Verklebung von Buchenholz für tragende Holzbauteile

The load bearing capacity of glulam could be enhanced by using lamella of European beech (Fagus sylvatica??L.). Essential requirements for the production of those beams are adhesives which allow for a durable und reliable bonding of lamella. The paper shows to what extent commercially available MUF and PU adhesives and adapted bonding parameters ensure gluing of European beech timber for load-bearing timber structures. Both beech wood containing heartwood and beech wood without heartwood were included in the study. With regard to technical approval, the chosen test methods were resistance to delamination according to EN??302-2 (2002) and shear test of glue line according to EN??392 (1995). From the two tested MUF systems it was evident, that delamination decreased by increasing the closed assembly time. The requirements of EN??301 (2006) were fulfilled for adhesive type??I and??II. Without any limitations beech wood containing heartwood could be glued using MUF-1. On the other hand, the results of the PU adhesive were insufficient. Light-optical micrograph examinations show that long assembly times secure the formation of a glue line. It was assumed that the curing process is retarded by beech wood. No relationship was found between the results of the shear strength test and the delamination test. For quality control of glulam the delamination test is recommended. Based on the positive results an application for technical approval of glulam consisting of beech was made. For this reason beech-glulam can be generally utilized in service class??1.     

Delaminierungseigenschaften und Scherfestigkeiten von verklebten rotkernigen Buchenholzlamellen

The paper deals with investigations on dry shear strength and delamination behavior of gluelines between beech wood laminations showing no or distinct red heartwood. The investigations covered 36 parameter combinations comprising besides the red heartwood two widths of the laminations, two different adhesive families and two delamination procedures related to verification of adhesive type I or II. The tests revealed no influence of red heartwood on shear strength which for all parameter combinations exceeded almost throughout 10 N/mm². In contrary thereto the delamination tests revealed a distinct dependency of the glueline integrity on red heartwood and width of the adherents. No parameter combination conformed to the requirements of adhesive type I. In case of laminations without red heartwood the requirements of adhesive type II were met. Gluelines between two laminations with red heartwood showed throughout very pronounced delamination above 50%. The integrity of gluelines versus delamination represents one of the essential normative requirements for adhesives used in glued load bearing wooden elements. This proof was not possible for gluelines of beech wood laminations with red heartwood when applying today’s European delamination test procedures and performance requirements developed and established for gluelines of softwood laminations.     

Bonding performance of heat treated wood with structural adhesives

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato ^®) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber.     

The evaluation of panel bond quality and durability of hem-fir cross-laminated timber (CLT)

To better use the second-growth wood resources in value-added applications, this work addressed the manufacturing aspects of cross-laminated timber (CLT) products from western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) (or hem-fir) harvested from coastal British Columbia, Canada. Small CLT billets (nominal 610 mm?×?610 mm) were made to examine CLT bond quality and durability through block shear and delamination tests. Two types of adhesives, single-component polyurethane (PUR) and emulsion polymer isocyanate (EPI) and two critical applied pressure parameters (0.28 and 0.83 MPa) were adopted to manufacture hem-fir CLT. It was found that the adhesive type and applied pressure significantly affected wood failure percentage (WFP) and delamination of hem-fir CLT. When PUR adhesive was used, CLT made at 0.83 MPa pressure yielded significantly higher WFP and lower delamination than that made at 0.28 MPa pressure. The results demonstrated that despite the fact that hem-fir lumber is not particularly specified in the current North American CLT standard, it could be used for manufacturing CLT with the required panel bond quality.     

Failure mechanisms in wood joints bonded with urea-formaldehyde adhesives

Wood joints bonded with urea-formaldehyde (UF) are weakened by cyclic swelling and shrinking. To study the failure mechanisms in UF-bonded joints, specimens were bonded with unmodified, modified (amine), or phenol formaldehyde adhesive and subjected to accelerated aging. Modification of the adhesive properties increased the cleavage fracture toughness and shear strength of bonded joints and improved the resistance of joints to cyclic swell-shrink treatment and accelerated moist-heat aging. Joints bonded with some modified urea-formaldehyde adhesives were as resistant to these treatments as joints bonded with phenol formaldehyde. Physical and mechanical origins of the improved adhesive performance were determined by microscopic analysis. Cure-shrinkage stresses precracked unmodified adhesive layers and damaged the wood interphase. The damaged interphase was especially susceptible to the effects of cyclic swelling and shrinking stresses. Certain modifiers reduced or eliminated cure-shrinkage cracking and damage to the wood. Moist-heat aging caused molecular scission in the bulk unmodified adhesive layer as revealed by the onset of shear cracking in the adhesive layer and erosion of exposed surfaces. Certain modifiers reduced or eliminated molecular scission and erosion responsible for adhesive weakening in moist-heat aging. We conclude that incorporating flexible amines in the adhesive structure improves the durability and stability of UF-bonded joints.     

