A method for preparing plywood by microwave heating pretreatment

Abstract

The embodiment of the present application discloses a method for preparing plywood pretreated by microwave heating, comprising the processes of moisture content control, gluing, aging, assembling, pressing and drying, wherein the moisture content control process comprises the following stages: stage one, making the initial moisture content of the veneer higher than the fiber saturation point of the wood; stage two, passing the veneer with covers on both sides through a microwave heating section; stage three, after leaving the microwave heating section, passing the veneer with covers on both sides through a slow-release operation section; wherein the microwave heating section makes the veneer reach a first temperature, the slow-release operation section has a second temperature, and the first temperature and the second temperature have a temperature difference of 47 to 57° C.; and gluing is applied to the veneer immediately after leaving the slow-release operation section.

Description

Technical Field

[0001] The invention relates to the technical field of plywood preparation technology, in particular to a plywood preparation method using microwave heating pretreatment. Background Art

[0002] Plywood is a man-made board made by gluing multiple veneers together. The production process of plywood generally includes moisture content control, gluing, aging, assembly, pressing, curing and other processes. Generally speaking, in the process of moisture content control, the veneer is kept at a low moisture content as much as possible, so that the adhesive applied to the surface of the veneer in the gluing process can penetrate into the veneer, and glue nails are formed in the pressing process, thereby helping to improve the interfacial bonding strength of the glue layer.

[0003] There are many prior arts using the above process. For example, in the Chinese patent database, the invention patent application with the publication number CN114474264A and the name "A method for preparing multi-layer plywood" records the technical solution of drying the moisture content of the veneer to 10~18% before gluing, and then applying adhesive on both surfaces of the veneer. The total amount of glue applied on the two surfaces of the veneer is 340~400g / ㎡. Another example is the invention patent application with the publication number CN102350716B and the name "Combined drying method suitable for industrial production of fast-growing poplar formaldehyde-free plywood". It records the technical solution of drying the moisture content of the veneer to 15~20% before gluing by solar energy or atmospheric drying, and then applying soybean adhesive on the surface of the veneer. The amount of glue applied on one side is 200~240g / ㎡.

[0004] In the existing bonding theory, the bonding strength of the adhesive to the wood is composed of the interfacial bonding strength of the adhesive layer (chemical bonding force) and the bonding strength of the glue nail (mechanical bonding force). Therefore, it is generally believed that the formation of glue nails can play a role in assisting bonding and improving the bonding strength. Based on this understanding, the existing technologies, such as the technical solutions listed above, all use a relatively low moisture content before gluing. Since the moisture content of the veneer before gluing is relatively low (lower than the fiber saturation point of the wood), after gluing, the moisture in the adhesive is driven by the moisture content gradient to move below the surface of the veneer, while carrying the adhesive into the surface of the veneer. After curing, this part of the adhesive forms glue nails in the pores or other pores of the wood.

[0005] However, because the pores or other pores on the surface of the wood need to be filled first, the formation of glue nails will increase the amount of glue applied while ensuring the thickness of the glue layer (interface bonding strength). A feasible solution to reduce the formation of glue nails and reduce the amount of glue applied is to carbonize the veneer. Of course, the original intention of carbonizing the veneer in the prior art is not to reduce glue nails or reduce the amount of glue applied, but to reduce the hydrophilicity of the veneer to improve the dimensional stability of the veneer. But obviously, as the hydrophilicity of the veneer decreases, the adhesive cannot or less penetrate into the surface of the veneer, which will inevitably lead to a reduction in glue nails and a reduction in the amount of glue applied. Corresponding to this phenomenon, the bonding performance of the carbonized veneer is inferior to that of the ordinary veneer, which further strengthens the impression of technicians in this field that the formation of glue nails can improve the bonding strength.

[0006] In the invention patent application with publication number CN103692505B in the Chinese patent database, entitled "A carbonized plywood and its manufacturing method", it is recorded that the double-sided glue amount of the carbonized veneer is 250~350g / ㎡, and the gluing performance of the carbonized veneer is improved by adding cellulase or hemicellulose complex enzyme to the soybean adhesive. The glue amount recorded in this invention patent application is obviously much lower than the single-sided glue amount of 200~240g / ㎡ (soybean adhesive) recorded in the invention patent application with publication number CN102350716B submitted by the same inventor. This proves to a certain extent that the formation of no or only a few glue nails can greatly reduce the necessary glue amount.

