Durable water-resistant low-cost guardrail wood-plastic composite material and preparation method thereof

Abstract

The invention provides a durable water-resistant low-cost guardrail wood-plastic composite material and a preparation method thereof, and belongs to the technical field of wood-plastic materials. The composite material is mainly formed by the low-cost wood-plastic core material and the complete wrapping coating, the plastic content in the core material formula is low, and the cost is low; and the coating layer is composed of a thermoplastic inner layer coating layer and a thermosetting outer layer coating layer. The content of polyethylene in the core material of the product is reduced, the use amount of wood flour is increased, and the effect of reducing the cost is achieved. Meanwhile, the inner layer adopts a special low-viscosity thermoplastic coating, the surface wetting and permeability of the wood-plastic core material are good, the finally formed inner layer coating is compact and complete, and on the basis of the inner layer coating, the thermosetting coating prepared by virtue of special modified resin and active diluting components is relatively good in toughness and water resistance after being cured and molded, so that the service life of the wood-plastic core material is prolonged. The mechanical property is excellent, and the static bending intensity performance of a final product is also obviously improved.

Description

Technical Field

[0001] This invention belongs to the field of wood-plastic composite material technology, specifically relating to a durable, water-resistant, low-cost wood-plastic composite material for guardrails and its preparation method. Background Technology

[0002] With the development of society, wood-plastic composite materials are increasingly used outdoors, such as walls, fences, and posts for factories, parks, riverbanks, seasides, and homes, as well as outdoor pavilions and swimming pool railings. These are collectively referred to as wood-plastic composite fences. Traditional wood-plastic composite fences are made from ordinary wood flour, calcium carbonate filler, polyethylene plastic, and additives. Unlike wood-plastic flooring, wood-plastic fence products do not have high requirements for mechanical properties, generally only needing a bending strength of 25 MPa or higher. Because they do not involve friction conditions, surface wear resistance is almost unrequired. However, they have very high requirements for outdoor water resistance. Due to the presence of wood flour, the prepared wood-plastic fence products are prone to absorbing moisture and water in outdoor or humid areas, leading to defects such as expansion, blistering, cracking, and mold growth, ultimately rendering the wood-plastic composite unusable and greatly reducing its service life outdoors. Improving its water resistance can significantly extend the service life of wood-plastic fence products. Meanwhile, considering that wood-plastic composite materials for guardrails have relatively low requirements for mechanical properties and surface wear resistance, reducing their cost is also a research direction for the industry.

[0003] To improve the water resistance and service life of wood-plastic composite materials for railings, a common method is to use co-extruded wood-plastic composites, adding a plastic co-extruded layer to the surface of the wood-plastic core. However, because the surface plastic used in the co-extruded layer is high-density polyethylene, its high high-temperature melt viscosity results in insufficient wettability with the core during the co-extruded process. This leads to defects in the gap and bonding strength between the co-extruded layer and the core. Wood powder is exposed on both transverse and longitudinal cut ends, which can cause moisture to seep in and blister and expand over time, leading to deformation and cracking of the wood-plastic composite. Moreover, two cut sections of the co-extruded wood-plastic material lack the co-extruded layer, allowing the wood powder to come into direct contact with water after molding, resulting in water absorption, expansion, and mold growth. This makes it impossible to achieve complete surface coverage of the wood-plastic composite, and the water resistance is also insufficient. Furthermore, the use of the co-extruded layer significantly increases the cost of raw materials and processes. Some studies have used adhesives to modify wood flour and inorganic fillers to enhance their water resistance, but this still cannot prevent wood flour from being directly exposed on the surface of the wood-plastic composite. It is difficult to completely coat the wood flour and inorganic fillers. When the wood flour is exposed to water for a long time, it will expand, which will lead to cracking of the wood-plastic composite and thus affect its service life. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a durable, water-resistant, low-cost wood-plastic composite material for railings and its preparation method. This alleviates the current problems of exposed wood flour and calcium powder on cut surfaces of wood-plastic composites, which are prone to absorbing moisture. It also addresses the issues of co-extruded composites where the high melt viscosity of the co-extruded layer limits the bonding force to the core material to the surface, creating gaps between the bonding layers. Furthermore, wood flour is exposed on both transverse and longitudinal cut surfaces, leading to prolonged exposure to moisture during use. This moisture penetration into the gaps causes blistering and cracking of the co-extruded layer. Simultaneously, it addresses the high cost issue of wood-plastic railing products, which do not require excessively high mechanical or abrasion resistance.

[0005] One of the objectives of this invention is to provide a durable, water-resistant, low-cost wood-plastic composite material for fences.

