A solid content determination experiment quantitatively measures the percentage of non-volatile matter in coatings, adhesives, emulsions, inks, and slurries using standardized gravimetric drying techniques. This article explains the experimental principle, applicable sample types, reference standards such as ISO 3251, ASTM D2369, and GB/T 1725, detailed testing procedures, calculation formulas, and typical industrial applications. It serves as a practical technical reference for laboratory analysts, quality control engineers, and formulation scientists.
Technical Overview
Solid content, also referred to as non-volatile matter content or solids content, is defined as the mass percentage of residue remaining after a liquid or pasty sample is heated under specified conditions until evaporation of volatile components is complete. The solid content determination experiment is one of the most fundamental and widely performed analyses in the coatings, adhesives, polymers, and battery materials industries, because it directly influences formulation cost, application properties, film formation, and volatile organic compound (VOC) compliance.
The experiment is based on the gravimetric (thermogravimetric) principle: a weighed quantity of sample is dispensed into a tared weighing dish, dried in an oven or rapid moisture analyzer under controlled temperature and time, cooled in a desiccator, and reweighed. The solid content is calculated using the following formula:
Solid Content (%) = (m2 - m0) / (m1 - m0) × 100
- m0 = mass of the empty weighing dish (g)
- m1 = mass of the dish plus sample before drying (g)
- m2 = mass of the dish plus dried residue after drying to constant weight (g)
It should be noted that the measured result is an "apparent" or "practical" non-volatile content. At excessive temperatures, thermally unstable resins may decompose or crosslink, while insufficient drying may leave residual solvent or moisture trapped in the film. Therefore, strict adherence to the temperature, time, and sample mass specified in the relevant standard is essential for accuracy and inter-laboratory reproducibility.
Testing Samples
Coatings and Paints
Waterborne and solventborne liquid coatings, including architectural paints, industrial coatings, primers, and varnishes. Typical solid content ranges from 30% for thin architectural paints to over 80% for high-solids industrial coatings. The applicable reference standards include ISO 3251, ASTM D2369, and GB/T 1725.
Adhesives and Sealants
Polymer emulsion adhesives, pressure-sensitive adhesive dispersions, hot-melt adhesives, and reactive sealants. Solid content determination controls batch consistency, open time, and final bond strength. Common reference standards include GB/T 2793 and ASTM D5095 for nonvolatile content of adhesives.
Emulsions and Polymer Dispersions
Styrene-butadiene latex, pure acrylic and styrene-acrylic emulsions, vinyl acetate-ethylene (VAE) emulsions, polyurethane dispersions, and epoxy emulsions. These materials serve as binders for coatings, textiles, paper, carpets, and construction products.
Lithium-Ion Battery Electrode Slurries
Cathode and anode slurries composed of active materials, conductive carbon, binders, and solvents (NMP or water-based systems). Solid content measurement is a critical process control parameter that affects coating weight, drying energy consumption, and electrode uniformity.
Printing Inks and Other Materials
Liquid inks, textile auxiliaries, paper coating colors, ceramic slurries, detergents, and cosmetic emulsions may also be characterized using the same gravimetric approach with method parameters adapted to the matrix.
Testing Items
Depending on customer requirements and product specifications, the solid content determination experiment may cover the following test items:
- Total solid content (non-volatile matter content) by oven drying or vacuum drying
- Volatile matter content, calculated as 100% minus solid content
- Water content by Karl Fischer titration (ASTM E203, GB/T 6283), used for water-specific quantification
- Conventional solid content by rapid infrared or halogen moisture analyzer for routine quality control
- VOC content estimation for regulatory declarations, in combination with density measurement (ASTM D3960 or equivalent)
- Constant-weight verification and duplicate precision checking according to the repeatability limits of the reference method
Testing Methods
Oven Drying Method (Primary Reference Method)
This is the standard gravimetric procedure described in ISO 3251, ASTM D2369, GB/T 1725, and GB/T 9272. A typical procedure is as follows:
- Condition and weigh a clean, flat aluminum or glass weighing dish to the nearest 0.1 mg using an analytical balance.
- Accurately weigh approximately 0.5 to 2 g of the homogenized sample into the dish, spreading it into a thin, uniform film.
