Determination of Non-Volatile Matter Content in Coatings: Principles, Standards, and Test Methods

Determination of non-volatile matter (solids) content is one of the most fundamental quality-control tests for paints, varnishes, and coating materials. This article explains the measurement principle, applicable standards such as ISO 3251, GB/T 1725, and ASTM D2369, sample requirements, the step-by-step gravimetric oven-drying procedure, calculation and reporting of results, and typical industrial applications of the test.

Technical Overview

Non-volatile matter (NVM) content, commonly referred to as solids content, is defined as the mass percentage of a coating sample that remains after volatile components such as solvents, water, and diluents have evaporated under specified drying conditions. It is a core quality parameter in the coatings industry because it directly affects film formation, application behavior, theoretical spreading rate, dry film thickness, and overall cost efficiency of a coating system.

The determination is almost universally performed by a gravimetric oven-drying method. A weighed test portion is dispersed in a dish, dried at an agreed temperature for an agreed time, cooled in a desiccator, and reweighed. The residue mass divided by the original sample mass gives the non-volatile content. The principal test standards include:

• ISO 3251 – Paints, varnishes and plastics – Determination of non-volatile-matter content
• GB/T 1725 – Chinese national standard for determination of non-volatile matter content of paints, varnishes and plastics, technically aligned with ISO 3251
• ASTM D2369 – Standard Test Method for Volatile Content of Coatings (volatile matter is obtained by difference)
• ASTM D5403 – Volatile content of radiation-curable materials (for UV/EB-curable products)

In addition to standalone quality control, the non-volatile content is an essential input for calculating volatile organic compound (VOC) content by the difference method described in ISO 11890-1, ISO 11890-2, GB/T 23985, GB/T 23986, and ASTM D6886, and for demonstrating compliance with regulations such as GB 18581, GB 18582, and Directive 2004/42/EC on the limitation of emissions of volatile organic compounds.

Testing Samples

Sampling should be carried out in accordance with ISO 15528 or GB/T 3186 to ensure that the laboratory receives a representative, homogeneous portion of the delivery. Samples must be mixed thoroughly (without entrapping air) immediately before the test portion is taken, since settling or separation of pigments and binders is a major source of error.

Solvent-Borne Coatings

Alkyd, epoxy, polyurethane, chlorinated rubber, nitrocellulose, and other solvent-reducible products typically contain 30–80% non-volatile matter. These samples dry readily at 105 ± 2 °C, and the released solvent vapors require a well-ventilated or explosion-protected oven.

Water-Borne Coatings

Emulsion paints, water-reducible varnishes, pigment dispersions, and latex binders are tested at the same nominal temperature, but longer drying times may be needed to remove retained water completely. Foaming during heating can be controlled by using a larger-diameter flat dish.

Resins, Binders, and Varnishes

Liquid and molten resins, solutions, and clear varnishes are tested with the same gravimetric approach. Product standards for individual resin families may prescribe specific temperatures and heating times different from the general 105 °C condition.

Specialty and Reactive Coatings

Radiation-curable (UV/EB) coatings, baking enamels, and chemically curing two-component systems may undergo crosslinking or incomplete volatile release under normal conditions; their non-volatile matter is determined under the curing or drying conditions specified in the relevant product standard, for example ASTM D5403 for radiation-curable materials.

Testing Items

The core determination and closely related measured quantities include:

• Non-volatile matter content, expressed as a mass fraction (%, m/m) of the sample
• Volatile matter content, obtained by difference (100% minus non-volatile content)
• Conventional solids content at a specified temperature and time, as defined by the product specification
• Supporting data used in derived calculations, such as density (ISO 2811 / GB/T 6750 / ASTM D1475) and water content (Karl Fischer titration per GB/T 6283 or ASTM D4017), which are needed to compute volume solids and VOC content

Testing Methods

Oven-Drying (Gravimetric) Method

The standard procedure according to ISO 3251, GB/T 1725, and ASTM D2369 is summarized below:

