Formaldehyde-free particleboard with improved cold-press adhesion and preparation method thereof

Abstract

The present invention provides a formaldehyde-free particleboard with improved cold-pressing adhesion and a preparation method thereof. The preparation method comprises mixing a polymer emulsion containing modified inorganic nanoparticles and a polyisocyanate with wood particles, then paving and forming the mixture of the polymer emulsion, polyisocyanate and wood particles, cold-pressing the mixture under a certain pressure into a slab with a certain shaping ability, and then performing a subsequent high-temperature hot pressing process to finally obtain the formaldehyde-free particleboard. The mixture formed in the preparation process of the present invention can have good looseness between the particles before pre-pressing (cold pressing) and good slab forming ability (cohesion) after pre-pressing. This adhesion is sufficient to ensure that the mixture slab after cold pressing can smoothly reach the subsequent hot pressing molding stage with a complete appearance. The preparation method of the present invention can improve the production efficiency and workability of formaldehyde-free particleboard.

Description

Technical Field

[0001] The invention relates to the technical field of particleboard preparation, in particular to a formaldehyde-free particleboard with improved cold-pressing adhesion and a preparation method thereof. Background Art

[0002] The most commonly used adhesives in the traditional wood-based panel industry include urea-formaldehyde adhesives, phenol-formaldehyde adhesives, and melamine-formaldehyde adhesives. Since these adhesives all use formaldehyde as a raw material, the wood-based panels produced from them often release formaldehyde during use, contributing to indoor air pollution. Using PMDI as an adhesive, on the other hand, eliminates this formaldehyde emission issue, eliminating its potential health hazards. Furthermore, isocyanate adhesives offer higher bonding strength, better water resistance, and improved deformation resistance. However, PMDI itself lacks adhesiveness, resulting in a lack of adhesion between the particles after mixing with wood shavings. In actual production, the cold-pressed slabs must pass through a series of discontinuous conveying devices before reaching the hot-pressing stage. This process requires the cold-pressed slabs to have a certain degree of firmness, meaning that the wood particles must adhere to each other after cold pressing. This ensures that the slabs maintain their shape and surface integrity during transport, preventing cracking, scattering, or breakage. This ensures that the hot-pressing process results in a smooth, well-formed board. Therefore, when using PMDI to prepare particleboard, it is often necessary to mix in a certain amount of tackifier to improve the adhesion of the mixture of wood particles and PMDI, so that the slab can pass smoothly through the discontinuous conveying device and finally obtain a complete and intact board.

[0003] However, in the particleboard production process, especially when using air-laying or diamond roller laying, if the wood particles after gluing are adhered into clumps or the wood particles contain a large amount of fluffy fibers (mainly from ground bark), it will cause the screen in the air-laying to be blocked or make it difficult for the diamond roller to separate the clumps, affecting normal production and reducing the quality of the particleboard. Therefore, the ideal situation is that the wood particles after gluing have a certain viscosity to make the fluffy fibers stick together into large particles, preventing the accumulation of hanging nets in the air-laying, and can maintain good looseness between the wood particles before cold pressing, so that they do not stick to each other into clumps. Only after cold pressing, the adhesion between the particles of the mixture is enhanced, and it has a certain cohesive force to maintain the prototype, that is, the molding strength, and will not easily scatter or break.

[0004] Currently, no patents addressing the use of PMDI as an adhesive in particleboard production address all of the above issues simultaneously. Existing patents are limited to providing adhesion to the mixture of isocyanate adhesives and wood particles, without considering the need for the mixture to have good bulkiness before cold pressing. For example, patent CN109321178 A discloses an adhesive that can improve the forming strength of PMDI particleboard mats. The adhesive comprises an acrylate copolymer emulsion, acetyl tributyl citrate, polydimethylsiloxane, and polyoxyethylene sorbitol ester. The functional groups in the acrylate copolymer emulsion undergo a prepolymerization reaction with the NCO groups in PMDI to form a network cross-linked structure and self-adhesive strength, effectively improving the mat forming strength of PMDI particleboard. Patent US008895643B2 discloses an aqueous dispersion for use as a tackifier in a mixture of PMDI and wood particles. This aqueous dispersion not only improves the adhesion between particles in the sizing mixture but also enhances the internal bonding strength of the board. The aqueous dispersion can be a PUD dispersion, polyvinyl alcohol resin, vinyl ester polymer, acrylic (ester) polymer, rubber resin, etc. Patent CN104960064B discloses an isocyanate-reactive graft polyol. The graft polyol comprises an isocyanate-reactive component and polymer particles. The graft polyol imparts excellent cold adhesion during the preparation of wood particle composite products and reacts with isocyanates to significantly improve the physical properties of the resulting wood particle composite products. However, the currently disclosed tackifier components for providing cold adhesion in isocyanate particleboards, similar to those described in the above-mentioned patents, are only limited to providing cold pressing adhesion during the preparation of wood particle composite products, or in addition, can simultaneously improve the physical properties of the final board material of the wood particle composite products (such as internal bonding strength, elastic modulus, moisture resistance, etc.). They do not take into account the problems of screen clogging caused by the airflow laying process when the wood particle material, such as wood particles, has a high content of velvety particles or the viscosity of the tackifier is very high, or the agglomeration of wood particles caused by the use of diamond rollers with a large number of grades, and the difficulty in grading causing production stoppages.

