Increased Heat Stability of Medical-Grade Vinyl Compounds Boosts Output without Tradeoff in Gamma Resistance

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ANAHEIM, CALIFORNIA, U.S.A., January 26, 1999-An advanced heat-stabilizer formulation for clear flexible vinyl compounds can enable medical device manufacturers to increase throughput beyond rates achievable with conventional PVC while yielding products with equivalent or better resistance to gamma irradiation, it was announced today at MD&M West by Peter M. Galland, Medical Industry Manager for Teknor Apex.

"With conventional heat stabilizers that meet FDA requirements for medical applications, the gamma resistance of vinyl falls off as heat stability increases," Galland said. "To avoid the discoloration and property losses caused by irradiation, manufacturers of medical products have had to employ gamma-resistant compounds with lower-than-desirable levels of heat stability. Our new stabilizer formulations not only end this tradeoff but actually yield somewhat better post-gamma color retention, already a Teknor Apex strength."

Manufacturers using compounds containing the new stabilizer package may increase productivity and reduce screen-pack changes, said Galland, because the formulation lowers the risk that the PVC polymer in vinyl compounds will degrade as a result of the heat and shear stress of melt processing.
Teknor Apex will make the improved stabilizer formulation available for new applications by incorporating it in new counterparts of existing grades of its Apex [registered trademark] vinyl compounds, adding the letter U to grade designations. (Apex 90-A471R-75 NT, for example, has the new counterpart Apex 90R-471U-75 NT.) Several improved compounds have been fully tested for compliance with the USP Class VI and cytotoxicity standards required of medical devices and their raw materials. They will be priced at a premium of 1 cent or less per pound over existing counterparts. "The increase will be easily offset by the boost in productivity obtained by the device manufacturer," Galland said.

While all polymers exhibit some degree of color shift and loss of strength upon exposure to gamma rays during medical sterilization, heat-sensitive PVC poses an additional challenge for manufacturers that process the material at very high speeds and temperatures (as in the extrusion of tubing at high throughputs) or under conditions that subject it to very high rates of shear (as in the injection molding of exceptionally thin-wall or complex parts).

"If processed under high heat and shear, the PVC polymer in conventional gamma-resistant vinyl may decompose sufficiently to yield defective products or cause frequent downtime for cleaning degraded material from the screen packs used to filter molten plastic during extrusion," Galland said. "By using compounds incorporating our improved stabilizer package, manufacturers should be able to boost productivity by reducing downtime or increasing line speeds."

To demonstrate the extent to which its new stabilizer formulation resolves such problems, Teknor Apex ran tests comparing two of its conventional gamma-resistant compounds with their reformulated counterparts. The results indicated the following improvements:

· Substantially greater heat stability. In tests of exposure to 180 °C heat, the new compounds took longer to yellow and degrade than their conventional counterparts-in one case, 46 minutes to the onset of degradation, as against 29 for the older compound.

· Equivalent or significantly less color shift, as measured by the delta in yellowness index, after exposure to gamma irradiation. A 75 Shore-A Durometer compound with the new stabilizer formulation exhibited 8 to 45% less change than its counterpart in tests of as-molded and aged test samples under two levels of gamma exposure