Progress in the research and applications of natural fiber-reinforced polymer matrix composites

2.1 Fiber surface treatment

The interfacial bonding between natural fibers and polymer matrices – a key technique for producing high-performance natural fiber composites – has been a major research focus in recent years. Kabir et al. [2] and Mohanty et al. [3] summarized different methods for the surface treatment of the natural fibers in detail. Li et al. [4] also elaborated on the surface treatment of natural fibers. Table 1 shows the different chemical treatments used to process the natural fibers. Among these, the alkali treatment is the most frequently applied method as it not only increases the surface roughness of fibers to improve the interfacial bonding performance between the fibers and resin matrices, but also improves the mechanical properties of the fibers. Bledzki and Gassan [5], [6] found that the tensile strength and modulus of jute yarn increased by 120% and 150%, respectively, after a 25 wt% NaOH solution treatment at 20°C for 20 min. Ray et al. [7] focused on the study of jute fibers and found that the strength and modulus of fibers increased by 46% and 68%, respectively, with 5 wt% NaOH solution alkali treatment for 6 h. Dai and Fan [8] reported that the alkali treatment reduced the diameter of fibers and increased the length-to-diameter ratio of fiber. As another important proven method, a silane coupling agent treatment has been widely applied in the surface treatment of glass fibers. In recent years, it has become a more popular technique in natural fiber modifications due to its ability to effectively improve the interfacial bonding. Xie et al. [9] prepared a detailed summary report on this method. Väntsi and Kärki [10] used a silane coupling agent to treat wood fibers.

New surface treatment methods have emerged rapidly one after another. For example, Yeh et al. [11] adopted maleic anhydride-grafted polypropylene (MAPP), maleated styrene-ethylene-butylene-styrene (SEBS-g-MA), along with alkali or silane coupling agents, to prepare rice husk/polypropylene (PP) composites. All the mechanical properties of the composites were improved. Fiore et al. [12] used sodium bicarbonate to process sisal fibers, which was a milder surface treatment method. They found that the tensile strength and modulus of sisal fibers increased by 138.5% and 63.2%, respectively, after 24 h of sodium bicarbonate treatment. Furthermore, the tensile strength and modulus of sisal fibers increased by 197.9% and 115.0%, respectively, after 120 h of treatment. Berthet et al. [13] used a drying method to process wheat straw fibers, and found that both the hydrophobicity of fibers and the interfacial compatibility with poly (hydroxybutyrate-co-valerate) improved. Ye et al. [14] used a self-assembly method to introduce the nanozinc oxide onto the surface of the wood fibers. Their results show that the interfacial bonding between the modified wood fibers and PP matrix improved. Meanwhile, the tensile and bending properties of the composites also improved.

2.2 Fiber agglomeration and dispersion

As natural fibers have poor compatibility with thermoplastic matrices, agglomeration can easily occur in the process of manufacturing different materials. The agglomeration influences the impregnation of the thermoplastic resin on the fibers and the improvement of the macromechanical properties of materials. Yam et al. [15] adopted the twin-screw mixing technique to study wood fiber-reinforced high-density polyethylene (HDPE) composites, and found that the tensile strength of the composites, which was created by the addition of wood fibers, was lower than that of pure polyethylene. They concluded that this was due to the uneven dispersion of wood fibers. Grande and Torres [16] reported that the strong hydrogen bonding between fibers made them hard to disperse: the more fibers were added, the more difficult the dispersion became. Peltola et al. [17] compared the enhancement effects of wood fibers and pulp fibers in the polylactide (PLA) and PP matrices, and found that the longer fibers are, the easier they will break.