[0005] However, in the actual production of PMDI-type formaldehyde-free particleboard, it is often encountered that the viscosity of the tackifier is too high, resulting in the particles sticking together into large clumps after gluing, or the particle raw materials used contain a large amount of fluffy substances formed after grinding bark, which causes the screen to be blocked, causing production to be stuck and reducing production efficiency. Summary of the Invention

[0006] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a preparation process with improved cold pressing adhesion of particles after sizing in the production of formaldehyde-free particleboard using polyisocyanate, which can maintain the looseness of the wood unit material before cold pressing and will not have a negative impact on the mechanical properties and other properties of the wood particle composite board.

[0007] Another object of the present invention is to provide such formaldehyde-free particleboard having improved cold press adhesion.

[0008] In order to achieve the above object of the invention, the present invention adopts the following technical solutions:

[0009] A method for preparing formaldehyde-free particleboard with improved cold press adhesion comprises the following steps:

[0010] 1) mixing polyisocyanate, polymer emulsion, and optionally water with wood chips to form a mixture;

[0011] 2) paving and shaping the mixture formed in step 1), cold pressing it into a slab with a setting ability, and then performing a high-temperature hot pressing process to finally obtain a formaldehyde-free particleboard;

[0012] The polymer emulsion contains hydrophobically modified inorganic nanoparticles, and the polymer emulsion is applied to the surface of the wood particles in an amount of 0.5% to 10% of the total weight of the absolutely dry wood particles.

[0013] In the present invention, the polymer emulsion and the polyisocyanate can be applied separately in any order, or can be mixed and applied simultaneously to the surface of the wood shavings. The bone-dry wood shavings generally refer to wood shavings with a water content of less than 1%.

[0014] In the present invention, the polymer emulsion is added in an amount of 0.5% to 10% of the total weight of the absolute dry wood shavings (with a water content of less than 1 wt%), for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, etc., preferably 1% to 5%. Within this addition amount range, the polymer emulsion has no negative effect on the mechanical properties of the wood composite board obtained after pressing.

[0015] In the present invention, a mixture of a polymer emulsion, a polyisocyanate, and wood shavings is paved and formed, then cold-pressed under a certain pressure into a slab with a certain degree of setting ability, and then hot-pressed at high temperature to ultimately produce a formaldehyde-free particleboard. The paving, cold-pressing, and hot-pressing processes are not particularly limited and may be referred to in the prior art, such as CN 109321178A.

[0016] The mixture of the polymer emulsion, polyisocyanate and wood shavings of the present invention maintains good looseness between the shavings after sizing before cold pressing, and only shows adhesion between the mixture particles after being subjected to a pressure greater than 0.01 MPa. This adhesion is sufficient to ensure that the cold-pressed mixture slab can smoothly reach the subsequent hot pressing molding stage with a complete appearance. In the present invention, this property is referred to as cold pressing adhesion, and its testing method will be described in detail below.

[0017] It will be understood by those skilled in the art that, in addition to polyisocyanate, polymer emulsion and water, components such as wax emulsion, fungicide or internal release agent may also be applied to the wood particles, which can be routinely selected by those skilled in the art based on the processing properties of the particleboard, etc.

[0018] In the present invention, the polymer is a homopolymer or copolymer obtained by free radical polymerization of monomers containing carbon-carbon double bonds; preferably, the monomers containing carbon-carbon double bonds are selected from one or more of ethylene, vinyl acetate, (alkyl) acrylic acid or acrylate (alkyl) monomers, and styrene monomers; more preferably, the monomers containing carbon-carbon double bonds are selected from at least two or more of C2-C8 ethylene, vinyl acetate, and (alkyl) acrylic acid or acrylate (alkyl) monomers.

[0019] In a preferred embodiment, the polymer emulsion comprises: 30% to 70% by weight of a carbon-carbon double bond copolymer based on the total weight of the emulsion components. Preferably, the copolymer is selected from one or a mixture of more than one of polyalkyl acrylate copolymers, alkylstyrene-alkyl acrylate copolymers, and alkyl acrylate-vinyl acetate copolymers; more preferably, 40% to 60% by weight of an alkyl acrylate-vinyl acetate copolymer.

