Biodegradable coated paper and preparation method thereof

Abstract

The invention discloses biodegradable coated paper and a preparation method thereof, and relates to the technical field of polymer materials. The coated paper comprises a polylactic acid compound, wherein the polylactic acid compound comprises the following components in percentage by weight: 2-40% of long-chain branched polylactic acid containing ZnO nanoparticles and 60-98% of polylactic resin. According to the biodegradable coated paper and the preparation method thereof provided by the invention, through introduction of ZnO nanoparticles, physical crosslinking points and nucleating points in a polylactic acid compound system are increased, and the mechanical property, crystallization property, hot water resistance and oil resistance of PLA can be remarkably improved, so that the prepared coated paper has good antibacterial property and excellent comprehensive performance.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and in particular to biodegradable coated paper and a preparation method thereof. Background Art

[0002] Biodegradable coated paper is a food packaging material made from paper as a raw material and coated on one or both sides with a biodegradable material. It has water-proof, oxygen-proof, and oil-proof properties and is widely used in the production of disposable food packaging products such as paper cups, paper bowls, and paper lunch boxes. Polylactic acid (PLA) is a commonly used biodegradable coated paper material. At present, there are still some technical difficulties in the production of PLA coated paper: (1) Low melt strength: The melt strength of pure PLA material is low, resulting in unstable film formation and easy rupture during processing, making it difficult to control the stability and thickness of the coating. (2) Slow crystallization rate: The crystallization rate of pure PLA material is slow, resulting in poor heat resistance, and water and oil resistance need to be improved. Therefore, it is very necessary to study the modification of polylactic acid materials. Common methods include synthesizing PLA with a certain topological structure, forming a stereocomplex of PLA, preparing composite materials based on PLA, etc.

[0003] Long-chain branched PLA is a PLA material with long chain branches. There are two main methods for its preparation: one involves using a multifunctional initiator to initiate ring-opening polymerization of PLA monomers during PLA synthesis; the other involves adding a branching agent (primarily a multifunctional compound that reacts with hydroxyl and carboxyl groups on the molecular chain) during PLA melt extrusion to form PLA with a branched structure. The formation of this branched structure promotes entanglement between molecular chains, effectively increasing PLA's melt strength and crystallization rate, thereby improving PLA's processing and mechanical properties. This approach holds great promise for application in PLA-coated paper packaging materials.

[0004] Among the many PLA composite materials, the complex formed by PLA and ZnO nanoparticles is a high-performance composite material. The introduction of ZnO nanoparticles significantly improves the mechanical properties and heat resistance of PLA, and has attracted much attention in the field of active food packaging. It has excellent antibacterial properties and can extend the shelf life of food. Therefore, developing PLA composite materials containing ZnO nanoparticles is a beneficial approach to obtaining antibacterial coated paper. Chinese patent CN108341940 uses metal oxide nanoparticles as a promoter to promote the transesterification reaction between multifunctional small molecule ester monomers and macromolecular polylactic acid, thereby preparing long-chain branched polylactic acid with high melt strength. The limitation of this method is that the multifunctional small molecule ester monomers are unstable during the high-temperature transesterification process, and degradation of the PLA molecular chain at high temperatures is inevitable.

[0005] Regardless of the synthesis and modification method used, the toxicity of the raw materials used in the preparation process, the mechanical properties, water and oil resistance, antibacterial properties and biodegradability of the material must be comprehensively considered to meet the application requirements of PLA coated paper materials in food packaging materials. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides a biodegradable coated paper and a preparation method thereof.

[0007] The specific technical solutions include the following:

[0008] In a first aspect, a biodegradable coated paper is provided, comprising a polylactic acid composite, wherein the polylactic acid composite comprises the following components in weight percentage: 2-40% of long-chain branched polylactic acid containing ZnO nanoparticles and 60-98% of polylactic acid resin.

[0009] Preferably, the number average molecular weight of the long-chain branched polylactic acid containing ZnO nanoparticles is 0.2-20,000.

[0010] Preferably, the molecular weight of the polylactic acid resin is 50,000-300,000.

[0011] Preferably, the weight percentage of the polylactic acid resin in the polylactic acid composite is 60%.

[0012] Preferably, the weight percentage of the polylactic acid resin in the polylactic acid composite is 80%.

[0013] Preferably, the weight percentage of the polylactic acid resin in the polylactic acid composite is 90%.

