The fineness and properties of theproduced fibers are comparable to cotton fibers.83

2.4.2.6.3 The Duralin Process

Another novel upgrading process for lignocellulose materials has beendeveloped to improve the poor environmental and dimensional stability of


these materials.120 This upgrading process, which was initially developed forwood, has also proved its applicability to natural fibers and has lent itself to the development of upgraded bast fibers, Duralin flax and hemp.121

The novel upgrading process developed by Ceres B.V. (Wageningen, The Netherlands) to treat flax and hemp consists of three steps, hydro-thermolysis, drying, and curing. Full-rippled (deseeded) straw fibers areused as raw material. The use of straw is beneficial for both strength andreproducibility (no dew retting required) of the treated fibers. In addition, avaluable by-product, the treated flax shives or hemp hurds, is producedfrom which products such as water-resistant chipboards can be made.122 TheDuralin treatment consists of a steam or water-heating step of the straw attemperatures above 160C for ~30 min in an autoclave. A drying step and aheating (curing) step above 150C for ~2 h follows the first step. During thistreatment, the hemicellulose and lignin are depolymerized into lowermolecular- weight aldehyde and phenolic functionalities, which are com-bined by the subsequent curing reaction into a water-resistant resin thatcements the cellulose microfibrils together. After the treatment the fibers caneasily be separated from the stem by a simple breaking and scutching oper-ation. The fibers obtained by these procedures are fiber bundles rather thanindividual fibers. The availability of an upgrading process for natural fiberscould remove one of the main restrictions for the successful application ofnatural fibers in high-quality engineering composites.

2.4.2.6.4 Enzyme Retting

Another alternative for producing high and consistent quality fibers isenzyme retting.123,124 This retting procedure uses pectin-degrading enzymesto separate the fibers from the woody tissue. The use of enzymes promotesthe controlled retting of the fiber crops through the selective biodegradationof the pectinaceous substances. Crimping, mechanically splitting the stalksof non- or only slightly retted plant straw prior to enzyme retting, enhancesenzyme penetration into the tissue and accelerates the degradation process.The actual retting process takes place in a tank. The enzyme activityincreases with increasing temperature up to an optimum temperature abovewhich the enzymes start to denature.125 The process lasts between 2 and 24hours. Enzyme retting can lead to undamaged individual fibers (withoutkink bands) with much higher intrinsic fiber strength.126 However, thus far this process has only progressed to pilot-scale experiments probablybecause of the high costs of the enzymes, the equipment, and the wastewatertreatment.55

2.4.2.6.5 Chemical and Surfactant Retting

Chemical and surfactant retting refers to all retting processes in which the fibercrop straw is submerged in heated tanks containing water solutions of sulfuricacid, chlorinated lime, sodium or potassium hydroxide, and soda ash (sodiumCopyright 2005 by Taylor & Francis

carbonate) to dissolve the pectin component. The use of surface-active agentsin retting allows the simple removal of unwanted noncellulose componentsadhering to the fibers by dispersion- and emulsion-forming processes.127

These processes reduce the duration of retting, depending on the process con-ditions, to a few minutes or up to 48 hours. Chemical retting produces high-quality fibers but adds costs to the final product.

2.5 Impact on Fiber Properties: Different FiberSeparation/Retting Procedures

Agronomic factors such as crop variety, seed density, soil quality, fertiliza-tion, location of the plantation and location of fibers on the stem of the plant,climate and weather conditions, and timing of the harvest (age of the plant)affect final fiber quality and their overall properties.128 Disturbances duringplant growth will affect plant structure and are also responsible for theenormous scatter of mechanical plant fiber properties. The number ofknobby swellings (Figure 2.15) and growth induced lateral displacement(Figure 2.16) present in plant fibers influence the tensile strength and elon-gation at break of the fibers obtained.

The mechanical properties of plant fibers do not only depend on the fac-tors previously mentioned. More importantly, the fiber separation processsignificantly determines fiber quality and its mechanical properties. Optimalbiological or chemical and physical retting procedures result in a much bet-ter and easier separation of the fibers from the woody core (Figure 2.17) and,

FIGURE 2.15Knobby swellings along flax fibers. (From Lampke, T., Technische Universitt Chemnitz,Lehrstuhl fr Verbundwerkstoffe, 2001, p. 64. With permission.)Copyright 2005 by Taylor & Francis

therefore, minimize the mechanical loading on the fibers (Figure 2.18a).Mechanical loads which affect the mechanical properties of the fibers areexperienced during the breaking, scutching, and hackling steps, especiallyduring green (mechanical) retting procedures (Figure 2.18b). Mechanicaloverstraining of the fibers during the mechanical processing steps of fiberseparation can result in the formation of kink bands (Figure 2.19) and splices(Figure 2.20) which dramatically lower the mechanical properties (tensileand compressive strength) of the fibers. Furthermore, mechanical rettingresults in much shorter fibers, which, in extreme cases, could be disadvanta-geous for further processing.

