Abstract
Film blowing of Poly(ethylene terephthalate) (PET) is challenging due its inherently low melt viscosity and poor melt strength. In this study, it is shown how the rheological properties of a commercial PET can be altered by reactive extrusion using either pyromellitic dianhydride (PMDA) or a multifunctional epoxy (Joncryl® ADR 4368) as chain extender, in order to improve the processing behavior during film blowing. The modified materials were characterized by shear and elongation rheometry and relevant processing characteristics, like melt pressure, bubble stability, and film thickness uniformity, were used to assess the influence of the type of modifier on processing and product performance. It is shown that PMDA is useful to increase the melt strength which leads to an improved bubble stability, while epoxy modified PET shows a reduced drawability that can cause problems at high take-up ratios. On the other hand, the epoxy modifier indicates a pronounced strain hardening during elongational deformation, and therefore leads to a better film thickness uniformity compared to the neat PET and the PET modified with PMDA. The differences with respect to processing performance are discussed and ascribed to the molecular structure of the materials.
Highlights
Polyester films used, e.g., for packaging, adhesive tapes, medical applications, or furniture surfaces, are generally produced by the cast-film technique with a subsequent biaxial stretching process
It is shown that pyromellitic dianhydride (PMDA) is useful to increase the melt strength which leads to an improved bubble stability, while epoxy modified Poly(ethylene terephthalate) (PET) shows a reduced drawability that can cause problems at high take-up ratios
The epoxy modifier indicates a pronounced strain hardening during elongational deformation, and leads to a better film thickness uniformity compared to the neat PET and the PET modified with PMDA
Summary
E.g., for packaging, adhesive tapes, medical applications, or furniture surfaces, are generally produced by the cast-film technique with a subsequent biaxial stretching process. These special molecular characteristics lead to a low melt strength which is of disadvantage for other processes, like blow molding or film blowing To overcome this deficiency high Mw resins ([η] = 0.7–0.85 dL/g [2]) are typically used for PET bottles manufactured by blow molding. While no films could be blown with the neat PET, the authors found an improved bubble stability with increasing PMDA concentration This has been correlated to an enhanced melt strength and is in line with investigations on polyethylene [14]. As the question concerning which of the two chain extenders is more suitable for film blowing applications is of high practical and industrial relevance, the aim of this contribution is to modify commercial PET with either PMDA or Joncryl and to compare the processing behavior. The molecular and rheological properties were analyzed and correlated to processing and end-product quantities, like extrusion pressure, bubble stability, and film thickness uniformity
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