Ultrahigh-viscosity foaming regulator for low-density PVC (polyvinyl chloride) wood-plastic composite material

Abstract

The invention relates to the technical field of PVC (polyvinyl chloride) foaming regulators, in particular to an ultrahigh-viscosity foaming regulator for a low-density PVC wood-plastic composite material. The core-shell type acrylate copolymer composite material is prepared from the following components in parts by mass: 40 to 90 parts of core-shell type acrylate copolymer particles, 2 to 40 parts of layered bimetal hydroxide, 0.2 to 20 parts of inorganic porous carrier with the average pore size of 0.3 to 50nm, 0.05 to 10 parts of inorganic nano nucleating agent or carbon nano material, 0.1 to 5 parts of beta-diketone or beta-dicarbonyl compound and 0.1 to 5 parts of coupling agent. A core layer of the component A comprises a crosslinking unit, and a shell layer of the component A comprises a polar or reactive unit. By constructing a multi-dimensional cooperative defense closed loop, the component A is used for chemically anchoring wood flour to prevent puncture, the component C is used for intercepting water and preventing bumping, the component B and the component E are used for cooperatively absorbing acid and preventing degradation, and the component D is used for promoting nucleation. And under the working conditions of high wood flour filling and high water content, micron-sized fine foaming, extremely low density and excellent bending strength can still be realized.

Description

Technical Field

[0001] This invention belongs to the field of PVC foaming regulator technology, specifically an ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites. Background Technology

[0002] To achieve weight reduction and cost optimization in PVC foamed products, sufficient melt strength and cell stability need to be maintained within the melt plasticization-foaming window. Publicly available technologies commonly employ acrylic processing aids / foaming regulators to improve melt elasticity and strength and suppress cell collapse. For example, published patent CN105254817A discloses a core-shell structured ultra-high molecular weight acrylic foaming regulator for improving PVC plasticization, melt strength, and fine, uniform cell structure.

[0003] However, when PVC is combined with wood flour / plant fiber to form a wood-plastic composite system, it faces a far more complex physical and chemical coupling and destructive environment than pure PVC foaming: First, the naturally bound water and small-molecule volatiles introduced by the wood flour are prone to instantaneous boiling during extrusion, resulting in extremely unstable gas release, causing co-occurring bubbles, coarse bubbles, and numerous pinholes on the surface. Second, the high proportion of large-size rigid wood flour fillers severely disrupts the continuous phase of the melt, and its sharp edges easily pierce the thin cell walls during biaxial stretching of the foam, leading to large-area collapse of the foam. Traditional pure linear ultra-high molecular weight acrylic additives often exhibit insufficient tear resistance due to molecular chain slippage when facing this "mechanical point stress" puncture. Finally, the high-filler system generates extremely strong mechanical frictional shear heat in the extruder, causing PVC to accelerate the release of hydrogen chloride at high temperatures and trigger "zipper-like" autocatalytic degradation, resulting in a sharp narrowing of the processing window and even carbonization and blackening of the material.

[0004] Existing technologies often attempt to address these problems by simply compounding higher molecular weight regulators or adding dehydrating agents or heat stabilizers. However, increasing the molecular weight inevitably leads to the powder itself being prone to agglomeration and clumping, making it difficult to disperse and de-entangle in the matrix in a short time. Simple physical compounding not only easily leads to phase separation in the screw extruder but also fails to simultaneously resist the simultaneous occurrence of multiple sources of "moisture boiling, hydrogen chloride degradation, and mechanical puncture" at each microscopic cell interface. Therefore, how to achieve high melt strength, puncture resistance, controlled moisture release, and in-situ acid absorption simultaneously at the microscopic interface under conditions of extremely high wood flour filling and high moisture content, through a multifunctional integrated material structure, to achieve lower density and higher closed-cell ratio, is a technical bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention proposes an ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites. This invention primarily addresses the technical problem in existing technologies of how to achieve lower density and more uniform cell structure in PVC wood-plastic composites under conditions of high wood flour content using a foaming regulator.

[0006] According to a first aspect of the present invention, an ultra-high viscosity foaming modifier for low-density PVC wood-plastic composites, said foaming modifier being a powder composition comprising, by weight, parts; Component A: 40-90 parts of core-shell acrylate copolymer particles; Component B: 2–40 parts of layered bimetallic hydroxide; Component C: 0.2–20 parts of inorganic porous support with an average pore size of 0.3–50 nm; Component D: 0.05–10 parts of inorganic nanonucleating agent or carbon nanomaterials; Component E: 0.1–5 parts of β-diketone compounds or β-dicarbonyl compounds; Component F: 0.1–5 parts coupling agent; Wherein: the core layer of component A is an acrylate phase containing crosslinked monomer units; its shell layer contains polar monomer units or reactive monomer units; component B and component D respectively coat or adsorb onto the surface of the particles of component A to form a composite precursor, and the composite precursor combines with component C to form the composite particle powder.

[0007] Preferably, component A is formed by at least two stages of emulsion polymerization; the core layer further comprises butyl acrylate units; the shell layer comprises polymethyl acrylate, polymethyl methacrylate, or copolymers thereof; and the mass ratio of the core layer to the shell layer is from 5:95 to 40:60.