[0006] The second objective of this invention is to provide a method for preparing this durable, water-resistant, low-cost wood-plastic composite material for railings.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0008] In a first aspect, the present invention provides a durable, water-resistant, low-cost wood-plastic composite material for fences, the wood-plastic composite material for fences comprising a core material and a coating layer; the coating layer comprises an inner coating layer and an outer coating layer, the thickness of the inner coating layer being 30-40 μm and the thickness of the outer coating layer being 105-120 μm; The core material comprises the following components in parts by weight: 60-65 parts wood flour, 19-23 parts high-density polyethylene, 4-6 parts calcium carbonate, 2-4 parts compatibilizer, 1.5-2 parts lubricant, 0.2-0.3 parts coupling agent, and 0.1-0.2 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating solution, which includes the following raw materials in parts by weight: 22-27 parts of low viscosity high chlorinated polyethylene resin, 1.2-1.5 parts of adhesive additive, 37-43 parts of toluene, 5-8 parts of 2-hexanone, and 0.05-0.07 parts of penetrant. The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 22-26 parts modified resin, 10-14 parts diglycidyl tetrahydrophthalate, 2.5-3.1 parts triethylenetetramine, 0.04-0.07 parts antioxidant, 0.06-0.08 parts ultraviolet absorber, and 0.1-0.15 parts leveling agent.

[0009] The following is a detailed explanation: Core material: Typical, but not limiting, parts by weight of wood flour are, for example, 60, 61, 62, 63, 64, or 65 parts. Preferably, the wood flour has a particle size of 80-100 mesh; Typical, but not limiting, parts by weight of high-density polyethylene are, for example, 19, 20, 21, 22, or 23 parts; Typical, but not limiting, parts by mass of calcium carbonate are, for example, 4, 5, or 6 parts. Typical, but not limiting, parts by mass of the compatibilizer are, for example, 2, 3, or 4 parts; Preferably, the compatibilizer is maleic anhydride-grafted high-density polyethylene; Typical, but not limited, parts by weight of the lubricant are, for example, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 parts; Preferably, the lubricant is stearic acid; Typical, but not limiting, mass fractions of coupling agents are, for example, 0.2, 0.25, or 0.3 parts; Preferably, the coupling agent is N-2-aminoethyl-3-aminopropyltrimethoxysilane (KH-792). Typical, but not limiting, parts by weight of color masterbatch are, for example, 0.1 or 0.2 parts; Preferably, the color masterbatch is yellow.

[0010] Inner layer immersion coating: Typical, but not limited, parts by weight of low-viscosity, highly chlorinated polyethylene resin are, for example, 22, 23, 24, 25, 26, or 27 parts; optional from type HCPE-L; Typical, but not limiting, parts by weight of the adhesive additive are, for example, 1.2, 1.3, 1.4, or 1.5 parts. Preferably, the adhesive additive is prepared by the following method: γ-glycidoxypropyltrimethoxysilane and 3-aminopropylsilanetriol are reacted in toluene solvent at a molar ratio of 1:0.5-0.65 at a reaction temperature of 90-95℃. The total mass ratio of γ-glycidoxypropyltrimethoxysilane and 3-aminopropylsilanetriol to toluene is 7-7.5:2.5-3. The reaction is stopped when the epoxy equivalent of the reactants reaches 600-635 g / mol, yielding a toluene solution of the adhesive with a solid content of 70-75%, which is used as an adhesive.

[0011] The amount of adhesive additive in the formula is calculated based on a solid content of 100%.

[0012] Typical, but not limiting, parts by mass of toluene are, for example, 37, 38, 39, 40, 41, 42, or 43 parts; Typical, but not limiting, parts by mass of 2-hexanone are, for example, 5, 6, 7, or 8 parts; Typical, but not limiting, mass fractions of the penetrant are, for example, 0.05, 0.06, and 0.07. Preferably, the penetrant is an alkali-resistant penetrant.

[0013] In some embodiments, the inner layer immersion coating solution is prepared by the following method: Add the prescribed amounts of toluene and 2-hexanone to dissolving vessel 1, heat to 102-105℃, add the prescribed amount of low-viscosity high-chlorinated polyethylene resin, stir at high temperature until completely dissolved, then cool to 70-80℃, add the prescribed amounts of binder and penetrant, stir thoroughly until completely dissolved into one phase, cool to room temperature, and discharge into dip coating tank 1.

[0014] Outer coating solution: Typical, but not limited, parts by weight of the modified resin are, for example, 22, 23, 24, 25, or 26 parts. Preferably, the modified resin is prepared by the following method: Diglycidyl adipate, diglycidyl dimerase, and xylene were added to a reaction vessel, and stirring was started. After uniform mixing, triphenylphosphine was added as a catalyst, and the temperature was raised to 115-120℃. Then, tetrabromobisphenol A and 2,3-dibromosuccinic acid were added, and the reaction was carried out at this temperature for chain extension polymerization. Samples were taken for testing. When the epoxy equivalent of the modified resin reached 960-1040 g / mol, the reaction was stopped, and the xylene solvent was removed under reduced pressure to obtain the modified resin. The volatile matter was controlled to be less than 1%, and its viscosity was 4800-5200 mPa·s. (70℃); wherein the molar ratio of diglycidyl adipic acid, diglycidyl dimerase, tetrabromobisphenol A, and 2,3-dibromosuccinic acid is 1-1.05:2-2.1:0.4-0.5:1.5-1.6, xylene accounts for 15-20% of the total mass of the material, and the amount of triphenylphosphine added is 0.08-0.11% of the mass of diglycidyl dimerase; the epoxy equivalent of diglycidyl dimerase is 450-480 g / mol; and the epoxy equivalent of diglycidyl adipic acid is 150-165 g / mol.