[0020] In the present invention, the polymer can be obtained by free radical polymerization of the following monomer components: styrene, vinyl acetate, alkyl acrylate, alkyl methacrylate, aryl acrylate and mixtures thereof; it may also contain one or more cross-linking monomers containing two or more functional groups, such as, but not limited to, (alkyl)acrylates containing one or more carbon-carbon double bonds or hydroxyl groups or carboxyl groups or amide groups or anhydride groups, alkyl (alkyl) acrylates, acrylic acid and mixtures thereof. The addition of a cross-linking agent component can improve the strength and toughness of the material, but it is not a necessary component.

[0021] The polymer emulsion can be prepared by adding modified inorganic nanoparticles to the polymerization reaction system before or during the polymer emulsion preparation reaction. At this time, the modified inorganic nanoparticles can also play a role in stabilizing the emulsion, that is, "PICKERING emulsion". It can also be dispersed into the polymer emulsion through physical blending after the polymerization reaction is completed. Regardless of the method, the expected application effect of the present invention can be achieved.

[0022] Among them, the polymer emulsion can provide good adhesion for wood particle materials of various forms and tree species. This adhesion is more obvious after being subjected to a certain pressure (>0.01MPa) and can maintain good looseness before pre-pressing. It is suitable for paving methods such as diamond roller paving with finer grading and air flow paving. When the particle unit material used contains a large amount of fine powder or fluffy particles (length and width less than 1mm), the preparation method of the present invention can effectively prevent the screen clogging caused by air flow paving, or the agglomeration of fine powder leading to defects on the surface of the wood composite product after hot pressing.

[0023] In some specific embodiments, the hydrophobically modified inorganic nanoparticles are selected from any one of hydrophobically modified nano-silica particles and hydrophobically modified nano-metal oxide particles; preferably, the particle size of the hydrophobically modified inorganic nanoparticles is between 1 nm and 100 nm.

[0024] In some specific embodiments, hydrophobically modified silica particles (particles) can include various coupling agents (such as silane coupling agents, titanate coupling agents, etc.) modified nano silica particles, polymer grafted modified nano silica particles, organic acid modified nano silica particles, alcohol ester modified nano silica particles, etc.; hydrophobically modified nano metal oxide particles (particles) can include, for example, hydrophobically modified nano zinc oxide particles, hydrophobically modified nano aluminum oxide particles, hydrophobically modified nano Fe2O3 particles, etc. The hydrophobic modification method can be selected from alkyl acid modification, organosilicon modification, polyethylene glycol modification, surfactant modification or other arbitrary modification methods to achieve the purpose of changing the surface hydrophilicity and hydrophobicity. The above method of hydrophobically modified inorganic nanoparticles is not limited to the above-mentioned method, and any other method for the purpose of changing the surface hydrophilicity and hydrophobicity of inorganic nanoparticles falls within the scope of the requirements listed in the present invention.

[0025] In some specific embodiments, the hydrophobically modified inorganic nanoparticles can be added before the polymer emulsion polymerization process, can be added during the polymerization process, or can be added after the polymerization reaction is completed. Regardless of the addition method, the modified inorganic nanoparticles can be stable in the polymer emulsion and will not stratify during long-term storage (more than 6 months at room temperature).

[0026] In some specific embodiments, the polymer emulsion comprises the following components, based on the total mass of the polymer emulsion:

[0027] 1) 30% to 70% by weight of a polymer obtained by free radical polymerization of one or more monomers containing carbon-carbon double bonds, preferably one or more mixtures selected from polyvinyl acetate polymers, poly(meth)acrylate polymers, butadiene-styrene polymers, polyvinyl acetate-(meth)acrylate polymers, aqueous PUD dispersions, and the like;

[0028] 2) 0.05% to 2% by weight of hydrophobically modified inorganic nanoparticles;

[0029] 3) The balance is water.

[0030] In a preferred embodiment, the polymer emulsion comprises:

[0031] a) hydrophobically modified inorganic nanoparticles accounting for 0.1% to 1% by weight of the total weight of the polymer emulsion; the hydrophobically modified inorganic nanoparticles are selected from hydrophobically modified nano-silica or metal oxide particles;

[0032] b) a polymer obtained by free radical polymerization of one or more monomers containing carbon-carbon double bonds, accounting for 45% to 60% by weight of the total weight of the polymer emulsion; preferably, the polymer is selected from at least one of polyvinyl acetate polymer, poly(meth)acrylate polymer, polyvinyl acetate-(meth)acrylate polymer, etc.;

[0033] c) The balance is water.

