Introduction

With the swift price ascent of petroleum based materials and serious threat to the environment, it has become a hot issue to cut down the use of plastic and initiate an environment-friendly society. (Osong et al. 2015) Petroleum products carry plenty of performance defects, whose case in point is poor tensile strength and dimensional stability placing restrictions as a sheet material on many fields. For the sake of solving the problem, researchers are now motivated to discover new alternatives. BF (Structure diagram is shown in Fig. 1) consisting of 73.8% cellulose, 12.5% hemicellulose, 10.1% lignin, 0.4% pectin and 3.2% water has the strong points of highly renewable, cheap, rapid growth, light weight, low density and high stiffness, which can place synthetic materials such as glass fiber in certain applications. (Bogoeva-Gaceva et al. 2007; Chaowana et al. 2013; Ku et al. 2011) BF reinforced polymer composites not only improve dimensional stability and mechanical strength but also diminish dissipation and protect the environment. (Zhang et al. 2018a, b) On this account, BF reinforced polymer composites are integrated into myriad industries including household, transport and composite manufacturing, to name a few. The products based on natural materials and polymers are competent to decrease the use of non renewable resources and become an ideal development direction. (Abdul Khalil et al. 2015; Abdul Khali et al. 2012; Hong et al. 2018; Besi et al. 2013; Saba et al.2015; Sánchez et al.2019).

Fig. 1
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reproduced with permission from John Wiley and Sons

The structure diagram of BFs.( Li et al. 2020) Fig. 1

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Recent work over the past several decades focused on diverse type of composites with regard to BF has been reported, the improvement of which is summarized in Table 1. The principal matter of BF reinforced polymers is that the BF is hydrophilic but the matrix is hydrophobic, which result in the incompatible interface, further leading to the uneven dispersion of fiber in matrix and poor interfacial adhesion. Hence, it’s extraordinary noteworthy that interfacial adhesion becomes an essential factor to improve mechanical properties. (Zakikhani et al. 2014) With respect to the mechanism of integrating BF and plastic, it can be separate into mechanical engagement mechanism, reaction binding mechanism and dissolution infiltration mechanism. (Orue et al. 2016; Rokbi et al. 2011; Srivabut et al. 2018 Li et al. 2013; Gu et al. 2009) When one surface comes into contact with the other, mechanical interlock will occur due to rough surface, which is the reaction mechanism of mechanical engagement. For the most part, the surface of BF is treated by various means to be attached with groove structures, which can better combine with the substrate in the form of friction force  (Zhang et al. 2018a, b; Cai et al. 2016) Furthermore, the superficial area magnifies with the accretion of coarseness, but there are still considerable holes that viscous liquid is unable to flow into, giving rise to interface debonding and stress concentration points. The principle of reactive bonding is that chemical interaction occurs between the components of the composite and chemical bond is formed at the interface, so strong interfacial bonding energy can be obtained theoretically. The chemical binding mechanism is mainly segmented into two cases. On the one hand, through a material that can act as a bridge reacts with polymer and BF chemically, at the same time, so as to improve the binding force between BF and polymer. On the other hand, one substance reacts with hydroxyl groups on BF to abate the number of hydroxyl groups thus improving the interfacial strength. The mechanism of dissolution and infiltration is based on the infiltration affinity of liquid resin on the surface of reinforcement material, namely physical and chemical adsorption, which will not be given unnecessary details in this paper.

Table 1 Different modification methods and corresponding reinforcement effect of BF
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Mechanical mechanism improves interface strength

The poor combination of BF and polymer leads to stress concentration, for which there are two mechanical ways to settle the dispute, as adding filler or physical structure in the composite so that the stress transfer or shift occurs under the action of external force to ameliorate the mechanical strength of compound material. The scheme of mechanical mechanism improving interface strength is shown in Fig. 2. Last but not least, the addition of physical structure contributes to the regularly distribution of fibers for restrictions on the movement space of fibers.

