Electrically and thermally conductive material/process innovations
Monday, January 18, 2016
Note: This is the second article of a three-part series covering conductive polymers (1) trends, (2) material/process advances and (3) applications.
Within a mixture of mature and simultaneously evolving technologies, there are numerous conductive polymer applied research projects underway throughout industry and universities. Those projects include both base resins and new conductive additives/structures, including multilayer extrusion.
For example, compounds of carbon black have historically had excellent price-performance ratios but their adverse effects on mechanical properties are a major drawback. Cabot has continued to develop its superconductive carbon black Vulcan Xcmax family that overcomes this problem by providing high conductivity at low carbon-black loadings.
Elsewhere, new thermally conductive materials that work separately or in combination with electrically conductive systems are advancing such applications as energy-harvesting possibilities to power miniature electronic devices.
By way of introduction, let's start with conductive-filled thermoplastics. Conductive and antistatic additives for compounded conductive polymers play an important role in electronic device development providing inexpensive electrostatic discharge (ESD) and electromagnetic interference or radio frequency interference (EMI/RFI) protection.
Most plastics have extremely high resistance to electron passage, greater than 1015 ohms, making them excellent insulating materials. However, plastics can be compounded with conductive modifiers to alter the polymer's inherent resistance. Electrically conductive thermoplastic compounds made from resins modified with conductive additives include: carbon-based (powder and fibers); metal-based (solids and coatings); all-polymerics (inherently conductive polymers, or ICPs).
By varying the percentage and type of conductive additive used, the plastic material's degree of electrical resistivity can be controlled. But using conductive filler to achieve polymer conductivity can often compromise processability, performance, part weight and total part economics. Loadings of conductive fillers as high as 50 percent by volume that are needed to achieve the desired conductivity may severely degrade mechanical properties.
Surface resistivity (ohm per square) from electrically insulating plastics to electrically conductive metals.
At a threshold concentration unique to each conductive additive/resin combination, resistance through the plastic is lowered enough to allow electron movement. Increasing additive content reduces interparticle separation distance. The critical loading threshold decreases dramatically with increasing aspect ratio (particle length to width) of the filler particles since longer particles cover a greater distance of the conductive pathway.
At a critical distance, the percolation point, resistance decreases dramatically and electrons move rapidly. Carbon-black-based concentrates and compounds, which offer an excellent price-performance ratio, continue to account for a major portion of conductive plastics applications.
Superconductive specialty carbon blacks are a benchmark starting point. Carbon blacks are important conductive additives. Applications use special conductive carbon blacks to provide electrical conductivity and prevent the risk of electrostatic discharge (ESD). These applications include electronics and electronics packaging, clean-room equipment, automotive fuel systems, semi-conductive cable, and industrial parts used in locations with explosion risks.
An improved line of superconductive specialty blacks for use in plastics applications has been commercialized under the trade name Vulcan Xcmax by Cabot Corporation. These products provide among the highest range of conductivities achievable with carbon blacks as well as excellent cleanliness and consistency.
The Vulcan XCmax family was developed for use in plastic applications requiring high conductivity at low carbon-black loadings in order to minimize the impact on reducing the underlying mechanical properties of a given polymer. The Vulcan Xcmax family is designed for use in wire/cable, antistatic flooring/systems, automotive fuel tanks/inlets, coatings and electrical/electronics products.
Vulcan Xcmax electrical cable and automotive fuel tank inlet applications.
Next, let's look at an additive yields significant thermal conductivity enhancement. Carbodeon has commercialized a thermal filler using nanodiamonds that increases the conductivity of thermally conductive polymers by 25 percent, providing significant performance increases for polymers used in electronics and light-emitting diode (LED) manufacture.
The thermal conductivity of the PA66 (Polyamide, or nylon 66) is increased by 25 percent averaged across all planes (in-plane and through-plane) in comparison to PA66 filled with 45 percent boron nitride. Roughly 44.9 percent by weight percent boron nitride and 0.1 percent uDiamond nano-diamond powder by Carbodeon is used. The increase in thermal conductivity is achieved without affecting the electrical insulation or other mechanical properties of the material.
Figure shows measured thermal conductivity values (W/mK) for PA-66 polymer reference material (two left bars). The middle bar shows thermal conductivity values for the same PA-66 polymer enhanced with 44.9 percent by weight percent Boron Nitride thermal filler material. The right side bar shows how thermal conductivity is further enhanced by replacing only 0.1 weight percent Boron Nitride with the same amount of uDiamond nanodiamond powder.
The performance improvements achieved result from the extremely high thermal conductivity (TC) of diamond, at around 2,000 W/mK (Watts per meter, degree Kelvin), the standard TC measurement. The key is to tune the surface chemistry of the diamond particles and mixing process to develop a nanocomposite in which the diamond is well interfaced to the polymer molecules.
The active surface chemistry inherent in detonation-synthesized nanodiamonds has historically presented difficulties in application of these promising 4-6 nanometer particles, making them prone to agglomeration. Carbodeon tunes that surface chemistry so that the particles are driven to disperse and to become consistently integrated throughout parent materials, especially polymers. The much-promised properties of diamond can therefore be imparted to the polymer at low, economic concentrations.
Finally, let's review technology trends in extruded multilayer conductive fuel line tubing. There is an increasing demand for conductive automotive fuel lines due to safety reasons. The flow of hydrocarbon and other nonpolar liquids over plastics can cause static electricity build-up. Any spark due to sudden electrical discharge in the fuel lines can cause an explosive effect.
Conductivity eliminates static electric charge build-up by dissipating the electric charge, avoiding risk of fuel ignition. Many global standards such as SAE J1645 require that supply and return lines for carrying liquid fuel shall have conductive properties to allow electrostatic discharge (ESD).
K Channel—Ube and Kuraray
Multilayer nylon conductive fuel line.
Ube and Kuraray jointly commercialized a new generation of multilayer tubing systems for fuel lines under the brand name Ecobesta-9T. Their multilayer tubing system based on heat stabilized Ubesta PA12 (PolyAmide. or nylon 12) outer layer, nonconductive PA 9T middle layer, and a specialty developed conductive PA 9T inner layer, provides lower permeation performance, excellent conductivity and negligible monomer/oligomer elution at a better cost/performance balance.
The recently developed conductive PA 9T uses highly conductive carbon black that allows lower carbon black loading. This conductive material provides outstanding extrusion processability, maintains excellent mechanical properties, and exhibits lower permeability to gasoline, especially to low alcohol-based types, together with excellent chemical resistance in contact with fuel and lower monomer/oligomer elution to prevent risks and problems from clogging in the fuel injector.
To minimize the environmental impacts of cars and comply with stringent emission regulations the fuel system also has to be equipped with barrier materials to minimize permeation of the fuel. The combination of Ubesta PA 12 outer layer and Genestar PA 9T inner layer fuel barrier material delivers on low emission requirements led the way here and with succeeding Sunbesta-ZV systems are now being commercialized by Ube, Kuraray and Asahi Glass (AGC).
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