How do coatings cure?

The inspector should be familiar with the types of curing mechanism of the coating he is working with. He should also be familiar with the correct curing and should be sure to check the coating manufacturers published Product Data Sheets for this information. A coating must be property of the applied coating cured before it is top coated and before it is placed into service. Premature failure will occur if the coating is top coated before the primer or intermediate have sufficiently cured. The uncured coating, if top coated, may blister because of entrapped solvents. This may result in lifting of coating film. Many coatings, such as coal tar, epoxies, have maximum recoat times. If the coating cures beyond the required period for top coating, the topcoat will not bound properly. It is especially critical in the applications of tank lining that the coating be sufficiently cured before being put into service. A differentiation must be made between “drying” and “curing. A coating may be dry to the touch and still may not be cured.

Following is the list of curing methods, together with the generic types of coatings using each method.

   Air oxidation	   Alkydes, epoxy, Ester
   Solvent Evaporation	   Vinyls, Chlorinated Rubbers, Bituminous (coal tar solution) Acylics.
   Chemical reactions	   Epoxies, Polyurethanes, Vinyl-Esters, Inorganic Zincs, (Hydrolysis)
   Heat Cure	   Silicones, High Bake Phenolics

The following is the description of each of the curing methods.

§  Air Oxydation

Coatings that cure by oxidation, or air drying, depends upon oxygen to penetrate the film after the solvents has evaporated. There is limit to the film thickness which can be achieved with this type of coating. Alkyds are examples of coating that cure by this method. If they are applied too thick, the solvent will evaporate from the surface, the oxygen will cure the top layer of the coating film, and it will be difficult for the solvent to evaporate from the lower layers of the coating. At the same time, oxygen will have difficulty penetrating the rest of coating film, and the coating will not cure properly.

Solvent Evaporation

Almost all coatings, unless they are 100 percent solids, will undergo solvent evaporation prior to curing. The solvent completely evaporates as the coating film curs, leaving the film of the resin and pigments on the surface.

§  Chemical Reactions

Coating that cure by the addition of a catalyst or hardener, or coatings that are chemically cured, are usually packaged in two or more containers, each container holding a carefully pre-measured amount of material. These proportions are very critical and, for this reason only complete units of any coating should be used. Do not attempt to proportion the different components of a multicomponent coating. Unless a chemically cured coating is mixed in the proper proportions and unless all the components are used, it is highly unlikely that the coatings will cure properly. Once these components are mixed together, the coating has a pot life, which refers to the length of time the coating material can be used. Material should not be used after its pot life has expired.

§  Heat Cure

These coatings require heat curing to obtain maximum stability and durability.