PROCESSING CONVENTIONAL PAPERS AND PULPS INTO HYDROPHOBIC AND WATER-RESISTANT PACKAGING MATERIALS AND TABLEWARE

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a metal-ion-modification method of converting a series of conventional papers (e.g., newspaper, copy paper, packing paper, tissue paper, kraft paper, etc.) to hydrophobic papers. The method includes immersing or spraying conventional papers into a class of metal ion solutions (e.g., Zr⁴⁺, Fe³⁺, Fe²⁺, Al³⁺, Y³⁺, Cu²⁺, Co²⁺, etc.) followed by air drying or oven drying. Such papers exhibit good water resistance properties and excellent hydrophobicity with water contact angle above 110°, as well as the good oleophilic property that can absorb a wide range of organic solvents and oils up to 10-50 times of its own weight.

Paper, which may consist of lignocellulosic fibers, is a class of useful material with many applications, including packaging, printing, decorating, filtering, and cleaning. However, the inherent hydrophilicity and hygroscopicity of paper make it with low water resistance and impede its wide applications. Therefore, it is important to endow hydrophobic character to paper products to overcome the hydrophilic and hygroscopic issues. Conventional paper hydrophobic modification methods include surface coating, adding hydrophobic sizing agents, fiber surface nano-engineering, and chemical surface modification. However, these methods have some drawbacks such as poor modification performance, expensive, poor recyclability. Thus, new and simple paper hydrophobic modification methods are needed.

Thus, one embodiment of the invention is a metal-ion-modification method for paper hydrophobic modification. We produced a series of metal ions (e.g., Zr⁴⁺, Fe³⁺, Fe²⁺, Al³⁺, Y³⁺, Cu²⁺, Co²⁺, etc.) modified hydrophobic papers, which exhibit high hydrophobicity (water contact angle >110°), good water resistance, and good oleophilic property (can remove oil from an oil/water mixture). These metal-ion-modified hydrophobic papers can be used as the new generation paper packaging materials, as well as a renewable adsorbent for oil spill cleaning.

Paper, made from cellulose, is a recyclable, biodegradable, and renewable material that plays a significant role in daily life. However, cellulose is a hydrophilic material, which restricts the widespread applications of cellulose papers. An embodiment of the invention provides a simple, effective, and environmentally friendly technique for converting conventional hydrophilic paper to hydrophobic paper through a metal-ion-induced modification process.

Various types of papers including paper towels, A4 paper, packaging papers, etc., are successfully modified by methods of the present invention, and high water contact angles (WCAs) above 120° are achieved. Moreover, the modified papers exhibit high stability and with maintained WCA even after the solvent washing (e.g., water, methanol, ethanol, etc.). Results from X-ray photoelectron spectroscopy, infrared spectroscopy, and scanning electron microscopy imply that the hydrophilic-hydrophobic transition is likely due to the coordination between multivalent metal ions and hydroxyl groups of cellulose.

Lignocellulosic paper is a crucial solution for preventing the increase in plastic packaging wastes; however, as stated above, the inherent hydrophilicity and poor water resistance impede its practical applications. The present invention includes a facile metal-ion-modification approach for converting conventional hydrophilic, low-wet-strength lignocellulosic paper to hydrophobic and water-resistant paper with high wet strength.

In one aspect of the invention, conventional paper is swelled a dilute multivalent metal ion (e.g., Fe³⁺, Zr⁴⁺) solution, followed by drying to induce the coordination interactions between metal ions and lignocellulosic fibers. The resulting hydrophobic paper exhibit a water contact angle up to 140°, good wet tensile strength up to 9.5 MPa, and a low water absorptiveness of <10 g/m², which are comparable to synthetic polymer films. The hydrophobic paper is stable for long-term storage and solvent wash. This metal-ion-modification approach also can be applied in the wood pulping process for the scalable production of hydrophobic fibers and papers.

