links here

NON-Drip Gloss Paints

Introducing Thixotropy into paint
By Ray Lewhite.

This article deals with the whole question of non-drip gloss finishes as it effects the "do-it-yourself" user. The viscosity of most present-day ready mixed gloss paints is independent of the rate of shear; consequently the pigments in these paints in time tend to settle in the tin, so that, before use the paint must be stirred to redisburse this pigment uniformity throughout the medium in which it was originally ground. With some paints this can be a lengthy procedure, and even while the paint is being used, the pigment will tend to settle once again. Also because of this nature, only very small brush loads of paint can be transferred to the working surface without some of it running off the brush and when painting a ceiling for, instance, this can be most unpleasant. Except in relatively experienced hands there is always the possibility of the paint running or "curtaining" after it has been applied.

Definition of Thixotropy

Non Drip Gloss Paints
With the advent of the non-drip gloss finish, these disadvantages have largely been overcome. This type of finish has been formulated so that the paint exhibits none of the characteristics of a Newtonian liquid and exists in what is known, scientifically, as the thixotropic state. Thixotropy is defined in British Standard Specification 2015, 1953, as ; "The flo properties of a material which undergoes a reduction in consistency when shaken, stirred or otherwise mechanically disturbed and recovers the original consistency when at rest". Many painters and decorators will have observed that some paints they have used do in fact exhibit limited thixotropy: this is mostly found in flat oil paints, certain undercoats and oil gloss paints, which appear to be quiet thick in the tin but which flow out to a certain extent under the brush without any apparent brush drag. In these cases thixotropy has been introduced into the paint, not by design, but because the formulation requires the use of certain pigments which bestows upon the paint a measure of thixotropy. Complete thixotropy cannot be developed by pigmentation alone, and the new non-drip finishes exhibit thixotropy for a combination of reasons. However before enumerating these reasons let us consider what are ideal paint properties. These have been stated to be:-

NON-Drip Gloss Paints

1. A firm gel giving non-spill and non-drip properties, rapid paint transference, and elimination of pigment settling.
2. Rapid breakdown when brushed to give easy brushing and join-up.
3. Retained fluid properties for a few minutes giving good elimination of brush marks.
4. Then set up to a gel, giving freedom from sagging or curtaining in thick films.

Meaning of "Gel"

Non Drip Gloss Paints
A "gel" is the technical word for a jelly, and the thixotropic state is exhibited by the non-drip finishes in this form. The logical development for conventional gloss finishes lay in the introduction thereto of the properties listed above. this has been attained by the introduction of alkyd resins exhibiting those properties coupled with a technique in formulation whereby the pigments employed enhance the gel structure.

NON-Drip Gloss Paints

These finishes are usually supplied ready for use in air-tight tins in a semi solid gel type consistency, and no thinners are necessary. Provided the contents of the tin have not been disturbed for approximately two hours before opening, it will be found possible to invert the open tin without removing any of the contents. In the larger size containers this may not be possible as the weight of the paint itself could cause the material to drop out. From this, it will be understood that the paint in the tin has no flow characteristics, and, indeed, on examination, it will be found to be in the form of a jelly (gel)

To use these paints it is only necessary to dip the brush into the paint and to withdraw it with the least amount of agitation. The thixotropic nature of the paint means that far greater quantities of paint can be loaded onto the brush than with normal type paints without fear of drips off the brush and consequently, this means that the paint can be applied far more quickly. It will also be found possible to paint awkward places far more easily without the risk of splashing.
When the paint is applied on a surface, under the shear of the brush, it is converted from the thixotropic to the liquid state, and it will therefore flow out on the surface free from brush marks. Once applied it will rapidly regain its gel structure, and thus the tendency to run or curtain will be minimised. In fact a much greater quantity of paint can be applied than with normal type finishes before the tendency to run is apparent. Owing to the thixotropic nature of the paint, there is little tendency for it to run off the brush and down the paint brush handle.

Paint Should Not Be Stirred Before Use

Non Drip Gloss Paints
The paint should not be stirred before use if full advantage is to be taken of the valuable properties bestowed upon it by virtue of its thixotropy. This does not mean that, if the paint has become fluid by, say, agitation in transport, it cannot be used in its fluid state. It can, but then will behave as a conventional material and its non-drip properties will not be apparent, although it still retains its non-run characteristics when applied to the working surface. If the paint is found to be liquid after any agitation, it must be allowed to stand up to about two hours until the semi-solid state reforms. In general the paints are characterized by slightly improved ease of brushing as compared with conventional finishes and their wet-edge properties are of the same order. The paints are normally supplied in a consistency suitable for brush application, but it is possible to use a roller. In the latter case, the paint may tend to liquefy in the tray and thereby lose its non-drip properties, but when applied by roller to a surface it will revert quiet quickly to the thixotropic state and therefore there will be no benefit from the fact that when properly applied it will be free from runs and sags. Work is at present in progress to produce a paint with thixotropic characteristics suitable for application by spray.

Paints Resistant to Steam

Non Drip Gloss Paints
It is possible to formulate paints for use on all interior and exterior surfaces, after proper surface preparation and the prior use, where necessary, of primer and undercoat, according to the nature of the surface being painted. No special preparation is required, only the normal preparation that is necessary prior to applying any type of paint. The paints are resistant to steam, condensation and atmospheric conditions and may be washed or scrubbed if necessary. Their characteristics avoid waste and reduce mess to a minimum.

Tins Should be Securely Closed After Use

NON-Drip Gloss Paints

After use tins should be securely closed to prevent loss of solvent and skinning: a skin will be found in a partially filled container, but this can easily be removed with a knife before using the paint again. Surface sweat due to alternate evaporation and condensation of the solvent may also take place in warm weather, but this is in no way detrimental and should not be stirred into the paint. Brushes should be washed out in clean turps substitute after use, or according to the manufacturers instructions. The paint may be stored indefinitely, preferably in an equable temperature: the pigments will not settle and the paint will remain in a fit for use condition. In warm summer months it may be found that the nature of the gel is more limp than in winter, but this is in no way detrimental. Generally these paints dry in about 6-8 hours under normal atmospheric conditions. When suitably formulated, on exterior work on a properly prepared surface, there durability is comparable with the best of materials based on conventional alkyd resins, and they will not rub off, crack or flake. Their covering capacity is 550 to 600 sq ft. per gallon approximately. It is almost certain that thixotropic undercoats and primers will follow as soon as manufacturers are sure that the finishes have been received by the user in the way they hoped.
Go Back to Home Page

Special Paint Finishes.