# Bitumen and Bitumen Modification: A Review on Latest Advances
## 3 Bitumen Chemical Modifiers
### Sulfur Modifier
An \"anti-gelling\" adjuvant with the formula R-X, where R is a C\({}_{2\text{-}50}\) monovalent organic radical and X is an acidic function (carboxylic, sulfonic, phosphoric) has been used. Other solutions were proposed by [PERSON] et al. and [PERSON] et al. [160; 161]. Many studies have shown that the use of sulfur as modifier improves the properties of some PmBs. In this respect it has been shown that sulfur enhances the storage stability, the elasticity, the deformation resistance, and some rheological properties of the PmBs [66; 119; 162]. From the rheological point of view, PmBs added with sulfur show a \"more elastic\" behavior compared to the ones without sulfur [66; 118; 163]. However, the use of sulfur as a chemical modifier has shown some drawbacks. First of all, the reaction of sulfur with polymer modifiers is based on the chemical reactions between sulfur and the double bonds of the polymer, so the use of sulfur is limited to PmBs modified by unsaturated polymers. The risk of obtaining nonhomogeneous PmBs during the polymer and sulfur addition into bitumen, is due to the difficulty of distributing the sulfur throughout the mixture. Moreover, local over-vulcanization of the polymer was detected. Replacing elemental sulfur with a more bitumen soluble polysulfide like di-hydrocarbyl polysulfide, represents a possible solution to this problem [164]. However, the emission of the hazardous health gas hydrogen sulfide during preparation of the bitumen/polymer mixes is the main drawback to the use of elemental sulfur. Hydrogen sulfide could be generated during sulfur vulcanization through the abstraction of hydrogen atoms in both bitumen and polymer modifiers, especially at high temperatures [165; 166; 167]. Some researchers argued that the gaseous emission is relatively small and most manufacturers know how to deal with the associated risks and dangers. The use of other sulfur-based agents as alternatives to elemental sulfur was introduced to eliminate, or at least reduce, such emissions. Compounds like benzothiazole sulfonamides, di-thiocarbamates, thiuram sulfates, morpholine sulfates, and caprolactam sulfates. Zinc mercaptobenzothiazole, zinc oxide, and dithiodimproline, as well as compositions without elemental sulfur, such as mercaptobenzothiazole, zinc oxide, mixed polythionorpholine zinc 2-mercaptoethanolhizole, and dithiodimproline, were reported [168; 169; 170]. Nevertheless, the undesired formation of hazardous hydrogen sulfide has been observed also using this sulfur source. Shell Oil Company [171] used a disulfide that does not release sulfur at high temperature mixing. It has been claimed that the disulfide, instead of a conventional source of free sulfur, enhances compatibility between the polymer and bitumen. Diaryl disulfides were firstly proposed. However, all these additives are so expensive that they have not, in general, been used commercially. Finally, the poor recyclability of sulfur PmBs is another problem with sulfur vulcanization. All these drawbacks limit the application of sulfur vulcanization in PmB.