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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2023 , Vol. 35 >Issue 12: 1783 - 1792

DOI: https://doi.org/10.7536/PC230501

Review

Fabric Coatings Based on Silicon Oxide Structure with both Flame-Retardant and Hydrophobic Properties

  • Xinchao Li 1 ,
  • Changlin Xia 2 ,
  • Mingjun Chen , 1, * ,
  • Ting Wang 1, 3 ,
  • Zhicheng Fu 1 ,
  • Jinni Deng , 1, *
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  • 1 School of Science, Xihua University,Chengdu 610039, China
  • 2 Yibin Hiest Fibre Limited Corporation,Yibin 644002, China
  • 3 Hubei Three Gorges Laboratory,Yichang 443000, China
*Corresponding author e-mail: (Jinni Deng);
(Mingjun Chen)

Received date: 2023-05-04

  Revised date: 2023-08-03

  Online published: 2023-09-10

Supported by

Natural Science Foundation of Sichuan Province for Outstanding Youth(2023NSFSC1955)

Science and Technology Program of Yibin(2022GY001)

West Light Talent Program of the Chinese Academy of Sciences(2022~2024)

Tianfu Qingcheng Plan of Sichuan Province(2019~2023)

Open and Innovative Fund of Hubei Three Gorges Laboratory(SK213005)

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Abstract

The flame retardancy of coatings on fabrics would always be destroyed and drastically reduced by daily use or routine maintenance because of their hydrophility. So functional coatings with both flame retardancy and hydrophobicity have become a research focus in fabric field. Coatings of silicon oxide compound with high heat resistance and low surface energy have presented good flame retardancy and hydrophobicity. In this paper, the latest research progress about fabric coatings with both excellent flame retardant and hydrophobic properties is described progressively by organosilicon, organosilicon/nano-silicon and polyhedral oligomeric silsesquioxane (POSS) system on the aspects of char formation, low surface energy, micro-nano structure and controllable multi-functionalization. The relationship between the structure of silicon oxide compound and the properties of flame-retardancy and hydrophobicity is deeply investigated. Finally, synergetic mechanism of flame-retardancy and hydrophobicity, enhancement of functional efficiency and service stability of coatings in complex environment are put forward as the future development of fabric coatings with both flame-retardancy and hydrophobicity. According to the requirements of some application scenarios of functional fabric materials, the hot spots are analyzed and prospected.

Contents

1 Introduction

2 Research progress

2.1 Organosilicon compounds

2.2 Organosilicon/nano-silicon

2.3 Polyhedral oligomeric silsesquioxane(POSS)

3 Conclusion and outlook

Key words： silicon oxide compound; flame-retardancy and hydrophobicity; micro-nano structure; controllable multi-functionalization

Cite this article

Xinchao Li , Changlin Xia , Mingjun Chen , Ting Wang , Zhicheng Fu , Jinni Deng . Fabric Coatings Based on Silicon Oxide Structure with both Flame-Retardant and Hydrophobic Properties[J]. Progress in Chemistry, 2023 , 35(12) : 1783 -1792 . DOI: 10.7536/PC230501

1 Introduction

With the continuous development of coating technology, flame retardant coating, as one of the most widely used functional coatings, has attracted wide attention of researchers[1]. Since Gay published the first monograph on systematic research and introduction of flame retardant technology in 1821, the research on flame retardant coatings has made great progress in high performance and high efficiency[2]. However, most flame retardants often contain hydrophilic ions or hydrophilic groups, especially in the field of textile applications, daily use and cleaning maintenance are easy to cause potential safety hazards of flame retardant coating failure[3,4][5]. Therefore, scholars at home and abroad have designed and modified flame retardants to prolong the service time of flame retardant coatings by endowing flame retardants with hydrophobicity.
In recent years, there has been a great deal of interest in the study of fabrics coated with silica to achieve good flame retardant properties[6⇓⇓~9]. It was found that the flame retardancy of silicon-oxygen system compounds, on the one hand, comes from the high bond energy of silicon-oxygen bond and good thermal degradation stability[10]; On the other hand, the silicon compound at high temperature can generate a carbon layer containing silicon (mainly silicon dioxide), which effectively prevents the escape of combustible substances, plays a role in heat insulation and oxygen insulation, and finally achieves the effects of flame retardancy, smoke suppression and low toxicity. At the same time, silicon-oxygen compounds are usually non-ionic, do not contain hydrophilic groups, have low surface energy, and can be crosslinked to form an ordered three-dimensional structure during reaction[11]. Moreover, nano-silica can construct a micro-nano rough structure on the surface, and obtain a super-hydrophobic functional surface through the synergistic effect of low surface energy and the micro-nano structure[12][13]. Therefore, the design and construction of the structure and composition of the new silicon-oxygen system plays a vital role in the development of coatings with both flame retardant and hydrophobic properties.
Therefore, in this paper, the flame retardant and hydrophobic functionalization of different structural silicone systems in fabric coatings were studied from three aspects: traditional silicone system, silicone/nano-silica hybrid system and polyhedral oligomeric silsesquioxane (POSS) system.The research progress of silicon-oxygen system flame retardant and hydrophobic bifunctional coating for fabric surface was introduced, and the possible development direction and prospect in this field in the future were put forward.

