Effects of microcrystallinity and morphology on physical aging of poly(ethylene terephthalate)

Abstract

Physical aging characteristics of poly(ethylene terephthalate) have been evaluated in relationship to volume fraction levels of crystallinity up to 25%. Changes in the enthalpies of relaxation, monitored at aging temperatures from 55 to 658C, are found to give good fits with the Cowie-Ferguson model. Overall equilibrium enthalpy of relaxation values decrease linearly with increased crystallinity. They increase with decreased aging temperature, providing extrapolated lower temperature results that are validated in terms of specific heat relationships. Activation energies for enthalpic relaxations are found to increase from 337 to 361 kJ/mole as crystallinity increases up to 25%. Overall relaxation endotherms are further resolved into contributions from interspherulitic and intraspherulitic amorphous regions. Interspherulitic, equilibrium enthalpies of relaxation decrease with increased levels of crystallinity, while intraspherulic values show corresponding increases. Characteristic relaxation times of the intraspherulic regions increase greatly, as levels of crystallinity increase; however, interspherulitic relaxation times decrease very slightly. Dynamic differential scanning calorimetry results show two glass transitions in the case of a 25% crystalline sample and a single transition for noncrystallized material.

Effects of microcrystallinity and morphology on physical aging and its associated effects on tensile mechanical and environmental stress cracking properties of poly(ethylene terephthalate)

“…The detailed effects of microcrystallinity and morphology on physical aging of poly(ethylene terephthalate) have been reported previously1 in terms of specific changes in the relaxation endotherms, observed with differential scanning calorimetry. It was found that overall equilibrium enthalpy of relaxation values decrease linearly with increased crystallinity and with increased aging temperatures.…”

“…PET is a widely used engineering plastics which will undergo physical aging when stored below its T g . Both amorphous and partially crystalline PET can undergo physical aging 1, 3. Mininni7 reported a correlation between the mode of sample failure and excess free volume by studying amorphous PET.…”

Stress cracking e ataque químico do PET em diferentes agentes químicos

“…Entretanto, o fato da molécula do líquido dificultar a absorção pelo polímero, não o impede de atuar como agente de ESC [6] . Zhou [11] & Moskala [8] utilizaram soluções aquosas de hidróxido de sódio como fluido ativo para o PET. O primeiro utilizou a concentração de 0,1% para avaliar a resistência ao ESC do polímero em função da cristalinidade, observando uma diminuição da resistência ao stress cracking com o aumento no grau de cristalinidade.…”

“…Uma das possíveis explicações, também válida para o presente estudo, seria fundamentada na hipótese de que as fissuras presentes em grande número poderiam mutualmente se anular, diminuindo os efeitos de concentração de tensão que existem em peças com poucas fissuras [30] . Uma correlação inversa entre os efeitos de stress cracking e o tempo de falha também foi observado por Zhou [11] . Uma outra questão a ser considerada no presente estudo é a possibilidade de haver reações de hidrólise nas moléculas de PET em presença de soluções de NaOH.…”

A numerical investigation of the effect of preform length for the fabrication of 1.5Lt PET bottle through the injection stretch blow molding process

“…The material parameters were taken by earlier SBM and experimental studies. [13], [19]. ANSYS POLYFLOW has also been employed for the investigation of PET SBM by another study [13].…”