Highly filled plastics take on heat and electricity

The ZBT fuel cell technology institute injection moulded this fan-gated bipolar plate already in 2003

Materials groups are exploring thermally and electrically conductive plastic compounds using low-cost mineral fillers. David Vink summarises developments in this feature for Plastics News Europe.

Flexible metering is important to encompass many additives, fillers and reinforcements that continue to “shift the borders of plastics materials into new and innovative application areas”, said Dr Roland Hingmann, vice president for structural materials processing at BASF.

His presentation at the 2016 Plastics Technology Colloquium, organised in February by the IKV plastics processing institute at RWTH University, Aachen, Germany, was one of the papers addressing highly filled thermally and electrically conductive plastic compounds.

Homogeneous dosing and thorough dispersion in gentle processing compounding extruders should retain properties of polymer matrices and added materials. This is “key to the central role fulfilled by compounding between raw material production and further processing of compounds into plastic parts”, Hingmann stated. This has become important as the possibilities of using new monomers as the basis for new polymers have already been largely exploited, he observed.

New compounds require many experiments, screening formulations in small quantities on laboratory equipment, followed by process development with a limited number of trials on scalable pilot plants to obtain “realistic product properties”. Scaled-up trials on full production lines allow determination of final product properties and economic processing costs.

IKV researcher Maximilian Adamy described thermally conductive, electrically insulating thermoplastic compounds prepared with conductive fillers. Fillers were compounded in the BASF B27 grade of PA6 on a Coperion ZSK26 Mc Megacompounder extruder. They included 3M’s Cooling Filler and Momentive’s TECO 2013-1016 CFX600 boron nitrides (BN), HC Starck’s Grade A aluminium nitride and a Silatherm special thermally conductive aluminium silicate from Quarzwerke.

Adamy and Thorsten Hilgers, product development manager in plastics at Quarzwerke, who made a presentation on Silatherm at the VDI plastics in automotive congress in March, were not able to tell Plastics News Europe how aluminium silicate became thermally conductive.

Adamy concluded that 40% overall filler content by combining 20vol% each of Silatherm and BN achieves 6 W/mK in-plane thermal conductivity. Silatherm (lower conductivity, with a price of ?3.20/kg) used with BN (higher conductivity, with a price of ?80/kg) cuts material cost for the same conductivity, or enables higher conductivity for the same material cost. Differing particle sizes and aspect ratios result in synergies improving conductive network formation, Adamy advised. Lower cost fillers with conductivity below BN and Silatherm, such as magnesium oxide, aluminium nitride or quartz, can also play similar if not so effective roles, Adamy said.

In his VDI congress paper, Hilgers said 1.5-2 W/mK thermal conductivity suffices for most applications. He showed PA6 filled with 65% Silatherm T 1360-400AST with similar conductivity levels in X and Z directions of respectively 1.075 W/mK and 1.044 W/mK. While 55% BN gave 5.3 W/mK level in the X direction, it was similar to Silatherm T in the Z direction at 1.04 W/mK. More than 65% BN filler content was not possible when producing granulate from extruded strands.

In a chart shown by Hilgers, Silatherm impact strength was higher at 57.4 J/m2 for 65% Silatherm in PA6 compared with 10.4 J/m2 for 55% BN filling. The latter is less abrasive (Mohs hardness 2), compared with a Silatherm compound (Mohs hardness 9).

Quarzwerke developed a new Silatherm Advance filler for light-coloured applications, filler thermal conductivity raised to 30W/mK from 12W/mK. It has higher density and aspect ratio, but similar conductivities in PA6 of 1.366 W/mK (X direction), 1.04 W/mK (Z direction).

Hilgers showed Silatherm based compound applications. An electric motor stator temperature sensor in Festool’s Planex LHS 225 drywall electric sander involves an Ensinger Tecacomp TC (thermally conductive) compound. A Bosch BLDC brushless DC motor heat-absorbent flange in an automotive heating fan unit is produced in a Lanxess PA6 compound, Durethan BTC75 H3.0 EF (Plastics News Europe April 2016).

Lanxess said, when introducing this 75wt% filled compound with a 65wt% version in 2013, that they involve a “special mineral” filler [clearly Silatherm], as an “alternative to compounds with boron nitride or abrasive aluminium oxide”. The company said both versions are much less expensive than BN based compounds and achieve “almost anisotropic” conductivity of respectively 1.5 and 1.0 W/mK. It referred to 35 kJ/m2 Izod impact strength for the 65% filled version as double that of aluminium oxide based compounds.

