Introduction to NR Rubber
NR, or Natural Rubber, is an elastomer obtained from the latex sap of the Hevea brasiliensis tree (rubber tree), primarily grown in tropical regions like Southeast Asia. It consists mainly of cis-1,4-polyisoprene, with a high molecular weight that contributes to its unique properties. First commercialized in the 19th century, NR was revolutionized by vulcanization (cross-linking with sulfur) discovered by Charles Goodyear in 1839, which enhances durability. It can be processed in latex, dry rubber, or compounded forms, often blended with fillers like carbon black, silica, oils, and antioxidants. Variations include ribbed smoked sheets (RSS), pale crepe, and technically specified rubber (TSR). NR is biodegradable and renewable but susceptible to supply fluctuations due to environmental factors.
Characteristics of NR Rubber
NR offers superior dynamic properties, making it ideal for applications requiring high elasticity. Here are its key characteristics:
- Mechanical Properties: NR has excellent tensile strength (20-30 MPa in vulcanized form), outstanding elongation at break (500-800%), and superior tear resistance. It exhibits high resilience ("snap" or rebound) and low hysteresis, leading to minimal heat build-up in dynamic applications like tires. However, it has moderate abrasion resistance without fillers and can crystallize under strain for added strength.
- Chemical Resistance: Fair resistance to water, alcohols, and dilute acids/alkalis, but poor resistance to oils, hydrocarbons, ozone, and oxidizing agents, which cause swelling or degradation. It requires antiozonants and antioxidants in formulations to mitigate cracking.
- Thermal Properties: Operates effectively from -50°C to +80°C (up to +100°C for short periods). It remains flexible at low temperatures but softens and degrades at high temperatures without heat stabilizers. NR has good low-temperature crystallization resistance in some strains.
- Durability and Aging: Good resistance to fatigue and flex cracking, but vulnerable to atmospheric aging, UV, and ozone without protective additives. It has low gas permeability and excellent adhesion to fabrics and metals, useful in composites.
- Processing and Cost: Easy to process via milling, extrusion, calendering, and molding. It has high green strength (uncured cohesion) and is cost-effective compared to synthetics, though prices vary with global supply. NR is biodegradable, making it environmentally friendlier.
- Other Properties: High electrical insulation, low specific gravity (0.92-0.95), and biocompatibility in purified forms. It can be vulcanized with sulfur, accelerators, or peroxides. However, it may cause allergic reactions due to proteins in latex form.
In formulations, NR is frequently blended with synthetics (e.g., SBR for better abrasion or BR for low-temperature performance) to balance properties and reduce costs.
Applications of NR Rubber
NR's natural elasticity makes it indispensable in vibration isolation and high-performance products:
- Automotive Tires and Components: Primary use in truck, aircraft, and off-road tires (up to 100% NR in some radials) for its resilience and low rolling resistance. Also in mounts, bushings, and seals.
- Industrial Products: Conveyor belts, hoses, and rollers due to its abrasion resistance and flexibility.
- Consumer Goods: Gloves, balloons, erasers, and footwear soles for elasticity and comfort.
- Medical and Healthcare: Surgical gloves, catheters, and condoms (latex form) for biocompatibility, though alternatives reduce allergy risks.
- Vibration and Shock Absorption: Engine mounts, seismic bearings, and sports equipment like tennis balls.
- Other Uses: Adhesives, foam mattresses, and agricultural products. It's also in earthquake-resistant building pads.
NR global production exceeds 13 million tons annually, with sustainable practices like agroforestry gaining traction. It's foundational in the rubber industry despite synthetics' rise.