Fig. 2
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Diagrammatic sketch of mechanical mechanism improving interface strength

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Filling treatment

In the interest of improving the interface compatibility, the fillers with outstanding mechanical properties are generally selected. If the fillers are evenly decentralized in the matrix, the fillers will crack and absorb energy under the action of external force, so as to debase the frequency of stress concentration.( Sliwa et al. 2012)Thus far, researchers have added materials such as halloysite nanotubes(HNT), (Smith et al. 2018; Ye et al. 2020; Liu et al. 2020; Abdullaha et al. 2019; Jin et al. 2020; Chee et al. 2021; Vahabi et al. 2018) nano silica,( Zuo et al. 2020; Zhou et al. 2020; Bino et al. 2020; Sinha et al.2020; Zhou et al. 2020) nano clay, (Adamu et al. 2020a, b; Chee et al. 2020a, b; Han et al. 2008; Liew et al. 2018; Patel et al. 2018; Velmurugan et al. 2008) calcium carbonate, (Essabir et al. 2017; Sugiman et al. 2020; Wang et al.2019; Wang et al. 2017a, b; ; Cheng et al. 2015) graphene oxide (Chen et al. 2017; Lu et al. 2016; Reale Batista et al. 2020; Wang et al. 2018; Wu et al. 2019; Zhang et al. 2017; Zhang et al. 2019) into the composites with the aim of improving the properties.

Jin et al. (2020) proposed a novel method to prepare flame retardant, as shown in Fig. 3a. HNT was added to polylactic acid (PLA) and BF surfaces under the action of positively charged chitosan (CS) and negatively charged tannic acid (TA) to prepare BF (60–80 mesh size)/PLA composites (BPC) to enhance thermal stability. It is because that the CS / TA complex forms a dense carbon sheet and HNT owns brilliant heat insulation ability during early combustion, as shown in Fig. 3b, which showed a flame retardant mechanism of free radical combined with gas phase and condensation. The composition of the composite is shown in Table 2. This is a highly meaningful research at present because the fire situation of residents in our country is endless. If it can be made into fireproof materials, (Guo et al. 2021) it will greatly reduce the frequency of large-scale fire. Respectively, Zuo and his coworkers (2020) proposed the crystallization process of composites with the same matrix material by extrusion of the mixture at 160 °C with a roller speed of 80 rpm for 10 min, as shown in Fig. 4c. After adding 1.5% nano silica, the interfacial compatibility reaches the optimal state and gets the best moisture resistance, bending and tensile strength, as shown in Fig. 4a b. A slight variation on the mechanism is that the improvement of mechanical properties of the former is via the large aspect ratio of HNT to transfer stress. The latter is that the stress concentration effect will be produced around the dispersed nanoparticles in the composite when subjected to external force. And then cracks will be generated and energy will be absorbed.

Fig. 3
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reproduced with permission from John Wiley and Sons. (Jin et al. 2020)

a Preparation of flame retardant. b Diagrammatic sketch of combustion mechanism about composites. Figure 3

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Table 2 Composition and content table of samples. Table 2
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Fig. 4
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reproduced with permission from Elsevier. (Zuo et al. 2020)

a Influence of various nano silica contents on contact angle and water-absorption quality of composites. b Influence of various nano silica contents on mechanical properties of composites. c,crystallization process of polylactic acid. Figure 4

