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Swedish University ofAgricultural SciencesDepartment of AgriculturalBiosystems and TechnologyP.O. Box 43SE-230 53 ALNARPSWEDENPhone: +46 - 40 41 50 00Fax: +46 - 40 46 04 21

Rapport 142Report

Alnarp 2006

Bio fibre technologyused for military applications– an overview

Christer WretforsBengt Svennerstedt

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PREFACE

Bio fibres, as annual plant fibres from flax and industrial hemp have a great potential as raw material for products in several industrial branches. There is a growing interest to use these renewable resources in branches like the automotive, building, packaging, paper and furniture industries. There have also been shown interest from the military sector to use bio fibre composites for different application purposes.

An overview of bio fibre technology used for military applications has therefore been carried out. This state of the art report is based on literature studies and information searching. The information work has been carried out within the project “Bio fibre technology for military applications”, which has been financed by the Swedish Defense Material Administration (FMV).

PhD-student Christer Wretfors, who belongs to the Biofibre Technology Research Group at the Department of Agricultural Biosystems and Technology, Swedish University of Agricultural Sciences in Alnarp has carried out the information searching and written most of the report. Associate Professor Bengt Svennerstedt, who is head of the Biofibre Technology Research Group at the Department of Agricultural Biosystems and Technology, Swedish University of Agricultural Sciences in Alnarp has completed and written some minor parts of the report.

Alnarp in May, 2006

Christer Nilsson, Professor

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CONTENT

PREFACE 3

CONTENT 4

SUMMARY 5

SAMMANFATTNING 6

1 INTRODUCTION 7 1.1 Background 7 1.2 Aim of the report 7 1.3 Materials and methods 8

2 NATURAL FIBRES – AN OVERVIEW 9 2.1 Bast fibres 9 2.2 Wood and wooden fibres 13 2.3 Animal fibres 13

3 MATRIX – BINDERS 15

4. BIO FIBRE COMPOSITES – AN OVERVIEW 18 4.1 Bast fibre composites 18 4.2 Wood fibre plastic and wood fibre cement composites 23 4.3 Cellulose fibre composites 25

5. APPLICATIONS FOR MILITARY USE 26 5.1 Uniforms and other textile products 26 5.2 Personal safety equipment 28 5.3 Military transport vehicles 35 5.4 Field housing and other constructions 36 5.5 Packaging 36 5.6 Weapon components and utility tools 36

6. DISCUSSION AND CONCLUSIONS 37

7. REFERENCES 38 Literature references 38Internet references 40

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SUMMARY

The annual global production of biological fibres from cultivated crops is today about 4 billion tons of which roughly 60 % comes from agricultural crops and 40 % from forests. In comparison, annual world production of steel is currently around 0,7 billion tons and plastic production is about 0,1 billion tons. The annual production of biological fibres in Sweden is about 11 million tons from forestry and very little from agriculture.

The renewed interest in industrial use of plant fibre based materials within industries world wide has led to development and production of many new natural fibre based products. Several companies in the European, North American and Japanese industry are interested to produce products based on natural fibre raw materials. Most of the fibres that are used come from short wood fibres but some parts come from long plant fibres as jute-, flax- and hemp fibres.

The bio fibre composites show many advantages. They are renewable and biodegradable, CO2 neutral (when burned), lightweight, exhibit good mechanical properties, good acoustic and thermal insulating properties. The raw materials are cheap and abundant. There are also disadvantages of bio fibre composites. One is the prone to water absorption. Most of the plant fibres are hygroscopic and the water absorption may be rather great. The absorption disadvantage may be controlled at an extra cost by different methods of impregnation e.g. heat treatments or chemical modification procedures as acetylation. Another disadvantage is the destructive effect of the alkaline pore solution on plant fibres. There are at least three strategies for controlling the alkaline degradation of fibre cement composites.

Six main fields of applications for military use have been identified, that might be of immediate interest utilizing bio fibre materials. These are:

- Uniforms and other textile applications - Personal safety equipment - Transport vehicles - Field housing and other constructions - Packaging- Weapon components and utility tools

“Smart textiles” include technologies such as intelligent fibres, interactive textiles and smart fabrics. Military applications include protective clothing and systems integration in the textile itself. The response of hemp and flax fabric reinforced polypropylene composites under ballistic impact has been investigated. The idea being to create a material for protection against secondary fragmentation from AP mines. In addition to the different composite materials, a steel-composite hybrid system was also tested. With the current generation of biocomposites for military transport vehicles, most applications would be interiors, such as various panels, dashboards and also textiles. The basic advantages would be the same as for the civilian industry. With the next generation of materials, we will also see more external and load bearing components being manufactured. Bast fibre and cellulose fibre based composites would make excellent packaging materials due to their light weight and biodegradability.

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SAMMANFATTNING

Den årliga världsproduktion av biologiska fibrer ligger idag på ca 4 miljarder ton av vilket ca 60 % kommer från jordbruket och ca 40 % kommer från skogsbruket. Den årliga världsproduktionen av stål och plast ligger jämförelsevis på ca 0.7 miljarder ton respektive 0.1 miljarder ton. I Sverige produceras idag årligen ca 11 miljoner ton träfibrer men mycket lite fibrer från ettåriga fiberväxter.

Det förnyade intresset att använda växtfiberbaserade material inom industrin över hela världen har lett till att åtskilliga företag i Europa, Nordamerika och Japan är intresserade att tillverka produkter baserade på växtfiberråvara. Den största andelen fiberråvara är korta träfibrer men viss råvaruandel kommer från långa fibrer såsom jute-, lin- och hampfibrer.

Biofiber kompositer har många fördelar. De är förnybara och nedbrytbara, CO2 neutrala, har låg vikt, uppvisar goda mekaniska egenskaper och goda isoleregenskaper (både ljudisolering och värmeisolering. Råmaterialen är billiga att framställa och finns i riklig mängd. Biofiber kompositer har även nackdelar. En av dem är känsligheten för vatten absorption. De flesta växtfiber är hygroskopiska och vattenabsorptionen är tämligen hög. Denna nackdel kan kontrolleras genom olika impregneringsmetoder, t.ex. värmebehandling eller kemisk behandling såsom acetylering. En annan nackdel är växtfiberns känslighet för alkaliska miljöer men det finns emellertid flera metoder att kontrollera den alkaliska nedbrytningen av växtfibermaterialet i fibercement kompositer.

I denna studie har sex huvudområden identifierats för användning av biofiber kompositer i militära sammanhang. Dessa är:

Uniformer och andra textila produkter Personlig säkerhetsutrustning Militära transportfordon Förråd och andra huskonstruktioner i fält FörpackningarVapenkomponenter och verktygsdelar

”Smarta textilier” omfattar teknologier, där man utnyttjar intelligenta fibrer, interaktiva textilier och smarta tyger. Militära användningsområden innefattar bl.a. integrering av interaktiva textilier i militära skyddskläder. Kompositer baserade på lin och hampfiber har undersökts under ballistisk och minsplitter påverkan. Även en stålfiber komposit har ingått i de ballistiska testerna. Militära transportfordon kommer precis som civila fordon med all säkerhet i framtiden att byggas med inrednings- och karossdelar av biofiberkompositer. Med nästa generation av biofiberkompositer kommer fler yttre och lastbärande delar att tillverkas. Biofiberkompositer kommer dessutom att kunna användas för militära förpackningar på grund av den lägre egenvikten och den biologiska nedbrytbarheten.

