EFFECTS OF END-PRESSURE ON THE FINGER-JOINT QUALITY OF BLACK SPRUCE LUMBER : A MICROSCOPIC ANALYSIS

1 Associate Professor, Departamento de Ingeniería en Maderas, Universidad del Bío-Bío, Concepción, Chile, 4081112 2 Professor and Dean respectively of the Dept. of Wood and Forest Sciences, Laval University, Quebec, QC, Canada, G1V 0A6. 3 Group Leader and Research Scientist, FPInnovations-Wood Products Division, Otawa, ON, Canada K1S 5R1. Corresponding author: cbustos@ubiobio.cl Received: 02.06.2011 Accepted: 04.08.2011 EFFECTS OF END-PRESSURE ON THE FINGER-JOINT QUALITY OF BLACK SPRUCE LUMBER: A MICROSCOPIC ANALYSIS


INTRODUCTION
Finger-jointed products are manufactured by taking short pieces of defect free kiln-dried lumber, machining a finger profile in each end of the pieces, applying an appropriate structural adhesive, and joint the pieces together under end pressure to form long length soft lumber.Finger-jointing is used in several Engineered Wood Products such as glulam and it is also commonly used for flange material in wood I-beams.Such structural applications require the wood and the glued joint to be strong enough to resist specified service loads in bending and tension.In principle, the finger-jointing process allows the removal of strength reducing defects, the increase of the surface area of the glue joint, which will produce a product with high engineering properties.The strength of the joints is controlled by specific process parameters such as moisture content and temperature of lumber, machining process parameters, several wood-related factors such as species, density, natural defects, as well as others that are related to the gluing process (Bustos et al. 2003a, 2004, Karastergiou and Ntalos 2005, St-Pierre et al. 2005, Dagenais and Salenikovich 2008, Vassiliou et al. 2009).Little information is available on the influence of end-pressure when using an isocyanate type of structural adhesive for finger-jointing black spruce.The effects of end-pressure and curing time on the performance of finger-jointed black spruce were investigated earlier by Bustos et al. (2003b) in order to determine the optimum conditions for the production of finger-joints for structural applications.Both curing time and end-pressure were found to have a statistically significant influence on the finger-jointing quality using isocyanate type of adhesives.After 5 hours of curing time, finger-joints made with isocyanate achieved more than 90% of the reference ultimate tensile strength based on a 24 hours curing time.Analysis also indicated that finger-jointed black spruce achieved its best performance at an end-pressure of 3.43 MPa.Lower or higher end-pressure can result in a lower tensile strength.
DOI 10.4067/S0718-221X2011000300007 The main purpose of end pressure is to bring the adjoining surfaces together so that the glue forms a thin and continuous film between these surfaces.This pressure also allows a uniform distribution of the adhesive and creates an optimum glueline thickness.Several authors have investigated the effect of the glueline thickness on strength of finger-joints (Groom and Leitchi 1994, River 1994, Sandoz 1984, Ebewele et al. 1979).Findings from these studies have indicated that the glueline thickness needs to be controlled in order to produce strong joints.Generally, thin glue lines lead to starved joints while thicker gluelines (i.e., beyond optimum thicknesses) may lead to stress concentration in the adhesive layer due to cure-shrinkage.Pressure must also be applied to force fingers together to form an interlocking connection, giving a certain immediate handling strength (Raknes 1980).While the increase of the endpressure up to a certain level gives a better contact of the fingers to obtain strong joints, cell damage or splitting of the finger root could be induced due to excessive pressure (Kutscha and Caster 1987, Marra 1984, Jokerst 1981).
Several opinions exist regarding the optimum pressure needed to produce high-strength finger-joints.The German standard DIN 68-140 (Deutsches Institut fuer Normung 1971) specifies minimum acceptable values for the different lengths of fingers.For example, 11.77 MPa for 10 mm fingers and 1.96 MPa for 60 mm fingers.The German standard also establishes that a minimum pressure of 0.98 MPa must be applied (Deutsches Institut fuer Normung 1971).Strickler (1980) stated that an end-pressure of about 2.76 MPa is near optimum for most of softwood species.Typically, with an increase in end-pressure, a better contact between the sides of the finger is obtained and the gap between the fingertip and root is reduced.Thus, as a higher end-pressure is applied, more locking efficiency and performance can be obtained up to the point where damage to the tips of fingers or splitting of the wood at the root occurs (Dawe 1965).Madsen and Littleford (1962) used end-pressures between 0 and 4.14 MPa with casein and phenol-resorcinol adhesives.Their results showed that 2.76 MPa end-pressure was adequate to facilitate curing and to develop optimum tensile strength.Juvonen (1980) also studied the effects of the end-pressure on joint strength but considering the geometry and size of the fingers.The end-pressure had a relatively small effect on joints with long fingers and, adequate joints could be obtained with a relatively low end-pressure.However, with short fingers, the effect of end pressure in the low pressure range was greater.As a result, a certain minimum pressure is recommended depending on the size of fingers.Ayarkwa et al. (2000) tested three end-pressures on finger-jointed African hardwoods.Results showed no significance of this parameter on Modulus of Rupture (MOR) and Modulus of Elasticity (MOE) in bending.On the other hand, Sandoz (1984) noted that a "back pressure" can be obtained during the phase of release of the pressure, which can cause separations and lead to adhesive-free gaps at the end of the finger.The key objective of this additional analysis is to complement the study carried out by Bustos et al. (2003b) on the effect of end-pressure on finger-joint quality in black spruce wood by providing a better understanding on how end pressure affects the finger-joint quality and be able to quantify that effect.