2 Research Progress

2.1 Traditional organosilicon system

Hydrophobic flame retardants of traditional organosilicon system are a kind of polymer hydrophobic flame retardants with organosilane as the main chain in chemical structure. Due to the strong bonding ability and high bond energy of siloxane and silicon-carbon bonds, polysiloxane compounds usually have low surface energy, thus showing good hydrophobic properties[14]. At the same time, polysiloxane can be decomposed into silicon dioxide by heating to form an inorganic thermal insulation layer, so the thermal stability of the coating material can be greatly improved.
Chen et al. Designed and synthesized a polyvinyl siloxane (PVS) structure (Scheme 1) with vinyltrimethoxysilane, and obtained a new hydrophobic coating for cotton fabrics by solution impregnation[15]. Due to the good thermal stability provided by the silica main chain structure, the carbon residue of the treated fabric increased from 1. 53% to 20. 14% compared with the raw cotton fabric at 700 ℃ in the air atmosphere in the thermogravimetric test. The water contact angle (WCA) of the fabric surface is 123 ° due to the low surface energy provided by the silicon-containing main chain and the methyl group of the side chain. On this basis, they used the synthesized PVS macromolecule as the raw material and added inorganic particle nano-titanium dioxide (nano-TiO2) as the crosslinking agent (Fig. 2)[16]. The addition of nano-TiO2 caused an increase in the surface roughness of the coating, and the enhanced WCA reached 158 °.Compared with the PVS film, the heat resistance of the composite film obtained by adding nano-TiO2 was improved.
图式1 PVS聚合物的合成[15]

Scheme 1 Synthesis of PVS[15]

图式2 nano-TiO2@PVS涂层的交联反应路线图[16]

Scheme 2 The crosslinking and reacting process of nano-TiO2@PVS coating[16]

Although the silicon-oxygen main chain can cooperate with the carbonization of fiber fabric in the thermal degradation process, thus improving the thermal stability of the coating material, the excellent flame retardancy is still difficult to achieve through a single silicon-oxygen bond. Therefore, in order to further improve the flame retardant performance, the simplest and most direct way is to add other kinds of flame retardants to the system. Intumescent flame retardants such as ammonium polyphosphate (APP) are widely used as additive flame retardants, which can play a synergistic effect of gas phase and condensed phase mechanisms, and have the characteristics of high flame retardant efficiency, low environmental pollution and low smoke content[17,18].
For example, Sun et al. Used the hydrolysis of vinyltrimethoxysilane (Figure 3) and montmorillonite (MMT) reaction to construct an organosiloxane network, in which APP was dispersed, and then directly impregnated to obtain cotton fabrics with hydrophobic and flame-retardant surfaces[19]. The limiting oxygen index (LOI) was found to be 28.3% and the WCA was 130.2 °. Based on the study of the effect of organosilicon network structure on the flame retardant and hydrophobic properties of coatings, they blended another organosilicon compound, methyltrimethoxysilane, with aluminum diethyl phosphinate and MMT to prepare composite coatings[20]. The results show that the coating on the fabric surface still has good hydrophobicity and flame retardancy: WCA reaches 120.3 °; Total heat release (THR) decreased by 85% in microcalorimetry (MCC) test; In the vertical burning test, the carbon residue was 4.5 inches long.
图式3 硅氧烷网络的反应路线图[19]

Scheme 3 Reaction route of siloxane network[19]

