GLUING OF EUROPEAN BEECH (FAGUS SYLVATICA L.) AND DOUGLAS FIR (PSEUDOTSUGA MENZIESII Mirb.) FOR LOAD BEARING TIMBER STRUCTURES

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1 GLUING OF EUROPEAN BEECH (FAGUS SYLVATICA L.) AND DOUGLAS FIR (PSEUDOTSUGA MENZIESII Mirb.) FOR LOAD BEARING TIMBER STRUCTURES Michael Schmidt 1, Markus Knorz 2 ABSTRACT: The bonding quality of beech and Douglas fir glulam bonded with MUF adhesives was determined and compared to glulam made of Norway spruce. By means of delamination tests according to EN and block shear tests according to EN 392 the durability and strength of the bondlines have been examined. Based on these test results further investigations on the stress distribution induced during the delamination test have been performed. For a better understanding of the curing process influenced by wood species a modified tack test is introduced. This method allows for real-time monitoring of the curing process. KEYWORDS: Glulam, MUF, adhesive, delamination test, block shear test, curing, modified tack test 1 INTRODUCTION 12 For decades Norway spruce (Picea abies L.) has been the main wood species used for load bearing purposes in central Europe. With global climate changes the forests in central Europe are facing a turning point. In many regions Norway spruce is regarded as not adapted to altering growth conditions. Therefore, measures are being taken to establish more resistant forests. Other species like European beech (Fagus sylvatica L.) or Douglas fir (Pseudotsuga menziesii Mirb.) which are more tolerant to altering growth conditions, are increasingly being planted. To ensure successful commercialization of beech and Douglas fir timber, innovative wood products for load bearing purposes should be developed. By using beech lamellas, the load bearing capacity of glulam can be enhanced, and joints can be designed with a lower number of fasteners [6]. On the other hand, Douglas fir could substitute Norway spruce in many applications. Furthermore, the natural durability could be a valuable advantage for increased use in outdoor applications. Essential requirements for the production of load bearing products such as glulam are adhesives that allow durable und reliable bonding of lamellas as well as the necessary finger joints. At present the most important type of adhesive for the production of glulam is melamine-ureaformaldehyde resin (MUF) [12]. In addition to good bonding qualities, MUF adhesives have a light-coloured glueline which is preferred by most customers. Of course, glulam made of Douglas fir has already been introduced into the market and is highly valued for its use in components which are exposed to weathering. However, glulam made of Douglas fir is mainly produced with phenol-resorcin-formaldehyde resin (PRF). Because MUF resins are well established and frequently used glues in Europe and to avoid frequent changes of the glue systems during production, the industry is demanding MUF systems which allow reliable bonding of different wood species. With regard to beech, several investigations have been carried out in the past years aimed at the glueability for use in load bearing timber structures [1, 2, 11, 14]. Regarding gluing characteristics of beech with and without, no consistent results could be determined from delamination and shear tests. An important test for an approval of adhesives is the resistance to delamination according to EN The corresponding American test method is specified in the standard ASTM D It is assumed that by passing the delamination test a bonded joint has good long-term durability [10,13]. Moreover, the delamination test is a method for production quality control besides the block shear test (EN 392). Therefore, this paper contains test results both from delamination and shear tests using the wood species Norway spruce (reference), beech and Douglas fir. Based on these results the delamination test method was reflected critically. Furthermore the influence of wood species on the curing process of MUF adhesives was examined by a new test method. 1 Michael Schmidt, Holzforschung München, Technische Universität München, Winzererstrasse 45, München, Germany. michael.schmidt@wzw.tum.de 2 Markus Knorz, Holzforschung München, Technische Universität München, Winzererstrasse 45, München, Germany. knorz@wzw.tum.de

2 2 OBJECTIVES One goal of this investigation was to describe to what extent two commercially available MUF adhesives can be used for the production of glulam made of beech and Douglas fir. With regard to requirements for a technical approval of beech glulam, the resistance to delamination according to EN was chosen as test method. Requirements for passing this test are specified in EN 301. Even though it is known that this test is primarily designed for Norway spruce, at present an adhesive for gluing beech also has to pass this test with the same requirements. Both beech with and beech without were included in the test series, because some important characteristics differ from white beech. In addition, comparative studies with Norway spruce were carried out. While the delamination test is supposed