Thermal Analysis of Toners

Introduction

Toners, as used in modern laser printers and photocopiers are, in fact, complicated mixtures that consist of a thermoplastic base material to which different ingredients such as flowing agents, pigments, UV-stabilizers and other additives have been mixed. The glass transition temperature of the base material and the melting temperatures and melting enthalpies of the additives are characteristic for the toner. These properties can be easily and reliably determined by thermal analysis, in particular with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). This article describes how the two techniques were used to measure a toner sample, and compares the results and information obtained. The work was done with a DSC821e and a DMA/ SDTA861e.

DSC Measurements

The sample was first heated at 10 K/min. Afterward it was cooled at 10 K/min and then heated a second time at 10 K/min. The first heating run shows two endothermic peaks that are not completely separated from each other. In addition, a slight shift of the baseline is noticeable. In the cooling curve, several exothermic peaks can be observed that appear shifted to lower temperature in comparison with the heating curve. Furthermore, a step-shaped displacement of the baseline occurs between 70 °C and 45 °C, i.e. in the same temperature range as the heating run. This indicates that a melting process and a glass transition overlap in the first heating run. In the second heating run of the same sample, only one clear peak is observed. This suggests that the second endothermic peak in the first heating run can be interpreted as an enthalpy relaxation peak that occurs due to enthalpy relaxation in the glassy state on heating (see also UserCom 10, page 13).

This interpretation can be confirmed by measuring new samples at different heating rates. Under these conditions, the melting peak should be more or less independent of the heating rate, but the enthalpy relaxation should shift to higher temperature with increasing heating rates. The experimental results are given in Figure 2. This displays the DSC curves obtained for the first heating runs at heating rates of 0.5, 10 and 150 K/min. 

The curves show that with increasing heating rates the enthalpy relaxation peak does in fact shift to higher temperatures, but that the melting point is observed practically unchanged at about 60 °C.

DMA Measurements

In dynamic mechanical analysis, a sample is subjected to a periodically changing sinusoidal force. In the linear (Hooke’s) region, this leads to a sinusoidal deformation of the sample. The deformation is, however, shifted with respect to time in comparison with the force exerted. The relationship of the force to the deformation amplitude, and the phase shift between the force and the deformation, allow one to obtain information on the molecular dynamics of the sample. Quantitatively, a dynamic mechanical analyzer yields the storage and loss moduli and the mechanical loss factor (damping).

An important parameter in DMA measurements is the period, i.e. the oscillation frequency of the sample. The frequency dependence of measurement curves often allows certain effects to be positively identified. For example, crystallization and melting are generally frequency-independent processes. In contrast, relaxation phenomena such as the glass transition are always frequency-dependent. If, in the case of the toner sample, we assume that the measured curve is due to the overlap of a melting process and a glass transition, then it should be possible to distinguish between the two processes from their behavior at different frequencies. The sample was a fine powder. One way to measure materials in powder form in a DMA is to press the powder to a cylindrical disk. The disks are then mounted in the shear clamp of the DMA.

Conclusions

Overlapping melting and glass transition processes can be easily separated with DSC and DMA. In DSC, the fact that the glass transition is dependent on the heating rate is used to separate the two effects. With DMA, separation is achieved because the frequency dependence of the two effects is different. In principle, both methods yield equivalent results. The sensitivity of the DSC with regard to glass transitions is of course appreciably lower than its sensitivity toward melting processes. The DMA, however, is much more sensitive toward changes in molecular mobility, which occur in melting processes and glass transitions to the same extent. The wide frequency range of the DMA/SDTA861e opens up exciting new possibilities for dynamic mechanical analysis.

^{Thermal Analysis of Toners | Thermal Analysis Application No. UC 152 | Application published in METTLER TOLEDO Thermal Analysis UserCom 15}