A flame-retardant molded silicone rubber and its vulcanization method

Abstract

This application discloses a flame-retardant molded silicone rubber and its vulcanization method. The silicone rubber comprises the following raw materials in parts by weight: 100 parts of raw silicone rubber, 1.5-3.5 parts of curing agent, 20-30 parts of flame-retardant filler, and 10-20 parts of flame retardant. The flame retardant comprises paraformaldehyde, an aminosilane coupling agent, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide in a molar ratio of 1-1.6:1:1-1.2. The paraformaldehyde and aminosilane coupling agent undergo nucleophilic addition to obtain an N-hydroxymethylsilane intermediate. The N-hydroxymethylsilane intermediate then undergoes a condensation reaction with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to obtain the final product. This application aims to solve the problems of poor compatibility between traditional flame retardants and the silicone rubber matrix, easy agglomeration and precipitation, which lead to deterioration of the mechanical properties and insufficient flame-retardant stability of silicone rubber.

Description

Technical Field

[0001] This application relates to the field of silicone rubber, and in particular to a flame-retardant molded silicone rubber and its vulcanization method. Background Technology

[0002] Silicone rubber, a special synthetic rubber with silicon-oxygen bonds as its main chain, exhibits excellent high and low temperature resistance, superior ozone resistance, weather resistance, and good electrical insulation properties due to its unique molecular structure. These characteristics have led to its widespread application in demanding fields such as aerospace, electronics, medical devices, and the automotive industry. Despite its many excellent properties, silicone rubber is an organic polymer material and still poses a flammability risk in high-temperature or open-flame environments. This severely limits its application in applications with extremely high safety requirements, such as printed circuit board (PCB) seals, fireproof and heat-insulating gaskets for new energy vehicle battery packs, rail transit seals, and fire barriers for high-rise buildings.

[0003] Currently, the most common method in the industry is to improve the flame retardant properties of silicone rubber by adding large amounts of various flame retardants to the silicone rubber matrix. Commonly used flame retardants mainly include phosphorus-nitrogen intumescent flame retardants (such as ammonium polyphosphate, melamine phosphate, and their compound systems) and organophosphorus flame retardants (such as DOPO and its derivatives). These flame retardants exert their flame-retardant effect by capturing free radicals in the gas phase, promoting char formation in the condensed phase, or decomposing and absorbing heat. However, these flame retardants differ significantly from the silicone rubber matrix in chemical polarity and solubility parameters, resulting in poor dispersion and a tendency to agglomerate within the silicone rubber matrix. Agglomeration of flame retardants not only causes localized stress concentration, deteriorating the physical and mechanical properties of the vulcanizate (such as tear strength and elongation at break), but also disrupts the material's uniformity, creating weak points in flame retardancy. Secondly, the poor interfacial compatibility between flame retardants and the matrix can easily lead to the gradual migration and precipitation of flame retardants to the surface during the storage or use of silicone rubber products (commonly known as "blooming"). This not only affects the appearance and surface properties of the products, but also leads to the gradual decay of the flame retardant effect, making it impossible to guarantee the long-term stability of the flame retardant performance. Summary of the Invention

[0004] This application provides a flame-retardant molded silicone rubber and its vulcanization method, aiming to solve the problems of poor compatibility between traditional flame retardants and silicone rubber matrix, easy agglomeration and precipitation, resulting in deterioration of the mechanical properties of silicone rubber and insufficient flame-retardant stability.

[0005] In a first aspect, this application provides a flame-retardant molding silicone rubber, comprising the following raw materials in parts by weight:

[0006] 100 parts of raw silicone rubber, 1.5 to 3.5 parts of curing agent, 20 to 30 parts of flame retardant filler, and 10 to 20 parts of flame retardant;

[0007] The raw materials for the flame retardant include paraformaldehyde, an aminosilane coupling agent, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide in a molar ratio of 1–1.6:1:1–1.2. The preparation method of the flame retardant includes the following steps: nucleophilic addition of paraformaldehyde and aminosilane coupling agent to obtain an N-hydroxymethylsilane intermediate; condensation of the N-hydroxymethylsilane intermediate with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to obtain a condensation product.

[0008] In any of the above technical solutions, the raw silicone rubber is selected from one or more of methyl vinyl silicone rubber, methyl vinyl phenyl silicone rubber, and methyl vinyl trifluoropropyl silicone rubber; preferably methyl vinyl silicone rubber.

[0009] In any of the above technical solutions, the vinyl content of the raw silicone rubber is 0.1% to 2.0%.

[0010] This application combines organic flame retardants and inorganic flame retardant fillers to achieve excellent flame retardant effects through a synergistic approach of chemical and physical flame retardancy. Specifically, by introducing organosilicon segments into the DOPO organic flame retardant molecule, a flame retardant with good compatibility with the silicone rubber matrix is ​​prepared. The siloxane segments in this flame retardant molecule exhibit good affinity with the silicone rubber backbone, thereby achieving uniform dispersion of the flame retardant in the matrix. This effectively avoids the aggregation and precipitation problems caused by poor compatibility of traditional flame retardants, ensuring stable flame retardant performance of silicone rubber during long-term use and preventing damage to the mechanical properties of silicone rubber due to localized stress concentration.

[0011] It is worth noting that while existing technologies graft DOPO onto silicone rubber molecular chains via chemical reactions can solve the compatibility problem, they inevitably disrupt the regularity of the silicone rubber molecular chains, leading to damage to the material's inherent properties such as flexibility and elasticity. In contrast, the proposed solution uses a modified flame retardant as a functional additive, maximizing the preservation of the silicone rubber molecular structure. This achieves good compatibility between the flame retardant and the matrix while avoiding negative impacts on the silicone rubber's intrinsic properties, resulting in a better balance between the material's flame retardant and mechanical properties.

[0012] It should be noted that in the preparation process of the flame retardant described in this application, the molar amount of paraformaldehyde mentioned is calculated based on the formaldehyde (CH2O) monomer produced after its depolymerization.

