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Keywords:

  • biocomposites;
  • biopolymer;
  • natural fiber;
  • thermoplastic;
  • thermosets

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

This century has witnessed remarkable achievements in green technology in material science through the development of natural fiber reinforced composites. The development of high-performance engineering products made from natural resources is increasing worldwide day by day. There is increasing interest in materials demonstrating efficient use of renewable resources. Nowadays, more than ever, companies are faced with opportunities and choices in material innovations. Due to the challenges of petroleum-based products and the need to find renewable solutions, more and more companies are looking at natural fiber composite materials. The primary driving forces for new bio-composite materials are the cost of natural fibers (currently priced at one-third of the cost of glass fiber or less), weight reduction (these fibers are half the weight of glass fiber), recycling (natural fiber composites are easier to recycle) and the desire for green products. This Review provides an overview of natural fiber reinfocred composites focusing on natural fiber types and sources, processing methods, modification of fibers, matrices (petrochemical and renewable), and their mechanical performance. It also focuses on future research, recent developments and applications and concludes with key issues that need to be resolved. This article critically summarizes the essential findings of the mostly readily utilized reinforced natural fibers in polymeric composite materials and their performance from 2000 to 2013.mame201300008-gra-0001

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

Due to the increasing environmental awareness, natural fiber composites are becoming more prevalent in use. In addition, the materials' relatively low cost and low density, acceptable specific properties, ease of separation, enhanced energy recovery, CO2 neutrality, biodegradability, and recyclable properties, have focused recent attention on natural fiber use in composites. The materials which are durable, reliable, lightweight, and with excellent mechanical properties, that are significantly better than those of traditional materials are fueling the growing demand for natural fiber in various industries such as automotive, building, and construction.

Recent study forecasts that in 2010, total global natural fiber composite materials market shipments topped 430.7 million pounds with a value of US$ 289.3 million and the market is expected to grow to US$ 531.3 million in 2016 with an 11% Compound Annual Growth Rate (CAGR) over the next 5 years.[1] The report also mentioned that increasing use of natural fiber composites in automotive applications is driving the market. Indeed, automotive is expected to remain the largest market through 2016. Several automotive components are now produced using natural composites, which are generally based on polypropylene (PP) resin and fibers such as flax, hemp, kenaf, or sisal. The automobile models, first in Europe and then in North America, featured natural fiber reinforced thermosets and thermoplastics in door panels, package trays, seat backs, and trunk liners. The application of natural fiber composites has increased and is gaining preference over glass fiber and carbon fiber. Because of natural fiber composites excel in most parameters except strength (the strength of glass and carbon fiber is higher compared to natural fiber) (Figure 1). The automotive industry's adoption of natural fiber composites is led by price, weight reduction, recycling, and marketing incentives rather than technical demands. The range of products is no longer restricted to interior and non-structural components such as door panels or rear shelves. The report summarized that the major drivers of the natural fibers are raw material source (natural fiber composites made with easily available renewable sources), properties (lighter weight, low energy consumption, and low cost product), volatility in oil prices (impacts substitute materials markets and natural fiber costs less), and environmental advantages with government supports.

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Figure 1. Comparison between natural fiber, glass fiber, and carbon fiber.

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Another trend is also remarkable that the concept of using biobased plastics as reinforced matrices for natural fiber composites is gaining more and more approval day by day. The developments in emerging biobased plastics are spectacular from a technological point of view and mirror their rapid growth in the market place. The average annual growth rate globally was 38% from 2003 to 2007. In the same period, the annual growth rate was as high as 48% in Europe. The worldwide capacity of biobased plastics is expected to increase from 0.36 million metric tonnes (2007) to 2.33 million metric tonnes by 2013 and to 3.45 million metric tonnes in 2020. The main product in terms of production volumes will be starch-based plastics, poly(lactic acid) (PLA) and polyhydroxyalkanoate (PHA).[2]

The growing importance of natural fiber composites is reflected by the increasing number of publications during the recent years including reviews, patents, and books. Satyanarayana et al.[3] represents the number of publications and patents on biodegradable lignocellulosic fiber based composites (1995–2007) found in the ISI database by entering the words “biodegradable, polymer, fiber” and it is seen that after year 2000, the number of publications and patents rose significantly (Figure 2).

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Figure 2. Overview of publications and patents on biodegradable lignocellulosic fiber-based composites from 1995–2007.

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This paper is mainly the summary of recently published comprehensive review paper,[4] which reviewed 525 papers on natural fibers in polymer composites from 2000 to 2010 and additionally adds very recent results from 2010 to till to date. This paper does not include natural fibers from animals (e.g., silk or wool) or cotton or man-made cellulosic fibers and also excludes wood fiber.

2 Reinforcing Fibers

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

The volatility in petroleum oil prices and their resources led to an increased awareness and our inevitable dependence on renewable resources has arisen. This century could be called the cellulosic century, because more and more renewable plant resources for products are being discovered. It has been generally stated that natural fibers are renewable and sustainable, but they are in fact, neither. The living plants are renewable and sustainable from which the natural fibers are taken, but not the fibers themselves.

There are thousands of different fibers in the world and in fact only few of these fibers have been studied. Most research has been carried out to study the potential use of natural fibers for technical applications and this paper has only covered the most widely studied and used natural fibers in reinforced composites materials. Among the most popular natural fibers; flax, jute, hemp, sisal, ramie, and kenaf fibers were extensively researched and employed in different applications. But nowadays, abaca, pineapple leaf, coir, oil plam, bagasse, bamboo, wheat straw, curaua, and rice husk fibers are gaining interest and importance in both research and applications due to their specific properties and availability.

