Erie, Pa. — Eastman Chemical Co. and Momentive Performance Materials Inc. are working together on a medical project: two-shot molding of liquid silicone rubber and thermoplastic, with Eastman's Tritan copolyester and Momentive's self-bonding LSR, called the Silopren 47X9 series.
They go together perfectly, like peanut butter and jelly. That was the title of a presentation by officials of the two companies at Penn State Erie's Innovation and Emerging Technologies Conference, June 21-22.
"Self-bonding to copolyester and Tritan is a big driver here," said Vincent Colarossi, Momentive's senior application development engineering for elastomers.
Both Tritan and Momentive LSR go into a range of products. Tritan, which Eastman introduced in 2007, goes into markets such as baby bottles, packaging, building products and electronic films for things like smartphones and e-readers. Eastman introduced the first Tritan medical applications in 2009, said Yubiao Liu, development association for medical applications.
Liu said Eastman has been working on two big medical areas for Tritan: fluid management and blood contact.
"For the fluid management components, because Tritan has a very good, clear toughness and chemical resistance," he said. Tritan also is durable, and it resists cracking during assembly processes such as sonic welding, and laser welding, bonding and marking, Liu said.
Liu added that Tritan copolyester also has good bonding strength, making it a match for fittings for medical tubing. And, he added, Tritan has very good color stability, when it undergoes Gamma/E-beam sterilization.
Liu and Colarossi tag-teamed during their 30-minute talk at the Penn State Erie conference. Colarossi jokingly apologized for using the PB&J analogy during the final presentation before lunch.
Momentive, based in Waterford, N.Y., is a silicone specialist. Eastman is based in Kingsport, Tenn.
Colarossi said the self-bonding nature of Momentive's Silopren LSR 47X9 series is the key for combining LSR with Tritan and other thermoplastics.
"We wanted one-step curing. We wanted a product that's not going to require any secondary operation, such as post-curing," he said.
The resulting hard-soft combinations in a single part are critical for many devices, such as respiratory devices and sealing gaskets and membranes on thermoplastic parts.
"So Eastman and Momentive started working on a project together. The total space is big [for two-component, LSR/thermoplastics products], and so we've got a lot of opportunities. We've got consumer electronics. We've got medical devices. We got wearable devices. And just general industrial applications," Colarossi said.
The presentation by Colarossi and Liu centered on medical products.
Colarossi gave a 101 primer on molding the two materials, which are in most ways direct opposites. LSR is a thermoset, and its liquid components get pumped through a cooled nozzle and runner manifold, then get heated inside the mold, then cured to a thermoset, before you can demold it. Thermoplastics like Tritan are the reverse: They need to be cooled in the mold before being demolded.
"The problem when trying to combine these materials is that you have opposing desires," Colarossi said. You can overmold LSR onto a component from engineering thermoplastics, using a rotating-plate mold that revolves between the injection units for the two separate materials. Or a robot can physically move the piece between two separate injection molding machines.
Colarossi said the standard way is do a mechanical interlock of the LSR to the thermoplastic. He said that could involve drilling holes, undercuts, or do plasma treating or a primer, before you apply the silicone.
"However, much more robust would be to have a chemical bond," he said. "Something where nothing's going to get in between that bond area. Something where you actually have a chemical bond formed."
Colarossi said the self-bonding LSR is made using chemical additives and adhesion promoters that are incorporated directly into the liquid silicone rubber, so the LSR can go directly over certain plastics without needing the special surface treatment or a primer.
"However, it's much more difficult to do this [self-bonding] and substrate selection becomes very critical in designing a project," Colarossi said.
Another challenge for an LSR that does its own bonding: making sure it doesn't bond to metal, as in the mold. If it did, you would never get the part out.
The bonding agent Momentive incorporates into the Silopren LSR 47X9 series adheres to the plastic, but not to metal. "So, as you put it in the mold and cure it, you're going to keep forming those chemical bonds across the plastic and the silicone," Colarossi said. "And when you release it, the trick is to have those bonds formed enough that it'll release from the metal and it'll release together. … So it's critical to have an almost immediate chemical bond developed."
Colarossi explained how they test the strength of the bond.
Liu said Tritan copolyester also is used in medical equipment, along with other materials. He has developed a four-step testing protocol that helps medical device manufacturers measure how different plastic materials perform in the "real world."
Medical housings can crack, discolor and become brittle and sticky after repeated exposure to aggressive disinfectants. Eastman officials introduced Liu's testing protocol June 9 at the Association for the Advancement of Medical Instrumentation Conference in Austin, Texas.
It's a big issue, as the medical world battles hospital-acquired infections. Clorox Healthcare, a maker of infection-control products, has endorsed Liu's four-step testing method.
Liu said Eastman is offering the protocol to the larger industry. "Eastman is trying to bring to the industry a standard way to evaluate chemical compatibility," he said.