Straw modification method and preparation method of wood-plastic composite material with high straw content and high water resistance

Abstract

The invention discloses a straw modification method and a preparation method of a wood-plastic composite material with high straw content and high water resistance. The modification method comprises the following steps: adding the straw powder into a high-speed mixer, adding the hydroxypropyl emulsion in a spraying manner, mixing to 90 DEG C, adding the polyisocyanate in a spraying manner, mixing to 110 DEG C, discharging, and cooling to obtain the modified straw powder. The invention provides a preparation method of a wood-plastic composite material, the special equipment comprises a parallel twin-screw extruder, the parallel twin-screw extruder is provided with a weightlessness type forced metering feeding device used for pushing modified straw powder into the parallel twin-screw extruder, and the modified straw powder and other raw materials are gradually mixed and plasticized in a machine barrel. The wood-plastic composite material prepared by the preparation method disclosed by the invention has relatively good water absorption resistance. Meanwhile, the problems that in the production process, when straw powder with low stacking density is mixed with other materials, mixing is not uniform, layering is serious, and when the straw powder enters a feeding opening of a parallel double-screw extruder, feeding is difficult, and stable production is difficult are solved.

Description

Technical Field

[0001] The invention relates to the field of composite materials, and in particular to a straw modification method and a method for preparing a high-straw-content and highly water-resistant wood-plastic composite material. Background Art

[0002] Wood-plastic composites are made by combining thermoplastics with plant fibers, so they have the dual properties of plastic and wood. Straw is a plant fiber with a very large output, but because the straw has polar hydroxyl groups inside and on the surface, especially when the straw content is high and the straw particles are large, the water absorption resistance is low and the water absorption expansion rate is large. As a co-extruded wood-plastic composite material with a core-shell structure, the shell plastic almost blocks the invasion of water molecules. However, after the cut end face and the hollow holes absorb water, the core layer expands, especially the cut end face absorbs water and expands significantly, which can even reach more than 10%. Therefore, the preparation of a high straw content and high water resistance wood-plastic composite material has great market application value.

[0003] When the straw is crushed, the bulk density is usually relatively small. When the conventional high-speed mixer is used to mix the materials (such as polyethylene, which is usually in granular form; lubricant, which is usually in powder form), the mixing is uneven and the stratification is serious. Moreover, when the raw materials with low bulk density enter the feeding port of the parallel twin-screw extruder, it is difficult to feed and stabilize the production. It is necessary to improve the mixing and feeding methods of the parallel twin-screw extruder. Summary of the invention

[0004] The purpose of the present invention is to provide a straw modification method and a method for preparing a wood-plastic composite material.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a method for modifying straw, wherein the modification method comprises adding straw powder into a high-speed mixer, starting the high-speed mixer, spraying and adding hydroxypropyl emulsion, mixing to 90° C. (friction heat generated during high-speed mixing may cause the material temperature to rise), spraying and adding blocked polyisocyanate, mixing to 110° C., discharging and cooling the material to obtain modified straw powder;

[0007] Taking the total mass of straw powder, hydroxypropyl emulsion and polyisocyanate as 100%, the mass percentage contents of straw powder, hydroxypropyl emulsion and blocked polyisocyanate are as follows: 70-99.8% of straw, 0.1-15% of hydroxypropyl emulsion and 0.1-15% of polyisocyanate.

[0008] Preferably, taking the total mass of straw powder, hydroxypropyl emulsion and polyisocyanate as 100%, the mass percentage contents of straw powder, hydroxypropyl emulsion and blocked polyisocyanate are: 80-95% straw, 2-6% hydroxypropyl emulsion and 5-15% polyisocyanate.

[0009] As a further preference, the mass dosage of the hydroxypropyl emulsion is 5% of the mass of the straw powder, and the mass dosage of the blocked polyisocyanate is 10% of the mass of the straw powder.

[0010] Preferably, the particle size of the straw powder is 5-60 mesh, more preferably 5-20 mesh.

