Anti-peeling multilayer heterogeneous wood veneer composite board and interface strengthening process thereof

Abstract

This invention discloses a peel-resistant multilayer heterogeneous wood veneer composite board and its interface strengthening process. The board comprises, from top to bottom, a hardwood veneer layer, a discontinuous phase gradient transition layer, and a softwood core layer; a 30–80 μm flexible resin pre-impregnated layer is provided on the side of the hardwood veneer layer near the transition layer. The transition layer is a resin-cured layer with a honeycomb-like closed-cell structure, and its porosity decreases gradually from the center to the edge. The transition layer has an array of anchoring protrusions formed by in-situ foaming on the side facing the core layer. The process steps include: instantaneous thermal activation at 140–160℃ only on the hardwood side; and a three-stage pulsed hot-pressing process after coating with an adhesive containing thermally expanded microspheres. This invention significantly improves the interfacial bonding strength and stability of the heterogeneous board through micro-mechanical interlocking and gradient modulus transition.

Description

Technical Field

[0001] This invention relates to the field of wood composite material processing technology, specifically to a peel-resistant multilayer heterogeneous wood veneer composite board and its interface strengthening process. Background Technology

[0002] In the manufacturing process of multi-layer wood composite boards, it is usually necessary to combine hardwood veneers of different densities with softwood cores. Due to the significant differences between hardwood veneers and softwood cores in terms of elastic modulus, dimensional stability under varying moisture content, and mechanical strength, large shear stresses are easily generated at the composite interface.

[0003] Existing adhesive bonding processes mostly employ uniform glue application followed by constant-pressure hot pressing, resulting in a relatively random distribution of the adhesive at the interface. For softwood cores with natural vessels, conventional adhesives, due to improper viscosity control, often exhibit over-penetration or under-penetration, making it difficult to form a stable mechanically interlocking structure at the interface. Furthermore, the abrupt change in rigidity at the interface can easily lead to peeling and damage of the boards from the bonding surface when subjected to fluctuations in ambient humidity or external impacts. Therefore, how to construct a composite structure that can effectively alleviate stress concentration and enhance interfacial bonding strength is a problem that needs to be solved in the field of heterogeneous wood veneer composites. Summary of the Invention

[0004] The present invention aims to provide a peel-resistant multilayer heterogeneous wood veneer composite board and its interface strengthening process, so as to solve the peeling risk caused by modulus mismatch at the interface of heterogeneous wood composites, as well as the problem of insufficient mechanical locking strength of traditional adhesive interfaces.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a peel-resistant multilayer heterogeneous wood veneer composite board, comprising a hard wood veneer layer, a discontinuous phase gradient transition layer and a soft wood core layer arranged sequentially from top to bottom;

[0006] The hardwood veneer layer has a flexible resin prepreg layer with a thickness of 30–80 μm on the side near the discontinuous phase gradient transition layer. The discontinuous phase gradient transition layer is a resin-cured layer with a honeycomb-like closed-cell structure, with an effective thickness of 120–250 μm. It contains micropores with an average pore size of 20–60 μm, and the porosity of the micropores decreases continuously or in stages from the center of the discontinuous phase gradient transition layer to its two wall edges along the thickness direction. The discontinuous phase gradient transition layer has an array of anchoring protrusions on the side facing the softwood core layer, which expand as it is foamed in situ and pressed into the natural vessels of the softwood core layer. The array of anchoring protrusions is integrally cured with the discontinuous phase gradient transition layer and extends along the natural vessels of the softwood core layer. The ends of the array of anchoring protrusions have an enlarged mushroom-shaped geometric feature, and their average extension depth into the interior of the softwood core layer is 50–200 μm.

[0007] Furthermore, the maximum diameter of the end of the array-type anchoring protrusion is greater than the neck diameter at its connection with the discontinuous phase gradient transition layer.

[0008] Furthermore, the porosity of the central region of the discontinuous phase gradient transition layer is 30%–55%, and the porosity of the regions near the edges of the hardwood veneer and softwood core layers is 5%–15%.

