INNOVATION IN WOODWORKING INDUSTRY AND ENGINEERING DESIGN, 1/2014 (5): 33 39

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1 INNOVATION IN WOODWORKING INDUSTRY AND ENGINEERING DESIGN, 1/2014 (5): DETERMINATION OF GLUE JOINT SHEAR STRENGTH OF THERMALLY MODIFIED PINE (PINUS SILVESTRIS) AND BIRCH (BETULA PENDULA) IN COMPARISON WITH THE UNMODIFIED WOOD Elena Vladimirova Moscow State Forest University Department of Technology of woodprocessing industries, 1st Institutskaya street, 1, , Mytischi, Moscow region, Russia Tel.: , ABSTRACT aim of this study was to determine glue joint shear strength of rmally modified pine (Pinus silvestris) and birch (Betula pendula) in comparison with unmodified wood and to look at possibilities of its simultaneous gluing on same equipment in same conditions. For experiments used two types of glue: polyvinylacetate glue (PVAc) Kleiberit and polyurethane glue Kleiberit PUR Afterwards glue joint ultimate strength in shear parallel to grain was measured. Key words: rmally modified wood, glue joint, shear strength, birch, pine INTRODUCTION In recent years woodworking industry shows interest in composite materials from wood. One of such interesting materials can be made from rmally modified wood. rmally modified timber can be glued toger or with unmodified wood. However re are many open questions about gluing of rmally modified wood for Russian manufactures. Among m are type of glue that can be used and difference of this process with classical gluing of unmodified wood. Some research of rmally modified wood gluing was made by Technical Research Centre of Finland (VTT). VTT has studied glueability of heat-treated wood with 1- component PVAc adhesives, 1- component polyurethane adhesives (PUR). glueability test was carried out in accordance with DIN strength of glue line was determined in accordance with EN 392 (block shear test). Heat-treated wood absorbs water and water-based adhesives, such as PVAc, slowly. That is why longer pressing times than normal are needed when using water-based adhesives. When one works with PVAc adhesives, water content in adhesive should be minimized. It is reported that PUR (polyurethane) adhesives work well with rmowood. Although all tests carried out with PUR adhesives have been successful, it must be kept in mind that curing reaction of PUR requires water. water can be absorbed from eir wood or surrounding air. required amount of moisture is dependent on adhesive, but if both wood and air are very dry, re exists possibility of unsuccessful gluing [Anonymous, 2003]. VTT gives some recommendations about gluing of rmally modified wood (trademark rmowood). We made tests with Russian rmally modified wood (trademark Vacuum Plus). aim of this study was to determine glue joint shear

2 DETERMINATION OF GLUE JOINT SHEAR STRENGTH OF 35 strength of rmally modified wood pine (Pinus silvestris) and birch (Betula pendula) in comparison with unmodified wood and to look at possibilities of its simultaneous gluing on same equipment in same conditions using two different types of glue. EXPERIMENTAL METHODS pine (Pinus silvestris) and birch (Betula pendula) wood specimens usedd in this study were obtained from wood producing manufacture Beavers Konakovo, Tver region, Russia. Birch andd pine woodd was harvested for company in spring of 2012 in Perm region, Russia. Boardss with dimensions 30х90х30000 mm (thickness x width x length) were taken after preliminary drying in convection c kiln (Fig.1). Drying took place during one month, at ºC. Humidity of timber after drying was 12 %. Figure 1: Boards after convective drying After that one half of boards were rmally modified with technology kiln brought to level of 175 ºC º for 96 hours. n for 12 hours boards were called VACUUMTERM (Russian trademark rmally modified at this temperature. Vacuum Plus). process of rmal Furr kiln was turned off and cooling modification was held in a cylindrical during 60 hours (Fig. 2). vacuum kiln SPVT 6. temperaturee in

3 36 ELENA VLADIMIROVA whole process of rmal modification took 7 days. After rmal modification, boards were visually evaluated for twists, cracks and or deformations. Only those boards that were free of defects were selected for furr testing. moisture content (MC) of rmally modified boards was measured to be about 4 %. untreated wood of same species was used as a control. specimens for all research were cut from rmally modified and unmodified boards, according to Russian standard GOST [GOST ]. Type of glue PVAc ± PUR Figure 2: Boards after rmal modification Table 1: Gluing characteristics birch and pine wood glue joint were glued with two types of glue. polyvinylacetate glue (PVAc), commercially named Kleiberit (D3) onecomponent glue and polyurethane glue (PUR), commercially named Kleiberit PUR (D4) - moisture curing, onecomponent reactive glue based on polyurethane, top group of water resistance. Prepared specimens were glued as recommended by manufacturers (Table 1). Viscosity Required by Time Open Time of climatic Press Glue under time of addition conditions, pressure spread glue al gluing temperature, pressure moisture content [mpa.s] [N/mm 2 ] [g/m 2 ] 20 [ C] 20 [ C] T [ C] MC, [%] 6-10 min 20 min 2-3 hours min 60 min 2-3 hours After that wood glued joints specimen were prepared (Fig. 3) according to GOST [GOST ]. Also, moisture content of specimens was determined [GOST ].

