Optimizing tire performance through proper chemical composition with FTIR

Optimizing Tire Performance through Proper Chemical Composition with FTIR — Application Note Summary

Significance of the topic:

Carbon black rubbers (CBRs) are central to tire construction because they improve durability and conduct heat away from the tread. Verifying the chemical composition of CBRs is therefore critical for optimizing tire performance, lifetime and safety. However, standard infrared transmission methods are largely unsuitable because CBRs are opaque; attenuated total reflectance (ATR) FTIR methods overcome this limitation but require careful handling and corrections to yield reliable, library-searchable spectra.

Objectives and overview of the study/article:

The application note demonstrates a practical workflow for analyzing CBRs by ATR-FTIR. It compares diamond and germanium (Ge) ATR crystals, explains the origin of spectral artifacts specific to CBRs, and shows how Thermo Scientific software tools (Advanced ATR Correction in OMNIC Specta and the multi-component search) enable robust identification of polymer components and additives in complex CBR samples.

Methodology and theory:

- ATR principle: infrared light undergoing internal reflection in a high-refractive-index crystal generates an evanescent wave that samples only a shallow depth of the adjacent material. The depth of penetration (dp) depends on wavelength, incidence angle, and the refractive indices of the crystal (nC) and sample (nS).
- Practical implications: When the sample refractive index approaches that of the crystal (nS ≈ nC), dp changes abruptly and ATR spectral features can be distorted or lost, producing artifacts and incorrect band ratios.
- Crystals compared: diamond (n lower relative to Ge, dp ≈ 2 µm at 1000 cm−1) versus germanium (higher n, dp ≈ 0.7 µm at 1000 cm−1). Because CBRs have high refractive indices near those of diamond, diamond ATR often produces spectral distortion; Ge produces cleaner, better-defined peaks for CBRs.

Použitá instrumentace (Instrumentation used):

- FTIR platform: Thermo Scientific Nicolet series (examples: iS20; experiments also possible on iS5, iS10, iS50).
- ATR accessories: diamond and germanium ATR crystals.
- Software: Thermo Scientific OMNIC Specta with Advanced ATR Correction and multi-component search algorithms.

Main results and discussion:

- Raw spectra: Diamond ATR spectra of CBR samples (e.g., 20% and 30% carbon black formulations) show spectral artifacts and distorted band shapes, particularly where strong absorptions alter the sample refractive index. Ge ATR spectra show clean, well-defined peaks without those artifacts.
- Advanced ATR Correction: The proprietary Advanced ATR Correction in OMNIC Specta accounts for the sample refractive index variation near absorption peaks (not just intensity scaling). Applying this correction to Ge ATR data produces spectra comparable to transmission spectra and suitable for searching against standard transmission libraries.
- Component identification: A simple library search on the corrected spectrum successfully identified the base polymer. A multi-component search further resolved a second component—identified as a silane slip aid—completing the spectral match and reproducing the composite spectrum closely. This illustrates the combined power of optimized hardware selection (Ge ATR) and advanced software processing for complex mixtures.

Benefits and practical applications of the method:

- Reliable analysis of otherwise intractable samples: Ge ATR combined with advanced correction allows FTIR analysis of opaque, high-index materials like CBRs that are problematic with diamond ATR.
- Use of transmission libraries: Advanced ATR Correction converts ATR spectra into forms compatible with transmission spectral libraries, expanding identification capabilities without requiring new reference collections.
- Mixture deconvolution: Multi-component search enables detection and quantification of minor additives (e.g., silane coatings) in composite rubber parts, useful for quality control, failure analysis, and formulation verification in automotive manufacturing.

Key takeaways:

• Choice of ATR crystal is critical: Ge is often superior to diamond for high-index CBRs due to shallower penetration depth and reduced artifacts.
• Spectral artifacts originate from index-of-refraction changes at strong absorptions; simple ATR corrections are insufficient.
• Advanced ATR correction algorithms that model refractive index variations restore spectral fidelity and allow use of transmission libraries.
• Software-assisted multi-component searches can reliably separate base polymer signals from additive signatures, enabling comprehensive compositional analysis.

Future trends and potential applications:

- Improved modeling: Continued development of ATR correction algorithms that better model complex refractive-index behavior and surface heterogeneity will increase spectral accuracy for challenging solids and composites.
- Integration with chemometrics: Coupling corrected ATR spectra with chemometric and machine-learning models will enable more sensitive detection and quantification of minor components and degradation products in rubber formulations.
- In-line and near-line QC: Compact FTIR-ATR systems with optimized crystals and automated correction/search routines could be deployed for rapid manufacturing quality control in tire and rubber production lines.
- Broader material classes: Lessons learned from CBRs apply to other highly absorbing or high-index materials (filled polymers, pigmented coatings, composites), expanding ATR-FTIR utility across industries.

Conclusion:

Analysis of carbon black rubbers by FTIR-ATR requires matching experimental hardware and processing to sample optical properties. Germanium ATR crystals provide a practical hardware solution for CBRs because of their higher refractive index and shallower penetration depth, avoiding the spectral distortions seen with diamond. When combined with Advanced ATR Correction and multi-component search algorithms in OMNIC Specta, ATR-FTIR becomes a powerful, library-compatible tool for identifying base polymers and minor additives in complex rubber systems, supporting formulation control and product performance optimization.

Reference:

1. Thermo Scientific Application Note AN01153.
2. Thermo Scientific Technical Note TN51506.