A flame-retardant coating based on PA-MEL flame-retardant curing agent, its preparation method and application

Abstract

This invention discloses a flame-retardant coating based on PA-MEL flame-retardant curing agent, using urea and formaldehyde as main raw materials, melamine and sodium lignosulfonate as additives, dioctyl phthalate (DOP) as a plasticizer, and phytic acid-melamine polyelectrolyte (PM) as a bio-based flame-retardant curing agent. The decomposition temperature at 5% by mass is 244.9℃, and the temperature at which the maximum decomposition rate is reached is 297.7℃. The char residue at 800℃ is 37.4 wt.%. The preparation method includes the following steps: 1. Preparation of phytic acid-melamine polyelectrolyte (PM); 2. Preparation of urea-formaldehyde resin emulsion (MUF); 3. Preparation of flame-retardant coating (SUF) based on PA-MEL flame-retardant curing agent. As an application of flame-retardant coatings for wood, it has flame-retardant properties. Wood coated with flame-retardant coatings passes the UL-94V-0 rating test in the UL-94 rating test. The char layer formed after complete combustion exhibits continuous and dense properties, with few and small pores. In the limiting oxygen index test, the limiting oxygen index is 32.1%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of flame-retardant materials, in particular to a flame-retardant coating based on a PA-MEL flame-retardant curing agent and a preparation method and application thereof. BACKGROUND

[0002] Wood has high flammability, which poses a fire safety hazard, so it is necessary to improve the flammability of wood. At present, a commonly used method to improve the flame retardancy of wood is to coat the surface of wood with a flame-retardant coating, which can isolate wood from fire and achieve the purpose of flame retardancy.

[0003] The current common preparation method of flame-retardant coating is to directly add a flame-retardant agent to the coating. Although the addition of traditional halogen flame-retardant agents has the advantages of good flame-retardant effect and low cost, they will produce substances harmful to the human body such as dioxins when burning. In order to reduce the use of halogen flame-retardant agents, phosphorus-based flame-retardant agents can be used as a substitute. Although phosphorus-based flame-retardant agents also have good flame-retardant effect and low cost, the use of phosphorus-based flame-retardant agents can also cause eutrophication problems. Therefore, the main research direction of current flame-retardant agents is bio-based flame-retardant agents.

[0004] Among bio-based flame-retardant agents, phytic acid-based flame-retardant agents are commonly used in flame-retardant coatings due to their green and renewable characteristics and high flame-retardant properties. For example, existing document 1 (Novel design and synthesis of bio-based polyelectrolyte complexes for enhancing the flame retardancy of epoxy resin [J]. Materials Chemistry and Physics 291 (2022) 126674) synthesized a bio-based polyelectrolyte complex PA-Ni-PEI by reacting phytic acid, nickel ions and polyethyleneimine, and added it as a flame-retardant agent to epoxy resin EP. Then, by using 4,4-diaminodiphenyl methane DDM as a curing agent, the flame-retardant performance of EP was improved, and the flame-retardant EP reached UL-94 V-1 level and the limiting oxygen index reached 27%. However, the technical solution has the problem that PA-Ni-PEI as an additive flame-retardant agent only plays a role in improving the flame-retardant performance, and the curing of epoxy resin still requires the additional use of a curing agent.

[0005] Similar situation also exists in the work of the inventor of the present application on the same application date, in the research on "a PCS-based flame-retardant urea-formaldehyde resin and its preparation method and application", it is found that adding phytic acid-based flame retardant phytic acid-chitosan composite polymer PCS to urea-formaldehyde resin can effectively improve the flame-retardant performance of urea-formaldehyde resin, but the added phytic acid-based flame retardant can only provide flame-retardant effect, and cannot play the role of curing agent of urea-formaldehyde resin, therefore, in this work, a composite curing agent needs to be additionally added to cure the urea-formaldehyde resin. For the above-mentioned technical solutions which need to add both flame retardant and curing agent, additional curing agent needs to be added, thereby increasing the raw material cost and improving the complexity of the production process.

