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Performance of Melamine as Flame Retardant Filler in SBR Rubber

Styrene-Butadiene Rubber (SBR) is one of the most widely used synthetic rubbers in the rubber industry, used in tires, hoses, seals, footwear and various industrial rubber products. To expand its application in fire-risk scenarios, adding efficient flame-retardant fillers is a common technical solution.

Melamine powder is a nitrogen-based, eco-friendly flame retardant with outstanding heat resistance, insulation and flame retardant properties. It has been widely used in plastics, coatings and polyurethane materials, while its application research in SBR rubber is relatively limited. This article presents a systematic study of SBR filled with different dosages of melamine, analyzing vulcanization characteristics, mechanical performance and flame-retardant effects. It also compares melamine with traditional fillers, such as clay and light calcium carbonate, providing practical references for the design of flame-retardant SBR formulas.

Experimental Materials & Test Formulas

Raw Materials

Main raw materials: SBR base rubber, industrial-grade melamine, zinc oxide (ZnO), stearic acid (SA), sulfur (S), accelerator CZ, white carbon black, N330 carbon black, clay and light calcium carbonate. All auxiliary materials are of industrial-grade standards.

Three Experimental Schemes

Three groups of formulas were designed to study the performance of melamine under different fixed reinforcing fillers:
  1. Scheme 1: 100 parts SBR + 30 parts white carbon black, melamine dosage as a variable
  2. Scheme 2: 100 parts SBR + 40 parts N330 carbon black, melamine dosage as variable
  3. Scheme 3: 100 parts SBR + 30 parts N330 carbon black, compare three fillers: melamine, clay and light calcium carbonate with different dosages

Sample Preparation & Testing Methods

All ingredients were mixed on an open mill following standard mixing procedures. After standing, rubber sheets were vulcanized on a plate vulcanizer at 150 °C for the measured vulcanization time, as measured by a vulcanometer.

All vulcanization properties, mechanical properties and oxygen index were tested in strict accordance with national standard test methods. Key test indicators include minimum torque (ML), maximum torque (MH), scorch time (T10), optimum cure time (T90), hardness, tensile strength, elongation at break, modulus at 300% elongation, tear strength, resilience, permanent set and oxygen index (LOI).

Test Results & Analysis

Performance of SBR Filled with Melamine + White Carbon Black

With 30 parts of white carbon black kept constant, melamine significantly accelerates the vulcanization rate of SBR.
  • Vulcanization characteristics: ML and MH increase gradually with rising melamine content. Optimum cure time (T90) decreases obviously, while scorch time (T10) stays stable, ensuring good processing safety. When the melamine dosage ranges from 25 to 45 parts, cure time tends to be consistent, which helps improve production efficiency. The weak alkalinity of melamine is the main reason for accelerating the vulcanization reaction.
  • Mechanical properties: Rubber hardness increases slowly as melamine dosage rises. Tensile strength first increases, then decreases, indicating that melamine is not suitable as a reinforcing agent for non-crystalline SBR. Elongation at break shows fluctuating values, reaching a high of 35 parts per melamine. Tear strength and resilience decline continuously with increased filler content.
  • Flame retardancy: The oxygen index rises steadily with more melamine added. Even a small amount of melamine can effectively improve flame resistance. At a 45 parts dosage, the rubber exhibits excellent flame-retardant performance, approaching self-extinguishing after flame removal.
Comprehensive evaluation: 25 parts of melamine deliver the best overall performance for the SBR compound filled with 30 parts of white carbon black.

Performance of SBR Filled with Melamine + Carbon Black

At 40 parts N330 carbon black, melamine also plays a remarkable role in promoting vulcanization.
  • Vulcanization characteristics: ML and MH increase with melamine addition. Scorch time rises first, then falls, and optimum cure time decreases slightly. The acceleration effect on vulcanization is weaker than that of the group with white carbon black.
  • Mechanical properties: Hardness increases continuously. Tensile strength, 300% modulus, permanent set and tear strength all show a trend of rising first, then falling. Elongation at break shows irregular fluctuations.
  • Flame retardancy: The oxygen index increases as the melamine dosage increases.
Comprehensive evaluation: When matched with 40 parts of carbon black, 25 parts of melamine is the optimal dosage for balanced performance.

Performance Comparison of Three Fillers

With 30 parts N330 carbon black fixed, melamine, clay and light calcium carbonate were compared under equivalent filler dosages:

Vulcanization performance: Melamine-filled compounds have higher MH and shorter T90, meaning faster vulcanization speed than clay and light calcium carbonate.

Mechanical performance:

  • Clay-filled SBR has higher tensile strength and elongation at break;
  • Light calcium carbonate provides better tear strength;
  • Melamine-filled rubber features higher hardness and superior resilience.

Flame retardant performance: The oxygen index of melamine-filled SBR is distinctly higher than that of the other two traditional fillers. This fully verifies melamine’s excellent flame-retardant properties.

For single filler performance:
  • 20 parts clay: best tensile strength
  • 25 parts clay: best elongation at break
  • 20 parts light calcium carbonate: best 300% modulus
  • 30 parts melamine: highest resilience and flame retardancy

Application Guidance

  • For general flame-retardant SBR products requiring balanced mechanical properties: Adopt the formula of SBR + 25 parts of melamine matched with white carbon black or carbon black.
  • For rubber products with high resilience and flame-retardant requirements, choose 30 parts of melamine as the main filler.
  • For products focused on tensile strength and elongation, prioritize clay; for products requiring high tear strength, select light calcium carbonate.
  • Melamine is highly recommended for rubber materials used in construction, electrical appliances and public facilities where flame retardancy is mandatory.

conclusion-Performance of Melamine as Flame Retardant Filler in SBR Rubber

  1. Melamine powder can effectively accelerate the vulcanization rate of SBR rubber. When combined with 30 parts white carbon black or 40 parts carbon black, 25 parts melamine gives the most balanced, comprehensive mechanical properties.
  2. Melamine powder acts as a functional filler rather than a reinforcing filler for SBR. Excessive addition will reduce tensile strength, tear strength and resilience of vulcanized rubber.
  3. Compared with clay and light calcium carbonate, melamine shows outstanding flame-retardant capability. Its high oxygen index makes it an ideal flame-retardant filler for rubber products.
  4. When using 30 parts of carbon black as the base filler, 30 parts of melamine produce SBR rubber with high resilience and good flame resistance, suitable for flame-retardant rubber products that require elasticity.

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