Modified red phenolic resin impregnated paper for concrete formwork and laminated plywood

Abstract

A preparation method of the modified red phenolic resin impregnated paper for the concrete formwork comprises the following steps: adding 0.8-1.2 wt% of nano silicon dioxide, 1.5-2.0 wt% of nano aluminum oxide and 0.5-1.0 wt% of tetrapod-like zinc oxide whiskers into a red phenolic resin glue solution, firstly shearing at a high speed of 800-1000r / min and pre-dispersing for 5 minutes, and then ultrasonically dispersing at a power of 300-500W for 20-30 minutes; wood pulp paper is soaked in the glue solution and then dried at 110-120 DEG C until the water content is 6-8%, and the modified red phenolic resin impregnated paper is prepared. The invention further discloses the red laminated plywood for the concrete formwork and a preparation technology of the red laminated plywood, through surface layer modification, core layer structure optimization and synergistic improvement of the adhesive and the preparation technology, the comprehensive performance of the plywood is improved, and the high-standard requirement for the formwork in building construction is met.

Description

Technical Field

[0001] This invention relates to the field of resin-impregnated paper technology, specifically to a modified red phenolic resin impregnated paper for concrete formwork and a laminated plywood. Background Technology

[0002] Concrete formwork is an indispensable temporary support structure in building construction, and its performance directly affects the surface quality of the poured concrete, construction efficiency, and project cost. Film-coated plywood is widely used in the field of concrete formwork due to its smooth surface, easy demolding, and reusability. Red film-coated plywood, in particular, is a mainstream product on the market because its eye-catching color facilitates on-site identification and classification management, and it also has a certain decorative appeal.

[0003] However, existing red-coated plywood for concrete formwork still has many shortcomings: First, the surface red coating is mostly made of ordinary phenolic resin impregnated paper, which has poor wear and scratch resistance. It is easily worn and faded during concrete pouring, demolding, and handling, resulting in a rough surface of the formwork, affecting the flatness of the subsequent concrete surface, and reducing the number of times the formwork can be reused. Second, the core layer is mostly made of single poplar or eucalyptus veneer. Poplar veneer has low density and insufficient strength, while eucalyptus veneer has poor toughness. The single-structure core layer is prone to deformation and delamination, affecting the overall load-bearing capacity of the plywood. Third, the adhesive selection is unreasonable, resulting in insufficient bonding strength between the surface, bottom, and core layers. The bonding between the veneers in the core layer is prone to delamination, especially in humid construction environments, where the poor water resistance is more prominent. Fourth, the hot-pressing parameters in the existing manufacturing process are not designed reasonably, and the adhesive is not fully cured, further aggravating the problems of formwork delamination and coating peeling, shortening the service life of the formwork. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of existing red film-coated plywood for concrete formwork, such as poor wear resistance, insufficient water resistance, easy deformation of the core layer, easy peeling of the film, and short service life. This invention provides a red film-coated plywood for concrete formwork and its preparation process. Through surface modification, core structure optimization, and synergistic improvement of adhesives and preparation processes, the overall performance of the plywood is improved, meeting the high standards required for formwork in building construction.

[0005] A red film-coated plywood for concrete formwork includes a surface layer, a core layer, and a bottom layer. The surface layer is modified red phenolic resin impregnated paper with added nano-silica, nano-alumina, and tetra-needle-zinc oxide whiskers. The core layer has a symmetrical laminated structure of eucalyptus layer-poplar layer-eucalyptus layer, with two layers of eucalyptus veneer and three or five layers of poplar veneer, resulting in a total of seven or nine veneer layers in the core layer. The bottom layer is undyed phenolic resin impregnated paper. The surface layer and core layer, as well as the bottom layer and core layer, are bonded with phenolic resin adhesive, with a wet adhesive application rate of 180-220 g / m². The veneers within the core layer are bonded with melamine-phenolic resin composite adhesive, with a wet adhesive application rate of 220-260 g / m².