[0011] The hydroxypropyl emulsion is a polyacrylic acid emulsion containing side hydroxyl functional groups, which is prepared by emulsion polymerization, and the side chain may contain fluorine or not. After drying, the hydroxypropyl emulsion has a dry matter weight remaining of 20-60%, of which the hydroxyl content accounts for 1-10% of the dry matter weight.

[0012] The blocked polyisocyanate is a compound containing two or more isocyanate groups (-NCO).

[0013] In a second aspect, the present invention provides a method for preparing a wood-plastic composite material, wherein the special equipment for the preparation method comprises a parallel twin-screw extruder, a conical twin-screw extruder, a single screw extruder and a co-extrusion die, and the conical twin-screw extruder and the single screw extruder are connected through a co-extrusion die, the parallel twin-screw extruder comprises a barrel and two parallel extruder screws arranged on the barrel, the extruder screws are connected to a drive motor through a gearbox; one end of the two extruder screws is connected to the gearbox, and the other end is flush with the end of the barrel and exposed at the end of the barrel; a weightless metering feeding integrated device and a weightless forced metering feeding device are respectively arranged at the feed port position of the extruder screw, and the weightless forced metering feeding device is provided with a weightless metering device, a drive motor and a feeding screw; the preparation method comprises the following steps:

[0014] (a) obtaining modified straw powder according to the modification method described in the first aspect;

[0015] (b) the modified straw powder is fed into a feeding screw through a weightless metering device, and a motor forces the straw powder into a parallel twin-screw extruder, and maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additive are fed into the extruder screw through a weightless metering feeding integrated device in proportion, so that the above raw materials are plasticized and granulated in the parallel twin-screw extruder to obtain a core layer material;

[0016] (c) adding the core layer material into a conical twin-screw extruder and extruding the core layer through a co-extrusion die; adding the shell layer plastic into a single screw extruder and extruding the shell layer through a co-extrusion die, wherein the shell layer plastic is evenly coated on the core layer through the co-extrusion die to obtain a co-extruded wood-plastic composite material with a core-shell structure;

[0017] Calculated by mass percentage, the contents of the components in the core layer material are: 50-75% of modified straw powder, 1-5% of maleic anhydride grafted PE, 47-16% of polyethylene, 1-2% of wood plastic lubricant, and 1-2% of anti-aging additive.

[0018] The wood-plastic lubricant described in the present invention refers to a lubricant specifically used for the production and processing of wood-plastic composite materials, and a commercially available product can be selected according to needs.

[0019] The anti-aging aid of the present invention can be an anti-aging aid commonly used in the production and processing of wood-plastic composite materials. The present invention specifically recommends that the anti-aging aid is selected from a combination of antioxidant 1010, antioxidant 168, light stabilizer 622, light stabilizer 944, and ultraviolet absorber UV531, preferably in a mass ratio of 1:1:1:1:1.

[0020] The shell plastic of the present invention may be a commonly used shell plastic of wood-plastic composite materials, such as at least one of polyethylene, polypropylene, and sarling resin.

[0021] In the present invention, the conical twin-screw extruder, single-screw extruder and co-extrusion die can all use conventional products.

[0022] Preferably, the two ends of the feeding screw are a feed port and a discharge port, respectively, and the discharge port is located in the feed tangent direction when the extruder screw rotates; the spiral from the feed port to the discharge port of the feeding screw gradually shrinks, and the groove depth of the screw groove becomes shallower. As a further preference, the feeding screw is tapered as a whole, the feed port screw diameter is large, and the discharge port screw diameter is small; the feed port end of the feeding screw is drive-connected to the drive end of the motor.

[0023] Preferably, both of the extruder screws have a core shaft arranged inside a barrel, and a plurality of threaded elements are penetrated through the core shaft in a building block-like manner, and the plurality of threaded elements include conveying threaded elements, kneading threaded elements, and SK threaded elements.