to reveal the long term durability of a bonded joint, the short term glue line strength can be investigated with a shear test. For quality control in glulam production facilities, both test methods may be used to determine the quality of bonding. This is due to an assumed correlation between resistance to delamination and block shear test results [1,17]. Therefore, another goal was to verify the supposed correlation between the results of the two methods. Based on the results of delamination tests, additional studies have been carried out. One approach was to closer observe the delamination test method. The test results depend, among others, on the wood species because of significant differences regarding their swelling, shrinking or drying characteristics. In relevant European standards for adhesives, these different characteristics of the wood species are not considered. Facing new wood species or modified wood for the production of glulam, it can be questioned whether the present delamination test is the appropriate test method. Several proposals have already been made, not knowing the magnitude of induced stresses, influenced by different wood species behaviour, versus bond strength. This study should contribute to this discussion by observing the drying behaviour of test specimen during a delamination test. Another approach was to describe gluing characteristics of the wood species involved. Understanding the differences between species and their specific interactions with adhesives is an important contribution for enhancing bonding of those species. Although curing is very important for the development of a bond, it is not well surveyed. Therefore, this study also pays attention to the curing behaviour depending on characteristics of the wood species. A new test method has been developed which allows for monitoring of real-time curing behaviour of wood adhesives. 3 EXPERIMENTAL SETUP 3.1 ADHESIVES AND BONDING OF BEAMS Two commercially available MUF systems, fulfilling all requirements of EN 301 have been used in this research. Both systems are two-component adhesives, which are used for the production of load bearing constructions made of softwood and can be utilized for interior or exterior applications. In the further course of the paper the adhesives are named MUF-1 and MUF-2. For the production of beams according to EN six flat sawn lamellas of different wood species were bonded in a climate chamber. The lamellas of Douglas fir, beech and Norway spruce had been stored for several weeks in standard climatic conditions (20 C / 65 % relative humidity). Lamellas of Douglas fir contained over the full cross-section. Both beech with and without red were included in the study. Seven beams consisting of lamellas containing red over the full cross section and twenty seven beams without were produced. Mean values for normal density (ρ12) and moisture content at the time of gluing are displayed in Table 1 for all lamellas. Before bonding, the lamellas were freshly planed. Table 1: Normal density (ρ12) and moisture content of lamellas Mean normal density (ρ12) in kg/m³ Mean moisture content in % Norway spruce Douglas fir Beech Table 2: Bonding parameters Wood species Norway spruce Douglas fir No. of beams Adhesive Mixing ratio Amount of glue in g/m² Pressure in N/mm² Closed assembly time in min. 12 MUF-1 100: , , 30, 45, 60, 75, 90 Beech white 16 Beech red MUF-1 100: , 60, 80, 90, Beech white 11 MUF-2 100: , 30, 45, 60, 75, 90

3 While MUF-1 was only tested for bonding of Douglas fir, Norway spruce and beech containing red, white beech lamellas were glued with both MUF systems. Applying glue, lamella assembly and pressing was performed in standard climate. For beech always a pressure of 1.2 N/mm² was applied, while for softwoods two different pressures (1.0 N/mm² and 0.7 N/mm²) were used. Due to the fact that results of the delamination test are sensitive to the bonding technology, varying closed assembly times were chosen. The parameters for bonding are given in Table 2. In total 62 beams were produced. After pressing and prior to cutting out the test specimens, the beams were stored at standard climatic conditions (20 C / 65 %) for at least seven days. Figure 1 gives an outline of sampling points of specimens from the beams. From each beam at least two specimens with a length of 75 mm were taken for the delamination test. For the block shear test, two sticks (50 mm x 50 mm) were cut out. Figure 1: Sampling of test specimen 3.2 BLOCK SHEAR TEST Block-shear specimens were tested for dry shear strength and wood failure according to EN 392. Before testing, the specimens were stored in standard climate (20 C / 65 % relative humidity). Shear strength at failure was calculated in N/mm². Wood failure in the shear area was estimated to the nearest 5 percent. For each beam two sticks were tested, so that for each bondline two shear strength and wood failure values were obtained. Since no significant differences between the two values for each bondline were found, mean values were calculated and assessed. 