Failure Analysis of Paper: Determination of the Chemical Identity of Impurities and Inclusions

Significance of the Topic

The quality and performance of paper products are critically influenced by microscopic impurities and inclusions. Identifying the chemical nature and distribution of these defects is essential for failure analysis, process optimization, and ensuring product consistency. Fast and accurate characterization methods help manufacturers reduce waste, improve printability, and maintain high standards in industrial and archival applications.

Study Objectives and Overview

This study demonstrates how FTIR microscopy can be applied to locate, identify, and map impurities and inclusions in paper. Two practical examples are presented: the analysis of a barely visible surface contamination and the investigation of a localized defect within the paper matrix. Emphasis is placed on minimal sample preparation, automated measurements, and comprehensive data processing workflows.

Methodology and Instrumentation

Measurements were performed with the Bruker LUMOS II FTIR microscope, which offers full automation and software control via the OPUS video wizard. Key features include:

• Attenuated total reflection (ATR) sampling for rapid contact measurements without extensive sample preparation
• Automated grid or mapping measurements with selectable lateral resolution down to 50–100 µm per spectrum
• Integration with OPUS spectroscopy software, including OPUS/SEARCH for library matching and ATR COMPLETE reference spectra
• Mixture analysis functions to deconvolute overlapping spectral contributions

Main Results and Discussion

Example 1 – Surface Contamination:
A 47×35 point grid covering 2.25×1.75 mm was acquired. Spectra from clean and contaminated regions revealed additional bands at 1100 cm–1 and 805 cm–1 in the defect. Library search and mixture analysis identified calcium carbonate and a modified silicate (Unisil) as contaminants. A chemical image generated by integrating the 1100 cm–1 band provided clear localization and semi-quantitative insight into coverage density.

Example 2 – Punctual Defect:
A 2×3 mm area was mapped at 100×100 µm resolution. Spectra showed typical cellulose and calcium carbonate signals, plus additional bands corresponding to a fatty acid ester (Struktol) and ethylene-vinyl acetate polymer (Evatane). A mixture analysis confirmed these components. A color-coded winner-takes-all (WTA) image displayed the defect distribution in relation to the filler, highlighting regions dominated by the polymeric inclusion versus calcium carbonate filler.

Benefits and Practical Applications

FTIR microscopy with the LUMOS II enables:

• Rapid localization and identification of microscopic defects in paper
• High lateral resolution mapping for quantitative distribution studies of fillers and coatings
• Minimal sample preparation and fully automated workflows suitable for routine quality control
• Accurate mixture analysis leveraging extensive spectral libraries

Future Trends and Possibilities

Advancements that can further enhance paper failure analysis include:

• Integration of machine learning for automated spectral classification and anomaly detection
• Expansion of reference libraries to cover emerging additives and recycled materials
• Higher spatial resolution through novel ATR crystal designs or coupling with nanoscale IR techniques
• Multi-modal imaging combining FTIR with Raman or hyperspectral optical methods

Conclusion

The Bruker LUMOS II FTIR microscope offers an efficient and reliable platform for failure analysis in paper production. Its automated ATR sampling, high-resolution mapping, and advanced data processing allow rapid chemical characterization of impurities and inclusions, supporting quality assurance and process development in the paper industry.

Reference

Application Note AN M137, Bruker Optics, 2021