[0024] As a further preference, the barrel is composed of sub-barrels connected in sections, and the first sub-barrel after the inlet is defined as zone one, and so on, zone two, zone three..., and the specific number of zones is defined by the industry; the loss-in-weight forced metering feeding device is arranged in zone three of the barrel;

[0025] A plurality of SK thread elements with large lead are arranged on the mandrel of the third zone of the barrel along the length direction of the mandrel, the pitch of the SK thread element reaches 1-2 times of the element diameter, and the ratio of the outer diameter to the inner diameter of the SK thread element is greater than 1.5;

[0026] The mandrel in the second zone of the barrel is provided with a plurality of kneading screw elements along the length direction of the mandrel, which are used for melting and mixing the materials;

[0027] A plurality of groups of conveying screw elements and kneading screw elements are arranged along the length direction of the mandrel starting from the mandrel of the fifth zone of the barrel, and the conveying screw elements and the kneading screw elements are arranged alternately;

[0028] Starting from the core shaft of the tenth zone of the barrel, only a conveying thread element is arranged along the length direction of the core shaft for conveying materials.

[0029] Preferably, the integrated loss-in-weight metering feeding device is composed of a plurality of loss-in-weight metering feeding devices, which are used to add maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additives to the parallel twin-screw extruder; a servo motor, a raw material storage barrel, and a feeding metering scale are provided at the upper end of the feed hopper of the plurality of loss-in-weight metering feeding devices, the mass of the required feeding is controlled by the speed of the servo motor, and the feeding mass is fed back and automatically fine-tuned by the feeding metering scale to ensure metering accuracy.

[0030] Preferably, the step (b) is specifically implemented as follows: start the parallel twin-screw extruder, preheat for 1-2 hours, and set the temperature as follows: barrel zone 1 130-150°C and zone 2 180-200°C, zone 3 130-150°C, zone 4 150-180°C, zones 5 to 10 180-200°C, zone 11 150-160°C, zone 12 130-150°C; after the preheating is completed, start the parallel twin-screw extruder main engine, the main engine speed is 400-500 rpm, start and adjust the weightless metering feeding integrated device and the weightless forced feeding metering device. The feeding quality makes the parallel twin-screw extruder main engine current reach 50-80% of the rated current, plasticize and granulate, cool, and obtain the core layer material;

[0031] Preheating a conical twin-screw extruder, a single-screw extruder and a mold for 2 hours, setting the temperature of the first and second zones of the conical twin-screw extruder to 160-200° C., the temperature of the third and fourth zones of the screw to 130-150° C., and the temperature of the confluence core to 130-140° C.; setting the temperature of the first zone of the single-screw extruder to 90-130° C., and the temperature of the second and third zones of the screw to 180-200° C.; setting the mold temperature to 140-160° C. for the core layer and 200-220° C. for the shell layer; after preheating, adding the core layer material to the conical twin-screw extruder to extrude the core layer through the mold, the main engine speed is 10-30 rpm, and the feeding speed is adjusted so that the current is 50-80% of the rated current; adding the shell layer plastic to the single-screw extruder to extrude the shell layer through the mold, so that the shell layer plastic is evenly coated on the core layer through the mold, and a co-extruded wood-plastic composite material with a core-shell structure is obtained.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The co-extruded wood-plastic composite material with a core-shell structure obtained by modifying the straw powder has a low water absorption expansion rate and good anti-water absorption performance.

[0034] (2) The parallel twin-screw extruder used in the present invention is designed to feed raw materials such as polyethylene, maleic anhydride grafted PE, wood plastic lubricant, anti-aging additives into the parallel twin-screw extruder through a weightless metering feeding integrated device, and gradually melt in the first and second zones of the barrel; the modified straw powder with low bulk density is metered by weightlessness and forced feeding to be pushed into the parallel twin-screw extruder, and the third zone of the barrel is provided with a screw element with a large lead and a deep screw groove, which is conducive to the modified straw powder entering the parallel twin-screw extruder smoothly and stably, and gradually mixing and plasticizing with other raw materials in the barrel. The present invention solves the problem of accurate metering of different raw materials, and at the same time solves the problem of uneven mixing and severe stratification when the straw powder with low bulk density is mixed with other materials (such as polyethylene, generally granular; lubricant, generally powdered) in the production process when high-content filling is used, and at the same time solves the problem of difficulty in feeding and difficulty in stable production when the straw powder with low bulk density enters the feeding port of the parallel twin-screw extruder. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a structural schematic diagram of a weightless forced metering feeding device, wherein 1: weightless metering device, 2: motor, 3: feeding screw, 4: parallel twin-screw extruder barrel, 5: parallel twin-screw extruder screw;

