TPE MAGAZINE n 03/2016
Cucchi, M. A. Ortenzi

Thermoplastic copolyester elastomers (TPC-ET) offer excellent mechanical properties and due to their partial polyester backbone chemical resistance superior to most other thermoplastic elastomers. A weakness of these high performance elastomers is their relatively slow hardening speed. SIPOL® developed modified copolyesters with improved crystallisation kinetics. The following article summarizes the route to these new grades by using DSC (Differential Scanning Calorimetry).

1 Introduction

Thermoplastic copolyester elastomers are segmented block copolymers obtained through the combination of rigid segments (polyesters) and soft segments (polyethers). These products are called TPC-ET according to ISO 14910‑1 but are also well known as COPE or TEEE. TPC-ET offer outstanding properties for what concerns the mechanical properties (tensile strength, tear strength, impact strength, and creep resistance) which, compared to most other thermoplastics, are much less affected by temperature variations. In addition, their partial “polyester” backbone provides these products with a chemical resistance which is superior to other common thermoplastic elastomers. Typically the crystalline part of a TPC-ET is a PBT short chain (or a modified PBT) and the amorphous component is a polyether based on polytetramethylene ether glycol (PTMEG). The ratio of the two components as well as the length of their polymer chains provides the product different combinations of hardness, melting point and other distinctive properties. The Sipolprene® range, developed and manufactured by SIPOL® SpA, covers hardnesses from 25 Shore D to 72 Shore D with melting points between 150 °C and 220 °C. One of the main weaknesses of TPC-ET is connected to their processing: their relatively slow hardening speed from the molten state to the solid leads to longer cycle times in injection molding because of the long cooling time in the mold. Similar limitations can be observed when TPC-ET is extruded.

2 Effect of different polyethers on the crystallization rate

Studies proved that hardening speed of a TPC-ET can be improved by using PTMEG with higher molecular weight. Disadvantage is a deterioration of the behavior at low temperatures. An alternative way to increase the hardening speed without compromising on the low temperature behavior is to use a different polyether than PTMEG. SIPOL® started working on the evolution of Sipolprene® 55200 (PTMEG-based TPC-ET, 52 Shore D) by testing various polyethers chemically different from PTMEG to develop a poly-ether-ester with increased crystallization rate. After several tests, a class of polypropylene glycols end-capped with polyethylene oxide, offering a good balance of reactivity, availability, and performance, was identified. The selection of the most efficient polyether grades, in terms of chain length and % end capped polyethylene oxide, has been done by conducting polymerization tests in a 2 l glass reactor at SIPOL® R&D Lab. The improvement of the crystallization rate was measured via DSC (Differential Scanning Calorimetry). The crystallization kinetics of semicrystalline polymers can be effectively studied by means of DSC. This technique is fundamental both for the study of dynamic and for the isothermal crystallization behavior. A quick indication on the speed of a molten polymer to rearrange its structure in crystalline and amorphous clusters is given by the difference between the melting point and crystallization temperature measured via DSC.