When customers go to the store, they need to know how many gallons of paint to buy to cover the area they wish to paint. They also want to know how many coats it takes to completely hide what they are painting over. The spreading rate of a paint is how to determine how “far” a paint will go by quantifying how much area is covered for a given quantity of paint. Sometimes to compare paints, a researcher will use the same spreading rate for two different paints and then compare the hiding power visually using the background of the drawdown chart as a guide or instrumentally using a spectrophotometer.
What is a display or spreading rate drawdown chart?
The display and spreading rate drawdown chart are used to display, to visually assess hiding or opacity, and to measure the spreading rate of paint. They are usually large charts that consist of a white and black diagonal pattern or a white and black checkerboard pattern. These patterns have a very strong visual impact, which emphasizes variations in film opacity or hiding. A few of the charts also have white and black areas that are large enough to measure opacity or contrast ratio using a spectrophotometer similar to the BYK-Gardner spectro-guide.
Spreading Rate Analyzed.
The spreading rate is defined by ASTM as the area covered by a unit volume of coating material frequently expressed as square feet per gallon, ft2/gal (m2/L). Hiding power (HP) goes hand in hand with the spreading rate (SR). The Paint and Coating Testing Manual defines hiding power quantitatively; it is the SR at which the film opacity is just sufficient to give complete hiding over the specified standard black-and-white substrate. The complete hiding point is determined visually in some test procedures and photometrically in others. The higher the hiding power, the thinner the paint film needs to be applied to achieve proper hiding.
Visual Wet Method
The operator starts by weighing the brush (or roller) and the can of paint together to get the initial weight. Using a spreading rate chart, make a brushout by brushing the paint inside the know area of the rectangle on the chart. The brushing continues in small increments until the operator can no longer see the contrast of the pattern on the chart. The brush and can of paint are then weighed together again to get the final weight. The weight of the paint on the chart is then calculated.
After the weight of paint is calculated. The spreading rate (SR) and by definition the HP can be calculated.
A problem with using the wet method is that all the calculations of spreading rate and hiding power only apply for wet paint. It doesn’t account for the properties of the paint when it is dry. And most people want to know the hiding power of the dry paint. As paint formulations have evolved over the years, it became more important to make evaluations on the dry paint because many new paints have hiding power that changes while the paint dries.
Krebs Method (From the Paint and Coating Testing Manual)
This method is used to calculate the relative hiding power of a test paint compared to a standard paint. Using the standard paint, 6 to 8 brushouts are made with varying Spreading Rates within a range of 400 to 800 ft2/gal (10 -20 m2/L). The spreading rates are controlled by using a syringe to accurately measure the paint and apply it to the chart. (Calculations to get the volume from the spreading rate can be found below in the ASTM D 344 method). A brushout is then made with the test paint at a median spreading rate. After dried, the test paint is compared to the group of standard brushouts. The two standard brushouts that bracket the test in contrast are determined. An interpolation is then made to estimate the spreading rate of the test paint. The relative hiding power is then calculated below.
ASTM D 344 Method
How to prepare a brushout
Chose a reasonable spreading rate between 400 to 800 ft2/gal (10 – 20 m2/L). Determine the density (D) of the paint being sure to use g/mL. Use the following equation to calculate the volume (V) in mL for the chosen spreading rate (SR).
Once the volume is known, the weight in grams can then be calculated from the following equation.
To begin the brush out, stir the paint well and condition the brush by dipping it in paint and working it out on a smooth surface. Draw up the calculated volume of paint into a syringe. Weight the syringe, brush and paint can. Using the syringe place the paint evenly over the test chart. Brush the paint out evenly within the spreading rate chart test area. Place the chart in the horizontal position free from dust at regular room temperature conditions and allow to dry. Weigh the empty syringe, brush, and paint can again. The difference is the actual weight (WA) of the paint and can be calculated below.
If there is a difference between the actual weight of the paint and the intended weight of the paint from the calculation above, do not try to correct it on the brushout. Instead, calculate the actual spreading rate (SRA) from the intended spreading rate (SR) using the equation below.
Evaluation – Qualitative
Do a brushout for both the test paint and comparison paint. View both brushouts side by side from 5 to 10 ft. Compare both brushouts. If the contrast of the black and white areas of the test brushout is determined to be less than, equal to or great than the comparison paint, then the hiding power of the test paint is considered to be better than, equal to, or poorer than that of the comparison.
If the actual spreading rate (SRA) of the lower brushout is lower than the comparison paint by more than 3%, do an additional brushout of either paint in order to eliminate the difference, then compare again.
Report the relative hiding power of the test paint as having better than, equal to, or poorer than the comparison paint.
Evaluation – Qualitatively
If the hiding power of the test brushout is not equal to that of the comparison paint, make more brushouts of the comparison paint at differing spreading rates. These new brushoust should vary by 15% for each step and should have contrasts above and below that of the test paint. These new brushouts of the comparison paint are now called standards.
Compare the test brushout to those of the standard to determine which standard matches the test brushout. If one does not match, choose the closest on each side of the test. So, there would be one standard of lower contrast (or lower spreading rate, SR1) and one standard of higher contrast (or higher spreading rate, SR2). Between these two standard determine fractionally where it lies by the nearest fourth compared to that of the lower contrast. Use the formula below to calculate the spreading rate of the comparison paint (SRC) that would match that of the test paint.
Now that you know the spreading rate of the comparison paint (SRC) that matches the contrast of the test paint and the Spreading Rate of the test paint (SRt), you can calculate the relative Hiding Power of the test paint (HPt) using the equation below. This relative hiding power is the value that is reported.
Some of the Spreading Rate Charts have a large black and white square that is large enough to place a spectrophotometer on in order to measure Contrast Ratio. 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 square area of the chart and then the Operate button while on the white square area. 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, 15th ed, 2012, p.569-590.