Solvents are difficult to understand because they all look alike in the can. One way to make sense of them is simply to learn what each does (see Understanding Solvents, Part I). A more sophisticated and very helpful method for understanding solvents is to organize them into their families.
There are five solvent families used in finishes:
- Petroleum distillates (mineral spirits, naphtha, etc.)
- Alcohols (methanol, denatured alcohol, etc.)
- Ketones (acetone, MEK, etc.)
- Esters (butyl acetate, amyl acetate, etc.)
- Glycol ethers (butyl cellosolve, butyl carbitol, etc.)
Within each family, the individual members have the same relationship to each other. The smaller molecules evaporate faster and the larger molecules evaporate slower. The larger molecules are also oilier.
The best way to illustrate this relationship is to break down the petroleum-distillate family, the one you are surely most familiar with. (See Chart.)
The smallest molecule in this family is methane, which is a gas at room temperature. Methane is so “fast-evaporating” that it has already evaporated at this temperature.
Following methane in evaporation rate are ethane, propane, butane, etc., up to heptane and octane, which are primary ingredients in gasoline, a very fast-evaporating liquid at room temperature. The next common distillations are naphtha and mineral spirits (commonly nicknamed “paint thinner”), naphtha being less oily and faster evaporating than mineral spirits.
Toluene (toluol) and xylene (xylol) make up the strong and smelly parts of naphtha and mineral spirits. Refineries remove the toluene and xylene to be sold separately and sell what is left as odorless mineral spirits. This less smelly mineral spirits has less solvent strength than regular mineral spirits, but it is usually strong enough to thin oils and varnishes and dissolve wax. Toluene and xylene are very fast-evaporating and dry (non-oily).
The next common petroleum distillate is kerosene, which is so oily that it barely evaporates at all at room temperature. Mineral oil is next, and it is a full-fledged oil that doesn’t evaporate. Paraffin wax is a solid at room temperature. It liquidifies at around 130 degrees.
Notice that petroleum distillates range all the way from a gas to a solid at room temperature. Other families don’t have quite this range.
The Other Families
Alcohols, ketones, esters, and glycol ethers work the same as petroleum distillates, with the smaller molecules being faster-evaporating and drier, and the larger molecules being slower-evaporating and oilier. Some of the faster evaporating alcohols, ketones and glycol ethers are sold to consumers. No esters are commonly available.
Within the alcohol family, methanol (wood alcohol) evaporates faster than ethanol (grain alcohol). Ethanol is the alcohol in beer, wine and liquor, and it is highly taxed. So a poisonous substance, often methanol, is added to make it undrinkable and avoid the tax. This product is sold as “denatured alcohol” or “shellac thinner.” Methanol itself is highly toxic and should not be used except outdoors or with good ventilation.
Within the ketone family, acetone evaporates faster than methyl ethyl ketone (MEK). All other ketones evaporate slower.
The molecules in the glycol-ether family are large and evaporate very slowly. This makes them good solvents for water-based finishes because they evaporate slower than water. First the water evaporates from these finishes, then the small amount of remaining glycol ether softens the droplets of emulsified resin so they stick together and form a film as the rest of the solvent evaporates.
An example of a glycol ether you may be familiar with is ethylene glycol monobutyl ether, which is commonly sold under its trade name, Butyl Cellosolve.
Butyl Cellosolve evaporates very slowly and is used as the most potent retarder for lacquer. You can buy this solvent separately to add to lacquers to slow their drying, or it may be included with other solvents and sold as lacquer retarder.
You can usually recognize a solvent by its suffix. Alcohols, which dissolve and thin shellac, have the suffix -ol. Ketones, which dissolve and thin lacquers, have the suffix -one. Esters, which dissolve and thin lacquers, have the suffix -ate (methyl acetate, amyl acetate, etc.). Glycol ethers, which dissolve water-based finishes and dissolve and thin lacquers, end in “ether.”
It’s helpful for understanding to group the solvents in their families. Once you understand this organization, lacquer thinner, which is made up of a number of solvents, makes more sense.