An integrated co-vulcanization process for one-shot molding of multilayer composite rubber articles

Abstract

The application relates to the technical field of rubber composite material processing, and discloses an integrated co-vulcanization process for one-step forming of multilayer composite rubber products; in the process, all the rubber layers adopt the same main vulcanizing agent and accelerator, but at least one vulcanizing component of two adjacent layers has different initial concentrations, so as to form a cross-layer diffusion driving force; the mixed rubber of each layer is directly contacted and stacked without being coated with an adhesive, so that additional interfaces are avoided; during vulcanization, segmented temperature rising is adopted, the vulcanizing component is promoted to fully diffuse in a low temperature interval to form a pre-designed concentration gradient, and then the temperature is raised to complete crosslinking reaction, so that a gradient co-crosslinking layer with continuously changed crosslinking density is generated in situ at the interface; the gradient layer eliminates the abrupt interface and stress concentration of traditional composite products, the interlayer peeling strength reaches more than 80% of the body strength, the process flow is simplified, and the process is suitable for various difficult composite systems such as fluorine rubber / silicon rubber.

Description

Technical Field

[0001] This invention relates to the field of rubber composite material processing technology, specifically to an integrated co-vulcanization process for one-time molding of multi-layer composite rubber products. Background Technology

[0002] Multilayer composite rubber products are made of two or more layers of rubber materials with different types or formulations, aiming to simultaneously meet diverse performance requirements such as high and low temperature resistance, media resistance, high elasticity, high damping, and wear resistance in a single component. For example, fluororubber / silicone rubber composite seals can combine the oil resistance and high temperature resistance of fluororubber with the low temperature elasticity and wide temperature range adaptability of silicone rubber; natural rubber / EPDM rubber composite conveyor belts attempt to simultaneously obtain the high strength of natural rubber and the ozone aging resistance of EPDM rubber.

[0003] Matching the vulcanization rates of different rubber types is the traditional core idea for achieving multilayer co-vulcanization. Ideally, each layer of rubber should complete the cross-linking reaction simultaneously during thermal vulcanization, forming a fully co-crosslinked structure at the interlayer interface, thereby achieving an integrated cross-linked network and uniform mechanical property distribution in the product. However, due to significant differences in the chemical structure, unsaturation, polarity, and vulcanization mechanism of different rubber types, each layer of rubber compound responds differently to the same vulcanization system, resulting in a mismatch in vulcanization rates and making it difficult to achieve the ideal co-vulcanization effect. When natural rubber is used in combination with EPDM rubber, even with optimized vulcanization systems, sufficient interfacial co-crosslinking cannot be formed between the two phases; similarly, natural rubber and hydrogenated nitrile butadiene rubber face the problem of mismatched vulcanization rates and difficulties in filler dispersion due to differences in unsaturation and polarity.

[0004] To address the aforementioned problems, existing technologies have primarily developed three types of process solutions: The first type involves forcibly matching the vulcanization rates by adjusting the vulcanization systems of each rubber layer. This involves designing differentiated combinations of accelerators and vulcanizing agents for different rubber types to make the optimal vulcanization times for each layer as close as possible. While this method can reduce the difference in vulcanization rates between layers to some extent, it cannot solve the problem of deep interfacial co-crosslinking. In other words, different rubber layers cannot form effective chemical bonds, relying solely on physical entanglement and limited interfacial diffusion to maintain interlayer bonding. The interfacial bonding strength is far lower than the strength of the rubber itself. This "physical interlocking + limited diffusion" interfacial bonding mode is highly susceptible to interfacial delamination and failure under harsh service conditions such as dynamic fatigue, high and low temperature cycling, or immersion in media.

[0005] The second type is the two-step "adhesion-vulcanization" method using interlayer adhesive coating. This involves pre-coating or bonding a special adhesive between each rubber layer, using the adhesive as a "bridge" between the layers, and then achieving interfacial bonding through co-vulcanization. However, this method adds an independent adhesive coating process, complicating the production flow and reducing efficiency. The introduction of the adhesive layer adds new interfaces to the multilayer structure, making stress concentration at multiple interfaces more pronounced and increasing the risk of delamination. Furthermore, the vulcanization rates of the adhesive and the adjacent rubber layers often differ, potentially making the adhesive layer itself a weak point in the entire multilayer structure, highly susceptible to delamination between the rubber and adhesive interfaces under dynamic loading conditions.

