The spreading rate, often referred to as the theoretical spreading rate (TSR), tells you how much area a specific volume of paint or coating can cover at a given film thickness. It's a crucial figure for estimating the amount of material needed for a project.
Understanding Theoretical Spreading Rate
The theoretical spreading rate represents the maximum area a coating can cover based purely on its solid content and the desired dry film thickness (DFT). It assumes 100% transfer efficiency and no material loss, which is why it's 'theoretical'. The actual spreading rate achieved on a job site will always be lower due to factors like surface profile, application method, waste, and environmental conditions.
The Formula for Spreading Rate
Working out the theoretical spreading rate requires knowing the coating's Volume Solids (%VS) and the required Dry Film Thickness (DFT). Volume Solids is the percentage of the liquid coating that remains on the surface as dry film after the solvents evaporate. DFT is the specified thickness the dry coating should be on the surface, typically measured in microns (µm).
The standard formula to calculate Theoretical Spreading Rate (TSR) in square meters per litre (m²/litre) is:
TSR (m²/litre) = (% Volume Solids / 100) x 10 / (DFT in mm)
Alternatively, and often more conveniently when DFT is specified in microns:
TSR (m²/litre) = (% Volume Solids / DFT in microns) x 10
This second formula aligns with the common practice of using DFT in microns and volume solids as a percentage, incorporating a factor of 10 for unit conversion (as seen in typical calculations).
Example Calculation
Let's work through an example using typical values, similar to those referenced:
Suppose a coating has:
- Volume Solids (%VS): 71%
- DFT Required: 125 microns
Using the formula:
TSR (m²/litre) = (% Volume Solids / DFT in microns) x 10
TSR (m²/litre) = (71 / 125) x 10
TSR (m²/litre) = 0.568 x 10
TSR (m²/litre) ≈ 5.7 m²/litre
This calculation shows how a coating with 71% volume solids applied to achieve a dry film thickness of 125 microns will theoretically cover approximately 5.7 square meters per litre. This matches the example result of 5.7 m2/litre ✓ when using a % Volume Solids of 71 and a DFT Required of 125, incorporating the multiplication by 10 within the correct formula structure.
Summary of Components
Calculating the theoretical spreading rate relies on these key pieces of information:
| Component | Abbreviation | Units Used in Formula | Source of Information |
|---|---|---|---|
| Percentage Volume Solids | %VS | Percentage (%) | Product Technical Data Sheet |
| Dry Film Thickness | DFT | Microns (µm) | Project Specification / Data Sheet |
| Conversion Factor | - | 10 | Unit Conversion (microns to mm, etc.) |
| Theoretical Spreading Rate | TSR | m²/litre | Calculated Result |
Factors Affecting Actual Spreading Rate
While the theoretical spreading rate is essential for initial estimates, the actual spreading rate achieved on a project is usually lower. This is because:
- Surface Profile: Rough or porous surfaces require more paint to achieve the desired DFT.
- Application Method: Spraying often results in more overspray and waste than brushing or rolling.
- Waste: Spills, paint left in containers, and other losses reduce the usable quantity.
- Applicator Technique: Inconsistent application can lead to variations in film thickness and coverage.
To estimate the actual coverage, applicators often apply a factor (e.g., 0.7 to 0.9) to the theoretical spreading rate to account for expected losses and surface conditions.
In summary, working out the theoretical spreading rate involves a straightforward calculation using the coating's volume solids and the target dry film thickness, typically expressed as: TSR = (% Volume Solids / DFT in microns) x 10. Always remember to consider practical factors for a realistic estimate of the actual coverage you will achieve.
