Butyl Rubber (IIR) / Bromobutyl (BIIR) / Chlorobutyl (CIIR)

Around the forties of the 20th century, butyl rubber (often just called “butyl”) was developed and brought to market by Stand Oil of New Jersey (now Exxon Mobil). The development of the thermoplastic polyisobutylene (PIB) by the German company BASF served as an important basis.

Butyl is generally a fairly flexible material, but in practice it is used primarily because of three special properties: its low gas permeability, its excellent damping over a wide temperature range, and its very good elasticity in cold conditions. The variants bromobutyl or chlorobutyl are frequently used, although for simplicity's sake we often just refer to "butyl".

In the following blog post, we will go into both the general and special properties of butyl, bromobutyl and chlorobutyl and list the most important fields of application.

While initially only pure butyl rubber (Isobutylene-isoprene rubber, IIR) was offered and used in the forties of the 20th century, modifications of the material with halogens took place in the following decades. Bromobutyl (BIIR) was used for the first time in the mid-fifties, while chlorobutyl (CIIR), which is still produced by Exxon Mobil, was launched in the early sixties.

In practice, butyl is mostly used in the form of bromobutyl (BIIR) or chlorobutyl (CIIR). Since bromine and chlorine are halogens, bromobutyl and chlorobutyl are also referred to as halobutyl rubbers (XIIR). Butyl is often used as a synonym, even though bromobutyl or chlorobutyl is actually used in the application.

Crosslinking can be done with sulfur, quinone dioxime, phenolic resins or zinc oxide, for example. In the case of bromobutyl (BIIR) and chlorobutyl (CIIR), crosslinking with peroxides is also possible. The crosslinking system is selected depending on the application. Zinc oxide or resin systems are used, for example, when better heat resistance is required. Quinone dioxime systems are actually no longer used because of their lead content.

Mechanical properties of butyl rubber (bromobutyl, chlorobutyl)

In terms of mechanical properties, the very low rebound resilience of butyl (bromobutyl, chlorobutyl) should be emphasized. The very low rebound resilience of butyl (bromobutyl, chlorobutyl) is synonymous with excellent damping properties, which are used in a wide range of applications. Due to its excellent damping properties, molded parts made of butyl (bromobutyl, chlorobutyl) are used, for example, as technical shock absorbers or vibration dampers. A special feature here is that the damping properties of butyl rubber (bromobutyl, chlorobutyl) are available over a very wide temperature range from around -40 degrees Celsius to +70 degrees Celsius (or -40 degrees Fahrenheit to +160 degrees Fahrenheit) without any major restrictions. Nitrile rubber (NBR) can achieve similar damping properties to butyl if the compound is suitably formulated. However, these are only possible in a much smaller temperature range compared to butyl.

Butyl compounds (bromobutyl, chlorobutyl) or molded parts made from them can be both very soft and relatively hard. Possible hardnesses range from 30 Shore A to 80 Shore A. However, finished parts made from very soft butyl compounds (bromobutyl, chlorobutyl) tend to be sticky.

The compression set of butyl (bromobutyl, chlorobutyl) is rather weak (i.e. high) compared to other rubber materials. This is especially true at rising temperatures. At cold temperatures, the compression set is decent.

The other mechanical properties such as tensile strength, tear strength or abrasion resistance are average compared to other elastomers. The elongation at break, on the other hand, is very good.

Thermal Properties of Butyl Rubber (Bromobutyl, Chlorobutyl)

The thermal resistance of butyl rubber (bromobutyl, chlorobutyl) is basically comparable to EPDM, tending to be perhaps a little worse. It is very dependent on the cross-linking system. If butyl rubber is crosslinked by sulfur, the heat resistance is somewhat weaker. If, on the other hand, quinone dioxime, phenolic resins or zinc oxide are used as crosslinking agents, the heat resistance increases. In general, the higher the isoprene content of the compound, the better the heat resistance. However, it should be noted that oxygen and ozone resistance decreases with increasing isoprene content. The heat resistance of butyl rubber with sulfur crosslinking is about +110 degrees Celsius or +230 degrees Fahrenheit (1,000 hours), and with resin crosslinking it is +150 degrees Celsius or +300 degrees Fahrenheit and higher.