Since the first plastic bag with handles was invented in 1965, plastic-based packaging has become increasingly indispensable in modern life because of its advantageous characteristics including low cost, lightweight, mechanically robust, durable, and adaptability. In 2020, approximately 367 million metric tons of plastics are produced worldwide. However, because of their non-biodegradability, more than three-quarters of plastics ended up as waste and discarded to the natural environment, causing severe detrimental impacts on the biosphere. To address this crisis, the development of sustainable packaging materials using renewable lignocellulosic resources is becoming an active research area. Paper, made of interlaced lignocellulosic fibers, is a long-established material since the second century A.D., has become an important packaging material because of its abundance and biodegradability. In 2020, global packaging paper consumption is expected to be over 169 million metric tons, which accounted for ˜40% of world paper production. However, the lignocellulosic paper suffers from poor water resistance and low wet strength because of the hydrophilic nature of lignocellulosic fibers, which greatly impedes its practical applications.

Lignocellulosic fiber is made of cellulose fibrils reinforced by amorphous hemicellulose and lignin macromolecules. Although lignin is hydrophobic, cellulose and hemicellulose are hydrophilic molecules with pronounced hygroscopic character and wetting because of their abundant accessible surface hydroxyl groups (OHs). To solve this issue, tremendous efforts have been applied to tune lignocellulosic materials from hydrophilic to hydrophobic by either physically or chemically blocking cellulose and hemicellulose surface OHs. For instance, surface covering or grafting hydrophobic compounds including paraffin wax, alkyl ketene dimer, alkenyl succinic anhydride, fatty acids, perfluoro esters, and silane coupling molecules onto cellulose and hemicellulose surface OHs are effective for producing hydrophobic lignocellulosic products (e.g., packaging paper and paper tableware). However, these approaches face several obstacles, including the use of expensive and/or hazardous chemicals, as well as the complex processing steps with associated high manufacturing costs. Alternatively, surface deposition of inorganic nanoparticles (e.g., TiO₂, SiO₂, and Al₂O₃) via dip or vacuum coating also induced the wettability change of lignocellulosic fibers. However, the high manufacturing cost originating from the expensive nanoparticle precursors and complex processing steps is still the main challenge for these approaches. In addition, the application of these nanoparticles modified papers and tableware for food packaging should be very cautious because the potential risks associated with nanoparticle leaking are unknown. Therefore, it is critical to develop a simple, scalable, and sustainable method for hydrophobic paper and lignocellulosic tableware production, in which the modification agents must be cost-effective and non-toxic, and at the same time, the process needs to be simple and can be easily integrated into the existing papermaking and wood pulping industries.

An embodiment of the present invention is a simple, cost-effective, and scalable metal-ion-modification approach that can produce hydrophilic papers with good water resistance and high wet strength from either conventional hydrophilic papers or lignocellulosic fibers. This method is called metal-ion-modification or metal-ion-induced surface nano-engineering (M^X+-SNE). In this process, conventional paper is swelled a dilute multivalent metal ion (e.g., Fe³⁺, Zr⁴⁺) solution, followed by drying to induce the coordination interactions between metal ions and lignocellulosic fibers. The resulting hydrophobic paper exhibit a water contact angle (WCA) up to 140°, good wet tensile strength up to 9.5 MPa, and a low water absorptiveness of <10 g/m², which are comparable to synthetic polymer films. Moreover, the modified hydrophobic papers exhibit high stability with the maintained high WCA between 120 and 130° either after solvent wash (e.g., water, methanol, ethanol, etc.) or long-term storage (i.e., 6 months). Furthermore, the coordinated metal ions are stable that will not leach out even immersed in water. Additionally, this approach also can be integrated into the existing paper-making process for the scalable production of hydrophobic fibers and papers.

One embodiment of the present invention is a method of imparting hydrophobic properties to a paper product that includes immersing the paper product into a metal ion solution, or spraying said paper product with a metal ion solution. In other embodiments the method includes a deionized water rinsing step after the immersing or spraying step. In other embodiments, the method includes drying the paper product to produce an hydrophobic paper product.

In one embodiment of the invention, the metal ion solution has a pH of about 1 to about 5.

4. The method of claim 1, wherein the hydrophobic paper product has increased water resistance, hydrophobicity and oleophilicity.

In one aspect of the invention, the paper product is a cellulose-based paper. In others, the paper product is a lignocellulosic fiber-based paper. In others, the paper product is chosen from A4, writing paper, Kraft packaging paper, newspaper, cardboard, or Kraft tissue paper.

In another embodiment of the invention, the metal ion solution comprises at least one of Zr⁴⁺, Fe³⁺, Fe²⁺, Al³⁺, Y³⁺, Cu²⁺, Zn²⁺, and Co²⁺. In another, the metal ion solution comprises at least one of Zr⁴⁺, Fe³⁺, and Y³⁺.

