Analysis of Resin Using FTIR and Thermal Analysis - "Silent Change" -

Importance of the Topic

The reliability of polymer-based parts in mechanical systems is often compromised when material compositions are altered without client approval, so-called silent changes. Such substitutions can lead to premature wear or failure, as demonstrated by resin motor gears in various industrial applications.

Objectives and Study Overview

This study aimed to identify the root cause of unexpected gear wear by comparing proper and failed resin motor gears. The investigation employed both spectroscopic and thermal techniques to characterize the resin type and its structural properties. Additionally, a comparative analysis of crystalline and amorphous PET samples was conducted to illustrate the methods’ versatility.

Methodology and Instrumentation

The analysis comprised three main techniques:

• Fourier-transform infrared spectroscopy (FTIR) for resin identification.
• Simultaneous thermogravimetric analysis and differential thermal analysis (TG/DTA) for thermal stability and melting behavior.
• Differential scanning calorimetry (DSC) to assess crystallinity in PET samples.

Instrumentation Used

• IRTracer-100 FTIR spectrophotometer (Shimadzu), DuraScope accessory, 4 cm−1 resolution.
• Simultaneous TG/DTA system (Shimadzu) for weight loss and thermal events.
• DSC unit (Shimadzu) for precise measurement of glass transition, crystallization, and melting points.

Main Results and Discussion

FTIR spectra of both proper and failed gears exhibited a strong absorbance between 1100 and 800 cm−1, consistent with polyacetal (POM) C–O–C stretching, but showed no clear distinction between homopolymer and copolymer forms.
The TG/DTA results revealed that the proper gear melted at 172.3 °C, while the failed gear melted at 166.9 °C. A higher onset decomposition temperature for the failed gear indicated greater thermal resistance. These findings suggest that the proper gear comprised a homopolymer POM with higher crystallinity, whereas the failed gear was a copolymer with lower crystallinity and reduced mechanical strength.
In the PET comparison, FTIR identified subtle differences in band intensities around 1400–1300 cm−1 and 1100–800 cm−1. DSC measurements showed the crystalline PET melting at 250.3 °C, while the amorphous PET exhibited a glass transition at 79.1 °C, crystallization at 159.6 °C, and melting at 249.5 °C, confirming DSC’s sensitivity to crystallinity variations.

Benefits and Practical Applications

The combination of FTIR and thermal analysis provides a robust approach for detecting silent material changes in polymer parts. FTIR offers rapid resin identification, while TG/DTA and DSC enable differentiation between polymer variants based on melting behavior and thermal stability. This protocol is valuable for quality control in manufacturing, failure analysis, and material certification.

Future Trends and Applications

Advancements may include hyphenated techniques such as TG-FTIR or DSC-FTIR for simultaneous molecular and thermal insights, integration of chemometric models for automated classification, and real-time monitoring systems for inline quality assurance. Emerging microspectroscopy and thermal imaging modalities could further enhance resolution and spatial analysis of polymer components.

Conclusion

The study confirmed that the unexpected wear of the resin motor gear resulted from an unauthorized switch to copolymer POM, which exhibits lower crystallinity and mechanical strength than homopolymer POM. The FTIR and thermal analysis workflow effectively detected these differences and can prevent failures due to silent material changes.

Reference

• Handbook of Polymer Analysis, p. 481–482
• Handbook of Polymer Analysis, p. 903