materials feature | Thermal conductivity
Thermal conductivities of various additives for plastics
progress with its product, despite the increasing competition. “We have discovered that our new grade PCTP30D works beautifully for a wide range of poly- mers, and has become our workhorse for plastic compounding applications,” says Neelam Kumar, product manager for CarboTherm fi llers at Saint-Gob- ain Ceramic Materials. The company markets this grade as a free-fl owing loosely agglomerated powder for cost-sensitive, high-volume applications.
Source: HPF/ Quarzwerke
Asked how the market is developing, Kumar says that the momentum that was started by the LED market has prompted developers to look at many new and novel applications where a thermally conductive and dielec- tric polymer can be used. “The range of BN in polymers has spread much beyond LED heat sinks, where it fi rst started,” she says.
As to the high price of BN, Kumar replies thus: “BN has always co-existed with competing materials in all the markets it serves. Like any other material, if BN meets the unique challenges of an application with its unique electrical, thermal and mechanical properties, customers will pay for the value proposition it brings.”
Carbon keeps it cool
Chart showing the infl uence of conductive fi llers on the mechanical properties of PA6 at 65 wt% loadings (with BN mechanically stable compounds could only be produced up to 55 wt%). Stiffness increases with the degree of fi lling, while impact strength falls. Finer products show higher impact strength. Modifi ed aluminosilicate provides the best impact strength Source: HPF/Quarzwerke
thermal conductivity along all three axes; in-plane conductivity is similar to that obtained with boron nitride, but through-plane conductivity is lower. So if higher through-plane conductivity is required, the two additives can be blended together. Compounders will probably want to try and use as little BN as possible, since it can cost upwards of €50/kg, whereas alumino- silicates sell for under €4/kg. Boron nitride shows the best through-plane thermal
conductivity, but is very anisotropic. Aluminosilicate provides in-plane thermal conductivity up to 2.3 W/m*K and up to 1.35 through-plane. Silatherm is available in different particle size
distributions and surface treatments adjusted for different polymer matrices. Meanwhile, one boron nitride supplier reports good
18 COMPOUNDING WORLD | February 2015
Daniele Bonacchi, a polymer application scientist at Imerys Graphite and Carbon (formerly Timcal), says that graphite is the best solution for thermally conduc- tive plastics when electrical insulation is not required. He says that graphite has proven to be more effective and economical than other solutions; it has a very high thermal conductivity and is affordable. “When graphite is added to a compound it gives an anisotropic thermal conductivity that can be maximized for an effi cient thermal management,” he says.
For products such as heat sinks for LED lamps, where ribs are usually present, high anisotropy can be an advantage as the thermal energy is more effi ciently removed away from the heat source. In the case of extruded heat exchangers such as cooling pipes, low aspect ratio graphite grades are preferred.
For weight-saving solutions, Imerys has developed a
special graphite called C-Therm that gives high thermal conductivity at low concentration. An alternative to graphite, when low conductivity is
targeted, is conductive carbon black. The high structure of conductive carbon black is important to decrease the number of thermal contacts that are detrimental to the fi nal conductivity, while the higher carbon black crystallinity improves the performance of the fi nal material. “The low thermal conductivity of the fi nal compound compared to the intrinsic conductivity of the fi ller can be explained as the phonons do not pass as easily through interfaces (particle-particle or particle-
www.compoundingworld.com
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