Solid content, or non-volatile matter content, is one of the most critical quality parameters for paints, varnishes, and coating raw materials, directly influencing film thickness, coverage rate, VOC compliance, and cost efficiency. Professional solid content determination combines standardized sampling procedures with gravimetric test methods defined in ISO 3251, ASTM D2369, and GB/T 1725. This article provides a comprehensive technical guide to sampling protocols, test conditions, calculation formulas, and practical applications for coating manufacturers, formulators, and quality control engineers.
Technical Overview
Solid content, also known as non-volatile matter content (NV), refers to the mass percentage of a coating that remains after the volatile components—solvents, water, and diluents—have evaporated under specified heating conditions. This parameter is fundamental to coating quality control because it determines the actual dry film build achievable from a given quantity of wet material, the theoretical spreading rate, and the volume of volatile organic compounds released during application and curing.
Accurate solid content determination depends on two equally important elements: representative sampling and standardized analytical testing. Poor sampling practice can invalidate even the most precise laboratory measurement, particularly for pigmented or multi-component systems that tend to settle during storage. For this reason, internationally recognized standards such as ISO 15528 and GB/T 3186 govern sampling procedures, while ISO 3251, ASTM D2369, and GB/T 1725 define the oven-drying gravimetric methods used to quantify non-volatile matter. Accredited third-party laboratories and in-house QC departments rely on the combination of these standards to deliver reproducible, comparable results across production batches and supply chains.
Testing Samples
Solid content testing applies to a wide range of liquid and semi-liquid coating materials, as well as their polymeric raw materials. The sampling approach must be adapted to the physical state and storage configuration of each product family.
Water-Based Coatings
Emulsion paints, latex coatings, water-reducible alkyds, and waterborne industrial coatings contain significant water fractions, typically 30 to 60 percent by mass. These products are prone to pigment settlement and creaming, making thorough homogenization prior to sampling essential. Samples are commonly drawn from production tanks, drums, or retail containers in accordance with ISO 15528 or GB/T 3186.
Solvent-Based Coatings
Alkyds, epoxies, polyurethanes, chlorinated rubbers, and other solvent-reducible systems contain organic solvents that volatilize during the test. Multi-component products such as two-pack epoxies and polyurethanes require separate sampling of base and hardener components, followed by testing of each component and, where relevant, of the mixed system at the specified pot-life stage.
High-Solids and Low-VOC Coatings
High-solids industrial coatings, typically exceeding 60 to 80 percent non-volatile content, and powder coatings, which are essentially 100 percent solid, are tested to verify formulation compliance with increasingly strict VOC regulations such as GB 30981 and comparable international frameworks.
Raw Materials and Intermediates
Resins, binders, polymer dispersions, curing agents, and pigment pastes are also characterized for non-volatile content. Raw material incoming inspection ensures consistency before compounding and supports accurate formulation calculations.
Testing Items
A solid content testing program typically encompasses the following analytical items, which may be performed individually or as part of a broader coating characterization package:
- Non-volatile matter content (solid content) by mass, expressed as a percentage of the original sample mass
- Volatile matter content, calculated as the complementary fraction of non-volatile matter
- Volatile organic compound (VOC) content, derived from solid content combined with water content and density measurements
- Water content of water-reducible coatings, determined separately to permit correct VOC calculation
- Density of the liquid coating, measured per ISO 2811 or ASTM D1475 and used for converting between mass-based and volume-based parameters
- Reproducibility verification through duplicate or triplicate determinations on parallel specimens
Testing Methods
Sampling Procedures
Sampling is performed in accordance with ISO 15528, GB/T 3186, and related national procedures. The objective is to obtain a sample that is statistically representative of the entire batch or delivery. Key operational steps include:
- Surface inspection of containers for skinning, contamination, leakage, or abnormal settlement before opening
- Full homogenization by mechanical stirring, rolling, or shaking for a duration sufficient to redisperse settled pigment; for large containers, propeller or drum-roller mixing is recommended
- Withdrawal of single samples at defined positions and depths using clean sampling tubes, thieves, or ladles of inert material
- Combination of single samples into an average (composite) sample, with retention of reference samples for arbitration purposes
- Transfer into clean, airtight, solvent-resistant containers filled to approximately 90 to 95 percent of capacity, labeled with product identification, batch number, sampling date, and sampler identity
- Storage in a cool environment away from direct sunlight, with testing scheduled promptly to avoid further settlement, evaporation, or skin formation
Typical minimum sample quantities range from 500 mL to 2 L depending on the number of test items requested; smaller quantities of 50 to 100 mL may suffice when only solid content is to be determined.