[0008] Preferably, the shell of component A contains a number of moles of shell monomers. The polar monomer unit or reactive monomer unit, wherein the reactive monomer unit is selected from at least one of glycidyl methacrylate, maleic anhydride, and maleic half ester. The polar monomer unit is selected from at least one of methacrylic acid, acrylic acid, and hydroxyethyl acrylate.

[0009] Preferably, component B is selected from at least one of magnesium aluminum hydrotalcite, zinc aluminum hydrotalcite, calcium aluminum hydrotalcite, and magnesium zinc aluminum hydrotalcite.

[0010] Preferably, component C is selected from at least one of 3A molecular sieve, 4A molecular sieve, mesoporous silica, activated alumina, silica gel, and zeolite.

[0011] Preferably, component D is selected from at least one of nano-calcium carbonate, nano-silica, titanium dioxide, montmorillonite, carbon nanotubes, and graphene.

[0012] Preferably, component E is selected from at least one of zinc acetylacetonate, acetylacetonate, and stearoylbenzoylmethane.

[0013] Preferably, the composite particle powder is formed by the following process: an aqueous dispersion containing the components B, D, E and F is introduced into the latex of component A, so that it is adsorbed on the surface of the particles of component A to form a composite precursor slurry, and then the composite precursor slurry is mixed with the dry powder of component C and spray-dried together to obtain the final product.

[0014] Preferably, the bulk density of the composite granular powder is 0.30 to 0.70 g / cm³. The beneficial effects of this invention are as follows:

[0015] 1. This invention utilizes the nanoporous effect of component C to convert up to 5% of the free moisture in wood flour into a controlled auxiliary foaming source. This eliminates the need for downstream manufacturers to perform energy-intensive deep drying processes lasting several hours, reducing production energy consumption while solving the surface pinhole problem caused by moisture boiling.

[0016] 2. In this invention, component B (LDH) not only passively absorbs hydrogen chloride, but its byproduct carbon dioxide after acid absorption provides a microscopically uniform nucleation driving force, and the exfoliated nanosheets provide a large number of heterogeneous nucleation interfaces. This increases the pore density and enhances the support of the pore walls.

[0017] 3. In this invention, due to the presence of the isolation layer, the ultra-high molecular weight component A does not self-aggregate in the dry mix. After entering the extruder, the isolation layer breaks down rapidly, allowing the long polymer chains to untangle with the PVC system in a very short time. This reduces the dependence on the extruder's length-to-diameter ratio, improves production efficiency, and broadens the equipment's applicability.

[0018] 4. In this invention, in-situ chemical bonding at the interface is achieved through shell-reactive monomers. This allows the material to maintain extremely low density while increasing its transverse bending strength, overcoming the technical defect of embrittlement during foaming of traditional wood-plastic composites and improving the toughness and strength of the foamed material. Detailed Implementation

[0019] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments. Example 1:

[0020] This embodiment provides an ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites. The specific components include: Component A (micro-crosslinked core-ultra-high viscosity linear shell copolymer, dry weight 750g): The core monomer (150g) includes: 140.0g butyl acrylate, 8.0g methyl methacrylate, and 2.0g ethylene glycol dimethacrylate (crosslinking agent); the shell monomer (600g) includes: 570.0g methyl methacrylate and 30.0g glycidyl methacrylate (containing epoxy-reactive monomers); Component B (LDH acid-absorbing heat-stabilizing component): 120.0g magnesium aluminum hydrotalcite powder; Component C (porous aqueous buffer carrier): 4A molecular sieve powder. (Average pore size 0.4nm) 80.0g; Component D (inorganic nanonucleating agent or carbon nanomaterial): precipitated nano-calcium carbonate (average particle size 80nm) 30.0g; Component E (synergistic inhibitory component): stearoylbenzoylmethane 10.0g; Component F (coupling / dispersion component): silane coupling agent 10.0g (approximately 10.5mL); Aqueous phase and auxiliary agent system (for emulsion polymerization of component A): deionized water 2000.0mL, sodium dodecyl sulfate (SDS emulsifier) ​​15.0g, potassium persulfate (KPS core layer initiator) 0.5g, sodium persulfate 0.4g~0.6g and sodium isoascorbate 0.4g~0.6g (shell redox initiation system).

[0021] The specific preparation process includes the following steps: Step 1: Preparation of component A high-elasticity micro-crosslinked core layer emulsion; In a 5000 mL four-necked flask equipped with a mechanical stirrer, condenser, constant-pressure dropping funnel, and thermometer, 2000.0 mL of deionized water and 15.0 g of SDS emulsifier were added. Stirring was started (200 rpm), and nitrogen was introduced to purge oxygen for 15 minutes. The water bath temperature was raised to 60 °C, and 150 g of pre-mixed core layer monomers (140 g butyl acrylate + 8 g methyl methacrylate + 2 g ethylene glycol dimethacrylate) were added to the flask all at once, and the mixture was stirred and emulsified for 30 minutes. Subsequently, 0.5 g of potassium persulfate initiator solution dissolved in 10 mL of water was injected into the system using a syringe. The polymerization reaction was carried out at 65 °C for 2 hours. The system exhibited a bluish, translucent emulsion, which is the micro-crosslinked core layer seed latex.

