Examination of the penetration of polymeric methylene di-phenyl-di-isocyanate (pMDI) into wood structure using chemical-state x-ray microscopy

The penetration behavior 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 resin and thin sections were prepared, cut from the transverse face of the wood composite. 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 bulkmore » concentration. This lower limit is the sensitivity limit imposed by photon statistics in the data.« less