Halogen-free flame-retardant rubber conveying belt

Abstract

The invention relates to a halogen-free flame-retardant rubber conveying belt and a preparation method thereof, and belongs to the field of rubber composite materials. The conveying belt comprises the following raw materials in percentage by mass: 55-65% of a rubber matrix (20-50nm EPDM / SBR blend), 5-10% of microencapsulated red phosphorus (the particle size is less than or equal to 10 microns), 12-15% of modified magnesium hydroxide, 4-6% of a pentaerythritol / melamine cyanurate compounded charring agent, 5-10% of a zinc molybdate / zinc borate compound (the mass ratio is 3: 1) and 3-8% of a maleic anhydride compatibilizer. The halogen-free, environment-friendly and high-flame-retardant performance is achieved, meanwhile, the tensile strength is kept to be larger than or equal to 16 MPa, and the problems that a traditional flame-retardant belt is large in toxicity and poor in mechanical property are solved.

Description

Technical Field

[0001] The invention relates to the technical field of conveyor belt manufacturing, in particular to a halogen-free flame-retardant rubber conveyor belt. Background Art

[0002] Flame retardant safety is crucial for rubber conveyor belts used in flammable environments such as mining and power generation. While traditional halogen-based flame retardant systems can meet basic flame retardant requirements, they release toxic bromine and chlorine-containing gases and dense smoke during combustion, making them incompatible with increasingly stringent environmental regulations (such as the EU RoHS Directive). While the industry has attempted to replace halogen systems with inorganic flame retardants such as magnesium hydroxide, high addition levels (typically >60%) lead to a surge in rubber compound hardness and a sharp decrease in flexural fatigue performance. Furthermore, due to poor compatibility between flame retardants and rubber, they are prone to migration and precipitation after long-term use, making it difficult to ensure flame retardant durability.

[0003] Halogen-free flame retardant technologies developed in recent years have primarily focused on phosphorus-nitrogen synergistic systems, such as ammonium polyphosphate (AMPP) and melamine. While these technologies can reduce smoke and toxic emissions, they face two major bottlenecks: First, high-efficiency flame retardants like red phosphorus are susceptible to moisture absorption and oxidation, posing a risk of explosion during processing and leaving residual phosphine gas. Second, balancing flame retardancy with mechanical properties often requires the addition of excessive amounts of charring agents, which can impair the fluidity of the rubber compound and lead to delamination or uneven thickness during conveyor belt calendering. Some technologies have attempted to improve dispersibility through nano-modification, but insufficient surface treatment of the flame retardant is sufficient to prevent migration during the high-temperature vulcanization stage.

[0004] Therefore, there is an urgent need to develop a new type of halogen-free flame-retardant conveyor belt. While ensuring the low-smoke, low-toxic and environmentally friendly characteristics, it can overcome the problems of mechanical property degradation and insufficient durability caused by high filling by optimizing the flame retardant structure design and process adaptability, and meet the long-term safe operation needs of high-risk scenarios such as underground coal mines. Summary of the Invention

[0005] The purpose of the present invention is to provide a halogen-free flame-retardant rubber conveyor belt, which solves the problems of high toxicity of halogen flame retardants, poor mechanical properties of traditional halogen-free systems due to high filling, and flame retardant migration failure.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions: A halogen-free flame-retardant rubber conveyor belt, the raw materials of which include, by mass percentage: Rubber matrix: 55-65%; Main flame retardant: 5-10%; Synergistic flame retardant: 12-15%; Carbon-forming agent: 4-6%; Smoke suppressants: 5-10%; Compatibilizer: 3-8%.

[0007] According to a preferred embodiment of the present invention, the rubber matrix is 20-50 nm.

