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

Abstract

The crystallinity affects the most of the properties of the products made of semi‐crystalline engineering polymers, such as poly (butylene terephthalate) (PBT) optical fiber loose buffer tubes. Precise control of crystallinity of PBT can be crucial for the quality of an optical fiber cable. Proper determination of crystallinity index from differential scanning calorimetry (DSC) measurements can be challenging for PBT as various processes occur between devitrification and melting that influence the initial crystallinity. Discrepancy between industrial and laboratory conditions leads to difficulties in transfer of laboratory‐scale procedures to technological processes. In this article, simple and time effective, small‐scale laboratory procedure based on Melt Flow Indexer was proposed for the preparation of samples with varying crystallinity indices to simulate industrial products of PBT extrusion, namely loose buffer tubes for optical fibers. Moreover, the conventional “linear” DSC method of characterization of PBT crystallinity index was revised with wide‐angle X‐ray scattering as the reference. Analysis showed the complexity of the thermal processes occurring in PBT melt‐extruded samples between Tg and Tm that strongly influenced the value of crystallinity index of PBT measured by DSC. These processes were identified and new semi‐empirical method of their extraction from the thermograms was proposed. Cold melting of the crystalline phase was found to have dominant contribution in the “pre‐melting” region that is between approx. 130°C and the onset of the fundamental melting peak, with only minor interrupting low‐intensity exothermic event. The contribution of this “pre‐melting” endotherm was more significant for the samples cooled at lower rates.