[0015] Typical, but not limiting, parts by weight of diglycidyl tetrahydrophthalate are, for example, 10, 11, 12, 13, or 14 parts. Typical, but not limiting, parts by mass of triethylenetetramine are, for example, 2.5, 2.6, 2.7, 2.8, 3.0, and 3.1 parts. Typical, but not limiting, parts by weight of antioxidants are, for example, 0.04, 0.05, 0.06, or 0.07 parts; Preferably, the antioxidant is antioxidant 1010; Typical, but not limiting, parts by mass of the ultraviolet absorber are, for example, 0.06, 0.07, or 0.08 parts; Preferably, the ultraviolet absorber is ultraviolet absorber UV-329; Typical, but not limiting, parts by weight of leveling agents are, for example, 0.1, 0.12, 0.14, and 0.15 parts; Preferably, the leveling agent is a butyl acrylate polymer.

[0016] In some embodiments, the outer layer immersion coating solution is prepared by the following method: Add the prescribed amount of tetrahydrophthalic acid diglycidyl ester to dissolving vessel 2, heat to 85-95℃, then add the prescribed amount of modified resin, antioxidant, ultraviolet absorber and leveling agent, stir at 85-95℃ until completely dissolved, cool to room temperature, add the prescribed amount of triethylenetetramine, stir evenly at room temperature, and then pour the material into dip coating tank 2.

[0017] It should be noted that the inner and outer dipping solutions need to be prepared and used immediately, and should be used up within 3 hours.

[0018] Secondly, the present invention provides a method for preparing the above-mentioned durable, water-resistant, low-cost wood-plastic composite material for railings, comprising the following steps: A. Add the formulated amounts of wood flour, calcium carbonate, high-density polyethylene, color masterbatch, compatibilizer, coupling agent, and lubricant to a mixer and mix thoroughly until homogeneous to obtain a premix. B. Add the premixed material to a parallel twin-screw extruder for extrusion granulation; C. The granulated material obtained in step B is put into a conical twin-screw wood-plastic profile extruder for extrusion molding; D. The extruded profile from step C is cooled and shaped, and then cut into rectangular wood-plastic semi-finished products according to the required length. Then, its six sides are lightly sanded to obtain the wood-plastic core material before coating (in addition to enhancing its aesthetics and decoration, the sanded interface also enhances the interface penetration and bonding of the subsequent coating). E. The wood-plastic core material obtained in step D before coating is placed into the inner layer of the coating solution in the dipping tank 1 for complete immersion. After being lifted and drained (without dripping for 30 seconds), it is sent to a vacuum oven for drying and shaping to ensure that it is fully dried and forms a film. The thickness of the dry film is controlled at 30-40 μm. F. Take the wood-plastic core material after the inner layer coating film obtained in step E out of the vacuum oven, cool it to room temperature, and then put it into the outer layer coating liquid of the dipping tank 2 for complete immersion. After being lifted and drained (without dripping for 30 seconds), send it into the high temperature oven for high temperature curing and molding to fully cure it into a film. The thickness of the cured film is controlled at 105-120μm. G. After curing, remove the wood-plastic composite material with the outer layer cured coating from the oven, cool it to room temperature, and package the resulting wood-plastic composite product.

[0019] It should be noted that the mass ratio of the core material to the inner layer immersion solution / outer layer immersion solution is generally 1:2-3 to ensure that the core material is fully immersed. The film thickness after coating can be controlled by adjusting the solid content and composition of the immersion solution.

[0020] Preferably, in step A, the stirring and mixing time is 0.5-1 hour; Preferably, in step B, the extrusion granulation process conditions include: barrel temperature of 190-200℃ and screw speed of 200-250rpm; Preferably, in step C, the extrusion molding process conditions include: barrel temperature of 205-210℃ and screw speed of 15-30 rpm; Preferably, in step D, the grinding depth is 0.02-0.05 mm; Preferably, in step E, the soaking time is 2-2.5 min; the vacuum degree of the vacuum oven is -0.098 to -0.099 MPa, the oven temperature is 90-95℃, and the drying time is 6-8 min; Preferably, in step F, the soaking time is 1-1.5 min; the high-temperature curing conditions are 40-45℃ / 2-3 min + 70-75℃ / 4-5 min + 95-98℃ / 8-10 min.

