Research Progress and Application of Natural Fiber Composites

ABSTRACT

Natural fiber composites have become a hot research topic all over the world, and have received strong support from governments. This paper first reviews the overall trend and development history of natural fiber composites, and analyzes their global market. Then the research highlights and situation of natural fiber composites are pointed out. Finally, the main industrial application fields of natural fiber composites are also summarized. In the future, High-performance and high value-added natural fiber composites are the important directions of global industrialization market. And it is necessary to further refine the key technologies behind the industrialization, and promote the green, intelligent and high-performance manufacturing of such composites, so as to play a positive role in national environmental protection and social sustainable development.

摘要

天然纤维复合材料已成为世界各国的研究热点，并得到了各国政府的大力支持. 本文首先回顾了天然纤维复合材料的总体趋势和发展历史，并分析了其全球市场. 然后指出了天然纤维复合材料的研究重点和现状. 最后，对天然纤维复合材料的主要工业应用领域进行了总结. 未来，高性能、高附加值的天然纤维复合材料是全球工业化市场的重要方向. 有必要进一步提炼工业化背后的关键技术，促进此类复合材料的绿色、智能和高性能制造，从而在国家环境保护和社会可持续发展中发挥积极作用.

KEYWORDS:

• Natural fiber composite
• biomass
• high performance
• industrialization
• application
• properties

关键字:

• 天然纤维复合材料
• 生物质
• 高性能
• 工业化
• 应用
• 性能

Introduction

In the 21st century, resource and environmental issues have become a major challenge confronting the human society, affecting the progress and future of the development of human society. Biomass resources are extremely abundant in nature. About 220 billion tons of biomass is produced in the world every year (Tahir, Saeed, and Ali 2023), and the yield of agricultural straw alone is up to more than 800 million tons per year in China (Chen 2018). However, about 10% of the straw is burned on the farmland, which wastes considerable biomass resources and has a great negative impact on the ecological environment of the surrounding areas. Therefore, it is urgent to make rational use of these biomass resources. In terms of comprehensive utilization, it is an excellent approach to prepare natural fiber composites from these biomass resources and thermoplastic resins such as polyethylene, polypropylene, polylactic acid, etc. This type of composites has broad market prospects and increasingly extensive application fields, such as furniture panels, garden landscape, outdoor construction and automobile industry. The development of this type of composites is of great significance for the protection of forest resources and the resource utilization of waste natural fibers in the agriculture and forestry industry and in line with the national circular economy and industrial development policy orientation. This is an important energy-saving and environmentally-friendly material encouraged by many countries. Although there are many review publications on natural fiber composites, this article highlights the current research hotspots and frontier direction, and also analyzes the global market of this kind of materials in the next few years.

The overall trend and development history of natural fiber composites

Natural fiber composites have become one of the research highlights in various countries around the world, and have received strong support from governments because of its low cost, low energy consumption, environmental friendliness, and sustainable development (Bajwa and Bhattacharjee 2016; Das et al. 2022; Kannan and Thangaraju 2022; Sekar et al. 2022; Vinod, Sanjay, and Siengchin 2023). Related research works in North America and European countries were carried out earlier. In North America, most of the researches were carried out by the United States while in Europe, most of the studies was carried out by Germany and the United Kingdom. In addition, the researches in Asian countries such as India were progressing rapidly. In recent years, China’s researches on natural fiber composites have received increasing attentions from the government and scientific research institutes, and such researches have been developing rapidly. On the other hand, the focuses of product application fields researched by various countries are somewhat different. European countries mainly develop natural fiber composites for automobiles, and most of the reinforcing fibers are hemp fibers; while North America mainly develops wood-plastic composite panels for municipal engineering, and most of the reinforcing fibers are wood fibers; India also focuses on automotive materials, hemp fibers are mostly used as reinforcing fibers; while most of the research in China now focuses on wood-plastic composite panels, and most of the reinforcing materials are wood flour and wood fiber (Sun 2010), which originate in the packaging industry and are used to solve the problem of quarantine of wooden packaging materials for export.

In addition, the global market of natural fiber composites is expected to grow at an annual average rate of 9.59%, and will reach a USD 41 billion net worth by 2025 (Zwawi 2021). Table 1 shows the statistical analysis of the consumption of natural fiber composites in major regions of the world from 2018 to 2021, and Table 2 shows the forecast of the consumption of natural fiber composites in major regions of the world from 2022 to 2026 (Beijing Yubo Zhiye Market Consulting Co., Ltd 2022). It can be seen from Tables 1 and 2 that North America, China and Europe are three important regions for the development of natural fiber composites.

