OIL BOUND DISTEMPERS -OBD

 

Post by – Gautam Shah

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Oil bound distemper is one of the most common and oldest oil and water dispersion coating system for masonry surfaces. These emulsion-distempers, are better then Calcimine. This is now an outdated technology in many parts of the world. Such  paint, however, are produced and used in many developing countries of the world. It is rather easy to procure the raw materials and produce it with very little use of power, tools or equipment. It is comparatively an eco-friendly system, as it has very small amounts toxic materials or petroleum solvents.

The Oil Bound Distempers are made by emulsifying Bodied drying oils or their alkyds with water in the presence of casein, glue dextrine (which also provide the film).

Oil-bound distemper are essentially emulsion formed between natural glues or gums mixed with oil in presence of water. The dispersion as emulsion helped near homogeneous mix, which did not allow separation of the ingredients, till its application as a coating. Such formulations were preceded by plain glue water mixes or egg whites. Distemper as emulsion and egg solution were used as interior surface finish and art media since historic times. The medium of  tempera paint with or without the egg, but with glue as the film forming agent, was formally termed ‘distemper’ paint, and commonly called tempera or poster paint. . 

Traditionally distemper or glue based interior paints were made with either animal hide glue or rabbit skin glue. Glue is heated and mixed with a paste of slaked (water soaked) whiting or calcium carbonate and pigment paste. Glue-based distempers were preferred as its pH was compatible with alkaline masonry and plaster surfaces. Oil paints were difficult and costly to produce, and peeled of from the alkaline surfaces. Distemper are water based systems and so dry out very fast, so preferred for not only interior surfaces but wood, paper and canvas  surfaces in art works. These are used for restoration of architectural heritage of 19th C later periods.

Water is mixed with casein, glue or dextrine and an alkali, such as lime or soap, is combined with the oil component such as the Bodied drying oils or their alkyds. The mixture is heavily agitated and emulsion is formed. Stabilizers (to preserve the state of suspension) and antibacterial agents are then incorporated for prolonged storage life.

The film is first formed due to evaporation of water, The Glue component, on removal of water provides the binding. The oil component then forms a latticed (porous) film on the top part of the surface. Oil oxidation of the oils may take more then 24 hours. The hardened film becomes moderately wipe-able or the commercial signature ‘washable distemper’ (compared with the Calcimine) .

After two or three coats over a period of 5/10 years, repainting becomes difficult. Fresh coat of OBD over old OBD softens it to some extent, and causes it to swell. The strength of coating is less than that of oil paint. During drying, the OBD film shrinks substantially and exerts a considerable pull over the underlying film.

Distempers are essentially masonry coatings so contain substantial amounts of, extenders as bodying agent. Extenders are low refractivity white mineral powders like Calcium Carbonate, Barytes, talc, etc. Extenders also reduce the need for White pigments (high refractivity powders) such Zinc Oxide, Titanium Dioxide. etc.

Oil Bound Distempers do not have rich hue colours. Majority of commercial shades are of pastel variety(white added tones). Due to a low amount of Oil or related constituents in the emulsion the finish is low gloss or dull (low sheen or matt).

Oil bound-distempers or OBD, are sold in viscous paste form, reducible with plain water or special thinning liquids, to brush-able viscosity. Proprietary thinning compounds are very light <oil in water> emulsions, often with additives such as silicone oil, stabilizers, plasticizer, and antibacterial agents and occasionally organic solvents.

Water-based latex paints

Oil bound distempers are difficult for re-coating purposes. Their water resistance is also not very good. So some time in 1950-1960s a range of Vinyl-based emulsion polymers began to replace the caseins and natural gums -the most moisture susceptible components. Later even the oils were eliminated. Resultant new product was known as the water-based latex paint. Latex is a term more used in USA, where all plastic emulsion products were also known as Latex systems. In most other countries Plastic emulsion paint is popular term.

Vinyl-based emulsion coating systems had good binding and slightly better water resistance, but their UV (solar light) resistance was poor. It made the film tacky -attracted moisture (on the breakdown of polymer chain). The product soon saw a replacement with Acrylic Polymer-based emulsions. Both the products were marketed as Latex or Plastic Distempers, fully washable distempers or synthetic distempers.

Water bound systems are favoured by films, drama and TV scenographers for set making and and scenery backdrops, for three reasons, Fast drying quality, matt (non reflective) surface and high level of opacity (presence of whiting offers better hiding power).

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