FIGURE 2.16Growth induced lateral displacement through a flax fiber bundle. (From Lampke, T., TechnischeUniversitt Chemnitz, Lehrstuhl fr Verbundwerkstoffe, 2001, p. 64. With permission.)

(a) (b)

FIGURE 2.17Optical micrographs (200:1) of cross sections through flax stalks after (a) 2 days of retting and(b) after 14 days of retting. (From Lampke, T., Technische Universitt Chemnitz, Lehrstuhl frVerbundwerkstoffe, 2001, p. 65. With permission.)


Besides the mechanical properties, the chosen retting procedure alsoaffects the fiber morphology, their surface composition and proper-ties,129,130 as well as their water uptake behavior.60 Not only the fiberappearance (Figure 2.21) but also the degree of disintegration, the fineness,and the amount of noncellulose components of the plant (flax) fibersdepend on the fiber retting/separation process. These fiber properties canbe adjusted within a broad range by the fiber retting and separationprocess.

FIGURE 2.19Optical micrograph of kink bands (see arrows) introduced in an elementary flax fiber bystandard isolating methods. (Adapted from Bos, H.L. and van den Oever, M.J.A., Proc. of the 5th International Conference on Woodfiber-Plastic Composites, Madison, 1999, p. 81.)

100 m

100 m(a) (b)

FIGURE 2.18Single (elementary) fiber (a) well prepared and (b) damaged due to excessive mechanicalstress during fiber separation. (From Kohler, R. and Kessler, R.W., Proc. of the 5th InternationalConference on WoodfiberPlastic Composites, Madison, 1999, p. 33. With permission.)


Figure 2.21 shows several original but differently retted and separated flaxfibers. Figure 2.21a shows dew-retted native flax fibers. The fibers are notwell separated and are still covered and glued together by noncellulosecompounds. Green flax fibers were separated from the nonretted raw flaxstraw using ultrasonification (US) (Figure 2.21b). During the separationprocedure the fiber bundles are partially separated. The micrographs revealpartially uncovered structures, comparable to those of the cellulose fibers(Figure 2.21c); however, other areas are still covered by noncellulose com-pounds (such as waxes or fats). STEX produces relatively smooth fibers(Figure 2.21d). However, the fiber structure is still not completely laid open(e.g., fiber waxes and fats are still attached to the surface). The STEX separa-tion process results in the dissolution of most cementing substances, leadingto a better disintegration of the remaining fiber bundles into elementaryfibers under the shear stress exerted during compounding in an extruder oran injection molding machine. In contrast, the fiber bundles obtained bypurely mechanical separation (green fibers) do not disintegrate duringcompounding, causing noticeably poorer mechanical properties of the com-posite.112,131

In general, plant fibers have very small specific surface areas (As0.5 m2/g),

just slightly bigger than the calculated geometric surface area (As,geo50.38m2/g) at a fiber diameter dwf14 m (compare Figure 2.21) and a density offlax fibers132 of w 51.47 g/cm3. Even though plant fibers have a very smallspecific surface area, the retting/separation process used affects the surfacearea. The higher the degree of disintegration and fineness, the larger is the

FIGURE 2.20Splices at the flax fiber surface after excessive mechanical processing. (From Lampke, T.,Technische Universitt Chemnitz, Lehrstuhl fr Verbundwerkstoffe, 2001, p. 65. Withpermission.)


total specific surface area measured in adsorption experiments applying theBrunauer, Emmet and Teller procedure (BET) (Table 2.3).

The water absorption of some flax fibers under different relative humiditywas also studied.60 Green flax fibers absorb much more water (averaged

FIGURE 2.21Scanning electron micrographs of differently separated flax fibers. (a) Retted flax fibers, native(raw) state. (b) Green flax, separated using ultrasonification. (c) Cellulose fibers. (d) STEX(DDA) flax. (From Bismarck, A. et al., Polym. Composites, 23, 872, 2002. With permission of theSociety of Plastics Engineers.)


MwCw5 43%) than dew-retted flax (MwCw5 27%). However, flax fibers obtainedfrom the novel Duralin separation process absorb much less water(MwCw5 19%). During the Duralin process, the noncellulose components of thebast fibers are depolymerized into low molecular-weight oligomers thatreact in a curing step to a water-resistant resin.

