CN116551798A

CN116551798A - Preparation method of plywood pretreated by microwave heating

Info

Publication number: CN116551798A
Application number: CN202310501881.4A
Authority: CN
Inventors: 王军锋; 汤正捷; 宋恋环; 黄腾华
Original assignee: Guangxi Zhuang Autonomous Region Forestry Research Institute
Current assignee: Guangxi Zhuang Autonomous Region Forestry Research Institute
Priority date: 2023-05-06
Filing date: 2023-05-06
Publication date: 2023-08-08

Abstract

The embodiment of the application discloses a plywood preparation method utilizing microwave heating pretreatment, which comprises the procedures of water content control, sizing, placing, assembling, pressing and drying, wherein the procedure of water content control comprises the following steps: stage one, making the initial water content of the single board be higher than the fiber saturation point of the wood; step two, the single board with covers on both sides passes through a microwave heating section; step three, after leaving the microwave heating section, passing the single board with covers on both sides through a slow release operation section; the microwave heating interval enables the single board to reach a first temperature, the slow-release operation interval has a second temperature, and a temperature difference between the first temperature and the second temperature is 47-57 ℃; and the veneer is immediately glued after leaving the slow-release operation interval.

Description

Preparation method of plywood pretreated by microwave heating

Technical Field

The invention relates to the technical field of a preparation process of plywood, in particular to a preparation method of plywood pretreated by microwave heating.

Background

The plywood is an artificial board manufactured by laminating and gluing a plurality of veneers. The production process of the plywood mainly comprises the procedures of water content control, sizing, placing, assembling, pressing, health preserving and the like. In general, in the procedure of controlling the water content, the veneer is in a state of low water content as much as possible, so that the adhesive coated on the surface of the veneer in the sizing procedure can penetrate into the veneer, and the adhesive nails are formed in the pressing procedure, thereby assisting in improving the interface bonding strength of the adhesive layer.

The prior art using the above process is more widely disclosed, for example, the invention patent application with publication number of CN114474264A in Chinese patent database and name of "a preparation method of multi-layer plywood" describes a technical proposal of drying the water content of the veneer to 10-18% before sizing, and then coating adhesive on both surfaces of the veneer, wherein the total sizing amount of both surfaces of the veneer is 340-400 g/square meter. In another example, the publication number is CN102350716B, the invention patent application entitled "Combined drying method suitable for industrial production of aldehyde-free plywood of fast-growing poplar" describes a technical scheme that the water content of a veneer is dried to 15-20% by solar energy or atmospheric drying before sizing, and then soybean adhesive is coated on the surface of the veneer, wherein the single-sided sizing amount is 200-240 g/square meter.

In the existing gluing theory, the gluing strength of the adhesive to the wood is jointly formed by the interface gluing strength (chemical bonding force) of the adhesive layer and the bonding strength (mechanical bonding force) of the adhesive nails, so that the formation of the adhesive nails is generally considered to have the functions of assisting in gluing and improving the gluing strength. Based on this recognition, prior art, such as the above-listed solutions, all employ relatively low pre-sizing water content. Because the water content of the veneer before sizing is relatively low (lower than the fiber saturation point of the wood), after sizing, the water in the adhesive is driven by the gradient of the water content to move below the surface of the veneer, and meanwhile, the adhesive is carried to enter below the surface of the veneer, and after the adhesive is solidified, nails are formed in pipe holes or other pores of the wood.

However, because the pores or other voids on the wood surface need to be filled first, the formation of the glue nails increases the amount of glue applied while ensuring the thickness of the glue layer (interfacial glue strength). One possible solution to reduce the formation of glue nails, with attendant reduction in sizing, is to char the veneer. Of course, the prior art is not intended to reduce the glue nails or the glue application amount, but to reduce the hydrophilicity of the veneer to increase the dimensional stability of the veneer. However, it is apparent that as the hydrophilicity of the veneer decreases, the adhesive does not or less penetrate below the surface of the veneer, which necessarily results in a reduction in the amount of nails and sizing. Corresponding to this phenomenon, the gluing performance of the carbonized veneer is inferior to that of a normal veneer, which further enhances the impression that the person skilled in the art can improve the gluing strength for the formation of the glue nail.

