A cross-linking agent is one of the important additives in the plastic industry. Cross-linking agents have an effect on the physical and mechanical properties of the polymers. In general, the curing or cross-linking process is an irreversible process. The cross-linking of polymer which is done by cross-linkers through covalent bond enables to control the physical and mechanical (i.e., impact and resistance of cracking) properties of the obtained polymer for commercial or scientific purposes. 

At the beginning of the entry, some important definitions as cross-link and cross-linking density were summarized. The types of cross-linking agents were discussed in detail in the entry. Then, some interesting items were discussed as there are cross-linking agents utilized in the plastic industry without external stimuli and others that need external stimuli (i.e., thermal or photo-irradiation, etc.), the latter of which are called latent curing agents.

 The common latent cross-linking agents based on imidazole are mentioned, and its working mechanism in the curing process is shown in the entry. The selection of the appropriate cross-linker was mentioned through discussion of the cross-linking agents based on the functional groups present on the polymer such as amine or anhydride groups. Consequently, the illustration of the function group should be present on the polymer needed to cure. For example, cross-linking agents based on polyamines were utilized with polymers that have epoxide, thiirane or methylene chloride, or ketone groups.

 The former cross-linking based on compounds have amino, the working mechanisms of each cross-linking agent were illustrated in detail. At the end of entries, cross-linking of important daily used plastic by different cross-linking agents was discussed such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PU), and polyepoxide (epoxy).