Dispersion Testing of Oxidized Carbon Nanotubes in Acrylic Resin: Methods, Indicators, and Standards

Dispersion testing of oxidized carbon nanotubes (O-CNTs) in acrylic resin is a critical quality-control step for manufacturers of conductive coatings, functional inks, antistatic finishes, and nanocomposite materials. By quantifying agglomerate size, distribution uniformity, and long-term stability, this testing verifies that acid-oxidized CNTs achieve true nanoscale dispersion rather than persistent bundling inside the acrylic matrix. This article outlines the sample types, test items, characterization methods, and international standards used by accredited third-party laboratories to evaluate CNT dispersibility in acrylic resin systems.

Technical Overview

Oxidized carbon nanotubes are multi-wall or single-wall CNTs whose surfaces have been functionalized, typically by mixed-acid treatment, to introduce carboxyl (-COOH), hydroxyl (-OH), and carbonyl (C=O) groups. These oxygen-containing groups improve wettability and interfacial adhesion in polar polymer matrices such as acrylic resins, where hydrogen bonding between the functionalized tube walls and the ester groups of acrylate or methacrylate copolymers promotes deagglomeration and inhibits re-bundling.

However, chemical compatibility alone does not guarantee a good dispersion. Strong van der Waals forces and tube-tube entanglement mean that CNTs frequently persist as agglomerates of tens of micrometers even after high-shear mixing. Because dispersion quality directly controls the electrical percolation threshold, mechanical reinforcement efficiency, optical clarity, and storage stability of the final acrylic formulation, systematic dispersion testing is required at every stage from raw powder to cured film. Dispersion testing therefore evaluates three dimensions: the size of residual aggregates, the spatial uniformity of the CNT network, and the temporal stability of the dispersion against sedimentation or flocculation.

Testing Samples

Neat Oxidized CNT Powder

Before compounding, the raw O-CNT material is characterized to confirm its oxidation degree, purity, ash content, and geometric parameters (diameter, length, aspect ratio). These baseline data are essential because a dispersion failure can originate from insufficient functionalization or excessive tube damage during oxidation, not only from poor mixing.

CNT-Acrylic Masterbatch or Liquid Concentrate

Concentrated dispersions, typically 1 to 10 weight percent CNT in acrylic resin, resin solution, or reactive diluent, are the primary sample type for dispersion evaluation. These are tested for agglomerate size distribution, fineness of grind, viscosity behavior, and accelerated storage stability.

Letdown Application Formulations

Masterbatches diluted to application concentrations in coating, ink, or adhesive formulations are tested to confirm that dispersion quality survives the letdown step and that no flocculation is triggered by solvent shock, pH shift, or interaction with other additives.

Cured Films and Cast Sheets

Free films, drawdowns, or coated substrates after full curing allow assessment of the dispersion state locked into the solid matrix, including cross-sectional morphology, surface defects, optical haze, and electrical uniformity.

Testing Items

Accredited third-party laboratories typically structure the test program around the following measurable indicators:

• Morphology and structure: aggregate and agglomerate size, individual tube diameter and length distribution, aspect ratio retention, and degree of exfoliation.
• Particle size distribution: D10, D50, and D90 values, distribution span, and the percentage of agglomerates exceeding a defined rejection threshold (for example, particles larger than the target dry-film thickness).
• Surface chemistry and oxidation degree: carboxyl and hydroxyl group density (Boehm titration), oxygen-to-carbon atomic ratio (XPS), characteristic FTIR absorbance bands, Raman ID/IG ratio, and thermogravimetric decomposition behavior.
• Dispersion stability: zeta potential, sedimentation rate, centrifugal stability ratio, and time-dependent changes in turbidity or UV-Vis absorbance.
• Rheological behavior: apparent viscosity versus shear rate, thixotropy loop area, and yield stress, which sensitively reflect the state of the CNT network.
• Film-level performance: surface roughness (Ra), haze and luminous transmittance, sheet resistance and its spatial uniformity, electrical percolation threshold, adhesion, hardness, and tensile properties.

Testing Methods

Microscopy and Image Analysis

Transmission electron microscopy, performed in line with ISO/TS 10797, and scanning electron microscopy provide direct visualization of individual tubes, bundles, and residual agglomerates. Optical microscopy of microtomed cross-sections or wet-film draws is used for rapid screening of large agglomerates, followed by quantitative image analysis statistics in accordance with ISO 9276-1. Atomic force microscopy complements these techniques for surface topography and tube diameter measurement.

