Research progress on basalt ﬁ ber - based functionalized composites

: Basalt ﬁ ber ( BF ) is a kind of high - performance ﬁ ber rising rapidly in recent years. BF is typically used in the ﬁ eld of structure engineering because of its high strength and high modulus. The preparation of BF - based composites ﬁ rst requires surface modi ﬁ cation of BF to improve the interfacial bonding between BF and the resin matrix. With the continuous deepening of the research on BF surface modi ﬁ cation, researchers have found that spe - cial surface modi ﬁ cation can obtain BF - based functiona - lized composites, and this ﬁ eld has received extensive attention in recent years. In this article, research work on BF - based functional composites in recent years are summarized and reviewed from the aspects of electro - magnetic shielding, water treatment, catalytic function and ﬁ re insulation. Finally, this article summarizes the BF surface modi ﬁ cation methods, and proposes the development trends and direction of BF - based functional composites.


Introduction
Fiber-reinforced polymer materials (FRPs) are a large branch of the field of composites. FRPs can be applied in many different fields based on various combinations of fibers and resin matrices [1][2][3]. In recent years, with the improvement of people's awareness of environmental protection, the greening of FRPs has gradually attracted the attention of researchers. For example, new types of bio-based fibers have been developed as they meet the performance requirements of composites to a certain extent [4][5][6]. Unfortunately, the poor weather resistance and narrow temperature range of bio-based fibers greatly limit their application in many scenarios. Therefore, another fiber material with excellent performance and environmental friendly production process, basalt fiber (BF), has entered the sight of researchers.
BF is made of natural volcanic rock-basalt, which is a high-performance fiber that has emerged in recent years. BF is not only environmentally friendly, but also harmless to life. Therefore, it can be combined with degradable polymers to prepare completely environmental friendly composites [7]. At the same time, BF has a wide temperature range, good weather resistance and excellent corrosion resistance. Based on the above advantages, BF-reinforced metal composites have been developed [8]. In addition, BF is also widely used to reinforce engineering plastics [9,10] and concrete [11,12] due to its excellent strength and modulus. Even in many studies, BF is used together with other fibers to prepare FRPs in order to obtain better properties [13,14]. Comparing all kinds of FRPs comprehensively, BF-based FRP shows better cost performance, especially compared with glass fiber [15][16][17].
When preparing FRP, BF needs to be surface modified in order to improve the interfacial bonding between BF and resin matrix. Many surface modification methods have been developed, such as surface coupling agent modification [18,19], surface coating [20], surface nanoparticle loading [9,21] and surface etching [22]. During the surface modification process, the researchers found that the functionalization of composites could be realized by special modification of BF. For example, the conductivity of BF can be improved by a new designed sizing agent system [23]. Similar modifications can transform BF from a single structural material into a functional material, thus greatly broadening the use scenarios of BF, and increasing the application value of the BF.
There have been many literature reviews on BF, which are basically focused on the surface modification methods of BF, the preparation and properties of BFbased composites. In recent years, the functionalization of BF-based composites has become a research hotspot, but there is no review report on this aspect so far. In this article, recent research work on preparation of BF-based functional composites by BF modification is summarized. The latest research progresses of BF-based functional composites in electromagnetic function [24,25], water treatment [26], catalytic reduction [27], fire insulation [28,29], sensing detection [30], antibacterial [31] and other aspects are reviewed ( Figure 1). Finally, the research trends of BF-based functional composites are discussed and prospected.

