Improvement of paper moisture permeability with hydrophobic modified nanocellulose/poly (lactic acid) composites

doi:10.21203/rs.3.rs-4984266/v1

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Improvement of paper moisture permeability with hydrophobic modified nanocellulose/poly (lactic acid) composites

https://doi.org/10.21203/rs.3.rs-4984266/v1

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Polylactic acid (PLA) and nanocellulose fibers (CNF) are promising biodegradable materials that exhibit great potential in healthcare and packaging applications. In this study, dissolved CNF (DMCNF) was obtained through the AlCl₃/ZnCl₂ solution system and subsequently grafted with γ-methylacryloxy propyl trimethoxy-silane (KH-570), and PLA/DMCNF coated paper was fabricated by the coating process. Scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermal analysis, water permeability, water contact angle, and tensile testing were employed to characterize the prepared coated paper. The results indicate that the dissolution of AlCl₃/ZnCl₂ can significantly facilitate the modification of CNF, and the addition of DMCNF has a crucial impact on the overall properties of polylactic acid coated paper. The increase of DMCNF can enhance the moisture permeability, hydrophobicity, and tensile properties of PLA-based coated paper. Notably, the moisture permeability of PLA/DMCNF coated paper was reduced by 72.6% compared to the base paper. Therefore, this research provides an effective approach for the enhancement of PLA-based coated paper.

Poly (lactic acid)

Nanocellulose

Inorganic salts

Moisture permeability

The increased use of polymer packaging materials in the last few decades has resulted in the generation of large amounts of non-degradable plastic waste, posing a serious environmental threat (Kumar et al.2019). Paper products have become a powerful alternative to plastic packaging due to their good biodegradability, low cost and scalable production (Wang et al.2021). However, the porous structure and surface hydrophilicity of paper (Rastogi et al.2015) make cellulosic paper still face some significant challenges in blocking water, grease and gas, which seriously limits the application of paper products in packaging materials (Huang et al.2023). In recent years, the use of degradable materials to improve the moisture permeability of paper has been popular among many researchers (Tian et al.2022; Ren et al.2017; Wang et al.2019).

Polylactic acid (PLA), is the most promising biodegradable polymer material (Shetty et al.2019). PLA has good film-forming air permeability, low water vapor permeability, gloss and transparency, and has a wide range of uses (More et al.2022; Swetha et al.2023), for example, in packaging materials, the healthcare industry and other fields. However, the shortcomings of PLA such as high brittleness, poor toughness and mechanical properties limit its application in many fields, and mixing cellulose nanofibers (CNF) as nanofillers with PLA is considered as a promising method to improve it. CNF has attracted extensive attention from scholars at home and abroad due to its excellent mechanical properties, barrier properties, and biocompatibility (Zinge et al.2020; Mousavi et al.2017). Processing nanocellulose and PLA as a layered system produces packages with better barrier properties, and a common method is to add a small amount of CNF to the PLA matrix to prepare composite films or as coatings (LafiaAraga et al.2021).

