The Impact of Log Heating on Veneer Quality and Plywood Performance
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International Scientific Conference on Hardwood Processing (ISCHP2017) The Impact of Log Heating on Veneer Quality and Plywood Performance Anti Rohumaa1*, Christopher G. Hunt2, Charles R. Frihart2, Jaan Kers3, Louis Denaud1, Mark Hughes4 1 LaBoMaP, Ecole 2US Forest Service, 3Department of Material 4Department of Nationale Supérieure Forest Products and Environmental Bioproducts and d'Arts et Métiers, Cluny, Laboratory, Madison, Technology, Tallinn Biosystems, Aalto 71250, France WI 53726, USA University of University, Espoo, Technology, Tallinn 02150 Finland 19086 Estonia ABSTRACT In veneer-based products manufacture, logs are commonly heated to soften the wood material prior to peeling. At a higher heating temperature, veneer properties like lathe checking and surface roughness generally decrease, but the literature has shown no clear effect of these changes on bond quality. One of the reasons could be that the evaluation methods used for roughness are not able to detect the depth of lathe checks or the integrity of the surface, which probably have a significant effect on bonding quality. In the present study silver birch (Betula pendula Roth) logs were used to prepare the veneer and a liquid phenol-formaldehyde (PF) resin was used in the bonding process. Lathe check depth, surface roughness, surface integrity and plywood bond strength were evaluated. This study indicated that the heating of the logs not only softened the wood during peeling, but also causes irreversible changes in the wood material which subsequently effects plywood bond strength development. During plywood bond testing according to SFS-EN 314, deep lathe checks in veneer significantly reduced the shear strength of PF bonded plywood, even though these checks are not mentioned in the standard. These findings not only confirm the importance of minimizing the depth of lathe checks for product quality, but also demonstrate how check depth could influence a test (SFS-EN 314) which was designed mainly for testing adhesive properties and evaluate adhesive cure. 1. INTRODUCTION In veneer-based product manufacture, the logs are commonly heated to soften the wood prior to peeling (Marchal et al. 2009, Dupleix et al. 2013) and to obtain smoother veneer with minimised severity of lathe checks (Dupleix et al. 2013). The formation of lathe checks during peeling has been well studied (Denaud et al. 2007, Tomppo et al. 2009, Pałubicki et al. 2010, Denaud et al. 2012, Dupleix et al. 2013, Antikainen et al. 2015, Darmawan et al. 2015, Pot et al. 2015). The literature shows that at higher peeling temperature the formation of deep lathe checks are reduced (Dupleix et al. 2013, Rohumaa et al. 2016a), which is beneficial since shallower checks are less detrimental to veneer strength perpendicular to the grain (Kontinen et al. 1992). However, several researchers have noticed that lathe checks also affect bond quality (Chow 1974, Neese et al. 2004, DeVallance et al. 2006, Rohumaa et al. 2013), but due to a lack of knowledge and consensus it is not clear how, and to what extent, the checks affect bonding quality. Generally, it has been shown that the veneer surface characteristics correlate with adhesive bond quality. For example, the roughness of wood has frequently been used as a parameter to predict adhesive bond quality (Aydin 2004, DeVallance et al. 2007). However, the measurement of the true roughness taking part in bonding is ambiguous (Baldan 2012) and this is probably also the reason why in the available literature contradictions can be found on the effect of roughness in the bonding process. It is generally known that surfaces that are too rough can be detrimental, where the adhesive cannot make intimate contact with the wood surface (Marra 1992). However, the real magnitude of the roughness affecting bonding is not properly defined. The main methods used today to evaluate surface roughness are based on stylus or laser displacement sensors (Sandak et al. 2004, Sandak and Negri 2005). These methods are based on a single roughness profile and cannot always adequately characterize the true surface roughness of veneer taking part in bonding since the parameters of lathe checks (depth and angle) cannot be measured with traditional surface roughness evaluation methods. Unfortunately, there are no surface roughness measurement techniques available which include the effect of lathe checks in bonding. * Corresponding author: Tel.: +358 415174661; E-mail: [email protected] 184 The Impact of Log Heating on Veneer Quality and Plywood Performance The bonding quality of plywood can be evaluated by testing saw kerfed specimens according the European standard SFS-EN 314. In this standard, percent wood failure (PWF) and shear strength are used in the assessment of bond quality. Generally, if the adhesive bond