The Chinese patent database with publication number CN103692505B, the invention patent application named as a carbonized veneer and a manufacturing method thereof records that the double-sided sizing amount of the carbonized veneer is 250-350 g/square meter, and the bonding performance of the carbonized veneer is improved by adding cellulase or hemicellulose complex enzyme into soybean adhesive. The amount of sizing described in this patent application was significantly reduced compared to the amount of single sided sizing described in the aforementioned patent application publication No. CN102350716B filed by the same inventor, which was 200-240 g/square meter (also soybean adhesive). This demonstrates to some extent that no or only very little formation of the glue pins can greatly reduce the necessary amount of glue.

In summary, because the pores or other pores on the surface of the wood need to be filled, the formation of the glue nails can increase the glue application amount on the premise of ensuring the thickness (interface bonding strength) of the glue layer, and the increase of the glue application amount can increase the production cost, and if aldehyde-containing glue such as phenolic resin glue is used, the formaldehyde release amount of the manufactured plywood can also be increased. The prior art can give a technical suggestion for solving the problem that the veneer is carbonized, but the gluing performance of the carbonized veneer is greatly reduced and an additive is needed, the cost of the couplant and the cellulose is very high, the curing condition of the cellulose is very harsh, and the process condition is difficult to control.

Disclosure of Invention

The invention aims to overcome the technical problems, and provides a method for preparing plywood pretreated by microwave heating.

In order to achieve the above object, the embodiment of the invention provides a method for preparing plywood pretreated by microwave heating, which comprises the steps of water content control, sizing, ageing, assembly, pressing and drying, wherein the water content control step comprises the following steps:

in the first stage, the initial water content of the veneer is higher than the fiber saturation point of the wood,

step two, the single board with covers on both sides passes through a microwave heating section,

step three, after leaving the microwave heating section, passing the single board with covers on both sides through a slow release operation section;

the microwave heating interval enables the single board to reach a first temperature, the slow-release operation interval has a second temperature, and a temperature difference between the first temperature and the second temperature is 47-57 ℃;

and the veneer is immediately glued after leaving the slow-release operation interval.

In some embodiments, the veneer has a thickness of 1.5 to 10mm.

In some embodiments, in the first stage, the veneer obtained by rotary cutting is cured for 30-40 min at 30-35 ℃ in an environment of 85-92% RH.

In some embodiments, in the microwave heating section, the temperature is 95-103 ℃, the time from the entrance of the veneer into the microwave heating section to the exit is 20-50 s, the first temperature is 92-101 ℃, the second temperature is 40-45 ℃, and the first temperature is 47-55 ℃ higher than the second temperature.

In some embodiments, the ambient relative humidity of the extended release operation zone is 90-94% rh and the time from entry of the veneer into the extended release operation zone to exit is 2-5 min.

In some embodiments, in the microwave heating section, the temperature is 40-45 ℃, the time from the entrance of the veneer into the microwave heating section to the exit is 15-30 s, the first temperature is 40-44 ℃, the second temperature is 95-103 ℃, and the first temperature is 50-57 ℃ lower than the second temperature.

In some embodiments, the time from the entrance of the single board to the exit of the slow release operation interval is 5-10 min.

In some embodiments, the time interval between the single board exiting the slow release working zone to the sizing step is less than 1.5 minutes.

In some embodiments, in the step of aging, the veneer is aged for 30-40 minutes at room temperature in an environment having a relative humidity of 55-60% RH.

In summary, compared with the prior art, the invention has the following beneficial effects:

1. the water content of the surface of the veneer is increased to a relatively high level, the adhesive hardly penetrates into the surface of the veneer due to driving force after sizing, and the veneer still has a relatively high temperature during sizing, so that the adhesive is pre-cured when heated or at least has reduced fluidity when being coated on the surface of the veneer, and the adhesive can be prevented from penetrating into the surface of the veneer, thereby reducing the consumption of the adhesive required for reaching the adhesive layer with the same thickness to a certain extent.