However, due to the highly polar surface and hydrophilicity of CNF (Koppolu et al.2019), CNF is not easy to be uniformly dispersed in PLA matrix due to the high number of exposed hydroxyl groups on the surface of the long chain of CNF. In the past decade, a number of researchers have attempted to hydrophobically modify CNF to improve its dispersibility. For example, Sethi et al (Sethi et al.2018) used lactic acid to ultrasonically esterify the hydroxyl groups on the surface of CNF under high temperature and pressure; Geng (Geng et al.2018) et al combined CNC with polyvinyl acetate; Niu (Niu et al. 2018) et al used rosin to modify CNF; Yin (Yin et al. 2017) prepared hydrophobic triazine derivatives and used them to modify CNC, and all of these methods had a dispersion have positive effects. In order to further enhance the modification effect of CNF, pretreatment of CNF for dissolution is considered. Currently, common cellulose solvents include N-methylmorpholine-N-oxide (NMMO), tetrabutylammonium hydroxide (TBAH), tetrabutylphosphine hydroxide (TBPH), LiCl/DMSO, NaOH/urea, ionic liquids, and aqueous inorganic salts (Chen et al. 2015; Lin et al. 2022; Shi et al. 2018; Singh et al. 2019). Inorganic salt aqueous solution has received wide attention due to its advantages of low cost, high efficiency, simple and mild treatment process (Zhang et al. 2019), Inorganic salt solvents are divided into pure molten salts and molten mixtures, etc. (Sun et al. 2022). The AlCl₃/ZnCl₂ system is a composite inorganic molten salt hydrate system with mild dissolution conditions, and current studies have shown that pure cellulose is well dissolved in this system at room temperature with a small degree of degradation; in particular, the solvent can be recycled (Xi et al. 2021). However, the potential of the AlCl₃/ZnCl₂ system to dissolve CNF before modifying it has not been explored.

In this paper, dissolution-modified CNF (DMCNF) was produced by dissolving CNF in AlCl₃/ZnCl₂ solution before Silane modification, and then PLA/DMCNF coated paper was produced by mixing it with PLA. The prepared coated papers were tested by scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermal analysis, moisture permeability, water contact angle and tensile properties. The effect of the addition of DMCNF on the moisture permeability of PLA/DMCNF coated paper was investigated with emphasis on the effect of the addition of DMCNF on the moisture permeability of PLA/DMCNF coated paper. This study provides an effective way to enhance the performance of PLA-based coated paper.

Materials

Nanocellulose, purchased from Guangxi Guilin Qihong Science and Technology Co; Polylactic acid, purchased from Zhejiang Haizheng Biomaterials Co. Zinc chloride, anhydrous ethanol and dichloromethane, purchased from Tianjin Zhiyuan Chemical Reagent Co. Aluminium chloride hexahydrate, purchased from Guangdong Guanghua Technology Co. γ-methylacryloxy propyl trimethoxy-silane (KH-570) and glacial acetic acid were purchased from Shanghai McLean Biochemical Technology Co.

CNF dissolution and modification

An aqueous solution of AlCl₃/ZnCl₂ (molar ratio ZnCl₂: AlCl₃: H2O = 0.9:0.1:3) was prepared in a three-neck flask. Cellulose equivalent to 1wt% of the solution was added to the prepared aqueous solution of AlCl₃/ZnCl₂ and stirred at room temperature until a transparent cellulose solution without residue was formed. Take the corresponding ethanol solution so that the ratio of water to ethanol is 1:4. After that, KH-570 was added to ethanol, and glacial acetic acid was added to adjust the pH of the solution, so that the pH value of the solution was stable at 3–4, and the obtained solution was fully stirred and put aside. After the CNF was completely dissolved, the prepared modified liquid was added to a three-neck flask and stirred in an oil bath at 80℃ for 12 h. Excess water and anhydrous ethanol were added to the solution for centrifugal treatment, and finally dissolved modified CNF was obtained after drying, which was denoted as DMCNF. At the same time, in order to verify that the process of dissolving CNF played a role in promoting the modification of CNF, CNF was directly modified, that is, no dissolving treatment was carried out. The modification operation was the same as the above process. The obtained CNF was cleaned with ethanol solution, centrifuged for three times, and dried to obtain modified CNF as a control and recorded as MCNF.

Preparation of PLA/DMCNF coated paper

DMCNF was placed into dichloromethane and subjected to ultrasonic operation for 30 min to obtain a solution of DMCNF dispersed in dichloromethane. PLA equivalent to 10 times the dry weight of DMCNF was added to the solution and mixed and dissolved with stirring at room temperature to obtain PLA/DMCNF coating. With the quantitative 83.5g/m² qualitative filter paper as the base paper, the coating was uniformly coated on the paper through the coating machine, and the coating speed was 10 mm/s, so as to produce PLA/DMCNF coated paper.

