LATHE CHECK FORMATION AND THEIR IMPACT ON EVALUATIONS OF VENEER-BASED PANEL BOND QUALITY Anti Rohumaa 1 , Christopher G. Hunt 2 , Mark Hughes 3 , Charles R. Frihart 4 , Jaan Kers 5 ABSTRACT: During the peeling of veneer, lathe checks as deep as 70 to 80% of the veneer thickness are formed. This study showed that during adhesive testing according to EN 314 deep lathe checks in birch (Betula pendula Roth) veneer significantly reduced the shear strength of phenol-formaldehyde (PF) bonded plywood, even though these checks are not mentioned in the standard. In addition, we show that specimens tested open can fail by a different mechanism than those pulled closed, especially when checks are deep. Lathe checks were also shown to influence bond strengths when using the Automated Bonding Evaluation System (ABES). These findings stress the importance of measuring lathe check depth and considering the orientations of checks during testing to get a better understanding of bond quality in veneer-based products. KEYWORDS: bond quality; lathe checks; plywood; shear strength; percent wood failure 1 INTRODUCTION 123 During peeling, lathe checks form on the veneer surface curving away from the knife and move through the veneer at an angle to the surface. The checks side of veneer has been named the ‘loose side’ and the opposite side is the ‘tight side’ (Figure 1). Lathe check parameters, their formation and measurement during peeling have attracted significant research interest [1-7]. It is known that the peeling settings are very important in obtaining high quality veneer [8,9] and the optimum settings can vary depending on the raw material [10]. Lathe check depth and frequency are correlated; deeper checks tend to be less frequent than shallower checks [12- 14]. Compression of the log just in front of the knife impacts the depth and frequency of lathe checks [11], and heating logs before peeling reduces the formation of deep lathe checks [5,15], which is beneficial since it has been shown that shallower checks are less detrimental to veneer strength perpendicular to grain [16]. Roughness of wood has been frequently used as a parameter to predict adhesive bond formation and quality, though measurement of the true topography taking part in bonding is ambiguous and the optimum surface topography for bonding varies also with adhesives [20]. Wood surfaces that are too rough sometimes prevent 1 Anti Rohumaa, Aalto University, School of Chemical Technology, Finland, anti.rohumaa@aalto.fi 2 Christopher G. Hunt, US Forest Service, Forest Products Laboratory, Madison, USA, cghunt@fs.fed.us 3 Mark Hughes, Aalto University, School of Chemical Technology, Finland, mark.hughes@aalto.fi intimate contact between the adhesive and with wood surface [17]. There is no clear limit for surface roughness, but according to Sellers [10], cited in [22], the maximum roughness depth for acceptable veneer bonding is about 0.5 mm. Traditional surface roughness measurement techniques might not adequately characterize the surface roughness of wood material relevant to bonding, because of a weak boundary layer [23], which is a result of wood processing and which can limit adhesive bond formation with intact wood. In this study we were interested in the weak boundary layer created by deep lathe checks or surface fractures, which prevent the surface from being well anchored to the bulk veneer. Wood processing parameters play an important role in the surface roughness and other parameters that ultimately impact bond performance [17]. Some studies have demonstrated that heating logs by soaking in hot water before peeling will decrease surface roughness [18,21], while at least one shows the reverse [19]. In general, the effects of log soaking prior to peeling have received little attention. Generally, the quality of an adhesive bond in plywood is evaluated by testing saw kerfed specimens in tensile shear. In the European Standard EN 314 “Plywood Bonding Quality” percentage wood failure (PWF) and shear strength are used in assessing adhesive bond 4 Charles R. Frihart, US Forest Service, Forest Products Laboratory, Madison, USA, cfrihart@fs.fed.us 5 Jaan Kers, Department of Polymer Materials, Tallinn University of Technology, Estonia, jaan.kers@ttu.ee