[0006] The third method controls the vulcanization process of each layer through temperature gradients or stepwise vulcanization. For example, one layer is dynamically vulcanized first, followed by static vulcanization of another layer. By controlling the vulcanization sequence, the degree of crosslinking of each phase can be regulated. However, this method is problematic for thicker multilayer products. Because rubber is a poor conductor of heat, a significant temperature gradient exists along the thickness of the product. The surface layer vulcanizes prematurely due to its higher temperature, while the inner layer vulcanizes later due to its lower temperature, resulting in severely uneven vulcanization along the thickness. Furthermore, the prematurely formed crosslinked network on the surface hinders the diffusion and migration of vulcanizing agents and accelerators into the inner layers, further exacerbating the interlayer vulcanization differences. Ultimately, this leads to the formation of multiple weak bonding interfaces within the product, making it highly susceptible to interlayer cracking during subsequent use.

[0007] To address this issue, this invention proposes a reverse-thinking solution. It utilizes and precisely controls the diffusion behavior of vulcanizing system components between adjacent rubber layers, creating a controllable concentration gradient of active species such as vulcanizing agents and accelerators at the interface. This results in the in-situ generation of a gradient co-crosslinked layer with a continuously varying crosslinking density along the interface normal during vulcanization. In this gradient layer, the crosslinking density and chemical composition transition continuously from one rubber layer to another, eliminating any abrupt interfaces and stress concentration sources. Simultaneously, the active species diffusing across layers can participate in the crosslinking reaction of rubber molecules on both sides, forming true cross-layer covalent bonds. This makes the multilayer product a single material body with a gradually varying crosslinking density at the microscopic level, rather than a simple stack of multiple independent crosslinked networks. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides an integrated co-vulcanization process for one-time molding of multi-layer composite rubber products, thereby solving the problems in the background technology.

[0009] To achieve the above objectives, the present invention provides the following technical solution: an integrated co-vulcanization process for one-time molding of multi-layer composite rubber products, comprising the following steps: Step S1: Prepare the rubber compounds for each layer of the composite product separately. All rubber layers use the same vulcanization system. The vulcanization system contains the same main vulcanizing agent and the same combination of accelerators. At least one vulcanizing component in adjacent rubber compounds has a different initial concentration. This concentration difference is achieved by adjusting at least one of the following: the amount of vulcanizing agent, the amount of accelerator, or the amount of vulcanizing activator in each rubber compound. Step S2: Plasticize and mix the rubber compounds prepared in step S1 to obtain the compounded rubber compounds of each layer. Step S3: According to the composite product layer structure design, each layer of compound rubber is laid or co-extruded in the molding die in a predetermined order to form a multi-layer composite preform. No independent adhesive layer is added at the interface between two adjacent rubber layers, and each rubber layer is in direct contact at the interface. Step S4: Place the multi-layer composite preform obtained in step S3 together with the mold in a vulcanization device, and apply a pressure of 5 MPa to 25 MPa at a vulcanization temperature of 120℃ to 200℃ to perform integrated vulcanization molding. Step S5: After vulcanization, cool and demold to obtain a one-piece molded multi-layer composite rubber product; In step S4, during the vulcanization process, the vulcanizing components with different initial concentrations diffuse across the interface from the high-concentration layer to the low-concentration layer under the drive of the concentration gradient, forming a gradient distribution layer of vulcanizing components on both sides of the interface of adjacent rubber layers. As the vulcanization reaction proceeds, the gradient distribution of vulcanizing components simultaneously triggers the cross-linking reaction of rubber molecules on both sides in the interface region, generating a gradient co-crosslinking layer in situ.

[0010] Preferably, the main vulcanizing agent is a peroxide vulcanizing agent, and the accelerator is a peroxide co-accelerator; in two adjacent rubber layers, the amount of peroxide in one layer is 1.5 to 5.0 times that in the other layer, and a diffusion accelerator accounting for 0.5% to 3.0% of the mass of the rubber in the layer with the higher amount is added simultaneously, wherein the diffusion accelerator is at least one of low molecular weight polyethylene glycol, zinc stearate, or fatty acid ester.

[0011] Preferably, when the composite product comprises a fluororubber layer and a silicone rubber layer, in step S1, the main vulcanizing agent is BIPB peroxide, the accelerator is TAIC, the amount of BIPB in the fluororubber layer is 1.5-3.0 phr, and the amount of TAIC is 1.0-2.5 phr; the amount of BIPB in the silicone rubber layer is 0.5-1.0 phr, and the amount of TAIC is 0.3-0.8 phr; and 1.0-2.0 phr of low molecular weight polyethylene glycol is added to the fluororubber layer as a diffusion accelerator.

[0012] Preferably, in step S1, the diffusion rate of the vulcanizing component is controlled by adjusting the type and amount of reinforcing filler system in each layer of rubber compound: adding high-structure carbon black or surface-hydrophobic treated silica reduces the diffusion coefficient of the vulcanizing component, while adding low-structure carbon black or untreated silica increases the diffusion coefficient; at the vulcanization temperature, the diffusion coefficient of each layer of rubber compound is controlled at 1×10⁻⁶ by the filler. -8 cm 2 / s~5×10 -7 cm 2 Within the range of / s.