FIGURE 2.21 (Continued)


2.6 Fiber Treatment and Modification

It is well known that the performance of composites depends on the proper-ties of the individual components and their interfacial compatibility. Fornumerous applications plant fibers have to be prepared or modified122,133

with the following considerations in mind:

l Homogenization of the properties of the fibersl Degree of elementarization and degummingl Degree of polymerization and crystallizationl Good adhesion between fiber and matrixl Moisture repellencel Flame-retardant properties

These properties can be partly influenced by different fiber separation pro-cedures, but subsequent fiber treatment processes are more influential.Several noncellulose components have to be removed to assure the compat-ibility of the plant fibers to the surrounding polymer matrix. Alkalization,washing, or boiling of the plant fibers in 2 to 10% sodium, potassium, orlithium hydroxide solutions leads to the removal of unwanted fiber compo-nents, which dissolve during the process. Alkalization, depending on theconcentration of the alkali and the process temperature, can significantlyimprove the fibers mechanical and surface properties. After naturalizingand thoroughly washing and drying the fibers, the following changes ascompared to the raw fiber material can be revealed:

l Fibers are purifiedl Unwanted fiber ingredients are largely removedl Fiber separation ability is increased

During the alkalization processes the wax components are saponified and, thereby removed from the fiber surface. The waxes sticking to the fiber

TABLE 2.3

Influence of Flax Fiber Retting/Separation Procedures on Fiber SurfaceArea, Fiber Surface Tension, and Water Uptake Behavior (MoistureContent)

Surface Area Surface Tension Maximum Moisture Content

Fiber (As/m2 g21) (f/mN m21) (MwCw /%)

Green flax 0.31 43Dew-retted flax 0.51 35.5 6 1.0 27US flax 0.75 36.3 6 0.7STEX flax 0.88 38.7 6 1.0Duralin flax 19


surfaces strongly influence the surface properties of cellulose-based materi-als. Most raw plant fibers are covered with hydrophobic noncellulose com-pounds (like waxes). It is, therefore, necessary to treat original fibers toremove these noncellulose components. Such a treatment increases the acces-sibility of surface groups (but not necessarily the specific fiber surface). Theseuncovered, reactive surface groups can be used in further chemical modifica-tion steps to increase the compatibility of natural fibers to nonpolar polymermatrices. Such further modification processes can be reactions with compati-bilizing agents such as carboxylic anhydrides, isocyanates, vinylsulfone,chlorotriazine systems,134 organo silanes, and compounds that containmethylol groups or grafting of MAH-graft PP.122

2.7 Bast Fibers

The shape and size of the stem of various bast fiber crops* are different but theyall contain varying amounts of fiber cells in the phloem (Figure 2.22 and

* For more digital photographs, schematics, and illustrations of fiber crops, see: Digital Flora ofTexas (DFT), Vascular Plant Image Library. Link in: http: //www.csdl.tamu.edu/FLORA/gallery.htm accessed on 26.07.2003; Kurt Stbers Online Library, link in: http: //caliban.mpiz-koeln.mpg.de/~stueber/stueber_library.html accessed on 26.07.2003, and index of 4246botanical images taken by Kurt Stber. Link in: http: //caliban.mpiz-koeln.mpg.de/~stueber/mavica/index.html accessed on 26.07.2003.

FIGURE 2.22A schematic comparison of the stalk cross sections of flax, nettle, and hemp. (From Scheer-Triebel, M. and Lon, J., Pflanzenbauwissenschaften, 4, 26, 2000. With permission of Verlag Eugen Ulmer, Stuttgart, Germany.)


Figure 2.23). Long individual fibers or long fiber bundles can be obtainedfrom many bast fiber crops at relative low cost. However, bast fibers have anonhomogeneous cell structure than do the much shorter wood fibers (aver-aged fiber length 2.7 mm), which are uniform, readily available, and inex-pensive. All bast fiber crops have a similar structure (see Figure 2.23).

(a)

(b)

FIGURE 2.23Micrographs of cross sections through the stems of (a) flax with bast fibers situated under thecuticula, (c) nettle (Urtica dioica) with bast fibers (arrow), and (d) hemp stalk cross sections(Micrographs (a), (b) and (d) from Lampke, T., Technische Universitt Chemnitz, Lehrstuhl frVerbundwerkstoffe, 2001, p. 22; and for (c) from Dreyling, G., Institut fr AngewandteBotanik, Universitt Hamburg, Institut fr Angewandte Botanik, AbteilungNutzpflanzenbiologie (Arbeitsgruppe Dreyling, Germany: http: //www.biologie.uni-hamburg.de/ianb/nb/urtica05.html#Text accessed on June 17, 2003.)