Particle Size Analysis

Laser diffraction per ISO 13320 delivers volume-based D10/D50/D90 values for the agglomerate population, while dynamic light scattering per ISO 22412 resolves submicrometer bundle populations. Sample dilution, sonication history, and refractive index assumptions must be documented, because CNTs are strongly light-absorbing and results are method-sensitive.

Spectroscopic Characterization

UV-Vis absorption spectroscopy, aligned with the principles of ISO/TS 10868, is used both to quantify dispersed CNT concentration through Beer-Lambert calibration and, through absorbance drift over time, to monitor re-agglomeration. Raman spectroscopy verifies the ID/IG ratio before and after dispersion to detect tube damage, while FTIR and XPS confirm that carboxyl and hydroxyl functionality is retained through processing. Thermogravimetric analysis quantifies functionalization level and inorganic residue.

Colloidal Stability Evaluation

Zeta potential measurement following ISO 13099 assesses electrostatic stabilization in waterborne acrylic systems. Accelerated centrifugation tests, elevated-temperature storage with periodic viscosity and particle size re-measurement, and gravity sedimentation observation with turbidity tracking provide practical stability rankings for shelf-life prediction.

Rheological Measurement

Rotational rheometry per ISO 3219 and rotational viscometry per ISO 2555 or ASTM D2196 generate flow curves and thixotropy data. A well-dispersed CNT-acrylic system shows a characteristic shear-thinning profile and a low-shear plateau reflecting a percolated but deagglomerated network; abrupt viscosity increases or unstable low-shear behavior signal flocculation.

Coating-Specific and Film-Level Methods

Fineness of grind is determined per ISO 1524 or ASTM D1210 for liquid coatings. Cured films are evaluated for haze and transmittance per ASTM D1003, surface resistance by four-point probe mapping, film thickness per ISO 2808, adhesion per ASTM D3359, and tensile properties of free films per ISO 527-3. Cross-sectional SEM of the cured film remains the definitive method for confirming the absence of through-thickness agglomerates. Sample preparation for the whole workflow follows the general principles of ISO 1513, and nanotechnology terminology is applied consistently with the ISO/TS 80004 series.

Applications

Dispersion testing of oxidized CNTs in acrylic resin supports a broad range of functional products:

• Conductive and antistatic acrylic coatings for electronic packaging, cleanroom flooring, ESD-safe work surfaces, and plastic housings.
• Electromagnetic interference shielding and radio-frequency attenuation coatings for electronics enclosures.
• Corrosion-protective acrylic coatings in which dispersed CNTs increase the diffusion-path tortuosity of the barrier layer.
• UV-curable acrylic inks, transparent conductive coatings, and conductive formulations for printed electronics.
• Photopolymer resins for additive manufacturing requiring tuned electrical or rheological behavior.
• Structural acrylic adhesives and reinforced acrylic composites with enhanced modulus, toughness, and fatigue resistance.

FAQ

Why are carbon nanotubes oxidized before dispersion into acrylic resin?

Acid oxidation introduces carboxyl and hydroxyl groups on the tube surfaces, improving wetting by the polar acrylic matrix, reducing bundle cohesion, and enabling hydrogen-bonding interactions with acrylate ester groups. This lowers the energy required for deagglomeration and slows re-bundling, resulting in finer and more stable dispersions than achievable with pristine CNTs.

What defines an acceptable dispersion quantitatively?

Acceptance criteria are application-specific, but typical requirements for coatings include a D90 below approximately 5 to 10 micrometers, no agglomerates larger than the specified dry-film thickness, fineness of grind within grade, sheet resistance uniformity within a defined tolerance band, and no significant viscosity increase or sedimentation after accelerated storage.

Can laser diffraction measure the diameter of individual nanotubes?

No. Laser diffraction reports an equivalent spherical diameter and is therefore suited to characterizing the agglomerate and bundle population rather than individual tube dimensions. Individual tube morphology is measured by TEM, SEM, or AFM, and the two data sets are interpreted together.

Which method best detects re-agglomeration over time?

No single method is sufficient in isolation. A combination of time-resolved UV-Vis absorbance, periodic particle size re-measurement, sedimentation or centrifugation testing, and low-shear rheology monitoring provides a reliable picture of colloidal stability, with cross-sectional microscopy of aged and cured films confirming the final state.

Is dispersion in the liquid resin enough, or must the cured film also be tested?

Both stages should be tested. A liquid dispersion can appear acceptable yet flocculate during solvent flash-off or curing, and agglomerates that are invisible in the liquid state may create surface defects or electrical non-uniformity in the film. Film-level optical, electrical, and cross-sectional evaluation closes the quality loop.