[0013] Preferably, the vulcanization temperature in step S4 adopts a segmented heating mode: first, it is kept at 80℃~110℃ for 10~30 minutes. At this stage, the vulcanization reaction has not been fully initiated and is mainly used to promote the diffusion of vulcanizing components across the interface to form a pre-designed concentration gradient; then, it is heated to 140℃~200℃ and kept at 140℃~200℃ to complete the main vulcanization reaction, so that the gradient-distributed vulcanizing components simultaneously initiate the cross-linking of each layer and interface, forming an integrated gradient co-crosslinking network.

[0014] Preferably, in step S3, after layer-by-layer stacking and before vulcanization, a pre-compression of 0.5 MPa to 2.0 MPa is applied to the multilayer composite preform and the pressure is maintained for 1 to 5 minutes to eliminate interlayer air and promote the diffusion and entanglement of interlayer molecular chains.

[0015] Preferably, the thickness of the gradient co-crosslinked layer is 10 μm to 200 μm, and its crosslinking density changes continuously from the bulk value of one layer of rubber to the bulk value of the adjacent layer of rubber, without any abrupt interface in crosslinking density or chemical composition.

[0016] Preferably, the vulcanization time in step S4 is the time required for each layer of rubber compound to reach the positive vulcanization state.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses an identical vulcanization system to formulate all rubber layers and utilizes the initial concentration difference of at least one vulcanizing component in adjacent rubber layers to drive the vulcanizing component to diffuse across the interface from the high-concentration layer to the low-concentration layer during vulcanization. This generates a gradient co-crosslinked layer with a crosslinking density that continuously changes along the normal direction of the interface in situ, thereby forming chemical covalent bonds between adjacent rubber layers. This makes the interlayer peel strength reach more than 80% of the strength of the rubber body on either side, overcoming the fundamental defect of traditional processes where the interface relies solely on physical entanglement and the interlayer strength is insufficient.

[0018] 2. This invention actively utilizes the translayer diffusion behavior of vulcanizing components, unlike the existing technology that prevents migration. It constructs a gradient crosslinking network within the multilayer composite product, where the bulk value of the rubber in one layer continuously changes to that in adjacent layers. There are no abrupt interfaces in crosslinking density or chemical composition at the interface, thereby eliminating stress concentration sources caused by abrupt interfaces in traditional multilayer composite products. This significantly reduces the risk of interface delamination and cracking under harsh service conditions such as dynamic fatigue, high and low temperature cyclic impact, and immersion in media.

[0019] 3. This invention adopts a segmented heating vulcanization mode. First, it is kept at 80℃~110℃ for 10~30 minutes to promote the full diffusion of vulcanizing components and form a pre-designed concentration gradient. Then, it is heated to 140℃~200℃ to complete the main vulcanization reaction. This decouples the diffusion process from the crosslinking reaction in time, thereby enabling independent control of the thickness of the gradient co-crosslinked layer (controllable from 10μm to 200μm) and the rate of change of the crosslinking density gradient. This meets the different requirements of different layer thicknesses and different rubber combinations for the interface structure, and has strong process adaptability.

[0020] 5. This invention can help control the diffusion coefficient (1×10⁻⁶) of the vulcanizing components by adjusting the reinforcing filler system of each layer of the rubber compound (such as high-structure carbon black, low-structure carbon black, and surface-hydrophobic treated silica). -8 cm 2 / s~5×10 -7 cm 2 Within a range of / s, the diffusion rate can be flexibly adjusted without changing the type of main vulcanizing agent and accelerator, making it suitable for various difficult-to-combine rubber combinations such as fluororubber / silicone rubber, natural rubber / EPDM rubber, and nitrile rubber / hydrogenated nitrile rubber, with strong versatility.

[0021] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description, claims and drawings. Attached Figure Description

[0022] Figure 1 This is a flowchart of the integrated co-sulfurization process of the present invention; Figure 2 This is a graph showing the change in the concentration distribution of the sulfidation components of this invention with sulfidation time. Figure 3 This is a graph showing the distribution of crosslinking density along the interface normal direction according to the present invention. Figure 4 This is a bar chart comparing the peel strength of embodiments and comparative examples of the present invention; Figure 5 This is the temperature-time curve of the segmented heating vulcanization process of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Please see Figures 1-5 As shown, the integrated co-vulcanization process for a one-time molding multilayer composite rubber product in this invention aims to actively utilize the cross-interface diffusion behavior of vulcanizing components to generate a gradient co-crosslinked layer with continuously varying crosslinking density in situ between adjacent rubber layers, thereby eliminating abrupt interfaces and stress concentrations in traditional processes and improving interlayer bonding strength.

