COMPOSITION of a COATING

by Gautam Shah ➔

Coatings are thin surfacing. A coating comes into being mainly through a phase-change of a material. At pre-application-stage a coating material is in various phases such as solid, liquid or vapour, or a combination thereof like: suspension, solution, dispersion, emulsion, thermoplastic compound, thixotropic compound, etc. During application it changes to a Liquid phase, but once applied, it settles down to a heavier phase, usually (but not necessarily) a Solid phase.

Coatings are not exactly considered to be adhered-finishes, because the coating film is formed and (mainly) bonded through a lower to higher phase conversion process.

Coating is a thin Surface Finishing Technique. A coating system has Four main components.

1. Film forming substances

2. Solvents and Diluents

3. Pigments and Extenders

4. Other additives

Film forming substances are the prime materials of a coating system. Even if other components are not present, a coating (Clear coating) can still be created. Most of the film forming substances are in liquid form or convert to a liquid phase just prior to application. Film forming substances provide a film that binds all the ingredients, and also develop inter-facial adhesion between the coating and the substrate.

Solvents and diluents, impart solubility to specific film forming substances, allow dispersion and adjust the viscosity.

Pigments are chief colourants to coloured coatings. In some clear as well as coloured coatings, Dyes (dispersed or dissolved) are added to provide specific tinge (staining) to the coatings. Other colourants include opacifiers, whitening agents, fluorescent phosphorescence agents. These besides providing colour, impart transparency translucency, opacity, fluorescence, phosphorescent and luminance to the film. Extenders or fillers are low refractivity materials, usually white, but not necessarily. These are added to provide bulk to the coating, economise the use of pigments.

Additives help in manufacturing, storing, applying, drying and maintenance of a coating. These include surfactant, anti skin agents, wrinkle agents, dryers (metallic compounds that helps in polymerization of resins), thixotropic agents, foaming and de-foaming agents, catalysts, odour suppressants, mildewcide, insecticide, fungicide, plasticizers, anti setting agents, etc.

A clear coating (often called Varnish) has no pigments or extenders. For a specific tinge (transparent staining) dyes or very low proportion of pigments are used.

Surface treatments: Coatings are generally capable of forming a film on the relevant surface. But to receive a coating, the surface may require some modification or treatment. Surface treatments form a very important section of coating technology. Surface treatments are specific for the surface to be coated, such as cleaning, roughening, smoothening, etching, etc., or moisture proofing, rust inhibiting, barriers, static arresters, etc. Other surface treatments are designed to facilitate the application or deposition of the surface forming component and its setting or drying.

Coatings are deposited on objects by many different techniques and in variety of conditions. Common coatings are applied at normal environmental conditions. However, high end coatings require conditions that are usually unavailable at construction sites such as exact environment and controlled application, drying and handling techniques.

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COLOURANTS -DYES and PIGMENTS

Post  -by Gautam Shah

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Colours -Biennale Venezia > Flickr Image by Strolic Furlan -Davide Gabino

Dyes and pigments are colourants. These substances impart a colour to a material. Pigments are nearly insoluble materials, resulting in a suspension, whereas the Dyes dissolve during application, losing their particulate structure, resulting in a solution. Often a pigment is made by attaching a dye molecule to an insoluble particle. The difference between the two, can be said due to the physical characteristics rather than chemical composition.

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Sun looking back at Trau > ART by Tivadar Kosztka Csontvary (1853-1919)

Dye molecules are comparatively smaller, so if each is presumed to be of pinhead size, relative size of a pigment particle would be of a football.

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Traditionally pigments are considered to be more light-fast than dyes. Light destroys coloured objects by breaking open electronic bonding within the colourant molecule. Light fastness or permanence and stability are desirable properties of colourants. Lower Dyes are very much vulnerable. Pigments that are not permanent are called fugitive, as they over-time fade out.

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Natural dyes for Alpaca Yarn, Peru, > Flickr image by Ken Bosma

Pigments change the colour of reflected or transmitted lights as a result of wavelength-selective absorption, wavelengths or parts of the spectrum are reflected or scattered. Pure pigments allow very little white light to escape, producing a highly saturated colour. However, a dye can only absorb light, and not reflect or scatter it. Pigments, can only subtract wavelengths from the source light, never add new ones, unlike the fluorescent or phosphorescent substances.

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Organic pigments have carbon compounds. Natural ones were of animal and vegetable origins. Synthetic organic pigments include, alizarin, azo-pigments (yellow, orange red), phthalocyanine (variants of blue, green colour) and quinacridone (red-violet).

Inorganic pigments (of mineral origins) are metal compounds compared to organic pigments. Examples of Natural inorganic pigments: Oxides, umbers, ochres and siennas. Pigments produced synthetically often have the same names. Other Synthetic inorganic pigments include, cadmium yellow/orange/red, cobalt blue and titanium white.

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Coatings are clear, dyed or pigmented. Clear-coatings are essentially, transparent to translucent in effect. Though, ‘water-white Clear Coatings are without any type of colourants. However, some tinted clear coatings or stainers include soluble colourants or dyes and often very minute quantity of non-soluble colourants or pigments. Dyes, are colourants soluble in water, organic solvents or oils. Clear coatings are used for where a very high level of transparency is required such as for the production of coloured metal foils and films, internal protective layer for cold-drink tins, coatings over paper, for flamboyant finishes in the paint industry and for wood stains (for floorboards and furniture). Dyes are also used for imparting colour to thermoplastics.

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Tinted Glass > Pixabay Image by mploscar

Pigmented coatings are mainly opaque. The opacity of the film, in a few cases, is inherent in the molecular lattice of the film forming medium, but mostly is imparted by additives (pigments and extenders) of crystalline nature. Pigments, natural or synthetics have a fine particulate matter, but these are in agglomerated state and require dispersion. Pigments are ground using ball-mill, roll-mill and attritor like equipments. Dispersion increases the spread of pigment or its covering or colouring capacity. Hiding capacity of pigment also depends on the particle size, shape and distribution, concentration, and also on the film thickness and clarity of the film forming substance. Pigments due to their particle size, shape and structure have greater capacity to reflect, refract and diffract the incident light, compared to extenders on volume basis.

Colour for Coatings

Additives with optical properties form the major and the quantitatively substantial part of a coating system, which otherwise consists of liquids or liquefiable film forming substances. Extenders, are low refractivity materials. Extenders have refractive index equal or lower than the pigments. Extenders provide bulk to the film, condition the viscosity, help even distribution of pigments, improve the brush-ability and define the gloss level and texture.

Pigment grinding machine

Extenders are mostly inorganic compounds like oxides, hydroxides, carbonates, silicates and sulphate, of metals like barium, calcium, aluminium and magnesium. Commercial extenders are asbestine, barytes, blanc fixe, china clay, diatomaceous silica, dolomite, mica, precipitated calcium carbonate, talc and whiting or chalks. Pigment and extender particles, both are held together by strong forces of attraction and are often enveloped by air moisture and other affinitive substances.

INDIA – Holi Festival of colours Radha playing with colours Painting of C-1788

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