In addition to the introduction of flame retardants, the introduction of fluorine-containing groups with low surface energy can also further enhance the hydrophobic properties of silicone coating systems. Yu et al. Constructed a fluorine-containing silane system with hydrophobic flame retardant function by introducing ethyl 2-cyanoacrylate and 1H, 1 H, 2H, 2H-perfluorooctyltrichlorosilane[21]. Then APP and fluorosilane were fixed on the surface of cotton fabric by impregnation and spraying respectively to prepare functional coatings for fabrics with excellent hydrophobic and flame retardant properties. Among them, APP has intumescent flame retardant effect, which can cooperate with the original fluorosilane to form carbon, so that the LOI of cotton fabric is as high as 31.1%. At the same time, the fluorosilane can provide effective hydrophobic protection of the APP surface, so that the WCA of the fabric surface is as high as 161 °.
Because of the synergistic flame retardant effect of nitrogen and phosphorus in APP, some researchers directly introduced nitrogen, phosphorus and other elements into the structure of organosilicon compounds through chemical reactions. Compared with the blending of APP, this chemical bond connection has a more homogeneous structure, which can effectively improve the flame retardant stability of coatings. Just as Lu et al. Synthesized nitrogen-containing heterocyclic aliphatic ether-terminated linear polysiloxane (PNPDMS) (Figure 4) by chemical reaction of 4-bromobutoxy-terminated polydimethylsiloxane (PDMS), phosphorus oxychloride and piperazine, and prepared flame-retardant hydrophobic cotton fabrics by direct impregnation[22]. Among them, the polymer skeleton containing nitrogen, phosphorus and silicon elements of PNPDMS provides good flame retardancy, and its LOI reaches 29.82% in the combustion test; The polysiloxane structure endows the fabric surface with good hydrophobic property, and the WCA of the fabric reaches 141.9 degrees. Similarly, they synthesized a phosphorus-containing and nitrogen-containing organic polysilane (PNCTSi) with a shape structure by potassium cis-tetramethylcyclotetrasiloxane, phosphorus oxychloride, methyltriethoxysilane, and piperazine (Figure 5)[23]. It was attached to the fabric by impregnation, and the shape network structure further improved the hydrophobic properties of the fabric surface: LOI was 29. 8%, WCA was 150 °.
图式4 PNPDMS的化学结构[22]

Scheme 4 Chemical formula of PNPDMS[22]

图式5 PNCTSi的化学结构[23]

Scheme 5 Chemical formula of PNCTSi[23]

It can be seen from the above study that the flame retardant mechanism of organosiloxane compounds is to form a composite layer of silicon oxygen and silicon carbon under high temperature, thus playing the role of heat insulation, oxygen insulation and preventing harmful gases from escaping. However, the flame retardant efficiency of this kind of flame retardant based on condensed phase mechanism is closely related to the quality and density of the carbon layer formed, and it is difficult to form a dense enough carbon layer only by using organosiloxane. At the same time, in order to improve the hydrophobic properties, especially to achieve superhydrophobic properties, in addition to relying on more low-surface-energy substances to endow the coating, the construction of surface micro-nano structures is essential according to the Wenzel model and Cassie model mechanism of superhydrophobic surface theory[24,25].