[0006] Preferably, the modified red phenolic resin impregnated paper has a thickness of 0.25-0.3 mm and a moisture content of 6-8% after drying; the modified red phenolic resin impregnated paper is prepared by adding 0.8-1.2 wt% nano-silica, 1.5-2.0 wt% nano-alumina, and 0.5-1.0 wt% nano-alumina to the red phenolic resin solution. Tetranephrite zinc oxide whiskers are first pre-dispersed at high speed of 800-1000 r / min for 5 min, and then ultrasonically dispersed at 300-500 W for 20-30 min. Blended coniferous sulfate pulp paper is impregnated with the adhesive solution and then dried at 110-120℃ to a moisture content of 6-8% to obtain modified red phenolic resin impregnated paper. The nano-silica has a particle size of 10-30 nm, the nano-alumina has a particle size of 50-80 nm, and the tetranephrite zinc oxide whiskers have a needle length of 10-20 μm and a needle diameter of 0.5-1 μm.

[0007] Preferably, the wood pulp paper is softwood bleached sulfate pulp paper with a basis weight of 120-150 g / m². This type of wood pulp paper has high fiber strength and uniform water absorption, and can fully absorb modified red phenolic resin solution, ensuring the wear resistance and toughness of the surface impregnated paper.

[0008] Preferably, the preparation method of the melamine-phenolic resin composite adhesive is as follows: liquid melamine-formaldehyde resin and liquid phenolic resin are taken at a mass ratio of 1:3-1:4 and mixed under stirring at 300-400 r / min at room temperature; 2.0-3.0 wt% polyethylene glycol, 0.3-0.5 wt% hexamethylenetetramine, and 0.1-0.2 wt% silicone defoamer are added to the mixed solution and stirred to disperse evenly; the solid content is adjusted to 40-45% with deionized water, cured at room temperature for 30 min, and filtered through a 120 mesh to obtain the composite adhesive; the free phenol content of the phenolic resin adhesive is ≤0.5%, the free aldehyde content is ≤0.3%, and the solid content is 45-50%, which reduces the release of harmful substances, meets environmental protection requirements, and improves the water resistance and stability of the bond.

[0009] Preferably, the moisture content of the eucalyptus veneer and the poplar veneer is 8-10%, and the cross angle of the veneer grain is 90°±5° during core layer assembly. This assembly method can effectively counteract the anisotropy of the veneer, reduce core layer deformation, and improve the overall flatness and structural stability of the plywood. The density of the eucalyptus veneer is ≥650kg / m³, and the density of the poplar veneer is ≥450kg / m³, ensuring that the core layer has sufficient strength and load-bearing capacity, balancing the requirements of strength and lightweight.

[0010] Preferably, the thickness of the bottom layer is 0.2-0.25mm. The undyed phenolic resin impregnated paper in the bottom layer can play a role in preventing moisture and delamination, protecting the core layer from external humid environment erosion, while reducing production costs and taking into account both practicality and economy.

[0011] This invention also discloses a method for preparing the above-mentioned red film-coated plywood for concrete formwork, comprising the following steps:

[0012] S1. Preparation of the surface and bottom layers: Prepare modified red phenolic resin impregnated paper and undyed phenolic resin impregnated paper for later use;

[0013] S2. Core layer assembly: After the eucalyptus veneer and poplar veneer are dried to a moisture content of 8-10%, they are assembled in a symmetrical structure of eucalyptus layer-poplar layer-eucalyptus layer. Phenolic resin glue is applied between the surface layer, bottom layer and core layer, and melamine-phenolic resin composite glue is applied inside the core layer to ensure uniform glue application without any missed areas or glue accumulation.

[0014] S3. Segmented hot pressing molding:

[0015] Pre-compression stage: Pre-compress at 0.6-0.8MPa and 80-90℃ for 10-15 minutes to allow the adhesive to initially impregnate and expel air, thus avoiding the formation of air bubbles during hot pressing, which would affect the bonding quality;

[0016] Gradient hot pressing stage: A three-stage gradient heating process is adopted, with a total hot pressing time of 1.2-1.6 min / mm based on the slab thickness.

