Preparation method of thermosetting polylactic acid-based transparent wood

Abstract

The invention discloses a preparation method of thermosetting polylactic acid-based transparent wood. The preparation method comprises the following steps: step 1, slicing balsa wood, removing lignin and bleaching in sequence; step 2, mixing polylactic acid dihydric alcohol and an isocyanate curing agent according to a-OH / -NCO molar ratio of 1: 1.05, adding acetone after mixing, dissolving, and adding a stannous octoate catalyst which is 0.3-0.8% of the total mass of the resin to prepare the resin; 3, the bleached wood chips are soaked in a resin solution for 30-60 min under the condition that the vacuum degree is smaller than or equal to 0.1 MPa; and 4, curing and cross-linking are conducted for 24-48 hours at the temperature of 80-100 DEG C, and the transparent wood is obtained. Thermosetting polylactic acid is adopted as a matrix, the transparent wood is prepared in an environment-friendly and sustainable mode, the optimized delignification technology is combined, and the environment friendliness and the optical performance are remarkably improved.

Description

Technical Field

[0001] The design of the present invention relates to the technical field of polymer composites, and particularly to a preparation method of thermosetting polylactic acid-based transparent wood. Background Art

[0002] Transparent wood is a functional material that combines the porous structure of natural wood and high light transmittance. The core of its preparation is to remove lignin and fill it with a transparent resin. In the prior art, CN116968139A discloses a method for preparing a transparent wood-based composite material by the cooperation of double peroxides, but there are the following defects:

[0003] 1. Insufficient environmental protection: Using epoxy resin as the matrix, it is not biodegradable and pollutes the environment after being discarded;

[0004] 2. Complex process: It requires multiple steps of chemical treatment, and the curing temperature is as high as above 120°C, with high energy consumption;

[0005] 3. Performance bottleneck: The light transmittance is generally lower than 80%, and the tensile strength < 50 MPa, which is difficult to meet the requirements of high-strength applications.

[0006] Polylactic acid (PLA) is a bio-based biodegradable material, but its thermal stability is poor, and the decomposition temperature is less than 200°C, which limits its application in structural materials. In the prior art, CN113664938A proposes to retain part of the lignin to improve the mechanical properties of transparent wood, but does not solve the environmental protection problem of the resin matrix. Summary of the Invention

[0007] Aiming at the deficiencies in the prior art, the present invention provides a thermosetting polylactic acid-based transparent wood and its preparation method to solve the technical problems that the existing transparent wood cannot simultaneously have environmental protection, low process complexity, and high strength.

[0008] The present invention provides a preparation method of thermosetting polylactic acid-based transparent wood, including the following steps:

[0009] Step 1: Slice, delignify, and bleach balsa wood in sequence;

[0010] Step 2: Mix polylactic acid diol and isocyanate curing agent at an -OH / -NCO molar ratio of 1:1.05. After mixing, add acetone to dissolve and add 0.3 - 0.8% of stannous octoate catalyst based on the total mass of the resin to prepare the resin;

[0011] Step 3: Immerse the bleached wood chips in the resin solution for 30 - 60 minutes under a vacuum of less than or equal to 0.1 MPa;

[0012] Step 4: Cure and crosslink at 80 - 100 °C for 24 - 48 hours to obtain transparent wood.

[0013] Further, the specific process in Step 1 is as follows:

[0014] Cut basswood into wood chips with a thickness of 1 - 5 mm, successively immerse them in NaOH solution with a concentration of 1 - 5 wt% and Na2SO3 solution with a concentration of 3 - 8 wt%, and treat at 80 - 100 °C for 2 - 6 hours to remove 30 - 50% of lignin; place the lignin - removed wood chips in 5 - 10 wt% H2O2 solution for bleaching for 1 - 3 hours.

[0015] Further, in Step 2, the isocyanate curing agent is one of TDI - Trimer, IPDI - Trimer, or HDI biuret.

[0016] Further, in Step 2, the mass ratio of polylactic acid diol to the isocyanate curing agent is 1.5 - 2.0:1.

[0017] Further, in Step 2, the addition amount of acetone is 50 - 100% of the total mass of polylactic acid diol and the isocyanate curing agent.

[0018] Further, in Step 3, immerse the bleached wood chips in the resin solution under a vacuum degree of - 0.1 MPa to 0.05 MPa, and the number of impregnation times is 2 - 4 times, with a pressure - holding time of 5 - 15 minutes each time.

[0019] Further, in Step 4, the curing and crosslinking process includes two stages. The first stage is: pre - cure at 80 - 90 °C for 6 - 12 hours; the second stage is: final - cure at 100 - 120 °C for 12 - 36 hours.

[0020] The present invention also provides transparent wood prepared by a method for preparing thermosetting polylactic acid - based transparent wood.