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In 2011, Kushwaha et al. (2011) made the conclusion that the tensile and flexural properties of epoxy resin / bamboo mat composites with montmorillonite were promoted by pressing for 24 h under a load of 170 KN. In 2012, V. Kumar et al. (2012) conducted in-depth research on bamboo epoxy hybrid composites with nano clay content of 0-10wt% in the hydraulic press under a pressure of 180 KN for 24 h. The composite materials filled with 3% nano clay increased from 80 and 6400 MPa to 112 MPa and 6825 MPa in terms of tensile strength and modulus. For 1wt% nano clay, the maximum impact strength was 60.43 kJ / m2. Slightly different from the results, Kim et al. (2012) discussed calcium carbonate strengthened BF and polyethylene / polypropylene composites prepared by twin-screw extruder. Compared with those without precipitated calcium carbonate (PCC), the tensile modulus and bending modulus were slightly increased, but the impact strength was decreased, as shown in Table 3. Subsequently, Adamu et al. (2019) reported the improvement of elastic modulus and fracture modulus of bamboo / polyvinyl alcohol / clay nanocomposites vacuum impregnated at 80 °C. In 2020, Chee et al. 2020a, b) made an expansion to compare the effects of nano clay and halloysite nanotubes on improving the tensile, bending, thickness swelling (shown in Fig. 5,a)and impact properties of complex material comprised of bamboo mat (B), woven kenaf (K) and epoxy. During preparation, the nanoclay/epoxy was subjected to a high shear speed of 10,000 RPM using a homogenizer and degassed after mixing under ice water for 30 min. And then the nano clay/epoxy /bamboo fiber mixture was cured at room temperature for 24 h, and then cured at 105 °C for 5 h. The results indicate that the organic modified montmorillonite (OMMT) reinforced hybrid nanocomposites reveal the best mechanical properties among all hybrid nanocomposites, followed by montmorillonite (MMT) and halloysite nanotubes.

Table 3 Mechanical performance of bamboo/PP/PE composites regarded to different content of PCC. Table 3
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Fig. 5
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reproduced with permission from Elsevier. (Chee et al. 2020a, b; Tolera et al. 2021)

a Thickness swelling of various composites. bThe storage modulus of composite formed by 4 layers of woven glass fiber (GGGG), 2 layers of woven glass fiber and 2 layers of garded ensete web (GGEE), and glass/ ensete /ensete/glass multi-layer (GEEG) and 4 layers of garded ensete web (EEEE) reinforced unsaturated polyester hybrid composites. Figure 5

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Wang and his co-workers (2020) studied graphene oxide reinforced BF/polypropylene composites. The preparation process is shown in Fig. 6a. When the content of graphene oxide is 0.05, 0.1wt%, graphene oxide forms more hydrogen bonds with mercerized BF, whose scanning electron microscopy (SEM) images is shown in Fig. 6d and e. Due to its special surface properties, the interfacial force is increased compared with the sample without graphene oxide, whose SEM images is shown in Fig. 6b and c. The tensile strength of untreated BF / polypropylene composites hiked from 31.9 to 35.9 MPa (increased by 12.6%) and the bending strength hiked from 42.9 to 53.1 MPa (an increase of 23.7%) after treatment, as shown in Fig. 6f, g and graphene oxide reinforced composites have achieved certain results. But due to the high price, we hold the view that there is little potential to be used in the market. Quite the opposite, in the light of saving energy and recycling resources, Jena et al.(2018) filled BF reinforced epoxy composites with industrial waste hollow beads by conventional hand lay-up technique at room temperature, which reduced the erosion wear rate. Ren et al. (2016) added a small amount of antioxidants to BF / maleated polypropylene composites, (Panaitescu et al.2015) increasing the thermal oxidation resistance of the composites with no significant negative impact on their properties. We think that in order to realize the recycling of waste, it is necessary to fill with industrial waste instead of some expensive raw materials on the basis of improving the properties of the composite.

Fig. 6
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reproduced with permission from John Wiley and Sons. (Wang et al. 2020a, b)

a Diagrammatic sketch of the preparation about the graphene oxide (GO) grafted onto alkali-treated BF (GO-ABF). SEM images of b, c, untreated BF, d, e 0.05, 0.1GO-ABF. f, g, tensile, flexural tests of various composites. 0.05, 0.1 and 0.3GO-ABF based on the GO concentrations of 0.05, 0.1 and 0.3 wt% respectively. Figure 6

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Reforming physical structure treatment

Most researchers focused on the modification of BF with coupling agent, ignoring the effect of reforming physical structure on improving the properties of composites. Some researchers established the finite element model and preliminarily confirmed that improving the physical structure can be deemed an adequate means to elevate the mechanical properties of composites. Subsequently, a growing quantity of researchers have directed energies to improve the physical structure. (Penellum et al.2018; Md Shah et al.2021).

