Preparation method of wood-plastic material
By modifying wood flour with polyisocyanate and adding a stepwise chain extender, the problems of poor interfacial compatibility between wood flour and PLA and low toughening efficiency of PLA were solved, achieving high mechanical properties and low water absorption of the material while maintaining good processing fluidity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HAINAN LIHUI NEW MATERIALS CO LTD
- Filing Date
- 2026-04-28
- Publication Date
- 2026-06-16
AI Technical Summary
Existing technologies suffer from poor interfacial compatibility between wood flour and PLA, low toughening efficiency of PLA and PBAT, and a decrease in PLA molecular weight due to the one-time addition of chain extenders, making it difficult to achieve optimal overall performance of composite materials.
Polyisocyanate-modified wood flour is used while retaining free isocyanate groups on the surface. It reacts in situ with the polylactic acid chain ends to form a urethane bond covalent link layer. By adding chain extenders in steps, PBAT active masterbatch is pre-prepared in the pre-mixing stage and chain extenders are added in the melt mixing stage to control reactivity compatibility and inhibit excessive local chain extension of PLA.
It significantly improves the mechanical properties of the material, reduces water absorption, and maintains the material's processing fluidity and overall performance.
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Figure SMS_1
Abstract
Description
Technical Field
[0001] This invention relates to the field of wood-plastic composite materials technology, specifically a method for preparing wood-plastic materials. Background Technology
[0002] Polylactic acid (PLA), a fully biodegradable polymer derived from renewable resources, possesses excellent mechanical properties and biocompatibility, making it one of the most promising green structural materials for industrialization. However, PLA's inherent brittleness and high cost limit its large-scale application. Introducing wood flour as a filler modifier into the PLA matrix can not only reduce material costs but also impart a natural wood-like texture to products, representing an important approach to expanding PLA's application scenarios.
[0003] Wood flour has a high concentration of hydrophilic hydroxyl groups on its surface, resulting in poor interfacial compatibility with the PLA matrix. The lack of effective chemical bonding between the two phases leads to weak interfacial bonding, insufficient mechanical properties, and high water absorption in the composite material. Current technologies often use silane coupling agents to treat the surface of wood flour. However, the interaction between silane coupling agents and both wood flour and PLA relies primarily on physical adsorption and a small amount of condensation reaction. The interfacial bonding is essentially still a physical bridging, offering limited improvement to mechanical properties and water resistance.
[0004] The insufficient toughness of PLA is typically addressed by introducing polybutylene adipate terephthalate (PBAT) for toughening modification. However, PLA and PBAT have significantly different solubility parameters, resulting in poor interfacial compatibility and limited toughening efficiency when directly blended. Existing technologies often employ chain extenders containing polyepoxy functional groups to reactively compatibilize the PLA / PBAT system. However, when these chain extenders are added all at once during the melt mixing stage, it can easily lead to excessive chain extension in certain areas, broadening the PLA molecular weight distribution and reducing processing fluidity. In the PLA / PBAT / wood flour ternary system, existing technologies lack a synergistic solution to address both wood flour interfacial modification and PLA / PBAT compatibility issues, making it difficult to achieve optimal overall performance of the ternary system. Summary of the Invention
[0005] This invention provides a method for preparing wood-plastic composite materials, which solves the problems of poor interfacial compatibility between wood flour and PLA, low toughening efficiency of PLA and PBAT, and the decrease in PLA molecular weight caused by the one-time addition of chain extenders in the prior art.
[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention discloses a method for preparing wood-plastic composite material, comprising: S1: Add wood flour to a 1-3% sodium hydroxide aqueous solution, soak at 60-80℃ for 1-2 hours, filter and wash until neutral, and then dry to obtain pre-activated wood flour; S2: Pre-activated wood flour and polyisocyanate with a functionality of not less than 2 are mixed and reacted at 50-70℃ for 1-3 hours under solvent-free conditions. The amount of polyisocyanate is 2-6% of the mass of pre-activated wood flour and the isocyanate groups are in excess relative to the hydroxyl groups on the surface of the wood flour. After the reaction, the wood flour is dried directly at 60-80℃ without washing to obtain modified wood flour with free isocyanate groups on the surface. S3: Polybutylene adipate terephthalate (PAD) and a styrene-acrylic acid copolymer chain extender containing polyepoxy functional groups are pre-mixed at 150-160°C for 2-4 minutes. The amount of chain extender is 0.3-0.8% of the mass of PAD, to obtain an active masterbatch of PAD containing free epoxy functional groups. S4: 100 parts by weight of polylactic acid, 20-50 parts by weight of modified wood flour obtained in S2, 15-35 parts by weight of poly(butylene adipate-terephthalate) active masterbatch obtained in S3, and 0.3-1.0 parts by weight of the chain extender are melt-blended in a twin-screw extruder at 160-175°C. During the melt-blending process, the free isocyanate groups on the surface of the modified wood flour react with the hydroxyl groups at the chain ends of the polylactic acid to form a urethane bond covalent linking layer between the modified wood flour and the polylactic acid. The mixture is then extruded and granulated. The ratio of the amount of chain extender directly added in S4 to the amount of chain extender added in S3 is 2:1 to 3:1.
