Coating Drying Weight Loss Test: Determination of Volatile and Non-Volatile Matter Content

The coating drying weight loss test is a gravimetric method that quantifies the mass a paint, varnish, or ink loses when it is dried under precisely controlled temperature and time conditions. The result defines the volatile matter content and its complement, the non-volatile (solids) content, which are essential for formulation control, film-thickness prediction, and VOC compliance with standards such as ISO 3251, ASTM D2369, and GB/T 1725. This article explains the test principle, applicable sample types, measured parameters, standard procedures, and typical industrial applications.

Technical Overview

A coating drying weight loss test, also referred to as a loss-on-drying determination, measures the mass of volatile material released by a coating specimen while it dries under standardized conditions. During film formation, water, organic solvents, coalescing aids, and other low-boiling constituents evaporate, leaving behind a dry film composed of binder, pigments, extenders, and non-volatile additives. By comparing the specimen mass before and after drying, the laboratory determines the volatile matter content, and by simple difference, the non-volatile matter content, commonly known as total solids.

The measured values are fundamental quality attributes. Solids content governs the practical spreading rate, the achievable dry film thickness per coat, packaging yield, and raw material cost. Volatile data also feed directly into the calculation of volatile organic compound (VOC) content required by environmental regulations, including GB 18582 for interior architectural coatings, GB 30981 for industrial protective coatings, GB 24409 for vehicle coatings, and GB 33372 for general coating products.

The principal reference methods for this test are ISO 3251 (Paints, varnishes and plastics — Determination of non-volatile-matter content), GB/T 1725 (the corresponding Chinese national standard, harmonized with ISO 3251), and ASTM D2369 (Standard Test Method for Volatile Content of Coatings). Supporting determinations include density measurement per ISO 2811-1 / GB/T 6750, water content determination per GB/T 6283 / ISO 760 / ASTM D4017 (Karl Fischer titration), and volume solids determination per ASTM D2697 or GB/T 9272.

It should be noted that the drying weight loss test is a physical evaporation procedure performed far below decomposition temperatures. It is distinct from loss-on-ignition or ash tests, and it does not measure binder crosslinking chemistry, only the mass balance of volatile versus non-volatile material.

Testing Samples

Waterborne Architectural Coatings

Interior and exterior emulsion wall paints, sealers, primers, textured finishes, and elastic waterproof coatings are the largest sample category. These products typically contain 30 to 60 percent water by mass, so the drying weight loss is substantial, and accurate water content determination by Karl Fischer titration is required to calculate the water-excluded VOC value in grams per liter.

Solvent-Borne Decorative and Wood Coatings

Alkyd enamels, nitrocellulose lacquers, acid-curing and polyurethane wood finishes, and chlorinated rubber coatings fall into this group. Here the volatile fraction consists almost entirely of organic solvents, so the volatile matter content closely approximates the solvent content of the formula.

Industrial, Protective, and Automotive Coatings

Two-pack epoxy and polyurethane systems, shop primers, coil coatings, electrodeposition baths, and automotive OEM and refinish materials are tested using drying conditions that match their real curing schedules, sometimes 125 °C or 150 °C as specified by the product standard. Two-component samples must be mixed at the specified ratio and weighed into the dish promptly to avoid moisture pickup and premature reaction.

Inks, Varnishes, and Powder Coatings

Printing inks and clear varnishes are explicitly within the scope of GB/T 1725 and ISO 3251. Powder coatings contain very little volatile matter, typically below 2 percent, and require adapted specimen preparation with a thin, well-spread layer to ensure complete release of volatiles during the heating period.

Testing Items

Depending on the purpose of the evaluation, a drying weight loss study may include the following measured and derived parameters:

• Volatile matter content, expressed as mass fraction in percent of the specimen.
• Non-volatile matter (total solids) content, expressed as mass fraction in percent.
• Water content of waterborne products, determined separately by Karl Fischer titration.
• VOC content in grams per liter, calculated from volatile matter, water content, and density.
• Mass loss at a specified stoving or curing schedule, for heat-cured industrial products.
• Weight-loss-versus-time drying curves at ambient conditions, for drying kinetics studies.
• Supporting measurements such as coating density (ISO 2811-1 / GB/T 6750) and volume solids (ASTM D2697 / GB/T 9272) where film-build calculations are required.

Testing Methods

Reference Oven-Gravimetric Procedure

The reference procedure specified by ISO 3251, GB/T 1725, and ASTM D2369 follows a gravimetric sequence with strict control of specimen preparation, temperature, and time:

• Condition clean, dry dishes of aluminum, glass, or tinplate in the oven, cool them in a desiccator, and weigh each to 0.1 mg to obtain the tare mass m0.
• Homogenize the coating sample by gentle stirring without entraining air, then weigh a test portion, typically about 1 g for ISO 3251 and GB/T 1725 or about 0.3 g for ASTM D2369, into the dish and distribute it evenly over the bottom.
• For waterborne or highly viscous products, use only the wetting aids or dispersing liquids expressly permitted by the governing standard, applied identically to any blank correction.
• Dry the specimen in a forced-ventilation oven at the specified condition, most commonly 105 ± 2 °C for 60 minutes per ISO 3251 and GB/T 1725, or 110 ± 5 °C for 60 minutes per ASTM D2369; products cured at higher temperatures may require 125 °C or 150 °C as specified, while heat-sensitive products may be dried at ambient temperature over desiccant to constant mass.
• Cool the dish in a desiccator, weigh it, then repeat short drying cycles until constant mass is reached, typically when two successive weighings differ by no more than about 1 mg, and record the final mass m2.
• Perform the determination in duplicate and verify that the two results agree within the repeatability of the method before reporting the mean value.

