SOLVENTS and THINNERS for coatings
Post 320 ⇒ by Gautam Shah →
Thinners are specific mixtures of different solvents to achieve desired viscosity for the film forming constituents of a coating system. Solvents are liquids that have power of dissolving or formulation with something.
Viscosity of a medium can be adjusted by including a low viscosity medium into a high viscosity material, or by solvents and diluents. Solvents dissolve by entering the inter-molecular space and changing the intermolecular forces. Diluents are non dissolving low viscosity substances, do not enter the inter-molecular spaces but extend the action of a solvent as a liquid to liquid-phase. Often in a multi medium formulation, one material that acts as a solvent to a particular medium, may act as a dilutent for the other medium. Solvents and diluents both increase the fluidity of a medium.
The solution of the film forming substances of a coating helps in manufacturing and application processes. Solvents or thinners are also required to clean up tools (brushes) and equipments (spray nozzles and containers) of application. These are also used for removal of patches, over-sprays and body parts.
Solvents convert resin and polymer molecules into smaller clusters, and it is a process of separating the molecules. In solution, the molecules of solute and the solvents are dispersed into each other.
Solvents on evaporation help in formation of a solid film. The coating may be deposited, and solvent evaporates to form a solid film (non-convertible system). Alternatively the coating undergoes one or many processes of chemical conversion (such as oxidation, chemical reaction on application of heat or catalyst reaction), while solvents get evaporated (convertible system).
Most solvents, including the most versatile one the water, evaporate at some temperature. There are two classes of solvents: Hydrocarbon (Petroleum) solvents and Chemical or oxygenated solvents, though these terms are overlapping due to complex process of manufacturing. Hydrocarbon solvents theoretically have only Carbon and Hydrogen but other substances such as Sulphur and heavy metals may be present as trace elements.
Coating formulations are in consideration of Solvents’ cost, flameability and the environmental effects. The solvent-power or solvency is very important aspect. The formulation must achieve a viscosity that is correct for manufacturing or application processes with a minimum amount of solvent. For a coating formulator, another important aspect is the rate of evaporation of solvent. If a solvent evaporates too rapidly, the applied coating will not get sufficient time to level out. Faster evaporation also induces early start for cross linking, and may seal the face, trapping part of the solvent. In spraying a fast evaporating solvent hinders even spray and may cause condensation of water around spraying nozzle and sprayed surface.
Thinners are mix of solvents and other carriers or non-solvent material. Thinners are formulated and proposed by the coating manufacturer, as competent authority they know what forms the coating. Often Thinners of different qualities are suggested such as for application and for cleaning of tools, equipments, patches and over-sprays. Some even provide specific thinners or special additives for monsoon seasons to counter effect of excess moisture and condensation.
For the later purpose, a well proportioned an economic blend of solvents and diluents, suitable for specific categories of coatings are marketed as Thinner or Reducer.
Following categories of solvents are used in coatings:
- Aliphatic hydrocarbons: white spirit (mineral turpentine), kerosene (superior and fuel grade)
- Aromatic hydrocarbons: benzene, toluene, xylene
- Alcohols: methanol, ethanol, butanol, isopropanol
- Chlorinated hydrocarbons: carbon tetrachloride
- Ketone: acetone, methyl ethyl ketone (MEK), methyl iso-butyl ketone
- Esters: methyl acetate, ethyl acetate, butyl acetate
- Ethers: methyl cellulose, cello-solve, cello-solve acetate
- Terpene: turpentine (genuine), di-pentene, pine oils