Sealing technology manual for the process industry - Basic materials testing

In sealing technology, the N and M processes are mainly used. The micro hardness measurements on seals are not directly comparable to the IRHD values, procedure N and the Shore A values on test plates. Volume change measurement Seals come into contact with a variety of media such as liq- uids and gases which influence the elastomer materials in different ways. A distinction is made between the chemical and physical effects of media. If an elastomeric compound is chemically affected, the properties are irreversibly changed. The splitting of the cross-linking points between the molecular chains leads to a softening and loss of elas- ticity of the material concerned. The formation of additional cross-linking points entails hardening leading to embrittle- ment of the material. Certain media can also attack the mo- lecular chains of the elastomer and destroy them. Physical processes can be roughly divided into two groups: a. Absorption of the medium by the elastomer b. Extraction of soluble components of the mixture from the elastomer Hardness, compression set (DVR) or the volume change of a material in use are important parameters which affect the sealing function but also the lifetime of the seal. Precise statements about the behavior of the material are there- fore essential. Freudenberg uses a variety of testing proce- dures and methods for this. Hardness tests One of the most important parameters in rubber technology is the hardness. The nominal hardness is specified for all Freudenberg materials in the materials designation before the base elastomer (e.g. 70 EPDM 291) and determined using various standardized procedures. In the process, the param- eter which is measured is always how far a given test speci- men that penetrates into the material under a defined force. Shore A pursuant to DIN 53 505 – The method according to DIN 53 505 was developed by Albert F. Shore in the 1920s and is the most commonly used method for measuring hardness in elastomers. The surfaces of the test specimen must be flat and coplanar. The value is read after a holding time of three seconds. The test according to Shore A is used mainly in materials development and in the clearance of mixture batches. It can only be used on molded parts if they have a sufficient thickness and flat supporting surfaces for the measurement. Shore A is suitable for hardnesses be- tween 10 and 90. Above 90 Shore, Shore D measurements must be made, which, however, are not applied in case of elastomers. IRHD pursuant to DIN ISO 48 – For hardness tests on seals, the IRHD (International Rubber Hardness Degree) pursuant to DIN ISO 48 is applied. Four procedures are described in this standard: 1.N (for hardnesses between 30 and 95 IRHD) 2.H (for hardnesses between 85 and 100 IRHD) 3. L (for the range from 10 to 35 IRHD) 4.M (micro hardness for the testing of small or thin-walled samples) BASIC MATERIALS TESTING APPENDIX PRODUCT PORTFOLIO MATERIALS RESISTANCE & PURITY ENGINEERING PLASTICS ELASTOMERIC MATERIALS TECHNICAL FUNDAMENTALS INTRODUCTION 21 Shore A pursuant to DIN 53505 – The method according to DIN 53505 was developed by Albert F. Shore in the 1920s

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