2.2 Organosilicon/nano-silica hybrid system

Scholars at home and abroad have taken the study of the surface structure of lotus leaf as a starting point to carry out more in-depth research on the construction mechanism of its surface superhydrophobicity and self-cleaning properties[26]. The results show that the combination of micro-nano structure and low surface energy material is a better choice for the construction of superhydrophobic surface. Surface micro-nano structures can be formed by incompatible multiphase mixing and microphase separation in the same substance (due to the incompatibility between polar, hydrophilic and hydrophobic molecules)[27⇓⇓~30]. Therefore, silicone/nano-silica hybrid system has received more attention as a new hydrophobic flame retardant coating system in the silicone coating system.
Organosilicon/nano-silica hybrid system is usually used to prepare organic-inorganic hybrid coatings by blending, self-assembly or in-situ reaction of organosiloxane after grafting modification of nano-silica in order to achieve good dispersion and functionalization of nanoparticles. At the same time, inorganic nano-silica shows certain advantages in hydrophobic and flame retardant properties: on the one hand, it shows the high heat resistance of inorganic compounds[31]. Inorganic nano-silica has better environmental safety than halogen, phosphorus and nitrogen flame retardants, and can form a silica protective layer on the surface of the polymer during combustion.At that same time, it can enhance the strength and compactness of the carbon lay, thus inhibiting the decomposition of the material, the release of combustible material and the transfer of heat, thus playing the role of condensed phase flame retardant to stop combustion[32,33]. On the other hand, highly dispersed silica nanoparticles can endow the coating surface with micro-nano rough structure, thus improving the hydrophobic properties of the coating surface to a greater extent[34,35]. Moreover, due to the differences in polarity and solubility of organic-inorganic systems, it is easy to produce micro-phase separation and form multi-scale micro-nano structures, which can further enhance the hydrophobicity of the surface.
For example, Wang et al. Used 3-aminoethylpropyl (diethoxy) methylsilane to modify and synthesize amino silica microspheres, then grafted 1H, 1H, 2H, 2H-perfluorodecyl trimethoxysilane on the surface, and finally connected with the surface of the fabric modified by epoxy compounds to construct a fabric with nano-roughness on the surface[36]. The nano-film covered by it plays a role in blocking heat and mass transfer in the process of combustion and thermal degradation, and the LOI reaches 20.9% in the combustion test. At the same time, the micro-nano rough structure provided by the silica sphere and the low surface energy of the fluorinated siloxane make the WCA as high as 160 °.
The introduction of fluorine to improve the hydrophobicity of the coating and nitrogen, phosphorus and other elements to enhance the flame retardancy are still widely used by researchers. High efficiency flame retardants containing nitrogen, phosphorus and other elements as blends can also play an excellent flame retardant effect in the coating[37,38]. In addition, the fluorine-containing structure has obvious advantages in improving the hydrophobic performance of the coating due to its low polarizability, strong electronegativity and low surface characteristics[39].
Zeng et al. Synthesized F-SiO2 with tetraethoxysilane and perfluorooctyltriethoxysilane, then added polyethyleneimine and APP flame retardant, and used PDMS for curing and crosslinking to prepare hydrophobic flame retardant coating[40]. The addition of F-SiO2 further enhances the flame retardancy of the coated fabric. In the MCC test, the heat release rate (HRR) and THR of the coated fabric are 50 W/G and 7.2 kJ/G, respectively. In the vertical burning test, the sample was self-extinguished immediately after the flame source was removed, and the charred length was only 8 cm. At that same time, the fluorine element in the nanoparticle of the F-SiO2 provides low surface energy, which is matched with the rough structure of the nanoparticle, so that the coating sample has super-hydrophobicity, and the WCA of the coating sample is great than 150 degrees.
Similarly, Guo et al. Synthesized a silicone hydrophobic flame retardant macromolecule using trimethylolpropane triacrylate, vinyltriethoxysilane, 1H, 1H, 2H, 2H-perfluorododecyl mercaptan, and 2-hydroxy-2-methylpropophenone as raw materials.9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and nano-SiO2 were added into the solution, and the hydrophobic and flame-retardant coatings were synthesized on different substrates by spraying and UV curing[41]. The flame retardancy is provided by DOPO, and the cloth treated with the coating burns slowly in the burning test of the alcohol lamp. Simultaneously fluorinated polymer chains and nano-SiO2 particles dispersed in a highly cross-linked network provide a low surface energy and a rough surface, ultimately endowing the fabric with excellent hydrophobicity with a WCA of 159 °.
Coincidentally, Xing et al. Immersed the fabric in a P, P-diphenyl-N- (3- (triethoxysilyl) propyl) phosphonamide (DPTES) solution synthesized from triethanolamine, dipalmitoylphosphatidylcholine, and 3-aminopropyltriethoxysilane to obtain a flame retardant coating[6]; Silica nanospheres obtained by catalyzing tetraethoxysilane by ammonium hydroxide are solidified on PDMS, and finally the silica nanospheres are coated on the surface of the flame-retardant treated cotton fabric to obtain a C-PDMS-SiO2 coating. The dehydration and carbonization of phosphorus-containing compound DPTES and the synergistic effect of siloxane and silica form a dense carbon layer on the surface of the fabric, thus realizing condensed phase flame retardancy. In which that silica ceramic lay in the carbon layer block heat transfer and inhibits the release of combustible gas products. The LOI of the treated fabric sample was 26%. The rough structure provided by nanosilica makes the coating surface superhydrophobic with a WCA of 154 °.
However, it is difficult to control the blending uniformity in the construction of hydrophobic and flame-retardant bifunctional coating system by the single coating method of blending system. Therefore, some researchers have used the multi-layer method to achieve the flame retardant and hydrophobic dual functionalization of the fabric surface. Yu and Du et al. Impregnated cotton fabrics with polyoxy-terminated silane phosphate polyurethane (SPPU)[42]. Based on the strong adhesion of polyurethane materials, a stable SPPU cotton fabric was made (Fig. 6). Then the mixture of tetraethoxysilane, methyltriethoxysilane and PDMS was sprayed on the surface of SPPU-C. Finally, it was placed in an ammonia chamber for catalytic treatment, and silica nanoparticles (SNPs) were grown in situ (Fig. 7) to obtain super-hydrophobic flame-retardant cotton fabric. Among them, the WCA of the fabric surface is as high as 160 °, and the LOI of the burning test is 28.1%.
图式6 SPPU聚合物的合成路线[42]