[0017] ① Core layer pre-curing section: Heat to 133-135℃, pressure to 1.2-1.4MPa, and maintain heat and pressure for 0.6-0.8min / mm to allow the melamine-phenolic resin composite adhesive inside the core layer to initially cure, laying the foundation for the strength of the core layer.

[0018] ② Gradient heating section: The temperature is raised to 142-144℃ at a uniform rate of 3-5℃ / min, and the pressure is maintained at 1.2-1.4MPa for 0.5-0.7min / mm to allow the adhesive to gradually and completely impregnate the core layer and achieve the initial bonding between the core layer and the surface and bottom layers.

[0019] ③ Heat preservation and curing section: Maintain 142-144℃, reduce pressure to 0.9-1.1MPa, and keep warm for 0.2-0.3min / mm to fully cure the adhesive and improve the bonding strength;

[0020] Secondary curing stage: Cool down to 100-110℃, maintain pressure at 0.8-1.0MPa, and hold for 3-5 minutes to achieve deep bonding between the surface layer, bottom layer and core layer, and avoid excessive cooling that could cause plywood deformation and delamination;

[0021] S4. Post-processing: After hot pressing, cut to standard size, grind the edges and corners to remove burrs, sand the surface until the flatness is ≤0.10mm / m, and obtain the finished product after passing inspection.

[0022] Advantages of this invention:

[0023] 1. Significantly Improved Surface Performance: The surface layer uses modified red phenolic resin impregnated paper with added nano-silica, nano-alumina, and tetra-needle zinc oxide whiskers. The three nanomaterials work synergistically, with nano-silica and nano-alumina improving the surface's wear resistance and hardness, and tetra-needle zinc oxide whiskers enhancing its toughness and scratch resistance, effectively solving the problems of easy wear and fading of existing red coatings. At the same time, the scientific preparation process of the modified red phenolic resin impregnated paper ensures that the nanomaterials are evenly dispersed and tightly bonded to the wood pulp paper. After drying, the moisture content of the surface layer is controlled at 6-8%, further improving the surface's stability and water resistance.

[0024] 2. Stable and high-strength core layer structure: The core layer adopts a symmetrical laminated structure of eucalyptus-poplar-eucalyptus layers. Eucalyptus veneer has high density and strength, while poplar veneer has good toughness and light weight. The combination of the two achieves a balance between strength and lightness. The symmetrical structural design, combined with the veneer grain crossing at 90°±5°, effectively offsets the anisotropy of the veneer, reduces core layer deformation, and improves the overall flatness and load-bearing capacity of the plywood. The total number of core veneer layers is 7 or 9, with uniform thickness and stable structure, making it suitable for concrete formwork with different thickness requirements.

[0025] 3. Strong adhesion and high water resistance: A specialized adhesive formulation is used. The surface, bottom, and core layers utilize phenolic resin adhesives with low free phenol and aldehyde content, resulting in high bonding strength and environmental friendliness. The core layer uses a melamine-phenolic resin composite adhesive with added polyethylene glycol and hexamethylenetetramine, significantly improving the adhesive's water resistance, toughness, and curing speed. Combined with a reasonable application rate (180-220 g / m² for the surface / bottom and core layers, and 220-260 g / m² for the core layer), the plywood layers are firmly bonded, preventing delamination and separation, making it particularly suitable for humid construction environments. Finished melamine-formaldehyde resin and finished phenolic resin are mechanically mixed at room temperature in a specific ratio, with a small amount of additives (toughening agent, curing agent, defoamer). The solid content is adjusted before use, eliminating the need for high-temperature copolymerization, resulting in a simple process with low equipment requirements.

[0026] 4. Scientific and controllable manufacturing process: The segmented hot-pressing molding process adopts a pre-pressing-gradient hot-pressing-secondary curing process. Pre-pressing removes air, and gradient heating ensures that the adhesive gradually impregnates and matures. Secondary curing achieves deep bonding of each layer and avoids hot-pressing defects. The hot-pressing parameters are precisely controlled, and the total hot-pressing time is calculated according to the thickness of the board blank, which is suitable for different specifications of products and ensures stable finished product quality. The post-processing steps of cutting, grinding, and sanding ensure that the flatness of the plywood is ≤0.10mm / m, which meets the high standard requirements of concrete formwork for surface flatness.