[0036] Figure 2 It is a structural schematic diagram of a parallel twin-screw extruder, wherein: 6: a parallel twin-screw extruder driving main motor, 7: a parallel twin-screw extruder gearbox, 8: a weightless metering feeding integrated device, 9: a straw weightless metering feeding device, 4: a parallel twin-screw extruder barrel;

[0037] Figure 3 It is a structural schematic diagram of a loss-in-weight metering feeding integrated device, wherein: 10: a feed hopper of the loss-in-weight metering feeding integrated device, 11: a servo motor of the loss-in-weight metering feeding device, 12: a raw material storage barrel, and 13: a feeding metering scale. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below through specific embodiments, but the protection scope of the present invention is not limited thereto.

[0039] If no specific conditions are specified in the examples of the present invention, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0040] The invention provides a method for preparing a wood-plastic composite material. The special equipment for the preparation method comprises a parallel twin-screw extruder, a conical twin-screw extruder, a single-screw extruder and a co-extrusion die, and the conical twin-screw extruder and the single-screw extruder are connected through the co-extrusion die;

[0041] like Figure 1 , Figure 2 and Figure 3 As shown, the parallel twin-screw extruder includes a barrel 4 and two parallel extruder screws 5 arranged on the barrel 4, and the extruder screw 5 is connected to the drive motor 6 through a gearbox 7; one end of the two extruder screws 5 is connected to the gearbox 7, and the other end is flush with the end of the barrel 4 and exposed at the end of the barrel 4; the feed port position of the extruder screw 5 is respectively provided with a weightless metering feeding integrated device 8 and a weightless forced metering feeding device 9, and the weightless forced metering feeding device 9 is provided with a weightless metering device 1, a drive motor 2 and a feeding screw 3, the modified straw powder enters the feeding screw 3 through the weightless metering device 1, and the motor 2 forces the straw powder into the parallel twin-screw extruder, and the maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additive enter the extruder screw 5 through the weightless metering feeding integrated device 8 in proportion.

[0042] Specifically, the two ends of the feeding screw 3 are respectively a feed port and a discharge port, and the discharge port is located in the feed tangent direction when the extruder screw 5 rotates; the spiral from the feed port to the discharge port of the feeding screw 3 gradually shrinks, and the groove depth of the screw groove becomes shallower; more specifically, the feeding screw 3 is conical as a whole, the feed port screw diameter is large, and the discharge port screw diameter is small; the feed port end of the feeding screw 3 is drive-connected to the driving end of the motor 2.

[0043] Specifically, the two extruder screws 5 each have a core shaft disposed inside the barrel 4 , and a plurality of threaded elements are penetrated through the core shaft in a modular manner, and the plurality of threaded elements include a conveying threaded element, a kneading threaded element, and an SK threaded element.

[0044] Specifically, the barrel 4 is composed of sub-barrels connected in sections, and the first sub-barrel after the inlet is defined as zone 1, and so on, zone 2, zone 3, etc. The specific number of zones is defined by the industry; the loss-in-weight forced metering feeding device 9 is set in zone 3 of the barrel;

[0045] A plurality of SK thread elements with large lead are arranged on the mandrel of the third zone of the barrel along the length direction of the mandrel, the pitch of the SK thread element reaches 1-2 times of the element diameter, and the ratio of the outer diameter to the inner diameter of the SK thread element is greater than 1.5;

[0046] The mandrel in the second zone of the barrel is provided with a plurality of kneading screw elements along the length direction of the mandrel, which are used for partial melting and mixing of materials;

[0047] A plurality of groups of conveying screw elements and kneading screw elements are arranged along the length direction of the mandrel starting from the mandrel of the fifth zone of the barrel, and the conveying screw elements and the kneading screw elements are arranged alternately;

[0048] Starting from the core shaft of the tenth zone of the barrel, only a conveying thread element is arranged along the length direction of the core shaft for conveying materials.

