Analysis of Discoloration and Coloration Using FTIR and EDX

Importance of the Topic

Understanding the chemical origins of discoloration and coloration in materials is critical for quality control across industries including paper manufacturing and plastic production. Rapid and non-destructive methods enhance root-cause investigations and reduce processing time without sample pre-treatment.

Aims and Overview of the Study

The article demonstrates a combined analytical approach to identify color-related contaminants and pigments in situ. The specific goals are:

• To determine the cause of minor discoloration in paper using FTIR spectral difference analysis.
• To investigate resin product discoloration via EDX elemental profiling.
• To identify coloring compounds in white and green resins using complementary FTIR and EDX techniques.

Methodology and Instrumentation

This study applies direct measurement without extraction, combining:

• Fourier transform infrared spectroscopy (FTIR) with ATR sampling (IRTracer-100 and MIRacle 10 diamond prism), 4 cm⁻¹ resolution, 40 scans, DLATGS detector.
• Energy-dispersive X-ray fluorescence (EDX) spectrometry (EDX-8000 with Rh target), operated up to 50 kV under vacuum, analysis diameters from 1 to 10 mm.

Results and Discussion

Case 1: Discolored paper – Difference ATR-FTIR spectra revealed a peak at ~1750 cm⁻¹ indicating a C=O bond. Spectral database matching identified paraffin oil containing a discoloration-causing additive adhered to paper.

Case 2: Discolored resin product – EDX profiles from discolored and normal areas showed 99.8% Al and 0.2% Fe exclusive to the defect region. This suggests contamination by aluminum alloy fragments, likely from a cutting edge.

Case 3: Resin coloration – FTIR confirmed polyethylene as the base polymer in both white and green resins. EDX detected Si, P, Al, and S in both samples (functional additives) and Cl and Cu only in green resin. Element ratios matched phthalocyanine green pigment composition.

Benefits and Practical Applications

The combined FTIR/EDX workflow offers non-destructive, rapid identification of organic and inorganic discoloration agents, streamlining quality assurance and failure analysis in material production.

Future Trends and Possibilities

Advancements may include integration with automated software for real-time spectral subtraction and library search, enhanced spatially resolved analysis, and expansion to other spectroscopic modalities for broader contaminant identification.

Conclusion

Direct ATR-FTIR difference spectroscopy effectively identifies organic discolorants, while EDX excels at detecting inorganic contaminants and pigments. Their complementary use provides a robust toolset for investigating material coloration issues without sample preparation.

Instrument Used

• IRTracer-100 FTIR spectrometer with MIRacle 10 ATR accessory
• EDX-8000 energy-dispersive X-ray fluorescence spectrometer with Rh target

References

• Shimadzu Application News No. X255