natural fibre as reinforcement for polymers: a review

Shehu U et al, SPJTS.2014,2.(1),238-253 ISSN 2321-4597

South pacific Journal of Technology and Science 238

NATURAL FIBRE AS REINFORCEMENT FOR POLYMERS: A REVIEW

Shehu U1*, Audu H.I1, Nwamara M.A1, Ade-Ajayi A.F1, Shittu U.M1, M.T. Isa1.2

1Petrochemical and Allied Department - Polymer Division

National Research Institute for Chemical Technology, Zaria, P.M.B 1052 2Department of Chemical Engineering, Ahmadu Bello University, Zaria

Abstract Attention is turning to natural fibres in composite material production because of the non-biodegradability and non-renewability of synthetic fibres traditionally in use. The non-biodegradability has effect on the environment and their non-renewability can make them scarce in the future. However, natural fibres are not without disadvantages of hydrophilicity and low mechanical properties. Therefore, chemical treatments have been suggested for the control of these setbacks. The objective of this work is to review different type of natural fibres, treatment and their applications, as polymer reinforcement. The review indicated that, among the various natural fibres considered, sisal fibre reinforced have high impact strength besides having moderate tensile and flexural properties compared to other lignocellulosic fibres such as jute, banana, bagasse, coir etc. Out of the different type of treatments suggested, biological treatment has been identified as a new alternative for chemical treatment of natural fibre for improve properties. The method is also identified as environmentally friendly when compared to other methods.

Key words: natural fibres, fibres treatments, properties and composites.

1.0 INTRODUCTION

In recent years, engineers and scientists have refocused their direction towards utilizing natural fibres as reinforcement in polymer for making low cost construction materials. Natural fibres are prospective reinforcing materials and their use until now has been more traditional than technical. They have long served many important purposes but the application of the material technology for the utilization of natural fibres as reinforcement in polymer matrix took place in comparatively recent years (Prankash et al., 2013). Economic and other related factors in many developing countries where natural fibres are abundant demand that scientists and engineers apply appropriate technology to utilize these natural fibres as effectively and economically as possible to produce good quality fibre reinforced polymer composites for housing and other needs (Kuruvilla et al., 1999). Natural fibres are elongated substances produce by plants and animals that can be spun into filaments, thread or rope, woven, knitted, matted or bonded, they form fabrics that are useful to society. Flax, hemp, jute, straw, wood fibre, rice husks, wheat, barley, oats, rye, cane (sugar and bamboo), grass reeds, kenaf, ramie, oil palm empty fruit bunch, sisal, coir, water hyacinth, pennywort, kapok, paper-mulberry, raphia, banana fibre, pineapple leaf fibre and papyrus are various types of natural fibres that can be used in plastics composites (H. Ku et al., 2011). According to a Food and Agricultural Organization survey, Tanzania and Brazil produce the largest amount of sisal. Henequen is grown in Mexico. Abaca and hemp are grown in the Philippines (Amar et al., 2005). The largest producers of jute are India, China, and Bangladesh while the largest producers of Sun Hemp are Nigeria, Guyana, Siera Leone and India. Presently, in Nigeria, Sun Hemp is mostly produced in the western part of the country (Chawla et al., 1987). The annual production of natural fibres in India is about 6 million tons as compared to worldwide production of about 25 million



tons (Saira et al., 2007). Natural fibres have the advantage that they are renewable resources, low densities, biodegradability, non-toxicity and good insulation property. However, the drawbacks of natural fibres are due to its poor wettability, high moisture absorption, and incompatibility with some of the polymeric matrices. The moisture absorption which is due to hydrophilic property of natural fibres adversely affects the mechanical properties such as flexural strength, flexural modulus and fracture toughness (Nguong et al., 2013).Natural fibres can be economically and ecologically useful alternatives to reinforcement fibres in polymeric composites, due to their low density and low cost in comparison to conventional fibres. Natural based fibres have been used as reinforcements for composite materials and give various advantages compared to conventional fibres (Mwaikambo et al., 1999). Natural fibre reinforced composites have great potential for use in engineering applications such as packaging, automotive industries etc (GeorgiosKoronis et al., 2012). A growing environmental awareness across the world has aroused interest in research and development of environmentally friendly and sustainable materials. Therefore, several researches in the use of natural fibres and the effect of treatment on their properties as reinforcement have been reported. Tabil L.G et al., (2007) reported that when sisal-polyester was treated with NaOH at room temperature, the tensile strength was increased by 4%. In another work of Cyras V.P et al., (2004) 10% NaOH was treated with sisal-polycaprolactone composites and it was reported that there was increased in elastic modulus than the untreated one. Pinkering et al., (2007) studied fungi treatment on Hemp fibre and reported that 22% higher composites strength was achieved in comparison to the untreated Hemp fibre. Moreover, for Hemp polypropylene thermoplastics, biological treatment showed 32% improvement in composite strength than chemical treatment (H. Wang et al., 2012). Doan et al. (2006) investigated the effect of maleic anhydride grafted Polypropylene coupling agents on the properties of jute fibre/Polypropylene composites. The addition of 2wt% maleic anhydride to Polypropylene matrices improved the adhesion strength with jute fibres and the mechanical properties of composites than the untreated jute fibre.

