Non-Linear Viscoelasticity Of (Unvulcanized) Natural Rubber, Derived Materials And Compounds

Natural Rubber (NR) is by far the most important elastic material but its  production specifications remain relatively poor when compared to synthetics elastomers. The so-called technically specified NR grades (i.e. TSR) are indeed characterized with respect to basic rubber test techniques, such as the Mooney viscosity, the (Wallace) Plasticity (Po) and the Plasticity Retention Index (PRI) but high elasticity grades like ribbed smoked sheets (RSS) are classified with respect to visual inspection criteria only. In addition NR exhibits by nature more variation than synthetic polymers, owing to inevitable production differences from around fifty different clones of Hevea Brasiliensis, which are further enhanced by climate, soil and other local affects, as well as seasonal effects on tapping. Quite paradoxically, Natural Rubber (NR) is nevertheless used in highly demanding applications such as tires, conveyor belts, hoses and other engineered parts. Such a situation clearly underlines the capabilities of current rubber engineering practices in compensating the variations of NR grades but calls also for a better understanding of the origin of the unique properties of Natural Rubber.

Most rubber processing operations occur at high rate of strain, and therefore it is essentially the non-linear viscoelastic response of rubber materials that is of interest. This prompted the development of advanced investigation testing, such as large amplitude oscillatory strain (LAOS) experiments, which over the last decade were developed into a powerful technique to document the non-linear viscoelastic behavior of polymer materials. The purposes of the presentation are first to briefly review and discuss the standard rheological tests for characterizing gum NR grades, second to describe the equipment, test protocols, data treatment and results modeling necessary to apply non-linear viscoelastic testing,  third to describe and discussed a number of LAOS experimental results obtained on various grades of Natural Rubber, on a few chemically modified NR materials and on several carbon black filled NR compounds.