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

Abstract

The invention relates to the technical field of plastic, in particular to a preparation method and application of low-temperature-resistant degradable plastic. The low-temperature-resistant degradable plastic is prepared from the following components in parts by weight: 80 to 90 parts of carboxylated polycaprolactone (C-PCL), 15 to 20 parts of modified lignin, 5 to 10 parts of nano cellulose, 2 to 3 parts of a cross-linking agent and 0.3 to 0.5 part of an antioxidant. The modified lignin is prepared by subjecting alkaline lignin to terephthaldehyde protection and hydroxylation and then grafting epoxy groups; the cross-linking agent is prepared from vanillin and hexamethylenediamine through a condensation reaction, and imine bonds contained in the cross-linking agent endow the material with a good degradation rate under an acidic condition. The low-temperature-resistant degradable plastic prepared by the invention has good low-temperature mechanical properties and excellent degradation speed under an acidic condition under the condition of ensuring a certain service life, and is suitable for being applied to agricultural mulching films in a cold environment.

Description

Technical Field

[0001] The present invention relates to the technical field of plastics, and in particular to a preparation method and application of low-temperature resistant and degradable plastics. Background Art

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

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

[0004] In view of this, the purpose of the present invention is to propose a preparation method and application of low-temperature resistant degradable plastics to solve the problems of poor mechanical properties, insufficient toughness and difficult to control degradation rate of existing plastics at low temperatures.

[0005] Based on the above purpose, 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, cross-linking 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°C, stirred for 2-4 hours, cooled to room temperature, and sodium bicarbonate is added. The mixture is stirred for 40-50 minutes. The filtrate is filtered and dried on a rotary evaporator. The obtained solid is then dissolved in 1,4-dioxane again, and ether is added until precipitation occurs. After precipitation for 1-3 hours, the mixture is filtered, washed, and dried to obtain terephthalaldehyde-protected lignin. (2) Phenol and the terephthalaldehyde-protected lignin obtained in (1) are mixed, concentrated sulfuric acid is added, the temperature is raised to 100-120°C, the reaction is carried out for 20-40 minutes, dimethyl sulfoxide and deionized water are added after cooling to room temperature, the pH is adjusted to 1 with sulfuric acid, and after precipitation, the hydroxylated lignin is filtered, washed, and dried; (3) Add the hydroxylated lignin in (2) to the NaOH solution and stir for 30-50 min. Then add epichlorohydrin and heat to 60-80 °C. React for 4-6 h. After cooling to room temperature, add dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain the modified lignin.

[0006] Preferably, the weight ratio of the alkaline lignin, terephthalaldehyde, 1,4-dioxane, hydrochloric acid and sodium bicarbonate in (1) is 20-28:35-45:120-130:3-5:4-8. The lignin after aldehyde protection can shield some active sites to a certain extent, reduce the overall reaction activity of the lignin molecule during subsequent hydroxylation, prevent the generation of a large amount of by-products due to excessive reaction, and help improve the purity and yield of the product.

[0007] Preferably, the hydrochloric acid in (1) refers to dilute hydrochloric acid with a concentration of 0.1M.

[0008] Preferably, the weight ratio of phenol, terephthalaldehyde-protected lignin, concentrated sulfuric acid, dimethyl sulfoxide and deionized water in (2) is 4-8:1:0.16-0.2:8-12:50-60.

[0009] Preferably, the concentrated sulfuric acid in (2) refers to sulfuric acid with a concentration of 98%.

[0010] Preferably, the sulfuric acid in (2) refers to dilute sulfuric acid with a concentration of 0.1 M.

[0011] Preferably, the weight ratio of the hydroxylated lignin, NaOH solution and epichlorohydrin in (3) is 1:10-14:0.3-0.5.

[0012] Preferably, the concentration of the NaOH solution in (3) is 0.1 M. This operation can deprotonate the hydroxyl groups on the surface of the hydroxylated lignin to form oxygen anions, increase the nucleophilicity, and facilitate the subsequent reaction.

[0013] Preferably, the concentration of the dilute hydrochloric acid in (3) is 0.1M.

