Calculating the density of a dry film is straightforward: it's the mass of the dry film divided by its volume.
As stated in the reference, "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."
This fundamental principle applies universally to density calculations, whether for liquids, solids, or dried coatings.
The Dry Film Density Formula
The calculation uses the basic density formula:
D = m / v
Where:
- D represents the Density of the dry film.
- m represents the Mass of the dry film.
- v represents the Volume of the dry film.
To find the density, you need to accurately measure both the mass and the volume of the dried coating sample.
Practical Considerations for Measurement
While the formula is simple, obtaining accurate measurements for a thin dry film can present practical challenges.
Measuring Dry Film Mass (m)
- The mass of the dried film is typically measured using a precise balance or scale.
- Often, the coating is applied to a substrate of known mass. The mass of the substrate with the dry film is measured, and then the original substrate mass is subtracted to find the dry film mass.
- Alternatively, a free film can be created and its mass measured directly.
Measuring Dry Film Volume (v)
Measuring the volume of a thin dry film is less direct than measuring mass. Volume is calculated from its dimensions:
v = Area × Thickness
- Area: The area covered by the dry film must be accurately known. This is usually straightforward if applied to a defined area or if a free film of a specific shape (like a circle or square) is created.
- Thickness: The dry film thickness (DFT) is a critical measurement. This is typically measured using instruments like:
- Micrometers (for free films or on non-magnetic substrates)
- Magnetic gauges (for coatings on ferrous substrates)
- Eddy current gauges (for coatings on non-ferrous metallic substrates)
- Ultrasonic thickness gauges (for coatings on various substrates, including non-metals)
Once the area and thickness are determined, the volume is calculated. Ensure that the units for area and thickness are consistent (e.g., both in millimeters or both in inches) to obtain volume in the correct units (e.g., cubic millimeters or cubic inches).
Example Calculation
Let's say you have a dry film sample with:
- Mass (m) = 0.15 grams
- Area = 10 square centimeters (cm²)
- Thickness = 50 micrometers (µm)
First, convert units to be consistent, e.g., centimeters.
- Thickness = 50 µm = 0.005 cm (since 1 cm = 10,000 µm)
Calculate Volume:
- Volume (v) = Area × Thickness
- v = 10 cm² × 0.005 cm
- v = 0.05 cm³
Calculate Density:
- Density (D) = m / v
- D = 0.15 g / 0.05 cm³
- D = 3 g/cm³
Here's a summary table:
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Mass | m | 0.15 | grams (g) |
| Area | - | 10 | cm² |
| Thickness | - | 0.005 | cm |
| Calculated Volume | v | 0.05 | cm³ |
| Calculated Density | D | 3 | g/cm³ |
Summary
In essence, calculating dry film density relies on the foundational physics principle: Density = Mass / Volume. By accurately measuring the dry film's mass and determining its volume (often derived from its area and thickness), you can calculate its density.
