Recent Advances in Urea–Formaldehyde Resins: Converting Crystalline Thermosetting Polymers Back to Amorphous Ones

Since their first synthesis in 1884, thermosetting and amorphous urea–formaldehyde (UF) resins have mainly been used as wood adhesives yet are known to be responsible for the release of formaldehyde, which contaminates indoor air and causes sick building syndrome. An easy and efficient way of reducing formaldehyde emissions is to synthesize UF resins with a low formaldehyde-to-urea (F/U) molar ratio (∼1.0). However, low molar ratio UF resins become crystalline polymers, as they form hydrogen bonds between linear molecules in the cured state, which inhibits the formation of a proper cross-linked structure and results in poor adhesion strength. Herein, recent advances in converting crystalline UF resins back to amorphous polymers through the blocking of hydrogen bonds are described, which consequently increases their cohesion, leading to a simultaneous improvement in their adhesion properties and formaldehyde emissions.