The use of microwave ashing technology for the determination of carbon black in SBR.

Carbon black is routinely added to elastomers in order to enhance physical properties. Styrene-butadiene rubber (SBR), used primarily in tires, benefits from the addition of carbon black through greater tensile strength as well as increased wear resistance. Since carbon black is added during the production of SBR, adequate means must be available for determining the amount of material added for quality control. Ideally, this would involve a rapid, accurate technique such that corrective action could be taken as soon as possible. While several methods exist for measuring the amount of carbon black in polymers, they have drawbacks. Commonly used methods are vacuum pyrolysis (ASTM D1416) and the Leco furnace. While both yield acceptable results, they require the use of reduced pressure and/or an inert gas purge. Time is also a factor with the Leco furnace requiring 18 minutes and vacuum pyrolysis over half-an-hour. Thermogravimetric analysis (TGA) can take up to one hour and requires the use of more sophisticated equipment. Measurement of carbon black content by chemical degradation with nitric acid is another technique, but it is laborious and can take up to four hours.

While conventional pyrolytic methods utilize electrical resistive heating, microwave furnaces use silicone carbide as the heating element. Absorption of over 90% of the microwave energy (ref. 1) by the silicone carbide results in the release of heat in excess of 1,000[degrees]C. The oven temperature is measured by means of a thermocouple and maintained by cycling the magnetron on and off. In order to provide an oxygen-free environment for pyrolysis, self-sealing quartz crucibles were used to contain the sample. As the pyrolysis gases are evolved, they are allowed to escape past the crucible lid. Once gas evolution ceases, the lid re-seats and does not allow air in to oxidize the carbon black.

This article attempts to demonstrate the use of a microwave furnace for determining the amount of carbon black in styrene-butadiene rubber. In addition to determining the optimum set of conditions for the test, other parameters such as bound styrene, extender oils and carbon black surface area were studied.

Experimental

Microwave furnace

Samples were analyzed using a CEM Microwave Ashing System-300 using self-sealing air-tight quartz crucibles. Prior to analysis, the crucibles were placed in a muffle furnace at 800[degrees]C for 10 minutes and then cooled in a desiccator. Analyses were performed by accurately weighing samples to 0.1 mg in the crucible. The crucible was then placed in the microwave at the appropriate temperature for the specified length of time. The crucible was then removed and cooled in a desiccator for 10 minutes prior to reweighing.

Vacuum pyrolysis

Measurements were carried out by placing weighed samples into preignited porcelain crucibles which were then loaded into a combustion boat. The boat was then placed into the combustion tube and purged with carbon dioxide for one minute, followed by evacuation to a pressure of < 22 mm Hg. Heat was applied to achieve 650[degrees]C which was maintained for 12 minutes. The heat was then removed and the combustion tube purged with carbon dioxide for 20 minutes. The combustion boat was carefully removed and the crucibles reweighed.

Leco furnace

Samples were analyzed using a Leco furnace fitted with a nitrogen purge. Leco Vycor insert crucibles were used which were ignited at 550[degrees]C for two hours prior to analysis. Samples were weighed to 0.1 mg in the Vycor insert and then placed into Leco quartz-enclosed carbon crucibles. The furnace was adjusted to 235 milliamps and the nitrogen purge started prior to insertion of the crucibles. After purging the sample for one minute, the sample was heated for five minutes followed by a seven minute purge. The crucibles were then removed, cooled in a desiccator for ten minutes and re-weighed.

Results and discussion

As with any analytical technique, the method parameters must be defined before meaningful data can be generated. In the case of quantifying the amount of carbon black in SBR using a microwave furnace, temperature, pyrolysis time and sample size are all critical parameters. Other factors, such as bound styrene, extender oil and carbon black surface area can also influence the test.