[0014] Preferably, the weight percentage of the long-chain branched polylactic acid containing ZnO nanoparticles in the polylactic acid composite is 2%.

[0015] Preferably, the weight percentage of the long-chain branched polylactic acid containing ZnO nanoparticles in the polylactic acid composite is 5%.

[0016] Preferably, the weight percentage of the long-chain branched polylactic acid containing ZnO nanoparticles in the polylactic acid composite is 10%.

[0017] Furthermore, the polylactic acid composite further comprises a chain extender, which is epoxidized soybean oil, and the weight percentage of the chain extender relative to the polylactic acid resin is 1-10%.

[0018] Preferably, the weight percentage of the chain extender relative to the polylactic acid resin is 2%.

[0019] Preferably, the weight percentage of the chain extender relative to the polylactic acid resin is 5%.

[0020] Preferably, the weight percentage of the chain extender relative to the polylactic acid resin is 10%.

[0021] Furthermore, the polylactic acid composite is composed of the following components in weight percentage: 10-30% of long-chain branched polylactic acid containing ZnO nanoparticles, 65-85% of polylactic acid resin and 2-5% of chain extender.

[0022] Preferably, the polylactic acid composite has a melt index of less than 4 g / 10 min (190° C., 2.16 kg), an elongation at break greater than 5%, and a tensile strength greater than 50 MPa.

[0023] Furthermore, the preparation method of the polylactic acid composite comprises the following steps: blending long-chain branched polylactic acid containing ZnO nanoparticles with polylactic acid resin, adding a chain extender, mixing and reacting under molten conditions, and placing the material in a high-pressure crusher for crushing after cooling to obtain the polylactic acid composite.

[0024] Preferably, in the method for preparing the polylactic acid composite, the heating and melting temperature of the mixing reaction is 150-200° C., and the mixing speed is 60-120 r / min.

[0025] Furthermore, the method for preparing the long-chain branched polylactic acid containing ZnO nanoparticles comprises the following steps:

[0026] ZnO nanoparticles are dissolved in a solvent and ultrasonically dispersed, and then lactide monomer is added and mixed, and then the solvent is removed; under nitrogen protection, polyol and catalyst are continuously added to the system to initiate ring-opening polymerization of the lactide monomer, and after cooling, chloroform is added to dissolve it, and after precipitation with methanol, long-chain branched polylactic acid containing ZnO nanoparticles is prepared in situ; the catalyst is an organic tin catalyst.

[0027] Preferably, in the method for preparing the long-chain branched polylactic acid containing ZnO nanoparticles, removing the solvent is specifically removing the solvent by distillation under reduced pressure.

[0028] The invention uses polyol as initiator, and initiates the ring-opening polymerization of lactide monomer under the action of catalyst in the presence of ZnO nanoparticles.

[0029] Preferably, in the method for preparing long-chain branched polylactic acid containing ZnO nanoparticles, the temperature of the ultrasonic dispersion is room temperature of 25-28° C., and the time of the ultrasonic dispersion is 1-2 h.

[0030] Preferably, in the preparation method of the long-chain branched polylactic acid containing ZnO nanoparticles, lactide monomer is added and mixed, and then heated to 85-88°C.

[0031] Furthermore, in the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles, the solvent is anhydrous dioxane, the polyol is one of trimethylolpropane and pentaerythritol, and the organic tin catalyst is stannous octoate.

[0032] Preferably, in the method for preparing long-chain branched polylactic acid containing ZnO nanoparticles, the particle size of the ZnO nanoparticles is 10-200 nm.

[0033] Furthermore, in the method for preparing long-chain branched polylactic acid containing ZnO nanoparticles, the weight amount of the ZnO nanoparticles added relative to the lactide monomer is 0.1-5%.

[0034] Furthermore, in the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles, the ZnO nanoparticles contain surface modification molecules, and the surface modification molecules are selected from one of a silane coupling agent, a titanate coupling agent, and stearic acid.

[0035] Preferably, in the method for preparing long-chain branched polylactic acid containing ZnO nanoparticles, the silane coupling agent is selected from one of triethoxyoctylsilane and aminosilane.

[0036] Preferably, in the method for preparing long-chain branched polylactic acid containing ZnO nanoparticles, the reaction temperature of the ring-opening polymerization is 150-200° C., and the reaction time is 1-6 hours.

[0037] Furthermore, the biodegradable coated paper also includes a base paper.

[0038] Preferably, the base paper is one of white cardboard, coated paper, kraft paper, and glassine paper.