2. In the second stage of the water content control process, relatively large water vapor pressure is formed inside the single plate, the water vapor pressure forms tiny local blasting at each part of the single plate, the internal tissue structure of the single plate is destroyed, and tiny internal cracks with huge quantity and distributed throughout the breadth and thickness of the single plate are formed, so that each single plate in the assembly material can be compressed under the cold pressing condition.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present application, and that other drawings may be obtained according to these drawings without inventive faculty for a person skilled in the art.

Fig. 1 is a schematic process route of the plywood preparation method of example 1 of the present application, which uses microwave heating pretreatment.

Fig. 2 is a schematic process route of the plywood preparation method of example 5 of the present application, which uses microwave heating pretreatment.

Detailed Description

In order to better understand the technical solutions in the present application, the following description will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only some embodiments of the present application, but not all embodiments. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments herein without making any inventive effort, shall fall within the scope of the present application.

Example 1

Referring to fig. 1, a method for preparing plywood pretreated by microwave heating is disclosed, wherein the processed object is a rotary-cut poplar veneer with the thickness of 3.5mm, and the method sequentially comprises the steps of water content control, sizing, ageing, assembly, pressing and drying, and the method is specifically described below.

The procedure of controlling the water content comprises three stages for controlling the initial water content, improving the temperature in the single plate and improving the water content of the surface layer.

The first stage is to control the initial water content of the veneer, and is to pass the veneer obtained by rotary cutting through a hot roller drying section or a cold/hot air drying section to quickly reduce the water content of the veneer. Rapidly reducing the water content of a part of the single boards to 40+/-3% to obtain single boards A0, in other words, the water content of the single boards A0 is higher than the fiber saturation point of the wood; the water content of the other part of the veneer was reduced to about 35% of the fiber saturation point, and the veneer of this part was called veneer A0'. The hot roller drying section or the cold/hot air drying section is in the prior art, and the roller temperature of the hot roller drying section is controlled to be 50-60 ℃; the cold/hot air drying section preferably performs hot air drying at 55 ℃, and if cold air drying at 25-30 ℃ is selected, the cold air speed should be properly increased. The water content control mode of the hot roller drying section or the cold/hot air drying section can facilitate assembly line operation, avoid carrying and turnover and improve production efficiency.

The second stage is to raise the temperature inside the board, and specifically, cover the two surfaces of the board A0 with covers suitable for microwave environment, and then pass the board A0 with covers on both sides through a microwave heating section to obtain a board A1 with the first temperature under the action of microwave heating. The cover on the bottom surface of the single board A0, which can be adapted to the microwave environment, is a conveyor belt of a conveyor, which is a plastic belt, a microwave belt or a molding belt. The cover which covers the surface of the single board A0 and can be suitable for the microwave environment is a closed plastic belt, a microwave belt or a molding belt which is tensioned and driven by a roller set consisting of a driving roller and a driven roller. The microwave heating section is a microwave heat tunnel in the prior art, a heat environment of 97+/-2 ℃ is formed in the tunnel after microwave heating, and the time from the entrance of the single board A0 into the microwave heating section to the departure of the single board A0 is 35s. If the heating time of 35s cannot be satisfied even if the length of the field, which can be used for placing the microwave heating section, is limited to the field, that is, the transmission speed is reduced, the single board A0 can be made to stay slightly in the microwave heating section. In any way, the total time of the single board A0 in the microwave heating section can reach 35s.

The single board A0 can reach a first temperature of 95±2 ℃ when heated in a thermal environment of 97±2 ℃ for 35 seconds, in other words, the material temperature of the single board A1 is 93 ℃ to 97 ℃ when leaving the microwave heating section. Meanwhile, as the principle of microwave heating is that microwaves formed by vibration penetrate the single plate A0 to enable water molecules in the single plate A0 to move along with the microwaves to generate heat energy, the purpose of heating the single plate A0 is achieved. Therefore, it can be considered that the inside of the single plate A1 leaving the microwave heating section has the first temperature everywhere. The first temperature is a range of values and not a particular value because the temperature difference is formed in the same thermal environment due to the difference in the heat-receiving properties of the wood throughout.