Changing the dispersion of fibers is an important task in improving the macromechanical properties of the natural fiber composites. The use of stearic acid and surface modifiers can ensure an even dispersion of fibers and improve the mechanical properties of the composite materials [18], [19]. Grande and Torres [16] studied the influences of stearic acid and mineral oils on fiber dispersion, and found that stearic acid was superior to oils in dispersing the fibers in matrices. Coutinho et al. [20] smeared a PP coating on the surface of natural fibers to improve their dispersion in resin matrices, after which the performance of composites was improved. Dalväg et al. [21] reported that stearic acid and paraffin can prevent the agglomeration of natural fibers, and that the impact strength and elongation at break of materials were improved. However, these chemical modifiers had no effect on the tensile modulus of the composites, and even produced a negative effect on the tensile strength. Raj and Kokta [22] pointed out that the dispersion effect of stearic acid resulted from their chemical bonding with the fibers. Moreover, about 3% stearic acid was effective enough for the dispersion of fibers, and a more condensed form of stearic acid had no greater effect on fiber dispersion. At present, with the help of computer simulation, people can easily determine the relationship between the dispersion of natural fibers and the mechanical properties of the composites [23], [24].

2.3 Interfacial transcrystallinity

The natural fibers have a different surface structure compared with other synthetic fibers, with rough surfaces and the capacity of heterogeneous nucleation. Table 2 provides a summary of the different natural fiber types that can induce the transcrystallinity structure in various crystalline polymers. Gray [25] reported the transcrystallinity of PP induced by natural fibers for the first time. In that work, he compared the effects of cotton fibers, bleached ramie fibers and several types of artificial cellulosic fibers on the crystallization of PP. He found that natural fibers generated transcrystallinity, whereas artificial ones cannot generate transcrystallinity. Zafeiropoulos et al. [26] conducted detailed studies on the crystallization of four types of flax with different surface properties in two types of PP. They concluded that transcrystallinity is determined not only by the type of fiber but also by the type of matrix. Felix and Gatenholm [27] studied the influence of cotton fibers on the crystallization of a PP system. The transcrystallinity underwent complete growth with a thickness of approximately 80–120 μm at 131°C for 15 min. The result of a single-fiber fragmentation experiment revealed that the transcrystallinity layer could improve the transmission efficiency of the interfacial shear stress by 40%–100%. Mi et al. [28] studied the influence of bamboo fibers on the crystallization of PP and MAPP systems, and concluded that the crystallization generated was stronger with bamboo fibers in the MAPP system than that in the PP system due to the stronger interfacial bonding in the MAPP system. Ning et al. [29] gave a detailed summary of the formation mechanisms of the interfacial transcrystallinity induced by various enhancement media, and determined that the effective control of interfacial transcrystallinity was an important new method to regulate interfacial bonding of composites.

2.4 Impact strength

The natural fiber composites have limited use due to their poor mechanical properties, particularly their low impact strength. Conventionally, the impact strength can be improved by adding rubber or elastomers, but the strength and modulus of materials would still be reduced inevitably. Ruksakulpiwat et al. [30] attempted to improve the impact properties of a vetiver grass/PP compound system by natural rubber (NR) and ethylene propylene diene monomer (EPDM), and found that the impact properties of the compounds improved significantly, whereas the tensile strength and modulus of materials decreased greatly with the increasing amount of rubber load. Rana et al. [31] found that the tensile and bending properties of materials decreased, whereas the impact strength increased after adding the impact modifiers by investigating the influence of different impact modifiers on the material properties of a jute/PP system. They also found that the paraffin modifiers were superior to elastomer modifiers. By examining the influence of different impact modifiers and MAPP on wood flour-filled PP materials, Oksman and Clemons [32] found that using SEBS-MA as an impact modifier yielded significant effects, whereas the modulus was inevitably reduced after adding the impact modifiers.

Hristov et al. [33] reported that the tensile strength and modulus of materials did not decrease and that the impact properties also did not increase after adding the compatibilizer MAPP and the impact modifier styrene-butadiene rubber (SBR) to the wood flour system. They also reported that the tensile strength and modulus of materials could not be reduced by the added SBR if favorable interfaces formed between matrices and fibers. Based on a study on the impact properties of three types of wood fiber/PP composites, Sudár et al. [34] reported that the added wood fibers improved the impact properties of composites, but only to a limited extent. However, the added elastomers significantly improved the impact properties of elastomer/PP binary composites, although they were unable to satisfactorily improve the impact properties of a wood fiber/elastomer/PP ternary composite system. Sudár et al. found that wood fibers might have caused surrounding gaps, which then affected the absorption of foreign impact energy from the matrix deformation. Thus, in considering the composite system and other properties, it could be inferred that further modification methods must be explored in order to improve the impact properties of the natural fiber composites.