[0006] In order to solve the above technical problems, a flame-retardant curing agent with both flame-retardant and curing functions can be prepared. For example, existing document 2 (Sustainable, high-performance, flame-retardant waterborne wood coatings via phytic acid based green curing agent formelamine-urea-formaldehyde resin [J]. Progress in Organic Coatings 162 (2022) 106597) prepares a phytic acid ammonium by controlling the molar ratio of phytic acid and urea, and uses the phytic acid ammonium as a flame-retardant curing agent to cure melamine-urea-formaldehyde resin, to obtain a flame-retardant coating. After the flame-retardant coating is applied to the surface of wood, the flame-retardant wood passes the UL-94 V-0 level, and the limiting oxygen index is as high as 42.4%. However, the technical problem of the technical solution is that the content of the curing agent used is more than 10 wt.%, while the content of the conventional curing agent is only 1-2 wt.%, obviously, the curing function of the flame-retardant curing agent in this technical solution cannot achieve the effect of the conventional curing agent, which directly leads to a substantial increase in raw material cost when using the flame-retardant curing agent.

[0007] Therefore, the technical problem existing in the prior art at present, i.e. the actual problem to be solved, is to prepare a flame-retardant curing agent with both flame-retardant and curing functions, and to achieve the effect of the conventional curing agent in the curing performance while ensuring the flame-retardant effect, i.e. to reduce the use amount of the flame-retardant curing agent, thereby reducing the raw material cost. SUMMARY

[0008] The purpose of the present application is to provide a flame-retardant coating based on PA-MEL flame-retardant curing agent and its preparation method and application.

[0009] In view of the problems existing in the prior art, the ion reaction between phytic acid and melamine is used first to prepare a phytic acid-melamine polyelectrolyte PM, which is used as a flame-retardant curing agent to cure and modify a urea-formaldehyde resin, and a bio-based flame-retardant coating is prepared, then the flame-retardant coating is coated on wood to improve the flame-retardant performance of the wood, and the problem of high usage of the curing agent is solved, specifically,

[0010] The method uses melamine and sodium lignosulfonate as additives, dioctyl phthalate as a plasticizer, and a phytic acid-melamine polyelectrolyte PM as a bio-based flame-retardant curing agent to achieve the following technical effects:

[0011] 1. Adding melamine optimizes the molecular structure of the urea-formaldehyde resin and improves the mechanical properties of the flame-retardant coating. In addition, melamine can block some hydrophilic groups of the flame-retardant coating, improving the water resistance of the flame-retardant coating;

[0012] 2. Adding sodium lignosulfonate improves the dispersibility of the flame-retardant curing agent in the flame-retardant coating, promotes curing, and reduces the usage of the curing agent. In addition, sodium lignosulfonate has a high carbon content, which can increase the carbon residue of the urea-formaldehyde resin and help improve the flame-retardant performance;

[0013] 3. Adding the plasticizer DOP improves the mechanical properties of the flame-retardant coating;

[0014] 4. Using the renewable and widely available bio-based materials phytic acid and melamine to synthesize an environmentally friendly bio-based flame-retardant curing agent PM, which is used to cure the flame-retardant coating and improve the flame-retardant performance, to solve the cost problem caused by the simultaneous addition of flame retardants and curing agents in conventional flame-retardant modification. In addition, it can replace the commonly used halogen curing agent ammonium chloride for urea-formaldehyde resin, avoiding the hazards that may be caused by halogen combustion.

[0015] In order to achieve the above-mentioned purposes, the technical scheme adopted by the present application is:

[0016] A flame-retardant coating based on a PA-MEL flame-retardant curing agent, using urea and formaldehyde as main raw materials, melamine and sodium lignosulfonate as additives, dioctyl phthalate DOP as a plasticizer, and a phytic acid-melamine polyelectrolyte PM as a bio-based flame-retardant curing agent. The temperature at a decomposition mass of 5% is 244.9℃, the temperature at the maximum decomposition rate is 297.7℃, and the carbon residue at 800℃ is 37.4wt.%.

[0017] A preparation method of a flame-retardant coating based on a PA-MEL flame-retardant curing agent, comprising the following steps:

[0018] Step 1, preparation of phytic acid-melamine polyelectrolyte PM, first, under certain conditions, melamine MEL is dissolved in deionized water to obtain MEL solution, at the same time, phytic acid PA solution is dissolved in deionized water to obtain PA solution, then, under certain conditions, PA solution is added dropwise into MEL solution to make MEL and PA fully react, after the reaction is completed, the obtained product is washed, then the filter residue is dried and ground, and PA-MEL, abbreviated as PM, can be obtained;

[0019] In the step 1, the molar mass ratio of MEL to PA is 6:1;

[0020] The stirring conditions of the MEL solution are that the dissolution temperature is 80-90℃, the dissolution stirring speed is 500-600rpm, and the dissolution stirring time is 30-60min;

[0021] The conditions for adding PA solution are that the dropping temperature is 80-90℃, the dropping stirring speed is 500-600rpm, the dropping stirring time is 1-1.5h, and the dropping rate is 0.5 drops / s;