Table 1. Statistics on consumption of natural fiber composites in major regions of the world from 2018 to 2021. Adapted and reproduced with permission from the ref (Beijing Yubo Zhiye Market Consulting Co., Ltd 2022).

Year	Consumption（ten thousand ton）
	North America	Europe	China	Japan	Southeast Asia	India
2018	156.4	80.6	83.2	39.8	32.5	30.1
2019	171.5	88.5	94.2	43.3	37.2	34.2
2020	181.2	93.9	104.1	45.6	41.1	37.5
2021	197.5	102.5	117.2	49.3	47.4	42.6

Table 2. Forecast of consumption of natural fiber composites in major regions of the world from 2022 to 2026. Adapted and reproduced with permission from the ref (Beijing Yubo Zhiye Market Consulting Co., Ltd 2022).

Year	Consumption（ten thousand ton）
	North America	Europe	China	Japan	Southeast Asia	India
2022	214.3	111.4	130.9	53.0	55.2	48.4
2023	233.0	121.4	148.4	57.1	64.4	55.8
2024	252.5	131.8	167.1	61.1	74.0	63.4
2025	271.7	141.5	184.4	65.3	83.3	71.4
2026	290.2	151.4	200.0	69.5	93.3	78.8

The earliest natural fiber composite was molded from thermosetting phenolic resin and wood fiber by Dr. Bend in 1907 (Liu 2007). At this stage, bast plant fibers are often used to fill thermosetting plastics such as phenolic resins and unsaturated polyesters. Composite products are mostly prepared by laminating technology and are mainly used as building materials. In the 1940s, the composite material technology played an important role in the national economic construction and national defense. At this stage, glass fiber reinforced resin was mostly used, and natural fiber was still only used as a filling material. In the 1990s, with the increasing depletion of resources on the earth and the increasing awareness of environmental protection by human beings, the researches on natural fiber reinforced composites gradually attracted the attention of scientists from all over the world.

In 1999, German scholars Bledzki and Gassan (1999) and Indian scholars Saheb and Jog (1999) systematically summarized the research situation of natural fiber composites, and introduced the development of this field in detail. In 2000, Swedish scientist Wågberg (2000) made a detailed discussion on the deposition and modification of polyelectrolyte on the surface of natural fiber, and pointed out the direction of functional modification of natural fiber composites. Mohanty, Misra, and Drzal (2001) and Li, Tabil, and Panigrahi (2007) discussed in detail the surface treatment methods of natural fibers and the changes in the mechanical properties of composites after treatment in 2001 and 2007, respectively. In 2008, New Zealand scholar Pickering (2008) published a book giving a comprehensive and detailed description of the development of natural fiber composites, covering many aspects such as fiber, matrix, interface modification, composite material preparation, and material mechanical properties. In recent years, many Chinese scholars have published representative research works (Ding et al. 2022; Yi and Li 2017), detailing the relevant characteristics of wood, bamboo, crop straw and other biomass, the molding process of natural fiber composites and the overview of their application fields.

Research highlights and situation analysis of natural fiber composites

Figure 1 shows the number of literatures published in the field of natural fiber composite materials searched by the Science Citation Index (SCIE) database in the past 20 years. It can be seen from Figure 1 that the number of literatures in this field has increased significantly with the years, reflecting that this field is increasingly becoming an important research hotspot.

Figure 1. Number of articles published in the field of natural fiber composites in the past 20 years.

According to the analysis of the literatures retrieved from the SCIE database, it can be seen that the high performance of natural fiber composites is the most concerned scientific issue, followed by the performance prediction of composites, and the foaming of composites and nanofiber composites are also areas of more concern. Other development focuses include flame retardant composites, fully degradable composites, antibacterial composites, 3D printing and other fields.

High performance of composites

Although natural fiber composites have significant economic and social benefits, the current mechanical properties of such composites are not ideal. Relevant literatures mainly focus on fiber surface modification, resin matrix modification, interface bonding control, hybrid reinforcement, fiber reinforcement methods, fiber dispersion, and composite material preparation technology in dealing with this problem.