MATERIALS AND METHODS
Experiments were carried-out with 38 mm by 64 mm, kiln dried planed black spruce studs coming from the Chibougamau region in the province of Quebec.The studs were placed in a conditioning room at 20°C and 65 percent relative humidity to reach equilibrium moisture content (EMC) of 11.8%.Defects were removed from pieces by cross-cutting to produce blocks varying in length between 20 and 91 cm of N° 2 and better grades (National Lumber Grading Authority 2000a).The defecting and sampling process was based on the Canadian National Lumber Grades Authority NLGA-SPS 1-2000 specifications for finger-jointed structural lumber (National Lumber Grading Authority 2000b).
A Conception RP 2000 finger-jointing machine with a lateral feed system, commonly used in the North American finger-jointing industry was used.The ends of the blocks were machined across their width in order to obtain horizontal finger-joints.A feather profile was selected due to its good mechanical Effects of end-pressure ...: Bustos et al. performance (Bustos et al. 2003b).A chip-load of 0.84 mm was obtained by setting the machine at 18.3 m/min feed speed, and 3500 rpm rotational speed, with 6 knife sets (bolts) per tool.The finger-joint geometry was 28.27 mm length, 0.76 mm tip width, and 6.69 mm pitch.Following finger-jointing, the blocks were glued with a structural adhesive formulation according to the technical recommendations supplied by the adhesive manufacturer.The structural adhesive was a two-component system consisting of an ISOSET UX-100 polyurethane prepolymer mixed with an ISOSET WD3-A322 emulsion polymer.These two components are mixed immediately prior to use.A single-face glueline application was used at a spread rate of 110 g/m 2 .The assembled joints were pressed at 20°C.The end-pressure ranged from 1.38 MPa to 4.90 MPa and was applied for 20 seconds.The finger-jointed lumber was cross-cut to produce test specimens of 2.44 m long.Their average basic density was estimated at 448 kg/m 3 .The specimens were mechanically tested after twenty-four hours of curing at room temperature.Scanning electron microscopy (SEM) evaluations were performed to quantify the effect of the endpressure on the finger-jointing of black spruce wood.One of the 2.4 m length finger-jointed lumber containing at least 5 joints was randomly selected in order to obtain the joint samples for SEM analysis.Twelve blocks of about 1 cm 2 of transverse face that includes the gluelines, were removed from the joints zone of the specimen.Seven blocks of 1 cm 2 of longitudinal face that includes gluelines were cut from specimens manufactured with low, medium and high end pressure (Figure 1).

Figure 1. Samples for SEM analysis taken from transverse and longitudinal surfaces
The blocks were prepared with a razor blade by carefully cutting either the end-grain surface or the longitudinal surface for SEM analysis.The blocks were then desiccated with phosphorus pentoxide over two weeks, mounted on a standard aluminum stub with silver paint, redesiccated and coated with gold palladium in a sputter-coater.Electron micrographs of representative sub-surfaces were taken for each of the end-pressure applied.The depth of damage and glueline thickness were evaluated at five systematically chosen points from each SEM micrograph.The gap at the tip of the finger (S) was also measured in each micrography taken in the longitudinal side of the surface.

Mechanical properties of finger-joints at different end-pressures
A summary of results reported by Bustos et al. (2003a) on the influence of end-pressure on the ultimate tensile strength (UTS) is given in table 1.These results were associated with the amount of end pressure applied during the manufacturing process.UTS increased slightly with the increase of pressure until it reached a maximum value at 3.43 MPa end pressure.After that, the UTS decreased as the end-pressure increased.Pressure in excess of 3.43 MPa will likely cause a reduction of the UTS, triggering splitting due to compression load at the finger root, even without the tips of fingers reaching the roots of the opposite fingers.On the other hand, an end-pressure of 1.35 MPa (196 psi) was enough for the joined wood to be handled even though it represented the lowest performance on the structural properties.Nevertheless, the mean UTS values in all tension tests were higher than those specified in SPS-1 2000 (National Lumber Grading Authority 2000b) for S-P-F group, grade No. 2 and better for finger-jointed structural lumber (Table 1).Detailed results are given in Bustos et al. (2003a).

Table 1. Summary of UTS values of finger-jointed black spruce lumber as determined from
tension tests with different end-pressure levels, Bustos et al. (2003a).

