Please activate JavaScript!
Please install Adobe Flash Player, click here for download

Sealing technology manual for the process industry

Temperature and fat resistance The temperature significantly affects the physical proper- ties of rubbery-elastic materials. If the modulus of elastici- ty (E-modulus/elastic modulus) is considered in relation to the temperature, a constant E-modulus at a high level and at a low temperature is visible, which identifies the glassy state. As the temperature rises, the area of the glass tran- sition is entered where the elastic modulus drops sharply. A constant level follows subsequently. The rubbery-elastic plateau, where the material displays elastic behavior, rep- resents its application range as a sealing material. If the temperature rises further, the modulus of elasticity drops, indicating aging or viscous flow of the material (diagram 1). The transition from the rubbery-elastic state to the glassy state is particularly important because in many cases it represents the lower limit of the possible operating tem- perature. Depending on the stress, the actual operating limit for the specific application is slightly higher (at the beginning of the glass transition range) or slightly lower (at the beginning of the glassy state). Simply stated, this means that in static applications elastomers can be used at lower temperatures than in dynamic applications. Diagram 2 shows an overview of the thermal application ranges of various types of elastomers. The grey areas indicate tem- perature application ranges to which standard types may only be exposed. Although fat causes problems for many sealing materials, it is in the traditional sense not an aggressive medium. In processing plants for the food industry, about 70% of the seals used are made of EPDM. Only if EPDM cannot be used, as is the case with products with a high fat content, must an alternative be found. The use of other materials may have disadvantages such as a change of the cleaning cycle, higher costs and additional storage overheads. In the past, EPDM was not used at all in the dairy industry. Today’s technologically advanced EPDM materials demonstrate a significantly better performance and are quite usable in many applications – despite fatty products. If EPDM seals are to be used, there are different upper temperature limits for various kinds of products containing fat: • Milk with 3.5% fat content can be processed up to a tem- perature of +80 °C (+176 °F). • For cream with 30% fat content, the temperature limit is already reached at +35 °C (+95 °F). • Butter with 82% fat content can only be processed up to a maximum of +8 °C (+46 °F). The diagram on page 35 illustrates the use of EPDM as a function of fat content and temperature. EPDM is applicable below the line; above it Fluoroprene® XP should be used if there are no restrictions regarding the cleaning process. –100 –50 0 50 100 150 200 250 300 Rubbers Temperature in °C FFKM VMQ FKM HNBR NBR EPDM Diagram 2 application range threshold range Materials resistance and purity 56 –100 –50050100150200250300

Pages Overview