Novel electromagnetic function materials
Electromagnetic shielding and adsorbing materials are research hotspots in the field of electromagnetics. Many novel shielding or absorption materials have been continuously reported, and BF-based functionalized composites have many brilliant achievements in this field. Mittal and Rhee [37] prepared a BF-based composite for electromagnetic shielding. They first supported nickel catalysts on the surface of BF fabric, and then grew carbon nanotubes (CNTs) on the fabric surface by chemical vapor  [26], (c) catalytic reduction [34], (d) fireproof [35] and (e) other applications [31,36].
deposition (CVD). Finally, they prepared CNTs modified BF fabric-based epoxy resin composite laminates by hand lay-up method. The results showed that the growth temperature of CNTs is positively correlated with the conductivity and shielding effectiveness of the laminated material. Based on the above work, Chang et al. [33] grew CNTs on the BF surface without additional catalyst. The study revealed that BF undergoes crystallization and forms nanoparticles on its surface that can catalyze CNTs growth when temperature reaches a critical point [38,39] (Figure 2). Meanwhile, the study also showed that the hydrogen flow rate would significantly affect the electromagnetic shielding effectiveness of laminated material. Yu et al. [40] sprayed MXene solution onto BF fabric and prepared BF fabric-based rubber composite. The composite also exhibited excellent electromagnetic shielding performance.
In addition to the above modification techniques for BF surface, some researchers have also used other methods to improve the electromagnetic shielding performance of BF-based composites. For instance, Liu et al. [41] directly filled graphene and graphite powder into BF-based composite, and when their contents were 10 and 20%, respectively, the composite had the best shielding performance. BF-based functionalized composites have also been applied in the field of electromagnetic wave adsorption. Some researchers have grown Fe 3 O 4 nanoparticles on the surface of BF by solvothermal method, which has made BF fabric composite laminates exhibit excellent microwave adsorption capacity [42]. Similar to the above study, He et al. [43] grew BaTiO 3 nanoparticles on the surface of BF fabrics and prepared composite laminates with poly (ether sulfone) as the matrix. Compared with the direct mixing of BaTiO 3 nanoparticles into the matrix, the surface-modified BF fabric-based laminate has better adsorbing properties [44]. This phenomenon explained that BF has the skeleton of the loaded nanoparticles, which can promote the uniform dispersion of nanoparticles in the matrix, thereby greatly improving the utilization efficiency of nanoparticles.
Other studies have also confirmed the above conclusions on improving the utilization efficiency of nanoparticles by BF. For example, Mittal and Rhee [45] loaded CNTs on the surface of BF and prepared corresponding composite. Compared with the directly dispersing CNTs in the resin matrix, the former has significantly higher utilization efficiency of CNTs ( Figure 3I). In the case of adding nanofillers directly to the matrix, the properties of the composites changed significantly only when the filler content exceeded the percolation threshold. In addition, Chen et al. [46] reported that the conductivity of BF-based composites increased only when the content of reduced graphene oxide (rGO) was higher than 0.24 wt% of the epoxy resin matrix ( Figure 3II).
From the above contents, we can conclude that BFbased composites have been studied extensively in the field of electromagnetic shielding and absorption. BF plays two main roles in these composites. On the one hand, functional nanoparticles can be uniformly dispersed in the resin matrix with BF as a skeleton. On the other hand, the BF skeleton can promote the formation of connected pathways between the nanoparticles, thereby improving the utilization efficiency of nanoparticles. Therefore, BF skeleton provides a new idea for the structure design of advanced electromagnetic shielding or absorbing materials.