Therefore, the objective of this work is to review different type of natural fibres, treatments and their applications, as polymer reinforcement

2.0 CLASSIFICATION OF NATURAL FIBERS

Natural fibres can be classified into animal fibres and plant cellulose fibres. Plants that produce natural fibres are categorized into primary and secondary depending on the utilization. Primary plants are grown for their fibres while secondary plants are plants where the fibres are extracted from the waste product. There are six major types of fibres namely; bast fibres, leaf fibres, fruit fibres, grass fibres, straw fibres and other types (wood and roots etc.). There are thousands of natural fibres available and therefore there are many research interests in utilization of natural fibres to improve the properties of composites. Fig 1 shows the classification of natural fibres.



Fig. 1 Classification of natural fibres (Sujan et al., 2013)

2.1 POTENTIAL PLANT FIBERS

2.1.1 Sisal

The true sisal from Agave sisalana is the most important of the leaf fibres in terms of quality and commercial use. Originating in the tropical west hemisphere, sisal has been transplanted to east Africa, Indonesia and Philippines. The sisal fibre coarse and strong, but compared with Abaca fibre it is inflexible, although with a relatively high elongation under stress. It is also resistance to salt water. The principal applications are in binders and baling twine and as a raw material for pulp for product requiring high strength. It was predicted between 1998 and 2010 that the global demand for sisal fibre and its products will decrease by a yearly rate of 2.3%. Joseph and Thomas (2003) prepared sisal fibre reinforced Polypropylene composites by melt-mixing and solution-mixing methods. The methods enhanced the tensile properties of the composites. The effect of fibre content and chemical treatments on the thermal properties of sisal/Polypropylene composites was also evaluated. It was found that treated fibre composites show superior properties compared to the untreated system. Differential scanning calorimetry (DSC) measurements exhibited an increase in the crystallization temperature and crystallinity, upon the addition of fibres to the polypropylene matrix. This is attributed to the nucleating effects of the fibre surfaces, resulting in the formation of transcrystalline regions. On increasing the fibre content, the melting peak of the Polypropylene component was shifted to higher temperatures suggesting a constrained melting.

2.1.2 Flax

The Flax fibre from the annual plant Linumusitatissimum has been used since ancient time as the fibre for linen. The plant grows in temperate, moderately moist climate, for example, in Belgium, France, Italy, Russia and Ireland. The boiled and bleached flax fibres contain almost 100% cellulose and it is the strongest of the vegetable fibres, highly absorbent but is particularly in extensible. Flax fibre is used in canvas, thread and twines, and certain industrial application such as fire hoses. Weyenberg et al., (2006) investigated the chemical modification of flax fibre by alkali treatment. The study concentrated on

Plant

Fruit

Coir

Cotton

Natural fibres

Animal

Wood pulp Straw

Corn

Rice

Feathers

Wool

Silk Leaf

Banana

Sisal

Bast

Jute

Kenaf

Hemp

Flax

Grass

Switchgrass

Indian grass

Bamboo



optimizing parameters, such as time and concentration of NaOH, to develop a continuous process for the treatment and fabrication of unidirectional flax fibre epoxy composites. The authors observed a specific improvement in the mechanical properties of the resulting material; treatment with 4% NaOH solution for 45 s increased the transverse strength of the composites by up to 30%. Wang et al., (2004) investigated the Effects of different chemical modifications on the properties of flax fibre reinforced rotationally molded composites. The chemical modifications carried out were mercerization, peroxide treatment, benzoylation, and peroxide treatment. The modified fibres were then extruded with the polymer matrix to form the composite and an improvement in interfacial adhesion was observed. Zafeiropoulos et al., (2007) investigated the effect of chemical modification on the tensile strength of flax fibres. The authors tried acetylation and stearation and found that the tensile strength of flax fibres did not exhibit any drastic improvement. The SEM examination of the fractured surfaces revealed that acetylated fibres exhibited a different mode of failure from the other fibres, suggesting that the treatment altered the bulk properties along with the surface properties.

