A bio-based flame retardant intumescent powder coating and a method of making the same

Abstract

This invention discloses a bio-based flame-retardant thermally expanding powder coating and its preparation method, relating to the field of coatings. It comprises the following components: bio-based epoxy resin, phenolic epoxy curing agent, ammonium polyphosphate, nano-titanium dioxide, zinc borate, expanded graphene, 8-12 parts of imidazole, benzoin, and a leveling agent. The bio-based epoxy resin is prepared by ring-opening copolymerization of cashew phenol glycidyl ether and linoleic acid prepolymer, followed by epoxy end-capping, with an epoxy equivalent of 900-1200 g / eq; the phenolic epoxy curing agent has an epoxy equivalent of 200-250 g / eq; and the expanded graphene has an initial expansion ratio of 200-300 times. The preparation method employs a two-step, staged mixing process. This invention, while maintaining zero VOC in powder coatings and the same film thickness, achieves for the first time the outstanding performance of 6H hardness, UL94V-0 flame retardancy, and 120 times expansion ratio through the systematic synergy of high epoxy equivalent bio-based resin, four-component complete flame retardant system, specific EG:APP ratio, and two-step process.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to a bio-based flame-retardant thermal expansion powder coating and its preparation method. Background Technology

[0002] Intumescent fire-retardant coatings are among the most widely used passive fire-retardant materials. Their fire-retardant mechanism works by rapidly foaming and expanding the flame-retardant components in the coating when exposed to fire or high temperatures, forming a porous, insulating carbonaceous foam layer that effectively isolates heat transfer and blocks oxygen, thereby slowing the spread of fire. Depending on the application, intumescent fire-retardant coatings can be categorized into decorative types and steel structure-specific types, with core performance indicators including fire resistance limit, expansion ratio, and hardness.

[0003] In recent years, fire-retardant powder coatings have been gradually replacing traditional solvent-based fire-retardant paints due to their advantages such as zero VOC emissions, no risk of flammability and explosion, and high material utilization (up to 95% or more). They have broad application prospects, especially in fields with stringent safety and environmental protection requirements, such as the "three-electric" systems of new energy vehicles and energy storage equipment.

[0004] However, existing intumescent fire-retardant powder coatings still have many shortcomings. The expansion ratio of existing intumescent coatings is typically only 10 to 30 times, with a maximum of no more than 50 times. Coatings based on the classic three-source system of ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (Mel) have expansion ratios limited by the carbon source's char-forming ability and the gas source's foaming efficiency. Lower expansion ratios result in a limited thickness of the carbonaceous foam layer, making it difficult to meet high-level fire resistance requirements for thermal insulation and flame retardancy. Furthermore, to achieve high hardness (e.g., above 5H), the coating needs a high cross-linking density, but high cross-linking density often leads to a decrease in the coating's softening and flowability at high temperatures, hindering expansion. While existing epoxy powder coatings can reach the UL94V-0 rating, their hardness is mostly in the FH range, making it difficult to achieve above 5H; and fire-retardant coatings with higher hardness typically have lower expansion ratios.

[0005] In summary, there is currently a lack of intumescent powder coatings that can simultaneously achieve high expansion ratio, high hardness, and excellent flame retardancy. Summary of the Invention

[0006] The purpose of this invention is to provide a bio-based flame-retardant thermally expandable powder coating and its preparation method. By using a bio-based epoxy resin with an epoxy equivalent of 900~1200 g / eq and a phenolic epoxy curing agent to construct a soft-segment-hard-segment microphase separation crosslinking network, a four-component flame-retardant synergistic system is constructed. Combined with a two-step stepwise mixing process with imidazole pre-dispersion, this invention solves the technical problems of existing intumescent fire-retardant coatings having low expansion ratios and difficulty in simultaneously achieving high hardness, high expansion, and high flame retardancy.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A bio-based flame-retardant thermal expansion powder coating, comprising the following components by weight:

[0009] Bio-based epoxy resin 620~780 parts, phenolic epoxy curing agent 88~115 parts, ammonium polyphosphate (APP) 90~110 parts, nano titanium dioxide 40~58 parts, zinc borate 35~45 parts, expanded graphene 270~325 parts, imidazole 8~12 parts, benzoin 1~5 parts, leveling agent 8~12 parts;

[0010] The bio-based epoxy resin is prepared by ring-opening copolymerization of cashew phenol glycidyl ether and linoleic acid prepolymer, followed by epoxy end-capping, with an epoxy equivalent of 900~1200 g / eq; the phenolic epoxy curing agent has an epoxy equivalent of 200~250 g / eq; and the expanded graphene has an initial expansion ratio of 200~300 times.

[0011] This invention is the first to combine a high epoxy equivalent (900~1200 g / eq) bio-based epoxy resin with a low epoxy equivalent (200~250 g / eq) phenolic epoxy curing agent in a specific ratio. The high epoxy equivalent resin provides long, flexible segments, forming a flexible framework with low crosslinking density; the phenolic epoxy curing agent, due to its multifunctional structure, forms locally rigid hard segments after curing, thereby constructing a soft-segment-hard-segment microphase separation crosslinking network. Existing bio-based epoxy resins generally have an epoxy equivalent of less than 600 g / eq, making it difficult to balance the contradiction between high hardness and high expansion. This invention, through a self-made high epoxy equivalent resin, enables the coating to possess both a flexible framework and rigid crosslinking points after curing, achieving a pencil hardness of 6H while ensuring sufficient softening and flow of the resin at high temperatures, providing a structural basis for an ultra-high expansion of 120 times.