For example, Mohanty, Misra, and Drzal (2001), Li, Tabil, and Panigrahi (2007), Gholampour and Ozbakkaloglu (2020) discussed in detail the surface treatment methods of natural fibers and the changes in the mechanical properties of composites after treatment. The fiber modification methods involved mainly include cold plasma, alkali treatment, silane treatment, acrylonitrile grafting treatment, benzoyl treatment, stearic acid treatment, acetylation treatment, peroxide treatment, permanganic acid treatment, isocyanic acid treatment, sodium hypochlorite treatment. Regarding the modification of the resin matrix, most articles adopt the introduction of active groups such as maleic anhydride groups, epoxy groups, and amino groups into the resin matrix, or the addition of macromolecular compatibilizers containing the above-mentioned active groups (Arbelaiz et al. 2020; Buetuen et al. 2019). The control of interface bonding is mainly achieved through fiber modification and resin matrix modification. Hybrid reinforcement is also used to strengthen the resin matrix by hybridizing with glass fiber, carbon fiber, carbon nanotube and other materials, and the effect of improving the mechanical properties of composites is significant (Santulli 2019). In addition, fiber dispersion mainly focuses on fiber modification, the addition of lubricants, and the improvement of mixing technology (Grande and Torres 2005; Raj and Kokta 1991). Good interface bonding and uniformly distributed fibers can lead to better mechanical properties of composites.

In terms of fiber reinforcement methods, the mechanical properties of composites prepared by different reinforcement forms vary significantly. The reinforcement methods for natural fiber reinforcement mainly include short fibers, long fibers, continuous fibers, fiber mats, etc. The reinforcement methods for natural fibers determine the molding and processing technologies of composites. At present, in the widely studied natural fiber reinforced thermoplastic composites, most of the processing technologies are extrusion molding and injection molding processes. In the preparation of high-performance composites, LFT (long fiber reinforced thermoplastics) technology (Kim and Park 2020), pultrusion technology (Angelov et al. 2007), NMT (natural fiber mat reinforced thermoplastics) technology (Chee et al. 2019) and film stacking technology (Manral and Bajpai 2020) have received lots of attentions. Thomason and Vlug (1996), Thomason et al. (1996), Thomason (2002) found that under a given certain fiber content and fiber orientation, with the increase of fiber length, the mechanical properties of fiber composites increased substantially. The change trend of various properties with length is shown in Figure 2. It can be seen that the improvement of the impact performance of composites is more dependent on the length of the fiber. The major disadvantage of natural fiber composites is that the impact performance is generally poor. Therefore, the development of long natural fiber composites has more advantages, which can comprehensively improve the mechanical properties of composites.

Figure 2. Relationship between fiber length and mechanical properties of glass fiber reinforced polypropylene composites. Adapted and reproduced with permission from the ref (Thomason 2002).

Functionalization of composites

The functionalization researches of natural fiber composites mainly focus on flame retardant, foaming and antibacterial properties. Most flame retardants are halogen-free flame retardants such as magnesium hydroxide, aluminum hydroxide, boric acid and zinc borate, ammonium polyphosphate, melamine phosphate, pentaerythritol, expandable graphite, and nano clay (Arao et al. 2014) Although the above flame retardants can improve the flame retardant properties of composites, most of them inevitably reduce the mechanical properties of composites. In addition, flame-retardant coating treatment of fibers and composites, addition of glass fibers can also improve the flame-retardant properties of composites (Chapple and Anandjiwala 2010).

Another major field of research on the functionalization of natural fiber composites is foaming technology, which can further reduce the density of composites and the cost of raw materials. At present, a larger number of studies focus on this topic. The method of adding a chemical foaming agent is often used to reduce the material density. However, the chemical foaming method needs to use a higher processing temperature to enable the foaming agent to be decomposed. The cell morphology in the material is not uniform, and the method is not environmentally friendly. Therefore, in recent years, more and more studies have focused on the field of physical foaming. The physical foaming method mostly uses supercritical N₂ and CO₂ injected into the system. Cells are formed while the molding pressure of the material is dropping. The physical foaming method is environmentally friendly and the cell sizes are relatively uniform (Villamil Jiménez et al. 2020). Figure 3 gives the principle of the foaming process. Figure 3(a) is pressure-induced batch foaming process. Temperature-induced batch foaming process is shown in Figure 3(b). Figure 3(c) is high pressure foam injection molding process. Low pressure foam injection molding process is presented in Figure 3(d).

Figure 3. Principle of the foaming process. Adapted and reproduced with permission from the ref (Villamil Jiménez et al. 2020).

