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. It is strongly associated with the production of thermosetting polymers. Curing can be initiated by heat, radiation, electron beams, or chemical additives. Characteristically curing entails an increase in viscosity or hardness.[1]

Structure of a cured epoxy glue. The triamine hardener is shown in red, the resin in black. The resin's epoxide groups have reacted with the hardener. The material is highly crosslinked and contains many OH groups, which confer adhesive properties.

Curing is initiated in many ways. To quote from IUPAC: curing "might or might not require mixing with a chemical curing agent."[2] 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.[1]

Curing induced by additives

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 mechanical and properties of the material.[3]

General representation of the chemical structure of vulcanized natural rubber showing the crosslinking of two polymer chains ( blue and green) with sulfur (n = 0, 1, 2, 3 …).

Paints and varnishes commonly contain oil drying agents; metal soaps which catalyse 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.

Simplified chemical reactions associated with curing of a drying oil. In the first step, the diene undergoes autoxidation to give a hydroperoxide. In the second step, the hydroperoxide combines with another unsaturated side chain to generate a crosslink. [4]

Curing without additives

In the case of concrete, curing entails the formation of silicate crosslinks. The process is not induced by additives.

In many cases, the resin is provided as a solution or mixture with a thermally-activated catalyst, which induces crosslinking but only upon heating. For example, some acrylate-based resins are formulated with dibenzoyl peroxide. Upon heating the mixture, the peroxide converts to a free radical, which adds to an acrylate, initiating crosslinking.

Some organic resins are cured with heat. As heat is applied, the viscosity of the resin drops before the onset of crosslinking, whereupon it increases as the constituent oligomers interconnect. This process continues until a 3-dimensional network of oligomer chains is created – this stage is termed gelation. In terms of processability of the resin this marks an important stage: before gelation the system is relatively mobile, after it the mobility is very limited, the micro-structure of the resin and the composite material is fixed and severe diffusion limitations to further cure are created. Thus in order to achieve vitrification in the resin, it is usually necessary to increase the process temperature after gelation. Cure monitoring methods give a significant insight to the chemical process and define process actions towards achieving specific quality indices of the cured resin systems.

When catalysts are activated by ultraviolet radiation, the process is called UV cure.[5]

References

  1. ^ a b Pham, Ha Q.; Marks, Maurice J. (2012), "Epoxy Resins", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a09_547.pub2
  2. ^ "curing". IUPAC Goldbook.
  3. ^ James E. Mark, Burak Erman (eds.) (2005). Science and technology of rubber. p. 768. ISBN 978-0-12-464786-2.CS1 maint: Extra text: authors list (link)
  4. ^ Ulrich Poth (2002), "Drying Oils and Related Products", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a09_055CS1 maint: Uses authors parameter (link)
  5. ^ Gregory T. Carroll, Nicholas J. Turro and Jeffrey T. Koberstein (2010) Patterning Dewetting in Thin Polymer Films by Spatially Directed Photocrosslinking Journal of Colloid and Interface Science, Vol. 351, pp 556-560 doi:10.1016/j.jcis.2010.07.070

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