|Evaporation from Paints|
Evaporation is the process by which a material, or part of it, is converted into a gas or vapour state. Drying of paints and coatings is considered to be the combination of processes by which volatile matter evaporates from an initially liquid film to produce and end product with the desired physical coating properties. In this discussion evaporation refers not only to volatile loss from the liquid film, but also from the semi-dry film.
1. Evaporation from Solvent-Based Paints and Coatings
Solvents for paints and coatings are selected so that initially they escape relatively quickly to prevent excess flow of the paint or coating, but sufficiently slowly to provide levelling and adhesion. The first 30% of solvent evaporates from typical alkyd coatings as quickly as the neat (pure) solvent, essentially at a constant rate depending on volatility. Later-stage solvent evaporation occurs several times more slowly and is rate-controlled by solvent diffusion to the surface of the paint or coating.
2. Evaporation from Water-Based Latex Paints
Water evaporates from a latex film in 3 stages:
a) an initial constant evaporation-rate stage during which the latex particles move freely. This initial evaporation is at close to the evaporation rate of water alone, and is controlled by temperature, humidity, evaporative cooling and surface air flow.
b) an intermediate stage when the latex particles come into irreversible contact with each other and the water evaporation rate drops rapidly to 5-10% of the initial rate. This state starts at approximately 30% water content by weight. This stage is complete when virtually all the water has evaporated and a nascent resin film has formed.
c) a final stage when water escapes the film very slowly by diffusion, after water content has decreased to approximately 10% by weight. The rate of drying is controlled by internal diffusion resistance. The dry surface film undergoes gradual coalescence and becomes increasingly homogeneous by diffusion of polymer chain ends acrodd inter-particle boundaries.
Solvent evaporation from latex paints and coatings is more complex.
Evaporation rates for solvents in latex paints and coatings are in the order of their volatilities for up to the first five hours. After, low volatility ethylene glycol has the fastest evaporation rate, since it is partitioned to continuous hydrophilic spaces between latex particles, where evaporation is controlled by volatility. Other solvents, notably the coalescents, partition to the resin phase, in which evaporation rates are diffusion-controlled.
The early drying behaviours of solvent components from a water-based latex coating are critical to the development of an acceptable film. Organic cosolvents may be added to speed up the drying process but rapid solvent loss may lead to polymer precipitation. The relative humidity during application is also a critical factor to solvent evaporation from these coatings. At high humidities water evaporation will be inhibited and the cosolvent will escale leading to a water-rich coating. At low humidities water evaporation will be enhanced, and a cosolvent-rich coating will remain.
3. Residual Solvents in Solvent-Based Coatings
Once a coating is touch dry, solvent evaporation rates are dependent on the solvent's diffusion coefficient in the film. This is a very slow process that is important to the curing of the paint becuase the residual solvent acts as a strong plasticizer to relieve film stress as it hardens. Some solvent may remain in the polymer film for years. Hard resin films retain significant amounts of solvents over long periods (greater than several weeks) while soft resin films retain little or no solvent after short periods (approximately one week). Solvent retention is reduced in coatings that include a plasticizer in the resin. The rate of solvent release can be increased by raising the temperature of the film.
4. Residual Offgassing in Water-Based Latex Paints
Latex paints contain coalescing solvents with much lower diffusivities than the solvents used in solvent-based paints. For latex paints, unlike solvent-based paints, the rates of all volatile losses were controlled by evaporative surface processes, and paint drying rates are comparable to those of the individial compounds within it. The development of cohesive films from latexes (at which point diffusive processes may take over) can take several tens of hours, and in some cases more than 200 hours.
|Record #24, revised 1/15/2001|
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