[0027] 5. High practicality and long service life: The red surface is highly identifiable, making it easy to classify and manage on the construction site, and it is not easy to fade or wear; the plywood has high overall strength, good stability, and strong water resistance, and can be reused many times, reducing construction costs; at the same time, the adhesive has low content of free phenol and free aldehyde, which meets environmental protection requirements and is suitable for the needs of modern green construction. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0029] <Example 1>

[0030] A red film-coated plywood for concrete formwork includes a surface layer, a core layer, and a bottom layer, with dimensions of 1830mm*915mm*14mm. Its specific structure is as follows:

[0031] Surface layer: Modified red phenolic resin impregnated paper, 0.25 mm thick, with a moisture content of 6% after drying; during preparation, 0.8 wt% nano silica (particle size 10 nm), 1.5 wt% nano alumina (particle size 50 nm), and 0.5 wt% tetraneedle-shaped zinc oxide whiskers (needle length 10 μm, needle diameter 0.5 μm) were added to the red phenolic resin slurry (201 phenolic resin). The mixture was first pre-dispersed at high speed of 800 r / min for 5 min, and then ultrasonically dispersed at 300 W power for 20 min. After impregnating 120 g / m² of bleached coniferous sulfate pulp paper with the slurry, it was dried at 110 °C to a moisture content of 6% to obtain the modified red phenolic resin impregnated paper.

[0032] Core layer: Symmetrical laminated structure of eucalyptus layer-poplar layer-eucalyptus layer. The eucalyptus layer consists of 2 layers of eucalyptus veneer (density 650kg / m³, moisture content 8%), and the poplar layer consists of 3 layers of poplar veneer (density 450kg / m³, moisture content 8%). The total number of core layers is 7. The cross angle of the veneer grain is 90° when the core is assembled.

[0033] Bottom layer: Undyed phenolic resin impregnated paper, 0.2mm thick.

[0034] Adhesives: Phenolic resin adhesive (free phenol content 0.5%, free aldehyde content 0.3%, solid content 45%) is used between the surface layer and the core layer, and between the bottom layer and the core layer, with a wet adhesive application rate of 180g / m². Melamine-phenolic resin composite adhesive is used between the single boards inside the core layer. During preparation, liquid melamine-formaldehyde resin (n-butanol etherified melamine-formaldehyde resin, solid content 60±2%) and liquid phenolic resin (2130 phenolic resin, solid content 75%) are mixed at a mass ratio of 1:3 and stirred at 300r / min at room temperature. 2.0wt% polyethylene glycol (molecular weight 380-420), 0.3wt% hexamethylenetetramine, and 0.1wt% silicone defoamer (model: BYK-024) are added. After stirring evenly, the solid content is adjusted to 40% with deionized water, cured at room temperature for 30min, and filtered through a 120-mesh filter. The wet adhesive application rate is 220g / m². The adhesive type is the same in subsequent embodiments.

[0035] The preparation method includes the following steps:

[0036] S1. Preparation of the surface and bottom layers: Prepare modified red phenolic resin impregnated paper and undyed phenolic resin impregnated paper according to the above parameters, and set them aside;

[0037] S2, Core layer assembly: Dry eucalyptus veneer and poplar veneer to a moisture content of 8%, and assemble them in a symmetrical structure of eucalyptus layer-poplar layer-eucalyptus layer. Apply phenolic resin glue between the surface layer, bottom layer and core layer, and apply melamine-phenolic resin composite glue inside the core layer. The glue is applied evenly.

[0038] S3. Segmented hot pressing molding:

[0039] Pre-compression stage: Pre-compress at 0.6 MPa and 80℃ for 10 min;

[0040] Gradient hot pressing stage: The total hot pressing time is 1.2 min / mm based on the slab thickness;

[0041] ① Core layer pre-solidification section: Heat to 133℃, pressure to 1.2MPa, heat and pressure hold for 0.6min / mm;

[0042] ② Gradient heating section: The temperature is increased to 142℃ at a constant rate of 3℃ / min, while maintaining a pressure of 1.2MPa for 0.5min / mm.

