Title of paper (Use 14 pt Arial bold)

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010Bonding of beech wood with an adhesive mixture made of liquefiedwood and phenolic resinAles Ugovsek 1 , Mirko Kariz 1 , Dr. Milan Sernek 11 Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Roznadolina, C. VIII/34, 1001 Ljubljana, SloveniaKeywords: adhesive, beech, liquefied wood, phenol-formaldehyde, shear strengthABSTRACTSolid wood is often bonded with synthetic adhesives, which contain formaldehyde or otherchemicals that are harmful to human health and the environment. The development ofenvironmentally friendly adhesives, which could be comparable to existing synthetic adhesives,has been the goal of many recent studies. Another option is to substitute part of the syntheticresin with an environmentally friendly material. The liquefaction of wood, which is a novelprocedure used to convert solid wood into the liquefied state, is a promising procedure for such apurpose. The objective of this research was, therefore, to develop an adhesive in which part ofthe synthetic resin is replaced by liquefied wood.A synthetic phenol-formaldehyde (PF) adhesive, mixed with different proportions of liquefiedwood, was used for the bonding of two beech wood lamellas. Liquefied wood made from poplarwas added in different proportions, ranging from 0 % to 100 %, with increments of 25 %. Thelamellas were bonded in a hot press at 180 °C for different press times. The test specimens weretested according to the standard EN 12765 and EN 205.It was found that the bond shear strength of the dry specimens increased if 25 % of the phenolformaldehydeadhesive was replaced by liquefied wood, but decreased if a higher proportion ofliquefied wood was used. When testing the specimens after immersion or boiling in water, thebond strength decreased rapidly in the case of the samples which had been bonded with adhesivemixtures containing more than 25 % of liquefied wood.INTRODUCTIONSolid wood and wood based composites are nowadays usually bonded with synthetic adhesives.These adhesives are mostly made of oil based derivatives. They are fairly cheap to manufacture,and can provide high quality bonds, but most importantly, they contain formaldehyde, which isharmful to human health and potentially carcinogenic. The problem of free formaldehyde,increased ecological awareness, ever stricter environmental requirements, the introduction ofspecial regulations about volatile organic compounds, and the rise in price of oil derivatives, arethe main reasons for study and development of new naturally-based adhesives. Throughouthistory, many adhesives have been based on natural materials, including tannin (BISANDA ET AL.,2003; VÁZQUEZ ET AL., 2002; LI ET AL., 2004a), lignin (VELÁSQUEZ ET AL., 2003; GOSSELINK ETAL., 2004), soy (HUANG AND SUN, 2000), and many others. Liquefied wood is another naturallybasedproduct, which can be introduced into the adhesive mixture (KOBAYASHI ET AL., 2000; LIET AL., 2004b).

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010Solid wood can be liquefied using different solvent systems. The first system uses phenols(ALMA AND BASTÜRK, 2006), whereas the second employs cyclic carbonates (XIE AND CHEN,2005), the third uses ionic liquids (HONGLU AND TIEJUN, 2006), the fourth introduces dibasicesters without hydroxyl groups (WEI ET AL., 2004), and the fifth system is based on polyhydricalcohols. Solid wood liquefied with polyhydric alcohols contains enough -OH groups, which canpotentially react with the PF resin, and take part in the adhesive mixture as a copolymer (BUDIJAET AL., 2009). The objective of this study was to determine the bond performance of specimensfrom beech wood bonded with an adhesive mixture made of liquefied wood and phenolic resin.EXPERIMENTAL METHODSSawdust (fractions 0,237 mm or smaller) of the black poplar (Populus nigra L.) was used for theproduction of liquefied wood (LW). Prior to the liquefaction process, the sawdust was dried in alaboratory oven (103 °C, 24 h). Glycerol (GLY) (p.a. grade, Kemika, Croatia) was used as thereaction reagent, and sulphuric acid (SA) (p.a. grade, Sigma Aldrich, Germany) was used as acatalyst. The mass ratio between the black poplar sawdust and the GLY was 1:3 (150 g ofsawdust and 450 g of GLY), and 3 % of SA, based on the GLY content, was added (13,5 g). Theliquefaction reaction was performed for 90 minutes in a glass laboratory reactor, which wasimmersed in an oil bath that had been preheated to 180 °C. After reaction, the reactor wasimmersed in cold water in order to quench the reaction. The liquefied product was then dilutedwith a mixture of 1,4-dioxane and water (4/1, v/v) and filtered through filter disks (Sartoriusfilter disks 388 grade/84/mm 2 ) to remove the insoluble parts of the wood. After filtration, themixture of 1,4-dioxane and water was evaporated under reduced pressure, obtained by using awater pump, in order to obtain the liquefied wood containing GLY.The adhesive mixtures were prepared according to Table 1. A commercially available syntheticPF adhesive (Fenolit d.d., Slovenia) was used to prepare the different PF-LW adhesive mixtures.Table 1: Preparation of the adhesive mixturesAdhesive mixture Weight portion of PF adhesive [%] Weight portion of LW [%]1 100 02 75 253 50 504 25 755 0 100Prior to bonding, all of the beech wood lamellas were planed in order to ensure a smooth and flatsurface. The two lamellas were then bonded together with the different adhesive mixtures (Table1). The adhesive mixtures were applied with a roller, using an application rate of 200 g/m 2 . Thepress temperature was 180 °C, and the pressure was 1 MPa. In the first part of experiment thepress time was 600 seconds for all the adhesive mixtures, whereas in the second part the presstime was adapted to the used adhesive mixtures (Table 2). The press times used in the secondpart of the experiment were determined on the basis of preliminary rheological measurements ofthe adhesive mixtures.