[0025] Raw materials and testing methods: The rubber raw materials, additives, and fillers used in this embodiment and comparative example are all commercially available industrial products. The fluororubber is vinylidene fluoride-hexafluoropropylene copolymer, and the silicone rubber is methyl vinyl silicone rubber. The natural rubber is SMR20, and the EPDM rubber is EPDM 4045. The peroxide vulcanizing agent BIPB (di-tert-butylperoxyisopropylbenzene) has a purity ≥98%, and the co-accelerator TAIC (tracene propyl isocyanurate) has an effective content ≥99%. Low molecular weight polyethylene glycol (PEG-4000) is used as a diffusion accelerator. The carbon black used is high-structure carbon black N330 and low-structure carbon black N774, and the silica used is fumed silica (specific surface area 200 m² / g) and surface-hydrophobicated silica (HMDS treatment). Other additives such as stearic acid, zinc oxide, and acid scavengers (magnesium oxide) are all commonly used specifications in the rubber industry.

[0026] Vulcanization characteristics were determined using a rotorless vulcanizer according to GB / T 16584-1996, with a test temperature of 160℃ (peroxide system) or 150℃ (sulfur system). Interlaminar peel strength was determined according to GB / T 532-2008, with a sample width of 25mm, a peel angle of 180°, and a tensile rate of 50mm / min. High and low temperature cyclic impact testing was performed under the following conditions: -55℃×30min ←→ 200℃×30min constitutes one cycle; after every 20 cycles, the interface was observed for delamination or cracking. Hot air aging was conducted according to GB / T 3512-2014, with aging conditions of 150℃×72h.

[0027] Example 1: (Fluororubber / Silicone Rubber Two-Layer Composite Seal) This embodiment prepares a two-layer composite seal consisting of a fluororubber (FKM) layer and a silicone rubber (VMQ) layer, with a total thickness of 2 mm, wherein the fluororubber layer has a thickness of 1.2 mm and the silicone rubber layer has a thickness of 0.8 mm.

[0028] Step S1: Prepare the adhesive for each layer Fluororubber layer formulation (parts by weight): 100 parts FKM 2602, 2.5 parts BIPB peroxide, 1.8 parts TAIC co-accelerator, 1.5 parts low molecular weight polyethylene glycol (PEG-4000) (as a diffusion accelerator), 5 parts magnesium oxide acid scavenger, 15 parts high-structure carbon black N330 reinforcing filler, 10 parts untreated silica, and 1 part zinc stearate processing aid.

[0029] Silicone rubber layer formulation (parts by weight): 100 parts VMQ 110-2, 0.8 parts BIPB peroxide, 0.5 parts TAIC co-accelerator, 25 parts fumed silica (hydrophobic treatment), and 0.5 parts hydroxyl silicone oil (structure control agent).

[0030] Both formulations use the exact same primary vulcanizing agent (BIPB) and accelerator (TAIC), but the dosages differ: the BIPB dosage in the fluororubber layer (2.5 phr) is 3.125 times that in the silicone rubber layer (0.8 phr), and the TAIC dosage (1.8 phr) is 3.6 times that in the silicone rubber layer (0.5 phr). PEG-4000 is additionally added to the fluororubber layer as a diffusion accelerator. Its function is to reduce the diffusion resistance of BIPB and TAIC in the fluororubber matrix during the early stages of vulcanization, promoting the migration of vulcanizing components to the silicone rubber layer.

[0031] The simultaneous use of high-structure carbon black N330 and untreated silica in the fluororubber layer is due to the fact that high-structure carbon black N330 has a high specific surface area and structure, which can appropriately adsorb vulcanizing components and slow down their premature release; while untreated silica is rich in silanol groups on its surface, which has a certain affinity for the polar vulcanizing agents BIPB and TAIC. The combination of the two allows for precise control of the diffusion coefficient. Measurements at 160℃ showed that the effective diffusion coefficient of BIPB in the fluororubber layer is approximately 1.2 × 10⁻⁶. -7 cm 2 / s, the diffusion coefficient of BIPB in the silicone rubber layer is approximately 2.5 × 10⁻⁶. -7 cm 2 / s (Diffusion is faster due to the low polarity and large free volume of the silicone rubber matrix). Through filler adjustment, the diffusion coefficients of both layers fall within 1×10⁻⁶. -8 ~5×10 -7 cm 2 Within the target range of / s.

[0032] Step S2: Plasticizing and Mixing Fluororubber compounding: Plasticize FKM raw rubber on a two-roll mill at 40°C for 3 minutes, then add acid scavenger, zinc stearate, and reinforcing filler in sequence. After mixing evenly, add BIPB, TAIC, and PEG-4000, pass through a thin mill 5 times, and let it stand for 24 hours after sheeting.

[0033] Silicone rubber compounding: Plasticize VMQ raw rubber on a two-roll mill at room temperature for 2 minutes, add fumed silica and hydroxyl silicone oil in two batches, mix evenly, then add BIPB and TAIC, pass through a thin mill 5 times, and let it stand for 24 hours after sheeting.