fails in the wood, it will most likely be accepted. It has also been shown that in testing according to EN 314, the results differ depending on whether the checks are pulled open or closed (Rohumaa et al. 2013). It has been reported that the shear strength of plywood with the checks pulled closed results in higher strength values than when the checks are pulled open, however, the magnitude of the strength difference varies from 14 up to 94% (Bethel and Huffman 1950, Koch 1965, Chow 1974, DeVallance et al. 2006). The variation in strength might be due to check depth, but in most published reports, the depth of checks is not measured or presented. The aim of this study is to understand the effect of soaking on veneer quality and to evaluate the effect of lathe checks in bonding process. 2. MATERIALS AND METHODS 2.1. WOOD MATERIAL Two freshly felled birch trees were sectioned into 6 logs nominally 1.2 m in length and completely immersed in water tanks heated to either 20ºC or 70ºC for 48h, or 70ºC for 48h followed by cooling until the core temperature reached 20ºC. From each tree the logs were selected for both soaking temperature in order to decrease the variability caused by raw material variation. Then the logs were rotary cut on an industrial scale laboratory lathe (cutting speed 100 m min-1, knife bevel angle 21°, compression rate 10%) manufactured by the Raute Corporation (Model 3HV66; Raute Oyj, Lahti, Finland) into veneer. The veneer was visually inspected and specimens were cut from the veneer ribbon with dimensions of approx. 900 mm by 400 mm, free from obvious defects such as knots or sloping grain. The veneers were subsequently dried at 160ºC in a laboratory scale veneer dryer (Raute Oyj, Lahti Finland) to achieve an average MC of 6%. 2.2. VENEER QUALITY EVALUATION A Mitutoyo Surftest 402 was used to evaluate the roughness of the veneer surface across the veneer grain. In the roughness measurements the cut-off length was 2.5 mm, sampling length was 12.5 mm and the detector tip radius was 5 μm. Roughness parameters Ra, Rmax and Rz were obtained (SFS-EN ISO4288 1998). Prior to bonding quality testing, the checks depths of each SFS-EN 314-1 (2005) test specimen were measured in the region where failure would occur. All plywood specimens were treated with the textile dye on one side of the test specimen (SFS-EN 314). The depths of all checks occurring between the saw kerfs were measured and the average check depth was calculated for each specimen. This method was developed and used by Rohumaa et al. (2013). The method significantly improves the correlation between the bonding strength of plywood and lathe check depth. The integrity of the veneer surface was evaluated with a Huygen internal bond tester (model 1314, Huygen Corporation, Wauconda, IL USA), which is commonly used to produce a high speed Z-direction rupture in paper and paperboard. In this study, the veneer was fixed between a stainless steel sample base and an aluminum angle using double-sided tape (P-02, Nitto Denko Corporation, Osaka Japan) by applying a constant pressure of 0.12 MPa for 5 s. After pressing, a pendulum, held in a horizontal position by an electro-magnet was released. The pendulum striking the vertical leg of the aluminium angle separated the tape from the veneer and allowed observation to be made of the attached wood particles on the tape surface. The method is described in detail by Rohumaa et al. (2016a). 2.3. ADHESIVE, BONDING PROCESS AND BOND QUALITY In the present study a liquid phenol-formaldehyde (PF) resin (Prefere 14J021, Prefere Resins Finland Oy, Hamina, Finland) with 49% solids content was used as the adhesive. Two different methods were used to evaluate bonding quality, the automated bonding evaluation system (ABES) and EN-314. In ABES testing, matched specimens, 20 x 117 mm2, were cut from the conditioned veneer sheets. For bonding, the resin was applied by a micropipette (HandyStep electronic, BRAND GMBH + CO KG, Wertheim, Germany) to an area of 5 x 20 mm2 at one end of the veneer specimens to give a resin spread rate of ~100 g m-2. After adhesive application, the veneer-resin assembly was placed in the ABES equipment (Adhesive Evaluation Systems, Inc., Corvallis, Oregon, USA) and hot pressing started immediately. Shear strength was measured after various pressing times ranging from 20 to 180 s. The platen temperature was 130°C and press pressure was 2.0 MPa. In total 400 specimens were prepared and at least 7 parallel samples were measured in each bonding group. 185 International Scientific Conference on Hardwood Processing (ISCHP2017) For testing according EN-314, the plywood was bonded with the same PF resin, with a spread rate of 155 g m-2. The 7-ply plywood was produced in a laboratory hot press. After lay-up, the panels were pre-pressed for 8 min at 0.8 MPa prior to hot pressing. The hot press time was 7 min, the platen temperature 128°C and press pressure 1.8 MPa.