[0007] In summary, because the pores or other pores on the surface of the wood need to be filled first, the formation of glue nails will increase the amount of glue applied while ensuring the thickness of the glue layer (interface bonding strength), and the increase in the amount of glue applied will increase the production cost. If phenolic resin adhesives or other formaldehyde-containing adhesives are used, the formaldehyde release of the prepared plywood will also increase. The existing technology can provide a technical inspiration to solve this problem by carbonizing the veneer, but the bonding performance of the veneer after carbonization is greatly reduced and additives need to be used. The cost of coupling agents and cellulase is very high, and the curing conditions of cellulase are very harsh, and the process conditions are difficult to control. Summary of the invention

[0008] The purpose of the present invention is to overcome the above technical problems, thereby providing a method for preparing plywood pretreated by microwave heating.

[0009] To achieve the above object, an embodiment of the present invention provides a method for preparing plywood pretreated by microwave heating, comprising the steps of moisture content control, gluing, aging, assembling, pressing, and drying, wherein the moisture content control step includes the following stages:

[0010] In the first stage, the initial moisture content of the veneer is higher than the fiber saturation point of the wood.

[0011] In the second stage, the single board with covers on both sides is passed through a microwave heating zone.

[0012] Stage 3: after leaving the microwave heating zone, the single board with covers on both sides passes through a slow-release operation zone;

[0013] The microwave heating section makes the single plate reach a first temperature, the slow-release operation section has a second temperature, and the first temperature and the second temperature have a temperature difference of 47-57°C;

[0014] The veneer is immediately glued after leaving the slow-release operation zone.

[0015] In some embodiments, the thickness of the single board is 1.5~10 mm.

[0016] In some embodiments, in the first stage, the veneer obtained by rotary cutting is cured in an environment with a temperature of 30-35° C. and a relative humidity of 85-92% for 30-40 minutes.

[0017] In some embodiments, in the microwave heating zone, the temperature is 95~103°C, the time from the single plate entering the microwave heating zone to leaving the zone is 20~50s, the first temperature is 92~101°C, the second temperature is 40~45°C, and the first temperature is 47~55°C higher than the second temperature.

[0018] In some embodiments, the relative humidity of the environment in the slow-release operation zone is 90-94%, and the time from when the single board enters the slow-release operation zone to when it leaves is 2-5 minutes.

[0019] In some embodiments, in the microwave heating zone, the temperature is 40-45°C, the time from the single plate entering the microwave heating zone to leaving the zone is 15-30s, the first temperature is 40-44°C, the second temperature is 95-103°C, and the first temperature is 50-57°C lower than the second temperature.

[0020] In some embodiments, the time from when the single board enters the slow-release operation interval to when it leaves is 5 to 10 minutes.

[0021] In some embodiments, the time interval between the veneer leaving the sustained-release operation interval and the gluing process is less than 1.5 minutes.

[0022] In some embodiments, in the aging process, the single board is aged in an environment at room temperature and a relative humidity of 55-60% for 30-40 minutes.

[0023] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. Raise the moisture content of the veneer surface to a relatively high level. After gluing, the adhesive has almost no driving force to penetrate into the interior of the veneer surface. Moreover, the veneer still has a relatively high temperature during gluing. Therefore, at the same time or a short time after the adhesive is applied to the surface of the veneer, the adhesive is pre-cured by heat, or at least its fluidity is reduced. This can also prevent the adhesive from penetrating into the interior of the veneer surface, thereby reducing the amount of adhesive required to achieve the same thickness of the adhesive layer to a certain extent.

[0025] 2. In the second stage of the moisture content control process, a relatively large water vapor pressure is formed inside the veneer. The water vapor pressure forms tiny local explosions in various parts of the veneer, destroying the internal organizational structure of the veneer and forming a huge number of tiny internal cracks throughout the width and thickness of the veneer, so that each veneer in the assembly material can also be compressed under cold pressing conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative labor.

