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Using non-toxic, low-volatile glyoxal to completely replace formaldehyde for preparing urea-glyoxal (UG) resin adhesive is a hot research topic that could be of great interest for the wood industry. However, urea-glyoxal (UG) resins prepared by just using glyoxal instead of formaldehyde usually yields a lower degree of polymerization. This results in a poorer bonding performance and water resistance of UG resins. A good solution is to pre-react urea to preform polyurea molecules presenting already a certain degree of polymerization, and then to condense these with glyoxal to obtain a novel UG resin. Therefore, in this present work, the urea was reacted with hexamethylene diamine to form a polyurea named HU, and then this was used to react it with different amounts of glyoxal to synthesize hexamethylenediamine-urea-glyoxal (HUG) polycondensation resins, and to use this for bonding plywood. The results show that the glyoxal can well react with HU polyuria via addition and schiff base reaction, and also the HUG resin exhibits excellent bonding strength and water resistance. The shear strength of the plywood bonded with this HUG at 160°C hot press temperature as high as 1.93 MPa, 2.16 MPa and 1.61 MPa, respectively, which meets the requirement of the China national standard GB/T 9846-2015 (≥0.7 MPa), and can be a good choice as a wood adhesive for industrial application. [ABSTRACT FROM AUTHOR]

This article presents the first applied results of using citric acid in combinations with a melamine-urea-formaldehyde (MUF) resin for bonding wood veneers. The chemical reactions involved are shown based on a MALDI ToF analysis of the reaction of the MUF resin with citric acid. The preliminary results of the physical and mechanical properties of the LVL prepared are also presented. Veneers from Populus sp were used to manufacture 5-layer laminated veneer lumber (LVL) of small dimensions. Five combinations of the amount of citric acid, MUF spread rate and pressing parameters were tested. LVL bonded with 20% of citric acid + 100 g/m² of MUF, hotpressed using a 3-step process with maximum 1.5 MPa of pressure yielded the board with better dimensional stability and mechanical properties. It could be concluded that citric acid in combination with MUF can be used for bonding wood veneer and the research should be continued to study further the parameters involved and to enhance the results. [ABSTRACT FROM AUTHOR]