[0049] Specifically, the weightless metering feeding integrated device 8 is composed of a plurality of weightless metering feeding devices, which are used to add maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additives to the parallel twin-screw extruder; the upper ends of the feed hoppers 10 of the plurality of weightless metering feeding devices are provided with a servo motor 11, a raw material storage barrel 12, and a feeding metering scale 13, the speed of the servo motor 12 is used to control the mass of the required feeding, and the feeding metering scale 13 is used to feedback and automatically fine-tune the feeding mass to ensure the metering accuracy.

[0050] Embodiment 1:

[0051] Preparation of modified straw powder: Add straw powder to a high-speed mixer, start the high-speed mixer, spray 5% of the weight of straw powder with hydroxypropyl emulsion (Chuanhua Zhilian Co., Ltd., model TF3201R, fluorine-free), mix to 90°C (friction heat generated during high-speed mixing will cause the material temperature to rise). Then spray blocked polyisocyanate (Jining Huakai Resin Co., Ltd., WB-908) in an amount of 10% of the weight of straw powder, mix to 110°C, discharge and cool for use.

[0052] The modified straw powder is added to the weightless forced metering feeding device 9, maleic anhydride grafted PE, polyethylene, wood plastic lubricant (Zhejiang Jieshangjie Chemical Co., Ltd., model H832), anti-aging additives (antioxidants 1010 and 168, light stabilizers 622 and 944, ultraviolet absorber UV531 in a mass ratio of 1:1:1:1:1) are added to the weightless metering feeding integrated device 8, and the metering feeding is set according to the weight ratio of modified straw powder, maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additive of 75%:3%:18%:2%:2%. The parallel twin-screw extruder is turned on for plasticizing and granulation, and the core layer material is added to the co-extrusion conical twin-screw extruder unit for extrusion molding.

[0053] The parallel twin-screw extruder was started and preheated for 2 hours. The temperature was set as follows: 150°C in zone 1 and 180°C in zone 2, 130°C in zone 3, 150°C in zone 4, 180°C in zones 5 to 10, 160°C in zone 11, and 150°C in zone 12. After the preheating was completed, the main machine of the parallel twin-screw extruder was started, the main machine speed was 400 rpm, the feeding quality of the weightless metering feeding integrated device 8 and the weightless forced metering feeding device 9 was started and adjusted, so that the main machine current of the parallel twin-screw extruder reached 80% of the rated current, plasticized and granulated, and cooled to obtain the core layer material of the wood-plastic composite material. Preheat the conical twin-screw extruder, the single-screw extruder and the mold for 2 hours. The temperature of the screw zone 1 and zone 2 of the conical twin-screw extruder is 180°C, the temperature of the screw zone 3 and zone 4 is 150°C, the temperature of the confluence core is 140°C, the temperature of the single-screw zone 1 is 90°C, the temperature of the screw zone 2 and zone 3 is 200°C, and the mold temperature is 160°C for the core layer and 200°C for the shell layer. After the heating is completed, the core layer material is added to the conical twin-screw extruder to extrude the core layer through the co-extrusion die. The main engine speed is 10 rpm. The feed speed is adjusted so that the current is 80% of the rated current. The shell layer plastic (polyethylene) is added to the single-screw extruder to extrude the shell layer through the co-extrusion die. The single screw speed is adjusted so that the shell layer plastic is evenly coated on the core layer through the die to obtain a co-extruded wood-plastic composite material with a core-shell structure.

[0054] The obtained wood-plastic composite material was tested by immersion in hot water at 98° C. for 24 hours, and the water absorption thickness expansion rate was 3.5%.

