Natural Fibres Reinforced Green Composites

Composites made from natural fi bres and various bio polymers have recently attracted great attention. In this paper, the reasons for the popularity of green composites are highlighted. The disadvantages and modifi cation methods of natural fi bres are then summarized. Additionally, biodegradable polymers and a brief survey of the research work are pointed out.


Introduction
A green or an eco/ecological composite may contain natural bres and natural polymers, or it can be a combination of natural bres and a biodegradable polymer matrix [1].Actually, natural raw materials have been used in composites since 1850.By combining shellac-resin and sawdust little boxes for photographs were produced until 1870.According to Müssig, in the 1920s and 1930s, the rst natural bre composites were manufactured in aircra construction with the aim of creating lighter parts for primary structures.In the late 1930s, Henry Ford promoted their application in the automotive sector [2].Automotive and aerospace industries still demonstrate an interest in using more natural bre reinforced composites [3].
e reasons for this great interest are of course the environmental advantages and biological degradability of natural bres.For example, if ax is burnt a er having been used, no residues remain [4].Natural materials like wood chips and our are also widely used because of their eco-friendly characteristics as llers within thermoplastics in automotive and building applications [5].Even though wooden materials are cheaper than natural bres, it is expected that in the near future, due to extensive native forest destruction, their price will increase to a point at which they will be no longer able to compete with natural bre composites. is is already the case for some native forest-depleted countries [6].Other advantages of natural bres in composite reinforcement are: natural bres are abundant and renewable [7]; production of natural bres results in lower envi-ronmental impacts compared to glass bre production [8]; lower weight of natural bre composites im-proves fuel e ciency and reduces emissions during the use phase of the component [9].e weight reduction when conventional composite materials are replaced with natural bre composites can reach even 26%.More ax bres have been progressively incorporated into natural bre

Izvleček
Kompoziti iz naravnih vlaken in različnih biopolimerov v zadnjem času zbujajo veliko pozornost.V članku podani pregled osvetljuje vzroke za priljubljenost zelenih kompozitov.Povzeti so slabosti naravnih vlaken, metode njihove modifi kacije in pregled biorazgradljivih polimerov ter kratek pregled izvedenih raziskav na tem področju.Ključne besede: naravna vlakna, kompozitni materiali, biorazgradljivi polimeri, ekologija automotive components that require 83% less energy and are 40% less expensive than glass bre components [10]; end of life incineration of natural bres results in energy and carbon credits; reduced tool wear [11]; they are safer especially in automobile interiors as the fractures of natural bre composites are not as sharp as the fractures of glass bre composites [12].Natural bres used in composite reinforcement are split into four categories; seed, bast, leaf and fruit.Cotton is a seed bre.e bast bre group includes kenaf, hemp and ax, while sisal may be considered a leaf bre.On the other hand, coir and banana are vegetable bres [13].According to Zampaloni 2007, bast bres exhibit superior exural strength and modulus of elasticity (MOE), whereas leaf bres show superior impact properties [3].Tensile strength and elastic modulus values of these bres are given in Table 1.
2 Modifi cation of natural fi bres in composite materials e characteristics of composites depend on: the properties of bre, the properties of matrix material, interfacial compatibility of bre and matrix.-Natural bres contain hydroxyl and other polar groups in their constituents [7].
is feature becomes a disadvantage when these bres are used in composite materials since they meet with non-polar polymers.Polar groups are hydrophilic which causes the absorption of water in natural bres. is nature leads to incompatibility and poor wettability in a hydrophobic polymer matrix and weak bonding in the bre/matrix interface [11].
e surface adhesion between the bre and the polymer plays an important role in the transmission of stress from the matrix to the bre.If bonding between the bre and the polymer matrix material is weak, then failure is inevitable in ultimate composite.e absorption of water in the pores and amorphous regions of natural bres serves to reduce interbrillar cohesion and to relieve internal bre stresses [15].Furthermore, the high moisture absorption property of natural bres makes them less attractive for exterior applications [7], decreases their dimensional stability [6] and tends to rotting [16].
ere are several methods existing which intend to improve surface adhesion between natural bre and polymer matrix material.e basic objective, however, remains the same: to remove surface contamination and to provide an intimate contact between the surfaces.e simplest way is to roughen the surface so as to enhance the contact area and facilitate mechanical interlocking [11].Modi cation methods are classi ed as physical and chemical.

Physical methods of modifi cation
Physical treatments change structural and surface properties of natural bres and thereby in uence their mechanical bonding with the matrix [6].Physical methods involve surface brillation, plasma, corona, dielectric barrier techniques etc. Plasma can be de ned as a partially ionized quasineutral gas.In this ionized gas there is the balance between the densities of negative and positive particles in macroscopic volumes and time.Plasma components include ions, electrons, UV and vacuum UV radiation.Plasma is thought to bring physical Tekstilec, 2016, 59(3), 237-243 modi cation on the surface through roughening the bre by the sputtering e ect, producing thus an enlargement of the contact area that increases the friction between the bre and the polymer.Corona and dielectric barrier techniques are nonthermal plasma treatments.Corona discharges are relatively low power electrical discharges that take place at or near atmospheric pressure.e corona is generated by strong electric elds associated with small diameter wires, needles, or sharp edges on an electrode.Corona treatment is a di cult technique to be used on 3D objects. is limitation has highlighted the need for alternative, more 3D 'compliant' surface modi cation techniques such as the ozone and/or ultraviolet radiation (e.g.UV) light [11].