3.3 DELAMINATION TEST The delamination test according to EN consists of three cycles of soaking of the test specimen, by means of vacuum and high pressure and a subsequent drying procedure. During soaking of the test specimen for 2.5 hours, shear forces typically arise from the swelling of the wood. Drying at high temperatures (65 ± 3 C) and low relative humidity (12.5 ± 2.5 %) causes a moisture gradient in the wood, which again leads to high stresses in the bondline. These stresses are partly perpendicular to the bondline due to the warping of the lamellas [10]. The test is intended to generate very high stresses by the wood itself. Because water as well as elevated temperature is involved, the bondline is also exposed to these degradative factors at the time these stresses are developing. If these stresses are higher than the strength of the bonded joint, usually delamination occurs between glue line and wood or within the glue line itself. These separations (delaminations) at the end grain faces are measured for length, added together and expressed as percentage of total length of glue line. To pass the test the total length of separations must be below a value specified by the standard. According to EN 301, the maximum delamination over all gluelines in a test specimen may not exceed 5%. 3.4 APPROACH FOR OBSERVING THE DELAMINATION TEST High stresses perpendicular to the bondline develop when a moisture gradient in the specimen is caused by drying. When the moisture content (MC) at the endgrain faces (e.g., positions 15, 21 as illustrated in Figure 2) falls below the fiber saturation point (FSP), the wood starts to shrink. At the same time the MC in the core (positions 16-20) is still well above the FSP, and the wood has its maximum dimensions. This results in tensile stresses perpendicular to the plane of the glue line. To put it simply it can be assumed that critical tensile stresses in the glue line appear in the shrunken area between the end-grain and the 30 % MC boundary. In areas with MC > 30 %, compression stresses act on the glue line. Further examinations were carried out observing differences in drying behaviour of Norway spruce and Douglas fir. Conclusions regarding the induced stress affecting the glue line should be drawn from this data. For the observation of the drying behaviour, the standardised test specimens as well as the drying parameters according to EN were used. The test specimens were glued with MUF-1 adhesive and 0.7 N/mm² pressure. To minimize the differences in drying characteristics within one wood species, five test specimens were taken out of one beam. The focus of this experiment was to determine the moisture content within a single lamella after certain drying times (4h, 8h, 12h, 16h, end of test). Due to expected differences in the drying behaviour depending on the position of the

4 lamellas, the upper and one of the middle lamellas were examined (see Figure 2). Besides that, the amount of absorbed water after soaking and the drying times were documented. Immediately after reaching the predefined drying times, the delamination of bondlines was measured. To determine the MC of the wood, both lamellas were then cut out. The lamellas were cut into small cubes and weighed, five cubes in width direction and seven cubes in length direction were obtained (see Figure 2). By means of oven-drying a mean value of moisture content for each cube could be determined and moisture content profiles could be created. Upper lamella Middle lamella time the probe is removed with a specified speed. The force required to separate the probe from the adhesive is measured. Tack is expressed as the maximum force (F max ) or energy dissipated in the process of debonding (w). For a better understanding of wood adhesives and their interactions with different wood species during bond formation, it was necessary to modify and adapt this method. In Figure 3 the devicee used for the adapted test method is illustrated. A usual rheometer (Physica MCR 301 Anton Paar) was altered for these modified tack measurements. In a special base plate (1) a freshly planed wood specimen (ws_1) was placed and fixed. Another freshly planed, circular wood specimen (ws_2) was fixed to a probe(2) with a screw. A constant diameter of 25 mm was chosen for ws_2. Until starting the tests ws_1 and ws_2 were stored in standard climate (20 C / 65 % relative humidity). During measurement the entire arrangement was covered with a cap (3), securing a constant temperature of 20 C (± 1 C). Figure 2: Test specimen for determination of drying behaviour during delamination test Figure 3: Device for the modified tack test 3.5 MODIFIED TACK TEST FOR MONITORING CURING Methods which allow for monitoring of real-time curing of an adhesive on a wooden surface are rare. A simple but still established method to measure tack is to spread adhesive on a wood surface, touch the adhesive after defined time intervals and assess the tack characteristics when removing the finger. This method has been used for decades for determining maximumm open assembly time of an adhesive system. It is obvious that this method is limited in reproducibility and reliability. Therefore, it seems to be evident that a better method