In the aspect of functional modification of natural fiber, there are also many researches on the layer by layer self-assembly modification technology based on polyelectrolyte. The introduction of polyelectrolytes and nanoparticles with different functions can endow natural fibers with unique and different functions (Chen et al. 2021; Wågberg and Erlandsson 2020). PDDA, PEI, PSS, PAH and PAA are mostly used as polyelectrolytes, wood fibers and cotton fibers are mostly used as natural fibers, and impregnation assembly process is mostly used as modification method. Table 3 systematically summarizes the modification methods and effects of polyelectrolytes on natural fibers (Sun et al. 2022). It can be seen that the flame retardancy, heat resistance, water resistance, antibacterial, coloring, conductivity, UV resistance and mechanical properties of the modified fiber can improve.

Table 3. Modification methods and effects of polyelectrolytes on natural cellulosic fibers. Adapted and reproduced with permission from the ref (Sun et al. 2022).

Fiber Type	Treatment Methods	Process Conditions	Modification Effects
Wood (Liriodendron tulipifera)	Immersive assembly	Polydiallyldimethylammonium chloride (PDDA) and montmorillonite (MMT) clay were used. The concentration of PDDA solution was about 0.5 wt%, the concentration of MMT was about 0.05 wt%, NaCl 0.5 M. PDDA treatment time was 20 min, and the pH was 10.5.	The surface of the fiber was covered with nanoscale PDDA-clay films, and the heat resistance of the fibers improved.
Wood	Immersive assembly	Poly(allylamine hydrochloride) (PAH) and Polyacrylic acid (PAA) were used, the addition amount was 30 mg/g fiber, NaCl 0.01 M, and the treatment time was 20 min. PAH and PAA were adsorbed at pH values of 7.5 and 3.5, respectively.	The water resistance of the fiber improved, and paper sheets from modified fibers showed improved mechanical properties.
Wood	Immersive assembly	Chitosan (CH) and poly (vinylphosphonic acid) (PVPA) were used. The addition amount was 10 mg/g fiber. The concentration of both solutions was 0.1 wt %, NaCl 0.01 M. The treatment time was 10 min, and the pH was 4.	The flame retardant effect improved, the phosphorus-containing thin films (20 CH/PVPA bilayers) reduced the peak heat release rate by 49%.
Wood	Immersive assembly	PAH and PAA were used. The addition amount was 30 mg/g fiber, NaCl 0.01 M, and the treatment time was 20 min (1) PAH at pH 7.5 and PAA at pH 7.5 and (2) PAH at pH 7.5 and PAA at pH 3.5.	The tensile properties of paper increased with the number of modified layers. The bonding force between fibers was also strengthened.
Wood	Immersive assembly	PAH and Poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were used. The concentration of both solutions was 0.0025 wt%. The addition amount was 5 mg/g fiber, NaCl 0.01 M, and the treatment time was 10 min. PAH and PEDOT:PSS were adsorbed at pH values of 7.5 and 3.5,respectively.	The conductivity of fiber and paper improved. The modified fibers had a much smoother surface.
Wood	Immersive assembly	PAH and Lignosulfonates-amine (LSA) were used. The concentration of PAH solution was 0.01 wt%, and the pH was 7.5. The concentration of LSA was 0.02 wt%, the pH was 12, and the treatment time was 15 min.	The water resistance of the fiber improved, and paper sheets from modified fibers showed improved mechanical properties.
Wood	Immersive assembly	Polyethylenimine (PEI) and Sodium hexametaphosphate (SHMP) were used. The addition amount was 30 mg/g fiber, NaCl 0.01 M. The treatment time was 30 min, with pH 9 for PEI, and pH 4 for SHMP.	The flammable character of wood fibers was reduced while the mechanical properties of the paper material were strengthened.
Wood	Immersive assembly	PEI, PSS and indium tin oxide (ITO) nanoparticles were used, and the concentrations of the three solutions were 0.2 wt%, 0.01 wt% and 0.01 wt%, respectively. The pH values were 6.5, 7.0 and 3.0 respectively. The treatment time was 20 min.	The conductivity of fiber and paper improved. The conductivity of coated paper improved by more than six orders of magnitude in both thein-plane and through-the-thickness directions.
Cotton	Immersive assembly	Branched PEI (pH 7 or 10) and MMT clay(0.2 or 1 wt %) were used. The PEI solution concentration was 0.1 wt %，The first dip into each mixture was for 5 min,and subsequent dips were for 2 min each.	This process could carry out flame-retardant treatment on cotton fiber fabrics, and improve the heat resistance to a certain extent without effects on the characteristics of fiber fabrics.
