Toluene (methylbenzene)
Toluene is a fast, strong aromatic solvent that has thinned and cleaned up coatings for a century, and one that regulations have steadily pushed out of many formulations.
Toluene is a benzene ring with one methyl group (molecular weight 92.14 g/mol). It is a clear, colorless liquid with a sweet aromatic odor, a boiling point of 110.6 °C and a flash point of about 4 °C.
Toluene dissolves a wide range of resins, including alkyds, vinyls, chlorinated rubber, acrylics and many epoxies, so it is used in solventborne coatings, thinners, adhesives and cleaners. It evaporates quickly, which speeds drying and helps early hardness. Dry, “urethane-grade” toluene can be used in polyurethane systems because it has no reactive hydrogens.
Toluene is highly flammable, can be fatal if swallowed and enters the airways, causes skin irritation and drowsiness, and is suspected of damaging the unborn child, with organ damage possible from prolonged exposure. ACGIH recommends a TLV-TWA of 20 ppm. It is a hazardous air pollutant and a VOC in the US, and restricted in many products. Use with ventilation, bonding and grounding, and suitable respiratory protection, and keep away from ignition sources.
Toluene evaporates faster; xylene is slower and gives more open time and flow. Both are aromatic solvents with strong solvency, and the right choice depends on drying speed, film build and regulations.
Only if the manufacturer allows it and the solvent is dry, urethane grade. Water in solvent reacts with isocyanate and causes bubbles. Always use the thinner the TDS specifies.
It is a hazardous air pollutant and a VOC, and it carries reproductive and neurological health concerns. Many formulators use exempt or less hazardous solvents or move to waterborne and high-solids systems.