[0022] Step 2: In-situ grafting growth of the ultra-high viscosity active shell of component A; The water bath temperature of the above reaction system was lowered to 50℃. A dropping method was used: 600g of the shell-shell mixed monomers (570g methyl methacrylate + 30g glycidyl methacrylate) were placed in a constant-pressure dropping funnel; simultaneously, 0.4g–0.6g of sodium persulfate and 0.4g–0.6g of sodium isoascorbate were dissolved in 20mL of water and placed in two other dropping devices. The dropping rate of the monomers and initiator was controlled to ensure uniform addition over 3 hours (approximately 180 minutes). The addition of chain transfer agents was strictly prohibited during the dropping process. After the dropping was completed, the system was heated to 70℃ for curing and held at that temperature for 1.5 hours. A small amount of gel was removed by filtration, yielding a core-shell structured ultra-high molecular weight acrylic latex with a solid content of approximately 27%. Calculations showed that the glycidyl methacrylate unit accounted for approximately 3.57 mol% of the total molar number of the shell-shell monomers.

[0023] Step 3: Preparation of composite slurry; In a stainless steel mixing tank equipped with a high-shear disperser (1500 r / min), 500 mL of deionized water was added, followed by the slow addition of 120.0 g of magnesium aluminum hydrotalcite, 30.0 g of nano-calcium carbonate, and 10.0 g of stearoyl benzoylmethane. After dispersion for 15 minutes, 10.0 g of silane coupling agent, pre-hydrolyzed in a small amount of water / ethanol, was slowly added dropwise. The shear reaction was continued for 40 minutes to form a stable aqueous dispersion of the inorganic functional components.

[0024] Subsequently, the prepared aqueous dispersion was slowly pumped into a reactor containing all of the A component latex (approximately 2700g of liquid) prepared in step 2. The stirring speed was reduced to 400r / min, and the mixture was mixed and kept warm for 1 hour to allow it to be adsorbed onto the surface of the A component particles, forming a uniform composite precursor slurry.

[0025] Step 4: Instantaneous composite and drying of porous carrier; 80.0g of 4A molecular sieve dry powder, which has been pre-activated by high-temperature roasting in a muffle furnace at 250℃ for 2 hours, is loaded into a spiral micro-feeder.

[0026] Start the centrifugal spray dryer (e.g., model B-290), set the inlet air temperature to 180℃, and control the outlet air temperature between 85℃ and 90℃. Start the slurry peristaltic pump to feed the slurry into the atomizing disc. The key step is to instantly mix 80.0g of activated molecular sieve powder into the atomizing disc at a proportional rate (matching the flow rate of the slurry's solid content) using a micro-feeder the moment the slurry enters the pre-mixing chamber.

[0027] The atomized droplets were rapidly dried in a drying tower, and the white powder collected below the cyclone separator was then collected. The final product, approximately 960g of a composite foaming regulator with excellent flowability and no caking, was obtained. Laser particle size analysis showed a median particle size (D50) of 85µm and a bulk density of 0.45g / cm³. Example 2:

[0028] Step 1: Preparation of component A high-elasticity micro-crosslinked core layer emulsion; 2200.0 mL of deionized water and 18.0 g of nonionic emulsifier (e.g., OP-10) were added to a 5000 mL reactor equipped with mechanical stirring, and the mixture was heated to 55°C under nitrogen protection. 160 g of premixed core layer monomers (butyl acrylate, methyl methacrylate, and trimethylolpropane trimethacrylate) were added all at once, and emulsified by high-speed stirring. Subsequently, 0.3 g of ammonium persulfate initiator solution was added, and the reaction was carried out at a constant temperature of 60°C for 2 hours to construct a highly elastic microsphere seed emulsion with extremely high internal cross-linking density.

[0029] Step 2: Graft polymerization of ultra-high molecular weight shells containing hydrogen bond arrays; The system temperature was fine-tuned to 45°C (low-temperature polymerization ensures ultra-high molecular weight). A dual-flow, uniformly added dropwise method was used: 640g of shell monomers (containing 575.0g of methyl methacrylate, 40.0g of hydroxyethyl acrylate, and 25.0g of methacrylic acid) and the remaining redox initiator (0.3g of ammonium persulfate + 0.6g of sodium bisulfite) were added dropwise to the core latex in two separate streams over 3.5 hours. Calculations showed that the polar monomer units (hydroxyethyl acrylate and methacrylic acid) accounted for approximately 9.43 mol% of the total molar amount of the shell monomers.

[0030] Because the system incorporates monomers containing free carboxyl groups (methacrylic acid) and hydroxyl groups (hydroxyethyl acrylate), the molecular chains readily undergo hydrogen bonding in the aqueous phase, causing the latex viscosity to spike and even "climb the pole." This step successfully ensured the stable growth of the ultra-high molecular weight hydrogen-bonded polymer in the emulsion state by relying on the steric hindrance of the nonionic emulsifier and the extremely low instantaneous monomer concentration. After the addition was complete, the temperature was raised to 70°C and maintained for 1.5 hours for curing.

