Preparation method and application of low-temperature-resistant degradable plastic

Abstract

The application relates to the technical field of plastics, in particular to a preparation method and application of low-temperature-resistant degradable plastics. The low-temperature-resistant degradable plastics contain, in parts by weight, carboxylated polycaprolactone C-PCL: 80-90 parts, modified lignin: 15-20 parts, nano-cellulose: 5-10 parts, crosslinking agent: 2-3 parts, and antioxidant: 0.3-0.5 parts. The modified lignin is prepared by grafting epoxy groups after the alkaline lignin is protected by p-benzene dicarboxaldehyde and hydroxylated; the crosslinking agent is prepared by condensation reaction of vanillin and hexanediamine, and the imine bond contained in the crosslinking agent endows the material with a good degradation rate under acidic conditions. The low-temperature-resistant degradable plastics prepared by the application have good low-temperature mechanical properties and excellent degradation speed under acidic conditions in the case of guaranteeing a certain service life, and are suitable for application to agricultural mulching films in cold environments.

Description

Technical Field

[0001] This invention relates to the field of plastics technology, and in particular to a method for preparing and applying a low-temperature resistant biodegradable plastic. Background Technology

[0002] With increasing environmental awareness, biodegradable plastics have become a research hotspot. Traditional plastics are extremely difficult to degrade in the natural environment, and the accumulation of large amounts of plastic waste has caused serious damage to the ecological environment. The emergence of biodegradable plastics has brought hope for solving the problem of "white pollution," but existing biodegradable plastics still have many problems.

[0003] In practical applications, many scenarios require plastics to have low-temperature resistance, such as agriculture and cold chain transportation in cold regions. However, common biodegradable plastics tend to become brittle and experience a decline in mechanical properties at low temperatures, failing to meet practical needs. For example, agricultural mulch films in cold regions, if lacking low-temperature resistance, are prone to cracking under low-temperature conditions. This not only fails to effectively insulate, retain moisture, and suppress weed growth but also increases the cost and labor intensity of replacing the mulch film. Furthermore, the degradation stability of some biodegradable plastics is difficult to balance in different environmental media. In natural environments, biodegradable plastics need to degrade within a certain service life, but existing products often exhibit premature degradation or require harsh degradation conditions. Summary of the Invention

[0004] In view of this, the purpose of this invention is to propose a method for preparing and applying a low-temperature resistant biodegradable plastic, so as to solve the problems of poor mechanical properties, insufficient toughness, and difficulty in controlling the degradation rate of existing plastics at low temperatures.

[0005] To achieve the above objectives, the present invention provides a low-temperature resistant biodegradable plastic comprising the following raw materials in parts by weight: carboxylated polycaprolactone (C-PCL): 80-90 parts, modified lignin: 15-20 parts, nanocellulose: 5-10 parts, crosslinking agent: 2-3 parts, and antioxidant: 0.3-0.5 parts.

[0006] The specific preparation method of the modified lignin is as follows:

[0007] (1) Under nitrogen protection, alkaline lignin, terephthalaldehyde, 1,4-dioxane and hydrochloric acid are mixed, heated to 80-90℃, stirred for 2-4h, cooled to room temperature, sodium bicarbonate is added, stirred for 40-50min, filtered, and the filtrate is dried on a rotary evaporator. The obtained solid is then dissolved again in 1,4-dioxane, and diethyl ether is added until precipitation occurs. After precipitation for 1-3h, the lignin protected by terephthalaldehyde is obtained by filtration, washing and drying.

[0008] (2) Mix phenol and the lignin protected by phenylenedialdehyde obtained in (1), add concentrated sulfuric acid, heat to 100-120℃, react for 20-40 min, cool to room temperature, add dimethyl sulfoxide and deionized water, adjust pH=1 with sulfuric acid, precipitate, filter, wash and dry to obtain hydroxylated lignin.

[0009] (3) Add the hydroxylated lignin from (2) to NaOH solution, stir for 30-50 min, then add epichlorohydrin, heat to 60-80℃, react for 4-6 h, cool to room temperature, add dilute hydrochloric acid to adjust pH to neutral, filter, wash and dry to obtain modified lignin.

[0010] Preferably, in (1), the alkaline lignin, terephthalaldehyde, 1,4-dioxane, hydrochloric acid and sodium bicarbonate are in a weight ratio of 20-28:35-45:120-130:3-5:4-8. The lignin protected by the aldehyde group can shield some active sites to a certain extent, reduce the overall reactivity of the lignin molecule during subsequent hydroxylation, prevent the generation of a large number of by-products due to excessively vigorous reaction, and help improve the purity and yield of the product.

