Environment-friendly high-strength PVC foam and preparation method thereof

Abstract

The present application belongs to the technical field of polymer foam materials, and particularly relates to an environment-friendly high-strength PVC foam and a preparation method thereof.The method provided by the present application comprises the following steps: synergistically modifying a metal hydroxide inorganic flame retardant by using a silane coupling agent and an epoxy vegetable oil, and then blending the modified metal hydroxide inorganic flame retardant with a part of a reactive polyol to prepare a flame retardant masterbatch; premixing raw materials such as PVC paste resin, chlorinated polyvinyl chloride, a heat stabilizer, melamine polyphosphate and the like, and then sequentially adding the remaining polyol, an epoxy resin, isocyanate, the flame retardant masterbatch, a foaming agent and inorganic fillers to prepare a paste; and performing mold pressing, secondary foaming and drying and shaping, so that the PVC foam is obtained.The PVC foam prepared by the present application has a closed cell rate of greater than or equal to 86%, uniform cells, and a compression strength that is significantly improved compared with that of a traditional process, and has the characteristics of high flame retardancy, high strength and high environmental protection, and is suitable for high-end fields such as wind power blades, rail transit and ships.

Description

Technical Field

[0001] This invention belongs to the field of polymer foam material technology, specifically relating to an environmentally friendly high-strength PVC foam and its preparation method. Background Technology

[0002] Rigid cross-linked PVC foam, due to its low density (40~250kg / m³), excellent thermal insulation, good chemical stability, and self-extinguishing properties, is widely used as the core material in "sandwich" structure composite materials in high-end fields such as wind turbine blades, rail transportation, shipbuilding, and building insulation. The global rigid PVC structural foam market has exceeded 8 billion yuan, with an annual growth rate of 6%~8%; a single wind turbine blade over 80m in length requires more than 15m³ of foam core material. 3 This places stringent requirements on the overall performance of foam, including its lightweight, high strength, and high flame retardancy.

[0003] Currently, the mainstream preparation route for rigid cross-linked PVC foam is the isocyanate chemical cross-linking method: using PVC paste resin as the matrix and isocyanate as the cross-linking agent, a semi-interpenetrating polymer network is formed during the molding stage, followed by a second foaming process through boiling in water to obtain the foam product. To meet the requirements for high flame retardant performance (oxygen index ≥28~32), existing technologies add flame retardants to the PVC foam system. For example, Chinese patent CN104031326B discloses a modified rigid crosslinked polyvinyl chloride flame-retardant foam, prepared from the following raw materials in the following weight ratios: 90-99 parts polyvinyl chloride paste resin, 1-10 parts chlorinated polyvinyl chloride resin, 3-15 parts crosslinking agent, 30-70 parts isocyanate, 9-24 parts acid anhydride, 4-8 parts azo blowing agent, 2-15 parts epoxidized soybean oil, 2-6 parts filler, and 5-30 parts flame retardant. The formula includes 5-30 parts of flame retardant, specifically selected from highly active phosphate esters, decabromodiphenyl ether, triaryl phosphate esters, and other organic flame retardants. It also indicates that by optimizing the formula, the foam can achieve a B1 flame retardant rating. However, these organic flame retardants have inherent drawbacks such as producing large amounts of toxic fumes and corrosive gases during combustion and poor environmental performance, thus their application is increasingly limited.