[0031] Step 3: Coupling of inorganic functional components with titanate esters; In a 5-liter high-speed dispersion tank, 100.0g of component B (zinc-aluminum hydrotalcite), 30.0g of component D (fumed silica), and 10.0g of component E (zinc acetylacetone) were dispersed in an appropriate amount of deionized water. 10.0g of component F (titanium ester coupling agent or fatty acid) was added dropwise, and the mixture was subjected to high-speed shearing for 45 minutes to achieve an oleophilic coating on the surface of the inorganic powder.

[0032] Subsequently, the prepared inorganic functional component aqueous dispersion was slowly pumped into a reactor containing the latex of component A prepared in step 2, and stirred at medium speed for 1 hour. Relying on the bridging effect of the titanate coupling agent, the inorganic powder was firmly adsorbed onto the surface of the polymer particles of component A, forming a composite precursor slurry.

[0033] Step 4: Mesoporous carrier blending and anti-caking co-granulation; 50.0 g of pre-activated mesoporous silica (component C) was mixed with the slurry just before entering the atomizing disc of the spray dryer via a metering feeder. The spray dryer inlet air temperature was set to 175°C and the outlet air temperature to 85°C. The intense instantaneous evaporation caused component D (fumed silica) to form an excellent "flowability barrier layer" on the outermost layer of the polymer particles. Approximately 975 g of composite foaming regulator dry powder was finally collected (median particle size D50 = 75 µm, bulk density 0.42 g / cm³). Example 3:

[0034] Step 1: Preparation of component A high-elasticity micro-crosslinked core layer emulsion; Add 2500.0 mL of deionized water and 12.0 g of sodium dodecylbenzenesulfonate emulsifier to a 5000 mL reactor equipped with a condenser and stirrer, and purge with nitrogen for 20 minutes. Heat to 65°C. Add 255.0 g of premixed core layer monomers (butyl acrylate, methyl methacrylate, allyl methacrylate).

[0035] Mechanism control point: Allyl methacrylate has two double bonds with different reactivity. The methacrylate double bond preferentially reacts to form a cross-linked network during core-layer polymerization, while the allyl double bond remains on the microsphere surface, providing chemical grafting sites for the subsequent shell layer. Adding 0.4 g of potassium persulfate solution and isothermal polymerization for 2 hours yields a cross-linked core latex with grafting sites.

[0036] Step 2: In-situ grafting growth of ultra-high molecular weight shells containing anhydride groups; The system was cooled to 45°C. A dual-flow addition method was used for the monomer and reducing agent: 595.0 g of shell monomer (containing 594.0 g of methyl methacrylate and 1.0 g of maleic anhydride) and 0.4 g of ascorbic acid (dissolved in a small amount of water) were added dropwise to the core latex at a uniform rate over 3 hours; the remaining 0.4 g of potassium persulfate was added to the reactor in advance in one batch; calculations showed that the reactive monomer unit (maleic anhydride) accounted for approximately 0.17 mol% of the total molar amount of the shell monomer.

[0037] During shell growth, the polymer chain segments in the shell undergo covalent polymerization with the allyl double bonds on the core surface, forming a true "core-shell chemical bond," which completely prevents "core-shell peeling" failure under the extremely high shear force of silucaria. After 1.5 hours of curing, a polymer latex is obtained.

[0038] Step 3: Silane coating of two-dimensional sheet nucleating agent; Add a small amount of water to a 5-liter mixing tank, then add 80.0g of magnesium zinc aluminum hydrotalcite (component B), 15.0g of organomontmorillonite (component D), and 5.0g of acetylacetone (component E). Add 10.0g of 3-methacryloyloxypropyltrimethoxysilane (component F) dropwise and disperse at high speed under shear for 1 hour.

[0039] Mechanism control point: The carbon-carbon double bonds of this silane coupling agent can form slight entanglements with residual free radicals or polymer chains. Subsequently, the above dispersion is slowly pumped into a reactor containing the latex of component A prepared in step 2, and stirred at medium to low speed for 1.5 hours to uniformly and firmly coat the organomontmorillonite nanosheets and magnesium zinc aluminum hydrotalcite around the polymer latex, forming a composite precursor slurry.

[0040] Step 4: Instantaneous blending and anti-caking granulation of zeolite powder; 40.0g of natural zeolite powder (component C), activated by calcination at 300°C, was quantitatively mixed with a composite slurry and rapidly atomized and dried the powder the instant it entered the atomizing disc of a spray dryer (inlet air 185°C, outlet air 90°C). Approximately 955g of powder with excellent flowability (median particle size D50 = 95µm, bulk density 0.48g / cm³) was collected.

[0041] Comparative Example 1: The difference between this comparative example and Example 1 is that the shell monomer of component A is only pure methyl methacrylate (MMA), without the addition of glycidyl methacrylate (GMA) reactive units.

[0042] Comparative Example 2 The difference between this comparative example and Example 1 is that no component C (inorganic porous carrier) was added during the preparation of the foaming regulator.

[0043] Comparative Example 3 The difference between this comparative example and Example 1 is that component B (layered bimetallic hydroxide LDH) was not added during the preparation of the foaming regulator.

[0044] Comparative Example 4 The difference between this comparative example and Example 1 is that the "co-spray drying" process was not used in the preparation method. Instead, the polymer powder of component A was directly mechanically and physically mixed with the powders of components B, C, D, and E at room temperature. Test example:

[0045] The foaming regulators prepared in Examples 1-3 and Comparative Examples 1-4 were mixed into the same PVC wood-plastic foaming base material at a fixed ratio of 8 phr (per 100 parts of resin).

