Thermal degradation of SBS in bitumen during storage: Influence of temperature, SBS concentration, polymer type and base bitumen

One of the major issues that manufacturers and suppliers of styrene-butadiene-styrene (SBS) modified bitumen face is erosion in its properties during storage and transportation. If appropriate controls are not in place, properties of SBS modified bitumen can erode significantly during storage and transportation, resulting in substandard quality of roads when constructed using such products. We report a comprehensive study of the effect of storage temperature, SBS concentration, polymer type and base bitumen on the properties of polymer modified bitumen during storage at elevated temperatures in sealed aluminium tubes. Our results show that storage temperature, SBS concentration and polymer type play a major role in the observed property erosion, while type of base bitumen has a lesser effect. The samples were stored in sealed aluminium tubes at ternperatures of 150 degrees C, 180 degrees C and 210 degrees C for up to 21 days. At 150 degrees C, the properties of PMB's remained intact even after storing for 21 days. But, significant erosion in conventional and rheological properties of SBS modified bitumen takes place within the first 3 days of storage at temperatures >= 180 degrees C. At 210 degrees C, the PMB properties degrade to the base bitumen values within 1 day. Gel permeation chromatography and rheological results clearly show degradation of SBS molecules. The property loss is most significant when the SBS concentration is between 3% and 6%, in the steep middle region of the S-curve (softening point vs. polymer concentration). 3% SBS content is close to the network formation threshold of the polymer molecules in bitumen. Conventional polymer grades such as D1101, D1184, LG411 and Elvaloy 4170 showed maximum erosion in properties, while D1192 grade (with higher vinyl content > 35%) showed significantly lower drop. The newer polymer grade introduced by Kraton, MD0243, remained completely stable at 180 degrees C even after 21 days. Among PMBs with different base bitumens, noticeable differences were observed only when the SBS concentration was in the steep middle region of the S-curve (3-6%). We have discussed all the results in the background of the S-curve, there by highlighting the major role played by SBS concentration in property erosion during storage. Our results have important implications for storage and handling guidelines for polymer modified bitumens.