The concentration of the metal ion may be from about 0.1 mM to about 3 nM, and any amount in-between. For example, the metal ion concentration may be from about 3 mM to about 80 nM. In another example, the metal ion concentration is from about 1 mM to about 1M.

In another embodiment of the invention, the ion solution immersion time may be between 30 seconds to about 12 hours, and any amount in-between. For example, the immersion time is between 1 minute to about 4 hours. In one aspect of the invention, the drying temperature of from 20-120° C. and a drying time of from 2 hours to 24 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams and photographs that illustrate the M^X+-SNE approach for paper's wettability transition. (1A) Schematic illustrates the self-assembly of pulp fibers' surface nanofibrils that induced by the coordination between metal ions and nanofibrils during the M^X+-SNE process. (1B) Photographs show wettability transaction of conventional KPP via the M^X+-SNE approach (60 mM metal ion immersion for 4 h followed by 2 h drying at 50)° ° C. The WCA of KPP in increased from 0° to 135° after the M^X+-SNE process. The blue drops are 20 ppm methylene blue solutions

FIG. 2 is a set of photographs show the fabrication of sustainable hydrophobic tableware through metal ion modification process.

FIGS. 3A and 3B are graphs that show the effect of metal ion modification time on the WCA of modified papers. WCA of KPP (3A) and A4 paper (3B) after immersion in 60 mM Fe³⁺and Zr⁴⁺ solution immersion for different periods and drying (100° C. for 2 h).

FIG. 4 is a graph that shows the WCA of KPP, A4 paper, and KTP after immersion in different solutions including H₂O, HCl (20, 50, and 100 mM), FeCl₃(60 mM), and ZrOCl₂(60 mM) solutions for 4 h and drying (100° C. for 2 h).

FIGS. 5A and 5B show the effect of metal ion concentration on the WCA of modified papers. WCA of KPP (A) and A4 paper (B) after immersion in Fe³⁺ and Zr⁴⁺ solutions of different concentrations for 2 h and drying (100° C. for 2 h).

FIGS. 6A and 6B Effect of drying temperature on the WCA of modified papers. WCA of KPP (A) and A4 paper (B) after immersion in 60 mM Fe³⁺ and Zr⁴⁺ solutions for 2 h and drying at different temperatures.

FIG. 7 shows the effect of paper type (A4, writing paper, Kraft packaging paper, newspaper, cardboard, Kraft tissue paper) on the effectiveness of metal ion modification. WCA of different types of papers after immersion in 60 mM HCl, Fe³⁺, or Zr⁴⁺ solutions and drying (100° C. for 2 h).

FIG. 8 shows the effect of metal ion type on the WCA of modified A4, KTP, and KPP. Metal ion concentration: 60 mM. Drying condition: 100° C. for 2 h.

FIGS. 9A, 9B, and 9C Hydrophobic stability of metal ion modified paper. (9A, 9B) WCA of Fe³⁺ (60 mM for 4 h) modified A4 after 5 h washing with different pH (A) and different organic (9B) solvents; (9C) WCA of the Fe³⁺ (60 mM for 4 h) the modified A4 after 1d to 6 months storage at ambient conditions.

FIG. 10 shows the comparison of the KPP origami boat and the Fe³⁺-KPP origami boat after water exposure.

FIGS. 11A and 11B show water absorptiveness of KPP before and after metal ion modification. (11A) Comparisons of Cobb values of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP measured at 60-300 s; (11B) Comparisons of water retention values of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP after water immersion for 1-162 h.

FIGS. 12A, 12B, and 12C show dry and wet strength of hydrophobicity paper. (12A) Photographs show mechanical performance of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP in both dry and wet status. (12B) Dry tensile strength of KPP, A4 paper, WP, and KTP before and after Fe³⁺ or Zr⁴⁺ modification. (12C) Wet tensile strength of KPP, A4 paper, WP, and KTP papers before and after Fe³⁺ or Zr⁴⁺ modification.

FIGS. 13A and B show performance metal ion modification enabled hydrophobic paper bag and tableware. (13A) Photographs show Zr⁴⁺-KPP packaging bag remain strong after water spraying. (13B) Photographs show water impermeability of Zr⁴⁺-pulp paper cup after filling 300 mL water.