[0021] Beneficial effects: This invention comprises a low-cost wood-plastic composite core material with low plastic content and a complete coating layer. The core material is composed of high-density polyethylene, wood flour, calcium carbonate, compatibilizer, lubricant, coupling agent, etc., with a low plastic content and low cost. The coating layer consists of a thermoplastic inner coating and a thermosetting outer coating. The core material of this invention reduces the polyethylene content and increases the amount of wood flour, achieving cost reduction. Simultaneously, the inner layer uses a special low-viscosity thermoplastic coating, which has good surface wetting and penetration of the wood-plastic composite core material, resulting in a dense and complete inner coating. Based on the inner coating, a thermosetting coating prepared with special modified resin, active diluent, and polyamine curing agent is applied. The cured coating exhibits excellent toughness and water resistance, as well as outstanding mechanical properties, significantly improving the static bending strength of the final product. The core material of this invention is coated with a double-immersion process, resulting in a complete sealing of the core material surface. The coating material used in this invention employs a specific formula and process to achieve coating thickness and bonding strength, resulting in complete coverage. This addresses the current problem of exposed wood flour and calcium powder on the cut surfaces of wood-plastic composites, which are prone to absorbing moisture. It also solves the problem that the high melt viscosity of the co-extruded layer in co-extruded composites causes the bonding force between the co-extruded layer and the core material to remain only on the surface, and the gaps between the bonding layers lead to prolonged exposure to moisture. This moisture entering the gaps causes blistering and cracking of the co-extruded layer, which greatly affects the service life of wood-plastic composites in outdoor humid environments.

[0022] The present invention has been described in detail above; however, the above embodiments are merely illustrative in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments. Detailed Implementation

[0023] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0024] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0025] The raw materials used in the examples, along with their manufacturers / suppliers, are as follows: Wood flour, 80-100 mesh, from Shijiazhuang Fenghua Mineral Products Co., Ltd. High-density polyethylene, model 8008, from Dushanzi Petrochemical Co., Ltd. Calcium carbonate, model SPSL-2, particle size 200-300nm, from Jiangxi Huaming Nano Calcium Carbonate Co., Ltd. The compatibilizer is maleic anhydride-grafted high-density polyethylene, sourced from Huangshan Beno Technology Co., Ltd. The coupling agent is N-2-aminoethyl-3-aminopropyltrimethoxysilane (KH-792). The lubricant is stearic acid; The color masterbatch used is yellow color masterbatch, model S10698, from Shanghai Jianuo Plastics Co., Ltd. Low-viscosity, high-chlorinated polyethylene resin, model HCPE-L, is from Shandong Gaoxin Chemical Co., Ltd. The penetrant, model OEP-70, comes from Jiangsu Haian Petrochemical Plant; Dimeric acid diglycidyl ester, model XY811, epoxy equivalent 450-480g / mol, from Anhui Xinyuan Technology Co., Ltd. Diglycidyl adipic acid, model XY816, epoxy equivalent 150-165 g / mol, from Anhui Xinyuan Technology Co., Ltd. The leveling agent, model WK502, is a butyl acrylate polymer and is sourced from Ningbo Weikai Chemical Co., Ltd. All other raw materials are conventional.

[0026] Determination of volatile content and solid content: Take 10g of sample, put it in a beaker, and then put the beaker into a forced-air drying oven for drying at 130℃ / 30min. Take it out, cool it to room temperature, and weigh it again. The reduced mass is the volatile substance. The volatile content is: (reduced mass / 10)×100%. The solid content is 100% - volatile content.

[0027] Epoxy equivalent: According to the method in GB / T 4612-2008 "Determination of epoxy equivalent of epoxy compounds in plastics".

[0028] Preparation Example 1: Preparation of Adhesive Additives 1 mol of γ-glycidoxypropyltrimethoxysilane and 0.6 mol of 3-aminopropylsilanetriol were reacted in toluene solvent at a reaction temperature of 90 °C. The total mass ratio of γ-glycidoxypropyltrimethoxysilane and 3-aminopropylsilanetriol to toluene was 7.5:3. The reaction was stopped when the epoxy equivalent of the reactants reached 627 g / mol, yielding a toluene solution of the adhesive additive with a solid content of 70%.

[0029] Preparation Example 2: Preparation of Modified Resin 1 mol of diglycidyl adipate, 2 mol of diglycidyl dimerase, and xylene were added to a reaction vessel and stirred. After mixing evenly, triphenylphosphine was added as a catalyst. Xylene accounted for 18% of the total mass of the materials, and the amount of triphenylphosphine added was 0.1% of the mass of diglycidyl dimerase. The temperature was raised to 120℃, and then 0.5 mol of tetrabromobisphenol A and 1.5 mol of 2,3-dibromosuccinic acid were added. The reaction was carried out at this temperature for chain extension polymerization. Samples were taken for testing. When the epoxy equivalent of the modified resin reached 982 g / mol, the reaction was stopped, and the xylene solvent was removed under reduced pressure to obtain the modified resin. The volatile matter was controlled to be less than 1%, and the viscosity was 4972 mPa·s (70℃).

[0030] Example 1 A durable, water-resistant, low-cost wood-plastic composite material for railings includes a core material and a coating layer; the coating layer includes an inner coating layer and an outer coating layer, the inner coating layer having a thickness of 35 μm and the outer coating layer having a thickness of 114 μm; The core material comprises the following components in parts by weight: 62 parts wood flour, 20 parts high-density polyethylene, 5 parts calcium carbonate, 3 parts compatibilizer, 1.6 parts lubricant, 0.3 parts coupling agent, and 0.15 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating liquid, which includes the following raw materials in parts by weight: 25 parts of low viscosity high chlorinated polyethylene resin, 1.3 parts of the adhesive additive of Preparation Example 1, 40 parts of toluene, 6 parts of 2-hexanone, and 0.06 parts of penetrant. The inner layer dip coating solution is prepared by the following method: the formula amount of toluene and 2-hexanone are added to the dissolving vessel 1, the temperature is raised to 104°C, the formula amount of low viscosity high chlorinated polyethylene resin is added, and the mixture is stirred at high temperature until completely dissolved. Then the temperature is lowered to 75°C, the formula amount of binder and penetrant is added, and the mixture is stirred thoroughly until it is completely dissolved into one phase. After the mixture is cooled to room temperature, it is discharged into the dip coating tank 1.

