Determination of the Oil Content in Elastomers by TGA

Introduction

An elastomer usually contains different polymer constituents as well as additives such as cross-linking agents, plasticizers, fillers, stabilizers, fire retardants, etc. The production process and physical properties of an elastomer depend mainly on the chemical composition of the elastomer.

The compositional analysis of elastomers is important both for manufacturers and purchasers of elastomers as well as for scientists engaged in research and development. The aim of such analyses is to assess the quality of the raw materials and the vulcanized products, to develop new formulations, and to optimize the different processing parameters during blending and vulcanization

The most important plasticizers used in elastomers are oils. They improve flow and processing behavior as well as the physical properties of the elastomers. The determination of the oil content in an elastomer is a difficult analytical task because the vaporization of the oil and the beginning of polymer pyrolysis often overlap. Accurate determination of the oil content therefore requires measurement conditions which ensure that the oil vaporizes at lower temperatures

The vaporization of the oil from the elastomer involves both a phase transition and a time-dependent transport process. Preferential vaporization is promoted by increasing the surface/volume ratio of the sample by using a low sample mass, and by reducing the ambient pressure. Low heating rates can also prolong the time available for vaporization without the temperature rising to the level at which pyrolysis begins.

In contrast, the decomposition temperature of a polymer is hardly influenced by the sample mass, heating rate, and pressure. It should therefore be possible to improve the separation of oil vaporization and polymer pyrolysis by optimizing these parameters and hence achieve a more accurate determination of the oil content. This is demonstrated below in the case of an NR/SBR elastomer blend.

Experimental Details

The measurements were performed using a TGA/SDTA851e . The composition of the material investigated is given in Table 1. The samples were measured in 30-µL aluminum oxide crucibles and the sample mass was approximately 20 mg. The change in mass of the sample was measured in the temperature range 50 °C to 625 °C at heating rates of 2, 10 and 30 K/min under nitrogen at normal pressure. A further sample was also measured at 10 K/min but at a pressure of 10 mbar (i.e. under partial vacuum). The measurements were performed under nitrogen because the determination of carbon black was not of interest in this study.

The Influence of Heating Rate on the Determination of Oil Content

The measurement curves of the sample recorded at heating rates of 2, 10 and 30 K/min at normal pressure are shown in Figure 1. Loss of mass occurs in three main steps.

Step 1 corresponds to the loss of volatile components such as moisture and oil. At 2 K/min (the lowest rate), this step can be divided into two different regions of mass loss, namely loss of moisture up to about 170 °C followed by vaporization of the oil up to 300 °C.

Steps 2 and 3 correspond to the pyrolysis of NR and SBR. The residue consists of inorganic additives and carbon black. The TGA curves were evaluated using the DTG curves. The results are summarized in Table 2.

The measurement curves show that the effects involving loss of mass shift to lower temperatures at lower heating rates. This results in better separation of the individual processes. The improved separation of oil vaporization and pyrolysis leads to the value for the measured oil content changing from 4.6% (at 30 K/min) to 8.4% (at 2 K/min). The latter result is much closer to the true value of the formulation of 9.1%. The increased resolution at low heating rates also allows the individual polymer constituents to be more accurately determined. The total mass loss of the pyrolysis steps (2 and 3) agrees well with the sum of the polymer constituents (25.5% + 7.2%) and vulcanization additives (4%), that is, 36.7%.

The Influence of Pressure on the Determination of Oil Content

The influence of pressure was investigated by measuring samples under nitrogen at 10 mbar and 1000 mbar at a heating rate of 10 K/min. The TGA and DTG curves are shown in Figure 2 and the results summarized in Table 3. The temperatures marked (M) are the midpoints of the step evaluation. The other temperatures are peak maximum temperatures of the DTG curves.

Conclusions

The oil content of elastomers can be determined by TGA. The best accuracy is achieved by performing measurements at low heating rates and/or under reduced pressure (vacuum).

Low heating rates improve the separation of different processes, in particular the separation of the vaporization of the oil and the pyrolysis of the polymer constituents. Measurement times are however significantly longer. Measurements under reduced pressure shorten the measurement time and allow the content of plasticizer (oil) and polymer constituents in elastomers to be accurately determined.

The separation of the residue into carbon black and ash requires measurements in an oxidizing atmosphere.

_{Determination of the Oil Content in Elastomers by TGA | Thermal Analysis Application No. UC 263 | Application published in METTLER TOLEDO Thermal Analysis UserCom 26}