Abstract
Hydrophobically modified ethoxylated urethanes (HEURs) are among the most widely studied class of waterborne polyurethanes. HEURs are amphiphilic polymers that are usually composed of a polyethylene glycol (PEG) backbone, typically end-capped with aliphatic alkyl chains. HEUR synthesis in the industry is a slow and energy-intensive step-growth polymerization process, due to heat and mass transfer limitations inherent in such highly viscous systems. We investigate for the first time the effect of microwave heating on both the solvent-free HEUR synthesis step and the initial pretreatment step of the reagents prior to the HEUR synthesis. We show that microwaves can drastically reduce the overall processing time, and thereby reduce total energy consumption by: A) faster melting of solid PEG and B) faster completion of the polymerization reaction, when operating with a novel rapidly rising transient temperature profile that cannot be reproduced by conventional heating. However, it was found that PEG pretreatment/dehydration by microwaves leads to faster degradation of PEG, compared to dehydration by conventional heating at the same bulk temperature, and should therefore be avoided.
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