Abstract: Introduces the classification of flame retardants and their flame-retardant mechanism, the interaction between various flame retardants, the miscibility characteristics of flame retardants and polymers, the surface modification methods and functions of flame retardants; Classes and precautions of flame retardants suitable for modified bitumen waterproofing membranes.
Key words: flame retardant; synergistic effect; antagonistic effect; flame retardant mechanism; asphalt base; waterproof membrane
Foreword
Waterproof membrane is a rollable sheet waterproof material, which is widely used in building walls, roofs, tunnels, roads and landfills.
According to statistics, in 2021, waterproof membranes accounted for 69.57% of the market share in waterproof materials (among them, SBS/APP modified asphalt waterproof membranes accounted for 37.09%, synthetic polymer waterproof membranes accounted for 9.92%, and self-adhesive waterproof membranes accounted for 22.56%. %), waterproof coatings accounted for about 25%, other new waterproof materials accounted for about 4.04%, and other waterproof materials such as fiberglass tire asphalt tiles accounted for about .45%. It is not difficult to see that hot-melt bitumen-based waterproofing membrane (SBS/APP modified bitumen waterproofing membrane) is still the mainstream product in waterproofing materials, occupying more than 1/3 of the market for waterproofing materials.
However, since the construction of the hot-melt waterproof membrane requires blowtorch heating, and the asphalt waterproof membrane material contains a large amount of asphalt and other combustibles, when the membrane is bombarded by welding slag, firecrackers, etc. or burns after being baked for a long time, Melting and dripping will also occur during burning. At the same time, some flammable gases released by the burning of asphalt will expand the fire trend, which is very likely to cause casualties and economic losses.
Fires caused by hot-melt construction of waterproof materials are not uncommon. For example, on November 11, 2008, the Jinan Olympic Sports Center fire was ignited due to waterproof materials, burning 3,150 square meters of composite roofs and causing a direct economic loss of 750,000 yuan. On November 11 of the same year , a fire broke out on the southeast side of the roof of the Jinan Olympic Sports Center Gymnasium, covering an area of 1,284 square meters; on October 24, 2010, a fire was caused by a blowtorch used for waterproofing in a middle school in Jilin; on April 19, 2013, the construction site of Yixing Business Building in Fengxian District, Shanghai; 2013 On July 3, 2013, Beijing house leaks were waterproofed and burned; on May 16, 2013, a restaurant in Xingtai caught fire due to improper waterproof construction; on September 25, 2014, the roof of Changchun Tongxin Garden was waterproof; on May 13, 2015 In Shaoxing, a printing and dyeing factory roof was repaired and caused a fire; fire accidents caused by waterproof engineering construction were frequent. Almost every year, fire accidents were caused by open fire construction of waterproof coiled materials, which not only caused the loss of property, but also threatened the safety of personnel. Therefore, while improving the construction method, it is imperative to improve the flame-retardant grade of the hot-melt waterproof membrane.
Classification of Flame Retardants
Flame retardant is a special chemical additive to improve the combustion performance of flammable and combustible materials. It is widely used in the flame-retardant processing of various decoration materials. At present, the most common distributions are roughly:
2.1 According to the composition elements
According to the classification of constituent elements, it can be divided into halogen flame retardants (such as decabromodiphenyl ether DBDPO, tetrabromobisphenol A, decabromodiphenylethane, etc.), phosphorus flame retardants (such as microencapsulated red phosphorus, polyphosphate, etc.), nitrogen-based flame retardants (such as melamine, melamine phosphate, etc.), but because the effect of a single flame retardant is not very obvious, phosphorus-halogen flame retardants, phosphorus-nitrogen flame retardants And other synergistic compound flame retardants came into being.
2.2 According to different components
According to the different components, it can be divided into two categories: organic flame retardants and inorganic flame retardants. The main components of organic flame retardants are organic substances, and the main products are halogen-based, phosphate esters, and halogenated phosphate esters. Inorganic flame retardants are currently the most used type of flame retardants. Its main components are inorganic substances. The main application products are aluminum hydroxide (ATH), magnesium hydroxide (MH), monoammonium phosphate, diammonium phosphate, Ammonium chloride, zinc borate, etc. Similarly, the combination of organic flame retardants and inorganic flame retardants has gradually become a self-contained system and is widely used in various fields.
2.3 According to the method of use
According to the method of use, it can be divided into additive flame retardants and reactive flame retardants. Among them, additive flame retardants are the most widely used, accounting for about 80%, including chlorinated paraffin, decabromodiphenyl ether, tetrabromobisphenol A-bis (2,3-dibromopropyl ether), decabromodiphenyl Ethane, etc.; reactive flame retardants include dibromoneopentyl glycol, tetrabromophthalic anhydride, vinyl bromide, etc.
Flame Retardant Mechanism
Flame retardants have the ability to prevent materials from burning or delay material combustion, so as to protect materials from fire, self-extinguishing or flames that are difficult to spread. Its flame-retardant mechanism has the following characteristics:
3.1 Endothermic effect
This class of flame retardants enhances the flame-retardant properties of polymers by increasing their thermal melting, allowing them to absorb more heat before reaching the thermal decomposition temperature. Representatives of such flame retardants are ATH and MH, and their specific flame-retardant mechanisms include:
(1) Dilution effect: ATH and MH combine with 34.6% and 31.0% crystal water in the form of chemical bonds respectively. The incorporation and dilution of a large amount of water vapor when heated changes the combustion limit of the mixed gas of organic combustible substances and oxygen, so Reduced likelihood of combustion occurring.