Calculation of Results

The volatile matter content wV, in percent by mass, is calculated as wV = (m1 − m2) / (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 dry residue. The non-volatile matter content is wNV = 100 − wV.

For VOC reporting in grams per liter, the water-excluded value for waterborne coatings is calculated as VOC = (wV − wW) × ρ × 10, where wW is the water content in percent by mass and ρ is the coating density in grams per cubic centimeter measured per ISO 2811-1 / GB/T 6750. For solvent-borne coatings containing negligible water, the VOC value approximates wV × ρ × 10. Products claiming very low VOC levels are typically verified additionally by gas chromatographic procedures such as ISO 11890.

Ambient Drying Weight-Loss Study

Beyond the standard oven determination, a drying weight-loss study can track volatilization behavior in real time. A film of specified wet thickness is applied to a glass plate, conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity, and weighed at defined intervals. The resulting weight-loss curve characterizes surface-dry and through-dry behavior, solvent retention, and coalescent release, which is valuable for formulation development and for predicting outgassing in enclosed spaces.

Quality Control of Results

Common sources of error include non-homogeneous samples, skin formation that traps solvent, insufficient drying to constant mass, moisture pickup by hygroscopic residues during weighing, and incorrect test-portion size leading to thick layers. Accredited third-party laboratories control these factors through calibrated balances and ovens, validated procedures, duplicate determinations, and participation in interlaboratory comparisons. Under such conditions, repeatability is typically within about 0.5 percent absolute for well-mixed liquid coatings.

Applications

Drying weight loss data support decisions across the entire coating life cycle, from raw material acceptance to regulatory compliance:

• Regulatory VOC compliance and eco-labeling against GB 18582, GB 30981, GB 24409, GB 33372, and comparable international requirements.
• Batch quality control and release testing to confirm that production solids match declared technical data sheet values.
• Formulation optimization, balancing solids content against cost, viscosity, application properties, and film build per coat.
• Calculation of wet-to-dry film relationship, theoretical spreading rate, and pigment volume concentration in research and development.
• Drying kinetics studies for waterborne systems, including coalescent retention and early rain resistance of exterior products.
• Technical documentation for tenders, product certification, and acceptance testing performed by accredited third-party laboratories.
• Failure analysis support, where unexpectedly low solids or retained volatiles may explain adhesion, drying, or odor problems.

FAQ

What exactly does the coating drying weight loss test measure?

The test measures the total mass of volatile material, consisting of water, organic solvents, coalescing agents, and other evaporating constituents, that leaves the coating during drying under specified conditions. It reports this as volatile matter content in percent, with the remainder reported as non-volatile matter or total solids.

Which standard should be used for testing?

ISO 3251 and GB/T 1725 cover paints, varnishes, and printing inks; ASTM D2369 is the widely used method for volatile content of coatings. The choice depends on the product type, the applicable purchase specification, and the market in which compliance must be demonstrated. Regulatory VOC values are then calculated according to the specific limit standard, such as GB 18582 or GB 30981.

What are the typical drying conditions?

The most common conditions are 105 ± 2 °C for 60 minutes under ISO 3251 and GB/T 1725, or 110 ± 5 °C for 60 minutes under ASTM D2369. Heat-cured industrial products may require 125 °C or 150 °C to match their real curing schedule, while heat-sensitive materials may be dried at ambient temperature over a desiccant until constant mass.

Is weight loss the same as VOC content?

No. Total volatile matter includes water, whereas regulated VOC content excludes water for waterborne products. Therefore, the water content must be determined separately, usually by Karl Fischer titration, and combined with volatile matter and density to calculate the VOC value in grams per liter. For solvent-borne coatings, volatile matter and solvent content are nearly identical by mass.

What precision can be expected?

For well-homogenized liquid coatings tested by a competent laboratory, duplicate determinations typically agree within about 0.5 percent absolute. Poorer agreement usually indicates inadequate mixing, premature skin formation, layers that are too thick, or incomplete drying to constant mass.

Can two-component and reactive coatings be tested?

Yes. The components should be mixed at the specified ratio and the test portion weighed immediately into the dish so that the drying condition reflects the intended curing schedule. Any small mass loss from reaction by-products is included in the measured volatile fraction, while the crosslinked network remains as non-volatile residue.

How should powder coatings be handled?

Powder coatings contain very low volatile levels, usually below 2 percent. A small, evenly spread test portion and a drying temperature consistent with the product standard are used to ensure complete release of volatiles, and particular care is taken with weighing speed because moisture pickup can bias the result.