The annealing and thermal analysis of poly(butylene terephthalate)

Abstract

When poly(butylene terephthalate) (PBT) is annealed, a second endotherm is often displayed in a subsequent scanning thermal analysis at a temperature below that of the original endotherm, and this new endotherm appears to grow with annealing at the expense of the original. This growth is not due to chemical changes, because the thermogram obtained before annealing is recovered after complete melting. But a physical change would also seem unlikely because the transformation of higher‐melting into lower‐melting crystals is generally prohibited by thermodynamics. Two hypotheses to explain the result were tested. The first is that higher‐melting crystals are not transformed into lower‐melting crystals. Instead, because of recrystallization during thermal analysis, the single endotherm that results without annealing overestimates the population of high‐melting crystals present before the analysis. This hypothesis was tested by extending to annealing a mathematical analysis previously used to describe the thermal scanning behavior of specimens crystallized at different cooling rates. Though most features of the thermograms obtained after annealing were able to be described, the decrease in the higher‐temperature endotherm concomitant with growth of the lower endotherm was not. The second hypothesis is that the transformation of higher‐melting to lower‐melting crystals during annealing is allowed because it is coupled to the crystallization of formerly amorphous material. The amount of such crystallization observed for PBT was found to be sufficient to satisfy thermodynamic requirements, suggesting that this hypothesis is correct. © 1994 John Wiley & Sons, Inc.

Crystallization Behaviors of Polypropylene/Polyamide-6 Blends Modified by a Maleated Thermoplastic Elastomer

“…Wilkinson et al 28 also found the same phenomenon in the PP/PA6/SEBS-g-MA ternary blends and attributed the shoulder peak to the secondary crystallization of PA6 component. [29][30][31] The extruded PA6 used in the study also exhibits this bimodal melting behavior under the same measurement condition. The conditions that reduce the mobility of chain segments after primary crystallization has occurred, such as rapid cooling 28 and crystallization at high degrees of undercooling, 2,28 can suppress the development of the low temperature shoulder and produce a melting endotherm with a single peak.…”

Stepwise annealing of poly(butylene terephthalate)

“…Various interpretations and/or explanations on the melting behavior of these semi-crystalline polymers have been proposed. Yeh and Runt, 5 and Robertson et al, 6,7 have shortly summarized the interpretation on the melting behavior of PET and PBT in their articles. Since PBT evidently results in the double melting peaks when observed by a differential scanning calorimeter (DSC), [5][6][7][8][9][10][11] PBT is a suitable polymer for an analysis of the melting behavior of the semi-crystalline polymers.…”

“…Many authors [5][6][7] have explained the double melting behavior of PBT by the melt-recrystallization model. In the model, the low-temperature endothermic peak has been explained to represent only a portion of the melting of original crystals.…”

“…DSC curves of PBT annealed samples have been reported by many authors. [5][6][7]9,10 Illers 9 performed long-time isothermal annealing (850 h) of PBT at 220°C after quenching from the melt at 270 to Ϫ10°C and obtained a high melting-temperature (233°C) sample. Though he performed long-time annealing, his value T m ϭ 233°C is not high in comparison with the equilibrium melting temperature (T°m) of PBT which has been suggested to be 245 Ϯ 2°C by Cheng et al 11 This result indicates that the long-time isothermal annealing is not an effective method to increase the melting temperature of PBT.…”

Mechanical and Thermo-Physical Properties of Short Glass Fiber Reinforced Polybutylene Terephthalate upon Aging in Lubricant/Refrigerant Mixture

“…After injection, all samples passed through an annealing process at 180°C during two hours, in order to erase thermal history and eliminate possible residual stresses arising from the molding process 3,17,18 . For the measurement of initial weight values before aging, the samples were rinsed quickly in acetone, wiped dry with a paper tissue and then immediately weighed using a high precision analytical balance.…”