Determination of Glass Transition Temperatures of Powder Disks by TMA - METTLER TOLEDO

Determination of Glass Transition Temperatures of Powder Disks by TMA

Introduction

The glass transition temperature (Tg) of a polymer film can easily be determined by TMA. In contrast, the reliable determination of the Tg of a powder sample by TMA is more difficult, especially compared with DSC. Powders and fine shavings can however be measured by first pressing disks of the material in a special die (Fig. 1). 

 

The disks can be pressed relatively easily at a defined and reproducible pressure by filling the special die described above with a known mass of powder and then placing it in a manually operated press whose spindle is connected to a torque wrench (Fig. 2). The pressure applied to the powder is varied by changing the torque setting.

The influence of the applied pressure (torque) and the mass of the powder on the Tg was investigated using TMA. The results were compared with Tg measurements of films cast from the same polymer. Pharmacoat® 606, a low viscosity hydroxy-propyl methyl cellulose (HPMC) from the family of non-ionic water-soluble cellulose ethers, was chosen as a model substance. Such cellulose ethers are used in many different types of pharmaceutical applications, for example as film coatings for drug formulations.


Conclusions

In summary, one can say that the deter-mi-nation of the glass transition temperature of HPMC using powder disks is a good alternative to using cast films. Both methods yield similar values as far as the glass transition temperatures (Tg1) and the melting or decomposition temperatures (Tg2) are concerned. It is however easier to press disks of constant thickness than it is to cast uniform films. The reproducibility of the values obtained with the HPMC disks is appreciably better. The applied pressure and sample masses used to prepare powder disks can be varied within wide limits, without any significant effect on the observed glass transition temperature values.

Determination of Glass Transition Temperatures of Powder Disks by TMA | Thermal Analysis Application No. UC 215 | Application published in METTLER TOLEDO Thermal Analysis UserCom 21