A Study on the Flame-retardant Property of Two-component Intumescent Fire-retardant Coatings

WANG Yan, LIU Zhan-jing, GUO Peng-chong, YANG Fan, LEI Ri-xiao (Shijiazhuang Paint Company, High Solids Coatings Technology Innovation Center of Hebei Province, Shijiazhuang 050051, Hebei, China)

Abstract: A kind of high solids two-component intumescent epoxy fire-retardant coatings was developed in this experiment. The effects of resin type, titanium dioxide content, flame-retardant filler and expansion system content on the flame-retardant property of the fire-retardant coatings are studied. The results show: (1) E51 epoxy resin and E20 epoxy resin used together in the twocomponent epoxy fire-retardant coatings with amino resin as foaming additive help increase the expansion ratio of the expansive layer, thereby improving the fire resistance limit. (2) Proper addition of titanium dioxide can improve the strength of the expansive carbon layer and realize heat insulation and flame retardancy. (3) The mixture of aluminum hydroxide and antimony trioxide can significantly reduce the amount of smoke, improve the strength of the expansive carbon layer and enhance the flame-retardant property. (4) The combination of expanded graphite and the ternary expansion system is selected to form a synergistic combination of graphite-structured carbon and graphite-like carbon layers and enhance the flame-retardant effect.

0 Introduction
intumescent fire retardant coating has It has the advantages of high expansion ratio, good flame retardant and heat insulation performance, and low cost. At present, it has been widely used in the construction industry, petroleum storage tanks, chemical pipelines and other industries. According to the components, it can be divided into two-component intumescent fire-resistant coatings and single-component intumescent fire-resistant coatings. One-component intumescent fire-resistant coatings are most widely used. Most of the film-forming base materials used are acrylic resins. As a thermoplastic resin, in the event of fire or high temperature, it cooperates with the expansion system to have a suitable softening temperature and viscosity to form a fire-resistant carbon layer with high expansion ratio and excellent flame retardancy. However, acrylic resin has the disadvantages of low mechanical strength and temperature sensitivity. . The expansion system of intumescent fire-resistant coatings often uses a ternary expansion system of dehydration to form a carbon catalyst (acid source), carbon forming agent (carbon source), and foaming agent (gas source). The system has a high expansion rate and thermal insulation and flame retardancy. Excellent, but the large number of hydroxyl groups contained in the carbon forming agent makes the ternary expansion system fire-resistant coatings have the disadvantage of poor water resistance.

In view of the above defects, in the aspect of film formation of fire-resistant coatings, a two-component epoxy resin was used in this experiment. The molecular structure is dense, the cohesion is strong, the mechanical strength is high, and the adhesion, heat resistance, and chemical resistance are excellent. The matching performance of topcoat is good. In terms of expansion system, this experiment uses expandable graphite and a ternary system. Expanded graphite is a physical expansion agent. At high temperature, the flake graphite expands under the action of interlayer forces to form a worm-like carbon body. Its expansion process It is a physical change, no toxic and harmful gas is generated, and has the advantages of excellent expansion and flame retardancy and high temperature oxidation resistance. It is matched with Al (OH) 3 / Sb2O3 synergistic flame retardant system to prepare high corrosion resistance and flame retardancy and high mechanical strength. Solid two-component epoxy intumescent fireproof coating.

1 Experimental part
1.1 Experimental materials
Epoxy resin (E51, E44, E20), Nanya epoxy resin; Modified polyamide epoxy curing agent, Shanghai Duer Chemical; Ammonium polyphosphate, Qingyuan Phosphorus Chemical; Melamine, Sichuan Meifeng Chemical Co., Ltd .; Pentaerythritol, Puyang Yongan Chemical Industry; Titanium Dioxide, Panzhihua; Decabromodiphenylethane, Antimony Trioxide, Zhengzhou Haorong Chemical Products; Expanded Graphite, Qingdao Tianheda Graphite; Dispersant, Antifoaming Agent, Beike; Bentonite, Anji County Shengli Bentonite .

1.2 Experimental instrument
electronic balance, TC3K, Changshu Shuangjie Testing Instrument Factory; QXD Scraper Fineness Meter, Shanghai Meiyu Instrument Equipment Co., Ltd .; Sand Mill, QSM-Ⅱ, Tianjin Kelian Material Testing Machine Co., Ltd .; High-speed mixer, SDF, Shanghai Weite Electric Co., Ltd .; Rapid test device for fire resistance of intumescent fireproof coatings, AVIC New Materials Technology Co., Ltd .; Steel ruler, film thickness meter, etc.