[0013] In any of the above technical solutions, the aminosilane coupling agent is a polyaminosilane coupling agent having at least two amino groups.

[0014] In any of the above technical solutions, the aminosilane coupling agent is a difunctional aminosilane coupling agent or a trifunctional aminosilane coupling agent.

[0015] In any of the above technical solutions, the polyaminosilane coupling agent is selected from any one or more of N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane, N-(β-aminoethyl)-γ-aminopropylmethyldiethoxysilane, bis(3-aminopropyl)tetraethoxydisiloxane, and N,N-bis(3-triethoxysilylpropyl)amine.

[0016] In any of the above technical solutions, the method for preparing the flame retardant further includes: subjecting the condensation product to a Michael addition reaction with an alkaline phosphate compound in an alkaline catalyst to obtain an addition product.

[0017] It should be noted that if the condensation product in the preparation method of the flame retardant does not undergo a Michael addition reaction, then the condensation product is the target product flame retardant; if the condensation product undergoes a Michael addition reaction with an alkenyl phosphate compound, then the addition product is the target product flame retardant.

[0018] In any of the above technical solutions, the molar ratio of the polyaminosilane coupling agent to the alkenyl phosphate compound is 1:0.2 to 0.5.

[0019] In any of the above technical solutions, the alkenyl phosphate compound is selected from any one or more of allyl phosphate, dimethyl vinyl phosphonate, and diethyl allyl phosphonate.

[0020] In the preparation of the above flame retardant, paraformaldehyde depolymerizes under heating conditions, releasing formaldehyde monomers. The primary amino group (-NH2) in the aminosilane coupling agent acts as a nucleophile, attacking the carbonyl carbon (C=O) of the formaldehyde molecule, undergoing a nucleophilic addition reaction to generate a silane intermediate containing N-hydroxymethyl (-N-CH2-OH). The hydroxymethyl group (-CH2-OH) of the N-hydroxymethyl silane intermediate undergoes dehydration condensation with the phosphorus atom of the PH bond in the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) molecule, forming a stable PC covalent bond, thereby grafting the DOPO structural unit onto the silane backbone. Finally, the secondary amino group (-NH-) in the polyaminosilane coupling agent that did not participate in the nucleophilic addition reaction acts as a nucleophile. Under the action of a basic catalyst, this secondary amino group initiates a nucleophilic attack on the β-carbon atom of the carbon-carbon double bond (C=C) of the alkenyl phosphate compound, achieving conjugated addition. After the reaction, the phosphorus atom is introduced into the final product molecule through a new CN bond.

[0021] In any of the above technical solutions, the preparation method of the flame retardant is as follows:

[0022] Under nitrogen protection, polyaminosilane coupling agent and paraformaldehyde were added to anhydrous dioxane to make the solid concentration 20-30% w / v and stirred until homogeneous. The reaction system was then slowly heated to 40-60°C and stirred for 1.5-2 hours to obtain a reaction system containing N-hydroxymethylsilane intermediate.

[0023] While maintaining nitrogen protection and a temperature of 40–60°C, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10- and Lewis acid catalyst were slowly added to the reaction system. After the addition was complete, the reaction temperature was raised to 100–105°C, and the reaction was stirred for 12–14 hours to obtain the condensation product.

[0024] Under nitrogen protection, an alkenyl phosphate compound and a basic catalyst were added to the condensation product, and the reaction was stirred at 70–90 °C for 4–6 hours. After the reaction was completed, the system was cooled to room temperature, and the dioxane solvent was removed by vacuum distillation to obtain the crude product.

[0025] The crude product was purified by precipitation by dissolving it in acetone and adding it dropwise into n-hexane under stirring to precipitate it. After filtration, it was dried under vacuum to obtain the final product.

[0026] In any of the above technical solutions, the alkaline catalyst is selected from triethylamine, diisopropylethylamine, or pyridine.

[0027] In any of the above technical solutions, the Lewis acid catalyst is zinc chloride.

[0028] When DOPO is modified using an aminosilane coupling agent, the introduction of siloxane segments improves compatibility, but at the same time dilutes the effective phosphorus content in the flame retardant. This may result in an increase in the amount of flame retardant required to achieve the same flame retardant rating, which in turn has an adverse effect on the mechanical properties of silicone rubber.

[0029] To address this issue, this application employs a polyaminosilane coupling agent for modification. The imino group of this agent can undergo a Michael addition reaction with alkenyl phosphate compounds, introducing additional phosphorus into the molecule. On one hand, the increased phosphorus content effectively compensates for the reduced flame retardant efficiency caused by the introduction of siloxane segments, allowing the modified flame retardant to maintain good compatibility while still exhibiting excellent flame retardant properties. On the other hand, since the increase in phosphorus content comes from internal molecular structure optimization rather than simply increasing the amount of flame retardant, better flame retardant effects can be achieved without significantly increasing the amount added, avoiding negative impacts on the material's mechanical properties.

[0030] In any of the above technical solutions, the flame-retardant filler is alumina-coated silica, the average particle size of the silica is 1 to 10 micrometers, and the thickness of the alumina is 0.1 to 0.5 micrometers.

[0031] In any of the above technical solutions, the alumina-coated silica is prepared by sol-gel method or hydrothermal method.

[0032] In any of the above technical solutions, the silica is fumed silica or precipitated silica.

[0033] In any of the above technical solutions, the curing agent includes a room temperature vulcanizing agent, a high temperature vulcanizing agent, and a room temperature vulcanizing catalyst in a mass ratio of 1:2 to 3:0.01 to 0.05;

[0034] The vulcanization method of the flame-retardant molding silicone rubber is as follows: the flame-retardant molding silicone rubber is placed in a mold and initially crosslinked for 20 to 60 minutes at a pressure of 0.3 to 1.0 MPa and a temperature of 15 to 35°C, and then molded for 5 to 30 minutes at a temperature of 150 to 180°C and a pressure of 5 to 15 MPa.

