Determining the oxidation index of a polyester-melamine coil-coating with an FTIR microscope - Top Analytica

Determining the oxidation index of a polyester-melamine coil-coating with an FTIR microscope

21.01.2025

Abstract

FTIR is a powerful and rapid technique for identifying molecular vibrations through absorption of electromagnetic radiation, often allowing for quick insights into molecular structure. In combination with light microscopy, the micro-scale applications of this technique can be expanded considerably. Herein, we discuss the functioning of the FTIR microscope in general, along with a practical use case in the determination of the oxidation index of a naturally weathered polyester-melamine coil-coating on metal, a system with real-world relevance across numerous industries.

Background

In FTIR, measurement time is saved through simultaneous data collection across a wavelength range and use of the Fourier transform in signal processing. As the longer wavelengths absorbed in IR-spectroscopy are less energetic compared to, for instance, those used in UV-Vis, the relevant states are vibrational rather than electronic in nature (see Fig 1). The twisting, bending, and stretching of various groups in given environments can be identified through wavenumber and peak shape. In the low wavenumber region, peaks specific to an analyzed molecule can be found. This region can also provide clues about larger-scale phenomena, such as crystallinity.1

Compared to a traditional FTIR machine, an FTIR microscope offers advantages in relating the molecular effects measurable in FTIR to larger-scale properties of the sample. With an FTIR microscope, one has an overview of the sample, and collecting spectra from specific defined points on the surface is simple. As such, visible regional defects in a sample can be identified and measured separately on a range of tens of micrometers. The ability to measure a spectrum in a small area is particularly beneficial in measurements of cross-sections of paint layers, identification of contaminants, and picking out, for instance, fibers in a larger matrix. Measured FTIR spectra are also commonly compared to libraries, both general and more specific user-generated ones to identify matching previously measured spectra.

Figure 1. A Jabłoński energy diagram of molecular states.2 After excitation, relaxation occurs through various transitions. Non-radiative transitions are indicated as open, red arrows and radiative transitions as filled arrows.

Weathering is driven by a range of different factors such as exposure to UV-radiation, humidity, temperature extremes and variations, pollution, various ions, and mechanical factors.3,4 As such, it is also affected by local soil and wind conditions.3 Weathering of polyester melamine coatings, among other effects, leads to an increase in the oxidation index,4 the calculation of which is covered by equation 1 below.

Methods

All samples were dried in a desiccator for 24 hours and six pairs of samples and references were measured. Representative regions were chosen, avoiding visible granules and excessive surface roughness in the naturally weathered polyester-melamine coatings (see Figure 2). Visually, the reference and weathered samples can be distinguished through the difference in color, with the reference being darker (see Figure 3). The oxidation index was calculated based on the change, from the reference to the weathered sample, in the area of the peak corresponding to NH and OH groups in the region 3750-2400 cm-1, as related to the stable CHX-peak in the region 3020—2680 cm-1 (see eqn. 1).3

(1).

Figure 2. Selecting a measurement locale in the FTIR microscope.

Further signal processing and calculations such as returning results in absorbance, smoothing, data tune-up, applying a predefined interactive baseline, normalization, area calculations, and returning the data to comma-separated-value-files, were automated through the macros of the Spectrum 10 software. The process of importing these data and calculating the six oxidation indices, as well as an average and a standard deviation, was also automated in a Microsoft Excel macro. All FTIR-measurements were performed on a PerkinElmer Spectrum Two FT-IR Spectrometer connected to a PerkinElmer Spotlight 200 FT-IR Microscopy System.

Results

The difference between the weathered samples and the references is apparent from the IR spectra, and the similar peak shape of the CH-peak across the board confirms the stability of this peak under weathering (see Figure 4). The increased peak area of the peaks related to hydroxyl groups indicates more oxidation of the weathered samples, as compared to the references, with an acceptable amount of variance within the weathered samples and almost none within the references. Given as a numerical value, the oxidation index of this particular weathered coating was found to be
 \mu_{\text{OHind}} = 0.48
with a standard deviation of
 \sigma_{\text{OHind}} = 0.08.

Figure 4. Spectra of weathered (red) and reference (blue) samples. In lighter red and blue the upper and lower bounds of the experimental data as an overestimate of the error. The small apparent peak around 2200 cm-1 is caused by disturbance from variation in gaseous CO2.

Conclusions

FTIR measurements of naturally weathered and reference samples of polyester-melamine coil-coated sheet metal were performed. Based on these, a procedure for automated signal processing, the calculation of relevant peak areas, and calculation of an oxidation index was developed and applied, yielding a value of 1.02 and a standard deviation of 0.104 for the oxidation index. Based on research by others, comparable values would be expected after 800 hours of artificial xenon arc weathering of the same material.5

While comparison to other naturally weathered samples would be of interest, their inherently greater variability would likely present a challenge. While a linear trend in an increasing oxidation index with exposure time for the material under exposure to xenon arc weathering has been shown by others,5 the same trend has yet to be demonstrated in naturally weathered samples. Future work should focus on investigating this relationship. In this work as in the present work, the advantages of an FTIR microscope, as compared to a traditional FTIR spectrometer, will aid in measuring representative spectra and ultimately enable the determination of accurate oxidation indices.

Keywords: IR microscope, vibrational spectroscopy, oxidation index, automated data processing, µFTIR, µ-FTIR


References

  1. Diem, M. Modern Vibrational Spectroscopy and Micro-Spectroscopy: Theory, Instrumentation, and Biomedical Applications; John Wiley & Sons, Inc.: Chichester, West Sussex, 2015.
  2. Kraufvelin, N. Synthesis and Characterisation of Hydrogel Based on Polyvinyl Alcohol and Guar Gum Benzoic Acid with a View toward Uses in Biomedicine, Åbo Akademi University, Turku, 2024.
  3. Chesworth, W. Weathering Systems. In Developments in Earth Surface Processes; Elsevier, 1992; Vol. 2, pp 19–40. https://doi.org/10.1016/B978-0-444-89198-3.50007-6.
  4. Wärnheim, A.; Edvinsson, C.; Sundell, P.-E.; Heydari, G.; Deltin, T.; Persson, D. Depth-Resolved FTIR-ATR Imaging Studies of Coating Degradation during Accelerated and Natural Weathering─Influence of Biobased Reactive Diluents in Polyester Melamine Coil Coating. ACS Omega 2022, 7 (27), 23842–23850. https://doi.org/10.1021/acsomega.2c02523.
  5. Wärnheim, A.; Persson, D.; Kotov, N.; Dobryden, I.; Telaretti Leggieri, R.; Edvinsson, C.; Heydari, G.; Sundell, P.-E.; Deltin, T.; Johnson, C. M.; Claesson, P. M. Nanomechanical and Nano-FTIR Analysis of Polyester Coil Coatings before and after Artificial Weathering Experiments. Progress in Organic Coatings 2024, 190, 108355. https://doi.org/10.1016/j.porgcoat.2024.108355.

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