Solid Content Testing: Non-Volatile Matter Determination for Coatings, Adhesives, and Emulsions

Solid content testing determines the mass percentage of non-volatile matter in coatings, adhesives, polymer emulsions, inks, and other liquid formulations, providing essential data for quality control, formulation optimization, and regulatory compliance. Accredited third-party laboratories perform this analysis in accordance with recognized standards such as ISO 3251, ASTM D2369, and GB/T 1725. This article explains the technical principles, applicable sample types, test methods, and industry applications of solid content measurement.

Technical Overview

Solid content, also referred to as non-volatile matter content or total solids, is defined as the mass percentage of residue remaining after the volatile components of a sample—typically water or organic solvents—have been evaporated under specified temperature and time conditions. It is one of the most fundamental parameters used to characterize liquid chemical products, because it directly reflects the effective quantity of film-forming, binding, or functional material delivered per unit mass of product.

The result is conventionally expressed as:

Solid Content (%) = (Mass of residue after drying ÷ Mass of original sample) × 100%

Accurate solids determination is critical for several reasons. Economically, most liquid products such as emulsions and adhesives are priced and traded on a solids basis, so even a small measurement error has direct commercial consequences. Technically, solid content governs application properties such as wet film thickness, drying time, coverage rate, bond strength, and final film formation. From a regulatory perspective, non-volatile matter data is a key input for VOC content calculations required by environmental regulations in the coatings and printing ink industries.

Representative standards governing this test include ISO 3251 (paints, varnishes and plastics), ASTM D2369 (volatile content of coatings), ASTM D1259 (nonvolatile content of latex), GB/T 1725 (Chinese national standard for non-volatile matter of paints), and GB/T 2793 (solid content of adhesives).

Testing Samples

Coatings and Paints

Water-based architectural coatings, solvent-borne industrial coatings, automotive refinish paints, anti-corrosion primers, powder coating raw materials, and specialty functional coatings. For multi-component systems, the base component is typically tested individually as well as in the mixed application ratio.

Adhesives and Sealants

Polyvinyl acetate (white glue), polyurethane adhesives and dispersions, epoxy adhesives, water-based pressure-sensitive adhesive emulsions, construction sealants, and contact adhesives. Solid content strongly influences open time, tack, and final bond strength for these products.

Polymer Emulsions and Latex

Pure acrylic, styrene-acrylic, vinyl acetate-ethylene (VAE), and styrene-butadiene emulsions, as well as fluoropolymer dispersions such as PTFE. Emulsion manufacturers and downstream compounders use solids data to control polymerization conversion and formulation consistency.

Inks and Printing Materials

Water-based and solvent-based printing inks, varnishes, overprint coatings, and inkjet fluids, where non-volatile content affects color strength, transfer, and drying behavior.

Chemical Auxiliaries and Process Chemicals

Textile finishing agents, leather chemicals, paper coating binders, sizing agents, water treatment flocculants and polymer solutions, surfactant concentrates, and agrochemical suspension concentrates.

New Energy Materials

Lithium-ion battery electrode slurries, binder dispersions, ceramic slurries, and electronic encapsulants, where solids content must be tightly controlled to ensure coating weight uniformity and electrode consistency.

Testing Items

Commonly requested test items related to solid content determination include:

• Total solid content (TSC) at a specified drying temperature
• Non-volatile matter content (NVC) per ISO 3251 / GB/T 1725
• Volatile content by mass and by volume (ASTM D2369)
• Water content by Karl Fischer titration (ISO 760 / GB/T 6283), used to distinguish water from organic volatiles
• Density and viscosity measurements as supporting data for converting solids between mass and volume bases
• Relevant supplementary items such as ash content and VOC content for full compliance characterization

Testing Methods

Oven Drying (Gravimetric) Method

This is the reference method for the majority of samples. A weighed quantity of the thoroughly mixed specimen is placed in a tared flat-bottomed dish, spread into a thin film, and dried in a forced-ventilation oven at a standard temperature—commonly 105 °C ± 2 °C for water-based products, or at product-specific temperatures such as 125 °C or 150 °C defined by the applicable standard. After the prescribed drying period, the dish is cooled to ambient temperature in a desiccator and reweighed. Drying is repeated until constant weight is achieved, typically defined as a mass change not exceeding a specified tolerance (for example, 0.01 g or 0.1% between successive weighings). The measurement is normally performed in duplicate and the results averaged, with a defined repeatability limit between parallel determinations.

