RESULTS FROM EXPERIMENTS WITH A NEW VENEER CONTACT DRYING TECHNOLOGY

RESULTS FROM EXPERIMENTS WITH A NEW VENEER CONTACT DRYING TECHNOLOGY Olli Paajanen1, Matti Kairi2 ABSTRACT: This paper introduces a new type of conta...
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RESULTS FROM EXPERIMENTS WITH A NEW VENEER CONTACT DRYING TECHNOLOGY Olli Paajanen1, Matti Kairi2

ABSTRACT: This paper introduces a new type of contact drying method that has been developed at Aalto University in Finland. The target in the new dryer development is shorter drying time and improved veneer quality. The paper presents experimental results from tests with this new drying method. The drying system consists of a hot upper plate, a cold bottom plate, a vacuum inside the drying chamber and a mechanical press. An experimental drying device was used in a study to compare the new method with the currently used drying method. KEYWORDS: Veneer drying, veneer dryer, contact drying, plywood manufacturing, LVL manufacturing

1 INTRODUCTION

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The use of rotary peeled veneer in high quality products applications is a restricted by the quality of the veneer. The quality of veneer is heavily affected by the peeling and drying processes, but most of the technical development in this area has concentrated on production efficiency. At Aalto University in Finland the quality of the veneer has been a primary goal in the drying research. A new type of veneer dryer has been in development since 2005.The idea is based on condebelt technique, used in board industry, where cardboard is dried between cold and hot press plates. [7] The application of this technique in veneer industry is interesting both from quality and energy efficiency viewpoints. This paper discusses the new type of contact drying technology that has been developed and patented at Aalto University. Prototype dryers have been built and used to conduct tests on drying times, drying quality and plywood and veneer properties.

2 BACKGROUND 2.1 PLYWOOD IN FINLAND AND THE MANUFACTURING PROCESS Plywood is the most important panel product in Finland. There are three companies producing plywood, with a total production of 1.4 million m3 in 2008 [1]. The share 1

Olli Paajanen, Department of Forest Products Technology, School of Chemical Technology, Aalto University, PO Box 16400 FI-00076 AALTO, [email protected] 2

Matti Kairi, Department of Forest Products Technology, School of Chemical Technology, Aalto University, PO Box 16400 FI-00076 AALTO, [email protected]

of exports was 80 %. Plywood products are used typically in construction and transportation applications. Especially in higher value applications the quality of the plywood is essential. Plywood boards are manufactured by laminating thin veneers together. The plywood manufacturing process consists of several parts. The process begins by veneer manufacturing. First the logs are soaked; then the veneer is peeled and cut to sheets. The veneer sheets are then dried and sorted. Then the veneers are laid up, glued and hot pressed. All parts of the manufacturing process have a role in the final product quality. The veneer manufacturing technology has progressed in many areas, but especially peeling technology has developed during the past decades. Modern lathe equipment can reach peeling speeds up to 300 m/min and still produce reasonable veneer quality. This means that the lathe capacity is not the capacity bottleneck in production. The next step in the process is veneer drying. As both the peeling and drying together have a big impact on veneer quality, the development of lathe technology increases the need for developing the drying process, both from quality and capacity perspective. Plywood and other veneer based products are facing strong competition from substituting products, such as other composite panels, which means that the traditional products must improve the product performance and quality. Several technologies have been researched in order to improve the industrial veneer drying process. 2.2 CONVECTIVE VENEER DRYING TECHNOLOGY Direct convective drying is the principal technology currently used in industrial veneer drying. The operating principle is presented in Figure 1.

