Research status and trend of wood material flame retardant technology

In recent years, with the gradual improvement of people's lives, the consumption of wood for architectural decoration has been increasing year by year, and it has also increased the hidden dangers of fire. Although wood is a flammable material, the use of flame retardant technology can make its application more extensive, which makes the fire safety of building decoration wood greatly improved [2].

Flame retardant researchers at home and abroad have carried out a lot of meticulous work in the research and development of wood material flame retardants, wood material flame retardant treatment technology, flame retardant performance test and wood material flame retardant mechanism, and have made a series of orders. The result of satisfactory people [3].

1 Research status

1.1 Flame retardant mechanism

Flame retardant, its essence is to delay, inhibit the spread of combustion and reduce the probability of occurrence of heat ignition, is a technology to fundamentally suppress and eliminate runaway combustion [4]. Flame retardants are chemicals that protect the material from ignition or the flame from spreading. Most of the wood materials used in construction, electrical and daily life are flammable materials, while wood materials have unique flame retardant mechanisms [3]. At present, the flame retardant mechanism of wood materials mainly has [5,6]: (1) gas dilution mechanism: thermal action causes the flame retardant to decompose to produce a hard-to-burn or non-combustible gas, diluting the concentration of flammable gas in the mixed gas, Reduce the concentration of oxygen on the surface of wood materials to achieve the purpose of flame retardant I (2) char formation mechanism: the flame retardant is decomposed by heat to produce acid or base groups with water absorption or dehydration effect, which promotes dehydration of cellulose to form heat insulation Carbonization layer; (3) Chain reaction inhibition mechanism (thermal mechanism): a gas generated by thermal decomposition of a flame retardant is used as a catalyst, and a chemical reaction of a flammable gas becomes a non-combustible or flame-retardant gas, thereby interrupting the chain of flammable gases. Reaction; (4) Covering mechanism: a variety of flame retardants form a fluid or foam on the surface of the wood when heated and melted, which hinders the escape of flammable gases such as CO and CH3 generated by thermal decomposition of wood, and is also isolated. The supply of heat and oxygen to achieve the purpose of flame retardant; (5) The theory of free radical trapping: at the pyrolysis temperature, the flame retardant releases free radical inhibitors, which can capture the free radicals emitted by wood burning, and The action produces incombustible materials, thereby destroying the chain growth mechanism in the combustion process; (6) Hydrogen bonding mechanism: phosphoric acid produced by thermal decomposition of flame retardants, -OH, -NH in sulfate, etc. and cellulose and lignin in wood The combination of hydrogen forms incombustible materials, inhibits the thermal decomposition of wood, thereby achieving the purpose of flame retardant; (7) Other mechanisms: the above various flame retardant mechanisms are not isolated, and a flame retardant often has various flame retardants. Mechanism, sometimes several functions at the same time, but have their own emphasis; In addition, there are synergistic effects between the two elements, which complement each other, strengthen and complement each other. Therefore, more than two kinds of compounds are generally used in the formulation of flame retardants. ingredient.