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1 INTRODUCTION

1.1 Background

With the appearance of synthetic materials at the beginning of the last century, synthetic based materials have steadily replaced bio-based products. As a result of this change in raw material utilisation, combined with an increase in energy and chemical demand, the world is now facing an ecological crisis. This crisis will greatly intensify with the expected growth in demand for industrial products in developing countries. Thus the world community is facing a challenge in having to decrease pollution levels, while at the same time significantly increasing industrial output. Such predictions have led to support for enhanced industrial use of renewable resources (bio-fibre) at the expense of non-renewable resources (glass- and mineral-fibres). Bio-fibres may therefore face a renaissance, not only for old industrial products but also for the manufacture of new types of materials and products.

The annual global production of biological fibres from cultivated crops is today about 4 billion tons of which roughly 60 % comes from agricultural crops and 40 % from forests. In comparison, annual world production of steel is currently around 0,7 billion tons and plastic production is about 0,1 billion tons. Sweden´s annual production of biological fibres is about 50 million tons from forestry and about 0,1 million tons from agriculture.

Taking the above figures in consideration, it is apparent that there should be more than sufficient volumes of agricultural fibres available globally for new products. The huge production of wood fibres in Sweden is mostly used for pulp and paper products. There is not a great supply of wood fibres for the manufacture of new types of materials and products. Therefore there is a potential of using agricultural fibres and the cultivation of agricultural fibres in Sweden has to increase to supply the industry with bio-fibres of sufficient quality.

1.2 Aim of the report

The aim of this report is to give a state of the art description on bio fibre technology used for military applications. The aim also includes a future forecast for new product areas within the military sector.

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1.3 Materials and methods

The report is carried out as a literary review and the information presented was found in published articles and books. Computers with internet access where also used to find relevant information by use of the search engines provided by the library of the Swedish University of Agricultural Science.

There are many concepts, which have to be explained in order to understand the meaning of several areas. Therefore some relevant concepts are defined in the following:

1. Bio fibre means in this report fibres from the plant kingdom including both annual plant fibres and wood fibres.

2. Natural fibres means in this report fibres from both the plant and the animal kingdom.

3. The Bio fibre composite concept consists of two components. “Bio fibre” means that the material is manufactured by renewable plant fibre resources. “Composite” means that it is a material, which has been made by combining at least two ground materials into a new material with special properties.

This report deals with three categories of fibres that are of specific interest for modern natural fibre materials; these are bast fibres, wood fibres and animal fibres. The report has been focused on military product application areas and adjacent areas. The main civilian product areas are not deeply discussed.

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2 NATURAL FIBRES – AN OVERVIEW

Natural fibre can be divided into several categories. Within plant fibres, we find several sub-categories such as bast fibres, wood fibres and seed hairs. This section deals with three categories of fibres that are of specific interest for modern bio fibre materials; these are bast fibres, wood fibres and animal fibres.

2.1 Bast fibres

Bast fibres exist in the inner bark or phloem of many fibre plants to provide structural rigidity to the stems. Just inside the phloem is a wood-like core material consisting short, fine fibres. In the middle of every fibre plant stem is a hollow core, which is shown in Figure 1.

Figure 1. Section of a fibre plant stem (USDA, 2005).

According to Figure 2, bast fibres occur in bundles, which run parallel to the stem between nodes. The fibre strands are composed of many smaller cells. The fibre width of the elementary fibres vary from 50-100 m for technical fibres to 4-10 nm for the micro fibres.

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Figure 2. Fibre section showing fibre diameter and multi structure of flax (Bos, 2005).

Cellulose is the main component of plant fibres and its molecules are held in a crystalline or para-crystalline lattice within the micro fibres, creating a structure of considerable tensile strength. In general, the primary cell walls of a plant contain 10-20 % cellulose, secondary walls up to 50 % and certain specialized cell walls, such as those of cotton fibres, contain up to 98 % cellulose (Keller et al, 2001). The more cellulose the better, since it is the strength of the cellulose we seek to exploit. Besides cellulose and hemi cellulose there are lignin and pectin compounds of the plant fibres. The lignin compound makes the stem rigid and the pectin compound glues the fibre bundles together. In general bast fibre strands are large with lengths varying between 100-400 cm for bundles and width varying between 0.5-5 mm. The physical and chemical characteristics of some bast fibres are shown in table 1.

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Table 1. Physical characteristics of flax, hemp and jute (Franck R.R, 2005).

Fibre type Length

mm

average

(range)

Width

mm

average

(range)

Cellulose

(%)

Hemi-cellulose

(%)

Lignin

(%)

Pectin(%)

Flax (bundles) (250-1200) (0.04-0.6) 68-85 10-17 3-5 5-10

Flax (single fibres) 33 (9-70) 0.019

(0.005-0.038)

Hemp (bundles) (1000-4000)

(0.5-5.0) 68-85 10-17 3-5 5-10

Hemp (single fibres)

25 (5-55) 0.025

(0.01-0.05)

Jute (fibre strands) (1500-3000)

- 70-75 12-15 10-15 1

Jute (single fibres) (2-5) 0.020(0.010-0.025)

The flax and hemp fibres are two very strong plant fibres and they are considered as two of the strongest bio-fibres. Measurements of dimensions, strength/stiffness and elongation are shown in table 2. Comparable values are also shown for synthetic fibres as glass, carbon and aramid. The values are shown as average values but there are great variations in the values at least for the plant fibres flax, hemp and jute.

Table 2. Dimensional and strength properties for flax and hemp fibres compared to synthetic fibres (Rowell et al, 1997). Fibre Density

(kg/m3) Strength(MPa)

Stiffness (GPa)

Elongation(%)

Flax 1 500 350 29 2.5Hemp 1 480 820 30 3.5Jute 1 500 580 26 1.5Glass 2 600 2 760 73 3Aramid 1 440 2 790 124 2.5Carbon 1 770 3 585 235 1.5

The hemp fibres show the greatest fibre strength and stiffness of the plant fibres. The plant fibres compared to the synthetic fibres glass and carbon have much lower density. If the specific strength is calculated as fibre strength in relation to density the hemp fibre (specific strength = 0.55) can be compared to glass fibre (specific strength=1.06).

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The potential for high strength using bast fibres is good. The maximum attainable strength for cellulose is 6-10 GPa; the highest values measured for single fibres are up to 5 GPa and practical fibre strengths in composites are 0.3 to 0.5 GPa. Assemblies of many fibre bundles show moderate average strength with often large scatter. Bast fibres have a large degree of non-uniformity, this applies to most of their characteristics such as chemical composition, crystallinity, surface properties, diameter, cross sectional shape, length, strength and stiffness.

Bast fibres consist of micro fibres with diameters in the order of a couple of nanometers. These cells are bonded together by an organic matrix, forming fibre bundles with a diameter of around 50-100 microns. Theoretically, if one could utilize components from further down in the microstructure, reinforcing elements of higher strength could be obtained. However, this is not easily achieved so today most of the fibres used are fibre bundles. These bundles have a relatively low strength of around 600-700 MPa. The actual fibre cells themselves have a much higher strength of approximately 1500 MPa. Even higher strengths could be achieved if methods for using micro fibres are developed, hence the current interest in this area..

Plant fibres exhibit many defects of various types contributing to the large non-uniformity. Structural defects relate to chemical composition, crystallinity, cracks and deformation faults. Geometrical defects relate to cros sectional shape, diameter and length. This means that better structural and geometrical control is needed to improve fibre performance (Lillholt, 2002).