The properties of natural fibers differ among cited works, because different fibers were used, different moisture conditions were present, and different testing methods were employed. The natural fiber reinforced polymer composites performance depends on several factors, including fibers chemical composition, crystalline cell dimensions, microfibrillar angle, defects, structure, physical properties, and mechanical properties, and also the interaction of a fiber with the polymer.

The hydrophilic nature of fibers is a major problem for all natural fibers if used as reinforcement in plastics. The range of the characteristic values, as one of the drawbacks for all natural products, is remarkably higher than those of glass-fibers, which can be explained by differences in the fiber structure due to the overall environment conditions during growth. The physical properties of each natural fiber are critical, and include the fiber dimensions, defects, strength, and structure. There are several physical properties that are important to know about for each natural fiber before that fiber can be used to reach its highest potential. A high aspect ratio (length/width) is very important in cellulose-based fiber composites as it gives an indication of possible strength properties. The fiber strength can be an important factor in selecting a specific natural fiber for a specific application. Fiber dimensions, defects, strength, variability, crystallinity, and structure must be taken into consideration.

Dittenber and GangaRao[5] compiled a cost per weight comparison between natural and glass fibers from several published papers shown in Figure 3. The variation of price for natural and glass fibers are depend on sources of geographic area.

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Figure 3. Cost per weight comparison between glass and natural fibers.

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It is also studied the comparison between the specific modulus of glass and natural fibers, a favorable comparison for several types of natural fibers, and additionally the wide range each type of fiber may have for the specific modulus obtained from published stiffness and density values (Figure 4). It is mentioned that most used natural fibers potential specific modulus are higher than glass fibers.

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Figure 4. Comparison of potential specific modulus values and ranges between natural fibers and glass fibers.

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There are many positive reasons to use natural fibers as reinforcing component in polymers, at the same time there are also several disadvantages, when preparing composite materials with these fibers. Aragones[6] summarized the pros and cons of selection of natural fibers for composite materials (Table 1).

Table 1. Pros and cons of selection of natural fibers for composite material
 ProsCons
Physical-mechanicalLow density, thus low weightFibers absorb moisture that causes swelling
Higher specific strength and stiffness than glassLower strength properties than glass fiber composites, particularly impact strength
Good thermal and acoustic insulating propertiesOdor generation due to degradation process
ProcessingNon-abrasive effect aver screws and other metaLtic partsThe maximum processing temperatures are limited, especially in relation to glass fiber
Nan-harmful processing, no tool wear and no skin irritationSome fibers need to be pelletized in order to increase the apparent density
EnvironmentalIt is a renewable resource, and is therefore an inexhaustible supplyRelatively low durability, due to fungus attack, weathering, etc.
Production energy is only 1/3 of that for glass fibersRelatively large price fluctuations due to harvest results or agricultural politics
The amount of CO2 that the plants absorb during their growth is the same as that given off when they are decomposedVariable quality, depending on unpredictable influences such as weather

The variation of properties of natural fiber relates to the lack of consistency of fiber qualities, which related to the location and time of harvest, processing conditions, as well as their sensitivity to temperature, moisture, and UV radiation. In addition, there are several factors that can influence fiber quality in each stage, and there are several different stages of production, as shown in Table 2.[5] The fiber quality is affected by the plant species, the crop production, the location, and the climate at the plant growth stage.

Table 2. Factors effecting fiber quality at various stages of natural fiber production
StageFactors effecting fiber quality
Plant growthSpecies of plant
Crop cultivation
Crop location
Fiber location in plant
Local climate
Harvesting stageFiber ripeness, which effects:
-Cell wall thickness
-Coarseness of fibers
-Adherence between fibers
and surrounding structure
Fiber extraction stageDecortication process
Type of retting method
Supply stageTransportation conditions
Storage conditions
Age of fiber

Baillie et al.[7] illustrated the properties of natural fibers (strength, thermal degradation, biological degradation, moisture absorption, and UV degradation) to their dependence on chemical constituents of the natural fibers (Figure 5). In all plant-based fibers, the basic chemical structure of cellulose is similar but they have different degrees of polymerization. The cell geometry of each type of celluloses varies with the fibers and these factors contribute to the diverse properties of the natural fiber.

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Figure 5. Properties of cellulose fibers and their dependence on chemical constituents.

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3 Modification of Natural Fibers

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

The main disadvantages of natural fibers in respective composites are the poor compatibility between fiber and matrix and their relatively high moisture absorption. Therefore, natural fiber modifications are considered leading to a change of the fiber surface properties to improve their adhesion with different matrices. An exemplary strength and stiffness could be achieved with a strong interface that is very brittle in nature with easy crack propagation through the matrix and fiber. The efficiency of stress transfer from the matrix to the fiber could be reduced with a weaker interface.

Extensive research was carried out and reported in the literature, showing the importance of the interface and the influence of various types of surface modifications on the physical and mechanical properties of natural fiber reinforced composites. The observed trend indicate a preference for the chemical modification (alkaline, silane, acetylation, benzoylation, acrylation and acrylonitrile grafting, maleated coupling, permanganate, peroxide, and isocyanate treatment) compared to physical modification (corona and plasma treatment). It has also been shown that maleated and silane treatment is becoming a choice method due to beneficial results.[8-10] Additive suppliers improved the additives with higher amounts of anhydride functional groups than previous grades (used in 1980s and 1990s), which create more sites for chemical links, resulting in significant performance improvement at low additive contents. Using coupling agents reduced the water absorption of the composites but has not resulted in decreased long-term performance.

