Curing is the mechanism that a specific coating uses to transform from the liquid state found in the bucket to a solid coating film. There are two broad classifications of curing mechanisms: Convertible and Nonconvertible. Understanding the curing mechanism is very important for the proper installation of the coatings and a guide to observe those factors that will affect the cure and ultimately the service life of the coating.
- Nonconvertible Coatings
Such coatings dry by simple solvent evaporation. No chemical reaction takes place. Binders are usually long chain polymers, which can interlock to form continuous films without chemical reaction. To facilitate film formation it is necessary to dissolve the polymers in appropriate solvents due to the inherently high viscosities of the polymers in use. This means that any usable paint will typically have a very low solids content.
The current legislative position with regard to the use of volatile organic compounds is having a major impact on the use of low solids (high solvent) coatings.
Physically drying paints can be redissolved in appropriate solvents as no chemical changes take place. This has advantages and disadvantages, the most important advantage being effectively an indefinite overcoating period with good intercoat adhesion. Solvents, in a freshly applied coat, will penetrate into the underlying paint film and, on drying, the layers will have become interlocked.
Another key feature of such systems is the ability to dry at a wide range of temperatures, the only variation being a change in drying time due to a change in the solvent evaporation rate. Over a wide application temperature range, physically drying systems will ultimately yield a film of the same properties.
There are two principal disadvantages associated with non-convertible coatings, the most important being the low solids levels of the paint produced. The use of high molecular weight polymers as film formers with their inherently high viscosities, results in the need to develop low solids coatings in order to achieve products which can be applied efficiently to substrates. This in turn leads to low film build, resulting in the need to apply several coats to achieve the film thickness necessary for the level of protection required. This can add extra time and cost to coatings projects.
Areas of concern for the coatings that cure by evaporation are:
- High surface temperature during application
- High wind flow over freshly coated surface
- Excessive film build
- Drying times (typically short)
- Freezing temperatures
- Ventilation in the area being painted
- Overcoating these materials can be problematic since the solvents in the overcoating material may soften or dissolve it.
Coalescence cure is a specialized case of evaporation cure. In these coatings, tiny particles of resin are dispersed in water with the aid of specialized additives called surfactants. When the water evaporates, the resin particles fuse (coalesce) together to form the film. Particle fusion is assisted by small quantities of organic solvents (coalescing solvents). These types of coatings are also known as latex or acrylic latex and are being used more and more.
- Convertible Coatings
Convertible coatings cure by one of several polymerization mechanisms, even when solvent evaporation is also involved. The resins used in convertible coatings undergo a chemical change as the cure progresses, so the resulting film is not readily re-dissolved in the solvent(s) used in application. These types of coatings are also known as thermoset materials. There are a number of chemical reactions that convertible coatings employ to form a protective film. Commonly chemistries used in convertible protective coatings areoxidation, chemical reaction (polymerization), hydration, fusion
Once the solvent evaporates from the film, these coatings cure by reaction with atmospheric oxygen. The main ingredient of the resin is a drying oil modified with synthetic molecules. Oxygen reacts with the oil portion of the resin, prompting a polymerization reaction known as oxidative cross-linking. This reaction can be accelerated by the addition (during manufacturing) of driers.
Early paints were based on natural oils, such as linseed oil. These are generally long chain aliphatic systems containing reactive CH2 groups between C=C double bonds, eg: CH2-CH=CH-CH2-CH=CH-CH2. The reactive CH2 reacts with oxygen free radicals in the atmosphere enabling crosslinked systems to be built. Reactivity is dependent upon the number of double bonds in the oil molecule. Oil molecules are fairly small, so drying to produce a dense film can take a long time. To reduce the drying time it is possible to modify the original oil to create larger basic molecules. The most common forms of modification are the alkyd resins. These are based on natural oils but can be modified to give a range of materials with very specific properties. Full dry/cure of these systems requires penetration of oxygen right through the film, hence applied film thickness tends to be low to prevent surface drying (skinning) only.
Chemically curing (polymerization) materials require the mixing of two components for film formation to occur. Polymerization basically means that a small molecule is transformed to a larger molecule by a variety of mechanisms. Polymerization is also referred to as cross linking. Individually neither component is capable of producing an acceptable paint film.
Once mixed, the two components react chemically. This process is irreversible.
Two pack materials have a finite usable lifetime during which application is practical. This can vary from seconds to days. This is termed the “pot life” of the product. There are a huge variety of possibilities for two pack systems, resulting in a range of products with broad or specific properties, e.g. light resistance, abrasion resistance, chemical resistance, corrosion resistance etc.
Chemical reaction curing coatings cover a vast range of chemistries such as epoxies, polyurethanes, polyureas, polyaspartics, polysiloxanes etc.
Like concrete, hydration coatings require some amount of water to complete their cure. Moisture-cured polyurethane is a hydration cured coating material. It must have some level of humidity in the surrounding air for it to cure. Another example is a solvent-based inorganic zinc coating based on an ethyl silicate resin. Upon application and solvent evaporation, water from the atmosphere reacts with the silicate to form silicic acid. The silicic acid reacts with the zinc pigment and polymerization proceeds until full cure.
Fusion is forced-heat curing. It is polymerization, but requires a particular temperature to complete the cure. Fusion-cured coatings may be single- or two-component materials. An example is fusion-bonded epoxy (FBE) commonly used to coat pipelines in the petrochemical industry.
- Capital Painting, Convertible Coatings, data of access: 29 August 2016, http://www.capital-painting.com/convertible-coatings/
- Capital Painting, Non-Convertible Coatings, data of access: 29 August 2016, http://www.capital-painting.com/curing-mechanisms-nonconvertible-coatings/
- Internationel Marine Coatings, What is Paint?, data of access: 29 August 2016, http://www.international-marine.com/paintguides/whatispaint.pdf