New ‘Long’ Carbon Nanotubes Show Promise In Thermoplastics, LSR

Novel hair-like nanotubes are said to “love thermoplastics” and to provide electrical conductivity at very low loadings.

Broad areas of potential applications in thermoplastics and thermosets—including liquid silicone rubber (LSR)—are envisioned for a new form of carbon nanotube (CNT) that’s said to be highly compatible with polymers. Miralon CNT sheets, yarn, and dispersion grade come from Nanocomp Technologies Inc. in Merrimack, N.H. The firm started in 2004 and moved to a larger facility in a former paper mill in 2012. It is currently in the pilot production phase, with capacity for around 0.85 metric ton per year. It is raising capital for a planned expansion to 8 m.t. in 2018 and potentially to more than 20 m.t. by 2021. Nanocomp co-founder, president, and CEO Peter Antoinette foresees broader potential for large-scale use of CNTs in existing plastics processes with his firm’s new technology. It uses ethanol and methanol as low-cost raw materials, which are vaporized in a furnace reactor at more than 800 C (1470F) with an oxygen-free atmosphere of pure hydrogen gas and iron as a catalyst. In this furnace, chemical vapor deposition precipitates billions of CNTs that have two unique properties.

First, Miralon CNTs are typically 1 to 10 mm long but only 3-5 nanometers wide, yielding an aspect ratio (length to diameter) of about 1 million to one. That compares with aspect ratios of 1000 to 10,000 to one for typical CNTs, which have similar thickness but lengths in the micron range (1/1000 of a millimeter). High aspect ratio is said to be an advantage over other CNTs, which are powder-like and reportedly tend to agglomerate when added to resins. Despite its greater L/D, Miralon is still invisibly small—roughly 10,000 times thinner than a human hair and 1000 times thinner than standard carbon fiber.

The second key difference is that, unlike powdered CNTs, which are produced as loose, individual fibrils, Miralon CNTs tend to entangle and intertwine with each other in the reactor; they also tend to stick together, forming an elaborate network (see photo above). This makes it easier to collect them out of the furnace and convert them inline to continuous fibers or sheets.

Miralon comes out of the furnace in a cloud of entangled CNTs called a “sock.” The sock can be wound around a drum, where it forms a felt and then can be densified into a nonwoven sheet, which can be slit into tape. Alternatively, the sock can be collected in a spinneret to form a continuous yarn made up of millions of entangled CNTs, each only a few millimeters long.

Miralon yarn can be used in composites and sheet can be made into prepregs or chopped into porous bundles of around 0.05 mm diam. × 1 mm long. Nanocomp offers the chopped “dispersed products” as either a “dry pulp” or as custom masterbatches on request. Chopped Miralon can be used in thermoplastic injection or compression molding, LSR injection molding, and compounding into 3D-printing filament. Nanocomp currently has 13 production lines—six for fiber and seven for sheets.

The company says that multi-millimeter-long Miralon CNTs have been classified by the U.S. EPA as “articles,” not “particles,” unlike CNT powders or loose tubes. This means they are too large to be inhaled or absorbed by the skin, according to Nanocomp. 

PROPERTIES OF MIRALON CNT
Current-generation Miralon yarns have a tensile strength of 0.8-1 GPa, though next-generation products in development will double those values. Miralon yarns also have a strain to failure of 3.5%. The new CNTs have a density of 0.8 g/cc, about half that of carbon fiber. Thus, the material’s specific tensile strength (per unit weight) is currently about half that of aerospace-grade carbon fiber or aramid fiber. But the next-generation product could match or exceed both of these.

When used in a composite, the current products already match or approach the strain to failure of aerospace carbon fibers such as IM7 and T1000. Those fibers may be stronger on their own, but suffer significant “knockdown” when incorporated into a composite. Miralon suffers no such knockdown, Antoinette asserts. He says NASA has produced thermoset composites with Miralon that have twice the specific strength of carbon-fiber composites. And, according to Antoinette, thermoplastics rein- forced with Miralon retain up to 98% of the fiber’s original strain to failure. “Our CNT loves thermoplastics,” he says, referring to good bonding with a variety of resins.

What’s more, Miralon yarn is very flexible, with high strength retention when tied into a knot, unlike carbon fiber. Because of its network structure, it also has a very high surface area, avail- able for bonding to resin and/or for transmitting heat or electricity. Its flexibility/ductility is also valuable in soft body armor and reportedly will not suffer fatigue fracture even at cryogenic temperatures. The CNTs are also said to be highly resistant to radiation, salt, moisture, and corrosive conditions.

Thermal and electrical conductivity are also outstanding properties of Miralon CNTs. According to Antoinette, as little as 0.02-0.05% Miralon can provide electrostatic dissipation (ESD) properties. In an adhesive application, Antoinette says, that compares with 0.15% graphene or 10% carbon fiber to provide ESD protection. EMI shielding can be provided by as little as 5% Miralon or a single layer of Miralon sheet 30-50 microns thick.

Antoinette says Miralon’s thermal conductivity can be appreciated by directing a torch flame at a composite sheet—the CNT fibers disperse the heat rapidly enough to resist melting or burning of the sheet. Miralon is also an infrared emitter. Antoinette demonstrates this with sheet of Miralon connected to an electric power source; a hand placed a few inches from the sheet feels emitted warmth almost immediately, but the sheet itself is barely warm to the touch. 

WHERE IT’S USED
Thermoset composites (epoxy and cyanate ester) with Miralon have been qualified for aerospace use. Miralon sheet is used in a composite radiation/ESD shield and protective EMI layer on thrusters and engine covers of the Juno spacecraft that reached Jupiter last July.

ABS filament containing dispersed Miralon in a 3D-printed bracket is “flying” on a spacecraft right now, Antoinette says. Thermoplastic honeycomb has been 3D printed with Miralon. He notes that 3D-printing filament can also be made to encapsulate continuous CNT fiber.

Nanocomp is working with a number of thermoplastic compounders, including HyComp LLC in Cleveland, to develop thermoplastic compounds with Miralon CNT, using resins such as PEEK. A compression molded Ultem PEI bracket with dispersed Miralon is currently used on a launch vehicle for space satellites. Antoinette says Siemens and various automotive companies are looking at injection and compression molded thermoplastic parts containing Miralon. The CNT sheet also has been infused with molten caprolactam to produce nylon 6 composite via in-situ polymerization.

And that’s not all: Antoinette claims that 0.5% dispersed Miralon can increase the heat resistance of LSR by 100° C.

In the current pilot-production stage, Miralon costs thousands of dollars per pound. As production scales up, Antoinette foresees that cost dropping by an order of magnitude in the next four to six years. 

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