2.1.3 Sunn

The stem of the herbaceous plant crotalariajuncea, called sunn provide a bast fibre. The plant is native to India and also grown in Bangladesh, Brazil and Pakistan. It has along tap root and grows to a height of up 5m. The white fibre is graded by colour, firmness, length, strength, uniformity and extraneous matter content. Sunn is used for canvas, paper, fishing net etc. Czigany (2006) manufactured the sunn fibre reinforced, Polyethylene matrix hybrid composites in the process of carding, needle-punching, and pressing. Hemp, glass, and carbon fibres were applied besides sunn fibre in these composites. In order to achieve a sufficient interfacial adhesion, the fibres were treated with the reaction mixture of maleic acid anhydride and sunflower oil. The hybrid effect in these composites was examined as a function of fibre content and fibre combination. The strength properties of hybrid composites improved owing to surface treatment and this was proven by mechanical tests and microscopic analysis, as well.

2.1.4 Hemp

Hemp is derived from plant cannabis originating in central China. It is also grown in central Asian and Eastern Europe. The stem is used for fibre; the seed for oil, leaves and flowers for drugs, among them is marijuana. The fibres are graded for colour, luster, spinning quality, density, cleanliness and strength. Hemp serves as substitute for flax in yarn and twine. Its earlier used in ropes has been replaced by leaf and synthetic fibres (Shibata et al., 2006). Arib et al. (2006) investigated the tensile and flexural behaviours of hemp fibre–low density polyethylene composites as a function of volume fraction. The tensile modulus and tensile strength of the composites were found to be increasing with fibre content in accordance with the rule of mixtures.

2.1.5 Abaca

The Abaca fibre is obtained from the leaves of banana-like plant. The fibre is also called Manila hemp from the port of its first shipment, although it has no relationship with hemp, a bastfibre. Abaca fibre is unique in its resistance to water, especially salt water, and it is used for marine ropes and cables although it is being replaced by synthetic fibres. It is the strongest of the leaf fibres and also the strongest among the papermaking fibres. It is also used for sausage casings and it is the preferred fibre for tea bags because of its high wet strength, cleanliness and structure which permits rapid diffusion of the tea extract. Shibata et al. (2007) fabricated the lightweight laminate composites made from Abaca and Polyester fibres by hand layup. The effects of the number of Abaca layers, heating time, and Abaca weight fraction on the flexural modulus of the composite specimen were investigated. The flexural modulus increased with increasing number of Abaca layers and heating time. The increase of the



number of Abaca layers contributed to homogeneous polyester dispersion in the composite board. This is because more Abaca layers caused better contact between Abaca and Polyester and prevented Polyester fibres from shrinking by heating. The increase of heating time contributed to better wetting between Abaca and Polyester.

2.1.6 Jute

This fibre is obtained from two herbaceous annual plants, Corchoruscapsularis originating from Asia and Africa. Countries such as Bangladesh, India, China and Brazil provide the best condition for the growth of jute. The plants are harvested by hand, dried in the field for defoliation and water retted for periods up to a month. Jute has traditionally been an important textile fibre, second only to cotton; however, jute is been steadily replaced by synthetic in the traditional high volume uses such as carpet backings and burlap fabrics and sacks. The strands are also used for twines, while kraft pulping of jute gives ultimate fibres for cigarette papers. Mwaikambo et al. (2000) showed that jute fibre has been used as reinforcement for conventional Polypropylene and maleic anhydride grafted Polypropylene resins. Treating the reinforcement with acetic anhydride and sodium hydroxide has modified the fabric (fibres). Thermal and mechanical properties of the composites were investigated. Results show that fibre modification gives a significant improvement to the thermal properties of the plant fibres, whereas tests on the mechanical properties of the composites showed poor tensile strength. Mercerization and weathering were found to impart toughness to the materials, with acetylation showing slightly less rigidity compared to other treatments on either the fibre or the composites. The modified Polypropylene improved the tensile modulus and had the least toughness of the jute reinforced composites.