[0012] Regarding the flame-retardant system, this invention constructs a four-component synergistic system consisting of ammonium polyphosphate (APP), nano-titanium dioxide, zinc borate, and expanded graphene, which is key to achieving 120-fold physical expansion. APP decomposes upon heating to generate polyphosphoric acid, which catalyzes char formation and releases non-flammable gases. Expanded graphene rapidly expands interlayer at high temperatures, generating mechanical thrust that pushes the molten char layer outwards. The two components work synergistically in a mass ratio of approximately 3:1, superimposing physical expansion and chemical foaming to increase the coating expansion ratio from the conventional 10-30 times to 120 times. Simultaneously, nano-titanium dioxide reacts with phosphorus oxides at high temperatures to form a Ti-PO ceramic phase, significantly enhancing the high-temperature strength of the char layer. Zinc borate melts to form a B2O3 glassy capping layer, which both physically isolates oxygen and fixes the char layer structure. The combined effect of these two components effectively prevents the collapse or peeling of the char layer after 120-fold expansion, ensuring the coating meets the UL94V-0 flame-retardant rating.

[0013] Imidazole has a dual function in this invention. On the one hand, it adsorbs onto the surface of APP, nano titanium dioxide and zinc borate, improving the interfacial compatibility between the filler and the resin matrix and ensuring uniform dispersion of the filler. On the other hand, at high temperature, imidazole forms an ionic complex with the decomposition products of APP, catalyzing the char formation reaction and improving the char formation efficiency.

[0014] Furthermore, the bio-based epoxy resin used in this invention is prepared by ring-opening copolymerization of cashew phenol glycidyl ether and linoleic acid prepolymer followed by end-capping. Its molecular structure contains a cashew phenol benzene ring and a long linoleic acid chain. The benzene ring structure provides a rigid char-forming framework, while the long linoleic acid chain imparts flexibility and high-temperature flowability to the resin, perfectly matching the dual requirements of intumescent coatings for sufficient carbon source and high-temperature expandability. This resin exhibits inherent compatibility with other components in this formulation, and its synergistic effect is irreplaceable by existing conventional bio-based resins.

[0015] Benzoin, acting as a degassing agent, decomposes and releases gas during the high-temperature curing stage, expelling air bubbles from within the coating. The leveling agent reduces the surface tension of the molten coating, promoting melt flow and forming a dense, defect-free coating. The combination of these two agents prevents large pores from forming in the expanded carbon layer, ensuring its uniformity and mechanical strength.

[0016] A method for preparing a bio-based flame-retardant thermally expanding powder coating includes the following steps:

[0017] S100: A portion of bio-based epoxy resin, ammonium polyphosphate, nano titanium dioxide, zinc borate, and imidazole are mixed evenly at high speed, then melt-dispersed, extruded, cooled, and crushed to obtain sheet material A.

[0018] S200: The remaining bio-based epoxy resin, the sheet material A, the phenolic epoxy curing agent, the leveling agent, benzoin, and expanded graphene are mixed evenly by high-speed stirring, and then melt-extruded, cooled, crushed, and ground to obtain the finished powder coating.

[0019] This invention employs a two-step, staged mixing and preparation process, breaking away from the conventional approach of traditional powder coatings where all components are melted and extruded in a single step. The first step involves mixing and melting a portion of the bio-based epoxy resin with ammonium polyphosphate, nano-titanium dioxide, zinc borate, and imidazole. This allows imidazole to preferentially adsorb onto the surface of the flame-retardant filler, forming a pre-dispersed composite micro-region of "imidazole-flame retardant." This improves the interfacial compatibility between the filler and the resin matrix and prevents imidazole from prematurely contacting all the epoxy resin and curing agent, thus avoiding pre-crosslinking during the extrusion stage. The second step involves adding the remaining bio-based epoxy resin, phenolic epoxy curing agent, and expanded graphene. Here, the expanded graphene maintains its complete layered structure because it does not come into contact with imidazole, allowing it to fully utilize its interlayer expansion thrust at high temperatures.

[0020] The preparation method of this invention, on the one hand, protects the expansion performance of expanded graphene, enabling it to provide strong mechanical thrust when the coating is exposed to fire, and, in conjunction with the chemical foaming effect of APP, increases the expansion ratio to 120 times; on the other hand, the pre-dispersion of imidazole avoids early cross-linking during the extrusion process, ensuring the storage stability and curing uniformity of the coating, thereby enabling the coating hardness to stably reach 6H and the flame retardant rating to pass UL94V-0; in addition, this preparation method also makes the flame retardant filler more uniformly dispersed in the resin matrix, avoiding carbon layer defects caused by local agglomeration, and ensuring the integrity and density of the carbon layer under ultra-high expansion ratio.

[0021] Furthermore, the preparation method of the bio-based epoxy resin includes the following steps:

[0022] Step 1: Cashew phenol is mixed with epichlorohydrin and reacted at 80-100°C for 4-6 hours in the presence of tetrabutylammonium bromide (TBAB) to obtain cashew phenol glycidyl ether.

[0023] Step 2: Add p-toluenesulfonic acid to linoleic acid and perform a prepolymerization reaction at 100-110°C for 2-3 hours under a nitrogen atmosphere to obtain linoleic acid prepolymer;

[0024] Step 3: Mix the cashew phenol glycidyl ether obtained above with linoleic acid prepolymer, add tetrabutylammonium bromide, and carry out ring-opening copolymerization reaction at 90~120℃ for 5~7 hours to obtain copolymer intermediate product;

[0025] Step 4: Add epichlorohydrin to the above copolymer intermediate, add tetrabutylammonium bromide, and carry out the end-capping reaction at 60~80℃ for 2~4 hours. After post-treatment, the bio-based epoxy resin is obtained.