Figure 1: Convective veneer dryer operating principle

Convective veneer dryers are large and expensive machine units. The typical capacity of the industrial veneer dryer is about 60 000 m3/a dry veneer. The convective dryers are effective, but there are quality issues and the drying method cannot be used to improve the quality of the veneer. Large energy consumption is also a problem with convective drying technology. Drying process consumes significant part of the heat and electricity is used in the plywood manufacturing process. The primary reason for the high energy consumption is the transfer of heat through air, convection. It is not very efficient, especially when compared to other heat transfer methods, such as conduction. [5] Modern veneer dryers are usually roller dryers, where the veneer sheets travel through the dryer once. The veneer moves between the rollers and the hot air is directed through nozzle boxes onto the both sides of the veneer. The roller dryers may have several sections with different conditions. The final stage of the drying is the cooling section where the veneers are cooled with air drawn from outside the dryer. The initial moisture content of the veneer has big variation, which means that the convective drying process is difficult to control accurately. Because of the variation in moisture content hot steam should be fed into the first part of the dryer. This both heats up the veneer quickly as well as evens out the moisture variation within the veneer sheet. However, as it is important to utilize the capacity of the dryer efficiently, this process is often incomplete, and wet patches are left on the veneer sheets. Also, in roller dryers the ends of the veneer sheet dry faster than the middle part of the veneer. As a result of this, the veneers buckle or have a convex shape. They also have internal stresses, and the ends of the veneer may crack, when the veneers are straightened in the stacks. There are two common types of veneer convective veneer dryers: so called web dryers and roller dryers. In web dryers the whole veneer mat is fed after peeling into the dryers, the mat travels through the dryer several times and the veneers are clipped into sheets after the drying. As the veneer is dried immediately after the peeling, this type of drying technology may have an impact on the colour of the veneer. The colour of veneer is very important for applications in furniture industry. Roller dryers are more flexible in operation, because the veneer is already cut into sheets before drying. This is important especially in the drying of spruce and other wood species that have a large difference in moisture content between sapwood and heartwood. From the viewpoint of veneer quality and especially strength, in drying process the moisture of the veneer should not be lower than 6…7 %. However, in practice

this is difficult to achieve, as material properties of the veneer vary a lot and also the initial moisture content is often rather uneven. Therefore also the final moisture content is not evenly distributed. Uneven moisture distribution inside a single veneer means that there are wet patches left in the veneer. These may cause problems in the hot pressing of plywood; so called steam blows may occur when the vapor pressure inside the board is too high. To avoid the stem blows, the target for the average moisture content in the drying process is set low, often approximately 4 %. As a result of this, large portion of the veneers are over dried close to 0 % moisture content. Over dried veneers are fragile and buckle often. They are difficult to handle and break easily in the following parts of the manufacturing process. Also the strength properties of the final product are reduced because of over drying. There is a strong need to develop more accurate drying processes, because the current drying technology is not optimal in this respect. 2.3 CONTACT DRYING TECHNOLOGY Contact drying of veneer has been researched quite extensively in the past. Especially in the 1970’s there was big interest to develop contact drying and there were several research programs, for instance by the Forest Products Laboratory in United States [1] and VTT in Finland [3]. The press drying was seen as a very promising technology [1]. There is also more recent activity in this area, indicated by publications and patents in this field of research. Contact drying offers several theoretical benefits in the drying of veneer. The most significant of these is probably the efficient transfer of heat in the process [5]. The contact drying operating principle is presented in Figure 2.

Figure 2: Contact drying technology

Contact drying can also provide good quality veneer. According to a 2007 review of Chinese drying technologies the contact drying is a promising technology to improve the properties of veneer [4]. According to Sandoe [10], platen drying has several advantages, such as rapid heat transfer, reduced shrinkage, even moisture content and reduced buckling, which explain why it has been investigated periodically in the past. Still, there are some major disadvantages. The veneer input into the dryer and fouling of plates have caused problems [11]. Platen dryers limit the veneer sizes and also the production capacity is an issue [10]. Higher temperatures could be used to speed the drying process, but this may lead to surface inactivation in veneer sheet. [10]. In order to have sufficient drying capacity, the platen dryer has to have a fast feeding

system. This type of machinery is rather complicated, meaning that the investment costs are higher. Despite the positive research results regarding the quality of the veneer, contact drying has not been adopted in large scale plywood manufacturing. The reason for this is probably combination of costs and operational problems in the industrial environment.