[0009] Furthermore, the density of the hardwood veneer layer is 0.65–0.90 g / cm³, and the density of the softwood core layer is 0.35–0.50 g / cm³.

[0010] Furthermore, the discontinuous phase gradient transition layer is formed by curing a modified isocyanate adhesive containing thermally expandable microspheres, wherein the mass fraction of the thermally expandable microspheres is 0.5%–1.5% of the total mass of the adhesive.

[0011] An interface strengthening process for preparing the above-mentioned peel-resistant multilayer heterogeneous wood veneer composite board includes the following steps: Step S1: Asymmetric thermal activation treatment, which involves performing an instantaneous heat treatment at 140–160°C on only the side of the hardwood veneer to be composited, so as to moderately degrade the hemicellulose on the surface of the hardwood veneer. Step S2: Single-sided coating, applying a modified isocyanate adhesive containing thermally expanding microspheres to the surface of the hardwood veneer layer or softwood core layer to be laminated. Step S3: Perform the three-stage pulse hot pressing curing process: Low-pressure initiation stage: Close the press and apply an initial pressure of less than 0.2 MPa at 140–160°C for 15–30 seconds to allow the thermally expanded microspheres in the adhesive to ignite and form initial bubble nuclei; Instantaneous high-pressure pulse stage: The adhesive system is measured online or offline. When the viscosity of the adhesive reaches 1.5–3.0 Pa·s, a pulse high pressure of 2.5–4.0 MPa is applied and held for 3–8 s to drive the semi-cured adhesive into the natural conduits of the soft wood core layer to form the anchoring protrusions and the mushroom-shaped geometric features. Medium-pressure setting stage: The pressure is rapidly released to a setting pressure of 0.8–1.2 MPa and held until the adhesive is completely cured, utilizing the microporous elastic rebound at the moment of pressure release to form the gradient distribution.

[0012] Furthermore, in step S3, the criterion for determining whether to enter the instantaneous high-voltage pulse stage is: to perform a pulse boosting action within a 5-10s time window after the end of the low-voltage initiation stage.

[0013] Furthermore, the modified isocyanate adhesive has a solid content of 65%–80% and a complete crosslinking time of 90–150 s at 150°C.

[0014] Furthermore, the heat treatment time in step S1 is 5–15 seconds.

[0015] Furthermore, in step S3, the pressure relief process of switching from the instantaneous high-pressure pulse stage to the medium-pressure shaping stage is completed within 1–2 seconds.

[0016] The beneficial effects of this invention are as follows: 1. By setting a flexible resin prepreg layer, the present invention achieves a smooth transition of modulus between the hardwood veneer layer and the transition layer, reducing the interface stress concentration on the high modulus wood side.

[0017] 2. This invention constructs a discontinuous phase gradient transition layer with a honeycomb-like closed-cell structure, thereby forming a structure similar to a "buffer pad" at the interface. The high porosity in the central region absorbs anisotropic deformation caused by changes in environmental humidity, thus enhancing the dimensional stability of the board.

[0018] 3. By controlling the viscosity window and pressure change in the three-stage pulse hot pressing process, this invention achieves controlled flow of adhesive in the natural conduits of soft wood core. The mushroom-shaped geometric features formed achieve microscopic mechanical interlocking, significantly improving the peel strength of the composite board.

[0019] 4. This invention achieves instantaneous exposure of the lateral polar functional groups of hardwood veneer through asymmetric thermal activation treatment, thereby improving the wettability and bonding force of the adhesive to the surface of dense wood.

[0020] 5. This invention achieves a porosity gradient distribution from the center to both sides by utilizing the micropore elastic rebound generated by pressure relief in the later stage of curing. This gradient density structure realizes the continuity of mechanical transmission between interfaces and reduces the probability of failure due to interface delamination. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the macroscopic structure of the composite material of the present invention; Figure 2This is a flowchart illustrating the interface strengthening process of the present invention.

[0022] In the diagram: 100, hardwood veneer layer; 110, flexible resin prepreg layer; 200, discontinuous phase gradient transition layer; 300, softwood core layer. Detailed Implementation

[0023] The embodiments of the present invention are described in detail below.