[0026] The present invention provides a method for preparing bio-based epoxy resin, which overcomes the technical limitation that the epoxy equivalent of existing bio-based epoxy resins is generally lower than 600 g / eq. By ring-opening copolymerization of cashew phenol glycidyl ether and linoleic acid prepolymer, long-chain flexible segments of linoleic acid are introduced into the molecular chain, effectively reducing the epoxy group density per unit molecular weight. Then, epoxy end-capping converts the terminal hydroxyl groups into epoxy groups, thereby precisely controlling the epoxy equivalent to a high value range of 900~1200 g / eq. Secondly, this method employs a two-step polymerization strategy. First, cashew phenol monoglycidyl ether is ring-opening copolymerized with a mixture of linoleic acid dimers / trimers to form a cashew phenol-fatty acid block structure. Then, epichlorohydrin is used to end-cap the copolymerization intermediate, achieving controllable adjustment of molecular weight (3000~8000) and softening point (80~110℃). The linoleic acid prepolymer is controlled at a dimer to trimer ratio of 2 to 3:1, which ensures that the copolymerization reaction has appropriate functionality and reactivity, and avoids excessive cross-linking or insufficient reaction.

[0027] The prepared bio-based epoxy resin contains a benzene ring skeleton of cashew phenol and a long carbon chain of linoleic acid in its molecular structure. After curing, it forms a flexible network with low cross-linking density, which not only ensures that the coating has excellent mechanical properties at room temperature, but also can fully soften and flow at high temperature. This provides an ideal melt environment for the physical extrusion of expanded graphene and the chemical foaming of APP, thereby synergistically achieving an ultra-high expansion ratio of 120 times.

[0028] Furthermore, the resin is derived from renewable biomass resources (cashew shell oil and vegetable oil), which meets the requirements of green and sustainable development. Moreover, its preparation process is mild, the catalyst is recyclable, and it is easy to scale up production, giving it outstanding industrial practical value.

[0029] Further, in step 1, the molar ratio of cashew phenol to epichlorohydrin is 1:1.1~1.5; the mass ratio of cashew phenol to tetrabutylammonium bromide (TBAB) is 1:0.01~0.05.

[0030] Current methods for synthesizing cashew phenol glycidyl ether typically employ a large excess of epichlorohydrin (molar ratio 1:3~1:5) to increase the reaction rate and epoxy value. This invention takes the opposite approach, controlling the molar ratio to a slight excess of 1:1.1~1.5. The aim is to selectively generate monoepoxy-terminated cashew phenol glycidyl ether, avoiding the formation of diepoxy byproducts. This ratio selection provides precise reaction sites for subsequent ring-opening copolymerization with linoleic acid prepolymer, a prerequisite for achieving the target epoxy equivalent (900-1200 g / eq).

[0031] Further, in step 2, the linoleic acid prepolymer includes dimer and trimer, with the content ratio of dimer to trimer being approximately 4~5:1; the mass ratio of linoleic acid to p-toluenesulfonic acid is 1:0.005~0.02.

[0032] In the composition of the oleic acid prepolymer of this invention, the ratio of dimer to trimer is controlled at 4-5:1. Secondly, the copolymerization reaction is predominantly linear, avoiding branching or micro-crosslinking caused by an excessively high trimer ratio, resulting in a narrower molecular weight distribution and significantly improved batch-to-batch stability. The linear structure helps to increase the epoxy equivalent, making it easier to reach the upper limit of 900-1200 g / eq, thereby reducing the crosslinking density of the cured network and ensuring sufficient softening and flowability of the coating at high temperatures.

[0033] Further, in step 3, the mass ratio of cashew phenol glycidyl ether to linoleic acid prepolymer is 1:0.6~1.4; the mass ratio of cashew phenol glycidyl ether to tetrabutylammonium bromide is 1:0.01~0.05.

[0034] This invention ensures that the proportion of fatty acid segments in the resin is appropriate by limiting the mass ratio to 1:0.6~1.4: if it is less than 1:0.6, the flexibility is insufficient, the high-temperature fluidity is poor, and the expansion ratio is difficult to increase; if it is more than 1:1.4, the resin is too soft, the hardness at room temperature decreases, and the char rate decreases.

[0035] Furthermore, in step 4, the end-capping reaction is monitored by infrared spectroscopy to detect the characteristic peak of hydroxyl groups in the copolymerization intermediate (wavenumber approximately 3450 cm⁻¹). -1 The disappearance of the characteristic peak is taken as the reaction endpoint, and the reaction is stopped when the intensity of the characteristic peak decreases to the level of background noise and no longer changes; the mass ratio of the copolymerization intermediate to the added tetrabutylammonium bromide is 1:0.005~0.03.

[0036] Furthermore, the mass ratio of the bio-based epoxy resin added in step S100 to the bio-based epoxy resin added in step S200 is 1:1 to 1.3.

[0037] In the two-step process, the ratio of the two resin components is a key parameter affecting the filler dispersion and the final coating performance. This invention limits the mass ratio of the first to the second component to 1:1 to 1.3, meaning the amounts of both components are roughly equal or the first component is slightly more abundant. If the proportion of the first component is too low, the imidazole and flame-retardant filler cannot be adequately coated, resulting in poor pre-dispersion. If the proportion of the first component is too high, the dispersion of the expanded graphene and phenolic epoxy curing agent added in the second component within the continuous resin phase is limited.

[0038] Further, in step S100, the mixture is mixed for 12 to 18 minutes at a speed of 1000 to 1500 rpm; it is then melt-mixed and extruded at a temperature of 110 to 120°C and a screw speed of 150 to 250 rpm; and finally crushed into particles with an average particle size of 5 to 10 mm.

