Curing Agents (Vulcanizing Agents)

Function of Curing Agents (Vulcanizing Agents)

The primary function of curing agents is to create cross-links between the long polymer chains of the rubber. These cross-links form a three-dimensional network structure, which gives the rubber its improved mechanical properties.

Here are some common curing agents and their functions in rubber, particularly relevant for shoe production:

1. Sulfur:
   • Function: The most common and historically significant vulcanizing agent. Sulfur atoms form bridges (sulfur cross-links) between rubber polymer chains. This process significantly enhances the rubber’s tensile strength, elasticity, and resistance to heat and chemicals.
   • Types: Elemental sulfur (powdered, sublimated, precipitated). Often used with accelerators and activators.
   • Considerations for Shoes: Widely used for natural rubber and many synthetic rubbers due to its effectiveness and cost-efficiency. However, in higher concentrations, it can lead to “sulfur bloom” (migration of sulfur to the surface), which can affect appearance and bonding.
2. Organic Peroxides (e.g., Dicumyl Peroxide (DCP), Benzoyl Peroxide):
   • Function: Peroxides decompose under heat to form free radicals, which then abstract hydrogen atoms from the polymer chains, leading to carbon-carbon cross-links.
   • Considerations for Shoes: Offer excellent heat resistance and low compression set. They are often used for specialty rubbers or where specific high-performance properties are required. They generally produce a more stable cure than sulfur alone but can be more expensive.
3. Metal Oxides (e.g., Zinc Oxide (ZnO), Magnesium Oxide (MgO)):
   • Function: While not primary cross-linking agents themselves for all rubbers, they are crucial activators in sulfur vulcanization. Zinc oxide, in particular, activates accelerators and participates in the formation of efficient sulfur cross-links. Magnesium oxide is also used as an activator, especially in halogenated rubbers like chloroprene (neoprene), where it can also scavenge acids formed during processing.
   • Considerations for Shoes: Essential for achieving optimal cure characteristics with sulfur-based systems. Zinc oxide also contributes to reinforcement and can help shield against UV degradation.
4. Resin Vulcanizing Agents (e.g., Alkyl Phenolic Resins, Epoxy Resins):
   • Function: These resins can vulcanize unsaturated carbon chain rubbers and butyl rubber, significantly improving their heat resistance. Epoxy resins are particularly effective for carboxyl rubber and chloroprene rubber, providing good flex resistance.
   • Considerations for Shoes: Used for specific applications where high heat resistance or particular adhesion properties are desired, especially in certain sole compounds.
5. Accelerators (e.g., Thiazoles like MBT, MBTS; Sulfenamides like CBS; Thiurams like TMTD; Dithiocarbamates; Guanidines):
   • Function: While not curing agents in themselves, accelerators are vital in all modern sulfur vulcanization systems. They speed up the vulcanization reaction, shorten the vulcanization time, reduce the required vulcanization temperature, and improve the efficiency of sulfur utilization, leading to better physical and mechanical properties of the vulcanized rubber. They achieve this by reacting with sulfur to form reactive intermediates that facilitate cross-linking.
   • Considerations for Shoes: Crucial for efficient production cycles and for achieving the desired balance of properties in shoe components (e.g., sole flexibility, abrasion resistance). Different accelerators offer varying scorch times (time before vulcanization begins) and cure rates, allowing for fine-tuning of the process.
6. Retarders/Inhibitors (e.g., Cyclohexylthiophthalimide (PVI)):
   • Function: Used to prevent premature vulcanization (scorch) during mixing and processing, extending the processing safety time.
   • Considerations for Shoes: Important for preventing issues during the mixing and molding of rubber compounds, ensuring a consistent and high-quality product.

Laboratory Testing Parameters Before Batch Production

Before a rubber compound goes into full batch production for shoe components (soles, midsoles, etc.), rigorous laboratory testing is crucial to ensure it meets the required specifications and will perform as expected.

Key parameters tested include:

1. Mooney Viscosity (ASTM D1646):
   • Purpose: Measures the flow characteristics of the uncured rubber compound. It indicates how easily the rubber can be processed (mixed, extruded, molded).
   • Relevance for Shoes: Critical for ensuring consistent processing on production lines, preventing issues like uneven filling of molds or poor extrusion.
2. Cure Characteristics / Rheometry (ASTM D5289):
   • Purpose: Evaluates the vulcanization behavior of the rubber compound. This is typically done using a Moving Die Rheometer (MDR). It measures:
     • Scorch Time (tS1, tS2): The time until the onset of vulcanization begins (when viscosity starts to increase). Crucial for processing safety.
     • Optimum Cure Time (Tc90): The time at which 90% of the maximum cross-linking has occurred. This dictates the ideal cure time in production.
     • Cure Rate: How fast the vulcanization reaction proceeds.
     • Minimum and Maximum Torque: Indicates the unvulcanized compound’s stiffness and the final cured compound’s stiffness/modulus.
   • Relevance for Shoes: Essential for setting accurate cure times and temperatures in presses and molds, ensuring proper vulcanization and preventing under or over-curing, which would negatively impact shoe sole performance.
3. Density/Specific Gravity (ASTM D792):
   • Purpose: Measures the density of the rubber compound. Helps confirm the uniformity of the mix and detect variations in filler content.
   • Relevance for Shoes: Ensures consistent weight and material usage, and contributes to predicting the final product’s physical properties.
4. Tensile Strength and Elongation at Break (ASTM D412):
   • Purpose: Measures the rubber’s ability to withstand pulling forces and how much it can stretch before breaking.
   • Relevance for Shoes: Directly relates to the durability and flexibility of the shoe sole, its resistance to tearing, and overall mechanical integrity.
5. Hardness (ASTM D2240 – Shore A, D, etc.):
   • Purpose: Measures the resistance of the rubber surface to indentation.
   • Relevance for Shoes: Determines the “feel” and stiffness of the sole, impacting comfort, cushioning, and grip. Different parts of a shoe (e.g., outsole vs. midsole) require different hardness levels.
6. Abrasion Resistance (e.g., DIN, Akront, NBS):
   • Purpose: Measures the material’s resistance to wear caused by friction.
   • Relevance for Shoes: Critically important for shoe outsoles, directly impacting the longevity and durability of the shoe.
7. Tear Strength (ASTM D624):
   • Purpose: Measures the force required to propagate a tear in the material.
   • Relevance for Shoes: Important for preventing rips and tears in high-stress areas of the sole.
8. Compression Set (ASTM D395):
   • Purpose: Measures the ability of the rubber to return to its original shape after being compressed for a period.
   • Relevance for Shoes: Especially important for midsole materials, which need to maintain their cushioning properties over time and repeated compression.
9. Specific tests for shoe components:
   • Flexibility/Bending Resistance: For outsoles, to ensure proper movement and prevent cracking.
   • Slip Resistance: For outsoles, often tested on various surfaces (wet, dry).
   • Adhesion Strength: Between different layers of the sole (e.g., outsole to midsole, upper to sole).

By meticulously testing these parameters, manufacturers can ensure that the rubber compounds used for shoe production will consistently deliver the desired performance, durability, and quality in the final product.