The scientists who develop new plastics have three levers they can pull when trying to add new functionality to their materials. They can tinker with the polymer molecule itself. They can impart new properties through filler and additive technology. Or they can do both, which is the tack GE Advanced Materials can now take with its LNP specialty compounds business.
Already known for plastics that offer "extras" like internal lubrication, thermal conductivity, and static dissipation, the LNP group plans to introduce more ground-breaking plastic compounds over the next few years. A couple of advances stand out. The company has developed "smart" plastics with tunable electrical properties that allow them to act as temperature controls. And the company is working to commercialize the next generation of plastic gear materials.
Can you turn a molded plastic part into a thermostat? Sure, if you mold it from thermoplastic compounds that have a positive temperature coefficient (PTC), says Anne Bolvari, a Ph.D. materials scientist and Americas technology manager for LNP specialty compounds.
The PTC effect, whereby electrical resistance increases with temperature, is a familiar one to many in the electronics industry. While many PTC materials are metals or ceramics, polymeric PTC materials exist, too, and have gone into sensors and thermistors for overcurrent protection. In these applications, the materials work like a fuse. When they hit a "trip" temperature, their microstructure changes, and their electrical resistance increases dramatically. Cool the materials below that trip temperature, and they regain their conductivity.
"We're still studying exactly how the effect works in our thermoplastic materials," Bolvari says. But she explains that conductive plastic compounds in general have been formulated so that their ceramic, graphite, or carbon fillers create conductive paths through the polymer matrix. With PTC compounds, the rising temperature somehow interrupts those pathways—by causing the base polymer to expand and instituting other changes in the plastic's microstructure. Once the material cools down, the conductive pathways "re- connect." Right now, GE is looking at compounds whose conductivity would switch on and off at a variety of temperatures.
GE Advanced Materials has a different take on polymeric PTC than those who make materials for sensor applications. According to Nitin Apte, an engineer and global product manager for LNP, the current work involves incorporating PTC into thermoplastics that can be injection-molded or extruded into complex, three-dimensional shapes. Earlier polymeric PTC materials, on the contrary, are usually deposited into thin films.
To come up with thermoplastic PTC compounds, GE material developers have to carefully match base resins to the right fillers and other additives. The company's current line-up of conductive materials includes compounds based on PPS, PP, liquid crystal polymers, nylons, PBT, and PEI. Bolvari can't say whether all these materials will work in PTC compounds. "Our big challenge is coming up with material and filler combinations that offer a sharp, repeatable transition point," Bolvari says.
And that challenge is ongoing. Apte acknowledges that the company is still at least a couple of years away from fully commercializing its PTC compounds. But these materials have already attracted some interest. GE has been working with a handful of development partners to evaluate the materials, particularly in automotive applications. Apte says that potential uses include mirror housings that heat up to melt ice and snow but shut off when they get too hot. The company is also looking at the materials to regulate the temperature of diesel fuel pre-heaters.
On-Off Plastics: LNP compounds may soon include injection-moldable materials whose electrical conductivity drops to nothing when the materials reach a predefined temperature. These "positive temperature coefficient" materials could act as thermostats in a variety of applications.
GE Advanced Materials has also focused some of its development efforts on creating new compounds for gears or other moving plastic components that suffer from friction and wear. "You might think that the portfolio of gear materials is mature," says Apte, noting that dozens of engineering thermoplastics materials already make good gears. But he believes the next generation of wear-resistant materials will come from improvements to internal lubricants.
As an alternative to applied lubricants, LNP and other material suppliers have long enhanced their wear-and-friction materials with internal lubricants such as PTFE, silicone, and other proprietary additives. That's nothing new. But GE has recently been working on a new kind of controlled release lubricant.
Like traditional internal lubricants, these controlled-release additives are dispersed throughout the polymer. Unlike traditional lubricants, though, the new ones remain "inert until activated by a combination of heat and friction," Bolvari says. She won't reveal too much more about these proprietary additives, other than to say they have a new proprietary form of lubricant.
Wear More: To make its portfolio of LNP wear-resistant materials even better, GE Advanced Materials is working on controlled-release internal lubricants that wear longer and leave the plastic with better mechanical properties.
Bolvari notes that early work on the controlled release materials shows that, depending on their base resin, they offer two to three times the wear performance of materials compounded with traditional lubricants. "That opens up a couple of doors," she says. One is that the controlled release lubricants could help bring plastics to new heights of wear performance. Or they could match the wear performance of existing grades but do so at lower loadings. "To get to highest levels of wear performance today, you really have to load up the plastic with fillers," she points out. But the higher the loadings, the greater the chance of reducing the material's mechanical properties. The new lubricants may steer clear of this trade-off.
As with the PTC compounds, the new controlled-release lubricants won't appear in fully commercial compounds for a couple more years. But GE Advanced Materials will soon be ready to evaluate the materials with development partners.