Research Journal of Engineering Sciences ___________________________________________ ISSN 2278 – 9472Vol. 2(3), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 46 Review Paper Natural Fiber as a substitute to Synthetic Fiber in Polymer Composites: A Review Begum K. and Islam M.A. Department of Chemical Engineering and Polymer Science, Shahjalal University of Science and Technology, Sylhet-3114, BANGLADESHAvailable online at: www.isca.in Received 12th February 2013, revised 21st March 2013, accepted 19th April 2013 Abstract This work presents a brief overview of the improvement of the mechanical properties (tensile and flexural strength and the corresponding modulus of elasticity) of natural fiber reinforced polymer materials. The mechanical strength of the natural fiber reinforced polymer composites (NFRPCs) has been compared with that of glass fiber reinforced polymer composites and it is found that for achieving equivalent mechanical strength of the material, the volume fraction of the natural fiber should be much higher than that of the glass fiber. The eco-friendly nature (emission, economy of energy) of the production of components of NFRPCs has also been briefly discussed. It is concluded that NFRPCs have already been proven alternative to SFRPCs in many applications in automotive, transportation, construction and packaging industries, and the production of natural fiber being labor-intensive, the NFRPC industry will create new employment and will contribute to the poverty alleviation program in developing countries. Keywords: Polymer composites, natural fiber, biodegradable composites, mechanical strength improvement, light composites. Introduction Fiber reinforced polymer materials are composites consisting of high strength fibers (reinforcement) embedded in polymeric matrices. Fibers in these materials are the load-carrying elements and provide strength and rigidity, while the polymer matrices maintain the fibers alignment (position and orientation) and protect them against the environment and possible damage. A pure polymer does not usually have requisite mechanical strength for application in various fields. The reinforcement by high strength fibers provides the polymer substantially enhanced mechanical properties and makes the fiber reinforced polymer composites (FRPCs) suitable for a large number of diverse applications ranging from aerospace to sports equipment. The FRPCs are developed primarily using synthetic fibers such as glass, carbon, aramid, Kevlar etc. Synthetic FRPCs have unique advantages over monolithic polymer materials. Besides high strength and high stiffness, these composites have long fatigue life and adaptability to the intended function of the structure. Additional improvements can also be realized in the synthetic FRPCs with regarding corrosion resistance, wear resistance, appearance, temperature-dependent behavior, environmental stability, thermal insulation and conductivity. Specific properties of these materials (such as high-stiffness, high-strength, and low-density as compared to metals) make the material highly desirable in primary and secondary structures of both military and civilian aircraft. These FRPCs are also used in various forms in the transportation industry. Ship structures incorporate composites in various forms. Kevlar or equivalent aramids have many applications in aerospace such as landing gear doors, aircraft cabin and jet engines. Although the SFRPCs possess exclusive mechanical strength, they have got some serious drawbacks such as high cost, high density (as compared to polymers), and poor recycling and non-biodegradable properties. For these reasons, over the last few years natural plant fibers reinforced polymer composites are increasingly gaining attention as viable alternative to SFRPCs2-4. In certain composite applications bio-fibers have shown to be competitive in relation to glass fiber. Natural fiber reinforced composites with thermoplastic matrices have successfully proven their qualities in various fields of application. The growing interest in using natural plant fibers as reinforcement of polymer-based composites is mainly due to their availability from renewable natural resources, satisfactorily high specific strength and modulus6, light weight, low cost and biodegradability. The biodegradability of the natural plant fibers may present a healthy ecosystem while the low costs and good performance of these fibers are able to fulfill the economic interest of industry. But still the mechanical strength of a natural fibers reinforced polymer composite (NFRPC) could not match that of a SFRPC and the natural fibers would not replace synthetic fibers in all applications. For the last decades, extensive research is underway in order to improve the mechanical properties of NFRPC, while Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 2(4), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 47 the intrinsic