When we evaluate paint, we ask, “how much paint must be applied to hide the substrate below it?” or “does this paint hide better than that paint?” Opacity is a paint’s ability to prevent the transmission of light in order to hide the substrate below it. Throughout the paint industry, the terms hiding, opacity and contrast ratio have frequently been used interchangeably. Hiding is a general term used to describe all of these concepts, including hiding power. However, hiding power also takes into account the spreading rate of paint and will not be discussed here. (More information on hiding power can be found in ASTM D 2805.)
What is an Opacity Drawdown Chart?
Opacity drawdown charts are made up of white and black areas that are large enough to make a reflectivity measurement with a spectrophotometer such as the BYK-Gardner spectro-guide. Opacity charts are used to make measurements for calculating Percent Opacity and the Contrast Ratio. BYK-Gardner byko-chart’s white and black areas have the tightest tolerances in the industry ensuring repeatable measurement batch after batch.
How to make a drawdown.
Using a byko-chart Opacity Drawdown Chart, make a drawdown manually at the rate of 6cm/s according to ASTM D 2805. Hold the charts flat with a vacuum pate or another suitable device. Each drawdown should be placed horizontally in a well-ventilated area to dry. On the second or third day (or your own specified time limit), use a spectrophotometer like the spectro-guide to measure the Y-tristimulus value for the coating over black and for over white on all your drawdowns. Then calculate the contrast ratio and/or opacity as shown below.
Contrast Ratio is defined in ASTM D 2805 as the ratio of the reflectance (Y-tristimulus value) of a film on a black surface to that of an identical film on a white substrate.
Opacity (%) is simply the Contrast Ratio multiplied by 100 to get a percentage, where 100% is complete hiding. Therefore, no differences can be seen on the drawdown between black or white for a 100% opacity measurement.
With a BYK-Gardner spectro-guide, you can skip this calculation completely and let the spectro-guide do the work for you. Simply select “Opacity” in the Option-Index pull down menu while in Difference Mode. Push the STD button while the spectro-guide in on the black and then the Operate button while on the white. Then, the spectro-guide automatically displays the Opacity (%).
The paint and coatings industry makes frequent use of these measurements. It is therefore very important to ensure that the drawdown charts below the paint film being evaluated are highly consistent. Unless your film is 100% opaque, the white and black areas of the chart are being measured. The consistency of the chart color is paramount to prevent erroneous paint batch rejections due to poor quality test charts. BYK-Gardner’s byko-charts have the tightest tolerances in the industry ensuring your accuracy every time.
Sources for instruments mentioned in this article:
Koleske, Joseph V., Paint and Coating Testing Manual, 14th ed, 1995, p.481-506.