[0014] Preferably, the cross-linking agent refers to vanillin-hexamethylenediamine polycondensate, and the specific preparation process is as follows: vanillin is added to ethanol, and then hexamethylenediamine is added, the temperature is raised to 70-90°C, the reaction is carried out for 10-14 hours, and the mixture is cooled to room temperature. The precipitate is filtered, washed, and dried to obtain the vanillin-hexamethylenediamine polycondensate. The reaction process is as follows: Formula (1) The product was characterized by HNMR analysis, HNMR (400 MHz, Chloroform-d) δ8.23 (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), 1.48-1.36 (m, 4H), vanillin, hexamethylenediamine and ethanol were in a weight ratio of 3-5:20-30:1-2.

[0015] Preferably, the cross-linking mechanism of the vanillin-hexamethylenediamine polycondensate is: Formula (2) In the formula, R represents lignin. Under high temperature, the epoxy bonds in the modified lignin open and undergo cross-linking reactions with the hydroxyl groups in the cross-linker. The longer cross-linker molecular chains interweave and weave with each other to form a cross-linked network.

[0016] Preferably, the hydrolysis mechanism of the imine bond in the vanillin-hexamethylenediamine polycondensate under acidic conditions is: Formula (3) The imine bond will undergo hydrolysis reaction under acidic conditions, breaking the imine bond and thus undoing the cross-linking structure, which can accelerate the degradation rate.

[0017] Preferably, the antioxidant is tocopherol or tea polyphenols.

[0018] Furthermore, the present invention also provides a method for preparing the above-mentioned low-temperature resistant degradable plastic, which specifically comprises the following steps: S1: adding modified lignin to an ethanol / water mixed solution, ultrasonically stirring for 20-30 minutes, then adding nanocellulose, ultrasonically stirring for 1-3 hours, spin-drying the ethanol, and freeze-drying to obtain a modified lignin-nanofiber composite powder; S2: C-PCL, the modified lignin-nanofiber composite powder obtained in S1, a crosslinking agent, and an antioxidant are uniformly mixed, added into a twin-screw extruder for melt extrusion, and then blow-molded by a blow molding machine to obtain a low-temperature resistant and degradable plastic.

[0019] Preferably, the weight ratio of the modified lignin to the ethanol / water mixed solution in S1 is 1:15-25.

[0020] Preferably, the ethanol / water mixed solution in S1 refers to a mixture of ethanol and deionized water in a weight ratio of 3:7.

[0021] Preferably, the parameters of the twin-screw extruder in S2 are: feeding section 130-150°C, plasticizing section 150-160°C, homogenizing section 160-170°C, die head 160-170°C, and screw speed 180-220rpm; at high temperature, the epoxy groups in the modified lignin react with the carboxyl groups in C-PCL, and the two are grafted and cross-linked 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 cross-linking agent, thereby being fixed, and a cross-linked network is also formed through physical entanglement of the long molecular chains of the cross-linking agent with C-PCL.

[0022] Furthermore, the present invention also provides the use of the above-mentioned low-temperature resistant and degradable plastic in agricultural mulch films in cold environments.

[0023] Beneficial effects of the present invention: 1. The low-temperature resistant degradable plastic prepared by the present invention has good low-temperature resistance through the reasonable proportion of specific raw materials, such as the synergistic effect of carboxylated polycaprolactone, modified lignin and nanocellulose. The modified lignin improves the dispersibility of nanocellulose on the one hand, and enhances the binding force between nanocellulose and carboxylated polycaprolactone on the other hand. In a low-temperature environment, the material can still maintain good flexibility and mechanical strength and is not easy to crack, thereby broadening the application range of the plastic and making it particularly suitable for related fields in cold regions.

[0024] 2. The low-temperature-resistant, degradable plastic prepared by the present invention incorporates a vanillin-hexamethylenediamine polycondensate as a crosslinking agent. This crosslinked structure further enhances the material's mechanical properties and low-temperature resistance. Furthermore, the imine bonds in the vanillin-hexamethylenediamine polycondensate undergo hydrolysis under acidic conditions, thereby unraveling the crosslinked structure. This characteristic makes the material's degradation process controllable. By adjusting the pH of the environment, the material's degradation rate can be precisely controlled, maintaining stable performance in everyday use environments. However, it can rapidly degrade in recycling processes or specific acidic environments, effectively reducing the residual time of plastic waste in the environment and reducing environmental pollution.