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010Table 2: Press times with different adhesive mixturesAdhesive mixture Press time-first part [s] Press time-second part [s]1 600 6002 600 6603 600 7204 600 12005 600 2400The PF-LW adhesive mixtures were tested according to the standard SIST EN 12765:2002. Thebonded samples were cut into specimens according to standard SIST EN 205. The specimenswere then conditioned for 1 week in a standard climate. The dimensions of the shear areas weremeasured for all the specimens. Specimens from each press time group were divided into foursubgroups for the different pre-treatments, prior to testing according to the standard SIST EN12765:2002. The first subgroup of specimens (pre-treatment 1) was tested in the dry state afterconditioning in a standard climate; the second subgroup of specimens (pre-treatment 2) wassoaked in cold water (20±5 °C) for 24 hours; the third subgroup of specimens (pre-treatment 3)was soaked in hot water (67±3 °C) for 3 hours, and then cooled in cold water for 2 hours; and thefourth subgroup of specimens (pre-treatment 4) was boiled for 3 hours, and cooled in cold waterfor 2 hours. The shear tests were carried out on a ZWICK/Z100 universal testing machineaccording to standard SIST EN 205, immediately after the pre-treatments had been performed.RESULTS AND DISCUSSIONThe shear strength values of the tested specimens bonded with different adhesive mixtures,tested after different pre-treatments and a press time 600 seconds, are shown in Table 3. Thewood failure of these specimens is shown in Table 4. The specimens bonded with adhesivemixture 2 (25 % of LW, 75 % of PF) exhibited higher shear strength than the specimens bondedwith the commercial PF adhesive after pre-treatment 1. All the other adhesive mixtures exhibiteda lower shear strength than that of the commercial adhesive. After pre-treatments 2, 3 and 4, onlythe test specimens bonded with adhesive mixture 2 exhibited comparable values to thecommercial PF adhesive. All the other values were much lower, and did not attain therequirements of the standard. According to the standard SIST EN 12765:2002, only thecommercial synthetic PF adhesive and the adhesive mixture with 25 % of added LW to the PFadhesive met the requirements for C1, C2 and C3 durability classes, whereas for the C4durability class only the PF adhesive was suitable.Table 3: Shear strength [N/mm 2 ] of the tested specimens after different pre-treatments and a press time of 600secondsAdhesive mixture Pretreatment 1 Pretreatment 2 Pretreatment 3 Pretreatment 41 11,92 7,89 7,64 7,542 13,5 7,58 7,18 6,963 9,61 4 0 04 7,38 1,72 0 05 0,56 0 0 0