needs to be developed, which allows for monitoring of real-time curing. Furthermore, it should be possible to describe the curing process not only on a wooden surface, but also between two assembled surfaces with a defined gap filled with adhesive. A method already introduced in other fields of adhesive research is the so-called probe tack test. It was especially developed to characterize pressure sensitive adhesives. In summary, the test procedure is to bring a probe in contact with an adhesive with a specified pressure. After having held this position for a certain By means of an electronically controlled motor it was possible to define an exact gap between the two wooden surfaces. After setting a zero gap between the two wood specimens adhesive was applied onto the wood specimen in the base plate. Then, a gap of 0.2 mm was adjusted between the two specimens. Thereby intimate contact between adhesive and wood was secured. To ensure that the narrow gap was completely filled with adhesive it was required that the adhesive squeezes out. After removing the squeezed out adhesive, the measurement of the closed assembly time started. In contrast to the briefly described method for characterizing pressure sensitive adhesives, no pressure was applied. Since MUF wood adhesives are aqueous adhesive solutions, the curing process is influenced by evaporation and absorption of the water by the wood. Additionally, the adhesive penetrates into the wood substrate. As a result, the initially defined gap of 0.2 mm will be continuously reduced. By means of normal force measurement it was possible to control and readjust the position of the probe. The readjusted position was related to the zero gap. Therefore, the shrinkage could be recorded. Thereby intimate contact

5 of adhesive and wood was ensured and an infiltration of air into the gap could be avoided. Knowing that grain orientation is influencing specific parameters of wood, like water absorption, it was determined that all specimen were flat sawn. After varying closed assembly times (30 minutes to 120 minutes) the two wood specimen were separated by driving the probe with ws_2 again in opposite direction. The velocity of the probe followed a logarithmical ramp starting at 0.02 mm/sec and ending at 1.0 mm/sec. The probe is connected to a force transducer which measures the forces during bond separation as a function of time. Data were recorded in periods of 0.02 second. The required tensile force showed a pronounced maximum and then decreased to zero at the moment where complete separation was achieved. The maximal required force F max allows for characterizing the development of curing. By integration of the force vs. distance curve during the separation period the energy w can be obtained according to Equation (1). (1) w: energy dissipated in the process of debonding F: force required for separating ws_1 and ws_2 h: separation distance h1: starting point of separation h2: end point of separation Investigations were carried out for Norway spruce, Douglas fir () and beech with and without red. The briefly described MUF-1 and MUF-2 system were tested with varying closed assembly times. The depending variables were maximum force F max and w. Figure 4: Mean delamination in % of total glue line length dependent on closed assembly time and type of adhesive; green bars MUF-1; blue bars MUF-2 In Figure 5 the results of the delamination test conducted for MUF-2, varying pressure and closed assembly times are illustrated for Norway spruce and Douglas fir. Satisfactory results were obtained for Norway spruce. Delamination was for all specimens below the required maximum delamination of 5%. This was not surprising, because this adhesive is already approved for the production of load bearing timber structures. But it is surprising that all Douglas fir specimens failed. Although there was no combination of parameters which realized satisfying results, less delamination was found when longer assembly times were chosen. Especially for specimens produced with 1.0 N/mm² pressure this trend was observed. The best results were achieved for a closed assembly time of 90 minutes and an applied pressure of 1.0 N/mm². Here the delamination was only about 7 %. 4 RESULTS 4.1 DELAMINATION TEST In Figure 4 the delamination of beech specimens is illustrated dependent on closed assembly time and adhesive. It is obvious that a prolongation of closed assembly time results in less delamination, while short assembly times caused excessive separations. No differences were found between beech lamellas containing red or not. Both MUF systems fulfilled the requirements according to EN 301 for gluing beech. Precondition is that longer assembly times have to be applied. Figure 5: Mean delamination in % of total glue line length dependent on closed assembly time, wood species and pressure

