A high-barrier flexible nanocellulose coating, coating and preparation method thereof

Abstract

The present invention belongs to the technical field of functional coatings, specifically a high-barrier flexible nanocellulose coating, coating and preparation method thereof that can be used for polylactic acid film materials. Using nanocellulose crystals as raw materials and sodium periodate as an oxidant, hydroxyl groups are oxidized to aldehyde groups to prepare dialdehyde nanocellulose. Dialdehyde nanocellulose is reacted with cysteine ​​to prepare thiol-modified nanocellulose. Then, CNC‑cys is mixed with polyethylene glycol diacrylate crosslinker and photoinitiator to form a water-based coating, and then subjected to knife coating, ultraviolet light irradiation crosslinking and room temperature drying processes to obtain a flexible, transparent nanocellulose coating. By adopting this process to carry out surface coating on polylactic acid film, a dense, transparent and flexible nanocellulose coating can be formed on the surface of polylactic acid, and finally a polylactic acid film material with good degradability, flexibility and high barrier is obtained, which can be used as a green packaging material in the fields of bio-health, food preservation, agriculture and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional coatings, and in particular relates to a high-barrier flexible nanocellulose coating, a coating and a preparation method thereof. Background Art

[0002] Polylactic acid (PLA), an aliphatic polyester made from lactic acid, exhibits excellent degradation and recyclability. In the natural environment, PLA rapidly decomposes into water and carbon dioxide, fully integrating into the ecological cycle without leaving any harmful residues. Therefore, PLA is an ideal choice for addressing the problem of plastic pollution. Furthermore, PLA's excellent gloss, transparency, and mechanical properties make it particularly popular in the field of green packaging. However, using PLA as packaging film also has some significant drawbacks. For example, PLA film has relatively limited barrier properties against water and oxygen, primarily due to the ester structure and low crystallinity of its molecular chains. Low crystallinity means that the molecular chains are not tightly packed, making it easily permeable to water and oxygen molecules. This, in turn, limits the performance of PLA in large-scale commercial applications such as food or pharmaceutical packaging, where high barrier properties are required.

[0003] To improve the barrier properties of polylactic acid (PLA), various strategies can be employed, such as modifying the molecular structure of PLA, adding barrier materials, using coating techniques, or compounding with other materials. Coating technology is an effective approach, not only enhancing the barrier properties of PLA but also maintaining its environmental and sustainability characteristics. In recent years, cellulose nanocrystals (CNCs) have gained increasing attention as a promising material. Cellulose nanocrystals are highly crystalline, one-dimensional nanorods extracted from natural cellulose, typically with a diameter of less than 100 nanometers. They possess a high aspect ratio and strength, along with excellent biocompatibility. These nanocrystals form a dense barrier layer that significantly blocks the permeation of water and oxygen, thereby significantly improving the barrier properties of PLA films. Furthermore, nanocrystals possess excellent optical and mechanical properties, which are crucial for improving the overall performance of the material. Coating PLA films with nanocrystals significantly enhances their water and oxygen barrier properties. Nanocrystal coatings can improve the film's mechanical strength, thermal stability, transparency, and gloss, giving the coated PLA films broad potential for applications in packaging, food preservation, and biomedicine. Especially in the field of food packaging, the improved films can better maintain the freshness of food, thereby extending its shelf life.

[0004] Although nanocellulose crystals have many advantages, they still face some challenges. For example, the high crystallinity and strong hydrophilicity of nanocellulose may cause the nanocellulose crystals to be brittle and have poor adhesion when used as a coating. Therefore, it is necessary to modify the nanocellulose crystals to improve their flexibility and adhesion to the PLA surface. The present invention uses a flexible polymer cross-linking strategy to simultaneously improve the flexibility and adhesion of the nanocellulose crystal coating, ensuring that the nanocellulose coating can fully utilize its barrier performance advantages, thereby improving the overall performance of the PLA film. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-barrier flexible nanocellulose coating, a coating and a preparation method thereof. Through a simple coating method, a high-barrier flexible nanocellulose coating is prepared, which not only enhances the oxygen barrier effect of the polylactic acid film, but also changes the flexibility and adhesion of the nanocellulose coating.