[0034] In some specific embodiments, the glass transition temperature of the polymer is -30°C to 60°C, preferably -30°C to 40°C, and more preferably -15°C to 45°C.

[0035] In a specific embodiment, the particle size of the polymer latex particles is 200 nm-2000 nm, preferably 500 nm-1500 nm.

[0036] In some specific embodiments, the polyisocyanate is selected from diphenylmethane diisocyanate (MDI), polymeric diphenylmethane diisocyanate (PMDI) or a combination thereof, preferably PMDI, with an NCO group content of 27% to 33%.

[0037] In some specific embodiments, the added mass ratio of the polymer emulsion to the polyisocyanate is between 0.1 and 5:1, for example, 0.1:1, 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, etc., preferably between 0.2 and 4:1. Within the addition ratio range, the polymer emulsion does not have any negative adverse effects on the mechanical properties of the wood composite board obtained after pressing.

[0038] On the other hand, the above-mentioned preparation method produces a formaldehyde-free particleboard with improved cold press adhesion.

[0039] According to the preparation method of the present invention, the initial adhesion of the wood chips after sizing can reach a very good level according to the initial adhesion test experiment described in the present invention. In the screen blocking test, common types of wood chips materials but not limited to those listed in the embodiments and comparative examples can be used to achieve the effect of completely non-blocking the screen.

[0040] Compared to currently disclosed inventions in related fields, the method for preparing formaldehyde-free particleboard of the present invention provides a process with adjustable cold-pressing adhesion, which can not only provide the blank with good cohesion after cold pressing, but also maintain the good looseness of the wood unit material before cold pressing, without any negative impact on the mechanical properties of PMDI-type wood particle composite products, and can greatly increase the pressing speed of wood particle composite products. In addition to being applicable to particleboard, it can also be applied to oriented strand board (OSB) and particle board, such as low-density particle board (LDF), medium-density particle board (MDF), high-density particle board (HDF), etc. The particleboard products described in the present invention can be in the form of other wood composite boards, not limited to those mentioned herein and in the embodiments.

[0041] Compared with the prior art, the preparation method of the present invention has the following beneficial effects:

[0042] Regarding the preparation process of wood particle composite products using PMDI as an adhesive, existing published patents only focus on the need to provide sufficient cold-pressed adhesion to the wood particle material after cold pressing, or to provide better mechanical properties or other properties after hot pressing, but ignore the problems of wood particle agglomeration caused by air-laying or fine-graded diamond roller paving processes, which leads to screen blockage or uneven grading. The formaldehyde-free particleboard preparation process of the present invention solves the above problems and is more tolerant to the morphology of the particle particles and the type of wood. Even particle materials containing a large number of fluffy particles will not cause screen blockage problems, nor will the slab stick to the conveyor belt due to excessive viscosity. At the same time, it will not have a negative impact on the mechanical properties of the particleboard obtained after hot pressing. The viscosity-enhancing composition provided by the present invention improves the stability of the particleboard preparation process and has good applicability to various types of particle materials, which is not addressed in the existing published patents.

[0043] The preparation method of the formaldehyde-free particle board of the present invention can not only provide the wood particle material after cold pressing with sufficient viscosity, but also maintain good looseness between the particles after gluing before cold pressing, thereby preventing the tendency of agglomeration, and the fluffy particle particles can be bonded together to prevent clogging due to static adsorption on the screen.

[0044] The preparation method of the formaldehyde-free particleboard of the present invention is applicable to most forms of wood particle raw materials, can improve the stability and production efficiency of the production equipment of the formaldehyde-free particleboard, and has a wide range of applicability to the shape and size of the particles and the type of wood. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 1 is a diagram of the internal material morphology of the particleboard before and after cold pressing in Example 1;

[0046] Figure 2 The following are actual pictures of the cold press adhesion test results of Example 1 and Comparative Examples 1, 2, and 5;

[0047] Figure 3 These are actual pictures of the screen blocking test results of Example 1 and Comparative Examples 1 and 4. DETAILED DESCRIPTION

[0048] In order to better understand the technical solution of the present invention, the following examples will further illustrate the method provided by the present invention, but the present invention is not limited to the listed examples, and should also include any other known changes within the scope of the claims of the present invention.

[0049] Unless otherwise specified, all raw materials in the present invention can be purchased from the market.

[0050] The following test methods are used in the present invention to characterize the various properties of the board making process and the final particleboard, including:

[0051] A) Cold Press Adhesion

[0052] Cold pressing adhesion refers to the adhesion between particles of a wood particle material after being mixed with PMDI and a tackifying composition. In the present invention, cold pressing adhesion is expressed by the ability of the slab formed after cold pressing to maintain its original shape. The specific test method is as follows:

[0053] The cold press adhesion test is conducted at a temperature of (35±1)°C and is evaluated using the "inclined push test method". The specific steps are as follows:

[0054] 1) Weigh 2 kg of wood pellets into a glue mixer. After stirring, use a spray gun to apply a certain amount of PMDI, tackifying composition, and water to the wood pellets in the glue mixer.

