Oscillatory disc rheometer

Abstract

Natural rubber (NR)-based elastomer, by virtue of its amenability to yield shear modulus at 3.5 kg/cm2 shear stress as low as 1.6–2.0 kg/cm2, is the most preferred elastomer for the realization of flex seals that are used for the thrust vector control of large rocket motors. Despite the utmost care taken during compounding, there exists a high degree of uncertainty in achieving required shear modulus primarily because of complexity involved in compounding to obtain high degree of homogeneity of the NR compound. Assessing the quality of NR compound is a tedious and time-taking process since it involves the preparation and testing of standard quadruple lap shear (QLS) specimens. Any non-conformance in shear modulus calls for fine-tuning of NR formulation; remaking of QLS specimens to recheck shear modulus leads to huge time delay and wastage of materials. The authors have meticulously generated oscillatory disc rheometer (ODR) data in comparison with QLS test results for various NR compounds and introduced ODR as an effective quality assurance tool. This article demonstrates how, based on ODR data, shear modulus can be predicted, thereby corrective actions can be taken in NR compound quickly and the appropriate vulcanization scheme can be prescribed for the given NR compound, which will prevent reversion and ensure optimum vulcanization, thereby eliminate the causes for shear modulus increase during storage of flex seals.