Butyl Rubber – Properties, Applications, and Uses
22/03/2022
Butyl rubber is a synthetic elastomer made by combining isobutylene and isoprene. It was the first rubber to be synthesized. It has good shock absorption characteristics and low moisture and gas permeability and is used in many commercial applications. This article briefly describes the material, how it is made, and some popular applications. Other elastomers include EPDM, nitrile, silicone, neoprene, etc., about which information may be found in our Types of Rubber article.
Fundamentals
Butyl rubber, or polyisobutylene, is a vinyl elastomer very similar to polyethylene and polypropylene in its structure, except that every other carbon atom is substituted with two methyl groups rather than one. It is made by a process called cationic vinyl polymerization from the monomer isobutylene. Usually, 1-2% isoprene is added to the isobutylene. The reaction is very fast so it is usually synthesized at very low temperatures. The addition of isoprene creates double bonds that allow the material to be crosslinked by vulcanization, just like natural rubber. This was an important step in making the original material useful during World War II as a substitute for natural rubber in the manufacturing of tires and tank treads. Polyisobutylene was first synthesized in 1931 and developed into butyl rubber in 1937. Curing rates improved in the 1960s with the development of halogenated, chlorinated, and brominated forms. These forms are often abbreviated as CIIR (for chlorinated isobutylene isoprene rubber) and BIIR (for brominated isobutylene isoprene rubber).
Put simply, vulcanizing is a process that ties all the rubber molecules together to form a single large molecule that does not melt as it gets warm and does not embrittle as it gets cold. Vulcanizing was invented by Charles Goodyear in 1839. It is a thermosetting process, so vulcanizing takes place after the product is formed.
Advantages
Butyl rubber is the only known elastomer that is impervious to gases. The material is flexible, with good room temperature damping characteristics. The material is biocompatible, resists many acidic and alkaline chemicals, ozone, heat, and weathering, and has good aging properties. It resists attack by phosphate ester hydraulic fluids and ketones but does not do well in the presence of mineral or petroleum-based fluids, hydrocarbons, or flame. It has good electrical insulating properties. Butyl rubber is usable between -50 and 250°F but its damping characteristics diminish at higher temperatures. It remains flexible at lower temperatures. Durometers range between 40 and 80 Shore A. A summary of butyl rubber’s mechanical strengths and weaknesses include:
- Compression set: fair
- Rebound rating: poor
- Flex cracking resistance: good to excellent
- Abrasion resistance: good to excellent
- Tear resistance: good
- Impact resistance: good
- Flame resistance: poor
- Weather resistance: excellent
- Sunlight resistance: excellent
- Ozone resistance: excellent
- Oxidation resistance: excellent
- Water resistance: very good
- Steam resistance: excellent
- Gas permeability: good
Applications
Due to its low gas and vapor permeability, butyl rubber is an important material in the manufacturing of tubeless tires, inner tubes, sports-ball bladders, glove-box gloves, etc. As a waterproofing material, it is applied as a liner in tanks and ponds. It is used as a patching material for membrane roofs and as a sealant for insulated windows. Combined with other chemicals, polyisobutylene makes oil and fuel additives and demisting agents for machining lubricants.
As a vibration dampener, butyl rubber is used for shock mounts, suspension bushings, and car- and truck-body mounts. Speaker cone edges are often made from butyl rubber today where once they were commonly made of foam. Stoppers for labware and medical equipment are manufactured from the material. Owing to its low permeability, it is used in making gas masks. Though not as soft and compliant as silicone rubber, butyl rubber is flexible enough that it can achieve a good face seal.
Butyl rubber is available as slabs, sheets, and tapes, and as an adhesive/sealant in tubes. The material can be molded (by transfer, injection, and compression molding processes) and extruded. It is used to make gaskets, hoses, O-rings, etc.
Compared with natural rubber, butyl rubber is costlier due to more complex processing requirements. Some anecdotal discussion exists regarding natural vs. butyl rubber motorcycle tire tubes, as the natural rubber is said to have better puncture resistance and tear strength but butyl rubber tubes can go longer between airings. Natural rubber, or polyisoprene, is also considered to be a sustainable resource, made from the sap of the Hevea brasiliensis tree, a native of South America. Polyisoprene may also be synthesized using the Ziegler-Natta polymerization process.
Butyl rubber, in its food-grade form, is used to make chewing gum. It has all but replaced the gum of the Chicle tree except in a few specialty, natural products. There have been some efforts to collect used chewing gum and recycle it into new products. Improper disposal of chewing gum is a chronic urban blight.
