Parallel Determination of Solid Content: Principles, Methods, and Precision Requirements

The parallel determination of solid content is a core quality-control procedure in which duplicate gravimetric measurements of non-volatile matter are performed on the same homogenized sample to verify precision, repeatability, and reliability. Guided by recognized standards such as ISO 3251, ASTM D2369, and GB/T 1725, this testing approach is widely applied to coatings, adhesives, polymer emulsions, and inks to ensure batch consistency, control formulation cost, and satisfy regulatory documentation requirements.

Technical Overview

Solid content, also referred to as non-volatile matter content, is the mass percentage of residue remaining after a liquid sample such as a paint, adhesive, or emulsion is dried under specified temperature and time conditions to remove water and volatile solvents. It is one of the most fundamental physical indicators used to evaluate product quality, calculate formulation cost, and assess volatile organic compound (VOC) contribution by difference.

A parallel test (parallel determination) means that at least two independent measurements are carried out on the same homogenized sample under repeatable conditions, that is, by the same operator, using the same apparatus and method, within a short time interval. The difference between the duplicate results is compared against the repeatability limit specified in the applicable standard. Only when the results agree within the permitted tolerance is the arithmetic mean accepted as the reported value. This practice suppresses random error, exposes operational or sampling problems, and provides statistical confidence in the final result.

Representative standards governing this testing include ISO 3251 and EN ISO 3251 for paints, varnishes, and plastics; ASTM D2369 for volatile content of coatings; GB/T 1725 for paints, varnishes, and printing inks; GB/T 2793 for adhesives; ISO 1625 for polymer dispersions; and ISO 124 or GB/T 8298 for rubber latices. Accredited third-party laboratories routinely apply these methods for certification and compliance purposes.

Testing Samples

Coatings and Paints

Waterborne, solvent-borne, high-solids, and radiation-curable coatings are the most common sample types. Depending on the product standard, specimens are typically dried at 105 ± 2 °C for a defined period, or until constant mass is achieved. The applicable references are ISO 3251, GB/T 1725, and ASTM D2369.

Adhesives and Sealants

Emulsion adhesives, solvent-based adhesives, pressure-sensitive adhesives, and sealant pastes are tested to confirm solids consistency between production batches and incoming shipments. GB/T 2793 and equivalent international methods describe the gravimetric determination for these materials.

Polymer Emulsions and Latices

Acrylic, vinyl acetate-ethylene (VAE), styrene-butadiene, and natural rubber latex dispersions are tested for total solids content using ISO 1625, ISO 124, GB/T 11175, or GB/T 8298. Because emulsions may skin on the surface, gentle homogenization before sampling is essential for obtaining agreeing parallel results.

Inks and Functional Fluids

Printing inks, textile auxiliaries, water-treatment polymer solutions, and battery electrode slurries are also evaluated for non-volatile fraction, since solids level directly affects viscosity, coverage, film build, and downstream processing performance.

Testing Items

The parallel solid content test typically covers the following determination items:

• Total solid content (non-volatile matter content), expressed as mass fraction in percent.
• Volatile matter content, obtained by difference (100 minus solids).
• Agreement check between parallel determinations, compared with the repeatability limit of the applicable standard.
• Optional supplementary data such as residue appearance, skinning, or gel formation after drying.

Testing Methods

Gravimetric Oven-Drying Method (Arbitration Method)

The gravimetric method is the reference (arbitration) procedure used for certification and dispute resolution. The required apparatus includes:

• Analytical balance with a readability of 0.1 mg.
• Forced-convection oven capable of maintaining the specified temperature within ± 2 °C.
• Flat-bottom weighing dishes, tins, or glass watch plates of resistant material.
• Desiccator with fresh desiccant for cooling specimens before weighing.