Cotton	Immersive assembly, Spray assembly	Positive alumina-coated silica and negativesilica nanoparticles were used. The concentration of both solutions was 0.2 wt %. The treatment time was 5 min, and the spray gun distance 6 cm.	The horizontal spray showed a very homogeneous silica coating when compared to vertical spray or dipping. A better flame retardant effect was achieved.
Cotton	Spray assembly	Graphene and vinylphosphonic acid were used. The concentrations of the graphene solutions were 1 wt% and 2 wt%, the concentration of the vinylphosphonic acid solution was 3 wt%, and the treatment time was 10 s.	This process could improve hydrophobicity, thermal stability, electrical conductivity, UV absorption and EMI shielding.
Cotton	Immersive assembly	CH and alginic acid sodium salt were used. The concentration of both solutions was 0.1 wt %, the pH was 5, and the treatment time was 5 min.	The surface water resistance and dyeability of the fiber could be accurately adjusted. Moreover, the treated fiber had potent antibacterial activity toward both Gram-positive and Gram-negativebacteria.
Cotton	Immersive assembly	CH and PEDOT:PSS/Graphene were used. The solution of CH was 3 wt%, PEDOT: PSS solution was 1 wt% PEDOT and 0.5 wt% PSS. The treatment time was 20 min.	This process couldimprove electrical conductivity，UV protection ability and laundering durability.
Cotton	Immersive assembly	PEI/melamine, and phytic acid were used, the concentration of PEI/melamine was 0.5 wt% and 3 wt%, the concentration of phytic acid was 3 wt%, the pH value of both solutions was 4, and the treatment time was 2 min.	This process could improve the flame retardancy and water repellency.The peak heat release rates decreased by more than 50%.
Cotton	Immersive assembly	CH, and pentasodiumtripolyphosphate and PSS were used, the solution concentration was 1 wt%, and the treatment time was 5 min.	The bacterial resistance of the fiber improved, and the water resistance could be precisely controlled. The air-permeability was reduced.
Ramie	Immersive assembly	Ammonium polyphosphate (APP) and PEI were used, the concentrations of both solutions were 0.4 and 0.9 wt %, the pH was 9, and the treatment time was 2 min	The specific surface area decreased with the number of deposited layers. The flame retardancy of the fiber improved.
Ramie	Immersive assembly, Spray assembly	APP and PEI were used, the concentration of both solutions was 0.9 wt%, the pH was 9, and the treatment time of Immersive process was 2 min. The spray gun distance was 55 cm.	The heat resistance of the fibers treated by the two methods improved, and the spray assembly had a better flame retardant modification effect than the immersive assembly.
Bagasse (Saccharum officinarum)	Immersive assembly	APP and CH were used, the concentration of both solutions was 0.5 wt%, and the treatment time was 10 min. The pH of chitosan solution was 3.5.	The flame retardant effect improved. The peak heat release rate and total heat release decreased by 64.6% and 13.2%, respectively after six-layermodification.
Sisal	Immersive assembly	CH and MMT were used, the concentration of CH solution was 0.2 wt %, the treatment time was 10 min, the concentration of MMT solution was about 0.5 wt %, and the treatment time was 30 min.	This process could significantly improve the flame retardant performance.
Rice (Oryza sativa)	Immersive assembly	The branched PEI and PAA were used. The concentration of the PEI solution was 0.5 wt %, the pH was 10, the concentration of PAA was 1 wt %, the pH was 3, and the treatment time was 3 min.	The heat resistance improved, and the mechanical properties of the prepared composite plates greatly improved.
Cellulose nanofibril	Immersive assembly	Branched PEI and PVPA were used, and the solution concentration was 0.1 wt%	This process could improve self-extinguishing and intumescent properties，not hamper optical transparency of the nanopapers.

Fully degradable composites

In recent years, due to the increasing maturity of the degradable matrix material preparation technology, the production cost has been declining year by year, and the research on the use of biomass fiber for enhancing the degradable matrix material has also received widespread attention. Typical degradable matrixes include PLA, PBS, PCL, and thermoplastic starch etc (Nurul Fazita et al. 2016). According to the latest statistics of the European Bioplastics Association, the global total output of bioplastics in 2016 was 4.156 million tons, and it will reach 6.111 million tons in 2021, while the biodegradable plastics will increase from 964,000 tons in 2016 to 1.26 million tons in 2021. Biodegradable plastics accounted for 23.1% of the total amount of bioplastics, and starch based plastics accounted for 10.3% of the total output; PLA, PBS, PBAT and PHA only account for 5.1%, 2.8%, 2.5% and 1.6% respectively (Zhu and Liu 2018).