.2.1.7Kenaf and Roselle

These are fibres derived from hibiscus cannabinus and H Sabdariffa respectively. Kenaf is grown for production in the People’s Republic of China, Egypt, and regions of the former USSR while Roselle is produced in India and Thailand. The plants are hand-cut, moved or pulled in developing countries while mechanised harvesting methods are under investigation in the United States. Kenaf fibres are considered as substitute for Jute and used in sacking, rope, twine, bags and as papermaking pulp in India, Thailand and the former Yugoslavia. Shibata et al., (2006) fabricated the lightweight laminate composites made from kenaf and Polypropylene fibres by press forming. The effects of the number of kenaf layers, heating time, and kenaf weight fraction on the flexural modulus of the composite specimen were investigated. The flexural modulus increased with increasing number of kenaf layers and heating time. The increase of the number of kenaf layers contributed to homogeneous Polypropylene dispersion in the composite board. This is because more kenaf layers caused better contact between kenaf and Polypropylene and prevented Polypropylene fibers from shrinking by heating. The increase of heating time contributed to better wetting between kenaf and Polypropylene.

2.2 CHEMICAL COMPOSITION OF NATURAL FIBERS Plant based natural fibres are lignocellulosic in nature. As stated by (Sujan et al.,2013), natural fibres (except cotton) are generally composed of cellulose, hemicellulose, lignin, waxes, and some water-soluble compounds, where cellulose, hemicelluloses, and lignin are the major constituents. Natural fibres generally contain 60-80% of cellulose, 5-20% lignin and moisture up to 20%. Cellulose is considered the major framework component of the fibre structure. It provides strength, stiffness and structural stability of the fibre. Hemicellulose occurs mainly in the primary cell wall and has branched polymers containing five and six carbon sugars (Fig. 2a) of varied chemical structures. Lignin is amorphous and has an aromatic structure (Fig. 2b). Pectin comprises of complex polysaccharides. Their



side chains are cross-linked with the calcium ions and arabinose sugars, (Kabir et al., 2012). With the processing temperature increases, the surface of cell wall of natural fibres will experience pyrolysis. Pyrolysis process is a chemical decomposition of organic material at high temperature with the absence of oxygen, (Sujan et al., 2013). The structural composition and properties of the fibres are summarized in Table 1 and 2. Table 1: Structural Compositions of natural fibres (Amar et al., 2005)

Fibers Cellulose (wt%)

Hemicelluloses (wt%)

Lignin (wt%)

Pectin (wt%)

Moisture (wt%)

waxes Microfibrillar Angle(Deg)

Flax 71 19.6 2.2 2.3 10 1.7 5-10

Hemp 72 20.1 4.7 0.9 9 0.8 26.2

Jute 66 17 12.5 0.2 13 0.5 8

Kenaf 51 21.5 10.5 3-5

Sisal 73 12 12 10 16 2 16

Henequen 78 4-8 13.1

Palf 76 8.85 11.8 14

Banana 64 10 5 11

Abaca 59 12.5 1 8

Oil palm EFB

65 19 42

Oil palm mesocarp

60 11 46

Cotton 88 5.7 0-1 8 0.6

Cereal straw

42 23 16 8

Table 2: Mechanical properties of natural fibres Fibre Density

(g/cm3) Diameter (µm)

Tensile strength (Mpa)

Young’s Modulus (Gpa)

Elongation at Break (%)

Jute 1.3-1.45 25-200 393-773 13-26.5 1.16-1.5 Hemp - - 690 - 1.6 Kenaf - - - - 2.7 Flax 1.5 - 345-100 27.6 2.7-3.2 Remie 1.0 - 400-938 61.4-128 1.2-3.8 Sunn - - 1.17-1.9 - 5.5 Sisal 1.45 50-200 468-640 9.4-22.0 3-7 Cotton 1.5-1.6 - 287-800 5.5-12.6 7-8 Kapok - - - - 1.2 Coir 1.15 100-450 131-175 4-6 15-40 Banana - - 1.7-7.9 - 1.5-9.0



HO

O OH3C - C - O

O OO

OH H H

H H

HH H

OH

OH

H H H

Fig 2a:structure of hemiceuose

OCH3o

H2C - c -OH- O -O

CH2

OCH3o

Fig 2b:structure of lignin

2.2.1 Advantages of natural fibres In recent times, natural reinforced fibres have attracted the attention of researchers because of their advantages over other established materials. Their main advantages may include: Environmentally friendly, fully biodegradable, abundantly available, renewable, cheap, low density, light compared to glass, carbon and aramid fibres and low abrasive in nature compare to synthetic fibres (Zini et al., 2011). 2.2.2 Disadvantages of natural fibres Although natural fibres and their composites are environmentally friendly and renewable (unlike traditional ceramics, metals and synthetic polymer fibres), they have several problems which include: Poor wettability, incompatible with some polymeric matrices and have high moisture absorption. Composite materials made with the use of unmodified plant fibres frequently exhibit unsatisfactory mechanical properties (Alamri et al., 2012). 3.0 TREATMENTS OF NATURAL FIBRES One of the major drawbacks associated with the use of natural fibres in composites is their high hydrophilic nature leading to low mechanical properties and delaminating. The reduction in mechanical properties may be due to poor interfacial bonding between resin matrices and fibres. Therefore, there is need to modify the fibre surface to make it more hydrophobic and well-suited with resin matrices (Abdelmouleh et al., 2004). To surmount the problems, a surface treatment or