properties of the natural fibers such as biodegradability and low specific gravity of the fibers remain unchanged. These researches include search of varieties of fibers from plant origin such as jute, Flax10, Coir11 etc. to be introduced in treated or untreated form in the polymer matrix. A number of review papers have also appeared describing different aspect of NFRPCs. Saheb and Jog reviewed the works on NFRPCs with special reference to the type of fibers, matrix polymers, treatment of fibers and fiber-matrix interface. Taj et. al.12 reviewed the properties, processing and application of polymer composites based on different types of natural fibers. Holbery and Houston13reviewed different aspects of the application of NFRPCs in automotive applications. Ticoalu et al.14 discussed the potential of NFRPCs for structural and infrastructural applications. This work aims at i. briefly summarizing the reports in the literature concerning the improvement of the properties of the polymer composites consisting of various types of polymers and natural fibers, and discussing the reproducibility and the long term preservation of the material properties, ii. presenting a short comparative analysis of the mechanical strength of NFRPCs and SFRPCs, and iii. throwing some light on the environmental aspect and the socio-economical impact of the substitution of synthetic fiber by natural ones in polymer composites in the developing countries. Finally, the potential of the agriculture sector of the developing countries in contributing to the NFRPCs production has been briefly described. Researches in search of an alternative to synthetic fiber Since the 1990s, researchers have begun to focus attention on NFRPCs in response to the increasing demand for environmentally friendly materials and the desire to reduce the cost of traditional fibers (i.e., carbon, glass and aramid) reinforced polymer composites. Besides ecological considerations natural fibers exhibit many advantageous properties which promote the replacement of synthetic fibers in polymer composites. They are a low-density material yielding relatively lightweight composites with high specific properties and therefore natural fibers offer a high potential for an outstanding reinforcement in lightweight structures. Natural fibers are derived from a renewable resource and do not have a large energy requirement to process, and are biodegradable15. These fibers also offer significant cost advantages and therefore the utilization of lightweight, lower cost natural fibers such as jute, flax, hemp, sisal, abaca, coir offer the potential to replace a large segment of the synthetic fibers in numerous applications. A number of thermosetting and thermoplastic polymers have been studied as binding materials inNFRPCs. Among the most studied thermosetting materials are epoxy resins and unsaturated polyesters, and among the thermoplastics, poly-olefins such as low and high density polyethylene as a well as polypropylene are the most studied. Different natural fibers have been introduced into the polymer compositions with a view to improve their mechanical properties. The reinforcement data of some NFRPCs have been collected from the literature and has been summarized below. Natural fibers reinforced thermoset composites The mechanical properties of a composite depend on the nature of the resin, fiber, resin-fiber adhesion, cross-linking agents and not the least on the method of the processing. Therefore, any improvement in the property is evaluated as compared to that of the polymer matrix undergone the same processing. The fibers are impregnated by the liquid resin usually at room temperature and then treated with some cross-linking agent for hardening. Usually with an increase in the fiber content in the composition, the tensile and flexural property gradually improves. Beyond certain limit of the fiber content, however, depending on the method of processing, the adhesion between the resin and the fiber decreases resulting in the decrease in the strength of final products. Epoxy resin has excellent adhesion to a large number of materials and could be further strengthened with the addition of fiber. Table-1 represents the improvements of the mechanical properties of some epoxy-based polymer composites. Table-1 Improvements of the mechanical properties of some epoxy based polymer composites Resin Reinforcing fibers Investigated mechanical properties Properties of the base polymer (MPa) Property improvement, % Corresponding Fiber content, % References Epoxy Banana Coconut Tensile strength 23.98 - 90 307.82 - 30 w 16 11 Banana Bagasse (treated) Coconut Flexural strength 53.38 - - 38 23.34 39.40 - 30 w 30 w 16 17 11 Pineapple-leaf (treated) Interfacial shear strength (IFSS) - 152 - 18 Banana Coconut Young’s modulus 1390 - 36 54.89 - 30 w 16 11 Banana Flexural modulus 1563.2 17.37 - 16 * Increment in relation to pure polymer (%), w = Fiber content in