[0004] Halogen-free flame retardants offer advantages such as good thermal stability, no toxic gas production, and low cost. Introducing halogen-free flame retardants into rigid cross-linked PVC foam systems to develop foam materials with both high flame retardancy and environmental friendliness is a significant development trend in this field. However, achieving high flame retardancy requires the addition of large amounts of inorganic flame retardants such as magnesium hydroxide and aluminum hydroxide, typically reaching 20-60 parts by weight of the PVC matrix. Due to the poor interfacial compatibility between the high-filling-content inorganic flame retardant particles and the PVC / epoxy resin organic matrix, particle agglomeration occurs during the paste mixing stage, and interfacial debonding forms microcracks during foaming when the foam walls are stretched. This leads to cell rupture, a sharp drop in closed-cell rate, severe cell unevenness, and a significant decrease in compressive strength. Manufacturers are forced to compromise between flame retardancy, mechanical properties, and cell uniformity, making it difficult to obtain foam products with high closed-cell rate, high uniformity, and high mechanical strength while maintaining a high oxygen index (≥30). This has become a core bottleneck restricting the promotion of this material in high-end applications such as wind turbine blades. Summary of the Invention

[0005] To address the technical problem that it is difficult to simultaneously achieve flame retardancy, mechanical properties, and cell uniformity in PVC foam in existing technologies, this invention provides an environmentally friendly high-strength PVC foam and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing environmentally friendly high-strength PVC foam includes the following steps: S1. Add silane coupling agent pre-dispersion to metal hydroxide inorganic flame retardant, stir and react at 75-85℃ for 15-25 min; then add epoxy vegetable oil and stir and react for 20-40 min, and dry to obtain modified metal hydroxide inorganic flame retardant; S2. The modified metal hydroxide inorganic flame retardant from step S1 is mixed with a portion of the reactive polyol at 60-80°C for 20-40 minutes to obtain the flame retardant masterbatch. S3. After premixing PVC paste resin, chlorinated polyvinyl chloride, heat stabilizer and melamine polyphosphate, add the remaining reactive polyol, epoxy resin and isocyanate in sequence and stir evenly. Then add the flame retardant masterbatch, foaming agent and inorganic filler from step S2 in sequence and stir evenly to obtain paste. S4. The paste from step S3 is then molded, foamed a second time, dried and shaped to obtain the final product.

[0007] In this scheme, an inorganic flame retardant is first synergistically organically coated with a silane coupling agent and epoxy vegetable oil, constructing a reactive organic shell containing epoxy groups and long organic chains on the particle surface. This allows the modified particle surface to carry residual epoxy functional groups and long-chain organic active hydrogen, which then bonds with reactive polyols and isocyanates during the subsequent molding and crosslinking stage, anchoring the flame retardant particles to the crosslinking network nodes. This transforms the flame retardant particles from "interfacial defect sources" into "network reinforcement points," reducing the inducing factors for foam wall debonding and cracking at the particle-matrix interface during foaming. Furthermore, by pre-coating the modified metal hydroxide-based inorganic flame retardant surface with partially reactive polyols, a multi-layered gradient interface structure is formed: "inorganic core → silane anchoring layer → long-chain vegetable oil layer → polyol hybrid layer → organic matrix." This structure reduces abrupt changes in physical properties and stress concentration at the phase interface, further enhancing the mechanical stability of the interface.

[0008] Preferably, in step S1, the metal hydroxide inorganic flame retardant is one or both of magnesium hydroxide and aluminum hydroxide.

[0009] Preferably, the inorganic flame retardant of the metal hydroxide is magnesium hydroxide.

[0010] Preferably, the particle size of the magnesium hydroxide is 2-5 μm.

[0011] Preferably, the silane coupling agent is an epoxy silane coupling agent.

[0012] Preferably, the epoxy silane coupling agent is KH560.

[0013] Preferably, the epoxidized vegetable oil is one or more of epoxidized soybean oil, epoxidized peanut oil, epoxidized rapeseed oil, epoxidized linseed oil, epoxidized cottonseed oil, and epoxidized sunflower seed oil.

[0014] Preferably, the epoxidized vegetable oil is epoxidized soybean oil.

[0015] Preferably, in step S1, the silane coupling agent pre-dispersion is obtained by dispersing the silane coupling agent in a mixed solvent, wherein the mixed solvent is a solvent composed of ethanol and water.

[0016] Preferably, the volume ratio of ethanol to water is 9-10:1.