compatible agents need to be applied prior to composite fabrication. These treatments (both chemical and biological) help in improving the mechanical properties of the fibre and hence making it more hydrophobic, (Munawar et al., 2007). 3.1 Chemical methods Different chemical treatments that have been used on natural fibres are: alkaline (Mercerization), silane, benzoylation, stearic acid, fatty acid derivative (oleoyl chloride) and triazine. According to Kabir et al., 2012, the major problems of natural fibre composites originate from the hydrophilic nature of the fibre and hydrophobic nature of the matrix. The inherent incompatibility between these two phases results weakening bonding at the interface. Chemical treatments on reinforcing fibre can reduce its hydrophilic tendency and thus improve compatibility with the matrix. Several research activities have been conducted to improve fibre adhesion properties with the matrix through chemical treatments. The followings are the reviews of different chemical treatments on the fibre and their effects on composite properties. 3.1.1 Mercerization treatment Sodium hydroxide (NaOH) is extensively used to treat the natural fibres in order to modify the molecular structure of the cellulose. It changes the orientation of highly packed crystalline cellulose order and forming an amorphous region where cellulose micro molecules are separated at large distances and the spaces are filled by water molecules (Saira et al., 2012).Mercerization leads to fibrillation which causes the breaking down of the composite fibre bundle into smaller fibres. It reduces fibre diameter, thereby increases the aspect ratio which leads to the development of a rough surface topography that results in better fibre/ matrix interface adhesion and an increase in mechanical properties (Giuseppe et al., 2010). Alkali sensitive hydroxyl (OH) groups present among the molecules are broken down, which then react with water molecules (HAOH) and move out from the fibre structure. The remaining reactive molecules form fibre cell–O–Na groups between the cellulose molecular chains. Due to this, hydrophilic hydroxyl groups are reduced and hence make the fibres to exhibit hydrophobic property. It also eliminates a certain portion of hemicelluloses, lignin, pectin, wax and oil covering materials (Alberta et al., 2010). For these reasons, the fibre surface becomes more homogeneous due to the elimination of microvoids and the stress transfer capacity between the ultimate cells improves. This increases effective fibre surface area for good adhesion with the matrix. High concentration of alkali leads to the excess delignification of the fibre which results in weakening or damaging of the fibres (Shafiullah et al., 2007). The chemical reaction of the fibre–cell and NaOH is represented in Scheme 1.

Fibre-cell-OH + NaOH Fibre cell-O-Na+ + H2O --------1 Scheme 1 Jacob et al., (2004) examined the effect of NaOH conc. (0.5, 1,2,4 and 10%) for treating sisal fibre reinforced and concluded that maximum tensile strength resulted from the 4% NaOH treatment at room temperature. Mishra et al., (2008) reported that 5% treated NaOH fibre reinforced polyester composites having better tensile strength than 10% NaOH treated composites. Because at high concentration there is delignification of natural fibre taking place and as a result damage of fibre surface. The tensile strength of composite decreased drastically after certain optimum NaOH concentration.



3.1.2 Silane treatment Silane is one of the Coupling agents which improve the degree of cross-linking in the interface region and offer a perfect bonding. The composition of silane forms a chemical link between the fibre surface and the matrix through a siloxane bridge. Hydrolysis, condensation and bond formation are the various stages undergo during the treatment process of the fibre. During hydrolysis process, in the presence of moisture and hydrolysable alkoxy group, silanol were formed. While one end of silanol reacts with the cellulose hydroxyl group during condensation process, the other end of the silanol reacts with matrix functional group to form a covalent bond (Fig3a).(Sreekala et al., 2000). This reaction enhances hydrocarbon chain which prevents swelling of the fibre by creating a cross-linked network because of covalent bonding between the matrix and the fibre. It also provides uniform distribution of molecules across the interface of the composite. For this reason, the properties of composite become stable and its fibre matrix adhesion improves.Ghasemi et al., 2010 investigated the effect of alkali (5% NaOH for 2 h) and silane (1% oligomericsiloxane with 96% alcohol solution for 1h) treatments on flexural properties of jute epoxy and jute polyester composites. For jute epoxy composites silane over alkali treatments showed about 12% and 7% higher strength and modulus properties compared to the alkali treatment alone. Similar treatments reported around 20% and 8% improvement for jute polyester composites.