weight (%) Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 2(4), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 48 Polyester composites Table-2 represents the improvements of the mechanical properties of some polyester-based polymer composites. Polyester has been studied more intensively than the epoxy resin, and the property enhancement is much higher with polyester.Thermoplastic composites and their mechanical properties: Unlike the liquid thermosetting resins, thermoplastic polymers are in solid state of aggregation and are mixed with the fibers in molten state. Various fibers have been used in thermoplastic polymer-based composites and the effect on the mechanical properties have been studied. The literature data on the improvement has been presented in Table-3 and 4. The thermoplastic polymers (PE and PP) investigated by different authors are with different molecular mass and their properties varies in a large range. Similar is the case with the resins. Different authors use different cross-linking agent and in different proportions. Thus the reference value for calculating the improvement in mechanical properties of the compositions described in the tables-1 through 4 varies from compositions to compositions. Still the trend in the improvement of the mechanical properties with the introduction of fiber in the composition is quite obvious. The extent of the improvement depends on the nature and strength of the resin/polymer and the fiber and also the interaction between them. Table-2 Improvements of the mechanical properties of some polyester-based composites Resin Reinforcing fibers Investigated mechanical properties Properties of the base polymer (MPa) Properties increment, % Corresponding Fiber contentw,v, %References Polyester jute (untreated) pineapple-leaf (untreated) okra (treated) Coir (untreated) Bagasse (treated) Tensile strength 250 22.9 28 - 10.6 900 176 135 30 152 60 v 40 w 27.61 v 25 w 65 w 19 20 21 22 23 Polyester Vinyl Ester pineapple-leaf coir(untreated) bagasse(treated) hemp(treated) Flexural strength 80.2 - - - 13 27 315 7 30 w 25 w 65 w 20 w 20 22 23 24 Polyester Vinyl Ester jute pineapple-leaf okra(treated) hemp(treated) Young’s modulus 4000 580 525 - 775 335 81 10 60 v 40 w 37.5 v 20 w 19 20 21 24 Polyester Vinyl Ester pineapple-leaf hemp (treated) Flexural modulus 1300 - 105 22.5 30 w 20 w 20 24 * Increment in relation to pure polymer (%), w = Fiber content in weight (%), v = Fiber content in volume (%) Table-3 Improvements of the mechanical properties of some polyethylene-based composites Polymer Reinforcing fibers Investigated mechanical properties Properties of the base polymer (MPa) Properties increment, % Corresponding Fiber contentw,v, % References LDPE Sisal (untreated) Sisal Wood(treated) Tensile strength 9.2 9.0 9.8 60 245 67 30 w 21.5 v 40 w 28 25 27 LDPE Sisal (untreated) Sisal (untreated) Wood (treated) Young’s modulus 140 140 350 458 853 272 30 w 21.5 v 40 w 28 25 27 HDPE Hemp Rice hulls Hardwood-A Hardwood-B 1070 555 181 349349 60w 60 w 60 w 60 w 26 LDPE- Low Density Polyethylene, HDPE-High Density Polyethylene * Increment in relation to pure polymer (%), w = Fiber content in weight (%), v = Fiber content in volume (%) Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 2(4), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 49 Table-4 Improvements of the mechanical properties of some PP-based composites PolymerReinforcing fibers Investigated mechanical properties Properties of the base polymer (MPa) Properties increment, % Corresponding Fiber contentw,v, %References PP sisal fiber( treated) bamboo(untreated) Pineapple-leaf(treated) Banana(treated) Abaca Bamboo(treated) Jute(treated) Tensile strength 35 22.5 24.50 24.50 - 24.50 - 27 56 43 34 45 106 120 30 w 50 w 20 v 15 v 30 w 50 v 60 w 28 29 31 32 33 34 9 Rice hulls Kenaf Pineapple-leaf(treated) Banana(treated) Abaca Bamboo(treated) Jute(treated) Flexural strength 19.43 19.43 38 38 - 38 - 130 260 53 48 35 82 100 50 w 50 w 10 v 15 v 30 w 50 v 60 w30 30 31 32 33 34 9 sisal fiber( treated) bamboo(untreated) Rice hulls(treated) Kenaf Pineapple-leaf(treated) Banana(treated) Bamboo(treated) Young’s modulus 498 1.8 869 869 560 560 560 140 160 214 505 32 83 192 30 w 50 w 50 w 50 w 20 v 15 v 15 v 28 29 30 30 31 32 34 Banana(treated) Bamboo(treated) Flexural modulus 1650 1650 125 150 15 v 50 v32 34 * Increment as compared to the properties of the pure polymer, w = Fiber content in weight (%), v = Fiber content in volume (%) Disadvantages of using natural fiber and the remedy Although natural fibers are obtained from renewable sources and the polymer composites based on them are environmentally friendly as compared to the SFRPCs, there are also some disadvantages, which are related to the utilization of unmodified/raw fibers in the preparation of the composites. These disadvantages are as quality variations, high moisture uptake and low thermal stability of the raw fibers35-37. High