[0027] Figure 1 This is a schematic diagram of the process route of the method for preparing plywood pretreated by microwave heating according to Example 1 of the present application.

[0028] Figure 2 This is a schematic diagram of the process route of the method for preparing plywood pretreated by microwave heating according to Example 5 of the present application. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of this application. Example 1

[0030] Reference Figure 1The method for preparing plywood using microwave heating pretreatment is shown. The processing object is a peeled poplar veneer with a thickness of 3.5 mm. The method includes the steps of moisture content control, gluing, aging, assembling, pressing and drying in sequence, as described in detail below.

[0031] The moisture content control process includes three stages for the purpose of controlling the initial moisture content, increasing the temperature inside the veneer, and increasing the surface moisture content.

[0032] The purpose of setting up the first stage is to control the initial moisture content of the veneer, specifically, the veneer obtained by rotary cutting is passed through a hot roller drying section or a cold / hot air drying section to quickly reduce the moisture content of the veneer. The moisture content of a part of the veneer is quickly reduced to 40±3% to obtain veneer A0. In other words, the moisture content of veneer A0 is higher than the fiber saturation point of the wood at this time; the moisture content of the other part of the veneer is reduced to about the fiber saturation point, about 35%, and this part of the veneer is called veneer A0'. The hot roller drying section or the cold / hot air drying section are both existing technologies. The roller temperature of the hot roller drying section should be controlled at 50~60℃; the cold / hot air drying section preferably uses hot air drying at 55℃. If cold air drying at 25~30℃ is selected, the cold air speed should be appropriately increased. The moisture content control method of the hot roller drying section or the cold / hot air drying section can facilitate assembly line operation, avoid handling turnover, and improve production efficiency.

[0033] The purpose of setting up the second stage is to increase the temperature inside the single board, specifically, both surfaces of the single board A0 are covered with a covering that can be applied to the microwave environment, and then the single board A0 with the covering on both sides passes through the microwave heating zone, and under the action of microwave heating, the single board A1 with the first temperature is obtained. The covering that can be applied to the microwave environment located on the bottom surface of the single board A0 is a conveyor belt of a conveyor, and the conveyor belt is a plastic belt, a microwave belt or a molded belt. The covering that can be applied to the microwave environment covered on the surface of the single board A0 is a closed plastic belt, a microwave belt or a molded belt that is tensioned and driven by a roller group composed of an active roller and a driven roller. The microwave heating zone is a microwave heat tunnel of the prior art. After microwave heating, a thermal environment of 97±2℃ is formed in the tunnel. The time from the single board A0 entering the microwave heating zone to leaving is 35s. If it is limited by the site, that is, when the transmission speed is reduced, the length of the site that can be used to place the microwave heating zone cannot meet the heating time of 35s, then the single board A0 can stay in the microwave heating zone for a while. Regardless of the method, it is sufficient as long as the total time of the single board A0 in the microwave heating zone can reach 35 seconds.

[0034] The veneer A0 can reach the first temperature of 95±2℃ when heated for 35s in a thermal environment of 97±2℃. In other words, the material temperature of the veneer A1 is 93~97℃ when it leaves the microwave heating zone. At the same time, since the principle of microwave heating is to use microwaves formed by vibration to penetrate the veneer A0 so that the water molecules inside it move and generate heat energy to achieve the purpose of heating the veneer A0. Therefore, it can be considered that the veneer A1 that leaves the microwave heating zone has the first temperature everywhere. The first temperature is a range of values ​​rather than a specific value because the heat resistance of different parts of the wood is different, resulting in temperature differences under the same thermal environment.

