Citation
(1998), "New hiding power meter", Pigment & Resin Technology, Vol. 27 No. 2. https://doi.org/10.1108/prt.1998.12927baf.003
Publisher
:Emerald Group Publishing Limited
Copyright © 1998, MCB UP Limited
New hiding power meter
New hiding power meter
The Rhopoint NOVO-PAC is believed to represent the first intelligent opacity meter of its kind. It uses the same hardware as a statistical reflectometer, but has specially written software for opacity calculations.
Opacity, also referred to as hiding power or contrast ratio, is a measure of how effectively a paint or coating will cover, or hide, the surface it is applied to. Pigments are one of the most expensive components used in paints and inks, so manufacturers are keen to use the minimum quantity possible, but not so little that the paint becomes transparent.
A well-recognised test method for measuring opacity of paints is ISO 2814. This requires that the material under test is spread as a 100µm film on a standard test substrate, such as the black-and-white card shown in Figure 1.
Once the coating has dried, a 45/0 reflectometer is used to make reflectance measurements. A minimum of four readings must be taken on the test material over each of the black and white areas of the card. The mean of the two sets of readings, black (Rb) and white (Rw), is then calculated. These parameters are then used to calculate hiding power according to the following equation:
Hiding power = Rb/Rw× 100 per cent
The greater the hiding power of the test material, the more similar it will appear over the two halves of the test card. This will lead to the difference between readings over the black and white areas becoming increasingly smaller until, at 100 per cent opacity, Rb and Rw are equal. However, 98 per cent opacity is generally considered as complete hiding.
Figure 1 Black and white card
The standard states that this procedure must be conducted on three test cards for each sample material, so there are 25+ individual calculations to be made for every sample tested. If you have a series of samples, this can be a very time consuming and error-prone task.
NOVO-PAC is said to be able to eliminate these error problems, and to save time. It is supplied complete with a paint spreader of the correct 100µm dimension and a flat glass bed to ensure that good test samples are prepared. The instrument software enables the user to take reflectance readings in any order over the two parts of the card, and more than four over each part may be taken if required. Once data has been gathered, the push of a button will instantly calculate and display the Rb and Rw values, as well as the hiding power. The data remains stored in the instrument, and the user is then ready to test the next sample. As NOVO-PAC is a statistical instrument, all data can also be down-loaded to a PC using NOVO-SOFT, Rhopoints' free Windows based data processing package.
For further details contact Rhopoint Instruments Ltd. Tel: +44 (0) 1424 214291; fax: +44 (0) 1424 730600; or E-mail: instruments@rhopoint.co.uk