[0006] To achieve the above objectives, the first aspect of the present invention provides a method for preparing a high-barrier flexible nanocellulose coating, comprising the following steps:

[0007] (1) using an oxidant to oxidize the nanocellulose dispersion to obtain a dialdehyde nanocellulose (DAC) dispersion;

[0008] (2) adding cysteine ​​to the dialdehyde nanocellulose dispersion and reacting in a water bath to obtain a thiol-modified nanocellulose (CNC-cys) solution;

[0009] (3) Adding polyethylene glycol diacrylate (PEGDA) crosslinker and photoinitiator to the thiol-modified nanocellulose solution and stirring evenly to obtain nanocellulose-based CNC-cys-PEG coating.

[0010] Furthermore, in step (1), the oxidant is sodium periodate, and the molar ratio of the monomer in the nanocellulose to the sodium periodate is 1:(1-4).

[0011] Furthermore, the mass concentration of the nanocellulose dispersion is 0.1wt%-22wt%, preferably 0.5-1wt%; the oxidation reaction is carried out in the dark, at a reaction temperature of 30-40°C, for 4-8h, preferably 6h, and at a stirring speed of 400r / min.

[0012] Furthermore, step (1) further comprises: after the oxidation reaction is completed, adding ethylene glycol to decompose the remaining sodium periodate for 10 min to 30 min, and then dialyzing in deionized water to obtain a dialdehyde nanocellulose dispersion; the mass ratio of the sodium periodate (referring to that added in step (1)) to the ethylene glycol is 1:(1.5-2).

[0013] Specifically, a 1wt% CNC dispersion was thoroughly mixed with an equal amount of deionized water in a flask. Sodium periodate was added, stirred, and heated in a water bath. During this process, the sodium periodate oxidized two reactive hydroxyl groups on the nanocellulose surface into aldehyde groups. After the reaction was complete, ethylene glycol was added to decompose the remaining sodium periodate, yielding a light yellow dispersion. This dispersion was then dialyzed to remove unreacted solvent and impurities.

[0014] Furthermore, in step (2), the molar ratio of the dialdehyde nanocellulose to cysteine ​​is 1:(1.8-2.2), preferably 1:2; the temperature of the water bath is 45-55°C, preferably 50°C, the reaction time is 4-8h, the stirring speed is 600r / min, and after the reaction is completed, the thiol-modified nanocellulose solution is dialyzed in deionized water to obtain.

[0015] Furthermore, in step (3), the molar ratio of the polyethylene glycol diacrylate (Mn=1000) to the monomer in the nanocellulose is 1:(10-20).

[0016] Furthermore, in step (3), the photoinitiator is Irgacure 2959 (2-hydroxy-2-methyl-1-[4-(2-hydroxyethoxy)phenyl]-1-propanone), and the mass of the photoinitiator is 8-12% of the mass of the nanocellulose in the nanocellulose dispersion in step (1), preferably 10%.

[0017] Dialdehyde nanocellulose reacts with cysteine ​​to synthesize thiol nanocellulose (CNC-cys). CNC-cys is then mixed with a crosslinker and a photoinitiator and irradiated with ultraviolet light for crosslinking, resulting in a CNC-cys-PEG gel. During the polymerization process, the thiol groups on the CNC-cys crosslink with the carbon-carbon double bonds at the ends of the PEGDA (Mn=1000) molecules, forming a high-density network structure. By adjusting the amount of PEGDA (Mn=1000), the degree of grafting can be controlled, resulting in the preparation of a nanocellulose gel coating.

[0018] A second aspect of the present invention provides a high-barrier flexible nanocellulose coating, which is obtained by curing the coating obtained by the preparation method.

[0019] A third aspect of the present invention provides a high-barrier polylactic acid film, comprising a polylactic acid film substrate and the above-mentioned high-barrier flexible nanocellulose coating coated on the polylactic acid film substrate.

[0020] Furthermore, the coating obtained by the preparation method described above is applied to a polylactic acid film substrate, then cured by ultraviolet light to obtain a hydrogel coating, and then naturally dried to obtain the high-barrier flexible nanocellulose coating; the thickness of the high-barrier flexible nanocellulose coating is 5-10 μm, and the specific thickness can be controlled by the scraper spacing; the water content in the hydrogel coating is 30-95 wt%.

[0021] The specifications of the base PLA film are: 20cm×20cm, and the thickness is: 45-50μm.

