TECHNOLOGY | THERMALLY CONDUCTIVE COMPOUNDS


Nabaltec says it is supporting the growing demand for thermally conductive and electrically insulating materials with new products that allow for very high filling levels while still providing good processability and good mechanical properties in the final compound. These include its Apyral HC (ATH) and Nabalox HC (Al2


O3 ) series. All grades


from both series are available from the company’s plant in Germany, while the Apyral HC series products are also produced at Nabaltec’s newest plant in Chattanooga, TN, US, which specialises in production of ground and viscosity-optimised as well as surface treated ATH grades. Data from Nabaltec (Figure 1) shows how


different surface treatments on a fine precipitated Apyral 40CD and Apyral HC 500 affect thermal conductivity (TC) in a filled TPU. To achieve a TC requirement of around 1 W/m·K, a filling level of 60 wt% can be seen to be sufficient. In this case Apyral 40CD with coating B is the recommended


option as it gives the best mechanical properties. Where a higher TC is required, the filling level needs to be increased. To reach 1.4 W/m·K, for example, a filler loading of 70 wt% is necessary and that requires the use of a product that is optimised for TC applications. Data in Figure 2 shows that Apyral HC 500 with coating A gives the best all round result of good TC, acceptable mechanical properties, and high flame retardancy (UL94 V-0 at 1.6 mm).


Surface effects The important role of surface treatments in thermal conductivity of thermoplastic compounds is emphasised by Ido Offenbach, Americas Segments Manager, Specialty Additives at Evonik. “Evonik organo-modified siloxane (OMS) surface treatment chemistry have proved to be essential to improve thermal conductivity in different markets and applications such as automotive, electronics, and appliances,” he says. Producers of aluminosilicate, aluminium oxide,


Figure 1: Thermal conductivity of TPU filled with 60 wt% and 70 wt% Apyral grades with different surface treatments Source: Nabaltec


and hydroxides of aluminium and other metals often use Evonik OMS surface treatment to increase the level of the filler loading in thermo- plastic compounds without compromising me- chanical properties or negatively impacting mould filling during injection, Offenbach says. Tegopren 6875 and Tegopren 6879 are two typical examples from the Evonik OMS product line. “We observed that the thermal conductivity value [W/m·K] of the filler that is added to the thermoplastic compounds is not enough for achieving the target value. Companies must also consider the part configuration (geometry), part thickness (especially if the part contains different thicknesses), and moulding conditions. Therefore, at Evonik we are using a quick screening method to evaluate the thermal conductivity behaviour of thermoplastic compounds via hot plate,” Offen- bach says. “We mould plaques with thicknesses of 2mm,


Figure 2: Mechanical properties of TPU compound filled with 60 wt% and 70 wt% Apyral showing effect on flame retardance (UL 94 V test at 1.6 mm) Source: Nabaltec


42 COMPOUNDING WORLD | August 2021


4mm, and 6mm and assemble them to achieve specimens 12mm thick. By doing that we can eliminate variable conditions that influence the thermal conductivity value. For example, a speci- men that was made with 2mm plaques of PA containing conductive black filler may show thermal conductivity of 1.46 [W/m·K] via hot plate, while a specimen that was made with 6mm plaques has a value of 2.34 [W/m·K]. The difference is due to air content between the plaques, despite the high pressure applied to bring the stacked samples together,” he explains. “We also found that applying conductive pastes


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