7 Types of Solvents for Coatings&Paints

25th January 2024
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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.

Aliphatic Hydrocarbons

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 Hydrocarbons

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.

Terpene

Derived from pine trees, terpene solvents are among the early solvents in use. Common examples include turpentine and dipentene.

Alcohols

Alcohols, Ketones, Esters, and Alcohol Ethers are commonly referred to as 'oxygen-containing solvents,' meaning solvents that contain oxygen atoms in their molecules. They have a wide range of dissolution and can dissolve most resins, often used in combination with other solvents.

  • Ethanol (ETHANOL), commonly known as 'alcohol,' is often mixed with other solvents as a diluent.
  • Isopropanol (IBA), with solvency and volatility similar to ethanol, has a strong odor. It is mainly used as a co-solvent for nitrocellulose and acetate fiber coatings.
  • n-Butanol (NBA), a colorless and transparent liquid with a distinctive aromatic odor, can be mixed with alcohols, ethers, benzene, and various organic solvents. A mixture of n-butanol and xylene is widely used in amino baking paints and epoxy resin paints. n-Butanol, as a co-solvent for nitrocellulose resin, has a 'anti-whitening effect' due to its high boiling point and slow evaporation, but its drawback is higher viscosity.

Ketones

Ketone solvents constitute another category of oxygen-containing solvents.

  • Acetone (ACT) is a strong solvent with a low boiling point and rapid volatility, making it an excellent solvent for volatile coatings. Due to its rapid evaporation and cooling effect, it can cause condensation of water vapor in the air on the surface of the coating, resulting in frosting and whitening of the coating surface. Therefore, it is often used in combination with low-volatile alcohols and alcohol ether solvents that have an anti-whitening effect.
  • Methyl Ethyl Ketone (MEK) - Similar solvency to acetone but slower evaporation. It is commonly used as a solvent for nitrocellulose, acrylic resin, vinyl resin, epoxy resin, and polyurethane resin.
  • Methyl Isobutyl Ketone (MIBK) has slower evaporation than MEK, strong solvency, and good performance. It is often mixed with other solvents.
  • Cyclohexanone (CHK) is a strong solvent with slow evaporation. It exhibits excellent solvency for various resins and is mainly used in polyurethane, epoxy resin, and vinyl resin coatings. It can improve film adhesion and make the coating smooth and aesthetically pleasing.
  • Isophorone (IP) - A light yellow liquid with a camphor-like odor, high boiling point, low hygroscopicity, slow evaporation, and good solvency. It can be mixed with most organic solvents and various nitrocellulose coatings.
  • Dipropylene Glycol - A colorless, odorless, transparent liquid commonly used to formulate antistatic diluents.

Esters

Ester solvents are also a type of oxygen-containing solvents. Most commonly used ester solvents in coatings are acetate esters, with a small amount of organic acid esters. Among these solvents, CAC is an extremely important solvent for PU coatings (slow-drying waterborne PU).

  • Ethyl Acetate (EAC) is a colorless and transparent liquid with a fruity fragrance. It can be mixed with most organic solvents and is capable of dissolving vegetable oil, glycerol rosin ester, nitrocellulose, chlorovinyl resin, and polystyrene resin. In coatings, it can be used as a solvent for nitrocellulose, ethyl cellulose, polyacrylic acid resin, and polyurethane resin.
  • n-Butyl Acetate (BAC) is a colorless liquid with a fruity fragrance, sparingly soluble in water, and less prone to hydrolysis. It can be mixed with alcohols, ethers, and other common organic solvents. It has good dissolving ability for vegetable oil, glycerol rosin ester, polyvinyl acetate resin, polyacrylic acid resin, chlorinated rubber, making it a commonly used solvent in nitrocellulose coatings, polyacrylic ester coatings, chlorinated rubber coatings, and polyurethane coatings.
  • Isobutyl Acetate (IBAC) - Similar properties to BAC but with a lower flash point, at 17.8℃.

Alcohol Ethers and Ether Esters Solvents

  • Ethylene Glycol Ethyl Ether, also known as glycol ethyl ether or ethanol solvent thinner, is a colorless liquid with a mild fragrance and belongs to the 'toxic' category. It can be miscible with water, alcohols, ethers, acetone, and various solvents, making it an excellent co-solvent for water-based coatings.
  • Ethylene Glycol Butyl Ether, also known as glycol butyl ether or butyl solvent thinner, is a colorless liquid with a slight fragrance. It is soluble in acetone, benzene, ethyl ether, methanol, and other organic solvents. It is an excellent solvent for nitrocellulose coatings and can act as an anti-whitening agent. Widely used in most coatings, it can prevent wrinkling, fogging, improve film flow, and enhance gloss; it also serves as an excellent co-solvent for water-based paints.
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