[0035] The purpose of setting up stage three is to increase the moisture content of the surface layer. Specifically, after the single board A1 leaves the microwave drying area, the covering is maintained, so that the single board A1 enters the slow-release operation area to obtain single board A2. The slow-release operation area is a relatively closed operation space. Here, relatively closed means that the operation space can keep its internal temperature and relative humidity relatively stable under the set conditions. The gas medium (air) inside the slow-release operation area has a second temperature of 42±2°C. Preferably, the gas medium (air) inside the slow-release operation area has a relative humidity of 92±2%. The time from single board A1 entering the slow-release operation area to leaving is 5.5~6min. In stage three, the moisture content of veneer A1 is higher than the fiber saturation point, that is, it has a certain amount of free water inside, and it has a higher temperature. Therefore, veneer A1 suddenly encounters cold when entering the slow-release operation range. The free water in the materials on the two surfaces of veneer A1 first condenses, and then, driven by the water vapor pressure gradient, the free water in the core of veneer A1, which may also include a certain amount of adsorbed water, migrates and gathers on the two surfaces of veneer A1. This migration trend takes 5.5 to 6 minutes to make the moisture content of the two surfaces of veneer A1 reach a higher level. Until the water vapor pressure is insufficient to continue to drive the migration of moisture, or the temperature of the core of veneer A1 drops to a lower level, generally below 60°C, the migration of moisture will stop. At this time, by detecting the layered moisture content of veneer A2, it can be found that the moisture content of the 0.3mm thick surface layer below the surface of veneer A2 can reach a level of 75~80%.

[0036] Compared with stage one and stage two, the single board A1 needs to stay in the slow-release operation zone for a longer time. In order to ensure the smooth flow of the production line, the production rhythm of stage three should be coordinated with stage one and stage two. The control of the production rhythm of stage three can be achieved by multi-track operation. For example, a stacking section is added between the microwave heating zone and the slow-release operation zone. Of course, the stacking section can also be under the environmental conditions of the slow-release operation zone. Specifically, the stacking section is composed of a plurality of conveying sections that can move up and down and stack each other. Each conveying section includes a conveyor whose conveyor belt is a plastic belt, a microwave belt or a molding belt and a closed plastic belt, a microwave belt or a molding belt located above the conveyor that can cover the surface of the single board A1. The conveying section is used to receive the single board A1 from the microwave heating zone and meet the requirements of maintaining coverage on the surface of the single board A1; the conveying section that receives the single board A1 sinks or lifts up so that the next conveying section can continue to receive the single board A1 from the microwave heating zone. About 5 to 6 layers of conveying sections can make the production rhythm of stage three compatible with stage one and stage two. Of course, the coordination and unification of production rhythm is not the purpose of the present invention, so it will not be described in detail.

[0037] The state of free water gathering on the two surfaces of veneer A2 can only last for a relatively short period of time. According to existing practical experience, this time is about 1 to 1.5 minutes. Then, the water will return to the core of veneer A2 driven by the moisture content gradient. Therefore, veneer A2 must be glued within 1 to 1.5 minutes.

[0038] In the gluing process, the adhesive used can be a cold-pressed adhesive or a heat-cured adhesive. In this embodiment, cold-pressed polyvinyl acetate adhesive is used as an example, and only one surface of the single board A2 is glued. Of course, according to the needs of the process, glue can be applied to both surfaces of the single board A2 at the same time. Since the single board A2 is still placed on the conveyor in stage three, the conveyor can be lowered or raised to a state horizontal to the conveyor line in the gluing process, and the single board A2 is directly conveyed to the conveyor line in the gluing process, and then the adhesive is applied to the two surfaces of the single board A2 by roller coating to obtain the single board A3.

[0039] If the adhesive you choose to use is a heat-curing adhesive, you only need to replace the adhesive in the glue pool. The gluing equipment and gluing method are the same as those for cold-pressed adhesives.

[0040] Through the above-mentioned process method, the moisture content of the surface of the veneer A2 is specifically increased to a relatively high level, and the pores on the surface of the veneer A2 are filled with free water. Then, since the macrocapillary system and the microcapillary system on the surface of the veneer A2 are filled with free water and bound water respectively, after the veneer A2 surface is glued, the adhesive has no driving force, or only a very weak driving force to penetrate into the interior of the surface of the veneer A2. Furthermore, since the veneer A2 still has a relatively high temperature during the glue application, even after the glue application is completed, according to practical statistics, the surface temperature of the veneer A2 still has a material temperature of about 45 to 50°C, so at the same time or a short time after the adhesive is applied to the surface of the veneer A2, the adhesive is heated and pre-cured, or at least its fluidity will be greatly reduced. In particular, when the adhesive used is a cold-pressed adhesive, the pre-curing effect is relatively good. Based on this situation, although a small amount of adhesive still enters the surface of veneer A2 in the form of molecular migration or transition to form glue nails, from a macroscopic point of view, the amount of adhesive consumed to form glue nails can still be ignored, so that the adhesive only forms a glue layer on the surface of veneer A2, thereby reducing the amount of adhesive consumed to achieve the same thickness of the glue layer to a certain extent. The specific glue layer thickness, glue application amount and bonding strength are shown in Table 1.