In 2016, Lanxess has meanwhile introduced a high light reflection, thermally conductive compound Durethan TP 723-620 with conductivity reaching 2.5 W/mK “along the flow trajectory”. It is not clear whether Silatherm Advance is used here.

PolyOne produces Therma-Tech thermally conductive polyamide compounds. The most recent example is the highly ribbed painted moulded heat sink (replacing metal), in the new larger and brighter 32.4W Orion LED spot lamps made by Sylumis, France.

An automotive LED light unit housing made with a PA66-based Therma-Tech compound won a 2013 SPE US materials category automotive award for replacement of a diecast aluminium heat-sink housing at Mars Automotiv, Turkey. The TT6600-5001 EC grade used has 2.5?5.5W/m/K through-plane thermal conductivity, depending on test method, and 19?21W/m/K in-plane. PolyOne says Therma-Tech saves 13% and 20% overall costs compared with aluminium in respectively LED and transportation lighting.

PolyOne showed the Sylumis and Mars Automotiv heat sinks at the 2016 VDI automotive plastics congress, alongside the 2nd place award in the “new surfaces for plastics parts” category of the 2016 European Plastics Innovation Awards, organised jointly by PlasticsEurope and the Society of Plastics Engineers (SPE).

PolyOne safety data sheets mention quartz, boron oxide and titanium oxide in various Therma-Tech compounds. A US patent 9243178 B2 assigned to PolyOne on 26 January 2016 refers to 20-37wt% of high resistivity (electrically insulating) pitch-based carbon fibre and 15-35wt% boron nitride in organic phosphinate flame retarded PA6 compounds with in?plane thermal conductivity of more than 8 W/mK.

Italian compounder Lati says its Laticonther 62GR/50, a 50% graphite filled PA6 compound with 10 W/mK thermal conductivity, substituted aluminium heat sinks in LED lamp units made by Vossloh Schwabe, Germany.

Andreas Cohnen of IKV pointed in his paper to brittleness and resulting poorer mechanical load?bearing of moulded highly graphite filled plastic fuel cell bipolar plates, compared with milled metal plates.

IKV addressed brittleness by blending 10-30wt% of the Keltan 2070P grade of EPDM from Lanxess into a Sabic PP 579S matrix in a Banbury-cam based Brabender 350 mixer to produce 60wt% and 80wt% graphite and expanded graphite filled compounds. This was done in a MC 15 Micro Compounder from Xplore Instruments (a DSM spin-off) and scaled up to a Coperion ZSK26 Mc co-rotating twin-screw extruder running at 10kg/h.

Mechanical testing on test bars, cut from compression moulded bipolar plates of dimensions 220mm x 200mm x 4mm, resulted in impact strength of 4.5 KJ/m2 (target 5 KJ/m2). Flexural strength of 22Mpa in 80wt% graphite filled PP was close to the > 25 Mpa target without EPDM, but fell to 16Mpa with 30wt% EPDM content. Cohnen showed microscopic photos illustrating different compound morphologies as EPDM content increases. At 0.17 Ωcm, electrical conductivity was 30% higher with expandable than standard graphite.

Cohnen said that in seeking further electrically and thermally conductive compound improvement, IKV will compound other plastic/elastomer/filler combinations with various process parameters in a co?rotating, intermeshing twin screw extruder. Hybrid filler systems based on graphite and carbon black may offer particularly good electrical properties through synergy between these fillers, he said.

As a researcher at the ZBT Centre for Fuel Cell Technology at Duisburg University (European Plastics News, December 2003), Marco Grundler referred in a presentation at the Lower Saxony Materials Technology Symposium in February 2015 to 80wt% graphite filled PA6 and PP for bipolar plates, as well as a special Ensinger Tecacomp PPS THE 3985V conductive PPS compound.

Grundler saw potential of continuous extrusion to produce large plastic bipolar plates cost effectively with areas in square metres, with surface profiles milled after extrusion. ZBT works here with semi?finished plastic products processor Centroplast Engineering Plastics. ZBT and Centroplast managing directors, Prof Angelika Heinzel and Ulrich Terbrüggen, have already presented a prototype of such a large bipolar plate in July 2013. So maybe extrusion will be the next plastic bipolar plate quantum leap?