The natural and processing induced defects in the fibre wall structure have an effect on the tensile properties of the fibre. Longer fibres have been proven to have lower tensile strength because they have a greater chance of containing defects. Plant fibres can therefore not be treated as homogenous structures but contain local defects resulting in heterogeneous stress and strain distributions throughout the fibre (Mott et al., 1996). The bast fibres have advantages as:

renewable resource with good supply good specific strength properties good sound and thermal insulation properties low density cheap fibres

The bast fibres have also disadvantages, which researchers in the bio composite field is working with. By additives and impregnation methods several of the disadvantages can be decreased

uneven quality moisture adsorption sensitive alkaline sensitive fire sensitive

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2.2 Wood and wooden fibres

Wood is a cellular material made up mainly of hollow elongated fibres. Wood fibres are composed principally of cellulose and are glued together by the lignin substance. The wood substance comprises approximately 50-60 % cellulose and 20-30 % lignin with the remaining 10-30 % being extractives or ash-forming minerals.

The moisture content of wood is important to all applications. In trees moisture content may range from 30 % to more than 200 %. The fibre saturation point varies between 25 % to 35 % for most of the commonly used species. This point represents an important characteristic, since moisture content variations below this point alter the physical and chemical properties of wood. Shrinkage and swelling occur when the moisture content is below or above the fibre saturation point (Dolby, et al. 1988).

Wood can be described as an orthotropic material with unique mechanical properties. In table 3 some mechanical property values are given.

Table 3. Mechanical properties for softwood and hardwood. Moisture content 12-15 % (Dolby, et al. 1988).

Density

Kg/m3

Tension

Strength

MPa

Compression

Strength

MPa

Bending

Strength

MPa

Shear

Strength

MPa

E-

Modulus

GPa

Softwood 450-590 90-110 40-50 65-90 7.5-10.0 110-120

Hardwood 490-750 60-140 40-60 60-125 8.0-12.0 100-170

2.3 Animal fibres

Animal fibres have traditionally been used for various textile applications. Popular fibres include wool, silk, cashmere and even camel hair, based on the natural resources available.

For more advanced and demanding applications, however, scientists are looking at exploiting the high strength and elasticity of spider silks. Spider silk polymers are among the toughest materials known (Volrath, 2002). They are extremely fine, tough, strong and extensible. While it was long thought that the silk was made up by protein crystals embedded in a protein matrix, where the crystals were responsible for the high strength and the matrix for the elasticity, it has now been shown that this model was over-simplified. It now appears that spider silks are not simple homogenous filaments but actually resemble microscopic climbing ropes with strands of micro fibrils interspersed with filled inclusion channels covered by several layers of coating. This is the case for the drag line silks for several common garden spiders.

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Depending on the function of the silk, other compositions might be utilized, as spiders are capable of producing many different kinds of silk. Currently there are several research groups investigating how spiders produce their silk. For instance, it has been found that the orb-web building spiders such as the Nephila spp. produce silks with an extendibility of up to 40 %, a tensile strength of 1.2 GPa and a large hysteresis of 50 %, indicating that the silks function as shock absorbers as well as structural elements.

If spider biopolymers could be produced comparably cheap and environmentally friendly, then they could become an interesting alternative to low-tech materials such as cotton and nylon, and high-tech materials such as KEVLAR and TWARON. One of the main industries researching spider silks is the medical industry. One interesting concept is the binding of colloidal magnetite particles to threads of dragline spider silk. Such mineralized fibres would maintain their high strength and elasticity, but could also be oriented by an external magnetic field.

An interesting method for making artificial spider silks has been developed in Canada by Nexia Biotechnologies. Genetically modified goats, specifically called BELE (Bread Early, Lactate Early), are capable of producing spider silk protein in their milk, which is then isolated and sold under the brand name Bio Steel. Currently, the material is being researched for use in nanotechnological applications (Nexia BioTechnologies, 2006).

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3 MATRIX – BINDERS

The matrix has several functions in the bio composite. It functions as a binder for the fibre reinforcement material and it gives protection against moisture and UV-light. In general, there are three categories of binders that are used in the manufacture of bio fibre composites today, these are:

synthetic polymers

biodegradable polymers

natural binding substances

Synthetic polymers are the most commonly used binders today, owing to the fact that early research was focused on replacing the synthetic fibres only and not the binders. The more commonly used synthetic polymers in biocomposites are polypropylene, HDPE, PVC, PS, epoxies and polyesters.

Also the thermal stability, as the fibres start to degrade at around 220 °C, restricts the choice of matrix polymers to conventional thermoplastics, such as PE, PP, PVC and PS (Kandachar, 2002). Another important factor is the viscosity of the polymer. For good impregnation of the fibre, the viscosity of the polymer melt should not exceed 100 Pas in case film stacking is chosen or 1000 Pas for co-mingled technique (Kandachar, 2002).

Both thermosetting resins and thermoplastics have been used as matrix materials. Composites with thermosets are usually stronger, while those with thermoplastics provide greater freedom for complex design shapes (Kandachar, 2002). The big advantage of using synthetic polymers as binders is that they are much cheaper than other alternatives. However, they are not environmentally friendly and as such they undermine, to a certain degree, the usage of the natural fibres themselves.

The current trend is to replace the synthetic polymers with renewable and biodegradable alternatives. The most common biodegradable polymers are:

PLA-Polylactic acid or polylactide

PGA-Polyglycolic acid or polyglycolide

PHB-Polyhydroxybuturate

The physical properties of these biopolymers are given in table 4. When judging these biopolymers as suitable matrixes for bio fibre composites the density and the temperature related properties seem to be the limiting criteria for the choice of suitable polymers. PGA is easily eliminated as its density and melt point are too high in order to be energy saving. PHB may be possible to use as its density and melt point are optimal for these polymers but the low glass transition temperature may be a problem. The best choice seems to be PLA, because polymer and composite densities are low, degradation behaviour, mechanical properties and transition temperatures are acceptable. The melt point of PLA is almost ideal in order to produce bio fibre composites.

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Biopolymer Density

(kg/m3)

Tensilestrength(MPa)

TensileModulus(GPa)

Elongation

(%)

GlassTransitionTemperature ( C)

MeltingTemperature( C)

PLA 1 210- 1 250

21-60 0.35-3.5 2.5-6 45-60 150-162

PGA 1 500- 1 707

60-100 6-7 1.5-20 35-45 220-233

PHB 1 180- 1 262

40 3.5-4 5-8 5-15 168-182

PLA is made from agricultural by-products using a fermentation process. It is produced from starch extracted from sugar beats, potatoes, maize or wheat. PLA has a melting point of 150-162 °C and is primarily suitable for textile and other applications that do not demand extreme performance. PLA is the only biopolymer, which is manufactured in a great industrial scale. The company Dow & Cargill produces about 180 000 ton PLA per year.

Figure 3. Synthesis of polylactic acid (Wikipedia, 2006).

PHB, or Polyhydroxybuturate, is produced by bacteria and has similar properties to PP. It is biodegradable and has a melting point between 168-182 °C. A new way of making PHB utilises genetically modified plants that produce the substance in their leaves. Approximately 14 % of the leaf dry mass is PHB. The main disadvantages of the substance is that it is expensive to produce and relatively brittle. It is currently manufactured by Monsanto under the brand name Biopol.

The tensile strength of PHB can be significantly increased if reinforced with 40 % jute fibre. However, electron microscopy shows voids between the fibre and matrix, indicating that the interfacial adhesion could be further improved.

Table 4 Physical properties of various biopolymers (Van de Velde & Kiekens, 2002).

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Figure 4. Chemical structure of polyhydroxybuturate, PH3B being themost common form (Wikipedia, 2006).