Determination of characteristic strength and stiffness values in glued laminated beams of Argentinean <Emphasis Type="Italic">Eucalyptus grandis</Emphasis> according to European standards

The present paper reports the results of an investigation regarding the determination of characteristic strength and stiffness values in glued laminated beams of fast-growth Argentinean Eucalyptus grandis according to the criteria established in European standards. An empirical research project with 100 beams in structural sizes and 191 finger-jointed laminations was carried out. Test results allow to analyse the mechanical property values of this glued laminated timber in comparison with those adopted by the strength class system established in the European standard EN 1194 (1999), and they reveal a very high ratio of modulus of elasticity to density. Test results were also compared with values obtained by means of the equations provided by EN 1194 (1999). The effectiveness of the criteria established in this European standard for determining mechanical properties in beams of this deciduous species is analysed through the discussion of test results and those obtained from calculations based on lamination properties.     

PVC/木粉复合材料胶接件拉伸剪切强度的研究

参照国家标准GB/T 7124-2008《胶粘剂拉伸剪切强度的测定（刚性材料对刚性材料）》检测PVC/木粉复合材料胶合试件的拉伸剪切强度,发现：PVC/木粉复合材料胶合试件的断裂均发生在粘接区域外的复合材料本身,因而无法比较胶黏剂的粘接强度。为了能比较胶黏剂胶接接头的粘接强度,实验中将GB/T 7124-2008中规定的试件的厚度从1.6 mm加大至4mm。实验结果表明：采用环氧树脂粘接的PVC/木粉复合材料的拉伸剪切强度值高于用J-39丙烯酸酯胶黏剂粘接的复合材料的拉伸剪切强度。     

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of wax-treated wood

The effects of hydroxymethylated resorcinol (HMR) on the tensile shear strength of wood joints treated with montan ester wax were studied with polyvinyl acetate (PVAc) and melamine formaldehyde (MF) adhesives. Untreated specimens which were bonded with MF and PVAc had similar bond strength properties under dry conditions, but MF bonded specimens’ tensile shear strength values were higher than that of PVAc glued specimens under wet conditions. With both adhesives tested, dry tensile shear strengths of wax-treated specimens were in a comparable range to those of untreated wood, while under wet conditions the strength values of wax-treated specimens were significantly lower than those of the controls. HMR priming increased the shear strength of PVAc bonded specimens under wet conditions and of MF-bonded specimens under dry and wet conditions. The effect on MF-bonded specimens, however, was much more pronounced under wet conditions.     

Integrity check of structural softwood glue lines: correspondence between delamination and block shear tests

In factory production control (FPC) of glued-laminated timber the integrity of glue lines according to product standards has to be checked by carrying out either delamination or block shear tests. The chosen test method should provide reliable and reproducible results, and from a practical point of view should be as cheap and easy to perform as possible. An extensive experimental study aiming at systematically comparing the outcomes of delamination and shear tests on structural softwood glue lines using different wood species and adhesives was carried out. In the delamination tests, the length of open glue lines was assessed after subjecting the test specimens to vacuum- and pressure-soaking and to subsequent drying. In the block shear tests, the shear strength and wood failure percentage were determined. Only if the benchmark values in the product standards were met or exceeded, the respective specimen passed the test and the glue lines were regarded as sound. As a main overall result it turned out that the outcomes of both test methods generally agree well, but will not necessarily lead to the same pass/fail conclusion in any case. Therefore, for FPC the choice of their application should be defined more specifically.     

A delamination test for structural wood adhesives used in thick joints

A test method was developed for the measurement of delamination of structural wood adhesives applied in thick joints. Large differences were observed within a range of 8 commercial epoxy-based products.     

A delamination test for structural wood adhesives used in thick joints

Subject A test method was developed for the measurement of delamination of structural wood adhesives applied in thick joints. Large differences were observed within a range of 8 commercial epoxy-based products.     

