Effects of curing time and end pressure on the tensile strength of finger-joined black spruce lumber Cecilia Bustos Mohammad Mohammad Roger E. Hernández ✳ Robert Beauregard ✳ inger-joints are commonly used to produce engineered wood products from short pieces of lumber. Such joints must have excellent mechanical performance. To be suitable for structural uses, a joint must be subjected to a proper end pres- sure following machining and adhesive application. To produce acceptable prod- ucts, technical parameters, such as ma- chining and gluing processes must be op- timized. The conditions of curing time and the pressure applied during joining play a major role in the gluing process and the final strength of the assemblies (CTBA 1973). Isocyanate-based adhesives such as polyurethane (PUR) are a viable alterna- tive for wood finger-jointing applica- tions. They are gaining acceptance in North America for a variety of structural applications (Verreault 1999, Chen 2000, Lange et al. 2000). PUR adhesives develop a high strength and cure at am- bient conditions. Hot-pressing or ra- dio-frequency treatments can be used to accelerate the curing process. Studies by Pagel and Luckman (1983, 1984) have shown that PUR-bonded joints did not fail in creep and had good water resis- tance. King and Chen (2001) investi- gated the performance of a two-part PUR adhesive for finger-joint applica- tion and for I-joist assembly using vari- ous curing times. They tested black spruce among other species. Results showed that the adhesive cured in a rela- tively short time, which makes it suit- able for finger-joint processing. How- ever, no information is available on the influence of end pressure using this type of adhesive for finger-jointing black spruce. Thus, end pressure and curing time need to be further investigated for FOREST PRODUCTS JOURNAL Vol. 53, No. 11/12 1 The authors are, respectively, PhD Student, Dept. of Wood and Forest Sciences, Laval Univ., Quebec, QC, Canada G1K 7P4; Research Scientist, Forintek Canada Corp., Quebec, QC, Canada G1P 4R4; Professor and Associate Professor, Dept. of Wood and Forest Sciences, Laval Univ., Quebec, QC, Canada G1K 7P4. The authors wish to thank the technicians from the Dept. of Wood and Forest Sciences at Laval University, as well as those from the Value Added Program and Building Systems Department at Forintek Canada Corp. for their techni- cal support. Acknowledgment is also made to the Canadian Forest Service for their financial support, and to the Bío-Bío University, Concepción, Chile, as well as Ashland Adhesives, for their valuable support. This paper was received for publication in May 2002. Article No. 9497. ✳Forest Products Society Member. ©Forest Products Society 2003. Forest Prod. J. 53(11/12): Abstract Finger-joined black spruce (Picea mariana (Mill.) B.S.P) specimens from eastern Canada were prepared to assess the effect of curing time and end pressure on the tensile strength of the joints. An isocyanate adhesive cured at room temperature and a feather joint configuration were used for this purpose. The joints were machined at a 18.3 m/min. (60 ft./min) feed rate, 3500 rpm rotation speed, and 0.86 mm (0.034 in.) feed per knife (chip load). A single-face glueline application was used at a spread rate of 110 g/m 2 . Four curing times (1, 2, 5, and 24 hr.) and six end pressures ranging from 1.38 MPa to 4.82 MPa (200 to 700 psi) applied for 20 seconds were tested. The results showed that curing time and end pressure have a statistically significant influence on the perfor- mance of structural finger-joints. After 5 hours of curing time, finger-joints made with isocyanate can achieve more than 90 percent of the reference ultimate tensile strength based on 24 hours of curing time. Analysis also indicated that finger-joined black spruce has the best performance at an end pressure of 3.43 MPa (498 psi). Lower or higher end pressure can result in a lower tensile strength.