Solid Content Testing by the Constant Weight Method: Principle, Standards, and Procedure

Solid content testing by the constant weight method is the standard gravimetric technique for determining the non-volatile matter content of paints, coatings, adhesives, polymer emulsions, and printing inks. By drying a precisely weighed specimen in a controlled oven and repeating drying, cooling, and re-weighing cycles until the mass stabilizes, the method delivers accurate and repeatable results aligned with ISO 3251, ASTM D2369, and GB/T 1725. This article explains the principle, applicable sample types, testing procedure, calculation formula, quality control points, and typical applications of the constant weight method.

Technical Overview

Solid content testing by the constant weight method (also referred to as gravimetric drying to constant mass or non-volatile matter determination) is a fundamental laboratory technique for quantifying the non-volatile fraction of liquid, paste, and dispersion materials. A precisely weighed specimen is heated in a temperature-controlled oven until volatile components, such as water, solvents, and diluents, evaporate. The specimen is then cooled in a desiccator and re-weighed, and the drying, cooling, and weighing cycle is repeated until the mass change between two consecutive weighings falls below a defined tolerance. At this point the specimen is considered to have reached constant weight, and the ratio of the final residue mass to the original specimen mass gives the solid (non-volatile) content as a percentage.

The constant weight approach differs from a single fixed-time drying step because it provides objective evidence that volatilization is complete, which improves repeatability and interlaboratory comparability. The procedure is harmonized with internationally recognized standards, including ISO 3251 (paints, varnishes and plastics, determination of non-volatile-matter content), ASTM D2369 (volatile content of coatings), ASTM D2832 (guide for determining volatile and nonvolatile content of paints and related coatings), ASTM D1644 (nonvolatile content of varnishes), ISO 124 (total solids content of rubber latex), GB/T 1725 (non-volatile matter of paints, varnishes and plastics), GB/T 2793 (nonvolatile content of adhesives), and GB/T 11175 (test methods for synthetic resin emulsions). Because the required equipment is simple and the principle is absolute and mass-based, the method is widely used by accredited third-party laboratories as the reference technique against which faster instrumental methods, such as infrared dryers and halogen moisture balances, are verified.

Testing Samples

Paints, Coatings, and Varnishes

Waterborne and solventborne architectural and industrial coatings, primers, topcoats, varnishes, electrodeposition materials, and related coating intermediates. Typical specimens of approximately 1 g are dried at 105 to 110 degrees Celsius in accordance with ISO 3251, GB/T 1725, or ASTM D2369, unless a product specification requires other conditions.

Adhesives and Sealants

Waterborne adhesives, solvent-based adhesives, reactive prepolymers, polymer dispersions for lamination and construction, and elastomeric sealants. Nonvolatile content is commonly determined per GB/T 2793 or the corresponding product standard, since solids level directly affects bond line thickness, coverage, and curing behavior.

Polymer Emulsions and Rubber Latices

Synthetic resin emulsions such as styrene-acrylic, pure acrylic, vinyl acetate, and VAE dispersions, as well as natural and synthetic rubber latices. Total solids content is determined according to ISO 124, GB/T 11175, or related latex standards and is a key indicator of polymerization conversion and batch consistency.

Inks, Resins, and Process Liquids

Liquid printing inks, resin solutions, binders, impregnation varnishes, textile finishing agents, paper and paperboard coating chemicals, cleaning formulations, and other dispersions in which the non-volatile fraction governs application performance and economic value.

Testing Items

The constant weight method is primarily used to report the following parameters:

• Non-volatile matter (solid) content, expressed as percent by mass
• Volatile matter content, calculated as 100 minus the solid content
• Total solids content of rubber latices and polymer emulsions
• Verification that drying has proceeded to constant mass (completeness of volatilization)
• Comparative assessment of incoming raw materials or finished batches against specification limits

Testing Methods

Apparatus and Materials

• Analytical balance with a readability of 0.1 mg (or 1 mg where the governing standard permits)
• Flat-bottom weighing dishes made of aluminum foil, glass, or stainless steel, typically 50 to 70 mm in diameter
• Forced-ventilation laboratory oven with temperature control within plus or minus 1 to 2 degrees Celsius
• Desiccator with an effective desiccant (for example, silica gel) for cooling specimens in a dry atmosphere
• Clean tweezers, gloves, and, where allowed by the standard, dried sand or glass fiber to disperse fluid specimens

Specimen Preparation

Homogenize the sample thoroughly and condition it to 23 plus or minus 2 degrees Celsius. Dry a clean dish to constant weight in the oven, cool it in the desiccator, and record its mass (m0). Add the specified quantity of specimen, typically 0.2 to 2 g depending on the standard, distribute it as a thin uniform film, and weigh the dish plus specimen (m1). For highly fluid specimens, a small quantity of dried sand or a short length of glass fiber may be added, if permitted, to increase the evaporating surface and prevent skinning.

