Size Exclusion Chromatography as a Method for Routine Check-Up and Quality Control of Industrial Thermoplastic Polyurethane Systems – Correlation between Molecular Weight and Mechanical and Thermal Properties

Abstract

A method has been developed which allows an accurate molecular characterization of linear prepolymers end-capped with diisocyanates which is correlated with the properties of the finally formed thermoplastic polyurethane elastomer (TPU). Therefore, this method is suitable for a fast routine check-up in quality control in the polyurethane (PU) processing industry. A homologous series of three toluene-2,4-diisocyanate (TDI) end-capped PU prepolymers with poly(oxytetramethylene) glycol (POTM) as soft segment (number-average degree of polymerization: 3, 4, and 7) have been characterized according to their molar mass and its distribution by means of size exclusion chromatography (SEC) calibrated with commercially available POTM standards. The validation of this method was carried out by SEC with double detection, using a combination of a concentration (differential refractive index detector, DRI) and an end group specific detector (ultraviolet detector, UV), which allows the determination of the absolute number-average molar mass of the PU prepolymers as well as their content of soft and hard segment. The values for the number-average molar mass were confirmed by osmometry. The PU prepolymers were reacted each with the same aromatic diamine to give the corresponding TPU elastomers which have been examined according to their thermal and dynamic behavior (transition temperature of soft and hard segment, temperature range of application) by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Also the hardness, tear strength and elongation at break could be related to the molecular structure, leading to a judgement for the efficiency of the final product for special fields of application.