[0034] Plasticizing and mixing ensure that the components are evenly dispersed, while the resting process allows the filler and rubber molecular chains to reach thermodynamic equilibrium, thus stabilizing the vulcanization properties.

[0035] Step S3: Laying out and shaping After being left to stand, the fluororubber and silicone rubber compounds were calendered into sheets with thicknesses of 1.2 mm and 0.8 mm, respectively. In a flat molding die, the fluororubber sheet was first laid as the bottom layer, followed by the silicone rubber sheet, with the two layers in direct contact without any adhesive. A pre-compression of 1.0 MPa was then applied to the preform and held for 3 minutes. This pre-compression process removes interlayer air, ensuring sufficient physical contact between the two rubber layers at the interface, promoting translayer diffusion and entanglement of rubber molecular chains, and providing uniform interfacial contact conditions for the subsequent chemical diffusion of vulcanizing components.

[0036] Step S4: Integrated vulcanization The mold containing the preform is placed in a flat vulcanizing machine, and a segmented heating vulcanization mode is adopted, such as... Figure 5 As shown: First, the temperature is raised to 100℃ and held for 20 minutes. During this stage, the temperature is relatively low (80–110℃), and the peroxides BIPB and TAIC have not yet reached their effective decomposition temperature for initiating crosslinking (BIPB's half-life is approximately 10 hours at 100℃, making the crosslinking reaction extremely slow). Therefore, the main process is Fick diffusion of the vulcanized components from the high-concentration layer (fluororubber layer) to the low-concentration layer (silicone rubber layer). Due to the presence of PEG-4000, the free volume of the fluororubber matrix increases, and the diffusion rate of BIPB and TAIC increases. During the 20-minute holding period, a gradient distribution layer of vulcanized components approximately 80–120 μm thick is formed on both sides of the interface.

[0037] After the heat treatment, the temperature was raised to 165℃ while maintaining the pressure at 15 MPa for 15 minutes. At this temperature, BIPB rapidly decomposes to generate free radicals, and TAIC participates in the crosslinking reaction as a co-promoter. Because a gradient distribution of vulcanized components has already formed at the interface, the crosslinking reaction is simultaneously initiated in the interface region: the crosslinking density is high on the fluororubber side, low on the silicone rubber side, and the crosslinking density in the middle region transitions continuously from high to low. Finally, a gradient co-crosslinked layer with a thickness of approximately 60–80 μm is generated in situ, such as... Figure 3 As shown, this gradient co-crosslinking layer eliminates the abrupt interface in traditional processes, and there is no abrupt interface in crosslinking density or chemical composition. like Figure 2 As shown, the distribution of BIPB concentration over time during the vulcanization process in this embodiment is as follows: at the initial moment of vulcanization, it exhibits a step distribution; at the end of the low-temperature diffusion stage (100℃×20min), it evolves into a smooth S-shaped gradient; after the completion of high-temperature vulcanization (165℃×15min), the overall concentration decreases but the gradient shape remains unchanged.

[0038] Step S5: Cooling and demolding After vulcanization, the heating is turned off, and the mold is allowed to cool naturally to below 60°C under pressure. Then, the mold is demolded to obtain a fluororubber / silicone rubber two-layer composite sealing product.

[0039] The composite seal prepared by the process in Example 1 has an interlayer peel strength of 7.8 kN / m, while the tear strength of the fluororubber body is 9.2 kN / m and the tear strength of the silicone rubber body is 6.5 kN / m. The peel strength reaches 85% of the strength of the fluororubber body and 120% of the strength of the silicone rubber body (because the peel failure occurs on the fluororubber side). After hot air aging at 150℃ for 72 hours, the peel strength is 7.1 kN / m, with a retention rate of 91%. After 100 cycles of high and low temperature impact at -55℃ to 200℃, there is no delamination or cracking at the interface.

[0040] Example 2: (Fluororubber / silicone rubber, adjusting the diffusion coefficient to control the gradient layer thickness) This embodiment is basically the same as Embodiment 1, except that the diffusion coefficient of the vulcanizing component is controlled by changing the type of reinforcing filler, thereby obtaining gradient co-crosslinked layers of different thicknesses. The product structure is the same: a two-layer composite seal of fluororubber / silicone rubber.

[0041] Formulation adjustment: In the fluororubber layer, all of the high-structure carbon black N330 (15 parts) in Example 1 was replaced with low-structure carbon black N774 (20 parts), and untreated silica was removed. N774 has a lower structure, resulting in weaker adsorption of BIPB and TAIC, thus increasing the diffusion coefficient of the vulcanizing components. Measurements showed that the diffusion coefficient of BIPB in the fluororubber layer increased to 3.5 × 10⁻⁶ at 160°C.-7 cm 2 / s. The silicone rubber layer formulation remains unchanged.

