Elastomers: Buna-S & Buna-N
Preparation • Properties • Applications
Elastomers are polymers that exhibit rubber-like elasticity, capable of significant deformation under stress and rapid recovery upon release. They are essential in industries requiring flexibility, durability, and resistance to environmental factors. Among synthetic elastomers, Buna-S (Styrene-Butadiene Rubber, SBR) and Buna-N (Nitrile Butadiene Rubber, NBR) are widely used due to their cost-effectiveness and tailored properties. Developed in the early 20th century, these materials were pioneered by German chemists during efforts to create alternatives to natural rubber.
Buna-S is a copolymer of butadiene and styrene, making it one of the most produced synthetic rubbers globally, with over 5.4 million tons processed annually. It closely mimics natural rubber in processing and performance but offers enhanced resistance to aging and abrasion.
Preparation
Buna-S is synthesized via free radical emulsion polymerization (E-SBR) or solution polymerization (S-SBR). The process involves:
- Monomers: 1,3-Butadiene (75% by weight) and styrene (25% by weight).
- Catalyst/Initiator: Sodium (Na) or peroxide at low temperatures (around 5°C for "cold rubber").
- Reaction: Copolymerization in aqueous emulsion → coagulation → washing → drying → vulcanization with sulfur.
The name "Buna-S" derives from "Bu" (butadiene), "Na" (sodium catalyst), and "S" (styrene).
Properties
| Property | Description/Details |
|---|---|
| Elasticity & Resilience | Excellent; bounces back to original shape after deformation. |
| Abrasion Resistance | Superior; ideal for high-friction environments. |
| Tensile Strength | High (≥10–15 MPa); good tear resistance. |
| Temperature Range | -50°C to +100°C; flexible across wide range. |
| Chemical Resistance | Good vs. water, dilute acids, alcohols; resistant to oxidation. |
| Limitations | Poor resistance to oils, fuels, ozone; swells in hydrocarbons. |
Applications
- Automotive: Tire treads & sidewalls (~50% of SBR use), conveyor belts.
- Electrical: Wire/cable insulation, tank linings.
- Consumer: Footwear soles, hoses, gaskets.
- Other: Adhesives, printing rollers, flooring.
Buna-N is a copolymer of butadiene and acrylonitrile, prized for its oil and fuel resistance. It contains 18–50% acrylonitrile (ACN), with higher ACN enhancing oil resistance but reducing flexibility. Developed in 1931 by IG Farben.
Preparation
Produced through emulsion polymerization:
- Monomers: Butadiene (50–82%) and acrylonitrile (18–50%).
- Catalyst: Free radical initiators (peroxides or redox systems).
- Process: Emulsion at 30–40°C → coagulation (CaCl2) → washing → drying → vulcanization.
Properties
| Property | Description/Details |
|---|---|
| Oil/Fuel Resistance | Excellent vs. petroleum oils, fuels, hydraulic fluids. |
| Tensile Strength | High (≥10–20 MPa); low compression set. |
| Abrasion Resistance | Superior; withstands mechanical wear. |
| Temperature Range | -40°C to +108°C (up to +150°C short-term). |
| Chemical Resistance | Strong vs. water, alcohols, silicone greases. |
| Limitations | Poor vs. ozone, ketones, esters, chlorinated solvents. |
Applications
- Automotive/Aerospace: Fuel/oil hoses, O-rings, gaskets, seals.
- Industrial: Conveyor belts, oil seals, V-belts, cable jacketing.
- Protective Gear: Non-latex gloves (medical, lab, food), protective suits.
- Other: Adhesives, synthetic leather, splash guards.
Comparison: Buna-S vs Buna-N
Buna-S (SBR)
- Composition: Butadiene (75%) + Styrene (25%)
- Key Strength: Abrasion & elasticity
- Temp. Range: -50°C to +100°C
- Primary Use: Tires, belts, insulation
- Cost: Low; general-purpose
Buna-N (NBR)
- Composition: Butadiene + Acrylonitrile (18–50%)
- Key Strength: Oil & fuel resistance
- Temp. Range: -40°C to +108°C
- Primary Use: Seals, hoses, gloves
- Cost: Moderate; oil-specialized
Summary: Buna-S is a durable, natural rubber substitute for mechanical applications. Buna-N is indispensable for oil-exposed environments. Both are vulcanized for enhanced performance and recyclable.