Curing (chemistry)

Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term curing can be used for all the processes where starting from a liquid solution, a solid product is obtained. Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term curing can be used for all the processes where starting from a liquid solution, a solid product is obtained. During the curing process, single monomers and oligomers, mixed with or without a curing agent, react to form a tridimensional polymeric network. In the first part of the reaction branches molecules with various architectures are formed, and their molecular weight increases in time with the extent of the reaction until the network size is equal to the size of the system. The system has lost its solubility and its viscosity tends to infinite. The remaining molecules start to coexist with the macroscopic network until they react with the network creating other crosslinks. The crosslinking density increases until the system reaches the end of the chemical reaction. Curing can be initiated by heat, radiation, electron beams, or chemical additives. To quote from IUPAC: curing 'might or might not require mixing with a chemical curing agent.' Thus, two broad classes are (i) curing induced by chemical additives (also called curing agents, hardeners) and (ii) curing in the absence of additives. An intermediate case involves a mixture of resin and additives that requires external stimulus (light, heat, radiation) to induce curing. The curing methodology depends on the resin and the application. Particular attention is paid to the shrinkage induced by the curing. Usually small values of shrinkage (2-3%) are desirable. Epoxy resins are typically cured by the use of additives, often called hardeners. Polyamines are often used. The amine groups ring-open the epoxide rings. In rubber, the curing is also induced by the addition of a crosslinker. The resulting process is called sulfur vulcanization. Sulfur breaks down to forms polysulfide cross-links (bridges) between sections of the polymer chains. The degree of crosslinking determines the rigidity and durability, as well as other properties of the material. Paints and varnishes commonly contain oil drying agents; metal soaps which catalyze cross-linking of the unsaturated oils of which they are largely comprised. As such, when paint is described as drying it is in fact hardening. Oxygen atoms serve the crosslinks, analogous to the role played by sulfur in the vulcanization of rubber. In the case of concrete, curing entails the formation of silicate crosslinks. The process is not induced by additives.