HQEE is an ideal chain extender for TPUs
The greater phase segregation and higher order of hydrogen bonding in the hard domain of HQEE-MDI polyurethanes are ideal for thermoplastic polyurethane (TPU) applications, leading to elastomers that exhibit the highest levels of mechanical properties and thermal performance. The Tg of a PTMEG 1000/MDI/HQEE TPU was reported as -48°C; the Tg of a PBA 1000/MDI/HQEE TPU was -30°C, thus indicative of a high degree of microphase separation.
In thermoplastic polyurethane (TPU) compositions, un-crosslinked HQEE-MDI domains can be melted and will flow at elevated temperatures to allow thermoplastic processing. The associated hard segment domains reform when cooled, to form tough elastomeric compositions. Based on DSC results, HQEE extension produces about a 30°C higher hard segment melting temperature compared to 1,4-BDO elastomers. HQEE-MDI systems often replace BDO-PPD TPUs to gain similar thermal and dynamic performance properties.
HQEE is the chain extender of choice in demanding polyurethane applications
Exemplary applications for HQEE include industrial wheels and tires; skateboard, in-line skate, and amusement park wheels; pipe linings and coatings; rollers, industrial roll covers, gaskets, oil-field seals, conveyor belts; oil field and mining equipment components, cyclone liners; and other high-performance end uses.
The excellent dynamic properties of PTMEG polyether-based HQEE-MDI elastomers are important in applications such as high performance wheels, rollers, and tires where low heat buildup (hysteresis) is important under dynamic flex conditions.
The HQEE-MDI elastomer series compares favorably with many of the performance attributes seen in MOCA-TDI systems. Furthermore, HQEE-MDI elastomers show improved dynamic properties, rebound, and tear strength.
The high melting point of HQEE requires higher processing temperatures
There are some challenges associated with processing HQEE due to its high melting point, ~98°C (~208°F), and the fact that HQEE does not supercool (HQEE will crystallize quickly below its melting point). Accordingly, it's important to uniformly heat and mildly agitate HQEE in the melting tank and preheat the prepolymer to 90°C (194°F) or above before mixing in HQEE. In addition, all HQEE transfer lines should be heated to 110°C (230°F) and insulated to prevent cold spots that might cause HQEE to crystallize in the lines and result in an inaccurate mix ratio. The temperature of the mold should be 110°C (230°F) or higher to prevent defects in the cast parts.
"Starring" on the surface of parts can be observed when processing HQEE. This phenomenon has been attributed to the phaseout of the reaction product of HQEE and free MDI monomer, especially on mold surfaces at temperatures lower than 110°C (230°F). It has been reported that low free MDI prepolymers exhibit a much lower tendency to starring because of the lower free MDI monomer content.