Fig (3a): Interaction of silanes with cellulosic fibres

3.1.3 Benzoylation treatment In benzoylation treatment, benzoyl chloride is used to decrease hydrophilic nature of the fibre and improves interfacial adhesion, there-by increasing strength of the composite. It also enhances thermal stability of the fibre (Bledzki et al., 2008). During benzoylation treatment alkali pre-treatment is used. At this stage, extractable materials such as lignin, waxes and oil covering materials are removed and more reactive hydroxyl (OH) groups are exposed on the fibre surface. Then the fibres are treated with benzoyl chloride. OH groups of the fibre are further replaced by benzoyl group and it attached on the cellulose backbone. This result in more hydrophobic nature of the fibre and improves adhesion with the matrix. The Possible reaction between cellulosic-OH and benzoyl chloride is given in Equation 2 and 3 (Joseph et al., 2000).

Fibre – OH + NaOH Fibre- O-Na+ + H2O -------------------------2



Fiber - O- Na+ + ClC - Fiber - O - Co

- + Nacl

--------------------------3Benzoylation of fibre improves fibre matrix adhesion, thereby considerably increasing the strength of composite, decreasing its water absorption and improving its thermal stability. Joseph et al. (2000) applied benzoyl chloride treatment on alkali pre-treated sisal fibre and reported higher thermal stability compared to the untreated fibre composites. A similar method was also applied by Wang (2004) to improve the interfacial adhesion of flax fibre and polyethylene (PE) matrix. The fibre was initially alkaline pre-treated in order to activate the hydroxyl groups of the cellulose and lignin in the fibre, then the fibre was suspended in 10% NaOH and benzoyl chloride solution for 15 min. The isolated fibres were then soaked in ethanol for 1 h to remove the benzoyl chloride and finally was washed with water and dried in the oven at 800C for 24 h. 3.1.4 Stearic acid treatment Stearic acid (CH3(CH2)16COOH) in ethyl alcohol solution is used to modify the fibre surfaces. The carboxyl group of stearic acid reacts with the hydrophilic hydroxyl groups of the fibre and improves water resistance properties(Wang et al.,2012).Apart from removing the amorphous constituents (pectin, wax oil,etc) from the fibre structure, this treatment facilitates fibre dispersion into the matrix to create better bonding at the interface and provides improved properties of the composites. Zafeiropoulos (2002) observed that treated flax fibres were more crystalline than the untreated ones and stearation decreased the fibre surface free energy. Kalaprasad et al. (2004) also reported that higher tensile strength and modulus properties were observed when sisal fibre was treated with stearic acid in ethyl alcohol solution. Torres et al., (2005) used 3% stearic acid treated sisal fibre reinforced in polyethylene composites and reported 23% higher shear strength properties compared to the untreated fibre composites. The reaction between fibre and stearic acid is presented in Equation (5) below. Fibre-OH + CH3(CH2)16COOH CH3(CH2)16COO-O-Fibre + H2O ---------------5 3.1.5 Fatty acid derivative (oleoyl chloride) treatment Fatty acid derivative is used as a coupling agent to modify fibre surface to improve wettability and interfacial adhesion characteristics. Oleoyl chloride is a fatty acid derivate which reacts with the hydroxyl groups (esterification) and grafted on the cellulose backbone of the fibre. During esterification, the molecules are penetrating inside the cellulose structure and reacts with the hydroxyl groups of the fibre and the matrix. Additionally, hydrophilic hydroxyl groups present on the external surface are removed by this treatment and make the fibre more hydrophobic. This improves the wetting and adhesion of fibre surface into the matrix and provides improved composite properties. Modification on jute fibre with dichloromethane and pyridine solvent under a dry nitrogen atmosphere results in more hydrophobic characteristics and increases composite properties (Wang et al., 2012). 3.1.6 Triazine treatment Triazine treatment is used for surface modification of the natural fibres. A triazine (C3H3N3) derivative (e.g. C3H3N3Cl3) has multifunctional groups in its structure. The reactive chlorines that are present in the heterocyclic ring reacts with the hydroxyl groups of the fibre by esterification and provide linkage



between the cellulose phase and coupling agent. The carbon-carbon double bonds form covalent bonds with the matrix by grafting. It also provides a secondary reaction with the hydrophilic hydroxyl groups of cellulose and lignin that provides better moisture resistance properties (Fig 3c) (Xue Li et al., 2006). This modification provides crosslinking between the cellulose (through hydrogen bond) and matrix. This results strong adhesion at the interface and improves composite properties (Reihmane et al., 2007). Mishra et al., 2002 used trichloro-s-triazine based different coupling agents to treat cellulose fibres and were reinforced in unsaturated polyester resin. Improved fibre matrix adhesion and higher moisture resistance properties were reported for the treated fibre composites.