The use of enzyme technology is becoming increasingly substantial for the processing of natural fibers. Currently, the use of enzymes in the field of textile and natural fiber modification is also rapidly increasing.[11] A major reason for embracing this technology is the fact that the application of enzymes is environmentally friendly. The reactions catalyzed are very specific and have a focused performance. In addition, enzyme technology could be cost effective, improved product quality compared to mostly use maleated and silane modification.

4 Matrices for Natural Fiber Composites

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

The composites' shape, surface appearance, environmental tolerance, and overall durability are dominated by the matrix while the fibrous reinforcement carries most of the structural loads, thus providing macroscopic stiffness and strength. The polymer market is dominated by commodity plastics with 80% consuming materials based on non-renewable petroleum resources. Governments, companies, and scientists are driven to find an alternative matrix to the conventional petroleum-based matrix through public awareness of the environment, climate change, and limited fossil fuel resources. Therefore biobased plastics, which consist of renewable resources, have experienced a renaissance in the past decades.

4.1 Petrochemical Based

The effects of the incorporation of natural fibers in petrochemical-based thermoplastics and thermoset matrixes were extensively studied. PP, polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC) were used for the thermoplastic matrixes. Polyester, epoxy resin, phenol formaldehyde, and vinyl esters were used for the thermoset matrices and are reportedly the most widely used matrices for natural fiber reinforced polymer composites.

4.2 Biobased

Many new polymers were developed from renewable resources, such as starch, which is a naturally occurring polymer that was re-discovered as a plastic material. Others are PLA that can be produced via lactic acid from fermentable sugar and PHAs, which can be produced from vegetable oils next to other biobased feed stocks and these are the mostly used biobased polymers in natural fiber reinforced composites. Other biobased polymers such as soy-based biodegradable resin (polyol derived from soybean oil), polycaprolactone (PCL) and polybutylene succinate (PBS) is also studied with natural fibers.

Though biobased polymers experiencing positive acceptance in composites area, but biobased polymers have their own advantages as well as drawbacks. The pro and contras of using biobased polymers are summarized below.

4.2.1 Advantages
  1. Sustainable bioplastics will help to reduce our dependency on oil
  2. Sustainable bioplastics are made from renewable sources
  3. Many of the bioplastics are biodegradable
  4. Bioplastics are used and promoted for recycled products
  5. Product costs are getting more competitive due to the increased oil prices
  6. Continued research will lead to improved production techniques and products that are more environment friendly.
4.2.2 Disadvantages
  1. There is a limited shelf life for some of the sustainable bioplastics
  2. Some bioplastics have a lesser performance factor when compared to petroleum-based plastics
  3. The manufacturing process of bioplastics still relies on petroleum based energy
  4. Studies have shown that various bioplastics manufacturing varies in its environmental impact
  5. There are concerns that sustainable bioplastics will upset existing recycling methods
  6. Since the production of bioplastics as market is fairly new, the manufacturing price tag is not as cost effective as the fossil fuel plastic production; this is based on existing price of oil.

The effects of reinforcing polylactide (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biopolymers and petrochemical PP on the mechanical performance of man-made cellulose, abaca and jute fiber composites were studied.[12] Figure 6 illustrates that biobased polymer composites exhibited higher tensile strength and modulus compared to petrochemical-based composites. It is also seen that PLA based man-made cellulose, abaca and jute fiber composites showed higher properties in comparison with PHBV and PP-based composites.

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Figure 6. a) Tensile strength and b) tensile modulus of PLA, PHBV, and PP composites with man-made cellulose, abaca, and jute fibers.

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Bledzki et al.[13] investigated the tensile properties of mostly used petroleum based polymer PP and biobased polymer PLA and PA 6.10 with natural fiber abaca (Figure 7). The biobased polymer composites clearly show better tensile strength and modulus than the petroleum-based PP abaca composites. In comparison between biobased PLA and PA 6.10, it is observed that PLA exhibits higher strength and modulus compared to PA-abaca composites.

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Figure 7. Tensile properties of abaca reinforced PP, PLA, and PA 6.10 composites.

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Kim et al.[14] performed the total volatile organic compound (TVOC) emissions from natural fiber (pineapple flour and cassava flour) reinforced biopolymer (PLA and PBS) composites and compared to mostly used synthetic polymer PE and PP (Figure 8). It is observed that natural fiber reinforced PLA and PBS composites exhibit significantly lower odor emissions, which is favorable for automotive interior components.

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Figure 8. Emission factor for the PLA and PBS-natural fiber composites, LDPE and PP (PLA-P: PLA with pineapple flour, PLA-C: PLA with cassava flour, similar for PBS).

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Ganster and Fink reviewed[15] the PLA for the production of bio- and nanocomposites. Attempts to improve PLA properties by reinforcing with lignocellulosic fibers (both wood and natural fibers) and man-made cellulosic fibers (rayon) are outlined. The mechanical properties of composites obtained from melt compounding are mainly considered, because this is the most industrially relevant properties.