[0022] The washing conditions are that the pH value of the washing liquid is between 4.0-5.0; the drying temperature is 60-80℃, and the drying time is 24-48h;

[0023] Step 2, preparation of urea-formaldehyde resin emulsion MUF, first, sodium hydroxide solution is added into formaldehyde solution to adjust the pH value of the solution, then, under certain conditions, raw materials are added in three stages to prepare urea-formaldehyde resin emulsion MUF, and the three stages are as follows,

[0024] The first stage is to add the first stage urea first, and then add melamine and sodium lignosulfonate under certain temperature conditions;

[0025] The second stage is to add acetic acid solution to adjust the pH value of the solution first, and then add the second stage urea under certain temperature conditions;

[0026] The third stage is to add sodium hydroxide solution to adjust the pH value of the solution first, and then add the third stage urea under certain temperature conditions, and stirring under certain conditions can obtain MUF;

[0027] In the step 2, the mass ratio of formaldehyde solution, total mass of urea, melamine and sodium lignosulfonate is 100:57:5.7;

[0028] The urea addition amount in the three stages satisfies the mass ratio of 37:12.4:7.6;

[0029] The pH value of the formaldehyde solution adjusted by adding sodium hydroxide solution is 8.0-8.5;

[0030] The conditions for adding raw materials in the step 2 are that the stirring speed is 400 rpm and the temperature is increased to 90 DEG C;

[0031] The conditions for the first stage are that the temperature is kept at 90 DEG C, and after the addition is completed, the stirring time is 30 min;

[0032] The conditions for the second stage are that the temperature is kept at 90 DEG C, and after the addition is completed, the stirring time is 10-15 min, so that the reaction reaches the end point;

[0033] The conditions for the third stage are that the temperature is kept at 90 DEG C, and after the addition is completed, the stirring time is 30 min;

[0034] The conditions for stirring after the addition is completed in the third stage are that the stirring temperature is 70 DEG C and the stirring time is 30 min;

[0035] In step 3, the preparation of the flame-retardant coating SUF based on the PA-MEL flame-retardant curing agent, MUF, dioctyl phthalate DOP and PM obtained in step 1 meet a certain mass ratio, DOP and PM are added to MUF, and after the addition is completed, mechanical stirring is carried out under the conditions, so that the components are uniformly mixed, and the flame-retardant coating based on the PA-MEL flame-retardant curing agent, namely SUF, is obtained;

[0036] In the step 3, the mass ratio of MUF, DOP and PM is 100:1:2;

[0037] The mechanical stirring conditions after the addition of DOP and PM are that the stirring speed is 500-600 rpm and the stirring time is 1.5-2 h.

[0038] The application of the flame-retardant coating based on the PA-MEL flame-retardant curing agent as the wood flame-retardant coating has the flame-retardant property, the wood coated with the flame-retardant coating passes the UL-94 V-0 level test in the UL-94 level test, the residual carbon layer formed after the complete combustion of the wood coated with the flame-retardant coating shows the continuous and dense property, the holes are few and small, and the wood coated with the flame-retardant coating has the limiting oxygen index of 32.1% in the limiting oxygen index test.

[0039] The technical effects of the green bio-based wood flame-retardant coating obtained in the application are tested as follows:

[0040] It can be known from the DSC test result that the DSC curve of the flame-retardant coating has only one curing peak, and the peak temperature is 84.1 DEG C. The curing behavior of the flame-retardant coating is single, and the lower peak temperature indicates that the flame-retardant coating can complete the curing in a lower temperature range, and the energy consumption required for curing is small. In addition, the curing behavior of the flame-retardant urea-formaldehyde resin is not affected by the use of PM as the curing agent.

[0041] TG test results show that: using PM as curing agent can improve the initial decomposition temperature and the temperature at which the maximum decomposition rate is reached of the material, that is, improve the thermal stability, and also can greatly improve the carbon residue. But with the increase of PM amount, the thermal stability of the material will decrease, therefore, the addition amount of PM cannot be too much.

[0042] Further SEM test results of the carbon residue carbon layer after complete combustion of the wood coated with the flame retardant coating show that: the flame retardant coating can improve the quality of the carbon residue carbon layer after combustion of the wood. Using PM as curing agent can improve the quality of the carbon residue carbon layer after combustion of the wood coated with the flame retardant coating, improve the ability of the carbon residue carbon layer to insulate oxygen and heat, and further improve the flame retardant performance.

