Optimizing tire performance through proper chemical composition with FTIR

Significance of the topic

Carbon black rubbers (CBRs) are a ubiquitous component of automotive tires and other rubber goods where they improve mechanical robustness and assist heat dissipation. Reliable chemical characterization of CBRs is essential for optimizing tire performance, durability and manufacturing quality control. Traditional transmission FTIR struggles with CBRs because of their opacity and high refractive index; attenuated total reflectance (ATR) FTIR, combined with appropriate corrections and data-analysis tools, provides a practical route to rapid, surface-sensitive compositional analysis that can support formulation verification and failure investigations.

Objectives and overview of the application note

This application note demonstrates an optimized workflow for analyzing carbon black rubbers by FTIR-ATR. The aims are to (1) explain the physical reasons for spectral distortion when using different ATR crystals, (2) show how an advanced ATR correction enables comparison with transmission libraries, and (3) illustrate how multi-component spectral searching can resolve composite formulations (e.g., base polymer plus silane slip aid). The study compares diamond and germanium ATR crystals and presents a complete analysis using Thermo Scientific OMNIC Specta software.

Methodology and analytical approach

The study acquires ATR spectra of CBR samples using FTIR spectrometers (examples: Nicolet iS20) with both diamond and germanium (Ge) ATR crystals. Spectra are baseline-corrected and processed in OMNIC Specta. Three processing conditions are compared: raw baseline-corrected ATR, traditional vendor ATR correction, and Thermo Scientific Advanced ATR Correction which models wavelength-dependent changes in the sample refractive index near strong absorptions. Resulting corrected ATR spectra are searched against standard transmission libraries first with a simple best-match search and then with OMNIC Specta’s multi-component search algorithm to separate overlapping contributions from multiple components.

Instrumentation used

• FTIR spectrometers: Thermo Scientific Nicolet family (examples cited: iS20; experiment also feasible on iS5, iS10, iS50).
• ATR crystals: diamond ATR accessory and germanium (Ge) ATR accessory compared.
• Software: OMNIC Specta for spectral processing, Advanced ATR Correction algorithm, and multi-component search capability.

Key theoretical points relevant to ATR of CBRs

• ATR probes the sample using an evanescent wave that penetrates a short distance (depth of penetration dp) into material in contact with the crystal. dp depends on wavelength, incidence angle and the refractive indices of the crystal and the sample (dp ∝ λ / {nC sqrt(sin^2θ – (nS/nC)^2)} in simplified form).
• If the sample refractive index approaches that of the ATR crystal (nS ≈ nC), the mathematical expression for dp can break down and ATR interaction is reduced or altered, producing spectral artifacts.
• Diamond has a relatively lower refractive index contrast with CBRs and yields larger dp (e.g., ~2 μm at 1000 cm−1), which, for high-index CBRs, can cause distorted bands and poor ratioing of crystal absorptions. Ge has a higher index, producing a shallower dp (~0.7 μm at 1000 cm−1) and cleaner ATR spectra for CBRs.

Main results and discussion

• Raw ATR spectra of CBRs collected on diamond showed pronounced spectral artifacts (distorted peaks and residual diamond bands) particularly near strong absorption regions, attributable to index-matching and resulting changes in the light path.
• Ge ATR spectra were substantially cleaner with well-defined peaks and absence of the diamond-related artifacts, due to Ge’s higher refractive index and shallower penetration depth.
• Applying the Advanced ATR Correction in OMNIC Specta produced spectra comparable to transmission data by explicitly accounting for refractive index changes near absorption maxima. This correction outperformed conventional ATR corrections that apply only simple intensity scaling.
• Simple library search of the corrected spectrum identified the base polymer successfully but left unexplained peaks. The multi-component search resolved the spectrum into two components: the primary polymer and a silane slip aid, providing a near-perfect composite match to the measured spectrum.

Benefits and practical applications

• Using Ge ATR with advanced correction yields reliable, transmission-comparable spectra for high-index, strongly absorbing samples such as CBRs without the need for complex sample preparation.
• The combined hardware-software workflow (appropriate crystal choice, advanced ATR correction, and multi-component spectral search) enables identification of formulations and minor additives on the surface or near-surface of rubber parts — valuable for QA/QC, incoming-material inspection, failure analysis, and formulation development.
• Fast, non-destructive ATR sampling with robust processing shortens analysis time and allows use of existing transmission libraries for identification, increasing laboratory efficiency.

Limitations and practical recommendations

• Diamond ATR is mechanically robust and chemically inert but may produce artifacts with materials that have refractive indices close to diamond. Evaluate sample refractive index when choosing the ATR crystal.
• Good contact between sample and crystal is essential; use appropriate pressure devices to ensure reproducible spectra.
• Advanced ATR corrections rely on models of index variation; validation with standards or complementary techniques is recommended when precise quantitative results are required.

Future trends and possible applications

• Improved modeling of wavelength-dependent optical constants and more sophisticated correction algorithms will further close the gap between ATR and transmission spectra for challenging materials.
• Integration of ATR-corrected spectra with machine learning and chemometric multi-component deconvolution will enhance detection of trace additives, contaminants and degradation products in complex polymer matrices.
• Portable and in-line ATR-FTIR systems with advanced corrections and automated multi-component search capability could enable real-time quality control on manufacturing lines for rubber and tire production.

Conclusion

For carbon black rubbers, germanium ATR combined with an advanced ATR correction produces clean, transmission-comparable spectra where diamond ATR often yields distorted results. When corrected spectra are searched using powerful multi-component algorithms such as OMNIC Specta’s, both the base polymer and minor additives (e.g., silane slip aids) can be reliably identified. Selecting the appropriate ATR crystal and applying advanced spectral corrections significantly improves the utility of ATR-FTIR for challenging high-index polymeric materials, supporting routine compositional analysis in tire and polymer laboratories.

References

1. Thermo Scientific Application Note AN01153: Details on Advanced ATR Correction methodology.
2. Thermo Scientific Technical Note TN51506: Information regarding silane additives and their spectral signatures.
3. Thermo Fisher Scientific product literature for Nicolet FTIR systems (iS5, iS10, iS20, iS50) and OMNIC Specta software (instrumentation noted in the application note).