- “Varnish is a homogeneous colloidal dispersion-solution of natural or synthetic resins in oil or thinner or both”.
- One uses it protective and decorative coating of suitable surfaces and it dries up by evaporation, oxidation and polymerization of its constituents, leaving behind a hard, transparent, glossy, lustrous and durable film.
Types:
- There are two main types of varnishes:
- Oil varnish
- Spirit varnish
1.Oil varnish:
- Oil varnish is a homogeneous solution of one or more natural or synthetic resins in a drying oil and a volatile solvent to which one adds some driers. The presence of oil reduces the natural brittleness of the pure resin film.
- This type of varnish dries up slowly by evaporation of the volatile solvent followed by oxidation and polymerization of drying oil. Such varnishes take longer period of about 24 hours for drying.
- Film produced is hard, durable and quite lustrous.
- For example copal varnish which is prepared by dissolving copal in linseed oil and mixing a quantity of turpentine.
- One uses it for exterior as well as interior works.
2.Spirit varnish:
- Spirit varnish is obtained by dissolving resin completely in volatile solvent such as methylated spirit. In this, spirit is used instead of oil.
- It dries up quite rapidly, but leaves behind a film which is brittle, so it has a tendency to crack or peel off. It can be avoided by addition of some plasticizers. Moreover, the film is easily affected by weathering.
- For example, spirit varnishes are prepared by dissolving shellac (resin) in the spirit or alcohol. Such varnishes are used for polishing wooden furniture.
Characteristics of a Good Varnish:
- A Good varnish should possess the following characteristics:
- It should dry very quickly.
- It should be soft and tender.
- Also, it should produce a protective film such as hard, tough, durable, quite resistant to wear and tear.
- It should produce glossy and shining film on drying.
- It should be able to adopt itself to contraction or expansion of coated material like wood due temperature variations.
- Also, it should give elastic film, which does not crack or shrink or peel off on drying.
- It should yield aesthetical appealing film.
- Lastly, it should yield film, whose colour does not fade or change on exposure to atmospheric weather.
Constituents of Varnish:
It contains following constituents:
- Resin:
- Natural resins-like shellac, copal, rosin etc.
- Synthetic resins-phenol-aldehyde, alkyds, urea formaldehyde etc.
- Drying oils: Linseed oil, tung oil, fish oil, dehydrated castor oil, soyabean oil etc.
- Thinner (or solvents): Turpentine, petroleum spirit, kerosene, xylol, tolyl, ethanol etc.
- Driers:Soaps of heavy metals e.g. these include Pb, Co and Mn naphtheantes, resinates,linoleates.
- Antiskining agent like tert – amyl phenol.
Uses:
- Varnishes are used:
- Firstly, For the protection of articles against atmospheric corrosion.
- Secondly, As brightening coat to painted surface. One first coats surface with paint and over then they applies a coat of varnish which prevents corrosion.
- Lastly, for improving the appearance and intensifying the ornamental grains of wooden surfaces.
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