1.3 Formula and preparation process of
fire-resistant coatings The reference formula of the two-component epoxy fire-resistant coatings for this experiment is shown in Table 1.

Table 1 Reference Formula

The preparation process of the two-component intumescent epoxy fireproof coating is as follows:
(1) Dispersion: The epoxy resin, defoamer, bentonite, and dispersant are sequentially added to the batch tank, and dispersed at 1 500 r / min for 15 minutes.
(2) Grinding: Add the dispersed materials to the grinding tank, and then add titanium dioxide, decabromodiphenylethane, antimony trioxide, melamine, pentaerythritol to grind to a fineness of 60 μm or less.
(3) After the fineness is qualified, add ammonium polyphosphate and expanded graphite with high stirring; then add toluene and butanol; stir evenly to obtain a two-component expanded epoxy fireproof coating.

1.4 Testing methods and standards
The properties of the coatings are tested in accordance with the national standard GB 14907—2018 “Fireproof Coatings for Steel Structures” and meet the requirements for high solid epoxy coatings in T / CNCIA 01005—2018.

2 Results and discussion
2.1 Effect of resin
2.1.1 Effect of epoxy resin
The epoxy resin reacts with the curing agent to form a macromolecule with a cross-linked network structure. It is a thermosetting resin and has excellent chemical resistance. The epoxy group, hydroxyl group, and ether bond contained in the epoxy resin easily wet the substrate and have excellent adhesion. The cohesive epoxy resin has good cohesion and excellent bonding strength. In this experiment, the influence of epoxy resins with different epoxy values ​​on the flame retardancy of intumescent fire retardant coatings was investigated. The results are shown in Table 2.

 Table 2 Effect of Epoxy Resin Type on Fire Resistance

It can be known from Table 2 that the fire resistance limit and expansion ratio of the two-component intumescent epoxy fireproof coating increase with the decrease of the epoxy value content. This is because the organic intumescent fire protection system is composed of a dehydration carbon-forming catalyst (acid source), a carbon-forming agent (carbon source), and a foaming agent (gas source). When the intumescent fire-resistant coating is heated by fire, the carbon-forming agent is Under the action of the catalyst, it is dehydrated into carbon, and the carbide forms an expanded porous structure under the action of the foaming agent, thereby achieving the effect of heat insulation and flame retardant. During the entire expansion process, the catalyst must be decomposed into acid, the carbon-forming agent is dehydrated into carbon, and the foaming agent is decomposed and foamed at the same temperature and speed. Otherwise, an expanded and heat-insulating carbon layer cannot be formed. Film-forming resin has a significant impact on the performance of intumescent fire-resistant coatings. It is necessary to ensure that the fire-resistant coating has excellent mechanical properties at room temperature. It also requires the coating to have excellent expansion performance and a dense carbonized layer at high temperatures in the event of fire. Epoxy resin It is a semi-finished thermoplastic resin, which is cross-linked and cured with a curing agent into a thermosetting resin. The softening temperature and softening viscosity of the film-forming material must be consistent with the decomposition and carbonization temperature of the ternary system to form a dense and uniformly porous, high-expansion carbonized layer. Therefore, the degree of cross-linking inside the film-forming system has an effect on the height, strength, and compactness of the expanded carbon layer, and then directly affects the fire resistance of the fire-resistant coating. Epoxy resin E51 has a higher cross-linking density than E44 and E20, and the coating film is brittle. In the event of fire, it does not match the expansion temperature and rate of the ternary system, which affects the expansion and foaming of the ternary system and crosslinks. The structure burns but does not form carbon. The expansion layer has a large number of large cells or voids, and the flame retardant effect is poor. The crosslink density of epoxy resin E20 is the smallest among the three resins, and the coating flexibility is better, but the strength and various physical and chemical properties of the coating will decrease, and the E20 resin has a larger VOC content than the E51 resin, so Considering the requirements of flame retardancy, VOC content and various physical and chemical properties, epoxy resin E51 and epoxy resin E20 are used in this experiment.

2.1.2 Effect of Amino Resin Addition
In order to meet the requirements of high solids epoxy coating in T / CNCIA 01005-2018 standard, epoxy epoxy coating in this experiment chose epoxy resin E51 as the main resin, but E51 resin formed a brittle film, and matched with E20 resin to improve flexibility. Different amounts of amino resin were added to the formulation, and the effect of amino resin content on fire resistance was investigated. The results are shown in Table 3.