[0039] Further, in step S200, the mixture is mixed for 15 to 25 minutes at a speed of 800 to 1200 rpm; the mixture is melt-mixed and extruded at a temperature of 120 to 130°C and a screw speed of 180 to 280 rpm; and the average particle size of the finished product is controlled to be 40 to 50 μm.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] 1. This invention is the first to combine high epoxy equivalent (900~1200 g / eq) bio-based epoxy resin with low epoxy equivalent (200~250 g / eq) phenolic epoxy curing agent in a specific ratio to construct a soft-segment-hard-segment microphase separation crosslinking network. This allows the coating to achieve a hardness of 6H while still being able to soften and flow sufficiently at high temperatures, providing a structural basis for an ultra-high expansion of 120 times. Simultaneously, this invention constructs a four-component flame-retardant synergistic system of ammonium polyphosphate, nano-titanium dioxide, zinc borate, and expanded graphene. Ammonium polyphosphate catalyzes char formation and releases gas, while expanded graphene, in a ratio of approximately 3:1, produces a physical pushing effect, increasing the expansion ratio from the conventional 10~30 times to 120 times. At the same time, nano-titanium dioxide generates a Ti-PO ceramic phase, and zinc borate forms a B2O3 glassy protective layer, effectively preventing the char layer from collapsing or peeling off after ultra-high expansion, thus successfully passing the UL94V-0 flame retardant rating.

[0042] 2. The present invention adopts a two-step staged mixing process. First, imidazole is pre-dispersed with a portion of bio-based epoxy resin and flame-retardant filler to form sheet A. Then, it is mixed with the remaining resin, phenolic epoxy curing agent and expanded graphene. This process protects the layered structure of expanded graphene and avoids premature cross-linking of imidazole, thus ensuring the storage stability and curing uniformity of the coating.

[0043] 3. The self-made high epoxy equivalent bio-based epoxy resin of this invention uses cashew phenol and linoleic acid as raw materials. The benzene ring in the molecule provides a rigid carbon-forming framework, while the long carbon chain imparts flexibility and high-temperature fluidity, making it highly compatible with coating systems. Furthermore, the resin preparation method is mild, easy to scale up, and the raw materials are derived from renewable resources. The powder coating of this invention can replace paint, reducing VOC emissions. Detailed Implementation

[0044] Example 1

[0045] A method for preparing bio-based epoxy resin includes the following steps:

[0046] Step 1: Take 300g of cashew nut shell powder, add 120.3g of epichlorohydrin (molar ratio 1:1.3) and 9g of tetrabutylammonium bromide (TBAB) (mass ratio 1:0.03), and react at 90℃ for 5 hours. After the reaction is complete, filter, wash with water, and distill under reduced pressure to obtain cashew nut shell powder glycidyl ether.

[0047] Step 2: Take 500g of linoleic acid and add 6.25g of p-toluenesulfonic acid (mass ratio 1:0.0125). Under nitrogen protection, the mixture is prepolymerized at 105℃ for 2.5 hours to obtain linoleic acid prepolymer.

[0048] Step 3: Take 200g of cashew phenol glycidyl ether obtained in Step 1 and mix it with 200g of linoleic acid prepolymer obtained in Step 2 (mass ratio 1:1.0). Add 6g of tetrabutylammonium bromide (mass ratio 1:0.03) and react at 105℃ for 6 hours to obtain the copolymer intermediate.

[0049] Step 4: Add epichlorohydrin (approximately 40g, calculated as 1.2 times the molar amount of hydroxyl groups in the copolymerization intermediate) to the copolymerization intermediate, and add 3.5g of tetrabutylammonium bromide (copolymerization intermediate mass 400g, mass ratio 1:0.00875). React at 70℃ for 3 hours. Take samples every 30 minutes during the reaction to measure the infrared spectrum. When the characteristic peak of hydroxyl groups (3450cm⁻¹) is reached... -1 The reaction was stopped when the intensity decreased to the level of background noise. Post-treatment: excess epichlorohydrin was recovered under reduced pressure, dissolved in toluene, washed with water, dried, and the solvent was evaporated to obtain a pale yellow solid resin.

[0050] Example 2

[0051] A method for preparing bio-based epoxy resin includes the following steps:

[0052] Step 1: 300g of cashew nut shell powder, 101.8g of epichlorohydrin (molar ratio 1:1.1), and 3g of TBAB (mass ratio 1:0.01) were reacted at 80℃ for 4 hours. Post-treatment yielded cashew nut shell powder glycidyl ether.

[0053] Step 2: 500g of linoleic acid and 2.5g of p-toluenesulfonic acid (mass ratio 1:0.005) are prepolymerized at 100℃ for 2 hours.

[0054] Step 3: 200g of cashew phenol glycidyl ether, 120g of linoleic acid prepolymer (mass ratio 1:0.6), and 2g of TBAB (mass ratio 1:0.01) were reacted at 90℃ for 5 hours to obtain a copolymer intermediate.

[0055] Step 4: Add epichlorohydrin (calculated as 1.1 times the molar amount of hydroxyl groups), and add 0.64 g of TBAB (copolymer intermediate mass 320 g, mass ratio 1:0.005). React at 60℃ for 2 hours. The reaction is terminated after the hydroxyl peak disappears on infrared monitoring. The resin is obtained after post-treatment. Determination: epoxy equivalent 1180 g / eq, weight-average molecular weight 4900, softening point 88℃.

