8th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING - 19-21 April 2012, Tallinn, Estonia EFFECT OF BIRCH VENEER PROCESSING FACTORS ON ADHESIVE BOND SHEAR STRENGTH Piirlaid, M.; Matsi, M.; Kers, J.; Rohumaa, A.; Meier, P. Abstract: The quality evaluation of the adhesive bond between layers of plywood material by standard testing methods is laborious and time consuming. To overcome these problems, novel Automated Bonding Evaluation System (ABES) as an alternative and effective testing method was used. ABES provides compact system which allows precise control of the pressing parameters and at same time evaluation of the shear strength properties of the bonds obtained. In this study important processing parameters of veneer manufacturing from birch logs were studied. The influence of birch logs soaking temperature, drying regime and conditioning of veneer sheets to the adhesive joint shear strength was determined. Preceding from the experimental test results the analysis of the technological parameters influence to plywood hot pressing cycles duration and curing cycles temperature was performed. Key words: birch veneer, plywood, adhesive bond shear strength, Automated Bonding Evaluation System 1. INTRODUCTION Plywood is most common material to be used in furniture, building and construction industries. The density of birch wood varies between 640-670 kg/m 3. Bending strength stays in range of 100-111 MPa, but compressive strength varies between 45-54 MPa [1]. Hardwood birch (Betula pendula) and phenol-formaldehyde resin (PF) have been used in plywood manufacturing process for a long time. Physical and mechanical properties will vary due to the different types of forest growths [1] as well as the weathering influence to the birch tree stem quality [2]. The aim of this study was to find out how silver birch veneer processing factors have an effect to phenol-formaldehyde bond strength development and thus affect the plywood quality. 2. MATERIALS AND METHODS 2.1 Birch veneer In current study silver birch (Betula pendula), which is typical European Birch, logs were used as a raw material for veneer peeling. All trees were timbered in wintertime in local forest nearby Tuusula in Finland. When the 300-350 mm diameter trees were trimmed and cut into 1300 mm long logs, they were numbered with the direction from the bottom to top. So the numbers were higher in top. Eight logs were pre-conditioned at room temperature before peeling. When the inner temperature was about 0 C the log was ready for soaking. Birch logs were soaked for 48 hours at different water temperatures (20 C, 40 C, 50 C, 70 C). With each temperature two logs were soaked. The veneer was peeled with speed of 100 m/min to a thickness of 0.8 mm by a giant lathe (Finnish company Raute) by rotary-cutting method. After peeling the logs and cutting the sheets the sheets were separated and dried with 40 C and 160 C. After drying the sheets were divided between two chambers to obtain different moisture contents 6% and 12%.
Most of the 900mm wide continuous veneer with 8mm thickness yield was cut to 900 x 450mm veneer sheets by hydraulic clipper (Fig. 1). parameters and at same time evaluation of the shear strength properties of the bonds obtained. ABES method has been used in scientific research papers to characterize the adhesive curing process [4-6]. Fig. 3. ABES die cutter Fig. 1. Cutting of the veneer sheets with hydraulic clipper For testing with ABES all sheets were punched to test strips according to cutting plan (see Fig. 2) with dimensions 117 x 20mm by special die cutter (Fig. 3) which was manufactured by Adhesive Evaluation Systems Inc. 2 3 3 Fig. 2. Cutting plan for birch veneer samples preparation ABES is miniature hot-pressing device which is equipped with a shear strength testing unit. It is a compact system which allows precise control of the pressing 4 1 5 4 5 The specimens were stored into two climate chambers that had different moisture contents. Room with MC 6% and room with MC 12% were used in this research. Taking the specimens out from the chambers to ABES room or on the contrary was made with using airtight plastic bags to keep the obtained conditions. 2.2. Adhesion tests As for an adhesive in plywood, phenolformaldehyde resin PF (Prefere 14J021) was used. As it is weather-resistant and also one of the cheapest resins, it is widely used especially in plywood manufacturing [2]. For preparation of adhesion test specimens pressing cycle time was varied (20s, 40s, 60s, 80s, 100s, 120s, 140s, 160s, 180s and 200s). With each pressing time seven parallel attempts were made. ABES is almost one square meters wide precise device which works with hydraulics and can be controlled from the computer. It presses and measures shear strength for overlapped veneer specimens. Basically two veneer strips with