In 2018, Zhao et al. (2018) proposed a new integrated material model with bamboo culm acting as skeleton, which approached the issue of irregular dimensions and hollow structure of bamboo. The finite element model proved that the bearing capacity of bamboo laminated material was 6 times lower than that with skeleton. After that, Gu et al. (2018) produced multilayer composites by film stacking method (Sharmin et al.2016) and prepared a composite of bamboo viscose fiber and maleic anhydride grafted polypropylene, whose flexural strength can reach 52.93 MPa. By stacking the multi-layer films, the fiber can be combined preferably with polypropylene matrix, heightening the surface strength. Negawo et al. (2021) found that different stacking ways of the same kind of fiber reinforced composites are diverse from each other in mechanical properties by vacuum-assisted resin transfer molding technique. Citing a case, the tensile properties of glass, ensete, ensete, glass sequence samples are about 7.5% higher than those of glass, glass, ensete, ensete sequence samples, as displayed in Fig. 5b. Consequently, different stacking methods of the same material will also engender a greater impact on the physical properties.(Aruchamy et al. 2020)Researchers should collect all kinds of physical properties parameters of various fibers including BF, and carry out mechanical property simulation analysis through software component finite element model, so as to explore the influence of different physical structures on the performance and improve the research efficiency.

Chemical mechanism improves interface strength

The reaction mechanism is that substance giving first place to coupling agents, one of which most frequently is silane, (Bogoeva-Gaceva et al. 2007; Abdelmouleh et al.2004; Abdelmouleh et al. 2005; Bengtsson et al. 2006; Fei et al. 2017) reacts with the group of BF to change physical properties. The interface compatibility is enhanced by coupling agents mainly via the reaction between functional groups possessed by coupling agent and hydroxyl group possessed by BF or entanglement with molecular chain of matrix.

Silane coupling agent treatment

In consideration of the low price, accessibility and high efficiency improvement, silane coupling agent has been selected such as aminoethyl-aminopropyl-trimethoxysilne (Kim et al. 2012) demonstrated in Fig. 7. 3-aminopropyl triethoxysilane is adopted to graft BF, (Panaitescu et al. 2016) which is hydrolyzed in ethanol solution to form silicon hydroxyl (Si–OH). (Gao et al. 2012) And then the hydroxyl on the surface of BF reacts with the hydrolyzed hydroxyl to form Si–O-C bond, as shown in Fig. 7. The bending strength of the untreated BF reinforced polypropylene composite by pressing of 4 MPa for 10 min at 180 °C is 26% lower than that treated by 3-aminopropyltriethoxysilane when content is 3%, which is more superior than that reported by Lee et al. (2009) (53.1 MPa). (Wang et al. 2020a, b) Liew et al. (2020) also obtained good results that Young's modulus of the composite treated by amino silane treatment and being placed in an electric hot press set at 180 °C and 3.45 MPa for 45 min can be increased to 6 times than that of the untreated composite. And Kushwaha (2010) trialed six different silane coupling agents to treat mercerized orthogonal bamboo strip mats (4.25*0.5 mm2) for the exact purpose of decreasing water absorption of composite materials. Aminopropyltriethoxysilane gets the best waterproof effectiveness among them. Meanwhile, two different diffusion behaviors were simulated and two water diffusion models based on Fick's theory were obtained. It provides a certain reference for the selection of coupling agent to solve the moisture absorption problem of composites reinforced by BF.

Fig. 7
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modified by typical methods (Kim et al. 2012; Dinesh et al. 2019; Liew et al. 2020; Kushwaha et al. 2010; Sun et al. 2016)

Reaction scheme of BF

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Other coupling agent treatment