N

N

N N

N

N

N

N

N

cl

cl cl clcl

clH2NR

Cellulose fibre

HN

R

oo

Cellulose fibre

H

Fig 3c:modification of cellulose fibres with trizine derivatives. 3.1.7 Biological Treatment Biological treatment has been recently considered as a promising alternative for surface modification of natural fibres. This biological treatment is environmental friendly and efficient. Biological treatment is used to remove non-cellulosic components (such as wax) from the fibre surface by the action of specific enzymes. White rot fungi produces extracellular oxidases enzymes that reacts with lignin constituents (lignin peroxidase) (Li Y et al., 2007). This causes the removal of lignin from the fibre. It also increases hemicelluloses solubility and thus reduce hydrophobic tendency of the fibre. In addition to this, fungi produce hyphane that creates fine holes on the fibre surface and produces a rough interface for better interlocking with the matrix (Jafari et al., 2007). The treatment process involves sterilization of the fibres in an autoclave for 15 min with 120oC. Afterwards, fungi are added proportionally with the fibre and incubate for 2 weeks at 27oC. Fibres are then sterilized, washed and oven dried. Pickering et al. (2007) studied fungal treatment on hemp fibre and reported that a 22% higher composite strength was achieved in comparison to the untreated one. Moreover, for hemp polypropylene thermoplastics, fungi over alkali treatment showed a 32% improvement in composite strength. 3.2 Natural fibre composites

Natural fibres are used to fill and reinforce both thermoplastics and thermosets, this represents one of the fastest-growing types of polymer additives. Fibre reinforced composites (FRC) contain



reinforcements having lengths much higher than their cross-sectional dimensions. Fibres are the load-carrying members, while the surrounding matrix keeps them in the desired location and orientation. Natural fibres themselves are cellulose fibre reinforced materials as they consist of microfibrils in an amorphous matrix of lignin and hemicelluloses. These fibres enjoy the right potential for utilisation in composites due to their adequate tensile strength and good specific modulus, thus ensuring a value-added application avenue (Maleque et a., 2007) . Lignocellulosic natural fibres, originated from different plant fibres are suitable raw materials for the production of a wide range of composites for Structures, properties and recyclability of natural fibre reinforced polymer composites different applications. When these fibres are incorporated into a matrix to form a composite, the matrix serves to bind the fibres together, transfer loads to the fibres, and protect them against environmental damage caused by elevated temperature and humidity. The matrix has a strong influence on several mechanical properties of the composite such as transverse modulus and strength, shear properties and properties in compression. The most common matrix materials for composites are thermoset and thermoplastic polymers. The combination of a plastic matrix and reinforcing fibres give rise to composites having the best properties of each component. Since the plastics are soft, flexible, and lightweight as compared to fibres, their combination provides a high strength-to-weight ratio for the resulting composite. The significant weight savings and the ease and low cost of the raw constituent materials make these composites an attractive alternative material to glass and carbon fibres (Kozłowski et al., 2008). Material scientists all over the world focus their attention on natural composites reinforced with jute, sisal, banana, coir, pineapple etc., primarily to cut down on the cost of raw materials. Recently, a lot of research has been done in this field (Khan et al., 2006). Khondker et al. (2006) have reported that the jute/PolyPropylene unidirectional composites, specimens with only 20% of jute fibre content, show remarkable improvement in tensile and bending properties when compared to those of the virgin PolyPropylene specimens. The improvements in the mechanical properties are broadly related to various factors, such as the wettability of resin melts into fiber bundles, interfacial adhesion, and uniform distribution of matrix-fibres and the lack of fibre attrition and attenuation during tubular braiding process. Unidirectional composites from filament wound non treated flax yarns and Polyethylene fabricated foils composite have exhibited axial stiffness and strength in the range 27–29 and 251 -321MPa, respectively. A modified version of the ‘rule of mixtures’, supplemented with parameters of composite porosity content and anisotropy of fibre properties, was developed to improve the prediction of composite tensile properties (Madsen B. et al., 2003). The tensile strength of hemp strand/Propylene composites can be as high as 80% of the mechanical properties of glass fibre/Poly Propylene composites (Mutje P. et al., 2007). Qiu et al. (2006) have reported that Poly Propylene with higher molecular weight revealed stronger interfacial interaction with cellulose fibres in the composites, compared with the lower molecular weight Poly Propylene; the composites derived from higher molecular weight of Poly Propylene exhibited stronger tensile strength at the same cellulose content. Georgopoulos et al. (2005) have investigated that the loading of low density poly ethylene with natural fibres leads to a decrease in tensile strength of the pure polymer. On the other hand, Young’s modulus increased due to the higher stiffness of the fibres. Although the properties of some blends are acceptable for some applications, further improvement is necessary, by optimizing fibre–polymer interface characteristics.

