Differentiation of Natural and Synthetic Polyisoprene by Combined Derivative Thermogravimetry and Gas Chromatography

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Abstract
A major objective of this study was to develop a general method by which NR and IR can be differentiated and quantitatively determined in unknown vulcanizates. From the above data, it is apparent that none of the thermal analysis methods—DTG, DSC or DTG-GC—can give an unequivocal answer alone, unless the type and loading of carbon black, the sulfur level and the accelerator system of the unknown compounds are known. The general type of carbon black can be determined using a recently published DTG method, and the loading is readily determined by DTG or an ashing experiment. Determination of the exact accelerator system remains a problem. The limitations of each method can be summarized: (1) The DTG thermogram will differentiate NR and IR provided the sample is vulcanized and contains carbon black. (2) The DSC will differentiate NR and IR on the basis of the observed exothermic heat, provided the sample is vulcanized. (3) In DTG—GC, the appearance of dimethylethylbenzene in the distillate is indicative of the presence of IR. For all vulcanized IR compounds, the presence of dimethylethylbenzene is accompanied by a low isoprene/3-methyl-cyclohexene yield. For the analysis of NR:IR blends, calibration curves such as those shown in Figure 7 can be constructed, but the above limitations still apply, i.e. filler loading and type, sulfur level, and accelerator system must be known.

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