Paint
Building Material Civil Engineering
Quick Revision of Paints Civil Engineering
Paints:
- It is the mechanical dispersion of one or more fine pigments in a medium (thinner + vehicle). When the paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes slow oxidation leading to the formation of a pigmented film.
Quick Revision of Paints Civil Engineering
Constituents of paint:
1. Pigment:
- It is an essential constituent of paint. It provides color and opacity, in addition to that imparting strength and aesthetic appeal to the paint.
- Pigments provide colour to the paint
E.g-
- White lead, Red lead, Zinc oxide, Iron oxide, etc
- Black – Lampblack, charcoal black
- Brown – Burnt umber, burnt sienna
- Blue – Persian blue, ultramarine
- Green – Chrome green, copper sulphate
- Red – Red lead, venetian red
- Yellow – Zinc chrome, raw sienna chrome yellow.
2. Vehicle or drying oils:
- It is a liquid that binds the pigment to the surface and protects pigment from decay e.g. linseed oil, dehydrated castor oil, etc.
E.g-
- Linseed oil, Tung oil, Poppy oil, Nut oil,etc
3. Extenders or fillers:
- The function of addition of filler in paint is that it improves the properties of the paint and mainly to reduce the cost. The main purpose is to increase pigment volume concentration.
- Magnesia, alumina, gypsum, silicate, barite are the commonly used fillers
E.g-
- Gypsum, Calcium carbonate, Aluminium silicate, Magnesium silicate, Talc, Diatomite, etc
4. Thinners or solvents:
- It is a volatile solvent, which is often added to paint which helps to adjust the consistency of the paint.
- Solvents of oil paints are used to thin the paints and increase the spread.
E.g-
- Turpentine, White spirit, Naptha, Benzine, Petroleum, etc
5. Drier:
- The main function of a drier is to increase the drying power of the vehicle. They act as oxygen-carrying catalysts.
E.g-
- Litharge, Red lead, Sulphate of manganese, etc
6. Plasticizer:
- They remain permanently in paints and varnishes. They improve the elasticity of the paint film which prevents cracking of the film.
7. Anti-skinning agents:
- These are chemicals added to paints to prevent gelling and skinning of the film is called plasticizers.
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Types of paints are
- 1. Oil paint: These paints contain white lead as base.
- 2. Enamel paint: This paint is prepared by adding white lead or zinc to varnish. It is desirable to provide titanium under coat. It may be used for exterior walls also.
- 3. Emulsion paint: It contains binding material like polyvinyl acetate and polystyrene. Cobalt and manganese are the pigments and driers. The paint becomes surface dry within 15 minutes and hardens in 2 hours. The surface is washable.
- 4. Cement paint: It consists of white or coloured cement as base. It is available in the form of powder, which is mixed with water and used.
- 5. Aluminium paint: It consists of finely ground aluminium particles in suspension, in spirit or oil varnish. It is visible in darkness.
- 6. Bituminous paint: It is manufactured by dissolving asphalt of vegetable bitumen in oil orpetroleum. It is black in colour. It is used for painting portions of wooden posts buried underground.
- 7. Synthetic rubber paint: It is prepared by dissolving chlorinated rubber in a solvent. It may be applied to concrete surfaces also.
- 8. Celluloid paint: It is prepared by dissolving celluloid sheets or nitro cotton in petroleum. Castor oil is added to improve adhesive property. It is used for painting vehicles.
- 9. Asbestos paint: It consists of fibrous asbestos. It is used for stopping leakages in metal roof,basements. It is used for painting gutters.
- 10. Plastic paint: It consists of plastic as a base and water as a thinner. It gives attractive colours. This is widely used for painting walls in auditoriums and show rooms.
- 11. Anticorrosive paint: It consists of linseed oil as vehicle and lead or zinc chrome as base. Finely ground sand is added as filler. It is black in colour and gives protection from corrosion.
Quick Revision of Paints Civil Engineering
Distemper
- A distemper is composed of base, carrier, colouring pigments and size.
- Water is used as a carrier.
- Whiting or chalk is used as a base.
Note:
- White lead is a base for paint
- Linseed oil is a carrier for paint
- Poppy oil is also a carrier for paint
Quick Revision of Paints Civil Engineering
Varnish:
- Varnish is nearly homogenous solution of resins in oil, alcohol or turpentine. Type of solvent used depends upon the type of resin used.
Quick Revision of Paints Civil Engineering
French Polish:
- It is a type of spirit varnish prepared by dissolving resin in Methylate spirit at room temperature for use on hardwood substance to hide the grain defects. The surface is made smooth by rubbing.
Wax Polish:
- It consists of bees wax dissolved in turpentine and is used for highlighting the grain over wooden surfaces. The polish is rubbed over the surface with rag until a bright appearance is obtained.
Quick Revision of Paints Civil Engineering
Varnishes are classified as follows-
1.Oil Varnish –
- uses linseed oil and takes 24 hours to dry. It is suitable both for interior and external works.
2. Spar Varnish –
- Derives its name from its use on spars and other parts of ships. It gives sticky effect in warm weather and is not used indoors.
3. Flat Varnish –
- Materials such as wax, metallic soap or finally divided silica when added to varnish produce a dull appearance on drying known as flat varnish.
4. Spirit varnish –
- it is resin dissolved in spirit. The examples are French polish, lacquer and shellac varnish. It dries very quickly.
5. Asphalt varnish–
- it is made by dissolving melted had asphalt in linseed oil with a thinner such as turpentine or petroleum spirit. It is used over shop fabricated steel works.
Bonus note
- Ultimate Bearing Capacity is given by
- qu = CNc + q’ Nq + 0.5 Bγ’Nγ
- C = 0 (As cohesionless soil)
- ∴ q’ = γ’D
Quick Revision of Paints Civil Engineering
Enamel Paint
- In Enamel paints varnish is used as vehicle where as in the common oil paints Linseed oil is used as vehicle.
- Enamel paint means any solvent-based paint that dries to a hard, vitreous (glass-like) shell. Solvent-based paints are also called oil-based paints, in contrast with water-based paints.
Quick Revision of Paints Civil Engineering
Carrier:
- It is an oily liquid in which the base and pigment are soluble. It facilitates the paint to be conveniently spread evenly over the surface by means of a brush. It acts as a binder for the base and cause it to stick to the surface.
- Oils most commonly used as vehicles are: Linseed oil, Poppy oil. Nut oil and Tung oil.
Quick Revision of Paints Civil Engineering
Driers:
- These are metallic compounds which when added to the paint in small quantities accelerate the process of drying of the paint. Driers have a tendency to affect the colour of the paint and also to destroy the elasticity of the paint.
- Litharge, manganese dioxide, lead acetate and cobalt are the usual driers, out of which litharge is most commonly used.
