Burning modes and flame retardant pathways of wool fabrics
[Abstract] The article discusses the chemical structure, fire protection mechanism, common fire protection technology and main evaluation methods of fire protection performance of wool, including the test method of heat release rate, an effective parameter for evaluating fire hazard, and looks forward to it. Progress trends in wool fireproof finishing technology.
Keywords: wool; fireproof finishing; fireproof test
Wool is a natural protein fiber with a relatively complex physical and chemical structure. It has a soft feel, firm texture, good elasticity and warmth retention. It is natural and soft, and is widely used in construction, clothing, home textiles and other fields. , This puts forward higher requirements for post-processing [1~2].
The limiting oxygen index (LOI) of wool is about 25, the flash point is 570~600℃, and the high combustion temperature is 680℃[1~4]. It is a natural flame-retardant fiber. Moreover, it will not melt or drip when burning, and the foamed ash produced has good insulation properties. These properties are closely related to its chemical structure. The moisture regain of wool is as high as 15% (relative humidity is 60%), and it contains 15%~16% nitrogen; 3%~4% sulfur; and 6%~7% hydrogen. . The high nitrogen content determines its good fireproof properties [1~2]. If it is subjected to fireproof finishing, higher-performance products can be developed, such as high-temperature protective clothing, aircraft decorative materials and blankets. , hoods of fuel vehicles, etc.[5~7].
1 Combustion mode and fire prevention methods
Wool fabric requires three elements to burn: fuel, heat, and oxygen.
To achieve the fire prevention effect, the above pattern needs to be broken,
① Remove the heat source. There is not enough heat to maintain combustion. Most organophosphorus fire retardants and AL(OH)3 belong to this mechanism. During combustion, catalytic dehydration takes away heat, making it difficult for the fiber to continue burning.
② Increase the decomposition temperature. The production of fire-retardant fibers through raw silk modification or fiber blended chemical fiber fabrics belongs to this mechanism.
③ Reduce contact with O2, that is, gas dilution theory. Halogen-based fire retardants have this mechanism. When burned, they produce hydrogen halide gas and dilute the oxygen around the fiber.
④ Reduce the generation of flammable gases and increase the formation of char. Most phosphorus-nitrogen fire retardants and metal complex fire retardants belong to this mechanism. When burning, carbon residue is produced, forming a heat insulation layer on the surface of the fiber to block the diffusion of heat to the fiber, thereby achieving the purpose of fire prevention.
2 Fire protection technology and fire performance testing
2·1 Fire prevention technology and fire prevention mechanism
The fire-proof finishing of wool fiber initially used inorganic boric acid, phosphoric acid and their salts. This finishing method is non-durable finishing, not resistant to water washing, and is only used for theater curtains, etc.; later, modified tetrahydroxymethylphosphorus chloride (THPC) and its derivatives and sulfamate were used to treat wool After fireproofing, the treatment can reach a semi-permanent level. Later, it progresses to the use of fluorine complexes and carboxylic acid complexes of titanium and zirconium to treat wool. This method can reach a permanent level. In order to meet higher requirements, wool was later developed. High-temperature resistant fabric blended with other flame-resistant fibers and chemical fiber fabrics. This article mainly introduces the durable fireproof finishing technology.
2·1·1 Chemical fiber fabric blended with fire-resistant fiber[13~14]
Wool is a natural flame-retardant fiber, and chemical fiber fabrics blended with other fibers can be made into high-temperature-resistant fabrics with good fire-proof properties, low prices, and simple finishing. They are widely used in protective clothing, bedding, interior decoration, carpets, etc. The high-temperature resistant fibers currently on the market include Nomex 1313. When the proportion of Nomex and wool blended chemical fiber fabrics exceeds 30%, the limiting oxygen index can reach more than 28, which has good fire resistance. When the content of high-temperature resistant phenolic fiber Kynol and wool blended chemical fiber fabric reaches 35%, the fabric becomes fire-resistant. Recently, the fire-resistant viscose fiber Visil has appeared on the European market. After research, it has been found that it is particularly easy to blend with wool fiber into chemical fiber fabrics. This fiber contains a high proportion of silica and metal, produces very little combustion smoke, and is non-toxic. Wool and its blended chemical fiber fabrics can increase the thermal decomposition temperature to achieve fire protection purposes. At present, this method has been widely used, and the Visil content of blended chemical fiber fabrics is about 60%.
