Optimizing Tire Performance through Proper Chemical Composition

Carbon black rubbers (CBRs) are most commonly utilized for automobile tires. The material effectively conducts heat away from the tread, thus enhancing reliance and increasing tire life. Making sure that CBRs have the correct chemical composition is a crucial part of optimizing the tire performance.

Traditionally, CBRs have been challenging to analyze by FTIR, because they are opaque even when cut very thinly. A measurement method which has been proven to analyze these difficult samples effectively is attenuated total reflectance (ATR).

Attenuated Total Reflection: Diamond and Germanium

In order to sample materials using Fourier Transform Infrared Spectroscopy (FTIR), ATR is now the technique of choice. The ease of the sampling device and simplicity of cleaning are big advantages. Yet, ATR devices change the appearance of the spectrum relative to what would be in transmission.

A thorough knowledge of the differences and the tools will increase the utility of ATR in the laboratory greatly. Light which passes between two different materials will alter its direction – which is how a magnifying glass works to focus the sun’s rays.

Using the index of refraction of the two materials, the alteration in direction is established. In the application of eyeglasses, high index of refraction materials such as polycarbonate can be utilized to create thinner lenses than lower index materials like glass, as the polycarbonate more effectively redirects the light.

Think of a rectangular fish aquarium as a way of understanding ATR. The far window is transparent when looking straight through it. Yet, the side windows look like mirrors, which is an effect caused by internal reflection.

Figure 1. Carbon black rubbers, such as this tire, represent a challenging sample for infrared analysis.

If a finger is pressed onto the side glass, the ridges of the fingerprint are visible, but not the valleys between them. To put it simply, viewed at this angle, the light only exits the side window slightly, and this “evanescent wave” interacts with the finger ridges which permits them to be visible to a human eye. The depth of penetration – the distance that the light travels out of the glass – is too shallow for the valleys of the fingerprint to be visible.

The same basic concept is true for infrared spectroscopy. Using the correct alignment, IR light can be driven to emerge slightly from an infrared transmissive crystal. Just like the fingerprint on the side of the aquarium, this light will interact with any sample in close contact with the crystal.

This is the reason that most ATR accessories include pressure towers, in order to press the sample against the crystal. The infrared light interacts with the sample at the crystal surface to generate the spectrum. The spectrum relies on the depth of penetration d_p which, consequently relies upon a number of factors. Mathematically, this is given by:

d_p = λ / {2 π n_C [sin²θ – (n_S/n_C)]^1/2}

If λ is the wavelength of the light, θ is the angle at which the light hits the crystals surface, and n_S and n_C are the respective index of refractions of the sample and crystal.

Two of the most common ATR crystals are germanium (Ge) and diamond. Diamond is extremely robust, being both chemically inert and hard, its dp is more than 2 microns at 1000 cm^-1. Albeit less so than diamond, Ge is also inert and hard. The main difference is the d_p, which is approximately 0.7 microns at 1000 cm^-1, because of the much higher index of refraction for Ge.

It is the aforementioned characteristic, the lower depth of penetration for Ge, which makes it perfect for carbon black rubbers. CBRs possess a high index of refraction which is near to that of diamond. If the sample index is too close to that of the crystal in the above equation, the ratio of indices is nearly 1, the square root term is negative and there is minimum to no ATR effect.

Thermo Scientific was the first to develop a proprietary advanced ATR Correction that considers every variable, enabling the utilization of transmission databases for comparison to ATR spectra for the first time.¹ The vital difference between older ATR corrections and the advanced correction is the recognition that the sample’s index of refraction alters (increases) near an absorption peak. Figures 2 and 3 show the effect of this.

Figure 2. Comparative raw spectra from 20 % CBR. The diamond ATR spectrum shows several problems, while the Ge ATR spectrum is clean.

Figure 3. Comparative raw spectra from 30 % CBR.

Figure 4. 30 % CBR on Ge. Bottom spectrum is baseline corrected only, the middle has the traditional ATR correction applied, and the top has the Advanced ATR Correction applied. Common scaled, offset for clarity.

These spectra were gathered using a FTIR spectrometer such as the Thermo Scientific™ Nicolet™ iS20 FTIR Spectrometer, utilizing a diamond and germanium ATR accessory. The sample index is approaching or exceeding the index for the crystal close to the strong absorption bands around 1400 cm^-1, and there is a sudden alteration in d_p.

This results in the spectra becoming badly distorted, and is not easily corrected. On the other hand, clean and well-defined peaks are shown in the Ge spectra in the two figures. This is due to the much higher index of refraction for Ge and the shallow depth of penetration.

In addition, the strong diamond absorption bands in the center of the spectrum no longer ratio accurately. When the light path through the crystal is the same in the background and the sample spectra, these diamond bands ratio out properly. Yet, the close match of the index of refraction between the crystal and the CBR is making the light path change, and the diamond bands no longer ratio.

In the spectral artifacts present around 2000 cm^-1 in the diamond spectra, the impact of this can be observed. Again, the Ge spectrum is completely clean. The total analysis of the CBR is performed by utilizing the powerful Thermo Scientific™ OMNIC™ Specta™ Software. First, the spectrum is baseline corrected and processed.

The (baseline corrected) spectrum pre ATR correction is shown in Figure 4, with the ATR correction supplied by most vendors, and with Thermo Scientific™ Advanced ATR Correction. The alteration in the latter is larger than just application of an intensity correction – the algorithm corrects the bands where strong absorbance changes the index of refraction. Now, the corrected spectrum can be searched against normal transmission libraries.

Figure 5. Simple search result for the top spectrum in Figure 4.The result is good, but there are extra peaks visible not in the result.

Figure 6. Multi-component search result from OMNIC Specta. The search result is now complete and agrees remarkably well with the spectrum.

A simple search produces the results in Figure 5. The match is good, as the base polymer was identified and a high match index is noted. Yet, visual examination shows that a number of peaks around 1000 cm^-1 are not shown in the search result – a second component is observed. OMNIC Specta’s unique multi-component search algorithm completes the analysis and this can be seen in Figure 6.

The powerful algorithm removes two components from this CBR – the silane slip aid, and the base polymer (notably, the same as the simple search).² The sample which is a car door insulation strip, utilizes a silane coating to stop the adhesion of the window to the insulation. It represents a nearly perfect match with the original spectrum, so the composite spectrum is visually remarkable.

Conclusion

Carbon black rubbers are a challenging IR sample. As the bands observed are distorted, diamond is not a universal solution for CBRs but Ge supplies clean peaks. Thermo Scientific produces all of the tools needed to complete the analysis, including OMNIC Specta’s multi-component search and Advanced ATR Correction. The software and hardware combine into a powerful analysis tool.

References and Further Reading

1. Thermo Scientific Application Note AN01153
2. Thermo Scientific Technical Note TN51506*Experiment can be conducted using the Thermo Scientific™ Nicolet™ iS5, iS10 or iS50 FTIR Spectrometer systems
3. *Experiment can be conducted using the Thermo Scientific™ Nicolet™ iS5, iS10 or iS50 FTIR Spectrometer systems

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.