Determination of Contrast Ratio, Opacity and Hiding Power


BS 3900-D4: ISO 2814
BS 3900-D7: ISO 6504/1
BS 3900-D11: ISO 6504/3

 Opacity Test

Scope and Field of Application

The terms "contrast ratio", "opacity" and "hiding power" are used interchangeably throughout the coatings industry but on this page, in order to simplify matters, only the term opacity will be used.

Opacity is defined as the ability of a coating to prevent the transmission of light. A practical example of this is the case where a yellow wall is painted using a red paint. The greater the opacity the red paint, the more efficient it will be at hiding the underlying yellow colour.

BS 3900-D4 and ISO 2814 are alternative names of a method for comparing the opacities of paints. It is the simplest of the three standard methods listed here.

BS 3900-D7 and ISO 6504/1 are alternative names of a method for determining the opacity of paints. It is mainly used for checking that coatings meet the requirements of the Ecolabel scheme.

BS 3900-D11 and ISO 6504/3 are alternative names of a method for determining the opacity of paints. In our experience it is most widely used method.

In addition to these standard methods, we also carry out an in-house method whereby paint is applied to a surface which includes a contrasting background. This gives a measure of both the opacity of the coating and its application properties.

With the exception of BS 3900-D7/ ISO 6504/1 when used to check Ecolabel paints, there are no pass/fail criteria associated with these standards. This is a matter of agreement between the parties concerned.

The actual standards are copyright-protected documents and we are not able to provide you with copies. If required however, you can easily obtain copies from the British Standards Institution.

 

Summary of Methods

The three BS standards all have a number of features in common but differ in detail. In all cases the thicknesses of the applied films and the temperature/humidity conditions under which they are dried/stored are specified.

BS 3900-D4: ISO 2814

This standard describes how a wet film of the test coating is applied to a black and white patterned substrate. After the film has dried, a spectrophotometer is used to measure the amount of light reflected from the overcoated black areas of the substrate. This is then expressed as a percentage of the amount of light reflected from the overcoated white areas.

A paint which is highly opaque will obscure the black and white areas to an equal extent. In this situation, equal amounts of light will be reflected from the overcoated black and white areas and consequently an opacity value of 100% will be obtained.

 

BS 3900-D7: ISO 6504/1

The method is based on equations (derived by Kubelka and Munk) relating the scattering and absorption coefficients of pigmented films to their opacity. In order to use these equations to obtain a value for opacity, it is necessary to know:-

The standard describes how to apply the test paint to clear polyester sheet in order to produce dry films ranging from 75 to 150 microns in thickness. A spectrophotometer is then used to measure the reflectance of each film and, if necessary, additional, thicker, films are produced until the reflectance values obtained are no longer dependent on film thickness. This can be regarded as the maximum reflectance of the film i.e the reflectance of the paint film at infinite thickness.

A set of paint films is then prepared at a thickness which the previous measurements have shown will give less than maximum reflectance. The reflectance of these is then determined with the coated polyester sheet placed over a black background. The dry film thicknesses of these films are then determined gravimetrically. This involves weighing a measured area of coated polyester before and after the coating is stripped from the substrate using solvent.

In order to calculate the corresponding wet film thicknesses, it is first necessary to determine the density and non-volatile content of the paint. These values, together with the dry film mass per unit area measured previously, enable the wet film thicknesses to be calculated.

The opacity of the paint can then be expressed as the spreading rate (in terms of square metres per litre) needed to give an opacity value of 98%.

 

BS 3900-D11: ISO 6504/3

It has been shown that, over a restricted range of film thicknesses, opacity is inversely proportional to spreading rate. Fortuitously, this restricted range of film thicknesses includes those commonly used for white and light-coloured paints. This standard utilises the relationship.

The methodology is similar to that used in BS 3900-D7: ISO 6504/1 in so much as the paint is applied to clear polyester sheet and the reflectance of the film measured. The same methods as those described in BS 3900-D7: ISO 6504/1 are used to determine the wet and the dry film thicknesses of the films.

Duplicate paint films at three different film thicknesses are prepared and their reflectance is measured when the film is placed over both a black and a white background. The reflectance over the black background is then expressed as a percentage of the reflectance over the white background. This value is the opacity of the film.

The spreading rate of each film (in terms of square metres per litre) is then calculated and a plot of opacity versus spreading rate is constructed. A linear relationship is assumed and the opacity at a spreading rate of 20 square metres per litre is read from the plot.

 

PRA In-House Method

The three standard methods described on this page all involve the use of laboratory equipment to apply paint to small areas of smooth non-absorbent substrates. This may not give a true indication of the opacity obtained when the paint is used for its intended purpose.

The PRA method involves an experienced painter applying the coating, using both a brush and a roller, to 1200 x 1200 mm panels. Depending on the nature of the paint, the panels can be either plasterboard or hardboard. Black and white cards are fixed to the panel before painting.

After the paint has dried, the cards are removed and a visual assessment is made of how well the paint has obliterated the black and white pattern.

The advantage of this method is that it gives a measure of opacity obtained at the natural spreading rate of the coating. It also has the ability to show up any tendency of the paint to produce patchy or uneven films, either of which will have an adverse effect on its opacity.

Sample Requirements

In our experience the methods described on this page are most commonly used for testing decorative rather than industrial paints. The BS methods could however be applied to industrial paints provided they were non-stoving systems.

The BS methods are only applicable to white or light coloured paints. They are not applicable to paints which are textured or those which fluoresce or have a metallic finish.

No such restrictions apply to our in-house method. Provided it can be applied by brush or roller to yield an air-drying film, then we can test it.

We require 250 ml of paint to test to any of the BS methods and 2.5 litres for our in-house test.

 

Accreditation

PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS) to carry out these tests.

 

Please contact Peter Collins for further details.

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