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010Table 4: Wood failure [%] of the tested specimens after different pre-treatments and a press time of 600 secondsAdhesive mixture Pretreatment 1 Pretreatment 2 Pretreatment 3 Pretreatment 41 80 92 82 732 91 77 62 713 83 5 0 04 9 0 0 05 0 0 0 0The shear strength of the specimens bonded with different adhesive mixtures, tested after differentpre-treatments and bonded with different press times, are shown in Table 5. The wood failurevalues of these specimens are shown in Table 6. As can be seen from Table 5, the values decreaseddrastically after the different pre-treatments. Different press times affected the shear strength valuesof adhesive mixtures 3 and 4. Due to the longer press times, adhesive mixtures 2, 3 and 4 attainedthe requirements for C1 durability class. All the other values decreased rapidly and did not meet therequirements for any of the durability classes according to the standard.Table 5: Shear strength [N/mm2] of the tested specimens after different pre-treatments and different press timesAdhesive mixture Press time [s] Pretreatment 1 Pretreatment 2 Pretreatment 3 Pretreatment 42 660 11,32 6,7 6,45 6,133 720 10,1 3,74 1,93 2,744 1200 10,12 5,41 0 05 2400 4,6 0 0 0Table 6: Wood failure [%] of the tested specimens after different pre-treatments and different press timesAdhesive mixture Press time [s] Pretreatment 1 Pretreatment 2 Pretreatment 3 Pretreatment 42 660 95 96 90 923 720 48 13 2 34 1200 93 11 0 05 2400 35 0 0 0CONCLUSIONSThe bond performance of specimens from beech wood, bonded with an adhesive mixture madeof liquefied wood and phenolic resin, was investigated. The bond shear strength of the dryspecimens increased if 25 % of the phenol-formaldehyde adhesive was replaced by liquefiedwood, but decreased if a higher proportion of liquefied wood was used. When testing thespecimens after immersion or boiling in water, the bond strength decreased rapidly in the case ofthe samples which had been bonded using adhesive mixtures containing more than 25 % ofliquefied wood. Longer press times contributed to the shear strength values for adhesivemixtures containing up to 75 % of liquefied wood. Up to 75 % of phenol-formaldehyde resin canbe replaced to attain the requirements of the standard for C1 class, whereas these adhesivemixtures are not suitable for other requirements. Longer press times contribute only in the caseof adhesive mixtures which are used for interior premises, where the moisture content does notexceed 15 %. It can be concluded that, in the case of solid wood to be used for non-structural

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010applications in dry conditions, up to 25 % of the synthetic phenol-formaldehyde resin can bereplaced by liquefied wood if satisfactory bonding is to be achieved.ACKNOWLEDGEMENTSThe authors acknowledge the financial support of the Slovenian Research Agency, throughProject J4-2177. Special thanks go to undergraduate student Aleš Ciber, for technical support ofthe investigations.

„Hardwood Science and Technology”The 4th Conference on Hardwood Research and Utilisation in Europe 2010REFERENCESALMA, H. M., BASTÜRK, M. A. (2006) Liquefaction of grapewine cane (Vitis vinisera L.)waste and its application to phenol-formaldehyde type adhesive. Industrial crops andproducts, 24, 171-176BISANDA, E. T. N., OGOLA, W. O., TESHA, J. V. (2003) Characterisation of tannin resin blendsfor particle board application. Cement and Concrete Composites, 25, 593-598BUDIJA, F., TAVZES, Č., ZUPANČIČ-KRALJ, L., PETRIČ, M., (2009) Self-crosslinking and filmformation ability of liquefied black poplar. Bioresource Technology, 100, 3316-3323GOSSELINK, R. J. A., SNIJDER, M. H. B., KRANENBARG, A., KEISERS, E. R. P., DE JONG, E.,STIGSSON, L. L. (2004) Characterisation and application of NovaFiber lignin. IndustrialCrops and Products, 20, 191-203HONGLU, X., TIEJUN, S. (2006) Wood liquefaction by ionic liquids. Holzforschung, 60, 509-512HUANG, W., SUN, X. S. (2000) Adhesive Properties of Soy Proteins Modified by Urea andGuanidine Hydrochloride. Journal of American Oil Chemist Society, 77(1), 101-104KOBAYASHI, M., TUKAMOTO, K., TOMITA, B. (2000) Application of Liquefied Wood to aNew Resin System-Synthesis and Properties of Liquefied Wood/Epoxy Resins.Holzforschung, 54, 93–97LI, G., QIN, T., TOHMURA, S., IKEDA, A. (2004b) Preparation of phenol formaldehyde resinfrom phenolated wood. Journal of forestry research, 15(3), 211-214LI, K., GENG, X., SIMONSEN, J., KARCHESY, J. (2004a) Novel wood adhesives fromcondensed tannins and polyethylenimine. International Journal of Adhesion & Adhesives, 24,327-333VÁZQUEZ, G., GONZÁLEZ-ÁLVAREZ, J., LÓPEZ-SUEVOS, F., ANTORRENA, G. (2002) Rheologyof tannin-added phenol formaldehyde adhesives for plywood. Holz als Roh-und Werkstoff,60, 88-91VELÁSQUEZ, J. A., FERRANDO, F., SALVADÓ, J. (2003) Effects of Kraft lignin addition in theproduction of binderless fiberboard from steam exploded Miscanthus sinensis. IndustrialCrops and Products, 18, 17-23WEI, Y., CHENG, F., LI, H., YU, J. (2004) Synthesis and properties of polyurethane resinsbased on liquefied wood. Journal of applied polymer science, 92, 351-356XIE, T., CHEN, F. (2005) Fast liquefaction of bagasse in ethylene carbonate and preparation ofepoxy resin from liquefied product. Journal of Applied Polymer Science, 98, 1961-1968