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Q: What is the difference between wet paint density, specific gravity and dry film density? How can I determine dry film density?

**A:** We are most familiar with wet density, which is commonly called weight per gallon in the United Sates. The wet density (D) is the mass (m) of the sample liquid divided by its volume (v), or as an equation D = m/v. It is very important to measure the wet density at a standard temperature, as the volume of a liquid can change with temperature for the D = m/v calculation. You can quite easily measure the wet density of a paint or any liquid using an ASTM method like ASTM D1475, for example.

This method measures the mass of the liquid in a standard volume container, normally made of stainless steel, called a pycnometer. In the U.S., a standard volume of 1 gallon is used to report the density values and to make comparisons easy. So water, for example, has a density or a weight per gallon of 8.34 pounds/gallon at 4°C.

Other regions of the world use metric units such as grams/ml or kg/liter for density. In the metric definition, 100 grams of water will fill a volume of 100 cm^{3} at 4°C. Therefore, water has a density of 1.0 grams/cm^{3} at 4°C. The density of water is often used as a standard value at 4°C, as this is its highest density.

Specific gravity is simply a ratio of two density values. It can be, for example, the ratio of the density of a sample liquid to the density of a reference liquid and, most often, the density of water is chosen as the standard reference. If you have water at 8.34 pounds/gallon and the reference liquid is water at 8.34 pounds/gallon, then the specific gravity of water will be 1.0 at 4°C. Strictly speaking, specific gravity is a dimensionless term that means there are no measurement units for this value as they cancel from the ratio of the density values. Thus, the specific gravity will be the exact same value if you start with English measurements or with metric measurements.

The specific gravity is often abbreviated as SG. If the specific gravity of a liquid is less than 1.0, it is less dense than water, and if the specific gravity is higher than 1.0, it is heavier than water. Hydrocarbon solvents, like heptane will have specific gravity values less than 1.0. Chlorinated solvents will typically have values greater than 1.0.

Dry film density is a bit more complicated. Simply put, dry film density is just the mass of the dry paint film divided by the volume of the dry paint film. So dry density follows the same density equation, D = m/v, as wet density. Imagine a paint film that is exactly 10 cm × 10 cm × 100 microns thick and thus has a volume of 1 cm^{3}. Weigh this film on a balance and get the mass of 1.4 grams, for example, then follow the equation of D = m/v to calculate the dry film density of 1.4 grams/cm^{3}. Of course, it is often impossible to get such an exact film sample, and the film thickness may not be uniform.

The dry density value can be determined via indirect methods using the Archimedes principle for determining volume displacement or the buoyancy effect. The ISO 3233 series has some helpful information on this topic, as does ASTM D2697. In addition, some balance manufacturers have density kits based on the Archimedes principle that are very helpful for making exact determinations. However, these methods also have limitations, especially with paints above the critical pigment volume concentration, when porosity, for example, may be a major factor.

Fortunately, there is a quick calculation method that provides good approximations of the dry film density if some key constants are known. This is an easy way to estimate the dry film density from the wet constants supplied by the paint manufacturer or from values on the Certificate of Analysis.

A good approximation of dry film density can be estimated based on the density of the wet paint, the density of the solvents and the weight of the solids. The limitation is that there is generally some shrinkage of the film during cross-linking and some loss of reaction volatiles, which especially impacts thermoset systems.

The general equation is:

Dry film density = [NV × Dp × Ds] / [100 × Ds – (100-NV) × Dp]

where NV = percent of NV solids measured per standard method, Dp = density of the wet paint measured per standard method, and Ds = density of the solvent (largest solvent present or often a calculated average of all solvents present).

For example, if you have a paint that measured at 39.1 percent solids and a wet paint density of 9.7 pounds/gallon and a solvent density of 8.1 pounds/gallon, the calculation for dry film density will be [39.1 × 9.7 × 8.1] / [100 × 8.1 – (100-39.1) x 9.7] = 14.0 pounds/gallon. This can easily be programed into an Excel spreadsheet and used as a dry film density calculator.

*Tim December is a technical expert for BASF Automotive Coatings Solutions. Visit basf.com.*