Thermal Analysis UserCom 26
Thermal Analysis UserCom 26; Table of Contents:
- Optimum choice of method and evaluation in DMA measurements of composites
New in our sales program
- New STARe Excellence products
- Liquid crystals as certified reference materials for the calibration and adjustment of DSC measuring cells in heating and cooling modes
- Thermogravimetric and calorimetric analyses of yeasts
- Determination of the oil content in elastomers by TGA
- Determination of calcium sulfate dihydrate and hemihydrate in cement
- Determination of oxidation stability by pressure-dependent OIT measurements
- Thermochromism of HgI2
Thermogravimetric and calorimetric analyses of yeasts
Flavors in foodstuffs are often volatile and can be lost during production or when the consumer prepares food products.
One possibility to reduce the loss of flavors during thermal processes (pasteurization, cooking) is to confine the volatile molecules in thermally stable particles such as empty yeast cells.
This article describes how we investigated the release of flavors encapsulated in yeast cells as a function of temperature. The measurements were performed using thermogravimetric analysis and differential scanning calorimetry. Interpretation of the measurement results provides a better understanding of the release mechanisms of the flavor molecules from the yeast cells.
Determination of the oil content in elastomers by TGA
The compositional analysis of elastomers is of major importance in quality control and product development. In this study, the oil content of an NR/SBR elastomer blend was determined using a TGA/SDTA851e system. The article investigates the extent to which the separation of the vaporization of the oil and the pyrolysis of the polymer can be improved by optimizing the heating rate and using vacuum. The results show that low heating rates improve the accuracy of the oil determination, but analysis times are of course longer. At reduced ambient pressure, however, the oil content can be determined both rapidly and with good accuracy.
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 an NR/SBR elastomer blend.
Determination of calcium sulfate dihydrate and hemihydrate in cement
Cement consists primarily of calcium silicates, calcium aluminates, and calcium aluminoferrites. Besides these main constituents, various additives are used in small quantities.
One important additive is gypsum in the form of the dihydrate (CaSO4 ·2H20) or the hemihydrate (CaSO4 ·½ H20). The gypsum is ground with the clinker and serves to retard the setting and hardening of the cement. Without gypsum, hydrated calcium aluminate crystallizes within about 10 min; gypsum retards this process by several hours or even days depending on the gypsum content. In the quality control of cement, it is therefore important to know the exact content of the CaSO4 dihydrate or hemihydrate.
Determination of oxidation stability by pressure-dependent OIT measurements
The pressure dependence of the oxidation induction time (OIT) of different materials is measured. The results show that the OIT obeys a power law in pressure and that the exponent provides additional information on the stability, tendency to oxidize, and previous damage. The prediction assumes that the oxidation reaction develops from reaction nuclei and then spreads out across the entire sample. This process was confirmed by high-resolution chemiluminescence measurements.
The determination of the oxidation induction time (OIT) is a method frequently used to estimate the oxidation stability of materials.
This is done by first heating the sample to a sufficiently high temperature under inert conditions (purging the DSC cell with nitrogen). After a brief thermal equilibration time, the purge gas is switched from nitrogen to oxygen. If the temperature has been chosen correctly there is no immediate change in the DSC signal.
However, after a certain induction time, the exothermic oxidation reaction begins. The OIT is the time at which the onset of the oxidation reaction occurs measured from the point when the gas is switched from nitrogen to oxygen.
The OIT depends very much on the current chemical state of a material. For example, chemical aging causes a significant decrease in the measured OIT.
An example of this is shown in Figure 1. This shows OIT measurements performed at 210 °C on polyethylene (PE) tubing that had been used to transport aqueous solutions of chemicals. Two samples, one from the outside and the other from the inside were measured.
The inside of the tubing had of course been in direct contact with the solutions.
The OIT of the material from the outside of the tubing is significantly longer than that from the inside. The OIT therefore provides information on the extent of damage of materials.
Figure 1. OIT measurements performed in a conventional DSC at atmospheric pressure. The material originated from a piece of PE tubing. The reaction temperature is 210 °C. The stressed (i.e.damaged) sample material originated from the inside of the tubing, the unstressed sample from the outside.
Thermochromism of HgI2
Thermochromism is the term used to describe a reversible change in the color of a substance as its temperature is varied. It is a property often encountered in both inorganic and organic compounds. The phenomenon has to do with temperature-dependent structural phase transitions in the substance whereby the orbitals of electrons that absorb light in the visible wavelength region of the spectrum are deformed.
This causes the absorption and reflection properties of a substance (and hence its color) to change. Most inorganic compounds of the light elements up to calcium are white. The reason for this is that atomic distances are short and the wavelength range in which absorption of light takes place (and hence “colors” occur) is in the ultraviolet region. This region is of course not accessible to the human eye. Colored compounds are therefore not encountered until they contain elements in which the d-orbitals are increasingly occupied; for example ZnS is white, CdS yellow and HgS black.
Thermochromic compounds are often used as optical temperature indicators where the colors distinguish between different temperature ranges.