Drying, Cooling, and Constant Weight Verification

Place the dish in the oven at the temperature and time specified by the governing standard. Common conditions include 105 plus or minus 2 degrees Celsius for waterborne products per ISO 3251 and GB/T 1725, and 110 plus or minus 5 degrees Celsius for 60 minutes per ASTM D2369; higher temperatures such as 125, 150, or 160 to 180 degrees Celsius are used for heat-stable or slow-drying materials when the product standard requires them. After the initial drying period, transfer the dish to the desiccator, allow it to cool to room temperature (typically 20 to 30 minutes), and weigh it. Return the dish to the oven for a further drying interval (commonly 30 to 60 minutes), cool, and re-weigh. Repeat the cycle until the difference between two consecutive weighings does not exceed the specified tolerance, commonly 0.5 to 1 mg for analytical work or up to 0.01 g where the standard and specimen size allow. Record the final mass (m2) as the constant weight.

Calculation

Non-volatile (solid) content in percent by mass = (m2 - m0) / (m1 - m0) x 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 weight. The volatile matter content is obtained by subtracting the solid content from 100 percent. Duplicate determinations are performed, and the two results should agree within the repeatability limit of the governing standard, commonly 0.5 percent absolute; the mean is reported together with the drying temperature used.

Quality Control and Interferences

• Thermal decomposition or post-curing of the residue inflates the apparent solid content; the drying temperature must not exceed the thermal stability of the binder
• Formation of a surface skin can trap volatiles and lower the result; spreading the specimen thinly and using dispersing aids where permitted minimizes this effect
• Hygroscopic residues absorb moisture during weighing; specimens should be weighed promptly and consistently after cooling
• Oxidation of unsaturated or solvent-free residues in a ventilated oven can cause mass gain; the standard-specified oven type and atmosphere must be followed
• All dishes should be pre-dried to constant weight before specimen addition, and balances should be calibrated and verified with certified reference weights

Applications

Constant weight solid content testing supports quality control, formulation development, and commercial transactions across many industries. Because the solids level of a coating, adhesive, or emulsion determines dry film thickness, coverage area, curing behavior, and cost per unit area, accurate non-volatile data are essential for both producers and users. Typical applications include:

• Batch release and in-process quality control of paints, varnishes, and coatings, ensuring consistency of solids, density, and application properties
• Adhesive and sealant formulation control, where nonvolatile content governs open time, bond line formation, and final performance
• Monitoring of polymerization conversion and batch-to-batch consistency in emulsion and latex production
• Input data for VOC content calculations in coatings and inks in combination with density and water content determinations
• Incoming inspection of purchased raw materials and outgoing inspection of finished goods against contractual solids specifications
• Reference verification of rapid moisture analyzers and infrared dryers used on production lines
• Technical support for dispute resolution and trade settlement based on independently verified solids values from accredited third-party laboratories

FAQ

What does constant weight mean in solid content testing?

Constant weight is the state reached when the difference between two consecutive weighings of a dried specimen, after identical drying and cooling cycles, no longer exceeds the tolerance defined by the governing standard. It is the objective criterion demonstrating that volatilization is complete and that the measured residue mass represents the true non-volatile fraction.

What drying temperature and time are used?

Typical conditions are 105 plus or minus 2 degrees Celsius per ISO 3251 and GB/T 1725, or 110 plus or minus 5 degrees Celsius for 60 minutes per ASTM D2369. Heat-stable or slow-drying products may require 125 to 180 degrees Celsius as specified in the relevant product standard. The chosen conditions must remove all volatiles without decomposing, crosslinking, or oxidizing the residue.

How long does a complete determination take?

A single determination, including the initial drying interval, cooling, and at least one verification cycle, commonly takes two to four hours. Highly solvent-rich, film-forming, or hygroscopic materials may require additional cycles before constant weight is achieved.

How does the constant weight method compare with a moisture analyzer?

Halogen and infrared moisture balances provide results in minutes and are well suited to routine screening and production monitoring. However, their heating profiles differ from oven drying, and their accuracy depends on method settings. The constant weight oven method remains the reference procedure used to validate and calibrate these rapid instruments.

Can the method be applied to reactive or thermosensitive resins?

Yes, with caution. For reactive systems such as curing prepolymers or oxidatively drying binders, the drying temperature is selected to remove volatiles while minimizing further reaction of the residue. Where necessary, reduced-temperature drying under vacuum or in an inert atmosphere may be agreed between the parties and documented with the result.

What repeatability can be expected?

With proper homogenization, thin specimen films, and a calibrated balance, duplicate determinations on homogeneous liquids typically agree within about 0.5 percent absolute, or within the repeatability limits stated in ISO 3251, ASTM D2369, or the applicable national standard. Poorer agreement usually indicates inadequate mixing, skin formation, or non-representative sampling.