2.5 Foaming technique

The foaming of natural fiber composites has become another major research field that is being studied by many researchers. Chemical foaming agents are usually added to reduce the density of natural fiber composites [35], [36], [37], [38]. However, the process is not environmentally friendly as it generates uneven foam holes in the materials. Therefore, in recent years, more and more studies have concentrated on physical foaming. In most studies, the supercritical N₂ and CO₂ are added to the system during physical foaming, but the foam holes are generated as the form pressure of materials decreases. Nevertheless, physical foaming is environmentally friendly as it generates foam holes with a more consistent size.

Turng et al. [39] conducted a foaming study on the natural fiber-reinforced nylon system using a microcellular foaming injection molding technique. The result showed that the natural fiber composites prepared by such a technique significantly reduced the yellowing effect found in those prepared by the traditional injection molding techniques. By preparing the composites of the wood fiber/HDPE system by using the physical foaming methods, Guo et al. [40] found that the small-sized foam holes that formed during the process were more even. Matuana et al. [41] investigated the foaming properties of the wood fiber-reinforced polyvinyl chloride (PVC) system using the supercritical CO₂ foaming technique. Their research findings indicated that the impact strength of the composites improved as the foaming degree increased, whereas the tensile modulus and strength of composites decreased slightly. Zhang et al. [42] studied the influences of different extruder parameters and contents of silica filling on the foaming properties of wood flour/PP composites, by using the supercritical CO₂ foaming technique. Their results showed that the optimized screw configurations and screw speed can lead to a more uniform foaming of composites, and that both wood fibers and silica filling could facilitate the foaming. Ding et al. [43] prepared wood fiber/PLA composites with an injection molding machine equipped with MuCell technology (Figure 1), and found that the foaming properties of composites with the added wood fibers were superior to that of the pure resin system. They attributed this phenomenon to the wood fibers serving as a foaming nucleating agent. They also reported that the introduced wood fibers increased the melt viscosity of the compound system. Thus, the foaming gas was easier to store within the composites with low susceptibility to leakage.

Figure 1:

SEM micrographs of PLA/PEG, PLA/NBSK/PEG and PLA/MDF/PEG composite foams with 15% void fraction [43]. NBSK and MDF stand for two types of natural fibers. (© 2016 Elsevier Ltd.)

The foaming technique can reduce the density and cost of the natural fiber composites but it also has an important defect, that is, all the mechanical properties of the materials decrease apart from the impact properties. This phenomenon may be due to the fact that a large number of created bubble holes extend in the direction of the forward propagation of the impact force, thus improving the capacity of composite material to resist impact. Therefore, further research must be carried out to explore the foaming methods that are capable of maintaining good mechanical properties and reducing the material density.

2.6 Studies on inflaming retardance

The flame retardancy of natural fiber composites has also aroused considerable attention. For example, Torres et al. [44] determined that the burning rate of sisal fiber-reinforced HDPE composite was more than twice that of pure HDPE. Therefore, the flame retardancy of natural fiber composites must be improved. Sain et al. [45] investigated the flame-retardant efficiency of magnesium hydroxide on the wood fiber/PP system through a horizontal burning test and a limiting oxygen index test. In that work, they retarded the flaming by adding retardants, such as boric acid and zinc borate. The results indicated that magnesium hydroxide reduced the flammability of materials, whereas flame retardants, such as boric acid and zinc borate could not yield any synergistic effect. In addition, the mechanical properties of materials decreased slightly upon the addition of the flame retardants. Li and He [46] investigated the influences on the flame retardancy of wood fiber composites of ammonium polyphosphate as well as a mixture of ammonium polyphosphate, melamine phosphate, and pentaerythritol. Limiting oxygen index tests indicated that the ammonium polyphosphate reduced the flammability of materials. However, the added ammonium polyphosphate and pentaerythritol both reduced the impact strength of the materials. Jang and Lee [47] studied the influences of several different flame retardants on the burning properties of prepared waste paper fibers/PP composites, among which, Saytex8010 and antimony trioxide had the highest synergistic flame-retardant efficiencies, whereas the magnesium hydroxide had no effect on this system. The mechanical properties of the composites reduced significantly with the addition of the thiamine pyrophosphate (TPP). An improvement in the flame-retardant abilities of natural fiber materials can also be achieved to a certain extent by the addition of glass fibers [48], [49].

