A cross-linking agent is a substance that is used to create chemical bonds between polymer chains, resulting in the formation of a three-dimensional network. This process is known as cross-linking and it is commonly used in the manufacturing of various materials such as plastics, rubbers, and resins to improve their mechanical properties like strength, durability, and thermal stability.

Cross-linking agents can be organic or inorganic compounds that have functional groups capable of forming covalent bonds with the polymer chains. These agents can be added during the polymerization process or applied to the finished material to induce cross-linking.

Some common examples of cross-linking agents include peroxides, isocyanates, sulfur compounds, and silanes. The selection of a cross-linking agent depends on the specific requirements of the material and the desired properties of the final product.