The bulk price for PHB is around 17-20 € per kilo. This can be compared to PP with a price of about 1-2 € per kilo. However, if large scale usage of PHB would occur, comments from suppliers indicate that the price would likely drop close to that of synthetic polyesters or polyamids but most likely never as low as that of polyolefins (Plackett & Andersen, 2002).

The second trend is to use natural substances as binders instead of synthetic or biodegradable polymers. These binders include substances such as starch, lignin, CNSL (Cashew Nut Shell Liquid), cardanol (destilled from CNSL) and also various blends such as BFL (Bio Fibre Lock).

Cardanol, a phenolic based by-product of the cashew nut industry, is obtained by distilling the Cashew Nut Shell Liquid which is the international name of an alkydphenolic oil contained in the spongy mesocarp of the “Acagiù” nut (Anacardium occidentalis). Only recently has cardanol been made available with an acceptable purity (about 90%) thanks to a new distillation process. A resole type pre-polymer is obtained through the polycondensation reaction between cardanol and formaldehyde in presence of a basic catalyst, in different molar ratios. From the resole pre-polymer, a thermosetting resin with a cardanol content ranging from 45% to 55% has been formulated adding an epoxy monomer and an acid catalyst. When reinforced with plant fibres, a fibre content of up to 70-75 weight-% can be achieved (Calo et al, 2003).

Bio Fibre Lock, is a 100 % natural binder from the Swedish research company BioCompoTech. It is primarily intended for the manufacture of the next generation of bio fibre composites based on long bast fibres (www.biocompotech.com).

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4. BIO FIBRE COMPOSITES – AN OVERVIEW

The Bio fibre composite concept is built by two components – “Bio fibre” and “Composite”. “Bio fibre” means that the material is manufactured by renewable plant fibre resources. “Composite” means that it is a material, which have been made by combining at least two ground materials into a new material with special properties.

4.1 Bast fibre composites

Within the last 20 years, the field of natural fibre research has experienced an explosion of interest, especially in the field of replacing glass fibre in composite materials. The main area of usage of plant fibre composites today is the automotive industry, where these materials are used mainly for interior applications. This is actually not a new idea, as early as the 1930’s scientists from Ford examined a variety of natural materials including cantaloupes, carrots, cornstalks, cabbages and onions in a search for materials from which to make an organic car body. Eventually, they found that soybean oil could be used to make high quality pain enamel and also moulded into a fibre based plastic. The company claimed that the material had ten times the shock resistance of steel and Henry Ford delighted in demonstrating the strength of the material by pounding a soybean boot lid with an axe. Unfortunately the material had a long curing time and suffered from moulding problems as was therefore not used in any production cars.

The technical advantages compared to glass fibre include reduced weight, good stiffness and strength, and similar volume fractions at reduced weight. The manufacture of these novel materials can easily be undertaken using the same processes as used for synthetic composites which include hand lay up, resin transfer moulding, vacuum infusion, injection moulding, compression moulding, pultrusion etc (Evans et al, 2002).

The environmental benefits of bast fibre composites include their ease of disposal, their carbon dioxide neutrality and the low energy requirement during their manufacture. Furthermore, the constituents are sustainable and based on biomass available throughout the world ((Evans et al, 2002).

The potential use of bio fibres is reduced because of their hydrophilic nature and the lack of sufficient adhesion between untreated fibres and the polymer matrix. As a result, considerable resources are directed at improving the mechanical performance of bast fibre reinforced composites, and in particular their durability, through optimization of the interfacial bond between the fibres and the polymer matrix. The variable quality of bast fibres, which is dependent on growing conditions and processing techniques, is an additional drawback when considering the use of these fibres in high performance technical applications (Evans et al, 2002).

According to Calleja (2003), one of the main advantages of the use of bast fibre composite materials is related to their intrinsic lightweight. In this way, labour and transport costs will be strongly decreased, and the total weight of the manufactured part will be minimised. The reduction of weight implies a considerable reduction of fuel consumption with the consequent respect to the environment.

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To achieve a strong composite material, not only strong fibres are required but also a good bonding between fibres and matrix. The practical strengths related to fibre bundles and to composites are of the order 30-60 % of the maximum recorded values for cellulosic fibres. The practical strength of cellulosic fibres is, in turn, of the order of 5-20 % of the maximum attainable strength

In view of the increasing burden of the atmosphere’s CO2 content, particular importance attaches to the processes that occur in energy conversion. Renewable raw materials, such as bast fibres, create a CO2 cycle, because plants absorb approximately the same amount of CO2 during their growth as a result of photosynthesis, which is subsequently liberated upon combustion or decomposition. A contribution to the reduction of the greenhouse effect could, therefore, be made, since no additional CO2would enter the atmosphere. In addition, the low density of bast fibres comparing to glass fibres will determine a lower combustible consumption with a lower gas emission. But probably more important is that the materials intended to be replaced by the natural fibre based composites are glass fibre composites. Although glass fibres, as plastics, are not intrinsically harmful to the environment, some of their products can be dangerous or, at least, environment “unfriendly”. The health hazards of dealing with glass fibres have been already mentioned.

A second drawback of these materials regards their recycling because it requires costly separation procedures. It is clear that from this point of view natural fibre offer more possibilities for a complete recycling allowing the development of «eco-waste» technologies and products. Although thermoplastic resins have been proposed recently as a more “clean” solution for mass production of composites parts, new processing routes have been also developed for the “clean” processing of the thermosetting matrix composites.

Thermosetting matrix- bast fibres composites, at the end of their life cycle, can be completely burned with energy recovered in a controlled way, safe for the environment. This behaviour does not apply to glass fibre composites. According to Peijs (2002), the advantages of bast fibre composites can be summarized in the following list, they are:

- renewable- abundant- cheap- lightweight- biodegradable- non-abrasive to processing equipment - CO2 neutral (when burned) - can be incinerated with energy recovery - show less concern with safety and health (no skin irritations) - exhibit good mechanical properties - good acoustic and thermal insulating properties An interesting development was reported from Daicel Chemical Industries in Japan.

By reacting wood pulp fibres in a solvent medium that does not fully penetrate the fibres and a subsequent hot pressing of the partially modified pulp, a semi-transparent polymer sheet is formed. During the hot pressing of the modified fibres, the cellulose ester surfaces of the fibres fuse together forming a continuous material.

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Bio composites have been developed by a number of laboratories using a wide range of biodegradable plastics like cellulose esters, PHB, PLA and starch derivatives and blends. The German Aerospace Center in Braunschweig has evaluated many bioplastics for de manufacture of degradable composites, from 1989 and onwards. Prototypes that have been developed together with industry partners include office chairs, panelling elements, pultruded support beams and safety helmets. However, due to economical reasons, very few of these products have been commercialized.

The ACRES group at the University of Delaware, USA, have developed a fibre-reinforced plastic hay baler door based on soybean resin using RTM techniques. The material involves chemically modifying soybean oil, a commodity that is 50 % cheaper than polyester and vinyl. It is possible to chemically “tailor” the material to provide specific processing and performance properties. Although the resin itself is not biodegradable, it could be formulated to be made such under certain conditions.

A similar technology has been developed by Ashland Speciality Chemicals in Ohio, USA. It is a resin called ENVIREZ 5000, based on soybean and corn. The material contains only 25 % renewable source content for making a polyester resin. Products are manufactured using SMC and one well known company using this technology is John Deere, where their newer model hay balers have exterior panels made from a soy bean plastic composite called HarvestForm, also developed by Ashland.