6 Table 3: Results of block shear tests Applied pressure in N/mm² No. of bondlines (strength values) shear strength in N/mm² wood failure in % mean standard deviation mean standard deviation Beech (358) Douglas fir Norway spruce (60) (60) (60) (60) BLOCK SHEAR TEST Table 3 gives a compilation of the block shear strength results of all bondlines, distinguished for species and pressure. Because for beech no relevant differences between the two MUF systems and for beech with and without were observed, the results were combined. The differences between the two softwoods and beech are ascribed to wood characteristics and are not the result of weak bondlines. The value of the wood failure shows clearly, that the failure did not occur in the bondline on most specimens. The achieved strength and wood failure values were satisfactory for all parameter combinations. Except for two bondlines in Douglas fir specimens and one bondline in a Norway spruce specimen, all glue lines fulfilled the requirements of EN 386. The fifth percentile value of the block shear strength was f v,bs = 14.8 N/mm² for beech, assuming a normal distribution. Also no differences could be found for the two pressures that were applied for bonding Douglas fir and Norway spruce. Therefore, these results are also combined. The fifth percentile values of block shear strength were f v,bs = 7.4 N/mm² for Douglas fir and f v,bs = 7.1 N/mm² for Norway spruce. 4.3 RELATIONSHIP BETWEEN DELAMINATION AND BLOCK SHEAR TEST For all species and examined MUF systems no correlation could be found between the test results from delamination and block shear test. Figure 6 illustrates the results for bondlines of Douglas fir and Norway spruce only. Bondlines which showed excessive delamination can not be identified by shear strength or wood failure. For bonded beech lamellas it was already shown, that there is no correlation [16]. Figure 6: Shear strength and wood failure of all glue lines joining Douglas fir and Norway spruce lamellas as a function of the results of the delamination test. 4.4 APPROACH FOR OBSERVING THE DELAMINATION TEST Due to differences in density, the test specimens made of Norway spruce and Douglas fir have different initial weights. After soaking, considerably higher absolute water absorption of Norway spruce was found as compared to Douglas fir. Despite the high amount of absorbed water, the drying rate of Norway spruce was much faster than that of Douglas fir. Whereas test specimens made of Norway spruce reached the targeted range of % of the initial weight within 16 h (NS_4) and 17 h (NS_5), respectively, test specimen DF_5 had to be dried for 23 h (see table 4) to achieve this level. The higher water absorption of Norway spruce in comparison to Douglas fir was also found on test specimens from delamination tests reported in paragraph 4.1. There, also a prolonged drying time for Douglas fir was observed.

7 Table 4: Soaking and drying characteristics of Norway spruce and Douglas fir within the delamination test Wood species Norway spruce (NS) Douglas fir (DF) Test specimen Drying time in h Weight before test in g Weight after soaking in g Water absorption in % Weight after drying in g Delamination in % NS_ NS_ NS_ NS_ NS_ DF_ DF_ DF_ DF_ DF_ Immediately after taking the test specimen out of the drying oven the delamination rate was determined. For both wood species first delaminations could be detected after 12 h drying time. While the maximum delamination for Norway spruce was within the requirements of EN 301 with 1.8 %, higher delamination values could be found for DF_5 (12.7 %) and DF_4 (15.9 %). For both wood species and each drying time, the upper and middle lamella were cut into cubes and the MC of each cube was determined. Exemplary, the MC levels after the five drying times are displayed for the crosssection in the center of the middle lamella in Figure 7 for Norway spruce and Figure 8 for Douglas fir (positions as illustrated in Figure 2). Analysing the graphs leads to the following conclusions: - Both the higher water absorption of Norway spruce and the slower drying rate of Douglas fir is revealed by the graphs. - For this cross-section, the MC level in the area of the end-grain of Norway spruce specimens decreases to a value below 30 % after a test duration between 12 h and 16 h. This decrease of MC occurs for the Douglas fir specimens between 8 h and 12 h drying. Therefore, tensile stresses on the bondline presumably affect the bondline for < 5 h in Norway spruce specimens and for > 11 h for Douglas fir. - After falling below 30 % MC, the slope of the MC gradient between the end-grain areas and the core is significantly higher for Douglas fir than for Norway spruce. This suggests that the distance between the end-grain and the 30 % MC boundary is smaller. Therefore high tensile stresses on the bondline are acting on a smaller area. Comparable differences in load duration and MC profiles between Norway spruce and Douglas fir were also found for other cross sections in the middle lamella. Figure 7: MC profiles after different drying times for centre area of the middle lamella of Norway spruce test specimens Figure 8: MC profiles after different drying times for centre area of the middle lamella of Douglas fir test specimens

