Introduction to IIR Rubber
IIR, or Butyl Rubber, is a copolymer of isobutylene (about 95-99%) and a small amount of isoprene (1-5%) to enable vulcanization. It was developed in the 1930s by Standard Oil (now Exxon) as a synthetic alternative during natural rubber shortages. The high isobutylene content creates a saturated backbone, contributing to its low permeability. IIR is produced via cationic polymerization in a slurry process and can be halogenated (e.g., Chlorobutyl or Bromobutyl) for improved cure rates and compatibility. It is compounded with fillers like carbon black or clay, oils, and curing agents (sulfur or resin systems). Grades include regular IIR for general use and halogenated versions for better adhesion and heat resistance. IIR is more expensive than general-purpose rubbers but essential where gas barrier properties are critical.
Characteristics of IIR Rubber
IIR stands out for its gas impermeability and environmental resistance, though it has limitations in mechanical strength. Here are its key characteristics:
- Mechanical Properties: IIR has moderate tensile strength (10-20 MPa), elongation at break (300-600%), and fair tear resistance. It offers excellent vibration damping and shock absorption due to high hysteresis, but low resilience compared to NR or SBR. It has good flex fatigue resistance but requires fillers for improved abrasion.
- Chemical Resistance: Excellent resistance to ozone, weathering, UV, water, dilute acids, bases, and polar solvents. However, it has poor resistance to hydrocarbons, oils, and non-polar solvents, which cause swelling. Halogenated IIR improves resistance to certain chemicals and heat.
- Thermal Properties: Operates from -50°C to +120°C (up to +150°C for halogenated grades). It maintains flexibility at low temperatures without crystallization and has good heat aging resistance, though it can degrade at very high temperatures.
- Durability and Aging: Superior impermeability to gases (e.g., air, oxygen) and vapors, making it ideal for barriers. It has excellent resistance to oxidation and corona discharge, with low water absorption. Compression set is moderate, improved in halogenated forms.
- Processing and Cost: Processed via extrusion, calendering, and molding, but has high viscosity requiring plasticizers. It has good green strength and adhesion in halogenated grades. IIR is costlier due to specialized production but durable, reducing long-term costs.
- Other Properties: Low gas permeability (10-100 times lower than other rubbers), good electrical insulation, and biocompatibility in pure forms. It can be vulcanized with sulfur (slow) or resins/metal oxides for faster cures. Specific gravity is low (0.91-0.92), and it's odorless with low toxicity.
In formulations, IIR is often blended with EPDM for better ozone resistance or NR for improved resilience, and requires antioxidants for extended life.
Applications of IIR Rubber
IIR's gas barrier and damping properties make it suitable for sealing and protective applications:
- Automotive and Tires: Primary use in tire inner tubes, tubeless tire liners, and curing bladders due to air retention. Also in seals, mounts, and diaphragms for vibration control.
- Pharmaceutical and Medical: Vial stoppers, syringe plungers, and closures for its impermeability to gases and biocompatibility, preventing drug contamination.
- Construction and Roofing: Membranes, sealants, and tapes for waterproofing and weatherproofing.
- Industrial Seals and Hoses: O-rings, gaskets, and hoses for chemical handling, vacuum systems, and air conditioning where low permeability is needed.
- Electrical and Electronics: Cable insulation and protective coatings for its dielectric properties and corona resistance.
- Other Uses: Sporting goods (e.g., basketball bladders), adhesives, and protective clothing. Halogenated IIR is used in tank linings and pharmaceutical packaging.
IIR production focuses on sustainability with recycled variants. It's a niche rubber with annual global output around 1.5 million tons, driven by automotive and pharma demands.