Paint Solid Content Testing: Standards, Methods, and Industrial Applications

Paint solid content testing measures the percentage of non-volatile matter in a coating, a critical indicator of film formation, theoretical coverage, cost efficiency, and VOC compliance. This article explains the testing principle, applicable standards such as ISO 3251, ASTM D2369, and GB/T 1725, common sample types, laboratory procedures, and practical applications of solids content analysis for waterborne, solvent-borne, powder, and specialty coating systems.

Technical Overview

Solid content, also known as non-volatile matter content, is the proportion of a coating that remains as a dry, adherent film after all volatile components, including solvents, water, and other evaporating substances, are driven off under specified drying conditions. It is expressed as a mass percentage of the original specimen and represents one of the most fundamental quality parameters for paints, varnishes, resins, polymer dispersions, and related products.

Testing is generally performed using the gravimetric difference method: a weighed specimen is heated at a standardized temperature until the volatile fraction has evaporated, and the residual mass is reweighed after cooling. The solid content is calculated from the mass ratio before and after heating. This parameter directly affects the following performance and commercial aspects of a coating:

• Achievable dry film thickness per unit of applied wet coating
• Theoretical spreading rate and material cost per square meter
• VOC content calculated by the difference method
• Batch-to-batch consistency and formulation control
• Drying behavior, sag resistance, and fullness of the applied film

The principal reference standards include ISO 3251 and ASTM D2369 for non-volatile or volatile content by oven drying, GB/T 1725 as the Chinese national standard technically aligned with ISO 3251, ASTM D2697 and GB/T 9272 for volume solids, and ISO 11890-1 for VOC determination by the difference method. Accredited third-party laboratories routinely perform these tests for quality control, regulatory compliance, and contractual verification.

Testing Samples

Solid content testing applies to virtually every category of liquid and liquid-precursor coating materials. Typical sample types submitted for analysis include the following:

Architectural Coatings

Interior and exterior emulsion (latex) paints, textured and elastic wall coatings, waterproofing membranes and coatings, primers, putties, and decorative finishes. These waterborne systems typically contain emulsion binders, pigments, fillers, and additives, and their solids level strongly influences hiding power and application economy.

Industrial and Protective Coatings

Epoxy, polyurethane, alkyd, acrylic, chlorinated rubber, and vinyl systems, including anticorrosive primers, zinc-rich coatings, marine coatings, floor coatings, and steel structure paints. For multi-component products, each component and the mixed system may be tested separately.

Automotive, Wood, and Coil Coatings

OEM and refinish paints, electrodeposition coatings, topcoats and clearcoats, UV-curable wood finishes, and coil and can coatings, where precise solids control is essential for consistent line application and film build.

Powder Coatings and Raw Materials

Thermosetting powder coatings, which are essentially 100 percent solids, are verified for volatile loss, while upstream materials such as resins, polymer emulsions, dispersions, binders, and additives are tested as incoming raw materials or for formulation development.

Testing Items

Depending on the purpose of the analysis and the product specification, the following test items are commonly carried out within the scope of solid content evaluation:

• Non-volatile matter content (mass solid content) of the coating or its individual components
• Volatile matter content, including solvent, water, and other evaporating species
• Volume solids of clear or pigmented coatings
• Water content determined by Karl Fischer titration, required for VOC calculation of waterborne products
• VOC content derived by the difference method according to ISO 11890-1 or equivalent procedures
• Comparison of solids values between mixed multi-component systems at their specified pot-life stages

Results are normally reported as mass percentages, with the test temperature, heating duration, specimen mass, and reference standard clearly stated, since these conditions directly affect the numerical outcome.

Testing Methods

Oven-Drying Gravimetric Method (ISO 3251 / GB/T 1725 / ASTM D2369)

This is the reference method for non-volatile matter determination. A clean weighing dish is dried to constant mass, a small specimen of typically about one gram is introduced and spread evenly, and the assembly is weighed to the nearest 0.1 mg. The specimen is then heated in a ventilated oven at 105 ± 2 °C for 60 minutes according to ISO 3251 and GB/T 1725, or at 110 ± 5 °C for 60 minutes according to ASTM D2369, which also specifies dispersing the specimen in a small volume of suitable volatile solvent to promote uniform film formation. After heating, the dish is cooled in a desiccator and reweighed, with the cycle repeated where constant mass is required. The solid content is calculated as: Solid content (%) = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the dish, m1 is the mass of the dish plus specimen before heating, and m2 is the mass of the dish plus residue after heating. Product standards may specify alternative temperatures, such as 125 °C or 150 °C, for particular binder chemistries.

