Storage stability is one of the main problems of polymer modified asphalts and the use of elemental sulfur is probably the most effective solution, even if not free from inconveniences. Here ten commercially available vulcanization accelerators were tested as potential alternative to sulfur and added to asphalts modified with SBS. Mixing asphalt and polymer at 180 [degrees]C, out of the 10 products two gave performance comparable to sulfur, while some of the other additives became effective only at mixing temperature of 210 [degrees]C and a few did not work at all. A qualitative correlation between the thermal stability and the effectiveness of the additives was attempted through the thermogravimetric analysis.
Polymer modified asphalts
Polymer-modified asphalts (PMA) were developed to enhance the performance and durability of asphaltic materials. Modification is normally achieved by simple mechanical dispersion of the polymer in the molten asphalt under high shear. Almost all common polymers have been tried as asphalt modifiers, but only a few are currently used in the industrial practice. This is because there is a relatively long list of requirements that have to be satisfied, including performance, costs and storage stability. The latter point is related to the thermodynamic instability of the asphalt/polymer blends which tend to separate into two macroscopically distinct phases during storage at high temperature under static conditions. The two phases are often identified as an asphaltene-rich phase (ARP) and a polymer-rich phase (PRP) [1-4] and can be easily observed by fluorescence microscopy (FM). Since PRP originates from a selective swelling of the polymer by the lighter asphalt components, it contains the major part of the light aromatics and appears bright in FM. In contrast, the not-fluorescing asphaltenes remain in ARP which constitutes the dark phase.
The problem of storage stability has been the subject of many studies during the last decades and among a long list of proposed solutions, probably the most effective one when using unsaturated thermoplastic elastomers like poly(styrene-b-butadiene-b-styrene) (SBS), is the use of sulfur as vulcanizing agent. Sulfur was used in asphalts even before the appearance of the first synthetic polymers , but the first patents suggesting its use to improve the storage stability of PMAs appeared in the seventies of the last century. In 1971 Petrossi  patented a process for preparing rubber-modified asphalt by adding sulfur and a catalytic quantity of a free-radical vulcanization accelerator. Maldonado et al. [7,8] suggested a similar recipe and added a few considerations about the procedure and the supposed effect of sulfur. The authors recommend adding the sulfur after the preparation of a homogeneous PMA and then to stir it further for 20-60 min to allow the sulfur to react. A premature loading of sulfur would have the inconvenience of "freezing" the system before having the desired morphology of the blend. Sulfur is supposed to create covalent linkages both between polymeric chains and between polymer and asphalt molecules. Therefore, it is not a "compatibilizer" intended as a compound that reduces...