[0008] The rubber matrix of this invention utilizes a blend of ethylene propylene diene monomer (EPDM) and styrene butadiene rubber (SBR) with particle sizes of 20-50 nm. The nanoscale dispersed phase forms a continuous network structure, and its high specific surface area enhances interfacial bonding, provides an elastic framework for the flame retardant system, and inhibits crack propagation. When heated, the phenolic resin coating of the primary flame retardant, microencapsulated red phosphorus (particle size ≤10 μm), melts and ruptures. The released red phosphorus decomposes into phosphorus oxide radicals (PO·), which capture hydrogen radicals (H·) and hydroxyl radicals (OH·) necessary for the combustion chain reaction in the gas phase, interrupting the free radical chain reaction. Simultaneously, the red phosphorus dehydrates in the condensed phase to form a polymetaphosphate glass layer, which covers the substrate and isolates it from oxygen. Magnesium hydroxide modified with a silane coupling agent, a synergistic flame retardant, consumes a large amount of combustion heat through a two-step endothermic reaction (endothermic decomposition into magnesium oxide and water at 340°C, and deep dehydration at 430°C). The released water vapor dilutes the concentration of combustible gas, and its nanosheet structure is directionally arranged in the rubber matrix, synergistically constructing a "skeleton reinforcement effect" with the carbon-forming agent. In this invention, the charring agent pentaerythritol is combined with melamine cyanurate (MCA) to form an intumescent flame retardant system. Pentaerythritol undergoes esterification and dehydration at 250-300°C to form a polyolefin backbone. Thermal decomposition of MCA produces ammonia, nitrogen, and triazine ring derivatives. The expansion of these gases drives foaming in the molten system, catalyzed by red phosphorus and magnesium hydroxide to form a dense honeycomb carbon layer (pore size ≤5μm, as measured by SEM). This carbon layer, containing aromatic fused rings and inorganic silicon magnesium oxide, provides dual thermal and oxygen insulation. In the smoke suppressant zinc molybdate / zinc borate complex (3:1 by mass), zinc molybdate catalyzes the dehydrogenation and crosslinking of rubber molecules, promoting graphitization of the carbon layer and enhancing barrier strength. Zinc borate melts at 500°C to form a glassy coating that seals cracks in the carbon layer and captures smoke particles. The synergistic effect reduces smoke density to less than 40% of that of conventional systems. The anhydride group of the compatibilizer maleic anhydride undergoes an esterification reaction with the hydroxyl group on the surface of magnesium hydroxide to form a chemical bond. Its polyolefin chain segment is entangled with the EPDM / SBR molecules to construct an "organic-inorganic bridging network", which increases the dispersion uniformity of the flame retardant to more than 95%, completely solving the interface defects and migration precipitation problems caused by high filling. The sulfur vulcanization system in this invention precisely triggers crosslinking during a staged vulcanization process: a 150°C pre-vulcanization stage promotes the slow formation of polysulfide bonds in the sulfur, preventing thermal decomposition of the flame retardant. A 170°C final vulcanization stage accelerates the formation of a CSC and C-S2-C crosslinking network, with the rubber molecular chains and the flame retardant covalently bonded via a compatibilizer. This ultimately creates a triple synergistic mechanism: gas-phase free radical quenching (red phosphorus), a condensed-phase carbon barrier (carbon former / magnesium hydroxide), and soot catalytic conversion (molybdenum-zinc complex). This achieves a synergistic multiplier effect with an oxygen index exceeding 31%, a smoke density below 185, and a tensile strength exceeding 15 MPa at a total flame retardant loading of 28-38%.

[0009] According to a preferred embodiment of the present invention, the main flame retardant is red phosphorus, and the particle size of the red phosphorus is ≤10 μm.

[0010] According to a preferred embodiment of the present invention, the synergistic flame retardant is magnesium hydroxide.

[0011] According to a preferred embodiment of the present invention, the carbon-forming agent is pentaerythritol compounded with melamine cyanurate.

[0012] According to a preferred embodiment of the present invention, the smoke suppressant is a zinc molybdate / zinc borate complex.

[0013] According to a preferred embodiment of the present invention, the mass ratio of zinc molybdate to zinc borate in the zinc molybdate / zinc borate composite is 3:1.

[0014] According to a preferred embodiment of the present invention, the compatibilizer is maleic anhydride.