[0043] Vertical combustion test results show that: coating the flame retardant coating can improve the UL-94 grade of the wood. Using PM as curing agent can improve the UL-94 grade of the wood coated with the flame retardant coating.

[0044] Limiting oxygen index test results show that: coating the flame retardant coating can greatly improve the limiting oxygen index of the wood. Using PM as curing agent can improve the limiting oxygen index of the wood coated with the flame retardant coating.

[0045] Therefore, the green bio-based wood flame retardant coating of the present application has the following advantages over the prior art:

[0046] 1. The bio-based flame retardant coating prepared in the present application has obvious flame retardant effect on wood, and the wood coated with the flame retardant coating has excellent flame retardant performance;

[0047] 2. The bio-based flame retardant curing agent PM used in the present application not only greatly improves the flame retardant performance of the flame retardant coating, but also can replace the use of urea-formaldehyde resin full-halogen curing agent ammonium chloride, eliminating the safety hazards caused by halogen combustion and being more environmentally friendly;

[0048] 3. The auxiliary agent melamine and sodium lignosulfonate used in the present application have the characteristics of low cost and environmental friendliness, and the modified urea-formaldehyde resin obtained by optimizing the molecular structure of urea-formaldehyde resin also has the characteristics of environmental friendliness, at the same time, the water resistance and dispersibility of the urea-formaldehyde resin are improved, and the amount of the curing agent is reduced. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 FT-IR graph of phytic acid-melamine polyelectrolyte PM prepared in Example 1;

[0050] Figure 2 FT-IR graphs of Example 1, Comparative Example 2, Example 2, and Example 3;

[0051] Figure 3DSC graphs of Example 1, Comparative Example 2, Example 2, Example 3;

[0052] Figure 4 TG graphs of Example 1, Comparative Example 1, Comparative Example 2, Example 3;

[0053] Figure 5 SEM graphs of Example 1, Comparative Example 1, Comparative Example 2, Example 3, carbon layer after combustion, wherein Figure 5 a is Comparative Example 1, Figure 5 b is Comparative Example 2, Figure 5 c is Example 1;

[0054] Figure 6 Vertical burning test graphs of Example 1, Comparative Example 1, Comparative Example 2, Example 2, Example 3, wherein Figure 6 a is before vertical burning test, Figure 6 b is after vertical burning test. DETAILED DESCRIPTION

[0055] The present application is further described in detail by way of examples with reference to the accompanying drawings. However, the present application is not limited to the examples.

[0056] Example 1

[0057] A preparation method of a flame-retardant coating based on a PA-MEL flame-retardant curing agent, comprising the following steps:

[0058] Step 1, preparation of phytic acid-melamine polyelectrolyte PM, first, 25.3 g of melamine MEL is dissolved in 400 ml of deionized water under the conditions of a dissolution temperature of 85°C, a dissolution stirring speed of 600 rpm, and a dissolution stirring time of 30 min to obtain a MEL solution, at the same time, 24.5 mL of 70 wt.% phytic acid PA solution is dissolved in 50 mL of deionized water to obtain a PA solution, then, the PA solution is added dropwise to the MEL solution under the conditions of a dropwise adding temperature of 85°C, a dropwise adding stirring speed of 600 rpm, a dropwise adding stirring time of 1 h, and a dropwise adding rate of 0.5 drops / s to make MEL and PA fully react, after the reaction is completed, the obtained product is washed until the pH value of the washing liquid is in the range of 4.0-5.0, then the filter residue is dried under the conditions of a drying temperature of 80°C and a drying time of 24 h and ground, and thus PA-MEL, abbreviated as PM, is obtained;

[0059] In order to prove the composition of PM, i.e. successful synthesis, FT-IR test is performed, and the test results are shown in Figure 1 It can be seen that PM contains both characteristic peaks belonging to MEL and characteristic peaks belonging to PA. The test results show that PM is successfully synthesized.