[0011] Preferably, the hydrochloric acid in (1) refers to dilute hydrochloric acid with a concentration of 0.1M.

[0012] Preferably, in (2), the phenol, lignin protected by terephthalaldehyde, concentrated sulfuric acid, dimethyl sulfoxide and deionized water are in a weight ratio of 4-8:1:0.16-0.2:8-12:50-60.

[0013] Preferably, the concentrated sulfuric acid in (2) refers to sulfuric acid with a concentration of 98%.

[0014] Preferably, the sulfuric acid in (2) refers to dilute sulfuric acid with a concentration of 0.1M.

[0015] Preferably, in step (3), the weight ratio of hydroxylated lignin, NaOH solution and epichlorohydrin is 1:10-14:0.3-0.5.

[0016] Preferably, the concentration of the NaOH solution in (3) is 0.1M. This operation can deprotonate the hydroxyl groups on the surface of the hydroxylated lignin to form oxygen anions, increase nucleophilicity, and facilitate the subsequent reaction.

[0017] Preferably, the concentration of dilute hydrochloric acid in (3) is 0.1M.

[0018] Preferably, the crosslinking agent refers to vanillin-hexanediamine condensate, and the specific preparation process is as follows: Vanillin is added to ethanol, then hexanediamine is added, the temperature is raised to 70-90℃, the reaction is carried out for 10-14 hours, cooled to room temperature, the precipitate is filtered, washed, and dried to obtain vanillin-hexanediamine condensate, and the reaction process is as follows:

[0019] Equation (1)

[0020] The product was characterized by ¹H NMR analysis. The ¹H NMR (400 MHz, Chloroform-d) values ​​were δ 8.23 ​​(d, J = 11.3 Hz, 2H), 7.27–7.14 (m, 4H), 6.96 (d, J = 8.4 Hz, 2H), 6.26 (s, 2H), 3.83 (s, 6H), 3.46 (t, J = 6.9 Hz, 2H), 3.29 (t, J = 6.9 Hz, 2H), 1.74–1.63 (m, 4H), and 1.48–1.36 (m, 4H). The vanillin, hexamethylenediamine, and ethanol were present in a weight ratio of 3–5:20–30:1–2.

[0021] Preferably, the crosslinking mechanism of the vanillin-hexanediamine condensate is as follows:

[0022] Equation (2)

[0023] In the formula, R represents lignin. At high temperature, the epoxy bonds in the modified lignin open and undergo a cross-linking reaction with the hydroxyl groups in the cross-linking agent. The longer cross-linking agent molecular chains interweave and weave with each other to form a cross-linking network.

[0024] Preferably, the hydrolysis mechanism of the imine bond in the vanillin-hexanediamine condensate under acidic conditions is as follows:

[0025] Equation (3)

[0026] Imine bonds undergo hydrolysis under acidic conditions, breaking the imine bonds and thus unlinking the cross-linked structure, which can accelerate the degradation rate.

[0027] Preferably, the antioxidant refers to one of tocopherol or tea polyphenols.

[0028] Furthermore, the present invention also provides a method for preparing the above-mentioned low-temperature resistant biodegradable plastic, specifically including the following steps:

[0029] S1: Add modified lignin to an ethanol / water mixture, ultrasonically stir for 20-30 min, then add nanocellulose, ultrasonically stir for 1-3 h, evaporate the ethanol, and freeze dry to obtain modified lignin-nanocellulose composite powder.

[0030] S2: After uniformly mixing the modified lignin-nanofiber composite powder obtained from C-PCL and S1, the crosslinking agent and antioxidant are added to a twin-screw extruder for melt extrusion and then blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0031] Preferably, the modified lignin and the ethanol / water mixed solution in S1 are in a weight ratio of 1:15-25.

[0032] Preferably, the ethanol / water mixed solution in S1 refers to a mixture of ethanol and deionized water in a weight ratio of 3:7.

[0033] Preferably, the parameters of the twin-screw extruder in S2 are as follows: feeding section 130-150℃, plasticizing section 150-160℃, homogenizing section 160-170℃, die head 160-170℃, and screw speed 180-220 rpm. At high temperature, the epoxy groups in the modified lignin react with the carboxyl groups in C-PCL, and the two are grafted and crosslinked together. At the same time, the epoxy groups in the modified lignin that do not participate in the reaction also react with the hydroxyl groups in the crosslinking agent, and are thus fixed. The long molecular chains of the crosslinking agent also form a crosslinked network through physical entanglement with C-PCL.