Till to date, the petroleum derived thermoplastics PP and PE are the two most commonly employed thermoplastics in natural fiber reinforced composites. Day by day, there is great interest in developing natural fiber composites with a thermoplastic rather than thermoset matrix, mainly due to their recyclability. Also the choice of a thermoplastic matrix fits well within the eco-theme of biocomposites, but there are some important limitations on the recyclability and mechanical performance of thermoplastics. Generally, the mechanical properties of thermosets are higher than the thermoplastics (lower modulus and strength). In addition, a dramatic loss in properties is observed above the glass transition temperature, which leads to decrease in other thermally sensitive properties such as creep resistance. On the contrary, thermoplastics show greater fracture toughness than thermosets and thus are more useful in resisting impact loads. Another remarkable change was the introduction of biopolymers in recent years with the aim of decreasing reliance on petroleum-based thermoplastics. The availability and outstanding mechanical properties of biopolymer PLA has led to this matrix system being one of the most thoroughly investigated in the biocomposites research area.

5 Processing Techniques

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

Generally, natural fiber reinforced plastic composites are manufactured by using traditional manufacturing techniques (designed for conventional fiber reinforced polymer composites and thermoplastics). The processing techniques include compounding, mixing, extrusion, injection molding, compression molding, LFT-D are suitable and investigated for natural fiber reinforced thermoplastic composites. On the other side, resin transfer molding (RTM) and SMC are implemented with thermosets matrix. Besides these processes, nowadays thermosets compression molding and pultrusion are investigated with natural fiber composites. Till to date, above mentioned techniques have been well developed and accumulated experience has proofed their successability for producing composites with controllable quality. Innovative technologies and process solutions should be intensively researched to get the high strength engineering composites required by new applications area.

There are some factors; such as fiber type, fiber content, fiber orientation, moisture content of fiber, which influence significantly the processing of natural fiber composites as well as the properties of final product. Therefore to select a suitable process to fabricate natural fiber composites, design, and manufacturing engineers would mainly focus on numbers of criteria including desired properties, size, and shape of resultant composites, processing characteristics of raw materials (both fibers and polymers: biobased or petroleum based), the production speed and the manufacturing cost. In addition, based on processing techniques, semi-finished product manufacturing; mat production, slivers, fiber yarns, fiber preparation (opening, mixing, and carding), and granule production are the important steps, which should be taken in account to production of natural fiber composites.

The influence of compounding processes (mixer-injection molding, mixer-compression molding, and direct compression molding) on the mechanical properties of the abaca fiber reinforced PP composites was investigated.[16] It represents that mixer-injection molding processes lead to higher tensile and flexural strength values compared to the other processes. It is also notable that direct compression process exhibits higher strengths compared to mixer-compression process. It seems that due to the agglomeration, the fiber breaks into lower length what plays a role in the compression molding process. It is also observed that after agglomeration (mixing) and injection molding process, composites showed very significant higher odor values compared to other processes. Compression molding process showed relatively lower odor concentrations, which is favorable for the automotive sector. It seems that injection molding process decomposes the composite materials more than compression molding process what results in higher odor concentration.

To date, injection molding, extrusion, compression molding, sheet molding, and RTM are the major manufacturing processes for natural fiber reinforced plastic composites. But new downstream and auxiliary equipment has been designed. Such as: unique heating and single or dual venting systems for in-line drying, high-intensity spray-cooling tanks, a variety of new configurations of feeding (gravimetric or vertical crammer) systems, combinations of extrusion-injection molding or extrusion-compression molding as well as screw, die, and mold design. Although the majority of natural fiber composites are produced today by the processes mentioned above, the manufacturers are improving the feasibility of using other processes like pultrusion and so on.

Based on final product, suitable manufacturing processes must be utilized to transform the materials to the final shape without causing any defect of products. The product size is a dominating factor for the preliminary assessment on a suitable type of manufacturing processes to be used. From the literature review, it is observed that for small to medium sized components, injection, and compression moldings are preferred due to their simplicity and fast processing cycle. In the case of large structures, they are typically manufactured by open molding.

6 Performance of Natural Fiber Composites

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

Tensile, flexural, and impact properties are the most commonly investigated mechanical properties of natural fiber reinforced plastic composites. Impact strength is one of the undesirable weak points of these materials in terms of mechanical performance. Besides these tensile, flexural, and impact properties, the long-term performance (creep behavior), dynamic mechanical behavior, compressive properties are also investigated for natural fiber composites.

Ramesh et al.[17] has developed sisal-jute-glass fiber reinforced polyester composites and compared their mechanical properties with sisal-glass fiber and jute-glass fiber reinforced polyester composites. It is seen that the incorporation of sisal and jute fiber with glass fiber can improve the properties and used as an alternate material for glass fiber reinforced polymer composites. Figure 9 illustrates the load versus the displacement graph for above-mentioned composites. It is observed that the displacement increases with the increase of load and above 14.2 mm displacement, there is a breaking exists. Based on the interfacial properties, internal cracks, and internal structure of the composites, it is indicated that sisal-jute-glass fiber reinforced composites showed better performance than the other type of composites tested.

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Figure 9. Comparison of load vs. displacement of sisal-glass fiber, jute-glass fiber, and sisal-jute-glass fiber reinforced composites.