Characterization

Scanning electron microscopy (SEM, Pharos GI) was used to observe the surface morphology of CNF and dissolved CNF. The samples were fixed on the sample stage with conductive adhesive, and after the samples were sprayed with gold for 60 s, the samples were placed under a voltage of 10 kV to observe the surface morphology of CNF, and photographs were taken to record the observations.

The samples were tested using a Fourier transform infrared spectrometer (FT-IR, IS50). The CNF, MCNF and DMCNF were dried at 100 ℃ for 6 h. The samples were tested in IR using the potassium bromide (KBr) pressing method, in which the samples and KBr were fully dried and then fully milled at a weight ratio of 1:100 and pressed. The scanning range was 4000 cm-1-500 cm-1, the number of scans was 32 and the scanning frequency was 4 cm-1 .

The surface chemical composition of the samples was determined using an X-ray photoelectron spectrometer (XPS, ESCALAB 250XI+). The samples were tested by drying CNF, MCNF and DMCNF at 100°C for 6 h to detect the difference in modification effect between post-dissolution modification and direct modification. The test conditions were as follows: voltage 12 kV, current 6 mA; full-spectrum scanning fluence energy of 150 eV with a step size of 1 eV; and narrow-spectrum scanning fluence energy of 50 eV with a step size of 0.1 eV.

X-ray diffractometry (XRD, A24A10) was used to test the samples after they were dried and ground into powder. The tests were performed using Cu-Kα rays at a wavelength of λ = 0.154 nm, a voltage of 40 kV and a current of 30 mA to scan the samples. The test conditions were a scanning diffraction angle 2θ ranging from 5° to 50° and a scanning rate of 6 °/min.

The tensile properties of the coated paper were tested using a universal materials testing machine (LS1). The samples were cut into 100 mm×10 mm strips and fixed in the middle of the tensile fixture, and the test conditions were at room temperature of 25 ℃, with the fixture's marking distance set to 50 mm, and the tensile was carried out at a rate of 5 mm/min. Five sets of parallel experiments were carried out on the same proportion of samples, and the tensile strength and elongation at break of the samples were obtained by taking their average values.

A new high-resolution field emission scanning electron microscope (FESEM, SU8020) and a coupled energy spectrometer (EDS) were used to observe the surface and section morphology of the coated paper. The cross sections were obtained by cutting the samples into 10 mm×50 mm strips, freezing them in liquid nitrogen and quenching them, then fixing the samples on the sample stage with conductive adhesive, spraying gold on the samples for 60 s, and then placing the samples at 10 kV to observe the surface morphology of CNF and taking photos to record the results; and the morphology of the samples captured by the SEM was subjected to surface scanning by the energy spectrum, and the scanning elements were C, O, and Si.

The water contact angle of the coated paper was tested using a Lückers contact angle meter (DSA-100). After taking 2 µL of water droplets to contact the surface, the contact angle was recorded after the droplets were stabilised for 30 s. The contact angle of the coated paper was measured by a Rückers contact angle meter (DSA-100).

The WVTR of the coated paper was tested using a moisture permeability tester (PERMATRAN-W3/61).The samples were dried in an oven at 60 ℃ for 6 h. For each sample to be tested, three round parallel samples were taken using a standard sampler with a diameter of 38 mm.The samples were placed in the instrument at a temperature of 23 ± 2 ℃ and a relative humidity of 90 ± 2%. At the end of the test the values were averaged to obtain the final WVTR.

