Solid content, also known as non-volatile matter content, is one of the most fundamental quality parameters of coatings, directly governing film formation, spreading rate, VOC emissions, and cost efficiency. This article explains how the solid content of coatings is determined in accredited third-party laboratories, covering applicable standards such as ISO 3251, ASTM D2369, and GB/T 1725, typical sample types, the gravimetric test procedure, and the practical applications of the results.
Technical Overview
Solid content refers to the percentage of non-volatile matter in a coating, that is, the mass fraction of resin binder, pigments, extenders, and other non-evaporating components remaining after volatile substances such as water and organic solvents are driven off under specified heating conditions. It is one of the most important technical indicators for liquid coatings, because it determines the amount of dry film deposited from a given quantity of wet coating.
Solid content can be expressed in two ways: mass solids (percentage by weight, most commonly reported) and volume solids (percentage by volume), the latter being essential for calculating the theoretical spreading rate and the conversion between wet film thickness and dry film thickness. The two values differ whenever the densities of the volatile and non-volatile fractions differ, which is almost always the case in real formulations.
The measurement principle is gravimetric: a test portion is weighed into a dish, heated in an oven at a standardized temperature for a defined period, cooled in a desiccator, and reweighed. The residue mass divided by the original sample mass, expressed as a percentage, is the non-volatile content. Although the principle is simple, the result is strongly influenced by drying temperature, time, specimen mass, and film thickness, so strict adherence to standard procedures is essential for comparable and reproducible data.
- ISO 3251 – Paints, varnishes and plastics; determination of non-volatile-matter content
- ASTM D2369 – Standard Test Method for Volatile Content of Coatings
- GB/T 1725 – Determination of non-volatile-matter content of paints, varnishes and binders for paints and varnishes (Chinese national standard, technically aligned with ISO 3251)
- ASTM D5403 – Standard Test Methods for Volatile Content of Radiation Curable Materials
- ISO 3233 / ASTM D2697 – supporting methods for determining volume solids from density measurements
Testing Samples
Waterborne Architectural Coatings
Emulsion-based interior and exterior wall paints, primers, textured coatings, and waterproofing membranes. These products typically show solid content in the range of 30% to 70% by mass, with water as the main volatile component. Careful homogenization before sampling is required because pigments and extenders settle rapidly in storage.
Solvent-Borne Industrial and Decorative Coatings
Alkyd enamels, nitrocellulose lacquers, polyurethane and epoxy systems, chlorinated rubber and other solvent-dispersed coatings. Because organic solvents evaporate quickly, samples must be kept in sealed containers and tested promptly after opening to avoid pre-evaporation losses that bias the result.
Automotive OEM and Refinish Coatings
Electrodeposition paints, primer surfacers, base coats, and clear coats, including multi-component (2K) products. For 2K systems, the solid content is determined on the correctly mixed and catalyzed material within the specified pot-life window, since the curing reaction itself consumes or releases volatile species.
Powder Coatings and Radiation-Curable Coatings
Powder coatings are essentially 100% solids, but non-volatile testing is still performed to quantify residual volatiles from extrusion. UV-curable liquids and inks contain reactive diluents that polymerize rather than evaporate, so they require modified procedures with controlled UV cure and post-heating per ASTM D5403.
Raw Materials
Resin solutions, polymer emulsions, dispersions, additives, and binders are routinely tested to verify supplier specifications and to provide input data for formulation calculations.
Testing Items
The core test item is the non-volatile matter content (solid content) by mass, expressed in percent. Depending on customer requirements, laboratories can extend the evaluation to related parameters:
- Non-volatile content by mass (%), per ISO 3251, ASTM D2369, or GB/T 1725
- Volatile content by mass (%), the complementary value to solid content
- Volume solids (%), derived from mass solids, coating density, and the density of the non-volatile fraction
- Water content within the volatile fraction, relevant to VOC calculations
- VOC content, per ISO 11890 or GB/T 23986, which uses the solid-content result as input data
Testing Methods
Standard Oven-Drying Gravimetric Method
A flat-bottomed glass, aluminum, or stainless-steel dish is dried to constant mass and weighed on an analytical balance with 0.1 mg resolution. A test portion of approximately 0.5 g to 1.5 g is dispensed into the dish and spread into a thin, uniform film; for highly viscous samples, a few drops of a suitable volatile solvent may be added to assist distribution. The specimen is then heated in a ventilated oven under standardized conditions, cooled in a desiccator, and weighed. Heating cycles of 15 to 30 minutes are repeated until the mass change between successive cycles is negligible, confirming that evaporation is complete.