Oven-Drying Gravimetric Method (ISO 3251 / GB/T 1725)
The gravimetric method is the reference procedure for liquid paints, varnishes, and polymer dispersions. The general sequence is as follows:
- Condition and weigh a clean flat-bottomed dish, disc, or foil container together with a stirring rod, recording mass m0 to the nearest 0.1 mg
- Introduce the specified test portion, commonly approximately 1 g for polymer dispersions or 0.2 to 1 g for paints depending on the standard, distribute it evenly, and weigh the assembly to obtain mass m1
- Place the container in a ventilated drying oven maintained at the temperature specified for the product type, typically 105 ± 2 °C, 125 ± 2 °C, 150 ± 2 °C, or 175 °C; recommended holding times range from 30 minutes to several hours depending on the binder chemistry
- For certain systems, pre-drying in a vacuum desiccator or gradual heating in a fume cupboard is required to prevent solvent flashing and splattering
- Remove the container, cool to room temperature in a desiccator over desiccant, and weigh; repeat heating, cooling, and weighing cycles until the change in mass between successive determinations does not exceed the specified tolerance, commonly 0.01 g or a fraction of a milligram
- Calculate non-volatile content as NV = (m2 − m0) / (m1 − m0) × 100 %, where m2 is the final constant mass
Two determinations are performed in parallel; if the difference between results exceeds the repeatability limit stated in the standard, typically on the order of 0.5 percent absolute for well-dispersed products, the entire test must be repeated.
ASTM D2369 Volatile Content Method
Widely used in North American practice, this method specifies a test portion of approximately 0.2 to 0.4 g for liquid coatings, distributed along a short section of drawn-out copper wire or in a flat dish. The specimen is heated at 110 ± 5 °C for 60 minutes, cooled in a desiccator, and weighed. Volatile content is calculated from the mass loss, and non-volatile content is obtained by difference. The method is referenced in VOC determination protocols that combine it with water content and density measurements.
Rapid and Complementary Techniques
For in-process quality control, halogen or infrared drying balances provide solid content results within 5 to 15 minutes and are calibrated against the reference oven method. Thermogravimetric analysis (TGA) may be used for research purposes to study volatilization profiles. However, contractual, regulatory, and certification purposes generally require results generated strictly in accordance with the reference gravimetric standards.
Quality Assurance of Results
Reliable results depend on calibrated analytical balances, temperature-verified ovens, trained operators, controlled cooling conditions, and validated test portion masses appropriate to the expected solid content. Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems provide independent verification when disputes arise between suppliers and purchasers.
Applications
Solid content determination and standardized sampling support decision-making throughout the coatings value chain:
- Manufacturing quality control: verification of batch conformity to internal specifications and detection of formulation or dispersion errors before release
- Incoming inspection: acceptance testing of purchased resins, dispersions, and finished coatings against supplier certificates of analysis
- VOC regulatory compliance: data input for VOC declarations under national and regional environmental regulations affecting architectural and industrial coatings
- Formulation development: benchmarking of competitive products, optimization of solids-to-solvent ratios, and cost modeling of raw material usage
- Contract and tender documentation: independent laboratory reports substantiating declared technical parameters for architectural, marine, protective, automotive, and wood coating projects
- Coverage and cost estimation: conversion of solid content and density into theoretical spreading rates and dry film thickness calculations for project planning
- Shelf-life studies: monitoring of storage stability through periodic re-testing of retained samples
FAQ
What is the difference between solid content and VOC content?
Solid content is the non-volatile residue remaining after heating under defined conditions, expressed as a mass percentage. VOC content is the mass of volatile organic compounds per unit volume or mass of coating, typically calculated by combining volatile content, water content, and density measurements. The two parameters are related but not identical, since water is volatile yet is not classified as a VOC.
Which drying temperature should be used for my coating?
The temperature and heating time are specified by the governing product standard or, in its absence, agreed between the parties. Common conditions include 105 °C, 125 °C, and 150 °C for waterborne dispersions and architectural paints, and up to 175 °C or higher for certain solvent-based and reactive systems. Using an inappropriate temperature can cause thermal degradation, which falsely inflates solid content, or incomplete volatilization, which falsely depresses it.
How much sample is required for testing?
For solid content alone, 50 to 100 mL of homogenized liquid coating is generally sufficient for duplicate determinations. When sampling a full batch, ISO 15528 and GB/T 3186 describe single samples, average samples, and reference samples, with typical total volumes of 0.5 to 2 L to accommodate additional test items.
Why is homogenization before sampling so important?
Pigments and dense fillers settle during storage, creating vertical concentration gradients within the container. A sample taken without re-dispersion may significantly misrepresent the true composition of the batch. Standards therefore require thorough mixing of the container before sample withdrawal, and any skin, crust, or foreign matter must be documented and, where appropriate, excluded or incorporated according to the specification.
What is the typical reproducibility of the method?
For properly homogenized samples tested by trained operators, repeatability within a laboratory is generally better than 0.5 percent absolute, and reproducibility between laboratories is typically within 1 percent absolute for liquid coatings. Discrepancies beyond these limits usually indicate sampling inconsistency, oven temperature deviation, or specimen splattering during heating.
How are powder coatings handled?
Thermosetting powder coatings are, by definition, essentially 100 percent non-volatile at room temperature, but they are still tested for volatile content—typically loss on heating at a specified temperature such as 105 °C or 60 °C under dedicated standards—to quantify residual moisture and degassing additives that can cause film defects during application.
Can an independent report be used for customs or tender purposes?
Yes. Test reports issued by accredited third-party laboratories conforming to ISO/IEC 17025, and clearly referencing the applied method such as ISO 3251, ASTM D2369, or GB/T 1725, are widely accepted for commercial settlement, regulatory submission, and technical documentation, provided the report identifies the batch, sampling protocol, and complete test conditions.