Effects of surface treatment and adhesives on bond performance and mechanical properties of cross-laminated timber (CLT) made from small diameter Eucalyptus timber

The feasibility of manufacturing cross-laminated timber (CLT) using fast-grown small diameter Eucalyptus timber was explored in preliminary studies. However, eucalyptus bonded with commercial one-component polyurethane adhesive usually provides poor resistance to delamination and shear force. The effects of priming surface of eucalyptus CLT lamella intended for a bondline and different adhesives on block shear strength (BSS), wood failure percentage (WFP), and rate of delamination (RD) of Eucalyptus CLT made from small diameter lumber were studied via block shear and cyclic delamination tests at both dry and wet states. Four different types adhesives: epoxy resin (EP), emulsion polymer isocyanate (EPI), phenol resorcinol formaldehyde (PRF), and polyurethane (PUR) adhesives were used to manufacture three-layer Eucalyptus CLT panels following adhesive manufacturers' instructions. Hydroxymethylated resorcinol (HMR) and hygroscopic organic solvent N,N-dimethylformamide (DMF) primers were adopted to priming surfaces of Eucalyptus CLT lamella intended for a bondline. The results indicated that eucalyptus CLT made of small diameter lumber with the four adhesives were comparable to commercial softwood CLT. The maximum wood failure percentage (WFP) and block shear strength (BSS) values of specimens bonded with PUR adhesive at dry state were 85.2% and 3.51 MPa whereas those values of specimens bonded with EPI adhesives at wet state were 58.2% and 1.62 MPa respectively. The minimum rate of delamination (RD) value was 7.6%, which was obtained from specimens bonded with PUR adhesive. The enhancing effects of HMR and DMF priming treatment on bond performance and mechanical properties of Eucalyptus CLT bonded with the above four adhesives did not show a clear trend because their effects depended on the coupling effects of the adhesive and primer. Generally, HMR priming treatment was effective to enhance bond performance and mechanical properties of eucalyptus CLT. After HMR priming, the WFP values of eucalyptus CLT at wet state bonded with PRF and PUR adhesives increased by 27.8%, 12.4% meanwhile the BSS values increased by 31.5%, 4.9%. The RD values of specimens bonded with PRF and PUR adhesives at wet state were close to 0.0% after HMR priming.