Mechanical behaviour of pre-stressed spruce timber–timber 2.5-mm-step grooved connections under shearing tests

A smart shear connection system was tested in order to be used in manufactured elements of a lattice of wooden slats and a cross offset. This type of floor element can present advantages both in terms of weight and suitable insulating filler (Adalberth et al. 2001; Kawasaki and Kawai 2006; Kermani and Hairstans 2006; Dodoo et al. 2014). This connection is made to provide a substitute for a glue connection. The shear connection proposed is made by double-sided grooving timber interfaces to form a cross section. A specifically designed test assembly was constructed to measure and guarantee homogeneous contact pressure in interface test specimens. Shear test results are presented and compared in terms of capacity and stiffness with glue connections. The principal conclusions are as follows: the allowable shearing resistance of the grooved timber–timber joints can approximately reach the resistance of glued connections. The stiffness of the joints is improved by increasing the pre-stress applied for holding contact in the assembly. The grooved timber-to-timber joints exhibit non-linear behaviour which dominates the behaviour of the whole system. Therefore, the behaviour of a unit connection must be determined to obtain the mean load-carrying capacity and stiffness of a system with grooved connections.     

Chinese wattle tannin adhesives suitable for producing exterior grade plywood in China

Wattle tannin adhesives were successfully formulated from tannin extracts, which were commercially produced from black wattle (Acacia mearnsii) bark in China. Using the Chinese wattle tannin adhesives a total of 153 plywood panels were prepared in order to study effects of pH, paraformaldehyde, glue spread, hot pressing temperature and time, and closed assembly time on bonding strength and their bonding quality was assessed according to the Chinese Standard for exterior grade plywood. Statistical analysis results revealed that Chinese wattle tannin can be used for producing exterior grade plywood under Chinese plywood factory conditions. One of the specific characteristics of these Chinese wattle tannin adhesives was that their use resulted in plywood of high quality bonding over the wide range of closed assembly time from 30 minutes to 16 hours. These Chinese wattle tannin adhesives have a great potential for commercialization in China.     

Thermal stability of glued wood joints measured by shear tests

The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.     

Experimental analysis on small-scale finger-jointed specimens at elevated temperatures

Finger joints have to meet strength performance requirements for the use in structural engineered wood products such as glued-laminated timber beams (glulam). Among these, the thermal stability of the adhesive used in the finger joint is an important criterion to determine the suitability of the connection. The influence of adhesives on the load-carrying capacity of glulam beams and finger-jointed members can be assessed by large-scale fire tests; however, there are obvious benefits in using small-scale specimens tested at elevated temperatures as an alternative. In Europe, there is currently no small-scale test available to test the fire performance of adhesives in structural timber members. The work presented in this paper addresses this issue and was supported by members of a European standardisation committee to develop such a small-scale test. This paper presents, as a first step, the results of a series of tensile tests on small-scale finger-jointed specimens tested at elevated temperatures. The tests were performed with different types of adhesives which are currently also tested in large-scale fire tests. In the small-scale tests here, structural adhesives showed nearly no weaknesses with increasing temperature up to 140 °C. Further, MUF and PRF adhesives showed mostly wood failure even up to temperatures of 220 °C. In the next step of this investigation, the results obtained from the small-scale tests at elevated temperature will be compared to large-scale fire test results in order to find a possible link between both testing methods.     

Development and optimization of a new formaldehyde-free cornstarch and tannin wood adhesive

The development and optimization of a new, environment-friendly adhesive made from abundant and renewable cornstarch and tannin is described in this study. At present, the production of wood composites mainly relies on petrochemical- and formaldehyde-based adhesives such as phenol-formaldehyde (PF) resins and urea-formaldehyde (UF) resins. Formaldehyde-free cornstarch plus tannin adhesives were evaluated for mechanical and physical properties (shear strength, rheological characterisation and thermogravimetric analysis). First, optimized conditions for the preparation of cornstarch-sodium hydroxide wood adhesives were determined. The parameters studied were the following: total solids content of the adhesives, sodium hydroxide concentration and sodium hydroxide/cornstarch volume ratio. The highest shear strength results appeared to be 0.5 sodium hydroxide/cornstarch volume ratio when using concentrations of cornstarch (65% p/v H₂O) and sodium hydroxide (33% p/v H₂O). The optimum cure temperature was 170 °C and the cure time 4 min. The addition of two types of tannin-based adhesives, Mimosa and Quebracho, with hexamethylenetetramine (hexamine) hardener to the cornstarch-sodium hydroxide formulation improved the shear strength and decreased the viscosity of adhesives.     

