Calorimetric analysis of the crystallization of PBT considering the dynamic cooling condition of real processing

The crystallinity of poly(butylene terephthalate) in mass‐scale extrusion products as seen by differential scanning calorimetry

“…After passing a local maximum of crystallization half times at about 115°C, heterogenous nucleation becomes prevailing 25,31,35 . Above 115°C, crystallization half times starts to decrease slightly to reach local minimum at about 130°C (about 1 s) and then increase monotonically with temperature to tens and then to hundreds of seconds passing to the temperature 200°C 25,31,35,43 . PBT grades containing a nucleating agent show similar, bimodal dependence of crystallization half times with reduced times of heterogenous nucleation and the transition point between two distinct mechanisms shifted toward slightly lower temperatures (ie, local maximum moved to around 110°C 3,45 or 95°C 35 ).…”

“…PBT crystallized from melt below the equilibrium temperature typically shows a “fundamental” apparent melting peak at temperatures below T m 0 (down to 180°C) and one or more additional “pre‐melting” peaks 25,31‐33,36‐42 . In the recent years, number of studies with use of fast scanning calorimetry variant of DSC technique gave deeper insight into the mechanisms and kinetics of these “pre‐melting” structural changes in PBT, for example see References 24,25,31,32,34,35,43,44. The decrease in T m of PBT “fundamental” melting peak (“fundamental” means here the peak associated with melting of crystals created on fast cooling from melt) can be explained by small crystallite sizes and their defect content that both reduces their thermodynamic stability 38,39 .…”

“…This tendency can be explained by fast formation of homogeneous crystal nuclei at low temperatures that starts even 20°C below T g and accelerates rapidly above T g 6,31‐33,37 . In pure PBT, homogenous nucleation mechanism is dominant in the range between T g and approximately 115°C, with crystallization half times in the order of magnitude fractions of seconds, with minimum at 75°C 25,31,35,43 . After passing a local maximum of crystallization half times at about 115°C, heterogenous nucleation becomes prevailing 25,31,35 .…”

“…Recrystallization of PBT was found to be much faster than initial crystallization from melt due to large difference in nucleation densities 31,32 . Critical rates of temperature change to suppress crystallization are estimated as 200‐2000°C s −1 on cooling the melt 3,25,31,32,35,43 and >100 000°C s −1 to avoid recrystallization of PBT on heating 3,25,31,32,44 …”

“…However, FSC allows only to analyze fine powder or (sub)micrometer slices of a material 3,45 that may be difficult to prepare and that may not give representative information for an industrial product. Furthermore, FSC requires to establish perfect thermal contact between the sample and the sensor that can be challenging for the first heating cycle 3,43,45 . To conclude, new paths to improve reliability of evaluation of crystallinity of PBT products with “conventional” DSC method are still desirable because it is well suited to industrial testing reality.…”