Types of Sealing Foam and Its Uses
by Duncan Geddes
Various types of materials are used as sealing and gasket materials in a number of sectors, some of which are:- automotive, aerospace and aeronautics, transportation, biotechnology, chemical industry, construction industry, electronics, food and beverages, oil and gas, water treatment and pharmaceuticals.
Types of sealing foam
One class of materials that find use in various sealing and gasket applications are sealing foams such as:
Polychloroprene (Neoprene) Rubber Foam
The monomer chloroprene (2-chlorobutadiene) can be obtained from acetylene and hydrochloric acid via monovinylacetylene, or through butadiene chlorination and dechlorination of 3,4-dichloro-1-butene. Many types and grades of polymers and copolymers of Polychloroprene are available and are known under the generic name Neoprene.
Neoprene foams have good oil and solvent resistance, but generally have less resistance than Nitrile foams. The presence of chlorine substituted double bonds makes the polymer quite un-reactive, resulting in good resistance to most chemicals, oxygen and ozone. The high chlorine content of the polymer results in products which are generally self extinguishing. Foam rubber items used in contact with oils, waxes, and greases as well as most aliphatic hydrocarbons are made from Neoprene.
Nitrile Rubber (NBR) Foam
Nitrile Rubber is a copolymer of acrylonitrile and butadiene. A number of NBR type copolymers are manufactured, the two monomers are randomly distributed and the main difference between the copolymers is related to the amount of acrylonitrile (ACN) in the resultant polymer. The ratio between acrylonitrile and butadiene can vary from approximately 18% to 50%. If a higher heat and chemical resistance is required, this is achieved with a high ACN content. Where superior mechanical performance at low temperature is required, a lower ACN content type nitrile rubber needs to be selected as the glass transiton temperature (Tg) is affected by the % of ACN in the polymer.
Because of the polar nature of ACN, the copolymers have good resistance to oils and hydrocarbons and are useful in applications where they come into contact with oil such as: carburettor diaphragms, fuel tanks, hoses and gaskets. Nitrile rubber is compatible with PVC (polyvinylchloride) at various blend ratios to give products which combine the oil/fat resistance and elasticity of nitrile rubber with the chemical stability of PVC.
Ethylene/Propylene rubbers are a random copolymer which generally contain about 35mol% of polypropylene, but rubber-like properties are shown by copolymers with a propylene content ranging from 30-60mol%. The ethylene/propylene copolymers being random copolymers do not contain any long chains of ethylene or propylene units so crystallisation cannot occur and they are completely amorphous. There is virtually no double bonds consequently vulcanisation by conventional techniques using sulphur is not possible and peroxides have to be used. This limitation is overcome by use of a third monomer in the copolymerisation process. The third monomer is a non conjugated diene: one of its double bonds takes part in the polymerisation process becoming incorporated in the main polymer chain, whilst the other double bond is left un-reacted in a side chain which is available for vulcanisation.
EPDM’s (ethylene/propylene diene monomers) are made from low cost monomers and therefore have commercial potential as general purpose rubbers. Several blends of EPDM with Neoprene are available giving products with the properties associated with Neoprene but with cost savings due to the lower cost of EPDM manufacture. EPDM rubber foams have good mechanical strength, ozone resistance, weathering properties, abrasion resistance, and thermal stability.
Silicone Rubber Foam
General purpose silicone elastomers are based on polydimethylsiloxanes, but special purpose products containing a small proportion of other groups other than methyl are also available. In order to obtain elastic properties it is necessary to crosslink the linear silicone macromolecules, this can be achieved in different ways. The most important methods are based on organic peroxides at high temperature and cross-linking with organometallic and organic silicone compounds at ambient temperature or temperatures slightly above ambient temperature.
The most important characteristic of vulcanised Silicone foams is their maintenance of physical properties over a large temperature range. General purpose material is serviceable over the approximate range -50 to 2000C but both ends of the range can be extended by the use of special purpose compounds. The mechanical properties at ambient temperature are inferior to those of other elastomers/rubbers. However the mechanical properties at elevated temperatures are much superior to other types of rubber foams.
General characteristics of silicone foams are:-
- – Wide service temperature range
- – High ignition resistance
- – Excellent resistance to attack by oxygen, ozone and sunlight
- – Low toxicity
- – Good electrical insulation properties
- – Low chemical reactivity and good chemical and weathering resistance
- – Lower tensile strength compared to other vulcanised rubbers at ambient temperature
- – Poor hydrocarbon, oil and solvent resistance (except for silicone polymers that have vinyl, fluorine, and nitrile substituent groups on the macromolecular chain)
A major use of silicone rubber is in applications where its high and low temperature properties, good ignition resistance and electrical insulation characteristics are utilised eg: Aircraft, Aerospace, and other transport areas such as cars, trains and buses.
Flexible Polyurethane (PU) Foam
There are two classes of PU foam, polyether and polyester. By selection of the appropriate reactants it is possible to manufacture a wide range of foam types having different densities, hardness, cell size, and porosities from 100% open cell (reticulated foam) to closed cell foams. Polyurethane foams can also be impregnated to give additional properties to the foam such as improved fire resistance, and water repellent properties. PU foam has good chemical resistance and has been found to perform well in fungal and microbial growth studies when compared to other polymers, and can be laminated to various materials/substrates. Due to its versatility and cost effectiveness PU foams are used in various sealing and gasket operations.