[0035] In any of the above technical solutions, the room temperature vulcanizing agent is selected from at least one of acetoxysilane, ketoxime silane, alkoxysilane or aminooxysilane; the room temperature vulcanizing catalyst is an organotin catalyst.

[0036] In any of the above technical solutions, the high-temperature vulcanizing agent is selected from at least one of 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,4-bis(tert-butylperoxyisopropyl)benzene or benzoyl peroxide.

[0037] Dicumyl peroxide (DCP) and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (bis2,5) are undesirable choices in this application due to their excessively high decomposition temperatures.

[0038] The flame-retardant molded silicone rubber of this application is mainly aimed at applications such as circuit board seals and new energy vehicle battery seals. These applications not only require materials with good flame-retardant properties but also have high requirements for thermal conductivity. Traditional processes add flame-retardant fillers and thermally conductive fillers simultaneously. However, when the two fillers coexist, the flame-retardant filler often interferes with the thermally conductive network formed by the thermally conductive filler, reducing the overall thermal conductivity efficiency. This application uses a core-shell structure filler with alumina-coated silica, which combines flame-retardant and thermally conductive functions. The silica mainly blocks combustion by constructing a continuous and dense inorganic barrier layer at the combustion interface, while the alumina shell constructs an effective thermal conduction path, avoiding mutual interference between different fillers and facilitating the formation of a continuous and complete thermally conductive network.

[0039] In traditional silicone rubber molding processes, high-temperature vulcanization systems are typically used to achieve rapid curing. This process requires high temperatures (150–180°C) and pressures (5–20 MPa). The high-temperature environment is essential to ensure the effective decomposition of peroxide-based high-temperature vulcanizing agents, thereby initiating the cross-linking reaction. However, the high-temperature process generates small-molecule volatiles that easily form bubbles within the product; therefore, high pressure must be applied to suppress and eliminate these bubbles. However, the enormous mechanical stress generated by high pressure conditions (typically exceeding 1 MPa) can easily damage the alumina coating layer on the filler surface, thereby disrupting the thermal conductivity network structure and ultimately affecting the material's thermal conductivity.

[0040] This application employs a two-step vulcanization process using a combination of room-temperature and high-temperature vulcanizing agents, effectively overcoming the aforementioned problems. First, preliminary molding is performed under low pressure, followed by room-temperature vulcanization to form a soft cross-linked structure with appropriate elasticity. This structure has a higher elastic modulus than completely unvulcanized silicone rubber, effectively buffering and protecting the alumina-coated silica filler. This allows the filler to be dispersed under high pressure during subsequent molding, significantly reducing the risk of damage to the alumina layer. In the subsequent high-temperature, high-pressure vulcanization stage, the soft cross-linked structure maintains a certain degree of fluidity and plastic deformation capacity, allowing the filler particles to fully adjust their position under pressure. The gaps between them are further compacted, forming a tighter contact and constructing a continuous, dense, and uniform thermally conductive network. Finally, complete curing occurs under high temperature and the action of a high-temperature vulcanizing agent, fixing the optimized filler distribution and obtaining a silicone rubber product with excellent thermal conductivity.

[0041] It is important to note that insufficient room temperature curing (e.g., too low a dosage of room temperature curing agent) results in a soft cross-linked structure with insufficient elasticity, limited protective effect, and inability to effectively prevent filler damage. Conversely, excessive room temperature curing (e.g., too high a dosage of room temperature curing agent) leads to a decrease in material elasticity and plasticity, weakening the protective effect on the filler and hindering the rearrangement and close contact of filler particles under high pressure, thus deteriorating thermal conductivity. By optimizing room temperature curing conditions, this application achieves a balance between protecting the integrity of the filler and promoting the formation of a thermally conductive network.

[0042] Secondly, this application provides a vulcanization method for flame-retardant molded silicone rubber, comprising the following steps:

[0043] According to the raw material ratio of flame-retardant molding silicone rubber described in any of the first aspects, fillers and flame retardants are added to silicone rubber and mixed to obtain a compound.

[0044] Add room temperature vulcanizing agent and high temperature vulcanizing agent to the compound and knead until uniform; then add room temperature vulcanizing catalyst and knead until uniform to obtain flame retardant molding silicone rubber; place the flame retardant molding silicone rubber in a mold and perform preliminary crosslinking for 20 to 60 minutes at a pressure of 5 to 8 MPa and a temperature of 15 to 35°C, and then mold for 5 to 30 minutes at a temperature of 120 to 180°C and a pressure of 15 to 20 MPa.

[0045] In summary, this application has the following beneficial effects:

[0046] This application employs a polyaminosilane-modified DOPO flame retardant and introduces additional phosphorus, ensuring both good compatibility between the flame retardant and the silicone rubber matrix while maintaining high flame retardant efficiency. Furthermore, the use of an alumina-coated silica core-shell filler design achieves an organic combination of flame retardancy and thermal conductivity, avoiding performance interference between different fillers. More importantly, the two-step vulcanization process, through room temperature pre-curing and high-temperature molding, protects the integrity of the filler structure and promotes the formation of a dense thermally conductive network. The final product is a silicone rubber material with stable flame retardant properties, good mechanical properties, and high thermal conductivity, meeting the stringent requirements for sealing materials in high-end fields such as electronics, electrical appliances, and new energy vehicles. Detailed Implementation

[0047] Preparation Example

[0048] Preparation Example 1-1, Flame Retardant, was prepared by following these steps:

[0049] Under nitrogen protection, 1.0 mol of N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane and 1.3 mol of paraformaldehyde (molar amounts based on formaldehyde monomer content) were added to a dry three-necked flask. Anhydrous dioxane was added to achieve a solid concentration of 25% w / v, and the mixture was stirred until homogeneous. The reaction system was slowly heated to 50°C and stirred for 1.5 hours to obtain a reaction system containing an N-hydroxymethylsilane intermediate. While maintaining nitrogen protection and a temperature of 50°C, 1.1 mol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 2.0 g of zinc chloride (catalyst) were added to the reaction system. After the addition was complete, the reaction temperature was slowly increased to 105°C, and the mixture was stirred for 13 hours to obtain the condensation product. Under nitrogen protection, 0.35 mol of dimethyl vinyl phosphate and 1.0 g of diisopropylethylamine were added to the condensation product, and the mixture was stirred at 80°C for 5 hours. After the reaction was complete, the system was cooled to room temperature, and the dioxane solvent was removed by vacuum distillation to obtain the crude product. The crude product was dissolved in 200 mL of acetone and slowly added dropwise to 1000 mL of n-hexane under vigorous stirring to precipitate a white precipitate. After filtration, the precipitate was dried under vacuum at 50 °C for 12 hours to obtain the flame retardant.

