1. Introduction
Paints and varnishes are important protective and decorative coatings used on various surfaces such as wood, metal, concrete, and walls. They protect the surface from corrosion, weathering, and microbial attack while providing aesthetic appeal.
2. Definition
2.1 Paint
Paint is a pigmented liquid or paste that, when applied to a surface, forms a thin solid film after drying. It consists of pigments, binders, solvents, and additives.
Key Point: Paint = Pigment + Binder (Vehicle) + Solvent + Additives
2.2 Varnish
Varnish is a transparent, protective finish that does not contain pigments. It is mainly composed of resins dissolved in oil or spirit solvents. Varnish provides a glossy, durable, and protective coating without hiding the natural texture or color of the substrate (especially wood).
3. Components of Paint
| Component | Percentage (approx.) | Function | Examples |
|---|---|---|---|
| Pigment | 10–30% | Provides color, opacity, and protection against UV and corrosion | TiO₂ (white), ZnO, Carbon black, Iron oxides, Chrome pigments |
| Binder (Vehicle/Non-volatile vehicle) | 20–40% | Forms the film, binds pigment particles, and adheres to the surface | Linseed oil, Alkyd resins, Acrylic resins, Epoxy resins, Polyurethane |
| Solvent (Thinner/Volatile vehicle) | 30–50% | Dissolves the binder, controls viscosity, and evaporates during drying | Turpentine, White spirit, Toluene, Xylene, Water (in water-based paints) |
| Additives | 1–10% | Improve specific properties | Driers (Co, Mn salts), Anti-skinning agents, Plasticizers, Fungicides, UV absorbers |
4. Chemistry of Paints and Varnishes
4.1 Drying Mechanisms
- Physical Drying (Evaporation): Solvent evaporates leaving behind the binder (e.g., nitrocellulose lacquers).
- Oxidative Drying: Unsaturated oils (linseed oil) react with atmospheric oxygen to form a cross-linked film. Catalyzed by driers (metal soaps like cobalt naphthenate).
- Chemical Curing (Polymerization): Two-component systems (e.g., Epoxy + Hardener, Polyurethane) where cross-linking occurs via chemical reaction.
4.2 Important Chemical Reactions
Oxidative Drying of Linseed Oil (Simplified):
R-CH=CH-R' + O₂ → Peroxides → Cross-linked polymer film
(Autoxidation followed by polymerization)
R-CH=CH-R' + O₂ → Peroxides → Cross-linked polymer film
(Autoxidation followed by polymerization)
4.3 Types of Resins Used
- Oil-based: Alkyd resins (modified with drying oils)
- Water-based: Acrylic emulsions, PVA (Polyvinyl acetate)
- Solvent-based: Epoxy, Polyurethane, Chlorinated rubber
5. Classification of Paints
- Based on Medium: Oil paints, Water paints (Emulsion/latex), Spirit varnishes
- Based on Use: Decorative, Protective (anti-corrosive), Marine, Heat-resistant, Fire-retardant
- Based on Finish: Gloss, Semi-gloss, Matte/Flat, Eggshell
6. Varnishes
Types of Varnishes
| Type | Composition | Characteristics | Applications |
|---|---|---|---|
| Oil Varnish | Resin + Drying oil + Solvent | Highly durable, glossy, slow drying | Wood furniture, floors, outdoor woodwork |
| Spirit Varnish | Resin + Alcohol (Spirit) | Fast drying, less durable | Indoor furniture, musical instruments |
| Turpentine Varnish | Resin + Turpentine | Good gloss and flexibility | General wood finishing |
| Polyurethane Varnish | Polyurethane resin | Excellent abrasion & water resistance | Floors, marine applications |
Learn about Chemistry of Paints in details
7. Applications of Paints and Varnishes
- Decorative: Interior & exterior walls, furniture, automobiles
- Protective: Anti-corrosive paints for ships, bridges, pipelines (e.g., Red lead, Zinc chromate primers)
- Industrial: Heat-resistant paints (silicone-based), Fire-retardant paints
- Specialty: Anti-fouling paints for ships, Luminescent paints, Anti-microbial paints
- Wood Protection: Varnishes for doors, windows, furniture to enhance grain and protect from moisture
8. Key Points for B.Sc./M.Sc. Examinations
- Paint is a dispersion of pigment in a vehicle.
- The film-forming material is called Binder.
- TiO₂ is the most widely used white pigment due to high refractive index and hiding power.
- Linseed oil is a drying oil containing linolenic and linoleic acids.
- Drying of oil paint involves autoxidation and polymerization.
- Varnish is a clear solution of resin in oil or solvent.
- Two important tests: Drying time and Hiding power.