[0055] 2) After the sizing is completed, take about 1.5 grams of wood shavings and use a moisture tester to test the moisture content of the wood shavings after sizing. If the moisture content is lower than the design range (in this test, the moisture content is uniformly (15±1)%), add water according to the test results; if the moisture content is higher than the design range, repeat steps 1) and 2) until the moisture content is within the design range.

[0056] 3) Weigh 500g of the glued wood particles and place them in a 30*30cm frame, evenly spread them out, and then squeeze them into shape;

[0057] 3) Cover the formed slab with steel plate and place it on the press. Place 8mm thick thickness gauges on both sides of the slab. Set the press pressure to 4.0MPa and the time to start cold pressing for 20s.

[0058] 4) Remove the cold-pressed slab and place it flat on a 45° wooden slope. Gently push the slab down the slope within 5 seconds and use a ruler to measure the width of the slab when it begins to break (i.e., the length of the first broken piece).

[0059] 5) Feel the looseness of the slab by hand and score it from 5 to 1 according to the looseness of the slab. 5 points means the slab is hard and can be lifted into large pieces, and 1 point means it is loose and non-sticky.

[0060] 6) Repeat steps 2-5 and repeat the test experiment 3 times.

[0061] The fewer the number of broken pieces of the slab, the longer the length at the beginning of the breakage, and the higher the looseness score, the better the cold pressing adhesion of the tackifier.

[0062] B) Agglomeration between wood pellet units (screen blocking test)

[0063] The stronger the agglomeration between the wood particle unit materials after gluing and before cold pressing, the more likely it is that small particles will agglomerate into large particles during subsequent paving, leading to screen clogging. In the present invention, the ability of the tackifier to agglomerate the wood particle unit materials before cold pressing is evaluated by characterizing the screen clogging of the wood particle materials after gluing.

[0064] 1) Weigh 3 kg of wood pellets after sizing;

[0065] 2) Turn on the fan, place the wind speed tester close to the center of the screen, and adjust the wind speed until the wind speed tester reading is stable at 3m / s;

[0066] 3) Evenly add the weighed glued wood pellets to the feed port of the blower, and observe the blockage of the screen while adding the material. If the wood pellets spray back from the feed port or the material is difficult to feed, stop feeding, take pictures and record the blockage of the screen.

[0067] Backsplash occurs when a large area of ​​mesh blockage occurs, causing the wood pellets to spray back in the opposite direction of the wind. The smaller the amount of material fed when backsplash occurs, the better the adhesion enhancer binds the wood pellets, making it more likely to cause mesh blockage during paving. The larger the mesh blockage area, the more serious the blockage.

[0068] C) Mechanical properties of wood particle composite products after hot pressing

[0069] The wood particle material with a gluing process of 30*30cm was laid and hot pressed at 220℃ and 4.0MPa for 95s to prepare wood particle composite products. The thickness of the board after hot pressing was controlled at 12mm and the density was 780g / cm 3 The mechanical properties of the wood particle composite products after hot pressing were evaluated with reference to the test methods for internal bonding strength, static bending strength and elastic modulus in the national standard GBT 17657-2013.

[0070] The technical solution of the present invention is further illustrated below with reference to specific embodiments, but does not constitute any limitation.

[0071] Example 1

[0072] 1) Preparation of polymer emulsion A

[0073] Weigh 26g polyvinyl alcohol 0588 type, 0.8g NaHCO3 and 1.21g KH570 modified nano-silica (purchased from Shandong Yunsheng Chemical Co., Ltd.) in a four-necked flask, then add 210g deionized water at the bottom of the kettle, heat in a water bath to 90°C, dissolve the polyvinyl alcohol under stirring, and disperse the modified nano-silica. Cool to 75°C, drip 300g vinyl acetate monomer, and simultaneously drip 42g of Na2S2O8 (mass concentration 2.4%) and Na2HSO3 solution (mass concentration 1.96%) 21g, the dropping time being controlled at 5-6 hours. After completion of the dropwise addition, add 5% tert-butyl hydroperoxide solution 5g and 2.5% hydrosulfite solution 5g with mass concentration and be incubated for 0.5 hour, then reduce to room temperature discharging.

[0074] 2) Gluing

[0075] Refer to the table below to weigh various raw materials (the amount of each raw material added in the table is the percentage of the weight of the absolutely dry wood chips) in a glue mixer. Among them, the poly-PMDI is Wanhua Chemical CW20 model. Use a spray gun to spray the polymer emulsion, PMDI and water simultaneously on the surface of the wood chips under stirring. Adjust the moisture content of the wood chips after gluing to about (15±1)%.

