Abstract
This paper reports a comparison of adhesion behavior of urea-formaldehyde (UF) with those of melamine-urea-formaldehyde (MU) resins in bonding wood by analyzing the results published in literatures. For this purpose, the adhesion behavior of UF resins prepared by blending low-viscosity resin (LVR) with high-viscosity resin (HVR) at five different blending and two formaldehyde/urea (F/U) molar ratios (1.0 and 1.2) was compared with those of two MUF resins synthesized by either simultaneous reaction (MUF-A resins) or multi-step reaction (MUF-B resins) with three melamine contents (5, 10, and 20 wt%). As the blending (LVR:HVR) ratio increased from 100:0 to 0:100, the viscosity and molar mass (Mw and Mn) of the blended UF resins increased while the gelation time decreased. The interphase features such as maximum storage modulus (E′max), resin penetration depth, and bond-line thickness of the UF resins increased to a maximum and then decreased as the blending ratio increased. In addition, both MUF-A and MUF-B resins also showed an increase in the Mw and Mn as the melamine content increased from 5% to 20%. However, the E′max, resin penetration depth, and bond-line thickness of the MUF resins decreased as the molar mass or melamine content increased. These results indicated that the adhesion of UF resins heavily depends on the interphase features while that of the MUF resins highly depends on the cohesion of the resins.
Highlights
Formaldehyde-based resins are still being used as adhesives for the manufacturing of wood-based composite panels such as particleboards (PB), oriented strand board (OSB), plywood, and medium-density fiberboard (MDF)
Details of gel permeation chromatography (GPC), differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), light microscopy (LM), confocal laser scanning microcopy (CLSM), and plywood properties (tensile shear strength (TSS)) of the UF and MUF resins are described in the publications [28,29]
The Mn and molecular weight (Mw) of MUF-A resins were lower than those of the MUF-A resins, indicating that more polymeric materials were formed in MUF-B resins and their molar mass increased at a higher melamine content
Summary
Formaldehyde-based resins are still being used as adhesives for the manufacturing of wood-based composite panels such as particleboards (PB), oriented strand board (OSB), plywood, and medium-density fiberboard (MDF). Low-molar-mass species are penetrated into wood tissues to form a strong interphase adhesion while high-molar-mass species are remaining to give a strong cohesive strength to the bond line. Dynamic mechanical analysis on PF resin-bonded wood composites showed that low-molar-mass species penetrated into wood while high-molar-mass species exerted the interphase adhesion [26]. He and Riedl [27] suggested that the storage modulus (E0 ) could be used for the evaluation of the rigidity of the resin network of amino resins. This paper reports a comparison of the adhesion behavior of UF and MUF resins in bonding wood by focusing on the interphase features such as resin penetration into wood, bond-line thickness and interphase strength, the adhesion strength, and the cohesion
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