[0024] The present invention discloses a peel-resistant multilayer heterogeneous wood veneer composite board, the overall structure of which is as follows: Figure 1 As shown, this is achieved through an asymmetric combination of a hardwood veneer layer 100, a discontinuous phase gradient transition layer 200, and a softwood core layer 300. The density of the hardwood veneer layer 100 is 0.65–0.90 g / cm³, and the density of the softwood core layer 300 is 0.35–0.50 g / cm³. A flexible resin prepreg layer 110 with a thickness of 30–80 μm is provided at the contact interface of the hardwood veneer layer 100 near the intermediate layer. The flexible resin prepreg layer 110 enables a gradual transition in the mechanical properties of the interface region, reducing stress concentration at the hardwood side interface.

[0025] A discontinuous phase gradient transition layer 200 is located between the hard and soft layers, with a thickness of 120–250 μm. Its structural feature is that the discontinuous phase gradient transition layer 200 is a resin-cured layer, with a porosity of 30%–55% in its geometrically central region and 5%–15% porosity in the peripheral regions near the hard veneer layer 100 and the soft core layer 300. The discontinuous phase gradient transition layer 200 provides a mechanical buffer space through the gradient distribution of porosity.

[0026] At the interface between the discontinuous phase gradient transition layer 200 and the softwood core layer 300, an array of anchoring protrusions is integrally cured with the discontinuous phase gradient transition layer 200. The array of anchoring protrusions extends along the natural vessel direction of the softwood core layer 300, with an extension depth of 50–200 μm. The ends of the array of anchoring protrusions have an enlarged mushroom-head geometric feature, thereby forming a stable mechanical interlocking structure at the interface.

[0027] The specific process flow is as follows: Figure 2 As shown. The heat treatment in step S1 degrades the hemicellulose in the hard side surface layer, improving the wettability of the adhesive. Step S3 performs a three-stage pulsed hot pressing. The low-pressure initiation stage induces the thermal expansion of microspheres to form bubble nuclei. When the adhesive viscosity reaches the range of 1.5–3.0 Pa·s, under pulsed pressure, the adhesive enters the natural vessels of the soft wood core layer and forms anchoring protrusions. Due to viscosity limitations, the adhesive flow rate decreases and undergoes lateral extrusion after entering a predetermined depth, forming a mushroom-shaped geometry. Finally, by rapidly depressurizing, a gradient pore distribution is formed using the rebound effect of the resin's internal micropores.

[0028] The present invention will be verified through specific embodiments below: Example 1 Oak with a density of 0.85 g / cm³ was selected as the hardwood veneer layer 100, and poplar with a density of 0.42 g / cm³ was selected as the softwood core layer 300.

[0029] Step S1: Perform instantaneous heat treatment at 155°C for 10 seconds on the oak side to be composited.

[0030] Step S2: Apply a modified isocyanate adhesive containing 1.0% by weight of thermally expanded microspheres.

[0031] Step S3: Hot pressing temperature 150°C; low pressure initiation stage pressure 0.15MPa, hold for 25s; when the viscosity reaches 2.2Pa·s, apply 3.5MPa pulse high pressure, hold for 5s; then release pressure to 1.0MPa within 1.5s until complete curing.

[0032] Observation results: A transition layer with an effective thickness of about 180 μm was formed, with a porosity gradient distribution of 45% to 10%, and the mushroom head anchoring depth was about 120 μm.

[0033] Example 2 It is basically the same as Example 1, except that the pulse high voltage in step S3 is set to 2.8MPa.

[0034] Observation results: The anchoring depth of the mushroom head decreased to about 70 μm, and the thickness of the transition layer was about 210 μm.

[0035] Comparative Example 1 (Conventional Constant Pressure Process) Same materials and adhesives as in Example 1, but without pulse hot pressing. Directly hot-pressed at constant pressure of 150°C and 1.2 MPa until cured.