A completely natural composite material can be produced using CNSL (Cashew Nut Shell Liquid). The chief constituents of the CNSL are anacardic acid, cardanol and cardol. Kapok/cotton fabric is soaked in caustic soda to modify the surface topography and used as reinforcement for formaldehyde polymerised cashew nut shell liquid resin. The caustic soda modifies and cleans the fibre surface of impurities (oils and waxes). The removal of the impurities exposes the micro-fibrils, resulting in a rough, textured fibre surface which binds more easily to the matrix. The resulting material shows good environmental stability. Although water disrupts the structural hydrogen bonds, resulting in a lower bond density leading to lower stiffness, it induces plastic characteristics, which in turn imparts an improvement in toughness. It has also been found that kapok fibre binds better to CNSL than cotton. This is due to the high quantities of lignin in kapok which results in an increase in hydroxyl groups. Cotton on the other hand has less than 1 % lignin content (Mwaikambo et al, 2002).

Bast fibre reinforced thermoset resins ca be made using vacuum infusion (Rodriguez et al, 2003). Two different plant fibres were tested, flax and jute. Flexural, tension and impact test were performed for the composites. Fire resistant (ASTM D3713-78), thermal degradation and water absorption (ASTM D570-81) was determined.

The composites reinforced with flax exhibited the best results on flexural tests, and lower impact energy, as consequence of the strong inter phase developed. Flax composites showed higher impact energy, due to the existence of effective energy dissipation mechanisms, like “pull out”. This was confirmed using SEM. None of the samples resisted the five seconds of exposition to a flame in the fire resistant test. All of them were completely consumed, and flax composites show the longest burning time.

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Sordal, a NASA licensee, is developing advanced honeycomb composite materials containing natural fibres for thermal protection systems (TPS) and structures that may be beneficial for many industrial sectors. Key advantages over synthetic fibres are that the structures will be recycle and biodegradable (Danver & Smart, 2003).

Foams made of biodegradable polyesters can also be reinforced with bast fibres. They are of particular interest in commercial applications including packaging, acoustic and thermal insulation, biomedical products, and sporting equipment. The foam density and the cell size distribution influence the final properties of the foam. A high density foam, that has a higher strength and Young’s modulus can be used for structural parts, while low density foams can be used in thermal and acoustic insulation as well as in packaging applications.

The final foam structure depends upon the relative importance of nucleation and growth rates of gas bubbles in the molten polymeric solution. Solubility and diffusivity of foaming agents as well as viscosity and surface properties of the polymeric solutions are among the most important system properties while temperature, pressure and time of foaming are among the most important process parameters. The initial morphology of the melt determines the formation of closed and/or open cell structure and this can be achieved by promoting heterogeneous nucleation by using nucleating agents, fibres and multiphase polymeric blends. A wide range of foam structures was obtained with various densities and cell size and morphology (Di Maio & Iannace, 2003).

With regards to composite manufacture, Cervera and Artal (2003) note that the use of bast fibres with thermoplastics in injection moulding process is limited by several reasons:1. Regularity of quality and supply of fibre

2. Processing problems: - Thermal degradation of fibres: bast fibres cannot be used with thermoplastics having melting points above 200 ºC, owing to the thermal degradation of the volatile compounds. - Material handling problem: The typical mixing machine cannot cope with fibre bales or fibre sheets and doesn’t even want to afford the problem of feeding a low density material into a extruder with all the associated problems.

3. Others: - Odours - Mechanical properties - Compatibility fibre matrix - Moisture absorption (fogging, low dimensional stability)

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In order to solve these problems and to provide improved properties to composites containing cellulosic and polymeric materials, different treatments can be used, such as: - Delignification: To prevent odours whose responsible are a high lignin content and other components (volatile), and to individualize fibres having a major regularity in the feeding. - Mercerization: To increase thermal resistance of fibres reducing their degradation in compounding process, which gives better mechanical properties at the final product.- Surface modification: To reduce hydrophilic behaviour of fibres with lower moisture absorption and to improve compatibility between fibres and matrix. - Debonding agents: In order to reduce hydrogen bonds between fibres and to facilitate an homogeneous distribution in the composite. - Mechanical treatment: Defibrator to produce a granulate easy to be fed to the extruder.

A new plant fibre composite, called the PTP-resin composite has been developed in Germany by the Faserinstitute in Bremen together with several industrial partners (Müssig, 2005). The different stages of the manufacturing process are shown in Figure 5. It is manufactured using SMC and contains approximately 28 % plant fibre. The composite requires a sixteen day maturation period after which it is ready for use.

Currently the PTP-resin composite is being tested for body components for buses in Germany.

Figure 5. Manufacture of the PTP plant fibre composite (Müssig, 2005).

Plant oil

Epoxy-enhanced triglyceride

Fermented alcohol Fermented carboxylic acid

Polycarbon acid anhydrides

Initiatior

Polymer material made of Triglycerides and Polycarbon acid anhydrides, PTP

Another bast fibre composite is being developed by the Swedish company BioCompoTech, located in southern Sweden. It is based on non-woven hemp fibre mats combined with an all-natural binder. According to the company, the material exhibits high strength and low weight, is biodegradable and based on renewable resources and also exhibits very good burning characteristics.

BioCompoTech have also developed a new series of hybrid composites, where natural fibres are combined with other materials such as carbon fibres and metals, allowing the materials to be tailored to specific needs.

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Figure 6. Welding mask in bast fibre composite from BioCompoTech (www.biocompotech.com).

Figure 7. Basic composite from BioCompoTech (www.biocompotech.com).

4.2 Wood fibre plastic and wood fibre cement composites

Fasalex is a wood product made of biological renewable raw materials manufactured by using the plastic technology process extrusion (Zodl, 2003). The material consists of natural fibres like wooden chips, rice husk, sugar cane, bamboo or straw (up to70 %), and also uses a thermoplastic biopolymer from maize, rice or corn in the materials. This is said to be a big difference to all the other fibre extrusion materials, and the biopolymer is also binding water in the extrusion system.

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One claimed advantage of the FASALEX material is that it is not necessary to dry the fibres. The granules can be produced with a humidity up to 15 % and this water stays in the granules as well as in the profiles.

The profiles have between 20 and 80 % surrounding humidity dimension stability because they are extruded with the same humidity as the surrounding environment. The granules always have a humidity content of up to 12 %. Fasalex is a wood material with the characteristic of wood. The profiles can be veneered and foliated with conventional hot melt lamination systems, also painting with water based lacquers and even powder-coating is possible. The factory price per kg granules is between €0.5 and €0.7 depending on the recipe and purchase quantity (Zodl, 2003).

Fibre boards are an interesting product group utilizing wood fibres (Thomsen et al, 2002). The bonding of fibres and particles by adhesives in fibre boards is accomplished by forming an adhesive matrix in which the fibres are cross-linked by mechanical entanglement or covalent bonding. However, bonding is also caused by auto-adhesion by the lignin in the wood. Under properly selected conditions of heat, pressure and moisture, wood fibres will bind through auto-adhesion. The binding achieved by auto adhesion is not sufficient to eliminate synthetic adhesives though.

Synthetic adhesives such as urea and phenol formaldehyde are commonly used in combination with hot pressing in order to obtain boards with good mechanical properties. Besides the environmental and health aspects, synthetic adhesives contribute significantly to the manufacturing cost of boards and panels. New adhesive systems are therefore considered. One such system being researched in Denmark, utilizes the effect of enzyme treatment by laccase to activate the phenol groups in lignin by oxidation, yielding phenoxy radicals, which are able to react with other formed radicals and produce stable bonds. Laccase is a peroxidase occuring in both insects and plants and catalyses biosynthesis and biodegradation of lignin through radical reactions.