[0043] ③ Insulation and curing section: Maintain 142℃, reduce pressure to 0.9MPa, and maintain temperature for 0.2min / mm;

[0044] Secondary curing stage: Cool down to 100℃, maintain pressure at 0.8MPa, and hold for 3 minutes;

[0045] S4. Post-processing: After hot pressing, cut to standard size, grind edges and corners, and sand the surface until the flatness is ≤0.10mm / m to obtain the finished product.

[0046] <Example 2>

[0047] A type of red film-coated plywood for concrete formwork, comprising a surface layer, a core layer, and a bottom layer, with dimensions of 1830mm*915mm*14mm, and its specific structure is as follows:

[0048] Surface layer: Modified red phenolic resin impregnated paper, 0.28 mm thick, with a moisture content of 7% after drying; during preparation, 1.0 wt% nano silica (particle size 20 nm), 1.8 wt% nano alumina (particle size 65 nm), and 0.8 wt% tetraneedle-shaped zinc oxide whiskers (needle length 15 μm, needle diameter 0.8 μm) were added to the red phenolic resin slurry (201 phenolic resin). The mixture was first pre-dispersed at 900 r / min high-speed shearing for 5 min, and then ultrasonically dispersed at 400 W power for 25 min; after impregnation of 135 g / m² of bleached coniferous sulfate pulp paper in the slurry, it was dried at 115 °C to a moisture content of 7% to obtain the modified red phenolic resin impregnated paper.

[0049] Core layer: Symmetrical laminated structure of eucalyptus layer-poplar layer-eucalyptus layer. The eucalyptus layer consists of 2 layers of eucalyptus veneer (density 700kg / m³, moisture content 9%), and the poplar layer consists of 5 layers of poplar veneer (density 500kg / m³, moisture content 9%). The total number of core layers is 9. The cross angle of the veneer grain is 92° when the core is assembled.

[0050] Bottom layer: Undyed phenolic resin impregnated paper, 0.22mm thick.

[0051] Adhesives: Phenolic resin adhesive is used between the surface layer and the core layer, and between the bottom layer and the core layer, with a wet adhesive application rate of 200g / m². Melamine-phenolic resin composite adhesive is used between the single boards inside the core layer. The melamine-phenolic resin composite adhesive is prepared by mixing liquid melamine-formaldehyde resin and liquid phenolic resin at a mass ratio of 1:3.5, stirring at 350r / min at room temperature, adding 2.5wt% polyethylene glycol, 0.4wt% hexamethylenetetramine, and 0.15wt% silicone defoamer, stirring evenly, adjusting the solid content to 42% with deionized water, curing at room temperature for 30min, and filtering through a 120-mesh filter; the wet adhesive application rate is 240g / m².

[0052] The preparation method includes the following steps:

[0053] S1. Preparation of the surface and bottom layers: Prepare modified red phenolic resin impregnated paper and undyed phenolic resin impregnated paper according to the above parameters, and set them aside;

[0054] S2, Core layer assembly: Dry eucalyptus veneer and poplar veneer to a moisture content of 9%, and assemble them in a symmetrical structure of eucalyptus layer-poplar layer-eucalyptus layer. Apply phenolic resin glue between the surface layer, bottom layer and core layer, and apply melamine-phenolic resin composite glue inside the core layer. The glue is applied evenly.

[0055] S3. Segmented hot pressing molding:

[0056] Pre-compression stage: Pre-compress at 0.7 MPa and 85℃ for 12 min;

[0057] Gradient hot pressing stage: The total hot pressing time is 1.4 min / mm based on the slab thickness;

[0058] ① Core layer pre-solidification section: Heat to 134℃, pressure to 1.3MPa, heat and pressure hold for 0.7min / mm;

[0059] ② Gradient heating section: The temperature is increased to 143℃ at a constant rate of 4℃ / min, while maintaining a pressure of 1.3MPa for 0.6min / mm.

