Film Blowing of Linear and Long-Chain Branched Poly(ethylene terephthalate)

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.

Preparation and Properties of Poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate)/Polyglycolic Acid (PETG/PGA) Blends

“…Figure 8 (A,B) show that the torque curves of neat PETG and neat PGA gradually shifted upward with an increase in ADR content. According to previous research [ 20 , 28 , 29 , 30 ], the multifunctional epoxide groups of the chain extender could react with both the carboxyl and hydroxyl end groups of many kinds of polyester. The resultant chain extension/branching reactions are responsible for the increase in melt viscosity of polyesters during melt processing.…”

Molecular structure and rheological properties of a poly(ethylene terephthalate) modified by two different chain extenders

“…This effect is related to the strongly branched molecules and to gel structures and should be beneficial for processing applications with distinct elongational components, when the uniformity of the processed items is considered. For films blown from a high viscous PET with a laboratory equipment, it has been shown that the modification with 0.4 wt% Joncryl improves the bubble stability and the uniformity of the film thickness 56 . However, a too large gel content may lead to a reduced extensibility of the polymer melt, 56 and optical properties of the films may be negatively affected.…”

“…For films blown from a high viscous PET with a laboratory equipment, it has been shown that the modification with 0.4 wt% Joncryl improves the bubble stability and the uniformity of the film thickness 56 . However, a too large gel content may lead to a reduced extensibility of the polymer melt, 56 and optical properties of the films may be negatively affected.…”

Effect of the Elongational Flow on the Morphology and Properties of Polymer Systems: A Brief Review

“…In this case, the polymer experiences non-isothermal elongational flow and the overall behavior of the material is determined by its rheological response, as well as by the variation of its rheological properties with temperature and by the rate of solidification (or of crystallization for semicrystalline polymers) [61]. The rheological behavior of a polymer melt subjected to non-isothermal elongational flow is typically evaluated through Rheotens (or other very similar devices) experiments, involving the application of a uniaxial extension under a constant tensile force on the molten filament [62,63]. During this kind of test, the tensile force needed for the elongation of the extruded filament is measured as a function of the draw ratio, obtaining tensile-force vs. drawdown-speed curves [64].…”