Elastollan

Medical grade TPU, Elastollan^® supplied by BASF (Lemförde, Germany), was received in raw pellets form. It is a polyether based TPU with polytetramethylene oxide (PTMO) as SS, 4,4-diphenylmethane diisocyanate (4,4-MDI) as HS and 1,4-butane diol (1,4-BDO) as a chain extender. Tetrahydrofuran (THF) of HPLC grade (≥99.8%) was supplied by Honeywell Riedel-de HaënTM (Seelze, Germany). Sodium Carbonate (Na₂CO₃) and salt tablets for Phosphate Buffer Solution (PBS) preparation were supplied by Sigma Aldrich (Steinheim, Germany). Ethanol of HPLC grade (≥99.9%) was supplied by Fisher Chemical (Geel, Belgium).

Material under investigation is TPU Elastollan^® (manufactured by Basf^®) 1190A and 1175AW in Shore hardness grades 90A and 75A, respectively. The TPU material was selected upon previous research of Bek [27 (link)], which revealed that this material exhibits sensitivity to pressure, which is an important material property for this research. In addition, the two grades of TPU material were selected because Bek’s research also revealed that 1190A material has higher sensitivity to pressure than 1175AW, which in turn means that the stiffness and energy absorption of the former is several decades better compared to the latter.
The TPU material was supplied already granulated, with the granules having the shape of an elongated spheroid. The typical size of a single granule is approximately 4.95 mm about the long axis and 3 mm about the shorter axis, and it weighs 0.03 g. The material was always stored in its original resealed package bag in a dry room. Other physical properties of the two materials can be found online on the manufacturer’s websites [49 ], but some relevant values are also presented here in Table 1.

We tested substrates of different polymer types (see Table 1). The polymers were purchased as granules. Elastollan was purchased from BASF SE (Ludwigshafen (Rhein), Germany), Styroflex from INEOS Styrolution Group GmbH (Fankfurt am Main, Germany), and all other polymers from Lubrizol (Wickliffe, OH, USA).
A heating press from VOGT Maschinenbau GmbH (Berlin, Germany) was used to produce flat samples. The temperature control to the target value for the upper and lower plates was carried out separately. The pressing process of the pre-dried granulate was carried out with the aid of intermediate layers of Polytetrafluorethylene-coated glass fabric Type 7069 from TACONIC (Rensselaer, NY, USA). Spacers between the glass fabrics were used to achieve a defined layer thickness of the flat samples. The processing temperature for the respective polymer is shown in Table 1.
After the press plates had reached the spacers, a residence time of 30 s was observed to ensure complete melting of the polymer. The intermediate layers were then removed and cooled in a water bath. After cooling, the sample could be removed easily from the intermediate layers.