What is the structural formula of elastomers?
Elastomers are a group of different materials and as such do not have a uniform structural formula. Elastomers have a specific carbon backbone that repeats itself in several thousand units. These main chains are added during vulcanization by the addition of Cross-linking chemical They are interconnected and thus retain their elastic properties.
The following table shows the structural formulas of various basic polymers with additional information in the linked knowledge articles:
| Rubber | Repetition unit (simplified) | Main chain |
|---|---|---|
| NR – Natural rubber | –[CH₂–C(CH₃)=CH–CH₂]ₙ– | unsaturated |
| CR – Chloroprene rubber | –[CH₂–C(Cl)=CH–CH₂]ₙ– | unsaturated |
| NBR – Nitrile-butadiene rubber | –[CH₂–CH=CH–CH₂]ₘ–[CH₂–CH(CN)]ₙ– | unsaturated |
| HNBR – Hydrated nitrile rubber | –[CH₂–CH₂–CH₂–CH₂]ₘ–[CH₂–CH(CN)]ₙ– | largely saturated |
| EPDM – Ethylene-Propylene-Diene rubber | –[CH₂–CH₂]ₖ–[CH₂–CH(CH₃)]ₘ–[Dien]ₙ– | saturated |
| FKM - Fluororubber | –[CH₂–CF₂]ₘ–[CF₂–CF(CF₃)]ₙ– | saturated, fluorinated |
| VMQ – Silicone rubber | –[Si(CH₃)₂–O]ₙ– | Silicon-oxygen |
| TPE-S (e.g. SEBS) | PS–[Ethylene/Butylene]–PS | Blockcopolymer |
What does the structural formula reveal about the properties?
The main chain determines how well an elastomer ages and what temperatures it can withstand. Natural rubber (NR) and nitrile-butadiene rubber (NBR) contain double bonds in the main chain, are therefore more susceptible to weathering and ozone and tend to form fine cracks under stretching. Ethylene-propylene-diene rubber (EPDM) has a saturated main chain and is therefore ozone and weather resistant. Silicone rubber (VMQ) is suitable due to a silicon-oxygen chain with a particularly wide temperature range.
The side groups determine how polar an elastomer is and how strongly it foams in oils. In nitrile-butadiene rubber (NBR), the polar nitrile group provides the typical resistance to mineral oils. More acrylonitrile improves the oil resistance of NBR, but makes the material stiffer in cold temperatures. In fluorocarbon rubber (FKM), the fluorine atoms shield the carbon chain and make the material resistant to many chemicals.
The crosslinking bridges determine how well an elastomer returns to its original shape after deformation. Peroxide-crosslinked EPDM is linked by direct carbon-carbon bonds and therefore has a lower residual compressive deformation than conventionally sulfur-crosslinked EPDM. The longer sulfur bridges in sulfur-crosslinked EPDM solidify when heated, resulting in the component remaining partially deformed.
