Characteristics of Phenol-Formaldehyde Adhesive Bonds in Steam Injection Pressed Flakeboard

Stephen E. Johnson, Frederick A. Kamke


A better understanding of the mechanisms involved in phenol-formaldehyde resin-wood bonding is needed to design adhesive systems that can adequately develop bond strength in a humid environment. This study was performed to determine how the molecular weight distribution of a liquid resole phenol-formaldehyde adhesive affects mechanical properties and adhesive flow in flakeboard bonded during steam injection pressing. The performance of three adhesives, differing only in molecular weight distribution, was studied. For all adhesives, mechanical properties of specimens located on the edge of the panel were found to be superior to those located in the center of the board. Excessive moisture present in the center of the mat was believed to be responsible for poor bonding. Edge internal bond strength improved with higher weight average molecular weight adhesive. Fluorescence microscopy and image analysis techniques were used to measure flow of adhesive into the wood substrate before and after exposure to a steam injection pressing environment. Flakes wetted with adhesive and not exposed to a pressing environment had more adhesive penetration with the lowest weight average molecular weight adhesive. Deeper and less concentrated adhesive penetration was measured in flakes exposed to a steam injection environment, with a smaller apparent difference between the three adhesives.


Phenol-formaldehyde;steam injection pressing;fluorescence microscopy;image analysis;wood bonding

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