[0060] ③ Insulation and curing section: Maintain 143℃, reduce pressure to 1.0MPa, and maintain temperature for 0.25min / mm;

[0061] Secondary curing stage: Cool down to 105℃, maintain pressure at 0.9MPa, and hold for 4 minutes;

[0062] S4. Post-processing: After hot pressing, cut to standard size, grind the edges and corners, and sand the surface until the flatness is ≤0.10mm / m to obtain the finished product.

[0063] <Example 3>

[0064] A type of red film-coated plywood for concrete formwork, comprising a surface layer, a core layer, and a bottom layer, with dimensions of 1830mm*915mm*14mm, and its specific structure is as follows:

[0065] Surface layer: Modified red phenolic resin impregnated paper, 0.3 mm thick, with a moisture content of 8% after drying; during preparation, 1.0 wt% nano silica (30 nm particle size), 1.8 wt% nano alumina (80 nm particle size), and 0.8 wt% tetraneedle-shaped zinc oxide whiskers (20 μm needle length and 1 μm needle diameter) were added to the red phenolic resin slurry (201 phenolic resin). The mixture was first pre-dispersed at 1000 r / min for 5 min by high-speed shearing, and then ultrasonically dispersed at 500 W for 30 min. After impregnating 150 g / m² of bleached coniferous sulfate pulp paper with the slurry, it was dried at 120 °C to a moisture content of 8% to obtain the modified red phenolic resin impregnated paper.

[0066] Core layer: Symmetrical laminated structure of eucalyptus layer-poplar layer-eucalyptus layer. The eucalyptus layer consists of 2 layers of eucalyptus veneer (density 750kg / m³, moisture content 10%), and the poplar layer consists of 5 layers of poplar veneer (density 550kg / m³, moisture content 10%). The total number of core layers is 9. The cross angle of the veneer grain is 88° when the core is assembled.

[0067] Bottom layer: Undyed phenolic resin impregnated paper, 0.25mm thick.

[0068] Adhesives: Phenolic resin adhesive is used between the surface layer and the core layer, and between the bottom layer and the core layer, with a wet adhesive application rate of 220 g / m². Melamine-phenolic resin composite adhesive is used between the single boards inside the core layer. During preparation, liquid melamine-formaldehyde resin and liquid phenolic resin are taken at a mass ratio of 1:4, stirred and compounded at 400 r / min at room temperature, and 3.0 wt% polyethylene glycol, 0.5 wt% hexamethylenetetramine, and 0.2 wt% silicone defoamer are added. After stirring evenly, the solid content is adjusted to 45% with deionized water, cured at room temperature for 30 min, and filtered through a 120 mesh to obtain the final product; the wet adhesive application rate is 260 g / m².

[0069] The preparation method includes the following steps:

[0070] S1. Preparation of the surface and bottom layers: Prepare modified red phenolic resin impregnated paper and undyed phenolic resin impregnated paper according to the above parameters, and set them aside;

[0071] S2, Core layer assembly: Dry eucalyptus veneer and poplar veneer to a moisture content of 10%, and assemble them in a symmetrical structure of eucalyptus layer-poplar layer-eucalyptus layer. Apply phenolic resin glue between the surface layer, bottom layer and core layer, and apply melamine-phenolic resin composite glue inside the core layer. The glue is applied evenly.

[0072] S3. Segmented hot pressing molding:

[0073] Pre-compression stage: Pre-compress at 0.8 MPa and 90℃ for 15 min;

[0074] Gradient hot pressing stage: The total hot pressing time is 1.6 min / mm based on the slab thickness;

[0075] ① Core layer pre-solidification section: Heat to 135℃, pressure to 1.4MPa, heat and pressure hold for 0.8min / mm;

[0076] ② Gradient heating section: The temperature is increased to 144℃ at a constant rate of 5℃ / min, while maintaining a pressure of 1.4MPa for 0.7min / mm.