[0034] Furthermore, the present invention also provides the application of the above-mentioned low-temperature resistant biodegradable plastic in agricultural mulch films in cold environments.

[0035] The beneficial effects of this invention are:

[0036] 1. The low-temperature resistant biodegradable plastic prepared by this invention, through the rational formulation of specific raw materials, such as the synergistic effect of carboxylated polycaprolactone, modified lignin, and nanocellulose, endows the material with good low-temperature resistance. Modified lignin improves the dispersibility of nanocellulose on the one hand, and enhances the binding force between nanocellulose and carboxylated polycaprolactone on the other. Under low-temperature conditions, the material can still maintain good flexibility and mechanical strength, and is not easy to crack, thus broadening the application range of plastics, especially suitable for related fields in cold regions.

[0037] 2. The low-temperature biodegradable plastic prepared by this invention incorporates vanillin-hexanediamine condensate as a crosslinking agent. On one hand, the resulting crosslinked structure further enhances the material's mechanical properties and low-temperature resistance. On the other hand, the imine bonds in the vanillin-hexanediamine condensate undergo hydrolysis under acidic conditions, thereby breaking the crosslinked structure. This characteristic allows for controllable degradation of the material. By adjusting the pH of the environment, the degradation rate can be precisely controlled, maintaining stable performance in everyday use environments while rapidly degrading during recycling or in specific acidic environments. This effectively reduces the residual time of plastic waste in the environment and lowers environmental pollution.

[0038] 3. The modified lignin and nanocellulose used in this invention are derived from natural renewable resources, which not only reduces dependence on non-renewable resources such as petroleum, but also allows these natural components to better integrate into the natural environment after material degradation, reducing negative impacts on the ecosystem and conforming to the concept of sustainable development. At the same time, the preparation method of the low-temperature resistant biodegradable plastic prepared by this invention is simple to operate, with mild and easy-to-control conditions in each step. It can be prepared by common processes such as ultrasonic stirring and melt extrusion, making it suitable for large-scale industrial production and providing strong technical support for the widespread application of the product. Attached Figure Description

[0039] Figure 1 The 1H NMR spectrum of vanillin-hexamethylenediamine condensate;

[0040] Figure 2 This is a diagram illustrating the crosslinking mechanism of vanillin-hexanediamine condensate.

[0041] Figure 3 This is a diagram illustrating the hydrolysis mechanism of imine bonds under acidic conditions. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0043] The reagent raw materials used in the embodiments of the present invention are sourced from the following sources:

[0044] Carboxylated polycaprolactone was purchased from Xi'an Ruixi Biotechnology Co., Ltd., with a purity of 97% and a Mn content of 45,000-50,000; alkaline lignin was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number L832292; nanocellulose was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number C909405, with an outer diameter of 10 nm and a length of 200 nm; terephthalaldehyde was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number P815782, with a purity of 98%; phenol was purchased from Shanghai Jizhi Biochemical Technology Co., Ltd., catalog number P3. 3741, purity 99.5%; epichlorohydrin was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., item number E808937, purity 99%; vanillin was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., item number V100115, purity 99%; hexamethylenediamine was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., item number H810896, purity 99%; tocopherol was purchased from Changzhou Assange Technology Co., Ltd., purity 95%; tea polyphenols were purchased from Shanghai Maclean Biochemical Technology Co., Ltd., item number T861565, purity 97%.

[0045] Example 1: A specific method for preparing a low-temperature resistant biodegradable plastic, comprising the following steps:

[0046] (1) Under nitrogen protection, 2.5 kg of alkaline lignin, 4.375 kg of terephthalaldehyde, 15 kg of 1,4-dioxane and 375 g of 0.1 M hydrochloric acid were mixed, heated to 80 °C, stirred for 2 h, cooled to room temperature, 500 g of sodium bicarbonate was added, stirred for 40 min, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved again in 1,4-dioxane, and diethyl ether was added until precipitation occurred. After precipitation for 1 h, the lignin protected by terephthalaldehyde was obtained by filtration, washing and drying.