[0031] The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 25 parts of the modified resin of Preparation Example 2, 12 parts of diglycidyl tetrahydrophthalate, 2.6 parts of triethylenetetramine, 0.05 parts of antioxidant 1010, 0.07 parts of ultraviolet absorber UV-329, and 0.14 parts of leveling agent.

[0032] The outer coating solution is prepared by the following method: Add the prescribed amount of tetrahydrophthalic acid diglycidyl ester to dissolving vessel 2, heat to 90°C, then add the prescribed amount of modified resin, antioxidant 1010, ultraviolet absorber UV-329 and leveling agent. After stirring at 90°C until completely dissolved, cool to room temperature, add the prescribed amount of triethylenetetramine, stir evenly at room temperature, and then pour the material into dip coating tank 2.

[0033] The method for preparing the above-mentioned wood-plastic composite material for guardrails includes the following steps: A. Add the formulated amounts of wood flour, calcium carbonate, high-density polyethylene, color masterbatch, compatibilizer, coupling agent and lubricant to a mixer and mix thoroughly for 1 hour. After mixing evenly, a premix is ​​obtained. B. The above-mentioned special premixed material is added to a parallel twin-screw extruder for extrusion granulation; the process conditions are: barrel temperature of 200℃ and screw speed of 200rpm. C. The granulated material obtained in step B is put into a conical twin-screw wood-plastic profile extruder for extrusion molding. The extrusion molding process conditions are: barrel temperature of 210℃ and screw speed of 20rpm. D. The extruded profile is cooled and shaped, and cut to the required length to obtain a rectangular wood-plastic semi-finished product. Then, its six sides are lightly sanded to obtain the wood-plastic core material before coating. The sanding depth is 0.03mm. In addition to enhancing its aesthetics and decoration, the sanded interface also enhances the interface penetration and adhesion of the subsequent coating. E. Place the wood-plastic core material prepared in step D, after surface polishing, into the dip coating tank 1 and immerse it completely for 2 minutes. After hanging it up and draining it (without dripping for 30 seconds), send it into the vacuum oven for drying and shaping for 7 minutes to allow it to dry fully into a film. The thickness of the dry film is controlled at 37 μm. The vacuum degree of the vacuum oven is -0.099 MPa and the oven temperature is 94℃. F. Take the wood-plastic core material with inner coating obtained in step E out of the vacuum oven, cool it to room temperature, and then put it into the dip coating tank 2 for complete immersion for 1 minute. After hanging it up and draining it (without dripping for 30 seconds), send it into a high-temperature oven for high-temperature curing (conditions: 41℃ / 2.5min + 74℃ / 4min + 96℃ / 9min) to fully cure it into a film. The thickness of the cured film is controlled at 110μm, and the oven temperature is 95℃. G. After curing, remove the wood-plastic composite material with the outer layer cured coating from the oven and cool it to room temperature to obtain the wood-plastic product.

[0034] Example 2 A durable, water-resistant, low-cost wood-plastic composite material for guardrails includes a core material and a coating layer; the coating layer includes an inner coating layer and an outer coating layer, the inner coating layer having a thickness of 38 μm and the outer coating layer having a thickness of 117 μm; The core material comprises the following components in parts by weight: 60 parts wood flour, 19 parts high-density polyethylene, 4 parts calcium carbonate, 4 parts compatibilizer, 1.5 parts lubricant, 0.25 parts coupling agent, and 0.1 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating liquid, which includes the following raw materials in parts by weight: 27 parts of low viscosity high chlorinated polyethylene resin, 1.2 parts of the adhesive additive of Preparation Example 1, 43 parts of toluene, 5 parts of 2-hexanone, and 0.07 parts of penetrant. The preparation of the inner layer immersion coating solution is the same as in Example 1.

[0035] The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 25 parts of the modified resin of Preparation Example 2, 11 parts of diglycidyl tetrahydrophthalate, 2.5 parts of triethylenetetramine, 0.07 parts of antioxidant 1010, 0.08 parts of ultraviolet absorber UV-329, and 0.1 parts of leveling agent.

[0036] The preparation of the outer layer immersion coating solution is the same as in Example 1.

[0037] The preparation method of wood-plastic composite material is the same as in Example 1.