[0025] 3. The modified lignin and nanocellulose used in the present invention are derived from natural renewable resources, which not only reduces dependence on non-renewable resources such as petroleum, but also after the material is degraded, these natural components can be better integrated into the natural environment, reducing the negative impact on the ecosystem, which is in line with the concept of sustainable development. At the same time, the preparation method of the low-temperature resistant degradable plastic prepared by the present invention is simple to operate, the conditions of each step are mild and easy to control, and it can be prepared through common processes such as ultrasonic stirring and melt extrusion, which is suitable for large-scale industrial production and provides strong technical support for the wide application of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the HNMR spectrum of vanillin-hexamethylenediamine polycondensate; Figure 2 This is the cross-linking mechanism diagram of vanillin-hexamethylenediamine polycondensate; Figure 3 Diagram of the hydrolysis mechanism of imine bond under acidic conditions. DETAILED DESCRIPTION

[0027] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to specific embodiments.

[0028] The sources of the reagents and raw materials used in the examples of the present invention are as follows: Carboxylated polycaprolactone was purchased from Xi'an Ruixi Biotechnology Co., Ltd. with a purity of 97% and an Mn of 45,000-50,000. Alkaline lignin was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a product number of L832292. Nanocellulose was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a product number of C909405, with an outer diameter of 10 nm and a length of 200 nm. Terephthalaldehyde was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. with a product number of P815782 and a purity of 98%. Phenol was purchased from Shanghai Jizhi Biochemical Technology Co., Ltd. with a product number of P3 3741, purity 99.5%; epichlorohydrin was purchased from Shanghai MacLean Biochemical Technology Co., Ltd., product number E808937, purity 99%; vanillin was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., product number V100115, purity 99%; hexamethylenediamine was purchased from Shanghai MacLean Biochemical Technology Co., Ltd., product 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., product number T861565, purity 97%.

[0029] Example 1: A specific preparation method of a low-temperature resistant biodegradable plastic, comprising the following steps: (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, added with 500 g of sodium bicarbonate, stirred for 40 min, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved in 1,4-dioxane again, and ether was added until precipitation occurred. After precipitation for 1 h, the lignin protected by terephthalaldehyde was obtained after filtering, washing, and drying. (2) 8.4 kg of phenol and 2.1 kg of terephthalaldehyde-protected lignin obtained in (1) were mixed, 336 g of 98% concentrated sulfuric acid was added, the temperature was raised to 100 °C, the reaction was carried out for 20 min, and after cooling to room temperature, 16.8 kg of dimethyl sulfoxide and 105 kg of deionized water were added, and the pH was adjusted to 1 with 0.1 M sulfuric acid. After precipitation, the mixture was filtered, washed, and dried to obtain hydroxylated lignin; (3) Add 2 kg of hydroxylated lignin from (2) to 20 kg of 0.1 M NaOH solution and stir for 30 min. Then add 600 g of epichlorohydrin and heat to 60 °C. React for 4 h. After cooling to room temperature, add 0.1 M dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain modified lignin.

[0030] (4) 1.5 kg of modified lignin was added to 22.5 kg of ethanol / water mixed solution (ethanol and water were mixed in a weight ratio of 3:7), ultrasonically stirred for 20 min, and then 500 g of nanocellulose was added and ultrasonically stirred for 1 h. After the ethanol was dried, the mixture was freeze-dried to obtain a modified lignin-nanofiber composite powder; (5) Add 180 g of vanillin to 1.2 kg of ethanol, then add 60 g of hexamethylenediamine, heat to 70 °C, react for 10 h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (6) 8 kg of C-PCL, 2 kg of the modified lignin-nanofiber composite powder obtained in (4), 200 g of the vanillin-hexamethylenediamine polycondensate obtained in (5) and 30 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set 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, the mixture was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0031] Example 2: A specific preparation method of a low-temperature resistant biodegradable plastic, comprising the following steps: (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 hours, cooled to room temperature, added with 625 g of sodium bicarbonate, stirred for 45 minutes, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved in 1,4-dioxane again, and ether was added until precipitation occurred. After precipitation for 2 hours, the lignin protected by terephthalaldehyde was obtained after filtering, washing, and drying. (2) 13.2 kg of phenol and 2.2 kg of terephthalaldehyde-protected lignin obtained in (1) were mixed, 396 g of 98% concentrated sulfuric acid was added, the temperature was raised to 110 °C, the reaction was carried out for 30 min, and after cooling to room temperature, 22 kg of dimethyl sulfoxide and 121 kg of deionized water were added, and the pH was adjusted to 1 with 0.1 M sulfuric acid. After precipitation, the mixture was filtered, washed, and dried to obtain hydroxylated lignin; (3) Add 2 kg of hydroxylated lignin from (2) to 24 kg of 0.1 M NaOH solution and stir for 40 min. Then add 800 g of epichlorohydrin and heat to 70 °C. React for 5 h. After cooling to room temperature, add 0.1 M dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain modified lignin.

