PAINT THINNERS – Part 2
Post 423 – by Gautam Shah
A Paint Thinner nominally changes a material’s solid phase into a liquid phase and heavier liquid into low viscosity liquid. Thinners act by dissolving, suspending the solids and by intervening into viscous liquids.
A thinner is a mix of different solvents that act as a thinning compound, suspending agent and a diluting material. A thinner is also conceived as accelerator or retarder of film formation of a coating. Thinners are used for cleaning of lubricants, machine cutting fluids, coolant materials, greases, waxes, etc. Thinners are used as stripping agent of dry or wet coatings (paint remover). A thinner sometimes may not strip a film of coating, but softens by affecting some of the constituents (through wetting, etching, etc.) of the film. Rest of the stripping act is accomplished with other chemicals, heat softening, singeing, mechanical stripping, scrapping or grinding.
Viscosity of a coating 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 by themselves 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 diluent for the other medium’.
Solvents and diluents both increase the fluidity of a coating medium. The fluidity of a coating medium is important for applications (spray, brush, etc.), flow or levelling properties, nature of drying and ‘curing’ of the film. Very high fluidity may not mean very low viscosity. Very thinned down coating material leads to separation of solids (like pigments) from liquids (resins, solvents, etc.) Excessively thinned down coating, on drying produces porous film (a film with marked solvent escape pores). Too much thinning often creates non-opaque or patchy (in terms of colourants) film. Solvents also affect the inter-molecular structure of resins affect their interlinking or polymerization, and thus the quality of the film.
Solvents and diluents are added during the manufacture and also prior to application. 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.
Thinning solvents included in a coating material, encourage separation and towards the gravity settlement of solids on storage. Nowadays thixotropic compounds are added to water based coatings (such as Plastic or Latex paints). These compounds create a false setting (thickening of liquid mass into a viscous paste), and with little stirring, the coating material gains the original consistency.
Constituents of thinners are nominally low boiling point temperature solvents. These evaporate at a faster rate but are affected by the temperature, moisture, movements of air, and the application process. In nominal weather, 25 ° C and 40% humidity, thinner evaporates at an average rate, but in hot weather thinner leaves the coating film before it has time to flow and level out. In case of moist weather (raining periods) the moisture in the air gets trapped in the film and cause blushes or whitish spots. This can be corrected by using retarder additive or slow drying thinner.
Coating applications require special thinner formulation. French polishes and lacquer coatings applied by rubbing pads need very thinned down and slowly evaporating thinner. Brush application of coatings needs film levelling time, high fluidity without reducing the viscosity. Spraying with compressed air creates cooling and moisture condensation and so prescribed quality of thinners must be used. Spraying by airbrushing (thin-narrow and a fine nozzle) for delicate work, need fast evaporating solvents.