[0076]

[0077] 3) Cold pressing

[0078] The surface layer and the core layer are weighed and laid in a ratio of 4:6 by mass. The weighed wood chips, the mixture of polymer emulsion and PMDI are placed in a 30*30cm fixed frame, evenly spread and moderately squeezed, then covered with a steel plate and placed on a press. An 18mm thick thickness gauge is placed on both sides of the slab, the pressure of the press is set to 4.0MPa, and the time is 20s to start pressing to form a formed slab; the cold pressing adhesion and screen blocking of the cold tackifying composition are tested with reference to the above-mentioned evaluation method.

[0079] 4) Hot pressed board

[0080] Hot pressing was performed at 220°C according to the set pressure curve process, and the hot pressing factor was selected as 6s / mm. After hot pressing, multiple plates were obtained and the mechanical properties of the plates were tested according to the above-mentioned method.

[0081] Example 2

[0082] 1) Preparation of polymer emulsion B

[0083] Weigh 175g of butyl acrylate, 120g of vinyl acetate, 5g of acrylic acid, and 1.72g of fatty alcohol-modified nano-silica (purchased from Shanghai Huijing Nano Technology) and mix them evenly. Add 0.65g of polyvinyl alcohol 1788, 1.9g of OP-10, 0.8g of NaHCO3, and 320g of deionized water to the reactor, heat to 80°C and stir until the polyvinyl alcohol is dissolved. Start adding the monomer mixed solution and a 3% sodium persulfate aqueous solution by mass. The monomer mixed solution is added dropwise for 4 hours, and the initiator solution is added dropwise for 4.5 hours. After the addition is completed, keep warm for 30 minutes and then cool and discharge.

[0084] 2) Pressed wood particle composite products b

[0085] Adjust the sizing ratio of each component as shown in the following table; refer to 3) in Example 1 for cold pressing and evaluation:

[0086] Shavings type CW20 / % polymer emulsion Moisture content / % surface layer Poplar surface shavings 4.1 1.1 16 core layer Poplar core wood shavings 2.4 0 3.2

[0087] 3) Hot pressed board

[0088] Hot pressing was performed at 220°C according to the set pressure curve process, and the hot pressing factor was selected as 7s / mm. After hot pressing, multiple plates were obtained and the mechanical properties of the plates were tested according to the aforementioned method.

[0089] Example 3

[0090] 1) Preparation of polymer emulsion C

[0091] 200g of vinyl acetate, 8g of polyvinyl alcohol 1788 type, 0.6g of NaHCO3, and 0.4g of sodium bisulfite were added to a reaction flask, and N2 was introduced and replaced with ethylene three times. The temperature was raised to 80°C and the polyvinyl alcohol was stirred until it dissolved. Ethylene was introduced and stirring was started. The reaction pressure was controlled at 3.5MPa and the reaction temperature was at 70°C. 30g of a 2.5% aqueous hydrogen peroxide solution was added dropwise for 3-4 hours. After the addition was complete, stearic acid-modified nano zinc oxide particles (purchased from Darcy Nanotechnology Co., Ltd., 60nm) were added and stirred for 20 minutes, and the temperature was lowered and the material was discharged.

[0092] 2) Pressed wood particle composite products c

[0093] The sizing ratio of each component was adjusted as shown below, and cold pressing and evaluation were performed with reference to 3) in Example 1.

[0094] Shavings type CW20 / % polymer emulsion Moisture content / % surface layer Poplar surface shavings 3.5 2.5 16.5 core layer Poplar core wood shavings 3.0 0 4.0

[0095] 3) Hot pressed board

[0096] Hot pressing was performed at 220°C according to the set pressure curve process, and the hot pressing factor was selected as 8s / mm. After hot pressing, multiple plates were obtained and the mechanical properties of the plates were tested according to the aforementioned method.

[0097] Example 4

[0098] 1) Preparation of polymer emulsion D

[0099] 200 g of butyl acrylate, 90 g of methyl methacrylate, 10 g of acrylic acid, 2.1 g of EPA073 (anionic emulsifier, purchased from Klein) and 6.51 g of cetyltrimethylammonium bromide-modified nano zinc oxide (purchased from Darcy Nanotechnology Co., Ltd.) were weighed and emulsified with 200 g of deionized water under shearing action to prepare a pre-emulsion; 20 g of Wanhua Chemical 8016 acrylic ester emulsion, 1.1g NaHCO3, 0.05g NaS2O8 are placed at the bottom of the kettle, heated to 85°C, and 50g of pre-emulsion and 5.3% mass concentration ammonium persulfate solution are added dropwise. The addition time is controlled within 4.5 hours. After the addition is completed, keep warm for 1 hour and then cool to room temperature and discharge.