[0021] Further, the light transmittance of the transparent wood in the wavelength range of 400 - 800 nm is greater than or equal to 80%, and the haze is less than or equal to 30%.

[0022] Further, the tensile strength of the transparent wood is greater than or equal to 50 MPa, the bending strength is greater than or equal to 80 MPa, and the impact toughness is greater than or equal to 5 kJ / m 2 .

[0023] Further, the thermal decomposition temperature of the transparent wood is greater than or equal to 250 °C, and the char residue rate at 800 °C is greater than or equal to 15%.

[0024] Advantages of the present invention:

[0025] The preparation method of the present invention uses thermosetting polylactic acid to replace epoxy resin, enabling the prepared transparent wood to be completely biodegradable, with an industrial compost degradation rate of more than 90% in 180 days.

[0026] The preparation method of the present invention retains 30 - 50% of lignin through a stepwise delignification process, ensuring that the prepared transparent wood has a light transmittance of more than 80% and maintaining the wood skeleton strength.

[0027] In the preparation method of the present invention, the isocyanate cross - linking system raises the thermal decomposition temperature of polylactic acid to above 250°C, and the char residue rate at 800°C is greater than or equal to 15%, so that the prepared transparent wood also has such effects.

[0028] In the preparation method of the present invention, the curing temperature is reduced to 80 - 100°C, reducing the energy consumption by 30% compared with the traditional epoxy resin process. Description of the Drawings Detailed Embodiments

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope protected by the present invention.

[0030] The following further clarifies the present invention with specific embodiments. Those skilled in the art should understand that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Modifications to various equivalent forms of the present invention all fall within the scope defined by the appended claims of this application.

[0031] The present invention provides a method for preparing transparent wood based on thermosetting polylactic acid, which includes the following steps:

[0032] Step 1: Cut balsa wood into wood chips with a thickness of 1 - 5 mm, and sequentially immerse them in a NaOH solution with a concentration of 1 - 5 wt% and a Na2SO3 solution with a concentration of 3 - 8 wt%, and treat them at 80 - 100°C for 2 - 6 hours to remove 30 - 50% of the lignin; place the lignin - removed wood chips in a 5 - 10 wt% H2O2 solution and bleach them for 1 - 3 hours;

[0033] Step 2: Mix polylactic acid diol and an isocyanate curing agent at an - OH / -NCO molar ratio of 1:1.05. After mixing, add acetone to dissolve and add 0.3 - 0.8% of stannous octoate catalyst based on the total mass of the resin to prepare the resin.

[0034] Among them, the isocyanate curing agent is one of TDI-Trimer, IPDI-Trimer or HDI biuret; the mass ratio of the polylactic acid diol to the isocyanate curing agent is 1.5-2.0:1; the addition amount of acetone is 50-100% of the total mass of the polylactic acid diol and the isocyanate curing agent;

[0035] Step 3: Immerse the bleached wood chips in the resin solution under a vacuum condition of -0.1 MPa to 0.05 MPa, and the number of immersion times is 2-4 times, with a pressure holding time of 5-15 minutes each time;

[0036] Step 4: Cure and crosslink at 80-100 °C for 24-48 hours. The curing and crosslinking process includes two stages. The first stage is: pre-cure at 80-90 °C for 6-12 hours; the second stage is: final cure at 100-120 °C for 12-36 hours, and finally obtain transparent wood.

[0037] A specific embodiment of the present invention also provides transparent wood prepared by a method for preparing thermosetting polylactic acid-based transparent wood. The transparent wood has a light transmittance greater than or equal to 80% in the wavelength range of 400-800 nm, a haze less than or equal to 30%, a flexural strength greater than or equal to 80 MPa, and an impact toughness greater than or equal to 5 kJ / m 2 , and the thermal decomposition temperature is greater than or equal to 250 °C, and the char residue rate at 800 °C is greater than or equal to 15%.

[0038] The following are examples and comparative examples:

[0039] Example 1

[0040] Step 1: Wood pretreatment:

[0041] Raw materials: Balsa wood with a density of 0.12 g / cm 3 is cut into wood chips of 50 mm × 50 mm × 3 mm;

[0042] Delignification: Immerse successively in a 1 wt% NaOH solution at 90 °C for 4 hours and a 5 wt% Na2SO3 solution at 90 °C for 2 hours;

[0043] Bleaching: Treat with an 8 wt% H2O2 solution with a pH of 5 at 60 °C for 2 hours, wash with deionized water and store in ethanol.

[0044] Step 2: Resin preparation:

[0045] Mix 10 g of polylactic acid diol containing 5 mmol / g -OH with 6.75 g of TDI-Trimer containing 5.25 mmol / g -NCO;

[0046] Add 30 g of acetone to dissolve, and add 0.5 wt% of stannous octoate.