[0039] In a second aspect, a method for preparing the biodegradable coated paper as described in the first aspect is provided, which is characterized in that it includes the following steps: drying and dehumidifying the polylactic acid composite as a polylactic acid coating layer in an oven, adding it to a coating extruder to obtain a molten resin; pressing the molten resin onto the base paper (base paper layer), achieving compounding under the action of pressure, and winding it after cooling to finally obtain the biodegradable coated paper.

[0040] The beneficial effects of the present invention are:

[0041] (1) The present invention provides biodegradable coated paper. By introducing ZnO nanoparticles, the physical crosslinking points and nucleation points in the polylactic acid composite system are increased, which can significantly improve the mechanical properties, crystallization properties, hot water resistance and oil resistance of PLA, so that the prepared coated paper has good antibacterial properties and excellent comprehensive performance.

[0042] (2) The polylactic acid composite provided by the present invention comprises the following steps: when a long-chain branched polylactic acid containing ZnO nanoparticles is mixed with a PLA resin, epoxy soybean oil is added, and the chain is extended by a chemical reaction between the terminal groups of the PLA molecular chain and the epoxy groups, thereby preventing the molecular weight of the PLA from decreasing and making the composite have good mechanical properties.

[0043] (3) In the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles provided by the present invention, the ZnO nanoparticles are introduced in situ during the synthesis of the long-chain branched polylactic acid, and the PLA molecular chains grow from the surface of the nanoparticles, so that the nanoparticles and PLA have good compatibility and are easy to disperse without agglomeration in the subsequent preparation process of the polylactic acid composite. In this grafted nanoparticle composite, the long-chain branched structure of PLA and the nanoparticles work together to form physical entanglement points, thereby improving the melt strength of the polylactic acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0045] Figure 1 This is a schematic structural diagram of the long-chain branched polylactic acid containing ZnO nanoparticles prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0046] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0047] It will be understood that when used in this specification and the appended claims, the terms “comprises” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0048] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used in the specification and appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0049] It should be further understood that the term "and / or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0050] In order to more fully understand the technical content of the present invention, the technical solution of the present invention is further introduced and illustrated in conjunction with specific embodiments below.

[0051] Example 1

[0052] A biodegradable coated paper comprises a base paper layer and a polylactic acid coated layer. The base paper layer is a base paper, specifically white cardboard. The polylactic acid coated layer comprises a polylactic acid composite composed of the following components by weight: 17.6% long-chain branched polylactic acid containing ZnO nanoparticles, 80% polylactic acid resin, and 2.4% chain extender.

[0053] In this embodiment, the preparation method of a polylactic acid composite comprises the following steps: 17.6 g of ZnO / LBPLA-1% (the aforementioned long-chain branched polylactic acid containing ZnO nanoparticles) is blended with 80 g of polylactic acid resin by heating at 60°C for 6 hours to remove moisture. 2.4 g of a chain extender (epoxidized soybean oil) is then added to the mixture, and the mixture is mixed and reacted in a melt state for 10 minutes. The heating and melting temperature of the mixing reaction is 170°C and the mixing speed is 80 rpm. After the mixture is cooled, it is crushed in a high-pressure crusher to obtain a polylactic acid composite. The molecular weight of the polylactic acid resin is 50,000.

[0054] Among them, adding epoxidized soybean oil during the blending process can effectively prevent the degradation of PLA molecular chains at high temperatures, making the polylactic acid composite have good mechanical properties.

[0055] In this embodiment, the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles includes the following steps: dissolving 0.6 g of ZnO nanoparticles in 60 mL of a solvent (anhydrous dioxane) for ultrasonic dispersion, wherein the particle size of the ZnO nanoparticles is 100 nm, the ultrasonic dispersion temperature is room temperature of 25° C., and the ultrasonic dispersion time is 1 hour; then, adding 60 g of lactide monomer, mixing, heating to 85° C., and then removing the solvent by reduced pressure distillation; under nitrogen protection, further adding 0.56 g of a polyol (trimethylolpropane) and 0.06 g of a catalyst (stannous octoate) to the system, using the polyol as an initiator, in the presence of the ZnO nanoparticles, initiating ring-opening polymerization of the lactide monomer under the action of the catalyst, the reaction temperature of the ring-opening polymerization being 160° C., and the reaction time being 2 hours; cooling, adding 500 mL of chloroform to dissolve the mixture, and precipitating the mixture with methanol to prepare long-chain branched polylactic acid containing ZnO nanoparticles in situ. The long-chain branched polylactic acid containing ZnO nanoparticles is recorded as ZnO / LBPLA-1% (wherein LBPLA is the abbreviation of long-chain branched polylactic acid, and the feed ratio of ZnO to LBPLA is 1%). For the polymerization product, the chloroform-soluble material was taken and the molecular weight was determined by gel permeation chromatography, and its number average molecular weight was 15,000. Figure 1 This is a schematic structural diagram of the long-chain branched polylactic acid containing ZnO nanoparticles prepared in Example 1 of the present invention.