[0017] In this scheme, the ethanol-water mixed solvent can effectively promote the hydrolysis and dispersion of silane coupling agents, improve their coating uniformity and reactivity on the surface of inorganic flame retardants, thereby enhancing the modification effect and further optimizing the foam's cell structure and mechanical properties.

[0018] Preferably, in step S2, the reactive polyol is castor oil polyol with a hydroxyl value of 110-130 mg KOH / g.

[0019] In this solution, castor oil polyol can form a stable pre-dispersion layer on the surface of the modified flame retardant, enhance the compatibility between the flame retardant and the matrix resin, reduce agglomeration, improve the fluidity of the paste, and ultimately improve the uniformity and compressive strength of the foam.

[0020] Preferably, in step S2, the mixing speed is 500~800 rpm.

[0021] Preferably, in step S4, the molding process specifically involves adding the paste into a metal mold and molding it for 10 to 30 minutes at a temperature of 170 to 180°C and a pressure of 10 to 20 MPa.

[0022] In this scheme, molding at 170~180℃ and 10~20MPa can ensure the initial formation of the cross-linked network and the completion of the ring-opening addition anchoring of the epoxy groups and isocyanates on the flame retardant surface.

[0023] Preferably, the secondary foaming specifically involves placing the foaming precursor formed after molding in a water bath environment and performing secondary foaming at 80~95℃ for 1~3 hours.

[0024] In this scheme, secondary foaming in an 80~95℃ water bath for 1~3 hours is beneficial for uniform expansion of the foam cells and avoids overheating and cracking.

[0025] Preferably, the drying and shaping process specifically involves drying the material after secondary foaming in a forced-air drying oven at 60-80°C for 2-6 hours.

[0026] This invention also provides an environmentally friendly high-strength PVC foam, prepared by the above method, comprising the following raw materials in parts by weight: 80-90 parts PVC paste resin; 10-20 parts chlorinated polyvinyl chloride (CPVC); 25-35 parts reactive polyol; 15-25 parts epoxy resin; 90-110 parts isocyanate; 12-18 parts foaming agent; 15-30 parts metal hydroxide inorganic flame retardant; 0.3-0.9 parts silane coupling agent; 0.75-2.4 parts epoxy vegetable oil; 10-15 parts melamine polyphosphate; 5-8 parts heat stabilizer; and 10-20 parts inorganic filler.

[0027] In this solution, the formulation of each raw material is reasonable, ensuring that the foam material is lightweight, high-strength, and highly flame-retardant: PVC paste resin provides good film-forming properties and cross-linking density, epoxy resin and isocyanate synergistically construct a semi-interpenetrating polymer network, improving the rigidity and thermal stability of the foam; metal hydroxide inorganic flame retardants are compounded with melamine polyphosphate, utilizing the dual mechanisms of heat absorption and cooling and expansion and oxygen isolation to jointly improve the flame-retardant performance.

[0028] Preferably, the average degree of polymerization of the PVC paste resin is 1000-2000.

[0029] Preferably, the epoxy resin is one or both of bisphenol A type epoxy resin or alicyclic epoxy resin.

[0030] Preferably, the heat stabilizer is a calcium-zinc composite stabilizer.

[0031] Preferably, the bisphenol A type epoxy resin is E51 or E44; the alicyclic epoxy resin is one or two of diglycidyl tetrahydrophthalate, diglycidyl 4,5-epoxycyclohexane-1,2-dicarboxylate, or 3,4-epoxycyclohexylcarboxylate.

[0032] Preferably, the isocyanate is one or more of polymethylene polyphenyl polyisocyanate (PAPI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI).

[0033] Preferably, the foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, foam leveling agent and nano-nucleating agent in a mass ratio of 50-60:15-20:10-15:4-6:3-5.