[0060] Step 2, preparation of modified urea-formaldehyde resin emulsion MUF, first, 20wt.% sodium hydroxide solution was added to 100g 37wt.% formaldehyde solution to adjust the pH value of the solution to 8.0-8.5, then, the temperature was raised to 90℃ under the stirring speed of 400rpm, after that, the raw materials were added in three stages to prepare the modified urea-formaldehyde resin emulsion MUF, the three stages were,

[0061] The first stage was to add 37g first-stage urea first, then 0.57g melamine and 5.7g sodium lignosulfonate SLS under the condition of keeping the temperature at 90℃, after the addition, the stirring was continued for 30min;

[0062] The second stage was to add 20wt.% acetic acid solution first to adjust the pH value of the solution to 4.5-5.0, then 12.4g second-stage urea, after the addition, the stirring was continued for 10min until the reaction reached the end point;

[0063] The judgment mark of the end point of the second stage reaction was that one drop of the solution was dropped into 30℃ water, and the dropped solution coagulated in the water without dispersing;

[0064] The third stage was to add 20wt.% sodium hydroxide solution first to adjust the pH value of the solution to 7.5-8.0, then 7.6g third-stage urea, after the addition, the temperature was adjusted, and the stirring was continued at the stirring temperature of 70℃ for 30min, and MUF was obtained;

[0065] Step 3, preparation of flame-retardant coating SUF based on PA-MEL flame-retardant curing agent, 1g DOP and 2g PM were added to MUF at the mass ratio of MUF, DOP and PM obtained in step 1 as 100:1:2, after the addition, mechanical stirring was carried out at the stirring speed of 600rpm for 2h to mix the components uniformly, and the flame-retardant coating based on PA-MEL flame-retardant curing agent, referred to as SUF, was obtained, the SUF prepared in specific example 1 was referred to as SUF-2.

[0066] In order to prove the composition of SUF-2, FT-IR test was carried out, and the test results were as shown in Figure 2 The test results showed that SUF-2 was composed of UF and SLS, that is, sodium lignosulfonate reacted with urea-formaldehyde resin emulsion.

[0067] In order to prove the curing characteristics of SUF-2, DSC test was carried out, and the test results were as shown in Figure 3As shown, the DSC curve of SUF-2 has only one peak, and the peak temperature is 84.1℃. The test results show that the curing behavior of the flame-retardant coating of SUF-2 is single, and the lower peak temperature indicates that the flame-retardant coating can complete curing at a lower temperature range, and the energy consumption required for curing is small.

[0068] In order to prove the thermal stability of SUF-2, TG test was carried out, and the test results are as shown in Figure 4 As shown in Table 1 and Table 1, the temperature at which the mass of SUF-2 decomposes by 5% is 244.9℃, the temperature at which the maximum decomposition rate is reached is 297.7℃, and the carbon residue at 800℃ is 37.4wt.%. The test results show that the high decomposition temperature indicates that SUF-2 has high thermal stability, and at the same time, it also has a high carbon residue.

[0069] Table 1 Thermal gravimetric test results

[0070]

[0071]

[0072] In order to prove the technical effect of SUF as a wood flame-retardant coating application, SUF was coated on the surface of wood to prepare flame-retardant wood SUFW for flame-retardant test.

[0073] The specific preparation method of SUFW is that, with a coating thickness of 0.3mm, SUF is coated on the surface of wood, and after coating is completed, drying is carried out under the condition that the drying time is 24h, and SUFW is obtained, and the SUFW prepared based on SUF-2 of Example 1 is named as SUFW-2.

[0074] TG test proves that SUF-2 has the characteristic of high carbon residue, in order to further prove the influence of high carbon residue on flame-retardant performance, the carbon residue layer after complete combustion of SUFW-2 is tested by SEM. The test results are as shown in Figure 5 As shown in c, the carbon residue layer formed after complete combustion of the flame-retardant wood SUFW-2 coated with SUF-2 shows continuous and dense properties, and has few and small holes. The test results show that the carbon residue layer of SUFW-2 is of good quality, which can effectively isolate oxygen and heat on the condensed phase, thereby improving the flame-retardant performance of wood.

[0075] In order to further prove the flame-retardant performance of SUFW-2, vertical combustion test and limiting oxygen index test were carried out. The test results are as shown in Figure 6 As shown in Table 2 and Table 2, SUFW-2 passed the UL-94V-0 level test; and the limiting oxygen index of SUFW-2 is as high as 32.1%. The test results show that the flame-retardant performance of SUFW-2 is excellent.

[0076] Table 2 Vertical burning test and oxygen index test results

[0077]

[0078] To prove the effect of the flame-retardant coating SUF-2 on the flame-retardant performance of wood, comparative example 1 is provided, which is pure wood without SUF coating.

[0079] Comparative example 1

[0080] A pure wood without SUF coating, referred to as pure wood.

[0081] The SEM test results of the residual carbon layer of the pure wood after complete combustion are shown in Figure 5 As shown in FIG. 8, the residual carbon layer of the pure wood is broken and has many broken fiber structures. In comparison with example 1, it can be seen that the residual carbon layer of the wood coated with SUF-2 becomes continuous and dense, with few and small holes, i.e., the quality of the residual carbon layer is greatly improved.

