SciMed Education
Absolute Vapor Pressure of Paints, Varnishes and Solvents
In Summary
Absolute vapour pressure measurement of paints, varnishes, and solvents is essential for understanding volatility, safety, and formulation performance. Using the Koehler RVP Pro, reliable and repeatable vapour pressure data can be obtained in accordance with relevant ASTM methods, supporting accurate product development and quality control.
Why is absolute vapor pressure testing required for paints, varnishes and solvents?
The absolute vapor pressure of a liquid describes the vapour pressure of the liquid portion itself, excluding the vapour pressure contributed by dissolved gases such as air. Regulatory frameworks, including the ADR and the European REACH regulation for registration, evaluation and authorisation of chemicals, require that paints, varnishes and solvents are classified and labelled according to their absolute vapour pressure.
Safety data sheets must list the absolute vapour pressure, and two temperatures are particularly important: at 20 °C, to be included in material data sheets, and at 50 °C, which must not exceed a specified limit for transport and environmental protection. Meeting these requirements helps organisations comply with transport rules and environmental regulations.
How do static methods determine the absolute vapor pressure of paints and varnishes?
The static vapour pressure method, defined in Commission Regulation EC No. 761/2009, measures the absolute vapour pressure by evacuating the sample at a reduced temperature to remove gases and then heating it so that vapours build and the equilibrium vapour pressure can be recorded.
This method can be applied to single‑ or multi‑component paints and varnishes, but when multi‑component samples are degassed, the evacuation temperature must be low enough not to alter the sample composition
What is the ASTM D2879 Isoteniscope method for the absolute vapor pressure of paints, varnishes and solvents?
The ASTM D2879 Isoteniscope method is based on the static method and determines the absolute vapour pressure of single‑component liquids and solids. It requires careful boiling and degassing to remove dissolved gases.
However, according to the European REACH regulation this method is not suitable for multi‑component samples, which limits its usefulness when dealing with varnishes or paints containing several constituents.
How does the ASTM D6378 Koehler Triple Expansion Method measure the absolute vapor pressure of paints and varnishes?
The ASTM D6378 Koehler Triple Expansion Method is a static‑based technique that measures the equilibrium vapour pressure of a liquid in a closed system and calculates the absolute vapour pressure directly using the ideal gas equation. By recording three total pressure values and subtracting the partial pressure of dissolved gas, the method yields the absolute vapour pressure.
It has several advantages for paints and varnishes: it is suitable for both single‑ and multi‑component samples, does not require sample preparation such as boiling or evacuation, does not alter the sample composition, performs tests quickly and automatically, includes automated cleaning and rinsing, and conforms to the latest vapour pressure standard ASTM D6378.
How were varnish and thinner samples tested with the RVP Pro to determine the absolute vapor pressure of paints and varnishes?
Vapour pressure measurements of varnishes and solvent-based systems can be performed using the Koehler RVP Pro, which is designed to accurately determine low vapour pressures in accordance with ASTM D6378.
Samples are introduced into a sealed measurement chamber where temperature is precisely controlled, allowing the instrument to determine equilibrium vapour pressure with high repeatability. This automated approach reduces operator variability and ensures consistent results across a range of sample types.
Measuring vapour pressure at different temperatures, such as 20 °C and 50 °C, provides valuable insight into the volatility and behaviour of paints, varnishes, and solvents, supporting formulation development, safety assessment, and quality control processes.
What were the results for the absolute vapor pressure at 20 °C and 50 °C for paints, varnishes and thinner samples?
The test results show that different varnishes and thinners have widely varying absolute vapour pressures. The absolute vapour pressure at 20 °C and 50 °C was calculated by subtracting the partial pressure of dissolved air from the total vapour pressure. The table below summarises the mean absolute vapour pressures for each sample type:
| Sample type | Mean absolute vapour pressure at 20 °C (kPa) | Mean absolute vapour pressure at 50 °C (kPa) |
|---|---|---|
| Brilliant varnish | 0.45 ± 0.03 | 1.86 ± 0.19 |
| Brilliant varnish | 0.52 ± 0.01 | 2.15 ± 0.01 |
| Matt varnish | 2.95 ± 0.30 | 5.82 ± 0.23 |
| Base varnish | 0.41 ± 0.06 | 1.36 ± 0.13 |
| Thinner | 1.91 ± 0.01 | 8.61 ± 0.01 |
These values demonstrate that matt varnish had the highest absolute vapour pressure among the varnishes tested, while the base varnish showed the lowest. The varnish thinner exhibited a much higher vapour pressure at 50 °C compared with the varnish samples, highlighting the effect of composition on volatility.
What conclusions about measuring the absolute vapor pressure of paints and varnishes can be drawn?
Accurate vapour pressure measurement is essential for evaluating the volatility and safety of paints, varnishes, and solvent-based systems. Using the Koehler RVP Pro, laboratories can obtain precise and repeatable results that support regulatory compliance, product development, and quality assurance.
By understanding how vapour pressure changes with temperature, manufacturers can optimise formulations, improve handling safety, and ensure consistent product performance across a wide range of applications.
What to do Next?
Speak to our specialists to learn more about measuring the absolute vapor pressure of paints and varnishes and find the right solution for your lab.
Page FAQ's
The absolute vapour pressure is the vapour pressure of the liquid itself, excluding the contribution from dissolved or fixed gases such as air
The main methods described in the application note are the static vapour pressure method, the ASTM D2879 Isoteniscope method and the ASTM D6378 Koehler Triple Expansion Method. The static method removes gases by evacuating the sample and then measures its equilibrium vapour pressure. The Isoteniscope method is intended for single‑component samples but is generally unsuitable for multi‑component paints and varnishes. The triple expansion method measures the absolute vapour pressure directly in a closed system and offers the advantage of minimal sample preparation.
The RVP Pro analyser implements the ASTM D6378 triple expansion method and can handle both single‑ and multi‑component samples. It requires little sample preparation, performs measurements quickly and automatically, and its cleaning system prevents sample contamination. The application note shows that this instrument produces highly repeatable results for brilliant, matt and base varnishes and for a varnish thinner.
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