Experimental Study on the Effect of Oxidized Carbon Nanotubes on Resin Curing Behavior

This technical article presents a comprehensive test program for evaluating how oxidized carbon nanotubes (o-CNTs) influence the curing behavior of thermosetting resins, covering specimen preparation, testing items, analytical methods, and practical applications. By quantifying changes in curing kinetics, gelation time, degree of cure, and glass transition temperature, formulators can determine optimal nanotube loading and oxidation levels for enhanced composite performance. All experimental approaches described here align with recognized standards from ISO, ASTM, and GB organizations and can be performed by accredited third-party laboratories.

Technical Overview

Oxidized carbon nanotubes (o-CNTs) are multi-walled or single-walled carbon nanotubes whose graphitic surfaces have been functionalized with oxygen-containing groups—typically carboxyl (-COOH), hydroxyl (-OH), and carbonyl (C=O)—through acid oxidation, ozone treatment, or plasma processing. When dispersed into thermosetting resin systems such as epoxy, vinyl ester, phenolic, or bismaleimide resins, these functional groups interact directly with the polymer network during cure.

The influence of oxidized carbon nanotubes on resin curing is dual-directional and strongly dependent on loading level. At low concentrations (typically 0.1–0.5 wt%), the carboxyl and hydroxyl groups can catalyze the epoxy–amine ring-opening reaction and provide hydrogen-bonding sites, which often accelerates curing and lowers the peak exotherm temperature. At higher loadings (above approximately 1 wt%), increased system viscosity, steric hindrance around the nanotube surfaces, and adsorption of curing agent molecules onto the nanotubes can restrict molecular mobility and retard the curing reaction. A systematic curing influence test is therefore essential to identify the threshold loading at which the catalytic effect transitions into a diffusion-controlled inhibition effect.

The test program described below measures cure enthalpy, kinetic parameters, gelation behavior, and post-cure thermomechanical properties, enabling engineers to make evidence-based decisions on nanotype selection, dispersion protocol, and curing cycle design.

Testing Samples

Neat Resin Baseline

An unfilled resin formulation consisting of the base resin (for example, a bisphenol-A diglycidyl ether epoxy) and its designated curing agent is prepared and cured under the manufacturer-recommended schedule. This baseline provides the reference curing enthalpy, peak temperature, activation energy, and glass transition temperature against which all nanocomposite samples are compared.

Oxidized CNT-Modified Resin Samples

A series of resin batches containing oxidized carbon nanotubes at graded loadings—commonly 0.1 wt%, 0.3 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% relative to the resin mass—is prepared. Each batch uses an identical dispersion protocol (for example, a combination of bath sonication and mechanical stirring for a fixed duration) to ensure that observed curing differences are attributable to nanotube loading rather than dispersion variability.

Control Samples with Pristine CNTs

To isolate the effect of surface oxidation itself, parallel samples containing pristine (non-oxidized) carbon nanotubes at matching loadings are prepared. Comparing oxidized versus pristine systems reveals whether functional groups, rather than the nanotube backbone alone, drive the observed curing kinetics changes.

Variable Oxidation Degree Samples

Where the study scope requires it, samples using o-CNTs with different oxidation intensities (mild, moderate, and strong acid treatment, verified by different carboxyl group densities) are included to correlate functional group concentration with curing acceleration or retardation.

Specimen Forms

• Small hermetic DSC pans containing 5–10 mg of uncured liquid mixture for calorimetric curing scans.
• Rheometer plates loaded with uncured paste for gelation and viscosity-build measurements.
• Cast and fully cured plaques (approximately 2–3 mm thick) for DMA, TGA, and mechanical characterization.
• Fractured coupon segments for SEM or TEM examination of nanotube dispersion quality.

Testing Items

The curing influence test evaluates the following measurable parameters for each sample group:

• Total curing enthalpy (ΔH) and normalized enthalpy per unit resin mass, indicating whether nanotubes alter the achievable degree of conversion.
• Curing characteristic temperatures: onset temperature, peak exotherm temperature, and finishing temperature from dynamic DSC scans.
• Degree of cure (α) as a function of time and temperature under isothermal and dynamic conditions.
• Apparent activation energy (Ea) determined by iso-conversional and multi-heating-rate methods.
• Curing kinetic parameters, including reaction order (n), autocatalytic exponent (m), and rate constants (k1, k2) fitted to the Kamal–Sourour autocatalytic model.
• Gel time and gel point, defined by the crossover of storage modulus and loss modulus in oscillatory rheometry.
• Viscosity evolution before gelation, reflecting processability and pot life changes.
• Glass transition temperature (Tg) of the fully cured network as an indicator of final crosslink density.
• Storage modulus, loss modulus, and tan delta profiles from dynamic mechanical analysis.
• Thermal decomposition onset temperature and residual char content from thermogravimetric analysis.
• Residual functional group conversion verified by FTIR, including the decay of the epoxy absorption band near 915 cm⁻¹.
• Residual acidity and moisture content of the o-CNTs, which can independently catalyze or interfere with the curing reaction.

Testing Methods

Differential Scanning Calorimetry (DSC)

Dynamic and isothermal DSC measurements are performed in accordance with ISO 11357 and GB/T 19466 for polymer curing characterization. Dynamic scans are run at multiple heating rates (5, 10, 15, and 20 °C/min) under nitrogen from ambient temperature to beyond the finishing temperature of the exotherm. Kinetic analysis follows the Kissinger and Ozawa methods (consistent with ASTM E698) to compute apparent activation energy, while the peak exotherm shift across loadings directly indicates acceleration or retardation of the cure. Glass transition temperatures of cured specimens are confirmed by modulated or second-scan DSC in line with ASTM E1356.

