TECHNOLOGY | THERMALLY CONDUCTIVE COMPOUNDS


Right:


Automotive LED lighting is a potential application area for thermally conductive compounds


additive and polymer suppliers, to compounders and OEMs,” says Van Bellingen. “Obviously, partnerships are necessary to innovate.” A good deal of development activity among additive suppliers is in carbon-based thermally conductive products, most notably graphite but also graphene. Graphite is widely used to provide thermal conductivity to polymer compounds due to its good cost/performance ratio, says Anna Ellett, Field Application Engineer, Polymers, at Imerys Graphite & Carbon. Typical final parts range from heat sinks for LEDs to geothermal pipes. The company supplies a wide range of both


natural and synthetic graphite powders, which Ellett says enables customers to fine-tune thermal conductivity and mechanical performance of compounds through selection of a grade with the most appropriate morphology and particle size distribution for the application. “With Imerys graphite additives, it is possible to achieve an in-plane thermal conductivity level higher than 20W/m.K or a through-plane level of 4W/m.K while maintaining good processability of the final compound,” she says. Variation in thermal conductivity in different directions is common when using thermally conductive additives as many are needle or plate-like in shape, leading to anisotropic behaviour. Timrex C-Therm products from Imerys are high


aspect ratio graphite powders, which are claimed to provide thermal conductivity at typically half the additive loading needed with standard graphite. Ellett says the lower loading gives greater flexibility to design formulations containing other ingredients such as glass fibres or minerals and to


reach the desired mechanical performance at a lower density. “The recently developed Timrex C-Therm MAX HD grade offers easier handling and extruder feeding due to its superior apparent [bulk] density, leading to higher production output of highly thermally conductive compounds,” she says.


Minimising trade-offs


Figure 1: Comparison of thermal conductivity and melt flow rate for various electrically insulative polypropylene-based formulations. Bends 1, 2 and 3 contain combinations of graphite and mineral fillers Source: Imerys


26 COMPOUNDING WORLD | August 2020


With any thermally conductive additive, there is a trade-off between thermal conductivity and processability. Ellett says that to minimise that trade-off Imerys minerals have been used in combination with graphites to achieve superior conductivity and fluidity in polymer compounds (Figure 1). The company has also developed polymer formulations that provide high thermal conductivity while remaining electrically insulating using carefully selected blends of appropriate grades of minerals and graphites. These could be used in applications such as electronic parts, where they may offer cost benefits over established electrically insulating options such as boron nitride and reduced abrasion compared to aluminium oxide, she says. Avanzare has developed a new multilayer pristine graphene — AvanThermal Conductive NCC-1701-A — for improving thermal conductivity in thermoplastics. “Its extremely high aspect ratio, higher than 10,000, due to its large lateral size and low thickness, makes it possible to create a thermal conductive network in different polymer matrices,” says Chief Commercial Officer Javier Pérez. Avanzare previously introduced AvanThermal Conductive 770, described as a “graphene related” material, for similar applications. Peréz says the new grade is more advanced, with improved dispersibility making it more efficient. High thermal conductivity in a thermoplastic


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IMAGE: IMERYS GRAPHITE AND CARBON


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