The cooling section of the dryer is presented in Figure 5. The efficient sealing of the dryer is a very important feature of the new drying system. In optimal configuration the pump runs only at the beginning of the drying cycle. This improves the efficiency of the system considerably. In practice this has been achieved with good success and reliable operation with the second version of the device.

2.4 CONDENSING DRYING TECHNOLOGY A new type of veneer dryer has been developed and patented at Aalto University in Finland. The operating principle is shown in Figure 3. Mechanical pressure Hot plate TH Veneer Porous material Metal wire

Vacuum pump

Cold plate with cooling TC

TH >> TC Pressure between plates

atmospheric pressure

Figure 3: New drying system

Figure 5: Cooling section of the dryer

The technology is based on contact or press drying principle, but there are several new components in the system, inspired by the condebelt technology, which have not been used together in this application before. A vacuum pump can be used to lower the pressure inside the dryer. The second important feature is the use of temperature difference in the dryer: the top plate is heated, but the lower plate located below the veneer is cooled. The cold plate has a wire surface, so that the water vapour can pass through it The temperature difference drives the water through the veneer. The evaporated water is condensed in the cooling section below the veneer. The use of vacuum reduces the drying time [5]. As a result of this, the drying time is reduced significantly compared to conventional driers. A prototype dryer is presented in Figure 4. The main parts of the dryer are indicated by the numbers.

Some drying curves obtained with the new device are presented in Figure 6. The traditional convective dryers are slower, almost 200 seconds is needed to reach below 5 % moisture content when using the same temperature range. With lower drying temperatures the drying times are consistently roughly twice longer when using the traditional drying method.

0.40 Initial moistre contents 0.84kg H2O/kg dm 1000mbar

Moisture content (kgH2O/kgdm)

0.35

0.95kg H2O/kg dm 250mbar (18 tests)

0.30

0.79kg H2O/kg dm 250mbar (6 tests)

0.25 200°C, 1000mbar

0.20

200°C, 250mbar

0.15 2

R = 0.9284

0.10 0.05

R2 = 0.903

0.00 0

20

40

60

80

100

120

Time (s)

Figure 6: Drying curves obtained with the first prototype dryer, using temperature 200 °C, with and without the use of vacuum [5]

Figure 4: The new drying device and its main parts: 1) Cooling water feed, 2) pressure release valve, 3) hot plate controls, 4) electrically heated hot plate, 5) press controls, 6) vacuum pump, 7) airtight cooling section

The drying curve shows that the removal of water is very rapid at the beginning of the drying process. This is probably due to the fast removal of free water in the beginning of the process [5]. As can be seen from the Figure, the use of vacuum accelerates the drying significantly. Overall, the total drying times are significantly shorter with the new device. The new dryer provides also improves the drying quality from moisture distribution perspective. The final moisture content variation within the veneer sheet is rather even compared to convection drying [6]. The veneers are also very flat and straight after drying, as is

typical to press dried veneers [1]. This can mean that also the plywood quality is improved. 2.5 VENEER AND PLYWOOD QUALITY The quality of veneer affects the properties of the final product, plywood and LVL. Many variables are present in the manufacturing process of plywood, already the storage and cutting time of the logs impact the veneer properties. Together with soaking, the peeling process has a big impact on the physical properties of the veneer. Surface roughness and depth of lathe checks are affected by the peeling process. Rotary peeling of veneer creates peeling checks or lathe checks on the veneer. The checks form on the loose side of the veneer when veneers are flattened from their natural curvature (Rohumaa 2011 ipps). Also great stresses are caused by the cutting forces during the peeling process. The depth of peeling checks varies greatly. A peeling check is presented in Figure 7. Dye has been used to indicate the cracks more clearly. As can be seen from the picture, some of the peeling cracks travel almost all the way through the veneer.

Figure 8: Peeling checks in the shear test. Source: [8]

The orientation of peeling checks in the rolling shear test affect the results obtained from the test. The specimens with opening peeling checks are significantly weaker [9]. If the difference between opening and closing specimens could be reduced, it would improve the properties of plywood in demanding structural floor applications. Figure 9 shows some actual photographs from shear strength tests. The photograph at the top shows a specimen with closing checks and the bottom photograph one with opening checks. Both are taken during the later stages of a test. The specimen with closing checks does not have a continuous or clear fracture plane, but in the bottom picture the checks open and there is rolling shear effect evident in the middle part of the specimen.