Rapid Infrared or Halogen Drying Method

Loss-on-drying analyzers equipped with infrared or halogen heaters can complete a determination in 10 to 20 minutes and are widely used for in-process factory control. Because heating is radiant rather than convective, results can deviate slightly from oven values, so the instrument parameters must be validated and calibrated against the reference oven method for each product family.

Vacuum Oven Method

For heat-sensitive binders, high-molecular-weight resins, and materials whose non-volatile fraction may partially decompose or oxidize at atmospheric drying temperatures, drying is performed in a vacuum oven at reduced pressure and moderate temperature. This lowers the effective boiling points of the volatiles and minimizes thermal degradation of the residue.

Karl Fischer Titration for Water Content

In waterborne coatings, the water fraction cannot be separated from organic volatiles by simple drying. Karl Fischer titration, performed either volumetrically or coulometrically on a suitably prepared extract, quantifies the water content precisely. This value is required for VOC calculation by the difference method, in which VOC equals the total volatile fraction minus the water fraction.

Volume Solids Determination (ASTM D2697 / GB/T 9272)

Mass solids alone do not indicate how much film volume a liter of coating will deposit, because pigment and binder densities differ. Volume solids methods coat metal panels, dry and cure the films under defined conditions, and determine the dry film volume by weighing the panels in air and immersed in water, applying Archimedes' principle. Volume solids are essential for calculating theoretical spreading rate and comparing the true economy of different products.

Applications

Solid content data serve multiple functions across the coatings value chain:

• Factory quality control and batch release, ensuring consistency between production lots and the technical data sheet
• Incoming inspection of purchased resins, emulsions, and additives against supplier specifications
• Formulation development and cost optimization in research and development laboratories
• Regulatory compliance demonstration for VOC limit standards, such as GB 18582 for architectural coatings and GB 30981 for industrial coatings, using the difference method
• Tender, contract, and dispute verification performed by accredited third-party laboratories
• Calculation of theoretical spreading rate, in which coverage (m²/L) equals volume solids multiplied by ten and divided by the specified dry film thickness in micrometers
• Process control for high-speed application lines, where solids variation affects film build, curing, and appearance

Typical reference ranges illustrate the practical spread of the parameter: architectural latex paints usually fall between 45 and 65 percent solids, solvent-borne industrial coatings between 40 and 80 percent, high-solids systems above 70 percent, UV-curable and powder coatings approaching 100 percent, and polymer emulsions commonly between 45 and 60 percent.

FAQ

Q: What is the difference between mass solids and volume solids?

A: Mass solids express the non-volatile fraction by weight, obtained directly from oven drying, while volume solids express the non-volatile fraction by volume, determined by dry film displacement methods. Because pigments and binders are denser than solvents and water, volume solids are always lower than mass solids for the same product, and only volume solids can be used to calculate theoretical spreading rate.

Q: Why do different standards specify different drying temperatures?

A: The drying condition must fully remove volatiles without decomposing or further curing the binder. ISO 3251 and GB/T 1725 commonly use 105 ± 2 °C, ASTM D2369 uses 110 ± 5 °C, and specific product standards may specify higher temperatures for systems containing high-boiling co-solvents. Results from different conditions are not directly interchangeable and the standard used must always be reported.

Q: How is solid content related to VOC content?

A: By the difference method, VOC content is obtained as the volatile fraction (100 percent minus solids) minus the water content determined by Karl Fischer titration. Accurate solids and water measurements are therefore prerequisites for defensible VOC declarations on product labels and safety data sheets.

Q: What precautions ensure accurate results?

A: Key practices include using a representative, homogenized specimen; spreading the specimen thinly and evenly; weighing quickly to avoid pre-evaporation loss; cooling in a desiccator before reweighing; checking for constant mass where required; and using vented ovens with stable, calibrated temperature control. For multi-component coatings, the mixing ratio and the state of the mixture tested must be documented.

Q: How long does a standard determination take?

A: A single oven determination typically requires about two hours including cooling and weighing, while validated rapid dryers can deliver results in under 30 minutes. Full reports from accredited third-party laboratories, including duplicate determinations and standard-specified precision checks, are generally issued within a few working days.