Sealing technology manual for the process industry

Compression set measurement/elasticity The compression set refers to the ability of a material, after a certain time of contact with the medium in question, to generate a sufficient restoring force on the sealing surfaces and thus ensure the tightness. In a newly manufactured seal, the compression set is a measure for assessing the vul- canization and the degree of cross-linking. The lower the compression set value, the better the cross-linking and the longer the expected life of the mixture. It should be noted that the compression set is not an absolute value, but must always be considered in comparison with the initial or tar- get values. The compression set is determined according to DIN ISO 815 using the following formula: DVR [%] = (h0 – hs ) / (h0 – h1 ) x 100 % With: h0 = height of the test specimen h1 = height of the compressed specimen hS = height of the relieved specimen A number of conditions must be met during the measure- ment. Thus, the time between the vulcanization of the test specimen and the compression set test should be at least 16 hours. The compression is defined and generally amounts to 25%. The sample is stored in its apparatus in air (or a me- dium) for a predetermined period of time at a specified tem- perature and removed from the apparatus after expiry of the testing period. 30 minutes after removal, the thickness is measured again. Please note that compression set values are only comparable if the parameters are identical: • Deformation (default: 25%) • Duration of the deformation (default: 24 or 72 hours) • Temperature during the deformation • Storage medium (standard: air) • Form of the test specimen Density measurement The density of vulcanized rubber is a specific characteristic. Through a simple, expeditious measurement of this para- meter, first conclusions regarding the compound used can be drawn. In the preparation of a mixture, the density value serves as an indication whether all components of the mix- ture were added properly. Similarly, the density measure- ment precedes all other tests in the analysis of unknown mixtures in order to limit the number of possible materials. In general, the density is measured using the buoyancy-flo- tation method. According to the “Archimedean principle”, a body experiences a buoyancy force when immersed in a liq- uid. This is calculated from the difference of the weight in air and the weight in liquid. Since the buoyancy force is equal to the weight of the fluid volume displaced by the body, the volume of the test specimen, regardless of its ge- ometric shape, can be derived. The specific density can be calculated on the basis of the volume determined in this way and the weight in air. ρ = [GL / (GL – GF )] x ρF With: ρ = density to be determined GL = weight in air GF = weight in fluid ρF = density of the fluid (in case of water approx. 1 g/cm3 ) F F Testing the compression set APPENDIX PRODUCT PORTFOLIO MATERIALS RESISTANCE & PURITY ENGINEERING PLASTICS ELASTOMERIC MATERIALS TECHNICAL FUNDAMENTALS INTRODUCTION 23

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