Vulcanization is a critical process that transforms raw rubber from a soft, plastic material into a tough, elastic one by forming cross-links between polymer chains. The choice of vulcanization system directly impacts rubber performance (e.g., elasticity, heat resistance, durability) and properties like odor or chemical stability. Below are the main types of rubber vulcanization systems, categorized by their core cross-linking agents:
#### 1. Sulfur Vulcanization System
The most traditional and widely used system, relying on sulfur (or sulfur donors) to form cross-links between rubber polymer chains.
- **Working principle**: Sulfur atoms react with unsaturated bonds in rubber (e.g., natural rubber, styrene-butadiene rubber, EPDM with diene monomers) to form polysulfide cross-links (-Sₓ-), connecting adjacent polymer chains.
- **Key components**:
- **Sulfur**: The primary cross-linking agent; particle size and purity affect reaction efficiency.
- **Accelerators**: Compounds (e.g., thiazoles, thiurams, sulfenamides) that speed up vulcanization and reduce sulfur dosage, preventing brittleness.
- **Activators**: Typically zinc oxide (ZnO) and stearic acid, which enhance accelerator activity and promote cross-link formation.
- **Advantages**: Low cost, easy operation, and versatile for most unsaturated rubbers (e.g., NR, SBR, EPDM).
- **Disadvantages**: May leave residual sulfur or volatile by-products (e.g., mercaptans), causing mild odors. Cross-links (polysulfides) are less heat-resistant than other systems.
#### 2. Peroxide Vulcanization System
Uses organic peroxides as cross-linking agents, suitable for both saturated and unsaturated rubbers (e.g., EPDM, silicone rubber, polyethylene).
- **Working principle**: Peroxides decompose at high temperatures to generate free radicals, which abstract hydrogen from polymer chains, triggering cross-linking via carbon-carbon (C-C) bonds.
- **Common peroxides**: Dicumyl peroxide (DCP), benzoyl peroxide (BPO), and di-tert-butyl peroxide (DTBP), chosen based on rubber type and processing temperature.
- **Advantages**:
- Forms stable C-C cross-links, improving heat resistance, chemical resistance, and compression set.
- Minimal odor (fewer volatile by-products) and lower risk of harmful residues, making it ideal for food-contact, medical, or odor-sensitive applications.
- **Disadvantages**: Higher cost than sulfur systems; may reduce rubber elasticity if overused.
#### 3. Metal Oxide Vulcanization System
Primarily used for rubbers with high halogen content (e.g., chloroprene rubber, chlorinated polyethylene).
- **Working principle**: Metal oxides (e.g., magnesium oxide, zinc oxide) react with halogen groups (-Cl, -Br) in the rubber to form ionic cross-links.
- **Key applications**: Neoprene (CR) and other halogenated rubbers, where oxides also act as acid acceptors to neutralize corrosive by-products.
- **Advantages**: Enhances flame resistance and oil resistance, critical for industrial seals and adhesives.
- **Disadvantages**: Limited to halogenated rubbers; may require higher temperatures for effective cross-linking.
#### 4. Quinone Dioxime Vulcanization System
A specialized system often used for butyl rubber (IIR) and EPDM, relying on quinone dioximes (e.g., p-quinone dioxime) as cross-linkers.
- **Working principle**: Reacts with active hydrogen in polymer chains to form stable cross-links, often with metal oxides (e.g., Pb₃O₄) as activators.
- **Advantages**: Produces rubber with excellent heat resistance and low permeability, suitable for tire inner tubes and chemical seals.
- **Disadvantages**: Higher cost and potential toxicity from heavy metal activators (e.g., lead compounds), limiting use in food/medical applications.
#### 5. Urethane Vulcanization System
A newer technology using isocyanates or urethane prepolymers to form cross-links, primarily for specialty rubbers (e.g., polyurethane rubber, nitrile rubber).
- **Working principle**: Reacts with hydroxyl or amino groups in polymers to form urethane linkages, offering precise control over cross-link density.
- **Advantages**: Produces rubber with high tensile strength, abrasion resistance, and customization options for hardness/elasticity.
- **Disadvantages**: High cost and sensitivity to moisture, requiring strict processing conditions.
### Summary
The choice of vulcanization system depends on rubber type, application requirements (e.g., heat resistance, odor), and cost. Sulfur systems dominate general-purpose uses, while peroxide systems are preferred for odor-sensitive or high-performance scenarios. Understanding these systems helps in selecting rubber products tailored to specific needs.