New insight into the use of latent catalysts for the synthesis of urea formaldehyde adhesives and the mechanical properties of medium density fiberboards bonded with them

Abstract

Recently, there has been a rapid growth in research and innovation in the synthesis of UF resins in the wood-based composites area. Much effort has gone into increasing their mechanical performance to extend the capabilities and application of this group of materials. This paper, for the first time, defines the research on the use of ammonium salts not as hardeners but in the synthesis of UF resins. The objective of this study was to investigated the effects of various types of catalysts (ammonium sulfate, ammonium chloride, sulfuric acid and hydrochloric acid) during UF resins synthesis on formaldehyde emission and characteristics of urea–formaldehyde (UF) adhesives for the manufacture of medium density fiberboard. These were investigated by 13C Nuclear Magnetic Resonance (13C NMR) and differential scanning calorimetry (DSC), and as a result some of the physical and mechanical properties of medium density fiberboard (MDF) panels bonded with different UF resins were evaluated. The resins structure examined by 13C NMR spectroscopy showed that a resin catalyzed by (NH4)2SO4 had a higher proportion of hydroxymethyl groups (mostly HNCH2OH) resulting in a higher level of formaldehyde emission. A resin catalyzed by NH4Cl gave the highest proportion of methylene groups. A resin catalyzed by H2SO4 yielded the highest proportion of methylene groups in di- or tri-substituted ureas but the lowest proportion of mono-substituted urea. It was of interest that (NH4)2SO4 catalyzed UF adhesive yielded the best overall mechanical properties for the medium density fiber board (MDF) bonded with it. Conversely, it also presented the shortest shelf life and gelled in about 15 days, which could be related to its high reactivity and shortest gel time. Medium Density Fiberboard (MDF) panels bonded with UF adhesives without any ammonium salt (thus with either H2SO4 or HCl) had lower IB strength, MOR and MOE. The HCl catalyst was considered to be chemically least active, yielding the lowest cohesive energy, and resulted in panels presenting a poor performance as shown by their lowest IB (internal bond) strength, modulus of elasticity (MOE), and modulus of rupture (MOR) and highest thickness swelling, with the exception of the free formaldehyde content.