[0056] The preparation method of the biodegradable coated paper of Example 1 comprises the following steps: drying and dehumidifying the above-prepared polylactic acid composite as a polylactic acid coating layer in an oven, adding the composite to a coating extruder to obtain a molten resin; corona treating the base paper, pressing the molten resin onto the base paper (base paper layer), achieving compounding under pressure through a cooling roller and a rubber pressure roller, and winding the composite after cooling to finally obtain the biodegradable coated paper.

[0057] The extrusion temperature of the laminating extruder is 180°C, the die head temperature is 180°C; the laminating speed of the laminating extruder is 100m / min, and the laminating weight of the laminating extruder is 20g / m 2 , the temperature of the cooling roller is 65℃, and the thickness of the polylactic acid coating layer is 10μm.

[0058] Example 2

[0059] Example 2 provides a biodegradable coated paper. The difference between Example 2 and Example 1 is that the long-chain branched polylactic acid component containing ZnO nanoparticles in the polylactic acid complex component is prepared by ZnO nanoparticles containing surface modification molecules (stearic acid), and the other conditions are the same.

[0060] The synthesis method for ZnO nanoparticles containing a surface-modifying molecule (stearic acid) is as follows: 5g of ZnO nanoparticles (100nm diameter) were added to 50mL of ethanol and dispersed by ultrasonication at room temperature (25°C) for 1 hour. 2g of stearic acid and 0.1g of sodium hydroxide were added to the mixture, and the mixture was heated to 60°C and stirred for 2 hours. After the reaction, the precipitate was washed several times with deionized water and ethanol, filtered, and dried at 70°C. The crude precipitate was then placed in a muffle furnace and heated at 300°C for 1 hour. After cooling naturally, the mixture was washed three times with ethanol / water and dried to obtain ZnO nanoparticles containing the surface-modifying molecule (stearic acid).

[0061] ZnO nanoparticles containing surface-modified molecules have good compatibility with polylactic acid resin and can prevent the degradation of the polylactic acid chain during polymerization and melt processing. This is because the active reaction sites on the ZnO surface are occupied by the surface-modified molecules, reducing their reactivity and thus inhibiting the chemical reaction between the nanoparticles and the polylactic acid molecules.

[0062] Example 3

[0063] Example 3 provides a biodegradable coated paper. The only difference between Example 3 and Example 1 is that the polylactic acid composite in Example 3 is composed of the following components in weight percentage: 10% long-chain branched polylactic acid containing ZnO nanoparticles, 85% polylactic acid resin and 5% chain extender.

[0064] Example 4

[0065] Example 4 provides a biodegradable coated paper. The only difference between Example 4 and Example 1 is that the polylactic acid composite in Example 4 is composed of the following components in weight percentage: 30% long-chain branched polylactic acid containing ZnO nanoparticles, 67% polylactic acid resin and 3% chain extender.

[0066] Example 5

[0067] Example 5 provides a biodegradable coated paper. The only difference between Example 5 and Example 1 is that the long-chain branched polylactic acid containing ZnO nanoparticles in Example 5 is ZnO / LBPLA-5% (wherein the feed ratio of ZnO to LBPLA is 5%).

[0068] Comparative Example 1

[0069] Comparative Example 1 provides a biodegradable coated paper. The only difference between Comparative Example 1 and Example 1 is that the polylactic acid composite in the coated paper of Comparative Example 1 is prepared using unmodified polylactic acid resin. All other conditions remain the same. Epoxidized soybean oil is still added during the preparation of the polylactic acid composite, but the long-chain branched polylactic acid containing ZnO nanoparticles is omitted.

[0070] Comparative Example 2

[0071] Comparative Example 2 provides a biodegradable coated paper. The only difference between Comparative Example 2 and Example 1 is that no chain extender is added during the preparation of the polylactic acid composite in the coated paper of Comparative Example 2, and the other conditions are the same.