[0032] (4) 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, and then 750 g of nanocellulose was added and ultrasonically stirred for 2 h. After the ethanol was dried, the mixture was freeze-dried to obtain a modified lignin-nanofiber composite powder; (5) Add 300 g of vanillin to 1.875 kg of ethanol, then add 112.5 g of hexamethylenediamine, heat to 80 °C, react for 12 h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (6) 8.5 kg of C-PCL, 2.5 kg of the modified lignin-nanofiber composite powder obtained in (4), 250 g of the vanillin-hexamethylenediamine polycondensate obtained in (5) and 40 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set 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, the product was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0033] Example 3: A specific preparation method of a low-temperature resistant biodegradable plastic, comprising the following steps: (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, stirred for 4 hours, cooled to room temperature, added with 857 g of sodium bicarbonate, stirred for 50 minutes, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved in 1,4-dioxane again, and ether was added until precipitation occurred. After precipitation for 3 hours, the lignin protected by terephthalaldehyde was obtained after filtering, washing, and drying. (2) 20 kg of phenol and 2.5 kg of terephthalaldehyde-protected lignin obtained in (1) were mixed, 500 g of 98% concentrated sulfuric acid was added, the temperature was raised to 120 ° C, the reaction was carried out for 40 minutes, and after cooling to room temperature, 30 kg of dimethyl sulfoxide and 150 kg of deionized water were added, and the pH was adjusted to 1 with 0.1 M sulfuric acid. After precipitation, the mixture was filtered, washed, and dried to obtain hydroxylated lignin; (3) Add 2.2 kg of hydroxylated lignin from (2) to 30.8 kg of 0.1 M NaOH solution and stir for 50 min. Then add 1.1 kg of epichlorohydrin and heat to 80 °C. React for 6 h. After cooling to room temperature, add 0.1 M dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain modified lignin.

[0034] (4) Add 2 kg of modified lignin to 50 kg of ethanol / water mixed solution (ethanol and water are mixed in a weight ratio of 3:7), ultrasonically stir for 30 min, then add 1 kg of nanocellulose, ultrasonically stir for 3 h, spin dry the ethanol, and freeze-dry to obtain modified lignin-nanofiber composite powder; (5) Add 300 g of vanillin to 1.8 kg of ethanol, then add 120 g of hexamethylenediamine, heat to 90 °C, react for 14 h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (6) 9 kg of C-PCL, 3 kg of the modified lignin-nanofiber composite powder obtained in (4), 300 g of the vanillin-hexamethylenediamine polycondensate obtained in (5) and 50 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set as follows: feeding section 150°C, plasticizing section 160°C, homogenizing section 170°C, die head 170°C, screw speed 220 rpm. After melt extrusion, the mixture was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0035] 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. The specific process is as follows: A specific preparation method of a low-temperature resistant degradable plastic includes the following processes: (1) Add 2 kg of alkaline lignin to 24 kg of 0.1 M NaOH solution and stir for 40 min. Then add 800 g of epichlorohydrin and heat to 70 °C. React for 5 h. After cooling to room temperature, add 0.1 M dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain modified lignin.

