Non-volatile matter (solid content) testing determines the percentage of residue remaining after volatile components evaporate from coatings, adhesives, inks, and resin products under controlled heating conditions. This gravimetric test is essential for quality control, formulation verification, VOC compliance, and cost management across the chemical and materials industries. This article explains the testing principles, applicable standards, sample types, procedures, and practical considerations for accurate solid content determination.
Technical Overview
Non-volatile matter content, commonly referred to as solid content, is defined as the mass percentage of a material that remains as residue after the volatile components—such as solvents, water, and diluents—have been driven off under specified temperature and time conditions. The result is expressed as a mass fraction (%) relative to the original specimen mass, and the volatile content is obtained by difference.
For liquid coating, adhesive, ink, and resin systems, the non-volatile content directly determines film-forming capability, application coverage, wet-to-dry film relationship, and overall formulation economics. It is one of the most frequently performed quality control tests in these industries and serves as a key input for VOC content calculations required by environmental regulations.
The test is typically performed using a gravimetric oven-drying method in accordance with recognized international and national standards, including:
- ISO 3251 — Paints, varnishes and plastics: Determination of non-volatile-matter content
- ASTM D2369 — Standard Test Method for Volatile Content of Coatings
- ASTM D1353 — Nonvolatile Matter in Volatile Solvents for Paint, Varnish, Lacquer, and Related Products
- GB/T 1725 — Determination of non-volatile matter content of paints and varnishes
- GB/T 2793 — Determination of non-volatile matter content of adhesives
Testing Samples
Paints, Coatings, and Varnishes
Solvent-borne, water-borne, high-solids, powder coating premixes, and radiation-curable coatings. Sample sizes are typically small (0.2 g to 1.0 g) to ensure complete evaporation of volatile components within the specified drying period.
Adhesives and Sealants
Water-based emulsion adhesives, hot-melt adhesives, solvent-based contact adhesives, and reactive sealants. These materials often require product-specific drying temperatures because some contain crosslinkers or plasticizers that behave differently under heat.
Printing Inks
Liquid inks, UV-curable inks, and paste inks. Inks containing reactive diluents may require modified procedures to account for monomers that polymerize rather than evaporate during heating.
Resins, Polymer Emulsions, and Latexes
Acrylic, vinyl acetate, styrene-butadiene, alkyd, epoxy, and polyurethane dispersions. Resin producers use solid content testing to monitor polymerization conversion and batch-to-batch consistency.
Other Chemical Products
Cleaning agents, surface treatment solutions, textile auxiliaries, paper coating chemicals, electroplating solutions, and intermediate chemical products where residue-on-evaporation is a specification parameter.
Testing Items
The core measurable parameters include:
- Non-volatile matter content (mass fraction, %) at a specified temperature and time
- Volatile matter content (%), obtained by difference from 100%
- Water content (auxiliary determination, e.g., Karl Fischer titration, required for VOC calculations of water-reducible coatings)
- Volume solids content (derived value combining non-volatile content and density measurements, used to compute theoretical spreading rate)
- VOC content (calculated from non-volatile content, density, and water content in accordance with regulatory methodologies)
Testing Methods
Gravimetric Oven-Drying Method (Primary Method)
The standard gravimetric procedure follows these general steps:
- Condition and weigh a clean, dry weighing dish (aluminum, glass, or stainless steel) to the nearest 0.1 mg; record as m0.
- Add a test portion to the dish by difference, typically 0.2 g to 1.0 g depending on viscosity and expected solids level; record total mass as m1. Distribute the specimen evenly over the dish bottom.
- For highly viscous samples, add a small quantity of a suitable volatile solvent to aid spreading, ensuring the solvent itself leaves no residue.
- Place the dish in a ventilated drying oven at the specified temperature—commonly 105 °C ± 2 °C for 60 minutes for paints, or a product-specified condition such as 125 °C, 150 °C, or 160 °C for certain resins and adhesives.
- Remove the dish, cool it to room temperature in a desiccator, and reweigh to the nearest 0.1 mg; record as m2.
- Repeat the heating, cooling, and weighing cycle until constant mass is achieved, typically when consecutive weighings differ by no more than 1 mg.
The non-volatile matter content is calculated as: 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 specimen before drying, and m2 is the mass of the dish plus residue after drying to constant mass.
