Determination of Non-Volatile Matter in Automotive Coatings: Standards, Methods, and Applications

Non-volatile matter (NVM), commonly known as solids content, is one of the most fundamental quality-control parameters for automotive coatings, directly affecting film build, hiding power, transfer efficiency, and VOC compliance. This technical article explains the principles, applicable standards (ISO 3251, GB/T 1725, ASTM D2369), sample types, test procedures, precision requirements, and typical applications of non-volatile matter determination for OEM and refinish automotive coatings.

Technical Overview

Non-volatile matter (NVM) is defined as the mass fraction of a coating that remains after volatile components, such as solvents, diluents, and water, have evaporated under specified heating conditions. For automotive coatings, which are applied as multi-layer systems consisting of electrodeposition (e-coat) primer, primer surfacer, basecoat, and clearcoat, the NVM value determines how much wet film must be applied to achieve a specified dry film thickness. It is therefore a primary input for spray application parameters, material consumption planning, and cost estimation.

Beyond process control, non-volatile matter determination is essential for regulatory compliance. VOC content declarations under GB 24409, EU Directive 2004/42/EC, and US federal emission rules are all calculated from the NVM value, often in combination with water content and density measurements. The test is performed gravimetrically using an oven-drying method in accordance with ISO 3251, GB/T 1725, or ASTM D2369, and is routinely carried out both by in-house paint laboratories and by accredited third-party laboratories.

Testing Samples

OEM Electrodeposition (E-Coat) Primers

Cathodic epoxy-based e-coat materials are tested both as supplied concentrates and as diluted bath samples. Bath solids monitoring is a daily process-control task in OEM paint shops. Because pigment settling occurs readily in these highly filled systems, thorough homogenization with a mechanical stirrer is essential before withdrawing the test portion. Elevated heating schedules are often specified for these bake-cured products.

Primer Surfacers and Stone-Chip Primers

Solvent-borne and waterborne polyester or polyurethane primer surfacers contain significant pigment and filler loadings. Their relatively high viscosity requires that the test portion be spread into a thin, uniform film in the dish to guarantee complete release of volatiles during heating.

Basecoats

Solid-color, metallic, and pearlescent basecoats, whether solvent-borne or the waterborne types now standard in OEM production, must be stirred until effect pigments are fully redispersed. Waterborne basecoats additionally require companion water-content determinations if the results are to be used for VOC calculations.

Clearcoats and Topcoats

One-component (1K) and two-component (2K) polyurethane or acrylic clearcoats are tested as supplied or as mixed systems. For 2K products, the components must be mixed at the specified ratio and the test portion weighed immediately, since NVM changes progressively as the crosslinking reaction advances within the pot life.

Automotive Refinish Coatings

Aftermarket refinish products, including primers, basecoats, clears, hardeners, and reducers, are tested both in the packaged state and as mixed with hardener and thinner, because VOC regulations for vehicle refinishing apply to the coating as applied.

Ancillary Automotive Materials

Seam sealers, structural adhesives, sound-damping coatings, and chassis protectants are frequently specified with minimum solids requirements and can be evaluated using the same gravimetric approach, subject to agreed heating schedules.

Testing Items

The principal measured item is the non-volatile matter content expressed as a mass percentage. In practice, NVM determination is commonly bundled with related measurements that together support VOC declarations and application engineering:

• Non-volatile matter content by mass (%, m/m), per ISO 3251, GB/T 1725, or ASTM D2369
• Water content of water-reducible coatings, per ASTM D3792 (gas chromatography) or ASTM D4017 (Karl Fischer)
• VOC content derived from NVM, per ISO 11890-1, GB/T 23985, or ASTM D3960
• Density for converting VOC to grams per liter, per ISO 2811-1, GB/T 6750, or ASTM D1475
• Volume non-volatile matter for theoretical spreading-rate calculations, per ASTM D2697

Testing Methods

Principle and Apparatus

The method is based on heating a weighed test portion of the coating in a tared dish until constant volatile loss is achieved, then weighing the dry residue. Required apparatus includes an analytical balance readable to 0.1 mg, a forced-ventilation oven controlled within approximately ±2 °C of the set point, flat-bottomed dishes of glass, aluminum, or stainless steel with a diameter of roughly 50 mm to 75 mm, and a desiccator with fresh desiccant for cooling. Samples are taken and conditioned in accordance with ISO 15528 or GB/T 3186, homogenized thoroughly, and equilibrated to laboratory temperature (typically 23 ± 2 °C) before testing.

Gravimetric Oven Method (ISO 3251 and GB/T 1725)

ISO 3251 and its Chinese counterpart GB/T 1725 describe the reference procedure used for most automotive coating specifications. The steps are as follows:

• Dry the dish, cool it in the desiccator, and record its mass (m0) to 0.1 mg.
• Add a test portion of approximately 0.2 g to 1.0 g, selected according to the expected solids level and viscosity, and spread it into a thin, uniform film; record the combined mass (m1).
• Heat the dish in the oven at the specified schedule, with the default condition of 105 ± 2 °C for 60 ± 5 min.
• Transfer the dish to the desiccator, allow it to cool to room temperature, and weigh (m2).
• Perform a duplicate determination and calculate the mean result.

The non-volatile matter content is calculated as NVM (%) = (m2 − m0) / (m1 − m0) × 100, and the result is reported to the nearest 0.1 % together with the heating schedule actually used.