CNF dissolution and Modified

Since the molar ratio of Al³⁺:Zn²⁺:H₂O is 0.1:0.9:3, in the AlCl₃/ZnCl₂/3H2O solvent system, Al³⁺ and Zn²⁺ are bound to only three water molecules in an unsaturated state (Sen et al.2016;Xu et al.2016), compared with Zn²⁺, Al³⁺ has a smaller ionic radius and higher charge density, a stronger attraction to water molecules, and a stronger hydration enthalpy is greater, and the corresponding hydration ion radius is smaller, Hence, unsaturated hydrated Al³⁺ has a higher propensity to permeate into the interior of cellulose and assail and undermine the intermolecular and intramolecular hydrogen bonds of cellulose fibers, followed by a large number of unsaturated hydrated Zn²⁺ into the relatively loose structure of the cellulose interior, which synergistically destroys some of the cellulose's hydrogen bonds, resulting in more hydrogen bonds to be further fractured, and making the cellulose chains uniformly dispersed in an aqueous solution forming a clear and transparent solution.

The process of silane modification is shown in Fig. 1a. First, the silane modifier KH-570 is hydrolyzed in an acidic environment, then undergoes a polycondensation reaction on its own, and finally reacts with CNF under heated conditions. The silyl group enables KH-570 to react with CNF, and the methacryloyloxy group acts as an organic group to interact with the solvent to improve the dispersibility of CNF.

Table 1

Contents of surface elements of CNF, MCNF and DMCNF.
Samples	C	O	Si
CNF	61.47	38.53	-
MCNF	64.77	32.57	2.65
DMCNF	54.79	38.27	6.99

In order to more intuitively reflect the influence of modification on CNF before and after modification and after dissolution, the surface morphologies of CNF, MCNF and DMCNF were observed, and the results were shown in Fig. 1. Figure 1b shows the shape of fibers in untreated CNF. Due to the large number of hydroxyl groups on the surface of CNF, it is easy to wrap, thus showing a flat ribbon structure. When CNF is modified, it can be found that the shape of fiber is more obvious and the distance between fibers is increased as shown in Fig. 1c, which is caused by the partial reaction of hydroxyl group on CNF after modification and the weakening of binding force. As can be seen from Fig. 1d, the shape of the fiber disappeared due to dissolution, and a multi-lamellar structure appeared, indicating that metal ions Zn²⁺ and Al³⁺ destroyed the hydrogen bond between CNF.

Figure 1e is the FT-IR diagram of CNF, MCNF, DMCNF and the KH-570 used. It can be seen from the figure that MCNF and DMCNF both show the characteristic peaks of cellulose, which are respectively O-H absorption peak at 3600 − 3100 cm^− 1 and C-H stretching vibration peak at 2892 cm^− 1. The O-H bending vibration peak of adsorbed water at 1618 cm^− 1, the C-O stretching vibration peak of the peak at 1066 cm^− 1, and the D-glucosyl -CH2 and C-H bending vibration peak of cellulose at 895 cm^− 1. However, it can be clearly seen from the figure that there is an extra peak between MCNF and DMCNF at 1732 cm^− 1 wave number, which is the C = O bond existing on silane coupling agent KH-570. This peak can be clearly seen from the infrared spectrum of KH-570, and the peak height of DMCNF is larger than that of MCNF. This also shows that the process of dissolution promotes the modification to a certain extent. The absorption peaks of 1237cm^− 1 Si-O-C and 1095cm^− 1 Si-O-Si in KH-570 overlap with cellulose C-O-C in the 1000–1200 cm^− 1 vibration band, which makes it impossible to easily show up in FT-IR spectra.