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Kenaf and lyocell (man-made cellulose) fiber reinforced PLA and poly(3-hydroxybutyrate) (PHB) composites were produced by compression molding process.[18] The use of different matrices leads to variable composite characteristics and provides a comparison of the mechanical characteristics of compression-molded 30% lyocell and 40% kenaf fiber reinforced PLA and PHB. The tensile and flexural test showed that 30% lyocell-PLA composites reached the highest tensile and flexural strength with 89 and 148 N · mm−2, respectively. The 30% lyocell-PLA composites also exhibited the highest tensile modulus with 9.3 GPa. The highest flexural modulus was measured for 40% kenaf-PHB composites with 7.1 GPa (Figure 10). The best impact strength was observed for lyocell-PHB composites with 70 kJ · m−2, whereas lyocell-PLA composites exhibited with 52 kJ · m−2. The investigation of the Shore D hardness resulted in a higher value for the PLA matrix with 81.5 and PHB achieved a hardness of 67.5. Due to the addition of fiber, the Shore D hardness increased up to 83.6 for lyocell-PLA composites and 73.1 for kenaf-PHB composites (Figure 11). Density measurements showed lower densities for the composites with higher fiber loads (kenaf-PLA and kenaf-PHB) in comparison to the theoretical density, which relates to a higher proportion of air inclusion in the composites. Thus results a negative effect to the mechanical composite characteristics.

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Figure 10. Flexural modulus of the PLA and PHB composites with lyocell and kenaf fibers (Box-whisker diagram with confidence intervals, different letters mean significant differences between the test samples, * means normal distributed).

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Figure 11. Shore D hardness of pure PLA and PHB, and their composites (Box-whisker diagram with confidence intervals).

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Zampaloni et al.[19] found that compression molded kenaf-PP composites have superior tensile and flexural strength when compared to other compression molded natural fiber composites such as other kenaf, sisal, and coir reinforced thermoplastics. With the aid of the elastic modulus data, it was also possible to compare the economic benefits of using kenaf composites instead of other natural fibers and E-glass. The manufactured kenaf maleated PP composites have a higher modulus/cost than sisal, coir, and even E-glass (Figure 12). Thus, they provide an option for replacing existing materials with a higher strength, lower cost alternative that is environmentally friendly.

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Figure 12. Comparison of modulus/cost for various natural fibers and glass fibers.

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Hybrid of kenaf/glass fiber reinforced epoxy composites was manufactured to enhance the desired mechanical properties for car bumper beams as automotive structural components.[20] Hybrid composites, which is fabricated by modified SMC method is tested and compared with a typical car bumper beam material (GMT-glass mat thermoplastic). Tensile strength and modulus of hybrid composites exhibited higher values than typical car bumper beam (Figure 13), which led to the potential utilization of hybrid natural fiber in some car structural components such as bumper beams.

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Figure 13. Tensile properties of hybrid (kenaf/glass) composites and GMT composites.

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Alves et al.[21] demonstrated the use of natural fiber composites, which is produced in developing countries, have presented several social, environmental, and economical advantages to design green automotive components. A structural frontal bonnet of an off road vehicle were manufactured with jute fibers to a replacement of glass fibers and through LCA method demonstrates the possibility to use natural fibers considering social, environmental, economical, and technical advantages (Figure 14). It is clearly observed that jute composites related to the four aspects, present the better overview performance than glass composites except technical aspects.

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Figure 14. Comparison between jute and glass composites based bonnet's aspects.

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Sobczak et al.[22] analyzed the mechanical performance profiles of various natural fiber reinforced PP composites considering them for a variety of potential applications, which compete with mineral reinforced (talc), short glass fiber (sgf), long glass fiber (lgf), and short carbon fiber (scf) reinforced PP property profiles of the latter materials are also included in the analysis. Figure 15 illustrated various natural fiber composites tensile strength and modulus comparing with talc, glass fiber, carbon fiber, and even with wood fiber reinforced composites. Natural fiber composites outperform PP-talc composites. Glass fiber composites (both sgf and lgf) clearly showed higher performance than natural fiber composite. Unusually, short carbon fiber composites for both in scientific investigations and for commercial products fall significantly, which might be short of rule of mixture.

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Figure 15. Ashby plot presenting the tensile strength vs. the Young's modulus of various PP compounds.

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To improve their performance to the desired level, still much work is to be done considering fiber processing, non-linear behavior, fiber-matrix adhesion, fiber dispersion, composite manufacturing with optimized processing parameters.

7 Developments and Applications

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

There are several major areas of interest behind the development of natural fiber composites, which provide the potential economic impacts, environmental impacts, and the ability of natural fiber composite to meet social, economic, and material needs all over the world. The advanced natural fiber reinforced polymer composite contributes to enhancing the development of biocomposites in regards of performance and sustainability. There are numerous research is going on all over the world, which reflects in recent enormous review papers focusing on overall performance of natural fiber composites,[23-27] influence of natural fibers on biodegradable polymers and their biodegradation,[28-31] chemical treatments of natural fibers and performance of chemically treated natural fiber composites,[32-35] mechanical and physical properties of natural fiber composites,[36] tensile properties,[37] tribology properties,[38] flame retardancy,[39] mechanical behavior of natural fiber woven composites,[40] natural fiber hybrid composites,[41] aspects of fatigue analysis of natural fiber composites,[42] and applications in automotive sector.[43] There are review papers also published on individual natural fibers and their suitability in composite materials, such as abaca/banana fiber,[44] oil palm fiber,[45] oil palm empty fruit bunch,[46] kenaf fiber,[47] hemp fiber,[48] bamboo fiber,[49] coir fiber,[50] bagasse fiber,[51] and cellulosic nanofibers.[52-54] Summerscales et al. has published a series of review papers on bast fibers and their composites with illustrating the natural fibers as reinforcements,[55] influence and their performance in composite materials,[56] and statistical models which already have been applied to natural fiber reinforcements for composite systems.[57]

Last 13 years natural fiber composites have created substantial commercial markets for value-added products especially in automotive sector. The automotive components with natural fiber reinforced composites can be expected to increase steadily with increased model penetration. All major vehicle manufacturers around the world now use natural fiber composites in various applications such as those listed in Table 3.[58] However, in order to be able to expand into other markets, such as commercial construction and consumer goods, composites need to achieve high-quality performance, serviceability, durability, and reliability standards.