Another method for fibre board production is graft polymerization of wood with hydroxyl-containing compounds like furfuryl alcohol and peroxides, such as hydrogen peroxide. This method is supposed to produce boards with similar mechanical properties to conventional MDF boards. By treating the wood fibres with hydrogen peroxide and ferro sulphate water swelling can also be reduced.

The traditional wood fibre cement board, sometimes called cementboard, is produced using up to 250 mm long fibre like particles called wood-wool or excelsior (Youngquist, J.A. 1999) but there are growing markets that use bagasse, rattan, coir fibre (coconut husk) (English, B.,Youngquist, J.A., Krzysik, A.M., 1994), and straw. The wood particles are blended with cement, formed into mats, and pressed to receive a density of between 0,48 kg/m3 (English, B.,Youngquist, J.A., Krzysik, A.M., 1994) and 1,0 kg/m3 (Youngquist, J.A. 1999).

The most traditional application for low density cementboard is roof decking due to its sound absorbing and fire resistant properties but other applications are blocks or panel for load bearing walls, partitions, concrete forms, exterior siding (English, B.,Youngquist, J.A., Krzysik, A.M., 1994).

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4.3 Cellulose fibre composites

The combination of cellulose fibres and polymer matrix in a composite is a good way to obtain structural load bearing materials and to exploit the strength of the cellulose fibres (Thomsen et al, 2002). Such composites have a high specific strength owing to the low density of the fibres.

The present interest in cellulose composites is based on three concepts:

the well-established principles for composite materials

the potential for improvement of the mechanical strength of the cellulosic fibres

the potentially very large resource of natural raw materials available and their renewability.

Processing a cellulose composite with a thermoset polymer implies a relatively long curing time but can be performed at room temperature or moderately increased temperatures (below 100 °C). If using a thermoplastic polymer, then the reaction temperature is higher, around 180 °C, which is close to the fibre temperature tolerance limit of 200 °C.

The department of Fiber and Polymer Technology at the Royal Institute of Technology in Stockholm has developed a method for modifying the surface of cellulose fibres so that they become conductive to electricity. The goal is to create interactive materials that can respond to external stimuli. The method, polymeric multilayers, creates multiple layers of oppositely charged polymers on the surface of the cellulose fibres. A number of different applications can be envisioned using multilayered cellulose fibres. For instance, magnetic particles could be applied in the various layers, creating a magnetic fibre. Another exciting application involves biochemical substances, such as proteins, that could move in and out of the fibre surface depending on the surrounding pH-level. Conductive papers that can function as sensors have already been developed using polymeric multilayers (Trogen, 2004).

Cellulose fibre reinforced cement has been known since the second world war but became a commercial technology in the beginning of the 1980’s (Coutts, 1988). Contents of up to 12% of the weight of the composite are not uncommon when adopting autoclave curing. The production is done most of the time using adapted Hatshek machines. The sheets can be autoclaved or not. Non-autoclaved products normally have small amounts of PVA fibres in addition to the cellulose ones.

The density of these products depend greatly on the fibre content and compactation stress, but it can be as low as 1.2 kg/dm3 (Coutts, 1988). The modulus of rupture (MOR) tends to slightly increase with the addition of the fibres and the toughness is also greatly improved. The major markets for cellulose reinforced cement is for boards (cladding, siding, flooring and roofing). Durability is a major concern, especially for uses involving roofs.

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5 APPLICATIONS FOR MILITARY USE

Six main fields of applications for military use have been identified, that might be of immediate interest utilizing natural fibre materials. These are:

- Uniforms and other textile applications - Personal safety equipment - Transport vehicles - Field housing and other constructions - Packaging- Weapon components and utility tools

5.1 Uniforms and other textile products

Before the Second World War, natural fibre textiles such as hemp and flax, were commonly used for textile applications. This often referred to as the “first generation” of textiles, based on thousand year old traditions in spinning and weaving of natural fibres. For instance can be mentioned, that the soldiers of the Russian Red Army worn uniforms made by hemp fibre fabrics.

After the Second World War however, a revolution of sort occurred, with the natural fibres being replaced by synthetic, petroleum based fibres like nylon, polyester and polypropylene. This was the “second generation” of textile fibres.

With the current interest in renewable materials combined with advances in the fields of nanotechnology, we are now moving towards the “third generation” of textiles, often called “smart textiles”. These include technologies such as intelligent fibres, interactive textiles and smart fabrics. Military applications include protective clothing and systems integration in the textile itself.

For instance, in 2001, the Defense Advanced Research Projects Agency (DARPA) started a project to accelerate the development of electronic textiles. It’s looking for, among other things, military uniforms that can adapt to different environments, providing on the move camouflage for soldiers.

One of the beneficiaries of the DARPA project is International Fashion Machines (IFM), which was founded by Maggie Orth to do fundamental research in this field. As part of her early research, Orth developed textiles for hanging artworks (IFM, 2006).

IFM and other companies are working on ways to weave touch sensors into fabrics. Orth built a musical jacket with a small keyboard woven into the sleeve. This has since become known as the MIT musical jacket. The jacket uses a combination of conductive and non-conductive textiles and is connected to a series of synthesizers that can be played remotely using the sewn buttons.

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IFM’s Electric Plaid is a textile with the ability to change colours like a computer display. The company claims that it is the only animated, reflective (non-lit)) colour change medium in the world. IFM has also created another technology called Stitch Switch, which is a textile touch sensor that could be used to control the Electric Plaid textile.

Figure 8. MIT musical jacket (MIT, 2006).

Figure 9. Touch sensitive fabric from International Fashion Machines (IFM, 2006).

Using other materials such as the conductive cellulose fibres discussed under cellulose composites, advanced medical systems could be integrated into uniforms. Since these fibres also can function as biosensors, the systems could be made to release biochemical substances, such as various pharmaceuticals, either when triggered by the person wearing the clothing or by environmental stimuli such as contact with blood or toxins.

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However, it will be interesting to see the durability of these new textile materials. Will they still be functional after prolonged usage and what happens if you pop them in the washing machine? So far the authors have not seen any reports on this aspect in the literature.

5.2 Personal safety equipment

On the civilian side, natural fibres are generating interest for personal safety equipment such as hard hats, bicycle helmets and shin guards. For the military, the main application of a plant fibre composite would be to support the currently used synthetic fibres (such as Kevlar) rather than replace them. The main benefits would be reduced costs and environmental impact.

The response of hemp and flax fabric reinforced polypropylene composites under ballistic impact has been investigated (Wambua et al, 2002). The idea being to create a material for protection against secondary fragmentation from AP mines. In addition to the different composite materials, a steel-composite hybrid system was also tested.

Polypropylene sheets were preheated in an oven at 140 °C together with natural fibre fabrics before compression moulding in a hot press at 6.4 bar pressure. The moulding temperature was maintained at 190 °C for 15 minutes, followed by forced cooling between cold plates of the press under the same pressure. The size of the demoulded specimens was 30 x 30 cm2.

The hemp and flax fabric-reinforced composites for ballistic tests were processed with 24 and 26 layers of fabric, respectively. The surface density was about 14.5 kg/m2

and the fibre volume fraction about 46 %.

For the steel-composite hybrids, 1.5 mm mild steel plates were used as facing and 0.8 mm used as backing and facing to the composites. They were then prepared by sandblasting and gluing of the plates and composites together with an Araldite 2011 glue. Curing was done in a hot press under clamping pressure at 100 °C for one hour. For mechanical testing, done in addition to the ballistic tests, the flax and hemp composites were processed at 37 % fibre volume fraction.