1.2 Wood material flame retardant treatment technology

In 1994, Li et al. [6] found that flame retardant treatment not only increased the flame retardancy of wood, but also increased the compressive strength of wood at high temperatures, helping to extend the time during which wood components were burnt at high temperatures. In the same year, Luo Wensheng et al [9] pointed out that the choice of flame retardant should fully consider the PH value of the flame retardant, the pyrolysis temperature, the particle size of the flame retardant and the chemical reaction between the flame retardant and the adhesive at high temperature. In 1995, Liu Yanji et al [10] studied the production process of WFR wood and flame retardant wood, plywood, flower board and MDF. Xu Yongji believes that there are four forms of combustion: spontaneous combustion, flaming combustion, fuming and red hot burning. In 1997, Chi Changyi [11] used flame retardant VDFP and impregnation method to treat fiberboard, and the flame retardant effect was better. Since 1999, Wang Qingwen, Liu Yingtao et al [12 ~ 16] systematically studied the flame retardant process of the new flame retardant FRW in various wood-based panels, and considered that the new wood flame retardant FRW is a kind of flame retardant, smoke suppression Phosphorus-nitrogen-boron composite high-efficiency flame retardant with various effects such as anti-corrosion, suitable for flame retardant treatment of wood and other cellulosic materials, has no effect on wood color, moisture absorption, physical mechanics and processing properties. There is no environmental pollution during production and use. Zhu Jiaqi et al [17] further explored the treatment effect of WFR as a flame retardant for rubber wood plywood. Yin Ning et al [18] added a flame retardant to the water-soluble wood preservative CCA to improve the flame retardancy while preserving the treated material. Hou Lun et al [19] analyzed the effects of the amount of flame retardant, water content after dipping of the veneer, the amount of glue applied, and the hot pressing temperature on the bonding properties and flame retardancy of the sheet. Xiao Zhongping [20] proposed a flame retardant infusion process that can be used to guide production practices. In 2003, Zhang Heping [21] introduced the ISO-ROOM fire test method and its test and research on the heat release rate of building decorative panels, and also studied the relationship between the heat release rate and other dynamics related parameters in the indoor combustion process; In the same year, Li Zhizhou et al [22]. The effects of temperature, time, pH and flame retardant concentration on the flame retardancy of eucalyptus wood were studied. Luo Wensheng et al [23] studied the combustion and heat transfer process of wood treated with flame retardant. In 2004, Chen Xuemei et al [24] studied the flame retardant system composed of dicyandiamide, phosphoric acid and boric acid, and obtained the first-grade flame retardant formula. In the same year, Zheng Chongwei et al [25] used urea-formaldehyde resin as the base material. Melamine is a foaming agent, aluminum hydroxide is used as a filler, and other additives are used to prepare an intumescent wood flame retardant coating with good flame retardant properties; Gu Bo et al. [26] analyzed hot pressing temperature, time and veneer impregnation. The effect of time on the flame retardant and environmental performance of the plywood, and the formaldehyde emission and the immersion peeling performance were examined.

1.3 Research and development of wood material flame retardants

Efficient flame retardant effect must rely on several synergistic agents or even elements. [3,4] Flame retardants are mainly elements of the third, fifth and seventh main groups of the periodic table, or their elements. Or a compound. UDFP and Al(OH)3 can synergistically delay the rise of temperature inside the wood, effectively inhibit the pyrolysis of wood, and release non-combustible gases, thus effectively delaying the burning of wood [3]. Sheng Yu et al [27] discussed the relationship between the flame retardant expansion properties of flame retardant plugging materials and the type and amount of carbonizing agent and the method of making the materials. In recent years, Chinese researchers have also been using inorganic metal compound flame retardants [28], phosphorus flame retardants [29], molybdate flame retardants [30], silicon flame retardants and nitrogen flame retardants [ 31] Several new types of halogen-free flame retardants have made great progress, laying a good foundation for the development of new wood material flame retardants. 80% of the fatal accidents caused by fires are caused by the suffocation of smoke and toxic gases released by the pyrolysis and combustion of building components and decorative materials, and most of those who have been burned are poisoned by suffocation. Burned to death [1]. In addition, smoke can reduce visibility and make people lose their way. Toxic gas stimulates the human respiratory system, poisoning people and losing consciousness, and ultimately affects people from the fire zone. Therefore, it is one of the key topics in the field of flame retardant research to study how to reasonably select flame retardant and flame retardant system and reduce the amount of smoke and toxic gas when burning materials. Since halogen-containing polymers generate a large amount of smoke and toxic and corrosive hydrogen halide gas when burned [35], Chinese researchers have studied new halogen-free flame retardants, such as inorganic metal compound flame retardants and phosphorus. A flame retardant, a molybdate flame retardant, a silicon-based flame retardant, and a nitrogen-based flame retardant have been intensively studied [31], and a series of halogen-free flame retardant reagents have been developed. At the same time, adding a proper amount of smoke suppressant to the flame retardant can greatly reduce the amount of smoke generated by the material, and the smoke suppressant is mainly composed of metal oxides and transition metal oxides [36].