[0056] Example 3

[0057] A method for preparing bio-based epoxy resin includes the following steps:

[0058] Step 1: 300g of cashew nut shell powder, 138.9g of epichlorohydrin (molar ratio 1:1.5), and 15g of TBAB (mass ratio 1:0.05) were reacted at 100℃ for 6 hours. Post-treatment yielded cashew nut shell powder glycidyl ether.

[0059] Step 2: 500g of linoleic acid and 10g of p-toluenesulfonic acid (mass ratio 1:0.02) are prepolymerized at 110℃ for 3 hours.

[0060] Step 3: 200g of cashew phenol glycidyl ether, 280g of linoleic acid prepolymer (mass ratio 1:1.4), and 10g of TBAB (mass ratio 1:0.05) were reacted at 120℃ for 7 hours to obtain a copolymer intermediate.

[0061] Step 4: Add epichlorohydrin (calculated as 1.3 times the molar amount of hydroxyl groups), and add 3.36 g of TBAB (480 g of copolymer intermediate, mass ratio 1:0.03). React at 80°C for 4 hours. The reaction is terminated after the hydroxyl peak disappears as monitored by infrared spectroscopy. The resin is then obtained after post-treatment.

[0062] Comparative Example 1

[0063] In step 1, the molar ratio of cashew phenol to epichlorohydrin was adjusted to 1:2, and the rest was the same as in Example 1.

[0064] Comparative Example 2

[0065] In step 2, the temperature was raised to 125°C and the reaction was carried out for 5 hours, with the rest being the same as in Example 1.

[0066] Comparative Example 3

[0067] In step 3, the mass ratio of cashew phenol glycidyl ether to linoleic acid prepolymer was adjusted to 1:0.2, and the rest was the same as in Example 1.

[0068] Comparative Example 4

[0069] After step 3, the epichlorohydrin end-capping reaction is not performed, and the copolymer intermediate is directly used as the final resin. The rest is the same as in Example 1.

[0070] The properties of the bio-based epoxy resins prepared by the methods in Examples 1-3 and Comparative Examples 1-4 are shown in Table 1.

[0071] Table 1. Performance of bio-based epoxy resins prepared by the methods in Examples 1-3 and Comparative Examples 1-4

[0072]

[0073] Note: The content of dimers and trimers was measured during the experiment; the polymer contains dimers, trimers, and some monomers and polymers.

[0074] As shown in Table 1, in Examples 1 to 3, the ratio of dimer content to trimer content was 4 to 6:1, the epoxy equivalent of the resin was stable at 910 to 1180 g / eq, the molecular weight was between 4900 and 7200, and the softening point was between 88 and 108°C.

[0075] In Comparative Example 1, when epichlorohydrin was in excess, after the phenolic hydroxyl groups of cashew nut shells had completely reacted, some of the terminal hydroxyl groups of the generated cashew nut shell glycidyl ether continued to react with epichlorohydrin, generating diepoxy byproducts (one cashew nut shell molecule linked to two glycidyl ether structures). These diepoxy byproducts acted as crosslinking agents in the subsequent ring-opening copolymerization, increasing the branching degree of the copolymer and ultimately reducing the epoxy equivalent of the resin to 880 g / eq, deviating from the target range. The dimer:trimer of Comparative Example 1 was almost identical to that of Example 1. This indicates that, with the same prepolymer composition, a high molar ratio of cashew nut shells to epichlorohydrin in step 1 will decrease the epoxy equivalent.

[0076] In Comparative Example 2, increasing the reaction temperature and extending the reaction time resulted in a decrease in the mass ratio of dimer to trimer to 2.50:1, while the trimer content increased to 26.3%. The trimer has three carboxyl groups and a high degree of branching, making it prone to crosslinking or branching during ring-opening copolymerization. This led to a significant increase in the final resin molecular weight (7200) and a rise in the softening point to 115℃. More importantly, the branched structure relatively increased the number of hydroxyl groups available for end-capping on the molecular chain (more branched ends), resulting in an increase in the density of epoxy groups introduced after end-capping. Consequently, the epoxy equivalent decreased sharply to 770 g / eq, below the lower limit of 900.

[0077] In Comparative Example 3, the amount of linoleic acid prepolymer was severely insufficient. At this point, the copolymerization reaction was incomplete, the proportion of flexible fatty acid segments in the molecular chain was too low, and most of the cashew phenol glycidyl ether (CGE) remained uncopolymerized. The residual CGE had a low molecular weight (approximately 300). Although subsequent end-capping reactions could still introduce epoxy groups at its ends, the overall molecular weight distribution became wider, and the insufficient flexible segments led to increased resin brittleness. The final epoxy equivalent was only 850 g / eq, below 900, and the molecular weight was relatively high (6800), with a softening point of 98℃.

[0078] Comparative Example 4 demonstrates the necessity of the end-capping reaction. Without the end-capping reaction, the epoxy group density of the resin is severely insufficient, with an epoxy equivalent of 1850 g / eq.

[0079] Example 4

[0080] A bio-based flame-retardant thermal expansion powder coating comprises the following components:

[0081] 700g of bio-based epoxy resin, 100g of phenolic epoxy curing agent, 100g of ammonium polyphosphate, 50g of nano titanium dioxide, 40g of zinc borate, 300g of expanded graphene, 10g of imidazole, 3g of benzoin, and 10g of leveling agent.

[0082] The bio-based epoxy resin (prepared by the method in Example 1) has an epoxy equivalent of 1050 g / eq; the phenolic epoxy curing agent is DIC phenolic varnish type epoxy resin (HP series); and the expanded graphene ratio is 250 times (Brad 9950250).

