A group of U.S. inventors might have found a way to make plastics conductive to electricity and heat without going through a laborious compounding process of mixing additives and resins.
The inventors have patented a method of attaching a polymer chain to a nano-scale metal particle and graphene, and that this composition can exhibit useful electrical, thermal and even magnetic properties. Their research avoided the conventional compounding of conductive additives with a polymer in an extruder, where the mixing action creates high temperatures and pressures that can affect other polymer properties such as strength.
“We can get performance comparable to conventional compounding,” claimed Ralph Locke, an inventor based in Fort Myers, Fla., who was part of the team that received a U.S. patent this summer.
Locke and colleagues spent five years on the project. They reacted nano-sized metal salts with graphene and polymer components to make new materials that have predictable electrical properties. The metal salts can be based on iron, copper, manganese or other transition metals. The polymeric components can be based on polyurethane ingredients such as polyols, copolyesters, polyacrylates and polysulfides.
Locke is in charge of chemistry for Mackinac Polymers LLC, a Fort Myers-based independent research company that applied for the patent, US Patent 9,074,053 issued July 7.
“We don't know of anyone else even remotely looking at this,” said Don Phillips, managing director for Mackinac Polymers, in a phone interview. “We are in the process of designing performance estimates for the materials.”
The researchers claim the new metallic polymers show conductivity 10-14 Siemens/meter to 4,700,000 S/m, within the range of compounds that are conventionally made conductive by mixing carbon particles or metal particles with a polymer in a compounding extruder. They seem to be able to incorporate nano-scale metal and graphene particles into a polymer molecule's backbone or in a side chain attached to the polymer backbone. This leads to a homogenous mixture, rather than a heterogenous one normally made for industry, they claim.
So far, the inventors have experimented mainly with thermosetting polymers such as polyurethanes and copolyesters, but short forays into thermoplastics show promise as well for resins such as polyolefins, Locke explained.
The inventors have met with undisclosed companies in resins, chemicals and other businesses that might be able to take the discovery to the commercial level. Usually the companies' first response is “it's not possible,” Locke said.
“It's very promising,” said Tom Nachtman, a polymer researcher and president of InstaCote Inc., an Erie, Mich.-based company that makes customized protective coatings for specialty industrial applications and nuclear shielding.
“I've seen it work in my own shop,” Nachtman said by phone. He was not involved in the Mackinac research but he did know some of the researchers from previous projects.
As is often the case in groundbreaking research, the inventors were looking in another direction when they realized the potential of their studies. Under contract with an undisclosed client, they were working on a way of making higher density acrylic polymers by adding metals. They started looking at lead salts but when they turned to bismuth salts they got reactions that generated heat in a mysterious way.
Their work met the original goals and then they decided to look at what happens with other metals such as iron, aluminum and silver under similar conditions. They established Mackinac Polymers to conduct the new avenue of research.
“Once we figured out we had something interesting we experimented with oligomers and prepolymers,” Locke recalled. “Now we can do the chemistry for many of the reactions by computer simulations.”
The most promising metal salt particle dimensions were in the 30 nanometer range. Most of the metal salts could be purchased in that form from specialty chemical houses because researchers buy them for a wide range of other nanomaterials projects. Most of the oligomers, which are short-chain polymers, and pre-polymers they studied also were commercially available.
The inventors see huge opportunity for the new conductive materials in applications where functional components or coatings add value to industrial, medical and other products. In addition to conductivity, the materials at different loading levels can act as anti-static agents in packaging. Magnetism is another property they introduced by orienting metallic nanoparticles in a specific way to give north and south poles. Useful conductive properties can be obtained with about 5 percent metal/graphene by weight on a polymer chain. Introducing excess conductive material can result in high electrical resistance, a promising property for capacitor applications.
Phillips said the metal/graphene do not leach out from the polymer. This opens the door to potential antimicrobial uses where silver is attached to the polymer chain to kill harmful organisms on contact. Another benefit is cost saving due to lower usage levels of conductive materials compared to compounding routes.
Early work indicates major mechanical properties such as flexural modulus can be retained in the new process. Initially Mackinac is focusing on automotive, packaging, electronics, medical and aerospace applications.
Locke cited conductive vinyl compounds as one example they explored. A conventional conductive vinyl compound might contain carbon black at a level of 30 parts per hundred parts resin prepared in a compounding extruder or other high-intensity mixer. The operation is messy, with carbon black powder getting on machinery surfaces, operator gloves and the factory floor. Locke said they attached the metal/graphene components onto the vinyl molecular chain and avoided the compounding mess.
A polymer researcher at the University of Akron said if the process works it could be advantageous if the new polymer/metal entity is stable in the presence of oxygen and if the product has beneficial properties besides conductivity.
“What special properties do the metal bring to the polymer,” Yu Zhu said in a phone interview. He said a neutral color could be a benefit since achieving conductivity with carbon black means a conductive compound only comes in one color — black.
Zhu said attaching a metal entity to a polymer chain is tricky and the new process could be more expensive than the cost for conventional conductive compounds.
The metal/graphene/polymer materials appear to survive conditions encountered in injection molding and extrusion, according to Locke. As well, it does not seem cost prohibitive.
The Mackinac team is not aiming to make and sell functionalized resin pellets. They would be open to licensing their techniques to companies willing to take it to a commercial stage.