[0050] Preparation Examples 1-2, flame retardants, were prepared according to the following steps:

[0051] Under nitrogen protection, 1.0 mol of N-(β-aminoethyl)-γ-aminopropyltriethoxysilane and 1.0 mol of paraformaldehyde (molar amounts based on formaldehyde monomer content) were added to a dry three-necked flask. Anhydrous dioxane was added to achieve a solid concentration of 20% w / v, and the mixture was stirred until homogeneous. The reaction system was slowly heated to 40°C and stirred for 2 hours to obtain a reaction system containing an N-hydroxymethylsilane intermediate. While maintaining nitrogen protection and a temperature of 40°C, 1.0 mol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 1.8 g of zinc chloride (catalyst) were added to the reaction system. After the addition was complete, the reaction temperature was slowly increased to 100°C, and the mixture was stirred for 14 hours to obtain the condensation product. Under nitrogen protection, 0.2 mol of dimethyl vinyl phosphate and 0.5 g of triethylamine were added to the condensation product, and the mixture was stirred at 70°C for 6 hours. After the reaction was complete, the system was cooled to room temperature, and the dioxane solvent was removed by vacuum distillation to obtain the crude product. The crude product was dissolved in 150 mL of acetone and slowly added dropwise to 800 mL of n-hexane under vigorous stirring to precipitate a white precipitate. After filtration, the precipitate was dried under vacuum at 45 °C for 15 hours to obtain the flame retardant.

[0052] Preparation Examples 1-3, flame retardants, were prepared according to the following steps:

[0053] Under nitrogen protection, 1.0 mol of N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane and 1.6 mol of paraformaldehyde (molar amounts based on formaldehyde monomer content) were added to a dry three-necked flask. Anhydrous dioxane was added to achieve a solid concentration of 30% w / v, and the mixture was stirred until homogeneous. The reaction system was slowly heated to 60°C and stirred for 1.5 hours to obtain a reaction system containing an N-hydroxymethylsilane intermediate. While maintaining nitrogen protection and a temperature of 60°C, 1.2 mol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 2.3 g of zinc chloride (catalyst) were added to the reaction system. After the addition was complete, the reaction temperature was slowly increased to 105°C, and the mixture was stirred for 13 hours to obtain the condensation product. Under nitrogen protection, 0.5 mol of allyl phosphate diethyl ester and 1.5 g of diisopropyl ethylamine were added to the condensation product, and the mixture was stirred at 90°C for 4 hours. After the reaction was complete, the system was cooled to room temperature, and the dioxane solvent was removed by vacuum distillation to obtain the crude product. The crude product was dissolved in 250 mL of acetone and slowly added dropwise to 1200 mL of n-hexane under vigorous stirring to precipitate a white precipitate. After filtration, the precipitate was dried under vacuum at 55 °C for 10 hours to obtain the flame retardant.

[0054] Preparation Examples 1-4, flame retardants, differ from Preparation Example 1-1 in that N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane is replaced with an equimolar amount of aminopropyltrimethoxysilane.

[0055] Preparation Examples 1-5, flame retardant, ungrafted allyl phosphate, the specific preparation steps are as follows:

[0056] Under nitrogen protection, 1.0 mol of N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane and 1.3 mol of paraformaldehyde (molar amounts based on formaldehyde monomer content) were added to a dry three-necked flask. Anhydrous dioxane was added to achieve a solid concentration of 25% w / v, and the mixture was stirred until homogeneous. The reaction system was slowly heated to 50°C and stirred for 1.5 hours to obtain a reaction system containing an N-hydroxymethylsilane intermediate. While maintaining nitrogen protection and a temperature of 50°C, 1.1 mol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and 2.0 g of zinc chloride (catalyst) were added to the reaction system. After the addition was complete, the reaction temperature was slowly raised to 105°C and stirred for 13 hours to obtain the condensation product. The system was cooled to room temperature, and the dioxane solvent was removed by vacuum distillation to obtain the crude product. The crude product was dissolved in 200 mL of acetone and slowly added dropwise to 1000 mL of n-hexane under vigorous stirring to precipitate a white precipitate. After filtration, the precipitate was dried under vacuum at 50 °C for 12 hours to obtain the flame retardant.

[0057] Preparation Example 2-1: Alumina-coated silica was prepared by the following method:

[0058] 100 g of fumed silica with an average particle size of 5 μm was dispersed in 500 mL of deionized water and sonicated for 30 minutes to form a homogeneous suspension. 78 g of aluminum nitrate nonahydrate was dissolved in 200 mL of deionized water and slowly added dropwise to the fumed silica suspension with stirring. The pH of the system was adjusted to 8.0 with ammonia, and the mixture was stirred at 60 °C for 4 hours. After the reaction, the mixture was allowed to stand for 12 hours to age. The solid product was separated by centrifugation, washed with deionized water until neutral, and then washed twice with ethanol. The product was dried at 100 °C for 6 hours and then calcined at 550 °C for 3 hours to obtain alumina-coated fumed silica. The obtained alumina-coated fumed silica was mixed with 10 g of hexamethyldisilazane and stirred at 120 °C for 3 hours. After the reaction, the mixture was washed three times with ethanol to remove unreacted hexamethyldisilazane and then vacuum dried at 80 °C for 4 hours to obtain surface-modified alumina-coated fumed silica with an alumina coating thickness of approximately 0.3 μm.

