Surface-densified wood and preparation method thereof

Abstract

The invention relates to the technical field of wood modification, and discloses surface-densified wood and a preparation method thereof. The preparation method of the surface-densified wood comprises the following steps that after wood is subjected to heating pretreatment, the surface of the wood is wetted and softened with water, then surface compression densification is conducted, and the surface-densified wood is obtained after cooling. According to the compressed and densified wood, the highest density peak value can reach 861 kg / m < 3 >, and the minimum distance between the density peak value and the surface of the wood can reach 0.241 mm. Compared with an unpreheated sample, the compressed and densified wood is greatly shortened, a densified area is closer to the surface, and the surface modification effect is more concentrated and remarkable. And by regulating and controlling the preheating temperature and the compression condition, the density peak value and the surface compactness degree can be further improved, and the surface hardness of the wood is remarkably enhanced and can reach 1337 N to the maximum.

Description

Technical Field

[0001] The invention relates to the technical field of wood modification, and more specifically to a surface densified wood and a preparation method thereof. Background Art

[0002] As a natural and renewable biomass material, wood has good processability and decorative properties and is widely used in many fields such as construction, furniture, panels, and decoration. However, natural wood also has inherent defects such as large fluctuations in mechanical properties, insufficient surface hardness and wear resistance, and easy damage, which limits its promotion and use in high-performance and high-durability application scenarios.

[0003] Wood surface densification can effectively improve the bending mechanical properties, surface hardness and wear resistance of wood while reducing the volume loss of wood, thereby reducing costs, by forming a densified layer on the wood surface. In addition, the time for wood surface densification is significantly lower than that for overall densification, so the production efficiency is high and it is easy to realize industrial production. It is a very promising technology for improving wood functions.

[0004] There are currently a variety of methods for preparing surface densified wood. Patent CN101966713B obtains densified wood by sealing the cross section of the wood with a hydrophobic material, then attaching a softener to the surface or shallow layer of the sealed wood and hot pressing the wood above the boiling point of the softener. However, this method requires the wood to be sealed, the processing steps are cumbersome, and the compression layer is difficult to control near the surface. Patent CN101214675B discloses a method for strengthening wood by hot pressing carbonization, adjusting the moisture content of the wood to 3-17%, planing it, and then hot pressing carbonizing it in a hot press at 160-260°C. However, hot pressing carbonization requires a long period of high temperature, high energy consumption, and the compression layer is difficult to control, which increases the production cost. Patent CN107457870A proposes a method for rapid and dense carbonization of the wood surface, fixing the wood to be treated and a titanium metal block on a fixture, pressurizing the wood to be treated and the titanium metal, and maintaining the pressure by high-frequency reciprocating vibration of the wood surface to be treated and the titanium metal to achieve rapid and dense carbonization of the wood surface. Although this method has a short production time, it requires a high-frequency vibration device and a titanium metal block, and the equipment cost is relatively high. At the same time, vibration and pressurization also require a lot of energy consumption. Patent CN101966713A discloses a wood densification method by first hydrophobically sealing the wood cross section, then applying a softener on the surface and performing hot pressing at a temperature higher than the boiling point of the softener. The key to this method is to prevent the softener from penetrating into the interior of the wood, thereby concentrating the compression effect on the surface layer and achieving wood surface densification. However, this method still has certain limitations in actual operation, such as the need to physically seal the wood cross section, which increases the complexity and cost of the processing procedure, and the sealing effect is greatly affected by the properties of the wood itself, and there is a risk of unstable permeability control. At the same time, this scheme relies on precise temperature control to ensure that the hot pressing process reaches the effective action temperature zone of the softener, has high requirements for equipment control accuracy, and does not systematically utilize the potential of thermal pretreatment to regulate the internal structure of the wood, which limits the further improvement of process efficiency and treatment layer control. Summary of the invention

[0005] The present invention provides a surface densified wood to overcome the defects of the above-mentioned prior art, such as complicated processing steps, low compression layer control accuracy, and reliance on high temperature or high frequency vibration equipment.

