Rubber linings are mainly used for protection against corrosion and/or erosion damage.
A wide range of rubbers and elastomers are available for lining vessels, tanks and piping.
Rubbers can also be made with anti-static properties to give a low surface electrical resistance.
Hard rubbers, i.e. hardness greater than Shore D 60, can only be applied by autoclave vulcanization, and therefore hard rubber lining is restricted to small equipment or components. Only soft rubbers can be applied on site.
Hard rubber linings can only be applied to rigid structures and they are also sensitive to large temperature fluctuations. Soft rubber linings remain elastic over a large temperature range, and consequently they can accommodate major deformation, vibrations and significant temperature changes.
With respect to safety aspects, pressure rating, etc. the regulations which apply to piping, equipment and structures are also valid for rubber-lined systems.
Material selection is determined by:
– service conditions (pressure, temperature, medium, etc.)
– manufacturing method
The following rubber types are used for lining purposes (classification according to ASTM D 1418):
– Isoprene or natural rubber (NR)
– Synthetic isoprene rubber (IR)
– Styrene-butadiene rubber (SBR)
– Chloroprene rubber (CR)
– Butyl rubber (IIR)
– Broom-butyl rubber (BIIR)
– Chloro-butyl rubber (CIIR)
– Nitrile-butadiene rubber (NBR)
– Ethylene propylene rubber (EP, EPDM)
– Urethane rubber (UR)
– Chlorosulphonated polyethylene (CSM)**
– Fluoro elastomer (FKM)*
* Commercially available under trade name “Viton” (DuPont product)
** Commercially available under trade name “Hypalon” (DuPont product)
Depending on the degree of vulcanization, rubbers can be classified as ‘soft’ rubber or as ‘hard’ rubber. The hardness of soft rubbers is expressed in Shore A, and the hardness of hard rubbers is expressed in Shore D (ASTM D 2240).
Hard rubbers (or Ebonites), i.e. with a hardness higher than Shore D 60, can be produced from NR or blends, e.g. NR/IR, NR/SBR and NR/IR/SBR.
Properties of Rubber
Each rubber material has a specific limit in terms of allowable service temperature and chemical resistance. The chemical resistance and temperature limits for continuous service of several rubber types are given below.
1. Natural rubber (NR)
Soft and hard natural rubber linings are suitable for handling most inorganic chemicals, with the exception of strong oxidising agents such as chromic and nitric acids. Natural rubber linings are also suitable for handling hydrochloric acid. Natural rubber is also resistant to most organic fluids, including alcohols and most esters. They should not be used in the presence of aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, mineral oils and certain vegetable oils. The allowable service temperature range is -40 °C to +80 °C.
The bond strength of NR linings on steel is excellent. The hardness is typically Shore A 55 for soft rubber and Shore D 75 for hard natural rubber.
2. Synthetic isoprene rubber (IR)
Isoprene rubber is a synthetic alternative form of NR, and has similar properties.
3. Styrene-butadiene rubber (SBR)
Styrene-butadiene rubber can be used for the containment of automotive brake fluids, alcohols and mixtures of alcohol and water. The allowable service temperature range is -30 °C to +80 °C. The hardness is in the same range as that of soft natural rubber (NR).
4. Chloroprene rubber (CR)
Chloroprene rubber is resistant to ozone and sunlight, and reasonably resistant to oils and chlorine. Special compounds are suitable for use with refrigerants (e.g. Freon 12 and 22).
The allowable service temperature range is -30 °C to +105 °C. Hardness is approximately Shore A 60.
5. Butyl rubbers (IIR, BIIR, CIIR)
Butyl rubbers have excellent tolerance to hydrochloric acid. Butyl rubber is resistant to ozone and sunlight, non-flammable hydraulic fluids, animal and vegetable oils, water, alcohols, ketones and acids. Butyl rubber should not be used in the presence of free halogens, petroleum oils or halogenated or aromatic hydrocarbons. The allowable service temperature range is -30 °C to +110 °C. Hardness is in the range of Shore A 55 to A 60.
6. Nitrile butadiene rubber (NBR)
Nitrile butadiene rubber (also known as BuNa-N) is a copolymer of butadiene and acrylonitrile. The acrylonitrile content must be at least 35% by mole to obtain good chemical resistance. Nitrile rubbers are resistant to petroleum-based hydraulic and lubricating oils, animal and vegetable oils, acetylene, alcohols, water, alkalis and fuel oils. Nitrile rubber should not be used for phenols, ketones, acetic acids, most aromatic hydrocarbons and nitrogen derivatives. The allowable service temperature is -35 °C to +80 °C. Hardness is approximately Shore A 60.
7. Ethylene propylene rubbers (EPDM / EPM)
Ethylene propylene rubbers are resistant to ozone and sunlight, oxidizing chemicals, non-flammable hydraulic fluids, pure aniline, fire extinguisher liquids, acids, hot water and steam. However, these rubbers are not resistant to mineral oils, petrol solvents and aromatic hydrocarbons. The allowable service temperature range is -40 °C to +150 °C. Hardness is typically in the range Shore A 40 to A 80.
8. Urethane rubber (UR)
Urethane rubber has excellent wear/erosion resistance and is chemically resistant to mineral oils, fuels and ozone. Urethane rubber should not be used for concentrated acids, ketones or chlorinated hydrocarbons, and shall not be used for water above 50 °C.
Otherwise, the allowable service temperature range is -40 °C to +70 °C. Hardness is typically in the range Shore A 50 to A 80.
9. Chlorosulphonated polyethylene (CSM)
Chlorosulphonated polyethylene is a highly wear-resistant synthetic rubber with excellent resistance to heat, ozone sunlight, oxidising media, sodium hypochlorite and sulphuric acid.
CSM rubber has also good resistance to most oils, lubricants and aliphatic hydrocarbons, but is unsuitable for use with esters and ketones. The allowable service temperature range is -35 °C to +80 °C. Hardness is approximately Shore A 60.
10. Fluoro-elastomers (FKM)
Fluoro-elastomers are copolymers of hexa-fluoro-propylene and vinyldiene fluoride. They are suitable for both high-temperature and vacuum applications. These materials have excellent resistance to oils, fuels, lubricants, carbon tetrachloride, most concentrated acids and many aliphatic and aromatic hydrocarbons such as toluenes, benzene and xylene.
They should not, however, be used with low molecular weight esters and ethers, ketones, certain amines and hot anhydrous hydrofluoric or chlorosulphonic acids. These materials are also resistant to ozone and sunlight and can be used in contact with many corrosive gases, e.g. bromine and chlorine. However, they are not resistant to ammonia or highpressure steam. The allowable service temperature range is -20 °C to +230 °C. Hardness is typically in the range Shore A 60 to A 90.