[0082] The vertical burning test and limiting oxygen index test results of the pure wood are shown in Figure 6 and Table 2. In the vertical burning test, the pure wood fails to pass the UL-94 rating test, and the limiting oxygen index is 20.5%. In comparison with example 1, it can be seen that the UL-94 rating of the wood coated with SUF-2 is improved from failing to pass the test to V-0, and the limiting oxygen index is greatly improved from 20.5% to 32.1%, with an increase of 156.6%, i.e., the test results show that SUF-2 significantly improves the flame-retardant performance of SUFW.

[0083] To prove the effect of the flame-retardant curing agent PM on the performance of the flame-retardant coating SUF and the flame-retardant wood SUFW, comparative example 2 is provided, which is a flame-retardant coating prepared without using PM as a curing agent, but using a conventional ammonium chloride as a curing agent.

[0084] Comparative example 2

[0085] A preparation method of a flame-retardant coating based on an ammonium chloride curing agent, the steps not specifically mentioned are the same as those in example 1, and the difference is that the step 1 is not required, and in the step 3, ammonium chloride is used instead of PM, and the obtained flame-retardant coating is named as SUF-0, and the further obtained flame-retardant wood is named as SUFW-0.

[0086] The DSC test results of SUF-0 are shown in Figure 3 As shown in FIG. 10, the DSC curve of SUF-0 has only one peak, and the peak temperature is 88.7℃. In comparison with example 1, it can be seen that the curing curve behaviors of SUF-2 and SUF-0 are consistent. The test results show that using PM as a curing agent does not affect the curing behavior of SUF.

[0087] The TG test results of SUF-0 are shown inFigure 4 As shown in Table 1, the temperature at which the decomposition mass of SUF-0 is 5% is 235.6°C, the temperature at which the maximum decomposition rate is reached is 275.1°C, and the amount of residual carbon at 800°C is 14.3wt.%. Compared with Example 1, it can be seen that the use of PM as a curing agent can increase the temperature at which the decomposition mass is 5% by 9.3°C, increase the temperature at which the maximum decomposition rate is reached by 22.6°C, and increase the amount of residual carbon at 800°C by 23.1wt.%, which proves that the use of PM as a curing agent can improve the thermal stability of SUF, and also can greatly increase the amount of residual carbon of SUF.

[0088] The SEM test results of the residual carbon layer of SUF-0 after complete combustion are shown in Figure 2. Figure 5 As shown in Figure 2b, the residual carbon layer of SUF-0 has more wrinkles and more large holes. Compared with Example 1, it can be seen that the use of PM as a curing agent can make the residual carbon layer of SUFW-2 more continuous and dense, with few and small holes. The test results show that the use of PM as a curing agent can improve the quality of the residual carbon layer of SUFW, thereby improving the flame retardant performance.

[0089] The vertical combustion test and limiting oxygen index test results of SUF-0 are shown in Table 2. Figure 6 As shown in Table 2, in the vertical combustion test, SUF-0 passed the UL-94 V-1 level test, and the limiting oxygen index was 27.2%. Compared with Example 1, it can be seen that the use of PM as a curing agent can increase the UL-94 level from V-1 level to V-0 level, and can increase the limiting oxygen index of SUFW-2 by 4.9%, which proves that the use of PM as a curing agent can improve the UL-94 level and greatly increase the limiting oxygen index, thereby greatly improving the flame retardant performance.

[0090] In order to prove the effect of the addition amount of PM on the performance of the flame retardant coating SUF and the flame retardant wood SUFW, Examples 2 and 3 are provided, which are flame retardant coatings with PM addition amounts of 1wt.% and 1.5wt.%.

[0091] Example 2

[0092] A method for preparing a SUF with a PM addition amount of 1wt.%, the steps not specifically stated are the same as those of Example 1, the difference is that in step 3, the addition amount of PM is 1wt.%, i.e. 1g of PM, and the obtained bio-based flame retardant coating is named as SUF-1, and the further obtained flame retardant wood is named as SUFW-1.

[0093] The DSC test results of SUF-1 are shown in Figure 3. Figure 3As shown in Figure 4, the DSC curve of SUF-1 has only one peak with a peak temperature of 88.5 °C. In comparison with Example 1, the curing curve behavior of SUF-2 and SUF-1 is consistent, but the peak temperature of SUF-2 is 84.1 °C, which is lower than that of SUF-1 (88.5 °C). The test results show that increasing the amount of PM does not affect the curing behavior of SUF, but can lower the curing temperature.