[0072] Performance Testing

[0073] (1) Melt Flow Rate (Melting Index): The melt flow rate of the polylactic acid composite of the coated paper of the Example / Comparative Example was measured using a melt flow rate tester. The melt flow rate was measured at 190°C for 5 minutes and then at 190°C and 2.16 kg.

[0074] (2) Tensile strength: The tensile strength of the polylactic acid composite of the coated paper of the embodiment / comparative example was tested using a universal testing machine according to GB / T 1040.1-2018 at a tensile rate of 10 mm / min.

[0075] (3) Elongation at break: The elongation at break of the polylactic acid composite of the coated paper of the embodiment / comparative example was tested using a universal testing machine according to GB / T 1040.1-2018 at a tensile rate of 10 mm / min.

[0076] (4) Thickness, fat resistance and leakage resistance of coated paper: The thickness, fat resistance and leakage resistance (water and oil resistance) of the coated paper of the embodiment / comparative example were tested with reference to the standard "GB / T36392-2018".

[0077] Performance tests were performed on Examples 1-5 and Comparative Examples 1-2, and the test results are shown in the following table:

[0078] Table 1 Performance test results of Examples 1-5 and Comparative Examples 1-2

[0079]

[0080] The test results in Table 1 show that when unmodified polylactic acid resin is used to prepare a polylactic acid composite and then used to prepare coated paper, the resulting coated paper (Comparative Example 1) exhibits poor performance test results. When no chain extender is added during the preparation of the polylactic acid composite, the resulting coated paper (Comparative Example 2) also exhibits poor performance test results.

[0081] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A biodegradable coated paper, characterized in that: The polylactic acid composite comprises the following components in weight percentage: 2-40% of long-chain branched polylactic acid containing ZnO nanoparticles and 60-98% of polylactic acid resin.

2. The biodegradable coated paper according to claim 1, wherein The polylactic acid composite further comprises a chain extender, which is epoxy soybean oil. The weight percentage of the chain extender relative to the polylactic acid resin is 1-10%.

3. The biodegradable coated paper according to claim 2, wherein: The polylactic acid composite is composed of the following components in weight percentage: 10-30% of long-chain branched polylactic acid containing ZnO nanoparticles, 65-85% of polylactic acid resin and 2-5% of chain extender.

4. The biodegradable coated paper according to claim 3, wherein: The preparation method of the polylactic acid composite comprises the following steps: blending long-chain branched polylactic acid containing ZnO nanoparticles with polylactic acid resin, adding a chain extender, mixing and reacting under molten conditions, and placing the materials in a high-power crusher for crushing after cooling to obtain the polylactic acid composite.

5. The biodegradable coated paper according to claim 1, wherein: The method for preparing the long-chain branched polylactic acid containing ZnO nanoparticles comprises the following steps: ZnO nanoparticles are dissolved in a solvent and ultrasonically dispersed, and then lactide monomer is added and mixed, and then the solvent is removed; under nitrogen protection, polyol and catalyst are continuously added to the system to initiate ring-opening polymerization of the lactide monomer, and after cooling, chloroform is added to dissolve it, and after precipitation with methanol, long-chain branched polylactic acid containing ZnO nanoparticles is prepared in situ; the catalyst is an organic tin catalyst.

6. The biodegradable coated paper according to claim 5, wherein: In the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles, the solvent is anhydrous dioxane, the polyol is one of trimethylolpropane and pentaerythritol, and the organic tin catalyst is stannous octoate.

7. The biodegradable coated paper according to claim 6, wherein: In the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles, the weight addition amount of the ZnO nanoparticles relative to the lactide monomer is 0.1-5%.

8. The biodegradable coated paper according to claim 7, wherein: In the preparation method of long-chain branched polylactic acid containing ZnO nanoparticles, the ZnO nanoparticles contain surface modification molecules, and the surface modification molecules are selected from one of silane coupling agents, titanate coupling agents, and stearic acid.

9. The biodegradable coated paper according to claim 1, wherein: Also includes backing paper.

10. The method for preparing the biodegradable coated paper according to claim 9, wherein: The method comprises the following steps: drying and dehumidifying a polylactic acid composite as a polylactic acid coating layer in an oven, and adding the composite into a coating extruder to obtain a molten resin; The molten resin is pressed onto the base paper, composited under pressure, and rolled up after cooling to finally obtain biodegradable coated paper.

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