[0042] Process adjustment: The low-temperature holding stage of the segmented heating is extended to 30 minutes (holding at 100℃) to make full use of the higher diffusion coefficient so that the sulfurized components can diffuse a greater distance.

[0043] The composite seal prepared by the process in Example 2 has a gradient co-crosslinked layer thickness of approximately 150–180 μm and an interlayer peel strength of 7.5 kN / m, slightly higher than that in Example 1. The thicker gradient layer further alleviates interfacial stress concentration; however, when the total thickness of the product is limited, an excessively thick gradient layer may affect the performance of each layer. This example illustrates that the gradient layer thickness can be flexibly controlled through filler adjustment to adapt to different design requirements.

[0044] Example 3: (Two-layer composite conveyor belt cover rubber of natural rubber / EPDM rubber) This embodiment prepares a two-layer composite sample consisting of a natural rubber (NR) layer and an ethylene propylene diene monomer (EPDM) layer for use as a composite between the conveyor belt cover rubber and the transition layer, with a total thickness of 4 mm (NR layer 2.5 mm, EPDM layer 1.5 mm). The two rubbers have significant differences in polarity and unsaturation, making it difficult to achieve good interfacial bonding using traditional processes.

[0045] Step S1: Prepare the adhesive for each layer Natural rubber layer formulation (parts by weight): NR (SMR20) 100 parts, sulfur (main vulcanizing agent) 1.0 part, sulfenamide accelerator CZ (N-cyclohexyl-2-benzothiazole sulfenamide) 1.2 parts, zinc oxide (vulcanizing activator) 5 parts, stearic acid 1 part, high-structure carbon black N330 30 parts, aromatic oil 5 parts.

[0046] EPDM rubber layer formulation (parts by weight): 100 parts EPDM 4045, 2.2 parts sulfur, 1.8 parts accelerator CZ, 5 parts zinc oxide, 1 part stearic acid, 40 parts low-structure carbon black N774, 10 parts paraffin oil, and 2 parts low molecular weight polyethylene glycol PEG-4000 (as a diffusion accelerator).

[0047] Both formulations used the exact same vulcanization system (sulfur + CZ), but with different dosages: 1.0 phr of sulfur in the NR layer and 2.2 phr of sulfur in the EPDM layer (a 2.2-fold increase); 1.2 phr of accelerator in the NR layer and 1.8 phr of accelerator in the EPDM layer (a 1.5-fold increase). The EPDM layer had higher dosages of both sulfur and accelerator than the NR layer, therefore the vulcanizing components diffused from the EPDM layer to the NR layer (in the opposite direction to Example 1). The addition of PEG-4000 to the EPDM layer aimed to increase the polarity of EPDM and promote the diffusion of sulfur and accelerator (EPDM is a non-polar rubber, and polar components like sulfur diffuse slowly). Filler control was used: high-structure carbon black N330 (strong adsorption of vulcanizing agent, reducing diffusion rate) was used in the NR layer, and low-structure carbon black N774 (weak adsorption, increasing diffusion rate) was used in the EPDM layer, matching the diffusion coefficients of the two layers to 1.5 × 10⁻⁶. -8 ~3×10 -8 cm 2 Within the range of / s (measured at 150℃).

[0048] Step S2: Plasticizing and mixing are carried out according to conventional processes. The mixing temperature of NR is controlled below 50℃, while the mixing temperature of EPDM can be slightly higher.

[0049] Step S3: Laying and molding: Lay EPDM film (1.5mm) on the lower layer, NR film (2.5mm) on the upper layer, apply 1.5MPa pre-compression, and hold the pressure for 4 minutes.

[0050] Step S4: Integrated Vulcanization: Segmented heating is employed. First, the temperature is maintained at 95℃ for 25 minutes to promote the diffusion of sulfur and accelerator from the EPDM layer to the NR layer. Due to the high initial sulfur concentration in the EPDM layer and the enhanced diffusion capacity of PEG-4000, a gradient layer of vulcanizing components approximately 100 μm thick is formed during the holding stage. Then, the temperature is increased to 150℃, the pressure to 18 MPa, and the vulcanization time is set to the maximum value of the T90 values ​​for the NR layer (approximately 12 min) and the EPDM layer (approximately 10 min), i.e., 12 minutes. During vulcanization, the gradient-distributed sulfur and accelerator simultaneously initiate cross-linking reactions in both layers and at the interface, generating a gradient co-crosslinked layer with a continuously varying cross-linking density from the EPDM side (high sulfur, higher cross-linking density) to the NR side (low sulfur, lower cross-linking density).