BF-based composites for water treatment
The earliest report of basalt-based water treatment materials can be traced back to 2010 by Kwon et al. [47]. They reported the treatment of wastewater containing heavy metal ions through the porous structure of basalt. Since then, research on the application of modified BF in water treatment has gradually emerged [48]. Rong et al. [49] grew MnO 2 nanosheets on the surface of BF fabric by hydrothermal method, and then coated with stearic acid to prepare oil-water separation fabric. The filtration fabric was tested for separation efficiency in a variety of oil-water mixing systems, and its optimal separation efficiency was as high as 97.2%. Meanwhile, the fabric was also endowed with ice resistance due to its hydrophobic surface. Yang et al. [26] also prepared an oil-water separation fabric. They first grew a layer of Co(OH) 2 nanosheet on the surface of BF fabric, and then coated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) by impregnation. The separation efficiency of this separation fabric reached 99.9% (Figure 4), and its separation efficiency for oil-in-water and water-inoil emulsion systems could reach 99.3%. In addition to these hydrophobic BF-based filter materials, there are also hydrophilic BF-based filter materials. For example, Cai et al. [50] grafted konjac glucomannan on the surface of BF to obtain a hydrophilic BF fabric, which is resistant to acid, alkali and organic solution, and can be used in harsh environments; meanwhile, Zhang et al. [51] coated polyvinyl alcohol on the surface of BF to obtain hydrophilic BF fabric, which still has excellent acid and alkali resistance, and also has self-cleaning ability, especially outstanding is that it also obtained more than 600 L·m −2 ·h −1 water treatment capacity. The detection, adsorption and degradation of harmful substances in water are other issues that need to be studied in the field of water treatment. Many researchers have delivered a lot of reports in this area, such as the detection of polycyclic aromatic hydrocarbons [52,53] and estrogens [54] in water, and the adsorption of phosphate [55] and bacteria [56] in water. Inspired by the above studies, some researchers have exploited catalytic performance of BF, and they expected BF can directly catalytic degradation of pollutants in water. For instance, ZnO nanorods grown on the surface of BF can degrade methylene blue in water [57], TiO 2 nanoparticles loaded on the surface of BF can photocatalytically degrade methyl orange [58,59], methyl red [60], ammonia-nitrogen [61,62] and so on. SiO 2 and some other metal oxides are the main components of BF, and the content of iron oxides (Fe 2 O 3 and FeO) are about 10 wt% [16,63]. It was reported that iron oxides in BF can be used to catalyze methane decomposition [64]. However, this catalytic reaction requires a high temperature, and the catalytic activity will decrease with the increase of carbon deposition. The catalytic activity of BF itself is very limited; therefore, loading nanomaterials on BF surface is the most common method to improve its catalytic performance. For example, Pd nanoparticles were loaded to catalytic oxidate phenylcarbinol [65], and Ag nanoparticles were loaded to catalyze the Suzuki-Miyaura coupling reactions [66], etc.
Reducing CO 2 into usable chemicals is considered an effective way to tackle the greenhouse effect. Kwak et al. [34] proposed to use oxides such as SiO 2 , MgO and CaO in BF to assist in the absorption of CO 2 , and then converted CO 2 to CO by ZnO and other nano-oxides loaded on the BF surface. However, the conversion efficiency was very low, only 5 μmol·g cat ·L −1 . In another report [68], researchers loaded TiO 2 crystal doped with different metal ions (Fe, Co, Ni and Cu) onto the surface of BF. They then mixed modified BF, cellulose and other materials to create a photocatalytically capable film. The CO 2 conversion efficiency of the film was as high as 360.5 μmol·g cat −1 ·L −1 (the transformation process of CO 2 is shown in Figure 5).

BF-based composites for fireproof and thermal insulation
BF is an excellent fire insulation material due to its extremely low thermal conductivity and thermal stability [69,70].
Yasir et al. [71] mixed BF, expandable graphite and epoxy resin, and then coated the mixture on the surface of the steel plate. The firing test shows that the coating can effectively prevent the temperature rise of steel plate. Thus, it could inhibit the failure of steel materials caused by high temperature. Rybinski et al. [72] prepared CNTs and BF synergistically reinforced ethylene-propylene-diene monomer composite. The results showed that carbon nanofillers could improve the thermal conductivity of the material and avoid local heat accumulation of the composite. At the same time, the flame-retardant properties of the composite were also improved due to the nonflammability of BF. Yang et al. [35] also used CNTs and BF to prepare flame retardant polylactic acid (PLA) composite. They blended BF, acidified CNTs, aluminum hypophosphite and PLA to form a composite plate. The limiting oxygen index (LOI) of the composite was evaluated, and its LOI increased to 31.0%, reaching V-0 level. In some fields, the flame retardancy and mechanical strength of materials are required at the same time. Xu et al. [70] prepared flame-retardant composites by coextrusion of Sb 2 O 3 nanoparticles, BF and polybutylene terephthalate. After testing, the flame retardancy of the sample reached V-0 level, and its LOI increased from 21.8 to 33.3%. Moreover, the tensile strength, Young's modulus and impact strength of composites increased by about 15, 60 and 20%, respectively.