(2) Barrier layer effect: the surface layer formed by Al2O3 and MgO with polymer residue carbon during combustion can prevent heat and material transfer between the condensed phase/gas phase interface. Compared with hydroxide, oxide has a higher Due to the high thermal melting, the temperature rise is small, and at the same time, it has a high specific surface area, so it has a high degree of activity and adsorption capacity, which helps to catalyze the formation of charcoal and protects the underlying polymer from degradation.
3.2 Coverage
Flame retardants can form a glassy or stable foam covering layer at high temperatures to isolate oxygen, and have the functions of heat insulation, oxygen isolation, and prevention of flammable gases from escaping, so as to achieve the purpose of flame retardancy. For example, when the organophosphorus flame retardant is heated, it can produce a cross-linked solid substance or a carbonized layer with a more stable structure. The formation of the carbonized layer prevents the further decomposition of the polymer on the one hand, and on the other hand prevents the thermal decomposition products inside it. Enter the gas phase to participate in the combustion process.
3.3 Inhibition chain reaction
Flame retardants can act on the gas phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the spread of the flame, reducing the flame density in the combustion zone, and finally reducing the combustion reaction speed until it terminates. For example, the evaporation temperature of halogen-containing flame retardants is the same or close to the decomposition temperature of the polymer. When the polymer is decomposed by heat, the flame retardant is also volatilized at the same time.
3.4 Otherwise gas suffocation
The flame retardant is decomposed by heat to produce other gases, which dilutes the concentration of combustible gases decomposed from combustibles to the lower limit of combustion. At the same time, it also has a dilution effect on the oxygen concentration in the combustion zone, preventing combustion from proceeding, and achieving flame retardancy.
Several issues to be considered in the selection of flame retardants
4.1 Environmental protection
Although flame retardants are gradually used in various industries, with the increasing awareness of global environmental protection, people have higher and higher requirements for fire safety and flame retardancy of products. Although some flame retardants can achieve the purpose of flame retardancy or delay combustion, they also release toxic and harmful gases and substances, which will harm people who have not been evacuated from the fire scene in time. left injury. For example, polybrominated diphenyl ethers (pentabromodiphenyl ether, octabromodiphenyl ether, decabromodiphenyl ether, etc.) will release dioxins when burned, which will affect the victim’s nervous system, The immune system and reproductive system, the consequences include mental decline, weakened immunity, cancer, reproductive difficulties and deformed children.
Therefore, it is the general trend to choose environmentally friendly flame retardants when using flame retardants, such as decabromodiphenylethane, brominated epoxy resin and brominated polystyrene in bromine series; red phosphorus in phosphorus series, resorcinol (Diphenyl phosphate); melamine, melamine cyanuric acid and melamine phosphate in the nitrogen system; and inorganic flame retardants such as MH and ATH are all environmentally friendly flame retardants. Can be prioritized when choosing.
4.2 Synergistic and antagonistic effects of compound flame retardants
Although flame retardants have the advantages of flame retardancy or delayed combustion, the flame-retardant effect is not obvious when most flame retardants are used alone. On the contrary, when two or more flame retardants are used in combination, the flame retardant effect will be greatly improved. . For example, phosphorus-based flame retardants are dehydrated and carbonized when they decompose, but this process must rely on the oxygen-containing groups of the polymer itself. Not good; but if it is used in combination with MH and ATH, it can produce a synergistic effect and obtain a good flame-retardant effect.
However, when the bromine-phosphorous compound flame retardant system and the bromine-antimony flame retardant system exist at the same time, they not only have no synergistic effect, but show an antagonistic effect. The reason is still under study. This shows that various flame retardants have their own characteristics. Compounding can make the flam- retardant effect more obvious, but it may also weaken the flame retardant performance, so it should be considered when choosing.
4.3 Compatibility issues and flame-retardant modification technology
Due to the polarity problem between the flame retardant and the polymer and the influence of hydrogen bonds, it is difficult to completely miscible the flame retardant and the polymer during production, thus affecting the flame-retardant performance of the product. Therefore, it is necessary to modify some flame retardants that are difficult to be miscible with polymers. Most of the flame-retardant modification technologies are coupling technology and microencapsulation technology.
For example, MH (magnesium hydroxide) and ATH (aluminum hydroxide) are often treated with silane coupling agents or titanate coupling agents to improve the wetting performance between the flame retardant and the polymer, and to strengthen and polymerize It can improve the adhesion of materials, form a special interface layer, and relieve the residual stress of polymers and flame retardants.
The promotion of microencapsulation technology not only enhances the stability of flame retardants, but also improves the compatibility with polymers. For example, the application of microencapsulated red phosphorus not only solves the problem of collision and flammability of red phosphorus during transportation, but also greatly improves the miscibility in flame retardant production.
Flame retardant and oxygen index testing for asphalt coils
After modifying the formula of elastomer-modified asphalt waterproofing membrane and adding various flame-retardant systems, the oxygen index was measured according to GB/T 10707-2008 “Determination of Combustion Properties of Rubber”
In the case of using only one flame retardant, the oxygen index of the product is small, and the flame-retardant effect cannot be achieved, and the flame-retardant effect cannot be shown in the case of single use of zinc borate and antimony trioxide, and phosphorus-bromine flame retardant The system has a certain resistance to the bromine-antimony flame retardant system at the same time.
epilogue
The consumption of modified asphalt waterproofing membrane is large in today’s society, but fires caused by construction are also frequent. Part of the reason is due to improper construction. The main reason is that hot-melt asphalt waterproofing membrane is easy to be caused by burning. Therefore, the flame retardancy of waterproof membranes will gradually be valued, and it is bound to be the general trend. However, due to the polarity and toxicity of the flame retardant itself, more consideration should be given to the selection. The key to the production of flame retardant and waterproof membranes is to choose one or several flame retardants that are flame retardant, environmentally friendly and have good compatibility.
