Ink Solid Content Determination: Principles, Test Methods, and Standards

Ink solid content determination measures the percentage of non-volatile matter—including resins, pigments, fillers, and additives—that remains after volatile solvents or water are evaporated under controlled laboratory conditions. As a cornerstone of ink quality control, this test directly affects printing consistency, film formation, drying behavior, VOC compliance, and overall cost efficiency. This article explains the technical principles, applicable standards, sample types, step-by-step test procedures, calculation methods, and practical applications of ink solid content analysis.

Technical Overview

Solid content, also referred to as non-volatile matter content, is defined as the mass percentage of an ink formulation that does not evaporate when a sample is heated under specified conditions. In practical terms, it represents the combined weight of film-forming resins, pigments, extenders, waxes, and functional additives that remain after solvents, water, and other volatile components are driven off. The result is calculated by comparing the mass of the dry residue to the mass of the original sample.

Solid content is one of the most fundamental parameters in ink manufacturing and printing. It determines how much dry film a given quantity of ink will deposit, which in turn governs color strength, gloss, abrasion resistance, adhesion, and coverage per unit weight. Because solvents are far cheaper than the resin and pigment fractions they carry, an unexpectedly low solids value can indicate over-dilution, incomplete batching, or raw material deviation, with direct commercial consequences for both ink producers and printers.

The determination is typically performed in accredited third-party laboratories or in-house quality control facilities using gravimetric oven-drying methods aligned with recognized standards such as ISO 3251 (non-volatile matter content of paints, varnishes, and plastics), ASTM D2369 (volatile content of coatings), GB/T 1725 (Chinese adoption of the ISO gravimetric method), and the GB/T 13217 series covering test methods for liquid printing inks.

• Solid content measures non-volatile residue as a mass percentage of the original sample.
• The result correlates directly with dry film thickness, color density, and yield per kilogram of ink.
• Standardized gravimetric drying ensures results are comparable between suppliers, laboratories, and production sites.
• Accurate solids data supports VOC declaration, emission calculations, and regulatory compliance.

Testing Samples

Solvent-Based Liquid Inks

Gravure and flexographic inks based on ethyl acetate, ethanol, isopropanol, toluene, or blended ester solvents typically show solid contents in the range of 15 to 45 percent depending on pigment loading and application. These low-viscosity systems dry rapidly and require careful control of oven temperature to prevent splattering of the thin sample film during the early stage of heating.

Water-Based Inks

Water-based flexo and gravure inks, used extensively in packaging and corrugated printing, generally contain 25 to 50 percent solids. Their slower evaporation profile demands longer drying times or slightly elevated temperatures, and trapped moisture must be fully removed before the residue is weighed. Water-based systems may also contain small amounts of coalescing aids that evaporate only partially at standard temperatures.

UV-Curable and Energy-Curable Inks

UV and LED-curable inks are formulated with reactive monomers and oligomers that polymerize rather than evaporate, so their non-volatile content is essentially 100 percent by design. Gravimetric determination is still performed to verify the absence of unwanted volatile contaminants, but the test does not simulate the cured film and must be interpreted accordingly.

Paste Inks for Offset Printing

High-viscosity sheetfed and heatset offset inks contain pigment, resin, and vegetable or mineral oils. Because the paste cannot spread into a uniform thin film on its own, samples are commonly dispersed in a small quantity of suitable solvent on the weighing dish before drying. Reported solids values for paste inks typically fall between 60 and 95 percent depending on the distillation range of the vehicle oils.

Raw Materials and Intermediate Products

Beyond finished inks, solid content testing is routinely applied to resin solutions, pigment dispersions, letdown varnishes, and grinding bases to verify incoming raw materials and monitor batch-to-batch consistency during production.

Testing Items

The core deliverable of the test is the non-volatile matter content, expressed as a percentage by mass, determined at a specified temperature and drying time. Depending on customer requirements, the scope of testing may be expanded to include the following related items:

• Non-volatile matter (total solid content) at 105±2°C or another agreed condition.
• Volatile organic compound (VOC) content, obtained by difference from the solids result.
• Water content determined specifically by Karl Fischer titration for water-based and solvent-borne systems.
• Thermal non-volatile residue for UV inks to screen for volatile impurities.
• Repeatability verification through duplicate determinations performed on parallel samples.

Testing Methods

Gravimetric Oven-Drying Method

The standard method follows the gravimetric principle: a weighed quantity of homogenized ink is placed in a tared dish, heated in a calibrated forced-air oven until volatile components are fully evaporated, cooled in a desiccator, and reweighed. The mass loss corresponds to volatile matter, and the remaining residue corresponds to solid content. Required equipment includes an analytical balance readable to 0.1 mg, flat-bottom aluminum or glass dishes, a temperature-controlled laboratory oven, a desiccator with fresh desiccant, and disposable syringes or pipettes for sample dosing.

