Thermoplastic and moisture‐dependent behavior of lignin phenol formaldehyde resins

Abstract

Bonding kinetics of thermosetting adhesives is influenced by a variety of factors such as temperature, humidity, and resin properties. A comparison of lignin‐based phenol formaldehyde (LPF) and phenol formaldehyde (PF) adhesive in terms of reactivity and mechanical properties referring to testing conditions (temperature, moisture of specimen) were investigated. For this purpose, two resins were manufactured aiming for similar technological resin properties. The reactivity was evaluated by B‐time measurements at different temperatures and the development of bonding strength at three different conditions, testing immediately after hot pressing, after applying a cooling phase after hot pressing, or sample conditioning at standard climate. In addition, the moisture stability of the two fully cured resins was examined. The calculated reactivity index demonstrated that LPF requires more energy for curing than PF. Further results indicate that lignin as substituent for phenol in PF resin has a negative impact on its moisture resistance. Additionally, the known thermoplastic behavior of lignin could also be detected in the behavior of the cured resin. This behavior is relevant for the adhesive in use and necessitates a cooling phase before testing the bonding strength development of lignin‐based adhesive systems. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48011.

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Sulfuric Acid-Catalyzed Dehydratization of Carbohydrates for the Production of Adhesive Precursors

“…The bond strength development for these resins is tested by measuring the tensile shear strength of two beech lamellas glued together at a 120 °C press temperature and varying press times. A more detailed description of the measurement method was published by Solt et al 20 All tests were conducted following standard procedures. 21…”

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“…The shear strength of the plywood was significantly lower than that of the PF, however, the plywood was suitable for exterior applications according to EN 314-1 [ 18 ]. Similarly, another publication reported that at 50% replacement of phenol by Kraft lignin the shear strength and especially the wet shear strength of plywood produced with LPF were inferior to those with PF resin [ 52 ]. These results were contradicted by another study, in which up to 50% Kraft lignin led to similar or even slightly improved performance compared to PF-bonded plywood [ 53 ].…”