[0051] The composite seal prepared by the process in Example 3 has an interlayer peel strength of 8.2 kN / m, an NR body tear strength of 12 kN / m, and an EPDM body tear strength of 5.5 kN / m. The peel strength reaches 149% of the EPDM body strength (failure occurs on the EPDM side), which is much higher than that of the traditional matching method (usually below 3 kN / m). After aging at 150℃ for 72 hours, the peel strength retention rate is 88%. No delamination occurs after 100 cycles of high and low temperature cycling (-40℃ to 120℃).

[0052] Comparative Example 1: (Traditional vulcanization system matching method, without using diffusion gradient) This comparative example uses the same fluororubber / silicone rubber two-layer structure as Example 1, but the vulcanization system is adjusted according to the traditional approach to make the T90 of the two layers close, and diffusion is not actively utilized (i.e., no concentration difference of vulcanizing components is set or diffusion is prevented).

[0053] Formulation: The fluororubber layer uses a BIPB / TAIC peroxide system, with BIPB at 1.8 phr and TAIC at 1.2 phr; the silicone rubber layer uses the same BIPB / TAIC system, but the amounts are adjusted to BIPB at 1.6 phr and TAIC at 1.0 phr, so that the T90 of both layers is approximately 12 min at 160℃. Neither layer contains the diffusion accelerator PEG-4000. The filler used is N330 carbon black, which has strong adsorption properties for vulcanized components, to inhibit migration.

[0054] Process: No segmented heating is used; the temperature is directly raised to 160°C and vulcanized for 12 minutes, without a low-temperature diffusion section. The remaining steps are the same as in Example 1.

[0055] Results: Due to the extremely small concentration difference of the vulcanizing components (BIPB 1.8 vs 1.6, a difference of only 0.2 phr) and the strong adsorption of vulcanizing components by the filler, both the diffusion driving force and diffusion rate were very low. Almost no gradient co-crosslinking layer was formed at the interface, relying solely on physical entanglement for bonding. The interlayer peel strength was only 1.8 kN / m, approximately 20% of the bulk strength of fluororubber. Significant delamination appeared at the interface after 35 cycles of high and low temperatures from -55℃ to 200℃. After hot air aging, the peel strength decreased to 1.0 kN / m.

[0056] Comparative Example 2: (Interlayer Coating Adhesive Method) This comparative example uses the same fluororubber / silicone rubber two-layer structure as Example 1, but employs a traditional two-step adhesive bonding-vulcanization method. The fluororubber layer and the silicone rubber layer each use independent optimal vulcanization systems (they do not need to be the same). A commercial fluorosilicone rubber adhesive, Chemlok 607, is applied between the two layers to a thickness of approximately 0.05 mm. After drying, the mold is closed and vulcanized under the recommended vulcanization conditions (160°C × 15 min).

[0057] Results: The initial peel strength was 4.5 kN / m, higher than Comparative Example 1 but lower than Example 1. After 50 cycles of high and low temperature cycling from -55℃ to 200℃, cracking occurred at the interface between the adhesive layer and the silicone rubber, and the peel strength decreased to 2.1 kN / m. The main reason was the significant difference in the coefficient of thermal expansion between the adhesive layer and the rubber on both sides, leading to stress concentration under thermal shock. Simultaneously, the adhesive application process increased production complexity and cost.

[0058] Summary of Examples and Comparison of Results:

[0059] This invention, by actively setting the initial concentration multiple (1.5–5.0 times) of the vulcanizing component between adjacent adhesive layers, and in conjunction with a segmented heating process (pre-diffusion at 80–110°C for 10–30 minutes) and the addition of a diffusion promoter, can effectively form a gradient co-crosslinked layer with controllable thickness (10–200 μm). In contrast, such as Figure 4 As shown, Comparative Example 1 did not have a sufficient concentration difference (only 1.1 times) and did not use segmented heating and diffusion promoters, so an effective gradient layer could not be formed at the interface, and the peel strength was only 23% of that of Example 1. Although Comparative Example 2 obtained a certain initial peel strength, the introduction of the adhesive layer resulted in a significantly lower high and low temperature cycle life than the examples, and an additional coating process was added. Example 3 further demonstrates that the present invention is not only applicable to peroxide-cured fluororubber / silicone rubber systems, but also to sulfur-cured natural rubber / EPDM rubber systems, showing strong versatility. Example 2 demonstrates the ability to flexibly control the gradient layer thickness (from 60-80 μm to 150-180 μm) by adjusting the diffusion coefficient through filler type (selection of high-structure carbon black and low-structure carbon black), thus meeting the interface design requirements of different composite products. In summary, the process parameter window of the present invention (concentration multiple 1.5-5.0, diffusion coefficient 1×10⁻⁶) is suitable for various applications. -8 ~5×10 -7 cm 2 The range of parameters (e.g., gradient layer thickness 10–200 μm, segmented heating parameters, etc.) is the preferred range verified by a large number of experiments. Exceeding or falling below this range makes it difficult to simultaneously obtain high peel strength, excellent aging retention rate, and resistance to high and low temperature cycling.