Fourier Transform Infrared Spectroscopy (FTIR)

In-situ or ex-situ FTIR tracks the disappearance of the epoxy group absorption band near 915 cm⁻¹ and the growth of hydroxyl and ether linkages. Comparing conversion curves between neat resin and o-CNT formulations validates DSC results and confirms whether carboxyl groups chemically participate in the network formation through esterification or epoxide ring-opening reactions.

Rheological Analysis

Oscillatory rheometry on uncured and curing systems measures complex viscosity evolution and identifies the gel point as the crossover of storage modulus (G') and loss modulus (G''), following the general principles of ASTM D4473. Shortened gel time indicates catalytic acceleration by surface functional groups, whereas extended gel time at high loadings signals mobility restriction and possible diffusion control.

Dynamic Mechanical Analysis (DMA)

Fully cured specimens are tested in three-point bending or dual-cantilever mode per ISO 6721 and ASTM D7028 to determine Tg from the tan delta peak, storage modulus, and crosslink density indicators. Shifts in Tg quantify whether o-CNTs enhance or impair final network perfection.

Thermogravimetric Analysis (TGA)

TGA conducted per ISO 11358 and ASTM E1131 establishes the thermal decomposition onset, degradation profile, and residual nanotube content, confirming both the thermal stability of the cured network and the actual nanotube loading incorporated into each specimen.

Morphological Verification

Scanning electron microscopy and transmission electron microscopy of fracture surfaces verify dispersion uniformity and detect agglomeration, which is critical for distinguishing true chemical effects on curing from artifacts caused by poor dispersion.

Data Interpretation Framework

• A downward shift in peak exotherm temperature and reduced activation energy at low loadings confirm a catalytic effect of oxygen-containing functional groups.
• Reduced total enthalpy and incomplete epoxy conversion at high loadings indicate curing inhibition and network defects.
• Combined DSC-FTIR-rheology convergence strengthens the reliability of conclusions regarding the reaction mechanism.

Applications

Test data on the curing influence of oxidized carbon nanotubes directly support engineering decisions across multiple industries:

• Aerospace composite manufacturing, where o-CNT-modified prepregs and film adhesives require precise cure-cycle windows to balance toughness enhancement with controlled exotherm and flow behavior.
• Electronic packaging and electrically conductive adhesives, in which accelerated curing at lower temperatures improves throughput while maintaining high glass transition temperatures for solder-reflow resistance.
• Anticorrosion and functional coatings, where shortened gel times and improved barrier properties from well-dispersed o-CNTs reduce application time on large steel and concrete structures.
• Automotive lightweight structures and battery enclosure resins, where optimized nanotube loading delivers simultaneous improvements in cure speed, mechanical reinforcement, and thermal stability.
• Wind turbine blade infusion resins, where viscosity build and gelation control directly affect mold filling and cycle time.
• Additive manufacturing photopolymer and thermoset inks, where nanotube-induced changes in curing kinetics influence print resolution and interlayer bonding.
• Quality control and incoming material qualification, where the curing influence test serves as a release criterion for nanomaterial batches and as evidence for compliance submissions to accredited third-party laboratories.

FAQ

Does oxidation of carbon nanotubes accelerate or retard resin curing?

Both effects can occur. At low loadings, carboxyl and hydroxyl groups on o-CNT surfaces typically accelerate the epoxy–amine reaction by catalytic and hydrogen-bonding mechanisms, lowering peak exotherm temperature and activation energy. At higher loadings, increased viscosity, steric obstruction, and adsorption of curing agent onto nanotube surfaces usually retard curing and reduce final conversion. The test program identifies the loading threshold where the mechanism transitions.

Why does the glass transition temperature change when oxidized CNTs are added?

Functional groups on o-CNTs can covalently or physically anchor into the network, increasing effective crosslink density and raising Tg when conversion is complete. However, if nanotubes hinder full conversion or trap unreacted species, plasticization and reduced crosslink density can lower Tg. DMA and second-scan DSC quantify this balance.

Which tests are essential for a minimum viable evaluation?

At minimum, multi-heating-rate DSC for enthalpy and kinetic parameters, oscillatory rheometry for gel time, and DMA for Tg of cured specimens. FTIR and TGA are strongly recommended to confirm conversion chemistry and actual loading.

Can residual acid from the oxidation treatment distort results?

Yes. Incompletely washed o-CNTs may carry residual sulfuric or nitric acid and adsorbed moisture that independently catalyze the cure. Thorough washing, neutralization, drying, and moisture verification by TGA or Karl Fischer titration are required before the test to ensure observed effects stem from bound functional groups.

How is dispersion-related error avoided?

A fixed, documented dispersion protocol—identical sonication energy, mixing speed, and duration for every batch—is applied to all samples, and dispersion quality is verified by electron microscopy. This prevents attributing curing differences to agglomeration artifacts rather than genuine chemical effects.

What standards govern this test program?

Primary references include ISO 11357, GB/T 19466, and ASTM E1356 for DSC and Tg, ASTM E698 for kinetic parameters, ISO 6721 and ASTM D7028 for DMA, ASTM D4473 for rheological gel behavior, and ISO 11358 and ASTM E1131 for thermogravimetric analysis. Testing can be conducted in-house or delegated to accredited third-party laboratories for independent certification.