[0059] Preparation Example 2-2: Alumina-coated silica was prepared by the following method:

[0060] 100 g of precipitated silica with an average particle size of 1 μm was dispersed in 400 mL of deionized water and sonicated for 40 minutes to form a homogeneous suspension. 52 g of aluminum nitrate nonahydrate was dissolved in 150 mL of deionized water and slowly added dropwise to the silica suspension with stirring. The pH of the system was adjusted to 7.5 with ammonia, and the mixture was stirred at 50 °C for 5 hours. After the reaction, the mixture was allowed to stand for 10 hours to age. The solid product was separated by centrifugation, washed with deionized water until neutral, and then washed twice with ethanol. The product was dried at 90 °C for 8 hours and then calcined at 500 °C for 4 hours to obtain alumina-coated silica. The obtained alumina-coated silica was mixed with 5 g of hexamethyldisilazane and stirred at 100 °C for 4 hours. After the reaction, the mixture was washed three times with ethanol to remove unreacted hexamethyldisilazane and then vacuum dried at 70 °C for 5 hours to obtain surface-modified alumina-coated silica with an alumina coating thickness of approximately 0.2 μm.

[0061] Preparation Example 2-3: Alumina-coated silica was prepared by the following method:

[0062] 100 g of fumed silica with an average particle size of 10 μm was dispersed in 600 mL of deionized water and sonicated for 20 minutes to form a homogeneous suspension. 130 g of aluminum nitrate nonahydrate was dissolved in 250 mL of deionized water and slowly added dropwise to the fumed silica suspension with stirring. The pH of the system was adjusted to 8.5 with ammonia, and the mixture was stirred at 70 °C for 3 hours. After the reaction, the mixture was allowed to stand for 14 hours to age. The solid product was separated by centrifugation, washed with deionized water until neutral, and then washed twice with ethanol. The product was dried at 110 °C for 5 hours and then calcined at 600 °C for 2.5 hours to obtain alumina-coated fumed silica. The obtained alumina-coated fumed silica was mixed with 20 g of hexamethyldisilazane and stirred at 150 °C for 2 hours. After the reaction was completed, the product was washed three times with ethanol to remove unreacted hexamethyldisilazane, and then dried under vacuum at 90°C for 3 hours to obtain surface-modified alumina-coated silica with an alumina coating thickness of approximately 0.5 μm.

[0063] Example

[0064] Example 1: A flame-retardant molded silicone rubber product is prepared according to the following steps:

[0065] 1000 g of methyl vinyl silicone rubber raw material (Dow Corning RBG-0614) with a vinyl molar content of 0.24-0.49% was added to a vacuum kneader and plasticized for 15 minutes at a vacuum of -0.08 MPa and a temperature of 50°C. Then, 255 g of alumina-coated silica filler from Preparation Example 2-1 and 150 g of flame retardant from Preparation Example 1-1 were added, and the mixture was kneaded for 30 minutes at 60°C and a vacuum of -0.09 MPa to obtain a compound. The compound was transferred to a two-roll mill, and 8.0 g of room temperature vulcanizing agent (methyltriacetoxysilane), 20.0 g of high temperature vulcanizing agent (1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane), and 0.25 g of room temperature vulcanizing catalyst (dibutyltin dilaurate) were added sequentially, and the mixture was kneaded until homogeneous. The uniformly mixed rubber compound is placed in a mold and initially crosslinked for 40 minutes at a pressure of 0.65 MPa and a temperature of 25°C to form a soft crosslinked structure. Then, the mold is transferred to a flat vulcanizing machine and molded at 160°C and a pressure of 10 MPa for 15 minutes. Finally, it is vulcanized in an oven at 180°C for 2 hours to obtain the final product.

[0066] Example 2: A flame-retardant molded silicone rubber product is prepared according to the following steps:

[0067] 1000 g of methyl vinyl silicone rubber raw material (Dow Corning RBG-0612) with a vinyl molar content of 0.13-0.20% was added to a vacuum kneader and plasticized for 20 minutes at a vacuum of -0.07 MPa and a temperature of 45°C. Then, 205 g of alumina-coated silica filler from Preparation Example 2-2 and 100 g of flame retardant from Preparation Example 1-2 were added, and the mixture was kneaded for 40 minutes at 55°C and a vacuum of -0.08 MPa to obtain a compound. The compound was transferred to a two-roll mill, and 4 g of room temperature vulcanizing agent (methyl tributanone oxime silane), 11 g of high temperature vulcanizing agent (1,4-bis(tert-butylperoxyisopropyl)benzene), and 0.05 g of room temperature vulcanizing catalyst (dibutyltin dilaurate) were added sequentially and kneaded until homogeneous. The homogeneous compound was placed in a mold and initially crosslinked for 60 minutes at a pressure of 1 MPa and a temperature of 15°C to form a soft crosslinked structure. The mold is then transferred to a flat vulcanizing machine and molded for 25 minutes at 150°C and 15MPa pressure. Finally, it is vulcanized in an oven at 170°C for 3 hours to obtain the final product.

[0068] Example 3: A flame-retardant molded silicone rubber product is prepared according to the following steps:

[0069] 1000 g of methyl vinylphenyl silicone rubber raw material (Dow Corning RBG-0702) with a vinyl content of 2.0% was added to a vacuum kneader and plasticized for 10 minutes at a vacuum of -0.09 MPa and a temperature of 55°C. Then, 300 g of alumina-coated silica filler from Preparation Examples 2-3 and 200 g of flame retardant from Preparation Examples 1-3 were added, and the mixture was kneaded for 25 minutes at 65°C and a vacuum of -0.10 MPa to obtain a compound. The compound was transferred to a two-roll mill, and 11.0 g of room temperature vulcanizing agent (vinyltriacetoxysilane), 23.0 g of high temperature vulcanizing agent (benzoyl peroxide), and 0.55 g of room temperature vulcanizing catalyst (dibutyltin dilaurate) were added sequentially and kneaded until homogeneous. The homogeneous compound was placed in a mold and initially crosslinked for 20 minutes at a pressure of 0.5 MPa and a temperature of 30°C to form a soft crosslinked structure. The mold is then transferred to a flat vulcanizing machine and molded at 180°C and 6 MPa for 10 minutes. Finally, it is vulcanized in an oven at 190°C for 1 hour to obtain the final product.