Scheme 6 Synthesis of SPPU[42]

图式7 SNPs粒子的制备路线[42]

Scheme 7 Synthesis of SNPs[42]

Multilayer method often requires strong bonding between layers to improve the service stability of the whole coating. Therefore, in order to further improve the interaction between functional coatings and achieve the stability of functional coatings, charge or hydrogen bonding between layers can be constructed to achieve more stable multi-functionalization of coatings[43⇓~45]. Zeng et al. Prepared DOPO-SiO2 particles (Scheme 8) using vinyltriethoxysilane, tetraethoxysilane and DOPO and dispersed them in a solution with added phytic acid (PA) and 1- [3- (trimethoxysilyl) propyl] urea (UPTMS)[46]. SFR-CF hydrophobic flame retardant coating was finally obtained by electrostatic interaction between PA and UPTMS. The phosphorus-containing and nitrogen-containing structures in PA and UPTMS cooperate with the silicon-oxygen structure to provide excellent flame retardancy. In the vertical burning experiment, the fabric self-extinguished immediately after the flame source was removed, and its coke length was only 5. 2 cm. At the same time, the low surface energy and micro-nano structure provided by the SiO2 particles promote the WCA of the fabric to reach 155.9 degrees.
图式8 DOPO-SiO2粒子的制备路线[46]

Scheme 8 Synthesis ofDOPO-Si2[46]

Similarly, Guo et al. Used the charge and hydrogen bonding between chitosan (CS) and PA to form polyelectrolyte complex (PEC), and then deposited HMDS-SiO2 particles synthesized by tetraethoxysilane and hexamethyldiphenylamine on the surface to prepare hydrophobic flame retardant coating[47]. The flame retardancy of the coating comes from the compact carbon layer formed by PEC, which hinders the transmission of oxygen and heat, and from the high heat resistance of the HMDS-SiO2 particles. In the vertical flame burning test, the coated fabric self-extinguishes 5 s after the flame source is removed. Furthermore, the HMDS-SiO2 creates a micro-nano structure on the surface of the fabric, and at the same time, with the low surface energy characteristics of siloxane, the coated fabric obtains super-hydrophobicity, and its WCA reaches 150 degrees.
In addition, the sol-gel method is also an important method for the preparation of organic-inorganic homogeneous hybrid coating systems under mild conditions. By controlling the blending components in the solution state, the uniform dispersion in the molecular scale and the stable network skeleton structure in the gel state are realized, so that the coating material with excellent flame retardancy and hydrophobicity is obtained[48]. Zeng et al. Synthesized PDMS-SiO2 particles by using tetraethoxysilane and hydroxyl-terminated PDMS, and then added APP flame retardant to prepare hydrophobic flame retardant coating by sol-gel method[49]. APP plays a major role in flame retardancy, and the PDMS-SiO2structure decomposes into stable inorganic SiO2 and carbon-silicon compounds at high temperature, which effectively inhibits the release of heat and flammable volatiles. In the MCC test, the peak heat release rate (PHRR) and THR of the fabric decreased by 69% and 67%, respectively, compared with the original fabric. In vertical combustion, the coke is 8.5 cm long and self-extinguished immediately after the flame source is removed; At the same time, the micro-nano structure was successfully constructed on the surface of cotton fabric by combining PDMS-SiO2 with APP, and the WCA of the coating surface was as high as 162 °.
It can be seen that the organic-inorganic hybrid silica system can provide good thermal degradation resistance at high temperature, reduce the surface energy, and endow the surface with micro-nano structure, thus realizing the dual functionalization of flame retardant and hydrophobic coatings. However, due to the complex composition of the system and many influencing factors, the stability of the flame retardant and hydrophobic functions needs to be further controlled and strengthened. So how to construct flame retardancy and hydrophobicity with stable performance and controllable structure in organic-inorganic silica system? Researchers at home and abroad have focused their research on polyhedral oligomeric silsesquioxane (POSS) systems with graftable functional groups and organic-inorganic hybrid structures.

