Module 5. Building construction materials and type of building construction

Lesson 32


32.1 Introduction

The purpose of painting is to protect the building and engineering materials from corrosion and rusting. This will greatly enhances the useful life of the building and equipments. Painting and coatings also decorative look to the building. The selection of appropriate paint and coating material is very important to get optimum result. The control of mold growth over the building and other parts of walls, ceilings, cold storages etc. is one of the essential requirements in dairy and food plants.

32.2 Painings

The objectives of painting are as under.

• It protects the surface from weathering effects and effect of other gases and fumes.
• It prevents decay of wood and wood based products.
• It prevents corrosion in metals used in dairy plants.
• It gives good appearance to the surface.
• Painting makes the surface smooth for easy cleaning.
• It makes the surface hygienically good, clean and attractive.

Paint is made of two broad components pigments or solid powders and vehicles (carriers), which transfer the pigment onto surface. Paint consists of base material, carrier, drier, colouring pigments and solvent. A base is a solid substance in a fine state which forms the bulk of paint. It forms opaque layer over the surface of the material to be painted. Vehicles are the liguid substances which hold the ingredients of paint in liquid suspension. Paints and other protective coatings deteriorate rapidly in dairy plants because of constant exposure to moisture, acid, alkali and high humidity. Painted surfaces showing evidence of deterioration must be attended immediately to maintain coating of the paint on the surfaces. It is noticed that break in colour film may cause similar failure surrounding the area. If timely maintenance is not carried out, then hygienic conditions will be adversely affected. It is possible to extend the life of painted surface inside the plant by minimizing condensation of steam/water vapour by installing mechanical ventilators.

Adjoining rooms at different temperatures differ in vapour pressure which causes moisture migration from one room to another. Under such conditions, moisture may penetrate the paint film on the colder surface. In order to minimize this effect, the wall of the warmer room must have an impervious paint film to provide a vapour barrier.

The surfaces to be painted should be cleaned and dried before applying paint coating. Apply rust inhibiting primer on bare unpainted steel before applying colour coating. If previously painted surface is to be repaired, clean the metal surface by using wire brush and then apply the colour. Concrete and plastered surfaces should be thoroughly cleaned by means of wire brush to remove loose paint before applying new paint.

Dairy products readily pick up solvent fumes from paint and therefore, it is necessary to avoid brushing or spray painting in room containing milk or milk products. During painting, every effort should be made to provide the best possible ventilation both to minimize product contamination and to reduce the nauseating effect of the paint fumes on the painters. Ventilation should be continued until the paint is dry and the room is free of solvent fumes.

32.3 Characteristics of Ideal Paint

The following are the ideal properties of paint.

• Good spreading power
• Low cost
• Ease of application
• Drying in reasonable time
• Form hard and durable surface
• No effect on the health of painters/workers
• No effect of weather
• Attractive and pleasing appearance
• No cracks on drying
• Produce uniform film

32.4 Types of Paints

32.4.1 Oil paint

It is oil based ordinary paint which is applied on wood, plywood, metal surfaces walls etc. It is necessary to apply oil primer before the application of two coats of oil paint. The surface to be painted should be free from moisture before the application of primer coat. These pains are available in glossy and mat finish variety to select as per the requirement. It can be applied with brush/spray painting or roller painting.

32.4.2 Plastic paint

It is water base colour which is commonly used for painting walls, ceilings, etc. It can be diluted with water. It is commonly applied with brush or roller. It is necessary to prepare the surface form the application of plastic paint. A primer coat of cement paint is applied on the surface and then wall putty is filled to make the surface smooth. The surface to be painted should be clean and rubbed with sandpaper/water paper to get very smooth surface. Subsequently, two coats of plastic paints are applied on the surfaces to be painted. Thousands of colour sheds can be prepared by addition of coloring agents in these paints. Colour suppliers have computerized system to add metered amount of coloring agents in the base of the colour. Hence, there is a wide range of colour sheds to select for the requirement. When the paint dries, film of binders, pigments and other solid is left on the surface. These paints are generally available in thick consistency and water is required to be added for uniform application on the surfaces. After drying of paint, surface can be washed.

