Examination of the Penetration of Polymeric Methylene Di-Phenyl-Di-Isocyanate (pMDI) into Wood Structure Using Chemical-State X-Ray Microscopy

Abstract

Summary The penetration behaviour of isocyanate-based wood resins was evaluated using x-ray microscopy. Aspen wood pieces were bonded together in a controlled manner. These were embedded in a methacrylate-based resinand thin sections were prepared, cut from the transverse face of thewoodcomposite. X-ray images of these sections were prepared at several selected x-ray energies to allow the isocyanate, cellulose, lignin and the embedding agent distributions to be mapped. The isocyanate resin was found to penetrate deeply into the wood. The resin enters large cell lumen, and wicks along the inner cell wall surfaces. The resin accesses connected cells via connecting pits, which become filled with the resin. The affinity of the isocyanate to the inner surfaces of the large cells is probably due to the hydrophobicity of these surfaces. Isocyanate resins do not penetrate into the smaller parenchyma and tracheid cells and indeed do not even wet the inner surfaces of these cells where isocyanate entry has been allowed due to damage of the cell at the macroscopic surface of the wood. If isocyanates penetrate into the wood-cell walls of the large cells, the concentration in the cell walls has been determined to be less than 2%of the bulk concentration. This lower limit is the sensitivity limit imposed by photon statistics in the data.

The interphase in phenol–formaldehyde and polymeric methylene di-phenyl-di-isocyanate glue lines in wood

“…Considering the slow curing of the resin in question, which takes several hours, diffusion of resin compounds over distances of several 100 mm seems plausible, the more so as it was found that significant amounts of melamine-formaldehyde resin may diffuse in wood over a distance of 100 mm within 60 min [31]. Frazier and Ni [16] argue that the molar volume of pMDI resins should be sufficiently small to diffuse into the cell wall, which could not be confirmed using chemical state X-ray microscopy [17] and UVmicroscopy (Fig. 11).…”

“…However, studying the swelling and plasticisation of wood by pMDI they found no evidence for penetration of pMDI. Accordingly, no pMDI compounds were detected in wood cell walls using chemical state X-ray microscopy [17].…”

Micromechanical properties of the interphase in pMDI and UF bond lines

“…The penetration behaviour is another issue assumed to contribute to the bonding performance of pMDI. Due to its low molecular weight and its extraordinary wetting properties, pMDI spreads wood surfaces easily (Roll 1993) as well as it penetrates deeply into the wood structure along cell lumen and microcracks (Buckley et al 2002;Gruver and Brown 2006;Kamke and Lee 2007;Zheng et al 2004). Furthermore, it has the ability to diffuse into wood cell walls, which results in a molecular mixing and probably in the occurrence of an interpenetrating network (IPN) of pMDI inside the cell walls as proposed by Frazier and Ni (1998), Bao et al (2003) and Marcinko et al (1998).…”

The research of chemical modification on fast-growing wood

“…Commonly, the diffraction peak at 2θ=17°, 22.5°, and 35° corresponded to the cellulose crystallographic planesⅠ 101 , Ⅰ 002 , and Ⅰ 004 respectively [7] [8] . Compared to the above statistic of cellulose crystallographic planes, the peaks position of modified wood almost unchanged, and the numerical values for X-axis (2theta axis) of modified wood sample were 16.8°, 22.2°and 35.2° which indicated the structure of cellulose unchanged after modification.…”