The general operating sequence is as follows:

• Homogenize the laboratory sample thoroughly without entrapping air, and condition it to ambient temperature.
• Dry the empty dish to constant mass, cool it in the desiccator, and record its mass (m0).
• Weigh an appropriate specimen, commonly around 1 g for coatings, into the dish and distribute it evenly; record the combined mass (m1). For highly fluid products, a small amount of clean dry sand or glass fiber may be added to increase surface area.
• Place the dish in the oven at the temperature and for the duration specified by the product standard, for example 105 ± 2 °C for 2 to 3 hours for many waterborne systems, or 110 °C for 60 minutes according to ASTM D2369.
• Transfer the dish to the desiccator, cool to room temperature, and weigh (m2).
• Repeat the drying-cooling-weighing cycle until the change between successive weighings does not exceed approximately 1 mg, confirming that constant mass has been reached.

Calculation

The solid content is calculated as: S = (m2 − m0) / (m1 − m0) × 100, where S is the solid content in percent by mass, 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 result of the accepted parallel determinations is the arithmetic mean, normally reported to 0.1 %.

Execution of the Parallel Determinations

At least two full determinations must be performed from the same homogenized sample, each with its own dish and independent weighing sequence. For high workloads, duplicate dishes may be placed in the oven simultaneously, provided thermal uniformity has been verified. The two results are then compared:

• If the absolute difference between duplicates does not exceed the repeatability limit defined by the standard, commonly around 0.5 % (m/m) for products with solids above roughly 40 %, the mean is accepted.
• If the difference exceeds the limit, a third determination shall be performed, and the cause of the discrepancy, such as poor homogenization, uneven specimen distribution, or oven temperature drift, shall be investigated.
• Outlying values that cannot be technically justified are rejected, and the mean of the valid determinations is reported together with the number of measurements taken.

Special Conditions and Rapid Alternatives

For heat-sensitive binders, unsaturated resins, or products containing reactive monomers, reduced-temperature drying, vacuum ovens, or inert-atmosphere ovens may be specified to prevent thermal degradation or polymerization that would falsely inflate the solids result. Infrared and halogen moisture analyzers are widely used for rapid in-process checks, but they must be validated against the oven method, and the oven method remains the reference for arbitration and certification.

Applications

Parallel solid content testing is applied throughout the chemical and materials value chain:

• Manufacturing quality control: monitoring batch-to-batch consistency in paint, adhesive, and emulsion production lines.
• Incoming material inspection: verifying supplier specifications for resins, dispersions, latex, and binder solutions.
• Research and development: comparing formulation variants and calculating theoretical solids during formula design.
• Commercial settlement: providing an objective basis for pricing and delivery acceptance between trading parties.
• Regulatory compliance and certification: supporting VOC declarations, environmental labeling dossiers, and test reports issued by accredited third-party laboratories.
• Process optimization: linking solids level to viscosity control, coating thickness, coverage rate, and drying energy consumption.

FAQ

Why are parallel determinations required for solid content testing?

A single measurement cannot reveal random errors caused by inhomogeneous sampling, weighing drift, or incomplete drying. Duplicate determinations quantify repeatability, ensure the result is representative, and are explicitly required by most international standards before a value can be reported.

How many parallel tests should be performed?

A minimum of two determinations from the same homogenized sample is standard practice. A third determination is added only when the first two results disagree beyond the permitted tolerance or when required by the contract or product specification.

What deviation between parallel results is acceptable?

The acceptance criterion is the repeatability limit of the applicable standard. As a common engineering guideline, duplicates should agree within approximately 0.5 % absolute for solids content above about 40 %, while low-solids products may be judged on a relative deviation basis. The specific limit of the cited standard always takes precedence.

Which drying temperature should be used?

The temperature and time must follow the product standard or specification. Typical conditions are 105 ± 2 °C for waterborne coatings and emulsions, and 110 °C for 60 minutes under ASTM D2369. Heat-sensitive materials may require lower temperatures, vacuum drying, or inert atmosphere to avoid degradation.

What are the common causes of disagreement between parallel results?

Frequent causes include insufficient sample homogenization, uneven specimen spreading leading to incomplete evaporation, skin formation trapping solvent, oven temperature non-uniformity, incomplete cooling before weighing, and moisture uptake from a saturated desiccator.

Can a moisture analyzer replace the oven method?

Fast infrared or halogen moisture analyzers are suitable for routine in-process screening once correlated with the oven method, but the gravimetric oven-drying procedure remains the reference method for arbitration, certification, and reportable quality data.