Basic components of varnishes and paints

Film of varnishes and paints coating is formed by binder, which is included in the composition of both priming and facing layers. For obtaining of painting mixtures there are used mineral binders (lime, cement, liquid glass); glues, which are made of materials of animal origin (bones, casein, leather); vegetable glues (starches, dextrin, flour), synthetic glues (carboxymethylcellulose). Films formed by water-soluble glues except casein, can be again diluted at water treatment that is why mixtures on their basis are not applied for the external painting of premises at high humidity of environment.

Drying oils and synthetic insoluble polymers are the binders for waterless mixtures. They provide the forming of water-resistant coating as well as cement and liquid glass. Drying oils are the binders for oil paints. They are divided into three groups: natural, compacted (seminatural) and artificial (synthetic).

Natural drying oils are the products of heat treatment at 150-300°C of vegetable oils with the addition of 2-4% of siccatives - manganous or cobalt salts of fatty acids for hardening accelerating. Compacted drying oils are obtained by the dissolution of vegetable oils, with the following compacting by oxidizing or polymerization. Synthetic drying oils are produced by heat or chemical treatment of polymers often with admixtures of vegetable oils. Such variety of drying oil is glyptal one.

All drying oils harden at the air conditions in the thin film during 12-24 h. Natural drying oils have the highest quality; they are used for the production of high quality oil paints for the external and internal painting of metal structures, roofs and joinery.

From synthetic polymers, which are used as binders for varnishes, paints and enamels, phenolformaldehyde, carbamide, perchlorovinyl, polyvinylacetate and other high molecular compounds are the most effective.

Insoluble binders are used in combination with organic solvents or as water emulsions. At first case there are obtained volatile resin mixtures, in other case - water-based mixtures. Organic solvents, as a rule, are toxic; their application requires special measures for safety and industrial sanitation. Water-based paints have no smell, harmless, they are easily applied at the surface, including moisture, and dry out rapidly. But they form less dense coatings, than volatile resin mixtures, less stable and frost-resistant; they can be subjected to microbiological destruction.

Table 18.2

Application of mineral pigments

Basic properties which determine the quality of binders for varnishes and paints are viscidity, density, colour, transparency, time of hardening, adhesion, hardness, flexibility, ability to be polished.

Painting materials, besides binders contain pigments - finely dispersed materials, non-dissolved in binder substance and solvent and capable in the mixture with them to form the opaque coatings of diverse colors and tints. At the selection of pigments there are considered the color, light- and the weathering resistance, resistance to the binder action, to oil absorption, and also resistance to the influence to hydrogen sulfide and other chemical compounds.

Pigments are divided into white, black and with different tints of grey, red, yellow, green, blue and brown. In Table 18.2 mineral pigments widespread in construction are shown.

Pigments of red, blue and yellow colors are considered as the basic. Other colors can be obtained by mixing pigments between itself in different proportions (Fig. 18.2). For example, green color is obtained at mixing of blue and yellow pigments, violet - red and blue, orange - red and yellow and so on. Ability of pigments to give at mixing the required colouring is called the painting ability or intensity.

Painting ability is determined by the whitenings, i.e., by mixing pigments with the consecutively growing dose of chalk for the aqueous, or white for the oil mixtures and by the comparative estimation of obtained paintings. The pigments of small intensity, for example ocher, become unobtrusive already at whitening 1:15, with the high intensity, for example azure keeps its tints at whitening to 1:2017 and more. Such intensive pigments for the dilution and the reduction of prices are mixed up with the fillers - chalk, kaolin, and other.

The second important characteristic of pigments is the spreading capacity or covering property - the dose of pigment, required for the complete coating previously laying on the surface layer of contrasting paint. The pigments of low intensity at the same time can have high spreading capacity. For example, ocher has large spreading capacity (60-90 g/cm²) with the small painting ability, azure - vice versa. Spreading capacity is increased with the increasing of the difference of the refractive indices of light by pigment and binder.

For the improvement of a series of properties and the reduction of prices of the painting mixtures fillers are used. For the aqueous painting mixtures by the simultaneously coloring substances and as fillers the chalk and lime can be used. For the savings of white in the oil paints barites is applied. In chalk-glue mixtures for the giving of the best coating ability kaolin is added. Into oil, silicate, emulsive and some other mixtures for increasing the water and weather resistance, mechanical strength talc is introduced as filler. Asbestos admixture attaches to the paints enhanceable fire resistance and the strength (Table 18.3).

Таble 18.3

Basic properties of fillers of the paintwork materials

Solvents and diluents are applied for reduction of paintwork material’s viscosity to the viscosity, required at the selected method of material spreading. Introduction of solvents retards the process of film formation; their excess can also result in the decline of the strength and density of coating, increasing of shrinkage and creep.

The volatile liquids with the boiling point at 50-200 °C are utilized as solvents (Table 18.4). According to the ability to dissolve this or other substance or only to dilute it until less viscidity, there are distinguished active solvents and substances which can be only diluents. The same solvent can be either active solvent or diluent for the different binders.

As diluents there are also applied drying oils, emulsions, adhesive solutions, which, unlike the solvents, contain film-forming materials and at the same time are applied for diluting of the pastes or dry inorganic paints.

Table 18.4

Basic solvents of the paintwork materials

Special admixtures can be added for adjustment of properties of the paintwork materials (except of the basic components - binders, pigments, fillers, solvents and diluents): siccatives, plasticizers, water-repellent agents, initiators, activators, hardeners, thixotropic agents, etc.

Siccatives accelerate drying of oil and enamel paints, varnishes and drying oils. As siccatives the solutions of lead- manganese salts of naphthenic acid or mixture of naphthenic acid with the drying or semi-drying oils in the gasoline or the turpentine are used.

Plasticizers give to the paint and varnish coatings the elasticity and enhanced resistance to the shock loadings. Hydrocarbons, their halogen derivatives, ethers, ketones, alcohols, amines and others belong to them. The most considerable at the production of paintwork materials are the following plasticizers: non-drying vegetable oils; various ethers - phthalates, sebacates, phosphates; resins of alkyd type, chlorinated paraffins and biphenyls.

It is possible to enhance the adhesion of coating film to the basement by the plasticizers; however the plasticization can decrease the hardness and enhance superfluously the plasticity of coating film. Plasticizers are tested on the strength, elasticity and other properties. Plasticizer should be colourless, odourless, untoxic, and resistant to the action of ultraviolet rays.

Date: 2015-12-18; view: 955