Measuring the Thermal Conductivity of Toner Powders
Measuring the thermal conductivity of powders is challenging with traditional steady-state and transient methods. Such methods provide inadequate containment of the loose material and are ill-suited for powders. Reproducible presentation the sample - with precise control of the compaction of the material - is of utmost importance, as compaction-associated densification of the material will significantly impact the effective thermal conductivity of the powdered material. There is no universal level of compaction recommended - as the compaction should be representative of the application conditions for the material to ensure a representative value for the thermal conductivity is obtained. Where it is necessary to precisely control the densification via controlling the compression of a sample, the TCi's Compression Test Accessory (CTA) provides unique capabilities.
Figure 1. A commercially-available toner cartridge.
Toner materials are a subset of powders with somewhat challenging thermal analysis requirements: they are a complex mixture of carbon black, polymeric carrier materials which may be low-melting plastics or waxes, and some metal salts. Toner cartridges (Figure 1) function in part by heating the toner up above the carrier’s melting point, which bonds the toner to the surface of the paper. Toner powders are extremely finely ground, which creates the potential for trapped air pockets to cause significant disruption in thermal conductivity as a function of pressure. Finally, the heat management involved in the system is complex: the toner is a mixture of materials with very different heat capacities and thermal conductivities. Its components are combustible so over-heating the device could prove hazardous. Finally, the temperatures at which toner cartridges operate range from ambient (when the material is cooled) up to the phase change temperature of the toner material’s carrier, which varies manufacturer to manufacturer. Understanding toner thermal conductivity as a function of temperature and pressure is thus important to toner cartridge design.
Figure 2. L: The C-Therm TCi’s Compression Test Accessory. R: Toner being tested for thermal conductivity using the Compression Test Accessory with the extension applicator inside a thermal chamber.
This combination of attributes challenging to thermal conductivity analysis and wide operational temperature range requires that toner materials be analyzed both under varying levels of compaction and under varying temperatures. A commercially-available toner material was analyzed for thermal conductivity using the C-Therm TCi Thermal Conductivity Analyzer. A Tenney Jr. Thermal Chamber was used to maintain sensor and sample at the desired analysis temperature. The Compression Test Accessory (CTA) with an extension applicator for use in the thermal chamber was employed to precisely observe the level of compaction pressure the toner material was under (Figure 2). The results of this analysis are pictured below:
Figure 3. Thermal conductivity of toner powder at two different temperatures under two different compaction forces.
As can be seen in Figure 3, compaction pressure does not have a significant impact upon the thermal conductivity of the toner at room temperature. However, at 75°C, the compaction pressure has a profound effect upon the thermal conductivity. Increasing the compaction force from 500 gf to 1400 gf causes a 26% increase in the thermal conductivity. This is attributable to the fact that the carrier material of this toner softens near 75°C, allowing the toner to be far more compressible than it is at room temperature, which in turn led to denser packing of the powder and to increased thermal conductivity. These results illustrate the impact of temperature and compaction force can have upon the thermal conductivity of powdered materials.