The third stage is to increase the water content of the surface layer, specifically, after the single board A1 leaves the microwave drying section, the cover of the cover is maintained, so that the single board A1 enters the slow-release operation section to obtain a single board A2. The slow release operation section is a relatively closed operation space, wherein the relatively closed operation space means that the operation space can keep the internal temperature and the relative humidity of the operation space under the set conditions relatively stably. The gaseous medium (air) inside the slow release operation zone has a second temperature of 42±2 ℃. Preferably, the gaseous medium (air) inside the sustained release working zone has a relative humidity of 92±2%rh. The time from the entrance of the single board A1 to the departure of the slow-release operation interval is 5.5-6 min. In the third stage, the water content of the single board A1 is higher than the fiber saturation point, i.e. the single board A1 has a certain amount of free water in the inside, and at the same time has a higher temperature, so that when the single board A1 enters the slow release operation interval, the free water in the materials of the two surfaces of the single board A1 is suddenly cooled, and then the free water in the core of the single board A1 is condensed firstly, and then is driven by the water vapor pressure gradient, and possibly also comprises a certain amount of absorbed water, and migrates and gathers to the two surfaces of the single board A1. This tendency to migrate requires a time of 5.5 to 6 minutes to enable the water content of both surfaces of the veneer A1 to reach a high level. Until the water vapor pressure is insufficient to continue driving the migration of moisture or the temperature of the core of veneer A1 is reduced to a lower level, typically below 60 c, the migration of moisture will cease. At this time, the water content of the surface layer 0.3mm thick below the surface of the veneer A2 was found to reach a level of 75 to 80% by detecting the layered water content of the veneer A2.

Compared with the first stage and the second stage, the single board A1 needs to stay in the slow-release operation interval for a long time, and in order to ensure the smoothness of the production line, the production beat of the third stage is coordinated to be suitable for the first stage and the second stage. The control of the tact of the stage three can be realized by a multi-track operation. For example, a laminating section is added between the microwave heating section and the slow release operation section, and of course, the laminating section can also be in the environment condition of the slow release operation section. Specifically, the stacking section is composed of a plurality of conveying sections capable of walking up and down and being stacked with each other, and each conveying section comprises a conveyor with a plastic belt, a microwave belt or a forming belt and a closed plastic belt, a microwave belt or a forming belt which is positioned above the conveyor and can cover the surface of the veneer A1. The conveying section is used for receiving the single board A1 from the microwave heating section and meeting the requirement of keeping coverage on the surface of the single board A1; the conveying section receiving 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 section. The conveying sections of about 5-6 layers can adapt the production tact of the third stage to the first stage and the second stage. Of course, the coordination of the tact is not an object of the present invention, and thus, a detailed description thereof will be omitted.

The state of free water accumulation on both surfaces of the veneer A2 can only be maintained for a relatively short period of time, which is approximately 1 to 1.5min summarized in the prior practical experience, after which the moisture will return to the core of the veneer A2 under the drive of the moisture content gradient. Therefore, it is necessary to size veneer A2 in 1 to 1.5 minutes.

In the step of sizing, the adhesive used may be a cold-press adhesive or a heat-curable adhesive, and in this embodiment, a cold-press-cured polyvinyl acetate adhesive is taken as an example, and only one surface of the veneer A2 is sized. Of course, both surfaces of veneer A2 may be simultaneously sized, depending on the needs of the process. In the third stage, the single board A2 is still placed on the conveyor, so that the conveyor can be lowered or lifted to a state of being horizontal to the conveying line in the gluing process, the single board A2 is directly conveyed to the conveying line in the gluing process, and then the adhesive is coated on the two surfaces of the single board A2 through roller coating, so that the single board A3 is obtained.

If the selected adhesive is a thermosetting adhesive, the adhesive in the adhesive pool is only needed to be replaced, and the sizing equipment, the sizing method and the cold-pressing adhesive are not two-way.