[0094] The TG test results of SUF-1 are shown in Figure 5 and Table 1. Figure 4 As shown in Figure 5 and Table 1, the temperature at which the mass of SUF-1 decomposes by 5% is 253.1 °C, the temperature at which the maximum decomposition rate is reached is 305.2 °C, and the amount of residual carbon at 800 °C is 32.8 wt.%. In comparison with Example 1, increasing the amount of PM by 1 wt.% can lower the temperature at which the mass decomposes by 5% by 8.2 °C, lower the temperature at which the maximum decomposition rate is reached by 7.5 °C, and increase the amount of residual carbon at 800 °C by 4.6 wt.%. The test results show that increasing the amount of PM can lower the initial decomposition temperature and the maximum decomposition rate temperature of SUF, i.e., decrease the thermal stability, but can increase the amount of residual carbon of SUF.

[0095] The vertical burning test and limiting oxygen index test results of SUFW-1 are shown in Figure 6 and Table 2. Figure 6 As shown in Figure 6 and Table 2, in the vertical burning test, SUFW-1 passed the UL-94 V-0 level test, and the limiting oxygen index was 29.1%. In comparison with Example 1, increasing the amount of PM by 1 wt.% did not affect the UL-94 level, but increased the limiting oxygen index by 3%. The test results show that increasing the amount of PM does not affect the UL-94 level of SUFW, but can increase the limiting oxygen index, i.e., improve the flame retardant performance.

[0096] Example 3

[0097] A method for preparing a SUF with a PM addition amount of 1.5 wt.%, the steps are not particularly specified and are the same as those of Example 1, except that in the step 3, the addition amount of PM is 1.5 wt.%, i.e., 1.5 g of PM. The obtained bio-based flame retardant coating is named as SUF-1.5, and the further obtained flame retardant wood is named as SUFW-1.5.

[0098] The DSC test results of SUF-1.5 are shown in Figure 7. Figure 3 As shown in Figure 7, the DSC curve of SUF-1.5 has only one peak with a peak temperature of 85.8 °C. In comparison with Example 1, the curing curve behavior of SUF-2 and SUF-1.5 is consistent, but the peak temperature of SUF-2 is 84.1 °C, which is lower than that of SUF-1.5 (85.8 °C). The test results show that increasing the amount of PM does not affect the curing behavior of SUF, but can lower the curing temperature.

[0099] TG test results of SUF-1.5 are shown in Table 1 Figure 4 As shown in Table 1, the temperature at which the decomposition mass of SUF-1.5 is 5% is 248.3°C, the temperature at which the maximum decomposition rate is reached is 312.2°C, and the amount of residual carbon at 800°C is 32.8wt.%. Compared with Example 1, it can be seen that increasing the amount of PM by 0.5wt.% makes the temperature at which the decomposition mass is 5% decrease by 3.4°C, the temperature at which the maximum decomposition rate is reached decrease by 14.5°C, and the amount of residual carbon at 800°C increase by 2.9wt.%. The test results show that increasing the amount of PM can make the initial decomposition temperature and the maximum decomposition rate temperature of SUF decrease, but can increase the amount of residual carbon of SUF.

[0100] Vertical burning test and limiting oxygen index test results of SUFW-1.5 are shown in Table 2 Figure 6 As shown in Table 2, in the vertical burning test, SUFW-1.5 passed the UL-94 V-0 level test, and the limiting oxygen index was 30.4%. Compared with Example 1, it can be seen that increasing the amount of PM by 0.5wt.% does not affect the UL-94 level, but the limiting oxygen index increases by 1.7%. The test results show that increasing the amount of PM does not affect the UL-94 level of SUFW, but can increase the limiting oxygen index, i.e. improve the flame retardant performance.

[0101] From Comparative Example 1, Comparative Example 2, Example 1, Example 2 and Example 3, the following conclusions can be drawn:

[0102] 1. The flame retardant performance of SUFW coated with SUF is greatly improved compared with pure wood without coating, and the flame retardant coating can effectively improve the flammable characteristics of wood, so that the wood has high flame retardant performance.

[0103] 2. Using PM as a bio-based flame-retardant curing agent can effectively improve the flame retardant performance of SUFW, and as the content increases, the flame retardant performance of SUFW will also increase. However, the increase of its content will gradually reduce the thermal stability of SUF, but the thermal stability after reduction is still higher than that of SUF cured with ammonium chloride, therefore, the amount of PM added should not be too much.