[0070] Example 4, a flame-retardant molded silicone rubber product, differs from Example 1 in that the flame retardant of Preparation Examples 1-4 is used in equal amounts instead of the flame retardant of Preparation Example 1-1.

[0071] Example 5, a flame-retardant molded silicone rubber product, differs from Example 1 in that an equal amount of the flame retardant used in Examples 1-5 is used instead of the flame retardant used in Example 1-1.

[0072] Example 6, a flame-retardant molded silicone rubber product, differs from Example 1 in that an equal amount of high-temperature vulcanizing agent (1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane) is used instead of room-temperature vulcanizing agent (methyltriacetoxysilane).

[0073] Example 7, a flame-retardant molded silicone rubber product, differs from Example 1 in that an equal amount of room temperature vulcanizing agent (methyltriacetoxysilane) is used instead of high temperature vulcanizing agent (1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane).

[0074] Example 8, a flame-retardant molded silicone rubber product, differs from Example 1 in that the curing agent dosage ratio is different, including: 14g room temperature vulcanizing agent (methyltriacetoxysilane) and 14.0g high temperature vulcanizing agent (1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane).

[0075] Example 9, a flame-retardant molded silicone rubber product, differs from Example 1 in that the curing agent dosage ratio is different, including: 5g room temperature vulcanizing agent (methyltriacetoxysilane) and 23.0g high temperature vulcanizing agent (1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane).

[0076] Example 10, a flame-retardant molded silicone rubber product, differs from Example 1 in that it does not undergo room temperature vulcanization but is directly vulcanized at high temperature. Specifically, the uniformly mixed rubber compound is placed in a mold, the mold is transferred to a flat vulcanizing machine, and molded at 160°C and 10 MPa pressure for 15 minutes. Finally, it is vulcanized in an oven at 180°C for 2 hours to obtain the product.

[0077] Example 11, a flame-retardant molded silicone rubber product, differs from Example 1 in that it does not undergo high-temperature vulcanization, but only room-temperature vulcanization, as follows: the uniformly mixed rubber compound is placed in a mold and initially crosslinked for 40 minutes at a pressure of 0.65 MPa and a temperature of 25°C.

[0078] Example 12, a flame-retardant molded silicone rubber product, differs from Example 1 in that the pressure during room temperature vulcanization (i.e., initial crosslinking) is 1.5 MPa.

[0079] Comparative Example

[0080] Comparative Example 1, a flame-retardant molded silicone rubber product, differs from Example 1 in that an equal amount of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is used instead of the flame retardant used in Preparation Example 1-1.

[0081] Comparative Example 2, a flame-retardant molded silicone rubber product, was prepared according to the preparation method of Example 1 in CN109233290A. The specific operation steps are as follows:

[0082] Step 1: Add methyl vinyl silicone rubber raw material (Dow Corning RBG-0614) with a vinyl content of 0.24-0.49% to a vacuum kneader and perform plasticizing under vacuum conditions. The plasticizing temperature is 150℃, the plasticizing time is 40min, and the rotor speed is 100rpm to obtain methyl vinyl silicone rubber plasticized compound.

[0083] Step 2: 52g of m-chloroperoxybenzoic acid and 255g of alumina-coated silica filler from Preparation Example 2-1 were sequentially added to the vacuum kneader in Step 1. Under vacuum conditions, the mixture was kneaded with 1000g of methyl vinyl silicone rubber plasticized rubber at a kneading temperature of 40℃, a kneading time of 60min, and a rotor speed of 65rpm. This allowed the vinyl groups on the methyl vinyl silicone rubber molecular chain to react with the peroxy groups on the m-chloroperoxybenzoic acid, resulting in epoxidized methyl vinyl silicone rubber.

[0084] Step 3: Add 150g of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to the vacuum kneader in Step 2, and knead it with epoxidized methyl vinyl silicone rubber under vacuum conditions. The kneading temperature is 100℃, the kneading time is 120min, and the rotor speed is 60rpm. During the kneading process, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide reacts with epoxidized methyl vinyl silicone rubber to obtain primary phosphorus-containing intrinsic flame-retardant methyl vinyl silicone rubber.

[0085] Step 4: Add the primary phosphorus-containing intrinsic flame-retardant silicone rubber to the filter press, filter it through a 500-mesh filter to remove impurities, and keep the temperature of the filter at 80°C to obtain phosphorus-containing intrinsic flame-retardant methyl vinyl silicone rubber.

[0086] Phosphorus-containing intrinsically flame-retardant methyl vinyl silicone rubber was placed in a mold and initially crosslinked for 40 minutes at 0.65 MPa pressure and 25°C to form a soft crosslinked structure. The mold was then transferred to a flat vulcanizing machine and molded at 160°C and 10 MPa pressure for 15 minutes. Finally, it was vulcanized in an oven at 180°C for 2 hours to obtain the final product.

[0087] Performance testing

[0088] Experiment 1: Flame retardant performance test

[0089] Test Procedure: Ten standard specimens, each 80mm × 10mm × 4mm in size, were cut from the vulcanized silicone rubber products of the various examples and comparative examples. Following the specifications in ASTM D2863-19, "Standard Test Method for Determining the Minimum Oxygen Concentration in Plastics (Oxygen Index Method)," the specimens were vertically fixed in the combustion chamber, with the top of the specimen at least 100mm below the top of the combustion chamber. The flow rate of the nitrogen-oxygen mixture was adjusted to 10-30 L / min, and the test was conducted at 23±2℃. The test started with the expected oxygen concentration, and the oxygen concentration was adjusted using an "up-down" method based on the combustion results until the limiting oxygen concentration was reached. The minimum oxygen concentration value was recorded when the continuous combustion time of the specimen did not exceed 3 minutes and the combustion length did not exceed 50mm. The average value of the 10 specimens was taken as the final LOI value.

