Solvents in Coatings

Most liquid coatings are a balance between the solids that stay on the surface and the volatile carrier that leaves. That carrier may be organic solvent, water or a mixture of both. Although solvents are not part of the finished film, they shape nearly everything that happens between opening the can and achieving full cure.

What solvents do

A solvent package performs several jobs at once:

• Dissolves the resin so the binder and curing agent can be manufactured and stored as stable liquids.
• Controls viscosity for the chosen application method, from brush to airless spray. See rheology and viscosity for how flow behavior is described.
• Wets the substrate by lowering surface tension, helping the coating penetrate the surface profile and pores.
• Governs open time and leveling while the film is still wet.
• Provides intercoat bite in some systems by slightly softening the previous coat.

Strictly, a true solvent dissolves the resin on its own, a latent solvent works only in combination, and a diluent is a cheaper liquid that reduces viscosity but cannot dissolve the resin alone. Too much diluent causes the resin to precipitate, a problem known as solvent kick-out.

The main solvent families

Protective coatings draw on a relatively small set of chemical families. Each is used for its solvency toward particular resins and for its evaporation characteristics.

Solvency and evaporation rate

Solvency

Whether a solvent dissolves a resin is commonly predicted with solubility parameters. The Hansen approach splits solvency into dispersion, polar and hydrogen-bonding components; a resin dissolves in liquids whose parameters fall close to its own. Formulators use this to build blends that keep the resin in solution through the entire drying process, not just in the can.

Evaporation rate

Relative evaporation rate is usually expressed against n-butyl acetate, assigned a value of 1. Acetone and MEK evaporate several times faster; xylene is a little slower than butyl acetate; glycol ethers and heavy aromatic naphthas are much slower. A typical blend contains:

• a fast fraction that flashes off between the gun and the surface, raising viscosity and resisting sags;
• a medium fraction that controls wet-edge time and leveling;
• a slow tail that keeps the film open long enough to flow and release trapped air, then leaves.

Temperature, airflow, film thickness and humidity all change real-world evaporation. As solvent leaves, the remaining solvent must still keep the resin dissolved; if the strong solvent escapes first, the film can turn hazy or weak.

Solvent-related defects

Many common coating failures trace back to solvent balance:

• Dry spray. Solvent evaporates before droplets reach the surface, leaving a rough, porous film. Hot, windy conditions and long gun distances make it worse. See dry spray and overspray.
• Sags and runs. Too much slow solvent or over-thinning keeps the film fluid too long.
• Blushing. Rapid evaporation cools the surface below the dew point, condensing moisture into the film.
• Popping and pinholes. A skin forms while solvent is still escaping, rupturing the surface.
• Solvent entrapment. Excess film build or premature topcoating traps solvent, causing softness, blistering or adhesion loss. See solvent entrapment.
• Lifting and wrinkling. Strong solvents in a topcoat attack a sensitive undercoat.

Waterborne and high-solids alternatives

Regulatory pressure and worker-exposure concerns have pushed the industry toward lower-solvent technologies:

• High-solids and 100%-solids coatings use lower-molecular-weight resins or reactive diluents to reach sprayable viscosity with little or no solvent. Many need heated or plural-component equipment.
• Waterborne coatings carry the binder as an emulsion or dispersion in water. They still contain small amounts of coalescing solvents that soften the polymer particles so they can fuse, then evaporate.
• Exempt solvents. Certain compounds, such as acetone and parachlorobenzotrifluoride, are excluded from the US federal VOC definition because of their low ozone-forming potential, although they are not free of health or flammability hazards.

Water evaporates slowly in high humidity and cannot evaporate below the dew point at all, so waterborne products have tighter environmental windows than many solvent-borne equivalents.

VOC content and regulations

Volatile organic compound (VOC) content is usually reported in grams per liter or pounds per gallon, excluding water and exempt compounds. Limits vary by country, state, air district and product category, and the figure on the label generally applies to the product as supplied or thinned per the label. The details are covered in VOCs and coating regulations.

Handling solvents safely

Most organic solvents are flammable, and their vapors are heavier than air, collecting in low areas, tanks and pits. Inhalation can cause dizziness and narcosis, and some solvents have specific organ toxicity. Practical controls include:

1. Read the SDS. Check flash point, exposure limits and required personal protective equipment for every product and thinner.
2. Ventilate. Keep vapor concentrations well below the lower explosive limit and below occupational exposure limits, especially in enclosed spaces.
3. Control ignition sources. Bond and ground containers and spray equipment, and use rated electrical equipment where flammable atmospheres may occur.
4. Protect workers. Use the respiratory protection and chemical-resistant gloves specified by the employer’s safety program.
5. Manage waste. Store solvent-soaked rags in closed metal containers and dispose of waste solvent according to local rules.

Follow the SDS, your employer’s safety program and local regulations for all solvent handling.

Frequently asked questions

Does solvent affect how long a coating takes to cure?

Yes. Solvent must leave before reactive coatings reach full properties, and retained solvent plasticizes the film. Low temperatures and thick films slow release considerably.

Can I clean equipment with the same solvent I thin with?

Often, but not always. Cleanup solvents are typically chosen for strong solvency and fast evaporation, which may be unsuitable for thinning. Check the product data sheet for both.

Why does my coating turn milky in humid weather?

Fast-evaporating solvent can chill the wet film below the dew point, so water condenses into it. A slower solvent blend, warmer surfaces or better humidity control usually prevents it.

Are waterborne coatings solvent-free?

Usually not completely. Many contain a few percent of coalescing or co-solvents, so they still have some VOC content and still require ventilation.