2·1·2 Metal complex treatment is currently the most widely used and mature technology for wool fireproofing finishing. It mainly uses titanium or zirconium fluorine complexes. Generally, titanium (zirconium)/fluorine molar ratio The ratio is 1:6 [15], such as K2TiF6 or K2ZrF6, under acidic conditions (generally the pH value is controlled around 2·5), the temperature is 70°C, and the treatment is for 30 minutes. This is the famous “Zirpro” process [1,3 ]. This technique can be performed before, after, or during the coloring process. Its fire prevention mechanism is that under acidic conditions, the amino groups on wool change into amino cations, which react with the anions of the fire retardant to produce adsorption. When burned, it can increase the formation of char and reduce the generation of flammable gases, thereby achieving the purpose of fire prevention[1 ,15~20]. The reaction formula is as follows (taking zirconium salt as an example):
W—NH2 (wool fiber) +H+→ W—NH+3, W—NH+3+ZrF2-6→W—NH+3·ZrF2-6.
In addition, after washing with water, ZrF2-6 is hydrolyzed into ZrOF2. When the fluorozirconate is heated and burned, the fluoride also gradually decomposes. ZrOF2 is also produced when the temperature is above 300°C. ZrOF2 is a very fine particle and cannot burn itself. Mixed with wool fiber or covering the surface of the fiber, it can prevent the large amount of flammable cracking gas from escaping in the event of fire, thus playing a fire prevention role.
Add tetrabromophthalic anhydride (TBPA) at about 10% to 15% (weight to fabric) during finishing; this additive can also react with wool fibers to further improve the flame retardancy. It has been verified that after finishing with fluorine complexes of titanium and zirconium, it can preventThe effect can withstand 50 times of hard water cleaning and dry cleaning, and is a durable fireproof finishing [5]. Studies have also found that fabrics treated with titanium complexes have better fire-proofing effects than fabrics treated with zirconium complexes, but they cause serious yellowing of the fabrics [5,9], so zirconium salt finishing is commonly used. The fire prevention effect of this technology is obvious to all. The wastewater after fire prevention contains a large number of heavy metal ions, and titanium and zirconium have caused environmental pollution [1]. In the near future, this technology will inevitably be replaced by environmentally friendly fireproof finishing technology.
2·1·3 Organophosphorus fire protection
Initially, derivatives of tetrahydroxymethylphosphonium chloride were used for finishing. This finishing agent has reducing properties. After finishing, the disulfide bonds of wool molecules are broken, resulting in a significant decrease in strength [2,21]. Currently, non-reducing organophosphorus finishing agents such as N-hydroxymethylmethoxyphosphoryl propionamide are used for fire-proof finishing of wool. This finishing agent can cross-link with wool fibers. After adding certain cross-linking agents, the degree of reaction is greatly increased, and the fireproof effect is obvious. After the fiber is heated, the fire retardant decomposes to generate phosphoric acid, which dehydrates the fiber and takes away part of the heat, making it difficult to continue burning. However, subsequent finishing will cause the fabric to feel stiff, so the amount of cross-linking agent must be strictly controlled. Rolling-baking-baking technology is adopted during finishing. Recommended dosage: 350 g/L fire retardant, 100 g/L cross-linking agent, 30 g/L phosphoric acid (85%). It can withstand 50 washes after finishing[2].