[0022] In general, the above technical solutions conceived by the present invention have the following advantages and beneficial effects compared with the prior art:

[0023] 1. The present invention uses nanocellulose crystals as raw materials and reacts them in an aqueous solution. First, sodium periodate oxidation is performed, and then a Schiff base reaction is further performed to introduce sulfhydryl groups. A PEGDA crosslinking agent is added in proportion. The coating is applied to the surface of a polylactic acid film using a coating machine, avoiding the problems of uneven coating and uneven thickness. Finally, a CNC-cys-PEG functional coating is obtained by ultraviolet light irradiation. Compared with the existing technology, the present invention has the advantages of being green and environmentally friendly, being carried out in an aqueous solution, not requiring the addition of any organic solvents, having a simple preparation process, and being low in cost. The coating also has a dense cross-linked network gel structure, a smooth surface, and a transparent coating with good flexibility and oxygen barrier effects.

[0024] 2. The present invention utilizes the thiol groups on CNC-cys to crosslink with the double bonds at the ends of the PEGDA molecules, forming a high-density network structure. By adjusting the amount of PEGDA, the degree of grafting can be controlled, thereby preparing a nanocellulose gel coating with excellent barrier properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Infrared spectra of cellulose nanocrystals (CNC), dialdehyde nanocellulose (DAC), thiol-modified nanocellulose (CNC-cys), CNC-cys-PEG coating and PEGDA.

[0026] Figure 2 These are the C1s XPS peak spectra of nanocellulose crystals (CNC), dialdehyde nanocellulose (DAC), thiol-modified nanocellulose (CNC-cys), and CNC-cys-PEG coatings.

[0027] Figure 3 XRD patterns of cellulose nanocrystals (CNC), dialdehyde nanocellulose (DAC), thiol-modified nanocellulose (CNC-cys), CNC-cys-PEG coating, and PEGDA.

[0028] Figure 4 This is the curve of the change of the molar ratio of nanocellulose and cross-linking agent PEGDA-solution viscosity.

[0029] Figure 5 The coating and UV-crosslinking process of CNC-cys-PEG coating.

[0030] Figure 6 The appearance and flexibility of unmodified nanocellulose and CNC-cys-PEG coating-modified PLA films.

[0031] Figure 7 SEM images of nanocellulose (a, b, c) and dialdehyde nanocellulose (d, e, f), CNC-cys (g, h, i) and CNC-cys-PEG coatings (j, k, l).

[0032] Figure 8 is the oxygen permeability of pure PLA film and PLA / CNC-cys-PEG coating composite film (cm 3 / (m 2 ·24h·0.1MPa))Test result chart.

[0033] Figure 9 This is a diagram illustrating the preparation principle and oxygen barrier performance of the high-barrier flexible nanocellulose coating of the present invention. DETAILED DESCRIPTION

[0034] 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 the following embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0035] In a specific embodiment, the nanocellulose ((C6H 10 O5) n The amount of monomer substance n = 0.00617 mol, diameter 10 nm, length 100-200 nm) content is 1 wt% (m 纯CNC =1g); the amount of PEGDA added is 5% n CNC (i.e. the ratio of the amount of monomer and PEGDA in nanocellulose n CNC :n PEGDA =20:1).

[0036] Example 1

[0037] 1 wt% nanocellulose dispersion (m 纯CNC=1g) was mixed thoroughly with an equal amount of deionized water in a flask. Protected from light, 0.00617 mol of sodium periodate was added, stirred at 400 rpm, and allowed to react in a 40°C water bath for 6 hours to oxidize the two active secondary hydroxyl groups on the nanocellulose surface to aldehyde groups. After the reaction was complete, ethylene glycol was added to decompose the remaining sodium periodate (the mass ratio of sodium periodate to ethylene glycol was 1:1.5) for 15 minutes. The resulting light yellow dispersion was then dialyzed against deionized water (molecular weight cut-off: 7000 kDa) for 72 hours.

[0038] According to Example 1, 1 wt% of nanocellulose dispersion (m 纯CNC =1 g) was mixed with an equal mass of deionized water to reduce the nanocellulose concentration to 0.5 wt%.