[0034] In this scheme, the composite foaming system can achieve uniform nucleation and stable growth of cells during the secondary foaming process. Sodium bicarbonate and citric acid synergistically regulate the foaming rate through chemical foaming, azodicarbonamide provides auxiliary foaming, nano-nucleating agents provide heterogeneous nucleation sites, and foam leveling agents reduce the surface tension of cells and inhibit cell merging and rupture, ultimately obtaining a foam structure with fine, uniformly distributed cells and a high closed-cell ratio.

[0035] Preferably, the foaming agent is one or more of methyl silicone oil, methyl phenyl silicone oil, or ethyl silicone oil.

[0036] Preferably, the nanonucleating agent is one or more of nano-hydrotalcite, nano-silica, and nano-montmorillonite.

[0037] Preferably, the inorganic filler is one or more of diatomaceous earth, wollastonite, and hollow glass microspheres.

[0038] Preferably, the inorganic filler has a particle size of 1250 mesh.

[0039] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes a synergistic process of "first modifying with silane coupling agent / epoxy vegetable oil coating, then pre-dispersing with polyol coating" to construct a multi-layered gradient interface structure on the surface of inorganic flame retardants. This transforms inorganic flame retardant particles from "interface defect sources" into "network reinforcement points," reducing foam wall debonding and cell rupture during the foaming process. This solves the contradiction between high flame retardancy rating and high cell integrity and high mechanical properties in cross-linked PVC foam materials.

[0040] The foamed material prepared by this invention can achieve a closed-cell rate of 86% to 90% under the condition that the total amount of flame retardant added is 25 to 45 parts. The compressive strength is significantly improved compared with the prior art, and the oxygen index is ≥32. It has environmental protection, high strength and high flame retardant properties, and is suitable for high-end structural core materials such as wind turbine blades, rail transit, and ships. Detailed Implementation

[0041] The technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0042] Unless otherwise specified, all raw materials used in the examples and comparative examples are commercially available.

[0043] Magnesium hydroxide, particle size 2-5μm; castor oil polyol SY752 produced by Shanghai Shuyu Chemical Co., Ltd., with a hydroxyl value of 115-125mgKOH / g; viscosity 100-200mPa·s / 25℃; PVC paste resin PSH-10 produced by Shenyang Chemical Co., Ltd., with a degree of polymerization of 1600-1800; chlorinated polyvinyl chloride (CPVC) CPVC-R207 produced by Shandong Gaoxin Chemical Co., Ltd.; melamine polyphosphate, particle size 5-30μm, nitrogen content ≥42%, phosphorus content ≥13%, decomposition temperature ≥350℃; diatomaceous earth, particle size 1250 mesh.

[0044] Example 1 This embodiment of environmentally friendly high-strength PVC foam and its preparation method includes the following steps: S1. Add 1.5 kg of magnesium hydroxide to a high-speed mixer, heat to 75°C, spray in KH560 ethanol-water dispersion (KH-560 dosage is 0.03 kg, solvent (ethanol to water volume ratio is 9:1) dosage is 0.225 kg), stir and react for 25 min; then add 0.075 kg of epoxidized soybean oil and stir and react for 40 min, heat to 105°C, dry and cool to obtain modified magnesium hydroxide.

[0045] S2. Place the modified magnesium hydroxide from step S1 and 1 kg of castor oil polyol in a planetary mixer and mix for 40 min at 60°C and 500 rpm to obtain flame retardant masterbatch. S3. Take 8 kg of PVC paste resin, 2 kg of CPVC, 0.5 kg of calcium-zinc composite stabilizer, and 1 kg of melamine polyphosphate and stir at low speed for 5 min. Add the remaining 1.5 kg of castor oil polyol, 1.5 kg of epoxy resin (of which E51 accounts for 70% by mass and diglycidyl tetrahydrophthalate accounts for 30% by mass), and 9 kg of isocyanate (polymethylene polyphenyl polyisocyanate PAPI) and stir for 10 min. Then, add the flame retardant masterbatch from step S2 and stir for 5 min. Add 1.8 kg of foaming agent and 1.0 kg of diatomaceous earth and stir for 10 min. Control the temperature to ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 50:15:10:4:3.

