Sorption Experiments with the TGA/DSC 1
The moisture content of a material has a large influence on its melting behavior, glass transition temperature, stability, and other properties. Sorption behavior also allows conclusions to be drawn about amorphous regions in a product. Knowledge of the moisture uptake and drying behavior of materials is particularly important in the food and pharmaceutical industries, for example to optimize production processes with regard to product quality or to define suitable storage conditions. In this article, we will show how such investigations can be performed using a TGA/DSC 1.
Introduction
The influence of relative humidity on the behavior of materials can be investigated by exposing a sample isothermally to a defined level of relative humidity (RH) for a certain time and continuously measuring its mass using a balance.
The measurement time should be long enough for the mass of the sample to reach a constant value. The RH is then increased in steps keeping the temperature constant and allowing sufficient time for the sample mass to stabilize each time at the new RH level. The equilibrium moisture content of the sample is determined for the different values of RH and plotted as a function of the RH.
This yields a so-called sorption isotherm. Under these conditions (equilibrium), the RH corresponds to the water activity (aw) of the water in the sample. Water activity is related to the partial pressure of water above the sample and is not the same as the water content. It is a measure of the relatively free or unbound water present in a raw material, food or product that is available to support processes such as non-enzymatic browning (Maillard reaction), fat oxidation, the growth of microorganisms such as molds or bacteria, and so on.
For example, foodstuffs are not affected by microbial attack until the water activity is greater than about 0.7. The moisture content that corresponds to a water activity of 0.7 can vary between a few percent and several tens of percent depending on the product. A sorption curve allows the corresponding water content in a product to be determined for a particular water activity.
Experimental Details
Sorption isotherms can be measured by attaching a specially designed humidity generator to a TGA/DSC 1 (Figure 1).
The humidity generator produces a well-defined gas flow with a precise RH. The gas flow from the humidity generator passes into the furnace chamber of the TGA/DSC 1 through a humidity interface attached to the outlet of the TGA furnace...
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Conclusions
The TGA/DSC 1 combined with a humidity generator is an inexpensive way to study the sorption and desorption behavior of different materials and products. To do this, the relative humidity in the furnace chamber of the TGA/DSC 1 is varied in steps or continuously under isothermal conditions by means of a humidity generator.
In the equilibrium state (the sample mass reaches a constant value at a certain relative humidity level), the relative humidity corresponds to the water activity of the water present in the product. The sorption isotherm can be used to estimate the optimum water content for storing a product. Knowledge of the relationship between water content and water activity is particularly important in the food and pharmaceutical industries in order to optimize production processes for product quality or to define suitable storage conditions.
Sorption Experiments with the TGA/DSC 1 | Thermal Analysis Application No. UC371 | Application published in METTLER TOLEDO Thermal Analysis UserCom 37