[0036] Observation results: The adhesive solution exhibits random seepage within the conduit, without obvious mushroom-shaped geometric features, and the pores in the transition layer are evenly distributed.

[0037] Comparative Example 2 (microspheres without thermal expansion) The process is the same as in Example 1, but thermally expanded microspheres are not added to the adhesive.

[0038] Observation results: The transition layer is a dense adhesive layer with no honeycomb-like closed-cell structure or gradient distribution.

[0039] The interface performance of the plates prepared in the above embodiments and comparative examples was tested, and the results are shown in the table below:

[0040] By analyzing the above experimental data, the following conclusions can be drawn: Regarding the interfacial mechanical properties, Example 1 achieved an interfacial peel strength of 2.45 MPa and a wood breakage rate as high as 95%, significantly superior to Comparative Example 1's 1.35 MPa and 45%. This demonstrates that the mushroom-shaped geometric features achieved by controlling the pulse pressure and viscosity window in this invention generate extremely strong micro-mechanical interlocking. When peeled under stress, the failure surface mainly occurs within the wood substrate rather than at the adhesive interface, proving that this invention achieves the beneficial effect of interfacial strength exceeding substrate strength.

[0041] Regarding the influence of process parameters, a comparison between Example 1 and Example 2 shows that when the pulsed high pressure decreased from 3.5 MPa to 2.8 MPa, the peel strength decreased by approximately 14%, and the anchoring depth decreased from 120 μm to 70 μm. This indicates that within a specific viscosity range, the magnitude of the pulsed pressure directly determines the degree of development of the anchoring protrusions; higher pulsed pressure helps the adhesive form a deeper and more fully-ended mushroom-shaped structure within the conduit.

[0042] Regarding environmental stability, neither Example 1 nor Example 2 exhibited delamination after 50 cycles of hot and cold cycling, while Comparative Example 1 showed a delamination rate of 12.5%. Based on these observations, it can be seen that the present invention, by setting a discontinuous phase gradient transition layer 200 with a gradient porosity distribution, effectively buffers stress at the interface. Under conditions of drastic temperature fluctuations, the high-porosity region at the center of the transition layer can absorb the stress from uneven dimensional changes between dissimilar wood materials through its own elastic deformation.

[0043] Regarding the microscopic mechanism, the results of Comparative Example 2 show that, in the absence of a honeycomb-like closed-cell structure formed by thermally expanded microspheres, even with the same hot-pressing process, the delamination rate and peel strength are still inferior to those of the example. This demonstrates that the coupling effect between the gradient microporous structure and the mushroom-head anchoring structure in this invention is key to achieving interface strengthening. The gradient pore layer ensures the continuity of mechanical transmission, while the anchoring protrusions provide extremely high static load-bearing capacity; together, they enhance the peel resistance of the composite board.

[0044] In summary, this invention effectively solves the stress concentration problem caused by modulus mismatch at the interface of heterogeneous wood composites by constructing a discontinuous phase gradient transition layer 200 with a specific gradient porosity distribution between the hardwood veneer layer 100 and the softwood core layer 300, and combining it with the flexible resin prepreg layer 110. Utilizing a pulsed hot-pressing process within a specific viscosity window, the adhesive forms an array of mushroom-shaped anchoring protrusions in situ within the natural vessels of the softwood core layer 300, achieving a strong micro-mechanical interlocking effect at the interface. Experimental results confirm that the composite board prepared by this invention not only has significantly improved peel strength but also maintains excellent structural integrity and dimensional stability under harsh thermal cycling conditions, providing a reliable technical solution for high-performance composites of heterogeneous woods.