[0077] ③ Insulation and curing section: Maintain 144℃, reduce pressure to 1.1MPa, and maintain temperature for 0.3min / mm;

[0078] Secondary curing stage: Cool down to 110℃, maintain pressure at 1.0MPa, and hold for 5 minutes;

[0079] S4. Post-processing: After hot pressing, cut to standard size, grind the edges and corners, and sand the surface until the flatness is ≤0.10mm / m to obtain the finished product.

[0080] <Comparative Example 1>

[0081] Commercially available red laminated plywood, dimensions 1830mm*915mm*14mm.

[0082] Performance Testing

[0083] The plywood of Examples 1-3 and Comparative Example 1 were subjected to performance tests. The test standards and results are shown in the table below:

[0084] Test Project Standard requirements Example 1 Example 2 Example 3 Comparative Example 1 Surface abrasion resistance (number of revolutions) ≥4000 5035 4986 5121 4113 Bond strength (MPa) ≥0.7 1.3 1.3 1.4 0.8 Water resistance (retention rate of adhesive strength after 24-hour immersion) ≥70% ≥85% ≥88% ≥89% ≥71% Number of times it can be reused - 21 22 24 ≤12 Static bending strength (average along the grain) ≥50 MPa 58.8 58.6 59.4 53 Static bending strength (average cross-sectional strength) ≥30 MPa 35.6 37.5 38.2 32 Elastic modulus (average along the grain) ≥6000 MPa 6880 6890 6930 6120 Elastic modulus (average along the grain) ≥4500 MPa 4890 4920 4950 4560 Moisture content 5-14% 9.4% 8.8% 9.1% 12.5% Impregnation and peeling performance The cumulative peel length between the impregnated paper and the plywood surface on each side shall not exceed 25mm. The cumulative peel length between the impregnated veneer and the plywood surface on each side does not exceed 15mm. The cumulative peel length between the impregnated veneer and the plywood surface on each side does not exceed 15mm. The cumulative peel length between the veneer impregnated paper and the plywood surface on each side does not exceed 14mm. The cumulative peel length between the impregnated veneer and the plywood surface on each side does not exceed 24mm.

[0085] The red film-coated plywood for concrete formwork of this invention has a reasonable structure, a simple and controllable manufacturing process, and requires only commercially available conventional raw materials, making it easy to mass-produce. The product has excellent comprehensive performance, strong wear resistance, water resistance, and structural stability, and can be reused many times, effectively reducing construction costs. The red surface facilitates identification and management on the construction site, and the adhesive meets environmental protection standards, adapting to the needs of modern green building construction. It can be widely used for formwork support in various concrete building constructions, and has extremely high industrial practicality and market promotion value.

[0086] The above embodiments are only for illustrating the technical solutions and features of the present invention, and are intended to enable those skilled in the art to implement them better. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention are within the scope of protection of the present invention. The embodiments not described in detail are prior art.

Claims

1. A modified red phenolic resin impregnated paper for concrete formwork, characterized in that, The preparation method is as follows: 0.8-1.2wt% nano-silica, 1.5-2.0wt% nano-alumina, and 0.5-1.0wt% tetrane needle-like zinc oxide whiskers are added to red phenolic resin adhesive. The mixture is first pre-dispersed by high-speed shearing at 800-1000r / min for 5min, and then ultrasonically dispersed at 300-500W for 20-30min. After impregnating wood pulp paper with the adhesive, it is dried at 110-120℃ to a moisture content of 6-8% to obtain modified red phenolic resin impregnated paper. Among them, the particle size of nano-silica is 10-30nm, the particle size of nano-alumina is 50-80nm, and the needle length of tetra-needle zinc oxide whiskers is 10-20μm and the needle diameter is 0.5-1μm.

2. The modified red phenolic resin impregnated paper for concrete formwork according to claim 1, characterized in that, The modified red phenolic resin impregnated paper has a thickness of 0.25-0.3 mm and a moisture content of 6-8% after drying.

3. The modified red phenolic resin impregnated paper for concrete formwork according to claim 1, characterized in that, The wood pulp paper is bleached sulfate pulp paper from softwood trees, with a basis weight of 120-150 g / m².