Japan, Europe and the United States and other countries have gradually deepened the research on the flame retardant mechanism of wood, and the development of high molecular polymers into flame retardants is the current research hotspot [37]. The three main footholds for polymer flame retardant design are: blocking the cooling, stopping the chemical reaction of the combustion and cutting off the heat source. The flame retardant mechanism of halogen-based flame retardants meets the requirements of these three. A small amount of use can impart good flame retardant effect to wood materials, and it is one of the most widely used and most effective flame retardants [29]. Wascott LD and other studies have found that in the initial stage of thermal decomposition of PVC, molybdenum compounds can promote the cross-linking reaction between molecules to form carbides, which cover the surface of the polymer to play the role of flame retardant and smoke elimination [30]. In the United States, there are already many silicone-based flame retardants with excellent flame retardancy and smoke suppression, such as SFR-100 produced by GE, USA, and block copolymers composed of polyester, polyimide and PDMS. Obvious flame retardant effect [12].

Halogen-free flame retardants must be developed because halogen-containing polymers generate large amounts of smoke and toxic, corrosive hydrogen halide gases when burned. Professor Andvey Moryganov of Russia first introduced a new halogen-free flame retardant Tezagran, whose main chemical component is a nitrogen derivative of alkylphosphoric acid, which is effective in flame retardant in both gaseous and solid state. The aspect is safe and has been widely used in the industrial production of cotton, hemp and other wood material flame retardants. Japan's NEC Corporation has developed highly flame-retardant plants that do not use halogens and phosphorus, and uses red phosphorus and ammonium polyphosphate to produce flame retardants that do not emit halogen gas when burned. Kyowa Kabushikikaisha discloses in its U.S. Patent No. 6,248,160 that its flame retardant compound has a red phosphorus weight of from 1.5 to 15 parts and an ammonium polyphosphate weight of from 10 to 70 parts [38]. In order to better suppress the generation of smoke during combustion, an appropriate amount of smoke suppressant can be added to the flame retardant, such as the US Firebrake zinc borate series, XP series, MolyFR molybdate series, tin and other elements of flame retardants, etc. [39]. Recently, many companies in western developed countries, such as Japan's Asahi Company, Daihachi Company, and Akzo, Nobel Company of the United States and Bayer Company of Germany, have applied for many patents on polyphosphate flame retardants [1]. It can be seen from the above-mentioned products sold abroad that China is slow in the development of flame retardants and slow in the innovation of flame retardant theory.

1.4 Flame retardant performance test

Thermal analysis is often used for the study of wood fire retardant because of its simplicity, speed and effectiveness. This method can also provide synergistic effects between test agents. Therefore, thermal analysis (TG, DTG, DSC) can be applied to the flame retardant effect of wood before and after the addition of Al(OH) in UDFP. The pyrolysis and combustion process of wood is analyzed from weight change and heat change to clarify Al. Flame retardant synergism of (OH)3 [3]. For example, Hu Yunchu et al. used the TG-DTA thermal analysis technique to determine the TGDTA-T curve of untreated and flame-retardant Chinese fir [32]. And the phenolic flame retardant treatment of charcoal yield and thermodynamic parameters in various stages of the pyrolysis process of Chinese fir [33]. TG, DTG and DSC were used to study the effects of different heating rates on the pyrolysis process, and the pyrolysis model of the samples was established to determine the kinetic parameters, which will help to deepen the research on the ignition mechanism, fire propagation mechanism and flame retardant mechanism. [34]. Other modern instruments have also been used to study the flame retardant mechanism of wood materials. For example, Fourier transform infrared light (FITR) can effectively study the flame retardant properties of the new flame retardant FRW [11-16]; using cone calorimeter (CONE), pyrolysis gas chromatograph (PYGC), differential scanning Modern analytical testing methods such as calorimeter (DSC) have studied the effect of silica on the flame retardant properties of intumescent flame retardant polyethylene, the synergy between flame retardant traces and the flame retardant mechanism, as well as boride and phosphorus. The flame retardant mechanism of compound treatment of wood [6].

2 Research trends

At present, all developed countries have paid great attention to the flame retardant treatment of wood materials, and it is increasingly recognized that the use of flame retardant wood materials is one of the strategic measures to prevent and reduce fires, and is related to "environment and humanity." "A major move." Therefore, the development of new flame retardants needs to be strengthened, which is highlighted in the following aspects.

