The most commonly used chain extenders with 4,4'-diphenylmethyl diisocyanate (MDI) terminated prepolymers are 1,4-butanediol (BDO) and hydroquinone bis (2-hydroxyethyl) ether (HQEE).
Compared to BDO as a chain extender, MDI-based polyurethane elastomers produced with HQEE have several distinct advantages. These include improved performance at elevated temperatures, higher hardness, increased tear strengths, and higher resilience.
In addition, MDI/HQEE systems replicate many of the performance attributes of MOCA/TDI systems. Processors find that HQEE/MDI elastomers exhibit better high-temperature and moisture-resistance properties compared to TDI/MOCA elastomers. For this reason, MDI/HQEE systems are often recommended as alternatives to the TDI /MOCA systems.
HQEE is widely used in a number of applications for MDI/HQEE elastomers, including:
Industrial wheels and tires.
Sport and amusement park wheels.
Pipe linings and coatings.
Industrial roll covers.
Rollers, gaskets, and seals.
Other high-performance end uses.
Despite HQEE's variety of applications and advantage over BDO, processors still face difficulties with this chain extender.
The challenges of using HQEE primarily spring from its processing characteristics. There are two main difficulties associated with HQEE:
HQEE has a high melting point (>98 °C / 208 °F).
HQEE does not supercool but will rapidly crystallize below its melting point.
This requires heating melting tanks and all process lines to ~110 °C and insulating the lines to prevent clogging. In addition, choosing HQEE precludes the possibility of hand-batching.
Furthermore, processors must preheat the prepolymer to around 100 °C before mixing it with HQEE. Otherwise, the HQEE will crystallize in the mix causing "starring" in the cured urethane elastomer. Any molds will also require preheating to above 110 °C.
Many polyurethane processors continue to rely on HQEE without knowing that these difficulties can be significantly reduced by a lesser-known, but effective, chain extender-HER.