Your Best Finish Starts With TTX!
Published

How to Determine Dry Film Density

Simply put, dry film density is just the mass of the dry paint film divided by the volume of the dry paint film, says BASF’s Tim December.

Tim December, BASF Automotive Coatings Solutions

Share

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 cm3 at 4°C. Therefore, water has a density of 1.0 grams/cm3 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 cm3. 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/cm3. 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.

ENGINEERED PAINT BOOTHS & FINISHING SOLUTIONS
Precision gear pumps
Your Best Finish Starts With Us!
Metal Pretreatment Technology
New Acid-Free Bright Nickel Process
plating and surface finishing additives
OptiCenter All-in-One OC11
Heatmax Heaters ad with immersion heaters
find masking products online
Gardner Intelligence
Pretreatment Washer and Finishing Equipment
Filtration Systems

Related Content

curing

Curing Oven Basics

Simply heating up the substrate does not cure the coating. There are many variables to consider when choosing the best cure oven for your application...

Read More
Liquid Coating

Pretreatment for Painting

Better adhesion, enhanced corrosion and blister resistance, and reduced coating-part interactions make pretreatment a must.

Read More
basics

Top Reasons to Switch to a Better Cleaning Fluid

Venesia Hurtubise from MicroCare says switching to the new modern cleaning fluids will have a positive impact on your cleaning process.

Read More
Anodizing

Anodizing for Bonding Applications in Aerospace

Anodizing for pre-prep bonding bridges the gap between metallic and composite worlds, as it provides a superior surface in many applications on aluminum components for bonding to these composites. 

Read More

Read Next

Parts Cleaning

A ‘Clean’ Agenda Offers Unique Presentations in Chicago

The 2024 Parts Cleaning Conference, co-located with the International Manufacturing Technology Show, includes presentations by several speakers who are new to the conference and topics that have not been covered in past editions of this event.   

Read More
Sponsored

Masking Solutions for Medical Applications

According to Custom Fabricating and Supplies, a cleanroom is ideal for converting, die cutting, laminating, slitting, packaging and assembly of medical-grade products.

Read More
sustainability

Episode 45: An Interview with Chandler Mancuso, MacDermid Envio Solutions

Chandler Mancuso, technical director with MacDermid Envio discusses updating your wastewater treatment system and implementing materials recycling solutions to increase efficiencies, control costs and reduce environmental impact.

Read More
Precision gear pumps