Applying TGA-IR to Your Deformulation Problems

Operators working with materials like rubbers, plastics, resins, pharmaceuticals, adhesives and packaging need to understand differences between competitive products, causes of failures or the origin of odors or potentially toxic off-gassing. Standard analytical procedures are often incapable of distinguishing subtle differences like the presence of low-concentration additives or contaminants. Further, the same components may be present in two different materials, but the process used to make the products – such as the processing temperature – may vary, causing the interactions between components in the material to differ. This can lead to poor performance, such as early material fatigue or complete material failure.

A complete analysis requires these materials to be deformulated – essentially torn apart to expose their underlying nature. Thermal Gravimetric Analysis (TGA) is a common tool for doing this. A temperature ramp is applied to the sample and the breakdown tracked through weight loss as components vaporize. While providing quantitative information through weight loss, this process does not give insights into the chemical identity of the off-gassing materials.

In TGA-IR, the off-gassing materials are directed through a transfer line to a gas cell, where the infrared light interacts with the gases. The spectra can be used to identify the materials. The FT-IR data set from the TGA-IR experiment is three dimensional – frequency versus intensity (the spectrum) versus time. To get an estimate of the size of the data file, consider that a typical TGA experiment ramps at 15 °C/minute from ambient to 900 °C then holds at 900 °C for 2 minutes. This results in a run time of approximately one hour.

In parallel, the typical FT-IR experiment collects 4 scans at 4 wavenumber resolution, resulting in one spectrum being collected about every 5 seconds. For a one hour run, this could result in around 720 spectra being acquired. Manual analysis of all these spectra would be overwhelming and highly inefficient. There are two critical pieces of information desired from the FT-IR data: the identity of the off-gassing materials and the time dependence of the gas evolution (called the profile) for each component. The former identifies what comes off the sample; the latter tracks when the component is released. Most of the time, “what is it?” is the main target. However, the profile conveys considerable information about how the material was produced. For instance, a shift of 30 °C in the evolution temperature for a component in a carbon black rubber may relate directly to the thermal robustness of that rubber and hence to the lifetime expectation when used in tire manufacture.

Complications

There are three complications: First, the quantity of spectra makes a complete manual analysis impractical, so the analyst is forced to select time regions to analyze. Second, the selection of those regions inherently risks missing information, which may or may not be critical to the analysis. Third, and most perplexing, much of the time multiple gases are being evolved simultaneously. This means the resulting spectra are complex, intertwined combinations, not single components.

The need for skilled analysts to perform this task, who can ensure both completeness and correctness of the results, has led to an underutilization of this important tool. Software is available that surmounts all three of the complications, giving fast, consistent and thorough results to operators of any skill level.

Read TGA-IR Analysis Using the OMNIC Mercury TGA Software to learn how any skill level of user can obtain a complete, thorough analysis for deformulation of rubbers, plastics and many types of compounded materials using TGA.