On Wednesday, November 20th 2019, Ms. Akansha Rathi has successfully defended her PhD thesis entitled 'Investigating Safe Process oils for Tire Tread Application'.
A tread compound is a complex mixture of rubber, process oil, fillers and a vulcanization system. The glass transition temperature (Tg) of the rubber is influenced by each of the constituents in the tire tread formulation. In this work, a combination of Dynamic Mechanical Analysis (DMA), Broadband Dielectric Spectroscopy (BDS) and Positron Annihilation Lifetime Spectroscopy (PALS) are used to estimate the Tg of the compounds. The DMA method can estimate the Tg of the compound at a fixed frequency (1 Hz). The BDS method can estimate the Tg at a wide range of frequencies (10-1 to 106 Hz) allowing the additional identification of smaller-scale motions. The PALS method can estimate the free volume at Tg of the rubber compounds.
The addition of process oil can improve the low temperature properties (decrease of Tg) of the tread compound. The process oil consists of low molecular weight molecules; its presence between the rubber chains has the effect of pushing the chains apart, resulting in a higher free volume. The most commonly used processing aid is a mineral oil-based Treated Distillate Aromatic Extract (TDAE). Nowadays, there is an increasing trend towards bio-based process oils due to increasing sustainability awareness. One such newly developed oil is the Vivamax 5000 (V5000) which is more polar than the conventional TDAE.
The overall goal of this thesis is to understand the distribution of process oil in typical tire tread compounds: Solution Styrene Butadiene Rubber (S-SBR) / Butadiene Rubber (BR) blend compounds. Using the BDS method, it was possible to measure a shift in the Tg of the individual S-SBR and BR components of the blend on addition of the TDAE or V5000. From these measurements, it was noted that the TDAE has a preference for the BR phase of the S-SBR / BR blend. While the V5000 tends to improve the overall miscibility in the blend which could be confirmed by the PALS measurement of the change in the free volume at Tg.
The basic knowledge developed from this study was further used to understand the 3 layer (chemically bound layer, physically bound layer and unrestricted layer) reinforcement mechanism of the silica-bifunctional silane filled blends. Finally, a method to predict Wet Skid Resistance (WSR) has been developed using the high frequency (~106 Hz) BDS temperature sweep measurements.