[0041] The purpose of the aging process is to reduce the temperature of the veneer A2 and average its internal moisture content. At the same time, since the veneer A2 has a relatively high temperature, the moisture content of the veneer A2 will be reduced to a certain extent during the aging process. Generally, it can be reduced to about the fiber saturation point, about 35%. Preferably, in the aging process, the veneer A2 is placed in a relatively dry natural environment, such as a temperature of 25-30°C and a relative humidity of 55-60%, and the aging time is about 30 minutes.

[0042] After the aging process is completed, four single boards A3 are taken and one single board A0' is placed on the top to form a stacked blank, thereby obtaining a blank material A4.

[0043] The assembly material A4 is sent to a multilayer press and pressed for a period of time to obtain a plywood A5. The multilayer press is a hot press or a cold press. The pressing plate temperature and pressing time during pressing are determined according to the adhesive used. The polyvinyl acetate adhesive used in this embodiment is a cold-pressing curing adhesive, so the assembly material A4 should be sent to a multilayer cold press and pressed for 40 to 45 minutes, and the pressure of the press is 2.2 to 2.5 MPa.

[0044] It is worth mentioning here that, as an additional technical effect, in the second stage of the moisture content control process, since the temperature of the veneer A0 is heated to a temperature close to or reaching the boiling point of water, a relatively large water vapor pressure is formed inside the veneer A0 under the pressure of the upper and lower covers. The moisture does not migrate in the veneer A0, and the water vapor pressure forms tiny local explosions everywhere in the veneer A0, destroying the internal organizational structure of the veneer A0, and forming a huge number of fine internal cracks that are distributed throughout the width and thickness of the veneer A0. The existence of these fine internal cracks will not affect the processes of gluing, aging and assembly, but in the pressing process, it makes each veneer A3 in the assembly material A4 easier to be compressed. Even in the absence of heating. For example, cold pressing for 40 to 45 minutes under the pressure conditions of 2.2 to 2.5 MPa in this embodiment can press the assembly material A4 with a thickness of 17.6 mm into a plywood A5 with a thickness of 15.8 mm, and the compression amount is about 10%. Therefore, by means of densification and compression, the mechanical strength of plywood A5 can be improved, and the application field of plywood A5 can be expanded.

[0045] In the drying process, the moisture content of the plywood A5 is reduced to about 16% by low-temperature kiln drying, for example, a maximum drying temperature of 50°C. Drying after the plywood is made is actually an inevitable process. Even in the prior art, the veneer is first dried to about 16% by kiln drying, and then glued, aged, assembled and pressed. Before using the finished plywood, the plywood needs to be dried twice to reduce the internal moisture content deviation of the board, so as to ensure that the moisture content is uniform everywhere inside the board. In this embodiment, the process of drying the veneer is replaced by the process of controlling the moisture content, so the production time is not actually extended. In the drying process, as the drying progresses and time passes, the plywood A5 undergoes a certain degree of thickness rebound. After the rebound, the thickness of the plywood A5 is 16.5 mm, and the actual compression is about 6.25%. Example 2

[0046] The difference between Example 2 and Example 1 is that the processing object is a rotary cut poplar veneer with a thickness of 2.0 mm. Since the thickness of veneer A0 is relatively thin, the moisture content of the surface layer 0.3 mm thick below the surface of veneer A2 after veneer A1 leaves the microwave drying zone can only reach a level of 65~70%.