[0046] S4. Add the paste to the metal mold and mold it for 30 minutes at a temperature of 170℃ and a pressure of 10MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 3 hours at 80℃. Then place it in a 60℃ forced-air drying oven to dry for 6 hours to obtain the foamed material.

[0047] Example 2 This embodiment of environmentally friendly high-strength PVC foam and its preparation method includes the following steps: S1. Take 2.5 kg of magnesium hydroxide and add it to a high-speed mixer. Heat it to 80°C and spray in KH560 ethanol-water dispersion (KH-560 dosage is 0.0625 kg, solvent (ethanol to water volume ratio is 9:1) dosage is 0.375 kg). Stir and react for 20 min. Then add 0.15 kg of epoxidized soybean oil and stir and react for 30 min. Heat it to 105°C, dry it and then cool it to obtain modified magnesium hydroxide.

[0048] S2. Place the modified magnesium hydroxide from step S1 and 1.2 kg of castor oil polyol in a planetary mixer and mix for 30 min at 70°C and 650 rpm to obtain flame retardant masterbatch. S3. Take 8.5 kg of PVC paste resin, 1.5 kg of CPVC, 0.6 kg of calcium-zinc composite stabilizer, and 1.3 kg of melamine polyphosphate and stir at low speed for 5 min. Add the remaining 1.8 kg of castor oil polyol, 2 kg of epoxy resin (E51 mass ratio of 60%, 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester mass ratio of 40%), and 10 kg of isocyanate (PAPI to HDI mass ratio of 2:1). Stir for 10 min. Then add the flame retardant masterbatch from step S2 and stir for 5 min. Add 1.5 kg of foaming agent and 1.5 kg of diatomaceous earth and stir for 10 min. Control the temperature ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 55:18:12:5:4.

[0049] S4. Add the paste to the metal mold and mold it for 20 minutes at a temperature of 175℃ and a pressure of 15MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 2 hours at 90℃. Then place it in a 70℃ forced-air drying oven to dry for 4 hours to obtain the foamed material.

[0050] Example 3 This embodiment of environmentally friendly high-strength PVC foam and its preparation method includes the following steps: S1. Take 3.0 kg of magnesium hydroxide and add it to a high-speed mixer. Heat it to 85°C and spray in KH560 ethanol-water dispersion (KH-560 dosage is 0.09 kg, solvent (ethanol to water volume ratio is 9:1) dosage is 0.45 kg). Stir and react for 15 min. Then add 0.24 kg of epoxidized soybean oil and stir and react for 20 min. Heat it to 105°C, dry it and then cool it to obtain modified magnesium hydroxide.

[0051] S2. Place the modified magnesium hydroxide from step S1 and 1.5 kg of castor oil polyol in a planetary mixer and mix for 20 min at 80°C and 800 rpm to obtain flame retardant masterbatch. S3. Take 9.0 kg of PVC paste resin, 1.0 kg of CPVC, 0.8 kg of heat stabilizer, and 1.5 kg of melamine polyphosphate and stir at low speed for 5 min. Add the remaining 2.0 kg of castor oil polyol, 2.5 kg of epoxy resin (E44 mass ratio of 55%, 3,4-epoxycyclohexyl carboxylate mass ratio of 45%), and 11 kg of isocyanate (PAPI, HDI, IPDI mass ratio of 2:1:1). Stir for 10 min. Then add the flame retardant masterbatch from step S2 and stir for 5 min. Add 1.2 kg of foaming agent and 2.0 kg of diatomaceous earth and stir for 10 min. Control the temperature ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 60:20:15:6:5.