[0007] As a preferred embodiment of the present invention, in S1, the wood flour particle size is 80-200 mesh; the drying temperature is 80-100℃, and the wood is dried until the moisture content is less than 0.5%.
[0008] As a preferred embodiment of the present invention, in S2, the polyisocyanate is at least one of hexamethylene diisocyanate trimer and polyphenyl polymethylene polyisocyanate.
[0009] As a preferred embodiment of the present invention, in S2, the amount of polyisocyanate used is 3-5% of the mass of the pre-activated wood flour; and the drying temperature is 60-70℃.
[0010] As a preferred embodiment of the present invention, in S3, the premixing is carried out in an internal mixer or a twin-screw extruder; the premixing temperature is 150-155°C and the time is 2-3 minutes.
[0011] As a preferred embodiment of the present invention, in S3, the content of free epoxy functional groups in the polybutylene adipate terephthalate active masterbatch is not less than 40% of the total amount of initial epoxy functional groups of the chain extender, and is determined by epoxy value titration.
[0012] As a preferred embodiment of the present invention, in step S4, the polylactic acid is dried under vacuum at 80°C until the water content is not higher than 0.02% before being added; the weight-average molecular weight of the polylactic acid is 100,000 to 250,000.
[0013] As a preferred embodiment of the present invention, in S4, the amount of modified wood flour is 30-40 parts by mass, the amount of polybutylene adipate terephthalate active masterbatch is 20-30 parts by mass, and the total amount of chain extender is 0.6-0.9 parts by mass.
[0014] As a preferred embodiment of the present invention, in S4, the temperature of each zone of the twin-screw extruder is set as follows: zone 140-150℃, zone 2 155-165℃, zone 3 165-175℃, zone 4 160-170℃, and zone 5 145-155℃; the screw speed is 120-160 rpm.
[0015] As a preferred embodiment of the present invention, in step S4, 0.5-2 parts by weight of a lubricant are added; the lubricant is at least one of stearic acid, calcium stearate or erucamide.
[0016] The beneficial effects of this invention are: 1. This invention uses polyisocyanate to modify wood flour and deliberately retains the free isocyanate groups on the surface, so that they react in situ with the polylactic acid chain ends during the melt mixing process, forming a urethane bond covalent link layer at the interface between wood flour and polylactic acid. This is fundamentally different from the physical bridging of silane coupling agents, which significantly improves the mechanical properties of the material and greatly reduces the water absorption rate.
[0017] 2. This invention adds the chain extender in two steps: first, during the pre-mixing stage, a pre-formulated poly(butylene adipate) terephthalate (PAT) active masterbatch is prepared; then, the chain extender is added supplementarily during the melt mixing stage. The ratio of the two steps is controlled between 2:1 and 3:1. This stepwise strategy effectively inhibits excessive local chain extension of polylactic acid while ensuring reactive compatibility, significantly improving melt index retention and maintaining the processing fluidity of the material. Detailed Implementation
[0018] To make the technical solution and beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.
[0019] The raw materials used in the following examples are as follows: Polylactic acid (PLA) with a weight-average molecular weight of 150,000 was dried under vacuum at 80°C for 12 hours before use, until the moisture content was no higher than 0.02%. Pine wood powder with a particle size of 100 mesh was pre-dried in an oven at 105°C before use; after S1 alkali treatment and subsequent drying, the moisture content must be below 0.5% before it can be used in subsequent steps. Polybutylene adipate terephthalate (PBAT) was a commercially available industrial-grade product with a melt index of 3 to 5 g / 10 min, tested at 190°C and 2.16 kg. The polyisocyanate used was hexamethylene diisocyanate trimer with an NCO mass fraction of approximately 21% and a functionality of 3. The chain extender was a styrene-acrylic copolymer containing polyepoxy functional groups, with an epoxy equivalent of approximately 285 g / mol and an epoxy functionality of 7 to 9. The lubricant was calcium stearate, industrial grade.