The mechanism of action of fire retardants is as follows:
Phosphorus-containing fire retardant is also a free radical scavenger. Using mass spectrometry technology, it was found that any phosphorus-containing compound will form PO· when the polymer burns. It can combine with hydrogen atoms in the flame area to suppress the flame. Its function can be expressed by the following formula:
PO·+H·→HPO·HPO·+H·→H2+PO·
This is the gas phase action mechanism of phosphorus-containing fire retardants.
2·1·4 Sulfamic acid-urea method
This technology mainly uses sulfamic acid as a fire retardant, which is rolled and baked in the presence of urea and then finished. During post-finishing, the fire retardant mainly reacts with the amino and hydroxyl groups contained in the amino acids on the wool fiber. The reaction is as follows :
RNH2 (wool) + NH2SO3H (sulfamic acid) = RHSO3H+NH3 ROH+NH2SO3H=ROSO3H+NH3 —SS—+NH2SO3H=—SNH2+—SSO3H
The formula of the fireproof finishing working fluid is 20% sulfamic acid, 20% urea, 60% water, and the emulsion rate is 97%. The process is padding working fluid → pre-baking at 120°C for 3 minutes → baking at 150°C for 2 minutes → washing with water → drying.
After finishing, it has no effect on the feel of the fabric and can withstand 50 washes. Since this method has dye-repellency to the fabric after finishing, finishing must be carried out after coloring [22].
2·2 Fire performance test
The test of fire protection performance is an important part of the research on fire protection finishing. Since the 1980s, countries around the world have successively developed many testing methods to evaluate fire protection performance [23~25]. Generally, the following main indicators are included [24]: ① Ease of ignition; ② Flame surface propagation speed; ③ Smoke visibility; ④ Toxicity of combustion products; ⑤ Corrosiveness of combustion products. Among them, ① and ② are the main evaluation indicators. Testing methods include measuring the burning extent, afterburning and smoldering time, and limiting oxygen index of materials. According to the relative position of the flame and the sample, it can be divided into vertical method, horizontal method and tilt method [23~25]. Currently commonly used methods are the vertical combustion method and the limiting oxygen index (LOI) method. The vertical combustion method is to clamp one end of the sample vertically, apply a prescribed gas flame to the free end of the sample, and evaluate the combustion performance of the sample by measuring the flaming combustion and flameless combustion time. The results given by this test method can be used for product quality control and material preselection, but cannot be used to evaluate fire risk. The limit oxygen index method is to maintain the low oxygen concentration required for sample combustion under specified test conditions in the oxygen and nitrogen mixed gas flow. This indicator is used to characterize the difficulty of materials being ignited. The measured data has good reproducibility and is suitable for process experiments, but it cannot measure the risk of fire [25~27].
In recent years, with the rapid progress of fire protection science and technology, a new testing method has emerged, which is to use a cone calorimeter to test the heat release rate (Heat Release Rate) of materials when they burn. The combustion heat release rate refers to the rate of heat release per unit area of the sample during the combustion process (kW/m2). Generally speaking, the more heat a material releases and the faster it burns, the greater the fire hazard. Therefore, this indicator is a fire protection performance parameter that can measure the risk of fire. In addition to measuring the heat release rate, the cone calorimeter can also provide parameters such as sample ignition time, mass loss rate, effective combustion heat, harmful gas content, etc. These parameters are useful for analyzing the comprehensive performance of fireproof materials, and then predicting the performance of materials and products. It plays an important role in combustion behavior in real fires [28~30].
3 Conclusion
Due to its structural advantages, wool itself is a flame-retardant fiber, and it is comfortable and warm to wear. It is a high-end fabric. Its application prospects are very broad. It is highly feasible to develop high-end products with high added value after fire-proof finishing. . With the improvement of people’s living standards, the requirements for the added value of textiles are getting higher and higher. “Environmental protection” and “comfort” have become basic requirements. Therefore, while fireproof finishing, environmental protection issues must be taken into consideration. For this reason, we must devote ourselves to the development of new environmentally friendly fire retardant agents in the future, and the original mature fire retardant finishing process must be further improved.
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