Polymerization cure rates using FT-NIR spectroscopy

Significance of the topic

Reaction rate and conversion are critical control parameters in polymer synthesis and adhesive curing because they determine process throughput, product performance and manufacturability. Rapid, non‑destructive analytical methods that track functional group disappearance in real time enable optimization of formulations, validation of curing protocols and quality control during production and R&D.

Objectives and study overview

This application note evaluates the capability of Fourier transform near‑infrared (FT‑NIR) spectroscopy to monitor polymerization cure kinetics of three commercially relevant adhesives. The study tracked spectral markers associated with reactive functional groups during cure for: an epoxy resin (amine‑cured), a methyl acrylate‑based adhesive (free‑radical initiated), and a rapid‑setting cyanoacrylate (moisture‑initiated anionic polymerization). The aim was to demonstrate temporal profiling of peak intensities to determine reaction progress and approximate cure completion times.

Methodology and instrumentation used

Samples were prepared according to manufacturer instructions and placed directly over the integrating sphere window of a Thermo Scientific Antaris II MDS FT‑NIR analyzer. Key acquisition and sample handling details:

• Spectral range: nominally 4000–10000 cm−1.
• Resolution: 4 cm−1; Norton‑Beer apodization.
• Integrating sphere geometry with internal gold flag used as background.
• Samples covered with a 0.5 mm polyethylene film; aluminum foil was used in some cases to promote beam reflection.
• Spectra were collected repeatedly over extended time periods (hours to days) and simple baseline correction was applied to measure peak heights at selected bands.

Main results and discussion

Epoxy resin (amine cure):
The epoxide absorption at 4528 cm−1 decreased steadily during monitoring. Baseline‑corrected peak heights showed a rapid decline during the first ~1000 minutes (~17 h) and approached a plateau by ~2000 minutes (~33 h), indicating near‑complete consumption of reactive epoxy groups within this period. This behavior is consistent with nucleophilic ring opening by amines and gradual network formation.

Acrylate (methyl acrylate copolymer; radical initiation):
The alkene first‑overtone C–H feature in the 6103–6234 cm−1 region (notably a band near 6167 cm−1) decreased markedly. The kinetics exhibited a biphasic profile: a rapid loss of peak intensity in the initial ~50 minutes followed by a slower decay extending to ~1200 minutes (~20 h). The shape of the time trace suggests at least two mechanistic regimes—fast chain propagation/termination early and slower diffusion‑limited or vitrification‑controlled conversion later.

Cyanoacrylate (moisture‑activated anionic polymerization):
The band at 4495 cm−1 (and additional bands at ~4742 and ~6208 cm−1) diminished quickly, with most change occurring in the first several hundred minutes and the process essentially complete by ~800 minutes (~13 h). This rapid profile aligns with the well‑known fast anionic polymerization of cyanoacrylates in the presence of moisture.

Across all systems, simple tracking of baseline‑corrected peak heights provided clear, quantitative trends that correlate with functional group consumption and cure progression. The differing time scales and kinetic shapes reflect chemistry (nucleophilic vs radical vs anionic) and physical effects such as diffusion and crosslinking density that slow conversion at advanced conversion.

Benefits and practical applications of the method

FT‑NIR monitoring in this context offers several practical advantages:

• Non‑destructive, minimal or no sample preparation required.
• Fast spectral acquisition enabling near‑real‑time kinetic profiles.
• Capability to monitor representative functional groups directly related to cure chemistry.
• Applicability for formulation screening, process optimization and quality control (end‑point detection, batch release).
• Feasibility for transfer to process analytical technology (PAT) implementations with fiber probes or inline sampling.

Instrumentation used

Instrumentation and key measurement parameters reported in the study:

• Thermo Scientific Antaris II MDS FT‑NIR analyzer with integrating sphere.
• Spectral range used for analysis: ca. 4000–10000 cm−1; acquisition resolution 4 cm−1.
• Norton‑Beer apodization and internal gold flag background for the integrating sphere.
• Sample handling: samples on 0.5 mm polyethylene film placed over the sphere; aluminum foil used to aid beam reflection where indicated.

Future trends and possibilities for use

Opportunities to extend and industrialize this approach include:

• Integration of chemometric and multivariate models (PLS, interval‑PLS) to convert spectral changes into quantitative conversion, residual monomer or crosslink density metrics.
• Deployment of ATR or fiber‑optic probes for inline/online monitoring within reactors or coating lines enabling true PAT control loops.
• Combining FT‑NIR kinetic data with rheology, calorimetry or mechanical testing to correlate chemical conversion with property development.
• Higher temporal resolution measurements for very fast chemistries and automated endpoint detection algorithms for manufacturing QC.
• Application to formulation development (additive effects, inhibitor levels, initiator efficiency) and lifetime/aging studies.

Conclusion

The Antaris II FT‑NIR system reliably tracked cure kinetics of three adhesive chemistries by monitoring the disappearance of characteristic near‑IR bands. Measured completion times were approximately 33 h for the amine‑cured epoxy, 20 h for the free‑radical acrylate system (with multiphase kinetics), and 13 h for the moisture‑activated cyanoacrylate. The study demonstrates FT‑NIR as an effective, low‑effort technique for kinetic monitoring, endpoint determination and process development in polymerization and adhesive curing applications.

Reference

Gabriela Budinova, Ivor Dominak, Todd Strother. Application note: Polymerization cure rates using FT‑NIR spectroscopy. Thermo Fisher Scientific, AN51548_E 03/22M. (Thermo Scientific Antaris II MDS FT‑NIR analyzer used in study.)