2.1 Smoke suppression and non-toxic gasification are the trend of developing new flame retardants

Generally, halogen-containing flame retardant materials will release a large amount of smoke and toxic and harmful corrosive gases such as hydrogen chloride, hydrogen cyanide and styrene in the event of fire, which will cause great damage to personnel and precision instruments. With the gradual development of the modern home environment, the source of hydrogen cyanide in fire smoke increases, making the fire smoke greatly increase the harm to people [1]. In this way, the damage caused by building fires to humans is also transformed by the physical damage of the past and the poisoning of pure carbon oxides to the poisoning of cyanide and other highly toxic gases. Therefore, research and development of halogen-free, low-smoke, non-toxic, non-corrosive flame-retardant polymer materials is of great significance. To this end, all countries in the world have placed smoke suppression and non-toxic gasification into the main position of flame retardant technology. The development of anti-smoke agents in China started late, and it is necessary to increase the research and development of anti-smoke agents. At present, foreign countries mainly develop micro-fine, surface modification, micro-encapsulation and synergistic synergy, while halogen flame retardants which generate a large amount of smoke and toxic and corrosive gases are gradually eliminated.

2.2 New flame retardants must adopt uniform standards

Improve the legal system of flame retardant technology and products, and develop a unified national standard for flame retardant technology and products, which meets the requirements for the development of new flame retardants [27]. At present, developed countries such as Europe, America, and Japan all have relatively complete methods for detecting flame retardant products, while China lacks the testing standards of the system, which restricts the development of flame retardant technology. Due to the imperfect testing standards and the different testing conditions and means, it is difficult for fire supervision departments and manufacturers to conduct reasonable evaluation and selection, and restrict the development of scientific research and production in China's flame retardant field. There are no instruments and standard methods for evaluating the burning properties of wood materials in China, and it is impossible to carry out research on wood material combustion theory and flame retardant mechanism. Most of the current researches have adopted the standards for evaluating the burning properties of plastics in Japan, the United States, the United Kingdom, and China. However, due to different testing conditions, the results are difficult to be used as the selection and evaluation basis for the design department and the fire supervision department [40]. Therefore, China should improve the evaluation and inspection standards of flame retardants as soon as possible, making it more conducive to the research of China's flame retardant field and improve the international competitiveness of related companies.

2.3 Functional compounding and superposition are requirements for new flame retardants

In modern flame retardant technology, the composite technology of flame retardants is an extremely important aspect. In recent years, it has been proposed that the flame retardant treatment of wood materials should reach five characteristics, namely, flame retardancy, smoke suppression, corrosion resistance, insect resistance (anti-mortal) and structural dimensional stability. In addition, it is also required to be inexpensive, non-toxic, and not pollute the environment, which makes the research on flame retardants greatly difficult [41]. A flame retardant component generally has a plurality of flame retardant mechanisms, but most of them have a certain aspect of emphasis; while two or more components have synergistic effects of complementing each other, reinforcing and complementing each other, so the flame retardant formulation generally Both types of composite components are used [5].

2.4 New flame retardants must be tested by the market

New flame retardants not only meet the requirements of efficient flame retardancy, but also the price can be accepted by consumers. For example, fire-retardant wood-based panels treated with high-efficiency flame retardants are still in their infancy due to high production costs. Only a few companies are testing and there are very few practical applications in the construction industry. In addition, the silicon-based flame retardant is a low-toxic and high-efficiency flame retardant material, which has received widespread attention as soon as it appears. However, at this stage, the high-polymer flame retardant containing silicon is limited in its application due to its high price and complicated manufacturing process. Most of them are in the laboratory stage, but the silicone flame retardant is highly efficient, low-toxic and non-toxic. The advantages of pollution, low smoke, low toxicity and low cost are among the many non-halogen flame retardant systems, which has opened up a new direction for flame retardant polymer materials and has broad application prospects [42]. For the newly proposed nano-flame retardant, due to its small particle radius, the agglomeration between the particles is strengthened, which makes it difficult to prepare and use, which is an unsolved problem.

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