[0036] (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, and then 750 g of nanocellulose was added and ultrasonically stirred for 2 h. After the ethanol was spin-dried, the mixture was freeze-dried to obtain a modified lignin-nanofiber composite powder; (3) Add 300 g of vanillin to 1.875 kg of ethanol, then add 112.5 g of hexamethylenediamine, heat to 80 °C, react for 12 h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (4) 8.5 kg of C-PCL, 2.5 kg of the modified lignin-nanofiber composite powder obtained in (2), 250 g of the vanillin-hexamethylenediamine polycondensate obtained in (3) and 40 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set 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, the product was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0037] 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 degradable plastic includes the following steps: (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 hours, cooled to room temperature, added with 625 g of sodium bicarbonate, stirred for 45 minutes, filtered, and the filtrate was dried on a rotary evaporator. The obtained solid was then dissolved in 1,4-dioxane again, and ether was added until precipitation occurred. After precipitation for 2 hours, the lignin protected by terephthalaldehyde was obtained after filtering, washing, and drying. (2) 13.2 kg of phenol and 2.2 kg of terephthalaldehyde-protected lignin obtained in (1) were mixed, 396 g of 98% concentrated sulfuric acid was added, the temperature was raised to 110 °C, the reaction was carried out for 30 min, and after cooling to room temperature, 22 kg of dimethyl sulfoxide and 121 kg of deionized water were added, and the pH was adjusted to 1 with 0.1 M sulfuric acid. After precipitation, the modified lignin was filtered, washed, and dried. (3) 1.75 kg of the modified lignin obtained in (2) was added to 35 kg of an ethanol / water mixed solution (ethanol and water were mixed in a weight ratio of 3:7), ultrasonically stirred for 25 min, and then 750 g of nanocellulose was added and ultrasonically stirred for 2 h. After the ethanol was dried, the mixture was freeze-dried to obtain a modified lignin-nanofiber composite powder; (4) Add 300 g of vanillin to 1.875 kg of ethanol, then add 112.5 g of hexamethylenediamine, heat to 80 °C, react for 12 h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (5) 8.5 kg of C-PCL, 2.5 kg of the modified lignin-nanofiber composite powder obtained in (3), 250 g of the vanillin-hexamethylenediamine polycondensate obtained in (4) and 40 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set 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, the product was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0038] 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 degradable plastic includes the following processes: (1) Add 300g of vanillin to 1.875kg of ethanol, then add 112.5g of hexamethylenediamine, heat to 80°C, react for 12h, cool to room temperature, filter the precipitate, wash, and dry to obtain a vanillin-hexamethylenediamine polycondensate; (2) 8.5 kg of C-PCL, 750 g of nanocellulose, 250 g of the vanillin-hexamethylenediamine polycondensate obtained in (1) and 40 g of tocopherol were mixed evenly and added to a twin-screw extruder. The parameters were set 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, the mixture was blow-molded by a blow molding machine to obtain a low-temperature resistant and biodegradable plastic.

[0039] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that vanillin-hexamethylenediamine condensation product is not added.

[0040] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the vanillin-hexamethylenediamine polycondensate is replaced by diisopropylbenzene peroxide.

[0041] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that the vanillin-hexamethylenediamine polycondensate is replaced by furan-maleimide.

[0042] Comparative Example 7: The difference between Comparative Example 7 and Example 2 is that the added amount of vanillin-hexamethylenediamine condensation product is reduced to 125 g.

[0043] Comparative Example 8: The difference between Comparative Example 8 and Example 2 is that the added amount of vanillin-hexamethylenediamine condensation product is increased to 500 g.

[0044] Performance testing: The low-temperature resistant degradable plastics prepared by the preparation methods described in Examples 1-3 and Comparative Examples 1-8 were extruded through a twin-screw extruder and then formed into strips using an injection molding machine. The strips had a size of 50 mm × 120 mm × 5.5 mm. 1. Elongation at break test at -40°C: Testing standard ISO527-2:2012, testing Examples 1-3 and Comparative Examples 1-4, the experimental results are shown in Table 1.

[0045] 2. Tensile strength test at -40°C: Testing standard ISO527-2:2012, testing Examples 1-3 and Comparative Examples 1-4, the experimental results are shown in Table 1.

[0046] 3. Low-temperature drop ball test at -40°C: After placing Examples 1-3 and Comparative Examples 1-4 in a -40°C environment for 5 hours, a low-temperature drop ball test was performed, and the height of the cracking of the specimens was recorded. The experimental results are shown in Table 1.

[0047] 4. Degradation experiment at different pH: Aqueous solutions with pH = 3, 5, and 7 were prepared respectively, and 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 continuously for six weeks. The experimental results are shown in Tables 2, 3, and 4.