[0020] This embodiment provides a method for preparing wood-plastic composite material, which is carried out according to the following specific steps.
[0021] Pine wood powder was added to a 2% sodium hydroxide aqueous solution (solid-liquid ratio approximately 1:10) and soaked in a 70°C water bath for 1.5 hours. After soaking, the solution was filtered and repeatedly washed with deionized water until the pH of the filtrate reached 6.5 to 7.5. The filtrate was then dried at 90°C until the moisture content was below 0.5%, yielding pre-activated wood powder. The purpose of the alkali treatment was to remove waxes, hemicellulose, and some lignin from the surface of the wood powder, fully exposing the active hydroxyl groups on the cellulose chains to provide sufficient reaction sites for subsequent isocyanate modification. The pre-activated wood powder was placed in a high-speed mixer and preheated to 60°C. Hexamethylene diisocyanate trimer was added at 4% of the pre-activated wood powder mass, and the mixture was sealed and reacted at 60-70°C for 2 hours. During the reaction, some isocyanate groups in the polyisocyanate react with the hydroxyl groups on the wood flour surface to form urethane bonds, grafting polyisocyanate molecules onto the wood flour surface. Because the polyisocyanate has a functionality of 3 and the dosage is designed to be in excess of isocyanate groups relative to the hydroxyl groups on the wood flour surface, unreacted free isocyanate groups remain on the wood flour surface after the reaction. After the reaction, without any washing step, the material is directly transferred to a forced-air drying oven and dried at 65°C for 2 hours to obtain modified wood flour with free isocyanate groups on the surface, which is then sealed and stored for later use. The washing step is deliberately omitted here to retain the free isocyanate groups on the wood flour surface, allowing them to further react with the hydroxyl groups at the PLA chain ends during subsequent melt mixing, forming a covalently linked layer of urethane bonds at the wood flour-PLA interface. If washing is performed, the content of free isocyanate groups will decrease significantly, and the degree of interfacial covalent bonding will be significantly weakened. The modified wood flour was characterized using infrared spectroscopy. A clear stretching vibration characteristic peak of free isocyanate groups was observed at 2270 cm⁻¹, confirming the presence of free isocyanate groups on the surface of the modified wood flour. PBAT and a chain extender were added to a twin-screw extruder at 0.5% of the PBAT mass and pre-mixed at 152°C for 2.5 minutes at a screw speed of 80 rpm. After extrusion, the mixture was granulated to obtain a PBAT active masterbatch. During the pre-mixing process, some epoxy groups on the chain extender underwent ring-opening addition reactions with the hydroxyl or carboxyl groups at the PBAT chain ends, moderately extending the chain of PBAT while retaining a considerable proportion of unreacted free epoxy groups in the chain extender. The content of free epoxy functional groups in the obtained PBAT active masterbatch, determined by epoxy value titration, was not less than 40% of the total initial epoxy functional groups of the chain extender, ensuring its activity in further reaction with the PLA chain-end functional groups during the subsequent melt mixing stage. The purpose of pre-formulated PBAT active masterbatch is to make PBAT reactive in advance, so that it can react efficiently with the functional groups at the chain end of PLA during the melt mixing stage, thereby improving the interfacial compatibility between PLA and PBAT. At the same time, by adopting a stepwise chain extension strategy, the loss of control of PLA molecular weight caused by the one-time addition of a large amount of chain extender is avoided, thus effectively protecting the molecular weight and processing flowability of PLA.In the following embodiments, the amount of each component is expressed in parts by mass. The amount of PLA is fixed at 100 parts by mass. The temperature of each zone of the twin-screw extruder is set as follows: zone 1 145°C, zone 2 160°C, zone 3 170°C, zone 4 165°C, and zone 5 150°C. The screw speed is 140 rpm.
[0022] The beneficial effects of the present invention are verified by the following embodiments.
