Paints are surface coatings generally suitable for site use, marketed in liquid form. Paints are surface coatings generally suitable for site use, marketed in liquid form

Paints are surface coatings generally suitable for site use, marketed in liquid form.

• Paints are surface coatings generally suitable for site use, marketed in liquid form.

• They may be used for one or more of the following purposes:

• To protect the underlying surface by exclusion of the atmosphere, moisture and insects.

• To provide a decorative easily maintained surface.

• To provide light- and heat-reflecting properties.

• To give special effects;

• for example, inhibitive paints for protection of metals; electrically conductive paints as a source of heat; condensation- resisting paints.

Painting constitutes a small fraction of the initial cost of a building and a much higher proportion of the maintenance cost.

• Painting constitutes a small fraction of the initial cost of a building and a much higher proportion of the maintenance cost.

• It is advisable to pay careful attention to the subject at construction stage.

• There are a number of situations in which restoration is both difficult and expensive once the original surface has failed and weather has affected the substrate;

• for example, clear film forming coatings on timber, and painting of steel.

• In other situations, access becomes more difficult later. Such situations merit special care.

• There are usually three stages in a painting system:

The function of the primer is to provide

• The function of the primer is to provide

• protection against corrosion/dampness and to provide a good key for remaining coats.

• The function of the undercoat is to provide

• good opacity (hiding power) together with a smooth surface, which provides a good key for the finishing coat. Undercoats usually contain large quantities of pigment to provide hiding power.

• Undercoats and priming coats do not in themselves provide an impermeable dirt-resistant coating.

This must provide a durable layer of the required colour and texture.

• This must provide a durable layer of the required colour and texture.

• most finishing coats were gloss finish and these tend to have the best resistance to dirt since they provide very smooth surfaces.

• Silk or matt finishes can be obtained if preferred and some paint types such as emulsions will not normally give the high gloss of traditional oil paints.

The types and proportions of paint constituents tend to be evolved by the manufacturer from experience rather than being designed from 'first principles'.

• The types and proportions of paint constituents tend to be evolved by the manufacturer from experience rather than being designed from 'first principles'.

• The main components of paint are:

• vehicle or binder
• pigment
• extender

• This is the fluid material in the paint which must harden after application.

• The hardening process may be due to one of the following:

• (a) Polymerisation by chemical reaction with air in the atmosphere. Such paints tend to form a film. They include ordinary ‘oil’ paints.

• (b) Coalescence of an emulsion: Emulsions are pre-polymerised into very small particles which are prevented form coalescing by an emulsifying agent.

• They set by water loss leading to 'breaking' of the emulsion.

• (c) Evaporation of a solvent: Solvents need to be volatile, hence they are often flammable.

Paints based on types (a) and (b) are described as convertible coatings because once set, they cannot easily be re-softened.

• Paints based on types (a) and (b) are described as convertible coatings because once set, they cannot easily be re-softened.

• Once weathered, application of new coatings to these paints therefore relies on the previous paint being roughened to provide a key.

• Paints based on type (c) are described as non-convertible since they can be re-softened by application of a suitable solvent.

• Subsequent coats also tend to fuse into previous coats and they do not form films.

The vehicle is largely responsible for the gloss and mechanical properties of the final coating.

• The vehicle is largely responsible for the gloss and mechanical properties of the final coating.

• Vehicles may be blended with:

• driers, which modify hardening properties;

• plasticisers, which increase the flexibility of the hardened film;

• solvents, which adjust the viscosity of the wet paint;

These are fine insoluble particles which give the colouring ability and body to the paint.

• These are fine insoluble particles which give the colouring ability and body to the paint.

• Primers and undercoats tend to have large proportions of pigment to produce opacity, while finish coats have low proportions, since to produce a gloss, the pigment should be beneath the surface.

• The particle size of pigments is very small in order that maximum colouring power is obtained by minimum thickness of material.

• Inorganic pigments such as titanium dioxide (white) have the best performance in respect of resistance to solvents, colour fastness and heat resistance, though organic pigments tend to produce the brightest, cleanest colours.

These can be added to control the flow characteristics and gloss of the paint with the added advantage of reducing the cost.