[0048] Table 1. Mechanical properties

[0049] Table 2. Weight loss test at pH=7

[0050] Table 3. Weight loss test at pH=5

[0051] Table 4. Weight loss test at pH=3

[0052] Data Analysis: From the experimental data in Tables 1, 2, 3, and 4, it can be seen that the low-temperature resistant degradable plastic prepared by the present invention still has good mechanical properties at low temperatures, has stable performance in daily use environments, and has a faster degradation rate in an acidic environment after recovery, and the degradation rate shows an accelerating trend with decreasing pH. Among them, the performance of Example 2 is the best. In terms of low-temperature elongation at break, this may be because the nanocellulose provides the material with good tensile resistance, and the modified lignin-coated nanofibers solve the problem of uneven dispersion caused by the surface hydrophilic effect of ordinary nanocellulose and poor compatibility with the C-PCL substrate, so that it can play a better role and improve the elongation at break of the plastic. At the same time, the cross-linked network formed by the vanillin-hexamethylenediamine condensation polymer as a cross-linking agent has a certain flexibility and will not excessively restrict the movement of the molecular chains. At low temperatures, the cross-linked network can buffer external forces to a certain extent, so that the molecular chains have enough room for activity to adapt to deformation during the stretching process of the material, avoiding brittle fracture, thereby improving the elongation at break at low temperatures.

[0053] In terms of low-temperature tensile strength, Example 2 also performed the best. This may be because during the preparation of the modified lignin, the alkaline lignin introduced a variety of functional groups, such as hydroxyl groups and epoxy groups, through multi-step reactions. 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 the epoxy groups can react with the carboxyl groups in C-PCL to cross-link, and can also cross-link with the hydroxyl groups in the vanillin-hexamethylenediamine condensation product to form a multiple cross-linked structure, thereby improving the tensile strength.

[0054] It can be seen from the low-temperature drop ball test that the plastic prepared in Example 2 has the best toughness at low temperatures. This may be because there is good interfacial interaction between the various components in Example 2. The modified lignin forms chemical bonds or strong physical interactions with the carboxyl groups of C-PCL and the hydroxyl groups on the surface of nanocellulose through various functional groups on its surface, such as hydroxyl groups and epoxy groups. When impacted by a low-temperature drop ball, this good interface enables stress to be effectively transferred between different components, avoiding material failure due to interface debonding. Nanocellulose, as a reinforcing phase, can transfer the impact force to the surrounding C-PCL and modified lignin, allowing the entire material system to withstand the impact together, thereby improving the material's impact resistance. At the same time, the cross-linked network structure also forms the material into a whole, enhancing the material's rigidity and integrity. When impacted by a low-temperature drop ball, the cross-linked network can quickly disperse the impact force to the entire material system, avoiding local stress concentration that causes material cracking, further improving the material's impact resistance at low temperatures.

[0055] Finally, it can be seen from the degradation experiments under different pH environments that in Example 2, vanillin-hexamethylenediamine condensation polymer is used as a cross-linking agent. Compared with the comparative examples using other common cross-linking agents, the stability is almost unaffected under daily environments. After recovery, the degradation rate under acidic conditions is much greater than that of the plastics prepared by using other common cross-linking agents in the comparative examples, and the degradation rate of Example 2 shows a positive correlation with the strength of the acidity. This can be explained by the hydrolysis reaction of the imine bond in the vanillin-hexamethylenediamine condensation polymer under acidic conditions. The hydrolysis of the imine bond causes the cross-linking structure to be destroyed, and the lignin and nanocellulose in the material are released, further accelerating the degradation of the material. At the same time, the hydrolysis rate of the imine bond is accelerated as the acidity becomes stronger, which explains why the degradation rate shows a positive correlation with the strength of the acidity. Finally, the present invention is used The addition amount of vanillin-hexamethylenediamine condensation polymer has also been proven to be the optimal choice. Too little or too much addition is not conducive to the degradation of plastics under acidic conditions. This is because, when less is added, the imine bond hydrolyzes during the degradation process, but due to the small amount, the macrostructure of the material is not destroyed, and the hydrolysis of the imine bond does not accelerate the degradation, thereby affecting the degradation rate; when too much is added, the cross-linking density of the material will increase significantly, and the highly cross-linked structure makes the molecular chains tightly connected, forming a dense spatial network. During the degradation process, whether it is degradation media such as water molecules and microorganisms, or small molecular products produced by the degradation reaction, it is difficult to diffuse inside the material. This results in the degradation reaction only being able to proceed in a limited area on the surface of the material, and the internal degradable components cannot contact the degradation medium in time, which greatly slows down the degradation rate.