[0047] After the aging process is completed, 8 sheets of veneer A3 are taken and 1 sheet of veneer A0' is placed on the top surface to form a blank, and the blank material A4 is obtained. Cold pressing is performed at a pressure of 2.2~2.5MPa for 40~45min, and the blank material A4 with a thickness of 18.8mm can be pressed into a plywood A5 with a thickness of 16.5mm, and the compression amount is about 12%. During the drying process, the thickness of the plywood A5 rebounds to 17.2mm, and the actual compression amount is about 8.51%. Example 3

[0048] The difference between Example 3 and Example 1 is that the adhesive used in the gluing process is a thermosetting phenolic resin adhesive. The assembly material A4 is sent to a multi-layer hot press, the hot pressing temperature is 150±2°C, the hot pressing time is 25~30min, and the table reading pressure of the press is 1.8~2.0MPa. The thickness of the plywood A5 after pressing is 15mm, and the compression amount is about 15%. This is because the moisture content of the veneer A3 of this embodiment is at the fiber saturation point level. After heating, the veneer A3 softens, and at the same time benefits from the small internal cracks in various places inside the veneer A3, which has more favorable conditions for compression, so it can use a smaller pressure and obtain a larger compression amount.

[0049] In the drying process, as the drying progresses and time passes, the plywood A5 rebounds to a certain extent. After the rebound, the thickness of the plywood A5 is 15.5 mm, and the actual compression is about 11.93%. Since the compression of this embodiment is not only formed by the yield strength of the material being less than the press pressure, but also by the material crushing caused by the tiny internal cracks inside the material, the compression rebound rate is very low even when the hot pressing time is only about 25 to 30 minutes and fixed in place.

[0050] The glue layer thickness, glue application amount and bonding strength of the prepared plywood A5 of this embodiment are shown in Table 1. Example 4

[0051] The difference between Example 4 and Example 3 is that the processing object is a peeled poplar veneer with a thickness of 5mm. Through the stage one of the moisture content control process, the moisture content of veneer A0 is reduced to 38±2%. In stage two, after microwave heating, a thermal environment of 99±2℃ is formed in the tunnel, and the time from veneer A0 entering the microwave heating zone to leaving is 48~50s. Veneer A0 can reach the first temperature of 94±2℃ after being heated in a thermal environment of 99±2℃ for 48~50s. In other words, the material temperature of veneer A1 is 92~96℃ when leaving the microwave heating zone. In stage three, the gas medium (air) inside the slow-release operation zone has a temperature of 39±2℃ and a relative humidity of 92±2%. At this time, there is a temperature difference of 55℃ between the first temperature and the second temperature. The time from veneer A1 entering the slow-release operation zone to leaving is 8.5~9min, and the moisture content of the surface layer 0.3mm thick below the surface of veneer A2 can reach a level of 78~85%.

[0052] Take two sheets of veneer A3 and place one sheet of veneer A0' on the top to stack and assemble to obtain assembly material A4. The assembly material A4 is sent to a multi-layer hot press, the hot pressing temperature is 170±2℃, the hot pressing time is 25~30min, and the pressure of the press is 1.4~1.6MPa. After pressing, the thickness of plywood A5 is 13.6mm, and the compression amount is about 10%.

[0053] In particular, in this embodiment, the ambient temperature of 39±2°C is close to the natural production environment. Considering factors such as equipment heat dissipation, the factory environment is higher than the normal room temperature. Therefore, the slow-release operation area of ​​this embodiment does not need to set up an additional closed operation space and control the temperature, and the gluing process can also be carried out in this environment, so that the interval time between the third stage of the moisture content control process and the gluing process can be better controlled.

[0054] During the drying process, as the drying progresses and as time goes by, the plywood A5 undergoes a certain degree of thickness rebound. After the rebound, the thickness of the plywood A5 is 14.2 mm, and the actual compression amount is about 5.3%.

[0055] The glue layer thickness, glue application amount and bonding strength of the prepared plywood A5 of this embodiment are shown in Table 1. Example 5

[0056] The difference between Example 5 and Example 3 is that the processing object is a peeled poplar veneer with a thickness of 10mm. Through the stage one of the moisture content control process, the moisture content of veneer A0 is reduced to 44±2%. In stage two, after microwave heating, a thermal environment of 103±2℃ is formed in the tunnel, and the time from veneer A0 entering the microwave heating zone to leaving is 45~47s. Veneer A0 can reach the first temperature of 99±2℃ after being heated in a thermal environment of 103±2℃ for 45~47s. In other words, the material temperature of veneer A1 is 97~101℃ when leaving the microwave heating zone. In stage three, the gas medium (air) inside the slow-release operation zone has a temperature of 52±2℃ and a relative humidity of 92±2%. At this time, there is a temperature difference of 47℃ between the first temperature and the second temperature. The time from veneer A1 entering the slow-release operation zone to leaving is 7.5~8min, and the moisture content of the surface layer 0.3mm thick below the surface of veneer A2 can reach a level of 87~90%.