[0052] S4. Add the paste to the metal mold and mold it for 10 minutes at a temperature of 180℃ and a pressure of 20MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 1 hour at 95℃. Then place it in an 80℃ forced-air drying oven to dry for 2 hours to obtain the foamed material.

[0053] Comparative Example 1 The comparative method for preparing environmentally friendly high-strength PVC foam includes the following steps: S1. Take 8.5 kg of PVC paste resin, 1.5 kg of CPVC, 0.6 kg of heat stabilizer, and 1.3 kg of melamine polyphosphate and stir at low speed for 5 min. Add 3 kg of castor oil polyol, 2 kg of epoxy resin (E51 mass ratio of 60%, 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester mass ratio of 40%), and 10 kg of isocyanate (PAPI to HDI mass ratio of 2:1). Stir for 10 min, then add 2.5 kg of magnesium hydroxide and stir for 5 min. Add 1.5 kg of foaming agent and 1.5 kg of diatomaceous earth and stir for 10 min. Control the temperature ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 55:18:12:5:4.

[0054] S2. Add the paste to a metal mold and mold it for 20 minutes at a temperature of 175℃ and a pressure of 15MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 2 hours at 90℃. Then, place it in a 70℃ forced-air drying oven to dry for 4 hours to obtain the foamed material.

[0055] Comparative Example 2 The preparation method of this comparative example of environmentally friendly high-strength PVC foam includes the following steps: S1. Take 2.5 kg of magnesium hydroxide and add it to a high-speed mixer. Heat it to 80°C and spray in KH560 ethanol-water dispersion (KH-560 dosage is 0.0625 kg, solvent (ethanol to water volume ratio is 9:1) dosage is 0.375 kg). Stir and react for 20 min. Then add 0.15 kg of epoxidized soybean oil and stir and react for 30 min. Heat it to 105°C, dry it and then cool it to obtain modified magnesium hydroxide.

[0056] S2. Take 8.5 kg of PVC paste resin, 1.5 kg of CPVC, 0.6 kg of heat stabilizer, and 1.3 kg of melamine polyphosphate and stir at low speed for 5 min. Add 3 kg of castor oil polyol, 2 kg of epoxy resin (E51 mass ratio of 60%, 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester mass ratio of 40%), and 10 kg of isocyanate (PAPI to HDI mass ratio of 2:1). Stir for 10 min, then add the modified magnesium hydroxide from step S1 and stir for 5 min. Add 1.5 kg of foaming agent and 1.5 kg of diatomaceous earth and stir for 10 min. Control the temperature ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 55:18:12:5:4.

[0057] S3. Add the paste to the metal mold and mold it for 20 minutes at a temperature of 175℃ and a pressure of 15MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 2 hours at 90℃. Then place it in a 70℃ forced-air drying oven to dry for 4 hours to obtain the foamed material.

[0058] Comparative Example 3 This comparative example of environmentally friendly high-strength PVC foam and its preparation method includes the following steps: S1. Place 2.5 kg of magnesium hydroxide and 1.2 kg of castor oil polyol in a planetary mixer and mix for 30 min at 70°C and 650 rpm to obtain flame retardant masterbatch; S2. Take 8.5 kg of PVC paste resin, 1.5 kg of CPVC, 0.6 kg of heat stabilizer, and 1.3 kg of melamine polyphosphate and stir at low speed for 5 min. Add 1.8 kg of castor oil polyol, 2 kg of epoxy resin (E51 mass ratio of 60%, 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester mass ratio of 40%), and 10 kg of isocyanate (PAPI to HDI mass ratio of 2:1). Stir for 10 min. Then add the flame retardant masterbatch from step S1 and stir for 5 min. Add 1.5 kg of foaming agent and 1.5 kg of diatomaceous earth and stir for 10 min. Control the temperature ≤40℃ to obtain the paste. The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, methyl silicone oil, and nano-hydrotalcite in a mass ratio of 55:18:12:5:4.