2.3 Cage polysilsesquioxane (POSS) system

POSS is a typical organic-inorganic hybrid structure in siloxane compounds. In the system, the inorganic skeleton of siloxane can provide high heat resistance, and POSS can form a dense silicon oxide film layer at high temperature, which can effectively prevent the escape of small molecules and molten droplets generated after the cracking of the polymer, and at the same time prevent oxygen and heat from being transferred to the inside of the polymer, thus improving the flame retardancy of the polymer material[50][51][52,53]. Moreover, the regularly arranged and enriched silicon-oxygen bonds make the POSS nano-three-dimensional structure relatively stable and controllable[54]; At the same time, other functional groups (such as hydrophobic, flame retardant and other groups) can be introduced into the R group on the outer side to realize the controllability of the overall structure and realize more functions such as self-cleaning, self-repairing, oil-water separation and the like. Therefore, it is a hot research field to obtain hydrophobic and flame-retardant coatings by POSS modification.
For example, Sun et al. Synthesized an F-POSS from 1H, 1H, 2H, 2H-perfluorodecanethiol, 2,2-dimethylolpropionic acid, and vinyl-POSS[55]. F-POSS can make the coating superhydrophobic (WCA up to 160 °), and when the superhydrophobic layer is destroyed, the fluorosilane grafted on F-POSS has mobility to realize the self-repair of superhydrophobicity. After that, the hydrophobic flame retardant coating with three-layer structure was obtained by successive deposition of branched polyethyleneimine (b-PEI), APP and F-POSS on the surface of the fabric. The F-POSS/APP/b-PEI coating makes the cotton fabric flame-retardant by creating a protective carbon layer. In the vertical burning test, the flame was suppressed 4 s after ignition and automatically extinguished after removal of the flame source, with a scorch length of only 10 cm, while the top of the fabric remained largely intact. At the same time, the micro-nano structure constructed by F-POSS can withstand more than 1000 times of friction (pressure 44. 8 kPa), and maintain its flame retardant and self-healing properties.
Dai et al. Designed a polymer containing fluorine and epoxy groups, poly (glycidyl methyl methacrylate-dodecyl heptafluoroethyl methacrylate), named P (GM-co-DM) (Scheme 9)[56]. Then the amino side group of the prepared octa (aminophenyl) silsesquioxane (OAPS) (Fig. 10) was crosslinked with the epoxy group of P (GM-co-DM), and finally a stable and dense coating was formed on the surface of the DOPO-pretreated cotton fabric. The rigid cage-like nanostructure of OAPS is composed of a stable silicon-oxygen framework, which endows OAPS with excellent heat resistance. In the MCC test, its HR decreased from 13.4 kJ/G to 12.0 kJ/G. The WCA in the hydrophobic test was 154.8 °. At the same time, the treated fabric can be washed by simple water to realize surface self-cleaning; After soaking in different solvents for 96 H, it can still maintain a water contact angle of 140 ° and has good chemical stability. The water/chloroform mixture can be separated quickly, and the separation rate is still more than 94% after 100 cycles. The fabric exhibits excellent and stable multifunctional characteristics.
图式9 P(GM-co-DM)的合成路线[56]

Scheme 9 Synthesis of P(GM-co-DM) [56]

图式10 OAPS的合成路线[56]

Scheme 10 Synthesis of OAPS [56]

Similarly, Hu et al. Synthesized the A-POSS structure using 3-aminopropyltriethoxysilane, and then complexed with phytic acid (PA) to obtain the A-POSS-PA coating (Figure 11)[57]. Then they used two-step spraying method to coat A-POSS-PA and TiO2@PDMS on the fabric surface to complete the construction of hydrophobic flame retardant coating. The introduction of A-POSS-PA improved the flame retardancy of the fabric, and the phosphorus-containing compound PA accelerated the dehydration and carbonization of the cotton matrix. Silicon-containing A-POSS can produce ceramic barrier on cotton to protect the formed carbon and cooperate with flame retardant. TiO2 can be used as a co-catalyst for phosphorus-containing PA. In the vertical burning test, the treated fabric was self-extinguished immediately after the ignition source was removed, and the carbon residue length was 7. 1 cm. The PHRR of the sample fabric was found to be 70% lower than that of the original fabric by WCC. The LOI of the coating reached 28% due to the ion complexation between the amino group introduced in A-POSS and the acidic group of PA, while the later introduction of PDMS and TiO2 did not affect it. At the same time, the siloxane structure provides a lower surface energy, and the WCA of the coating surface reaches 153 °.
图式11 A-POSS-PA的合成路线[57]

Scheme 11 Synthesis of A-POSS-PA[57]

It can be seen that the POSS system can endow the coating with stable micro-nano structure and excellent heat resistance due to its unique cage structure and rigid silicon-oxygen skeleton. At the same time, the R group which is easy to graft can realize functional modification to realize the stable and multifunctional characteristics of the coating. However, the problems of low yield and difficult separation and purification of POSS materials limit its further development, especially for large-scale applications in the industrial field, more innovations and breakthroughs are needed in the preparation technology.