Volatile Content Method (ASTM D2369)

ASTM D2369 measures volatile content directly using a test specimen of approximately 0.3 g heated at 110 ± 5 °C for 60 minutes in a forced-draft oven. The volatile fraction equals the mass loss divided by the original specimen mass, and the non-volatile matter is obtained as 100 minus the volatile percentage. This method is widely referenced in North American automotive specifications and in US federal VOC calculation procedures for refinish coatings.

Heating Schedules for Bake-Cure Products

Automotive OEM coatings are stoved materials, and the heating schedule materially influences the result. In addition to the default 105 °C condition, commonly agreed schedules include 125 ± 2 °C for 60 minutes, 150 ± 2 °C for 30 minutes, and, for electrodeposition primers and other high-bake products, temperatures up to about 175 °C. Hotter or longer schedules generally produce slightly lower NVM values by driving off retained solvent, whereas excessively severe conditions risk thermal degradation. The schedule must therefore be fixed by product specification, agreed between the parties, and clearly stated in the test report.

Two-Component (2K) Systems

For 2K polyurethane clearcoats and primers, the components are mixed at the manufacturer-specified ratio, stirred for the stated induction time, and the test portion is weighed immediately. The time between mixing and weighing must be recorded because the reaction consumes part of the volatile content profile over the pot life. Testing the individual components separately is also useful for incoming inspection of the hardener and base components.

Waterborne Coatings and VOC Calculation

For waterborne basecoats and primers, NVM alone does not define VOC, because water is a non-VOC volatile. The water content is determined separately by gas chromatography (ASTM D3792) or Karl Fischer titration (ASTM D4017), and the VOC mass fraction is then calculated as 100 minus the NVM minus the water content, with any exempt solvents also deducted. Multiplying the VOC mass fraction by the coating density measured per ISO 2811-1 or ASTM D1475 yields VOC in grams per liter, which is the unit used in GB 24409, EU Directive 2004/42/EC, and comparable regulatory limits.

Precision and Quality Control

Properly executed duplicate determinations on homogeneous automotive coatings typically agree within roughly 0.5 % absolute, and the mean is reported. Principal sources of error include volatile loss or skinning during sample handling and weighing, non-uniform film thickness in the dish, moisture uptake by hygroscopic residues during cooling, and excessive oven loading that disturbs temperature uniformity. Verified balances, calibrated ovens, fresh desiccant, and consistent operator technique are the core requirements for reliable results, whether testing is performed in-house or by accredited third-party laboratories.

Applications

Non-volatile matter data are used throughout the automotive coatings value chain, from resin and pigment suppliers to OEM paint shops and body refinishers:

• Batch release and incoming material certification against purchase specifications for e-coat, primer, basecoat, and clearcoat products
• Daily bath solids monitoring and replenishment control in electrodeposition lines
• Calculation of theoretical spreading rate and wet-film requirements from combined mass solids, density, and volume solids data
• Demonstration of compliance with VOC content limits in GB 24409, EU Directive 2004/42/EC, and US federal refinish coating rules
• Formulation development and solids optimization for high-solids and waterborne coating platforms
• Material cost accounting and coverage estimation for warranty, tendering, and technical dispute resolution

FAQ

What exactly counts as non-volatile matter in an automotive coating?

Non-volatile matter is everything that remains in the dish after heating under the specified conditions: binders, pigments, extenders, additives, and any retained reaction components. It is an operational definition, meaning the measured value depends on the agreed heating temperature and time, which is why these conditions must always accompany the result.

Which standard should be selected: ISO 3251, GB/T 1725, or ASTM D2369?

GB/T 1725 is technically aligned with ISO 3251 and both use comparable gravimetric oven procedures, while ASTM D2369 specifies 110 °C and reports volatile content directly. The choice is normally dictated by the product specification, the customer's market region, or the applicable regulatory framework; results from the methods are closely comparable when heating schedules are harmonized.

Why do different heating conditions give different results for the same paint?

Higher temperatures and longer dwell times remove more strongly retained solvent and can approach the onset of resin degradation, lowering the measured NVM. Conversely, mild schedules may leave traces of high-boiling solvent behind, inflating the result. Automotive stoving enamels are therefore often tested at schedules closer to their cure conditions, such as 150 °C or higher for e-coat materials.

How should two-component clearcoats be sampled and tested?

Mix the base and hardener at the exact specified ratio, allow the stated flash-off or induction time if required, weigh the test portion immediately, and record the elapsed time since mixing. Because the isocyanate-hydroxyl reaction proceeds continuously, delayed testing understates the volatile fraction and overstates the solids of the applied coating.

How is NVM used to calculate VOC for a waterborne basecoat?

First determine NVM by the gravimetric method, then determine water content by ASTM D3792 or ASTM D4017. The VOC mass percentage equals 100 minus NVM minus water content, and multiplying by density gives VOC in g/L. This combined procedure is the basis of declarations required by GB 24409, ISO 11890-1, and EU Directive 2004/42/EC.

What are typical solids ranges for automotive coatings?

Indicative mass solids ranges are roughly 15 % to 25 % for e-coat bath liquids (with concentrates considerably higher), 30 % to 45 % for waterborne basecoats, 40 % to 60 % for solvent-borne basecoats, and 50 % to 65 % for high-solids clearcoats. Exact values depend on chemistry, pigment loading, and the specified test conditions, so specification limits rather than generic ranges should always govern acceptance.

Can the same method be used for powder coatings used on automotive parts?

Yes, gravimetric loss-on-heating principles apply to powder primers and trim powders, but these materials are near-100 % solids by nature and are instead controlled by parameters such as gel time and melt flow. Any NVM-type check on powders follows a separately agreed schedule appropriate to the thermosetting chemistry.