[0059] S3. Add the paste to the metal mold and mold it for 20 minutes at a temperature of 175℃ and a pressure of 15MPa. After cooling and demolding, the foamed precursor is obtained. Place the foamed precursor in a water bath environment and foam it for 2 hours at 90℃. Then place it in a 70℃ forced-air drying oven to dry for 4 hours to obtain the foamed material.

[0060] Performance testing The properties of the foamed materials prepared in Examples 1-3 and Comparative Examples 1-3 were tested, and the test results are shown in Table 1.

[0061] Table 1 Performance Test Results

[0062] As can be seen from the data in Table 1, the densities of the foamed materials in Examples 1-3 are 80, 150, and 200 kg / m³, respectively. 3 The corresponding compressive strengths were 1.5, 3.8, and 5.2 MPa, respectively. The foam prepared by this invention exhibited excellent mechanical properties at different density levels, with the highest being 200 kg / m³. 3 Its compressive strength reaches 5.2 MPa, far exceeding that of conventional rigid cross-linked PVC foam (typically 2.0~3.0 MPa at the same density). This meets the requirements for wind turbine blade core materials (80~200 kg / m²). 3 The material meets the comprehensive requirements of being lightweight and high-strength. Its oxygen index reaches 32, 34 and 36 respectively, all of which meet the requirements of B1 grade flame retardant materials (oxygen index ≥32) in GB8624-2012.

[0063] Compared to Example 2, steps S1 and S2 were omitted in Comparative Example 1, meaning that the magnesium hydroxide flame retardant was not modified with silane coupling agent and epoxidized soybean oil, nor was it pre-dispersed and coated with castor oil polyol; step S2 was omitted in Comparative Example 2, meaning that the magnesium hydroxide flame retardant was only modified with silane coupling agent and epoxidized soybean oil, without pre-dispersing and coating with castor oil polyol; and step S1 was omitted in Comparative Example 3, meaning that the magnesium hydroxide flame retardant was not modified with silane coupling agent and epoxidized soybean oil, but only pre-dispersed and coated with castor oil polyol. The compressive strength and closed-cell rate of the foamed material obtained in Example 2 were 3.8 MPa and 90%, respectively, both significantly higher than those of Comparative Example 1 (1.8 MPa, 62%), Comparative Example 2 (2.5 MPa, 75%), and Comparative Example 3 (2.1 MPa, 68%). This demonstrates that the present invention, through a synergistic process of "first silane coupling agent / epoxy vegetable oil coating modification, then polyol pre-dispersion coating treatment," successfully constructed a multi-layer gradient interface structure on the flame retardant surface, effectively inhibiting foam wall debonding and cell rupture during the foaming process, increasing the closed-cell rate to over 90%, and doubling the compressive strength.

[0064] The oxygen index of the foamed material obtained in Example 2 was 34, while the oxygen index of Comparative Examples 1-3 was only 26-29 with similar total flame retardant addition. This indicates that interface defects caused uneven distribution of flame retardant, which reduced flame retardant efficiency.

[0065] In summary, this invention successfully resolves the contradiction between high flame retardancy and high cell integrity and mechanical properties in rigid cross-linked PVC foam by constructing a multi-layer gradient interface structure. With a total flame retardant addition of 25-45 parts, the closed-cell rate can reach 86%-90%, the compressive strength is significantly improved compared to existing technologies, and the oxygen index is ≥32. It combines environmental friendliness, high strength, and high flame retardancy, making it suitable for high-end structural core materials in wind turbine blades, rail transportation, and shipbuilding.

