Solid content determination is one of the most fundamental quality-control tests for paints, coatings, and polymer dispersions, quantifying the non-volatile matter remaining after volatile components evaporate under controlled heating conditions. This article explains the test principles, internationally recognized methods such as ISO 3251, ASTM D2369, and GB/T 1725, sample handling, calculation procedures, and why this parameter is critical for formulation development, VOC compliance, and cost control.
Technical Overview
Solid content, also referred to as non-volatile matter content, is the mass fraction of a coating sample that remains after volatile components such as solvents, water, and diluents are evaporated under specified conditions. The residue consists of film-forming resins, pigments, extenders, and non-volatile additives, which together determine the dry film that the coating can deliver on a substrate.
The measurement is performed gravimetrically: a weighed quantity of the coating is dispersed in a preconditioned dish, heated in a ventilated oven at a standard temperature for a defined period, cooled in a desiccator, and reweighed. The ratio of residue mass to original sample mass, expressed as a percentage, is the solid content. Two related quantities must be distinguished: mass solids (percentage by weight) and volume solids (percentage by volume), the latter being the key input for calculating theoretical spreading rate and dry film thickness.
Solid content data underpin nearly every downstream calculation in coatings technology, including volatile organic compound (VOC) content, practical coverage, film build, formulation cost, and batch-to-batch consistency. Major internationally recognized standards for this test include:
- 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 plastics
- ASTM D2697 — Standard Test Method for Volume Nonvolatile Matter in Clear or Pigmented Coatings
- GB/T 9272 — Determination of volume nonvolatile matter of paints and varnishes
Testing Samples
Solvent-Borne Liquid Coatings
Alkyd primers, epoxy paints, polyurethane topcoats, chlorinated rubber coatings, and other solvent-reducible systems. These samples are typically diluted with a small amount of suitable solvent to ensure uniform spreading in the weighing dish and complete release of trapped volatiles during heating.
Waterborne Coatings
Emulsion-based architectural paints, water-reducible industrial coatings, and latex systems. Waterborne samples require gentle initial spreading and adequate oven ventilation so that skinning does not entrap water. Foaming and floating must be avoided during sample preparation.
Powder Coatings
Thermosetting epoxy-polyester, pure epoxy, and polyurethane powder formulations. Because powder coatings contain almost no volatile solvent, the test verifies the very low loss-on-drying value and detects residual moisture or low-molecular-weight volatiles from raw materials.
Raw Materials and Intermediates
Resins, polymer dispersions, emulsions, curing agents, and pigment pastes are tested by the same gravimetric principle to verify supplier specifications and support formulation calculations.
Testing Items
Typical test items and deliverables for a solid content determination include:
- Non-volatile matter content (mass fraction, %), tested to ISO 3251, GB/T 1725, or ASTM D2369
- Volatile matter content (%), reported as the complement of mass solids per ASTM D2369
- Volume solids (%), determined by displacement methods per ASTM D2697 or GB/T 9272, or calculated from formulation density data
- Supporting measurements such as density, which are required for converting between mass solids, volume solids, and VOC content
- Derived results such as theoretical spreading rate and VOC content for regulatory declarations
Testing Methods
Oven Gravimetric Method (Primary Method)
The reference procedure described in ISO 3251, GB/T 1725, and ASTM D2369 follows these steps:
- Condition a flat-bottomed glass, aluminum, or stainless steel dish to constant mass in the oven, then cool it in a desiccator and weigh it accurately to 0.1 mg.
- Add approximately 0.5 g to 2 g of the homogenized, stirred sample into the dish, spreading it evenly. Weigh the dish plus sample immediately to prevent solvent loss errors.
- For viscous solvent-borne coatings, add a small quantity of specified solvent and swirl to form a thin, uniform layer, which promotes complete evaporation.
- Heat in a forced-ventilation oven at 105 ± 2 °C for 1 hour (ISO 3251, GB/T 1725) or at 110 ± 5 °C for 60 minutes (ASTM D2369). Products with specified baking schedules may be tested at their cure temperature where the standard permits.
