Solvents used in coatings, except for water, are generally volatile organic solvents. Classified by compound type, they can be divided into seven categories: aliphatic hydrocarbon solvents, aromatic hydrocarbon solvents, alcohol solvents, ketone solvents, ester solvents, alcohol ether and ether ester solvents, and substitute hydrocarbon solvents.
The chemical composition of aliphatic hydrocarbons mainly consists of linear hydrocarbons and is derived from petroleum distillation.
- Petroleum ether is a low-boiling fraction of petroleum, consisting of lower alkanes; it is now less commonly used in coatings.
- 200# paint solvent oil is a mixture composed of C4 to C11 alkanes, alkenes, cycloalkanes, and a small amount of aromatic hydrocarbons. The primary components are pentane, hexane, heptane, and octane, etc. Straight-run gasoline obtained directly from crude oil distillation is essentially free of alkenes, while catalytically cracked gasoline contains a considerable amount of alkenes. Gasoline used as a solvent must be free from cracked fractions and tetraethyl lead.
Aromatic hydrocarbon solvents are currently the most widely used in the industry, and they can be classified into two major types: coking aromatic hydrocarbons and petroleum aromatic hydrocarbons. Coking aromatic hydrocarbons are obtained through the distillation of coal tar, while petroleum aromatic hydrocarbons are obtained from petroleum products through platinum reforming, catalytic cracking oil, and toluene disproportionation oil distillation.
- Benzene: Used in combination with butyl acetate, acetone, and butanol; benzene vapor is highly toxic to the human body and tends to be phased out.
- Toluene: Due to its rapid volatility (about three times that of xylene), toluene is rarely used as a solvent. Currently, it is mainly used as a component in mixtures for vinyl and chlorinated rubber coatings. It is also used as a diluent in nitrocellulose coatings.
- Xylene: Xylene is insoluble in water but can be mixed with ethanol, ether, aromatic hydrocarbons, and aliphatic hydrocarbon solvents. Due to its strong solubility and moderate volatility, it is the main solvent for short-oil alkyd resins, vinyl resins, chlorinated rubber, and polyurethane resins. It is also used as a solvent for asphalt and petroleum asphalt, and can be used as a diluent in nitrocellulose coatings. Adding 20% to 30% n-butanol to xylene can increase its solvency for amino resin paints and epoxy resins. As xylene can be used for both air-drying and baking coatings, it is currently the most widely used and largest quantity solvent in the coating industry.
Derived from pine trees, terpene solvents are among the early solvents in use. Common examples include turpentine and dipentene.