The drying and hardening of Coatings

Drying terminology

There are numerous test methods and international standards used to determine the drying stages of coatings, depending on the coatings type and its use, with few of them using the same methodology. To avoid unnecessary confusion I have used the following terms throughout as a generalisation, unless otherwise noted.

• Touch Dry: When the coating film is dry to a slight touch with the finger but without applied pressure.
• Hard Dry: When the coating can be handled without any significant marking. Note that the ultimate hardness may, in fact, take several hours, or days, to be finally realised.

Evaporative drying

When a coating is first applied to the substrate, the solvent begins to evaporate from the surface. The rate at which the solvent evaporates is determined by the type of solvent(s) contained in the coating and the ambient conditions (the higher the temperature and the more windy the conditions, the faster the solvent will evaporate). Eventually, most of the solvent will have evaporated from the coating surface leaving a coating film behind that is often (but not always) 'touch dry'.

As the solvent evaporates from the coating surface the viscosity of the coating begins to rise, the rate at which the viscosity rises dependent on the evaporation rate of the solvent(s) chosen for the coating. If the viscosity rise is slow then there is the possibility that the applied coating might 'run' or 'sag' if applied to vertical, or inclined, surface. On the other hand, if the viscosity rises too quickly, then there is a possibility that the coating might not 'flow' such that the coating is uneven, with reduced gloss and variable thickness.

Additionally, the method of application of the coating also dictates the solvent used. For spray application, the solvent evaporation should be rapid, as required by industrial applications, for fast 'handling' of the finished item. Whereas for coatings applied by brush or roller, the solvent should not evaporate too quickly such that the coating dries on the brush or roller whilst in use.

Hard drying

Once the solvent has evaporated from the coating, the film left behind might be the finished coating which is the case with those based on thermoplastic resins which dry from solvent evaporation only. However, the problem with all coatings that dry by solvent evaporation alone is that they may be re-solubilised using the same, or similar, solvents that the coating originally contained. The upshot of which is that these coatings are of limited use where they may encounter solvents, including liquids, such as oils, grease and other fluids, that may be encountered in many applications.

There are other, secondary reactions that occur in those resins that belong to the thermoset class of polymers.

Oxidative drying

With resins based on drying, and semi drying, oils and fatty acids, whilst the coating is drying by solvent loss, it is also absorbing oxygen from the atmosphere. The absorbed oxygen begins to form 'bridges' (cross linking) between the oil molecules contained in the alkyd polymer, physically increasing the polymer molecular mass such that, eventually, the coating film is no longer soluble in the solvents used in the original coating. The process of oxidative drying is slow, and so is the hardening rate of the coating film.

Room Temperature curing

Certain polymers are designed to react with specifically designed hydroxyl containing polymers, at room, or slightly elevated, temperatures. These systems are often called 'twin pack, two pack, or simply '2K' coating systems by virtue of the coating being supplied in two separate containers. Specified portions of the two components are mixed and must be utilised within a set length of time, known as the 'pot life', after which time the coating will have thickened to such an extent that it is no longer useable. The rate at which the reaction proceeds is temperature dependent, higher temperatures resulting in a shorter pot life, and vice versa. The rate at which hardening of the coating proceeds is very system dependent. it can take from a few minutes through to several hours, or even days to achieve the maximum hardness.

Oven Curing

In some instances, when the cross-linking component is stable in combination with the coating polymer at room temperature, the coating is supplied already containing the cross-linking component, as a single pack. Once the coating has been applied to the substrate, and after a short 'flash-off' time to allow most of the solvent to evaporate, the coated article is either placed in a closed oven to effect the cross-linking reaction, or through a 'continuous' oven (and oven where the article is passed through the heated area on conveyors belts). Temperatures used in either of these ovens range from ≥ 120°C to ≤ 180°C