Bast fiber crops have rigid herbaceous stalks containing nodes at regularintervals that are fluted or channeled. From the inside to the outside thestalks have a hollow core, except at joints, followed by the pith, the cam-bium, the phloem or parenchyma, the cortex, and finally the protective layerepidermis with the waxy cuticula (Figure 2.24).

The pith is composed of a thick woody tissue that supports the plant.After harvesting this area produces the hurds or shives that can compriseup to 60 to 75% of the total mass. The cambium is the differentiating layer.

(c)

(d)

FIGURE 2.23 (Continued)


It is the pith on the inside which produces bast fiber cells, and it is bark onthe outside. Short chlorophyll-containing cells and long bast fiber cellsmake up the phloem or parenchyma. The cortex is a thin wall of cells thatproduce no fibers but contain chlorophyll. The epidermis is the thin outsideprotective layer of plant cells.

2.7.1 Flax (Linum usitatissimum L., Linaceae) Fibers

Flax is one of the bast fibers grown in temperate regions. Flax for millenniahas provided many important products such as fibers for textiles, oil seed,and paper and pulp. Flax has been cultivated for nearly 10,000 years and itsfibers, i.e., linen, are reported to be the oldest textile known, dating back toearly Mesopotamian times.3,127 Textile flax is primarily grown in Europe,Argentina, India, China, and the Commonwealth of Independent States (thestates of the former Soviet Union).135

The flax plant consists of the root, stalk, and branches carrying the seed cap-sules. The central portion of the stalk is delimited at the bottom by the cotyle-donary node and at the top end by the bottom of the branches (Figure 2.25a).Flax is dicotyledonous. In 80 to 110 days the plant grows to heights between80 and 150 cm (Figure 2.25 and Figure 2.26). Only the central portion (up to75% of the plant height) can be used to produce bast fibers. The flax fiber

FIGURE 2.24The structure of a bast fiber crop stem: a cross section of a flax stem. (Micrograph fromLampke, T., Technische Universitt Chemnitz, Lehrstuhl fr Verbundwerkstoffe, 2001. WithPermission.)


bundles are between 60 and 140 cm long and their diameter ranges from 40to 80 m. The color of the flax fibers varies from light blond to gray. The fibers are very strong but also flexible. Flax is a particularly inextensiblefiber. It stretches only slightly as tension increases. Within the small degreeof extension, flax is an elastic fiber. Flax fiber properties are controlled by the molecular fine structure of the fibers which are affected by growing con-ditions136 and the retting procedure that is applied.137 A nature flax cell wall(for composition see ref. 138) consists of ~7075% cellulose, 15% hemicellu-loses and 1015% pectic material, about 2% lignin, and 2% waxes. Thedecomposition temperature of flax fibers is above 200C,64 as can be seen in Figure 2.27. Flax fibers lose strength gradually on exposure to sunlight.

FIGURE 2.25(a) Schematic of a flax plant: 1, branches; 2, central portion of stalk; 3, cotyledonary node; and 4, root. (From the Gesamtverband der Deutschen Versicherungswirtschaft e.V.: http: //www.tis-gdv.de/tis_e/ware/f_inhalt0.htm accessed on June 24, 2003. With permission.) (b) A flowering flax plant. (From Seigler, D.S., http: //www.life.uiuc.edu/plantbio/263/image/Flax_Bs.jpeg accessed on July 5, 2003. With permission.)


Flax can withstand dilute weak acids, but is attacked by hot dilute acids orcold concentrated acids. It has good resistance to alkaline solutions.

Flax fibers are quite expensive because of the many labor-intensive pro-duction steps. Still, flax fibers are used as reinforcement for high value-added

FIGURE 2.26Photograph of flax plantation. Depending upon the flax variety, it can be cultivated for production of flax fibers or linseed oil. (Courtesy of www.inaro.de.)

0

20

40

60

80

100

200 250 300 350 400 450

Temperature (C)

Mass (

%) 4 K/min

5 K/min

7 K/min

10 K/min

12 K/min

15 K/min

FIGURE 2.27Thermogravimetry STEX (DDA) flax fibers in helium atmosphere at a heating rate of 415 K/min.


composite products in fields where the composites are exposed only to amedium stress range (Figure 2.28a,b),139 especially for interior automotivecomponents (Figure 2.29).140

2.7.2 Hemp (Cannabis sativa L., Cannabaceae) Fibers

Hemp (Figure 2.30) is another famous bast fiber crop that is having animpressive comeback after being legislated out of existence.141 The floweringtops, and to a lesser extent the leaves of hemp and hemp varieties produceresin secretions containing the narcotic 9- tetrahydrocannabinol (THC, aditerpene containing a hydroxyl functionality), for which marijuana andhashish are famous. Hemp cannot be used as a narcotic since it produces virtually no THC (less than 1%), whereas marijuana varieties producebetween 3 and 20% THC.142