Recently, Subasinghe and Bhattacharyya [50] investigated the influences of three types of ammonium polyphosphate flame retardants on PP/kenaf composites, and found that ammonium polyphosphate (APP) with surface treatment had a higher flame-retardant efficiency. Meanwhile, all three flame retardants had some negative effects on the tensile and bending properties of the composites. Arao et al. [51] investigated the flame retardancy of the three flame retardants, including APP, melamine polyphosphate (MPP) and aluminum hydroxide on wood flour/PP composites. They discovered that the composites can self-extinguish with 10 wt% APP. Arao et al. also concluded that the added flame retardants reduced the mechanical properties of the composites. The above analyses indicate that flame retardants can improve the flame-retardant properties of composites; however, they also inevitably reduce the mechanical properties of the composites. Thus, the operating requirements for the final products must be considered comprehensively.

2.7 Biodegradable resin matrix

As the preparation techniques for the degradable matrix materials have matured and the production costs have decreased in recent years, extensive attention has been given to studies on natural fiber-reinforced degradable matrix materials for further cost reduction. The representative degradable matrices include PLA, polybutylene succinate (PBS), polyhydroxybutyrate (PHB) and thermoplastic starch. Figure 2 gives detailed classifications of the different biodegradable polymers [52]. Yu et al. [53] studied the influence of PLA grafted with maleic anhydride (PLA-g-MA) on the mechanical properties of ramie/PLA composites. They reported that the tensile strength of composites reached 64.3 MPa, bending strength reached 112.4 MPa and impact strength reached 7.1 kJ/m² when 3% maleic anhydride was added at a fiber weight fraction of 30%. Sawpan et al. [54] investigated the hemp/PLA short fiber composites by injection molding methods, and found that the limit of weight fraction was 30%. They also reported that it was difficult to add any more fibers due to the low liquidity of the compound system. Afterwards, Sawpan et al. prepared the hemp/PLA long fiber composites by lamination mold pressing, and found that the alkali-treated long fiber composites had the highest mechanical properties when the fiber weight fraction was 35%. The Young’s modulus was 12.6 GPa and the impact strength was 7.4 kJ/m² when the tensile strength of composites reached 85.4 MPa. Muthuraj et al. [55] and Gamon et al. [56] investigated the influences of different processing techniques on the properties of composites, and reported that the length and form of fibers changed to varying degrees after processing.

Figure 2:

Classifications of the different biodegradable polymers [52]. (© 2015 Elsevier Ltd.)

Recently, extensive reviews of the literature on the natural fiber-reinforced biodegradable resin matrix composites have been published. For example, Terzopoulou et al. [57] discussed in detail a series of issues, such as the preparation methods of aliphatic polyester and bast-fiber composites, the selection of fiber and resin matrices and the improved methods of interfacial bonding. Gurunathan et al. [52] also summarized the characteristics of various biodegradable resin matrices, a variety of natural fibers and the application fields of composites. Additionally, Abdul Khalil et al. [58] discussed the bamboo fibers and their composites in detail. Soroudi and Jakubowicz [59] also investigated the biodegradable resin matrices and the recycling of their composites. Mokhothu and John [60] summarized the hygrothermal aging of natural fiber composites.