moisture uptake is the major drawback of the natural fibers. This phenomenon weakens the interfacial bonding between the polymer matrix and fiber and causes deterioration of the mechanical properties. The high moisture sensitivity of some fiber such as lingo-cellulosic fiber causes even the dimensional instability38-39 and limits the use of natural fiber as reinforcement in composite materials. In order to overcome this problem and ultimately to improve the fiber-matrix adhesion, in many cases, a pre-treatment of the fiber surface or the incorporation of surface modifier is required during the composite preparation. Many investigations have been reported in the literature on the influence of various type of chemical treatment on the physical and mechanical properties of NFRPCs32-34, 40, 41. Different treatments of the fibers give significant improvements in tensile and flexural strength, ranging from 10 to 120%9,17whereas for tensile and flexural modulus, the improvements were from 80 up to 214%21, 28, 30. The moisture absorption can be reduced substantially by the chemical treatments of fibers. Various types of chemicals such as, alkali (sodium hydroxide), isocyanate, KMnO(permanganate), CTDIC (cardanol derivative of toluene diisocyanate), peroxide, enzyme etc. have been used for the treatment and a considerable change in the mechanical and physical properties of the composites have been obtained42. Tensile strength of NFRPCs as compared to some SFRPCs The tensile strength of the fibers (synthetic or natural) is much higher than that of the polymer. Therefore, it is expected that Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 2(4), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 50 the fiber reinforced polymer composites will show higher tensile strength than the polymer itself. Such improvement in mechanical strength is really achieved and the strength is usually found to increase with an increase in the fiber contents, although in most cases the strength of the composites does not strictly follow additive rule. The tensile strength of some NFRPCs has been compared with that glass fiber reinforced composites in figure-1. It should be noted that the polymer matrix used by different authors is not exactly the same. Although the matrix bears the general name ‘polypropylene’, due to different molecular mass distribution and preparation condition, it shows different characteristics. But still the data in figure-1 provides information about rough estimation. Thus as the figure-1 shows, the glass fiber yields much higher tensile strength in polypropylene (PP)-based composites than a number of natural fibers do. The same desired level of tensile strength (at some low level of strength) could be achieved for glass fiber reinforced composite as well as for NFRPCs; but the required fiber content for the purpose is much higher in latter composition than that in the first. For example, to achieve a tensile strength of 48 MPa, the required fiber content is 12 wt% for glass fiber, and 15wt%, 35wt%, 38wt%, and 38wt% for pineapple, abaca, kenaf, and aspen fiber respectively. Such approach to achieve high desired tensile strength is limited by the maximum natural fiber content that can be introduced in a given composition. Mueller and Krobjilowski43 on the other hand, claim to have achieved almost identical tensile strength for PP composites with glass and Flax fiber. They showed that to achieve the tensile strength of 28 MPa, the required fiber content is 30wt% for glass fiber and 33wt% for flax fiber. For equivalent strength performance, the specific gravity NFRPCs is lower than that of SFRPCs and hence the NFRPCs will be much lighter than the corresponding SFRPCs. Thus, although the NFRPCs do not still match SFRPCs concerning mechanical strength, as lighter material, they can meet the standard of other applications43, 44. Recently, hybrid composites reinforced by plant-synthetic fibers in a single polymer has been studied and enhanced mechanical strength have been achieved45. Industrial application of NFRPCs at this stage Due to the low density and satisfactorily high specific properties of natural fibers, the composites based on natural fibers have very good implications in the automotive and transportation industry. There are numerous examples of the use of NFRPCs in automotive applications. In the past decade, natural fibers such as hemp, flax, jute, kenaf etc. have been introduced as reinforcement in both thermoplastic and thermoset polymer-based composites and have found extensive applications in transportation (automobile and railway coach interior, boat etc), construction as well as in packaging industries worldwide50-54. Polymer composites reinforced by flax, jute, kenaf, and sisal fibers are used to make the interior and exterior automotive body panels5, 55-57. Figure-1 Tensile strength of composites based on different fiber materials and PP. The data were collected