Table 3: Selected Properties of Natural and Synthetic Fibres. (Westman et al., 2010)

Fibre

Density (g/cm3)

Tensile Strength (MPa)

Specific Tensile Strength (MPa)

Elastic Modulus (GPa)

Specific Elastic Modulus (GPa)

Cotton 1.5-1.6 400 250-267 5.5-12.6 3.5-8.1 Kenaf 1.45 930 641 53 36.5 Sisal 1.5 511-635 341-423 9.4-22 6.3-14.7



E-glass 2.5 2000-3500 800-1400 70 28 Carbon 1.4 4000 2857 230-240 164-171

Table 4: Physico-Mechanical Properties of Some Natural fibre/Polyester Composite Lamination V.K Mathur 2005

Property Sisal/Polyester (resin~50wt%)

Jute/Polyester (resin~70wt%)

Coir/Polyester (resin~70wt%)

Density (g/Cm3) 1.05 1.22 1.4 Water absorption, 24 h(%) 3-4 1.09 3-4 Swelling thickness (%) 5 - 5-6 Tensile strength (MPa) 40-50 66.01 20.40 Elongation (%) 4-6 2.31 0.4-1.0 Tensile Modulus (GPa) 2.13 4.42 1.2-2.0 Flexural Strength (MPa) 77 93.80 41.54

TABLE 5: Selected Mechanical Properties of Some Composites. Ramesh 2013.

Composites Tensile strength (MPa)

Flexural load (KN)

Displacement (mm) Impact Strength (Joules)

Glass fibre + Sisal fibre composite 176.20 2.3 11.2

18

Glass fibre + Jute fibre Composite 229.54 2.1 12.3 10 Glass fibre + Jute fibre + Sisal fibre Composite

200.00 3.0 14.2 12

3.3 Applications of natural fibre composites

Natural fibres composite are used for manufacturing the following

Automotive: The lightweight, low cost natural fibres offer the possibility to replace a large portion of the glass and mineral fillers in several automotive interior and exterior parts. In the past decade, natural fibre composites with thermoplastic and thermoset matrices have been embraced by European car manufacturers and suppliers for door panels, seat backs, headliners, package trays, dashboards, and interior parts. Natural fibres such as kenaf, hemp, flax, jute, and sisal are providing automobile part reinforcement due to such drivers as reductions in weight, cost, and CO2, less reliance on foreign oil sources, recyclability, and the added benefit that these fibre sources are “green” or eco-friendly (Ghassemieh, 2011).

Aerospace structures: The military aircraft industry has mainly led the use of polymer composites. In commercial airlines, the use of composites is gradually increasing. Space shuttle and satellite systems use graphite/ epoxy for many structural parts (Lekakou et al., 1999).

Building and construction industry: Panels for partition and false ceiling, partition boards, wall, floor, window and door frames, roof tiles, mobile or pre-fabricated buildings which can be used in times of natural calamities such as floods, cyclones, earthquakes, etc (Josmin et a.,2012).



Other areas of application of natural fibre composites includes; marine, Sports goods, bulletproof vests and other armor parts, Chemical storage tanks, pressure vessels, piping, pump body, valves, biomedical applications, electrical application and so on. (Lekakou et al., 1999)

4.0 Conclusion

The following can be drawn from the review. Treated fibres have mechanical and physical properties than untreated fibres. Flax fibres treated with NaOH are found to perform better in composites than those treated with benzoyl chloride and peroxide. Biological treatment has been recently considered as a promising alternative for surface modification of natural fibres and works are few in this area. The review also indicate that, among the various natural fibres considered, sisal fibre reinforced have high impact strength besides having moderate tensile and flexural properties compared to other lignocellulosic fibres. References

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