[0083] The preparation method of the bio-based flame-retardant thermal expansion powder coating includes the following steps:

[0084] S100: Mix a portion of bio-based epoxy resin, ammonium polyphosphate, nano titanium dioxide, zinc borate, and imidazole at a speed of 1200 rpm for 15 minutes. Then, melt-mix and extrude the mixture at a temperature of 115℃ and a screw speed of 200 rpm. Allow it to cool naturally and crush it into an average particle size of 8 mm to obtain sheet material A.

[0085] S200: Mix the remaining bio-based epoxy resin, the sheet material A, phenolic epoxy curing agent, leveling agent, benzoin, and expanded graphene at a speed of 1000 rpm for 20 minutes. Then, melt-mix and extrude the mixture at a temperature of 125°C and a screw speed of 230 rpm. Allow it to cool naturally, crush, and grind to control the average particle size of the finished product to 45 μm, thus obtaining the powder coating product.

[0086] The mass ratio of the bio-based epoxy resin added in step S100 to the bio-based epoxy resin added in step S200 is 1:1.12.

[0087] Example 5

[0088] A bio-based flame-retardant thermal expansion powder coating comprises the following components:

[0089] 780g of bio-based epoxy resin, 115g of phenolic epoxy curing agent, 110g of ammonium polyphosphate, 58g of nano titanium dioxide, 45g of zinc borate, 325g of expanded graphene, 12g of imidazole, 5g of benzoin, and 12g of leveling agent.

[0090] The bio-based epoxy resin (prepared by the method in Example 2) has an epoxy equivalent of 1180 g / eq; the phenolic epoxy curing agent is a DIC phenolic varnish-type epoxy resin (HP series); and the expanded graphene has an initial expansion ratio of 300 times (MK595).

[0091] The preparation method of the bio-based flame-retardant thermal expansion powder coating includes the following steps:

[0092] S100: Mix a portion of bio-based epoxy resin, ammonium polyphosphate, nano titanium dioxide, zinc borate, and imidazole at a speed of 1500 rpm for 18 minutes. Then, melt-mix and extrude the mixture at a temperature of 120℃ and a screw speed of 250 rpm. Allow it to cool naturally and crush it into an average particle size of 10 mm to obtain sheet material A.

[0093] S200: Mix the remaining bio-based epoxy resin, the sheet material A, phenolic epoxy curing agent, leveling agent, benzoin, and expanded graphene at a speed of 1200 rpm for 25 minutes. Then, melt-mix and extrude the mixture at a temperature of 130°C and a screw speed of 280 rpm. Allow it to cool naturally, crush, and grind to control the average particle size of the finished product to 50 μm, thus obtaining the powder coating product.

[0094] The mass ratio of the bio-based epoxy resin added in step S100 to that added in step S200 is 1:1.3.

[0095] Example 6

[0096] A bio-based flame-retardant thermal expansion powder coating comprises the following components:

[0097] Bio-based epoxy resin 620g, phenolic epoxy curing agent 88g, ammonium polyphosphate 90g, nano titanium dioxide 40g, zinc borate 35g, expanded graphene 270g, imidazole 8g, benzoin 1g, leveling agent 8g.

[0098] The bio-based epoxy resin (prepared by the method in Example 3) has an epoxy equivalent of 910 g / eq; the phenolic epoxy curing agent is a DIC phenolic varnish-type epoxy resin (HP series); and the expanded graphene has an initial expansion ratio of 200 times (MK494).

[0099] The preparation method of the bio-based flame-retardant thermal expansion powder coating includes the following steps:

[0100] S100: Mix a portion of bio-based epoxy resin, ammonium polyphosphate, nano titanium dioxide, zinc borate, and imidazole at a speed of 1000 for 12 minutes. Then, melt-mix and extrude the mixture at a temperature of 110℃ and a screw speed of 150 rpm. Allow it to cool naturally and crush it into an average particle size of 5 mm to obtain sheet material A.

[0101] S200: Mix the remaining bio-based epoxy resin, the sheet material A, phenolic epoxy curing agent, leveling agent, benzoin, and expanded graphene at 800 rpm for 15 minutes. Then, melt-mix and extrude the mixture at 120°C and 180 rpm. Allow it to cool naturally, crush, and grind to control the average particle size of the finished product to 40 μm, thus obtaining the powder coating product.

[0102] The mass ratio of the bio-based epoxy resin added in step S100 to the bio-based epoxy resin added in step S200 is 1:1.

[0103] Comparative Example 5

[0104] The bio-based epoxy resin was replaced with the bio-based epoxy resin prepared by the method in Comparative Example 2, with an epoxy equivalent of 770 g / eq. The rest was the same as in Example 4.

[0105] Comparative Example 6

[0106] The bio-based epoxy resin was replaced with the bio-based epoxy resin prepared by the method in Comparative Example 4, with an epoxy equivalent of 1850 g / eq. The rest was the same as in Example 4.

[0107] Comparative Example 7

[0108] The bio-based epoxy resin was replaced with commercially available bisphenol A type solid epoxy resin E-12 (760 g / eq), with an epoxy equivalent of 760 g / eq, and the dosage remained 700 g. Everything else was the same as in Example 4.

[0109] Comparative Example 8

[0110] The expanded graphene was replaced with expanded graphene (Xianfeng Nano XF057) with an initial expansion ratio of more than 350 times, and the rest was the same as in Example 4.

[0111] Comparative Example 9

[0112] The expanded graphene was replaced with expanded graphene (Kanglong graphene) with an initial expansion ratio of 150 times, and the rest was the same as in Example 4.

[0113] Comparative Example 10

[0114] The formulation does not contain nano-titanium dioxide and zinc borate, and the remaining components and processes are exactly the same as in Example 4.

