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 performances elastomers is their relatively slow hardening speed. Sipol (Mortara, Italy) developed modified copolyesters with improved crystallisation kinetics
Features of TPC-ET
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.
Generally 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 (Mortara, Italy), 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.
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 behaviour at low temperatures. An alternative way to increase the hardening speed without compromising on the low temperature behaviour 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 percent end capped polyethylene oxide, has been done by conducting polymerization tests in a two litres 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 behaviour. 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.
Injection molding tests and mechanical characterization
The tests resulted in a development product (Sipolprene 55211). To carry out tests to confirm the quicker crystallization rate of the development product indicated by DSC, the experimental formulation developed in the glass reactor was scaled up. Sipolprene 55211 and Sipolprene 55200 were processed under similar conditions on the same injection molding machine and trying to achieve the fastest cycle time during the injection molding process (Table 1).
Table 1 Injection molding conditions
|Injection molding machine||Oima Stratos 400-90|
|Clamping force (t)||90 ton|
|Nozzle||Open with 3 mm Ø|
|Mold||Tensile bars (ISO 527)|
|Melt temperature (°C)||235|
|Mold temperature (°C)||30|
|Injection speed (%)||50|
|Holding pressure (bar)||30|
The shortest cooling time achieved for Sipolprene 55200 was 14 seconds, while with the experimental product (Sipolprene 55211) it was possible to reduce the cooling time to 11 seconds, an improvement of 21%. The full cycle time went accordingly from 22 seconds for Sipolprene 55200 to 19 seconds achieved for Sipolprene 55211 (Table 2).
Table 2 Cooling and cycle times
|Sipolprene 55211||Sipolprene 55200|
|Cooling time measured (s)||11||14|
Table 3 shows the comparison of the mechanical properties of the two products. The chosen polyether causes a moderate reduction of tensile/flexural properties but also leads to a substantial improvement of low temperature impact strength. It has also been observed that the change of polyether leads to a higher melting point of the polymers even if the hardness is the same.
Table 3 Physical and mechanical properties
|Method||Sipolprene 55211||Sipolprene 55200|
|Density (g/cm3)||ISO 1183||1.21||1.19|
|Hardness Sh D (instantaneous)||ISO 868||54||52|
|Melting temperature (°C)||ISO 3146||215||198|
|Strength at break (MPa)||ISO 527||39||43|
|Elongation at break (%)||ISO 527||570||650|
|Izod Impact Strength (notched at -40°C, J/m)||ASTM D256||120||135|
|Water absorption (%)||(24h immersion at 23°C)||2.10||0.30|
The investigation of the crystallization kinetics was carried out at the Laboratory of Materials and Polymers (LaMPo), Università degli Studi di Milano (Italy). Being dependent on cooling rates, dynamic crystallization is generally used to assess the ability of a polymer to start the crystallization process in non-equilibrium conditions (i.e. the process is not under thermodynamic equilibrium): since crystallization is an exothermal process, related to the growth of crystals from the polymer melt, higher crystallization temperature indicates that the polymer has the tendency to start nucleation of crystals sooner than polymers with lower Tc.
Another important issue in the dynamic crystallization studied via DSC is related to the shape of the crystallization peak: narrow peaks indicate polymers that have a fast crystallization process (highly desired in injection molding). On the contrary, a broad crystallization peak indicates that crystallization is slow and occurs over a wide range of temperatures, thus making it necessary to increase cycle times. On this regard, Sipolprene 55200 and Sipolprene 55211 were compared using the same thermal cycle:
1 Dynamic segment: from 25 °C to 250 °C @ 10 °C/min;
2 Isothermal segment: 250 °C for 5 min;
3 Dynamic segment: from 250 °C to 25 °C @ 10 °C/min;
4 Isothermal segment: 25 °C for 5 min;
5 Dynamic segment: from 25 °C to 250 °C @ 10 °C/min.
Segments 1 and 2 were performed in order to relax internal stresses of the polymer and to let all crystalline fraction melting. The significant part of the analysis is therefore related to segments 3 and 5. The results are shown in Pictures 1 and 2. Tc of Sipolprene 55211 (Picture 1) is more than 40 °C higher than Tc of Sipolprene 55200 (Picture 2), even if Tm is only 16 °C higher. This is not the only positive feature of the new experimental Sipolprene, since also crystallization enthalpy, directly correlated to the crystalline fraction of the polymer, is higher. This indicates that Sipolprene 55211, under dynamic conditions, can crystallize more than Sipolprene 55200 and it explains why in the new polymer there is only a moderate reduction of tensile/flexural properties and why toughness at low temperatures is maintained. Isothermal crystallization, on the other hand, is used to assess the thermal properties of polymers under thermodynamic conditions.
No thermal stress is provided: the same temperature is maintained for a set period of time and the polymer is free to release all the energy related to the crystallization process over time. This implies that isothermal crystallization is used to determine crystallization kinetics and “behavior” of polymers. One of the most used and reliable equations for this purpose is the Avrami equation:
1 – Xc = e[-Kc*tn]
Log[-ln(1-Xc)] = nLog t + Log Kc
Where Xc is the crystalline fraction, Kc the crystallization kinetics and h is the “Avrami” number, depending on the kind of crystallization phenomenon observed.
A first study over isothermal crystallization of Sipolprene 55211 compared to Sipolprene 55200 was conducted to determine the temperature ranges allowing crystallization of the two polymers and ongoing studies will lead to the determination of crystallization parameters according to Avrami equation. Thermal cycles for isothermal crystallization studies were as follows:
1 Dynamic segment: from 25 °C to 250 °C @ 20 °C/min;
2 Isothermal segment: 250 °C for 5 min;
3 Isothermal segment: 30 min @ set temperature T3 for analysis
The difference between the two polymers is evident: Sipolprene® 55211 starts a slow isothermal crystallization even at T3 = 185 °C (Picture 3), a temperature that is far too high to observe any thermal phenomenon with Sipolprene 55200.
Lowering the isothermal temperature by only 5 degrees, to T3 = 180 °C (Picture 4), makes the crystallization phenomenon extremely fast and partially invisible because of the transition between the two heating programs of the DSC instrument.
On the contrary, in order to observe a very slow crystallization with Sipolprene 55200, similar to the one observed at T3 = 185 °C with Sipolprene 55211, temperature must be lowered almost 20 degrees to T3 = 167.5 °C (Picture 5). This difference is a direct proof of the high tendency of Sipolprene 55211 to crystallize in comparison to Sipolprene 55200.
The partnership between Sipol and LaMPo, Università degli Studi di Milano, who verified the effectiveness of the crystallization rate increase, lead to the commercial product Sipolprene 55211 which is now part of the Sipolprene TPC-ET family. In addition, the route of changing polyether to enhance the identified crystallization rate has a general validity which is currently used by Sipol to extend the range of hardnesses.
In addition Sipolprene 55211 and the other products under development using this system (all grades will be identified by “1” as last digit in the product code) provide some cost reductions in the raw materials used that enhance the cost/performance ratio of these TPC-ETs.