SEM analysis of transverse glueline faces
SEM analysis on transverse sections of specimens showed signs of damaged cells close to the gluelines.Crushed and collapsed cells were observed near or at the glueline.In general, the depth of damaged cells (D) was more pronounced as the end-pressure increased from 1.3 MPa to 4.9 MPa (Fig. 2 and Fig. 3).Variability in the depth of damage observed could be affected by the presence, or not, of latewood or earlywood in the glueline.The random growth ring orientation observed in the different joints could have also contributed to this variability.Tracheids near at the glueline were highly compressed at higher end-pressures.Rays were also compressed and deformed.As expected, the damage in latewood (LW) was less severe than in earlywood (EW), which is owing to the higher density of LW.The maximum depth of cell damage (53 μm) was reached at 4.9 MPa of end-pressure.This represents 90% more cell damage than that observed at a low end-pressure of 1.3 MPa (28 μm).On the other hand, at low end-pressures (1.3 MPa), air bubbles inside the glueline were observed, which can also explain the lower quality of the finger-joints.Pellicane (1994) observed a similar effect on the mechanical strength of finger-joints, which lead to a considerable reduction in the tensile strength.The mechanical fracture of a joint can be initiated by a discontinuity of the adhesive due to stress concentrations in the joints.According to River (1994), an air bubble in the glueline is an example of geometrical discontinuity inside the joint.

SEM analysis of longitudinal glueline faces
The SEM analysis of longitudinal surfaces of black spruce finger-joints showed mainly the effect of the end-pressure on the gap at the tip of the fingers (S).The results are shown in figures 4 and 5.As can be seen, the gap size did not exceed the maximum value required (1600 μm) by the SPS 1-2000 standard.Results confirm that the gaps at the tip of fingers decrease as the end-pressure increases.This behaviour is more important at lower pressures (i.e. from 1.3 MPa to 3.7 MPa).At higher end-pressures, this effect decreases slightly until reaching a gap size at the tip of the fingers of 53 μm at 4.9 MPa.This can be explained in terms of a greater incompressibility of the adhesive within the reduced gaps.The formation of air bubbles appeared clearly for the low end-pressures due to excess of gap because this is part of a discontinuity within the bonded joint.Cracks were also observed at the ends of the joints when the end-pressure was increased (Figure 5F).Tension stress perpendicular to the grain due to excessive axial compression loading exerted by the end-pressure could cause cracks in wood.River (1994) studied the fracture of adhesive-bonded wood joints and established that the fracture of wood and bonded joints and materials begins at a geometric or material discontinuity, where displacement of the adherents (due to external or internal stress) creates the greatest stress concentration and where either the adherent or the adhesive is the weakest.Also, the finger-joints have a flat portion of each finger tip that represents a small butt joint and geometric as well as material discontinuity.This makes this type of joint less efficient in terms of their mechanical performance.Some examples of the geometric discontinuities in adhesive-bonded wood joints are the square-cut ends of overlapped adherents, voids at the tips of fingers in finger-joints, voids, etc.In the feather joint configuration, the rigid fingers are inserted into a pre-cut finger-joint piece.The fingers are pushed into the precut piece due to the end-pressure applied, and can cause splitting along the plane of the precut piece.When the finger-tip is pushed further into the precut specimen, the crack propagates.Figure 5F shows a crack occurred at 4.9 MPa of end-pressure applied.Cracking was initiated at the finger corner and propagated along the grain.As the joint was loaded by the end-pressure, some energy was probably stored within the interface of the adhesive and wood.Thus, micro cracking in the wood surrounding the crack tip could have occurred.Once cracking began, the crack front then advanced and dissipated the stored energy until this energy drops at a lower level.A rapid propagation of the crack can occurs when the stored energy reaches the critical level (strain concentration around the finger-tips) and the wood grain is parallel to the end-pressure applied.The fracture appears to follow the grain.According to Smith et al. (2003), three types of failure can be distinguished microscopically in wood, inter-cell failure (IC) that occurs at the middle lamella and represents the separation of cells, intra-wall failure (IW) that refers to failure within the secondary wall, and trans-wall failure (TW) when fracture Tension tests were performed according to ASTM D 198-99 (American Society and Materials 1997) and SPS-1 2000 (National Lumber Grading Authority 2000b) requirements using an MTS Metriguard testing machine, model 412.Testing and data acquisition were controlled with software developed by FPInnovations (MTS Systems Corporation 1999).The ultimate tensile strength (UTS) was determined.

Figure 2 .
Figure 2. Effect of end-pressure on depth of cell damage of finger-jointed black spruce lumber.Values are mean and standard error of the mean.

Figure 3 .
Figure 3. SEM micrograph of cell depth damage of finger-jointed black spruce wood at end-pressure of A) 1.3 MPa and B) 4.9 MPa.

Figure 4 .Figure 5 .
Figure 4. Gap size at the finger tip in black spruce finger-joint according to the end-pressure applied.Values are mean and standard error of the mean