TPEs developed by GLS provide 10 times better oxygen-barrier performance than conventional TPEs.
No material is flawless, and all can benefit from some kind of improvement. This concept is what keeps materials scientists in their labs late at night. It is also a concept contributing to the flood of developments in medical plastics.
Offering design flexibility, useful properties, and a range of possible processing operations, plastics often play the role of problem solver in medical applications. Recent obstacles that have tripped up design engineers include the phasing out of lead in products, sterilizing barrier packaging, and improving the oxygen-barrier performance of thermoplastic elastomers. In each of these cases, various plastic types and compounds have provided the elusive answers.
Getting the Lead Out
Since lead landed on the list of banned materials laid out in the European Union’s Restriction of Hazardous Substances (RoHS) directive, manufacturers have been scrambling to replace the illicit material. But this task has not been easy, especially in the case of x-ray shielding applications.
Shielding of medical devices and equipment that produce gamma rays and x-rays is necessary in order to protect nearby people and sensitive electronic components. Responding to this need, GE Plastics (Pittsfield, MA; www.geplastics.com) developed a line of thermoplastic materials with high specific gravity (HSG) that can replace lead in many x-ray shielding applications. LNP Thermocomp HSG composites enable shielding as effective as lead without its associated hazards, or the hot spots or leakage it can produce, according to the company.
GE Healthcare recently turned to the lead-replacement material for use in one of its x-ray machines. The company employed an LNP Thermocomp composite within a collimator, which absorbs stray radiation and limits x-ray exposure.
Benefits of the composite were threefold, according to Clare Frissora, market director, healthcare, GE Plastics. “Lead typically needs to be machined and stamped in order to insert this type of component; here, it’s a secondary operation,” she says. “When you can transition that machining, stamping, and secondary operation into one step of injection molding, you streamline the process and the total system cost has the opportunity to be reduced.”
Frissora adds that the plastic material’s ability to be injection-molded enables design freedom that was nonexistent when using lead. It can also facilitate uniform shielding and may offer better performance than lead, she says. “When [engineers] have gone to this type of injection-molded part, it has enabled tighter tolerance specifications and more part-to-part consistency, which then enhances the performance of the component in the larger x-ray machine,” she says.
In collaboration with custom injection molder Thogus Products Co. (Avon Lake, OH; www.thogus.com), GE Plastics is developing new grades of the thermoplastic that match the specific gravity of lead. The companies are also researching the creation of an elastomeric grade with high elongation for even greater design flexibility than is offered with the current composite.
Clearing the Hurdle
A composite by GE Plastics can replace lead in collimators for x-ray shielding applications.
Originating these lead-replacement composites was not the only project keeping GE Plastics busy. The company has found a use for its Lexan HP Polycarbonate resins in multiple-layer barrier packaging applications.
Terminal sterilization of packaging can present problems in terms of material selection. Some materials cannot withstand the high heat of autoclave sterilization, while others are subject to breakage. Lexan can undergo autoclave sterilization with no adverse effects and is tough; however, the material does not have the barrier capabilities needed for packaging applications.
The conundrum continues: Is there any material that features barrier capabilities and can withstand autoclave sterilization? Perhaps one material cannot live up to all these standards. But GE has discovered that a combination of materials may.
Coinjection blow molding of sterilizable Lexan with such barrier materials as amorphous nylon or cyclo olefins can help overcome this material obstacle. The result of the process is an interior barrier layer sandwiched between two exterior layers of Lexan. Essentially, a protective jacket of Lexan encases the barrier material, according to Frissora.
“The challenge then becomes, how do we process this? Easier said than done,” she says. “You have to blow all these layers at the same thickness distribution. And your barrier is only as good as the thinnest spot. You need uniform interior and exterior layers to ensure a predictable barrier.
“We’re not a molder; however, one thing we do in our business is help our customers learn how to use our materials, so we added the coinjection blow molding expertise,” Frissora adds.
Putting Up Barriers
While GE Plastics is laboring to improve barrier packaging, GLS Corp. (McHenry, IL; www.glscorporation.com) has been working to enhance oxygen-barrier performance. The company has developed custom-formulated TPEs that it claims provide 10 times better oxygen-barrier performance than conventional TPEs. Although the materials were initially engineered to suit the needs of the food-packaging industry, the company quickly realized that they could benefit medical device applications as well.
The company asserts that its product offers numerous advantages over thermosets, which have traditionally reigned as the material of choice for barrier applications. Associated qualities such as clarity and cleanliness are desirable to manufacturers and end-users alike, according to the supplier. Cleanliness is ensured via elimination of halogens and heavy metals often employed in curing thermoset rubber, which can leach.
Owing to its inert nature, the TPE can be used for tubing applications. It can also act as a substitute for silicone. Though valued for its rubbery properties and clarity, silicone is viewed as a poor material for barrier applications. Design engineers could use the TPE in lieu of silicone in order to improve barriers in devices, according to Raj Varma, commercial innovation manager for GLS.
Suitable also as a replacement for butyl rubber, potential medical device applications for the TPE include stoppers, gaskets, and syringe plungers. And while the TPEs exhibit barrier performance comparable to that of butyl rubber, they do not require the multiple-step production demanded by butyl rubber and thermosets which can compromise quality, Varma says. He points out that if each step yields 99% accuracy, quality and accuracy have diminished somewhat significantly after undergoing several steps of the process.
“But with this [GLS] material, you injection mold or extrude it, and the part coming out is usable. And if you try overmolding, you’ve consolidated all the steps,” he says. “As the [industry] goes more toward disposables, this [material allows] you to pretty much complete your disposable product without having to go through the various steps of assembly.”
Among its most beneficial characteristics is the material’s ability to be overmolded, according to the company. Consolidation of parts can save money. Moreover, many disposable-device manufacturers are injection molders and extruders. Using the TPEs enables these companies to mold parts in-house instead of contracting out the service, thereby reducing costs and allowing for increased control over manufacture, Varma says. Moldability also enables flexibility and design freedom, he adds.