Paint produced by the architectural paint industry must perform on a wide range of substrates. Some examples are previously coated wallboard, from glossy to flat, wood trim, plaster, uncoated wallboard, stucco, etc. The porosity of these substrates varies greatly, yet the paint must maintain the same color and gloss appearance no matter the substrate.
What is a Penopac drawdown chart?
To test both the Penetration Resistance and Opacity of a film in one drawdown, the byko-chart Penopac Drawdown Chart was created. Using a single drawdown chart to do two tests reduces the time spent performing multiple drawdowns and saves money on the purchase of drawdown charts. BYK-Gardner’s stringent quality control during the production process ensures that the byko-charts have the most consistent color and gloss in the industry.
The Penopac Drawdown Chart consists of a coated and uncoated area used to test the uniformity of film properties, such as the appearance of color and gloss, over both areas. One of these properties is often referred to as penetration resistance. The coated and uncoated areas on the chart create extreme testing conditions. The uniformity of a paint film is quantitatively measured with a spectrophotometer and glossmeter such as the BYK-Gardner spectro-guide and/or micro-gloss.
In order to test such properties as the opacity, hiding, or contrast ratio, the Penopac drawdown chart also consists of a white and black area. This test is done just like using an opacity drawdown chart. Y-reflectance measurements are made using a spectrophotometer.
How to make a drawdown.
Using a byko-chart Penopac Drawdown Chart, make a drawdown manually at the rate of 6cm/s using a drawdown bar film applicator according to ASTM D 2805. Hold the chart flat with a vacuum plate 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), evaluate your drawdowns as shown below.
Penetration resistance analyzed.
To analyze penetration resistance, use a spectrophotometer to take color readings using the color scale of your choosing over the coated and uncoated areas with the same color background. Subtract the uncoated from the coated readings to get the difference (or ∆) for each colormetric component. For example, using the L*, a*, b* color system:
∆L* = L*uncoated – L*coated
∆a* = a*uncoated – a*coated
∆b* = b*uncoated – b*coated
The closer your ∆ value is to zero, the smaller the difference is between the coated and uncoated areas for that colormetric component. At zero, your penetration resistance is at its best possible value. Conversely, when the ∆ value is large, the penetration resistance is considered poor.
Now, evaluate your coated and uncoated areas for gloss, using the appropriate angle for the paint you are evaluating (e.g. 60°). If you are using a BYK-Gardner spectro-guide, you can take your gloss measurements at the same time as you take your color measurements. Otherwise, using a gloss meter, take gloss readings over the coated and uncoated areas. Evaluate the difference (∆) between the coated and uncoated area just like with the color components:
∆G60° = G60°, uncoated – G60°, coated
As with the color components, small ∆ values means there is less difference there is between the coated and uncoated areas for that gloss angle. The penetration resistance is at its best when the ∆ value for that gloss angle is zero. Conversely, ∆ value is large, the penetration resistance is poor at that gloss angle.
For both color and gloss, paint manufacturers try to get the ∆ values as close to zero as possible.
To evaluate Opacity, use a spectrophotometer to measure the Y-tristimulus value for the coating over black and over white. 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, and the coated and uncoated areas of the chart itself are being measured, not just your coating. The consistency of the chart color and gloss 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.
US Federal Specification TT-P-29