At present, the type of solvent used in the coating industry is aromatic hydrocarbon solvent. Because of its complex source and classification, combined with the names inherited in history and the names that have been standardized in recent years, it has caused confusion to some extent.

According to different sources, aromatic hydrocarbons can be divided into two categories: coking aromatics and petroleum aromatics. The coking aromatics are obtained from the fractionation of coal tar, and the petroleum aromatics are obtained from the rectification of petroleum products by platinum reforming oil, catalytic cracking oil and toluene disproportionated oil.

Isopropyl alcohol manufacturers coking aromatics and oil aromatics according to the number of their carbon atoms, further divided into light aromatics and heavy aromatics, generally C8 (including C8) below called light aromatics, C8 above, mainly C9 ~ C10 components called heavy aromatics. Light aromatic solvents for coking aromatics, including coking benzene, coking toluene, coking xylene and solvent naphtha. The light aromatics of petroleum aromatics include petroleum benzene, petroleum toluene and petroleum xylene.

1. Benzene

Industrial benzene is a colorless transparent liquid, with the unique smell of aromatic hydrocarbons, the impurities contained are mainly aromatic homologues, thiophene and saturated hydrocarbons, and need to be refined when necessary. Benzene is insoluble in water (dipole moment is zero), in addition to glycerol, ethylene glycol, diethylene glycol, 1, 4-butanediol and other polyols, can be miscible with ethanol, chloroform, carbon tetrachloride, carbon disulfide, acetic acid, acetone, toluene, xylene and aliphatic hydrocarbons and most other organic solvents. Benzene can dissolve rosin, glycerol resin, glycerol alkyd resin, vinyl resin, styrene resin, acrylic resin and other synthetic resins. Polyvinyl acetate resin partially dissolved in benzene; Ethyl cellulose and benzyl cellulose are soluble in benzene. Cellulose acetate and nitrocellulose are insoluble in benzene.

The main use of benzene in coatings is to use butyl acetate (or ethyl acetate), acetone and butanol together as a diluent for nitro paint, because benzene vapor is highly toxic to the human body, it is now replaced by other solvents and tends to be eliminated.

2. Toluene

Industrial toluene is a colorless transparent liquid that has a benzol-like odor and may sometimes contain a small proportion of aliphatic hydrocarbons of the same boiling range. Similar to benzene, toluene is insoluble in water and can be miscible with methanol, ethanol, chloroform, acetone, glacial acetic acid and benzene. Toluene can dissolve drying oil and other vinyl resins and alkyd resins except vinyl chloride. Adding methanol and ethanol to toluene can increase the solubility of cellulose acetate.

Because toluene has a faster volatilization rate (about 3 times that of xylene), it is rarely used as a solvent, and is currently mainly used as one of the components in the mixed solvent of ethylene coatings and chlorinated rubber coatings. It is used as a diluent in nitrocellulose coatings.

3. Xylene

Xylene is a mixture of 3 isomers, including ortho, meta and para-xylene, and any one of the 3 isomers is not suitable for use as a solvent in paint alone.

Industrial mixed xylene series colorless transparent liquid, with a unique aroma of aromatic hydrocarbons, sometimes emit weak fluorescence. Due to different sources, it is divided into petroleum mixed xylene and coking xylene. The former can be divided into 3° mixed xylene and 5° mixed xylene according to different distillation ranges. The latter can be divided into 3°, 5° and 10° xylene, other parameters are basically the same.

According to different sources and processing routes, the content of three isomers in mixed xylene is also different. The following table lists the composition of petroleum mixed xylene prepared by three routes. At the same time, the mixed xylene often contains ethylbenzene, a small amount of toluene, tritoluene, aliphatic hydrocarbons and sulfides.