Assessment process of a new structural PUR adhesive

Assessment process of a new structural PUR adhesive Mario Marra 1, Martino Negri 2, Stefano Guercini 3 and Ario Ceccotti 4 ABSTRACT: Engineered timber-based elements used nowadays in timber buildings are based on various types of structural adhesives. The adhesives have to fulfil many requirements; among others they must not unduly reduce strength and stiffness of an element. Also the creep properties and performance at elevated temperatures must be taken into account. To ensure safe and durable glued joints and products (buildings), European standards are focused on a given set of requirements. KEYWORDS: One component polyurethane adhesive, structural adhesive classification, PUR 1 INTRODUCTION 123 For many years several types of adhesives for load bearing structures have been designed and developed, but the admittance of these adhesives has been hindered. The reason for this is a lack of demonstrated long-time performance and the lack of a testing, approval and classification system [1]. Two new European standards on glue laminated timber [2] and cross laminated timber [3] establish the use of phenolic (PF) and aminoplastic (MF) resins, one component polyurethane (PUR) and emulsion isocyanate adhesives (EPI). Thermosetting polycondensates (PF and MF) have been classified by a standard [4] since 1992, while the PUR and EPI classification process is very recent [5]. Casein adhesives, the oldest glue for structural timber, are classified by a specific standard [6] but are not mentioned in glulam and CLT requirements because they are not used anymore by the timber industry. These European standards establish a classification for adhesives according to their suitability for use in load 1 Mario Marra, CNR IVALSA National Research Council, all Adige, Italy. Email: marra@ivalsa.cnr.it and University of Padova, Department of Land, Environment, Agriculture and Forestry, Viale dell Università, 16, 35020 Legnaro, Italy. Email mario.marra@studenti.unipd.it 2 Martino Negri, CNR IVALSA National Research Council, all Adige, Italy. Email: negri@ivalsa.cnr.it and FOXLAB Via Mach 1, 38010 San Michele all Adige, Italy 3 Stefano Guercini, University of Padova, Department of Land, Environment, Agriculture and Forestry, Viale dell Università, 16, 35020 Legnaro, Italy. Email: stefano.guercini@unipd.it 4 Ario Ceccotti, CNR IVALSA National Research Council, all Adige, Italy. Email: ceccotti@ivalsa.cnr.it and FOXLAB Via Mach 1, 38010 San Michele all Adige, Italy Acknowledgements: to Jim Mehaffey for reviewing bearing timber structures in defined climatic conditions. They specify performance requirements for such adhesives for the industrial manufacturing of loadbearing timber structures. According to these standards the performance requirements are applied to the adhesive, but not to the structure nor to the performance of adhesives for wood-based panel production. The use of these standards is primarily intended to assess or control the quality of adhesives. Two types of adhesives, Type I and Type II, are classified according to their suitability for use in different climatic conditions. Type I is for exposure to high temperature and weather conditions, while Type I and II are for the interior of buildings and for exterior applications protected from weather. Adhesives for structural purposes must produce joints of such strength and durability that the integrity of the bond is maintained throughout the expected life in the required service class according to Eurocode 5 [7]. All adhesives must meet the requirements after longitudinal tensile shear [8], delamination [9], perpendicular tensile strength [10] and shear strength after shrinkage [11] tests. Furthermore, one component polyurethane and emulsion isocyanate adhesives adhesive must meet the acceptance criteria of two different creep tests: static load in compression shear [12] and creep deformation in bending shear [13]. The purpose of this study is to analyze the bonding performance of a new PUR adhesive recently introduced in the market and to classify it, focusing on Type I adhesive. 2 MATERIAL AND METHODS 2.1 ONE COMPONENT POLYURETHANE ADHESIVE The two primary components of polyurethane adhesives are polyols and isocyanates. One component adhesives are synthesized from polyetherpolyole with an