BF-based composites for other applications
The service environment and external loads of composites can cause cracks and other defects. Therefore, it is necessary to develop composites with self-monitoring function. Wang et al. [36] prepared conductive BF/epoxy composites by mixing carbon nanofibers, epoxy resin and BF together. The percolation network based on carbon nanofibers brings self-monitoring function to composites. However, the amount of nanofibers needed to form the conductive network in this composite is high, so the nanofiber utilization efficiency of this method is low.
Hao et al. [73] deposited a layer of pyrolytic carbon on BF surface by CVD method, and then prepared epoxy resin composites for tensile text. The resistivity of the composite changed with the increase of the load, and they demonstrated the potential of modified fiber in self-monitoring composite by a simple model. Piezoelectric materials were also used in BF-based composites to give them sensing functions. For instance, a novel BF sizing agent could be prepared by mixing piezoelectric function BaTiO 3 nanocrystals into epoxy resin. This sizing agent not only endowed the composite with sensing and energy harvesting properties, but also improved the interlaminar shear strength of the composite [74]. Furthermore, BF can also be seen in antibacterial materials [75]. To solve the problem of bacterial growth in water supply pipes, Liu et al. [31] prepared modified BF/high-density polyethylene (HDPE) antimicrobial composites. They grafted silver ion glass micro-beads onto the surfaces of BF and blended with HDPE. Antibacterial experiments showed that when the amount of glass beads grafted was more than 1.5 wt%, the composite could almost completely inhibit the growth of Escherichia coli. In a similar study, Saleh et al. [76] pressed basalt powder into a thin film and then grafted ZnO nanorods on it. The final result showed that the antibacterial rate of the modified film against E. coli was 100%.
In addition to the above functional materials, BF is also used in many other functional materials, such as BFbased anti-corrosion materials [77][78][79][80], thermal conductivity materials [81,82], damping materials [83,84], absorbing materials [85], etc. As a high-performance fiber, BF not only has high strength and high modulus, but also can prepare various functional composites after surface modification. In the future, BF-based composites will definitely become a research hotspot in the field of composites.

Conclusion and prospect
As a new type of high-performance fiber, BF is regarded as a good substitute for carbon fiber and glass fiber by many researchers in the field of structure engineering [17,23]. While BF surface modification is used to enhance the mechanical strength of composites, more functional properties of composites have also attracted people's attention. Among many BF-based functionalized materials, part of them utilized the intrinsic properties of BF, such as catalysis, adsorption, flame retardant and thermal insulation functional materials. Indeed, more BF-based functional materials are obtained by surface modification of BF, such as electromagnetic shielding and water treatment functional materials. Among many surface modification methods, nano-scale modification is the first choice due to its nano-effect. Meanwhile, the micro-nano hierarchical structure can be obtained by surface modification of micro-sized BF with nanomaterials, which can effectively improve the utilization efficiency of nanomaterials [33,37,40]. In future study, new methods for BF surface modification will surely be developed continuously. The application of BF in more new functionalized materials will emerge. For examples, the authors believe that BF has great potential for applications such as marine anti-corrosion materials and UVresistant materials based on its high weatherability and corrosion resistance. At the same time, based on the strength of BF, we also hope to obtain new functional materials with a balance between special functions and mechanical properties.
Acknowledgments: The authors would like to acknowledge financial support from the Major Science and Technology Project of Sichuan Province. Author contributions: Wencan Tao: writingoriginal draft, writingreview and editing; Bin Wang: writingreview and editing, writingtranslating; Nuoxin Wang: writing review and editing; Yifan Guo: writingreview and editing; Jinyang Li: writingreview and editing; Zuowan Zhou: writingreview and editing. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest:
The authors state no conflict of interest.