Effect of Amino Resin Content on Fire Resistance Table 3 Effect of Amino Resin Content on Fire Resistance

As can be seen from Table 3, the addition of a certain amount of amino resin can increase the expansion ratio and fire resistance of fire-resistant coatings. This is because amino resins act as part of the foaming agent in organic intumescent fire-resistant coatings. They decompose at high temperatures to produce gases such as ammonia, dilute the oxygen concentration in the air, and play a role in promoting foaming and helping to increase expansion. The expansion ratio of the layer, thereby increasing the fire resistance. However, when the amino resin content is too high, it will affect the mechanical strength of the coating, and its content is more suitable at 3%. .

2.2 Effect of the added amount of titanium dioxide
titanium dioxide as Intumescent paint is used, can be classified according to crystal form rutile titanium dioxide and anatase titanium dioxide, anatase titanium dioxide of rutile type is better thermal stability Titanium Dioxide. The rutile titanium dioxide has a more hexahedral structure, which is easier to disperse and uniform than anatase titanium dioxide, and the agglomerates formed by it are more uniform. In this experiment, rutile titanium dioxide was used to study the effect of different titanium dioxide content on the fire resistance limit. The experimental results are shown in Table 4.

Table 4 Effect of Titanium Dioxide Content on Flame-retardant Property

It can be seen from Table 4 that when the titanium dioxide is added in an amount of 3%, the fire resistance limit is low, the strength of the expansion layer is low, and the carbonized layer is loose and shedding. As the titanium dioxide content increases, the strength of the expanded carbon layer increases and the fire resistance limit increases; when the titanium dioxide content is too high, although the strength of the expanded layer increases, the fire resistance limit decreases. This is because at high temperature, titanium dioxide can react with ammonium polyphosphate to form white inorganic substances such as titanium pyrophosphate, which covers the surface of the carbon layer, improves the strength of the expanded carbon layer, and plays a role of heat insulation and flame retardancy. When the content of titanium dioxide is low, the expanded carbon layer is loose, easy to crack and fall off, and the adhesion to the substrate is poor, and the fire resistance limit is reduced; but when the titanium dioxide content is high, it will affect the expansion and foaming of the ternary system and reduce the fire resistance Therefore, the content of titanium dioxide is preferably 9%.

2.3 Effect of antimony trioxide and aluminum hydroxide
Antimony trioxide is a synergistic flame retardant, which is used in combination with other flame retardants to produce a synergistic effect. In the early stage of combustion, the first step is melting, forming a protective film on the surface of the material to isolate the air, and reducing the combustion temperature through an internal endothermic reaction; at high temperatures, antimony trioxide is vaporized, diluting the concentration of oxygen in the air, thereby becoming flame retardant effect. However, due to the higher melting point of antimony trioxide, the effect of single group use is not obvious, and it is often used in conjunction with other flame retardants. In this experiment, the effect of Al (OH) 3 / Sb2O3 compounding system on the flame retardancy of fire-resistant coatings was studied using the compounding of aluminum hydroxide and antimony trioxide. The experimental results are shown in Table 5.

Effect of Al (OH) 3 / Sb2O3 Compound System on Flame Retardant Properties Table 5 Effect of Al (OH) 3 / Sb2O3 System on Flame-retardant Property

It can be known from Table 5 that the addition of an appropriate amount of aluminum hydroxide and antimony trioxide can improve the flame retardancy of fire-resistant coatings. This is because aluminum hydroxide decomposes and decrystallizes water when exposed to high temperatures. It can dilute the oxygen concentration in the air and slow down the reaction. On the other hand, the water that has evolved evaporates and absorbs heat, which reduces the surface temperature of the polymer and plays a flame retardant role. In addition, the product of aluminum hydroxide decomposition covers the surface of the polymer to block oxygen and reduce the amount of smoke to play a certain flame retardant effect. However, the surface structure of aluminum hydroxide makes it difficult to disperse, easy to agglomerate, and the effect of compounding with antimony trioxide is obvious. It can reduce the amount of smoke, improve the strength of the expanded carbon layer, and has excellent flame retardancy. Reducing the content of aluminum hydroxide and alleviating the decline in mechanical properties caused by excessive content of aluminum hydroxide, the effect is best when the content of aluminum hydroxide is 4% and the content of antimony trioxide is 2%.