In general, it can be said that butyl (bromobutyl, chlorobutyl) is typically used in a temperature range from -60 degrees Celsius (-75 degrees Fahrenheit) to +130 degrees Celsius (270 degrees Fahrenheit).

With regard to cold temperatures, butyl rubber (bromobutyl, chlorobutyl) shows very good performance values. The Tg is around -70 degrees Celsius or -95 degrees Fahrenheit which is roughly on a par with natural rubber. The elasticity of butyl rubber (bromobutyl, chlorobutyl) is largely retained at low temperatures, which makes the material special compared with many other elastomers.

It should also be emphasized that the excellent damping properties of butyl rubber (bromobutyl, chlorobutyl) are retained over a wide temperature range (-40 degrees Celsius to +70 degrees Celsius or -40 degrees Fahrenheit to +160 degrees Fahrenheit).

Chemical resistance of butyl rubber (bromobutyl, chlorobutyl)

A special feature of butyl rubber (bromobutyl, chlorobutyl) is its extremely low gas permeability. In this respect, butyl (bromobutyl, chlorobutyl) is clearly superior to other elastomers. Compared to natural rubber, for example, the gas permeability of butyl rubber (bromobutyl, chlorobutyl) is lower by a factor of about 10.

The weathering and ozone resistance of butyl (bromobutyl, chlorobutyl) are very decent overall, but remain below the values of EPDM. In general, it can be said that EPDM has displaced butyl (bromobutyl, chlorobutyl) in this area, provided that no other special properties of butyl (bromobutyl, chlorobutyl) are required.

Butyl rubbers (bromobutyl, chlorobutyl) exhibit good to very good resistance to dilute acids and alkalis and to polar solvents such as alcohol. Also good to very good is the resistance to acetone, glycerol, methyl ethyl ketone, phthalates, vegetable oil and esters.

In addition, butyl (bromobutyl, chlorobutyl) exhibits good compatibility with hot water and superheated steam up to about +130 degrees Celsius or +270 degrees Fahrenheit.

However, resistance to fuels, oils, greases, hydrocarbons and concentrated oxidizing acids is unsatisfactory.

Fields of application of butyl rubber (bromobutyl, chlorobutyl)

The areas of application of butyl rubber (bromobutyl, chlorobutyl) result in particular from its special properties in terms of damping as well as gas impermeability, where butyl is superior to other rubber materials. In this respect, articles made of butyl (bromobutyl, chlorobutyl) are used especially where these properties are required.

An important area of application for butyl rubber (bromobutyl, chlorobutyl) is tire tubes or air tubes of both automobile and bicycle tires, as well as the inner layers of tubeless tires. In general, butyl (bromobutyl, chlorobutyl) is very suitable for hoses used for conveying gases due to its low gas permeability. This property is also used in the manufacture of special protective clothing (protective gloves, protective suits) and in the production of vacuum seals.

Due to its excellent damping properties, butyl (bromobutyl, chlorobutyl) is also used in the manufacture of technical shock absorbers, vibration dampers and other damping elements and buffer springs.

Butyl rubber (bromobutyl, chlorobutyl) is also used as a material for diaphragms, seals, O-rings or flange closures.

Furthermore, plugs or caps made of butyl (bromobutyl, chlorobutyl) are used in the pharmaceutical industry and also in the medical sector.

Overview of the properties of butyl rubber (bromobutyl, chlorobutyl)

In conclusion, here is a recap of the properties of butyl rubber (bromobutyl, chlorobutyl).

Remember, this is only a general guide and not to be used for your ultimate selection of materials. The individual properties of blends can be positively and negatively influenced by targeted formulation and as such may be different from the information presented here.

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For more details about properties or chemical resistance, or if you have a query about a particular application, please do not hesitate to contact us.  

If you have a question about this blog post or would like us to discuss a particular aspect of elastomers in an upcoming blog, please email us on info@hepako.de   

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