Quality assessment of glued ash wood for construction engineering

Abstract

Delamination resistance and tensile shear strength (TSS) are essential for load-bearing timber structures. Thus these two factors were investigated on industrially bonded ash wood (Fraxinus excelsior L.) to check for the suitability of adhesively bonded ash wood as a building material. Two melamine urea formaldehyde (MUF) resins, two polyurethanes (PUR), one emulsion polymer isocyanate and one phenol resorcinol formaldehyde resin were taken for bonding. Face milled and planed surface series were made to highlight potential differences. For PUR, an additional series with dimethylformamide primed surfaces was also made. The influence of the mixing ratio and the closed assembly time were analysed for one MUF system. The samples for the TSS were tested in dry and wet conditions. 80 % of the tested series met the standard requirements (EN 15425; EN 301) in dry condition, whereas only 30% passed in wet condition. None of the adhesives tested were able to pass the delamination test. No distinct influence of the different parameters studied is notable for most of the adhesive systems, only extended CATs and lower MRs seem to improve the bond quality of MUF. In addition, chemical analyses were performed to find evidence for the poor bonding performance. It was found that acidic extractives, fatty acid content and pH of ash fell within the range of beech and spruce wood, with only formic acid being an exception with an amount four times higher than the other two wood species.