8 4.5 MODIFIED TACK TEST FOR MONITORING CURING Figure 9 shows three representative curves which illustrate the required tensile forces for debonding 0 N F N ,9 79, ,05 80,1 80,15 80,2 80,25 80,3 min 80,4 Zeit t Figure 9: Representative curves of tensile forces for debonding beech, Norway spruce and Douglas fir after 80 minutes assembly time specimens of Douglas fir, Norway spruce and beech without after 80 minutes assembly time. While for debonding beech a maximum tensile force of F max N was recorded, Norway spruce had a peak at a maximum tensile force of F max -42.8N and Douglas fir at F max N. The new method indicated for F max and w show highly reproducible values. Besides F max and w more information could be obtained from the shape of the force vs. time curve, which is however not subject of this paper. In Table 5 the recorded mean F max and mean dissipated energy w are given for all specimens and assembly times. It is clear that F max increases with increasing closed assembly times and MUF-2 is a faster curing adhesive system. Furthermore Table 5 reveals that F max and w are affected by wood species. Compared to Norway spruce and white beech, which were always characterized by the highest values, the recorded F max and w for Douglas fir were lower for all assembly times. This shows clearly that the curing process of MUF-adhesives is retarded for Douglas fir. Also, the curing process seems to be slowed down for beech containing red. F max and w were staying always remained behind values for white beech. Table 5: Overview tack tests performed with Norway spruce, Douglas fir and beech Assembly time in min MUF-1 MUF-2 Wood species number of F max,mean in number of F w specimens N/mm² mean in J max,mean in specimens N/mm² w mean in J Norway spruce Douglas fir beech white beech Norway spruce Douglas fir beech white beech Norway spruce Douglas fir beech white beech Norway spruce Douglas fir beech white beech Norway spruce Douglas fir beech

9 5 DISCUSSION 5.1 DELAMINATION AND BLOCK SHEAR TEST The current procedure of an adhesive approval for softwoods requires a delamination test according to EN This test has to be carried out with Norway spruce. After approval, the adhesive may be used for gluing other softwoods regardless of wood characteristics. For gluing hardwoods, additional delamination tests with the respective wood species have to be passed. Although this test and the standardised requirements, respectively, are not designed for hardwoods, both MUF systems fulfilled the requirements. For gluing beech certain production parameters have to be considered though. These positive results contributed to a technical approval for the production of glulam consisting of beech. Since 2009, beech glulam can therefore be generally utilized for indoor application in Germany. For satisfying bonding of beech a prolongation of assembly times is recommended. Also beech glulam consisting of lamellas containing red can be produced. As expected, all delamination tests for Norway spruce specimens glued with an approved MUF showed good test results. In contrast, no test specimens made of Douglas fir met the requirements according to EN 301, even though no excessive delamination was observed. Due to a low number of test specimens these results have only preliminary character. However, the trend of a better resistance to delamination with longer assembly times could also be seen for Douglas fir, in particular with an applied pressure of 1.0 N/mm². These results indicate that an adjustment of certain parameters for gluing wood species other than Norway spruce may contribute to better and more durable bonds in terms of delamination. The assumed correlation between results of delamination and block shear test could not be observed. Regarding block shear strength all wood species showed high bonding strength. High wood failure percentages also suggest good quality of bondlines. With the exception of three bondlines, the requirements of EN 386 were fulfilled. Glue lines with high delamination could not be identified with the block shear test. For the production control of beech glulam this has already been considered in the corresponding technical approval. 5.2 OBSERVATIONS WITHIN THE DELAMINATION TEST A more detailed examination of water absorption and drying characteristics revealed significant differences between Norway spruce and Douglas fir. On the one hand, the lower drying rate of Douglas fir leads to longer stress exposure of glue lines. On the other hand steeper gradients of MC suggest considerably higher stress. For an appraisal of different test results this has to be considered. Although the results of the delamination test depend on wood species, it is still considered as a reasonable test. It is a well established method for determining durability of glue lines. Also the test identifies weak bondlines caused by incorrect bonding. However, the test method is based more on experience than on scientific background. 