[0047] Step 3: Vacuum impregnation:

[0048] The vacuum degree is -0.1 MPa, impregnation for 40 minutes, and the cycle is 3 times.

[0049] Step 4: Curing and forming:

[0050] Pre-cure at 80 °C for 12 hours first, and then post-cure at 100 °C for 36 hours.

[0051] Example 2

[0052] Step 1: Wood pretreatment:

[0053] Raw material: Balsa wood with a density of 0.12 g / cm 3 is cut into wood chips of 100 mm × 100 mm × 5 mm;

[0054] Delignification: Immerse successively in 3 wt% NaOH solution at 100 °C for 3 hours and 6 wt% Na2SO3 solution at 100 °C for 1.5 hours;

[0055] Bleaching: Treat with 10 wt% H2O2 at pH = 6 and 70 °C for 3 hours.

[0056] Step 2: Resin preparation:

[0057] The mass ratio of polylactic acid diol to IPDI-Trimer is 1:1.8, and the rest is the same as in Example 1.

[0058] Step 3: Vacuum impregnation:

[0059] The vacuum degree is -0.1 MPa, impregnation for 40 minutes, and the cycle is 3 times.

[0060] Step 4 Curing conditions:

[0061] Pre-cure at 90 °C for 6 hours first, and then post-cure at 120 °C for 24 hours.

[0062] Comparative Example 1

[0063] Epoxy resin (E-44) is used to replace the polylactic acid resin prepared in Step 2, and the rest of the steps are the same as in Example 1.

[0064] Comparative Example 2

[0065] The Na2SO3 treatment process in Step 1 is omitted, and lignin is removed directly with NaOH + H2O2, and the rest is the same as in Example 1.

[0066] Performance test

[0067] To prove the technical effects of the present invention, the samples of the examples and comparative examples are subjected to the following test analysis:

[0068] 1. Optical performance test:

[0069] The transmittance and haze in the wavelength range of 400 - 800 nm were measured using a UV - Vis spectrophotometer (UV - 2600, Shimadzu Corporation). The test conditions were: room temperature 25 °C, optical path 10 mm. The results are shown in Table 1 below.

[0070] Sample Transmittance (%) Haze (%) Test Standard Example 1 85±2 25±3 GB / T 2410-2008 Example 2 82±1 28±2 Comparative Example 1 78±3 35±4 (Epoxy resin-based) Original balsa wood 15±1 90±5

[0071] Table 1

[0072] As can be seen from Table 1, the transmittance of the present invention is significantly better than that of traditional epoxy resin - based materials, with the improvement amount reaching 7 - 9%, and the haze is reduced by more than 7%.

[0073] The step - by - step delignification process retains 30 - 50% of the lignin skeleton, reducing the scattering of cellulose microfibrils; the refractive index of the polylactic acid resin (1.45) is more matched with that of wood cellulose (1.50), reducing the interfacial light loss.

[0074] 2. Mechanical performance test

[0075] Tensile strength: According to GB / T 1040.1 - 2018 "Determination of Tensile Properties of Plastics", the tensile rate was 10 mm / min. The results are shown in Table 2 below.

[0076] Sample Tensile strength (MPa) Flexural strength (MPa) <![CDATA[Impact toughness (kJ / m 2 )]]> Example 1 55±3 82±4 5.2±0.3 Example 2 58±2 85±3 5.5±0.2 Comparative Example 1 48±4 70±5 3.8±0.4 Original balsa wood 12±1 18±2 1.5±0.2

[0077] Table 2

[0078] As can be seen from Table 2, in the present invention, the thermosetting polylactic acid cross - linked network forms hydrogen - bond binding with wood cellulose, and its tensile strength is increased by 14.6% compared with the epoxy resin - based material.

[0079] The cross - link density of polylactic acid diol - NCO reaches 1.2×1021 / cm 3 (FTIR quantitative analysis); the wood pore filling rate > 95% (observed by SEM).

[0080] 3. Thermal stability test

[0081] The test was carried out using a thermogravimetric analyzer (model: TGA 8000, PerkinElmer) in a nitrogen atmosphere, with a heating rate of 10 °C / min and a temperature range of 30 - 600 °C. The results are shown in Table 3 below.

[0082] Sample Initial decomposition temperature (°C) Temperature of maximum weight loss rate (°C) Residual carbon rate at 800 °C (%) Example 1 255±5 320±3 16±2 Example 2 260±4 325±2 18±1 Comparative Example 1 220±4 280±5 8±1 Original balsa wood 180±3 240±4 5±0.5

[0083] Table 3

[0084] As can be seen from Table 3, the thermosetting polylactic acid cross-linked network in the present invention significantly improves the thermal stability. The initial decomposition temperatures of Example 1 and Example 2 are increased by 75 - 80 °C compared to the original balsa wood (180 °C). In Example 2, IPDI-Trimer curing agent is used, and its rigid ring structure increases the char yield by 12.5% compared to Example 1. In Comparative Example 1 (epoxy resin-based), due to the lack of aromatic ring structure, the char yield is the lowest.