3 Conclusion and prospect

Silicon-oxygen system compounds have become the main research direction of flame retardant and hydrophobic bifunctional coatings for textiles because of their good thermal insulation and low surface energy. In particular to an organic-inorganic hybrid system, wherein the combination of organic silicon and inorganic silicon dioxide can effectively control the heat resistance of the coating and construct a rough structure with micro-nano size on the surface, thereby further improving the flame retardancy and hydrophobicity of the coating; POSS system can perfectly integrate organic and inorganic silicon systems, and its structure is controllable, and it is easy to achieve functionalization through grafting, so it has also attracted the attention of researchers nowadays.
However, there are still many difficulties in the application of silicone system compounds in the functionalization of flame retardant and hydrophobic fabrics, because: (1) whether there is an inhibition or promotion interaction between flame retardant and hydrophobic functional groups in the same system.The mechanism research is still lack of sufficient experimental data support, and the exploration of this scientific problem needs to be supplemented and improved. (2) How to minimize the amount of coating materials in the system to achieve high flame retardant and strong hydrophobic effect is an urgent technical problem to be solved; (3) As a functional coating material, how to improve its adhesion with fabrics or other substrates, and how to stably and lastingly display flame retardant and hydrophobic functions are the application problems faced by functional coatings today.
Therefore, based on the problems of silicon-oxygen system in fabric coating, combined with the development process of hydrophobic flame retardant coating, three research directions of silicon-oxygen compound in fabric coating can be proposed: mechanism expansion, performance improvement, and service stability.
In recent years, the research on single hydrophobic or flame retardant mechanism has been very mature, so it is an inevitable and potential direction to explore the synergistic mechanism between hydrophobic and flame retardant in functional coatings[58,59]. For example, Zhang et al. Treated cotton fabric with chitosan/sodium polyborate polyelectrolyte composite aqueous solution to obtain a flame retardant layer, and then used PDMS tetrahydrofuran solution to construct the superhydrophobic property of cotton fabric surface[60]. Although the methyl group of PDMS increases the heat release rate structurally, the coverage of siloxane reduces the smoke density and increases the strength of the char layer. At the same time, the original hydrophilic polyelectrolyte composite coating is coated on the surface of siloxane, and the micro-nano rough structure of chitosan effectively improves the hydrophobic performance of cotton fabrics. Therefore, the incorporation of PDMS synergistically enhanced the flame retardant and hydrophobic properties of the coating. Compared with before and after the introduction of siloxane, the total smoke release (TSP) was reduced from 0.48 m2 to 0.15 m2, and the smoke suppression performance was significantly improved. The WCA of the coating was increased from 130 ° to 152 ° compared to the fabric covered with silicone only. The synergistic effect between the flame retardant and the hydrophobic coating is obvious, and the comprehensive safety performance of the fabric is effectively improved.
In addition, hydrophobic flame retardant coatings are developing towards high performance and high efficiency[61][62]. For fabrics, high efficiency means that less flame retardant and hydrophobic agents are used to reduce the coating weight gain rate of the fabric surface without affecting the flame retardant and hydrophobic properties. For example, APP is used alone as a flame retardant and has strong hydrophilicity. When the mass ratio is 5%, the LOI is 26%, while after the microencapsulation of polysiloxane and polyborosiloxane, the mass ratio can be reduced to 3% under the same flame retardant performance, and the integrity of the carbon residue is improved, and the water resistance of APP is enhanced[63]. High performance is reflected in the indicators of hydrophobicity and flame retardancy: WCA reaches more than 150 °, which can be considered as superhydrophobicity; If the LOI of the modified coated fabric is more than 27%, it can be considered as a flame retardant fabric, and if it is more than 30%, it can be preliminarily considered that the flame retardant performance of the fabric is very superior. For example, Guo et al. Prepared hydrophobic flame-retardant cotton fabrics by sequentially depositing a novel multi-component flame-retardant phosphonyl octaaminopropyl POSS (PPA-POSS) and a fluorine-free imidazole skeleton (ZIF-67 @ PDMS) superhydrophobic coating wrapped by polydimethylsiloxane[64]. The optimized flame retardant sample with 6.2 wt% PPA-POSS exhibited a LOI of 34% and self-extinguishing ability. The HRR, THR and average effective heat of combustion (av-EHC) values of the fabric with the hydrophobic flame retardant coating were reduced by 51.4%, 56.2% and 68.4%, respectively, compared with the untreated fabric. Its WCA is as high as 159 °.
Moreover, maintaining the lasting stability of performance in various complex service environments is the primary key to the development of functional coating applications[65⇓⇓⇓~69]. Table 1 shows the service stability studies of hydrophobic flame retardant coatings in recent years. It can be found that the fabric with flame retardant and superhydrophobic functions is further endowed with wear resistance and self-cleaning properties, which is an effective strategy to improve the service time of the surface coating of the fabric, and is also a necessary development direction to promote the application of functional coatings in multiple environments.
表1 疏水阻燃涂层的服役稳定性研究