[0055] Embodiment 2:

[0056] This embodiment refers to Embodiment 1, wherein:

[0057] Preparation of modified straw powder: Add straw powder to a high-speed mixer, start the high-speed mixer, spray 2.5% hydroxypropyl emulsion by weight of straw powder, and mix to 90° C. Then spray blocked polyisocyanate in an amount of 5% by weight of straw powder, mix to 110° C., discharge and cool for later use.

[0058] Add modified straw powder, maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additives to a weight-loss weighing scale, and set the metering feeding according to a weight ratio of 65%: 3%: 28%: 2%: 2%. Start the parallel twin-screw extruder for plasticizing and granulation, and add the core layer material to the co-extrusion conical twin-screw extruder-single screw extruder unit for extrusion molding.

[0059] Other conditions are the same as in Example 1.

[0060] The obtained wood-plastic composite material was tested by immersion in hot water at 98° C. for 24 hours, and the water absorption thickness expansion rate was about 2%.

[0061] Embodiment 3:

[0062] This embodiment refers to Embodiment 1, wherein:

[0063] Preparation of modified straw powder: Add straw powder to a high-speed mixer, start the high-speed mixer, spray 1.5% hydroxypropyl emulsion by weight of straw powder, and mix to 90° C. Then spray blocked polyisocyanate at 3% by weight of straw powder, mix to 110° C., discharge and cool for later use.

[0064] Add modified straw powder, maleic anhydride grafted PE, polyethylene, lubricant, and anti-aging additives to a weight-loss weighing scale, and set the metering feeding according to a weight ratio of 65%: 3%: 28%: 2%: 2%. Start the parallel twin-screw extruder for plasticizing and granulation, and add the granulated material to the co-extrusion conical twin-screw extruder-single screw extruder unit for extrusion molding.

[0065] Other conditions are the same as in Example 1.

[0066] The obtained wood-plastic composite material was tested by immersion in hot water at 98°C for 24 hours, and the water absorption thickness expansion rate was 2.4%.

Claims

1. A method for modifying straw, characterized in that: The modification method comprises adding straw powder into a high-speed mixer, starting the high-speed mixer, spraying hydroxypropyl emulsion, mixing to 90° C., spraying blocked polyisocyanate, mixing to 110° C., discharging and cooling to obtain modified straw powder; Taking the total mass of straw powder, hydroxypropyl emulsion and polyisocyanate as 100%, the mass percentage contents of straw powder, hydroxypropyl emulsion and blocked polyisocyanate are as follows: 70-99.8% of straw, 0.1-15% of hydroxypropyl emulsion and 0.1-15% of polyisocyanate.

2. The modification method according to claim 1, characterized in that: Taking the total mass of straw powder, hydroxypropyl emulsion and blocked polyisocyanate as 100%, the mass percentage contents of straw powder, hydroxypropyl emulsion and blocked polyisocyanate are as follows: 80-95% of straw, 2-6% of hydroxypropyl emulsion and 5-15% of blocked polyisocyanate.

3. The modification method according to claim 1, characterized in that: The mass dosage of the hydroxypropyl emulsion is 5% of the mass of the straw powder, and the mass dosage of the blocked polyisocyanate is 10% of the mass of the straw powder.

4. The modification method according to claim 1, characterized in that: After the hydroxypropyl emulsion is dried, the weight of the dry matter remains 20-60%, wherein the hydroxyl content accounts for 1-10% of the weight of the dry matter.