[0015] The present invention also provides a method for preparing the flame-retardant conveyor belt, comprising the following steps: S1, put 55-65% of the rubber matrix (EPDM / SBR blend with a particle size of 20-50 nm) into an internal mixer, heat to 80°C and masticate for 3 minutes; add 3-8% of maleic anhydride compatibilizer and mix for 2 minutes; S2: Add 12-15% of a surface-modified magnesium hydroxide synergistic flame retardant (treated with a silane coupling agent), 5-10% of a microencapsulated red phosphorus primary flame retardant (particle size ≤10μm, coated with phenolic resin), and 5-10% of a smoke suppressant (zinc molybdate / zinc borate complex, mass ratio 3:1) in sequence, control the temperature to ≤110°C, and mix for 8 minutes. Finally, add 4-6% of a charring agent (a compound of pentaerythritol and melamine cyanurate) and a sulfur vulcanization system, cool to below 90°C, mix for 4 minutes, and then release the tablets. S3: The mixed rubber was laminated to the polyester canvas reinforcement layer (upper / lower cover rubber thickness ratio 1.6:1) via a calender, pre-vulcanized at 150°C / 15 MPa for 25 minutes in a flat vulcanizer, and then final vulcanized at 170°C / 20 MPa for 12 minutes. After spray cooling, the edges were trimmed and aged at room temperature for 72 hours.

[0016] The beneficial effects of the present invention are: The present invention fundamentally resolves the contradiction between environmental protection, safety, and mechanical properties in traditional rubber conveyor belts through an innovative halogen-free flame retardant formula and segmented process design. The conveyor belt completely abandons halogen flame retardants and uses red phosphorus and magnesium hydroxide to form a core flame retardant system. During combustion, it does not release highly toxic gases such as dioxins, significantly reduces smoke density, and completely eliminates hazards to the environment and human health. Zinc molybdate and zinc borate are compounded in a specific ratio, catalyzing the formation of a dense carbon layer in the flame, effectively isolating oxygen and adsorbing smoke particles. This allows the conveyor belt to produce only thin white smoke under high-temperature fire sources, significantly improving fire escape visibility. While ensuring high flame retardancy, the close integration of the nano-scale rubber matrix and surface-modified fillers achieves a breakthrough improvement in the material's mechanical properties. The maleic anhydride compatibilizer promotes uniform dispersion of the flame retardant within the rubber network, preventing embrittlement caused by high filler content. The conveyor belt combines strength and toughness with flexible flexibility, exhibiting excellent tear and wear resistance during long-term operation. The segmented vulcanization process precisely controls the cross-linking reaction, forming a stable anchoring structure for the flame retardant within the rubber compound. This structure exhibits no migration or precipitation even after extreme environmental testing, completely overcoming the short lifespan and rapid performance degradation of traditional halogen-free systems. The final product demonstrates excellent comprehensive performance in harsh scenarios such as underground mines and chemical tunnels: it quickly self-extinguishes when exposed to open flames without sparks, and maintains high tensile strength and low dynamic heat generation under high-load conditions. DETAILED DESCRIPTION

[0017] The following specific implementation methods are only used to further illustrate the present application and should not be understood as limiting the scope of protection of the present application. Technicians in this field may make some non-essential improvements and adjustments to the present application based on the above application content.

[0018] 1. Implementation Example 1 60% of EPDM rubber and styrene-butadiene rubber (mass ratio 70:30) with a particle size of 30nm were put into an internal mixer and plasticized at 80℃ for 3 minutes; 5.5% of maleic anhydride compatibilizer was added and mixed for 2 minutes; 13.5% of magnesium hydroxide modified with a silane coupling agent, 7.5% of red phosphorus (particle size 8μm), and 7.5% of zinc molybdate / zinc borate complex (mass ratio 3:1) were added in sequence and mixed at 105℃ for 8 minutes; finally, 3.5% of pentaerythritol and 1.5% of melamine cyanurate were added to form a carbonizing agent, 0.6% of sulfur and 0.3% of accelerator DM were added, the temperature was lowered to 88℃, mixed for 4 minutes and sheets were produced. The mixed rubber was laminated to flame-retardant polyester canvas (upper covering rubber thickness 4.8mm / lower covering rubber thickness 3.0mm) via a four-roll calender, and vulcanized at 150℃ / 15MPa for 25 minutes in a flat vulcanizer, then switched to 170℃ / 20MPa for 12 minutes, spray-cooled, trimmed, and aged at room temperature for 72 hours.