FIGURE 2.28(a) Green-Line suitcases for a variety of instruments. (Courtesy of Jakob Winter GmbH,Satzung, Germany and Kompetenzzentrum Strukturleichtbau, TU Chemnitz, Germany. From Odenwald, S. and Griesmann, G., Proc. 10. Internationale Tagung Stoffliche Verwertungnachwachsender Rohstoffe, 2002, Chemnitz, p. 157. With permission.) (b) Biobased helmets for various applications. (From Schuberth Helme GmbH, Braunschweig, Germany: http: //www.riko.net/html/produkte/produkte_2.php3 accessed on July 21, 2003.)


FIGURE 2.29Photograph of flax fiber composite product. Flax fibers find applications as reinforcement forinterior door panelings for passenger cars to replace glass fibers. (Courtesy of www.inaro.de.)

FIGURE 2.30Schematic (a) and photograph (b) of a hemp plant. (Courtesy of www.inaro.de.)


Hemp is native to Central Asia and has been cultivated for more than12,000 years.83 Wild hemp and cannabinoide hemp varieties are typicallydioecious. However, examples of monoecious plants, single plants havingboth male and female inflorescence, can also be found within any givenpopulation.143 Planting and producing of hemp crops has many advantagesfor farmers. Hemp needs almost no or minimal herbicides, pesticides,fungicides, and fertilizer. The plants have impressive growth rates so thatthey quickly cover the ground and, therefore, suppress weeds and somesoil-borne pathogens. Hemp also restores nutrients to the soil, which arethen available to the next crop planted in rotation. The plant is very deeprooting and is a beneficial break crop because it cleans the ground and pro-vides a good disease break while helping soil structure. The different hempvarieties grow to heights between 1.2 and 5 m135 and have bast contents inthe stalk from 28 to 46%.144 The male plants (fimble hemp) ripen earlier andmust be harvested earlier. The female plants are more highly branched andbear denser foliage. All monoecious varieties have the same rate of devel-opment. The dioecious male plants yield particularly fine fibers. Dioecioushemp varieties yield better textile fibers, while the monoecious varieties arepreferred by the pulp and paper industry.143 Strands of hemp fibers may be1.8 m or longer. The individual or elementary fibers are on average 13 to25 mm long. Elementary fibers are thick-walled and polygonal in cross sec-tion, with joints, cracks, swellings, and other irregularities on the surface.The whitish to yellow hemp fibers can be up to 50 mm in length, are highlywater-resistant, and have good tensile strength. Hemp fibers are coarser ascompared to flax and difficult to bleach. The fibers have an excellent mois-ture resistance and rot only very slowly in water.83 Hemp fibers have hightenacity (about 20% higher than flax) but low elongation at break. The tex-tile industry keeps using hemp fibers because of their excellent properties.Hemp textiles are resistant to water and mechanical stress, insensitive dur-ing washing, and moths will not attack them.83

Products made from hemp fibers include specialty paper, textiles,construction materials, plastics and composites, food, medicine, and fuel(Figure 2.31).

2.7.3 Jute (Corchorus capsularis, Tiliaceae) Fibers

Jute fibers are the most important plant fibers alongside cotton. Jute (Figure 2.32) is native to the Mediterranean from where it spread throughout the Near and Far East. Jute was used by humans since prehistoric times. Thecrops are herbaceous annuals that grow to 2 to 3.5 m in height, with a stalk2 to 3 cm in diameter. Jute is grown entirely for its fibers. The plants thrivein hot and humid climates. Jute is the most versatile, ecofriendly, natural,durable, and antistatic fiber available. Today most jute is produced in thedelta formed by the Ganges River and Bramhaputra River in India andBangladesh. Corchorus capsularis, known as white jute, and C. olitorius,


known as dark jute, are grown in India, Bangladesh, Thailand, and China aswell as Brazil. Jute plantations, fiber processing, and spinning and weavingmills all make a very important contribution to the economies of several ofthese countries.

Individual jute fibers have a polygonal cross section and vary in size.Owing to these irregularities in the thickness of the cell walls the fibers vary greatly in strength. Jute fibers are long (1.5 to 3 m), lustrous, andresilient. Jute is sensitive to chemical and photochemical attack, but resistantto microorganisms. The natural color of jute fibers is brown. Jute fibers arestrong but brittle and have a low extension to break (about 1.7%). The highlignin content (up to 20%) causes brittleness of the fibers.145 The fibers lose

FIGURE 2.31Hemp products in front of a hemp plantation. (Courtesy of www.inaro.de.)


tensile strength on exposure to sunlight and have little resistance to moistureand acids. Jute is the most hygroscopic plant fiber; however, if kept dry, jutewill last indefinitely. The tensile strength of jute fibers is lower compared toflax or hemp.

