Irradiation induced crosslinking in thermoplastic polyurethanes for structural applications

Heat treatment of steels and other structural metals is common practice to enhance the performance of the material without drastically increasing the materials cost. Therefore, from one material bought a great number of properties can be derived downstream of the material supplier. Furthermore, it is possible to restrict the improvement to certain areas where it is needed. Polymers are another material class, which can be used for structural applications. Benefits like good corrosion resistance and good mechanical properties are only some reasons for the use of polymers instead of metals. Especially for thermoplastic materials there are only a few ways to fit the mechanical properties to application requirements, e.g. by using reinforcing materials. Additionally, it is possible to use polymer blends or switch to a completely different thermoplastic polymer. All three strategies involve major effort either for the user of the material or for the material supplier. This effort is mainly concerned with finding the right processing parameters to use the reinforced thermoplastic or to find the right blend composition. Additional to those efforts the material cost usually increases. Nevertheless, many manufacturers downstream of the raw material supplier often face changed loading scenarios, which have to be incorporated under cost and time limitations.
One possible solution for slight changes in the materials performance is the use of irradiation to change material properties by crosslinking in the solid state. As irradiation sources gamma radiation and high velocity electrons can be used to induce crosslinking. This strategy is widely used to improve the performance of insulation materials like PE in the electronic industry. Three main factors contributed to the widespread application in this industry. One is the highly scalable irradiation process, which can be applied with little additional cost and in large quantities. Therefore, the raw material and manufacturing process can be unchanged for products with enhanced performance requirements compared to those in standard applications. The only difference is an additional processing step. However, the main reason for a broad application in the electronics industry is a better creep resistance of a low-cost polymer like polyethylene. Even though this strategy is long known, only very few applications of irradiation enhanced polymers can be found for mainly structural applications. Even though in this type of application additional benefits could be realized by localized property modification of the polymer (e.g. locally enhanced toughness to restrict crack initiation). However, not all polymers can be modified, and only slight changes are possible. The presented work investigates the irradiation modification of thermoplastic polyurethane (TPU) for structural applications. Some research can be found on the irradiation effects on rubbery TPU’s but rarely on materials with higher stiffness. It is the aim to show the potential benefits of irradiation induced crosslinking in structural polymers. The investigated material is an ether-type TPU. For this material class it is sometimes possible to induce crosslinking without the use of any additional crosslinking agents. This makes the material class to a potential candidate for irradiation modification after the injection molding, for organo sheets (continuous fiber reinforced thermoplastic sheets) or the final product without any changes in earlier processing steps. A wide variety of mechanical and thermal tests like differential scanning calorimetry (DSC), dynamic thermal mechanical analysis (DTMA) were used to better understand the material changes under irradiation. Static and fatigue experiments have been undertaken to show the influence of irradiation on the mechanical properties. The comparison of different dose levels shows the competing effects of crosslinking and chain scission, which can lead to deterioration of the material as well as a property improvement. In addition to the mechanical characterization, the material was evaluated with regard to the impregnation behavior of glass fiber fabrics as prerequisite for organo sheet manufacturing. The results promote further research into the use of irradiation crosslinking for thermoplastic polymers in structural applications.