The rubber process analyzer was commercialized by Monsanto Rubber Instrument and Equipment company in the early 1990s. In its initial version, the RPA was able to perform only oscillatory-type tests, which varied in frequency and strain. Later, it offered a controlled strain–stress relaxation test. Only recently and not for all models has a controlled steady shear test been made available. Using this type of test, the instrument has successfully measured steady shear viscosity with high repeatability without correction. The results fit well with other rheometers or viscometers when no-slip conditions are ensured. The closed-boundary configuration prevents edge fracture, as commonly experienced with open-boundary rheometers (DMA) on high-viscosity, high-elasticity materials. A comparison of results using grooved dies (no-slip) and polished dies (slip) readily provides wall slip velocity under constant pressure. The results of wall slip versus shear stress follow a power-law function per the Navier slip law [F(V) = −k(Vr)e]. This method separates the shear rate from pressure effects on wall slip. It questions pressure-driven flow instruments (capillary rheometer), which use pressure measurements for shear stress calculations, and prevents an easy and controlled change of the die surface roughness.

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