Effect of adhesive type and glue-line thickness on the fatigue strength of mortise and tenon furniture joints

The fatigue strength of rectangular mortise and tenon rubberwood furniture joints made from polyvinyl-acetate (PVAc) and urea-formaldehyde (UF) adhesives were compared. The results showed that the allowable design stress for PVAc adhesive joints could be set at 30 % of its ultimate bending moment as compared to 25 % for the UF adhesive joint. Further, PVAc adhesive joints have large working tolerances for glue-line thicknesses which make it suitable for mass production furniture manufacturing.     

Effect of tannin on increasing UF adhesive performance at high temperature investigated by TMA and TGA analysis

Tannins have been largely studied as wood additives and wood adhesives. In a perspective of a desirably increased utilization of natural raw material in the wood timber industries, this study provides further and specific information on the use of condensed tannins to improve the heat resistance of urea–formaldehyde (UF) resins. The results obtained by thermomechanical and thermogravimetric analyses show that high percentage (33 and 50 %) of quebracho tannin in UF-tannin blend dramatically improves the overall UF thermal resistance, but 50 % of tannin decreases the maximum value of modulus of elasticity (MOE) of the bonded joint. The best balance between mechanical performance and decreased degradation at high temperature is obtained with 33 % of quebracho tannin. The MOE value of UF + 33 % of quebracho tannin at 260 °C is seven times greater than MOE of UF control. Thermograms show that quebracho tannin has a beneficial effect on increasing the resistance of glue-mix to degradation induced by temperature. When the content of tannin in the glue-mix is increased up to 50 %, the resin degrades completely at about 670 °C, i.e. more than 50 °C higher than neat UF resin. This improvement is related mainly to the resorcinol-type ring structure of tannin and the alkaline pH of UF + quebracho tannin glue mix that confer a higher resistance against joint degradation to UF even at elevated temperatures.     

Properties of laboratory made yellow poplar (Liriodendron tulipifera) laminated veneer lumber: effect of the adhesives

Four different commercial resin formulations namely cross-linked polyvinyl acetate (XPVAc), melamine urea formaldehyde (MUF), urea formaldehyde (UF) and melamine formaldehyde (MF) were used to produce laminated veneer lumber (LVL) from yellow poplar veneers in the laboratory. Physical and mechanical properties were evaluated using ASTM (D 1037, D 906, D 5456) standards and compared. Internal bond, tensile shear and block shear strengths of specimens subjected to accelerated (boiled water) and cyclic (repeated cold water) conditions were also determined. Strength properties of yellow poplar LVL were improved compared with the solid wood. The properties of XPVAc bonded LVL were superior or comparable to the thermosetting adhesives. This study clearly indicated that cross-linked PVAc can be used to manufacture LVL with acceptable properties.     

Colourless formaldehyde-free urea resin adhesives for wood panels

Amino resin precursors prepared by the addition of a new, colourless, non-volatile and non-toxic aldehyde, dimethoxyethanal (DME), to urea gave resins for boards that while able to harden were underperforming due to the lower reactivity of DME in relation to formaldehyde. Urea reacts with one and even two molecules of DME to form UDME and U(DME)₂ (called DU) but the subsequent cross-linking reaction to form bridges between two ureas, although existing as observed by CP MAS ¹³C NMR, was too slow at temperatures lower than 140 °C to be of significance for wood panel adhesives. However, addition of 20% isocyanate (pMDI) contributed to cross-linking of DU by its reaction with pMDI to also form urethane bridges, their existence being confirmed by CP-MAS ¹³C NMR. The adhesive resins so formed had excellent performance, were colourless, and produced boards that satisfied well the requirements of the relevant norms for interior panels (EN 120 and EN 312). The results were good enough to decrease the proportion of pMDI to 14% at pressing times starting to be of significance for industrial panel products. Formaldehyde emission, by perforator method was down exclusively to the formaldehyde produced by heating the wood chips. The panel emission was sufficiently low to even satisfy the relevant F^**** JIS A 5908 Japanese standard (JIS A 5908, 1994 ). These adhesives are colourless, as UF resins.