[0023] Example 1: The components used were 100 parts PLA, 30 parts modified wood flour, 25 parts PBAT active masterbatch, and 1 part lubricant; the total amount of chain extender was 0.7 parts, with 0.2 parts added during the pre-mixing stage and 0.5 parts added directly during the melt mixing stage, in a ratio of 2.5:1. The dried PLA, modified wood flour, PBAT active masterbatch, chain extender, and lubricant were added to a twin-screw extruder according to the above proportions. The mixture was melt-mixed, extruded, and granulated according to the above temperature and speed parameters. The resulting granules were dried at 80°C for 4 hours for later use. Mechanical properties and other test results are shown in Table 1.
[0024] Example 2: This example differs from Example 1 in that the modified wood flour dosage is 40 parts, the PBAT active masterbatch dosage is 20 parts, and the total chain extender dosage is 0.8 parts, of which 0.23 parts are added during the pre-mixing stage and 0.57 parts are added directly during the melt mixing stage, with a dosage ratio of 2.5:1. The remaining steps are the same as in Example 1. Mechanical properties and other test results are shown in Table 1.
[0025] Example 3: This example differs from Example 1 in that the modified wood flour dosage is 20 parts, the PBAT active masterbatch dosage is 30 parts, and the total chain extender dosage is 0.6 parts, of which 0.17 parts are added during the pre-mixing stage and 0.43 parts are added directly during the melt mixing stage, with a dosage ratio of 2.5:1. The remaining steps are the same as in Example 1. Mechanical properties and other test results are shown in Table 1.
[0026] Control Group 1: This control group differs from Example 1 in that it uses unmodified isocyanate-treated wood flour instead of modified wood flour, and is used directly after only alkali treatment; ordinary PBAT is used instead of PBAT active masterbatch, and no pre-mixing is performed; 0.7 parts of chain extender are added all at once during the melt mixing stage. The remaining steps are the same as in Example 1. Mechanical properties and other test results are shown in Table 1. Under the conditions of no modification treatment and no step-by-step chain extension design, the performance of this control group is at the lowest level, and it can be used as a benchmark.
[0027] Control Group 2: This control group differs from Example 1 in that it uses silane coupling agent KH-550 modified wood powder instead of isocyanate modified wood powder. The preparation method of silane modified wood powder is as follows: pre-activated wood powder is added to an ethanol-water mixture containing 3% silane coupling agent KH-550 (ethanol to water volume ratio 95:5), soaked at 60℃ for 2 hours, filtered, and dried to obtain silane modified wood powder. Ordinary PBAT is used instead of PBAT active masterbatch, and pre-mixing is not performed; 0.7 parts of chain extender are added all at once during the melt mixing stage. The remaining steps are the same as in Example 1. Mechanical properties and other test results are shown in Table 1. Infrared spectroscopy characterization of the obtained composite material showed no characteristic peak at 2270 cm⁻¹ before and after mixing, and no new urethane bond peak appeared at 1700 to 1720 cm⁻¹, confirming that no covalent bond is formed between the silane modified wood powder and PLA, and the interfacial bonding is a physical bridge, which is fundamentally different from the Example group.
[0028] Control Group 3: This control group differs from Example 1 in that ordinary PBAT is used instead of PBAT active masterbatch, and premixing is not performed; 0.7 parts of chain extender are added all at once during the melt mixing stage; the modified wood flour is still an isocyanate modified product. The remaining steps are the same as in Example 1. Mechanical properties and other test results are shown in Table 1. Compared with Example 1, the mechanical properties of this control group show a significant decrease, indicating that the PBAT active masterbatch pre-processing step is indispensable for improving the compatibility of PLA and PBAT phases.
[0029] Control Group 4: This control group differs from Example 1 in that no chain extender was added during the pre-mixing stage, and PBAT was not pre-activated; all 0.7 parts of the chain extender were added at once during the melt mixing stage. The remaining steps were the same as in Example 1. Mechanical properties and other test results are shown in Table 1. Compared with Example 1, the melt index retention rate of this control group was significantly reduced, indicating that the one-time addition of a large amount of chain extender caused excessive local chain extension reaction of PLA during extrusion, resulting in a significant decrease in molecular weight and impaired processing fluidity; the contribution of the stepwise addition strategy to the protection of PLA molecular weight is thus confirmed.
[0030] The materials obtained from Examples 1 to 3 and each control group were subjected to performance tests according to the corresponding standards, and the results are shown in Table 1.
