PET is a thermoplastic polymer that is extensively used in the production of packaging material for applications such as drawn fibers, bottles, and stretched films. Its industrial applicability is largely based on the fact that it undergoes strain induced crystallization on deformation just above its glass transition temperature Tg. Crystallization imparts increased density, stiffness, dimensional stability, and resistance to permeability. However, the crystallization process and the mechanical behavior of PET above its Tg is highly dependent on factors such as temperature, strain rate, and the mode of deformation. This makes it necessary to have a reliable material model that can be used in FEM simulations to predict its mechanical behavior.
This thesis is aimed at achieving two goals:
i) to mechanically characterize three PET-PCT blends that have not been previously tested (PET00, PET1.5, and PET12) and to do a comparative study of the five PET-PCT blends. This was done by testing five PET-PCT blends over a range of temperatures and strain rates in uniaxial compression and plane strain compression modes.
ii) to modify the Dupaix-Krishnan constitutive model to predict the occurrence and effects of strain induced crystallization in PET. This involved testing PET and PETG under load-hold conditions to identify the criteria that induce crystallization in PET. Subsequently the material model was modified by incorporating these criteria.
Monotonic tests were conducted for the five PET-PCT blends (PET00, PET1.5, PET3.5, PET12, and PETG) at temperatures of 90C and 100C and strain rates of 0.1/s, 0.05/s, and 0.005/s in uniaxial compression and plane strain compression. The experimental results were then fit to the Dupaix-Boyce constitutive model for these five blends. The model was able to successfully capture the dependence of the material behavior on temperature, strain rate, and strain state above Tg for the five materials. The experimental results were also useful in making a comparison of the mechanical behavior of the five materials to each other. This showed that the behavior of the low PCT content materials were different from that of the high PCT content materials at conditions that favored crystallization.
Load hold experiments were conducted on PET00, PET3.5 and PETG at temperatures of 90C and 100C, and strain rates of 0.1/s and 0.005/s in both uniaxial and plane strain compression. The results obtained were similar to that of the monotonic tests, as they showed that while PET00 and PET3.5 crystallized at certain favorable conditions, PETG did not. Therefore, it was found that the load hold condition was not one of the factors that lead to crystallization. Crystallization occurred in PET only when all of the following conditions were met: i) high strain rates of 0.1/s and above, ii) temperatures of 90C-100C, iii) plane strain compression and iv) after a certain amount of deformation. Based on these findings, changes were made to the Dupaix- Krishnan material model to improve its ability to predict the occurrence and effects of strain-induced crystallization on the large strain deformation behavior of PET near Tg.