[0047] (2) Mix 8.4 kg of phenol and 2.1 kg of phenylenedialdehyde-protected lignin obtained in (1), add 336 g of 98% concentrated sulfuric acid, heat to 100 °C, react for 20 min, cool to room temperature, add 16.8 kg of dimethyl sulfoxide and 105 kg of deionized water, adjust pH=1 with 0.1 M sulfuric acid, precipitate, filter, wash and dry to obtain hydroxylated lignin;

[0048] (3) Add 2 kg of hydroxylated lignin from (2) to 20 kg of 0.1 M NaOH solution, stir for 30 min, then add 600 g of epichlorohydrin, heat to 60 °C, react for 4 h, cool to room temperature, add 0.1 M dilute hydrochloric acid to adjust pH to neutral, filter, wash and dry to obtain modified lignin.

[0049] (4) Add 1.5Kg of modified lignin to 22.5Kg of ethanol / water mixed solution (ethanol and water are mixed in a weight ratio of 3:7), stir ultrasonically for 20min, then add 500g of nanocellulose, stir ultrasonically for 1h, evaporate the ethanol, freeze dry, and obtain modified lignin-nanocellulose composite powder.

[0050] (5) Add 180g vanillin to 1.2Kg ethanol, then add 60g hexamethylenediamine, heat to 70℃, react for 10h, cool to room temperature, filter to precipitate, wash, and dry to obtain vanillin-hexamethylenediamine condensate.

[0051] (6) Mix 8 kg of C-PCL, 2 kg of the modified lignin-nanofiber composite powder obtained in (4), 200 g of vanillin-hexamethylenediamine condensate obtained in (5) and 30 g of tocopherol evenly, and add them to a twin-screw extruder. Set the parameters as follows: feeding section 130°C, plasticizing section 150°C, homogenizing section 160°C, die head 160°C, screw speed 180 rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0052] Example 2: A specific method for preparing a low-temperature resistant biodegradable plastic, comprising the following steps:

[0053] (1) Under nitrogen protection, 2.5 kg of alkaline lignin, 4.17 kg of terephthalaldehyde, 13.02 kg of 1,4-dioxane and 417 g of 0.1 M hydrochloric acid were mixed, heated to 85 °C, stirred for 3 h, cooled to room temperature, 625 g of sodium bicarbonate was added, stirred for 45 min, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved again in 1,4-dioxane, and diethyl ether was added until a precipitate appeared. After precipitation for 2 h, the lignin protected by terephthalaldehyde was obtained by filtration, washing and drying.

[0054] (2) Mix 13.2Kg of phenol and 2.2Kg of phenylenedialdehyde-protected lignin obtained in (1), add 396g of 98% concentrated sulfuric acid, heat to 110℃, react for 30min, cool to room temperature, add 22Kg of dimethyl sulfoxide and 121Kg of deionized water, adjust pH=1 with 0.1M sulfuric acid, precipitate, filter, wash and dry to obtain hydroxylated lignin;

[0055] (3) Add 2Kg of hydroxylated lignin from (2) to 24Kg of 0.1M NaOH solution, stir for 40min, then add 800g of epichlorohydrin, heat to 70℃, react for 5h, cool to room temperature, add 0.1M dilute hydrochloric acid to adjust pH to neutral, filter, wash and dry to obtain modified lignin.

[0056] (4) Add 1.75Kg of modified lignin to 35Kg of ethanol / water mixed solution (ethanol and water are mixed in a weight ratio of 3:7), stir ultrasonically for 25min, then add 750g of nanocellulose, stir ultrasonically for 2h, evaporate the ethanol, freeze dry, and obtain modified lignin-nanocellulose composite powder.

[0057] (5) Add 300g vanillin to 1.875Kg ethanol, then add 112.5g hexamethylenediamine, heat to 80℃, react for 12h, cool to room temperature, filter to precipitate, wash, and dry to obtain vanillin-hexamethylenediamine condensate.

[0058] (6) Mix 8.5Kg C-PCL, 2.5Kg modified lignin-nanofiber composite powder obtained in (4), 250g vanillin-hexamethylenediamine condensate obtained in (5) and 40g tocopherol evenly, and add them to a twin-screw extruder. Set the parameters as follows: feeding section 140℃, plasticizing section 155℃, homogenizing section 165℃, die head 165℃, screw speed 200rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain low-temperature resistant and biodegradable plastic.

[0059] Example 3: A specific method for preparing a low-temperature resistant biodegradable plastic, comprising the following steps:

[0060] (1) Under nitrogen protection, 3 kg of alkaline lignin, 4.82 kg of terephthalaldehyde, 14 kg of 1,4-dioxane and 536 g of 0.1 M hydrochloric acid were mixed, heated to 90 °C and stirred for 4 h. After cooling to room temperature, 857 g of sodium bicarbonate was added and stirred for 50 min. After filtration, the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved again in 1,4-dioxane and ether was added until precipitation occurred. After precipitation for 3 h, the lignin protected by terephthalaldehyde was obtained by filtration, washing and drying.