[0046] The base material formula is as follows: 100 parts PVC resin; 45 parts poplar wood powder (80 mesh, not deeply dried, actual moisture content 5.0%); 4.0 parts calcium-zinc composite stabilizer; 1.0 part PE wax; 0.5 parts OPE wax; 0.8 parts calcium stearate; 0.6 parts AC foaming agent.

[0047] Molding process: The mixture is hot-mixed at 115°C and cold-mixed at 45°C, and then extruded using a conical twin-screw extruder with the die temperature set at 175°C-185°C.

[0048] Test items and specific test methods: Density and apparent porosity testing: Density was determined according to the ASTM D792 water displacement method to evaluate the effect of foaming modifier on melt strength and weight reduction.

[0049] Average cell size test: The extruded board is brittlely fractured in liquid nitrogen. After the cross section is sprayed with gold, image analysis software is used to count no less than 100 cells and calculate the average diameter to evaluate the cell fineness and bubble formation defects.

[0050] Transverse bending strength test: According to GB / T24148 or ASTMD790 standards, a three-point bending test is performed using a universal testing machine with a loading speed of 5 mm / min to evaluate the macroscopic mechanical bearing capacity of the material.

[0051] Static thermal stability time: Take 2.0 g of the dry mixture and place it in a test tube. Place the test tube in a 180°C oil bath and place a moistened Congo red test paper at the mouth of the tube. Record the time (in minutes) for the test paper to change from red to blue to evaluate the system's ability to inhibit the degradation of PVC by dehydrochlorination.

[0052] Surface roughness (Ra): Using a portable surface roughness meter, multiple measurements were taken on the skin layer surface of the extruded board and the average value was obtained to evaluate the degree of surface pinholes and pitting defects caused by moisture boiling.

[0053] The test results for the above test items are shown in the table below. The table below shows the performance test results of the adsorbents prepared in Examples 1-3 and Comparative Examples 1-4;

[0054] By comparing and analyzing the data in the table, it can be seen that Examples 1-3 prepared by the technical solution of the present invention, even under harsh conditions containing up to 5.0% free moisture and 45 parts of wood flour, still achieved low density (0.40-0.46 g / cm³), micron-sized uniform pores (48-63 μm), good bending strength (22.8-27.1 MPa), good surface smoothness, and long thermal stability time. All indicators are superior to the comparative examples, confirming that the multi-component synergistic system of the present invention achieves better overall technical effects.

[0055] Comparative Example 1 did not introduce the GMA reactive monomer into the shell. Its flexural strength decreased significantly, reaching only 14.2 MPa, far lower than the 24.3 MPa of Example 1.

[0056] This data directly demonstrates the irreplaceability of the reactive monomers in the first group. In Comparative Example 1, pure linear acrylates only provide viscosity, while wood flour, as a polar foreign substance, becomes a stress concentration point (foam breakage source) during foaming and stretching, resulting in weak mechanical properties. Example 1 utilizes the in-situ etherification reaction between the epoxy groups of GMA and wood flour to "chemically anchor" the wood flour into a reinforcing skeleton within the continuous phase. Beneficial effects: At extremely low density (significant weight reduction), not only is mechanical strength not lost, but the bending resistance of the board is increased by nearly 70% through interfacial bridging, meeting the high-strength application requirements such as load-bearing and impact resistance.

[0057] Comparative Example 2 had component C (porous molecular sieve) removed. Its average pore size increased dramatically to 185 μm, its density soared to 0.58 g / cm³ (due to foam collapse), and its surface roughness reached 6.5 μm (due to dense pinholes on the surface).

[0058] The mechanism and beneficial effects confirm that this data strongly supports the core innovation of the "porous aqueous buffer carrier" in the claims. In Comparative Example 2, the 5% free water carried by the wood flour instantly boiled at the die, and the strong vapor pressure directly ruptured the thin pore walls, resulting in severe bubble formation and surface cracking. Example 1 utilizes the nanopores of molecular sieves (steric hindrance allows only water molecules to enter, blocking macromolecules) to precisely retain water in the melt and transform it into a physically foaming source with slow gas release.

[0059] It completely eliminates the collapse and pinhole defects caused by high moisture content wood flour, so that manufacturers do not need to spend a lot of electricity to completely dry the wood flour, which greatly reduces energy consumption costs, while obtaining the ultimate smooth skin of pure PVC boards.

[0060] Comparative Example 3 removed component B (LDH hydrotalcite). Its static thermal stability time was drastically reduced to 16 min (compared to 42 min in Example 1).

[0061] The large amount of wood flour added in wood-plastic extrusion leads to extremely high mechanical shear heat. In Comparative Example 3, PVC rapidly dechlorinated under strong shear, undergoing autocatalytic degradation, causing the melt viscosity to collapse during the critical foaming period. In Example 1, LDH absorbed the destructive hydrogen chloride in situ and irreversibly through interlayer ion exchange, completely interrupting the degradation chain reaction.