5.3 MODIFIED TACK TEST Results of delamination tests for bondlines in beech and Douglas fir glulam revealed a positive trend with longer assembly times. Therefore, it was assumed that curing has an impact on the results. To understand how the curing process is influenced by wood species a new test method was introduced. This approach for monitoring real-time curing behaviour indicated satisfying results for analysis or screening of wood-glue combinations. Although this study has only the character of preliminary investigations, it can be assumed that this method is reliable and reproducible. Therefore it seems to be an enhancement compared to common methods. For an appraisal of the results it has to be considered that curing of MUF adhesives consists of a combination of physical and chemical processes. First of all, physical reasons for the observed results should be discussed. MUF-adhesives are water-based systems, containing 30-50% of water. During the curing process this water has to be absorbed by the wood substrate or released into the surrounding climate. In case of assembled lamellas water absorption by the wood is the main pathway. It is already known that the capacity of water absorption of beech is much higher compared to Norway spruce or beech containing [15]. For Douglas fir the capacity of water absorption seems to be much lower [4]. This difference might contribute to a retarded curing, which was observed. Another factor influencing curing are specific chemical properties of wood species. The hardening process of the two MUF systems is initiated by acid (hardener). The speed of the polycondensation reaction is significantly influenced by the amount of acid added to the adhesive and the resulting ph-value of the system [5]. For interactions taking place between wood and adhesive during the curing process it can be assumed that the phvalue of the wood and the capacity of buffering acid and protons, respectively, influence curing. Limited knowledge of chemical properties, which are able to influence the curing process, is available. Therefore further research is ongoing to identify those properties and to understand the influences on curing behaviour of wood species. 6 OUTLOOK Studies on tensile stresses occurring during delamination test will be expanded to other wood species. Based on

10 this data a model of stress distribution will be developed by means of finite-element methods. More information regarding the induced stresses in glue lines could contribute to an ongoing discussion in Europe. Based on investigations regarding stress distribution depending on wood species, the requirements should be adjusted. The corresponding American standard ASTM D-2559 for instance distinguishes between softwood and hardwood. A 60 % higher delamination is allowed for hardwoods to pass the test in comparison to softwood. The modified tack test allows for observing the curing process in detail. Advanced studies will be started in order to understand interactions between adhesives and wood. In particular, focus will be put on chemical characteristics of wood surfaces. REFERENCES [1] Aicher S., Reinhardt H.-W.: Delaminierungseigenschaften und Scherfestigkeiten von verklebten rotkernigen Buchenholzlamellen. European Journal of Wood and Wood Products, 65: , [2] Aicher S., Ohnesorge D.: Shear strength of glued laminated timber made from European beech timber. European Journal of Wood and Wood Products, DOI /s , Published online: 28 January [3] ASTM D-2559: Standard Specification for Adhesives for Structural Laminated Wood Products for Use Under Exterior (Wet Use) Exposure Conditions [4] Boehme, C., Hora, G.: Water Absorption and Contact Angle Measurement of Native European, North American and Tropical Wood Species to Predict Gluing Properties. Holzforschung, 50: ,1996 [5] Dunky M., Niemz P.: Holzwerkstoffe und Leime Technologie und Einflussfaktoren. Springer-Verlag Berlin Heidelberg, 2002 [6] Ehlbeck J., Werner H.: Softwood and hardwood embedding strength for dowel-type fasteners. CIB- W18/ Meeting 25 Sweden, [7] EN 301: : Adhesives, phenolic and aminoplastic, for load-bearing timber structures - Classification and performance requirements [8] EN 302-2: : Adhesives for load-bearing timber structures - Test methods - Part 2: Determination of resistance to delamination [9] EN 392: : Glued laminated timber - Shear test glue lines [10] Frihart, C.R.: What does moisture-related durability of wood bonds mean? In: Proceedings of the Final Conference on COST E34 Bonding of Timber, , 2007 [11] Frühwald A., Ressel J. B., Bernasconi A.: Hochwertiges Brettschichtholz aus Buchenholz. Forschungsbericht Bundesforschungsanstalt Forstund Holzwirtschaft Hamburg, [12] Mack H.: Der europäische Markt für Brettschichtholz (BSH). In: Wiener Leimholz Symposium 2006, 2006 [13] Marra A. A.: Technology of wood bonding Principles in practice. Van Nostrand Reinhold New York, [14] Ohnesorge D., Richter K., Becker G., Aicher S.: Adhesion behaviour of glued laminated timber from European Beech. In: Proceedings of the Final Conference on COST E34 Bonding of Timber, , 2007 [15] Schmidt M.: Verklebung hochfester Laubhölzer für tragende Holzbauteile. In: Doktorandenkolloquium Holzbau Forschung+Praxis, , [16] Schmidt M., Glos P., Wegener G.: Verklebung von Buchenholz für tragende Holzbauteile. European Journal of Wood and Wood Products, 68:43-57, [17] Zeppenfeld G., Grunwald D.: Klebstoffe in der Holz- und Möbelindustrie. DRW-Verlag Leinfelden-Echterdingen, 2005.