[0061] (2) Mix 20Kg of phenol and 2.5Kg of phenylenedialdehyde-protected lignin obtained in (1), add 500g of 98% concentrated sulfuric acid, heat to 120℃, react for 40min, cool to room temperature, add 30Kg of dimethyl sulfoxide and 150Kg of deionized water, adjust pH=1 with 0.1M sulfuric acid, precipitate, filter, wash and dry to obtain hydroxylated lignin;

[0062] (3) Add 2.2 kg of hydroxylated lignin from (2) to 30.8 kg of 0.1 M NaOH solution, stir for 50 min, then add 1.1 kg of epichlorohydrin, heat to 80 °C, react for 6 h, cool to room temperature, add 0.1 M dilute hydrochloric acid to adjust pH to neutral, filter, wash, and dry to obtain modified lignin.

[0063] (4) Add 2Kg of modified lignin to 50Kg of ethanol / water mixed solution (ethanol and water are mixed in a weight ratio of 3:7), stir ultrasonically for 30min, then add 1Kg of nanocellulose, stir ultrasonically for 3h, evaporate the ethanol, freeze dry, and obtain modified lignin-nanocellulose composite powder.

[0064] (5) Add 300g vanillin to 1.8Kg ethanol, then add 120g hexamethylenediamine, heat to 90℃, react for 14h, cool to room temperature, filter to precipitate, wash and dry to obtain vanillin-hexamethylenediamine condensate.

[0065] (6) After mixing 9Kg C-PCL, 3Kg modified lignin-nanofiber composite powder obtained in (4), 300g vanillin-hexanediamine condensate obtained in (5) and 50g tocopherol evenly, add them to a twin-screw extruder. Set the parameters as follows: feeding section 150℃, plasticizing section 160℃, homogenizing section 170℃, die head 170℃, screw speed 220rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain low-temperature resistant and biodegradable plastic.

[0066] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the modified lignin is replaced with alkaline lignin, and then epichlorohydrin is grafted onto it. The specific process is as follows: A specific method for preparing a low-temperature resistant biodegradable plastic includes the following steps:

[0067] (1) Add 2 kg of alkaline lignin to 24 kg of 0.1 M NaOH solution, stir for 40 min, then add 800 g of epichlorohydrin, heat to 70 °C, react for 5 h, cool to room temperature, add 0.1 M dilute hydrochloric acid to adjust pH to neutral, filter, wash and dry to obtain modified lignin.

[0068] (2) 1.75 kg of modified lignin was added to 35 kg of ethanol / water mixed solution (ethanol and water were mixed in a weight ratio of 3:7), ultrasonically stirred for 25 min, then 750 g of nanocellulose was added, ultrasonically stirred for 2 h, the ethanol was evaporated, and then freeze-dried to obtain modified lignin-nanocellulose composite powder.

[0069] (3) Add 300g vanillin to 1.875Kg ethanol, then add 112.5g hexamethylenediamine, heat to 80℃, react for 12h, cool to room temperature, filter to precipitate, wash and dry to obtain vanillin-hexamethylenediamine condensate;

[0070] (4) Mix 8.5Kg C-PCL, 2.5Kg modified lignin-nanofiber composite powder obtained in (2), 250g vanillin-hexamethylenediamine condensate obtained in (3) and 40g tocopherol evenly, and add them to a twin-screw extruder. Set the parameters as follows: feeding section 140℃, plasticizing section 155℃, homogenizing section 165℃, die head 165℃, screw speed 200rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain low-temperature resistant and biodegradable plastic.

[0071] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the modified lignin is not grafted with epichlorohydrin. The specific process is as follows: A specific method for preparing a low-temperature resistant biodegradable plastic includes the following steps:

[0072] (1) Under nitrogen protection, 2.5 kg of alkaline lignin, 4.17 kg of terephthalaldehyde, 13.02 kg of 1,4-dioxane and 417 g of 0.1 M hydrochloric acid were mixed, heated to 85 °C, stirred for 3 h, cooled to room temperature, 625 g of sodium bicarbonate was added, stirred for 45 min, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved again in 1,4-dioxane, and diethyl ether was added until a precipitate appeared. After precipitation for 2 h, the lignin protected by terephthalaldehyde was obtained by filtration, washing and drying.