[0006] Another object of the present invention is to provide a method for preparing surface densified wood.

[0007] In order to solve the above technical problems, the technical solution of the present invention is as follows:

[0008] A method for preparing surface densified wood comprises the following steps: pre-treating the wood by heating, moistening and softening the wood surface with water, compressing and densifying the surface with equipment, and obtaining the surface densified wood after cooling.

[0009] The method of the present invention preheats the wood before compressing it, so that the interior of the wood is first heated to a higher state. In this high temperature state, moisture is immediately applied to the surface of the wood for softening treatment, and then hot pressing is performed. Due to the high internal temperature, a temperature gradient from the inside to the outside is formed during the hot pressing process, which inhibits the migration and diffusion of moisture to the inside, so that the moisture is mainly retained and acts on the surface area of ​​the wood, thereby effectively limiting the softening and compression reactions to the surface. This temperature distribution mechanism ensures that the compression layer is concentrated on the surface, improving the localization and efficiency of densification.

[0010] Preferably, the wood before the heating pretreatment is bone-dry wood.

[0011] Furthermore, the temperature of the heating pretreatment is 100-200° C., and the treatment time is 0.5-2 h.

[0012] Preferably, the temperature of the heating pretreatment is 100-150°C.

[0013] Furthermore, the moisture content of the wood after being moistened and softened with water is 4% to 10%.

[0014] Preferably, the moisture content of the wood after being moistened and softened with water is 10%.

[0015] Preferably, the wet softening method comprises brushing, wiping or covering the wood surface with a layer of water-containing paper.

[0016] The moisture content will affect the softening effect of wood. The wood of the present invention is pre-treated by heating, and the wood surface is quickly moistened and softened with water, so that the high temperature inside the wood can be maintained. The moisture content after moistening and softening by different methods is basically the same.

[0017] Furthermore, when surface compression and densification is performed, the preheating temperature (compression temperature) of the equipment is 90 to 150° C., and the preheating time is 10 to 60 seconds.

[0018] Preferably, the preheating temperature of the equipment is 120-150°C.

[0019] Preferably, the preheating temperature of the equipment is 120° C. and the preheating time is 30 seconds.

[0020] Furthermore, when the surface is compressed and densified, the compression time is 8 to 15 minutes.

[0021] Preferably, the compression time is 10 to 15 minutes.

[0022] Furthermore, during surface compression and densification, the pressure is 3 to 5 MPa and the compression feed speed is 1 to 3 mm / min.

[0023] A surface densified wood is prepared by the preparation method.

[0024] Furthermore, the thickness of the wood softening compression layer is less than 3 mm.

[0025] Preferably, the thickness of the wood softening and compressing layer is 1 to 2 mm.

[0026] Furthermore, the density peak of the surface densified wood is less than 0.5 mm away from the wood surface.

[0027] Furthermore, the surface hardness of the surface densified wood is greater than or not less than 940N.

[0028] The present invention innovatively performs high-temperature pretreatment on the wood surface before densification, and then quickly softens the wood surface by coating the surface with a softener. This treatment method can form a temperature distribution pattern in which the wood is low on the outside and high on the inside, and can effectively prevent moisture from penetrating from the wood surface to the inside during the hot pressing process, and control the compression of the softened layer on the wood surface to achieve the effect of densification of the wood surface. This high-temperature pretreatment method can achieve efficient batch processing of wood, greatly improving the speed of wood pretreatment, and moisture is coated on the surface as a softener, shortening the softening time of the wood. Since the pretreatment optimizes the thermal responsiveness of the wood, the subsequent hot pressing time is also significantly shortened, further improving the overall process efficiency. The present invention can achieve efficient and rapid surface densification of wood by adjusting the process and the wood compression process, which is of great significance for solving the current problems of slow production efficiency and small scale of high-value wood.

