Curing of an acrylate – Rheology with simultaneous FTIR spectroscopy

Significance of the topic

The control of curing kinetics and mechanics is central to adhesive development and application. Accurate knowledge of pot life, open time, cure speed and the time to reach maximum bond strength determines manufacturability, application procedure and end-use performance. Combining macroscopic mechanical information with molecular-level chemical changes provides a comprehensive view needed to design formulations, troubleshoot batches and optimize processing conditions.

Objectives and study overview

This application note demonstrates simultaneous measurement of rheological and chemical changes during the curing of a two-component acrylate adhesive. The aim is to record, on the same sample and under identical conditions, (1) time-resolved viscoelastic properties (G', G", |η*|, phase angle) and (2) chemical conversion of functional groups by FTIR, enabling direct correlation between mechanical transitions and chemical reaction progress.

Methodology and test protocol

A consumer-grade two-component acrylate was mixed per manufacturer instructions and a portion transferred to the rheometer. The experimental protocol was optimized to capture the rapid initial cure:

• Internal time reset at moment of mixing to avoid timing offsets between runs.
• Upper geometry pre-positioned at 10 mm prior to loading to shorten lift travel and start time.
• No thermal or mechanical equilibration delay: the oscillatory time sweep begins immediately when the measuring gap is reached.
• Oscillatory time sweep under controlled-deformation (CD) mode using a small amplitude within the linear viscoelastic range to maintain signal quality across large modulus changes.
• Simultaneous FTIR spectra collected approximately every 13 s over a 25 min rheology run (≈115 spectra).

This setup captures the earliest and fastest changes in both rheological and spectroscopic signatures, which are critical for short pot-life systems.

Used instrumentation

• Thermo Scientific HAAKE MARS Rheometer with oscillatory measurement capability.
• Rheonaut module (ATR-FTIR cell with dedicated IR detector and temperature control) developed by Resultec for integration with the rheometer.
• Thermo Scientific Nicolet iS20 FTIR spectrometer (data collected on a previous-generation spectrometer is also noted).
• Thermo Scientific OMNIC Spectroscopy Software with OMNIC Series add-on for time-resolved 3D spectral visualization and extraction of absorbance-time profiles.

Main results and discussion

Rheological observations:

• The freshly mixed adhesive is primarily viscous (G" > G'; phase angle ≈ 70°).
• Gel point (G' = G" or δ = 45°) occurs at ~3.2 min, after which elastic behavior predominates as a network forms.
• By 10 min the phase angle falls to ≈3° and G' reaches a near plateau; full mechanical maturation continues slowly over 12–24 h.

Spectroscopic observations and correlation:

• FTIR band at 1637 cm⁻¹ (C=C of acrylate monomer) decreases monotonically, indicating monomer consumption via radical polymerization.
• Band at 1241 cm⁻¹ (ester C–O/C=O features associated with growing polymer) increases, reflecting polymer formation.
• Time-aligned plots show the initial rapid increase of moduli coincides with the fastest monomer depletion and polymer bond formation.
• After G' plateaus (~10 min) monomer consumption slows markedly due to reduced mobility in the solidifying network; polymer-related band growth persists but at a reduced rate, implying intramolecular or diffusion-limited reactions dominate late-stage curing.

Interpretation:

• Simultaneous datasets reveal the mechanistic link between chemical conversion and mechanical stiffening—early cure is controlled by monomer conversion and network build-up, while late cure is limited by reduced molecular mobility and local reorganization.
• These insights inform targeted formulation adjustments (e.g., monomer content, initiator level, temperature control) to tune open time and final bond properties.

Benefits and practical applications

Key advantages of the combined rheology–FTIR approach demonstrated here:

• Identical-sample, time-synchronized mechanical and chemical data eliminate uncertainties from separate sample preparations and conditions.
• Faster overall workflow and reduced sample handling compared with separate analyses.
• Direct, quantifiable links between molecular conversion and macroscopic properties facilitate rational formulation development, process optimization and troubleshooting of out-of-spec batches.
• Relevant for R&D, QC of adhesive production, and application engineering where precise control of working time and final properties is needed.

Future trends and potential applications

Extensions and developments likely to increase the impact of simultaneous rheology–spectroscopy include:

• Integration with additional in situ probes (Raman, UV–Vis, dielectric) to capture complementary chemistries and reaction pathways.
• Temperature-ramp and multi-step cure protocols to simulate real processing conditions and reveal thermally activated pathways.
• Real-time chemometric and kinetic modeling to extract rate constants and predictive cure models for process control and formulation screening.
• Automated routines and AI-driven analysis to accelerate formulation optimization and quality control decisions.
• Application to multifunctional adhesives, composites, coatings and additive manufacturing resins where coupled chemical/mechanical evolution defines performance.

Conclusion

Simultaneous measurement of rheological properties and FTIR spectra using the HAAKE MARS Rheometer combined with the Rheonaut ATR module provides a powerful method to answer both what happens (mechanical evolution) and why it happens (molecular conversion) during adhesive curing. The approach yields time-resolved, co-registered datasets that improve formulation development, troubleshooting and process design while reducing analysis time and sample-related variability.

Reference

Thermo Fisher Scientific. Curing of an acrylate – Rheology with simultaneous FTIR spectroscopy. Application Note No. V254. 2020.