Investigation and Identification of Constituents of a Rubber Compound

The composition and glass transition temperatures of a rubber compound were investigated by TGA and DSC. The tests revealed that the sample was a blend although it was labeled pure NBR (nitrile-butadiene rubber). This article describes the measurements performed to identify the different constituents of the elastomer.

Introduction

The thermal properties, polymer content and glass transition temperature of a rubber compound assumed to consist only of NBR were determined by TGA and DSC. DSC analysis however revealed two glass transitions, which was also confirmed by DMA. The result shows that the material was actually a blend of two different elastomers. 

A TGA instrument is often coupled to an infrared spectrometer (FTIR) or a mass spectrometer (MS) in order to understand particular decomposition reactions better and to identify the different gases produced in decomposition reactions during the TGA analysis. In some cases it is difficult to precisely identify the gaseous decomposition products using MS or FTIR because they originate from the decomposition of complex polymers or bitumen. Often, it is almost impossible to identify gases that are simultaneously evolved. 

In this particular example, a TGA was interfaced to a GC/MS system (a gas chromatograph coupled to a mass spectrometer) in order to positively identify the main constituents of the rubber compound. Other possible reasons for coupling a TGA to a GC/MS system could be for the analysis of product defects, the evaluation of replacement parts from a different supplier, or the investigation of possible patent infringements with a new product. 

To obtain the maximum amount of information about the composition of the gases released during the TGA analysis, an IST16 storage interface was installed between the TGA and the GC/MS system. The IST16 interface allows 16 different fractions of evolved gases to be collected and stored at user-defined times or furnace temperatures during a TGA analysis.

The gas fractions are then injected into the gas chromatograph. Using this arrangement, the decomposition products are first separated on a GC column and then identified in the mass spectrometer and, if desired, quantified. The TGAIST16-GC/MS combination is shown in Figure 1.

Experimental Details

DSC

The DSC measurement was performed using a METTLER TOLEDO DSC  1 equipped with an FRS5 sensor and an Intracooler. About 15 mg of the rubber compound was heated from –90 to 25 °C at a heating rate of 10 K/min. The sample was measured in a 40-µL aluminum crucible using a purge gas flow rate of 50 mL/min nitrogen. 

DMA

The DMA measurement was performed using a METTLER TOLEDO DMA 1 equipped with a liquid nitrogen cooling system. A sample specimen 6 mm wide, 2 mm thick and 10 mm free length was measured in the single cantilever mode from –100 to 25 °C at a heating rate of 1 K/min and frequencies of 1 and 10 Hz. The displacement amplitude was 20 µm.

TGA

The TGA measurement was performed using a METTLER TOLEDO TGA/DSC 1 equipped with a DTA sensor. A sample of about 15 mg was heated in a 70-µL aluminum oxide crucible from 35 to 650 °C at 10 K/min (purge gas: nitrogen, 50 mL/min)

The sample was then cooled to 300 °C at 10  K /min (purge gas nitrogen: 50 mL/min). At 300 °C, the purge gas was switched to air at 50 mL/min. The sample was then heated to 1000 °C at 10 K/min. The measurement curves were corrected using blank curves.

TGA-IST16-GC/MS

A sample specimen of about 40 mg was analyzed between 40 and 600 °C at a heating rate of 10 K/min in a 70-µL aluminum oxide crucible using protective and reactive gas flow rates of 10 mL/min and 20 mL/min nitrogen. A TGA/DSC 1 was coupled to an Agilent 7890B GC / 5977A MS system using the IST16 storage interface. The TGA outlet was heated to 160 °C to prevent condensation of the pyrolysis gases. The GC injector was at a temperature of 300 °C with a split ratio of 11:1.

The temperature program in the GC oven was as follows: isothermal at 50 °C for 5 minutes, heating at 10 K/min from 50 to 300 °C, then isothermal at 300 °C for 5 minutes. A 30 m x 0.32 mm x 0.25 µm HP-5ms GC column with a carrier gas flow rate of 1.2 mL/min was used for gas chromatographic separation. The measurement mode was set to detect masses of m/z 33 to 450 using an MS detector amplification setting of 1. The temperature of the IST16 oven and transfer lines was 300 °C.

Measurements and Results

DSC Figure 2 displays the first and second DSC heating runs. The first heating run is normally used to eliminate the thermal history of the sample. The second heating run is therefore characteristic for the actual material. The sample shows a glass transition at –8 °C that can be assigned to NBR. 

 Normally NBR exhibits a glass transition between –40 and 0 °C depending on the content of acrylonitrile, which can be between 5 and 50%. In addition to this transition, there are signs of a weak glass transition at –50 °C. To confirm this result, a DMA analysis was performed.

DMA

Figure 3 shows the DMA storage modulus (E’), loss modulus (E’’) and loss factor (tan delta) curves measured at frequencies of 1 and 10 Hz as a function of temperature. All the curves were obtained from the same measurement.

Summary and Conclusions

This article describes how different constituents of a rubber compound that allegedly consisted only of NBR rubber were identified. A DMA analysis confirmed the DSC results that the sample is a blend of two elastomers because two glass transitions were identified. TGA analysis was able to separate the carbon black produced in the pyrolysis of the polymer from the carbon black filler. This enabled the content of carbon black filler in the sample to be determined.

The FTIR-ATR analysis of the residue left after decomposition of the sample at 650 °C exhibited IR absorption bands typical for SiO₂ and CaCO₃.

The combination of a TGA instrument with an IST16 storage interface and a GC/MS system allowed the gaseous decomposition products formed during TGA analysis to be collected and stored before they were separated and identified by GC/MS.

This greatly facilitated the interpretation because it enabled the emission profiles of the decomposition gases to be plotted as a function of furnace temperature. The results showed that NBR was the main constituent of the elastomer but that the sample contained a second elastomer constituent which was clearly identified as NR. NR is responsible for the second glass transition at –50 °C.

Besides this, sulfur and 2-mercaptobenzothiazole were detected, both of which are used as vulcanizing agents or vulcanization accelerators.

_{Investigation and Identification of Constituents of a Rubber Compound | Thermal Analysis Application No. UC 425 | Application published in METTLER TOLEDO Thermal Analysis UserCom 42}