Polyester is an engineering polymer that is commonly used for food and beverage packaging applications due to its strong mechanical properties, high transparency and strong permeability resistance. However, the as-polymerized polyesters are low in tensile properties such as tensile strength and toughness due to their low average molecular weights immediately after polycondensation synthesis. In order to enhance the tensile properties, a second stage synthesis, e.g.,solid state polymerization and reactive extrusions with the use of chain extenders, is employed after the initial polycondensation process. The use of chain extenders has been shown to speed up the second stage polymerization reaction. In this study, four different concentrations of chain extender (0%, 0.15%, 0.25% and 0.5%)...[ Read more ]

Polyester is an engineering polymer that is commonly used for food and beverage packaging applications due to its strong mechanical properties, high transparency and strong permeability resistance. However, the as-polymerized polyesters are low in tensile properties such as tensile strength and toughness due to their low average molecular weights immediately after polycondensation synthesis. In order to enhance the tensile properties, a second stage synthesis, e.g.,solid state polymerization and reactive extrusions with the use of chain extenders, is employed after the initial polycondensation process. The use of chain extenders has been shown to speed up the second stage polymerization reaction. In this study, four different concentrations of chain extender (0%, 0.15%, 0.25% and 0.5%) has been used to investigate the effect of chain extender on microrheology of polyesters, besides, we aim to study whether it is advantageous to introduce chain extenders during the polycondensation process such that the second stage polymerization can either be shortened or removed.

First, effect of chain extender concentration on the dynamic rheological response of the PET resin was studied. Dynamic frequency sweeps show that branching effects introduced in the PET resin leads to a significant shear thinning effects, and the relaxation time constants in the polymer resin are enhanced as shown in the step-strain stress relaxation behavior. The shear-thinning response implies that it is possible to achieve higher molecular weight during a one-stage polymerization process. Additionally, the elasticity of the PET resins is improved as shown in the Cole-Cole plots.

Second, whilst the effect of chain extenders shows a lower crystallinity both on the static crystallization behavior and stress-induced crystallization of PET resins, it results in due to the restriction of chain mobilities. Furthermore, the reduction in Tm and Tcc, while the increasing in Thc and Tg are observed after original PET being chain extended by different concentrations of chain extenders. As to the thermal stability, PET with the addition of 0.5% chain extender owns 7°C higher degradation temperature compare with original PET.

Third, the enhanced elasticity leads to enhanced drawability of the resin after glass transition. This means that the chain-extended PETs are able to form large bubbles for blowing into bottles during the bottle blowing process. However, for bottle applications, high-crystallinity is necessary for both mechanical and permeability resistances; we find that higher drawn ratio (5:1) could be applied to obtain similar crystallinity in order to fulfill the mechanical requirement.

In summary, the use of chain extenders during polymerization is effective in enhancing molecular weights of PET during polymerization due to the stronger shear thinning effects induced by the branching effects. The branching effects are also responsible for the enhanced elasticity of resin after glass transition and higher drawability. In the meantime, higher orientation can make up lower stress-induced crystallization of chain extended PET during bottle blowing process.