- Place the dish in a forced-ventilation oven at 105 ± 2 °C (or at the temperature specified by the product standard, such as 120 °C or 150 °C for high-boiling systems).
- Dry for the specified time, typically 1 to 3 hours, then transfer the dish to a desiccator, cool to room temperature, and reweigh.
- Repeat drying cycles of 15 to 30 minutes until the change in mass between successive weighings is no greater than 0.005 g (or the constant-weight criterion defined in the standard).
- Calculate the solid content using the gravimetric formula and report the mean of duplicate determinations.
Vacuum Drying Method
For heat-sensitive materials such as certain polymer dispersions, thermoplastic resins, or solvent-containing adhesives, drying is performed in a vacuum oven at reduced pressure (for example, below 5 kPa) and a moderate temperature (40 to 80 °C). This lowers the boiling point of the volatiles and prevents thermal degradation of the residue.
Rapid Infrared or Halogen Moisture Analyzer Method
A thermogravimetric moisture analyzer integrates a halogen or infrared heating source with a precision balance, delivering a solid content reading within 5 to 20 minutes. This method is widely used for in-process quality control; however, its parameters must be validated against the oven reference method for each specific product matrix.
Karl Fischer Titration (Complementary Method)
When water is the only significant volatile component, the water content can be determined precisely by Karl Fischer titration according to ASTM E203 or GB/T 6283, and the total solid content can be obtained indirectly. This approach is particularly useful for low-water systems and for distinguishing water from organic volatiles.
Key Factors Affecting Accuracy
- Drying temperature and duration must match the product standard; excessive temperature causes decomposition or retention of volatiles through skin formation.
- Sample mass and film thickness influence the evaporation rate; excessive thickness may entrap solvent.
- Humidity and condensation during cooling must be excluded by using a desiccator and consistent weighing intervals.
- Duplicate determinations should agree within the repeatability limit of the standard, typically within 0.5% absolute for well-controlled systems.
Applications
The solid content determination experiment supports a broad range of industrial and regulatory applications:
- Incoming raw material inspection for emulsions, resins, and dispersions supplied to coating and adhesive manufacturers.
- Batch-to-batch quality control and release testing in production of paints, adhesives, inks, and battery slurries.
- Formulation development and cost optimization, where solid content determines the calculated dry film thickness and raw material efficiency.
- Coating and electrode manufacturing process control, ensuring correct wet film application weight and predictable drying loads.
- VOC compliance evaluation and environmental reporting, where non-volatile content is a key input for VOC calculation.
- Independent verification testing performed by accredited third-party laboratories for supplier qualification, customer disputes, and product certification.
FAQ
What is the difference between solid content and non-volatile content?
The two terms are often used interchangeably. Strictly speaking, "non-volatile content" is the experimentally measured residue under defined drying conditions, while "solid content" may refer to the theoretical amount of non-evaporating components. Because measured values depend on temperature and time, test reports should always state the drying conditions.
Which drying temperature should be used?
The temperature is specified by the governing product or method standard. For most waterborne coatings, 105 ± 2 °C for one hour with subsequent constant-weight checks is standard; for solventborne or high-boiling systems, 120 to 150 °C may be specified. Heat-sensitive polymers require vacuum drying at lower temperatures.
How is "constant weight" confirmed?
The sample is repeatedly dried, cooled, and reweighed at short intervals. When the mass change between two successive weighings is below the limit defined in the standard (commonly 0.005 g or 0.1% of sample mass), constant weight is achieved and the final mass is recorded.
How much sample should be tested?
Most gravimetric standards specify 0.5 to 2 g of sample spread into a thin film. Too little sample increases relative weighing error, while too much sample prolongs drying and may trap solvent beneath a surface skin.
Which standards are most commonly referenced?
ISO 3251 and GB/T 1725 for paints and varnishes, ASTM D2369 for volatile content of coatings, ASTM D5095 and GB/T 2793 for adhesives, and ASTM E203 or GB/T 6283 for Karl Fischer water determination.
Can a moisture analyzer replace the oven method?
A calibrated halogen or infrared moisture analyzer is acceptable for routine internal quality control, provided its drying parameters have been correlated with the oven reference method for the specific product. For contractual, certification, and dispute purposes, the oven method according to the recognized standard is generally preferred.