Solvents in Paints and Coatings

26th December 2023

The role of solvents in paints and coatings is often underestimated, with many considering them as volatile components that simply evaporate and do not bring any effect the final dried coating.

In reality, factors such as the solvency and evaporation rate of various solvents have a substantial impact on the production, storage, application, as well as the overall performance of paints and coatings. Solvents can affect properties including the gloss, adhesion, and surface condition of the coating film.

What are the uses of solvents?

Solvents comprise approximately 30%-50% in the coating formulation, playing an important role in various aspects of the coating process.

Dissolving and Dispersion: The main function of solvents is to dissolve and disperse various components including resins, pigments and additives in the coating, adjusting viscosity to facilitate easy application. After the application of coatings, the resin undergoes a curing (or drying, hardening) process as solvents evaporate, ultimately resulting in the formation of a solid film.

Role of solvents in paints and coatings

Enhancing Storage Stability: Solvents contribute to increased stability during the storage of coatings.

Improving Film Appearance: Solvents play a role in enhancing the appearance of the coating film, including aspects such as gloss and fullness.

Increasing Wetting of Substrates: They enhance the wetting ability of the coating on the substrate, improving adhesion.

Optimizing Volatility: By providing a reasonable evaporative rate, solvents impart optimal flow and leveling characteristics to the coating.

Different resin series can only dissolve in specific active solvents. In a single oil coating formula, various resins are often used, so a combination of different solvents is often used to achieve optimal results.

Types of solvents

There are various types of organic solvents, which can be categorized into 10 major classes based on their chemical structures:

  • Aromatic hydrocarbons: Benzene, toluene, xylene, etc.
  • Aliphatic hydrocarbons: Pentane, hexane, octane, etc.
  • Alicyclic hydrocarbons: Cyclohexane, cyclohexanone, methylcyclohexanone, etc.
  • Halogenated hydrocarbons: Chlorobenzene, dichlorobenzene, dichloromethane, etc.
  • Alcohols: Methanol, ethanol, isopropanol, etc.
  • Ethers: Ethyl ether, epoxy propane, etc.
  • Esters: Methyl acetate, ethyl acetate, propyl acetate, etc.
  • Ketones: Acetone, methyl ethyl ketone, methyl isobutyl ketone, etc.
  • Diol derivatives: Ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, etc.
  • Others: Acetonitrile, pyridine, phenol, etc.

Common solvents arranged in order of polarity: Water (highest polarity) > Formamide > Acetonitrile > Methanol > Ethanol > Isopropanol > Acetone > Dioxane > Tetrahydrofuran > Methyl ethyl ketone > n-Butanol > Ethyl acetate > Ether > Isopropyl ether > Dichloromethane > Chloroform > Bromoethane > Benzene > Chloropropane > Toluene > Carbon tetrachloride > Carbon disulfide > Cyclohexane > Hexane > Heptane > Kerosene (lowest polarity).

polarity of compounds

Factors to consider when choosing a solvent

1.Solubility refers to the ability of a solvent to disperse and dissolve solutes. The chosen solvent should exhibit excellent solubility with the primary film-forming materials, demonstrating a strong capability to reduce viscosity. This transformation allows solid or viscous substances to become thinned liquids suitable for spraying or brushing.

2.The evaporation rate of a solvent must align with the formation of the coating film, particularly for volatile coatings. The solvent's evaporation rate directly influences the drying speed of the film, the ease of application, and the overall quality of the coating.

3.The chemical properties of the solvents should be stable, ensuring no chemical reactions occur with the various components of the coating. Additionally, the solvent should possess low toxicity and high safety performance.