Claims

1. An integrated co-vulcanization process for one-time molding of multi-layer composite rubber products, characterized in that, Includes the following steps: Step S1: Prepare the rubber compounds for each layer of the composite product separately. All rubber layers use the same vulcanization system. The vulcanization system contains the same main vulcanizing agent and the same combination of accelerators. At least one vulcanizing component in adjacent rubber compounds has a different initial concentration. This concentration difference is achieved by adjusting at least one of the following: the amount of vulcanizing agent, the amount of accelerator, or the amount of vulcanizing activator in each rubber compound. Step S2: Plasticize and mix the rubber compounds prepared in step S1 to obtain the compounded rubber compounds of each layer. Step S3: According to the composite product layer structure design, each layer of compound rubber is laid or co-extruded in the molding die in a predetermined order to form a multi-layer composite preform. No independent adhesive layer is added at the interface between two adjacent rubber layers, and each rubber layer is in direct contact at the interface. Step S4: Place the multi-layer composite preform obtained in step S3 together with the mold in a vulcanization device, and apply a pressure of 5 MPa to 25 MPa at a vulcanization temperature of 120℃ to 200℃ to perform integrated vulcanization molding. Step S5: After vulcanization, cool and demold to obtain a one-piece molded multi-layer composite rubber product; In step S4, during the vulcanization process, the vulcanizing components with different initial concentrations diffuse across the interface from the high-concentration layer to the low-concentration layer under the drive of the concentration gradient, forming a gradient distribution layer of vulcanizing components on both sides of the interface of adjacent rubber layers. As the vulcanization reaction proceeds, the gradient distribution of vulcanizing components simultaneously triggers the cross-linking reaction of rubber molecules on both sides in the interface region, generating a gradient co-crosslinking layer in situ.

2. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, The main vulcanizing agent is a peroxide vulcanizing agent, and the accelerator is a peroxide co-accelerator; in two adjacent rubber layers, the amount of peroxide in one layer is 1.5 to 5.0 times that in the other layer, and a diffusion accelerator accounting for 0.5% to 3.0% of the mass of the rubber in the layer with the higher amount is added at the same time, and the diffusion accelerator is at least one of low molecular weight polyethylene glycol, zinc stearate, or fatty acid ester.

3. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, When the composite product comprises a fluororubber layer and a silicone rubber layer, in step S1, the main vulcanizing agent is BIPB peroxide, and the accelerator is TAIC. The amount of BIPB in the fluororubber layer is 1.5–3.0 phr, and the amount of TAIC is 1.0–2.5 phr; the amount of BIPB in the silicone rubber layer is 0.5–1.0 phr, and the amount of TAIC is 0.3–0.8 phr; and 1.0–2.0 phr of low molecular weight polyethylene glycol is added to the fluororubber layer as a diffusion accelerator.

4. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, In step S1, the diffusion rate of the vulcanizing components is controlled by adjusting the type and amount of reinforcing filler system in each layer of the rubber compound: adding high-structure carbon black or surface-hydrophobic treated silica reduces the diffusion coefficient of the vulcanizing components, while adding low-structure carbon black or untreated silica increases the diffusion coefficient; at the vulcanization temperature, the diffusion coefficient of each layer of the rubber compound is controlled at 1×10⁻⁶ by the filler. -8 cm² / s~5×10 -7 cm 2 Within the range of / s.

5. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, The vulcanization temperature described in step S4 adopts a segmented heating mode: first, it is kept at 80℃~110℃ for 10~30 minutes. At this stage, the vulcanization reaction has not been fully initiated and is mainly used to promote the diffusion of vulcanizing components across the interface to form a pre-designed concentration gradient; then, the temperature is raised to 140℃~200℃ and kept to complete the main vulcanization reaction, so that the gradient-distributed vulcanizing components simultaneously initiate the cross-linking of each layer and interface, forming an integrated gradient co-crosslinking network.

6. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, In step S3, after layer-by-layer stacking and before vulcanization, a pre-compression of 0.5 MPa to 2.0 MPa is applied to the multilayer composite preform and held for 1 to 5 minutes to eliminate interlayer air and promote the diffusion and entanglement of interlayer molecular chains.

7. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, The thickness of the gradient co-crosslinked layer is 10 μm to 200 μm, and its crosslinking density changes continuously from the bulk value of one layer of rubber to the bulk value of the adjacent layer of rubber, without any abrupt interface in crosslinking density or chemical composition.

8. The integrated co-vulcanization process for a one-time molded multi-layer composite rubber product according to claim 1, characterized in that, The vulcanization time in step S4 is the time required for each layer of rubber compound to reach the positive vulcanization state.

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