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Sealing technology manual for the process industry

Einführung Every volume change, whether swelling or shrinkage, is as- sociated with changes in the physical properties such as hardness, elasticity, tensile strength and elongation at break. Therefore, the determination of the volume change after a defined storage period (time and temperature) is a parameter in order to measure and evaluate the stability of elastomeric compounds in the media used. Note: In the food industry, not only must the production media (e.g. beer, mineral water, dairy products etc.) be tested for their compatibility with the sealing material, but so too must the cleaning and sterilization media used in the pro- cesses. Sequence of the tests according to DIN ISO 1817: 1. The volume of the test specimen (the seal or a portion thereof) is determined 2. The test specimen is stored in the medium according to the standard or to customer specifications 3. At the end of the storage period (and after cooling down), the volume of the specimen is measured again 4. The result is expressed as a percentage of the initial state Both changes can take place separately but also in parallel. These processes are detected by measuring the change in vol- ume occurring thereby. If an increase in volume is measured, the absorption of medium takes precedence, while a decrease in volume means a predominant loss of mixture components. Swellingprocesses are usually reversible, which means that the absorbed medium can also leave the elastomer again when switching to another product. The size of the volume change depends on five factors: 1. Type of medium 2. Composition of the elastomeric compound 3. Interaction parameters (temperature, time) 4. Geometric form (thickness) of the seal 5. Stress condition of the seal (in the case of stretched parts, the impact is greater, in the case of compressed parts lower) Due to the cross-linked structure of the elastomers, swelling is limited, i.e. it no longer changes after reaching a limit value. Technical fundamentals 22

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