[0038] Example 3 A durable, water-resistant, low-cost wood-plastic composite material for guardrails includes a core material and a coating layer; the coating layer includes an inner coating layer and an outer coating layer, the inner coating layer having a thickness of 32 μm and the outer coating layer having a thickness of 108 μm; The core material comprises the following components in parts by weight: 65 parts wood flour, 22 parts high-density polyethylene, 5 parts calcium carbonate, 2 parts compatibilizer, 2 parts lubricant, 0.3 parts coupling agent, and 0.2 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating liquid, which includes the following raw materials in parts by weight: 24 parts of low viscosity high chlorinated polyethylene resin, 1.4 parts of the adhesive additive of Preparation Example 1, 40 parts of toluene, 7 parts of 2-hexanone, and 0.06 parts of penetrant. The preparation of the inner layer immersion coating solution is the same as in Example 1.

[0039] The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 23 parts of the modified resin of Preparation Example 2, 13 parts of diglycidyl tetrahydrophthalate, 2.8 parts of triethylenetetramine, 0.06 parts of antioxidant 1010, 0.07 parts of ultraviolet absorber UV-329, and 0.12 parts of leveling agent.

[0040] The preparation of the outer layer immersion coating solution is the same as in Example 1.

[0041] The preparation method of wood-plastic composite material is the same as in Example 1.

[0042] Example 4 A durable, water-resistant, low-cost wood-plastic composite material for railings includes a core material and a coating layer; the coating layer includes an inner coating layer and an outer coating layer, the inner coating layer having a thickness of 37 μm and the outer coating layer having a thickness of 110 μm; The core material comprises the following components in parts by weight: 62 parts wood flour, 20 parts high-density polyethylene, 4 parts calcium carbonate, 4 parts compatibilizer, 1.8 parts lubricant, 0.2 parts coupling agent, and 0.2 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating liquid, which includes the following raw materials in parts by weight: 23 parts of low viscosity high chlorinated polyethylene resin, 1.5 parts of the adhesive additive of Preparation Example 1, 37 parts of toluene, 5 parts of 2-hexanone, and 0.05 parts of penetrant. The preparation of the inner layer immersion coating solution is the same as in Example 1.

[0043] The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 26 parts of the modified resin of Preparation Example 2, 14 parts of diglycidyl tetrahydrophthalate, 3 parts of triethylenetetramine, 0.07 parts of antioxidant 1010, 0.06 parts of ultraviolet absorber UV-329, and 0.11 parts of leveling agent.

[0044] The preparation of the outer layer immersion coating solution is the same as in Example 1.

[0045] The preparation method of wood-plastic composite material is the same as in Example 1.

[0046] Comparative Example 1 Other conditions are the same as in Example 1, except that no adhesive is used for the inner layer of the coating.

[0047] Comparative Example 2 Other conditions are the same as in Example 1, except that only the inner layer of the coating is used for coverage, and the outer layer of the coating is not used for coverage.

[0048] Comparative Example 3 Other conditions are the same as in Example 1, except that the inner coating layer is not used, and the outer coating layer is applied directly.

[0049] Comparative Example 4 Other conditions were the same as in Example 1, except that an equal mass of brominated epoxy resin (model EC-1500, epoxy equivalent 769 g / mol, softening point 102 °C, from Jiangsu Xingsheng Chemical Co., Ltd.) was used instead of the modified resin.

[0050] Comparative Example 5 Other conditions were the same as in Example 1, except that an equal mass of E-12 epoxy resin (model WHDW, epoxy equivalent 744 g / mol, softening point 88℃, from Huangshan Wuhuan Technology Co., Ltd.) was used instead of the modified resin.

[0051] Comparative Example 6 Everything else is the same as in Example 1, except that a wood-plastic composite core material is used as Comparative Example 6, and no inner or outer coating is used.

[0052] Comparative Example 7 The rest is the same as Comparative Example 6, except that the core material is wood-plastic composite material, and the amount of high-density polyethylene is increased from 20 parts to 26 parts, and the amount of wood flour is reduced from 62 parts to 50 parts.

[0053] Comparative Example 8 Using the product of Comparative Example 7 as the core material, and then using high-density polyethylene as the surface layer, co-extruded wood-plastic composite material was obtained (the mass ratio of surface layer to core material was controlled at 1:20), and the product was obtained by length determination and cutting.

[0054] Comparative Example 9 Example 1 in CN201910586171.X is used as Comparative Example 9.

[0055] Performance testing: The static bending strength (three-point bending), water resistance, and damp heat resistance tests were all conducted in accordance with GB / T17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The static bending strength (three-point bending) was tested according to section 4.7; the water resistance was tested according to section 4.52, and the test conditions for the resistance to hot water were 50℃ and full immersion, the test conditions for the resistance to boiling water were normal pressure and full immersion, and the test conditions for the resistance to high pressure water were 0.1 MPa pressure and full immersion; the damp heat resistance test was conducted according to section 4.50.

[0056] The performance of the products obtained from the above embodiments and comparative examples is shown in Table 1 below.