4. A film-coated plywood for concrete formwork, comprising a surface layer, a core layer, and a bottom layer, characterized in that, The surface layer is a modified red phenolic resin impregnated paper with added nano-silica, nano-alumina and tetra-needle zinc oxide whiskers. The core layer is a symmetrical laminated structure of eucalyptus layer-poplar layer-eucalyptus layer, with the eucalyptus layer consisting of 2 layers of eucalyptus veneer, the poplar layer consisting of 3 or 5 layers of poplar veneer, and the total number of veneer layers in the core layer being 7 or 9. The bottom layer is undyed phenolic resin impregnated paper; The surface layer and core layer, as well as the bottom layer and core layer, are bonded with phenolic resin adhesive, with a wet adhesive application rate of 180-220 g / m². The single boards inside the core layer are bonded with melamine-phenolic resin composite adhesive, with a wet adhesive application rate of 220-260 g / m².

5. A film-coated plywood for concrete formwork according to claim 4, characterized in that, The preparation method of the melamine-phenolic resin composite adhesive is as follows: liquid melamine-formaldehyde resin and liquid phenolic resin are taken at a mass ratio of 1:3-1:4 and mixed at room temperature with stirring at 300-400 r / min; 2.0-3.0 wt% polyethylene glycol, 0.3-0.5 wt% hexamethylenetetramine, and 0.1-0.2 wt% organosilicon defoamer are added to the mixed solution and stirred to disperse evenly; the solid content is adjusted to 40-45% with deionized water, cured at room temperature for 30 min, and filtered through a 120 mesh to obtain the composite adhesive.

6. A film-coated plywood for concrete formwork according to claim 4, characterized in that, The moisture content of the eucalyptus veneer is 8-10%, the moisture content of the poplar veneer is 8-10%, and the cross angle of the veneer grain is 90°±5° when the core layer is assembled.

7. A film-coated plywood for concrete formwork according to claim 4, characterized in that, The thickness of the bottom layer is 0.2-0.25 mm.

8. A red film-coated plywood for concrete formwork according to claim 4, characterized in that, The density of the eucalyptus veneer is ≥650kg / m³, and the density of the poplar veneer is ≥450kg / m³.

9. The manufacturing process of a film-coated plywood for concrete formwork according to any one of claims 4-8, characterized in that, Includes the following steps: S1. Surface and bottom layer preparation: Prepare modified red phenolic resin impregnated paper and undyed phenolic resin impregnated paper; S2. Core layer assembly: After the eucalyptus veneer and poplar veneer are dried to a moisture content of 8-10%, they are assembled in a symmetrical structure of eucalyptus layer-poplar layer-eucalyptus layer. Phenolic resin glue is applied between the surface layer, bottom layer and core layer, and melamine-phenolic resin composite glue is applied inside the core layer. S3. Segmented hot pressing molding: Pre-compression stage: Pre-compress at 0.6-0.8MPa and 80-90℃ for 10-15 minutes to allow the adhesive to initially impregnate and expel air; Gradient hot pressing stage: A three-stage gradient heating process is adopted, with a total hot pressing time of 1.2-1.6 min / mm based on the slab thickness. ① Core layer pre-solidification section: Heat to 133-135℃, pressure to 1.2-1.4MPa, and maintain heat and pressure for 0.6-0.8min / mm; ② Gradient heating section: Heat to 142-144℃ at a uniform rate of 3-5℃ / min, maintain pressure of 1.2-1.4MPa, and continue for 0.5-0.7min / mm; ③ Heat preservation and curing section: Maintain 142-144℃, reduce pressure to 0.9-1.1MPa, and maintain heat for 0.2-0.3min / mm; Secondary curing stage: Cool down to 100-110℃, maintain pressure at 0.8-1.0MPa, and hold for 3-5 minutes to achieve deep bonding between the surface layer, bottom layer and core layer; S4. Post-processing: After hot pressing, cut to standard size, grind the edges and corners, and sand the surface until the flatness is ≤0.10mm / m to obtain the finished product.