[0066] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing environmentally friendly high-strength PVC foam, characterized in that, Includes the following steps: S1. Add silane coupling agent pre-dispersion to metal hydroxide inorganic flame retardant, stir and react at 75-85℃ for 15-25 min; then add epoxy vegetable oil and stir and react for 20-40 min, and dry to obtain modified metal hydroxide inorganic flame retardant; S2. The modified metal hydroxide inorganic flame retardant from step S1 is mixed with a portion of the reactive polyol at 60-80°C for 20-40 minutes to obtain the flame retardant masterbatch. S3. After premixing PVC paste resin, chlorinated polyvinyl chloride, heat stabilizer and melamine polyphosphate, add the remaining reactive polyol, epoxy resin and isocyanate in sequence and stir evenly. Then add the flame retardant masterbatch, foaming agent and inorganic filler from step S2 in sequence and stir evenly to obtain paste. S4. The paste from step S3 is then molded, foamed a second time, dried and shaped to obtain the final product.

2. The preparation method according to claim 1, characterized in that, In step S1, the inorganic flame retardant of the metal hydroxide is one or two of magnesium hydroxide and aluminum hydroxide; the silane coupling agent is an epoxy silane coupling agent; and the epoxidized vegetable oil is one or more of epoxidized soybean oil, epoxidized peanut oil, epoxidized rapeseed oil, epoxidized linseed oil, epoxidized cottonseed oil, and epoxidized sunflower seed oil.

3. The preparation method according to claim 1, characterized in that, In step S1, the silane coupling agent pre-dispersion is obtained by dispersing the silane coupling agent in a mixed solvent, wherein the mixed solvent is a solvent composed of ethanol and water; the volume ratio of ethanol to water is 9-10:

1.

4. The preparation method according to claim 1, characterized in that, In step S2, the reactive polyol is castor oil polyol.

5. The preparation method according to claim 1, characterized in that, In step S4, the molding process specifically involves adding the paste into a metal mold and molding it for 10-30 minutes at a temperature of 170-180℃ and a pressure of 10-20MPa. The secondary foaming process specifically involves placing the foamed precursor formed after molding in a water bath environment and performing secondary foaming at 80-95℃ for 1-3 hours. The drying and shaping process specifically involves drying the material after secondary foaming in a forced-air drying oven at 60-80℃ for 2-6 hours.

6. An environmentally friendly high-strength PVC foam, characterized in that, The preparation is made by the method according to any one of claims 1-5, comprising the following raw materials in parts by weight: 80-90 parts of PVC paste resin; 10-20 parts of chlorinated polyvinyl chloride; 25-35 parts of reactive polyol; 15-25 parts of epoxy resin; 90-110 parts of isocyanate; 12-18 parts of foaming agent; 15-30 parts of metal hydroxide inorganic flame retardant; 0.3-0.9 parts of silane coupling agent; 0.75-2.4 parts of epoxy vegetable oil; 10-15 parts of melamine polyphosphate; 5-8 parts of heat stabilizer; and 10-20 parts of inorganic filler.

7. The environmentally friendly high-strength PVC foam according to claim 6, characterized in that, The average degree of polymerization of the PVC paste resin is 1000-2000; the epoxy resin is one or two of bisphenol A type epoxy resin or alicyclic epoxy resin; the heat stabilizer is a calcium-zinc composite stabilizer; the isocyanate is one or more of polymethylene polyphenyl polyisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate.

8. The environmentally friendly high-strength PVC foam according to claim 6, characterized in that, The foaming agent is obtained by grinding and mixing sodium bicarbonate, citric acid, azodicarbonamide, foam leveling agent and nano-nucleating agent in a mass ratio of 50-60:15-20:10-15:4-6:3-5.

9. The environmentally friendly high-strength PVC foam according to claim 8, characterized in that, The foaming agent is one or more of methyl silicone oil, methyl phenyl silicone oil, or ethyl silicone oil; the nanonucleating agent is one or more of nano hydrotalcite, nano silica, or nano montmorillonite.

10. The environmentally friendly high-strength PVC foam according to claim 6, characterized in that, The inorganic filler is one or more of diatomaceous earth, wollastonite, and hollow glass microspheres.

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