The ballistic tests were done at the Royal Military Academy in Brussels following NATO STANAG 2920 ballistic standard (NATO, 1992). The weapon used was a Block Monometric Cannon Interchange gun (BMCI) and the Fragment Simulating Projectile (FSP) was a 5.56 mm calibre weighing 1.1 grams. The propellant used was a Ball powder 0.50 inch blank and the twist generated was seven inches per revolution. The tests were done at 22 °C. The shots were fired at a distance of 10 metres from the target and the exit velocity, in case of full penetration, was measured with a Doppler Radar Antenna. As for the STANAG 2990 standard, the V50 was determined from the average of six readings, the three highest non-penetrating and the three lowest penetrating velocities, all within a variation of 40 m/s.

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Figure 10. BMCI gun (Wambua, 2006).

The results of the ballistic tests are shown in Table 1. Whereas the plain flax composites tested did not meet the criteria set by the NATO STANAG 2920 standard (V50 = 450 m/s), the composite mild steel hybrids attained a V50 of at least 466 m/s. Despite the low V50, the flax composite panels were earmarked for field tests since the envisaged threat and the secondary fragmentation from blast AP mines is considered less than that posed by primary fragmentation from fragmentation mines. Most of the secondary fragmentation is usually of much lower density and larger diameter than metallic primary fragments. The irregular shape and larger surface area presented to the armour material further decreases the possibility of its complete penetration of the material.

Table 5. Results of ballistic testing (Wambua, 2006). Sample VF = 46% Sample Code Total Thickness

(mm) Surface Density (kg/m2)

V50(m/s)

Flax composite F26 12.9 14.5 312Flax steel faced hybrid F26S 14.4 26.3 466

Flax steel faced and backed hybrid

SF26S 14.5 26.7 576

Plain steel PS 1.5 11.8 264

Table 6. Effect of fibre volume fraction on the flexural modulus of flax composites (Wambua, 2006). Fibre Volume Fraction (%) Flexural Modulus (GPA) 46 5.22 ± 0.13 52 5.83 ± 0.31 55 6.47 ± 0.18 58 5.71 ± 0.16

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The ballistic performance of composites can be improved by processing to high fibre volume fractions. Depending on the application, the most suitable material for ballistic protection provides a good balance between weight, comfort, cost and the level of protection. While it is possible to use very low resin contents in synthetic fibre (e.g., Kevlar, glass) composites, natural fibre composites presented wetting problems at high fibre volume fractions. A flexural (three-point bending) test was conducted to monitor the bonding at the interface so as to ensure the mechanical integrity of the composite panel. The flexural modulus of the flax composites increased with increasing fibre volume fraction up to Vf = 55 percent, then showed a decrease at Vf = 58 percent as demonstrated in Table 2. A fibre volume fraction of 52 percent was utilised in the processing of samples for the field tests. The increase in the fibre volume fraction resulted in a decrease in the composite thickness and surface density (which in turn cause a reduction in the V50). With the said fibre volume fraction (52 percent), the composites presented a good balance between weight, thickness and V50. Table 7 presents the parameters of the composite and composite/steel hybrid solution for this research work. These two materials were used for the field tests.

Table 7. Flax composite and composite/steel hybrids optimised for field tests (Wambua, 2006).

Code Material Flax Fabric Layers

VF(%)

Thickness(mm)

SurfaceDensity(kg/m2)

V50(m/s)

F26FT Flaxcomposite 23 52 9.9 11.3 280

SF26SFTComposite/Steelhybrid

26 + steel plates(2 x 0.8 mm)

52 11.6 24.2 489

Simulated AP mines containing C4 explosives were utilised in the field tests carried out at the NATO test zone at the Houthalen-Helchteren shooting field in Belgium. The panel was placed in front of a wooden support fixed to the ground as illustrated in Figure 5. The test was conducted using 35 g, 70 g and 150 g of C4 explosives (see Figure 6) to simulate the small, medium and large AP mines. The explosives were placed in the ground at a distance of 30 and 50 cm (50 cm only for plain composites) from the target and covered with different kinds of projectiles, such as stones, to increase the amount of secondary fragmentation and to simulate demining accidents. The results of the field tests are as summarised in Table 8.

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Figure 11. An illustration of the field test setup (Wambua,2006).

Table 8. Summary of field test (Wambua, 2006). FieldTestNumber

Composite Type (flax)

Test Setup Distance from Mine (cm)

Mass of C4 Explosives(g)

Result

1a hybrid fixed/noback support

30 70 slightdebonding

1b hybrid fixed/backsupport

50 70 minor frontscratches

1c hybrid fixed/noback support

30 150 thrown 5 m, slightdebonding

1d hybrid fixed/backsupport

50 150 thrown 2 m, no debonding

2a composite fixed/backsupport

50 35 surface dents,fibre fracture

2b composite fixed/backsupport

50 70 fibre fracture,surface dents, crack at rear

Composite/steel hybrids. Two samples were tested at a time at 30 cm and 50 cm respectively from the simulated mine. The distances were measured from the face of the panels to the centre of the simulated mine. These distances were shorter than the representative field operating distances from a mine to the sternum derived from field measurements by deminers, i.e., 65–70 cm. The differences in the distances may produce significant blast effects (pressure has been found to fall as the inverse cube of the distance from the blast), but it was assumed that the differences in the distances selected did not produce a significant change in the velocities of the secondary fragmentation.

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The simulated AP mines used for composite steel hybrids contained 70 g and 150 g of C4 explosives to mimic common medium and large AP mines used in many countries. The explosives were placed in two casings of a PMN mine, which is the largest AP mine, and covered. After detonating high explosive material, almost 100 percent of the energy liberated is transformed into blast energy, which propels secondary fragmentation at high velocities.

Figure 12. Complete debonding of the front steel plate after detonation of 150 g of C4 at 30 cm from the simulated mine (Wambua, 2006).

Figure 13. Front face of the composite/steel hybrid after detonation of 150 g of C4 at 50 cm away (Wambua, 2006).

Tests using 70 g of C4. Tests on sample SF26SFT, the composite/steel hybrid placed 30 cm from the simulated mine, revealed a small debonding between the front 0.8 mm mild steel plate and the composite at one corner. There was no noticeable damage on the front or rear surface of the material.

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Apart from small surface scratches on the front side steel plate, there was no visible damage on sample SF26SFT tested at 50 cm from the simulated mine. Tests using 150 g of C4 Sample SF26SFT, placed 30 cm from the simulated mine, showed complete debonding of the steel plate on the front side of the composite/steel hybrid system. The debonded steel plate had small dents on the surface. A small debonding between the backside steel plate and the composite was seen at one corner of the composite/steel hybrid system. There was, however, no penetration on the material by a projectile.

Figure 14. Front damage on the composites after detonation of 35 g of C4 at 50 cm away (Wambua, 2006).

Flax fabric reinforced polypropylene composites—tests using 35 g of C4. Figure 14 demonstrates damage that occurred on the front side of the composite panel after detonation of 35 g of C4 explosives. Numerous surface dents and fibre fractures as a result of projectile hits are clearly visible. One projectile caused barely visible damage at the back side of the composite. No complete projectile penetration was observed.

Flax fabric reinforced polypropylene composites—tests using 70 g of C4. Visual observation after detonation of the 70 g of C4 explosives indicated fibre failure and numerous dents (sizes ranging from small to large) on the front side of the flax composites as shown in Figure 15 (below). Several areas of visible damage were observed at the back of the plate. Figure 16 (below) indicates a nearly transverse crack at the back of the composite; however, none of the projectiles was very close to the actual penetration.

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Figure 15. Crack at the back of the composite panel after detonation of the 70 g of C4 explosives (Wambua, 2006).