[0090] Experiment 2: Physical and Chemical Properties Test

[0091] Test Procedure: Following JISK 6251:2017 "Vulcanized or thermoplastic rubbers—Determination of tensile stress-strain properties", at least three dumbbell-shaped specimens (Type 1) with a thickness of 2.0 ± 0.2 mm were cut from the vulcanized silicone rubber samples of each example and comparative example. The specimens were conditioned for 24 hours at a standard temperature of 23 ± 2 °C and a relative humidity of 50 ± 5%. An INSTRON universal testing machine was used, with the tensile speed set to 500 ± 50 mm / min. The maximum tensile force at fracture was recorded, and the tensile strength (unit: MPa) was calculated. Simultaneously, the elongation at fracture was recorded, and the elongation at break (unit: %) was calculated. The result was the arithmetic mean of at least three specimens. Following ISO 34-1-2022 "Determination of tear strength of vulcanized or thermoplastic rubbers", right-angled specimens with a thickness of 2.0 ± 0.2 mm were prepared and conditioned for 24 hours at a standard environment. Five specimens were prepared for each group. Using an INSTRON universal testing machine, the tensile speed was 500±50 mm / min. The maximum force during the tearing process of the specimen was recorded, and the tear strength was calculated.

[0092] Experiment 3: Anti-migration and precipitation performance test

[0093] Sample preparation: Three samples with dimensions of 100mm×100mm×2mm were cut from the vulcanized silicone rubber products of each embodiment and comparative example.

[0094] Test procedure: Place the sample in an oven at 80±2℃ for 168 hours, then remove it and cool it under standard conditions for 1 hour. Observe whether there are white or yellow precipitates on the sample surface.

[0095] Experiment 4: Thermal conductivity test

[0096] Test Procedure: Following the steps of each embodiment or comparative example, prepare three circular specimens with a diameter of 12.7 mm and a thickness of 2-4 mm, ensuring the parallelism error between the two sides does not exceed 0.02 mm. Refer to ASTM E1461-13, "Standard Test Method for Determination of Thermal Diffusivity by Flash Method," and use a laser flash thermal conductivity meter at 25°C. First, measure the thermal diffusivity of the specimen; simultaneously, measure the specific heat capacity of the specimen using a thermogravimetric analyzer and the density of the specimen using a density balance. Calculate the thermal conductivity using the formula λ = α × p × Cp, where λ is the thermal conductivity, α is the thermal diffusivity, p is the density, and Cp is the specific heat capacity.

[0097] Table 1. Performance Test Results

[0098]

[0099] Analysis of experimental results:

[0100] Compared to Example 1, Example 4, which replaced polyaminosilane with monoaminosilane, showed poorer performance in the LOI index, but little change in mechanical properties. This indicates that the flame retardant molecules of the monoaminosilane-modified product have good compatibility with the silicone rubber matrix and no significant negative impact on mechanical properties and migration resistance. The decrease in flame retardancy may be because monoaminosilane can only provide one reaction site and is consumed in the reaction with formaldehyde, making it impossible to introduce additional phosphorus through the Schiff base reaction. Example 5, without grafted allyl diethyl phosphate, showed poor performance in the LOI. This indicates that the lack of introduction of additional phosphorus has a significant negative impact on flame retardant efficiency, and its oxygen index is close to that of Example 4. This may be because both allyl diethyl phosphate and polyaminosilane are key features for introducing additional phosphorus. Without the addition of allyl diethyl phosphate, the unreacted remaining amino groups in the polyaminosilane cannot be fully utilized, resulting in a low effective phosphorus content in the flame retardant molecules and insufficient flame retardant efficiency.

[0101] Compared to Example 1, Example 6, which completely replaced the room-temperature vulcanizing agent with a high-temperature vulcanizing agent, exhibited poorer thermal conductivity. This indicates that the lack of a soft cross-linked structure formed during room-temperature vulcanization negatively impacts the construction of the thermally conductive network. This may be because the absence of buffer protection during the room-temperature vulcanization stage makes the alumina coating layer more susceptible to damage during high-pressure molding, disrupting the thermal conductivity pathway and weakening heat transfer efficiency. In Example 7, where the room-temperature vulcanizing agent completely replaced the high-temperature vulcanizing agent, the silicone rubber could not be effectively vulcanized. The original room-temperature vulcanized structure was severely damaged after high-temperature and high-pressure treatment, leading to a sharp deterioration in material properties. Therefore, this material lacks practical application potential, and the LOI, physicochemical properties, and thermal conductivity data measured under these circumstances are not comparable.

[0102] Compared to Example 1, Example 8, using equal amounts of room temperature vulcanizing agent and high temperature vulcanizing agent (1:1), showed a slight decrease in thermal conductivity. This indicates that a relatively excessive amount of room temperature vulcanizing agent has a slight negative impact on thermal conductivity. This may be because excessive room temperature vulcanizing agent leads to premature formation of soft cross-linking structures and a high cross-linking density, reducing the material's elasticity and plasticity. This weakens the protective effect on the filler and hinders the filler particles from approaching each other under high pressure, thus deteriorating thermal conductivity. Example 9, using a smaller amount of room temperature vulcanizing agent (room temperature vulcanizing agent: high temperature vulcanizing agent = 1:4.6), showed poorer thermal conductivity. This indicates that insufficient room temperature vulcanizing agent has a negative impact on thermal conductivity. This may be because the soft cross-linking structure formed during the room temperature vulcanization stage lacks elasticity, providing limited buffering protection for the filler, and the alumina coating layer is still easily damaged during high-pressure molding.