In order to more clearly show the difference between post-dissolution modification and direct modification, XPS tests were carried out on the three samples of CNF, MCNF and DMCNF, and the three elements of C, O and Si were scanned, and the element broad spectrum obtained was shown in Fig. 1f. It can be seen from the broad spectrum in the figure that the three samples all have peaks at 532.5eV and 286.2eV, which represents the presence of O 1s and C 1s respectively. Moreover, it can be obviously seen that MCNF and DMCNF have two more peaks than CNF in the range of 100–200 eV. This is a typical peak of Si 2s and Si 2p, which indicates that CNF has reacted with KH-570, and the modification has been successful. According to the corresponding contents of each element in Table 1, it can be seen from the table that after the direct modification of CNF, the content of Si element only accounts for 2.65%, while the content of Si in the dissolved modified CNF can reach 6.99%, which is 2.64 times of the original modification. This is because the hydrogen bond between CNF makes CNF and CNF intertwined. When CNF is directly modified, the modifier cannot or only a small amount can contact and react with the -OH bond on CNF to achieve the purpose of modification. However, after the dissolution of CNF, part of the hydrogen bond formed by CNF will be destroyed. As a result, more sites on CNF can be provided for the modifier reaction, and the modified effect of dissolved CNF is better, and the Si content in CNF is increased.

In order to characterize the crystal structure changes of dissolved nanocellulose, XRD tests were conducted on CNF, MCNF and DMCNF, and the results were shown in Fig. 1g. It can be seen from the figure that the crystal pattern of the directly modified CNF is consistent with that of the original nanocelluloses, with obvious crystallization peaks at 2θ = 15.9°, 17.1° and 23.2°, which is consistent with the typical diffraction crystal plane of cellulose type I (11 ̅0), (110) and (200) (Yang et al.2015), indicating that the direct modification of CNF does not affect its crystal structure. The crystal shape of the regenerated CNF after dissolution modification is obviously changed, and obvious crystal peaks appear at 2θ = 11.6° and 20.3°, belonging to (11 ̅0) and (020), which are consistent with the diffraction crystal plane of cellulose type II (Yang et al.2011). This indicates that metal ions will enter the crystalline zone of nanocellulose during the process of dissolution, destroying the original hydrogen bond structure (Zhang et al.2018), resulting in the change of its crystal type. This also suggests that dissolution has the effect of untangling cellulose chains that are entangled by hydrogen bonds, which is consistent with FTIR results.

Tensile testing of PLA/DMCNF coated paper

Table 2

Tensile properties of PLA/CNF coated paper with different coating quantities.
Coating quantity(g/m²)	Stress(MPa)	Strain(%)
0	21.31	2.76
10.20	39.11	4.67
12.98	35.81	4.88
13.74	37.75	5.54
14.36	34.78	4.43
14.85	30.53	6.36

The tensile test of coated paper with varying coating amounts was conducted to investigate the impact of coating amounts on the mechanical properties of the paper. The results are presented in Fig. 2, and the specific parameters are displayed in Table 2. The tensile strength and elongation at break of uncoated base paper were respectively 21.31 MPa and 2.76%. With the increase of the coating amount, the tensile strength initially rises and then declines, and the elongation at break gradually increases on the whole. When the amount of coating is small, the coating solution first fills the pores of the paper, enabling the coating solution to penetrate among the fibers. Since the PLA/DMCNF coating solution has a certain viscosity, it functions as an adhesive between the pores of the paper, enhancing the bond strength among the fibers and causing the tensile strength and elongation at break to increase. When the coating amount is 10.20 g/m², the maximum strength is 39.11 MPa, which is 83.5% higher than that of the base paper. The elongation at break was 4.67%, which rose by 69.2%. When the coating amount is 12.98 g/m², the tensile strength decreases, which might be attributed to the uneven dispersion of DMCNF in the PLA matrix, resulting in the formation of large or small holes in the coating, thereby damaging the overall continuity and integrity of the film and presenting a decline in the tensile property of the prepared film. After the coating amount increases to 14.36 g/m², when the pores between the paper fibers are completely filled, a resin layer will be formed on the paper surface, and the tensile strength and tensile stress at this time are determined by the formed resin layer and the fiber-resin mixed layer. During the stretching process, the force generated by the tensile test can be transferred from the matrix to the DMCNF particles, causing these particles to deform before dispersing into the matrix. In this case, the elongation at break of the coated paper can be enhanced. When the coating amount is 14.85 g/m², it reaches the highest of 6.36%, which is 2.3 times that of the base paper, significantly strengthening the mechanical properties of the paper.