Figure 16. Flax composite front side damage after detonation of 70 g of C4 explosives (Wambua, 2006).

These tests show that in order to protect a deminer against secondary fragmentation from small to medium AP blast mines, a natural fibre composite protective material with a V50 less than the standard 450 m/s may be sufficient. For large AP mines, a composite steel hybrid system may provide the required protection, but care should be taken to prevent possible injury from steel plates in case they debond.

The results from the study show that composites based on natural fibres can be alternative materials for anti-ballistic protection against secondary fragmentation in situations of detection and clearance of AP (blast types) landmines. The performance per areal weight, both in terms of V50 and critical absorbed energy at penetration, reaches the highest value when the natural fibre composites are covered at the front and back with thin (0.8-mm) steel plates. It is most probable that even better solutions exist using high-tech aramid fibres or special ballistic steels, but these materials are very expensive for people in developing countries threatened by landmines. Composite materials based on readily available natural fibres and commodity polypropylene, faced with cheap mild steel plates, could be a possible alternative.

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5.3 Military transport vehicles

Since the civilian automotive industry has been leading the field of natural fibre composite development, it is not surprising that it is also perhaps the most promising area for future military automotive applications.

With the current generation of biocomposites, most applications would be interiors, such as various panels, dashboards and also textiles. The basic advantages would be the same as for the civilian industry. With the next generation of materials, we will also see more external and load bearing components being manufactured.

One new advantage not seen in the civilian industry would be the ability to repair damaged composites in the field. Some materials under development would allow the patching of composites using locally available fibre sources, in a way similar to plastic padding. In an emergency, a solider could even simply gather the plant material himself, for instance nettles, allow it to sun dry and then extract the fibres by hand grinding the plants, followed by applying a proper binder to the fibres and patching up the damaged area.

An example of an early natural fibre composite used for a military application is the “Gordon-Aerolite”, a composite of unidirectional, unbleached flax yarn impregnated with phenolic resine and hot pressed. The “Gordon-Aerolite” was used in aircraft fuselages during World War II (Evans et al, 2002).

Natural fibre based honeycomb cores could be used as base materials for panels and structures in many applications and for many industrial segments. By way of example, in aerospace, honey comb cores are used, among other things, for the passenger walk ways and cargo spaces, the overhead stowage bin compartments, galley’s, heads, bulkheads, nose cone, blocker doors and control flaps. In the electronics and medical area, honeycomb cores are used to make panels for “clean rooms”. Other applications are for airflow profiling in ductwork, refrigeration and related applications (Danver & Smart, 2003).

Remote controlled unmanned vehicles would also be good candidates for bio fibre materials since the minimum requirements on the materials would be lower compared to manned vehicles. A cheap, light and mass produced unmanned surveillance vehicle that is biodegradable and with several of the electronic systems integrated in the composite hull could be a reality within the next couple of years.

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5.4 Field housing and other constructions

In the civilian industry there is a growing interest for cheap, light and safe housing alternatives suitable for foreign aid. A similar concept could also be applied to military housing, where light weight plant fibre based panels could be used for housing modules. If based on the next generation of composites, they would be durable, fire proof and biodegradable. Add a fibre with a modified conductive surface that is capable of mimicking its surroundings in order to create an ultra-modern version of the camouflage tent.

The Sordal company are also looking at military applications for their honeycomb materials, of special interest are temporary roads and buildings which may later be composted and stay at the point of use (Danver and Smart, 2003).

5.5 Packaging

Bast fibre and cellulose fibre based composites would make excellent packaging materials due to their light weight and biodegradability. They are already in use in the civilian industry, for instance, Nokia have been packaging their higher end mobile phones in a material based on flax fibre.

It is possible that with future advances in barrier technology, plant fibre composites could also be used for distributing food and drinks. For instance, the interior surface could clad with a thin glass layer, something already done today with certain plastic beer bottles.

5.6 Weapon components and utility tools

The high specific strength of plant fibre composites would make them an option for use in certain components such as rifle stocks and knife handles. Suitable materials would be bast fibre composites or wood plastics, depending on the weight requirements.

The greatest challenge would be to ensure that the components are all weather capable but today several materials exist, that have a high moisture tolerance and with a proper surface coating most natural fibre materials can be toughened up although it might compromise the eco-friendliness. It is less likely that we will see natural composites used for demanding applications such as gun barrels due to the extreme demands on such materials.

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6. DISCUSSION AND CONCLUSIONS

The bio fibre composites show many advantages. They are abundant, renewable, biodegradable, cheap, non-abrasive to processing equipment, CO2 neutral (when burned), can be incinerated with energy recovery, show less concern with safety and health (no skin irritations), lightweight, exhibit good mechanical properties, good acoustic and thermal insulating properties.

There are also disadvantages of bio fibre composites. One is the prone to water absorption. Most of the plant fibres are hygroscopic and the water absorption may be rather great. The absorption disadvantage may be controlled at an extra cost by different methods of impregnation e.g. heat treatments or chemical modification procedures as acetylation. Another disadvantage is the destructive effect of the alkaline pore solution on plant fibres. There are at least three strategies for controlling the alkaline degradation of fibre cement composites. This can be accomplished by using low alkaline cement, protecting the fibres against the alkaline water by fibre impregnation and protecting the whole composite from water.

Improving the properties of plant fibres can be interesting from the technological point of view. Modifications that result in an increase of the elastic modulus, an increase in dimensional stability when absorbing or releasing water or in a longer service life under different conditions are particularly interesting. Improvements can be obtained by (a) chemical modification; (b) densification by thermo-mechanical treatment; (c) densification by impregnation.

There is a large knowledge base in the civilian industry concerning natural fibre materials that can be adapted to military applications. This brief literature review has shown that there are many possible military applications for natural fibre based materials. Using the materials available today in the civilian industry, most of the applications would be relatively low-tech, such as panels, boxes, textiles and other non-load bearing products.

There could however be substantial gains from both an environmental and an economical perspective from using bio based materials. With the next generation of bio fibre based composites, more advanced applications involving tailor-made smart materials will come into use, both in textiles and in the design of natural fibre composites.

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7. REFERENCES

Literature references

Bos H.L., 2005. Naturfaserverbundwerkstoffe für industrielle Anwendungen. Technische Universiteit Eindhoven, The Neatherlands, PDF-presentation, Agrotechnology and Food Innovations,Wageningen, UR, Germany, 2005, http://www.agrofibrecomposites.com/BiogeneRohstoffe.pdf.

Calleja N., Elizetxea C., Rubio A., 2003. Samples preparation, mechanical characterization and recycling of natural fibre composites. Proceedings of the 1’st International Workshop on Natural Fibre Composites, 19-21 October, 2003, Ischia, Italy.

Calò E., Maffezzoli A., Mele G., Tarzia A., Zurlo S., 2003. Cardanol based composite reinforced with natural fibres, Proceedings of the 1’st International Workshop on Natural Fibre Composites, 19-21 October, 2003, Ischia, Italy.

Cervera S. F., Artal G., 2003. Natural fibre treatments to improve their performance in composite reincorcement. Proceedings of the 1’st International Workshop on Natural Fibre Composites, 19-21 October, 2003, Ischia, Italy.

Coutts R.S.P., 1988. Wood fibre reinforced cement composites. Swamy, R.N. Ed., Natural Fibre Reinforced Cement and Concrete. London, Blackie, (1988), pp. 1-61.

Danver D., Smart R. P., 2003. Honeycomb cores based on natural fibres. Proceedings of the 1’st International Workshop on Natural Fibre Composites, 19-21 October, 2003, Ischia, Italy.

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