Temperature

Literature references for polyethylene microwave applications along with some initial experimentation indicated that acceptable results could be obtained at 650[degrees]C for seven minutes, so this time was chosen as the starting point. In order to determine the effect of temperature, therefore, a known sample was pyrolyzed for seven minutes at different temperatures. The point at which no additional weight loss occurred was taken to be the optimum temperature. In the study, this point was reached at 650[degrees]C figure 1). Above this temperature, the apparent amount of un-pyrolyzed sample increases, probably due to the formation of trace metal oxides. The optimum temperature was determined to be 650[degrees]C.

Pyrolysis time

This was another critical parameter which was investigated. In order to determine the optimum time, samples were pyrolyzed at 650[degrees]C for varying amounts of time. The point at which a consistent value was obtained was considered to be the optimum analysis time. In this case, seven minutes appeared to be long enough to adequately pyrolyze the samples (figure 2).

Sample size

In this study, sample sizes of 0.5 and 1.0 grams were used with both giving good results (table 1). It was decided to use the larger sample size for this test. Having determined the optimum conditions for the method, the effect of several polymer parameters was investigated.

Bound styrene

SBR is produced having a wide range of % bound styrene, from 10-85%. Samples spanning this range were analyzed by both vacuum pyrolysis and the microwave furnace in an effort to determine if comparable results could be obtained with both methods. The vacuum pyrolysis is considered to be the accepted method and therefore a ratio of the data was plotted to measure any deviation from the accepted value (figure 3). The plot of the data yields an essentially flat line over the entire range, indicating that the methods give essentially the same values with very little deviation of the ratio from unity.

Extender oils

Aromatic and napthenic oils are frequently added to SBR to enhance the physical properties, with concentrations ranging from none to 70 phr. Samples containing extender oils over this range were analyzed. A ratio of the data was plotted (figure 4) which resulted in a flat line over the range tested. This would indicate that extender oils do not have any detrimental effects on the microwave method relative to the accepted method.

Carbon black surface area

It has been reported that carbon black will absorb microwave radiation and generate heat in direct proportion to its surface area (ref. 2). A study was undertaken to determine to what extent this might influence the test. Samples containing carbon black with a high surface area (N-110, 140[m.sup.2]/g) as well as samples with a low surface area (N-550, 43[m.sup.2]/g) were analyzed at varying temperatures. No detectable differences could be seen between the samples. While the carbon black present in the samples may indeed be exposed to microwaves in the furnace, the potential heat generated is too small to be of any consequence under the experimental conditions.

Comparison of Leco furnace with microwave

The results obtained by the conventional Leco furnace were compared to the microwave method and are summarized in table 2. The data indicate that die Leco furnace had a higher standard deviation, possibly due to less consistent heating than is possible with the microwave.

Table 2 - comparison of Leco and microwave
furnaces

                                           Leco      Microwave

Min. value                                 33.2      33.93
Max value                                  34.2      34.05
Average (10 samples)                       33.59     33.99
Standard deviation                          0.28     0.039

Conclusion

A microwave furnace can be used to rapidly and accurately determine the amount of carbon black in SBR without the need for an inert gas purge or reduced pressure. Results obtained are comparable to the standard vacuum pyrolysis method. This method allows for faster analysis time than the conventional methods. Use of the microwave furnace is applicable to SBR polymers with any amount of bound styrene or extender oil. The method can also be used with carbon blacks of varying surface areas.

References

[1.] Kingston, H.M. and Jassie, L.B., "Introduction to microwave sample preparation," American Chemical Society (1988). [2.] Clark, M.S., paper no. 4, presented at a meeting of the Rubber Division, American Chemical Society, Cleveland, Ohio, October 23-26, 1979.

Acknowledgements

"Improved aging and UV resistance of TPES derived from diimide HSBRs" is based on a paper given at the 1994 Society of Plastics Engineers Annual Technical Conference. "Property enhancement of melt processible rubber by blending thermoplastics" is based on a paper given at the October, 1994 Rubber Division meeting.

"Thermoplastic elastomer based on ionomer" is based on a paper given at the May, 1995 Rubber Division meeting. "The use of microwave ashing technology for the determination of carbon black in SBR" is based on a paper given at the May, 1995 Rubber Division meeting.