Function of Protective Agents
Antioxidants
The primary function of antioxidants in rubber is to protect against oxidative degradation caused by exposure to oxygen, heat, and UV light. Rubber aging due to oxidation can lead to a loss of elasticity, strength, and ultimately, cracking and breakdown of the material.
Here’s how antioxidants work:
- Inhibiting Free Radical Reactions: During rubber aging, free radicals are generated. Antioxidants react with these free radicals, neutralizing them and breaking the chain reactions that lead to polymer degradation.
- Preventing Peroxide Formation: They can destroy hydroperoxides and peroxides, which are highly reactive species that accelerate the oxidation process.
- Delaying Thermal Oxidative Aging: Heat significantly accelerates oxidation. Antioxidants improve the rubber’s resistance to high temperatures, helping to maintain its structural integrity.
- Prolonging Lifespan: By slowing down the aging process, antioxidants extend the service life of rubber components in shoes, ensuring they remain flexible and strong.
- Maintaining Mechanical Properties: They help preserve essential mechanical properties like tensile strength and elasticity, which are vital for the performance and comfort of shoes.
Common antioxidants include aminic and phenolic compounds. Some are designed to be low-migration to maintain their effectiveness over time.
Antiozonants
Antiozonants protect rubber specifically from ozone cracking, a severe form of degradation caused by atmospheric ozone. Ozone attack is particularly aggressive on unsaturated rubbers, especially under mechanical stress (e.g., bending or stretching). This often manifests as visible cracks on the surface of rubber products.
Here’s how antiozonants work:
- Forming a Protective Barrier: Many antiozonants (like certain waxes) migrate to the surface of the rubber, forming a physical barrier that prevents direct contact between ozone and the rubber. This is particularly effective under static conditions (e.g., storage).
- Reacting with Ozone: Chemical antiozonants (such as p-phenylenediamines – PPDs) react with ozone before it can attack the rubber polymer chains. They essentially “scavenge” the ozone, sacrificing themselves to protect the rubber.
- Repairing Surface Damage: Some antiozonants can also react with the initial products of ozone attack, helping to “reseal” small cracks and prevent their propagation.
- Synergistic Effects: Often, antioxidants and antiozonants are used together as they can have synergistic effects, providing more comprehensive protection against various forms of degradation. Many antiozonants also offer some antioxidant properties.
Antiozonants are crucial for shoe soles and other rubber parts exposed to outdoor conditions, where ozone levels can be significant.
Laboratory Testing Parameters Before Batch Production
Before initiating batch production of rubber shoes, rigorous laboratory testing of the rubber compound is essential to ensure it meets the required performance specifications. These tests help to confirm the quality and consistency of the raw materials and the compound formulation.
Key laboratory testing parameters for rubber before shoe batch production include:
- Rheological Properties (Cure Characteristics):
- Mooney Viscosity (ASTM D1646): Measures the flow characteristics of the uncured rubber compound. This indicates how easily the rubber can be processed (mixed, extruded, molded). Too high a viscosity can make processing difficult, while too low can lead to issues like “scorch” (premature curing).
- Moving Die Rheometer (MDR) or Oscillating Disc Rheometer (ODR) (ASTM D5289 or ISO 3417): These tests assess the curing behavior of the rubber. Key parameters obtained include:
- Scorch Time (tS1 or tS2): Time to the onset of vulcanization. Crucial to prevent premature curing during processing.
- Optimum Cure Time (t90 or tc90): Time to achieve 90% of the maximum torque, indicating the optimal vulcanization time for desired properties.
- Maximum Torque (MH) and Minimum Torque (ML): Indicate the stiffness of the cured rubber and the processability of the uncured rubber, respectively.
- Cure Rate: How fast the rubber cures.
- Physical and Mechanical Properties (on Cured Rubber):
- Hardness (Shore A or D) (ASTM D2240): Measures the resistance of the rubber to indentation. This is critical for shoe soles for comfort, grip, and durability.
- Tensile Strength and Elongation at Break (ASTM D412): Measures the maximum stress the material can withstand before breaking and how much it can stretch before breaking. Essential for the durability and flexibility of the shoe.
- Tear Strength (ASTM D624): Measures the resistance of the rubber to tearing. Important for preventing rips and prolonging the life of the shoe, especially in areas subjected to stress.
- Abrasion Resistance (ASTM D1630): Measures the rubber’s resistance to wear caused by rubbing or friction. Highly relevant for shoe outsoles that experience constant contact with surfaces.
- Resilience/Rebound (ASTM D2632): Measures the ability of the rubber to return to its original shape after deformation. Important for energy return and shock absorption in shoes.
- Compression Set (ASTM D395): Measures the permanent deformation of rubber after being subjected to a compressive force for a period. Relevant for cushioning and maintaining the shape of shoe components.
- Aging Properties:
- Accelerated Aging (Heat Aging) (ASTM D573): Samples are exposed to elevated temperatures for a defined period to simulate long-term aging. After aging, the mechanical properties (tensile strength, elongation, hardness) are re-measured and compared to unaged samples to assess the effectiveness of antioxidants.
- Ozone Resistance (ASTM D1149): Samples are exposed to a controlled ozone environment under static or dynamic conditions. The time to crack formation and the severity of cracking are observed to assess the effectiveness of antiozonants. This is crucial for outdoor footwear.
- Density/Specific Gravity (ASTM D792): Measures the density of the rubber compound. This helps in quality control to ensure consistency in the mix and detect any variations in filler content.
- Chemical Composition (less frequent for every batch but important for new formulations):
- Thermogravimetric Analysis (TGA): Determines the composition of the rubber compound by measuring weight loss as a function of temperature, identifying the amount of polymer, carbon black, fillers, etc.
- Fourier-Transform Infrared Spectroscopy (FTIR): Identifies the types of polymers and additives present in the compound.
By performing these tests before batch production, manufacturers can ensure that the rubber compound has the desired properties for shoe production, leading to consistent quality, performance, and durability of the final footwear products.
