Determination of Solid Content by the Oven Drying Method

Solid content testing by the oven drying method is a fundamental gravimetric technique used to determine the non-volatile fraction of coatings, adhesives, emulsions, inks, and chemical products. Governed by internationally recognized standards such as ISO 3251, ASTM D2369, and GB/T 1725, this method provides essential data for quality control, formulation accuracy, and cost management. This article explains the testing principle, applicable sample types, operating procedures, calculations, precision control, and typical industrial applications of oven-based solid content determination.

Technical Overview

Solid content, also known as non-volatile matter content or total solids, is defined as the mass percentage of residue remaining after a test portion has been heated under specified conditions until all volatile components, including water, organic solvents, and other volatile substances, have been driven off. It is one of the most fundamental quality parameters for liquid and pasty products, directly influencing film formation, bonding strength, coverage rate, viscosity control, cost accounting, and contractual specifications between suppliers and buyers.

The oven drying method is a direct gravimetric approach. A known mass of sample is placed in a pre-weighed dish, dried in a ventilated oven at a standardized temperature (commonly 105 ± 2 °C, with alternative temperatures of 110 °C, 120 °C, or 150 °C specified for particular product families), cooled in a desiccator, and reweighed. The ratio of dry residue mass to the original sample mass, expressed as a percentage, gives the solid content. Major standards governing this method include:

• ISO 3251 — Paints, varnishes and plastics: determination of non-volatile-matter content
• ASTM D2369 — Standard Test Method for Volatile Content of Coatings
• ASTM D1644 — Standard Test Methods for Nonvolatile Content of Varnishes
• ASTM D4758 — Standard Test Method for Nonvolatile Content of Latexes
• GB/T 1725 — Determination of volatile matter and solids of paints, varnishes and plastics
• GB/T 2793 — Determination of solids content of adhesives
• GB/T 6284 — Determination of water content in chemical products by loss of mass on drying

Accredited third-party laboratories routinely perform this test as part of incoming material inspection, in-process quality control, finished product release testing, shelf-life studies, and failure analysis programs.

Testing Samples

Coatings, Paints, and Varnishes

Waterborne and solvent-borne architectural paints, industrial coatings, primers, topcoats, varnishes, and lacquers are the most frequently tested sample categories. Solid content values typically range from 30% to 60% for waterborne architectural paints and can exceed 70% for high-solids industrial systems.

Polymer Emulsions and Dispersions

Styrene-acrylic emulsions, pure acrylic emulsions, vinyl acetate homopolymer and copolymer emulsions, styrene-butadiene latexes, polyurethane dispersions, and EVA dispersions are tested to verify polymer loading, which normally falls between 40% and 60%.

Adhesives and Sealants

White emulsion adhesives based on polyvinyl acetate, waterborne pressure-sensitive adhesive lattices, solvent-borne adhesives, and construction sealants are analyzed to confirm formulation consistency and to calculate solids applied per unit area.

Battery Materials and Functional Slurries

Lithium-ion battery electrode slurries containing active materials, binders, and conductive agents dispersed in water or N-methylpyrrolidone are tested because solids loading directly determines coating areal density, electrode thickness, and cell energy density. Ceramic slurries and electronic pastes are similarly evaluated.

Inks, Resins, and Chemical Auxiliaries

Printing inks, epoxy and unsaturated polyester resins, textile sizing and finishing agents, surfactant solutions, water treatment polymers, and other liquid chemical products can all be characterized using the oven method.

Testing Items

Core measurable parameters delivered by this method include:

• Solid content (non-volatile matter content) of coatings per ISO 3251, ASTM D2369, or GB/T 1725
• Solid content of adhesives per GB/T 2793
• Nonvolatile content of latexes and emulsion polymers per ASTM D4758
• Volatile fraction derived from mass loss, supporting VOC compliance evaluation
• Loss on drying of general chemical products per GB/T 6284
• Estimated water content by difference for waterborne systems containing negligible organic volatiles

Testing Methods

Apparatus and Materials

• Forced-ventilation laboratory oven maintaining the specified temperature with uniformity of ±2 °C
• Analytical balance with 0.1 mg readability, calibrated and draught-shielded
• Flat-bottom weighing dishes made of aluminum, tin, or glass
• Desiccator charged with fresh silica gel desiccant
• Clean tongs, glass rods, or droppers for sample transfer, plus optional glass beads or quartz sand to suppress skinning