- ISO 3251 / GB/T 1725: typically 105 °C ± 2 °C for 1 hour, unless the product standard specifies otherwise
- ASTM D2369: 110 °C ± 5 °C for 60 minutes in a forced-ventilation oven
- Alternative conditions (for example 125 °C, 150 °C, or 180 °C) may be applied where the relevant product specification or customer agreement defines them
Calculation
The non-volatile content is calculated as NV = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the empty dish, m1 is the mass of the dish plus specimen before heating, and m2 is the mass of the dish plus residue after drying to constant mass. The result is reported to one decimal place, normally as the mean of at least two determinations that agree within the repeatability limit of the method.
Special Cases and Corrections
For oxidatively drying binders such as alkyds, oxygen uptake during heating can partially offset mass loss and, in extreme cases, produce apparent solid content exceeding 100%; the drying regime must therefore be strictly controlled. For thermally sensitive or reactive systems, reduced temperatures with extended times, vacuum drying, or dedicated procedures such as ASTM D5403 for radiation-curable materials are applied. Powder coatings are tested with larger specimens to improve representativeness of the low volatile fraction.
Factors Affecting Accuracy
- Insufficient drying temperature or time leads to residual solvent and an overestimated solid content
- Excessive temperature can cause thermal degradation or loss of low-boiling coalescents and plasticizers, biasing the result downward
- Thick specimen films trap solvent, so the sample must be spread thinly and evenly
- Incomplete homogenization of settled or separated samples produces non-representative specimens
- Moisture uptake during cooling and weighing is prevented by using a desiccator and rapid, consistent weighing practice
Applications
Solid-content data are used throughout the coatings value chain. In manufacturing, routine batch testing verifies conformance to internal specifications and detects formulation or process deviations such as over-dilution. For incoming inspection, resin and emulsion solid content confirms supplier declarations and provides the basis for cost-per-kilogram-of-solids comparison.
In formulation and application engineering, mass solids combined with density yield volume solids, which convert wet film thickness to dry film thickness (dry film thickness = wet film thickness × volume solids) and determine theoretical spreading rate in square meters per liter. This is critical for estimating paint consumption in architectural projects, industrial maintenance, and automotive finishing lines.
Solid content is also a key input for regulatory VOC compliance calculations under standards such as GB 30981, ISO 11890, and the EU Decopaint Directive, where the VOC value is derived from the volatile fraction and density of the product. In commercial disputes or batch acceptance testing, results from accredited third-party laboratories using standardized methods provide technically defensible evidence for buyers, suppliers, and certification bodies.
FAQ
What is the difference between solid content and volume solids?
Solid content (mass solids) is the percentage of non-volatile matter by weight, obtained directly by the oven-drying gravimetric method. Volume solids express the non-volatile fraction by volume and are calculated from mass solids together with the density of the coating and of the non-volatile fraction. Volume solids are used for spreading-rate and film-thickness calculations, while mass solids are the direct laboratory measurement.
Which standard should be selected for testing?
ISO 3251 and GB/T 1725 are technically equivalent and widely accepted for most liquid coatings, drying at 105 °C for 1 hour. ASTM D2369 is the reference method in North America, drying at 110 °C for 60 minutes. The choice should follow the product specification, purchase contract, or target market regulation; results from different drying regimes are not directly interchangeable.
How much sample is needed and how should it be packaged?
Typically 100 g to 250 g of homogeneous liquid coating in a sealed, full container is sufficient. The sample must be protected from evaporation, moisture, and temperature extremes, and should be shipped promptly. For multi-component systems, components and hardener must be supplied so the laboratory can mix them according to the specified ratio before testing.
What accuracy can be expected?
Under well-controlled conditions, duplicate determinations should normally agree within about 0.5% absolute. Published precision data for ISO 3251 and ASTM D2369 indicate repeatability limits of approximately 0.3% to 0.4% and reproducibility limits of approximately 0.6% to 0.8% absolute for typical coating types.
Why can oxidatively drying coatings show unusual results?
Air-drying binders such as alkyds absorb oxygen during crosslinking, which adds mass and can partially compensate for solvent loss. This is why the drying temperature and time must follow the standard exactly, and why results for such systems should always be interpreted together with the declared test conditions.
Can powder coatings be tested for solid content?
Yes. Although powder coatings are nominally 100% solids, residual volatile content is measured to quantify traces of water and processing volatiles, using a modified gravimetric procedure with a larger specimen mass to improve sensitivity.
What is the typical turnaround time?
The oven-drying determination itself is completed within one day, including conditioning, drying cycles, and calculation. Most accredited third-party laboratories report results within 3 to 7 working days from sample receipt, depending on the number of standards applied and whether complementary tests such as VOC content or density are ordered together.