Claims

1. A process for the preparation of a flame retardant coating based on PA-MEL flame retardant curing agent, characterized by: With urea, formaldehyde as the main raw material, melamine, lignin sulfonate sodium as the auxiliary agent, dioctyl phthalate DOP as the plasticizer, phytic acid-melamine polyelectrolyte PM as the bio-based flame-retardant curing agent, comprising the following steps: Step 1, preparation of phytic acid-melamine polyelectrolyte PM, first, melamine MEL is dissolved in deionized water to obtain MEL solution, at the same time, phytic acid PA solution is dissolved in deionized water to obtain PA solution, then, PA solution is added to MEL solution at a temperature of 80-90 DEG C to make MEL and PA fully react, after the reaction is completed, the obtained product is washed, then the filter residue is dried and ground to obtain PA-MEL, which is abbreviated as PM; In step 1, MEL and PA satisfy the molar mass ratio of 6:1; In step 1, the stirring conditions of MEL solution are that the dissolution temperature is 80-90 DEG C, the dissolution stirring speed is 500-600 rpm, and the dissolution stirring time is 30-60 min; The washing conditions are that the pH value of the washing liquid is between 4.0-5.0; the drying temperature is 60-80 DEG C, and the drying time is 24-48 h; Step 2, preparation of urea-formaldehyde resin emulsion MUF, first, sodium hydroxide solution is added to formaldehyde solution to adjust the pH value of the solution to 8.0-8.5, then, under the condition of stirring speed of 400 rpm, the temperature is raised to 90 DEG C, and the raw materials are added in three stages to prepare urea-formaldehyde resin emulsion MUF, the three stages are as follows, The first stage is to keep the temperature at 90 DEG C, first add the first stage urea, then add melamine and lignin sulfonate sodium SLS, and continue stirring for 30 min after the addition is completed; The second stage is to keep the temperature at 90 DEG C, first add acetic acid solution to adjust the solution pH value to 4.5-5.0, then add the second stage urea, and stir for 10-15 min after the addition is completed to ensure that the reaction reaches the end point; The third stage is to keep the temperature at 90 DEG C, first add sodium hydroxide solution to adjust the solution pH value to 7.5-8.0, then add the third stage urea, and stir for 30 min at a stirring temperature of 70 DEG C to obtain MUF In step 2, the urea addition amount in the three stages satisfies the mass ratio of 37:12.4:7.6; In step 2, the mass ratio of formaldehyde solution, total mass of urea, melamine and lignin sulfonate sodium is 100:57:0.57:5.7; Step 3, preparation of flame-retardant coating SUF based on PA-MEL flame-retardant curing agent, MUF, dioctyl phthalate DOP and PM obtained in step 1 satisfy a certain mass ratio, DOP and PM are added to MUF, and mechanical stirring is carried out under certain conditions to make the components uniformly mixed to obtain the flame-retardant coating based on PA-MEL flame-retardant curing agent, which is abbreviated as SUF; The temperature at which the obtained flame-retardant coating based on the PA-MEL flame-retardant curing agent has a decomposition mass of 5% is 244.9°C, and the temperature at which the maximum decomposition rate is reached is 297.7°C; the amount of residual carbon at 800°C is 37.4 wt.%; In the step 3, the mass ratio of MUF, DOP and PM is 100:1:

2.

2. The method of claim 1, wherein: The conditions for dropping the PA solution are that the stirring speed is 500-600 rpm, the stirring time is 1-1.5 h, and the dropping rate is 0.5 drop / s.

3. The method of claim 1, wherein: In the step 3, the mechanical stirring conditions after the addition of DOP and PM are completed are that the stirring speed is 500-600 rpm, and the stirring time is 1.5-2 h.

4. The method of claim 1, wherein: The application of the obtained flame-retardant coating based on the PA-MEL flame-retardant curing agent as a wood flame-retardant coating has flame-retardant properties, and the wood coated with the flame-retardant coating passes the UL-94 V-0 level test in the UL-94 level test.

5. The method of claim 1, wherein: The application of the obtained flame-retardant coating based on the PA-MEL flame-retardant curing agent as a wood flame-retardant coating, the residual carbon layer formed after the complete combustion of the wood coated with the flame-retardant coating exhibits continuous and dense properties, and has few and small holes.

6. The method of claim 1, wherein: The application of the obtained flame-retardant coating based on the PA-MEL flame-retardant curing agent as a wood flame-retardant coating, the wood coated with the flame-retardant coating has an limiting oxygen index of 32.1% in the limiting oxygen index test.

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