Jutes silky texture, its biodegradability, and its resistance to heat and fire make it suitable for use in industries as diverse as fashion, travel, lug-gage, furnishings, and carpets and other floor coverings. It is now possibleto visualize jute not only as a major textile fiber, but also as a raw materialfor nontextile products. Jute fibers are suitable as reinforcement for parti-tions, paneling, false ceilings, and other furniture. Mohanty and Misra146

have reviewed jute fiber-reinforced thermoset, thermoplastic, and rubber-based composites. Extensive studies have been carried out to fabricatejute/epoxy, jute/polyester, and jute/phenol-formaldehyde composites forapplications such as low-cost housing materials, silos for grain storage, andsmall fishing boats.147

FIGURE 2.32Schematic (a) and photograph (b) of a jute plant. ((a) from the Gesamtverband der DeutschenVersicherungswirtschaft e.V.: http: //www.tis-gdv.de/tis_e/ware/f_inhalt0.htm; photograph(b) from Seigler, D.S.: http: //www.life.uiuc.edu/plantbio/263/image/Corchorus.jpeg accessedon July 5, 2003.)


2.7.4 Kenaf (Hibiscus cannabinus L., Malvaceae) Fibers

Kenaf (Figure 2.33) is an annual canelike crop originating in Asia andAfrica. Kenaf is fast growing and reaches heights up to 2.4 to 6 m in 5 months. Furthermore, it has the highest carbon dioxide absorption of anyplant (1 ton kenaf absorbs 1.5 tons of atmospheric CO2), a valuable tool inthe prevention of global warming.148 The stems of kenaf are generallyround and contain thorns, which, depending on variety, vary in size fromsmall to large. Kenaf contains two fiber types: long fibers situated in thecortical layer and short fibers located in the ligneous zone. Kenaf elemen-tary fibers are short, between 1.5 and 6 mm, and polygonal.149 Their sur-face is striated and irregular. The lumen varies greatly in thickness atdifferent points in the cell, sometimes disappearing altogether. Kenaf is apale-colored fiber that contains less noncellulosic materials than jute. Kenaffibers are coarse, brittle, and difficult to process. They have a breakingstrength similar to that of low-grade jute and are weakened only slightlywhen wet.150

The kenaf whole stalk and outer bast fibers have many potential specificuses, including paper, textiles, and composites. The use of kenaf fibers is alsoof particular significance from the standpoint of environmental friendliness.Historically, kenaf fibers were used to manufacture rope, twine, and sack-cloth. Now, various new applications for kenaf products are emerging,including those for paper products, building materials, absorbents, and feedand bedding for livestock. It is certain that still other options will alsoemerge. These options will involve issues ranging from basic agriculturalproduction methods to marketing of kenaf products.151 Kenaf fibers are envi-sioned for applications as reinforcing fibers for polymers.

2.7.5 Ramie (Boehmeria nivea L. and Boehmeria viridis, Urticaceae)Fibers

Ramie (Figure 2.34) is considered one of the oldest cultivated fiber crops ofEast Asia and is cultivated mainly in Indonesia, China, Japan, and India.Ramie is a hardy perennial crop and can be harvested up to six times peryear. The plant grows to heights of 1.2 to 2.5 m.152,153 Technical ramie fibersare long (1.5 m or more). The fibers obtained from the outer part of the stalkare the longest and one of the strongest fine textile fibers. The diameter of theelementary fiber varies from 10 to 25 m. They are flat and irregular inshape, with a thick cell wall, and taper to rounded ends. The primary cellwall is often lignified, and this aspect is responsible for the low hygroscopiccharacter of the fibers.154

Ramie fibers have been used as a textile fiber for centuries due to theirexcellent fiber characteristics.154 Ramie fibers are very fine and silk-like, nat-urally white in color, and have a high luster. Other advantages of ramiefibers are good resistance to bacteria, mildew, and insect attack. The fibersare stable in alkaline media and not harmed by mild acids. The fibers have


FIGURE 2.33(a) Kenaf plant. (Courtesy of www.inaro.de.) (b) Cross section of a kenaf stalk. (Courtesy ofUSDA/American Kenaf Society.)


an exceptional strength that even increases slightly when wet.153 Because ofexcellent fiber properties, ramie fibers have a high potential as reinforcingfibers for polymer composites.