- Cool the dish in a desiccator for 20 to 30 minutes and reweigh. Repeat the heating, cooling, and weighing cycle until the change in mass between successive weighings does not exceed 0.01 g or the specified constant-mass criterion.
- Run duplicate determinations and report the mean; the difference between duplicates should fall within the repeatability limit of the standard, typically about 0.5% absolute for well-mixed liquid samples.
Calculation
The non-volatile matter content is calculated from the measured masses: Non-volatile content (%) = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the empty dish, m1 is the mass of the dish plus the original sample, and m2 is the mass of the dish plus residue after heating to constant mass. Results are reported to one decimal place together with the test temperature and duration.
Volume Solids Determination
Volume solids are measured by applying films of the coating to metal panels, curing them under defined conditions, and determining the volume of the dry film by water displacement or by mass and density measurement, following ASTM D2697 or GB/T 9272. The result is essential because two coatings with identical mass solids can differ substantially in volume solids due to pigment and resin density differences, directly affecting coverage.
Rapid Instrumental Methods
Infrared dryers, halogen moisture balances, and microwave drying ovens shorten the determination to minutes and are widely used for in-process quality control. Such rapid methods must be validated against the oven reference method and correlated for each product family, because heating mode and intensity can slightly alter the result.
Factors Affecting Accuracy
Key sources of error include incomplete homogenization of settled pigments, sample loss during weighing, skinning of the surface that traps volatiles, insufficient oven ventilation, condensation during cooling, and prolonged exposure of hygroscopic residues to laboratory air. Strict adherence to weighing sequence, desiccator cooling, and constant-mass verification minimizes these errors.
Applications
Solid content determination is applied throughout the coatings value chain:
- Manufacturing quality control: verifying each production batch against internal specifications and confirming correct dosing of solvents and raw materials.
- Incoming material inspection: checking resins, dispersions, and intermediates delivered by suppliers before release to production.
- Formulation and R&D: comparing binder systems, optimizing solvent levels, and calculating the theoretical solids of formulations from component data.
- Regulatory compliance: serving as the base input for VOC content calculations under regulations such as GB 30981 and GB 18582 in China, the EU Directive on the limitation of VOCs in decorative paints, and US environmental rules, supported by practices such as ASTM D3960 and ISO 11890.
- Coverage and economics: deriving theoretical spreading rate and dry film thickness for specification writing, and evaluating applied cost per square meter.
- Contract and dispute testing: accredited third-party laboratories perform determinations to internationally recognized standards for product certification, tender documents, and quality disputes.
FAQ
What is the difference between mass solids and volume solids?
Mass solids express the non-volatile fraction by weight, obtained directly from the gravimetric oven test. Volume solids express the non-volatile fraction by volume and require density data or a displacement test. Volume solids govern the actual film thickness and spreading rate achievable per liter, so both values are routinely reported.
Which heating temperature should be used for the test?
The standard conditions are 105 ± 2 °C for 1 hour under ISO 3251 and GB/T 1725, and 110 ± 5 °C for 60 minutes under ASTM D2369. The chosen standard, temperature, and time must always be stated in the report, because results obtained under different conditions are not directly comparable.
How is solid content related to VOC content?
VOC content is derived from the volatile fraction together with density, water content, and exempt-compound data. A higher measured solid content corresponds to a lower VOC value for the same product, which is why regulators and buyers frequently require the determination to be performed by accredited third-party laboratories using validated methods.
How much sample is required and what repeatability can be expected?
A few grams of homogenized coating are sufficient for duplicate determinations. Under well-controlled conditions, repeatability within a laboratory is typically around 0.5% absolute, while reproducibility between different laboratories may reach 1% to 2% absolute depending on the product type and standard applied.
Can the oven method be used for moisture-curing or thermally sensitive coatings?
For products that react or crosslink at the test temperature, the measured residue includes reaction by-products released during curing, so results represent non-volatile matter under the specific test conditions rather than true formulation solids. In such cases, complementary methods such as volume solids measurement or formulation-based calculation are recommended for a complete assessment.