[0029] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0030] The present invention introduces appropriate heat pretreatment before the wood is compressed to form a high temperature environment inside the wood, thereby effectively preventing moisture from migrating into the wood during the subsequent compression process, and causing the compression effect to be concentrated on the surface of the wood. The compressed densified wood of the present invention can achieve a maximum density peak of 861kg / m 3 , and the minimum distance between the density peak and the wood surface can reach 0.241mm, which is significantly shorter than that of the unpreheated samples. The dense area is closer to the surface, and the surface modification effect is more concentrated and significant. At the same time, by adjusting the preheating temperature and compression conditions, the density peak and surface density can be further improved, and the surface hardness of the wood can be significantly enhanced, up to 1337N. This method not only improves the surface properties and mechanical performance of the wood, but also optimizes the accuracy of compression control. It has the advantages of strong adjustability of process parameters, good controllability of the treatment layer, low equipment requirements, and low energy consumption. It is suitable for promotion and application on an industrial scale and has significant technical and economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the cross-sectional density of the surface densified wood in Examples 1 to 3 and Comparative Example 1;

[0032] Figure 2 is the cross-sectional density of the surface-densified wood in Examples 4 to 5;

[0033] Figure 3 The peak density and position of wood at different heating pretreatment temperatures;

[0034] Figure 4 The surface hardness of wood at different heating pretreatment temperatures;

[0035] Figure 5 The peak value and position of wood density at different hot pressing preheating temperatures and times;

[0036] Figure 6 The surface hardness of wood at different hot pressing preheating temperatures and times. DETAILED DESCRIPTION

[0037] The present invention is further described below in conjunction with the accompanying drawings and specific examples, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0038] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0039] Example 1

[0040] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0041] S2, placing the fir sample in an oven at 100°C for heating pretreatment for 2 hours;

[0042] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0043] S4. Put the heated wood with surface moisture into a 150°C hot press, and compress it after preheating for 60 seconds. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and the pressure is maintained for 10 minutes after reaching the specified thickness, and then cooled to room temperature through water.

[0044] Example 2

[0045] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0046] S2, placing the fir sample in an oven at 150°C for heating pretreatment for 2 hours;

[0047] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0048] S4. Put the heated wood with surface moisture into a 150°C hot press, and compress it after preheating for 60 seconds. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and the pressure is maintained for 10 minutes after reaching the specified thickness, and then cooled to room temperature through water.

[0049] Example 3

[0050] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0051] S2, placing the fir sample in an oven at 200°C for heating pretreatment for 2 hours;

[0052] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0053] S4. Put the heated wood with surface moisture into a 150°C hot press, and compress it after preheating for 60 seconds. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and the pressure is maintained for 10 minutes after reaching the specified thickness, and then cooled to room temperature through water.

[0054] Example 4

[0055] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0056] S2, placing the fir sample in an oven at 150°C for heating pretreatment for 2 hours;

[0057] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0058] S4. Put the heated wood with surface moisture into a 150°C hot press, and compress it after preheating for 10s, 30s, and 60s respectively. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and after reaching the specified thickness, the pressure is maintained for 10min, and then cooled to room temperature through water.

[0059] Example 5

[0060] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0061] S2, placing the fir sample in an oven at 150°C for heating pretreatment for 2 hours;

[0062] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0063] S4. Put the heated wood with surface moisture into a 120°C hot press, and compress it after preheating for 10s, 30s, and 60s respectively. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and after reaching the specified thickness, the pressure is maintained for 10min, and then cooled to room temperature through water.

[0064] Example 6

[0065] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0066] S2, placing the fir sample in an oven at 150°C for heating pretreatment for 2 hours;

[0067] S3. Apply water to both sides of the wood surface to make its moisture content greater than 4% (about 10%).

[0068] S4. Put the heated wood with surface moisture into a 90°C hot press, and compress it after preheating for 10s, 30s, and 60s respectively. The hot pressing pressure is 5MPa, the compression feed speed is 3mm / min, the thickness of the thickness gauge is 8mm, and after reaching the specified thickness, the pressure is maintained for 10min, and then cooled to room temperature through water.