[0062] It provides a large thermal redundancy for the high-temperature extrusion process (the processing window is greatly widened), eliminates the serious "zinc burning (blackening)" accident caused by local friction overheating, and ensures long-term continuous and stable production; Comparative Example 4 had the same components as Example 1, but instead of using "spray drying co-granulation", it was physically dry-mixed. Its overall performance was inferior to that of Example 1 (density 0.53 g / cm³, pore size 135 μm, strength 16.8 MPa).

[0063] This comparative data strongly responds to the obviousness review opinion regarding the potential "patchwork of existing components." During physical mixing, ultra-high molecular weight powders are prone to self-agglomeration and clumping, and undergo "phase separation" with other inorganic powders in the screw, resulting in uneven performance across the microstructure. The "armor" formed by spray drying in Example 1 not only completely solves the problem of polymer powder bridging and clumping by relying on the outer inorganic nanoparticles, but also forcibly achieves the "synchronous release" of anti-boiling (C), acid absorption (B), nucleation (D), and reinforcement (A) functions in every micro-region of the PVC melt.

[0064] This greatly shortens the deentanglement and dispersion time of ultra-high molecular weight regulators in resin, enabling the composite material to exert a perfect synergistic effect even in injection molding foaming processes with extremely short material residence times.

[0065] Working mechanism: Because the low-density wood-plastic composite foam system is a complex, multi-phase, multi-field coupled extreme environment; it involves large-scale mechanical shearing and tensile puncture of rigid wood flour, accompanied by thermodynamic high-temperature boiling of free moisture in the wood flour, and superimposed with the chemical autocatalytic degradation of PVC caused by strong frictional heat. Traditional single-function foaming regulators, such as pure acrylic ester additives, can only passively provide viscosity, resulting in the instantaneous collapse of the melt network when faced with the simultaneous outbreak of the above-mentioned multi-source destructive forces. Macroscopically, this manifests as large-area collapse of cells, perforation, inability to reduce density, and a narrow processing window. In this technical solution, firstly, component A not only provides physical entanglement in the continuous phase through its ultra-high molecular weight backbone, but its specially introduced polar or reactive units in the shell also undergo ring-opening etherification reactions with the hydroxyl groups on the surface of wood flour at high temperatures, chemically anchoring the wood flour into a reinforcing skeleton and providing a puncture-resistant three-dimensional topological reinforcement network. During this process, component F (coupling agent) further acts as an interfacial bridge between inorganic particles and organic resin, avoiding stress concentration during multiphase mixing. Secondly, component C utilizes steric hindrance to physically trap the boiling water molecules released from the wood flour, converting them into gently releasing micro-gas reservoirs. Simultaneously, component B (such as magnesium aluminum hydrotalcite) undergoes a solid-gas phase ion exchange reaction with the hydrogen chloride released from PVC degradation.

[0066] During this process, the trace amounts of water vapor slowly released from the micropores of component C act as a medium for ion exchange, greatly promoting the migration and exchange kinetics of interlayer ions in component B, making the acid absorption reaction more rapid and instantly maintaining the melt viscosity of PVC; at the same time, component E (β-diketone compounds) locks in the catalytic activity of free metal ions through strong coordination chelation, and together with component B, constructs redundant thermal stability protection; and the carbon dioxide byproduct generated by the acid absorption reaction of component B, together with the water vapor released by component C, constitutes a uniform auxiliary physical foaming motive force.

[0067] Furthermore, component D (such as nano-calcium carbonate) and component B (nanosheets) which undergo in-situ exfoliation after absorbing hydrogen chloride provide numerous microscopic interfaces within the high-elasticity melt network of component A. These rough interfaces lower the Gibbs free energy barrier for bubble nucleation, resulting in an explosive increase in the number of bubble nuclei in the initial stage. The exponential increase in pore density allows the mechanical stress at the tip of the wood flour to be evenly distributed among thousands of tiny pores, thereby achieving macroscopic mechanical stability at low density. Through the synergy of the above mechanisms, this invention achieves stable free foaming of low-density PVC wood-plastic composites with high wood powder filling, with no large or collapsed bubbles in the cross-section, resulting in a uniform micro-foamed structure; it greatly expands the temperature and shear processing window of the extruder, reducing the scrap rate; due to its extremely high tolerance to moisture and degradation, it allows downstream enterprises to use inexpensive wood powder with higher moisture content and without deep drying, significantly reducing raw material and drying energy costs.

[0068] Because traditional foaming regulators are purely linear macromolecules, their molecular chains are prone to irreversible slippage under stretching conditions; at the same time, wood flour particles are large and have rigid sharp corners. This causes the thin cell walls, which are only a few micrometers thick, to be easily mechanically pierced by the wood flour tips during low-density foaming and expansion, resulting in bubble formation and large-area collapse.

[0069] This technical solution completely solves the problem by constructing a micro-crosslinked core-ultra-high viscosity linear shell topology: the outer layer of ultra-high molecular weight polymethyl methacrylate or polymethyl acrylate segments, due to their large mean square radius of gyration, can undergo rapid physical entanglement with the PVC continuous phase, providing the necessary melt viscosity; the inner layer, due to the introduction of flexible butyl acrylate segments with low glass transition temperature, combined with a three-dimensional covalent network formed by bifunctional groups, becomes an infusible, highly elastic physical anchor point at the processing temperature. When the wood powder tip pierces the cell wall, this crosslinked network deforms according to the principle of high molecular entropy elasticity, rapidly converting the sharp point stress applied by the wood powder into the surface stress of the crosslinked network deformation and dissipating it, giving the film extremely strong tear resistance, thereby completely eliminating the collapse of bubbles caused by wood powder puncture.