FIG. 14 shows the effect of metal ion concentration on the Fe and Zr contents of hydrophobic KPPs

FIG. 15 shows the biodegradability of KPP and hydrophobic KPPs under outdoor condition (no-soil contact). From left to right, samples are KPP, Zr⁴⁺-KPP, and Fe³⁺-KPP.

FIG. 16 shows a comparison of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP after 10 d natural weathering. KPP is hydrophilic and was wetted by dew, while Fe³⁺-KPP and Zr⁴⁺-KPP are hydrophobic and dew drops remain spherical shape on their surfaces.

FIG. 17 shows the biodegradability of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP under outdoor soil-contact environment.

FIGS. 18A and 18B show (18A) The recyclability of hydrophobic paper and tableware; and (18B) the comparison of Zr⁴⁺-KPP (left) and recycled Zr⁴⁺-KPPs after the first (middle) and second (right) round recycling process.

FIGS. 19A, 19B, and 19C show FTIR spectra of (19A) conventional A4, WP, KTP, KPP, and cardboard, and (19B) Fe³⁺- and (19C) Zr⁴⁺-modified papers. Green, blue, and red line marked peaks represent cellulose, hemicellulose, and lignin vibration bands, respectively. Black line labeled peaks represent calcium carbonate (CaCO₃) vibration bands. CaCO₃is the filler material that been used of increase the brightness of A4 and WP.

FIGS. 20A, 20B, and 20C show FTIR spectra comparison of (20A) cellulose nanofiber (CNF), (20B) xylan, and (20C) softwood kraft lignin before and after Fe³⁺- and Zr⁴⁺ modification.

FIGS. 21(A-L), are SEM images of pristine A4 (21A), KTP (21B), and KPP (21C) papers; (21D-F) SEM images of hydrophobic A4 (21D), KPT (21E), and KPP (21F) papers modified with 1 wt % FeCl₃solution; (21G-I) SEM images and their corresponding EDS elemental mapping images of hydrophobic A4 (21G), KPT (21H), and KPP (21I) papers. Scale bars are 20 μm.

FIG. 22 shows high-magnification (2,5000×) SEM views of pulp fiber (A) before and (B) after Fe³⁺ ion modification.

FIG. 23 shows water drops on non-modified nanocellulose films and Zr4+, Al3+, and Fe3+-modified nanocellulose films. The photographs were taken after the water drops were added to the films' surface for 5 minutes. After metal ion modification, the water resistance of nanocellulose film is increased.

FIG. 24 shows the water and oil resistance of lignin-containing nanocellulose film (s) before and (b) after Zr⁴⁺ modification. Nanocellulose 0.12 g and 0.12 g of lignin-containing film was modified using 60 mM (200 ml) for 4 h and dried at 80° C. for 3 h. The photographs were taken after the water and oil drops were added to the films' surface for 20 minutes. The lignin-containing nanocellulose show both excellent water and oil resistance performance.

DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to the following detailed description of the invention and the Examples and Figures included herein.

Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this pertains. The references disclosed are also individually and specifically incorporated by reference herein for the material contained in them that is discussed in the sentence in which the reference is relied upon. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein may be different from the actual publication dates, which can require independent confirmation.

As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Additionally, ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed.

Experimental
Materials

Different types of papers, including A4 copy paper, kraft packaging paper (KPP), kraft tissue paper (KTP), writing paper (WP), newspaper, and cardboard, were purchased from Amazon.com. Nanocellulose was purchased from the Process Development Center at the University of Maine (Orono, ME, USA), which was originally produced from a cellulose mechanical defibrillation process. Xylan was purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). Softwood kraft lignin was obtained from Ingevity. Other chemicals include zirconyl chloride octahydrate (ZrOCl₂·8H₂O, ≥98%), iron (III) chloride nonahydrate (FeCl₃·9H₂O, ≥99%), aluminum chloride nonahydrate (AlCl₃·9H₂O, ≥99%), zinc (II) chloride hexahydrate (ZnCl₂·6H₂O, ≥99%), methanol, ethanol, isopropanol, toluene, and hexane were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA).