[0019] Example 2 The specific implementation method is the same as that of Example 1, except that after 55% of the rubber matrix (EPDM / SBR=60 / 40) is masticated, 8% of maleic anhydride; 15% of magnesium hydroxide, 10% of red phosphorus, 5% of zinc molybdate / zinc borate (3:1); 3.2% of pentaerythritol and 1.8% of MCA as carbonizing agents; and 0.9% of the sulfur system are added; and vulcanization is carried out using the same process.

[0020] Example 3 The specific implementation method is the same as that of Example 1, except that: the rubber matrix is 65% (EPDM / SBR=80 / 20), maleic anhydride is 3%; magnesium hydroxide is 12%, red phosphorus is 5%, zinc molybdate / zinc borate is 10%; carbonizing agent is 4.5%; and sulfur system is 0.8%.

[0021] Comparative Example 1 The specific implementation method is the same as that of Example 1, except that the main flame retardant is replaced by 10% of decabromodiphenyl ether and the synergist is 5% of antimony trioxide. The rest is the same as that of Example 1.

[0022] Comparative Example 2 The specific implementation method is the same as that of Example 1, except that 15% of uncoated red phosphorus (particle size 50 μm) is added, the smoke suppressant is changed to 5% of single zinc molybdate, and the staged vulcanization (160° C.×30 min) is cancelled.

[0023] Comparative Example 3 The rubber matrix is micron-grade (>1μm) EPDM 60%, compatibilizer is absent, and magnesium hydroxide is added 25%.

[0024] 2. Performance Testing The conveyor belts prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests according to the following method: Test Method: Oxygen index was tested according to GB / T 10707 (sample dimensions: 120 mm × 10 mm × 3 mm); roller friction testing was conducted according to EN ISO 340 (sample: 200 mm × 100 mm, steel cylinder diameter: 450 mm / 500 N pressure / 30 min); smoke density (Ds) was determined using the flame combustion method according to GB / T 8323 (sample: 25 mm × 25 mm × 3 mm); tensile strength was determined using Type I dumbbell specimens prepared according to GB / T 528 (tensile speed: 500 mm / min); abrasion was determined using the Akron method according to GB / T 9867 (load: 26.7 N / 1.61 km); mobility was determined according to ISO 4649 (percentage of precipitates after aging at 70°C for 168 h); toxic gases were detected using an EN 45545-2 cone calorimeter combined with FTIR.

[0025] Performance test results: Table 1: Performance test results of various embodiments and comparative examples

[0026] As shown in Table 1, the halogen-free flame-retardant conveyor belts of the present invention demonstrate significant breakthroughs in all three dimensions: environmental safety, flame retardant efficiency, and mechanical durability. Regarding environmental performance, Examples 1-3 produced no detectable toxic gases during combustion tests (HCN / CO / PH3 concentrations were below the instrument detection limit). However, Comparative Example 1, due to its use of decabromodiphenyl ether, released 520 ppm of hydrogen bromide (10 times the EN 45545-2 limit), while Comparative Example 2, uncoated with red phosphorus, produced 8 ppm of phosphine (a highly toxic gas). This demonstrates that the synergistic effect of red phosphorus and the molybdenum-zinc composite smoke suppressant completely eliminates the toxicity risk of conventional systems. Smoke density data further supports this: the average value for the Example group was 172 (only 36% of that of Comparative Example 1). The key to this is the catalytic formation of a dense ceramic carbon layer by the zinc molybdate / zinc borate (3:1) condensed phase, effectively isolating oxygen and adsorbing smoke particles, improving fire escape visibility by more than 2-fold.

[0027] The oxygen index, a core indicator of flame retardancy, and the results of roller friction reveal the scientific nature of the formulation design. Example 2, thanks to the highly effective synergistic effect of 10% red phosphorus and 15% magnesium hydroxide, achieved an oxygen index of 35.1% (exceeding the 30% requirement of the coal mine MT914 standard), and exhibited no sparks during the entire roller friction process. In contrast, in Comparative Example 2, due to the uncoated and excessive addition of red phosphorus (15%), the oxygen index plummeted to 28.1% and ignited, highlighting the necessity of particle size control (≤10μm) and the coating process. Notably, although Example 3 only used 5% red phosphorus, the smoke suppressant was increased to 10% to compensate for the flame retardant effect, and the oxygen index remained at 31.5%, demonstrating that the 5-10% elastic range in the claimed range can adapt to the needs of different scenarios.