Through the above-mentioned technological method, specifically improve the moisture content of the surface of single board A2 to the relatively higher level, in order to fill the pore of the surface of single board A2 with the free water, and then, because big capillary system and microcapillary system of the surface of single board A2 are filled with the free water, binding water separately, therefore after sizing the surface of single board A2, the adhesive has no driving force, or only very weak driving force permeates into the surface of single board A2. Further, since the veneer A2 still has a relatively high temperature during sizing, even after sizing is completed, the surface temperature of the veneer A2 still has a material temperature of about 45 to 50 ℃ through practical statistics, so that the adhesive is pre-cured when heated or at least its fluidity is greatly reduced while or after being coated on the surface of the veneer A2. Particularly in the case of cold-pressed adhesives, the pre-curing effect is relatively good. Based on this, although a small amount of adhesive still enters the inside of the surface of the veneer A2 in the form of molecular movement or transition to form a tack as a result, the consumption of the adhesive to form the tack can be neglected macroscopically, so that the adhesive forms a glue layer only on the surface of the veneer A2, thereby reducing the consumption of the adhesive required to reach the glue layer of the same thickness to some extent. Specific bond line thicknesses, amounts of sizing, and bond strengths are shown in table 1.

The setting of the setting process aims at reducing the temperature of the veneer A2 and averaging the water content in the veneer A2, and at the same time, since the veneer A2 has a relatively high temperature, the water content of the veneer A2 can be reduced to a certain extent during setting, and can be reduced to about the fiber saturation point, generally about 35%. Preferably, in the step of aging, veneer A2 is placed in a relatively dry natural environment, such as a temperature of 25-30℃, a relative humidity of 55-60% RH, and allowed to age for about 30 minutes.

After the setting process is completed, 4 single boards A3 are taken, and 1 single board A0' is placed on the topmost surface to laminate a blank, so as to obtain a blank material A4.

The assembly material A4 was fed into a multi-laminator for pressing for a period of time to obtain plywood A5. The multi-layer press belongs to a hot press or a cold press, and the temperature of a pressing plate and the pressing time are determined according to different used adhesives during pressing. The polyvinyl acetate adhesive used in this example is a cold-press curing adhesive, so the assembly material A4 should be sent to a multi-layer cold press for pressing for 40-45 min, and the gauge pressure of the press is 2.2-2.5 MPa.

It should be noted here that, as an additional technical effect, in the second stage in the water content control process, since the temperature of the single plate A0 is heated to be close to or reach the boiling point of water, a relatively large vapor pressure is formed inside the single plate A0 under the pressing of the upper and lower covers. The moisture does not migrate in the single plate A0, the steam pressure forms tiny local explosion at all parts of the single plate A0, the internal tissue structure of the single plate A0 is destroyed, and tiny internal cracks which are huge in quantity and spread all the parts of the width and thickness of the single plate A0 are formed. The presence of these fine internal cracks does not affect the sizing, setting and assembly steps, but in the pressing step, the individual veneers A3 in the assembly material A4 are more easily compressed. Even without heating. For example, the assembly material A4 having a thickness of 17.6mm can be pressed into the plywood A5 having a thickness of 15.8mm by cold pressing under a pressure of 2.2 to 2.5MPa for 40 to 45 minutes, and the compression is about 10%. Therefore, by means of densification and compression, the mechanical strength of the plywood A5 can be improved, and the application field of the plywood A5 is expanded.

In the drying step, the water content of the plywood A5 is reduced to about 16% by low-temperature kiln drying, for example, at a drying temperature of 50 ℃. The drying after the plywood is made is an unavoidable procedure in practice, and even after the veneer is dried to about 16% by kiln drying in the prior art, the veneer is glued, aged, assembled and pressed, and before the made plywood is used, the plywood is required to be dried for the second time so as to reduce the deviation of the water content in the board, thereby ensuring that the water content in the board is uniform and consistent all over. In this embodiment, the step of drying the veneer is replaced with the step of controlling the water content, so that the production time is not prolonged in practice. In the drying step, the plywood A5 had a certain degree of thickness rebound with the progress of drying and the lapse of time, and the thickness of the plywood A5 after rebound was 16.5mm, and the actual compression was about 6.25%.