Figure 7: A peeling check

The peeling checks affect the physical properties of the veneer and veneer based products, such as the strength of the board and the durability of the board surface and coatings. The properties of the veneer surface have an impact on the plywood bonding. The wettability of the surface, or contact angle test, is commonly used to test the properties of veneer surface. The hydrophobic properties of the veneer surface affect the bonding process. There are also more advanced methods to evaluate the bonding properties and the chemical composition of the veneer surface, but the contact angle test is commonly used as it is a rather simple test to conduct. The properties of the final product, plywood, are also a very important matter. Especially the strength of the plywood is essential in many demanding applications, such as the challenging plywood structures used inside different types of liquid natural gas (LNG) tanker ships. The strength of plywood is usually evaluated by measuring the bending strength and the bond quality of the plywood. One of the common testing standards for bond quality is the EN 314, which is a plywood rolling shear test. There the quality of the veneer plays a very important role. Figure 8 shows the test specimen and how the peeling check orientation affects the shear test [8].

Figure 9: Peeling check breakage during the rolling shear test. Source: [9].

The difference in shear strength values is significant depending on the orientation of the checks in the specimen. In a large plywood study the specimens with closing checks were 30 % stronger in shear strength test compared to specimens with opening checks. Especially with opening checks the check depth has a very big effect on strength, which also means that it is very important to measure the peeling check depth from shear specimens [9]. The dimensional stability of plywood is needed in many high value applications. However, the dimensional stability of plywood is not always in control of the manufacturers. Even though the dimensional stability is essential in many applications, there are no European standards that are used specifically in plywood products. The current standard EN 13647:2002 is about “wood and

parquet flooring and wood paneling and cladding” and therefore more general in nature. Previously there have been a number of national standards used to quantify the dimensional stability of plywood, but according to the Finnish Standards Association (SFS) there are no current standards available.

3 MATERIALS AND METHODS The new drying method was compared to convective drying in a number of tests. The results discussed in this paper are from several test series. The first tests were done using a small scale prototype. As these showed promising results, a new, larger and improved dryer was built. Some of the first results from these tests are also included for comparison. These tests and the equipment used are described next. 3.1 EXPERIMENTAL DRYERS USED IN THE TESTS The first prototype drying device had a size of 600 x 600 mm. The second, improved dryer has 1200 x 1200 mm plates, so it can be used to dry industrial size veneer. The basic operation principle of the dryers is the same, but some technical features are improved in the second dryer. One of these is the more efficient removal of water. The use of porous material between the top of the cooling section and the lower surface of the veneer was necessary in the first prototype, as the removal of water from the cooling section was not efficient enough. The water condensated already on the first wire surface. A pulp sheet was used to reduce the condensation problem and get more consistent test environment. The use of pulp may have an effect on test results obtained with the first dryer, even if the material is very porous and it was regularly changed. One of the targets for the next generation device was to improve the water removal and remove the need to use the porous material between the veneer and the wire section. This has been accomplished. Some pressure is needed to achieve good contact between the veneer and the hot plate. Because of this, the wire section leaves a small, but visible mark on the lower surface of the veneer. It may have an effect on veneer properties. Further work is needed to reduce the impact of wire surface; adjusting the press settings or alternative wire material can provide better results in this respect. The current prototype dryer has a size of 1200 mm x 1200 mm, which means that industrial scale veneer can be dried. The vacuum pump and especially the sealing system are very efficient, as the pressure inside the drying chamber can be set as low as 0.15 bar absolute pressure. The hot top plate is electrically heated and it can reach temperatures over 200 °C. Also a third dryer was used in the test series. This dryer is a traditional convective dryer and it was used to produce reference veneer for the tests. The device is a laboratory scale dryer, manufactured by Raute. The dryer simulates industrial equipment, although the veneers are dried one at the time. The veneer is attached