Vacuum Oven Method
For heat-sensitive materials such as certain acrylic solutions, silicone products, and reactive systems that may degrade, crosslink, or lose low-molecular-weight fractions at elevated temperatures, drying is performed in a vacuum oven at reduced pressure and moderate temperature (typically 40 °C to 80 °C). This lowers the evaporation temperature of volatiles and minimizes thermal alteration of the residue.
Rapid Thermogravimetric (Infrared/Halogen Analyzer) Method
Loss-on-drying moisture and solids analyzers use infrared or halogen heating to dry a specimen on an integrated balance, delivering results within 5 to 15 minutes. These instruments are widely used for in-process quality control, but the drying parameters must be validated and correlated against the standard oven method for each product type before results are accepted for specification purposes.
Practical Considerations for Accuracy
- Prevent spattering of low-viscosity or high-solvent samples by using a larger-diameter dish or pre-evaporating at room temperature or under reduced pressure before full heating.
- Avoid skin formation, which traps solvent beneath the surface, by spreading the specimen thinly or using a glass rod or wire spiral in the dish.
- Use specimens that are homogeneous and representative; mix thoroughly before sampling while avoiding entrained air.
- Control oven temperature uniformity and recovery time, as frequent door opening causes erratic results.
- Always cool dishes in a desiccator to prevent moisture uptake by hygroscopic residues.
- Perform duplicate determinations; typical repeatability for a properly executed test is within approximately 0.5% absolute for solids levels above 40%.
Applications
Non-volatile matter and solid content testing supports critical business and technical decisions, including:
- Manufacturing quality control: verifying batch consistency, detecting formulation errors, and confirming completion of polymerization or condensation reactions.
- Incoming material inspection: validating supplier certificates of analysis for resins, dispersions, and intermediates before production use.
- Regulatory compliance: providing the non-volatile input required for VOC content calculations under environmental regulations governing architectural coatings, industrial coatings, and consumer chemical products.
- Formulation development: comparing theoretical solids derived from formulation mass balances against measured values to identify volatile losses, retained solvent, or moisture.
- Cost and yield management: calculating solids-based pricing, determining required application quantities, and estimating wet film thickness needed to achieve a specified dry film thickness.
- Customer specification verification: demonstrating conformance to contractual and industry specification limits for solids content.
FAQ
What is the difference between non-volatile matter content and solid content?
The two terms are generally interchangeable in practice. Non-volatile matter content refers to the standard oven-drying residue under specified conditions, while solid content is the common industrial term for the same measurement. Note that the measured value is method-dependent—it represents what remains under the chosen temperature and time, which may differ slightly from the "true" polymeric solids in reactive or heat-sensitive systems.
Why does the measured value differ from the theoretical formulation solids?
Common causes include retained solvent trapped in the film, residual moisture in raw materials, evaporation of low-molecular-weight components such as coalescents or plasticizers, and incomplete drying due to skin formation. Persistent deviations above 1% to 2% typically justify a review of both the formulation mass balance and the drying conditions.
Which drying temperature and time should be used?
Unless the product specification or customer standard states otherwise, 105 °C ± 2 °C for 60 minutes is the default condition for most paints and varnishes per ISO 3251 and related standards. Product-specific standards may require 125 °C, 150 °C, or 160 °C for certain resins, adhesives, and polymer dispersions to ensure complete removal of higher-boiling components.
What does "constant mass" mean in the test procedure?
Constant mass is achieved when the difference between two consecutive weighing cycles after additional heating is negligible—commonly 1 mg or less for typical specimen sizes. Drying to constant mass confirms that all volatile components have been removed and the residue value is stable.
Can a rapid moisture analyzer replace the oven method?
A validated infrared or halogen analyzer can be used for routine in-process control provided its parameters have been correlated with the standard oven method for the specific product. Official certifications of analysis, dispute resolution, and regulatory submissions should rely on the standard gravimetric oven method.
How are the results reported?
Results are reported as non-volatile matter content in mass percent, rounded conventionally to one decimal place, together with the drying temperature, drying time, specimen mass, and the standard followed. Duplicate results and their mean are reported, with the deviation between duplicates indicated.
How long does the test take?
A single gravimetric determination typically takes 2 to 3 hours including heating, desiccator cooling, and constant-mass verification, with results usually available within one working day. Same-day turnaround is standard for routine quality control testing.