2.4 Impact of expanded graphite and ternary system on flame retardancy
C—N’s ternary expanded flame retardant system has poor mechanical properties and poor water resistance. In this experiment, a two-component epoxy resin was selected to improve mechanical strength and water resistance. performance. However, the two-component epoxy resin is a thermosetting resin, and its cross-linked structure affects the softening of the resin. In the event of fire, it does not match the expansion temperature and speed of the ternary system, and the fire retardancy of the fire retardant coating is poor. In this experiment, the ternary expanded flame retardant system was used in combination with expanded graphite to study the effects of the content of expanded graphite and the ternary system on the fire resistance. The experimental results are shown in Table 6.

Table 6 Effect of Expanded Graphite Content on Flame-retardant Property

It can be seen from Table 6 that as the content of expanded graphite increases, the fire resistance of the coating gradually increases, but when the content of expanded graphite is too high, the fire resistance of the coating decreases. This is because expanded graphite is a physical expansion agent. Under high temperature conditions, flake graphite expands under the action of interlayer forces to form a worm-like carbon body. A large number of carbon layers play the role of high temperature resistance, heat insulation and flame retardant. However, when the content of expanded graphite is high, the expansion volume is too large, and the expanded carbon layer is too loose, which causes the strength to decrease and fall off, which does not play a good barrier role. The content of expanded graphite is preferably 5%. For two-component epoxy fire-resistant coatings, the curing system is an amine curing agent, which has a chemical reaction with ammonium polyphosphate, which affects the service life and flame retardancy of two-component epoxy fire-resistant coatings. In this experiment, melamine formaldehyde resin-coated ammonium polyphosphate was used. Its storage stability was excellent. Ammonium polyphosphate was coated with melamine formaldehyde resin, which did not have chemical reaction with ammonium polyphosphate. It had excellent flame retardancy and was coated with melamine formaldehyde resin. Ammonium polyphosphate has very low water solubility and its coating has excellent water resistance.

Pentaerythritol is used as a carbon-forming agent in a ternary system. When it is exposed to high temperature, it is dehydrated to char under the action of a catalyst. However, pentaerythritol has a certain water solubility and the coating has poor water resistance. Dipentaerythritol was selected in this experiment, and its char formation effect and water resistance were better than pentaerythritol.
The influence of the content of the ternary system on the fire resistance and the strength of the expanded carbon layer was studied. The specific experimental results are shown in Table 7.

 Table 7 Effect of Ternary System Content on Flame-retardant Property

It can be known from Table 7 that when the ternary system content is low, the fire resistance limit is low, the expanded carbon layer is loose and easy to fall off, and the thermal insulation and flame retardancy performance is poor. It is suitable when the ternary system content is 36%. This is because there are two types of carbon in the expanded carbon layer structure: graphite structure carbon and graphite-like structure carbon. The carbon layer formed by expanded graphite is a graphite structure carbon, and the graphite structure carbon is a layered structure. There is a lack of cross-linking between layers. The strength of the carbon layer is low. The carbon layer formed by the ternary system is a graphite-like carbon. Cross-linking exists [6]. The two carbon layer structures work in coordination. When the ternary system content is 36%, an expanded carbon layer with strong flame retardant effect is formed. The expanded carbon layer after combustion is shown in Figure 1.

Fig. 1 Expanded Carbon Layer Structure

2.5 Fire-resistant coating properties
Through the discussion of the above factors, a two-component epoxy fire-resistant coating with excellent intumescent flame retardancy was prepared. The fire resistance and comprehensive physical and chemical properties of the coating were tested. The test results are shown in Table 8.

3 Conclusions
Through the above experiments, the following conclusions can be drawn:
(1) Epoxy E51 and epoxy E20 are used in conjunction with the two-component epoxy fire retardant coatings. The addition of amino resins can help foaming and help Increase the expansion ratio of the expansion layer, thereby increasing the fire resistance.
(2) When the titanium dioxide is added in an amount of 9%, the strength of the expanded carbon layer can be increased, and it can play the role of heat insulation and flame retardancy.
(3) The compounding of aluminum hydroxide and antimony trioxide can significantly reduce the amount of smoke, increase the strength of the expanded carbon layer, and enhance the flame retardancy.
(4) The combination of expanded graphite and ternary expansion system is used to form a carbon layer of graphite structure carbon and graphite-like structure carbon to synergize and enhance the flame retardant effect.

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