Surface morphology of PLA/DMCNF coated paper

Figure 3 shows the surface and section morphologies of coated paper with different coating amounts. It can be clearly seen from the morphology that the surface porosity of the base paper is high. After being coated with PLA/DMCNF coating solution, the surface porosity of the paper was significantly reduced (Sundar et al.2020). As the amount of coating increases, the pores between the paper fibers are filled (Fig. 3 (B) (C)). When the coating amount was further increased, the surface fiber structure gradually became less obvious when the coating amount was 13.74 g/m² and 14.36 g/m² (Fig. 3 (D) (E)), but there were still a few pores. However, when the coating amount reaches 14.85 g/m², no fiber structure can be observed at all on the surface (Fig. 3 (F)), and continuous layers formed on the paper can also be seen from the profile of Fig. 3 (f). At this time, there are three layers of fiber layer, fiber-resin mixed layer and resin layer on the entire coated paper. This also confirms the effective adhesion of PLA/DMCNF coatings on paper.

Permeability analysis of PLA/DMCNF coated paper

Water vapor permeability is affected by material thickness, temperature and humidity. In addition to the influence of external factors on the moisture permeability of the material, the molecular structure of the material itself also has an influence on the moisture permeability. The addition of CNF will improve the moisture permeability of the material to a certain extent, this is because the nanofiller can extend the diffusion path of water vapor in the material, which is the main reason for improving the barrier properties of composites (Angellier-Coussy et al. 2013). The cross section of coated paper at a coating amount of 14.85 g/m² and the EDS scans for Si elements are shown in Fig. 4a, b. The penetration and distribution of CNF in the paper can be determined more intuitively by scanning the Si elements.

It can be seen from Fig. 4a that when the coating amount is 14.85g /m², a PLA/DMCNF coating layer is formed on the surface of the coated paper. From Fig. 4b, it can be seen that most of the CNF is distributed in the coating layer on the surface of the paper, but some of it still penetrates into the inside of the paper. The water vapor penetration diagram shown in Fig. 4c can be obtained from Fig. 4b. If there is no nano-filler CNF, the water vapor penetration in the paper can be approximately simulated into the path① process, and when CNF is added to the material due to its high length-diameter ratio and cross-linking in the material, the water vapor penetration process becomes tortuous, as shown in the simulation of path ②. It can be clearly seen from the figure that the water vapor transmission length of the ② process is significantly higher than that of the ① process, because with the extension of the gas diffusion path, the gas barrier performance of the material will be improved, and the gas diffusion resistance will increase or the gas diffusion rate will slow down. The penetration of oxygen and water vapor molecules depends on the pores in the film (the main way of gas diffusion). At the same time, due to the flexibility of the molecular chain, the fibrillar molecules in CNF are prone to entanglement, thus forming a more dense and tortuous diffusion path, thereby extending the diffusion path of the gas, which can reduce the water vapor transmission rate to a certain extent, thereby improving the moisture permeability of the coated paper.

Influence of coating quantity

As can be seen from Fig. 5a, the hydrophobicity of uncoated base paper is extremely poor. When the PLA/CNF coating solution was coated on the surface of the paper, it was found that the paper had hydrophobic properties, and the contact Angle of the material increased with the increase of the coating amount. This is because with the increase of the amount of coating, the pores of the paper are gradually filled and the fibers are covered by the coating solution, so that the holes on the surface of the paper are reduced. This results in an increase in the surface uniformity of the paper and a decrease in the permeability of water and air. PLA and the silane modification of CNF give the coated paper hydrophobicity. It can be seen from the figure that when the coating amount increased to 13.74 g/m², the contact Angle stabilized at 120.9°, which indicates that at this time, the surface of the paper has basically no hydrophilic fibers exposed on the surface, which is consistent with the surface topography in Fig. 3.