Table 1 Durability of hydrophobic flame retardant coating

Coating composition Flame retardance Hydrophobicity(WCA) Durability ref
organosilicon compound +nano-SiO2+
DOPO		 Burn slowly Above 150° 500 cycles adhesion test 41
phosphate polyurethane + nano-SiO2 +
polydimethylsiloxane		 LOI is 28.1%, self-extinguishing 160° undergoing 1000 cycles of abrasion, 60 min of ultrasonic washing and 50 standard machine washing cycles 42
flame retardancy-dopamine +
silver-polydimethylsiloxane		 LOI is 29.8%, and self-extinguishing Above 150° 50 standard washing cycles 65
fluoroalkyl silanes + Sb2O5 +
Al(OH)3		 LOI is 45.1%, and self-extinguishing 152° 1000 washing cycles 66
N,N-dimethyloctadecyl
phosphate acrylamide		 LOI is 21%, and self-extinguishing 157° 30 laundering cycles 67
diethylenetriamine penta(methylene-
phosphonic acid) + Fe3+		 self-extinguishing 155.6° 12 laundering cycles 68
Nano-SiO2 + APP + fluorinated
alkyl silane		 self-extinguishing 156° 20 cyclic cross hatch tape peel tests, 12 cycles cyclic abrasion, 30 cycles tape-peeling and 30 washing cycles 69
Finally, aiming at the application scenarios of fabric materials is also a new idea for the design of flame retardant hydrophobic silicone coating.
Oil and gas applications are in harsh and high-risk environments, which require materials not only to have flame retardant properties, but also to be hydrophobic and oleophilic or hydrophobic and oleophobic[70⇓~72]. Fu et al. Immersed cotton fabrics in a mixture of functionalized Co0.2Mg0.8Fe2O4(FCMFO) nanoparticles, vinyl-terminated polydimethylsiloxane (VPDMS), trimethylolpropane triacrylate and 2-hydroxy-2-methylacetophenone, and prepared fabrics with flame-retardant and superhydrophobic functions using a photo-curing method[73]. Its water contact angle is 157.1 °, and it has high oil/water separation efficiency (98.7% for dichloromethane/water) and high oil flux (71 506 L·m-2·h-1 for dichloromethane/water). Oil/water separation efficiency and oil flux remain 96.4% and 64 012 L·m-2·h-1 even after 20 separation cycles; At the same time, the fabric can still maintain its original shape after burning for 45 s, which fully demonstrates that the fabric material treated by the functional coating has potential application value as an oil-water separation material.
Daily outdoor protection is another extension of fabrics, which requires not only hydrophobic function, but also certain flame retardancy and ultraviolet absorption to ensure human safety and health[74,75]. For example, Bi and Quan et al. Used layer-by-layer self-assembly (LbL) method to prepare hydrophobic flame retardant cotton fabric with WCA of 153. 7 ° by using CS, APP and TiO2-SiO2-HMDS, and its burning length was reduced from 30 cm without treatment to 5. 7 cm, and it was self-extinguishing from the fire[76]. In MCC, the THR is reduced from 13. 4 kJ/G to 5. 5 kJ/G, which greatly reduces the fire risk, and the method shows potential application value in the field of wearable fabrics.
Wearable electronic device is an important hot spot in the research of fabric functionalization. Intelligent fire fighting equipment plays an important role in fire warning, reducing casualties and property losses. In addition to hydrophobic and flame retardant, intelligent fire fighting equipment also depends on excellent physical structure and good conductivity[77⇓⇓~80]. Song et al. Deposited polyethyleneimine (PEI), APP and titanium carbide (MXene) on cotton fabric by LbL method, and finally used silane elastomer for surface hydrophobization treatment to prepare cotton fabric with intelligent fireproof function[81]. Compared with the untreated fabric, the PHRR and THR were reduced by 42% and 43%, respectively, and the LOI reached 39%. On top of this, the treated fabric shows excellent electromagnetic interference (EMI), shielding and motion sensing capabilities, and the multifunctional smart fire-resistant fabric is expected to be applied to the next generation of fire uniforms.
Scientific development is changing with each passing day. Starting from the structure, the new functions of hydrophobic flame retardant silicone system compounds are carefully explored, which will provide new strategies and open up new directions for the application and development of flame retardant and hydrophobic silicone system coatings for fabrics.

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