[0070] During operation, the main component of wood flour, cellulose, contains a large number of polar natural hydroxyl groups, while the PVC matrix is ​​weakly polar. This thermodynamic incompatibility creates an extremely fragile "sea-island" interface. Under the bidirectional tensile force generated by the expansion of the foaming gas, this interface is highly susceptible to microscopic peeling, becoming the primary source of defects leading to cell rupture. This technical solution addresses the aforementioned technical deficiencies by copolymerizing 0.1–10 mol% of glycidyl methacrylate or maleic anhydride in the linear shell of component A.

[0071] On the one hand, when the first group (such as GMA or maleic anhydride) is used, its side chain contains highly reactive epoxy groups or anhydride groups. In the microenvironment of the extruder at high temperatures of 170℃-190℃ and strong shear force of the screw, the epoxy groups in the regulator shell will undergo in-situ ring-opening etherification with the hydroxyl groups on the surface of the wood fiber:

[0072] Where R is the acrylate polymer backbone; It is a wood flour phase; It consists of highly reactive epoxy groups suspended on the side chain; This represents the high processing temperature of 170℃−190℃ inside the extruder; This represents the intense mechanical shearing and friction generated during twin-screw extrusion. This extreme microenvironment of "heat + force" provides ample activation energy for cross-phase chemical reactions; it bridges and anchors the originally physically repelled phase interfaces through covalent bonds.

[0073] On the other hand, when the second group is selected, its highly polar carboxyl and hydroxyl groups can form a dense three-dimensional hydrogen bond array at the interface between the continuous phase network and the wood flour, providing strong physical entanglement. The aforementioned chemical bonding or strong polar hydrogen bonding results in a tighter connection between one end of the foaming regulator and the wood flour, while the other end becomes entangled in the PVC network. This transforms the wood flour impurities, which would normally cause bubble breakage, into a strong and resilient pore-reinforced framework. This interfacial strengthening mechanism endows the material with excellent resistance to stress cracking, allowing it to maintain structural integrity even in high-strength energy-absorbing structures such as crash barriers.

[0074] Because the wood-plastic composite system contains a large amount of filler, the material generates intense frictional heat during extrusion; and the PVC molecular chain contains structural defects such as allyl chloride. This makes PVC extremely prone to autocatalytic degradation reactions involving the removal of hydrogen chloride, resulting in decreased melt strength and viscosity during the critical foaming period, an extremely narrow processing window, and even carbonization due to "burning". This technical solution addresses the aforementioned technical problems by incorporating magnesium-aluminum layered bimetallic hydroxides (LDHs, such as magnesium-aluminum hydrotalcite) into the regulator powder. LDH has a unique layered crystal structure with exchangeable carbonate ions between the layers. When PVC is heated and releases free hydrogen chloride, which has a catalytic destructive effect, LDH undergoes a gas-solid phase reaction, absorbing and neutralizing it through interlayer anion exchange.

[0075] Not only does it completely eliminate the autocatalytic degradation of PVC through dechlorination, maintaining high viscosity, but its reaction byproduct, carbon dioxide, can also serve as an extremely uniform auxiliary physical foaming agent. Simultaneously, the LDH interlayers expand and peel off after absorbing chloride ions, and the peeled nanosheets form excellent heterogeneous nucleation sites in situ within the melt.

[0076] Natural wood flour contains bound water and small volatile molecules that are difficult to remove through conventional drying. This causes the moisture to vaporize and boil violently when the material leaves the extruder die and the pressure drops sharply. This uncontrollable high-energy vapor pressure directly ruptures the weak cell walls, resulting in large voids in the cross-section of the part and dense pinholes on the surface. This technical solution addresses this issue by using a porous carrier (such as 3A / 4A molecular sieves) with an average pore size between 0.3nm and 50nm.

[0077] The dynamic diameter of a water molecule is about 0.26 nm, while the mean square radius of gyration of ultra-high molecular weight acrylate molecular chains is as high as tens of nanometers.

[0078] During high-temperature mixing, due to the absolute steric hindrance effect, the huge polymer chains cannot enter the micropores of the molecular sieve, thus ensuring the unobstructed flow of the pores.

[0079] At this point, the silica-alumina framework inside the molecular sieve precisely captures high-energy water molecules that are moving violently in the melt through hydrogen bonds. When the material is extruded through the die, these micropores can effectively capture and analyze the high-energy water, transforming the originally instantaneously boiling steam into a controlled gaseous force that is released smoothly in nanoseconds, completely eliminating boiling and tearing. Even in extremely high humidity environments (such as in the processing of deep rock support templates), it can still ensure that the pores are fine and uniform.

