# Bitumen and Bitumen Modification: A Review on Latest Advances
## 3 Bitumen Chemical Modifiers
### Sulfur Modifier  
The early applications of sulfur as a modifier involve its use in large quantities without other materials, such as polymers [115; 116]. Some commercial products, named as \"sulfur extended bitumen\" (e.g., Shell Thiopave(r), Shell Brands International AG, Baar, Switzerland [52]) are obtained using this methodology, in which many parameters should be considered. Among the factors that control the reaction between sulfur and bitumen, the mixing temperature plays a determinant role. Below 140 \({}^{\circ}\)C sulfur can be incorporated into the bitumen molecules or can form hydrogen sulfide via dehydrogenation reaction. Higher temperatures allow C-S bond formation promoting a dynamic vulcanization of bitumen. It induces bridge formation within aromatic and naphthenic components modifying the chemical composition and colloidal structure of bitumen. Some authors also suggested that sulfur could self-polymerize when added to the binder [156]. However, nowadays, sulfur is added in very small quantities to bitumen where the polymers are the main modifiers. Some studies [120; 121] have also shown that sulfur vulcanization made the PmBs more susceptible to oxidative aging and concluded that it is not a good idea to use sulfur as a sole additional modifier. The first studies that used a small quantity of sulfur to increase the polymer/bitumen interaction date from the early 1970s when [PERSON] proposed a method for preparing a rubber-modified bitumen. He blends bitumen with rubber (natural or synthetic) at 145 to 185 \({}^{\circ}\)C and after slightly reducing the temperature (125 to 160 \({}^{\circ}\)C) added 0.3 to 0.9% (by the weight of rubber) of sulfur. A free-radical vulcanization accelerator is also added to aid vulcanization [157]. Later on, [PERSON] [158] proposed a different procedure starting with the preparation of a homogeneous PmB by stirring bitumen and SBS for 2 h at 130 to 230 \({}^{\circ}\)C and then adding 0.1 to 3 w/w% of sulfur and stirring for another 20-60 min. However, the exact reaction of sulfur-polymer-bitumen system is not completely clear. It was supposed that sulfur crosslinks the polymer molecules and links polymer and bitumen through sulfide and/or polysulfide chemical bonds. This assumption comes from the well-known process of vulcanization of unsaturated rubbers. Further investigation on this topic was carried out by [PERSON] et al. [117; 118]. They prepared a low-viscosity homogeneous bitumen/SBS/sulfur blend by heating it to a temperature of 120 \({}^{\circ}\)C. This limits the occurrence of reactions between bitumen and sulfur or the vulcanization of SBS. Finally,an increase in the temperature in the range of 150 to 180 \({}^{\circ}\)C allow these reactions to start. Possible evidence of this fact is that, unlike the two binary systems, an increase of the torque to maintain a constant rpm for the ternary bitumen/SBS/sulfur blend was necessary. The authors also hypothesized a vulcanization of both the unsaturated rubber and the functionalized bitumen components by the aid of sulfur, which creates interconnections between the polymer and bitumen. It has been claimed that these connections increase blend storage-stability by linking the polymer with the bitumen components through covalent chemical bonds. Later, [PERSON] et al. [159] reported more indirect evidence for vulcanization. In this study a detailed rheological characterization of bitumen/SBS/sulfur and bitumen/SB/sulfur blends was performed. The importance of the homogeneity of the blend before sulfur addition has been also highlighted. In this respect a fast vulcanization, and consequently a useless material, could be observed if a too early sulfur addition is carried out. An appropriate sulfur bridge distribution is also important for a good chemical compatibility. To avoid freezing the blend's morphology at an earlier stage, good polymer dispersion and swelling must be achieved before network formation. Finally, the risk of gelation must be avoided.