Based on the reaction mechanism of hydroxyl group with carboxyl group, hydroxyl group, benzyl group and other functional groups, coupling agents other than silane coupling agents are also opted. The anhydride in functional ethylene propylene rubber forms carboxyl group under certain conditions that reacts with functional group of BF, by which esterification and hydrogen bonding endow it with interfacial binding force, as analyzed in Fig. 7. (Dinesh et al. 2019) The coupling effect of different content of methylacrylamide on bamboo strips/expoxy has been investigated by hot pressing. (Kushwaha et al. 2010) It is proved that when the coupling agent content is 1%, the coupling effect reaches the best and the decomposition temperature is also improved. Compared with the former methods, a more efficacious and environmental technology has been discovered. (Chen et al. 2020) A polyepoxy soybean oil interface layer, which is generated by in-situ polymerization of epoxy soybean oil monomer, connected with the BF and PLA matrix by hot compression is formed, enhancing the interfacial adhesion, as a consequence of which, the elongation at break of the composite increased by 110.1% with the addition of 0.5% epoxy soybean oil. The graft copolymerization of benzyl group can also enhance the compatibility between BF and medium density polyethylene, whose reaction scheme is displayed in Fig. 7. Sun et al. (2016) get the results that the mechanical strength of benzylated BF (40 ~ 60mesh particle size) / medium density polyethylene made at 120 °C under a pressure of 2 MPa for 3 min is higher than that of untreated BF / medium density polyethylene at the same mass fraction. The copolymerization grafted by benzyl group can also enhance the compatibility between BF and medium density polyethylene. As shown in Fig. 8, the C/O atomic ratio increased from 0.86 to 2.87 with representative SEM micrographs of untreated and benzylated bamboo flour, which further explains that the grafting degree of the benzyl groups was significantly enhanced. Jin compared three different kinds of coupling agents, i.e., γ—methoxylpyrimethylsilane (KH570), ethanediolato titanate (HY311) and distearoyl isoproxy aluminum (HYA1). (Jin et al. 2019) The results showed that the order of coupling agents for improving the interfacial compatibility was (KH570) > (HY311) > (HYA1). It is worth noting that the mechanical properties of the composites reach the best level at the content of 2%.

Fig. 8
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reproduced with permission from John Wiley and Sons

SEM micrographs: a, b, untreated bamboo flour (× 50), (× 800), c, d, benzylated bamboo flour (× 50), (× 800). e, energy dispersive X-ray spectra of unmodified bamboo flour (UBF) and benzylated bamboo flour (BBF). (Sun et al. 2016) Fig. 8

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Physical and chemical reactions take place simultaneously to improve interface strength

The polarity of BF and the non polarity of plastic make the interface compatibility poor, causing inferior performance of composite. For one thing, the fibers can be treated by physical methods to acquire groove and remove wax, greasy ingredients and lignin in BF can be partially removed to increase potential reaction sites, such as alkali treatment, plasma treatment and so on. For another, it can reduce hydroxyl and reduce polarity of BF, as shown in Fig. 9.

Fig. 9
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modified by physical and chemical reactions simultaneously

Diagrammatic sketch of interface of BF and matrix

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Alkali and acid treatment

After alkali treatment, the wax and greasy ingredients in bamboo are supposed to be removed so that the cross-linked ester bond between xylan and lignin will be destroyed to change the lignin content, increasing the porosity and potential reaction sites, as shown in Fig. 7. Simultaneously, lessening the number of hydroxyl group and surface tension of bamboo strips could be instrumental in increasing the effective contact zone between BF and matrix, promoting the even spreading of resin on bamboo strips. Suwan et al. (2020) treated the waste bamboo with potassium hydroxide and carried out the elemental chemistry analysis and scanning electron microscope imaging, as shown in Fig. 10 d, e. It turned out that the content of carbon made a reduction and oxygen augmented after the reaction, fully confirming that alkali treatment changed the surface of the original waste bamboo. Sánchez et al. (2019) also conducted atomic force microscopy images of fiber, as shown in Fig. 10 a, b. For these reasons, alkali treatment is considered to be the most common method to improve BFs.(Ku et al. 2011; Teli et al. 2018).

