Medium density boards made of groundwood fibres: an analysis of their mechanical and physical properties

The use of stone groundwood (SGW) pulp in paper manufacturing has constantly decreased in the last decades due to the increasing recycling of paper. The production capacity for SGW fibres is however still available in many paper mills, which could be exploited for alternative applications, like for example medium density fibreboards (MDF). Thin SGW MDF of 800 kg/m3 in density, 2 mm in thickness and bonded with polydiphenilmethan-4,4′-diisocyanate (PMDI) or urea–formaldehyde (UF) adhesives were produced with varying resin contents, raw fibres moisture content and hot-pressing parameters. Their mechanical characteristics, dimensional stability and optical properties were investigated and compared to MDF made of coarser Asplund fibres. SGW MDF bonded with PMDI adhesive showed higher internal bonding (IB = 1.5–2.6 N/mm2), lower thickness swelling (TS = 22–24%) and whiter surfaces (distance from white E = 28–30) than Asplund MDF (IB = 1.3, TS = 42%, E = 42). The possible development of covalent links between SGW fibres and PMDI adhesive was indicated by Fourier transform infrared spectroscopy (FTIR). Bending properties of SGW MDF remained however inferior to Asplund MDF. SGW MDF might represent a suitable product for non-load bearing applications requiring low moisture susceptibility and the possibility for surface decoration, for example flooring and furniture elements.