[0100] 2) Pressed wood particle composite products

[0101] Adjust each Shavings type CW20 / % polymer emulsion Moisture content / % surface layer Poplar surface shavings 2.1 8.0 18.5 core layer Poplar core wood shavings 4.5 0 2.8

[0102] 3) Hot pressed board

[0103] Hot pressing was performed at 220°C according to a set pressure curve process, with a hot pressing factor of 8.5s / mm. After hot pressing, multiple plates were obtained and the mechanical properties of the plates were tested according to the aforementioned method.

[0104] Example 5

[0105] 1) Preparation of polymer emulsion E

[0106] Weigh 190g of butyl acrylate, 100g of styrene, 10g of acrylic acid, 1.9g of LCN407 (nonionic emulsifier, purchased from Klein) and 200g of deionized water and emulsify them under shearing to prepare a pre-emulsion; put 20g of Wanhua Chemical 8056 acrylic ester emulsion, 1.1g NaHCO3, 0.05g NaS2O8 are placed at the bottom of the kettle, heated to 85°C, and 50g of pre-emulsion and 5.3% mass concentration of ammonium persulfate solution are added dropwise. The addition time is controlled within 4.5 hours. After the addition is completed, 8.95g of KH550 modified nano-silica is added and dispersed and kept warm for 1 hour, and finally cooled to room temperature and discharged.

[0107] 2) Pressed wood particle composite products

[0108] Adjust each Shavings type CW20 / % polymer emulsion Moisture content / % surface layer Poplar surface shavings 5.0 4.0 17.2 core layer Poplar core wood shavings 3.7 1.0 2.6

[0109] 3) Hot pressed board

[0110] Hot pressing was performed at 220°C according to the set pressure curve process, and the hot pressing factor was selected as 4s / mm. After hot pressing, multiple plates were obtained and the mechanical properties of the plates were tested according to the aforementioned method.

[0111] Example 6

[0112] The polymer emulsion and PMDI in Example 1 were sequentially applied to wood chips and mixed evenly, with other conditions remaining unchanged, to verify the effect of the sizing order.

[0113] Comparative Example 1

[0114] The KH570 modified nano-silica in Example 1 was removed, and the other conditions remained unchanged.

[0115] Comparative Example 2

[0116] The amount of KH570 modified silica in Example 1 was adjusted to 12.5 g, and the other components and reaction conditions remained the same as in Example 1.

[0117] Comparative Example 3

[0118] Commercially available tackifier EVA emulsion F was used instead of polymer emulsion A, and other conditions remained the same as in Comparative Example 3 to test cold pressing adhesion, mesh blocking, and board performance after hot pressing.

[0119] Comparative Example 4

[0120] Wood pellets without polymer emulsion, only 4.5% CW20, and constant moisture content were used to test cold pressing adhesion, mesh blocking, and board properties after hot pressing.

[0121] Table 1 Comparison of cold pressing adhesion and network blocking between the embodiment and the comparative example

[0122]

[0123]

[0124] Figure 1 The figure shows the state of wood particles sprayed with PMDI and polymer emulsion before and after cold pressing. It can be seen that before cold pressing, the polymer emulsion 3 wrapped by the modified nanoparticles 1 is coated on the surface of the wood particles 2, preventing adhesion between the mixture particles and maintaining good looseness between the particles. After being subjected to higher cold pressing pressure, the polymer emulsion and the wood particles can be well contained together to form a mixture of polymer emulsion and wood particles 4, and the polymer emulsion provides sufficient adhesion between the particles.

[0125] As can be seen from Table 1, the wood particle mixtures with the modified nanoparticle polymer emulsion do not have any screen clogging, indicating that the degree of agglomeration between the wood particles before cold pressing is small and they can easily disperse and pass through the screen under the force of air blowing. The cold pressing adhesion is greatly improved compared with that of Comparative Example 4, proving that the wood particle board has good cohesion after cold pressing, has a good ability to maintain its original shape, and has good adhesion between particles. Figure 2 and Figure 3 From the above, the initial tack of Comparative Example 1 after removing the modified nano-silica on the basis of Example 1 is improved, but the screen blocking situation deteriorates and the screen blocking area becomes larger, indicating that removing the modified nano-particles aggravates the agglomeration between the wood particles after sizing, which has a negative impact on screen blocking, while being beneficial to the improvement of cold-pressing adhesion. A similar conclusion can also be seen from Comparative Example 2, so the modified nano-particles in the tackifying composition need to be controlled within the required scope of the present invention. Exceeding the required scope will cause the cold-pressing adhesion of the tackifying composition to decrease. When the sizing order is changed and the polymer emulsion and PMDI are applied to the wood chips in sequence, the cold-pressing adhesion and screen blocking situation of Example 6 are similar to those of Example 1, indicating that the sizing order has no effect on cold-pressing adhesion and screen blocking.