to a frame, which moves the veneer up and down in the airflow inside the device. A large fan circulates air dryer and the airflow is directed onto the veneer through boxes with holes. The maximum veneer size is approximately 450 x 900 mm. Also a steam generator can be connected to the Raute dryer to provide more realistic drying conditions. To monitor the drying conditions during the process, a Vaisala probe can be used. It provides accurate information conditions (temperature and air humidity) during the drying process. 3.2 THE WOOD MATERIAL As there were several tests were conducted at different dates, several batches of veneer used in the tests. The wood material was birch (Betula Pendula), acquired from industry. Large volume of veneer was used in order to reduce the impact of material properties variation. The first test series peeled from 9 large bolts. The later series were done using 6 bolts and in the latest on going test series there are 10 logs. In all test series, the veneer was manufactured using laboratory equipment. The soaking temperature used in all tests was 40 °C. The peeling was done using a refurbished industrial scale Raute lathe. The thickness of the veneer was 1.55 mm. The veneer was dried using the dryers described earlier. 3.3 VENEER AND PLYWOOD TESTS Different veneer tests were done to compare the veneer dried with different dryers. The tests described below provide information about the bonding properties of veneer and the dimensional changes occurring during the drying process. This information may explain if there are differences in the properties of the final product. Veneer shrinkage was assessed by measuring the width of the veneer sheet once and thickness of the veneer several times. Approximately 20 veneers were measured in each group in the latest tests. The dimensional measurements are commonly used to provide information regarding the radial and tangential shrinkage of veneer sheets [see e.g. Sandoe]. The veneer contact angle was measured using CAM 200 device. The purpose of the test was to compare the wetting of veneer surfaces. In the test a drop of water is dropped on the tested surface. The cross section of the droplet is recorded with a camera over time. The contact angle is measured between the droplet and the tested surface and this tells about the hygroscopicity of the surface. The contact angle measurements were repeated on average eight times from each piece of veneer. Altogether there were 80 veneers that were used in contact angle tests. The veneers were dried with 200 °C temperature. The peeling checks were measured to see, if there were differences in the peeling checks in different veneer groups. The measurements were done using microscope and image measuring and analysis software. Dye was used highlight the checks in specimens. A large number of veneers were used in the test; the different groups

have between 108 and 707 checks measurements in this comparison. As already mentioned, the properties of the plywood are also a very important matter. Unfortunately this paper does not include the results from plywood tests, as the latest test veneer and plywood test panels were not ready at the moment of writing. However, the results should be available for presenting in the WCTE 2012 conference. The tests conducted will include tests on strength, bonding quality, as well as dimensional stability. Figure 10 shows the rig used in the dimensional stability tests, which are done according to EN 13647. The plywood board is place on to the aluminium frame and held tight with clamps from one side. Then the deformation is measured with a gauge. The frame can be used to measure different deformation types (warp, bow, and cupping). The test boards are subjected to cyclic humid changes during the test period. The dimensional changes are recorded several times.

A B C

4 RESULTS AND DISCUSSION The veneer shrinkage was measured from veneers dried with the newer dryer. The shrinkage was small in tangential direction but large in radial direction, which is logical considering that pressure has to be used to maintain the contact between the hot plate and veneer. The average values were 2.6 % in radial direction and 12.2 % in tangential direction when using 200 °C temperature and 2.2 % and 12.1 when using 170 °C temperature. The results were similar to initial tests with the first dryer prototype, despite the fact that no porous material was used this time, indicating that it probably did not affect this test. Also the convective dryer results were similar to previous research. The veneer shrinkage obtained with the new drying system was also similar to results from research previous contact drying research in similar temperature range. Forsen found that in contact drying using 180 °C temperature with 1.5 mm birch veneer, the tangential shrinkage was 1.4 % and radial 10.63 %. With a convective dryer using 190 °C the respective shrinkage values were 7.7 % and 5.15 % [3]. Sandoe found that platen drying resulted in reduced shrinkage in veneer sheet width, but the thickness shrinkage offset that, meaning that overall the veneer volume was more or less similar in platen drying compared to conventional drying [10]. In the case of the new drying device, more research is needed to see how the pressure settings and temperature affects the veneer shrinkage during drying and if the volume change could be minimized. Also the spring back effect should be researched. The contact angle measurements were done from veneers dried with 200 °C. The results indicate that there are no major differences between different veneers, although there is some variation in results.

