Determination of Solid Content in Single-Wall Carbon Nanotube Slurries: Methods, Standards, and Tech

Solid content determination is a critical quality control test for single-wall carbon nanotube (SWCNT) slurries used in lithium-ion battery conductive agents, conductive inks, coatings, and composite materials. Accurate measurement of non-volatile matter ensures formulation consistency, precise dosing in downstream processes, and reliable product performance. This article provides a comprehensive technical overview of SWCNT slurry solid content testing, covering sample preparation, gravimetric drying methods, thermogravimetric analysis, applicable standards such as ISO 3251, ASTM D2369, and GB/T 1725, and practical guidance for achieving reproducible results.

Technical Overview

Single-wall carbon nanotube (SWCNT) slurries are dispersions of carbon nanotubes in aqueous or organic media, typically stabilized by surfactants, dispersants, or polymeric binders. These slurries are widely used as high-performance conductive additives in lithium-ion battery electrodes, conductive inks for printed electronics, transparent conductive films, antistatic coatings, and functional composites. Because SWCNT slurries are often supplied at very low nanotube concentrations (frequently below 1 wt%), even small errors in solid content measurement can lead to significant deviations in formulation, conductivity, viscosity, and electrode microstructure.

Solid content, also referred to as non-volatile matter content, is defined as the mass percentage of residue remaining after the volatile components (water, organic solvent, and volatile additives) are completely removed under specified drying conditions. The result is calculated as: Solid content (%) = (m2 − m0) / (m1 − m0) × 100, where m0 is the mass of the empty weighing vessel, m1 is the mass of the vessel plus the sample before drying, and m2 is the mass of the vessel plus the dried residue.

Several factors make SWCNT slurry testing technically demanding. The high viscosity and strong tendency of nanotubes to aggregate or settle require careful homogenization before sampling. Solvent can be trapped inside nanotube bundles, extending the time required to reach constant weight. In addition, the dispersants and binders used in the slurry are non-volatile at drying temperatures and are therefore included in the measured solid content; if the effective nanotube fraction must be separated from the dispersant fraction, complementary techniques such as thermogravimetric analysis are required. The gravimetric determination itself is generally performed with reference to established non-volatile-matter standards such as ISO 3251, ASTM D2369, GB/T 1725, and GB/T 6284, adapted to the specific characteristics of nanotube dispersions.

Testing Samples

Aqueous SWCNT Dispersions

Water-based dispersions are the most common commercial form of SWCNT slurry, typically containing 0.05 to 1.5 wt% nanotubes stabilized by surfactants such as sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium cholate, or nonionic dispersants. These low-concentration samples require a relatively large specimen mass (2 to 5 g) so that a sufficient and weighable dry residue is obtained.

Solvent-Based SWCNT Slurries

Solvent systems using N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), ethanol, or isopropanol are used in applications requiring compatibility with polymer binders or printing processes. The higher volatility and lower surface tension of these solvents allow faster drying, but flammability and solvent recovery must be managed, and slightly elevated drying temperatures may be applied where justified by validation data.

SWCNT Conductive Pastes for Lithium-Ion Batteries

Battery-grade SWCNT pastes combine nanotubes with carboxymethyl cellulose (CMC) or other binders in water or NMP, typically at solid levels of 3 to 6 wt%. These pastes are highly viscous and thixotropic, so thorough mechanical stirring or planetary mixing before sampling is essential to obtain a representative specimen.

High-Concentration Masterbatches

Concentrated SWCNT masterbatches, with solid content above 10 wt%, are supplied for in-house dilution by end users. For these materials, a smaller specimen mass (0.3 to 1 g) spread into a thin film is preferred to ensure complete solvent removal within a reasonable drying time.

Testing Items

A comprehensive solid content evaluation of SWCNT slurries typically includes the following test items:

• Total solid content (non-volatile matter content) by gravimetric drying
• Volatile matter content, calculated as 100% minus the solid content
• Moisture content of the slurry by loss on drying or Karl Fischer titration where applicable
• Dry residue mass per unit mass of slurry, reported to 0.01% or better
• Ash or residue-on-ignition testing to estimate inorganic catalyst residues associated with the nanotubes
• Constant-weight verification of the dried residue to confirm completeness of solvent removal

Testing Methods

Oven Drying Gravimetric Method (Reference Method)

The oven drying method is the reference procedure, aligned with the principles of ISO 3251, ASTM D2369, GB/T 1725, and GB/T 6284. The typical workflow is as follows:

• Homogenize the slurry thoroughly by rolling, shaking, or mechanical stirring, taking care not to introduce air bubbles.
• Dry a clean glass, aluminum, or stainless-steel weighing dish to constant weight, cool it in a desiccator, and record its mass (m0).
• Accurately weigh 1 to 3 g of homogenized slurry into the dish, spreading it as thinly and evenly as possible, and record the mass (m1).
• Dry the sample in a forced-convection oven at 105 ± 2 °C for water-based slurries, or at a validated temperature of 105 to 120 °C for solvent-based systems, for 2 to 4 hours.
• Cool the dish in a desiccator for 30 minutes and weigh. Repeat drying cycles of 30 to 60 minutes until the change in mass between successive weighings is 0.5 mg or less (constant weight).
• Record the final mass (m2) and calculate the solid content as (m2 − m0) / (m1 − m0) × 100.