2.7.6 (Stinging) Nettle (Urtica dioica L., Urticaceae) Fibers

The stinging nettle (Figure 2.35) is a perennial, cosmopolite, dioecious plantthat grows to heights of 2.8 m and contains nonlignified bast fibers in thebark (see Figure 2.22 and Figure 2.23).155,156 The fibers of the stinging nettlehave a remarkable tensile strength and fineness (Figure 2.36) at an averagelength of about 4 cm. Growing nettles as a source of fibers, food, and phar-maceuticals dates back to 3000 B.C.157,158 The nettle fiber yield of the wildnettle (35%) was increased by breeding and through selection proceduresto 14%. During the Middle Ages and the great wars (World Wars I and II),the extremely durable nettle bast fibers were used to produce various tex-tiles (nettle cloth). However, after the war (1945), use of nettle stoppedcompletely. Nowadays, nettle fibers can be used as reinforcing fibers forpolymer composites.159,160

FIGURE 2.34Schematic (a) and photograph (b) of a ramie plant. (Schematic (a) from the Gesamtverbandder Deutschen Versicherungswirtschaft e.V.: http: //www.tis-gdv.de/tis_e/ware/f_inhalt0. htmaccessed on July 5, 2003; photograph (b) from Seigler, D.S.: http: //www.life.uiuc.edu/plantbio/263/image/Boehmeria.jpeg accessed on July 5, 2003.)


2.8 Leaf Fibers

2.8.1 Sisal (Agave sisalana, Liliaceae) Fibers

Sisal (Figure 2.37) is native to Mexico and Central America, but is nowwidely grown in tropical countries in Africa, the West Indies, and the FarEast. The plant grows to a height of about 2 m. The Mayans and Aztecs usedsisal fibers, extracted from the sword-shaped leaves of the plant, to manu-facture crude fabrics.161 The fibers are extracted from the fresh leaves bydecorticators, then washed and sun dried. Currently, decorticators are fullyautomatic, to which the plant leaves are fed crossways on a conveyor belt.The machines remove the leaf tissue by crushing, scraping, and washing.162

FIGURE 2.35The stinging nettle and its fibers. (From Brstmayr, H.: http: // ifa-tulln.boku.ac.at/BPNEU/Homepage/Deutsch/BPdeFAN.htm accessed on July 5, 2003.)


The sisal fiber (Figure 2.38) is a hard fiber extracted from the leaves of thesisal plant and is one of the four most widely used plant fibers. It accountsfor almost half the total production of plant fibers. Nearly 4.5 million tons ofsisal fiber are produced every year throughout the world. Tanzania andBrazil are the two main producing countries.163

The length of sisal fiber varies between 0.6 and 1.5 m and its diametersrange from 100 to 300 m. The fiber is actually a bundle of hollow sub-fibers. Their cell walls are reinforced with spirally oriented cellulose in ahemicelluloses and lignin matrix. The composition of the external surface ofthe cell wall is a layer of ligninaceous material and waxy substances thatbond the cell to its adjacent neighbors.164 Hence, this surface does not formstrong bonds with a polymer matrix.

Sisal fibers are smooth, straight, yellow, and are easily degraded in saltwater. The tensile properties of sisal fiber are not uniform along its length.The fibers extracted from the root or lower part of the leaf have a lower ten-sile strength and modulus but a higher fracture strain. The fibers become

FIGURE 2.36Scanning electron microscopy of elementary nettle fibers (Urtica dioica). (From Dreyling, G.,Institut fr Angewandte Botanik, Universitt Hamburg, Institut fr Angewandte Botanik,Abteilung Nutzpflanzenbiologie, Arbeitsgruppe Dreyling, Germany: http: //www.biologie.uni-hamburg.de/ianb/nb/urtabb16.html accessed June 17, 2003.)


stronger and stiffer at midspan, and the fibers extracted from the tip havemoderate properties. The tensile strength, modulus, and toughness of sisalfibers decrease with increasing temperature.165

FIGURE 2.37(a) Schematic of a sisal plant, and (b) sisal plants in Cuba. (From the Gesamtverband derDeutschen Versicherungswirtschaft e.V.: http: //www.tis-gdv.de/tis_e/ware/f_inhalt0.htmaccessed on July 5, 2003.)