5. A method for preparing a wood-plastic composite material, characterized in that: The special equipment of the preparation method comprises a parallel twin-screw extruder, a conical twin-screw extruder, a single-screw extruder and a co-extrusion die, and the conical twin-screw extruder and the single-screw extruder are connected through the co-extrusion die, the parallel twin-screw extruder comprises a barrel (4) and two parallel extruder screws (5) arranged on the barrel (4), and the extruder screws (5) are connected to the drive motor (6) through a gearbox (7); one end of the two extruder screws (5) is connected to the gearbox (7), and the other end is flush with the end of the barrel (4) and exposed at the end of the barrel (4); the feed port position of the extruder screw (5) is respectively provided with a weightless metering feeding integrated device (8) and a weightless forced metering feeding device (9), and the weightless forced metering feeding device (9) is provided with a weightless metering device (1), a drive motor (2) and a feeding screw (3); the preparation method comprises the following steps: (a) obtaining modified straw powder by the modification method according to any one of claims 1 to 4; (b) the modified straw powder is passed through a weightless metering device (1) into a feeding screw (3), a motor (2) forces the straw powder into a parallel twin-screw extruder, maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additive are passed through a weightless metering feeding integrated device (8) in proportion into the extruder screw (5), and the above raw materials are plasticized and granulated in the parallel twin-screw extruder to obtain a core layer material; (c) adding the core layer material into a conical twin-screw extruder and extruding the core layer through a co-extrusion die; adding the shell layer plastic into a single screw extruder and extruding the shell layer through a co-extrusion die, wherein the shell layer plastic is evenly coated on the core layer through the co-extrusion die to obtain a co-extruded wood-plastic composite material with a core-shell structure; Calculated by mass percentage, the contents of the components in the core layer material are: 50-75% of modified straw powder, 1-5% of maleic anhydride grafted PE, 47-16% of polyethylene, 1-2% of wood plastic lubricant, and 1-2% of anti-aging additive.

6. The preparation method according to claim 5, characterized in that: The two ends of the feeding screw (3) are respectively a feed port and a discharge port, and the discharge port is located in the feed tangent direction when the extruder screw (5) rotates; the spiral from the feed port to the discharge port of the feeding screw (3) gradually shrinks, and the groove depth of the screw groove becomes shallower.

7. The preparation method according to claim 6, characterized in that: The feeding screw (3) is tapered as a whole, the feeding port screw has a large diameter, and the discharge port screw has a small diameter; the feeding port end of the feeding screw (3) is drivingly connected to the driving end of the motor (2).

8. The preparation method according to claim 5, characterized in that: The two extruder screws (5) each have a core shaft arranged inside the barrel (4), and a plurality of threaded elements are penetrated on the core shaft in a modular manner, wherein the plurality of threaded elements include a conveying threaded element, a kneading threaded element, and an SK threaded element.

9. The preparation method according to claim 8, characterized in that: The barrel (4) is composed of a series of sub-barrels connected together, and the first sub-barrel after the inlet is defined as zone 1, and so on, zone 2, zone 3, etc.; the weightless forced metering feeding device (9) is arranged in zone 3 of the barrel; A plurality of SK thread elements with large lead are arranged on the mandrel of the third zone of the barrel along the length direction of the mandrel, the pitch of the SK thread element reaches 1-2 times of the element diameter, and the ratio of the outer diameter to the inner diameter of the SK thread element is greater than 1.5; The mandrel in the second zone of the barrel is provided with a plurality of kneading screw elements along the length direction of the mandrel, which are used for melting and mixing the materials; A plurality of groups of conveying screw elements and kneading screw elements are arranged along the length direction of the mandrel starting from the mandrel of the fifth zone of the barrel, and the conveying screw elements and the kneading screw elements are arranged alternately; Starting from the core shaft of the tenth zone of the barrel, only a conveying thread element is arranged along the length direction of the core shaft for conveying materials.

10. The preparation method according to claim 5, characterized in that: The weightless metering feeding integrated device (8) is composed of a plurality of weightless metering feeding devices, and is used to add maleic anhydride grafted PE, polyethylene, wood plastic lubricant, and anti-aging additives to the parallel twin-screw extruder; the upper ends of the feed hoppers (10) of the plurality of weightless metering feeding devices are provided with a servo motor (11), a raw material storage barrel (12), and a feeding metering scale (13); the mass of the required feeding is controlled by the rotation speed of the servo motor (11), and the feeding metering scale (13) is used to feedback and automatically fine-tune the feeding mass to ensure metering accuracy.

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