Curing of an acrylate – Rheology with simultaneous FTIR spectroscopy

Importance of the topic

The timing of adhesive curing—pot life, open time, time for adjustments and time to reach maximum bond strength—directly determines process design, operator actions and end-use performance. Reliable, high-resolution information about both mechanical evolution and concurrent chemical changes during cure is essential for formulation development, quality control and troubleshooting. Combining rheology and molecular-level spectroscopy on the same sample removes ambiguity introduced by separate measurements and enables a mechanistic understanding of curing kinetics and structure formation.

Objectives and study overview

This application note demonstrates simultaneous measurement of rheological properties and FTIR spectra during curing of a consumer-grade two-component acrylate adhesive. The combined approach aims to record what happens mechanically (viscoelastic transition) while simultaneously revealing why it happens chemically (monomer consumption and polymer formation) on the identical sample under identical conditions. The work uses an oscillatory time-sweep protocol to follow cure dynamics on the HAAKE MARS rheometer equipped with the Rheonaut module coupled to an FTIR spectrometer.

Methodology and test protocol

The test was explicitly optimized for curing systems where early-time dynamics and handling reproducibility are critical. Key methodological elements:

• Time-zero reset: The internal time is reset at the instant the two components are mixed outside the rheometer to avoid offsets caused by loading variability.
• Minimized setup delay: The upper geometry is prepositioned to a 10 mm gap before sample loading to reduce lift travel and shorten the interval to reach the measuring gap.
• Immediate start: The oscillation time sweep begins as soon as the measuring gap is reached without prior thermal/mechanical equilibration to capture the fastest cure stages.
• Rheology settings: An oscillatory time sweep in controlled deformation (CD) mode was used with a small amplitude inside the linear viscoelastic range (LVR) to maintain optimal signal-to-noise as moduli change by orders of magnitude. Evaluation parameters include storage modulus (G’), loss modulus (G”), complex viscosity (|η*|) and phase angle (δ).
• Simultaneous spectroscopy: FTIR spectra were recorded roughly every 13 s, giving 115 spectra over the 25 min rheology run, enabling time-resolved chemical monitoring aligned with rheological events.

Instrumentation used

The experimental setup comprised the following main instruments and software:

• Thermo Scientific HAAKE MARS Rheometer (mechanical rheology).
• Rheonaut module (temperature-control module integrated with an ATR cell and its own IR detector) enabling in-situ ATR-FTIR on the rheometer sample.
• Thermo Scientific Nicolet iS20 FTIR spectrometer (FTIR data acquisition).
• OMNIC Spectroscopy Software with OMNIC Series add-on for chronological 3D spectral visualization and extraction of time-dependent absorbance profiles.

Main results and discussion

Rheological observations:

• Initial state: The freshly mixed acrylate behaves mainly viscous with G” > G’ and phase angle δ ≈ 70° (pure viscous = 90°, pure elastic = 0°).
• Gel point: A crossover (G’ = G” or δ = 45°) occurs at approximately 3.2 min, marking the transition to an elastic-dominated network where macroscopic movement and join adjustment must already be finished.
• Elastic plateau: By 10 min δ falls to about 3° and G’ approaches an almost constant plateau, indicating the material has achieved its practical final stiffness; however, slow residual curing continues beyond this window, with full strength developing over 12–24 h.

The FTIR time-resolved chemistry:

• 115 spectra collected at ~13 s intervals allow tracking of functional-group concentration changes.
• The peak at 1637 cm⁻¹, assigned to the C=C stretch of the acrylate monomer, steadily decreases, directly indicating monomer consumption via radical polymerization.
• The band at 1241 cm⁻¹, associated with ester linkages (O=C–O–C) in the forming polymer, increases over time, confirming polymer formation.

Combined interpretation:

• Correlation of rheology and FTIR shows the initial rapid rise of moduli corresponds to rapid monomer consumption—network formation driven by free-monomer reactions.
• After G’ plateaus (~10 min), the rate of monomer consumption decreases markedly due to reduced diffusivity in the developing solid network. The ester-bond growth continues but at a slower rate; the polymer-band increase proceeds roughly twice as fast as the residual monomer decrease, indicating intramolecular and network internal rearrangements become relatively more important in the late cure stage.
• This combined dataset explains both the timing constraints for assembly (open time before gel) and the mechanistic drivers of final-property development, enabling targeted formulation or process changes (e.g., adding reactive monomer vs. raising cure temperature to increase mobility).

Benefits and practical applications

Advantages of simultaneous rheology–FTIR on the same sample:

• Direct alignment of mechanical and chemical time scales eliminates ambiguity caused by running separate tests on different samples.
• Reduces sample preparation steps, improves throughput and lowers analysis cost and variability.
• Enables mechanistic insight that supports rational formulation optimization, troubleshooting of out-of-spec batches, and tailored process parameter selection (mix ratios, temperature, fillers, inhibitors).
• Provides quantitative markers (gel time, modulus evolution, spectral band kinetics) that can be used in QC specifications and to guide processing windows in manufacturing.

Future trends and potential applications

Potential extensions and developments building on this combined approach include:

• Integration with inline or at-line process analytical technology (PAT) for manufacturing control and closed-loop optimization of cure schedules.
• Application of multivariate and chemometric methods to extract kinetic parameters and predictive models from coupled rheology–spectroscopy datasets.
• Coupling to other spectroscopies (Raman, NIR) or thermal analysis for complementary information on different functional groups or phase changes.
• Use with a wider range of curing chemistries (epoxies, polyurethanes, silicones, UV-cured systems) and filled formulations to inform additive and filler effects on cure kinetics and mechanics.
• Real-time decision-making in robotic assembly or automated dispensing where precise open time control is required.

Conclusion

Simultaneous rheological and FTIR measurements using a Rheonaut-equipped HAAKE MARS rheometer and FTIR spectrometer deliver a comprehensive picture of adhesive curing by coupling macroscopic mechanical evolution with molecular-scale chemical changes. The approach captures critical processing time points (e.g., gel time), explains mechanisms driving modulus build-up, and supports targeted formulation and process interventions. The methodology increases data quality, reproducibility and efficiency compared with separate measurements and has broad applicability for adhesive development, quality control and process optimization.

References

• Thermo Fisher Scientific, Application Note: Curing of an acrylate – Rheology with simultaneous FTIR spectroscopy, No. V254, 2020. (Describes experiments using HAAKE MARS Rheometer, Rheonaut module and Nicolet iS20 FTIR; Rheonaut developed by Resultec for Thermo Fisher Scientific.)