Identifying the Composition of a Rubber Compound

Identifying the Composition of Rubber Compound

Image Credit: NETZSCH-Gerätebau GmbH

Carbon black is an essential and indispensable product for the polymer industry. Whether used as a strengthener in rubber mixtures, conductive agent or UV stabilizer, pigment for plastics, carbon black depends on occurrence, particle size, and structure. Therefore, it is critical to evaluate and certify its level and precise dispersion to avoid severe effects on the final material costs and mechanical properties.

Carbon black is a pure elemental carbon composed of colloidal particles produced by partial combustion or thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions. As previously mentioned, it is typically used in rubber, tires, and plastic products, printing inks and paints, and it is similar to the properties of a specific surface, size and structure of its particles, conductivity and color.

The traditional method for evaluating each property is included in standards, where the determination is carried out by pyrolysis of a sample at 550 °C in nitrogen followed by calcination in a muffle furnace at 900 °C.

Carbon black content is measured from the difference in mass before and after calcination. Properties are evaluated using an electric tube furnace. However, this method makes it impossible to differentiate between pyrolytic soot (formed during pyrolysis) and added carbon black.

This article discusses an alternative technique that will be helpful not only in terms of speed, efficiency, and performance, but will also lead the way where today’s supreme analytical technology is concerned.

A promising alternative to a tube or muffle furnace, thermogravimetric analysis (TGA) is considered to be primed for determining carbon black content and the various types of added carbon black in a compound. Using TGA, weight changes can be quantified as well as the system being able to record changes of a few micrograms within a material as a function of temperature or time.

This technique can be applied to various areas, including studies of volatility, reliable and accurate compositional analysis, determination of decomposition mechanisms, moisture content and ash content. In addition to temperature, the purge gas is yet another variable that can radically affect mass-change results. When fluctuating the purge gas in TGA throughout the analysis, it may be possible to isolate the additives from the mass of a polymer.

The following example shows that carbon combustion occurs in several steps. The measurement firstly demonstrates the enabled separate pyrolytic soot from added carbon black. Two types of added carbon blacks in NBR are then recognized by using an adequate temperature-time program.

Identifying the Composition of a Rubber Compound

Image Credit: NETZSCH-Gerätebau GmbH

The measurement was carried out under N2 atmosphere up to a temperature of 600 °C. The plasticizer was discharged at 332 °C (DTG peak), while the NBR decomposed at 454 °C (DTG peak). At 600 °C, the measurement was programmed to reduce the temperature of the sample down to 300 °C. The atmosphere was then modified to O2.

Throughout the heating under O2, an observation can be made of three mass-loss steps. The first step illustrates the burning of pyrolytic carbon and the two subsequent steps are included because of the burning of two different types of carbon black. Typically, the pyrolytic carbon has a much higher surface area, which delivers earlier combustion. The two added carbon blacks also vary in particle size and therefore burn out at different temperatures.          

Thermogravimetry is a dependable method for a thorough analysis of rubber compounds. Moreover, a vacuum-tight thermobalance allows for the possibility to distinguish quantitatively between its pyrolytic soot and different types of added active carbon blacks.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.

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