Example 2

Example 2 differs from example 1 in that the object to be processed is a rotary-cut poplar veneer having a thickness of 2.0mm, and since the thickness of veneer A0 is relatively thin, the moisture content of the surface layer 0.3mm below the surface of veneer A2 after veneer A1 leaves the microwave drying zone can only reach a level of 65 to 70%.

After the setting process is completed, 8 single boards A3 are taken, and 1 single board A0' is placed on the topmost surface to laminate a blank, so as to obtain a blank material A4. The assembly material A4 with the thickness of 18.8mm can be pressed into the plywood A5 with the thickness of 16.5mm by cold pressing for 40-45 min under the pressure condition of 2.2-2.5 MPa, and the compression amount is about 12%. In the drying step, the thickness of the plywood A5 was rebound to 17.2mm, and the actual compression was about 8.51%.

Example 3

Example 3 differs from example 1 in that the adhesive used in the sizing procedure is a heat-curable phenolic resin adhesive. And (3) conveying the assembly material A4 into a multi-layer hot press, wherein the hot pressing temperature is 150+/-2 ℃, the hot pressing time is 25-30 min, and the surface reading pressure of the press is 1.8-2.0 MPa. The thickness of the pressed plywood A5 was 15mm and the compression was about 15%. This is because the water content of the veneer A3 of this embodiment is at the fiber saturation point level, and the veneer A3 softens after being heated, and at the same time benefits from the tiny internal cracks in the veneer A3, and has conditions that are more conducive to compression, so that a smaller pressure can be used, and a larger compression amount can be obtained.

In the drying step, the plywood A5 had a certain degree of thickness rebound with the progress of drying and the lapse of time, and the thickness of the plywood A5 after rebound was 15.5mm, and the actual compression was about 11.93%. Since the compression of the present embodiment is not only formed in the material with a yield strength smaller than the press pressure but also is greatly formed in the material with a small internal crack, the compression spring rate is very low even in the case of in-situ fixation for a hot press time of about 25 to 30 minutes.

The glue line thickness, glue amount and glue strength of the plywood A5 produced in this example are shown in table 1.

Example 4

Example 4 differs from example 3 in that the object to be processed is a rotary-cut poplar veneer having a thickness of 5 mm. In the first stage of the step of controlling the water content, the water content of the single board A0 is reduced to 38±2%. In the second stage, after microwave heating, a thermal environment of 99+/-2 ℃ is formed in the tunnel, and the time from entering the microwave heating section to leaving the single board A0 is 48-50 s. The single board A0 can reach a first temperature of 94+/-2 ℃ when heated for 48-50 s in a thermal environment of 99+/-2 ℃, in other words, the material temperature of the single board A1 is 92-96 ℃ when leaving the microwave heating section. In the third stage, the gas medium (air) in the slow release operation section has a temperature of 39+/-2 ℃ and a relative humidity of 92+/-2%RH, and at this time, a temperature difference of 55 ℃ exists between the first temperature and the second temperature. The time from entering the slow-release operation interval to leaving of the single board A1 is 8.5-9 min, and the water content of the surface layer with the thickness of 0.3mm below the surface of the single board A2 can reach the level of 78-85%.

Taking 2 single boards A3, and placing 1 single board A0' on the topmost surface to laminate and assemble to obtain an assembled material A4. And (3) conveying the assembly material A4 into a multi-layer hot press, wherein the hot pressing temperature is 170+/-2 ℃, the hot pressing time is 25-30 min, and the surface reading pressure of the press is 1.4-1.6 MPa. The thickness of the pressed plywood A5 was 13.6mm and the compression was about 10%.

In particular, in this embodiment, the environment temperature of 39±2 ℃ is already relatively close to the natural production operation environment, and the factory environment is higher than the normal room temperature in consideration of the factors such as heat dissipation of the equipment. Therefore, the slow-release operation interval of the embodiment does not need to additionally provide a closed operation space and control the temperature, and the sizing process can be performed in the environment, so that the gap time between the stage three of the water content control process and the sizing process can be well controlled.

