COMPOSITIONAL ANALYSIS OF TYRES USING THERMOGRAVIMETRY

Introduction:

In view of the push to find alternative energy sources to reduce dependence on fossil fuels, waste tires serve as a source of high quality fuel, having a high calorific value that is roughly similar to that of coal, yet their heterogeneity posses issues with regard to complete burnout and emissions. Tyre elastomeric materials consist of four main constituents: a) oil or plasticizers b) polymer c) carbon black and d) inner filler. The particular end-use application of a given elastomeric material will determine the nature of its specific composition. The levels of the various components will have a major effect on the resulting end-use characteristics of the elastomer.

The compositional determination of the tyre constituents have always been a challenge to the quality assurance people. The most complicated part is to separate the oil or plasticizer in the elastomer. The isothermal TGA method permits excellent separation of the oil from the polymer without having to resort to vacuum or extraction techniques. The other decomposition steps could also be distinguished clearly if proper heat treatment conditions are employed under appropriate gaseous environment.

Experimental:

The following experimental conditions were used to characterize the compositional properties of the tire elastomer.

Instrument:         STA 6000

Heating rate:       20 ˚C/min from 30 to 900 ˚C

Pan:                      Open platinum pan in ceramic holder

Purge gases:       Nitrogen to 700 ˚C and air to 900 ˚C (30 mL/min)

Sample mass:     Approximately 30 mg

Results and Discussion:

Displayed in Figure 1 to 3 are the TGA results obtained on the different tire elastomers using the conventional TGA approach. These results were generated by heating the sample at a constant rate of 20 ˚C/min. The purge gas was changed from nitrogen to air at 700˚C. The TGA results on the tire elastomer shows that there are several weight losses between 300 and 900 ˚C. The first weight loss at about 300 ˚C is that of the oil and is not decomposition, but the evaporation or volatilization of the oil. The weight loss occurring at about 400 – 500 ˚C is the thermal degradation of the polymer. The third weight loss occurs in the range of 700 – 750 ˚C where decomposition of carbon back occurs in presence of oxygen leading its oxidation to Carbon di-oxide. The material that resides after removal of carbon black is the innert filler which may be metallic or ceramic. The composition of three commercial tyres derived from this study may be summarized as follows:

ca1

ca2

ca3

ca4

Conclusion:

Thermogravimetric Analysis under optimized condition provides the best possible resolution, or separation, of weight loss events. For elastomers, the technique permits the quantitative separation of the critical components: oil, polymer, carbon black and inert filler.

* The work has been carried out at MSK Central Laboratory, Kolkata by Ms. Sutapa Pal and Dr. Saswati Ghosh.
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