Solid content testing is a fundamental quality control procedure used to determine the non-volatile matter content in coatings, adhesives, emulsions, and other chemical products. This comprehensive testing procedure guide covers sampling requirements, standard testing methods based on ISO, ASTM, and GB standards, calculation formulas, and practical applications across industries. Understanding the correct solid content inspection protocol ensures accurate batch evaluation, consistent product quality, and reliable trade settlement.
Technical Overview
Solid content, also known as non-volatile matter content, refers to the mass percentage of residue remaining after a specified amount of sample has been dried under controlled temperature and time conditions. It is one of the most critical quality parameters for liquid chemical products such as paints, adhesives, emulsion polymers, inks, and resin solutions. The solid content value directly influences film formation thickness, bonding strength, coverage rate, and the economic value of the product.
A standardized solid content testing procedure ensures that measurements are repeatable, reproducible, and comparable between laboratories, production batches, and suppliers. The most widely referenced standards include ISO 3251 for paints, varnishes and plastics, ASTM D2369 for volatile content of coatings, GB/T 1725 for non-volatile matter content of coatings in the Chinese national standard system, and GB/T 2793 for adhesives. These gravimetric methods share the same fundamental principle: weigh a sample, evaporate the volatile components under defined conditions, and weigh the non-volatile residue.
Accurate solid content determination depends on several controlled variables, including drying temperature, drying duration, sample mass, container type, and the precision of the balance used. Deviations from the specified procedure can lead to systematic errors, which is why a formal inspection protocol must define every step of the operation, from sampling through calculation and reporting.
Testing Samples
Coatings and Paints
Liquid coatings, including solvent-borne paints, water-borne paints, varnishes, and industrial protective coatings, are the most common sample category. Non-volatile content in coatings determines the dry film thickness achievable per unit of wet film applied, making it essential for cost estimation and application planning.
Adhesives and Sealants
Water-based and solvent-based adhesives, hot-melt adhesives, and sealant compounds require solid content measurement to verify formulation consistency. For adhesives, GB/T 2793 provides the applicable gravimetric procedure, and results correlate directly with bond line formation and gap-filling performance.
Emulsion Polymers and Latex
Polymer emulsions such as styrene-butadiene latex, acrylic emulsions, vinyl acetate emulsions, and polyurethane dispersions are tested to confirm polymerization conversion and solids concentration. GB/T 11175 describes testing methods applicable to emulsions used in paints and related applications.
Inks and Resin Solutions
Printing inks and resin solutions rely on non-volatile content measurement to control viscosity behavior, transfer properties, and final deposit weight. ASTM D1259 is a recognized method for the nonvolatile content of resin solutions.
Sample Requirements and Conditioning
- Sample quantity: typically 50 to 200 g of representative material taken from the batch or container.
- The sample must be thoroughly mixed before testing to ensure homogeneity; gently stir to avoid entrapping air.
- Condition the sample to laboratory temperature, usually 23 ± 2 °C, before weighing.
- Avoid contamination from moisture, dust, or solvent loss during storage and transfer.
- Record the batch number, sampling date, and sample description on the test report.
Testing Items
The core testing item under this procedure is the solid content (non-volatile matter content), expressed as a mass fraction percentage. Depending on the product specification, related items may also be evaluated:
- Solid content (non-volatile matter), expressed in % by mass.
- Volatile content, the complementary fraction, also expressed in % by mass.
- Moisture content where required, determined separately by Karl Fischer titration for products where water is the primary volatile component.
- Density and viscosity, often tested alongside solid content for a complete quality profile.
- Repeatability check, performing duplicate determinations and verifying that results agree within the specified tolerance.
Testing Methods
Principle
A weighed quantity of the sample is placed in a preconditioned, constant-weight container and dried in a ventilated oven at a specified temperature for a specified time. After cooling in a desiccator, the container is weighed again. The mass of residue relative to the original sample mass, multiplied by 100, gives the solid content percentage.
Apparatus and Materials
- Analytical balance with a readability of 0.1 mg.
- Flat-bottomed dishes made of aluminum, glass, or stainless steel, approximately 50 to 75 mm in diameter.
- Forced-draft or gravity-convection drying oven capable of maintaining the specified temperature within ± 2 °C.
- Desiccator with an active desiccant such as silica gel.
- Disposable droppers or glass rods for sample transfer.
- Thermometer or calibrated temperature monitoring device for the oven.