[0073] (2) Mix 13.2Kg of phenol and 2.2Kg of phenylenedialdehyde-protected lignin obtained in (1), add 396g of 98% concentrated sulfuric acid, heat to 110℃, react for 30min, cool to room temperature, add 22Kg of dimethyl sulfoxide and 121Kg of deionized water, adjust pH=1 with 0.1M sulfuric acid, precipitate, filter, wash and dry to obtain modified lignin;

[0074] (3) Add 1.75Kg of the modified lignin obtained in (2) to 35Kg of ethanol / water mixed solution (ethanol and water are mixed in a weight ratio of 3:7), stir ultrasonically for 25min, then add 750g of nanocellulose, stir ultrasonically for 2h, evaporate the ethanol, freeze dry, and obtain modified lignin-nanocellulose composite powder.

[0075] (4) Add 300g vanillin to 1.875Kg ethanol, then add 112.5g hexamethylenediamine, heat to 80℃, react for 12h, cool to room temperature, filter to precipitate, wash and dry to obtain vanillin-hexamethylenediamine condensate.

[0076] (5) Mix 8.5 kg C-PCL, 2.5 kg modified lignin-nanofiber composite powder obtained in (3), 250 g vanillin-hexamethylenediamine condensate obtained in (4) and 40 g tocopherol evenly, and add them to a twin-screw extruder. Set the parameters as follows: feeding section 140°C, plasticizing section 155°C, homogenizing section 165°C, die head 165°C, screw speed 200 rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain low-temperature resistant and biodegradable plastic.

[0077] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that no modified lignin is added. The specific process is as follows: A specific preparation method of a low-temperature resistant biodegradable plastic includes the following steps:

[0078] (1) Add 300g vanillin to 1.875Kg ethanol, then add 112.5g hexamethylenediamine, heat to 80℃, react for 12h, cool to room temperature, filter to precipitate, wash and dry to obtain vanillin-hexamethylenediamine condensate;

[0079] (2) Mix 8.5 kg C-PCL, 750 g nanocellulose, 250 g vanillin-hexamethylenediamine condensate obtained in (1) and 40 g tocopherol evenly, and add them to a twin-screw extruder. Set the parameters as follows: feeding section 140°C, plasticizing section 155°C, homogenizing section 165°C, die head 165°C, screw speed 200 rpm. After melt extrusion, blow molding is performed by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0080] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that vanillin-hexanediamine condensate was not added.

[0081] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that vanillin-hexanediamine condensate was replaced with dicumyl peroxide.

[0082] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that vanillin-hexanediamine condensate was replaced with furan-maleimide.

[0083] Comparative Example 7: The difference between Comparative Example 7 and Example 2 is that the amount of vanillin-hexanediamine condensate added is reduced to 125g.

[0084] Comparative Example 8: The difference between Comparative Example 8 and Example 2 is that the amount of vanillin-hexanediamine condensate added is increased to 500g.

[0085] Performance testing:

[0086] The low-temperature resistant biodegradable plastics prepared according to the methods described in Examples 1-3 and Comparative Examples 1-8 were extruded using a twin-screw extruder and then injection molded into test strips with dimensions of 50 mm × 120 mm × 5.5 mm.

[0087] 1. Elongation at break test at -40℃: Test standard ISO527-2:2012, test examples 1-3 and comparative examples 1-4, the experimental results are shown in Table 1.

[0088] 2. Tensile strength test at -40℃: The test standard is ISO527-2:2012. Test examples 1-3 and comparative examples 1-4 were tested. The experimental results are shown in Table 1.

[0089] 3. Low-temperature drop ball test at -40℃: After placing Examples 1-3 and Comparative Examples 1-4 in an environment of -40℃ for 5 hours, a low-temperature drop ball test was conducted, and the crack height of the samples was recorded. The experimental results are shown in Table 1.

[0090] 4. Degradation experiments at different pH values: Aqueous solutions with pH values ​​of 3, 5, and 7 were prepared respectively. The samples prepared in Examples 1-3 and Comparative Examples 5-8 were placed in the experimental solutions. Samples were taken weekly, and the weight loss rate (%) of the samples was recorded for six consecutive weeks. The experimental results are shown in Tables 2, 3, and 4.