Table 3. Automotive models, manufacturers, and components using natural fiber composites
ModelManufacturerComponents
A2, A3, A4, A4 Avant, A6, A8, Roadstar, CoupeAudiSeat back, side and back door panel, boot lining, hat rack, spare tire lining
C5CitroenInterior door paneling
3, 5, 7 seriesBMWDoor panels, headliner panel, boot-lining, seat back, noise insulation panels, molded foot well linings
Eco EliseLotusBody panels, spoiler, seats, interior carpets
Punto, Brava, Marea, Alfa Romeo 146, 156FiatDoor panel
Astra, Vectra, ZafiraOpelInstrumental panel, headliner panel, door panels, pillar cover panel
406PeugeotFront and rear door panels
2000 and othersRoverInsulation, rear storage shelf/panel
Raum, Brevis, Harrier, CelsiorToyotaDoor panels, seat backs, floor mats, spare tire cover
Golf A4, Passat Variant, BoraVolkswagenDoor panel, seat back, boot-lid finish panel, boot-liner
Space star, ColtMitsubishiCargo area floor, door panels, instrumental panels
Clio, TwingoRenaultRear parcel shelf
Mercedes A, C, E, S class, Trucks, EvoBus (exterior)Daimler-BenzDoor panels, windshield/dashboard, business table, piller cover panel, glove box, instrumental panel support, insulation, molding rod/apertures, seat backrest panel, trunk panel, seat surface/backrest, internal engine cover, engine insulation, sun visor, bumper, wheel box, roof cover
PilotHondaCargo area
C70, V70VolvoSeat padding, natural foams, cargo floor tray
Cadillac Deville, Chevrolet TrailBlazerGeneral MotorsSeat backs, cargo area floor
L3000SaturnPackage trays and door panel
Mondeo CD 162, Focus, freestarFordFloor trays, door panels, B-piller, boot liner

Besides automotive sector, recent research is also focusing on natural fiber composites in construction sector. Building materials based on renewable resources like natural fibers and their reinforcement in cement based materials are evaluated. Reinforcement suitability of jute fiber,[59-61] coir fiber,[62-64] corn stalk,[65] flax fiber,[66] and hemp fiber[67] were investigated in fiber cement composites. Pacheco-Torgal and Jalali reviewed the natural fiber reinforcement in cementitious building materials focusing on fiber characteristics, properties and the description of the treatments that improve their performance, compatibility between the fibers and the cement matrix, influence of natural fiber on cement properties, and the properties and durability performance of cementitious materials reinforced with natural fibers.[68]

Kalyankar and Uddin[69-71] studied the manufacturing and structural feasibility of natural fiber-reinforced polymeric structural insulated panels for panelized construction, mainly focuses on the manufacturing feasibility and structural characterization of natural fiber reinforced structural insulated panels (NSIPs) using jute fiber reinforced polypropylene (NFRP) laminates as skin. The natural fibers were bleached before their use as reinforcement.

Another major advancement lies within the establishment of nanotechnology (i.e., reinforcing as well as producing nanocrystalline cellulose from natural fibers). Natural fibers consist of approximately 30–40% cellulose and about half of that is crystalline cellulose. The nanocrystalline cellulose may be only one-tenth as strong as carbon nanotubes but it costs 50–1 000 times less to produce.

Zainuddin et al.[72] treated kenaf fibers with NaOH, bleached with sodium chlorite and acetic buffer solution, and subsequently acid hydrolyzed to obtain cellulose nanocrystals (CNCs). CNCs obtained after acid hydrolysis were characterized by TEM images (Figure 16). It is seen that CNCs are in needle-like shapes and some were still agglomerated. It is found that the diameter of individual CNCs was 12 ± 3.4 nm and the length was between 70 and 190 nm with an aspect ratio (L/D) of 13.2.

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Figure 16. TEM micrographs of CNCs from kenaf under different magnifications: a) 13 000× and b) 35 000×.

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The above-mentioned CNCs from kenaf fiber were then mixed with cassava starch to develop biocomposites using a solution casting method. Figure 17 represents the tensile modulus values as a function of reinforced fiber content. It is observed a clear tendency of increasing stiffness for all of the composites with increasing fiber content. Generally, the significant increase was attributed to the similarity between the chemical structures of cellulose and starch. An increasing trend in the elastic modulus of the films was observed with the addition of nanocellulose regardless nanocellulose content. CNCs reinforced composites showed the highest stiffness compared to other treated kenaf fiber reinforced composites and it was 326.1 MPa at 6 wt.% CNCs content. It is also notable that aggregates of CNCs in the matrix contributed to this phenomenon and hence restricted fiber content reinforcement.

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Figure 17. Effects of fiber treatments on tensile modulus of kenaf fiber reinforced starch composites.

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Eichhorn et al.[53] provided a comprehensive overview of recent progress made in the area of cellulose nanofiber and their composites. The isolation methods of cellulosic nanofibers (nanowhiskers, nanofibrils) with details of their structure are given. The processing and characterization of cellulose nanocomposites and new developments in the area, with particular emphasis on applications were described. The review paper highlighted the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibril-based composites. It also covered the applications and new advances of cellulose nanofibers to reinforce adhesives, to make optically transparent paper for electronic displays, to create DNA-hybrid materials, to generate hierarchical composites and for use in foams, aerogels, and starch nanocomposites and the use of all-cellulose nanocomposites for enhanced coupling between matrix and fiber.