Standard Operating Procedure

• Step 1: Clean the dish, pre-dry it in the oven, cool it in the desiccator for approximately 30 minutes, and record the mass of the empty dish (m0).
• Step 2: Homogenize the sample thoroughly, accurately weigh approximately 1 g (or the mass specified by the governing standard) into the dish, and record the combined mass (m1). Spread the sample into a thin, even layer; add glass beads or sand for liquids prone to surface skin formation.
• Step 3: Place the dish into the oven at the specified temperature, typically 105 ± 2 °C. For highly volatile solvent-borne samples, gentle pre-drying or a brief infrared pre-treatment may be applied to prevent splattering and sample loss.
• Step 4: Dry for the specified period, commonly between 30 minutes and 3 hours depending on the standard and product type, then transfer the dish to the desiccator, cool for approximately 30 minutes, and weigh.
• Step 5: Return the dish to the oven for an additional short drying cycle, cool, and reweigh. Repeat until constant weight is achieved, meaning the change between successive weighings does not exceed the tolerance stated in the standard (commonly 0.01 g or stricter). Record the final mass (m2).
• Step 6: Perform duplicate or triplicate determinations in parallel. Results must agree within the repeatability limit of the standard, commonly 0.5% absolute for coatings, before the mean value is reported.

Calculation

Solid content (%) = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the empty dish, m1 is the mass of the dish plus sample before drying, and m2 is the mass of the dish plus dry residue after drying to constant weight. The volatile content is obtained as 100 minus the solid content, and results are typically reported as the arithmetic mean of valid replicates to one decimal place.

Error Sources and Precision Control

• Surface skin formation can trap solvent beneath the film; mitigate with thin layers, beads, or sand
• Splattering during heating causes sample loss and falsely low solids readings
• Excessive oven temperature can oxidize or thermally degrade resins, artificially raising the result; always use the temperature specified for the product family
• Overly thick films may retain solvent and yield falsely low values
• Hygroscopic residues gain moisture during weighing; keep cooling and weighing times consistent
• Balance drift, buoyancy effects, and inadequate desiccant maintenance introduce systematic bias; calibrate equipment regularly

Method Variants

Infrared and halogen rapid moisture analyzers offer faster turnaround for routine in-process checks, while vacuum-oven drying at reduced pressure and moderate temperature suits heat-sensitive polymers. Nevertheless, the ventilated oven drying method remains the referee technique for certification and dispute resolution because of its simplicity, low equipment cost, and direct traceability to international standards.

Applications

Oven-based solid content determination supports decision-making across the entire product lifecycle:

• Raw material acceptance testing of emulsions, resins, and intermediates against supplier certificates of analysis
• In-process control of dilution ratios and batch-to-batch consistency in paint, adhesive, and ink manufacturing
• Formulation and cost optimization by linking applied solids to coverage, dry film thickness, and material consumption
• Electrode slurry control in lithium-ion battery production, where solids loading governs areal capacity and electrode density
• Supporting VOC declarations and environmental compliance documentation
• Trade settlement and contractual verification between buyers and suppliers
• Shelf-life and storage stability studies monitoring solvent loss or sedimentation effects

FAQ

What is solid content and why does it matter?

Solid content is the percentage of non-volatile material left after all water and solvents evaporate. It determines how much useful film, binder, or active material a given quantity of product delivers, so it directly affects application performance, cost per square meter, and contract compliance.

What temperature and drying time are used?

Most standards specify 105 ± 2 °C, while ASTM D2369 specifies 110 °C for approximately one hour for coatings. Certain products require 120 °C, 125 °C, or 150 °C. The exact temperature, sample mass, and drying duration must always follow the governing product standard or technical data sheet.

What does "constant weight" mean?

Constant weight is reached when successive drying, cooling, and weighing cycles change the mass by no more than the tolerance defined in the standard, typically 0.01 g or less. This confirms that volatile removal is complete and the residue mass is stable.

How is solid content different from moisture content?

Solid content measures the non-volatile residue, whereas moisture content measures water specifically. In simple waterborne systems with negligible organic volatiles, the two are complementary: moisture approximately equals 100% minus solids. For precise water quantification, Karl Fischer titration may be used alongside the oven method.

Is the oven method suitable for every sample?

For heat-sensitive or readily oxidizable materials, a vacuum oven at lower temperature or shortened drying schedules are recommended to avoid degradation bias. Highly volatile or foaming samples require careful pre-drying and anti-splatter measures.

How long does a test take and how are results reported?

A single determination, including cooling and repeated weighing, typically takes two to four hours. Results are reported as the average of at least two valid parallel determinations, expressed to one decimal place, together with the applied standard and drying conditions.