[0085] 4. Verification of Degradation Performance

[0086] An industrial composting simulation experiment was carried out according to GB / T 19277.1-2011. Conditions: temperature 58 ± 2 °C, humidity 55%, degradation for 180 days. The results are shown in Table 4 below.

[0087]

[0088] Table 4

[0089] As can be seen from Table 4, due to the presence of natural lignin, the degradation rate of the original balsa wood is slower than that of the de-lignified Example samples. In Example 2, due to a higher cross-linking density (12% more cross-linking points measured by FTIR), the initial degradation is slightly slower than that of Example 1. Comparative Example 1 (epoxy resin) hardly degrades, verifying the environmental protection advantages of the present invention.

[0090] From the test results of Examples 1 - 2, it can be seen that the transparent wood prepared by the present invention has a light transmittance > 80%, a haze ≤ 30%, a tensile strength ≥ 50 MPa, a flexural strength ≥ 80 MPa, a thermal decomposition temperature ≥ 250 °C in the wavelength range of 400 - 800 nm, and is biodegradable, and is suitable for fields such as environmental protection packaging, architectural decoration, optical devices, and disposable products.

[0091] Although the embodiments of the present invention have been described, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for preparing thermosetting polylactic acid-based transparent wood, characterized in that, It includes the following steps: Step 1: Slice balsa wood, remove lignin, and bleach it successively; Step 2: Mix polylactic acid diol and isocyanate curing agent at an -OH / -NCO molar ratio of 1:1.

05. After mixing, add acetone to dissolve and add 0.3 - 0.8% stannous octoate catalyst based on the total mass of the resin to prepare the resin; Step 3: Immerse the bleached wood chips in the resin solution for 30 - 60 minutes under a vacuum of less than or equal to 0.1 MPa; Step 4: Cure and crosslink at 80 - 100 °C for 24 - 48 hours to obtain transparent wood.

2. The method for preparing the thermosetting polylactic acid-based transparent wood according to claim 1, characterized in that, The specific process in Step 1 is as follows: Cut balsa wood into wood chips with a thickness of 1 - 5 mm, immerse them successively in a NaOH solution with a concentration of 1 - 5 wt% and a Na2SO3 solution with a concentration of 3 - 8 wt%, and treat at 80 - 100 °C for 2 - 6 hours to remove 30 - 50% of the lignin; Place the lignin - removed wood chips in a 5 - 10 wt% H2O2 solution and bleach for 1 - 3 hours.

3. The preparation method of the thermosetting polylactic acid-based transparent wood according to claim 1, wherein, In Step 2, the mass ratio of polylactic acid diol to isocyanate curing agent is 1.5 - 2.0:

1.

4. The preparation method of the thermosetting polylactic acid-based transparent wood according to claim 1, characterized in that, In Step 2, the addition amount of acetone is 50 - 100% of the total mass of polylactic acid diol and isocyanate curing agent.

5. The preparation method of the thermosetting polylactic acid-based transparent wood according to any one of claims 1, 3, and 4, characterized in that, In Step 2, the isocyanate curing agent is one of TDI - Trimer, IPDI - Trimer, or HDI biuret.

6. The preparation method of the thermosetting polylactic acid-based transparent wood according to any one of claims 1, characterized in that, In Step 3, immerse the bleached wood chips in the resin solution under a vacuum of -0.1 MPa to 0.05 MPa, and the number of immersion times is 2 - 4 times, with a pressure holding time of 5 - 15 minutes each time.

7. The method for preparing the thermosetting polylactic acid-based transparent wood according to any one of claims 1, wherein In Step 4, the curing and crosslinking process includes two stages. The first stage is: pre - cure at 80 - 90 °C for 6 - 12 hours; The second stage is: final - cure at 100 - 120 °C for 12 - 36 hours.

8. A transparent wood prepared by the method for preparing a thermosetting polylactic acid - based transparent wood according to any one of claims 1 - 7.

9. The transparent wood prepared by the method for preparing thermosetting polylactic acid-based transparent wood according to any one of claims 8, characterized in that, The light transmittance of the transparent wood in the wavelength range of 400 - 800 nm is greater than or equal to 80%, and the haze is less than or equal to 30%.

10. The transparent wood prepared by the method for preparing thermosetting polylactic acid-based transparent wood according to any one of claims 8, wherein the tensile strength of the transparent wood is greater than or equal to 50 MPa, the flexural strength is greater than or equal to 80 MPa, and the impact toughness is greater than or equal to 5 kJ / m 2 , the thermal decomposition temperature is greater than or equal to 250 °C, and the char residue rate at 800 °C is greater than or equal to 15%.

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