excess of di- or poly-isocyanates. The main isocyanates used for this class of adhesives are 4,4 diphenylmethane-diisocyanate (MDI) [14]. It is often assumed that the strong bonds achieved with polyisocyanates are the result of a higher amount of covalent bonds between the adhesive and the lignocellulosic substrate [15]. The adhesive is cured by reacting with active hydrogens present on the surface of wood or in the ambient moisture [16]. Such adhesives complying with standard EN 15425 [5] must be analyzed in accordance with the tests illustrated below and must meet the performance requirements listed in chapter 3. 2.2 TENSILE TEST The tensile shear test is conducted by applying a longitudinal tensile force to a single lap joint with thin and thick glue lines between two rectangular 20x10 mm wooden adherents. The joints are strained to rupture. Samples 150x20x10 mm (Fig. 1) made of beech (Fagus sylvatica L.) are cured for seven days in a standard atmosphere (20 C and 65 RH). After curing and before testing, samples are treated according to Table 1. Ten valid replicates for each treatment are necessary. Load and wood failure are recorded. from that kind of wood. Samples are subjected to an impregnation and a drying procedure. The samples are soaked by water applying alternating vacuum and high pressure. This treatment is repeated twice. Then they are dried rapidly at low humidity in a high velocity air stream. Impregnation and drying are repeated three times. The extent of open glue lines, the delamination, as a result of these treatments is measured and compared with the total length of glue lines on both end-grain faces of the test pieces. The delamination is expressed in percentage and is calculated for each of the test pieces. 2.4 PERPENDICULAR TENSILE STRENGTH TEST The perpendicular tensile test is carried out on two glued beech laminated specimens. From each of the two specimens eight test pieces are cut with dimensions 50x50 mm and 60 mm long and with a rectangular lap joint 25x50 mm thick 0,5 mm (Fig. 1). The first series is submitted to climatic treatments as defined in Table 2 four times and the second one is left untreated. Then they are strained to failure by a transverse tensile load. Load and wood, fibres and adhesive failure are recorded. Table 2: Climatic treatment prior to tensile strength test Duration Temperature Relative humidity 24 h 50 C 87,5 8 h 10 C 87,5 16 h 50 C 20 Figure 1: Tensile shear test (left) and perpendicular tensile test (right) specimens Table 1: Climatic treatment prior to tensile shear test Designation A1 7 days in standard atmosphere A2 4 days soaking in water 20 C Samples tested in the wet state A3 4 days soaking in water 20 C Recondition Samples tested in the dry state A4 6 h in boiling water 2 h soaking in water 20 C Samples tested in the wet state A5 6 h in boiling water 2 h soaking in water 20 C Recondition Samples tested in the dry state A7 72 h in 70 C Samples tested hot 2.3 DELAMINATION TEST The delamination test is performed on two pairs of laminated specimens with six lamellae glued at minimum (5 min.) and maximum (45 min.) closed assembly time. From each of the four specimens two test pieces 75 mm wide, 150 mm long and 180 mm tall are cut. They are made of spruce (Picea abies Karst.). If the adhesive is specifically for hardwoods or treated wood, the adhesive must also be tested on specimens made 2.5 STRENGTH TEST AFTER SHRINKAGE The shear strength test after shrinkage is performed on three spruce crosswise double jointed 100x100 mm specimens with a 0,5 mm thick glue line (Fig. 2). Samples are submitted to a dry storage treatment in a climate of 40 C temperature and 30 relative humidity and an air speed of 0,7 m/s. Each specimen attains a moisture content reduction of nine percentage points from around 17 to 8. Then they are strained to failure by a compressive shear force. Also in this case load and wood failure are recorded. Figure 2: Shear strength test after shrinkage specimen 2.6 STATIC LOAD TEST IN COMPRESSION The static load test is performed on six beech glued test pieces that are subjected to a constant compression shear load under a series of three different climates as defined in Table 3. Each sample is made by three 16 mm thick lamellae trimmed every 28,5 mm along the grain to obtain a specimen that contains 6 pairs of bond lines of 1 290 mm². Total dimensions are 48x50,8x133,6 mm (Fig. 3). Samples are loaded in axial compression by a

calibrated spring at 3 870 N equivalent to a shear stress of 3 in the bond area throughout the climate treatment cycles. The amount of deformation is measured after all the climate cycles, across 12 inscribed lines made before the test by a razor blade perpendicular to every bond line. The measurement, immediately after unloading, was made with an Olympus light microscope with 200 times magnification and a Mitutoyo micrometer 0,001 mm resolution. Figure 3: Static load test in compression shear specimen (left) and load device (right) Table 3: Climatic treatment foreseen for the static load test Duration Temperature Relative humidity 14 h 70 C 10 14 h 20 C 85 14 h 50 C 75 2.7 CREEP DEFORMATION TEST IN BENDING The creep deformation test is carried out on five pairs