LONG-TERM STABILITY OF RUBBER HARDNESS REFERENCE BLOCKS

Abstract- Methods for hardness measurement on elastomers are characterized by a set-up where indenters with various defined geometries are pressed under defined test forces into the surface of testing material. The hardness value represents the resistance of the material to the rigid indenter and is calculated from the indentation depth of the indenter in the to be tested material. The test methods IRHD and Shore are the most common methods used in the field of rubber hardness measurements. Appropriate hardness measuring instruments are spread world wide. A periodic verification has to be carried out to assure the accuracy of these measuring instruments. Direct and indirect verification methods are recommended to be used. For the indirect method rubber hardness reference blocks are used. This indirect verification method is especially appropriate for the daily check of hand held rubber hardness testers. On the other side the direct verification of test force, depth measuring system and indenter geometry is suited for the periodic calibration of rubber hardness testers. For the rubber hardness reference blocks their accurate hardness reference value and the long-term stability are the most important characteristics. In this paper the long-term behaviour of hardness of rubber blocks which at present are available on the market and are made by different producers is presented. For the measurements the Shore and IRHD hardness standard devices in the PTB have been used. In order to determine the relationship between hardness and time under the specified conditions a regression analysis of the obtained measurement results was carried out. The results of this analysis can be used for the improvement of quality assurance systems in the field of rubber hardness measurements