Standard Operating Procedure

The typical procedure consists of the following sequential steps:

• Homogenize the ink sample thoroughly by gentle stirring, avoiding air entrainment.
• Dry the clean dish, cool it in the desiccator, and record its mass as the tare weight.
• Introduce approximately 0.3 to 1.0 g of sample for fluid inks, or a smaller dispersed quantity for paste inks, and record the combined mass.
• Distribute the sample into a thin, uniform film; where necessary, add a few drops of compatible solvent to assist spreading.
• Pre-dry briefly at a lower temperature if splashing is expected, then dry at 105±2°C, or the temperature specified by the product standard, for two to three hours.
• Cool the dish in the desiccator to ambient temperature and weigh.
• Repeat heating, cooling, and weighing cycles until the change in mass between successive weighings is no greater than the acceptance limit, typically 1 mg or 0.1 percent relative.

Calculation of Solid Content

Solid content is calculated using the following relationship: solid content (%) = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the empty dish, m1 is the mass of the dish plus the original sample, and m2 is the mass of the dish plus the dry residue after constant weight is achieved. Volatile content is then obtained by subtracting the solid content from 100 percent. Duplicate determinations should agree within the repeatability limits stated in the governing standard, and the mean is reported.

Method Considerations for Different Ink Systems

Low-boiling solvent inks benefit from a staged heating profile or a slightly reduced initial temperature to prevent foaming and loss of sample. Water-based systems require extended drying to eliminate residual moisture trapped beneath a skin that may form on the surface. Paste offset inks must be pre-dispersed in solvent to expose the full surface area to the oven atmosphere. For UV-curable inks, the gravimetric result reflects thermal stability rather than cure conversion and should be clearly labeled as such in the test report.

Complementary Analytical Techniques

For faster throughput or more detailed characterization, laboratories may supplement the oven method with thermogravimetric analysis (TGA), which records mass loss as a continuous function of temperature and helps identify decomposition onset; Karl Fischer titration, which quantifies water selectively in the presence of organic solvents; and rapid infrared moisture balances, which are widely used for in-process checks once they have been calibrated against the reference oven method.

Applications

Solid content determination serves multiple functions across the ink supply chain. Ink manufacturers use it for incoming inspection of resin solutions and dispersions, in-process control during batching and letdown, and final conformity verification before release. Printers rely on solids data to calculate actual consumption, predict coverage, and troubleshoot defects such as slow drying, blocking, weak color strength, or insufficient film build. Regulatory teams use the volatile fraction derived from the test to support VOC declarations, emission inventories, and compliance with environmental labeling schemes. Research and development groups apply the method when optimizing formulations, evaluating alternative raw materials, and demonstrating batch-to-batch consistency during scale-up or transfer between production sites.

• Verification of finished ink conformity against specification sheets and certificates of analysis.
• Cost analysis comparing usable solids delivered by competing ink systems.
• Root-cause investigation of print defects linked to over-thinning or incomplete dispersion.
• Supporting documentation for VOC and environmental compliance reporting.
• Formulation optimization and stability studies in research and development programs.

FAQ

Q: What temperature and drying time are used for ink solid content testing? A: The most common condition is 105±2°C for two to three hours, followed by repeated drying cycles until constant weight is reached. Product standards or customer specifications may define alternative temperatures, such as 100°C or 120°C, particularly for systems with unusual solvent profiles.

Q: What solid content values are typical for different ink types? A: Indicative ranges are 15 to 45 percent for solvent-based liquid inks, 25 to 50 percent for water-based inks, 60 to 95 percent for offset paste inks, and close to 100 percent for UV-curable inks. Exact targets depend on pigment loading and application requirements.

Q: Is solid content the same as non-volatile matter? A: Yes. The two terms are used interchangeably in most standards and both describe the mass percentage remaining after volatiles are evaporated under specified conditions. Volatile matter is the complementary value obtained by difference.

Q: How is constant weight defined in practice? A: The sample is considered at constant weight when the mass change between two consecutive drying and cooling cycles falls below a defined tolerance, commonly 1 mg. This confirms that evaporation is complete and the residue is stable.

Q: Can the test be performed quickly on the production floor? A: Infrared moisture balances and halogen dryers provide results in ten to twenty minutes and are suitable for routine process monitoring, provided they are periodically validated against the full gravimetric oven method as the referee technique.

Q: How much sample is required and what is the typical turnaround? A: A representative sample of 50 to 100 grams of ink is generally sufficient. The gravimetric determination itself, including duplicate runs and constant-weight confirmation, is usually completed within one working day.

Q: Why do UV inks still undergo this test if they are 100 percent solids? A: Gravimetric determination on UV inks verifies that no unexpected volatile impurities, residual monomer contaminants, or moisture are present, and it provides a useful quality fingerprint even though the result should theoretically approach 100 percent.