of glued bending specimens with bondline thickness 0,3 mm made in spruce. Each sample is made by two 25 mm thick lamellae and the total dimensions are 50x50,3x600 mm. The first series bonded with the adhesive to be tested is compared with the second one glued with phenolic-resorcinol adhesive (PRF) conforming to the requirement of adhesive Type I. The bending tests must be performed as four point bending tests with 4 000 N load (Fig. 4). Bending specimens are subjected to the constant load for at least 26 weeks. If the relative creep requirement is fulfilled the test is completed. In case the requirement is not met, the loading shall continue until 52 weeks. Throughout this period the conditions varying weekly from humid (20 C 85) to dry (45 C 40) climates. 3 RESULTS 3.1 EUROPEAN PUR CLASSIFICATION AND PERFORMANCE REQUIREMENTS PUR adhesives complying with standard EN 15425 [5] must meet the performance requirements illustrated in the next tests. 3.2 TENSILE TEST The mean shear strength and percentage of wood failure of the six series of thin joint specimens are shown in Table 4, while thick joint results are listed in Table 5. Tests after conditioning in a standard atmosphere (A1) with thin joints presented percentages of wood failure always around 100. This indicated that the bonding performance was good. Glued thick joints showed a wood failure of 62 and a shear strength value that fulfilled the requirements. Reconditioning after soaking in water (A3 and A5) and at 70 C (A7) with thin and thick joints, showed a percentage of wood failure around 60 with shear strength values above the limit. The bonding performance after water immersion in a wet state (A2 and A4) illustrated that wood failures of thin joint bonds were 0, whereas it raised up to 16 and 37 respectively for thick glue lines. In both cases, the required thresholds were reached. It was shown that PUR has values of tensile shear, above 12 for dry and hot conditions, slightly above 6 for soaking and boiling treatments in a wet state and again 12 for the same treatments in dry state. Table 4: Shear strength and wood failure average and classification requirements for PUR Type I thin joint Requirement Shear Strength Wood failure A1 10 12,6 (1,0) 100 (0) A2 6 6,2 (1,3) 0 (0) A3 8 12,5 (1,3) 60 (30) A4 6 6,0 (1,0) 0 (0) A5 8 11,8 (1,2) 58 (44) A7 8 12,9 (1,9) 68 (34) Table 5: Shear strength and wood failure average and classification requirements for PUR Type I thick joint Figure 4: Creep deformation test in bending shear Requirement Shear Strength Wood failure A1 9 10,3 (1,8) 62 (49) A2 5 7,4 (0,6) 16 (22) A3 7,2 11,1 (2,3) 73 (34) A4 5 6,4 (0,5) 37 (40) A5 7,2 11,1 (1,9) 66 (43) A7 6,5 9,5 (1,8) 63 (43)

3.3 DELAMINATION TEST The delamination of each specimen are shown in Table 6. The threshold of 5 of maximum delamination admitted was fully satisfied (Fig. 4). Table 6: Delamination Minimum closed assembly time Specimens Delamination 1 2,7 (1,8) 2 1,0 (1,2) Maximum closed assembly time Specimens Delamination 1 4,0 (2,7) 2 0,0 (0,0) 3.5 STRENGTH TEST AFTER SHRINKAGE The wood shrinkage due to a reduction in moisture content can have negative effects on the glue joint (Fig. 5). The results in Table 8 show any negative effect for this adhesive. The mean shear strength was 75 above the standard requirement of 1.5. The joint failure occurred mainly in the wood (83) proving the good quality of the adhesive. Table 8: Average shear strength and wood failure Strength Wood failure 2,6 (0,3) 83 (28,9) Figure 4: Delamination test: no open glue lines in this specimen 3.4 PERPENDICULAR TENSILE STRENGTH TEST The mean perpendicular tensile strength of the treated series must be greater than 5 and greater than 80 of the average tensile strengths of the untreated samples. Table 7 shows the mean perpendicular tensile strengths and percentages of wood failure for treated and untreated samples. The average of the tensile strengths of the treated and untreated samples was identical. This result indicates that the adhesive has not changed due to climate treatments. Moreover, the type of failure that occurred in the untreated samples was in the wood (35), while in the treated samples was found predominantly in the wood (95). Temperature and humidity cycles have showed an effect on the resistance of the wood rather than of the adhesive. Table 7: Perpendicular tensile strength and wood failure for treated and untreated samples Samples Strength Wood failure Treated 5,1 (0,5) 95 (14,1) Untreated 5,1 (0,6) 35 (33,4) Figure 5: Image of the rupture of the specimen during the shear strength test after shrinkage 3.6 STATIC LOAD TEST IN COMPRESSION The mean creep deformation of each specimen and of all samples are shown in Table 9. The threshold of 50 µm of maximum admitted deformation was fully satisfied. Table 9: Creep deformations Specimens Deformation µm 1 31 (18) 2 38 (37) 3 32 (23) 4 32 (16) 5 29 (23) 6 32 (25) Mean 32 (24) 3.7 CREEP DEFORMATION TEST IN BENDING The creep deformation in bending shear is determined by the ratio of the PU relative creep value (k def PU) and the PRF value (k def PRF). The relative creep value k def is calculated using the formula (1):