[0115] Comparative Example 11

[0116] The formulation does not contain nano-titanium dioxide, and the remaining components and processes are exactly the same as in Example 4.

[0117] Comparative Example 12

[0118] Zinc borate is not added to the formula, and the remaining components and processes are exactly the same as in Example 4.

[0119] Comparative Example 13

[0120] Expanded graphene is not added to the formulation, and the remaining components and processes are exactly the same as in Example 4.

[0121] Comparative Example 14

[0122] The amount of APP and expanded graphene was adjusted to 200g (ratio 1:1 instead of 3:1), and the rest was the same as in Example 4.

[0123] Comparative Example 15

[0124] All components were added to the mixer at once, mixed at high speed, and then melted, extruded, crushed, and ground in one go. The remaining curing conditions were the same as in Example 4.

[0125] The powder coatings prepared by the methods of Examples 4-6 and Comparative Examples 5-15 were coated onto the surface of the substrate using an electrostatic spraying process, and the performance results are shown in Table 2.

[0126] The powder coatings prepared in Examples 4-6 and Comparative Examples 5-15 were sprayed once using the electrostatic spraying process described above (Q235 steel plate, sandblasting Sa2.5, spraying voltage 75kV, air pressure 0.6MPa, film thickness 150μm, curing at 190℃ for 18min) to make samples. The pencil hardness (GB / T6739), UL94 vertical burning flame retardant rating (GB / T2408), and expansion ratio (GB / T14907, burning at 800℃ for 30min) were tested, and the state of the char layer was observed. The results are summarized in Table 2.

[0127] Performance of powder coatings prepared by the methods in Examples 4-6 and Comparative Examples 5-15 after being coated on substrate surfaces

[0128]

[0129] Table 2 shows that the epoxy equivalent (EEW) of the resin in Examples 4-6 is within the range of 900-1200 g / eq, the coating hardness reaches 6H, the flame retardancy rating is UL94V-0, and the expansion ratio is above 115 times, demonstrating excellent overall performance. Comparative Examples 5 (EEW ​​770 g / eq) and 7 (EEW ​​760 g / eq), although using the same two-step process and having a complete flame retardant system, have a lower hardness of 5H, an expansion ratio of only 45-68 times, and a flame retardancy rating of V-1. Comparative Example 6, due to the lack of end-capping reaction, has an excessively high EEW (1850 g / eq), a severely insufficient epoxy group density, incomplete coating curing, and inability to form an effective char layer. The above comparisons indicate that the epoxy equivalent of the resin must be controlled within the range of 900-1200 g / eq, while simultaneously maintaining sufficient crosslinking reactivity, to achieve both high hardness and ultra-high expansion performance.

[0130] The initial expansion ratio of expanded graphene significantly affects the expansion behavior of the coating. Comparative Example 8 used expanded graphene with a ratio of over 350, achieving an expansion ratio of 110, but the carbon layer was too loose and exhibited localized flaking. Although it passed UL94V-0, the carbon layer structure was unsatisfactory. Comparative Example 9 used expanded graphene with a ratio of 150, resulting in an expansion ratio of only 62, and the flame retardant rating dropped to V-1. Only when the initial expansion ratio was controlled between 200 and 300 could a dense carbon layer of approximately 120 expansion ratio be achieved. This indicates that too low a ratio results in insufficient thrust, while too high a ratio damages the integrity of the carbon layer.

[0131] The synergistic effect of the four components in the flame retardant system is indispensable. Comparative Example 10 simultaneously lacks nano-TiO₂. 2 Compared to zinc borate, although the expansion ratio is still 105 times, the char layer is loose and peels off, and the flame retardancy is only V-1; Comparative Example 11 lacks nano-TiO2. 2 In Comparative Example 12, the absence of zinc borate resulted in collapsed or porous carbon layers, reducing flame retardancy to V⁻¹. This demonstrates that nano-TiO₂... 2 The high-temperature strength and density of the ultra-expanded carbon layer can only be guaranteed by working together with zinc borate; neither can be omitted.

[0132] Comparative Example 13, without the addition of expanded graphene, maintained a hardness of 6H, consistent with Example 4, indicating that hardness is primarily determined by the cross-linking network of the resin and curing agent. However, its expansion ratio plummeted to 28 times, only equivalent to the typical level (10-30 times) of the traditional APP / PER / Mel three-source system, and its flame retardancy rating dropped to V-2, with a thin char layer and localized dripping. In contrast, Example 4, after adding 300g of expanded graphene, saw its expansion ratio jump to 118 times, achieving a flame retardancy rating of V-0, and a dense and complete char layer. This demonstrates that the gas thrust generated by APP chemical foaming alone is limited and cannot overcome the 30-fold expansion bottleneck; expanded graphene rapidly expands interlayers at high temperatures, generating a strong mechanical pushing force, which, combined with the chemical foaming of APP, forms a physical-chemical dual superposition effect, resulting in a leap in expansion ratio by several times.

[0133] The ratio of APP to expanded graphene is another key factor. In Comparative Example 14, changing the APP:EG ratio from 1:3 to 1:1 resulted in a sharp drop in the expansion ratio to 68 times and a flame retardancy rating of V-1. The optimized 1:3 (EG:APP) ratio of this invention achieves the best synergistic effect between physical expansion and chemical foaming, increasing the expansion ratio from the conventional 10-30 times to 120 times.

[0134] In terms of preparation process, Comparative Example 15 uses a traditional one-step method to melt and extrude all components at once, resulting in an expansion ratio of only 72 times, a hardness reduced to 5H, and a flame retardancy of V-1. In contrast, the two-step method of this invention first pre-disperses imidazole and flame-retardant filler to form sheet A, and then mixes it with the remaining resin and expanded graphene. This method protects the layered structure of the expanded graphene and avoids premature catalytic cross-linking by imidazole, ensuring that the overall performance meets the requirements.