In terms of fiber selection, at present, researches mainly focus on wood fiber, hemp fiber and bamboo fiber, and bagasse fiber, reed fiber and straw fiber are also studied. In addition, some other plant fibers are also concerned. In terms of the selection of degradable resin matrix, PLA resin has been studied more because of its superior comprehensive performance.

The research focus in this direction is mainly on fiber surface modification, interface binding regulation, synthesis and modification of degradable resin matrix, selection of fiber reinforcement methods, preparation process of composite materials, biodegradability, flame retardancy and heat resistance of composite materials (Mohanty, Misra, and Drzal 2002). At present, the bottleneck hindering the development of fully degradable composites is mainly the price of resin matrix and the brittleness of composites as a whole (Siakeng et al. 2019). Therefore, the low-cost and toughening modification of fully degradable composites are the current research and development focus.

Nanofiber composites

Recently, natural nanofiber materials have received widespread attention as a new material. The research highlights presented in this aspect primarily include: properties and applications of nanofibers, extraction and surface modification of nanofibers, processing methods of nanofiber composites, performance characterization of nanofiber composites, nanofiber reinforced biodegradable resin matrix, and the reinforcement effect of nanofibers in polymers etc (Hao et al. 2020; Zhu et al. 2020).

The key technologies in the field of nanofiber composites are the extraction of nanofiber reinforcements and the dispersion technology in the composites. In terms of the extraction of nanofiber reinforcement, currently, mechanical methods (high-pressure homogenization, micro jet or ultra-fine grinding methods), enzymatic hydrolysis or chemical pretreatment combined with mechanical methods are mainly used to peel the natural cellulose and reduce the fiber size. Among them, the mechanical method requires high energy consumption and has low enzymatic hydrolysis efficiency, while the nano cellulose obtained by chemical pretreatment combined with mechanical method has the advantages of small diameter, high transparency, large length diameter ratio and high elastic modulus, and is still the main production method at present. However, the catalysts used in this method are toxic and expensive, which is one of the main factors that restrict the production cost of nano cellulose to decrease significantly.

In terms of dispersion of nanofiber reinforcement, the current research focuses on chemical modification of nanofiber, addition of lubricant, improvement of mixing process, etc. (Mandeep, Praveen, and Santosh 2021; Sharma et al. 2019; Shi et al. 2021), among which the mixing process of composite materials plays an important role in the dispersion of nanofiber. The mixing process is mainly solution mixing and melt blending. The energy consumption of solution mixing is lower, and the dispersion is more uniform, but the performance of composite materials is lower and there is a problem of waste liquid treatment. However, the melt blending method has higher energy consumption and is more difficult to disperse fibers, the composites with good dispersion have higher performance and no waste liquid problem. In addition, in the process of preparing nanofiber composites, the directional arrangement of fibers has a significant effect on the properties of the composites. Figure 4 systematically summarized the common preparation methods for aligned nanofibers.

Figure 4. Preparation of aligned nanofibers. Reproduced with permission from the ref (Zhu et al. 2020).

3D printing of composites

Due to low production costs, 3D printing technology has also received extensive attention in natural fiber composites. The current researches focus on the preparation of filaments for printing, the form of fiber reinforcement, the improvements of the mechanical properties of composites after printing, and the development of printing equipment. The main matrix materials used are PLA and ABS. In terms of 3D printing process, the methods mainly include Fused Deposition Modeling (FDM), Laminated Object Manufacturing (LOM), Stereo Lithography Apparatus (SLA), and Selective Laser Sintering (SLS). The interface bonding of the composites prepared by LOM is good, but it can not be cured completely locally; The composites prepared by SLA show weak mechanical properties; SLS mainly has the disadvantages of long processing time and preheating operation. Nowadays, the FDM 3D printing technique is widely used in the manufacture of natural fiber composites, but a series of problems such as large pores between layers, poor fiber and matrix compatibility, and fiber feeding difficulties are expected to be resolved (Balla et al. 2019). In the preparation of composite filament, the current research focuses on the composite process of fiber and resin matrix, improvement of interface bonding, rheological control of composite system, mechanical properties of filament, etc. In terms of fiber reinforcement form, powder fiber reinforced resin is adopted by the vast majority at present. Although the printing process is simple, there is a problem that the mechanical properties are generally not high. Recently, to solve the problem of poor mechanical properties of composites, FDM 3D printing of continuous natural fiber/PLA composites has also been becoming a trend (Duigou et al. 2019; Zhang et al. 2020), as shown in Figure 5.