[0103] Example 10, by omitting the room temperature vulcanization stage and directly performing high-temperature vulcanization, exhibited poor thermal conductivity. This indicates that omitting the room temperature vulcanization stage has a significant negative impact on the construction of the thermally conductive network. The reason may be that without the buffer protection of the soft cross-linked structure formed during room temperature vulcanization, the filler particles are more easily damaged under high pressure, and the interparticle gaps are larger. Example 11, by omitting the high-temperature vulcanization stage and only vulcanizing at room temperature, cannot effectively vulcanize and mold the silicone rubber. Therefore, this material has no practical application potential, and the data measured under this condition, such as LOI, physicochemical properties, and thermal conductivity, are not comparable. Example 12 exhibited poor thermal conductivity. The reason may be that, before the formation of soft cross-links, the high initial pressure will damage the alumina coating layer.

[0104] Compared to Example 1, Comparative Example 1, which directly used unmodified DOPO, showed significantly poorer performance in terms of LOI, mechanical properties, and migration resistance. This may be because DOPO has a large polarity difference from silicone rubber, making it prone to agglomeration and precipitation, forming stress concentration points. Simultaneously, the uneven distribution of flame-retardant elements severely affects the overall performance of the material. Comparative Example 2, which grafted DOPO onto the silicone rubber molecular chain, also showed poor performance in flame retardancy, mechanical properties, and thermal conductivity. This may be because, firstly, no additional phosphorus was introduced into the DOPO in Comparative Example 2, resulting in a relatively low phosphorus content and decreased flame retardancy; secondly, the epoxidation and grafting reactions disrupted the regularity of the silicone rubber molecular chain, impairing its inherent mechanical properties; and thirdly, the grafted DOPO groups consumed the vinyl groups in the raw rubber and created strong steric hindrance, significantly reducing the room temperature fluidization efficiency, hindering the formation of the soft cross-linked elastic structure, and failing to provide effective buffering protection for the filler, leading to deterioration in thermal conductivity.

[0105] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A flame-retardant molded silicone rubber, characterized by, The raw materials include the following components by mass: 100 parts of raw silicone rubber, 1.5-3.5 parts of curing agent, 20-30 parts of flame-retardant filler, and 10-20 parts of flame retardant, wherein the flame-retardant filler is alumina-coated white carbon black; The raw materials of the flame retardant include paraformaldehyde, amino silane coupling agent and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide in a molar ratio of 1-1.6:1:1-1.2; the preparation method of the flame retardant includes the following steps: making the paraformaldehyde and the amino silane coupling agent undergo nucleophilic addition to obtain an N-hydroxymethyl silane intermediate; the N-hydroxymethyl silane intermediate is condensed with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to obtain a condensation product; the condensation product is subjected to Michael addition reaction with an alkenyl phosphate ester compound under an alkaline catalyst to obtain an addition product; the amino silane coupling agent is a polyamino silane coupling agent having at least two amino groups; and the preparation method of the flame retardant is as follows: Under nitrogen protection, the polyamino silane coupling agent and the paraformaldehyde are added to anhydrous dioxane to make the solid concentration 20-30% w / v, and stirred uniformly; the reaction system is slowly heated to 40-60℃, and stirred for 1.5-2 hours to obtain a reaction system containing the N-hydroxymethyl silane intermediate; Under nitrogen protection and at a temperature of 40-60℃, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and a Lewis acid catalyst are slowly added to the reaction system, and after the addition is completed, the reaction temperature is raised to 100-105℃, and stirred for 12-14 hours to obtain a condensation product; Under nitrogen protection, the alkenyl phosphate ester compound and the alkaline catalyst are added to the above condensation product, and stirred at 70-90℃ for 4-6 hours; after the reaction is completed, the system is cooled to room temperature, and the dioxane solvent is removed by distillation under reduced pressure to obtain a crude product; The crude product is purified by a precipitation method, dissolved in acetone, and added dropwise to n-hexane under stirring to precipitate, filtered, and vacuum dried to obtain the flame retardant; The curing agent includes room temperature vulcanizing agent, high temperature vulcanizing agent and room temperature vulcanizing catalyst in a mass ratio of 1:2-3:0.01-0.05; the vulcanization method of the flame-retardant molding silicone rubber is as follows: the flame-retardant molding silicone rubber is placed in a mold, and subjected to preliminary crosslinking at a pressure of 0.3-1.0 MPa and a temperature of 15-35℃ for 20-60 minutes, and then subjected to molding at a temperature of 150-180℃ and a pressure of 5-15 MPa for 5-30 minutes.

2. The flame-retardant molded silicone rubber according to claim 1, characterized by The molar ratio of the polyamino silane coupling agent to the alkenyl phosphate ester compound is 1:0.2-0.

5.

3. The flame-retardant molded silicone rubber according to claim 1, characterized by The average particle size of the white carbon black in the alumina-coated white carbon black is 1-10 microns, and the thickness of the alumina is 0.1-0.5 microns.

4. The flame-retardant molded silicone rubber according to claim 1, characterized by The room temperature vulcanizing agent is at least one selected from acetoxy silane, ketoxime silane, alkoxy silane or aminoxysilane; and the room temperature vulcanizing catalyst is an organic tin catalyst.

5. The flame-retardant molded silicone rubber according to claim 1, characterized by The high temperature vulcanizing agent is at least one selected from 1,1-bis(tert-butyl peroxy)-3,3,5-trimethylcyclohexane, 1,4-bis(tert-butyl peroxyisopropyl) benzene or benzoyl peroxide.

6. A method of curing a flame-retardant molded silicone rubber, characterized by, The method includes the following steps: The raw material ratio of the flame-retardant molded silicone rubber according to any one of claims 2-5 is mixed by adding the flame-retardant filler and the flame retardant into the silicone rubber to obtain a mixed rubber; the room temperature vulcanizing agent and the high temperature vulcanizing agent are added into the mixed rubber and opened to obtain uniformity; the room temperature vulcanization catalyst is added and opened to obtain uniformity to obtain the flame-retardant molded silicone rubber; the flame-retardant molded silicone rubber is placed in a mold, and is primarily cross-linked for 20-60 minutes under the pressure of 0.3-1.0 MPa and the temperature of 15-35℃, and then is molded for 5-30 minutes under the temperature of 150-180℃ and the pressure of 5-15 MPa.

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