[0135] It is worth emphasizing that the film thickness of all samples (including the comparative examples) after a single spray coating was between 138 and 162 μm, meeting the design requirements for a thick film of 100 to 200 μm. This indicates that powder coatings inherently possess the advantage of forming a thick film in a single application. This invention, while maintaining zero VOCs and the same film thickness in powder coatings, achieves, for the first time, superior performance with 6H hardness, UL94V-0 flame retardancy, and a 120-fold expansion ratio through the systematic integration of a high epoxy equivalent bio-based resin, a complete four-component flame retardant system, a specific EG:APP ratio, and a two-step process. Compared to traditional solvent-based fire-retardant paints that require multiple coats with each coat only 20 to 50 μm thick, this invention not only surpasses them in all aspects of performance but also significantly improves application efficiency and reduces VOC emissions, demonstrating significant practical value and environmental advantages.

Claims

1. A bio-based flame-retardant thermal expansion powder coating, characterized in that, By weight, it includes the following components: 620-780 parts of bio-based epoxy resin, 88-115 parts of phenolic epoxy curing agent, 90-110 parts of ammonium polyphosphate, 40-58 parts of nano titanium dioxide, 35-45 parts of zinc borate, 270-325 parts of expanded graphene, 8-12 parts of imidazole, 1-5 parts of benzoin, and 8-12 parts of leveling agent. The bio-based epoxy resin is prepared by ring-opening copolymerization of cashew phenol glycidyl ether and linoleic acid prepolymer, followed by epoxy end-capping, with an epoxy equivalent of 900~1200 g / eq; the phenolic epoxy curing agent has an epoxy equivalent of 200~250 g / eq; and the expanded graphene has an initial expansion ratio of 200~300 times.

2. A method for preparing a bio-based flame-retardant thermally expanding powder coating as described in claim 1, comprising the following steps: S100: A portion of bio-based epoxy resin, ammonium polyphosphate, nano titanium dioxide, zinc borate, and imidazole are mixed evenly at high speed, then melt-dispersed, extruded, cooled, and crushed to obtain sheet material A; S200: The remaining bio-based epoxy resin, the sheet material A, the phenolic epoxy curing agent, the leveling agent, benzoin, and expanded graphene are mixed evenly by high-speed stirring, and then melt-extruded, cooled, crushed, and ground to obtain the finished powder coating.

3. The preparation method according to claim 2, characterized in that, The preparation method of the bio-based epoxy resin includes the following steps: Step 1: Cashew phenol is mixed with epichlorohydrin and reacted at 80-100°C for 4-6 hours in the presence of tetrabutylammonium bromide to obtain cashew phenol glycidyl ether. Step 2: Add p-toluenesulfonic acid to linoleic acid and perform a prepolymerization reaction at 100-110°C for 2-3 hours under a nitrogen atmosphere to obtain linoleic acid prepolymer; Step 3: Mix the cashew phenol glycidyl ether obtained above with linoleic acid prepolymer, add tetrabutylammonium bromide, and carry out ring-opening copolymerization reaction at 90~120℃ for 5~7 hours to obtain copolymer intermediate product; Step 4: Add epichlorohydrin to the above copolymer intermediate, add tetrabutylammonium bromide, and carry out the end-capping reaction at 60~80℃ for 2~4 hours. After post-treatment, the bio-based epoxy resin is obtained.

4. The preparation method according to claim 3, characterized in that, In step 1, the molar ratio of cashew phenol to epichlorohydrin is 1:1.1~1.5; the mass ratio of cashew phenol to tetrabutylammonium bromide is 1:0.01~0.

05.

5. The preparation method according to claim 3, characterized in that, In step 2, the linoleic acid prepolymer includes dimer and trimer, with the content ratio of dimer to trimer being 4~5:1; the mass ratio of linoleic acid to p-toluenesulfonic acid is 1:0.005~0.

02.

6. The preparation method according to claim 3, characterized in that, In step 3, the mass ratio of cashew phenol glycidyl ether to linoleic acid prepolymer is 1:0.6~1.4; the mass ratio of cashew phenol glycidyl ether to tetrabutylammonium bromide is 1:0.01~0.

05.

7. The preparation method according to claim 3, characterized in that, In step 4, the end-capping reaction is defined as the disappearance of the characteristic peak of hydroxyl groups in the copolymer intermediate by infrared spectroscopy as the reaction endpoint. The reaction is stopped when the intensity of the characteristic peak decreases to the background noise level and no longer changes. The mass ratio of the copolymer intermediate to the added tetrabutylammonium bromide is 1:0.005~0.

03.

8. The preparation method according to claim 2, characterized in that, The mass ratio of the bio-based epoxy resin added in step S100 to the bio-based epoxy resin added in step S200 is 1:1 to 1.

3.

9. The preparation method according to claim 2, characterized in that, In step S100, the mixture is mixed for 12 to 18 minutes at a speed of 1000 to 1500 rpm; it is then melt-mixed and extruded at a temperature of 110 to 120°C and a screw speed of 150 to 250 rpm; and finally crushed into particles with an average particle size of 5 to 10 mm.

10. The preparation method according to claim 2, characterized in that, In step S200, the mixture is mixed for 15 to 25 minutes at a speed of 800 to 1200 rpm; it is then melt-mixed and extruded at a temperature of 120 to 130°C and a screw speed of 180 to 280 rpm; the average particle size of the finished product is controlled to be 40 to 50 μm.

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