[0056] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention is limited to these examples. Within the scope of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

Claims

1. A low-temperature resistant degradable plastic, characterized in that: The invention comprises the following raw materials in parts by weight: carboxylated polycaprolactone C-PCL: 80-90 parts, modified lignin: 15-20 parts, nanocellulose: 5-10 parts, cross-linking 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°C, stirred for 2-4 hours, cooled to room temperature, and sodium bicarbonate is added. The mixture is stirred for 40-50 minutes. The filtrate is filtered and dried on a rotary evaporator. The obtained solid is then dissolved in 1,4-dioxane again, and ether is added until precipitation occurs. After precipitation for 1-3 hours, the mixture is filtered, washed, and dried to obtain terephthalaldehyde-protected lignin. (2) Phenol and the terephthalaldehyde-protected lignin obtained in (1) are mixed, concentrated sulfuric acid is added, the temperature is raised to 100-120°C, the reaction is carried out for 20-40 minutes, dimethyl sulfoxide and deionized water are added after cooling to room temperature, the pH is adjusted to 1 with sulfuric acid, and after precipitation, the hydroxylated lignin is filtered, washed, and dried; (3) Add the hydroxylated lignin in (2) to the NaOH solution and stir for 30-50 min. Then add epichlorohydrin and heat to 60-80 °C. React for 4-6 h. After cooling to room temperature, add dilute hydrochloric acid to adjust the pH to neutral. Filter, wash and dry to obtain the modified lignin.

2. The low-temperature resistant degradable plastic according to claim 1, characterized in that: The weight ratio of the alkaline lignin, terephthalaldehyde, 1,4-dioxane, hydrochloric acid and sodium bicarbonate in (1) is 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 degradable plastic according to claim 1, characterized in that: In the (2), the weight ratio of phenol, terephthalaldehyde-protected lignin, concentrated sulfuric acid, dimethyl sulfoxide and deionized water is 4-8:1:0.16-0.2:8-12:50-60, wherein 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 degradable plastic according to claim 1, characterized in that: The weight ratio of the hydroxylated lignin, NaOH solution and epichlorohydrin in (3) is 1:10-14:0.3-0.5, the concentration of the NaOH solution is 0.1M, and the concentration of the dilute hydrochloric acid is 0.1M.

5. The low-temperature resistant degradable plastic according to claim 1, characterized in that: The cross-linking agent refers to vanillin-hexamethylenediamine condensation polymer, and the specific preparation process is as follows: vanillin is added to ethanol, then hexamethylenediamine is added, the temperature is raised to 70-90°C, 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 the vanillin-hexamethylenediamine condensation polymer, wherein the weight ratio of vanillin, hexamethylenediamine and ethanol is 3-5:20-30:1-2.

6. The low-temperature resistant degradable plastic according to claim 1, characterized in that: The antioxidant refers to one of tocopherol or tea polyphenols.

7. The method for preparing the low-temperature resistant degradable plastic according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1: adding modified lignin to an ethanol / water mixed solution, ultrasonically stirring for 20-30 minutes, then adding nanocellulose, ultrasonically stirring for 1-3 hours, spin-drying the ethanol, and freeze-drying to obtain a modified lignin-nanofiber composite powder; S2: C-PCL, the modified lignin-nanofiber composite powder obtained in S1, a crosslinking agent, and an antioxidant are uniformly mixed, added into a twin-screw extruder for melt extrusion, and then blow-molded by a blow molding machine to obtain a low-temperature resistant and degradable plastic.

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

7.

9. The method for preparing low-temperature resistant degradable plastic according to claim 7, characterized in that: The parameters of the twin-screw extruder in S2 are: feeding section 130-150°C, plasticizing section 150-160°C, homogenizing section 160-170°C, die head 160-170°C, and screw speed 180-220rpm.

10. Use of the low-temperature resistant degradable plastic obtained by the preparation method according to any one of claims 7 to 9 in agricultural mulch films in cold environments.

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