[0069] Example 7

[0070] S1. Sample preparation, specific steps: fir wood is processed into samples of 11mm×50mm×120mm (R×T×L) and then sanded. The samples are dried in an oven at 103°C to absolute dryness and sealed for storage;

[0071] S2, placing the fir sample in an oven at 200°C for heating pretreatment, the treatment time is 0.5h;

[0072] S3. Brush water on both sides of the wood to make its moisture content 4% to 10%.

[0073] S4. Put the heated wood with surface moisture into a 150°C hot press, and compress it after preheating for 60 seconds. The hot pressing pressure is 3MPa, the compression feed speed is 1mm / min, the thickness of the thickness gauge is 8mm, and after reaching the specified thickness, the pressure is maintained for 15 minutes, and then cooled to room temperature through water.

[0074] Comparative Example 1

[0075] Comparative Example 1 is similar to Example 1, except that the sample in Comparative Example 1 is not pre-treated by heating and is at room temperature.

[0076] Comparative Example 2

[0077] Comparative Example 2 is similar to Example 1, except that the heating pretreatment temperature of the sample in Comparative Example 2 is 80°C.

[0078] Comparative Example 3

[0079] Comparative Example 3 is similar to Example 1, except that the heating pretreatment temperature of the sample in Comparative Example 3 is 250°C.

[0080] Comparative Example 4

[0081] Comparative Example 4 is similar to Example 4, except that the hot pressing temperature of the sample in Comparative Example 4 is 80°C.

[0082] Comparative Example 5

[0083] Comparative Example 5 is similar to Example 4, except that the hot pressing temperature of the sample in Comparative Example 5 is 200°C.

[0084] Detection Methods

[0085] 1. Measurement of volumetric density (VPD)

[0086] The density distribution on the cross section of the wood was measured using an X-ray profile densitometer. The X-ray measured the density of the wood at an interval of 0.05 mm along the radial direction of the wood on the cross section of the wood. The measurement size of the wood was 8 mm × 50 mm × 50 mm (R × T × L).

[0087] 2. Surface hardness

[0088] The wood surface hardness test is measured according to the international standard ISO 13061-12 (ISO 2017), in which an indenter with a diameter of 11.28 mm is pressed into the wood sample to a compression depth of 2.82 mm and a compression speed of 4 mm / min, and the maximum force reached during the compression process is recorded. The calculation formula for the wood surface hardness is as follows:

[0089] Hw=K×F

[0090] Where: F is the maximum load when the indenter is pressed into a depth of 2.82 mm, N; K is the coefficient when the indenter is pressed into a depth of 2.82 mm, which is equal to 4 / 3.

[0091] Analysis

[0092] according to Figure 1 and Figure 2 In each embodiment, an obvious high-density area is formed on the surface of the wood, while the density of the middle part remains at a low level, and the thickness of the softened dense layer caused by compression is controlled within 3 mm, which further verifies that the method of the present invention can achieve shallow and precise densification processing, and the compression layer does not spread to the interior of the wood.

[0093] According to Table 1 and Figures 3-4 The results show that after preheating, the peak density of the compressed wood in Examples 1 to 3 is about twice as far away from the surface as the density peak of the sample without preheating (Comparative Example 1), indicating that the high temperature inside the wood can effectively prevent moisture from migrating into the wood during the compression process. The appropriate heat pretreatment temperature can make the wood have a high density peak and make the position of the density peak close to the wood surface. However, when the heating pretreatment temperature is increased, although the distance from the wood density peak to the surface is still closer than that of the control sample, it will improve the surface hydrophobicity, reduce the moisture content of the wood surface after water is applied, and reduce the softening effect of the wood surface. At the same time, high temperature will also degrade the components in the wood, resulting in a decrease in physical properties such as density peak and surface hardness.