In the drying step, the plywood A5 had a certain degree of thickness rebound with the progress of drying and the lapse of time, and the thickness of the plywood A5 after rebound was 14.2mm, and the actual compression was about 5.3%.

Example 5

Example 5 differs from example 3 in that the object to be processed is a rotary-cut poplar veneer having a thickness of 10mm. In the first stage of the step of controlling the water content, the water content of the single board A0 is reduced to 44±2%. In the second stage, a thermal environment of 103+/-2 ℃ is formed in the tunnel after microwave heating, and the time from entering the microwave heating section to leaving the single board A0 is 45-47 s. The single board A0 can reach a first temperature of 99±2 ℃ when heated for 45 to 47 seconds in a thermal environment of 103±2 ℃, in other words, the material temperature of the single board A1 is 97 to 101 ℃ when leaving the microwave heating section. In the third stage, the gas medium (air) in the slow release operation section has a temperature of 52+/-2 ℃ and a relative humidity of 92+/-2%RH, and at this time, a temperature difference of 47 ℃ exists between the first temperature and the second temperature. The time from the entrance of the single board A1 to the departure of the single board A2 is 7.5-8 min, and the water content of the surface layer with the thickness of 0.3mm below the surface of the single board A2 can reach the level of 87-90%.

Taking 1 single board A3, and placing 1 single board A0' on the topmost surface to laminate and assemble, thus obtaining assembled material A4. And (3) conveying the assembly material A4 into a multi-layer hot press, wherein the hot pressing temperature is 170+/-2 ℃, the hot pressing time is 25-30 min, and the surface reading pressure of the press is 1.8-2.0 MPa. The thickness of the pressed plywood A5 was 16mm and the compression was about 20%. In the drying step, the plywood A5 had a certain degree of thickness rebound with the progress of drying and the lapse of time, and the thickness of the plywood A5 after rebound was 16.8mm, and the actual compression was about 16%.

Example 6

Example 6 differs from example 1 in that in the second stage of the step of controlling the water content, the microwave heating section is a microwave heat tunnel of the prior art, a heat environment of 42±2 ℃ is formed in the tunnel after microwave heating, and the time from entrance of the single board A0 into the microwave heating section to exit is 20s. The single board A0 can reach a first temperature of 42±2 ℃ when heated in a thermal environment of 42±2 ℃ for 20 seconds, in other words, the material temperature of the single board A1 is 40 ℃ to 44 ℃ when leaving the microwave heating section. And step three, under the condition of keeping the coverage of the cover, enabling the single board A1 to enter a slow-release operation interval to obtain a single board A2. The slow release operation interval of the embodiment is a thermal channel formed by a plurality of continuous thermal roller pairs which are arranged up and down, and the temperature of the thermal roller is 100+/-2 ℃. In another possible embodiment, the slow release operation section is formed by a pair of hot pressing plates, and the temperature of the hot pressing plates is 100+/-2 ℃. When the surface of the single board A1 is suddenly heated, the moisture migrates to the side with higher temperature under the action of capillary force, so that the moisture is gathered on the surface of the single board A2. At this time, the water content of the surface layer 0.3mm thick below the surface of the veneer A2 may reach a level of 65 to 67%.

According to the observation in practice, the moisture migration speed caused by the capillary force is significantly lower than the water vapor pressure, so the temperature difference between the first temperature and the second temperature in this embodiment should be larger, and the time from entering the slow-release operation interval to leaving the single board A2 is longer than that in embodiments 1 to 4, and at least 5min is required according to the thickness of the single board. The water content of the surface layer 0.3mm thick below the surface of the veneer A2 was also lower than that of examples 1 to 4. Of course, the time of the single board A2 in the slow-release operation section cannot be too long, and the long time enters the hot press drying processing state, so that the water content of the single board A2 is reduced.

Another difference is that in stage two of the procedure of controlling the water content, the single board A0 is not subjected to the high temperature treatment without evaporation of moisture, and the inside of the single board A0 maintains the complete structural morphology, so that the densification and compression of the assembly material A4 cannot be realized in terms of the cold pressing process. However, the process of this example can also be implemented to some extent for the purpose of reducing the sizing amount.