[0028] The mechanical properties data confirm the core value of the nano matrix and the compatibilizer. Example 1 uses a 30nm rubber matrix + 5.5% maleic anhydride, with a tensile strength of 17.2MPa and a wear loss of 0.59cm 3 , compared with 9.3MPa and 1.48cm of Comparative Example 3 (micron-sized matrix + no compatibilizer) 3 Improvements of 85% and 150%, respectively, are achieved. The microscopic mechanism lies in the formation of a continuous network structure by nano-EPDM / SBR (20-50nm), where the carboxyl groups of maleic anhydride bond with the hydroxyl groups on the surface of magnesium hydroxide. This reduces the flame retardant loading to one-quarter of that of conventional processes (compared to the 25% loading in Comparative Example 3), thus avoiding embrittlement caused by high loading. Mobility data further demonstrates the durability advantage: the average mobility of the Example group is 4.1% (after 168 hours of aging), significantly lower than the 18.9% in Comparative Example 2. This means that within a five-year service life in coal mines, the flame retardant efficiency degradation can be controlled to less than 8%.

[0029] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A halogen-free flame-retardant rubber conveyor belt, characterized in that: In terms of mass percentage, the raw materials include: Rubber matrix: 55-65%; Main flame retardant: 5-10%; Synergistic flame retardant: 12-15%; Carbon-forming agent: 4-6%; Smoke suppressants: 5-10%; Compatibilizer: 3-8%.

2. The flame-retardant conveyor belt according to claim 1, characterized in that: The rubber matrix is 20-50 nm.

3. The flame retardant conveyor belt according to claim 1, characterized in that: The main flame retardant is red phosphorus, and the particle size of the red phosphorus is ≤10 μm.

4. The flame retardant conveyor belt according to claim 1, characterized in that: The synergistic flame retardant is magnesium hydroxide.

5. The flame retardant conveyor belt according to claim 1, characterized in that: The carbonizing agent is pentaerythritol compounded with melamine cyanurate.

6. The flame retardant conveyor belt according to claim 1, characterized in that: The smoke suppressant is a zinc molybdate / zinc borate complex.

7. The flame retardant conveyor belt according to claim 6, characterized in that: The mass ratio of zinc molybdate to zinc borate in the zinc molybdate / zinc borate composite is 3:

1.

8. The flame retardant conveyor belt according to claim 1, characterized in that: The compatibilizer is maleic anhydride.

9. The method for preparing a flame-retardant conveyor belt according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1, put 55-65% of the rubber matrix (EPDM / SBR blend with a particle size of 20-50 nm) into an internal mixer, heat to 80°C and masticate for 3 minutes; add 3-8% of maleic anhydride compatibilizer and mix for 2 minutes; S2: Add 12-15% of a surface-modified magnesium hydroxide synergistic flame retardant (treated with a silane coupling agent), 5-10% of a microencapsulated red phosphorus primary flame retardant (particle size ≤10μm, coated with phenolic resin), and 5-10% of a smoke suppressant (zinc molybdate / zinc borate complex, mass ratio 3:1) in sequence, control the temperature to ≤110°C, and mix for 8 minutes. Finally, add 4-6% of a charring agent (a compound of pentaerythritol and melamine cyanurate) and a sulfur vulcanization system, cool to below 90°C, mix for 4 minutes, and then release the tablets. S3: The mixed rubber was laminated to the polyester canvas reinforcement layer (upper / lower cover rubber thickness ratio 1.6:1) via a calender, pre-vulcanized at 150°C / 15 MPa for 25 minutes in a flat vulcanizer, and then final vulcanized at 170°C / 20 MPa for 12 minutes. After spray cooling, the edges were trimmed and aged at room temperature for 72 hours.

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