[0094] According to Table 1 and Figures 5-6 By comparing the results of Example 2 with Examples 4 to 6, it can be seen that the compression temperature and preheating time of wood also have a great influence on the density distribution of wood. By adjusting the compression temperature and preheating time of wood, the density peak of wood can be increased and the density peak can be made closer to the wood surface, thereby increasing the surface hardness of wood. However, when the compression temperature is too low, the energy input is insufficient and the density peak is low; when the compression temperature is too high, due to the enhanced moisture migration, aggravated thermal damage to the surface layer and uneven structural response, the softening and compression area shifts inward, and the surface hardness and density peak are far away from the surface.

[0095] In Comparative Example 2, the wood was pre-heated at only 80°C, the internal temperature rise was insufficient, and the overall thermal responsiveness of the wood was low, which made it difficult to establish a clear internal heat and external moisture temperature gradient during the subsequent compression process. The results show that the density peak of Comparative Example 2 is only 702kg / m 3 , and the density peak is as far as 0.845mm from the surface, and the compression layer is offset inward, indicating that the softened compression area is not effectively concentrated on the wood surface, which ultimately causes the surface hardness to decrease (1011N), indicating that low-temperature pretreatment is difficult to effectively inhibit the migration of moisture to the inside, and is not enough to activate the structural response of the wood cell wall. Comparative Example 3 uses an excessively high pretreatment temperature of 250℃. Although the peak density is shortened from the surface, the density peak drops to 672kg / m 3 , the surface hardness dropped to 917N. This shows that high temperature causes thermal degradation or carbonization of the wood surface, which increases hydrophobicity, reduces water adsorption and softening efficiency, and destroys the cell wall structure, causing it to lose good compression deformation ability, thus affecting the densification quality.

[0096] Comparative Example 4 was pretreated at 150°C and then hot-pressed at 80°C, resulting in a peak density of only 685 kg / m 3 , the surface hardness is only 932N. Insufficient hot pressing temperature leads to limited energy input, which cannot effectively drive the directional compression of the softened layer, resulting in insufficient compression and insufficient densification. In Comparative Example 5, the hot pressing temperature is too high, causing uncontrolled moisture migration and thermal damage to the surface layer, thereby affecting the distribution and densification effect of the compression layer, resulting in a decrease in the density peak, a backward position, compression failure or a decrease in surface performance.

[0097] Table 1 Density peak distribution and surface hardness of the embodiments and comparative examples

[0098]

[0099]

[0100] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preparing surface densified wood, characterized in that: The following steps are involved: After the wood is heated for pretreatment, the surface of the wood is moistened and softened with water, and then the surface is compressed and densified by equipment, and the surface densified wood is obtained after cooling.

2. The method for preparing surface densified wood according to claim 1, characterized in that: The temperature of the heating pretreatment is 100-200° C., and the treatment time is 0.5-2 hours.

3. The method for preparing surface densified wood according to claim 1, characterized in that: After being moistened and softened, only the surface of the wood contains water, with a moisture content of 4% to 10%.

4. The method for preparing surface densified wood according to claim 1, characterized in that: When performing surface compression and densification, the preheating temperature of the equipment is 90 to 150°C and the preheating time is 10 to 60 seconds.

5. The method for preparing surface densified wood according to claim 1, characterized in that: When the surface is compressed and densified, the compression time is 8 to 15 minutes.

6. The method for preparing surface densified wood according to claim 1, characterized in that: When the surface is compressed and densified, the pressure is 3 to 5 MPa and the compression feed speed is 1 to 3 mm / min.

7. A surface densified wood, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 6.

8. The surface densified wood according to claim 7, characterized in that: The thickness of the wood softening compression layer is less than 3 mm.

9. The surface densified wood according to claim 7, characterized in that: The density peak of the surface densified wood is less than 0.5 mm away from the wood surface.

10. The surface densified wood according to claim 9, characterized in that: The surface hardness of the surface densified wood is greater than or not less than 940N.

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