MUF Resins Improved by Citric Acid as Adhesives for Wood Veneer Panels

Abstract

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/m 2 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.

Recent Developments in Advanced Lignin-based Adhesives and Binders

“…Citric acid has been promoted as a direct binder for wood [36][37][38] and also as an additive improving wood binding by traditional or new adhesives [39][40][41][42]. While in direct bonding of wood substantial esterification of the polymeric carbohydrate constituents of wood does occur leading to cross-linking between contact surfaces [38], the reaction although less immediately noticed occurs extensively also with lignin [38].…”

Recent progress of biomass in conventional wood adhesives: a review

“…78 This ability makes citric acid a promising candidate for wood adhesion purposes. Del Menezzi et al 79 used citric acid combined with a MUF resin to glue wood veneer. During the bonding process of wood veneer, the reaction between citric acid and MUF adhesive revealed that the carboxylic acid functional group of citric acid was esterified with the CH 2 OH group of hydroxymethylated UF oligomer, and a bridge was formed by reacting with some amino groups of melamine.…”