[0057] Take a piece of veneer A3 and place a piece of veneer A0' on the top surface to stack and assemble to obtain assembly material A4. Send assembly material A4 to a multi-layer hot press, the hot pressing temperature is 170±2℃, the hot pressing time is 25~30min, and the pressure of the press is 1.8~2.0MPa. After pressing, the thickness of plywood A5 is 16mm, and the compression is about 20%. In the drying process, as the drying progresses and the time passes, the thickness of plywood A5 rebounds to a certain extent. After rebound, the thickness of plywood A5 is 16.8mm, and the actual compression is about 16%.

[0058] The glue layer thickness, glue application amount and bonding strength of the prepared plywood A5 of this embodiment are shown in Table 1. Example 6

[0059] The difference between Example 6 and Example 1 is that in the second stage of the moisture content control process, the microwave heating zone is a microwave heat tunnel of the prior art. After microwave heating, a thermal environment of 42±2°C is formed in the tunnel. The time from the single board A0 entering the microwave heating zone to leaving is 20s. Single board A0 can reach the first temperature of 42±2°C by being heated in a thermal environment of 42±2°C for 20s. In other words, the material temperature of single board A1 is 40~44°C when leaving the microwave heating zone. Stage three is to make single board A1 enter the slow release operation zone under the condition of keeping the cover covered to obtain single board A2. The slow release operation zone of this embodiment is a hot channel composed of a plurality of continuous upper and lower hot roller pairs, and the temperature of the hot roller is 100±2°C. In another feasible embodiment, the slow release operation zone is composed of a pair of hot pressing plates, and the temperature of the hot pressing plates is 100±2°C. When the surface of single board A1 is suddenly heated, under the action of capillary force, the moisture migrates to the side with higher temperature, so that the moisture gathers on the surface of single board A2. At this time, the moisture content of the surface layer 0.3 mm thick below the surface of the veneer A2 can reach a level of 65~67%.

[0060] According to the observation results in practice, the water migration speed caused by the capillary force is significantly lower than the effect of water vapor pressure. Therefore, the temperature difference between the first temperature and the second temperature of this embodiment should be larger, and the time from the veneer A2 entering the slow-release operation interval to leaving is longer than that of Examples 1 to 4. According to the thickness of the veneer, at least 5 minutes is required. In addition, the moisture content of the surface layer below 0.3 mm thick on the surface of the veneer A2 can reach a moisture content level lower than that of Examples 1 to 4. Of course, the time that the veneer A2 spends in the slow-release operation interval cannot be too long. If it is too long, it will enter the hot press drying processing state and the moisture content of the veneer A2 will decrease.

[0061] Another difference is that in the second stage of the moisture content control process, the veneer A0 does not undergo high temperature treatment without water evaporation, and the veneer A0 maintains a complete structural form inside, so in terms of the cold pressing process, it is impossible to achieve the densification and compression of the assembly material A4. However, in any case, the process of this embodiment can also achieve the purpose of reducing the amount of glue applied to a certain extent.

[0062] The glue layer thickness, glue application amount and bonding strength of the prepared plywood A5 of this embodiment are shown in Table 1. Example 7

[0063] The difference between Example 7 and Example 6 is that the adhesive used in the gluing process is a heat-cured phenolic resin adhesive. The assembly material A4 is sent to a multi-layer hot press, the hot pressing temperature is 150±2°C, the hot pressing time is 25-30 minutes, and the pressure of the press is 1.8-2.0 MPa. The thickness of the plywood A5 after pressing is 17 mm, and the compression amount is about 4%.