The PLA/DMCNF paint was coated on a paper with a base paper of 83.5g /m² by the coating method, and the water vapor transmission rate of the coated paper was tested under different coating amounts, as shown in Fig. 5a. Due to the porous structure of the base paper itself and the extreme hydrophilicity of the fiber, the water vapor transmission rate of the base paper is very high. After the coating of PLA/DMCNF coating solution, due to the penetration of the solution and the formation of a continuous coating, the coating is directly covered in the micropores on the surface of the filter paper, increasing the resistance of water vapor penetration. Therefore, with the increase of the coating amount, WVTR gradually decreases. When the coating amount is 14.85 g/m², WVTR decreases gradually. Its WVTR is 210.1911 g/(m²·24h), which is 72.6% less than the base paper. Figure 5b shows that PLA/DMCNF coated paper has excellent hydrophobicity (WVTR of 210.1911 g/(m²·24h), WCA of 120.9°), which is higher than that of some previous PLA-based coated papers (WVTR of 265.11-403.89 g/(m²·24h)). The WCA is 74-115.5°).

Influence of temperature and humidity

From Fig. 6a, it can be seen that under the same environmental conditions, the water vapor transmission rate of the coated paper decreased significantly compared with that of the original paper, which is due to the fact that the PLA/CNF coating fills the micropores in the paper and the CNF in the coating plays a certain role in path obstruction. With the increase of temperature, the water vapor transmission rate of both the original paper and the coated paper increased significantly, which may be due to the hydrophilic properties of the paper itself, making the original paper in the test process because the paper is wetted so that water vapor can pass through a large number of; for the coated paper, the increase of temperature increases the swelling of the coating and promotes the rate of molecular transfer, so that the water molecules are more likely to enter into the PLA/DMCNF coated paper. in the coated paper.

It can be seen from Fig. 6b that under the same environmental conditions, the water vapor transmission rate (WVTR) of the coated paper decreased significantly compared with that of the original paper, and the WVTR gradually decreased with the increase of the coating amount, which was attributed to the fact that the micropores in the paper were filled by the PLA/CNF coatings and the CNF in the coatings played a certain role in the pathway obstruction. The water vapor transmission rate (WVTR) of both virgin paper and coated paper with different coating amounts increased significantly with increasing humidity, probably due to the increasing density of water vapor when the humidity increased, and the pressure difference between the two sides of the test paper increased, which would result in more water vapor passing through the coated paper with greater pressure.

In this paper, dissolved CNF(DMCNF) was obtained by AlCl₃/ZnCl₂ solution system and then grafted with γ-methylacryloxy propyl trimethoxy-silane (KH-570). PLA/DMCNF coated paper was prepared by composite coating process with PLA. By comparing the chemical structure and crystal structure of CNF, MCNF and DMCNF, it is shown that the dissolution promotes the modification of CNF obviously, and the addition of DMCNF improves the moisture permeability, hydrophobicity and tensile properties of PLA-based coated paper. In particular, when the coating amount is 14.85 g/m², the water contact Angle of the coated paper is 120.9°, giving the paper hydrophobicity, and the moisture permeability of the coated paper is reduced by 72.6% compared with the base paper. This will be conducive to the development of PLA-based coated paper in the packaging field.

Author contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Hongzhe Chu, Zeyan Chen, Yang Liu, Jifeng Huang, and Yujie Yang. The first draft of the manuscript was written by Xiaoguang Xu and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

FundingThis research was supported by the National College Student Innovation and Entrepreneurship Program Fund (No. 202310593073).

Data availabilityData will be made available on request.

Declarations

Ethical approval No results of studies involving humans or animals are reported

Competing interests The authors declare no competing financial interests

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Improvement of paper moisture permeability with hydrophobic modified nanocellulose/poly (lactic acid) composites

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