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

Abstract

Poly(ethylene terephthalate) samples with volume fraction levels of crystallinity up to 27% have been evaluated in terms of physical aging, tensile mechanical properties, and environmental stress cracking failures. Specific enthalpy recovery values were found to correspond with transitions from ductile to brittle modes of tensile mechanical failures, exhibited by samples with each level of crystallinity. For samples exposed to aging temperatures from 45 to 60 C, these critical enthalpies decrease linearly with increased crystallinity. The environmental stress cracking behavior of semicrystalline PET was found to depend on both crystallinity levels and physical aging. Samples with higher levels of crystallinity undergo stress cracking failures at lower critical stresses than their less crystalline counterparts. Physical aging, before environmental stress cracking exposure, further decreases these critical stress values and leads to much shorter critical times for failures. It has been shown that the interspherulitic amorphous portion of the material is primarily responsible for the differences in tensile mechanical and environmental stress cracking behaviors, observed as a result of aging and exposure. This amorphous region becomes progressively more restricted with increasing crystallinity as well as with increased aging. It, therefore, requires shorter exposure times to exhibit brittle tensile mechanical failure and environmental stress cracking rupture.

A Study on Physical Aging of Semicrystalline Polyethylene Terephthalate below the Glass Transition Point

“…Zhou et al (2008) studied the primary degree effect of crystallinity (X c ) of polyethylene terephthalate on the crystallinity of polymer chains during aging. They observed that in the c-PET samples with X c >0.2, the rate of crystallinity increases more slowly than in the samples with X c = 0.1 -0.2 during aging [9]. In this study, the effect of physical aging on the glass transition point and crystallization process of semicrystalline polyethylene terephthalate at two aging temperatures, 25 and 45ºС, was investigated.…”

Accelerated physical ageing of poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate)

“…[8][9][10] This polymeric compaction by segmental polymer movement results in reduced free volume, and thus increases density of the amorphous phases. 11,12 Physical ageing is only observed below the glass transition temperature (T g , indicated as onset throughout this paper) where the amorphous phases are solidied out of equilibrium. Thus, heating the material above T g erases physical ageing, presented as enthalpic relaxation, determinable by differential scanning calorimetry (DSC).…”

Low Cost, Durable Seal

“…Generally, as polymers age they tend to become more brittle and this could affect any long-term application of the material [10]. Analysis of the mechanical behavior, such as elastic modulus, tensile strength and strain measured at various aging times and conditions, is important to understand for materials that require reliability and or durability in their applications [11]. The aging characteristics of a material can be evaluated as a function of time, environment and strain by prescribing strain on samples subject to various environments and temperatures.…”