Rubber Processing

Introduction

• Rubber processing shares similarities with plastic production but differs in some aspects.
• The rubber industry is dominated by tire production for vehicles.

Natural Rubber (NR)

• Source: Hevea brasiliensis trees, mainly in Southeast Asia.
• Historically held a large share (over 40%) of the elastomer market.
• Latex: Commercially known form of NR.
• Chemical composition: Polyisoprene (C5H8)_n (about 30% of latex).
• Most latex is coagulated to produce "dry rubber" for products like tires.

Properties of Natural Rubber

• Excellent tensile, tear, and dynamic (flexibility) properties.
• Poor resistance to solvents, oil, and ozone.
• Epoxidation can improve resistance by introducing reactive groups and polarity.
• Epoxidation is more economical and environmentally safe in latex form.

Synthetic Rubbers

• Developed to replicate or enhance NR properties.
• Production: Primarily from petroleum using polymerization techniques.
• Supplied as large bales, unlike TP and TS polymers (pellets or liquid resins).

Common Synthetic Rubbers

• Butadiene Rubber (BR): Oil-resistant copolymer.
• Nitrile Rubber (NBR): High-strength and oil-resistant.
• Styrene-butadiene Rubber (SBR).
• Silicone Rubber (SiR).

Silicone Rubbers

• Composition: Silicone polymer containing silicon, carbon, hydrogen, and oxygen.
• Properties: Excellent low-temperature flexibility and high-temperature resistance (service temperatures from -70 °C to 250 °C).
• Application: pressure-sensitive adhesives.

Basic Steps in Rubber Goods Production

1. Production of the rubber itself (natural or synthetic).
2. Processing into finished goods: Compounding, mixing, shaping, and vulcanizing.

Compounding

• Rubber is always mixed with additives.
• Chemicals are added for vulcanization.
• Fillers:
  • Reinforcing fillers: Enhance mechanical properties.
  • Non-reinforcing fillers: Reduce costs.
• Compounding is crucial for tailoring rubber properties, cost, and processability.

Vulcanization

• Definition: Cross-linking of viscoelastic polymers.
• Effect: Prevents the rubber from permanently deforming; it returns to its original shape.

Vulcanization Process

• Process: Heating rubber for a specific time to create chemical connections.

Reasons for Vulcanization

• Increases stiffness and strength while maintaining extensibility.
• Reduces flow by joining long-chain molecules at tie points.
• Soft rubber: 1-2 cross-links per 1000 mers.
• Higher cross-link density increases stiffness (e.g., hard rubber).

Carbon Black in Rubber

• Most important reinforcing filler.
• Composition: Colloidal form of carbon from thermal decomposition of hydrocarbons (soot).
• Effects: Increases tensile strength, abrasion, and tear resistance; provides UV protection.
• Consequence: Most rubber parts are black.

Filament Reinforcement in Rubber

• Purpose: Reduces extensibility while retaining other properties.
• Examples: Tires, conveyor belts.
• Typical Materials: cellulose, nylon, and polyester.
• Other Materials: Fiber-glass and steel (e.g., steel-belted radial tires).
• Application: Added during shaping, not mixing.

Other Fillers and Additives in Rubber

• China clays: hydrous aluminum silicates (Al2Si2O5(OH)4); less reinforcement than carbon black, used when black color is undesirable.
• Other polymers: styrene, PVC, and phenolics.
• Recycled rubber: Usually 10% or less.
• Additional additives: antioxidants, fatigue and ozone-protective chemicals, coloring pigments, plasticizers and softening oils, blowing agents (for foamed rubber), mold release compounds.

Shaping Processes for Rubber Products

• Four basic categories: Extrusion, Calendering, Coating, Molding and casting.
• Some products (e.g., tires) require multiple processes and assembly.

Rubber Extrusion

• Method: Screw extruders.
• L/D ratio: Typically 10 to 15 (lower than for thermoplastics) to reduce premature cross-linking.
• Die swell: Occurs in rubber extrudates due to the polymer's “memory”.
• State: Rubber is unvulcanized at this stage.

Calendering

• Process: Rubber stock passes through decreasing gaps between rotating rolls.
• Thickness: Rubber sheet thickness is slightly greater than the final roll gap due to die swell.

Roller Die Process

• Definition: Combination of extrusion and calendering.
• Outcome: Better quality than either process alone.

Molding Processes for Rubber

• Principal processes: Compression molding (most important for tire manufacture), transfer molding, and injection molding.
• Curing: Vulcanization occurs in the mold in all three processes, unlike previous methods where vulcanization is separate.

Moulded Rubber Products

• Examples: shoe soles and heals, gaskets and seals, suction cups, and bottle stops.
• Foamed rubber parts and tires are also commonly produced by moulding.

Tires and Other Rubber Products

• Tires: Approximately 75% of total rubber usage.
• Carcass: Internal structure made of rubber-coated cords (plies).
• Cords: Made of nylon, polyester, fiber glass, or steel to reinforce the rubber.

Rubber Belts

• Application: Conveyor and pulley systems.
• Manufacturing: Fabrics are coated by calendering, assembled to required thickness, and vulcanized via continuous or batch heating.

Rubber Components in Footwear

• Includes: Soles, heels, rubber overshoes, and certain upper parts.
• Production: Molded parts are made by injection molding, compression molding, and special techniques.