NEW TYPES OF PAINTI
TYPES OF PAINT
IN
the last fifteen years, the development of many new resins and some new pigments has made it possible to produce a number of new types of paint and to improve many of the older ones. In this highly technical and changing field it is difficult for the user to keep abreast of developments and to assess the claims made for the various new products. This Digest offers some guidance by describing briefly the nature, properties and uses of the principal new types and comparing them with those of some older paints.
The account, however, is necessarily incomplete, since each of the groups described includes many formulations. Occasionally, too, a paint intended for average use may need modification to made it suitable for special conditions of surface or exposure. In case of doubt, it is always advisable to consult the manufacturers.
Paint consists essentially of a pigment, a binding medium and a thinner to make the mixture suitable for application by brush, roller, spray or dipping. After application, the paint undergoes changes which convert it from a fluid to a tough film which binds the pigment. The nature of these changes varies with different types of paint. Some, such as size-bound distemper, or bitumen. chlorinated rubber
cellulose paints, simply lose the thinner by evaporation; the resulting film then remains soluble in that thinner, and, when applying further coats of such paints, care must be taken not to pull up the previous coat.
or
With most paints containing dry. ing oils, part at least of the change on drying is due to reaction of the oil with oxygen from the air. The resulting film is no longer soluble in the original thinner (usually white spirit) but it can be swollen and softened by organic solvents of the type used in paint removers.
resist many
A third type of paint is supplied in two parts. These are mixed, just before application, and combine chemically in a short time to form a very tough, resinous material. Such paints can be designed to solvents and other chemicals but do not necessarily have greater resistance than other paints to normal weathering in- fluences. Because of their resistance to chemicals they may be difficult to remove, except mechanically.
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Finally, in emulsion paints and oil- bound distempers, the binding ma- terial is emulsified, that is, dispersed as fine globules in an aqueous liquid. After the paint is applied. the water evaporates and the globules coalesce to form a tough water-resistant film. Sometimes such paints can be re- moved by steam stripping but more usually it is necessary to use organic solvents to swell the resin or varnish binder.
Within each of these four groups, the special properties required of a paint for any given purpose -- its working properties, appearance, re- sistance to the weather and protective qualities all depend on many other factors in the formulation and manufacture of the paint, notably the nature and the proportions of the pigments used. The preparation of the surface for painting and the skill in application, too, are impor- tant in determining the performance of any paint. For general guidance on the painting of different materials and the selection of paint types, reference may be made to the British Standard Code of Practice C.P.231. 'Painting'; however, this is now under revision and does not as yet cover all the types discussed in this Digest.
EVALUATION
The performance of new types of paint cannot be evaluated fully until there has been some year's experience of their use under practical condi- tions. Accelerated tests, which have their uses in evaluating modifications
of established types may be mislead ing when applied to paints of an entirely new character. Practical trials in the field are needed to establish such matters as the per- formance to be expected under site conditions (which may involve indif ferently prepared surfaces or adverse weather during application) and the way in which a weathered film will take fresh coats of pain. Until such experience is gained a new type of paint should be accepted with caution and used at first only where pre mature failure would have no serious
consequences.
In the following text and in the tables various recently introduced materials are compared with a few established paints. Good as many of the new paints are, they are often less tolerant of bad preparation of surfaces and bad weather conditions than the better known types.
PROPERTIES AND USES Priming points for metals
In this field the chief advance has been the introduction of new pig ments able to inhibit the corrosion of iron and steel. In the protection of metals, good preparation of the surface and the application of an adequate total thickness of paint are still essential: the use of a rust inhibiting primer does not obviate the need for these measures, though it helps to delay the onset and restrict the spreading of corro sion and so is a valuable additional safeguard.
THE HONG KONG & FAR EAST BUILDER-VOLUME 17, NUMBER 3