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| Wood Finishers News - EXTRA |
| Wood Finishing Tips from M.L. Campbell |
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Solvents and Lacquers
By: Rich Clark
Thinners and retarders are critical tools, which can help applicators produce quality finishes. To do this a finisher should have an understanding of what solvents are and how they work. Solvents are liquids, which are mixed with solid materials. One of three things can happen when liquids are mixed with solids:
(1) If the attraction between the liquid and solid molecules are less than the attraction between solid molecules then the liquid will do no more than separate the solid particles. This is referred to as a suspension.
(2) If the attraction between the liquid and solid molecules is greater than the attraction between the solid molecules the liquid will penetrate the solid structure. The solid molecules on the surface will break away from the rest of the solid molecules. Eventually all the individual solid molecules will be dispersed within the liquid and not be visible. This is known as a solution.
(3) A chemical reaction can occur that will result in a mixture which will behave as either (1) or (2).
Lacquers and paints are generally a combination of solutions and suspensions. The polymer or film former is usually in solution. The pigments and flattening agents are generally in suspension. The ability of a liquid to dissolve a solid is known as its solvency. The solvency of a liquid depends as much on the solid to be dissolved as it does on the liquid itself. Solvency can be characterized in a number of ways, including if the solvent can dissolve a useful amount of a given polymer say 20% non-volatile for straight nitrocellulose. If it does, that solvent is said to dissolve the given polymer type, in this case nitrocellulose. Another way to characterize solvency is to make up solutions of a given solid with different solvents. Then compare the viscosities of the solutions. The solvent which produced the lowest viscosity solution has the highest or most potent solvency for that solid type. Solvency is the most important attribute of a solvent. With lacquers there are three broad categories of solvency, active, latent and diluent. Active solvents possess the highest solvency power, with latent solvents considerably less and diluent solvents do not dissolve nitrocellulose at all. Diluents for nitrocellulose may be efficient solvents for other solids within the coatings such as the alkyd.
Another factor influencing the use of a particular solvent is its volatility or evaporation rate. There is a relative measure of how fast the solvent evaporates into the air. N-Butyl Acetate is set at one as a convention and all solvents compared to it. Acetone has a evaporation rate of seven?, this means it evaporates seven times faster than N-Butyl Acetate. This value is of course an incomplete picture of a solvents evaporation tendency within a coating film, as there are several other factors that come into play such as the relative humidity and temperature. Both of these dramatically affect the evaporation profile of solvents. Another is the affinity the solvent has for the polymer itself as this attraction will compete with the solvents desire to evaporate into the air. Yet another influence will be the steric hindrance or the affects of the polymers and other solids in the film physically blocking or retarding the solvent from leaving the film.
There are several other solvent attributes, which influence their use in coatings. Such as odor, toxicity, flash point and cost. All these factors come into play when formulating a coating. These should be kept in mind when thinning a coating. The manufactures recommendations should be followed when reducing a coating precisely because of the interplay amongst the various components of the coating. If a lacquer is thinned with to much diluent solvent the nitrocellulose will precipitate out of solution. If this occurs during drying the film may become hazy or have localized areas with chunks of nitrocellulose, which will not form a proper film. This will result in less then optimal film properties such as poor chemical or moisture resistance.
Solvents are not inert bystanders within a coating. They play an active role and have influence over many of the coatings attributes, such as dry time, cure response and application characteristics. It is therefore important to use the appropriate thinners and retarders in a responsible way. As a general rule, as little solvent should be added as possible to achieve the desired affect.
| Solvent |
Evaporation Rate |
Solvency on Nitrocellulose |
| N-Butyl Acetate |
1.0 |
Active |
| Toluene |
1.8 |
Diluent |
| Ethanol |
1.7 |
Latent |
| MEK |
5.7 |
Active |
| Xylene |
0.86 |
Diluent |
| Acetone |
14 |
Active |
Rich Clark is the Laboratory Manager for M.L. Campbell. He has a B.A. in biochemistry and an M.B.A. Rich has 15 years experience formulating various coatings and has been with M.L. Campbell for the past ten years.
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