How to precisely identify the composition of a rubber compound

How to precisely identify the composition of a rubber compound

Carbon black is a virtually indispensable product for the polymer industry, whether in rubber mixtures as a strengthener, pigment for plastics, conductive agent or UV stabilizer, depending on the occurrence, particle size and structure. Therefore, it is crucial to evaluate and confirm its level and precise dispersion in order to avoid  adverse effects on the final material costs and mechanical properties

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

The conventional method for evaluating each of these properties is mentioned 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. Finally, the carbon black content is calculated from the difference in mass before and after calcination. Generally,  the properties are evaluated using an electric tube furnace. However, with this method it is not possible to distinguish between  pyrolytic soot (formed during pyrolysis) and added carbon black.

But is there an alternative? Yes, there is one. Let’s  talk about a technique that will not only be helpful in terms of efficiency, speed and performance but will also be at the forefront with the best analytical technology today.

Thermogravimetric analysis (TGA) is considered as promising alternative to a tube or muffle furnace for determining carbon black content and different types of added carbon black in a compounds. Using TGA,  weight changes can be quantified, capable of recording changes of a few micrograms within a material as a function of temperature or time. This method can be applied to various areas such as reliable and accurate compositional analysis, determination of decomposition mechanisms, studies of volatility, moisture content and ash content. In addition to temperature, the purge gas is another variable that can significantly affect mass-change results. When varying the purge gas in TGA during the analysis, it is sometimes even possible to separate the additives from the mass of a polymer.

The following example illustrates that the combustion of carbon occurs in several steps. The measurement allows firstly to separate pyrolytic soot from added carbon black. Then two types of added carbon blacks in NBR are identified by using a suitable temperature-time program.

Up to 600° C the measurement was done under N2 atmosphere up to a temperature of 600°C. The plasticizer was released at 332°C (DTG peak) and the NBR decomposed at 454°C (DTG peak). At 600°C, the measurement was programmed to cool the sample down to 300°C. Then the atmosphere was changed to O2. During the following heating under O2, one can observe three mass-loss steps: the first step shows the burning of pyrolytic carbon and the two following steps are due to the burning of  two different types of added carbon black. Usually, the pyrolytic carbon has a higher surface area which leads to earlier combustion. The two added carbon blacks also differ in particle size and therefore burn out at different temperatures.

Thermogravimetry is a reliable method for the investigation of rubber compounds. In addition, a vacuum-tight thermobalance makes it possible to distinguish quantitatively between its pyrolytic soot and different types of added active carbon blacks.

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