Monitoring the UV Cure Process of a Polymer Based Ink by FT-IR

Significance of the topic

The UV curing state of polymer-based inks directly affects final product appearance, mechanical properties and durability. Fast, surface-selective analytical methods that quantify degree of cure are therefore essential for quality control (QC), process optimization and troubleshooting in printing and coatings manufacture. Fourier transform infrared spectroscopy (FT-IR) with attenuated total reflectance (ATR) provides a practical, rapid approach to monitor chemical changes during photopolymerization at the ink–substrate interface.

Objectives and overview of the study

This application note demonstrates an FT-IR/ATR method to quantify the percent cure of a UV-curable ink screen-printed onto Mylar film. The study aims to: (1) identify spectral markers that report on free monomer versus cured polymer, (2) develop a simple ratio metric using an internal standard peak, and (3) show a calibration relationship between that spectral metric and percent cure to support QC and process optimization.

Methodology

Sample handling and measurement approach:

• Ink was screened onto Mylar film and exposed to controlled UV irradiation to produce samples with various cure levels.
• No chemical preparation was required; the inked side of the Mylar was pressed directly onto the ATR crystal to maximize surface sensitivity.

Spectral acquisition and processing:

• FT-IR spectra were acquired at 4 cm-1 resolution using 32 co-added scans (≈40 s acquisition).
• ATR sampling geometry (multi-bounce horizontal ATR) was chosen to preferentially sample the ink surface and reduce Mylar bulk contributions.
• A simple baseline correction point at 895 cm-1 was applied and a software macro computed the ratio of peak heights 810 cm-1 / 830 cm-1 for each spectrum.

Instrumentation used

The analysis employed the following equipment and software:

• Thermo Scientific Nicolet FT-IR spectrometer.
• Smart Multi-Bounce horizontal ATR accessory with a zinc selenide (ZnSe) crystal.
• OMNIC Macros\Basic software to perform baseline correction and calculate the 810/830 cm-1 peak-height ratio automatically.

Main results and discussion

Key spectral findings and interpretation:

• The band at 810 cm-1 corresponds to free acrylate monomer in the uncured ink. Its intensity decreases as polymerization proceeds.
• The 830 cm-1 band remains essentially unchanged during cure and therefore serves as an internal standard to normalize sample-to-sample variation.
• By calculating the ratio of the 810 cm-1 to 830 cm-1 peak heights (with baseline correction at 895 cm-1), a monotonic decrease in this ratio was observed with increasing percent cure. Spectra autoscaled on the 830 cm-1 absorbance show a clear drop in 810 cm-1 intensity from uncured to cured samples.
• A linear relationship was obtained between the ratioed spectral metric and independently determined percent cure (reported up to 87% in the example series), enabling quantitative estimation of cure state from single spectra.

Practical considerations and limitations:

• ATR sampling effectively minimizes signal from the Mylar substrate, but the measurement remains surface-biased and is most sensitive to the near-surface extent of cure.
• Accurate percent-cure assignments depend on a representative calibration set and consistent sampling/contact pressure on the ATR crystal.
• Matrix effects (pigments, additives, film thickness) can affect absolute band intensities; use of the internal standard helps mitigate but may not fully remove these influences.

Benefits and practical applications of the method

This FT-IR/ATR approach offers several advantages for industrial QC and R&D:

• Rapid, non-destructive measurements with minimal sample preparation.
• High surface sensitivity appropriate for thin film inks and coatings.
• Simple spectral metric (810/830 cm-1 ratio) that can be automated for high-throughput screening.
• Enables process optimization by linking percent cure to desired physical properties, supporting tuning of UV dose, formulation and production parameters.

Future trends and applications

Areas where this methodology can be extended or improved:

• Integration into in-line or at-line process control using portable or fiber-coupled FT-IR/ATR probes for real‑time monitoring.
• Application of chemometric models (multivariate calibration, PLS) to incorporate more spectral information and improve robustness against formulation variability.
• Use of ATR imaging or micro-ATR mapping to assess spatial heterogeneity in cure across printed patterns.
• Combination with other process sensors (UV radiometry, temperature, mechanical testing) to build multidimensional control strategies.
• Advances in detector and accessory design to increase signal-to-noise, reduce measurement time, and extend usable spectral range for alternative chemistries.

Conclusion

FT-IR with multi-bounce ATR sampling provides a fast, practical and surface-sensitive method to quantify UV cure in polymer-based inks. Monitoring the 810 cm-1 band (free acrylate) relative to a stable 830 cm-1 internal standard yields a reproducible spectral metric that correlates linearly with percent cure. The approach supports routine QC, formulation development and process optimization with minimal sample handling and straightforward automation.

References

Thermo Fisher Scientific, Application Note AN50752: Monitoring the UV Cure Process of a Polymer Based Ink by FT-IR, 2007.