from Somjate and Gunter46, Xue et al.47, Rungsima et al.48, A.K Bledzki et al.10and Rajeev et al49 Abaca fiber reinforced polypropylene composite has got remarkable and outstanding interest in the automobile industries owing to low cost availability, high flexural and tensile strength, good abrasion and acoustic resistance, relatively better resistance to mould and very good resistance to UV rays58. Comparison of the environmental impacts between the NFRPCs and SFRPCsProduction of natural fibers causes less severe environmental impacts as compared to that of synthetic fibers. Natural fibers cultivation depends mainly on solar energy, and for the fiber production, processing and extractions, relatively small amount of fossil fuel energy is required. On the other hand, the production of synthetic fiber depends mainly on fossil fuels and needs nearly ten times more energy as compared to natural fiber. As a result, the pollutant gas emissions to the environment from synthetic fiber production are significantly higher than that from the natural fiber production44, 59. Both the energy-consumption and the emissions for the production of polymers used as matrix in composites are significantly higher than those for the production of natural fibers. For example, PP production requires about 20 times more energy than natural fiber production and correspondingly the emissions are also higher. Therefore the reduction of polymer content at the expense of increase of natural fiber in the composition will improve the environmental performance of NFRCs compared to pure polymer and SFRCs as well60. Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472 Vol. 2(4), 46-53, April (2013) Res. J. Engineering Sci. International Science Congress Association 51 Socio-economic impact of NFRPCs in developing countries The production of synthetic fiber is capital-intensive, and that of the natural fiber is labor-intensive. In most part of the world, the vegetable and fruit plants are being harvested manually and the natural fiber extraction machine can easily be operated by unskilled labor. Therefore, the urban people as well as rural uneducated people can be appointed for the production and extraction of natural fiber. Sometimes some natural fibers (e.g. jute, coconut) can be extracted manually. Thus the production and extraction of natural fibers involve huge human resources. A ‘natural fiber production’ project is quite appropriate for developing and moderately developed countries as it could create a lot of employment opportunities in urban and rural sectors. Potential of South-East Asia for contribution in NFRPCs production The most common natural fibers which exhibit moderately high mechanical strength and are used for the reinforcement of polymer composites are jute, rice husk26, bamboo34, coconut22, banana32, flax, hemp49, pineapple48, sisal41 and wheat husk61. Most of these products are cultivated extensively in this region, but most of their wastes are abundant and do not have any useful utilization. If the entrepreneurs take initiatives for the collection of these agriculture wastes for NFRPCs, the new application of natural fibers may present an economic interest for the whole agricultural sector. A lot of employment opportunities in urban as well as in rural sectors would be created by this natural fiber project and the poverty alleviation program will be enhanced. Conclusion NFRPCs have already been proven alternative to SFRPCs in many applications in automotive, transportation, construction and packaging industries. Ongoing researches find varieties of natural fibers, which improve the mechanical strength of polymer composites. Natural fibers result in lighter composite materials as compared to SFRPCs with equivalent mechanical strength. Natural fibers are biodegradable and their productions are associated with lower emission than that in the production of synthetic fiber. Also high natural fiber contents in composites at the expense of polymer itself results in the economy of energy in wide aspect, since the production of polymer is more energy-consuming than that of the natural fiber. Production of natural fiber is labor-intensive and hence NFRPC industry will create new employment and will contribute to the poverty alleviation program in a number of developing countries.AcknowledgementThe financial support from the Ministry of Science and Technology, Bangladesh (Grant No. EAS-7/582/1(5)/2012) for conducting this research is highly appreciated. The author Khurshida Begum is thankful to the Ministry of Science and Technology, Bangladesh for providing her with NST fellowship (Grant No. 363(105)/2011) for this research purpose. References 1.Isaac M. and Ishai D.O. Engineering Mechanics of Composite Materials, (Oxford, New York), 3-13 (2006) 2.Coutinho F.M.B. and Costa T.H.S., Performance of polypropylene-wood fiber composites, Polym. 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