Chemical methods of modifi cation
e compatibility and dispersability of bre and matrix can be improved by developing a hydrophobic coating of a compatible polymer on the surface of a ller before being mixed with polymer matrix.Generally, coupling agents facilitate the optimum stress transfer at the interface between the bre and the matrix.Coupling agents are molecules having two functions.e rst is to react with OH groups of cellulose and the second is to react with functional groups of the matrix [7].
ere are many types of coupling agents including surface-active agents and reactive chemistries, at times referred to as functional modi ers.Surfaceactive agents are materials that increase interfacial adhesion by acting as a solid surfactant and that do not form covalent bonds to the polymer matrix.Materials that form covalent bonds to either thebre or the polymer matrix can be reacted in-situ during processing [17].
e selection of a coupling agent that can combine both strength and toughness to a considerable degree is important for a composite material.e most common coupling agents are silane, isocyanate and titanate based compounds, the chemical composition of which allows them to react with the bre surface, which forms a bridge of chemical bonds between the bre and the matrix [7].
ose options (physical and chemical modi cations) are not attractive, even in terms of cost-bene t ratio, when the composites are designed for low strengthlow cost applications such as transit panels, because of the signi cant cost increase that might result from the use of chemicals or treatment apparatus [6].

Matrix materials
Matrix materials are divided into two categories as thermosets and thermoplastics.ermosets cannot be melted once they have been cured since they are chemically crosslinked.But these matrices generally have quite good mechanical properties.Most common thermoplastics have low transition temperature, giving them quite low sti ness at room temperature and above.e low Tg also makes them very brittle at low temperatures, however, the lack of cross-linking is one of the big advantages of thermoplastics which makes them reshapeable and re-meltable [14].ermoplastics also o er many advantages.One of the advantages of thermoplastic matrix composites is their low processing costs.Other advantages are the design exibility and the ease of moulding complex parts.Simple methods such as extrusion and injection moulding are used for the processing of these composites [18].Most commonly used thermoset polymers in natural bre reinforced composites are polyester, epoxies and vinylester.While natural bres have been traditionally used to ll and reinforce thermosets, natural bre reinforced thermoplastics, especially polypropylene composites, have attracted greater attention due to their added advantage of recyclability [8].Other thermoplastics like polyethylene (PE) and polystyrene (PSI) have also been used in ecological composites.No matter which polymer is used, high temperatures must be avoided due to the possibility of bre degradation.When working with natural bres, the processing temperature must be below 200°C [18].
is also restricts natural bre composites to relatively low temperature applications [19].Low twisted yarns display a very low strength when tested dry in the air and, therefore, they cannot be used in processes such as pultrusion or textile manufacturing routes.On the other hand, by increasing the level of twist, a degradation of the mechanical properties is observed in impregnated yarns (e.g.unidirectional composites) similar to o -axis composites.erefore, an optimum twist should be used to balance processability and mechanical properties [20].On the other hand, strands of natural bres are usually heavily twisted, and that can hinder in ltration or impregnation of the resin matrix into thebre strands [21].

Biodegradable polymers
There are many different polymers of renewable materials.Biodegradable plastics can be based on natural or synthetic resins.Natural biodegradable plastics are based primarily on renewable resources (such as starch) and can be either naturally produced or synthesized from renewable resources [22].PLA is a good example of thermoplastic biopolymer that is used today.Its basic monomer is lactic acid, which is derived from starch by fermentation.PLA is and was frequently used for biodegradable packing materials.However, numerous tests have shown that PLA is also suitable as a matrix for embedding bres in composites [23].Other studied biopolymers have been soy-oil based epoxy, starch, polycaprolactone (PCL), polyhydroxybutyrate (PHB), modi ed cellulose, acetic acid, polyester amide, natural rubber and gluten [22,24].Polyesters play a predominant role as biodegradable plastics due to their potentially hydrolysable ester bonds.Biodegradable polyesters which have been developed commercially and are in commercial development are PHA -polyhydroxyalkanoates, PHHpolyhydroxyhexanoate, PHB -polyhydroxybutyrate, PHV -polyhydroxyvalerate, PLA -polylactic acid, PCL -polycaprolactone, PBS -polybutylene succinate, PBSA -polybutylene succinate adipate, AACaliphatic-aromatic copolyesters, PET -polyethylene terephthalate, PBAT -polybutylene adipate/terephthalate and PTMAT -polymethylene adipate/terephthalate [22].
e problems with most of these polymers have been poor commercial availability, poor processability, low toughness, high price and low moisture stability [25].

Preforms
Natural bres such as ax are usually avialable as short bres.ese short bres are suitable for injection moulding process and for producing non woven mats.In both processes, short bres are randomly orginised which causes relatively poor mechanical properties in resulting composites [20].
ere are several studies available concerning the mechanical properties of green composites.A brief survey of the research work published in the eld of natural bre reinforced composites is given in Table 2.According to the literature, preform types have been generally used in the form of nonwoven/ bre mat.

Conclusion
e aim of this article is to give a short overview of the reasons for the usage of green composites, disadvantages of natural bres and their modi cation methods, matrix materials and research activities associated with these composite materials.e research showed that there are many types of natural bres and biopolymers available for green composites all of which have di erent characteristics and bene ts.Although modi cations and biopolymers increase the cost of ultimate composite, their environmental returns are undeniable.

Table 2 :
Literature survey of green composites