Sustainability assessment methodology, such as Life Cycle Analysis (LCA) has been developed to evaluate the environmental effect of parameters such as raw materials, energy consumption, and ultimate disposal or recycling characteristics. Several researchers have attempted to quantify the environmental improvements natural fibers offer over synthetic fibers. Akil et al.[47] represented the life cycle of all biocomposites (Figure 18) and for further assessment of natural fiber composites, LCA should be done for all products.

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Figure 18. Life cycle of natural fiber composites.

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Natural fiber composite's LCA was performed to find environmental impacts related to the composite materials and their boundary conditions are evaluated (Figure 19). A frontal bonnet of vehicle were manufactured with jute fiber reinforced composites and its boundary conditions is the entire life cycle of the bonnets made of composite materials and their influence for whole vehicle, from the extraction of raw materials, over production processes, and the use phase to the end of life of the vehicle. The boundaries of LCA include all of needed transports as well as the infrastructure to apply the treatments at jute fibers and to produce the bonnets even to dispose them.[21]

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Figure 19. Boundary conditions of the LCA of a frontal bonnet (manufactured with jute fiber reinforced composites).

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Le Duigou et al.[73] investigated the environmental impact and simplified LCA of flax fiber reinforced PLA composites and compared their properties to conventional glass fiber reinforced polyester composites. Their analysis was based on four steps: Goal and Scope definition, Life cycle Inventory (LCI), Life Cycle Impact Assessment (LCIA), and Interpretation. The evaluation of greenhouse gas emissions in kg of CO2 equivalent are lower for the biocomposite as carbon dioxide is trapped during photosynthesis (for both flax fibers and the maize used to produce PLLA). The environmental indicators were calculated with the CML 2000 method and it is illustrated that the biocomposites appear to be an attractive alternative to conventional glass/polyester composites. The natural fiber reinforced composites reduced the majority of the environmental impact indicators, such as global warming (−70%), photochemical oxidation (−60%), and human toxicity (−80%). On the contrary, natural fiber composites result in a transfer of pollution from global to local impacts [eutrophication (+50%) and fresh water toxicity (+20%)].

The application of life cycle assessment (LCA) methodology in order to explore the possibility of improving the eco efficiency of glass fiber composite materials by replacing part of the glass fibers with hemp mats were investigated.[74] Besides the exploration of eco-efficiency of biocomposites, it is also provided the LCI data on composites. LCA was performed of two different elbow-fittings made of glass fiber/epoxy vinylester resin composite and hybrid (glass fiber-hemp)/vinylester resin composite considering human health, ecosystem quality, resources, global energy requirement, global warming point, and agricultural land occupation and significant environmental benefits of using hemp mats were found. According to their Life Cycle Costing (LCC), which was carried out from cradle to grave, the material costs reduction was significant for the hemp fiber reinforced elbow formulation.

Figueirêdo et al.[75] investigated the life cycle assessment of cellulose nanowhiskers which was extracted from unripe coconut fiber and cotton fibers. Nowadays it is really important to know the LCA of cellulosic nanofibers, which needs many chemical processes to extract the nanofibers from different natural fibers. This study focused on the environmental performance of cellulose nanowhiskers production processes and the product systems encompassed fiber, electricity, and chemical production processes. LCA was performed considering the environmental aspects: such as, energy, water, and emissions present in liquid effluents [chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen, nitrate, total phosphorus, phenols, furfural, and hydroxymethylfurfural (HMF)] and life cycle impact was also assessed for climate change, water depletion, eutrophication, and human toxicity impact categories. Figure 20 illustrated the system boundary of extraction nanowhiskers from unripe coconut fiber based on their production process. They suggested further research to improve the environmental performance of nanofiber production systems before scaling up the results from the laboratory to industry with emphasize focusing on improving yield efficiency, reducing energy and water use during the extraction of nanowhiskers, and recovering substances from effluents possessing market value.

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Figure 20. System boundary of extraction of cellulosic nanowhiskers from unripe coconut fibers.

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As technology improves for biocomposites reinforced with natural fibers to provide enhanced material and product characteristics, the products will become more diverse and enter markets that as of yet are unexplored. Today natural fiber reinforced biocomposites are found extensively in automotive sectors. By the time, natural fiber composite materials and associated design methods are sufficiently mature to allow their widespread use, e.g., as construction materials. The development of methods, systems, and standards could see natural fiber composite materials at a distinct advantage over traditional materials. There is a significant research effort underway to develop natural fiber composite materials and explore their use as construction materials, especially for load bearing applications.