[0031] Table 1 Performance Test Results
[0032] As shown in Table 1, Examples 1 to 3 outperformed the control groups in all test indicators, demonstrating the effectiveness of the synergistic effect of the three core designs of the present invention: isocyanate covalently modified wood flour, PBAT active masterbatch pre-preparation, and stepwise addition of the chain extender. Compared with control group 1, Example 1 showed an approximately 110% increase in impact strength, an approximately 64% increase in flexural strength, an approximately 65% increase in tensile strength, and a approximately 60% decrease in water absorption. Compared with control group 2, Example 1 showed significant improvements in all mechanical properties and water absorption, confirming that chemical bonding interfaces are superior to physical bridging. Compared with control group 3, Example 1 showed comprehensive improvement in mechanical properties, further illustrating that the PBAT active masterbatch pre-preparation step cannot be omitted. The difference between control group 4 and Example 1 was mainly reflected in the melt index retention rate. The stepwise addition strategy increased the retention rate by approximately 12 percentage points, fully supporting the rationality of the stepwise addition ratio design of the chain extender in the present invention.
[0033] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A method for preparing a wood-plastic composite material, characterized in that, include: S1: Add wood flour to a 1-3% sodium hydroxide aqueous solution, soak at 60-80℃ for 1-2 hours, filter and wash until neutral, and then dry to obtain pre-activated wood flour; S2: Pre-activated wood flour and polyisocyanate with a functionality of not less than 2 are mixed and reacted at 50-70℃ for 1-3 hours under solvent-free conditions. The amount of polyisocyanate is 2-6% of the mass of pre-activated wood flour and the isocyanate groups are in excess relative to the hydroxyl groups on the surface of the wood flour. After the reaction, the wood flour is dried directly at 60-80℃ without washing to obtain modified wood flour with free isocyanate groups on the surface. S3: Polybutylene adipate terephthalate (PAD) and a styrene-acrylic acid copolymer chain extender containing polyepoxy functional groups are pre-mixed at 150-160°C for 2-4 minutes. The amount of chain extender is 0.3-0.8% of the mass of PAD, to obtain an active masterbatch of PAD containing free epoxy functional groups. S4: 100 parts by weight of polylactic acid, 20-50 parts by weight of modified wood flour obtained in S2, 15-35 parts by weight of poly(butylene adipate-terephthalate) active masterbatch obtained in S3, and 0.3-1.0 parts by weight of the chain extender are melt-blended in a twin-screw extruder at 160-175°C. During the melt-blending process, the free isocyanate groups on the surface of the modified wood flour react with the hydroxyl groups at the chain ends of the polylactic acid to form a urethane bond covalent linking layer between the modified wood flour and the polylactic acid. The mixture is then extruded and granulated. The ratio of the amount of chain extender directly added in S4 to the amount of chain extender added in S3 is 2:1 to 3:
1.
2. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S1, the wood flour particle size is 80-200 mesh; the drying temperature is 80-100℃, and the wood is dried until the moisture content is less than 0.5%.
3. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S2, the polyisocyanate is at least one of hexamethylene diisocyanate trimer and polyphenyl polymethylene polyisocyanate.
4. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S2, the amount of polyisocyanate used is 3-5% of the mass of the pre-activated wood flour; the drying temperature is 60-70℃.
5. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S3, the premixing is carried out in an internal mixer or a twin-screw extruder; the premixing temperature is 150-155°C and the time is 2-3 minutes.
6. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S3, the content of free epoxy functional groups in the polybutylene adipate terephthalate active masterbatch is not less than 40% of the total amount of initial epoxy functional groups of the chain extender, as determined by epoxy value titration.
7. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S4, the polylactic acid is added after being dried under vacuum at 80°C until the water content is not higher than 0.02%; the weight-average molecular weight of the polylactic acid is 100,000 to 250,000.
8. The method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S4, the amount of modified wood flour is 30-40 parts by weight, the amount of polybutylene adipate terephthalate active masterbatch is 20-30 parts by weight, and the total amount of chain extender is 0.6-0.9 parts by weight.
9. A method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S4, the temperature of each zone of the twin-screw extruder is set as follows: Zone 1 140-150℃, Zone 2 155-165℃, Zone 3 165-175℃, Zone 4 160-170℃, Zone 5 145-155℃; the screw speed is 120-160 rpm.
10. A method for preparing a wood-plastic composite material according to claim 1, characterized in that, In S4, 0.5-2 parts by weight of a lubricant are also added; the lubricant is at least one of stearic acid, calcium stearate or erucamide.