dimensions 117 x 20mm, will be bonded together by hot pressing from one end of the strips and afterwards pulled apart from the other end of these strips. ABES working process is illustrated on Fig. 4. Fig. 4. ABES working process [4] Pressure used in ABES adhesion test specimens preparation was 2.1 MPa. 3. RESULTS AND DISCUSSION 3.1. Optimal Amount of Resin In order to find the optimal amount of PF - resin, which penetrates the specimens most effectively initial tests were made with different quantities from 40 to 145 g/m 2. As the curing curves had almost the higher values and lower variance of parallel tests at this point, the optimal amount of PF resin for bonding the veneer proved to be 100 g/m 2. In addition, it also gave best technological results, as it was possible to cover the overlapping area homogenously and the hot press plates remained clean of resin residues after pressing (Fig. 5). 3.2 Processing factors effect on shear strength of plywood Log soaking temperature (LST), moisture content (MC), drying temperature (DT) and Based on the Figs. 6 and 7 it was found that MC has great effect to bond strength development. While comparing the MC of 6% with MC of 12%, MC 6% had better values in every case. As the bond between the adhesive and the surface is not as strong with 12% as with 6%, it could be that with 12% of MC there is more water left in veneer cell pores. Drying temperature has noticeable effect to bond shear strength development in curing process with PF resin. Drying with temperature 40 C the strength results are quite similar, but 160 C of drying temperature has an opposite effect. Higher drying temperature will clearly divide measured results of tested strips treated with different soaking temperatures. In fact temperature 40 C gains better strength values with soaking under 50 C. Over 50 C of soaking temperature strength values are better with 160 C of drying temperature instead. As the material is less moistened it needs less drying as well. Soaking temperature has considerable effect to the bond shear strength. The higher soaking temperature is the better is PF resins bond shear strength values. The curing process is rapider, when soaking temperature is higher. Could be that more moistened material will be less damaged in producing process. In comparison of pressing time 180s and 200s, it appeared that pressing has an effect on bond development, but the time effect depends on how the material has been treated before testing. The higher soaking temperature is used the shorter pressing time is needed. Fig. 5. Optimal amount of PF-resin
*Soaking temperature 70 C, drying 40 C/160 C, moisture content 6% and 12% Fig. 6. Shear strength of hot pressed birch veneer adhesive joints 4.57 3.94 5.10 4.30 5.07 5.03 4.98 4.06 4.80 5.45 4.41 4.93 5.86 6.30 5.81 5.65 6% 6% 12% 12% 6% 6% 12% 12% 6% 6% 12% 12% 6% 6% 12% 12% 40 C 160 C 40 C 160 C 40 C 160 C 40 C 160 C 40 C 160 C 40 C 160 C 40 C 160 C 40 C 160 C 20 CS 40 CS 50 CS 70 CS Fig. 7. Average shear strength results with hot pressing time 180s 4. CONCLUSION The influence of plywood processing factors to the bond strength development between birch veneer and phenolformaldehyde resin, was studied using Automated Bonding Evaluation System as an alternative testing method. The biggest effect to bond strength development has soaking temperature, followed by moisture content and then drying temperature. As a result of the study it was discovered that pressing time depends on soaking temperature and therefore has also important role. According to the results, the higher soaking temperature is used the shorter curing time is needed.
5. REFERENCES 1. Kasesalu A. Comparative data about silver- and lowland birch wood technical properties, Proceedings of Estonian Agriculture Academy, 41. Tartu: EPA, 1965 2. Ilisson, T; Metslaid, M; Vodde, F; Jogiste, K; Kurm, M, Storm disturbance in forest ecosystems in Estonia, Scandinavian Journal of Forest Research, 2005 20, 6, pp. 88-93 3. Prefere 14J021. Fenolihartsi vanerin liimaukseen. Käyttöturvallisuustiedot. Dynea Chemicals OY. 2003 4. Heinemann C., Lehnen R., Humphrey P.E. Kinetic Response of Thermosetting Adhesive Systems to Heat: Physico-Chemical Versus Mechanical Responses, In: Proc. 6th Pacific Rim Bio-Based Composites Symposium, Portand/USA, 10.-13.11.2002. Corvallis: Oregon State University 2002, Vol. 1, pp. 34-44. 5. Ferra, JMM, Ohlmeyer, M., Mendes, AM.; Costa, MRN, Carvalho, LH., Luisa H., Magalhaes, FD., Evaluation of urea-formaldehyde adhesives performance by recently developed mechanical tests, International Journal of Adhesion and Adhesives, 31, 3, 2011, 127-134 6. Chowdhury, MJA., Humphrey, PE., The effect of acetylation on the shear strength development kinetics of phenolic resin-to-wood bonds, Wood And Fiber Science, 1999 31, 3 pp. 293-299