Measurement of Dynamic Water Sorption Processes by Modified TGA

Introduction 

This article describes the modification of a TGA instrument to measure dynamic water vapor sorption processes (Dynamic Vapor Sorption, DVS). Instrumentation of this type allows the change in mass of a sample in powder form to be measured as a function of relative humidity and time [1]. This is of particular interest because many water-soluble and powdered foodstuffs are moisture sensitive. If such foodstuffs are stored under conditions where the humidity is too high, they can become lumpy, undergo phase changes and recrystallization, lose aroma or go bad due to the growth of molds. The DVS method described here is one of many methods that can be used to characterize the behavior of powder systems with regard to humidity [1].

Compared with conventional methods, the DVS technique has the advantage that it is rapid and less labor-intensive [1]. With a DVS instrument, it is possible to measure a complete sorption isotherm quasi fully automatically with a single sample, and at the same time follow the sorption process dynamically, i.e. as a function of time. A commercially available DVS instrument, however, costs several times more than a standard TGA instrument. We therefore decided, together with METTLER TOLEDO, to modify a TGA instrument to a DVS instrument so that it could be used for either DVS or TGA measurements as desired. An additional advantage is that important thermodynamic quantities such as the enthalpy of sorption can be determined from a combined DVS/TGA measurement. This is not directly possible with conventional DVS instruments.

TGA Instrument Modifications 

A METTLER TOLEDO TGA/SDTA851e / LF1100 was chosen for the construction of the DVS instrument. The large gas outlet makes this model especially suitable for the modifications required. A specially built water vapor generator equipped with a humidity sensor (Rotronic AM3 hygrometer, Rotronic AG) was connected to this gas outlet. The humidity sensor is located in the furnace chamber and is positioned close to the crucible holder with the sample. The humidified gas is passed from the water vapor generator via silicone rubber tubing directly into the furnace chamber. 

The furnace chamber in fact serves as a controlled environmental chamber. It is maintained at 25 °C by means of a thermostat (Fig. 1) during the DVS measurement. The measurement signal of the humidity sensor is transferred to a PC and regulates the water vapor generator so that defined humidity steps can be set. Technical details of the DVS instrument are listed in Table 1.

This type of humidity-sorption measurement places special demands on the thermobalance:

• The humidity sensor with its 4-pole cable must be located in the furnace chamber
  
• The humidified gas must be supplied without condensation or cooling occurring
  
• The exhaust gas should flow out of the cell in a defined way without condensation so that further analysis, e.g. online GC-MS, is possible
  

To meet these requirements, a special furnace lid with O-ring seal and wide-diameter stainless steel outlet tube (internal diameter 6.8 mm) was made. The outlet tube is maintained at a constant temperature by a heat exchanger. 

Acknowledgements 

The authors wish to thank the many people involved in this work at Givaudan Dübendorf AG: in particular, Mr. Ch. Montagner and R. Graf of the Creation Tools group for the design and construction of the water vapor generator, and Mr. J. Bouwmeesters of the Delivery Systems group for performing the sorption experiments. Finally, we owe our thanks to the management of Givaudan Dübendorf AG for their financial support for the development of this instrument.

_{Measurement of Dynamic Water Vapor Sorption Processes by Modified TGA | Thermal Analysis Application No. UC 172 | Application published in METTLER TOLEDO Thermal Analysis UserCom 17}