• These can be added to control the flow characteristics and gloss of the paint with the added advantage of reducing the cost.

• Because they are not involved in the colouring process they have a particle size larger than that of the pigment.

• Oil Paints and Varnishes

• Well established and are the most widely used paints for general purposes including painting of wood and metals.

• They were traditionally based on linseed oil but modem oil paints are manufactured.

• Once hardened, alkyd resin-based paints behave as thermosetting plastics, being resistant to solution in oils from which they were formed.

• Unfortunately the hardening process continues slowly with time and these paints tend to become brittle over a period of years, especially if exposed to sunlight.

• This leads to cracking, especially when they are applied to substrates with high movement tendency such as timber.

Oil- or alkyd-based paints are made by reacting organic acids with alcohols such as glycerol.

• Oil- or alkyd-based paints are made by reacting organic acids with alcohols such as glycerol.

• If an alkali such as calcium hydroxide contacts an oil-based film, there is a tendency to revert to glycerol with the production of the corresponding salt, which in this case is a soapy material - hence the name ‘saponification’.

• This leads to the breakdown of films and formation of a scum.

• Hence, oil-containing paints should not be used on alkaline substrates such as asbestos cement, concretes, plasters or renders based on Portland cements, especially when new or if there is a risk of dampness.

• Alkali-resistant primers, such as PVA (polyvinyl acetate) emulsion paints, should be applied.

widely used in interior decorating.

• widely used in interior decorating.

• Examples: polyvinyl acetate (PVA) emulsion which are suitable for application to new cement or plaster.

• water-miscible advantage, although, on drying, coalescence of polymer particles occurs, resulting in a coherent film with moderate resistance to water.

• The film is, not continuous, so that the substrate can, if necessary, dry out through the film.

• Acrylic emulsions for painting of timber have now been produced, including a form which results in a medium gloss finish.

• Emulsions must have a certain amount of thermal energy to coalesce, hence, there is a ‘minimum film formation temperature’ (MMMF) for each type.

• Typical MMMF values: for PVA 7°C and for acrylic copolymers, 9°C. They should not be used below these temperatures.

• These are solvent-based paints.

• Plasticisers added to give elasticity,

• synthetic resins are added to give a gloss

• Drying usually occurs rapidly, but well-ventilated areas are essential and the paint is highly flammable.

• Most suited to spray application.

• These properties, together with the fact that the paints give off a penetrating odour, tend to restrict the use of cellulose paints to factory application.

• High-quality finishes can be obtained and the resulting coat has good resistance to fungal attack and to chemicals, including alkalis.

Intended primarily for protection of metals used externally.

• Intended primarily for protection of metals used externally.

• Sunlight softens the paint, though resistance can be improved by use of aluminium in the final coat.

• Chlorinated-rubber paints have similar properties. Some uses are based on their resistance to alkalis.

Ferrous metals

• Ferrous metals

• Steel forms the largest bulk of metals used in building and is one of the most difficult to maintain.

• The best time to paint steel is immediately after production, though mill scale (iron oxide film produced during hot rolling) should be removed because:

• It behaves cathodically to the bare metal and may lead to local corrosion.
• It may eventually flake off due to differential movement.

Grit-blasting may be applied to remove any corrosion.

• Grit-blasting may be applied to remove any corrosion.

• Inhibitive primers, which interfere with the corrosion process should water be present, include red lead (lead oxide), zinc dust, zinc chromate and zinc phosphate.

• Red lead, though toxic, is still preferred in ‘safe’ situations because it is fairly tolerant of poorly prepared surfaces and is amenable to application in thick coats.

They also have superior chemical resistance.

• They also have superior chemical resistance.

• Due to their toxicity, lead-based primers are not recommended for use in domestic situations.

• Where a decorative finish is required, alkyd or aluminium paints may be used.

• Red oxide is an effective primer and is used both in undercoats and finishing coats.

• Special (‘prefabrication’) primers about 15-20 µm thick are often applied to steel soon after production to afford weather protection prior to and during fabrication.

• These have good weathering resistance without the need for further paint coats.

Generally, the wetter the situation and the more aggressive the climate or atmosphere, the more coats should be given.

