Single and double bubble tubular film extrusion of polybutylene terephthalate

An experimental study of film formation and structure development of polybu tylene terephthalate (PBT) in single and double bubble tubular film extrusi on processes is presented. PBT was found largely stable in the first stage. In the second stage, the bubble was unstable and biaxial inflation was ach ieved only with a blow-up ratio of one. The sources of second bubble instab ilities were related to the crystalline nature of the first bubble being in flated in the second stage. The structure and properties in the films varie d substantially with processing conditions. While the first bubble had poor ly ordered alpha-structure, the double bubble possessed oriented polymorphi c texture. The second deformation that induced high film-line stress favore d the formation of a stable beta phase. The mechanical behavior of the film s reflected the crystal perfection as well as orientation in the films.