2.8 Nanofiber

Recently, nano materials have attracted wide attention as a type of new matrix material. Two types of nano-structure reinforcements are suited to the special structures of natural fibers. One is called the nanowhisker, which is a nanoscale cellulose crystalline region separated by special techniques, and the other is a cellulose microfibril with a diameter of 2–20 nm and a length of several microns. Many researchers have used the abovementioned natural fiber nano reinforcements for resins [61], [62], [63], [64], [65], [66], [67], [68], and the composites showed superior mechanical performance and transparency. For example, Nakagaito et al. [66] found that the transparency of the composite remained ideal when the content of microfibrils reached approximately 70% (Figure 3).

Figure 3:

(A) Scanning electron micrograph of 14-times high-pressure homogenizer-treated pulp. (B) Appearance of 53 μm-thick homogenizer-treated pulp/acrylic resin sheet (left) and 60-μm-thick bacteria cellulose/acrylic resin sheet (right) [66]. (© 2005 Springer Ltd.)

Nano natural fibers have served as key areas of research over the past 10 years, with nearly 6000 research articles published [69]. Numerous researchers have summarized the important progress of a range of subjects, including the nature and applications of nanofibers [70], [71], [72], [73], the extraction and surface modification of microfibrils [74], [75], the processing methods for nanofiber composites [69], [76], the characterization of the properties of nanofiber composites [77], the nanofiber-reinforced biodegradable resin matrices [78], the enhancement of nanofibers in polymers [79], the production and applications of nanofiber aerogel [80] and many more.

3.1 Automotive industry

Since the 1990s, the natural fiber composites have been widely adopted by various countries supporting automotive industries. The main products include interior door panels, trunks, roofs, seat backboards, dashboards and similar parts (Figure 4) [82], [83]. A market survey conducted by the NOVA Institute in Germany indicated that the automotive industry’s need for natural fibers has exhibited rapid growth over the past few years and will continue to increase in future. Automotive components made of natural fiber composites grew by approximately 50% from 2000 to 2005. The automobile industry’s annual demand for natural fibers grew to more than 45,000 tons in 2005 [84]. Currently, the automotive industry is still the main area where natural fiber composites are most widely applied. Numerous researchers have played a positive role in evaluating the applications of natural fiber composites in automobiles in their respective studies [81], [85], [86], [87], [88], [89], [90], [91], [92], [93].

Figure 4:

Flax, hemp, sisal, wool and other natural fibers are used to make 50 Mercedes-Benz E-Class components [82]. (© 2006 Springer Ltd.)

The biggest driving force for the application of natural fiber composites in automobiles comes from the resulting light weight of automobiles and the easier recycling of materials [94]. However, the development of the industry has brought increasingly strict requirements on recycling automotive materials. According to 2005 data, 85% of all vehicle components in Europe were made of recyclable materials, and the European Community’s 2000/53/EC “End of Life Vehicles” programs mandated that this percentage must reach 95% by 2015. Taking a similar approach, the Japanese government required that 88% of each vehicle should be recyclable by 2005, and this percentage must reach 95% by 2015 [82]. The automotive manufacturers are permitted to enter the market only if the above standards are satisfied, and this requirement has driven them to identify solutions as quickly as possible. Consequently, it is practical for the industry to adopt natural fiber materials as much as possible. Natural fiber-reinforced thermoplastic composites can be recycled repeatedly with high performance and then used in circumstances not requiring high performance. If the performance becomes too poor, the composites can be incinerated to provide fuel and energy, thus resulting in fewer waste residues relative to glass fiber composites.

3.2 Household appliances

In recent years, the rapid increase in the price of raw materials has resulted in higher prices for matrix resins. Thus, many countries are gradually studying and developing other substitute materials for household appliances. The composites, often made by adding cheap filling and other different functional substances into the expensive resin matrices, have become effective substitutes for the shells of household appliances. The cost of materials for appliances can be significantly reduced by adding low-cost natural fibers into resin matrices. Furthermore, natural fiber-reinforced thermoplastic composites are light, low-cost, pollution-free and do not have any negative effects on human health. They also comply with the development trends of household appliances. Therefore, a broad application prospect for natural fiber composites has emerged in the household appliance industry. For example, in November 2015, Tchibo (DE) produced a wood-plastic composite bedside table clock, which looked more natural and appealed to many consumers [95].