• Generally, the wetter the situation and the more aggressive the climate or atmosphere, the more coats should be given.

• In extreme situations or where extended life without maintenance is required, protection is only likely to he achieved by:

• impregnated wrappings,
• bituminous or coal-tar coatings,
• thick, factory-formed films or
• prior treatment, such as galvanising.

Zinc and aluminium are the non-ferrous metals most likely to require surface coatings and each provides a poor key for paint, unless surface treatment is first carried out.

• Zinc and aluminium are the non-ferrous metals most likely to require surface coatings and each provides a poor key for paint, unless surface treatment is first carried out.

• Zinc, in particular, reacts with most oil-based paints, forming soluble salts which reduce adhesion.

• Zinc should be degreased with white spirit, followed by roughening of un-weathered surfaces with emery paper.

• Primers containing phosphoric acid are available for this; they often also contain an inhibitor, such as zinc chromate.

• Other suitable primers contain calcium plumbate, zinc dust or zinc oxide.

Wood

• Wood

• Protect as soon as possible after the manufacturing process is completed since the surface is quite rapidly affected by weathering/ultraviolet light, as a result of which the paint adhesion properties significantly deteriorate.

• A primer is essential to penetrate and yet block the pore structure.

• Undercoats are unsatisfactory here, since they often do not penetrate the wood and may flake off later.

Lead-based primers have been replaced by newer types such as aluminium and acrylic water- borne primers, as well as the conventional ‘solvent based’ primers.

• Lead-based primers have been replaced by newer types such as aluminium and acrylic water- borne primers, as well as the conventional ‘solvent based’ primers.

• Acrylic primers are tolerant of higher moisture contents in the timber, though experience shows that permeability is too high for use in single coats as a protection for joinery timber exposed on site prior to installation.

• Where exposure of primed timber is a possibility, it is recommended that the specification should require that priming paints comply with the appropriate Standard, since factory-based primers, in particular, often do not comply with the 'six months' exposure test requirements of current standards.

• Undercoats contribute to the film thickness and therefore protection, though the final coat provides the bulk of the protection.

• rapid drying

• • rapid drying

• • ease of application

• • ease of cleaning equipment, brushes, etc.

• • good durability, particularly cracking resistance

Possible problems of acrylic paints include:

• Possible problems of acrylic paints include:

• they may inhibit hardening of fresh putty,
• they may be affected by rain during drying, and
• gloss levels are not as good as those of conventional alkyd paints.

• Some paints are claimed to be microporous or to accommodate wood movement, or both.

• Tests on microporosity have shown that many such paints do 'breathe', though often not to a markedly greater extent than conventional paints.

There is some evidence that, since cracking and blistering are less likely in such paints, they should have greater life.

• There is some evidence that, since cracking and blistering are less likely in such paints, they should have greater life.

• However, such paints would not be able to overcome high moisture contents caused by water admission at defective joints or breaks in the paint film.

• In some cases, there may be a possibility of higher resultant moisture contents in the wood, due to extra penetration through microporous paints during wet spells.

• Such porous paints produce a ‘sheen’ rather than the high-gloss finish which may be preferred by some clients.

• Hence, although there may be advantages in some situations, the need for adequate preparation for painting, together with good maintenance, still applies.

• Varnishes are essentially drying oils/resins with little or no pigment, which enhance the natural colouring and grain of timber.

• Formulations vary greatly,

• some being suited to external use, though the main problem with all varnishes is that exterior surface maintenance must be done carefully.

• Cracking or peeling of varnish very quickly leads to bleaching or staining of underlying surfaces due to exposure to water and/or ultraviolet light.

• Once affected, it is difficult to restore wood to its fomer state.

Most plastics do not require painting, and paint coats, once applied, cannot be removed by normal techniques.

• Most plastics do not require painting, and paint coats, once applied, cannot be removed by normal techniques.

• Paints, on the other hand, will reduce the rate of degradation of plastics such as polyethylene.

• Adhesion is poor unless the surface is first roughened to give a mechanical key.

• The impact strength of some plastics, such as PVC, may be adversely affected, if painted, by migration of solvent into the paint.