Course : B.E & B. Tech Subject: Applied Chemistry Unit: I. Paint: Paint is a mechanical dispersion of one or more pigments in a vehicle.

Transcription

1 Course : B.E & B. Tech Subject: Applied Chemistry Unit: I PROTECTIVE COATINGS More than You Know! Syllabus: Protective Coatings Introduction Constituents, Functions and Uses of, Paints Varnishes Lacquers Dr. K. SIVAKUMAR Department of Chemistry SCSVMV University Paint: Paint is a mechanical dispersion of one or more pigments in a vehicle. Vehicle: Vehicle is a liquid consisting of non-volatile film forming material, drying oil and a highly volatile solvent thinner. When paint is applied on a metal surface, the thinner evaporates while the drying oil slowly oxidizes forming a dry pigmented film. 1

2 Requisites (Characteristics) of a good paint: A good paint should possess the following characteristics, It should be fluid enough to spread easily on the metal surface. It should have a high covering power, corrosion and water resistance. It should not get cracked on drying. It should form a tough, uniform, adherent and impervious film. It should give a glossy film, decent and stable colour on the metal surface. Constituents of paint and their functions: Pigments: Pigments are solid, colour producing substances. It comprises 60-80% of the weight of paint film. Different types of pigments Colour of pigments White Black Red Blue Green Brown White lead Example Carbon black Chrome red Prussian blue Chromium oxide Brown umbre 2

3 Functions of pigments: It provides colour and opacity to the film. It provides strength to the film. It provides resistance to paint against abrasion/wear. It protects the film by reflecting harmful UV rays. It gives impermeability of paint film to moisture. A good pigment should be, chemically inert, non-toxic, free mixable with vehicle and cheap. Drying oil or Vehicle Drying oil is the film forming, non-volatile constituents of paint. These are glyceryl esters of high molecular weight fatty acids present in animal and vegetable oils. Examples: Linseed oil, Dehydrated castor oil etc., 3

4 Functions of drying oil or vehicle Drying oil form a protective film by the oxidation and polymerization of the vegetable oil. They hold the pigment particles together on the metal surface. They impart water repellency, toughness, durability and waterproofness to the film. Thinners (solvents) Thinners are solvents that are added to paint to reduce viscosity of paints, so that they can be easily applied on metal surface. Examples: Turpentine, benzene, kerosene, petroleum spirit, naphtha, etc., Functions of thinners It is used to dissolve oil, pigments etc., It reduces the viscosity of the paint. It increases the elasticity of the paint film. It increases penetrating power of the vehicle. It evaporates readily and helps drying of film. 4

5 Driers Driers are oxygen-carrier catalysts. They accelerate the process of drying of oil film by oxidation, polymerization and condensation. Examples: Resinates, Tungstates and Naphthenates of Co, Pb, Mn, and Zn. Functions of driers Driers provide oxygen, which is essential for the oxidation, polymerization and hydrogenation of drying oil. Fillers or Extenders Fillers are white or colorless and finely divided solids having low refractive indices. Examples: Talc, CaCO3, China clay and Gypsum gypsum clay Functions of fillers It reduces the cost of the paint. It prevents shrinkage and cracking of the film. talc It increases the durability of paint. It retards the settling of pigment. It modifies the shades of the pigment 5

6 Plasticizers Plasticizers are added to the paint to give elasticity and to prevent cracking of the film. It reduces the brittleness and increases the adherences of film on the surface of metal. Examples: Triphenyl phosphate, tributyl phthalate. Anti skinning agents Anti skinning agents are added to the paint which prevent gelling and skinning of the paint film. Example: Polyhydroxy phenols. 6

7 Plasticizers Plasticizers are added to the paint to give elasticity and to prevent cracking of the film. It reduces the brittleness and increases the adherences of film on the surface of metal. Examples: Triphenyl phosphate, tributyl phthalate. Varnish is a homogeneous colloidal solution of natural or synthetic resin in thinner oil or both. When Varnish is applied on a metal or wood surface, the it dries by evaporation, oxidation and polymerization giving a transparent, durable and glossy film on surface. 7

8 Requisites (Characteristics) of a good varnish: A good varnish should possess the following characteristics, It should be soft and tender. It should produce a glossy and shinning film on drying. It should adopt itself to contraction and expansion due to temperature variations. It should dry quickly. It should not shrink / crack after drying. Types of Varnishes: 1. Spirit Varnish 2. Oil varnish 8

9 Oil varnish: Oil varnish is produced by dissolving natural or synthetic resins in a drying oil and volatile solvent. Oil varnish dries up by evaporation of volatile solvent followed by oxidation and polymerization of drying oil. It is used for exterior as well as interior works. Example: Copal varnish (copal + linseed oil + Turpentine). Spirit Varnish: Spirit varnish is produced by dissolving natural or synthetic resins in a completely volatile solvent. Spirit varnish dries up by evaporation of solvent. It is used for polishing wooden furniture. Example: Shellac in alcohol 9

10 Constituents of varnish and their functions: Resins: Natural or synthetic resins can be used. Resins give elasticity, good adhesion, colour and glossiness to varnish film. Example: Phenol-formaldehyde and vinyl resins. Drying oils: They dry by oxidation and polymerization. Example: Linseed oil, Fish oil and Soybeans oil. Solvents / Thinners: Solvents are used for dissolving various constituents of varnish. They reduce viscosity and help in drying process. Example: Turpentine, Ethyl alcohol, Kerosene. Constituents of varnish and their functions:.. Driers: Driers are added to enhance the drying rate of varnish film. Example: Co, and Pb slat of linoleates, resinates and naphthenates. Anti skinning agents: They prevent the skinning and peeling off of varnish. Example: Tertiary amyl alcohol. 10

11 Uses of varnishes: To protect articles against corrosion. Used as a brightening coat to painted surface. Used for improving appearance of wood surface. LACQUERS Lacquers are colloidal dispersion of cellulose derivative, resins and plasticizers in solvent or in diluents or both. Lacquers dries quickly in air by evaporation of solvent and diluents forming a transparent hard glossy and waterproof film. 11

12 LACQUERS Constituents, examples and functions of lacquers Constituents Example Functions Resins Alkyd, Copal Cellulose derivative Cellulose nitrate Plasticizers Dibutyl phthalate Solvents Ethyl acetate Diluents Benzol, Toluol Provide adhesion and to increase thickness of film. Provide hardness and to increase durability to film. To reduce brittleness and to increase elasticity of film. To dissolve film forming constituents. To reduce viscosity of medium LACQUERS Uses of lacquers Used for interior decorations like painting woodwork and furniture. Used in the preparations of artificial leather goods. Used is giving finishing coat to automobile bodies. 12