[0064] In the drying process, as the drying progresses and time passes, the thickness of plywood A5 rebounds to a certain extent. After the rebound, the thickness of plywood A5 is 17.2 mm, and the actual compression is about 2.27%. Since the compression of this embodiment is only formed when the yield strength of the material is less than the pressure of the press, even if the compression is only fixed in situ for about 25 to 30 minutes of hot pressing time, it is impossible to achieve compression fixation, resulting in the compression achieved in the pressing process during the drying process or due to the passage of time. Based on this situation, for practical considerations, a low pressure of 1.0 to 1.2 MPa should be used.

[0065] The glue layer thickness, glue application amount and bonding strength of the prepared plywood A5 of this embodiment are shown in Table 1.

[0066] Among them, control group 1 and control group 2 are both Class III (NC) moisture-resistant composite plywood, with 5-layer and 3-layer composite structures respectively. In order to reflect the influence of bonding strength, control groups 1 and 2 are also bonded with phenolic resin adhesive. The thickness of the glue layer is the actual thickness of the glue layer measured by peeling the glue layer before assembly and after pre-curing.

[0067] Table 1. Glue layer thickness, glue application amount and bonding strength of the obtained plywood of each embodiment

[0068] Group adhesive Glue layer thickness (mm) Glue application amount (g / ㎡) Bonding strength (MPa) Elastic modulus (MPa) Example 1 Polyvinyl acetate adhesive single-sided sizing 0.1 103 1.2093 7109.4 Example 2 Polyvinyl acetate adhesive single-sided sizing 0.1 107 1.1929 7596.2 Example 3 Phenolic resin adhesive single-sided sizing 0.1 98 1.3972 7344.9 Example 4 Phenolic resin adhesive single-sided sizing 0.1 100 1.4155 7818.5 Example 5 Phenolic resin adhesive single-sided sizing 0.1 97 1.3835 8705.3 Example 6 Phenolic resin adhesive single-sided sizing 0.1 113 1.3903 6595.0 Example 7 Phenolic resin adhesive single-sided sizing 0.1 115 1.4139 6714.3 Control group 1 Phenolic resin adhesive single-sided sizing 0.1 120 1.4107 6708.0 Control group 2 Phenolic resin adhesive single-sided sizing 0.1 125 1.4374 7501.0

[0069] The above description is intended to be illustrative and not limiting. Upon reading the above description, many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art. Therefore, the scope of the present teachings should not be determined with reference to the above description, but should be determined with reference to the appended claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the preceding claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed application subject matter.

Claims

1. A method for preparing plywood by microwave heating pretreatment, comprising the steps of moisture content control, gluing, aging, assembling, pressing and drying, characterized in that: The process of moisture content control includes the following stages: In the first stage, the initial moisture content of the veneer is higher than the fiber saturation point of the wood. In the second stage, the single board with covers on both sides is passed through a microwave heating zone. Stage 3: after leaving the microwave heating zone, the single board with covers on both sides passes through a slow-release operation zone; In the microwave heating zone, the temperature is 95-103° C., the time from the single board entering the microwave heating zone to leaving the microwave heating zone is 20-50 seconds, and the microwave heating zone makes the single board reach a first temperature, which is 92-101° C.; The relative humidity of the slow-release operation zone is 90-94%, and the time from when the board enters the slow-release operation zone to when it leaves the slow-release operation zone is 2-5 minutes. The slow-release operation zone has a second temperature, the second temperature is 40-45° C., and the first temperature is 47-55° C. higher than the second temperature. The veneer is immediately glued after leaving the slow-release operation zone.

2. The method for preparing plywood by microwave heating pretreatment according to claim 1, characterized in that: The thickness of the single board is 1.5-10 mm.

3. The method for preparing plywood by microwave heating pretreatment according to claim 1, characterized in that: In the first stage, the single board is cured in an environment with a temperature of 30-35° C. and a relative humidity of 85-92% for 30-40 minutes.

4. The method for preparing plywood by microwave heating pretreatment according to claim 1, characterized in that: The time interval between the veneer leaving the slow-release operation interval and the gluing process is less than 1.5 minutes.

5. The method for preparing plywood by microwave heating pretreatment according to claim 1, characterized in that: In the aging process, the single board is aged in an environment at room temperature and a relative humidity of 55-60% for 30-40 minutes.

Images