Figure 5. The production of 3D-printable continuous flax fiber reinforced plastic prepreg filaments. Adapted and reproduced with permission from the ref (Zhang et al. 2020).

Prediction of mechanical properties of composites

Due to the complexity of natural fibers and the diversity of preparation processes for composites, the use of experimental methods for performance measurement is time-consuming and laborious, while model prediction can quickly obtain the mechanical properties of composites, exhibiting significant advantages. The most commonly used prediction methods currently in published articles are the law of mixing, Halpin-Tsai and Kelly-Tyson empirical equations (Beckermann and Pickering 2009; Tham et al. 2019). Recently, it has been reported in some studies that, in response to the complexity of the natural fiber’s structure, it is proposed that attention should be paid to the properties of the components of the natural fiber, such as the properties of cellulose, hemicellulose, and lignin, so as to meet the high-precision requirements of performance prediction (Ciesielski et al. 2020). In response to the relatively high dispersibility of the mechanical properties of natural fibers, many scholars have employed the Weibull distribution model to characterize the tensile properties of the fibers (Andersons, Poriķe, and Spārniņš 2011). In addition, multiscale modeling methods have also been widely used to improve the accuracy of prediction of mechanical properties of natural fibers and composites (Sun et al. 2014), as shown in Figure 6.

Figure 6. Schematic diagram of multiscale modeling for mechanical properties of natural fiber. Adapted and reproduced with permission from the ref (Sun et al. 2014).

At present, the performance prediction mainly focuses on the prediction of tensile properties of composite materials, while the prediction of other properties is less. In the prediction of tensile properties, the prediction of tensile modulus is superior to the prediction of tensile strength in terms of both the quantity and accuracy of research. The prediction accuracy of longitudinal tensile modulus is better than that of transverse tensile modulus. The prediction accuracy of Poisson coefficient is good, but the prediction accuracy of shear modulus is poor (Potluri et al. 2018). It is worth noting that in the prediction of the tensile properties of natural fiber composites, the influence of interface bonding has not been paid enough attention. Most studies regard interface bonding as close bonding for performance prediction. This has a small deviation in the prediction of longitudinal modulus, but has a greater impact on the prediction of transverse modulus, elastic coefficient and tensile strength. In recent years, Li and other scholars have deeply predicted and analyzed the influence of interface bonding on the mechanical properties of sisal fiber composites, and discussed the interface load transfer mechanism in detail based on the multi-level interface parameters generated by the unique multi-level structure of sisal fiber (Li et al. 2020a, 2020b).

In addition, among the mechanical properties of natural fiber composites, the low impact property is a major drawback that hinders their wide application. Therefore, the improvement of impact performance and its performance prediction should also be focused. However, the current literature on prediction of impact properties is relatively limited (Abu Seman, Ahmad, and Md Akil 2019).

Frontier research

Frontier fields refer to strategic and key new technologies that may bring revolutionary influence or have significant application prospects to the development of natural fiber composites. For example, in terms of seawater desalination and energy conversion, Chen G. et al. (2019, 2021) prepare wood fiber membrane with high density structure, high strength and high ionic conductivity through chemical modification of natural wood fiber (Figure 7).

Figure 7. Wood fiber membrane with high strength and ionic conductivity. Reproduced with permission from the ref (Chen G. et al. 2019).

An efficient and expandable solar steam generator has been developed on the surface of graphite sprayed basswood, with an efficiency of about 80% (Li et al. 2018). In response to global warming and energy crisis, Li et al. (2019) report a high-performance structural material that can automatically cool down, the radiation cooling effect is as high as 10°C, and the specific strength is comparable to titanium alloy. In addition, a new transparent material was prepared based on the selective delignification and epoxy resin penetration process, which shows great potential in the application of energy-saving buildings (Mi et al. 2020). In terms of flexible electronics and optical devices, by removing lignin from wood in situ, and then cold pressing, Zhu et al. (2018) prepared isotropic transparent paper with transparency of about 90%, which is environmentally friendly and biodegradable, showing excellent application value in flexible electronics, optical devices and other fields. In terms of biomedicine and medical diagnosis, soft gel materials (such as hydrogels and ionic gel) prepared from fibrillar natural cellulose have good biocompatibility and can be used in a series of advanced bioengineering fields, such as wound dressings, tissue engineering, drug delivery, medical diagnosis, intelligent sensors and electronic skin, and have extremely important commercial value (Li et al. 2021). In addition, natural fiber conductive composites, bionic intelligent materials, screen display materials and other aspects have also received certain attention (Al-Oqla et al. 2015).