Figure 10: Testing rig for evaluation dimensional stability of plywood. In the points A, B and C the panel is propped on the rig frame

Figure 11: Contact angles measured from veneers dried in 200 °C temperature

The contact angle values were measured from both sides of the veneer, but the variation is much higher on the loose side of the veneer due to the checks and surface roughness. The results shown here are from the tight side of the veneer. The peeling checks were measured from veneers dried with the first prototype dryer. The peeling checks are similar in all groups, as can be seen from the Figure below.

Figure 12: Peeling checks percentage measured after drying with 200 °C convection dryer and the new dryer, both with and without vacuum

The plywood properties will be discussed in more detail in the conference presentation. Especially results from the cyclic plywood stability tests should be interesting.

5 CONCLUSIONS The new drying method discussed in this paper reduces the drying time approximately 50 % and the quality of the veneer does not differ significantly from the veneers dried using current technology. Especially the straightness of the veneer is considerably improved compared to traditional technology. This is due both to improved moisture distribution and the pressing effect during the drying process. However, the settings of the prototype driers are not yet optimised for best veneer quality, especially with the final product quality in mind. More tests are being conducted at the time of the writing and these are planned to be included in the conference presentation. The shrinkage of veneer is an important consideration, because it impacts the veneer volume in the production. With the new system the shrinkage was found to be similar to conventional contact drying, which is not surprising considering the use of pressure during drying. The total volume of the veneer is not reduced, as the increase in radial shrinkage is offset by the reduced tangential shrinkage. This result is consistent with earlier platen drying research. The contact angle values and peeling check depths indicate that the veneer quality is not negatively affected by the new drying method. Overall the new drying system is efficient and produces good quality veneer. The conference presentation will include latest test results and include also information about the properties and quality of plywood manufactured from veneers dried using the new system.

REFERENCES [1] Baldwin, R. Plywood Manufacturing practices, second revised edition. Miller Freeman Publications Inc., San Fransisco, 1981.

[2] Finnish Forest Research Institute, Finnish Statistical Yearbook of Forestry, 2009. [3] Forsen H. Viilun kontaktikuivaus [Veneer press drying]. Technical Research Centre of Finland, Research Reports 605. Espoo. 1989. [4] Gu Lianbai. Recent Reseacrh and Development in Wood Drying Technologies in China. Drying Technology, 25: 463-469. 2007. [5] Holmberg H., Lahti P., Paajanen O. and Ahtila P. An experimental study on drying times in a contact drying of veneer. Proceedings of 8th World Congress of Chemical Engineering. Montreal, Canada. August 23-27, 2009. [6] Lahti P., Paajanen O., Holmberg H. and Kairi M. Introducing a new method of veneer drying. In proceedings of the 11th International IUFRO Wood drying conference. Skellefteå, Sweden. January 1822, 2010. [7] Lehtinen, J. Condebelt Board and Paper Drying. Drying Technology, 16(6): 1047-1073, 1998. [8] Marra, A.: Technology of wood bonding: principles in practice. Van Nostrand Reinhold, New York, 1992. [9] Rohumaa A., Logren J., Hughes M. Peeling checks and their effect on phenol formaldehyde bonded birch plywood. In proceedings of the International Panel Products Symposium 2011, pages 57-66, 2011. [10] Sandoe M.D., Wellons J.D., Parker R.J., Jokerst R. Gluability of platen dried veneer of Douglas-fir. Forest Products Journal, Vol. 33, No.7/8 July/August 1983. [11] Usenius A., Siimes H. Tulevaisuuden puunkuivausmenetelmät, loppuraportti (in Finnish, Future drying technologies of wood, final report). VTT, Espoo. 1991.

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