[0126] Table 2 Mechanical properties of wood particle boards after hot pressing of the examples and comparative examples

[0127]

[0128]

[0129] As shown in Table 2, the wood particle composite material containing the polymer emulsion of the present invention exhibits improved mechanical properties compared to those of Comparative Example 4, with a reduced expansion rate after water absorption. Experiments also revealed that increasing the amount of polymer emulsion added improves the mechanical properties of the hot-pressed board. Furthermore, Table 2 also shows that when the amount of modified nanoparticles added exceeds the range required by the present invention, the mechanical properties of the resulting wood particle composite material decrease.

[0130] Although the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description is not intended to limit the present invention. Those skilled in the art will appreciate that, based on the teachings of this specification, modifications or adjustments may be made to the present invention. Such modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for preparing formaldehyde-free particleboard with improved cold pressing adhesion, characterized in that: The following steps are involved: 1) mixing polyisocyanate, polymer emulsion and wood shavings to form a mixture; 2) paving and shaping the mixture formed in step 1), cold pressing it into a slab with a setting ability, and then performing a high-temperature hot pressing process to finally obtain a formaldehyde-free particleboard; The polymer emulsion comprises hydrophobically modified inorganic nanoparticles, and the polymer emulsion is applied to the surface of the wood shavings in an amount of 0.5% to 10% of the total weight of the absolutely dry wood shavings; The polymer in the polymer emulsion is a homopolymer or copolymer obtained by free radical polymerization of monomers containing carbon-carbon double bonds; Before cold pressing, the mixture of the polyisocyanate, polymer emulsion and wood shavings in step 1) has essentially no adhesion or significant adhesion between particles. Adhesion between polymer particles only occurs after being subjected to a pressure greater than 0.01 MPa.

2. The preparation method according to claim 1, characterized in that The monomer containing a carbon-carbon double bond is selected from one or more of ethylene, vinyl acetate, (alkyl) acrylic acid or acrylate (alkyl) and styrene monomers.

3. The preparation method according to claim 2, characterized in that The monomer containing a carbon-carbon double bond is selected from at least two or more of C2-C8 ethylene, vinyl acetate, and (alkyl) acrylic acid or acrylate (alkyl) monomers.

4. The preparation method according to claim 1, characterized in that The hydrophobically modified inorganic nanoparticles are selected from any one of hydrophobically modified nano-silicon dioxide particles and hydrophobically modified nano-metal oxide particles.

5. The preparation method according to claim 4, characterized in that The particle size of the hydrophobically modified inorganic nanoparticles is between 1 nm and 100 nm.

6. The preparation method according to any one of claims 1 to 5, characterized in that Based on the total mass of the polymer emulsion, the polymer emulsion comprises: (1) 30%-70% polymer; (2) 0.05%-2% hydrophobically modified inorganic nanoparticles; (3) The remainder is water.

7. The preparation method according to claim 6, characterized in that The polymer is selected from at least any one of polyvinyl acetate polymer, poly(alkyl)acrylate polymer, and polyvinyl acetate-(meth)acrylate polymer.

8. The preparation method according to claim 6, characterized in that Based on the total mass of the polymer emulsion, the polymer emulsion comprises: (1) 45%-60% polymer; (2) 0.1%-1% hydrophobically modified inorganic nanoparticles; (3) The remainder is water.

9. The preparation method according to any one of claims 1 to 5, characterized in that: The glass transition temperature of the polymer is between -30°C and 60°C.

10. The preparation method according to claim 9, characterized in that The glass transition temperature of the polymer is between -15°C and 45°C.

11. The preparation method according to claim 1, characterized in that The polyisocyanate is selected from diphenylmethane diisocyanate MDI, polymeric diphenylmethane diisocyanate PMDI or a combination thereof, and the NCO group content is 27% to 33%.

12. The preparation method according to claim 11, characterized in that The polyisocyanate is polymeric diphenylmethane diisocyanate PMDI.

13. The preparation method according to any one of claims 1 to 5, characterized in that: The added mass ratio of the polymer emulsion to the polyisocyanate is between 0.1 and 5.

14. The preparation method according to any one of claims 1 to 5, characterized in that: The added mass ratio of the polymer emulsion to the polyisocyanate is between 0.2 and 4.

15. Formaldehyde-free particleboard with improved cold pressing adhesion obtained by the preparation method according to any one of claims 1 to 14.

Images