[0080] High-viscosity acrylic latexes exhibit extremely strong surface tack above their glass transition temperature. This results in strong interdiffusion of polymer chains between polymer particles during powder post-processing (drying, packaging) in chemical plants and during hopper feeding in downstream wood-plastic composite enterprises, leading to severe powder adhesion, clumping, bridging during feeding, and even difficulty in rapid dispersion in dry mixes. This technical solution solves the problem by spray drying and co-granulating component D (inorganic nanoparticles) with component A to form a microscopic composite structure with component A as the core and inorganic nanoparticles densely coated on its surface. During the high-temperature atomization moment in the spray drying tower, the rapid evaporation of water on the surface of the droplets generates a strong capillary contraction force; rigid inorganic nanoparticles with different surface charges (such as nano-calcium carbonate) undergo self-assembly under the drive of electrostatic action, migrate to the outer layer and densely coat the surface of soft polymer particles, forming a dense inorganic isolation layer. In its macroscopic powder state, this insulating layer physically severs the contact and diffusion paths of macromolecular chains between adjacent polymer particles, imparting flowability to the powder. However, when it enters the extruder and is sheared and heated, the coating layer rapidly breaks down, releasing an extremely high-viscosity polymer network and numerous nucleation sites, achieving a balance between flowability and high viscosity.

[0081] When using environmentally friendly calcium-zinc stabilizers, zinc chloride, a byproduct formed after absorbing hydrogen chloride, is a Lewis acid with extremely strong electron acceptor properties. This causes it to strongly catalyze the removal of adjacent chlorine and hydrogen atoms from PVC molecular chains, resulting in the material instantly turning black and cross-linking within minutes (i.e., the malignant "zinc burning" phenomenon). This technical solution addresses the issue by introducing a small amount of β-diketone compounds (such as stearoylbenzoylmethane SBM); β-diketone compounds exhibit keto-enol tautomerism at processing temperatures. The oxygen atom in its enol form carries a lone pair of electrons, enabling it to undergo a strong coordination complexation reaction with electron-deficient zinc ions. Molecular-level effects:

[0082] The resulting chelate forms a highly stable six-membered ring spatial structure, completely sealing the empty orbitals of zinc ions and eliminating Lewis acid catalytic activity. The hydrogen chloride released in the reaction is then captured again by LDH in the component. This redundant design provides ultimate thermal stability during processing.

[0083] The embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A high-viscosity foaming modifier for low-density PVC wood-plastic composites, characterized in that: The foaming regulator is a powder composition, comprising, by mass parts; Component A: 40-90 parts of core-shell acrylate copolymer particles; Component B: 2–40 parts of layered bimetallic hydroxide; Component C: 0.2–20 parts of inorganic porous support with an average pore size of 0.3–50 nm; Component D: 0.05–10 parts of inorganic nanonucleating agent or carbon nanomaterials; Component E: 0.1–5 parts of β-diketone compounds or β-dicarbonyl compounds; Component F: 0.1–5 parts coupling agent; Wherein, the core layer of component A is an acrylate phase containing cross-linked monomer units; its shell layer contains polar monomer units or reactive monomer units; component B and component D respectively coat or adsorb onto the surface of the particles of component A to form a composite precursor, and the composite precursor combines with component C to form a composite particle powder.

2. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 1, characterized in that: Component A is formed by at least two stages of emulsion polymerization; the core layer further comprises butyl acrylate units; the shell layer comprises polymethyl acrylate, polymethyl methacrylate, or copolymers thereof; the mass ratio of the core layer to the shell layer is from 5:95 to 40:

60.

3. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 2, characterized in that: The shell of component A contains a certain number of moles of shell monomers. The polar monomer unit or reactive monomer unit, wherein the reactive monomer unit is selected from at least one of glycidyl methacrylate, maleic anhydride, and maleic half ester. The polar monomer unit is selected from at least one of methacrylic acid, acrylic acid, and hydroxyethyl acrylate.

4. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 3, characterized in that: The core layer contains crosslinked monomer units formed by polymerization of crosslinked monomers or branched monomers, wherein the crosslinked monomers or branched monomers are selected from at least one of allyl methacrylate, ethylene glycol dimethacrylate, and trimethylolpropane trimethacrylate.

5. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 4, characterized in that: Component B is selected from at least one of magnesium aluminum hydrotalcite, zinc aluminum hydrotalcite, calcium aluminum hydrotalcite, and magnesium zinc aluminum hydrotalcite.

6. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 5, characterized in that: The C component is selected from at least one of 3A molecular sieve, 4A molecular sieve, mesoporous silica, activated alumina, silica gel, and zeolite.

7. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 6, characterized in that: The D component is selected from at least one of nano-calcium carbonate, nano-silica, titanium dioxide, montmorillonite, carbon nanotubes, and graphene.

8. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 7, characterized in that: The E component is selected from at least one of zinc acetylacetone, acetylacetone, and stearoylbenzoylmethane.

9. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 8, characterized in that: The composite granular powder is formed by the following process: an aqueous dispersion containing the components B, D, E, and F is introduced into the latex of component A, so that it is adsorbed on the surface of the particles of component A to form a composite precursor slurry. Subsequently, the composite precursor slurry is mixed with the dry powder of component C and spray-dried together to obtain the final product.

10. The ultra-high viscosity foaming regulator for low-density PVC wood-plastic composites according to claim 9, characterized in that: The bulk density of the composite granular powder is 0.30–0.70 g / cm³.

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