Fig. 10
figure 10

reproduced with permission from Elsevier. (Sánchez et al. 2019; Suwan et al. 2020)

a, b, c Atomic force microscopy images of untreated treatment, alkaline treatment, plasma treatment of BF. d, e SEM of untreated bamboo, alkaline treated bamboo. f, characteristic absorption bands. Figure 10

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Das and his coworkers (2006) soaked bamboo strips (100*15* (1.1–1.5) mm) in different concentrations of sodium hydroxide solution. When the fiber filling amount was constant at 25%, the mechanical properties enhanced with the increase of mercerization degree until the alkali concentration was up to 20%. The reinforcement material treated with 16–20% concentration of caustic soda obtained the maximum performance improvement, and the elastic modulus reached 2200 MPa. When the alkali concentration was more than 20%, all the properties on the mechanical side failed to get refined. Of equal significance, Sánchez et al. (2019) improved mechanical behavior of Guadua fiber, which is soaked in water for three days, then crushed by mechanical method, and immersed in borax and boric acid salts at a concentration of 3%, reinforced composites after alkali treatment. Their crystallinity index and average roughness were 3.3 and 114.3% higher than those of untreated fibers. Alkali treatment was used to work in coordination with other measures. For instance, Li et al. (2020) softened hollow bamboo with high pressure steam, immersed bulk bamboo in boiling aqueous solution of NaOH and Na2SO3 for 12 h and then densified by hot pressing. The densified bamboo also showed 310% increase in tensile modulus, which was significantly higher than other reported materials such as natural polymers, plastics, steel and alloys. It can be made into furniture such as bed board and wardrobe and a series of high-density products after antibacterial treatment. Due to the excessive damage of fibers caused by various treatment methods, K. venkata et al. carried out a work to treat fibers with suitable concentrations of NaOH, NaHCO3and silane and found that the other two had better modification effect than NaHCO3 (Venkata Chalapathi et al. 2020).

There are also some works on the modification of BFs with organic acid. For example, Zhu et al (2021) grafted natural rosin acid onto the bamboo powder after hydrothermal pretreatment reinforced biomass matrix by esterification reaction, which significantly improved the specific surface area. And Fajardo et al. (2021) prepared bamboo flour (250 μm) derived from manufacturing product residue /polypropylene composites by treating BFs with organic acid as tricarboxylic acid, hexadecanoic acid and dodecanoic acid respectively, increasing about 190% in tensile strength all due to the esterification of acid rosin in a heterogeneous system after removing the lignin and hemicllulose, which is summarized in Table 1. These works opened up a green and feasible way to prepare bio based materials with ideal thermoplastic properties.

Plasma treatment

As one of the common means to modify the interfacial compatibility, plasma treatment is an ionized gas comprised of positive, negative and neutral particles. The surface characteristics of the material will be changed by charged particles after material is exposed to the gas along with reacting chemically. No matter what type of plasma (argon, methane, etc.) (Fiorelli et al. 2011; Scalici et al. 2016; Sinha et al. 2009; Omar Faruka et al. 2012) and time length of treatment and particular conditions, not only the roughness of the fiber can be changed, but also the hydrophobicity. For example, Sánchez et al. monitored the morphology of the fibers under plasma treatment, as shown in Fig. 10 a, c. (2019).

Due to the plasma bombardment, the molecular bonds on the surface of BFs will be destroyed, resulting in considerable number of breaks, pits and small particles on the surface of BFs. Xu et al. (2006)used natural BFs with a fineness of 6.01 dtex and a length of 70–160 mm to be treated with argon gas. This kind of defect is about 30–50 nm deep, with the diameter of 100–200 nm. Sánchez et al. (2021) reported that the mechanical strength of the panel, pressing at room temperature made of unidirectional fibers, extracted by mechanical means and treated by plasma is superior to that treated by 5% alkali and its Young's modulus is in compliance with 6439 MPa, which is 83% higher than that of alkali treatment. What's slightly flawed is that the graphite/bamboo/ polyimide composites treated by plasma show low friction coefficient. (Sun et al. 2018).

Ozone treatment

Recently, the high reactivity of ozone treatment promotes the removal of lignin for modifying BFs without damaging the cellulose of the material. (Pujokaroni et al. 2020; Mamleeva et al. 2021) Although ozone treatment is rarely applied in the study of surface modification of fibers, whose several advantages involving the efficiency of methods for removing impurities and low waste, because the ozone is decomposed in the process or destroyed in terminal process. (García-Cubero et al. 2010; Sever et al. 2011) It turned out that the tensile strength the treated BF were more excellent than that of the untreated BF, and the tensile strength raised from 571 to 599 MPa.(Sánchez et al. 2021).