Vacuum Drying Method

For heat-sensitive materials such as certain polymer dispersions, reactive adhesives, or biopolymer solutions, drying under reduced pressure at a lower temperature (for example 40–60 °C) minimizes thermal degradation and premature crosslinking of the residue. This approach is selected when standard oven conditions are known to cause decomposition or further reaction of the non-volatile fraction.

Karl Fischer Titration (Indirect Determination)

When the volatile phase consists essentially of water, coulometric or volumetric Karl Fischer titration provides a highly precise water content value, from which solid content is calculated as 100% minus the water fraction. This method is particularly useful for samples containing volatiles that are difficult to remove completely by drying, or where very low measurement uncertainty is required. It should be noted that the oven-drying result and the KF-derived result differ whenever non-aqueous volatiles are present, so the two methods are complementary rather than interchangeable.

Rapid Infrared and Halogen Moisture Analysis

Halogen-lamp moisture analyzers determine mass loss upon drying automatically and are widely used for in-process quality control where speed matters. Because the heating profile differs from a conventional oven, results should be validated against the gravimetric reference method and the correlation established for each specific product type.

Microwave Drying Method

Microwave-assisted drying offers very short analysis times for high-throughput screening of emulsions and slurries. As with infrared methods, it is generally treated as a rapid secondary technique calibrated against the applicable standard oven method.

Regardless of the technique selected, critical factors for accuracy include homogenization and degassing of the sample before weighing, use of an appropriate specimen mass that forms a sufficiently thin film, prevention of skin formation that traps solvent, control of oven temperature uniformity, and complete cooling in a desiccator before each weighing.

Applications

Solid content testing supports a wide range of industrial and commercial objectives:

• Manufacturing quality control: batch release, polymerization conversion monitoring, and formulation consistency for emulsion, adhesive, and coating producers
• Incoming material inspection: verification of supplier specifications by compounders and downstream users
• Research and development: solids-adjusted formulation design, solids-build comparison of competitive products, and dilution calculations
• Regulatory compliance: input data for VOC and hazardous air pollutant content declarations in the coatings, ink, and adhesives sectors
• Battery and electronics manufacturing: control of slurry solids and viscosity for electrode coating uniformity
• Trade settlement: independent solids verification for products bought and sold on a non-volatile matter basis
• Failure analysis: investigating defects such as insufficient film build, poor adhesion, or slow drying traced to out-of-specification solids

FAQ

What is the difference between solid content and non-volatile matter content? The two terms are used almost interchangeably. "Non-volatile matter" is the preferred expression in paint and coating standards such as ISO 3251 and GB/T 1725, while "solid content" is more common in the adhesives, emulsion, and polymer industries. Both describe the residue mass fraction after evaporation of volatiles under defined conditions.

Which drying temperature is used? The temperature is specified by the applicable product standard or agreed between the parties. Common conditions include 105 °C for water-based systems, 125 °C or 150 °C for coatings tested to ASTM D2369 or GB/T 1725, and reduced-temperature vacuum drying for heat-sensitive materials. The chosen condition must always be reported together with the result, because residues can undergo further mass loss or oxidation at elevated temperatures.

How much sample is required? Typically 10–50 g of homogeneous liquid sample is sufficient for duplicate gravimetric determinations. For Karl Fischer water content, quantities in the range of 0.1–1 g per determination are usual, depending on expected water level.

Why can oven-drying and Karl Fischer results differ? Oven drying measures total mass loss, which may include organic solvents and traces of decomposed or released components in addition to water, while Karl Fischer titration responds selectively to water only. A significant gap between the two results is itself diagnostic information indicating the presence of non-aqueous volatiles.

How is result accuracy ensured? Accredited third-party laboratories perform parallel determinations with controlled weighing uncertainty, verify constant weight, use calibrated balances and ovens, apply repeatability limits defined in the standards, and report the method, drying condition, and individual replicate values together with the final average.

What is a typical turnaround time? Standard gravimetric testing, including duplicate determination and report issuance, is commonly completed within three to five working days after sample receipt; rapid moisture analysis methods can provide results within one day when required for urgent screening.