[0039] Example 2

[0040] The dialyzed dialdehyde nanocellulose (DAC) dispersion was mixed with 0.0123 mol of cysteine ​​and stirred in a 50°C water bath at 600 rpm for 6 hours. This introduced two thiol groups into the DAC, resulting in thiol-containing nanocellulose, forming a light yellow, transparent solution. The solution was then dialyzed against deionized water (molecular weight cutoff: 7000 kDa) for 72 hours.

[0041] Example 3

[0042] According to n CNC :n PEGDA =20:1, PEGDA (Mn=1000) crosslinker was added to the thiol nanocellulose solution, and 0.1g of photoinitiator Irgacure 2959 was added and stirred evenly to prepare a coating. This coating was then applied to a polylactic acid film substrate (size: 20cm×20cm, thickness: 45-50μm). The coating thickness (2-10μm) was controlled by adjusting the gap between the scraper and the PLA film. The coating was then irradiated with a 10W 365nm UV lamp for 15-20 minutes to obtain a hydrogel crosslinked coating with a water content of 95wt%. The coating was then air-dried for 2 hours to obtain the high-barrier flexible nanocellulose coating.

[0043] In the embodiment, the relevant test contents are as follows:

[0044] like Figure 1 , the functional groups of nanocellulose (CNC), dialdehyde nanocellulose (DAC), thiol nanocellulose (CNC-cys) and CNC-cys-PEG coating (i.e., cross-linked high-barrier flexible nanocellulose coating) were characterized using Fourier transform infrared spectrometer IS10. Figure 1The spectrum of DAC shows a distinct absorption peak at 3343 cm⁻¹, attributed to the -OH group; the peak at 2904 cm⁻¹ is characteristic of C-H vibration; and the peak at 1032 cm⁻¹ represents the stretching vibration of CO. In the spectrum of DAC, the peak at 1724 cm⁻¹ corresponds to the absorption of the aldehyde group (C=O), indicating that the hydroxyl group has been successfully oxidized to an aldehyde group. After graft copolymerization, the spectrum of CNC-cys-PEG shows significant enhancement of the ester C=O peak and the methylene C=H peak. Furthermore, the peak at 1637 cm⁻¹ is characteristic of the C=C peak of the PEGDA molecule, while the peak at 1291 cm⁻¹ is attributed to the presence of the ether C=C bond.

[0045] like Figure 2 XPS C1s peak analysis results show that peaks A (283.5 eV), B (285 eV), and C (286.7 eV) correspond to the C-OH, CC / CH, and COC / C=O signals, respectively, present in CNC, DAC, CNC-cys, and CNC-cys-PEG. During oxidation, the conversion of C2-C3 bonds within CNC results in the conversion of hydroxyl groups to aldehyde groups. This change is reflected in the C1s data as a decrease in peak B and an increase in peak C. Peak D (287.8 eV) is attributed to the O=CO signal of the carboxylate group. Furthermore, peak E (283.2 eV) further confirms the presence of the CSC / C=N group.

[0046] like Figure 3 , from CNC to CNC-cys-PEG, the nanocellulose crystal structure did not change at all and still maintained the cellulose type I structure.

[0047] like Figure 4 , studies have shown that the effect of the molar ratio of nanocellulose to PEGDA on the viscosity of the solution changes significantly, which in turn affects the coating's coating processing performance. The results show that as the PEGDA content increases, the solution viscosity gradually increases; conversely, as the PEGDA content decreases, the solution viscosity decreases. At the same time, the nanocellulose / PEGDA molar ratio affects the flexibility and barrier properties of the CNC coating. However, when the PEGDA content is low, the flexibility of the CNC-cys-PEG coating is poor. When the PEGDA content is high, the voids inside the CNC-cys-PEG coating increase, and the barrier properties of the coating also decrease. The purpose and advantage of the present invention is to use a smaller amount of flexible polymer to improve the brittleness and adhesion of the CNC coating while ensuring the barrier properties of the coating.

[0048] like Figure 5 , described the PLA membrane coating process of CNC-cys-PEG coating (see Figure 5 a and b), and after UV irradiation (see Figure 5Experiments show that the CNC-cys-PEG coating has good adhesion to the PLA film, and the surface is uniform and transparent after coating (see Figure 5 (d)

[0049] like Figure 6 The pure nanocellulose coating is relatively fragile and has poor adhesion to the PLA substrate. However, the addition of PEGDA cross-linking makes the nanocellulose coating more flexible and more resistant to cracking, while significantly improving the adhesion between the nanocellulose coating and the PLA film.