Other methods improve interface strength

It is reported that there are considerable number of other methods to improve the compatibility of BF and matrix. Adding maleic anhydride (Chattopadhyay et al. 2011) and polyester amide polyol (Saxena et al. 2003) as typical compatibilizers in the composite will improve the mechanical performance. Guan et al. (2013) have manufactured toughness BF reinforced phenolic resin modified by poly (vinyl alcohol) with different contents (5, 10 and 20%) with a pressure of 2 MPa at 140 °C. A new repair mechanism explored by fluorescence characterization that compared with rigid phenolic resin, toughness phenolic resin adhesive machine has changed from traditional pure mechanical locking to automatic repair of interface sliding and cross winding has been proposed. What’s similar but more novel, Hong et al. (2018) proposed an eco-friendly method that can modify all natural fibers to improve the interfacial compatibility. Polypolyamine (PDA) and octadecylamine (ODA) can be firmly attached to the fiber (40–60 mesh) surface, which provides abundant hydrophobic long-chain alkyl. The reaction mechanism and preparation process is shown in the Fig. 11. It can significantly reduce the free energy and increase the roughness by coating on BF surface, which directly promotes the mechanical properties of PDA/ODA/BF/polybutylene succinate matrix. (Song et al. 2016; Meng et al. 2017) By attaching hydrophobic groups to improve the hydrophobic ability of BF, the self-cleaning ability of BF can be effectively improved.

Fig. 11
figure 11

modified by PDA coating and ODA grafting. Figure 11 reproduced with permission from Shuangbao Zhang. (Hong et al. 2018)

The process of BFs

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Application

The composite made of BF reinforced plastics is a good decoration material. Due to the unique hollow structure of BF, BF/polyurethane composite has a broad prospect in the preparation of composite materials to absorb odor and reduce the content of volatile organic compounds. (Ning et al. 2020) With its light weight and high strength, under the condition of good interfacial, it can also be used to prepare sports goods and electronic and electrical products, such as snowboard, golf club, computer shell, etc. (Sanjay et al. 2016) BF is tightly wrapped by polymer, including hydroxyl, so composite has the advantages of small shrinkage and distortion, waterproof, insect resistance, mildew resistance and excellent engineering properties, which is a pretty good light material in the automotive industry. In recent years, a number of automotive BF composites have been developed and applied all over the world. At present, polypropylene and other thermoplastic resin reinforced by BF gets good interfacial adhesion. Automobile door panel, coat rack, ceiling, back-up cabin side panel and other automotive interior materials prepared by non-woven process and hot pressing process have been mature in industrial application. (Zhang 2020) BF reinforced plastics composites are also widely used in construction, transportation, light industry and other fields. Unidirectional continuous BF reinforced thermosetting composites have excellent mechanical properties, which have the potential to replace glass fiber reinforced resin composites in wind turbine blade materials, highway fence materials, ship materials and other fields.(Liu et al. 2012) We think BF reinforced composites can be used in the fields of flexible electronic sensors, such as motion detection system including sweat glucose detection system, and aerospace, such as landing gear and seat in the future. As one of the substrates of flexible sensors, elastomer often faces the problem of low tensile strength, which limits its use. If the problem of uniform distribution of BF in the substrate can be solved, its tensile strength can be effectively enhanced.

Conclusion

In this review, we emphasize that a feasible way to modify the interface between BF and polymer is plasma treatment on the score of short period, high efficiency, convenient operation and clean process. Alkali treatment is suitable for industrial production to deal with mass of BF. On this basis, the coupling agents and nano fillers are also more common. However, too many fillers will lead to high possibility of stress caused by uneven distribution in the composite. Especially graphene oxide, of whose drawbacks are expensive and not suitable for mass production and market launch. A significant challenge in composites is the distribution of fibers, which has not yet been solved absolutely, leading a consequence of the poor performance. A way to distribute the fibers evenly to be invented should be called for, as weaving the fiber into a certain kind frame structure. We suggest that a series of physical structures can be extracted or constructed from waste materials, which can helpfully enhance the mechanical performance of hybrid compounds, so as to save resources and recycle.