Investigation of urea-melamine-formaldehyde (UMF) resin penetration in medium-density fiberboard (MDF) by high resolution confocal laser scanning microscopy

A confocal laser scanning microscope (CLSM) was used to investigate the distribution and penetration of urea-melamine-formaldehyde (UMF) resin in the fiber when injected through blowline blending in a medium density fiberboard (MDF) pilot plant. Samples were prepared with respect to industrial parameters and were collected at the dryer's end. The samples were later dyed in a Dye Star-Brilliant Red solution (0.01%) and rinsed with distilled water to remove excess. The samples were scanned with the CLSM to build three-dimensional reconstructions of MDF fiber cross-sections. With proper lenses and optimized CLSM settings, it was possible to obtain fiber reconstructions with a resolution greater than the laser wave length (514 nm). The Zeiss CLSM built-in software image analyzer enabled to rebuild them in rotation on any of the three axes with up to 64 images per rotation. The resin penetration sites were identified using this software option. The penetration sites were numerous and well dispersed. The largest openings (lumen, pits and cracks) were responsible for most of the resin lost by over-penetration. The presence of resin in the cell walls (detected with the CLSM) proves their porosity without giving much information about the resin concentration. Finally, the atomic force microscope (AFM) enabled to recreate the finest surface details for these fibers samples. It was found that the fibrils aggregates orientation and size can influence the resin penetration and distribution. It was concluded that the porous structure of wood fibers and their affinity to water enable the resin to penetrate through capillary force. This phenomenon is stimulated by the high pressure, saturated steam, turbulent flow and heat. When injected in the pilot plant blowline, UMF resin was uniformly distributed over the fiber surface (22.5% coverage). The resin penetration was however important and also occurred in nanometric defaults of the wood fibers. Thus, industrial panels made out of very porous, damaged or small fibers will need more resin to fill the gaps in order to make a strong board.