Some of the organizations involved in the application and development of electric appliances made of natural fiber composites include Kareline Oy in Finland, NEC in Japan, the Research Center of Biological Materials at Wageningen University in the Netherlands and JER Envirotech in Canada. The main products include shells of cell phones, laptops and small electrical appliances, as well as a variety of articles of daily use – from children’s toys to household drinking cups. Most of the resin matrices used are common thermoplastics. The latest trend is to manufacture the shells of cell phones and computers with biological resin matrices and natural fibers. Toward this end, the Japanese company NEC manufactured shells of new types of laptops by adding kenaf into polylactic acid. Several models of the LaVie laptop range introduced by NEC in 2004 used this new material. NEC then announced in 2006 that this material was also used in cell phones (specifically, the Foma N701iECO) (Figure 5) [96]. However, the costs of these products remain high and the biological resin matrices have to be modified to allow the composite shells to last for the expected phone lifetime. Therefore, these products still face substantial technical problems that hinder their widespread application.

Figure 5:

Bioplastic phone from NEC [96].

3.3 Construction materials

Early studies and developments of natural fiber composites and their applications were undertaken with the idea of aiding the disposal of increasing amounts of plant wastes. The wood-plastic composites filled with natural wood flour were the same natural fiber composites studied earlier, which usually consisted of filled waste wood flour or short wood fibers and thermoplastic matrices. Such wood-plastic composites are chiefly used in building materials, such as outdoor bedboards, park benches, fences, indoor decorative boards and building templates, all of which are the major products made of natural fiber composites used in China [8]. Most non-Chinese construction companies have invested in natural fiber composites in order to meet the enormous demands of the construction material market. For example, the Indian government encouraged the development of Madras House and grain elevator buildings with jute fiber-reinforced polyester resins [6]. In North America, the Trex, TimberTech and AERT companies provided most of the decking products in the construction material market [97].

3.4 Packaging materials

The manufacture of packaging materials is another new application field for natural fiber composites. Most of the resins used in packaging materials are very expensive, degradable resins. The natural fibers added to degradable resins can significantly reduce the cost of materials as well as improve the strength, rigidity and thermal deformation of the materials without affecting their degradability. Therefore, the application of natural fibers enjoys remarkable advantages in the manufacture of packaging materials. Currently, natural fiber composites are mainly applied in disposable products in packaging. For example, Biosphere Industries applied a composite made of starch and some grass fibers in tableware and plates. Their composite is capable of degrading in less than 40 days, thus making it a perfect material for disposable tableware [98]. Earthcycle applied composites made of palm fibers and edible resins in fruit trays and flowerpots in nursery gardens [99]. The natural fiber-reinforced PLA has also been applied in packaging materials to a certain extent.

3.5 Boards of musical instruments

There is a growing shortage of timber for musical instruments, and this is due to the increasing scarcity of forest resources around the world and the implementation of environmental conservation policies. The issue has drawn increasing attention to the substitution of natural fiber composites for common wood materials in manufacturing the boards of musical instruments, and this is mainly due to the similar structures of natural fibers and wood fibers. Phillips and Lessard [100] studied the application of flax fiber composites in guitar boards and compared them with the properties of Sitka spruce, finding that such composites can substitute for Sitka spruce when producing guitar boards. The German company Jakob Winter manufactured violin boards and saxophone cases using hemp or flax fiber and PP matrix by mold pressing or injection molding, thus creating a type of musical instrument board with promising market prospects (Figure 6) [101]. In addition, the American company Blackbird manufactured guitar boards with a composite of linen fiber fabric and bio resins. The resulting boards showed improved resistance to temperature and humidity as well as satisfactory acoustic characteristics compared with the conventional wood boards [102].

Figure 6:

Cases for musical instruments made of quickly renewable, biobased natural fiber composites [101].