Thermal stability screening test

The first aim of a thermal stability screening test (e.g., DSC/DTA) is to obtain data on the potential for exothermic decomposition and on the enthalpy of decomposition (AHd). These data, together with the initial theoretical hazard evaluation, are used in reviewing the energetic properties of the substance (Box 4) and the detonation and deflagration hazards of the substance (Boxes 7 and 8). The screening tests also provide data on the thermal stability of the substance or mixture, on the runaway potential, on the oxidation properties, and to a lesser extent, on the kinetics of the reaction (Box 10). 

The equipment is quite adequate for screening purposes. In its simplest form (i.e., a glass tube in an oven), it is a relatively low cost technique that can be assembled with standard laboratory equipment. However, the simple test set-up provides no quantitative thermal data for scale-up purposes, but only T0 values. The more advanced instruments like the SEDEX and SIKAREX, which are also isoperibolic calorimetry equipment, acquire specific thermal stability data that can be used for scale-up. Furthermore, the small autoclave tests provide gas evolution data. 

OECD Guideline for Testing of Chemicals, "Screening Test for Thermal Stability and Stability in Air," No. 113, Organization for Economic Cooperation and Development, Paris, France (1981). 

Five of nine respondents to the CSB survey frequently use both screening and more sophisticated approaches, including adiabatic calorimetry, to determine the thermal stability or compatibility of process materials. Seven of nine respondents use screening alone for chemical reactivity testing. The most often used testing objectives are ... 

To determine thermal stability using screening tests. 

DSC is a basic screening test and should be applied to all chemicals and mixtures unless the thermal stability has already been clearly established. 

A wide variety of metal oxides were screened by Keller and Bhasin in their pioneering investigation into methane dimerization. Since then, many catalysts have been developed and tested (2-6). However, due to the thermal stability of methane, even in the presence of oxidizers, conversion is extremely low. To date, total yields of C2 products of 15 to 207. 2, 4) are considered high. 

Our physical testing program is concerned with two main areas, thermal stability and reaction calorimetry. The thermal stability testing is broken down into two phases, initial screen and followup tests. The initial screen is intended to quickly identify any thermally unstable materials in a process. The follow-up tests examine in more detail any significant instability detected in the initial screen. 

Another reason that isothermal heating methods are used in the initial screen is to identify materials that have time dependent thermal stability. These materials have a thermal decomposition that does not follow a simple Arrhenius relationship in which the reaction rate increases exponentially with an increase in temperature. Instead an extended induction period is required before the decomposition becomes detectable. An example of this behavior is shown in Figure 2. The DTA isothermal test recorder traces of methane sulfonic acid, 3,7-dimethyloctyl ester at different test temperatures are shown. The induction time varies from less than 1 hr. at 180 C to 46 hr. at 130 C. As with this compound, it is not unusual that once decomposition is detected it proceeds very rapidly, releasing all of the heat in a short period of time. Dynamic heating methods do not indicate if this type of thermal instability is present if it is, the initial detection temperature from dynamic tests will be grossly misleading as to the thermal stability of the material. 

A pressure DSC test (semi-closed sample pan, encapsulated in air, under 500 psig of N2) and a non-pressure DSC test (semi-closed sample pan, encapsulated in air, at ambient pressure) are used in our initial screen for several reasons. The pressure DSC allows the thermal stability of liquids to be examined near, at, or above their boiling points. It also suppresses the evaporation of volatile materials from the sample, which can hide an exotherm. This can be especially important when testing evaporation residues. A comparison of the results of both tests gives an indication of the effect of pressure on the decomposition of the material. It will also give an indication if oxidation is an... 

During our initial screen, we also check the thermal stability of the sample in the presence of stainless steel. Accidents have occurred when processes that were no problem in glass equipment were either scaled-up in, or switched to stainless steel equipment. We perform this check during our isothermal DTA test by running duplicate samples and adding stainless steel powder to one of them. We use the isothermal test because it allows longer contact time between the sample and the stainless steel. 

The eoneern over the performance of negative plates in VRLA batteries has resulted in renewed interest in the influence and mechanisms of organic additives and extensive research programmes have been carried out under the auspices of the ALABC. This work has included an assessment of 34 materials, five of which were synthetie organie compounds that were identified to have the potential to act as effective expander components in lead-acid batteries [32]. Preliminary screening tests for stability in acid, impurities and thermal stability, followed by studies of potentiostatic transients, impedance plots, and cyclic voltammograms [33], have... 

It was outlined in chapter 2 in detail that screening tests primarily have the purpose, to provide a first characterization of the safety relevant substance properties as part of the basic assessment. It was further explained that the determination of the thermal stability of a substance is of the greatest importance. The most fi-equently used methods for this puipose are those that investigate thermal stability using very small amounts of sample material only. The most widely used test equipments to perform such investigations are the DTA ( difference thermal analysis ) and DSC ( differential scanning calorimetry). 

The results of the hazardous chemical evaluation are used to determine to what extent detailed thermal stability, runaway reaction, and gas evolution testing is needed. The evaluation may include reaction calorimetry, adiabatic calorimetry, and temperature ramp screening using accelerating rate calorimetry, a reactive system screening tool, isoperibolic calorimetry, isothermal storage tests, and adiabatic storage tests. 

Martin (1983) examined many commercially available polymers, including HPAMs, polysaccharides and other polymers. He developed a very wide range of screening tests which examine most of the properties discussed above, including thermal stability and shear stability. He showed that some of the modified acrylamide polymers exhibit improved performance compared with conventional HPAM, in so far as these modified materials generate higher viscosities in brine. In this work, a very large number of new... 

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