Duplicate determinations should be performed, and results should typically agree within 0.2 to 0.3 percentage points (absolute). If the difference exceeds this tolerance, the test should be repeated with fresh homogenized samples.

Vacuum or Inert-Atmosphere Drying

Vacuum drying at 60 to 90 °C under reduced pressure is recommended for slurries containing temperature-sensitive dispersants, polymeric binders, or high-boiling solvents that are difficult to remove at atmospheric pressure. Drying under nitrogen may be specified to eliminate any possibility of oxidative degradation during extended drying, although SWCNTs themselves are stable in air well above typical drying temperatures.

Rapid Moisture Analyzer (Halogen Balance)

For routine in-process quality control, halogen moisture analyzers provide solid content results within 10 to 20 minutes. The instrument dries the sample on a built-in balance while continuously monitoring mass loss. Before use in release testing, the analyzer method must be correlated against the oven reference method for the specific slurry formulation, and the heating program (temperature and auto-stop criteria) must be validated to prevent premature termination while solvent remains trapped in nanotube agglomerates.

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis, performed in accordance with the principles of ISO/TS 11308 for SWCNT characterization, measures mass change as the sample is heated under a controlled atmosphere. TGA distinguishes the volatile fraction, dispersant decomposition, nanotube oxidation, and inorganic ash residue within a single experiment, making it the preferred technique when the effective nanotube fraction must be separated from the total non-volatile content. It also provides residual catalyst metal content, which is an important impurity indicator for battery applications.

Applications

Solid content data for SWCNT slurries support quality control and process development across multiple industries:

• Incoming material inspection and supplier qualification for lithium-ion battery conductive agents, where accurate solid content directly determines the nanotube loading in electrode formulations.
• Electrode slurry formulation and mixing control, enabling precise calculation of nanotube, binder, and solvent quantities to hit target rheology and conductivity.
• Conductive ink and printed electronics manufacturing, where non-volatile content governs film thickness, line resistivity, and printability.
• Coatings and antistatic film production, ensuring consistent percolation network formation and surface resistivity.
• Logistics, cost accounting, and regulatory documentation, since product value and volatile organic content are commonly expressed per unit of solids.

FAQ

What is the typical solid content range of commercial SWCNT slurries?

Dilute aqueous dispersions for battery applications typically contain 0.1 to 1 wt% nanotubes, battery-grade conductive pastes commonly range from 3 to 6 wt%, and concentrated masterbatches may exceed 10 wt%. The exact value depends on the supplier grade and should always be verified by testing upon receipt.

What drying temperature should be used?

A temperature of 105 ± 2 °C is the standard condition for water-based slurries and aligns with the reference conditions of ISO 3251 and GB/T 6284. Temperatures up to 120 °C may be used for solvent-based systems if validated. Temperatures above approximately 150 °C should be avoided for routine solid content testing because some organic dispersants begin to lose mass, which would bias the result low.

Does the dispersant or binder count as solid content?

Yes. By convention, solid content is total non-volatile matter and includes surfactants, dispersants, and binders. If the true nanotube weight fraction is required, thermogravimetric analysis or an orthogonal technique such as UV-Vis-NIR absorption spectroscopy should be used to quantify the nanotube component separately.

Why is constant weight important, and how is it judged?

Carbon nanotube networks can retain solvent within bundles, so a single drying period may underestimate volatile loss. Constant weight, defined as a mass change of no more than 0.5 mg between successive drying and weighing cycles, confirms that all removable volatiles have been eliminated.

How much sample should be tested?

For dilute dispersions, 2 to 5 g is recommended so the dry residue is large enough to weigh accurately. For pastes and masterbatches, 0.3 to 1 g spread into a thin, uniform film dries faster and more completely.

Can a rapid moisture analyzer replace the oven method?

It is suitable for routine in-process checks once its heating program has been validated and correlated against the oven reference method for the specific formulation. Release testing and dispute resolution should rely on the oven drying gravimetric method or on analysis by an accredited third-party laboratory following recognized standards such as ISO 3251, ASTM D2369, or GB/T 1725.