Under Steam And Pressure

Only a few elastomers are suitable for manufacturing molded parts that are resistant to hot water and steam. Processing these elastomers is often difficult and the molded parts sometimes need to be manually finished. The solution comes in the form of the new liquid silicone rubber ELASTOSIL® LR 3020/60.

Espresso machine in a café in New York City, USA: the Italian Achille Gaggia patented the principle of the portafilter espresso machine in 1938, whereby hot water is forced through the coffee granules at high pressure. This method of preparing coffee aims to extract the ingredients, aroma and flavor as fully as possible.

Espresso machines and fully automated coffee machines have to withstand a great deal in their lifetime: at a high pressure of up to 10 bar, they press boiling water of more than 100 °C through the ground coffee – and do so hundreds of times a day if used in the catering industry. Withstanding this level of permanent thermal stress requires the machines to be sufficiently robust – that applies to the housing as well as the tubing and seals. In practice, hot water boiler systems as used in district heating systems or for heat transfer in industrial plants even operate at water temperatures of up to 180 °C – and with gauge pressures of 15 bar and more.

“At higher temperatures, as is the case with hot water or hot, pressurized steam, the water molecules have so much energy that they chemically attack the silicon-oxygen bonds of the silicone polymer.”

Dr. Thomas Frese Applications Engineer at WACKER

Extremely hot water and steam affects rubber-elastic seals unlike virtually any other medium. This applies to organic elastomers as well as to silicones. Only a few materials withstand long-term exposure to highly pressurized water that is over 100 °C – the technical term for this is “medium temperature hot water systems” (MTHWS) – and with pressurized superheated steam. Poor resistance often causes molded seals to fail, ultimately resulting in the failure of the equipment in which the seals are integrated.

The rubber grades that can be used to manufacture MTHW- and steam-resistant elastomer molded parts include ethylene propylene diene (EPDM) rubber, perfluorinated rubber (FFKM) and specific types of fluorinated rubbers (FKM) optimized for contact with these media, and specialist solid silicone rubbers. For reasons of cost and environmental protection, seal manufacturers prefer to seek suitable alternatives to fluorinated materials.