[0057] Table 1

[0058] As shown in Table 1, the product of this invention uses a core material prepared with a relatively low proportion of plastic components (high-density polyethylene) and a relatively high content of wood flour. This core material is then coated with two layers to finally produce a wood-plastic composite product for railings. Due to the significantly reduced amount of plastic components, the raw material cost is lower. However, thanks to the two coatings, the static bending strength can still reach over 33 MPa, and the appearance is smooth and even with complete coating coverage. Most importantly, the wood-plastic composite product exhibits excellent water resistance. The two coatings (inner thermoplastic layer, outer thermosetting layer) completely block external water. Over long periods, the product's appearance remains largely unchanged after being boiled in warm water (1200h), boiling in normal pressure water (360h), and boiling in high pressure water (240h), with the mass increase rate being less than 0.1%, indicating excellent long-term water resistance. It also performs well in terms of long-term resistance to damp heat (960h). This invention demonstrates superior mechanical properties and durable water and damp heat resistance through a specific manufacturing process, achieved with a low-cost wood-plastic core material formulation. This significantly enhances the long-term outdoor performance of wood-plastic composite materials for railings. Furthermore, the two coatings of this invention are applied via immersion, making it particularly suitable for irregularly shaped wood-plastic composite materials for railings, such as those with internal pores. The coating can be uniformly formed within the numerous pores, achieving durable waterproofing and preventing mold and mildew.

[0059] In Comparative Example 1, the absence of an adhesive additive resulted in a slight deficiency in the bonding strength between the inner coating and the core material, leading to slight blistering during prolonged high-pressure boiling. Comparative Example 2, lacking an outer thermosetting coating, exhibited a significant decrease in water resistance. Comparative Example 3, by directly employing a relatively high-viscosity thermosetting coating instead of a low-viscosity thermoplastic inner coating, suffered from poor surface wetting of the wood-plastic core material, resulting in uneven and incomplete coating coverage and ultimately reduced water resistance. Comparative Examples 4 and 5, using low-softening-point brominated epoxy resin and E-12 epoxy resin instead of the modified resin prepared from low-viscosity, highly hydrophobic dimeric acid diglycidyl ester and adipate diglycidyl ester, respectively, resulted in poor bonding between the outer and inner coatings and a rough coating surface, significantly reducing water resistance. Comparative Example 6, using only the wood-plastic core material from Example 1, exhibited insufficient mechanical properties and water resistance. Comparative Example 7 increased the plastic component (high-density polyethylene) content and reduced the amount of wood flour. After reducing the plastic content to the level currently found in commercially available wood-plastic composite flooring, the mechanical properties and water resistance are improved compared to Comparative Example 6. Comparative Example 8 is a co-extruded wood-plastic product obtained by using a relatively high plastic content as the core material in Comparative Example 7, and then co-extruded with high-density polyethylene. The water resistance of the product is improved, but because the co-extrusion is continuous, there is only a co-extruded layer on four sides. There is no co-extruded layer at the cut section, and the water resistance at the cut section is obviously insufficient. In addition, the co-extruded layer is directly co-extruded with high-density polyethylene, resulting in excessively high melt viscosity, which leads to poor bonding force with the core material. There is a gap between the core material and the surface layer, which allows water to seep in, resulting in defects such as insufficient water resistance at the cut section and the surface layer interface. Comparative Example 9 is Example 1 in CN201910586171.X, which uses a higher plastic content (plastic content reaches 59%) and uses a high-cost fluoroplastic component (perfluoroethylene-propylene resin). Its water resistance is relatively good, but the cost is much higher than that of the product of this invention.

[0060] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and substance defined by the claims of the present invention; and such modifications or substitutions are still within the scope defined by the claims of the present invention.

Claims

1. A durable, water-resistant, low-cost wood-plastic composite material for railings, characterized in that, The wood-plastic composite material includes a core material and a coating; The coating layer includes an inner coating layer and an outer coating layer. The thickness of the inner coating layer is 30-40 μm, and the thickness of the outer coating layer is 105-120 μm. The core material comprises the following components in parts by weight: 60-65 parts wood flour, 19-23 parts high-density polyethylene, 4-6 parts calcium carbonate, 2-4 parts compatibilizer, 1.5-2 parts lubricant, 0.2-0.3 parts coupling agent, and 0.1-0.2 parts color masterbatch. The inner layer of the coating is formed by dip coating with an inner layer immersion coating solution, which includes the following raw materials in parts by weight: 22-27 parts of low viscosity high chlorinated polyethylene resin, 1.2-1.5 parts of adhesive additive, 37-43 parts of toluene, 5-8 parts of 2-hexanone, and 0.05-0.07 parts of penetrant. The outer coating layer is formed by dip coating with an outer coating solution, which comprises the following raw materials in parts by weight: 22-26 parts modified resin, 10-14 parts diglycidyl tetrahydrophthalate, 2.5-3.1 parts triethylenetetramine, 0.04-0.07 parts antioxidant, 0.06-0.08 parts ultraviolet absorber, and 0.1-0.15 parts leveling agent.

2. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The particle size of the wood flour in the core material is 80-100 mesh; The compatibilizer is maleic anhydride-grafted high-density polyethylene; The lubricant is stearic acid; The coupling agent is N-2-aminoethyl-3-aminopropyltrimethoxysilane.

3. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The adhesive aid in the inner layer immersion coating solution is prepared by the following method: γ-glycidoxypropyltrimethoxysilane and 3-aminopropylsilanetriol were reacted in toluene solvent at a molar ratio of 1:0.5-0.65 at a reaction temperature of 90-95℃. The total mass ratio of γ-glycidoxypropyltrimethoxysilane and 3-aminopropylsilanetriol to toluene was 7-7.5:2.5-3. The reaction was stopped when the epoxy equivalent of the reactants reached 600-635 g / mol, yielding a toluene solution of the adhesive additive with a solid content of 70-75%.

4. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The penetrant in the inner layer immersion coating solution is an alkali-resistant penetrant.

5. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The inner layer immersion coating solution is prepared by the following method: Add the prescribed amounts of toluene and 2-hexanone to dissolving vessel 1, heat to 102-105℃, add the prescribed amount of low-viscosity high-chlorinated polyethylene resin, stir at high temperature until completely dissolved, then cool to 70-80℃, add the prescribed amounts of binder and penetrant, stir thoroughly until completely dissolved into one phase, cool to room temperature, and discharge into dip coating tank 1.

6. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The modified resin in the outer coating solution is prepared by the following method: Diglycidyl adipate, diglycidyl dimerase, and xylene were added to a reaction vessel, and stirring was started. After uniform mixing, triphenylphosphine was added as a catalyst, and the temperature was raised to 115-120℃. Then, tetrabromobisphenol A and 2,3-dibromosuccinic acid were added, and the reaction was carried out at this temperature for chain extension polymerization. Samples were taken for testing. When the epoxy equivalent of the modified resin reached 960-1040 g / mol, the reaction was stopped, and the xylene solvent was removed under reduced pressure to obtain the modified resin. The content of the active ingredient is controlled within 1%, and the viscosity at 70℃ is 4800-5200 mPa·s; the molar ratio of diglycidyl adipic acid, diglycidyl dimerase, tetrabromobisphenol A, and 2,3-dibromosuccinic acid is 1-1.05:2-2.1:0.4-0.5:1.5-1.6, xylene accounts for 15-20% of the total mass of the material, and the amount of triphenylphosphine added is 0.08-0.11% of the mass of diglycidyl dimerase.

7. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The antioxidant in the outer coating solution is antioxidant 1010; The ultraviolet absorber is UV-329; The leveling agent is a butyl acrylate polymer.

8. The durable, water-resistant, low-cost wood-plastic composite material for guardrails according to claim 1, characterized in that, The outer coating solution is prepared by the following method: Add the prescribed amount of tetrahydrophthalic acid diglycidyl ester to dissolving vessel 2, heat to 85-95℃, then add the prescribed amount of modified resin, antioxidant, ultraviolet absorber and leveling agent, stir at 85-95℃ until completely dissolved, cool to room temperature, add the prescribed amount of triethylenetetramine, stir evenly at room temperature, and then pour the material into dip coating tank 2.

9. A method for preparing a durable, water-resistant, low-cost wood-plastic composite material according to any one of claims 1-8, characterized in that, Includes the following steps: A. Add the formulated amounts of wood flour, calcium carbonate, high-density polyethylene, color masterbatch, compatibilizer, coupling agent, and lubricant to a mixer and mix thoroughly until homogeneous to obtain a premix. B. Add the premixed material to a parallel twin-screw extruder for extrusion granulation; C. The granulated material obtained in step B is put into a conical twin-screw wood-plastic profile extruder for extrusion molding; D. The extruded profile from step C is cooled and shaped, and then cut into rectangular wood-plastic semi-finished products according to the required length. Then, its six sides are sanded to obtain the wood-plastic core material before coating. E. The wood-plastic core material obtained in step D before coating is completely immersed in the inner layer of the coating solution in the dipping tank 1. After being lifted and drained, it is sent to a vacuum oven for drying and shaping to ensure that it is fully dried and forms a film. The thickness of the dry film is controlled at 30-40μm. F. Take the wood-plastic core material after the inner layer coating film obtained in step E out of the vacuum oven, cool it to room temperature, and then put it into the outer layer coating liquid of the dip coating tank 2 for complete immersion. After being lifted and dried, it is sent to the high temperature oven for high temperature curing and molding, so that it can be fully cured into a film. The thickness of the cured film is controlled at 105-120μm. G. After curing, remove the wood-plastic composite material with the outer layer cured coating from the oven, cool it to room temperature, and package the resulting wood-plastic composite product.

10. The preparation method according to claim 9, characterized in that, In step A, the mixing time is 0.5-1 hour; In step B, the extrusion granulation process conditions include: barrel temperature of 190-200℃ and screw speed of 200-250rpm; In step C, the extrusion molding process conditions include: barrel temperature of 205-210℃ and screw speed of 15-30 rpm; In step D, the grinding depth is 0.02-0.05mm; In step E, the soaking time is 2-2.5 min; the vacuum degree of the vacuum oven is -0.098 to -0.099 MPa, the oven temperature is 90-95℃, and the drying time is 6-8 min; In step F, the soaking time is 1-1.5 min; the high-temperature curing conditions are: first, treat at 40-45℃ for 2-3 min, then at 70-75℃ for 4-5 min, and then at 95-98℃ for 8-10 min.

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