Observations of Die Swell Behavior of Filled Elastomers Measured Automatically with a New “Die Swell Tester”

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Abstract
A new instrument has been developed to measure the elastic memory of rubber or plastic compounds by automatically indicating the die swell of the material when a small amount is extruded. It has the potential to be used as a quality control instrument in a manufacturing plant. The new Die Swell Tester measures the time (in sec.) necessary to extrude a fixed length of extrudate at a constant volumetric flow rate. The time measurement is directly proportional to the die swell when expressed as an area ratio, and it can be made equal to the die swell by choosing the extrudate length judiciously. Furthermore, it is measured electronically and has a digital readout in hundredths of a second. Measurements made on tire tread compounds using the Die Swell Tester correlate well with die swell measurements made using a more conventional gravimetric method. The die swell of filled elastomers generally decreases with increase in filler loading and structure. When a composite of fixed filler loading is mixed, its die swell first increases due to the dispersing of filler, but it eventually reaches a maximum and begins to decrease. The composite viscosity, on the other hand, decreases monotonically as it is mixed. These variations can be explained by taking into account changes in the amount of rubber occluded by filler agglomerates and simultaneous degradation of free rubber during the mixing process. The time to reach the peak in die swell is a simple macroscopic measure of the filler dispersion rate for a given elastomer-carbon black composition.

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