[0050] like Figure 7 Scanning electron microscopy (SEM) observation of the example products showed that the CNC, DAC and CNC-cys samples were fragmented (see Figure 7 The surface density of CNC-cys-PEG after grafting was significantly improved (see Figure 7 The prepared CNC-cys-PEG coating has good adhesion to the PLA film, and the SEM cross-sectional images show that the coating thickness is 2.0 to 10.0 μm (see Figure 7 (middle k).

[0051] like Figure 8 Comparative test results show that the oxygen barrier performance of PLA film (thickness: 45-50μm) is relatively average. However, the oxygen permeability of PLA composite film (coating thickness: 2-10μm) coated with CNC coating is significantly reduced, indicating that the nanocellulose coating effectively improves the barrier performance of PLA film.

[0052] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a high-barrier flexible nanocellulose coating, characterized in that: The following steps are involved: (1) using an oxidant to oxidize the nanocellulose dispersion to obtain a dialdehyde nanocellulose dispersion; (2) adding cysteine ​​to the dialdehyde nanocellulose dispersion and reacting in a water bath to obtain a thiol-modified nanocellulose solution; (3) Adding polyethylene glycol diacrylate crosslinking agent and photoinitiator to the thiol-modified nanocellulose solution and stirring evenly to obtain a nanocellulose-based coating; the molar ratio of the polyethylene glycol diacrylate to the monomer in the nanocellulose is 1: (10-20).

2. The method for preparing the high-barrier flexible nanocellulose coating according to claim 1, wherein In step (1), the oxidant is sodium periodate, and the molar ratio of the monomer in the nanocellulose to the sodium periodate is 1:(1-4).

3. The method for preparing the high-barrier flexible nanocellulose coating according to claim 1, wherein: The concentration of the nanocellulose dispersion is 0.1 wt%-22 wt%, the oxidation reaction is carried out in a dark environment, the reaction temperature is 30-40° C., and the reaction time is 4-8 hours.

4. The method for preparing the high-barrier flexible nanocellulose coating according to claim 1, wherein: The concentration of the nanocellulose dispersion is 0.5-1 wt %.

5. The method for preparing the high-barrier flexible nanocellulose coating according to claim 2, wherein: Step (1) further comprises: after the oxidation reaction is completed, adding ethylene glycol to decompose the remaining sodium periodate for 10 min to 30 min, and then dialyzing in deionized water to obtain a dialdehyde nanocellulose dispersion; the mass ratio of the added sodium periodate to the ethylene glycol is 1:(1.5-2).

6. The method for preparing the high-barrier flexible nanocellulose coating according to claim 1, characterized in that: In step (2), the molar ratio of the dialdehyde nanocellulose to cysteine ​​is 1:(1.8-2.2), the temperature of the water bath is 45-55°C, the reaction time is 4-8 hours, and after the reaction is completed, the thiol-modified nanocellulose solution is dialyzed in deionized water to obtain the thiol-modified nanocellulose solution.

7. The method for preparing the high-barrier flexible nanocellulose coating according to claim 1, characterized in that: In step (3), the photoinitiator is Irgacure 2959, and the mass of the photoinitiator is 8-12% of the mass of the nanocellulose.

8. A high barrier flexible nanocellulose coating, characterized in that: The coating is obtained by curing the coating obtained by the preparation method according to any one of claims 1 to 7 through ultraviolet light.

9. A high barrier polylactic acid film, characterized in that: The invention comprises a polylactic acid film substrate and the high-barrier flexible nanocellulose coating according to claim 8 coated on the polylactic acid film substrate.

10. The high barrier polylactic acid film according to claim 9, characterized in that: The coating obtained by the preparation method according to any one of claims 1 to 7 is applied to a polylactic acid film substrate, and then cured by ultraviolet light to obtain a hydrogel coating, and then naturally dried to obtain the high-barrier flexible nanocellulose coating; The thickness of the high-barrier flexible nanocellulose coating is 5-10 μm; the thickness of the polylactic acid film substrate is 45-50 μm; and the water content of the hydrogel coating is 30-95 wt%.

Images