materials feature | Thermal conductivity
Surface analysis of thin film made using the University of Michigan’s thermally conductive polymer
Exploring molecular engineering for
thermal management
A University of Michigan (U-M) research team has made a plastic blend that is claimed to conduct heat 10 times better than its conventional counterparts. Rather than using additives to increase thermal conductiv- ity, they focused on optimising the molecular structure of the materials. “Researchers have paid a lot of attention to designing polymers that
conduct electricity well for organic LEDs and solar cells, but engineering of thermal properties by molecular design has been largely neglected,” says Kevin Pipe, U-M associate professor of mechanical engineering. He led the project with Jinsang Kim, associate professor of materials science and engineering. Heat energy travels through substances as molecular vibrations. For
heat to efficiently move through a material, it needs continuous pathways of strongly bound atoms and molecules. Otherwise, it gets trapped, meaning the substance stays hot. “The polymer chains in most plastics are long and don’t bind well to each other,” says Pipe. “When heat is applied to one end of the material, it causes the molecules there to vibrate, but these vibrations, which carry the heat, can’t move between the chains well because the chains are so loosely bound together.” The researchers devised a way to strongly link long polymer chains of
polyacrylic acid (PAA) with short strands of polyacryloyl piperidine (PAP). The new blend relies on hydrogen bonds that are 10-to-100 times stronger than the forces that loosely hold together the long strands in most other plastics. “We improved those connections so the heat energy can find continuous pathways through the material,” Kim says. “There’s still a long way to go, but this is a very important step we made to understand how to engineer plastics in this way. Ten times better is still a lot lower heat conductivity than metals, but we’ve opened the door to continue improving.” ❙
www.umich.edu
24 COMPOUNDING WORLD | February 2015
products is occurring. “Downstream understanding of how to use these compounds has come a long way in that time,” he says. “People are now coming to us with designs that are much more appropriate for thermo- plastics, they understand that they can’t use them as drop-in replacements for metals.” RTP is also doing increasing business in replace- ment, not of metals, but of non-conducting thermoplas- tics where just a moderate level of thermal conductivity is very useful for thermal dissipation. Without going into specifics, he says motor housings are a potential market for this type of materials replacement. Numerous other independent compounders are also emphasizing thermally conductive compounds. Ensinger, for example, offers compounds based on a wide range of polymers and with different additives depending on the level of conductivity required, the processability, and price, and also the level of electrical conductivity required. It says components made of Tecacomp TC provide thermal conductivity of between 1 and 25 W/(mK). Lehmann & Voss is another active supplier, with its
Luvocom range. It cites applications ranging from heat exchanger plates in a polypropylene compound through to heat sinks for hospital operating theatre lights in a compound based on polyetherimide. Other major polymer suppliers are also making
moves in conductive compounds, either through internal development or through acquisitions. Last October, for example, Celanese acquired the assets of Cool Polymers, which as the name suggests, was a compounder focusing strongly on such materials. Celanese said at the time that Cool Polymers’ technical capabilities in the LED market “will allow for immediate customer growth while continuing to advance Cela- nese’s engineered materials business across thermal management and electrical conductivity polymer applications.” Cool Polymers’ portfolio includes thermally conductive thermoplastics and elastomers with and without electrical conductivity. Lanxess added Durethan TC thermally conductive
polyamides to its range last year. Its first two grades are easy-flow polyamide 6 variations, Durethan BTC65 H3.0 EF and BTC75 H3.0 EF. Their high thermal conductivity is based on reinforcement with 65% and 75% of a “special” mineral, details of which are not disclosed. “Both materials display a very good balance between high thermal conductivity, outstanding mechanical properties and good processing behaviour,” says Detlev Joachimi, head of Durethan product development at the company, adding, “They have been approved by two international automotive suppliers”. The thermal conductivity (determined by the
www.compoundingworld.com
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