• Prepare flat dishes (aluminum, glass, or stainless steel) of approximately 50–70 mm diameter. Clean, dry, cool in a desiccator, and weigh each dish to the nearest 0.1 mg.
• Add a test portion of approximately 1 g (0.2–1 g depending on viscosity and expected solids; heavier portions for low-solids products, as specified in the standard), spreading it evenly over the bottom of the dish. Weigh the dish plus sample to the nearest 0.1 mg.
• Place the dish in a forced-ventilation oven maintained at 105 ± 2 °C for 60 ± 2 min, unless a different temperature and time are specified in the product standard or agreed between the parties (common alternatives include 120 °C, 125 °C, 150 °C, or simulated baking schedules for industrial stoving coatings).
• Remove the dish, cool it to room temperature in a desiccator, and reweigh. Return the dish to the oven for further short drying cycles (15–30 min), cooling and reweighing after each cycle, until constant mass is achieved (typically when the difference between two consecutive weighings does not exceed about 1 mg).
• Perform two determinations on separate test portions from the same sample.

Calculation and Expression of Results

The non-volatile matter content is calculated as the mass of residue after drying divided by the mass of the test portion before drying, multiplied by 100 to give a percentage. The reported result is the mean of the two determinations, expressed to the precision stated in the standard (commonly to the nearest 0.1% or 0.05%). If the difference between the duplicate results exceeds the repeatability limit given in the standard, the entire procedure must be repeated on fresh test portions.

Precision and Quality Control

Precision depends strongly on product type, homogeneity, and drying behavior. Repeatability limits of roughly 1–2% (absolute mass fraction) between duplicate determinations and reproducibility limits of a few percent between different laboratories are typical for liquid coatings. Good practice includes calibration of the analytical balance, verification of oven temperature uniformity, use of a desiccator with active desiccant, and strict adherence to the specified temperature and time, all of which are routinely checked in accredited third-party laboratories.

Safety and Practical Considerations

• Solvent vapors released during drying are flammable; only ventilated or explosion-proof ovens should be used, and sample sizes should be kept small.
• Skin-like film formation on water-borne samples can trap water; spreading the sample thinly and using dishes of adequate diameter improves moisture release.
• Hygroscopic residues must be weighed promptly after cooling to avoid moisture pickup.
• For reactive systems, the drying condition should reflect the intended curing schedule, because thermal exposure can change the chemistry of the residue.

Applications

• Production quality control and batch release for solvent-borne, water-borne, and industrial coatings
• Incoming inspection of resins, binders, and coating raw materials against purchase specifications
• Formulation development, solids adjustment, and calculation of theoretical spreading rate and achievable dry film thickness
• VOC compliance testing by the difference method under regulations such as GB 18581, GB 18582, and Directive 2004/42/EC
• Contractual and trade verification of coating solids between manufacturers, applicators, and end users
• Support of eco-labeling, green building certification, and product environmental declarations that require verified solids and VOC data

FAQ

What is non-volatile matter content in a coating?

It is the mass fraction of the coating that remains after the volatile components have evaporated under specified drying conditions. In everyday language it corresponds to the solids content of the paint, including binders, pigments, extenders, and non-volatile additives.

Which standard should be used for the determination?

ISO 3251 and GB/T 1725 are the primary gravimetric methods for paints, varnishes, and plastics, while ASTM D2369 is the widely used North American equivalent for determining volatile content by difference. Radiation-curable materials are handled under ASTM D5403. The choice usually follows the product specification or the regulatory market of the coating.

What drying temperature and time are normally applied?

The default condition is 105 ± 2 °C for approximately 60 minutes in a ventilated oven. However, the product standard always takes precedence, and baking enamels or reactive systems may require higher temperatures or simulated stoving schedules that reflect real curing conditions.

How is constant mass confirmed?

After the initial drying period, the dish is reheated for short additional cycles, cooled, and reweighed each time. When the change between two consecutive weighings falls below the small tolerance defined in the standard (about 1 mg), constant mass is considered achieved and the final mass is recorded.

How does non-volatile content relate to VOC content?

VOC content calculated by the difference method equals 100% minus the non-volatile content, minus the water content, adjusted for density. Accurate non-volatile data are therefore a prerequisite for reliable VOC declarations and regulatory compliance of both solvent-borne and water-borne coatings.

What is the difference between mass solids and volume solids?

Mass solids is the result of the gravimetric test described in this article, expressed as a percentage of mass. Volume solids additionally requires density measurements of the coating and its non-volatile residue, and it is the parameter used directly to compute wet-to-dry film thickness and theoretical spreading rate.