32.4.3 Aluminum paint

It consists of very finely ground aluminum suspended in a medium composed of a quick drying spirit varnish or slow drying oil varnish, according to the requirement. It protects iron and steel from corrosion far better than any other paint. It is widely used for painting marine pillars (supports), oil storage tanks, gas tanks, etc. It also resists heat to a certain extent, so it is applied to radiators, hot water pipes. It is also good for decorative purpose.

32.4.4 Anticorrosive paint

This paint consists of oil and strong drier with chromium oxide or lead or zinc chrome as pigment.

32.4.5 Emulsion paint

It contains binding material such as polyvinyl, synthetic resins etc. This paint is easy to apply and it dries within 2 hours. The surface of the paint is tough and it can be cleaned by washing with water. It is advisable to make the surface smooth before application of paint. A primary coat of cement paint is applied followed by two coats of emulsion paint.

32.4.6 Enamel paint

This paint is available in different colours. It contains white lead or zinc white, oil, spirit and resins. It dries slowly and forms hard durable surface which is not affected by acids, alkalis, fumes, etc.

32.5 Varnishes

It consists of resins dissolved in volatiles. It is made by dissolving the heated resins in hot oils and adding turpentine. Varnishes are available as transparent or translucent. The oil oxidizes to form a tough protective film. Depending on the solvent used, varnishes are classified as under.

• Oil varnishes
• Spirit varnishes
• Turpentine varnishes
• water varnishes

Linseed oil is used as solvent in oil varnishes while methylated spirit is used as solvent in spirit varnishes. Spirit varnishes dry quickly but it is not durable. It is used for furniture. In water varnishes, shellac is dissolved in hot water and required quantity of either ammonia or borax or potash or sods is added so that shellac is dissolved. Varnishes are applied by using smooth fine brush.

32.6 Painting for Mold Prevention

It is necessary to carry out painting on building and equipment by adopting recommended procedure in order to make the surface which is not suitable for mold growth. It is desirable to use paint containing a fungicide. The fungicide should be such that it is effective, non toxic, impart no odour or flavour to food products and be economical. Solubilized copper quinolinolate has been found to be an effective fungicide for paints, especially when combined with the paint during the manufacturing process. The surfaces containing mold growth may be treated with a hypochlorite solution containing 0.5 to 1.0% chlorine to prevent a general contamination of the area during the cleaning process. Many patented mold inhibitors are available to use as per the recommendation.

32.7 Painting Problems/ Failures

The basic reasons for the defects in painting are due to (i) atmospheric conditions (ii) defective surfaces and (iii) Incorrect painting methods.

  1. Alligatoring: It occurs due to application of relatively fast drying coat over one which is too soft. The reasons for soft under coat could be use of too much oil, use of unsuitable oil which dries to a soft film or due to insufficient drying time before another coat is applied.
  2. Blistering and peeling: Blistering and peeling is caused by moisture penetration behind the film of paint on the wooden surface and plaster. The change in temperature causes vaporization of moisture which increases the volume. This causes a blister. This may also happen if the seasoning of wood is not done properly.
  3. Cracking and scaling: This happens when paint becomes too brittle as it ages and then it begins to break. The wood expands and contracts due to moisture absorption which may break the film of colour. However, in the long run, the elasticity of the paint decreases. In order to overcome this, more elastic paints of higher grade should be used. In order to repaint the surface, first the old paint should be removed. This can be done by scrapping, using a sand paper or a wire brush, blow torch and scrapper or by using chemical solvents. (liquid paint removal).
  4. Running and sagging: Use of too much oil results in sagging down and applying too much thick coat also results in sagging. Application of paint to very glossy a surface would also result in sagging. It is necessary to maintain proper viscosity and surface finish of the surface to be painted.
  5. Wrinkling: Formation of wrinkles is due to improper drying of paints applied on the surface. This happens when surface dries quickly leaving undried paint below.
Last modified: Wednesday, 26 September 2012, 10:44 AM