Sisal fiber-reinforced polymers166 are used for interiors of cars.167 Forinstance, Georgia Composites developed a product that is made of partiallyconsolidated recycled poly propylene (PP) reinforced with sisal. The com-posite is produced on a double-belt laminating press. The sisal-reinforced PPcan be used to boost the performance of Woodstock-type materials. Whenthe two are compression molded together in a laminate, the sisal/PP signif-icantly increases tensile modulus and strength for applications such as door-panel inserts and trunk liners.25

2.8.2 Henequen (Agave fourcroydes, Liliaceae) Fibers

Henequen (Figure 2.39) is a close relative of the sisal plant. Its fibers are com-monly used in the manufacture of textile products. It was used by theMayans (in ancient Mexico) to make string, hammocks, clothing, and rugs.Today the henequen (and sisal) industry is concentrated in the tropicalregions of Africa, Central and South America, and Asia (particularly China).Henequen (and sisal) fibers are produced in some of the poorest areas of the

FIGURE 2.38Original, untreated (standard) sisal fiber, plan view (magnification 3400). (From Bismarck, A. et al., Green Chem., 3, 100, 2001. With permission of The Royal Society of Chemistry.)


FIGURE 2.39(a) Processed henequen fibers from Mexico. (From Baltazar Y Jimenez. With permission.) (b) Scanning electron microscopy of original, untreated henequen fibers, plan view (magnification 350.)


world. In many cases the fiber crop production is the only source of incomeand economic activity in these areas. Therefore, fiber production can con-tribute significantly to the efforts to reduce poverty and provide ruralemployment to nearly 6 million people.168

Henequen fibers consist from ~60% cellulose, 25% hemicelluloses, 8%lignin, and 2% waxes. The fibers have a variable diameter, being larger atthe butt end and smaller at the tip end of the fiber. Also, the diameterchanges for different fibers cultivated in other locations. The fiber cross sec-tion changes from a beamlike shape at the butt end to a rounded form atthe tip end of the fiber. Like sisal, henequen fibers are smooth, straight, yel-low, and easily degraded in salt water. Compared to other leaf fibers,henequen has low elongation at break and a low modulus. Henequen fibershave a relatively high tenacity which makes them suitable as reinforcementfor polymers. The attraction of henequen fibers is due to toughness andresiliency.169,170

2.8.3 Pineapple (Anannus comosus, Bromeliaceae) Leaf Fibers (PALF)

Pineapple is usually cultivated for its fruit. Pineapple bran, the fruit residueafter juicing, is high in vitamin A and is used in cattle feed. Pineapple leaffibers (PALF) are obtained from the leaves of the pineapple plant (Figure 2.40).The plant is largely cultivated in tropical countries. Its cultivation in India issubstantial (about 91,000 ha) and still increasing. The pineapple crop has avery short stem which first produces a rosette of fibrous leaves. The leavesare about 91 cm long, 5 to 7.5 cm wide and sword shaped, dark green incolor, and bear spines of claws on their margins. From them strong, white,and silky fibers can be separated. So far, pineapple leaves are mostly beingwasted simply because of the lack of knowledge about their economic uses.Traditionally, PALF yarns have been used to manufacture fabrics, carpets,mops, and curtains.

PALF is a multicellular lignocellulosic fiber consisting mainly of cellulose(7082%), polysaccharides and lignin. The fibers have a ribbon-like structureand consist of a vascular bundle system present in the form of bunches offibrous cells, which are obtained after mechanical removal of all the epider-mal tissues. PALF is of fine quality, and unlike jute, its structure is withoutmesh. The fiber is very hygroscopic. The superior mechanical properties ofPALF are associated with its high cellulose content and comparatively lowmicrofibrillar angle (14).171 Both flexural and torsional rigidity of PALF arecomparable with jute fibers. An interesting characteristic of PALF is that thefiber bundle strength of PALF decreases by 50% when wet, but the yarnstrength increases by about 13%.

The presence of high amounts of lignin and other waxy substances onPALF causes dyes to fade more rapidly when compared to cotton. PALF hasan additional problem of difficult dye penetration due to its relatively highcoarseness.172


FIGURE 2.40(Top) Photograph of a pineapple plantation in Hawaii. (Courtesy of Regina Hessenmller-Lampke.) (Bottom) Pineapple plant. (Courtesy of Dominik Wieder, Schwertberg, Austria.)


2.8.4 Abaca (Musa textilis Nee, Musaceae) Fibers

The abaca plant (commonly known as Manila hemp), a relative of the banana,grows to heights of 3 to 4 m (see Figure 2.41). It is native to the Philippines,but was also introduced to Indonesia and Central and South America. ThePhilippines are the worlds biggest supplier of abaca fibers and products.

Abaca is a hard fiber obtained from the leaf sheaths. The fibers are sepa-rated from the plants in much the same way as sisal fibers. Abaca is consid-ered to be the strongest of all plant fibers.173 Its tensile strength is three timeshigher than cotton and twice that of sisal. Furthermore, abaca is far moreresistant to degradation in salt water than most other plant fibers. Abaca is alustrous fiber and yellowish white in color. The technical fibers are 2 to 4 mlong. The single fibers are relatively smooth and straight and have narrowpointed ends. The single abaca fi

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