Industrialization of natural fiber composites and their applications

Natural fiber composite material has become one of the materials with the highest growth rate of demand among all composites in recent years. Industries are chiefly concentrated in the construction materials and automobile industries.

Wood-plastic composites with natural wood powder as the filling material are the natural fiber composites that has been studied earlier. This type of wood-plastic composite material is mainly used in building materials, such as outdoor decking, park seats, guardrails, interior decorative panels, and building formworks, etc. Many companies aim at the huge demand in the building materials market when they are starting to invest in the field of natural fiber composites. For example, Trex, US Plastic Lumber and AERT provide most of the products in the building materials market in the North America.

Natural fiber composites with bast fibers as reinforcements have a large number of applications in the automotive industry. Automobile manufacturers such as Mercedes-Benz, BMW, Audi, and Volkswagen have taken the forefront in the world in the application of natural fiber composites to automobile manufacturing. The main products include car door inner panels, luggage compartment, ceiling, seat back panels, coat racks, dashboards, etc. (Pickering, Efendy, and Le 2016). Due to the environmental protection standards in Europe, the requirements of lightweight cars and higher recyclability of materials, the European automotive industry’s demand for natural fiber has increased linearly in the past few years. Such demand will continue to increase substantially in the future.

Natural fiber composites also have certain applications in the shell of household appliances. The companies involved in this field mainly include Kareline Oy (Finland), NEC (Japan), the Biomaterials Research Center of Wageningen University (Netherlands) and JER Envirotech (Canada). The products mainly include mobile phone shells, laptop shells, and small electrical appliance shells (Nurul Fazita et al. 2016). Most of the resin matrixes used are common thermoplastics. One of the latest trends is the use of bio-resin matrix and natural fiber to fabricate shells of mobile phones and computers.

Another new application field of natural fiber composites is packaging materials. Almost all resins used in packaging materials are biodegradable resins, which are relatively expensive. The addition of natural fibers to the biodegradable resins can significantly reduce the cost of the materials, and can also increase the strength, rigidity and thermal deformation of the material without affecting the material degradability.

Due to the increasingly scarce global forest resources and the implementation of environmental protection policies, wood for musical instruments is increasingly scarce (Bucur 2016). Because of the structural similarity between natural fiber and wood fiber, the natural fiber composites are employed to fabricate musical instrument panels instead of the wood material, which has been attracting increasing attention recently. For example, Phillips and Lessard (2009, 2011) found that flax fiber composites can replace spruce to fabricate guitar panels by studying the application of flax fiber composites in guitar panels and comparing their properties with the properties of spruce wood. The American Blackbird company uses natural fiber mat and biomass resin to fabricate guitar panels, which have more advantages over traditional wooden panels in terms of temperature and humidity and achieve satisfactory acoustic properties.

Conclusions

High-performance, high-value-added natural fiber composites will be an important orientation of the global industrialization market. Natural fiber composites with bast fiber as reinforcement have higher mechanical properties and have important applications in the automotive industry and the aviation industry. In addition, the use of natural fiber composites to replace wood materials to make musical instrument panels and electrical appliance shells has higher added value, which has been increasingly attracted attention recently.

In the future, the supply stability and performance stability of natural fiber need to be guaranteed. It is also necessary to conduct in-depth investigations on the status quo of the industrialization of high-performance and high-value-added products of natural fiber composites, further refine the key core technologies for the industrialization, deploy high-tech in advance, promote the industrialization of key technologies, and promote the green, intelligent and high-performance manufacture of such composites.

Highlights

• Global market of natural fiber composites analyzed.

• Research highlights of natural fiber composites reviewed in detail.

• Frontier research of natural fiber composites discussed.

• Applications of natural fiber composites summarized.

• Prospects of natural fiber composites pointed out.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Natural Science Foundation of Hebei Province [E2018208161]; Project of Hebei University of Science and Technology [2021YWF18].