w(t) (t) = 1 w (0) kdef (1) where w(t) = deflection at time t, w(0) is the initial deflection immediately after initial loading, (actually measured 1 min after the loading of the specimen). The average creep deformation ratio of the 4 last climate cycles of all bending specimen pairs evaluated after 26 weeks shall not be greater than 1,12 or 1,15 after 52 weeks. The test is still in progress; however, the average creep deformation ratio after 7 weeks was 1,09. This value is lower than the admitted threshold. The literature does not provide any information on whether the ratio increases or decreases during the trial, so we are not able to define the creep behavior of a PU adhesive yet. 4 CONCLUSIONS This study revealed that the tensile shear strength of bonded specimens depended on the adhesive system used for bonding. In a dry state the PUR adhesive performed with good strength values, while in wet condition the PUR adhesive had sufficient performance. However, the PUR adhesive met the requirements for bonding, especially in reconditioned, warm and thick gap joints. Delamination, perpendicular tensile strength and shear strength after shrinkage tests were fully satisfied. Also the creep deformation for static load in compression shear was fulfilled by the PUR adhesive. The following conclusions can be drawn from the results of the study focused on the polyurethane adhesive tested, which has been recently introduced in the market: the high bonding strength and particularly the good creep performance (frequently a weak point for commercial PUR structural adhesives) that can be achieved indicate that one component polyurethane adhesive has great potential in load bearing timber structures. Other research is ongoing about the potential improvement of the bonding performance by means of small changes in the adhesive composition such as in the bonding process itself. [6] CEN. EN 12436. Adhesives for load-bearing timber structures Caseine adhesives - Classification and performance requirements. Brussels, 2001. [7] CEN. EN 1995-1-1. Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings. Brussels, 2008. [8] CEN. EN 302-1. Adhesives for load-bearing timber structures - Test methods - Part 1: Determination of bond strength in longitudinal tensile shear strength. Brussels, 2004. [9] CEN. EN 302-2. Adhesives for load-bearing timber structures - Test methods - Part 2: Determination of resistance to delamination. Brussels, 2004. [10] CEN. EN 302-3. Adhesives for load-bearing timber structures - Test methods - Part 3: Determination of the effect of acid damage to wood fibres by temperature and humidity cycling on the transverse tensile strength. Brussels, 2004. [11] CEN. EN 302-4. Adhesives for load-bearing timber structures - Test methods - Part 4: Determination of the effects of wood shrinkage on the shear strength. Brussels, 2004. [12] CEN. EN 15416-2. Adhesives for load bearing timber structures other than phenolic and aminoplastic Test methods - Part 2: Static load test of multiple bondline specimens in compression shear. Brussels, 2007. [13] CEN. EN 15416-3. Adhesives for load bearing timber structures other than phenolic and aminoplastic Test methods - Part 3: Creep deformation test at cyclic climate conditions with specimens loaded in bending shear. Brussels, 2007. [14] G. Zeppenfeld. Klebstoffe in der Holz- und Möbelindustrie, Fachbuchverlag Leipzig. 1991. [15] M. Dunky, T. Pizzi, M. Van Leemput, editors. Wood Adhesion and Glued Products. Working Group 1: Wood Adhesives. State of the Art Report. COST Action E13, 1st Edition., 2002. [16] A. Pizzi, K. L. Mittal editors. Handbook of Adhesive Technology. Marcel Dekker, Inc., 2st Edition., 2003 REFERENCES [1] C. J. Johansson, T. Pizzi, M. Van Leemput, editors. Wood Adhesion and Glued Products. Working Group 2: Glued Wood Products. State of the Art Report. COST Action E13, 1st Edition., 2002. [2] CEN. pren 14080. Timber structures - Glued laminated timber and glued solid timber Requirements. Brussels, 2011. [3] CEN. pren 16351. Timber structures - Cross laminated timber Requirements. Brussels, 2011. [4] CEN. EN 301. Adhesives, phenolic and aminoplastic, for load bearing timber structures. Classification and performance requirements. Brussels, 1992. [5] CEN. EN 15425. Adhesives - One component polyurethane for load bearing timber structures - Classification and performance requirements. Brussels, 2008.