Thermal Analysis UserCom 7
Thermal Analysis UserCom 7; Table of Contents:
- Measuring specific heat capacity
New in the sales program
- Furnace expander
- Decomposition of ammonium perchlorate
- Characterization of soil samples
- Precise measurements of the specific heat capacity
- Safety investigations in glass crucibles
- Determination of curing behavior
- Platinum crucibles in thermogravimetry
Decomposition of ammonium perchlorate
The kinetics of the thermal decomposition of the cubic ammonium perchlorate (AP) has been extensively studied. The reported effective activation energies vary from about 371 to 2602 kJ mol-1. The mechanistic interpretation of the values is also different. The confusing character of the kinetic information is not surprising for the process that is known3 to be a tangled interplay of various chemical (solid-state decomposition, reaction of gaseous products with the solid, gas-phase reactions) and physical (diffusion, sublimation, adsorption-desorption) processes. The effective activation energy of the thermal decomposition of AP is a composite value determined by the activation energies of these processes as well as by their relative contributions into the overall decomposition rate. If these processes have different activation energies, the effective activation energy shows a variation with the temperature. The kinetics of such multi-step processes cannot be characterized by a single constant value of the activation energy.
1 Brill, T. B.; Brush, P. J.; Patil, D. G. Combust. Flame 1993, 94, 70.
2 Kishore, K; Pai Verneker, V. R.; Krishna Mohan, V. Thermochim. Acta 1975, 13, 277.
3 Jacobs, P. W. M.; Whitehead, H. M. Chem. Rev. 1969, 69, 551.
Characterization of soil samples
Soil plays a central role in agriculture and forestry and also in problems concerning environmental protection, waste utilization and many ecological problems. One of the most important characteristics of soil is the organic soil substance (in the original article in German, ‘organische Bodensubstanz’ OBS). It is the basis of plant nutrition and is of decisive importance for many chemical, physical and biological soil properties. It influences the behavior of pollutants substances and plays a key role in the material cycle of ecosystems. For several decades, people have been looking hard for ways that are generally applicable for its characterization. Up until now all attempts at determining the quality of soil have proven to be unsatisfactory. The reasons are the extremely large soil-specific variety of material components, the heterogeneity of transformation processes and the continuously changing adaptations of soil organisms. Because of this, methods and results of the analysis of OBS and its components are hardly comparable with each other. They have not been applicable to current tasks of soil protection and are not suitable tackling future projects in ecological (e.g. risk assessment of the use of genetically changed organisms in agriculture). A thermogravimetric method for the assessment of the quality of organic substance in soil has now been developed as part of a federal research project at the Technical University in Berlin.
Precise measurement of the specific heat capacity
The specific heat capacity cp is one of the most important quantities used to characterize the thermodynamic properties of materials in the condensed phase. This is especially true for the static and dynamic changes in properties in structural phase transitions and glass transitions. Prominent examples of such transition phenomena that are in the forefront of current research activities are ferroelectric and ferromagnetic phase transitions, transitions in liquid crystals and superconductors, glass transitions in structural and classical (canonical) glasses etc. The transition temperatures of interest lie typically between a few degrees Kelvin and many hundreds of degrees Kelvin. The cp investigations in the region of phase changes can be roughly divided into two classes: costly and time-consuming precision measurements using adiabatic or ‘alternating current’ (AC) calorimetry, and survey measurements using conventional DSC. The purpose of this work is to show that ‘temperature modulated’ DSC (TMDSC) provides an excellent alternative to the above mentioned precision methods.
Safety investigations in glass crucibles
Previously, safety investigations were often performed in glass crucibles. Glass has the advantage of a noncatalytic surface. In addition, the sample can be inspected visually after the measurement. Unfortunately the glass crucibles (ME 27 812) are too high for the DSC821e cell. A furnace expansion with a special lid (known as the DSC furnace expander ME 51 140 735) is now available, so that the 100 μl glass crucibles can be used once again.
Determination of curing behaviour
During curing, a duroplast is at first liquid or can at least undergo plastic deformation. With increasing crosslinking it becomes gel-like or rubber-like before it finally hardens. The molecules form a three-dimensional network throughout the entire reaction mass. The gelation of normal epoxy resins occurs with a conversion of about 65 %. The gelation time is important for the processing of duroplasts, because afterwards the material can no longer be plastically formed. An adhesive, for example, can only be used up until just before the gelation (the gelation time is also known as the ‘pot life’).
Thermal analysis includes several different techniques that can characterize the curing behavior of such systems: reaction kinetics (curing time and curing temperature) as well as the measurement of certain physical properties before, during and after the curing. DSC is the technique normally used to follow the crosslinking reaction by measuring the heat evolved, and then afterwards, to measure the resulting glass transition temperature. DSC can not, of course, determine the increase of the viscosity or the modulus of elasticity during the curing, or the expansion coefficient of the cured resin.
Thermomechanical analysis (TMA) is then the method of choice, especially if bending measurements are performed. Ideally both techniques, i.e. the measurement of the heat of reaction and of bending are combined. This has has been realized in the METTLER TOLEDO TMA/SDTA840.
Platin crucibles in thermogravimetry
A potential customer asked us to measure the decomposition profile of dishwashing powder (a mixture of silicates and organic surfactants), in order to check the suitability of our thermobalance for his application. The initial TGA measurements were performed under nitrogen in 70 μl alumina crucibles in the tempearture range 40 °C to 600 °C at a heating rate of 10 K/min. The result, a weight loss step between 200 °C and 300 °C as well as a significant residue, was presented to the customer. His comments were that our system did not show the results be expected and that it was not as sensitive as other competitive instruments. Spurred on by this reaction, we performed further measurements at reduced heating rates, using smaller sample sizes and lighter aluminum crucibles. Without success. Finally our experts in Schwerzenbach gave us the decisive tip - to test for the possible catalytic effect of platinum crucibles. A new series of experiments was started that infact yielded the expected results. The catalytic effect of platinum led to the decomposition being completed 30 ° C lower and made three decomposition steps visible. Today this customer has a Mettler-Toledo instrument...