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

Significance of the Topic

Modern paper consists of various fibers, fillers and additives that define its mechanical, optical and printing properties. Defects such as contaminants or inclusions often evade visual detection, yet they can lead to product failure and quality issues.

Aims and Overview of the Study

This work demonstrates the application of FTIR microscopy for rapid identification and spatial mapping of impurities and punctual defects in paper. Two case studies highlight the detection of surface contamination and the analysis of a localized defect, emphasizing sensitivity and resolution.

Instrumentation Used

• LUMOS II FTIR microscope (Bruker) with automated ATR measurement
• OPUS spectroscopy software and OPUS/SEARCH for spectral evaluation
• ATR-COMPLETE spectra library for reference spectra
• Digital libraries for mixture analysis

Methodology

The ATR-enabled FTIR microscope collects infrared spectra with lateral resolution down to 50 μm. Two measurement schemes were applied:

• Grid measurement on a 2.25×1.75 mm area (47×35 points, 50×50 μm each) to locate and characterize a barely visible contamination.
• Mapping measurement on a 2×3 mm area (100×100 μm resolution) to identify and image a punctual defect and filler distribution.

Spectral analysis involved library search and mixture deconvolution to assign chemical identity to each spectrum. Chemical images were generated by integrating diagnostic bands and applying Winner-Takes-All color coding.

Main Results and Discussion

Case Study 1: Surface Contamination

• The contaminant spectrum exhibited bands at 1100 cm-1 and 805 cm-1.
• Mixture analysis identified calcium carbonate and a modified silicate (Unisil).
• Chemical imaging of the 1100 cm-1 band revealed the contaminant distribution with high contrast.

Case Study 2: Punctual Defect

• Spectrum of the defect contained cellulose bands plus additional bands indicating a fatty acid ester (Struktol) and ethylene-vinyl acetate polymer (Evatane).
• Winner-Takes-All color mapping of the 1738 cm-1 carbonyl band distinguished the defect (yellow) from calcium carbonate filler (red).

Benefits and Practical Applications

• Non-destructive and minimal sample preparation with ATR technique.
• Rapid localization and identification of microscopic defects in paper.
• Quantitative mapping of fillers and coatings to assess homogeneity.
• Valuable tool for quality control, failure investigation and research in paper production.

Future Trends and Opportunities

• Integration of AI-driven spectral analysis for automated defect classification.
• Expansion of comprehensive spectral libraries to cover new paper additives.
• Advancements in spatial resolution and coupling FTIR with microscopy modalities.
• In-line monitoring of paper production for real-time quality assurance.

Conclusion

FTIR microscopy with automated ATR measurement and advanced spectral tools enables precise chemical identification and mapping of impurities and inclusions in paper. This approach enhances failure analysis workflows and supports quality control in the pulp and paper industry.

References

• Bruker Application Note AN M137: Failure Analysis of Paper: Determination of the Chemical Identity of Impurities and Inclusions, Bruker Optics, 2021.