4 DETERMINATION OF GLUE JOINT SHEAR STRENGTH OF 37 Figure 3: Dimension specimens according to GOSTT a blank for specimens, 2 - cut, 3 - handlingg allowance calculation of t minimum specimens were mountedd in a test machine number of specimens wass carried out ZD10/90 (Fig. 4, Fig. 5). device with according to GOST att a confidence level of 0.95 and a relative accuracy of specimen average of 5 %. specimen set re inn was placed on support platform of testing machine so that axis of punch h tool coincides with number of 38 specimens for each wood axis of charging device testing species was accepted. glue joint breaking tests were machine. specimen was loaded at a rate of continuously movingg loading head conducted in accordance with GOST testing machine (0,60 ± 0,15) mm / min [GOST ].

5 38 ELENA VLADIMIROVA Figure 4: Device for testing Figure 5: 5 Test machine ZD10/90 strength of glue joints, according to RESULTS AND DISCUSSION Glue joint ultimate strength while requirements of GOSTT [GOST shearing along grain (τ) is calculatedd in ], should be nott less than 6.0 MPa, MPa (kgf/cm 2 ), rounded up to t 0.1 MPaa (1 with an average of not less than 8.0 MPa. kgf/cm 2 ) using formula tests continued until complete destruction of o specimen. result P, (1) should be arithmetic mean of tensile b l strength of all a tested specimens (of one where P breaking load, N (kgf); type), type of failure was also fixed. b width of shearing area of Table 2 presents results obtained for specimen, m (cm); shear strength. l length of shearing area of specimen, m (cm). Table 2: Test results Type of glue PVAc, UM* PVAc, TM** PUR, UM PUR, TM PVAc, UM Failure loadd P, N [kgf] 4,90 3,47 4,83 2,11 7,19 Ultimate strength, МPА [kgf/cm 2 ] Pine 7,84 5,69 7,87 3,44 Birch 11,54 Moisture M content, [%] destruction of wood, [%]

6 DETERMINATION OF GLUE JOINT SHEAR STRENGTH OF 39 PVAc, TM PUR, UM PUR, TM 4,60 8,24 2,11 7,63 13,24 3, * UM unmodified wood ** TM rmally modified wood During test, jointss failed in three different ways (Fig. 6). With PVAc and PUR glue, in unmodified pine and particularly in unmodified birch, failure was usually 100% in wood. With PVAc glue, in rmally modified wood both in birch and pine showed some combination of o both types of failure. With PUR glue, in rmally modified wood both particularly in birchh failure was 85 % along glue line andd combined. Glue failure Wood failure Combination Figure 6: types of failure average value of failure loadd in experiments with unmodified pine wood was 7,84 МPА (PVAc) and 7,87 МPА (PUR) and with unmodified birch wood was 11,54 МPА (PVAc) and 13,24 МPА (PUR). rmally modified wood glued with PVAc has average value of failure load 7,63 МPА birch and 5,69 МPА pine. All results, except for last conform to standards specified in GOST [GOST ]. It seems likely that it should be taken into account that rmally modified wood absorbs slowly water and water based glues, g such h as PVAc. That is why sometimes longer pressing times than normal is needed and results can be better. rmallyy modifiedd wood glued with PUR has e values of failure load 3,47 МPА birch b and 3,44 МPА pine that

7 40 ELENA VLADIMIROVA significantly below standards specified in GOST [GOST ]. re appear to be that when using PUR glues, it has to be kept in mind, that hardening reaction of PUR needs water. water can be absorbed eir from wood or surrounding air. If both wood and air are very dry, gluing may fail [Anonymous, 2003]. From se data we can conclude that moisture of rmally modified wood is not enough to get a proper gluing joint. And simultaneous gluing of rmally modified and unmodified wood with using two different types of glue needs to take additional measures. In particular recommendations of FTA, should take into account. From this it follows that for a complete reaction (crosslinking) of PUR glue, and to achieve desired curing glue joints in articles moisture content of wood should be 12 ± 3%, or rmally modified specimens should be moisturized additionally to rise moisture content of surface. To clarify recommendations for gluing of rmally modified wood it is reasonable to continue research in this direction. CONCLUSIONS results found in present study allow drawing following conclusions: 1. average value of failure load in experiments with unmodified wood conform to standards specified in GOST rmal modification reduces equilibrium moisture of content, and shear strength of wood glued with PVAc and PUR glues. 3. results show that re are some differences in processes of gluing and strength characteristics of unmodified and rmally modified wood. 4. simultaneous gluing of rmally modified and unmodified wood with using two different types of glue needs to take additional measures. 5. It should be taken into account that rmally modified wood absorbs slowly water and water based glues, such as PVAc. That is why sometimes longer pressing times than normal is needed and results can be better. 6. When using PUR glue for a complete reaction (crosslinking), and to achieve desired strength of glue joints moisture content of wood should be 12 ± 3%, or rmally modified wood should be moisturized additionally to raise moisture content of surface. REFERENCES 1. Anonymous (2003). rmowood Handbook, Finnish rmowood Association, c/o Wood Focus Oy, P.O. Box 284 (Snellmaninkatu 13), FIN Helsinki, Finland. 2. GOST Glued massive wood. Methods for determination of glued joint of ultimate strength while shearing palong grain. 3. GOST Wood. General requirements for physical and mechanical tests. 4. GOST Wood. Methods for determination of moisture content. 5. GOST Wooden laminated structures. General specifications. 6. Sergeev D.V. Determination of glued joint of ultimate strength while shearing palong grain of rmally modified and unmodified pine wood. Scientific papers of Moscow State Forest University master students, technology and equipment for woodprocessing, Issue 365, 2013, 63 р.