Polyethylene terephthalate (PET) is most commonly used in medical devices such as sutures and implantable textiles. Polybutylene terephthalate (PBT), an alternative thermoplastic polyester, is also used in medical devices such as injection molded connectors for pulse oximeters, tips for electrosurgical instruments and clips for breathing masks.
Polyesters are produced by reacting terephthalic acid with alcohol. In the case of PET the alcohol is ethylene glycol, whereas in PBT the alcohol is butylene glycol. Otherwise, the chemistry is similar. Both are semi-crystalline polymers comprised of up to 50% orderly molecular regions (crystals) when cooled from a melt.
When successfully crystallized, PET has better strength and stiffness, including at elevated temperatures, than PBT. PET is also made from less expensive raw materials and is typically manufactured on a larger scale; resulting in a polymer that is generally less expensive.
However, the rate of crystallinity between these two polymers is a key determining factor for applications. The molecular structure is less dense in PET than PBT. This slows the crystallinity rate of PET when cooling and allows for controlled orientation of the molecules in fiber and film processing. This orientation provides maximum strength for sutures and fibers used for textile manufacturing.
PBT has been successful due to its ability to crystallize very rapidly. Fast crystallization allows injection-molding with short cycle times and higher productivity. Complete crystallization also leads to better dimensional stability of molded parts and higher production yields.
PBT is more likely to compete with other fast crystallization polymers, such as nylon 6 (polyamide), than PET for injection molded parts. Nylon 6 also has a similar melt point, good chemical resistance and good mechanical properties. However, nylon is more likely to absorb moisture resulting in mechanical properties that can change with humidity. The dimensions of a nylon component may also change with humidity, which can hinder functionality of a device. PBT absorbs very little moisture and offers consistent mechanical properties and dimensions at various levels of humidity.
Sources:
Scheirs, J; Long, T. Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. John Wiley & Sons. Sept. 2005.
Sepe, M. ‘PBT and PET Polyester: The Difference Crystallinity Makes’. Plastics Technology. October 2012.
Sepe, M. ‘PBT & PET Polyester: Part 2 The Performance Factor’. Plastics Technology. November 2012.