[0091] Table 1. Mechanical Properties

[0092]

[0093] Table 2. Weight loss rate test at pH=7

[0094]

[0095] Table 3. Weight loss rate test at pH=5

[0096]

[0097] Table 4. Weight loss rate test at pH=3

[0098]

[0099] Data Analysis:

[0100] As can be seen from the experimental data in Tables 1, 2, 3, and 4, the low-temperature biodegradable plastic prepared by this invention still possesses good mechanical properties at low temperatures, exhibits stable performance in everyday use environments, and degrades rapidly in acidic environments after recycling, with the degradation rate accelerating as the pH decreases. Example 2 shows the best performance. Regarding the low-temperature tensile strength, this may be due to the excellent tensile properties provided by nanocellulose. The modified lignin-coated nanofibers solve the problems of uneven dispersion and poor compatibility with C-PCL substrates caused by the surface hydrophilic effect of ordinary nanocellulose, allowing it to function better and improving the low-temperature tensile strength of the plastic. Simultaneously, the cross-linking network formed by vanillin-hexamethylenediamine condensate as a cross-linking agent has a certain degree of flexibility and does not excessively restrict the movement of molecular chains. At low temperatures, the cross-linking network can buffer external forces to a certain extent, allowing the molecular chains sufficient space to adapt to deformation during stretching, avoiding brittle fracture, thereby improving the low-temperature tensile strength.

[0101] In terms of low-temperature tensile strength, Example 2 also performed best. This may be because during the preparation of modified lignin, alkaline lignin was introduced with a variety of functional groups, such as hydroxyl and epoxy groups, through multiple steps. These functional groups enhanced the interaction between lignin and other raw materials. For example, the large number of hydroxyl groups introduced can form a stronger adsorption on the surface of nanocellulose, and epoxy groups can react with the carboxyl groups in C-PCL for crosslinking, or they can crosslink with the hydroxyl groups in vanillin-hexanediamine condensate to form a multi-crosslinked structure, thereby improving tensile strength.

[0102] The low-temperature drop ball test showed that the plastic prepared in Example 2 exhibited the best toughness at low temperatures. This is likely due to the excellent interfacial interactions between the components in Example 2. The modified lignin, through its surface functional groups such as hydroxyl and epoxy groups, formed chemical bonds or strong physical interactions with the carboxyl groups of C-PCL and the hydroxyl groups on the surface of nanocellulose. During low-temperature drop ball impact, this excellent interface allows stress to be effectively transferred between different components, preventing material failure caused by interfacial debonding. Nanocellulose, as a reinforcing phase, can transfer the impact force to the surrounding C-PCL and modified lignin, enabling the entire material system to withstand the impact and improving its impact resistance. Simultaneously, the cross-linked network structure also makes the material a unified whole, enhancing its rigidity and integrity. When subjected to low-temperature drop ball impact, the cross-linked network can quickly disperse the impact force throughout the entire material system, preventing localized stress concentration that could lead to cracking and further improving the material's impact resistance at low temperatures.

[0103] Finally, degradation experiments under different pH conditions showed that, compared to the comparative example using other common crosslinking agents, the vanillin-hexanediamine condensate used in Example 2 exhibited almost no impact on stability under normal conditions. After recycling, the degradation rate under acidic conditions was significantly higher than that of the plastics prepared using other common crosslinking agents in the comparative example. Furthermore, the degradation rate of Example 2 showed a positive correlation with the strength of the acid. This can be explained by the hydrolysis of imine bonds in the vanillin-hexanediamine condensate under acidic conditions. The hydrolysis of imine bonds leads to the destruction of the crosslinked structure, releasing lignin and nanocellulose from the material, further accelerating the degradation. Simultaneously, the hydrolysis rate of imine bonds increases with increasing acidity, explaining why the degradation rate shows a positive correlation with the strength of the acid. Finally, the present invention… The optimal amount of vanillin-hexamethylenediamine condensate added has also been proven. Adding too little or too much is detrimental to the degradation of plastics under acidic conditions. This is because when adding too little, the imine bonds hydrolyze during the degradation process, but due to the small amount, the macroscopic structure of the material is not destroyed, and the hydrolysis of the imine bonds does not accelerate the degradation, thus affecting the degradation rate. When adding too much, the cross-linking density of the material will increase significantly. The highly cross-linked structure makes the connection between molecular chains tight, forming a dense spatial network. During the degradation process, neither water molecules, microorganisms and other degradation media, nor small molecule products generated by the degradation reaction can diffuse inside the material. This results in the degradation reaction only taking place in a limited area on the surface of the material, and the internal degradable components cannot come into contact with the degradation media in time, greatly slowing down the degradation rate.

