Thermal analysis techniques for rubbers.

Thermal analysis has been applied to rubbery material for many years, and most practitioners are at least aware of the techniques. However, there have been some recent developments in thermal analysis that have increased its usefulness for rubbers. In this article, we will review the techniques of thermal analysis and how they apply to rubbers, highlighting the new approaches available.

Differential scanning calorimetry (DSC) looks at the changes in the heat capacity of a material and has had limited utility for rubbers in the past. This was due to the glass transitions (Tg) of crosslinked materials being often weak, and broad transitions that were hard to detect. The technique of fast-scan DSC or HyperDSC, however, makes seeing these transitions easier. HyperDSC involves heating a sample at ramp rates of 100-500[degrees]C/minute. This allows the detection of weak glass transitions in many highly filled or crosslinked rubbers. As the glass transition is linked to both degree of cure and many material properties, the ability to detect weak Tgs is useful in studying material properties.

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Rubber, as we know, contains more than just polymer. Therefore, constituent analysis is often needed to address quality or competitive issues. Oil and extenders are used to improve some properties, and carbon black or other filters are used to adjust others. The amount of these fillers can be determined by thermogravimetric analysis (TGA), as shown in figure 2. Variations in gas uses, heating rate and operating conditions can be used to explore a material's make-up. While this technique is well known in the rubber industry, recently turnkey TGA-FTIR or TGA-MS systems allow determination of exactly what is evolved. This leads to more precise understanding of the materials in a rubber formulation. Hyphenated techniques allow one to understand how much of an initial weight loss is water, oils or solvents.

Finally, tuning the material properties of the rubber is the whole reason why things like fillers, extenders, tougheners, etc., are added to a rubber blend. Dynamic mechanical analysis (DMA) applied a sinusoidal distortion to a sample and from that calculates its elastic and viscous properties. Not only does DMA allow one to see the transitions in materials, as shown in figure 3, but it gives modulus data too. This allows one to see the strength of a material above and below the TG, as well as estimate the crosslink density. Using a photoadapter, one can study how UV light affects material properties and tunes antioxidant packages. DMA has also been coupled with IR and MS to study material changes from applied strain and temperature programs.

Finally, rubbers are often studied in the DMA using the technique of time-temperature superposition. This technique works on the Boltzman principle of time-temperature equivalence and allows the prediction of material responses at frequency ranges well outside those measured in the laboratory. Since frequency is the inverse of time, lifetime predictions can also be made. This technique can be tricky, as it follows a set of assumptions, and these should be checked by a Cole-Cole or wicket plot. However, it is often a value tool for estimating performance in rubbers, and modem software makes it very accessible to the average user.

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by Kevin P. Menard and Peng Ye, PerkinElmer