[0045] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A peel-resistant multi-layer heterogeneous wood veneer composite board, characterized in that, It includes a hardwood veneer layer (100), a discontinuous phase gradient transition layer (200), and a softwood core layer (300) arranged sequentially from top to bottom. The hardwood veneer layer (100) has a flexible resin prepreg layer (110) with a thickness of 30–80 μm on the side near the discontinuous phase gradient transition layer (200). The discontinuous phase gradient transition layer (200) is a resin-cured layer with a honeycomb closed-cell structure and an effective thickness of 120–250 μm. It contains micropores with an average pore size of 20–60 μm, and the porosity of the micropores decreases continuously or in stages from the center of the discontinuous phase gradient transition layer (200) to its two wall edges along the thickness direction. The discontinuous phase gradient transition layer (200) facing the soft wood core layer (300) has an array of anchoring protrusions formed by in-situ foaming expansion and pressing into the natural vessels of the soft wood core layer (300); the array of anchoring protrusions are integrally cured with the discontinuous phase gradient transition layer (200) and extend along the natural vessels of the soft wood core layer (300); the ends of the array of anchoring protrusions have an enlarged mushroom-shaped geometric feature, and their average extension depth into the interior of the soft wood core layer (300) is 50–200 μm.

2. The anti-peeling multi-layer heterogeneous wood veneer composite board according to claim 1, characterized in that, The maximum diameter of the end of the array-type anchoring protrusion is greater than the neck diameter at its connection with the discontinuous phase gradient transition layer (200).

3. The peel-resistant multi-layer heterogeneous wood veneer composite board according to claim 1, characterized in that, The porosity of the central region of the discontinuous phase gradient transition layer (200) is 30%–55%, and the porosity of the regions near the edges of the hard veneer layer (100) and soft core layer (300) is 5%–15%.

4. The anti-peeling multi-layer heterogeneous wood veneer composite board according to claim 1, characterized in that, The density of the hardwood veneer layer (100) is 0.65–0.90 g / cm³, and the density of the softwood core layer (300) is 0.35–0.50 g / cm³.

5. The peel-resistant multi-layer heterogeneous wood veneer composite board according to claim 1, characterized in that, The discontinuous phase gradient transition layer (200) is formed by curing a modified isocyanate adhesive containing thermally expandable microspheres, wherein the thermally expandable microspheres account for 0.5%–1.5% of the total mass of the adhesive.

6. An interface strengthening process for preparing peel-resistant multilayer heterogeneous wood veneer composite boards as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step S1: Asymmetric thermal activation treatment, which involves performing an instantaneous heat treatment at 140–160°C on only the side of the hardwood veneer layer (100) to be composited, so as to moderately degrade the hemicellulose on the surface of the hardwood veneer layer (100). Step S2: Single-sided coating, applying a modified isocyanate adhesive containing thermally expanding microspheres to the surface to be laminated on the hard veneer layer (100) or soft core layer (300). Step S3: Perform the three-stage pulse hot pressing curing process: Low-pressure initiation stage: Close the press and apply an initial pressure of less than 0.2 MPa at 140–160°C for 15–30 seconds to allow the thermally expanded microspheres in the adhesive to ignite and form initial bubble nuclei; Instantaneous high-pressure pulse stage: The adhesive system is measured online or offline. When the viscosity of the adhesive reaches 1.5–3.0 Pa·s, a pulse high pressure of 2.5–4.0 MPa is applied and held for 3–8 s to drive the semi-cured adhesive into the natural conduits of the soft wood core layer (300) to form the anchoring protrusions and the mushroom-shaped geometric features. Medium-pressure setting stage: The pressure is rapidly released to a setting pressure of 0.8–1.2 MPa and held until the adhesive is completely cured, utilizing the microporous elastic rebound at the moment of pressure release to form the gradient distribution.

7. The interface strengthening process according to claim 6, characterized in that, In step S3, the criterion for determining whether to enter the instantaneous high-voltage pulse stage is: to perform a pulse boosting action within a 5-10s time window after the end of the low-voltage initiation stage.

8. The interface strengthening process according to claim 6, characterized in that, The modified isocyanate adhesive has a solid content of 65%–80% and a complete crosslinking time of 90–150 s at 150°C.

9. The interface strengthening process according to claim 6, characterized in that, The heat treatment time in step S1 is 5–15 seconds.

10. The interface strengthening process according to claim 6, characterized in that, In step S3, the pressure relief process of switching from the instantaneous high-pressure pulse stage to the medium-pressure shaping stage is completed within 1–2 seconds.

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