8 Focusing on Future Research

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

In the future, natural fiber composites will see increased use in structural applications. Various other applications depend on their further improvements. But there are still a number of problems that have to be solved before these composites become fully competitive with synthetic fiber composites. Natural fiber composites are sustainable and could be fully recyclable, but could be more expensive if fully biobased and biodegradable and they are extremely sensitive to moisture and temperature. If a proper matrix is used, natural fiber composites could be 100% biodegradable, but their biodegradation can be difficult to control. Natural fiber composites exhibit good specific properties, but there is high variability in their properties. Their weaknesses can and will be overcome with the development of more advanced processing of natural fibers and their composites. However, fully environmental superiority of natural fiber composites compared to synthetic fiber composites is still questionable because of their relatively excessive processing requirements, which in turn consume more energy. Therefore, future research should be focused on the following issues:

  1. Fiber extraction should provide more elemental and technical fibers for effective embodiment into composite matrix to avoid the fiber variability
  2. The improvement of the interfacial properties between the fiber and the matrix should be achieved
  3. Overcome moisture absorption related to long term durability (temperature, humidity, and UV radiation), fiber and matrix modification, fire resistance, properties and durability, hybridization, and manufacturing and processing optimization subject to specific final products
  4. Biodegradability and life cycle assessment (LCA) should be thoroughly researched
  5. Composites, matrix (thermoplastic and thermosets), additives, coupling agents made from renewable resources should be developed. Search for new and improved biopolymers to replace petroleum-based polymers should be continued to fully meet with future environmental goals
  6. Multidisciplinary research, involving agricultural, biotechnology, polymer, and composite manufacturing aspects should be carried out
  7. Composite manufacturing technologies should be developed and adopted with new biobased polymer
  8. Current trend and expansion of nanocomposites, extensively research could be focused on cellulosic nanofibers as well as inorganic nanofillers (e.g., nanoclays) incorporation in natural fiber reinforced composite materials.

9 Conclusion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies

The uses of natural fiber as reinforcing agent in polymeric composites were reviewed from the point of view of status, structure, performance, surface treatments, and applications. The field of natural fiber reinforced composites research has experienced an explosion of interest, particularly with regard to its comparable properties to glass fibers within composites materials. It should be mentioned that natural fiber reinforced composite materials are gaining increasing importance in automotive, construction, aerospace, and other industrial applications due to their lighter weight, competitive specific strength and stiffness, improved energy recovery, carbon dioxide sequestration, ease and flexibility of manufacturing, and environmental friendliness as well as their renewable nature. It is also observed that the market scenario for composite applications is changing due to the introduction of newer biobased and biodegradable polymers.

Nanotechnology shows numerous opportunities for improving biocomposite products by providing nanotechnology-based coatings to increase water uptake, reduce biodegradation, and volatile organic compounds and even flame resistance. The use of nanocrystalline cellulose is being explored for a variety of uses since it is stronger than steel and stiffer than aluminum. Nanocrystalline cellulose reinforced composites could soon provide advanced performance, durability, value, service-life, and utility while at the same time being a fully sustainable technology.

Green composite materials could be the material revolution of this century focusing on sustainability, “cradle-to-grave” design, industrial ecology, eco-efficiency, and green chemistry, which may guide the development of a new generation of green materials. The success of natural fiber reinforced polymeric composites will be dependent upon on appropriate processing techniques, modification of fibers to improve the adhesion between fiber and the biopolymer, matrix modification and after treatment to improve performance as well as long-term durability and fire retardancy.

Biographies

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Reinforcing Fibers
  5. 3 Modification of Natural Fibers
  6. 4 Matrices for Natural Fiber Composites
  7. 5 Processing Techniques
  8. 6 Performance of Natural Fiber Composites
  9. 7 Developments and Applications
  10. 8 Focusing on Future Research
  11. 9 Conclusion
  12. Biographies
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    Dr. Omar Faruk completed his B.S. and M.S. in Chemistry at the University of Chittagong, Bangladesh. With a DAAD (German Academic Exchange Service) scholarship, he joined at University of Kassel, Germany. He achieved his PhD in Mechanical Engineering at 2005. He worked at the Department of Forestry, Michigan State University, USA as a Visiting Research Associate from 2006 to 2009. Since 2010, he is working at the Centre for Biocomposites and Biomaterials Processing, University of Toronto, Canada.

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    Prof. Andrzej Bledzki studied at the Technical Universities in Poland and Germany. In 1988 he took on the professorship of Polymer Processing in the department of Mechanical Engineering at the University of Kassel. From 1994 on, Prof. Bledzki was the head of the endowed professorship Polymer and Recycling Technology, which was sponsored by industry and belonged to the Institute of Materials Engineering of the University of Kassel. At present, he continues to work at the University of Kassel in the position of a “Senior Professor” and is also active at the West Pomeranian University of Technology, Szczecin, Poland. Prof. Bledzki has been honoured numerous times. He was, inter alia, a scholarship holder of the German Humboldt-Foundation (1984/85).

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    Prof. Hans-Peter Fink studied physics at the University of Rostock and made his PhD thesis (1977) on X-ray scattering of glasses. From 1975 to 1992 he was employed at the Institute for Polymer Chemistry in Teltow-Seehof, since 1992 he is working at the Fraunhofer Institute for Applied Polymer Research in Potsdam-Golm. Since 2006 he is the Director of that Institute. His research activities are centered on biopolymers, cellulose fibers, composites, and on different types of carbon fibers. Additionally he holds adjunct professorships for biopolymer science at the Universities of Potsdam and Kassel.

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    Prof. Mohini Sain is Dean and professor at Faculty of Forestry, University of Toronto. He specializes in advanced nancellulose technology, biocomposites and bio-nanocomposites. He is cross-appointed to the Department of Chemical Engineering and Applied Chemistry. He is a fellow of Royal Society of Chemistry, UK. Besides, he is also an adjunct professor of the Chemical Engineering Departments at the University of New Brunswick, Canada; King Abdulaziz University, Jeddah Saudi Arabia; University of Guelph, Canada, University of Lulea, Sweden, Honorary Professor at Slovak Technical University and Institute of Environmental Science at the University of Toronto, and collaborates with American and European research institutes and universities.