[0104] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A low-temperature resistant biodegradable plastic, characterized in that, The raw materials include the following parts by weight: carboxylated polycaprolactone (C-PCL): 80-90 parts, modified lignin: 15-20 parts, nanocellulose: 5-10 parts, crosslinking agent: 2-3 parts, antioxidant: 0.3-0.5 parts; The specific preparation method of the modified lignin is as follows: (1) Under nitrogen protection, alkaline lignin, terephthalaldehyde, 1,4-dioxane and hydrochloric acid are mixed, heated to 80-90℃, stirred for 2-4h, cooled to room temperature, sodium bicarbonate is added, stirred for 40-50min, filtered, and the filtrate is dried on a rotary evaporator. The obtained solid is then dissolved again in 1,4-dioxane, and diethyl ether is added until precipitation occurs. After precipitation for 1-3h, the lignin protected by terephthalaldehyde is obtained by filtration, washing and drying. (2) Mix phenol and the lignin protected by phenylenedialdehyde obtained in (1), add concentrated sulfuric acid, heat to 100-120℃, react for 20-40 min, cool to room temperature, add dimethyl sulfoxide and deionized water, adjust pH=1 with sulfuric acid, precipitate, filter, wash and dry to obtain hydroxylated lignin. (3) Add the hydroxylated lignin from (2) to NaOH solution, stir for 30-50 min, then add epichlorohydrin, heat to 60-80℃, react for 4-6 h, cool to room temperature, add dilute hydrochloric acid to adjust pH to neutral, filter, wash and dry to obtain modified lignin; The crosslinking agent refers to vanillin-hexanediamine condensate. The specific preparation process is as follows: Vanillin is added to ethanol, then hexanediamine is added, the temperature is raised to 70-90℃, the reaction is carried out for 10-14 hours, the temperature is cooled to room temperature, the precipitate is filtered, washed, and dried to obtain vanillin-hexanediamine condensate. The vanillin, hexamethylenediamine, and ethanol are present in a weight ratio of 3-5:20-30:1-2.

2. The low-temperature resistant biodegradable plastic according to claim 1, characterized in that, In (1), the alkaline lignin, terephthalaldehyde, 1,4-dioxane, hydrochloric acid and sodium bicarbonate are in a weight ratio of 20-28:35-45:120-130:3-5:4-8, and the hydrochloric acid refers to dilute hydrochloric acid with a concentration of 0.1M.

3. The low-temperature resistant biodegradable plastic according to claim 1, characterized in that, In (2), phenol, lignin protected by terephthalaldehyde, concentrated sulfuric acid, dimethyl sulfoxide and deionized water are in a weight ratio of 4-8:1:0.16-0.2:8-12:50-60. Concentrated sulfuric acid refers to sulfuric acid with a concentration of 98%, and sulfuric acid refers to dilute sulfuric acid with a concentration of 0.1M.

4. The low-temperature resistant biodegradable plastic according to claim 1, characterized in that, In step (3), the hydroxylated lignin, NaOH solution and epichlorohydrin are in a weight ratio of 1:10-14:0.3-0.5, the concentration of NaOH solution is 0.1M and the concentration of dilute hydrochloric acid is 0.1M.

5. The low-temperature resistant biodegradable plastic according to claim 1, characterized in that, The antioxidant refers to either tocopherol or tea polyphenols.

6. The method for preparing the low-temperature resistant biodegradable plastic according to any one of claims 1-5, characterized in that, Includes the following steps: S1: Add modified lignin to an ethanol / water mixture, ultrasonically stir for 20-30 min, then add nanocellulose, ultrasonically stir for 1-3 h, evaporate the ethanol, and freeze dry to obtain modified lignin-nanocellulose composite powder. S2: After uniformly mixing the modified lignin-nanofiber composite powder obtained from C-PCL and S1, the crosslinking agent and antioxidant are added to a twin-screw extruder for melt extrusion and then blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

7. The method for preparing the low-temperature resistant biodegradable plastic according to claim 6, characterized in that, The modified lignin and ethanol / water mixed solution in S1 are in a weight ratio of 1:15-25. The ethanol / water mixed solution refers to the mixture of ethanol and deionized water in a weight ratio of 3:

7.

8. The method for preparing low-temperature resistant biodegradable plastic according to claim 6, characterized in that, The parameters of the twin-screw extruder in S2 are as follows: feeding section 130-150℃, plasticizing section 150-160℃, homogenizing section 160-170℃, die head 160-170℃, and screw speed 180-220rpm.

9. The application of the low-temperature resistant biodegradable plastic obtained by the preparation method according to any one of claims 6-8 in agricultural mulch films in cold environments.

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