Antec Correspondent: Comparison of Polycarbonate and Copolyester Resins Reveals How Physical Properties Affect Mouldability

At the largest technical conference in the plastics industry—Antec 2013, held on April 21-25, 2013 in Cincinnati, USA—a presentation was made which provided results of a comparison of the mouldability of polycarbonate and copolyester. The presentation was made by Ian Menego, an engineer at the US subsidiary of German polycarbonate manufacturer Bayer MaterialScience.

The paper was entitled Material Properties and Their Influence on Moulding Productivity and Efficiency of Medical Resins and was held as part of the Injection Moulding session of the conference.

The press release announcing the paper is as follows. Both physical properties and processability play key roles when injection-moulded parts are mass produced. When producing medical parts, a consistent, high degree of accuracy is also vital. It is relatively simple to adjust a process to account for differences in molecular weight once that process is optimised for a specific polymer. It is much more complex to adjust the process for changes in polymers.

Bayer MaterialScience engineers Ian Menego and Mark Yeager along with Bayer scientist Dr Pierre Moulinie performed a study to determine the processability of three resins and the effect their physical properties have on their mouldability.

The study looks at three transparent resins—a standard medical grade polycarbonate, a high-heat polycarbonate and a medical copolyester resin—moulded into a part with precise features typical in medical part designs. Through this process, the authors considered the physical properties of the materials and how they influenced their cycle times. They also compared the variability of moulded parts and the energy consumed during production. These factors serve as bases for insights into the relative total costs to manufacture medical devices using each of the three resins.

Menego explained that in their efforts to determine the optimal cycle time for each resin, they discovered that the cycle time depends more on the modulus of a material—particularly at the cooling temperature—than on the material’s heat resistance or viscosity. Due to their high stiffness properties, both the polycarbonate and high-heat polycarbonate resins achieved very rapid cycle times. On the other hand, a copolyester with a lower modulus required a much longer cycle time.

“We observed greater weight variability in the medical copolyester that we tested,” said Menego. “We feel that it’s at least partially because of the greater viscosity fluctuations that copolymers undergo as well as the greater viscosity fluctuations with temperature change.”

The study’s final experiment investigated the energy consumption of each of the three resins as they were processed for use in a medical device. Energy consumption for the copolyester resin was found to be greater than the two polycarbonate resins, potentially due to the copolyester’s relatively longer cycle time. “Going against conventional wisdom,” explained Menego, “increasing the polycarbonate resins’ melt temperatures actually reduces the energy consumed per part.”