Procedure
First, clean the weighing dish and dry it in the oven for the specified time. Cool it in the desiccator and weigh it accurately to obtain the tare mass. Using a dropper, add the specified amount of sample to the dish. For low-viscosity samples, weigh approximately 1 to 2 g; for high-viscosity samples, distribute the material evenly across the bottom of the dish. Weigh the dish with sample to record the initial mass.
Place the dish in the oven set to the specified temperature, commonly 105 ± 2 °C for water-based products and 120 ± 2 °C or higher for solvent-borne products, depending on the governing standard and product specification. Typical drying times range from 30 minutes to 3 hours. For materials prone to skinning, pre-drying at a lower temperature or adding a small amount of solvent may be required to ensure complete evaporation.
After the drying period, remove the dish using clean tongs, cool it in the desiccator for the specified time, typically 15 to 30 minutes, and weigh it again. If a constant-weight requirement applies, repeat the drying, cooling, and weighing cycle until the change in mass between successive weighings does not exceed the specified limit, commonly 0.5 mg.
Calculation
The solid content is calculated as the mass of residue divided by the mass of the original sample, multiplied by 100. The mass of residue equals the final dish-plus-residue mass minus the tare mass of the dish, and the sample mass equals the initial dish-plus-sample mass minus the tare mass. Express the result to one decimal place, and report the mean of duplicate determinations when both results fall within the specified repeatability limit.
Precision and Quality Control
- Perform duplicate determinations on independent sample portions; the difference between results should not exceed the repeatability limit stated in the governing standard.
- Verify balance calibration at regular intervals with certified reference weights.
- Monitor oven temperature distribution and record calibration status.
- Run periodic control samples with known solid content to detect systematic drift.
- Retain all raw weighing data with the test record for traceability.
Safety Precautions
Solvent vapors released during drying may be flammable or toxic. Operate the oven in a well-ventilated area or under an exhaust hood, and never exceed the temperature limits for flammable solvents. Wear heat-resistant gloves when handling hot dishes and use tongs to transfer containers. Dispose of solvent-containing waste in accordance with local environmental regulations.
Applications
Solid content testing procedures are applied across a wide range of industrial and commercial contexts:
- Incoming material inspection, verifying that raw materials such as resins, emulsions, and dispersions meet purchase specifications before production release.
- In-process quality control, monitoring solids concentration during mixing, dilution, or polymerization to keep the process within control limits.
- Finished product release, confirming that outgoing batches comply with declared technical data sheet values.
- Trade settlement, since many liquid chemical products are bought and sold on a dry solids basis, accurate determination has direct commercial consequences.
- Regulatory compliance and environmental reporting, where volatile organic compound (VOC) content calculations often require an accurate non-volatile fraction.
- Research and development, comparing formulation variants and evaluating the effect of additives on solids and application properties.
- Dispute resolution, where accredited third-party laboratories may perform referee testing according to the same standardized protocols.
FAQ
What is the difference between solid content and non-volatile content?
The two terms are used interchangeably in most standards. Both refer to the mass percentage of material remaining after volatile components, solvents and water, are driven off under specified drying conditions.
Which drying temperature should be used?
The drying temperature depends on the governing standard and the product type. Common conditions include 105 ± 2 °C for water-based coatings per ISO 3251 and GB/T 1725, 110 ± 5 °C per ASTM D2369 for coatings, and higher temperatures for products containing high-boiling solvents. Always follow the temperature stated in the product specification or the referenced standard.
How much sample should be weighed?
Most standards specify approximately 1 to 2 g for liquid samples. Using too much sample produces a thick film that traps solvent and causes low results; using too little reduces weighing accuracy. The sample should be spread evenly to maximize surface area for evaporation.
Why do duplicate results sometimes disagree?
Common causes include non-homogeneous samples, uneven sample distribution in the dish, incomplete drying, moisture uptake during cooling, or balance instability. Thorough mixing, consistent technique, and proper desiccator cooling minimize variability.
Can solid content be measured by methods other than oven drying?
Yes. Alternatives include infrared balance dryers and halogen moisture analyzers for rapid routine checks, though results must be correlated with the standard oven method for release testing. Karl Fischer titration determines water content specifically and is used when water and solvent must be distinguished.
How should results be reported?
Report the individual and mean values to one decimal place, along with the test standard used, drying temperature and time, sample identification, and the testing date. This documentation ensures that results are traceable and reproducible across laboratories.