Example 7

Example 7 differs from example 6 in that the adhesive used in the sizing procedure is a heat-curable phenolic resin adhesive. And (3) conveying the assembly material A4 into a multi-layer hot press, wherein the hot pressing temperature is 150+/-2 ℃, the hot pressing time is 25-30 min, and the surface reading pressure of the press is 1.8-2.0 MPa. The thickness of the pressed plywood A5 was 17mm and the compression was about 4%.

In the drying step, the plywood A5 had a certain thickness rebound with the progress of drying and the lapse of time, and the thickness of the plywood A5 after rebound was 17.2mm, and the actual compression was about 2.27%. Since the compression of this embodiment is only formed at a material yield strength less than the press pressure, even if the fixation of the compression is not achieved by the in-situ fixation for a hot press time of about 25 to 30 minutes alone, the compression amount achieved during the drying process or in the pressing process is substantially all rebounded due to the passage of time. Based on this, a low pressure of 1.0 to 1.2MPa should be used for practical reasons.

Wherein, control group 1, control group 2 are III class (NC) moisture-resistant composite plywood, and the structure is 5 layers and 3 layers composite structure respectively. In order to embody the influence of the bonding strength, the control groups 1 and 2 are bonded by using a phenolic resin adhesive. The glue layer thickness is the actual glue layer thickness obtained by stripping the glue layer before assembly and after pre-curing.

TABLE 1 glue line thickness, glue size and glue strength of veneer A2 of each example and plywood produced

The foregoing description is for purposes of illustration and is not intended to be limiting. Many embodiments and many applications other than the examples provided will be apparent to those of skill in the art upon reading the above description. The scope of the present teachings should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated herein by reference for the purpose of completeness. The omission of any aspect of the subject matter disclosed herein in the preceding claims is not intended to forego such subject matter, nor should the applicant be deemed to have such subject matter not considered to be part of the subject matter of the disclosed application.

Claims

1. The preparation method of the plywood pretreated by utilizing microwave heating comprises the procedures of water content control, sizing, ageing, assembling, pressing and drying, and is characterized in that the procedure of water content control comprises the following steps:

2. The method for preparing plywood pretreated by microwave heating according to claim 1, wherein the thickness of said veneer is 1.5-10 mm.

3. The method of producing plywood according to claim 1, wherein in the first stage, the veneer is cured at 30 to 35 ℃ in an atmosphere of 85 to 92% rh for 30 to 40 minutes.

4. The method of manufacturing plywood pretreated by microwave heating according to claim 1, wherein in the microwave heating section, the temperature is 95-103 ℃, the time from when the veneer enters the microwave heating section to when it leaves is 20-50 s, the first temperature is 92-101 ℃, the second temperature is 40-45 ℃, and the first temperature is 47-55 ℃ higher than the second temperature.

5. The method for preparing plywood pretreated by microwave heating according to claim 4, wherein the ambient relative humidity of said slow-release operation zone is 90-94% RH and the time from entering said slow-release operation zone to leaving said veneer is 2-5 min.

6. The method of manufacturing plywood pretreated by microwave heating according to claim 1, wherein in the microwave heating section, the temperature is 40-45 ℃, the time from when the veneer enters the microwave heating section to when it leaves is 15-30 s, the first temperature is 40-44 ℃, the second temperature is 95-103 ℃, and the first temperature is 50-57 ℃ lower than the second temperature.

7. The method for preparing plywood pretreated by microwave heating according to claim 6, wherein the time from entering the slow-release operation interval to leaving of the veneer is 5-10 min.

8. The method for preparing plywood pretreated by microwave heating according to claim 1, wherein the time interval between the step of leaving the slow-release operation zone and the step of sizing said veneer is less than 1.5min.

9. The method for producing plywood pretreated by microwave heating according to claim 1, wherein in said step of aging, said veneer is aged for 30 to 40 minutes in an atmosphere of 55 to 60% rh at room temperature.