## 3 Sulfur Cake
### Processing and Modification Technologies
#### 3.2.1 Chemical Modifications  
The chemical modification of sulfur cake significantly enhances its structural and functional properties, primarily through sulfur polymerization and interaction with inorganic components such as calcium hydroxide and calcium sulfate. Sulfur polymerization occurs via two dominant mechanisms: cyclization and crosslinking. The data in Table 4 provides the chemical modification mechanisms and their effects on the features of the sulfur cake. Elemental sulfur undergoes ring-opening polymerization, producing linear polysulfide chains at certain (160-180 \({}^{\circ}\)C) temperatures. These chains can be stabilized through inverse vulcanization, a process where unsaturated organic crosslinkers (e.g., di- or tri-vinyl monomers) react with sulfur to form thermally and chemically stable copolymers [38]. Figure 2: Schematic outline of three major techniques for modifying sulfur cake, including thermal treatment, chemical purification, and additive-assisted enhancement. The mechanical properties (specifically crosslinking density) are very dependent on the crosslink structure. For example, the crosslinkers with stiff aromatic center cores produce sulfur polymers which have a stronger tensile load (up to 5.1 MPa) and a higher thermal stability (glass transition temperature (Tg) exceeding 100 \({}^{\circ}\)C) [39] than the flexible compositions utilizing aliphatic crosslinkers, which result in a more elastic material. According to measurements, this ratio between sulfur and crosslinker should be from 70:30 to 80:20 by mass, which optimizes the rigidity of the structure and process ability. When residual calcium hydroxide (Ca(OH)\({}_{2}\)) and calcium sulfate (CaSO\({}_{4}\)) are present in sulfur cakes, they are used in two possible ways, predominantly as a source of neutralization and also as a type of chemical reactant in a modification reaction. Calcium hydroxide may also be combusted with hydrogen sulfide (H\({}_{2}\)S) or organosulfur intermediates to form calcium sulfides, which fix sulfur species and inhibit emissions [40]. In addition, CaSO\({}_{4}\) can facilitate thermochemical sulfate reduction (TSR) at elevated temperatures (250-280 \({}^{\circ}\)C), which creates agent sulfide, such as H\({}_{2}\)S or polysulfide-free radicals [42]. These radicals are capable of spurring on additional sulfur cross-linking reactions which affect the end polymer network. \begin{table} \begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Types** & **Mechanism** & **Operating T\({}_{\circ}\)C** & **Polymer Yield/Product Features** & **Optimal Composition/Ratio** & **Reaction Products** & **Limitations** & **Ref.** \\ \hline Sulfur polymerization (cyclization) & Ring-opening of S\({}_{\text{g}}\) to form linear polyvinylsalide chains & High linear polyvinylsalide the chains & High linear polyvinylsalide the thermally unstable unstable & Linear sulfur system & Poor thermalsalide the chains (-S-S-)\({}_{\text{n}}\) & Poor thermalsalide strength, oxidation-sensitive & [38] \\ \hline Sulfur polymerization (crosslinking/inverse vulcanization) & Radical-mediated copolymerization with organic crosslink & 180–220 & Thermally stable copolymers with Tg \(>\) 100 & Crosslinked polysalide 80:20 wt\% & Requires precise crosslinker (stable) & Requires precise crosslinker design; may emit residual monomers & [39] \\ \hline Sulfur polymerization (crosslinking/inverse vulcanization) & Radical-mediated copolymerization with organic crosslink & & & & & \\ \(\text{Ca(OH)}_{2}\) & mediated copolymerization with organic crosslink & & & & & \\ \(\text{Ca(ON)}_{2}\) & \(\text{Molecular}\) crosslinkers (e.g., divinylbenzene) & & & & & \\ \(\text{Na(ON)}_{2}\) & \(\text{Molecular}\) crosslinkers (e.g., divinylbenzene) & & & & & \\ \hline \hline \end{tabular}