Metal Ion Modification

In a typical metal ion modification process, three pieces of paper samples (4×4 cm²) were immersed in 100 ml of 60 mM metal ion solution. After a set period of immersion, the paper samples were taken out from the metal ion solution and rinsed with deionized water to remove the free metal ions on the paper surface. Then, the modified papers were dry in an oven at a set temperature. Different metal ions including Na⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, and Zr⁴⁺ were tested. The immersion time was between 1 min and 4 h, and the metal ion concentration ranged from 1 mM to 1M. The drying temperature of 25, 60, 80, 100, and 120° C. were tested for Zr⁴⁺ and Fe³⁺ modified papers, and the drying time ranged from 2 h to 24 h depending on the temperature. For paper samples used for mechanical testing and paper bag fabrication, large-sized paper samples and a large volume of metal ion solutions were used.

Paper pulps were also used as a feedstock to test the applicability of metal ion modification on pulp fiber's wettability transition. Prior to modification, wet pulps (200 g/L) were prepared by dispersing 20 g of kraft packaging paper into 100 mL of distilled water, vigorously blending the suspension at 1000 rpm for 3-4 min, and followed by filtration. For pulp modification, 4 g of wet pulps were added into 100 mL 60 mM metal solution and set for 4 h. Then, modified pulps were separated from the metal ion solution through vacuum filtration to yield a compressed pulp sheet. The pulp sheet was rinsed with plenty of distilled water to remove the surface metal ions and followed by oven drying the sheet at 50° C. for 2 h to obtain the dried modified pulp sheet.

For paper model compound modification, approximately 400 mg nanocellulose, kraft lignin, or xylan were dispersed into 200 mL of 60 mM ZrOCl₂·8H₂O or FeCl₃solution for 4 h with magnetic stirring. Then, the modified model components were filtrated out and washed with 200 mL of deionized water through vacuum filtration (the pore size of the filter paper was 20-25 μm). The collected filtrates were vacuum dried at 80° C. for 2 h.

For comparison purposes, A4, KPP, and KTP papers were also immersed in 0-0.1 M HCl solution for 4 h, followed by oven dried at 50° C. for 2 h.

Water Contact Angle

The water contact angle (WCA) of conventional paper, modified hydrophobic paper, as well as films obtained from vacuum filtered pulps and model compounds are determined by the sessile drop method. Briefly, a 6 μl of water droplet was placed on the surface of paper or film samples. After 1 min, a photograph was taken and the WCA was measured using ImageJ software. Six droplets were analyzed for each sample.

Stability Test of Hydrophobic Paper

To evaluate the stability of hydrophobic paper, Fe³⁺ modified A4 paper (60 mM FeCl₃for 4 h immersion followed by 80° C. oven drying) was immersed into 100 ml of aqueous HCl solutions (pH from 0 to 14), methanol, ethanol, isopropanol, toluene, and hexane for 5 h and then dried in an oven at 50° C. After that, the sample's WCA was measured. Moreover, Fe³⁺ modified A4 paper was conditioned in the ambient condition (i.e., ˜25° C., ˜65% humidity) for up to 6 months, and the WCA of the conditioned sample was measured after a certain period.

Surface Morphological Analysis

The morphology and structure of the modified and nonmodified pulp fibers, paper, and model compounds were characterized by scanning electron microscopy (SEM, JEM-6110 LV, JOEL). The samples were sputter-coated with 30 nm platinum and operated at an accelerating voltage of 5 kV. The element distribution of samples was characterized using an energy dispersive X-ray spectroscopy (EDS) that was equipped on the SEM, the accelerating voltage was 15 kV.

Fourier transform infrared spectrometer (FT-IR) analysis of FT-IR spectrum of paper and model compounds were recorded in the PerkinElmer Spectrum Two spectrometer. This technique identified the specific functional group in lignocellulosic fiber that tends to form a coordination bond with the metal ion. The spectrometer was operated in an attenuated total reflectance (ATR) mode, and the scanning range is 450-4000 cm⁻¹. An average of 20 scans with a 4 cm⁻¹resolution was used. The spectra will be obtained by baseline-corrected using Spectrum Quant software through the “data tune-up” function.

X-ray photoelectron spectroscopy (XPS, Thermo esca lab 250Xi, USA) was used to analyze the surface elemental information and further confirm which functional group tends to form a coordination bond with the metal ion.

Total Metal Ion Content and Metal Ion Leachability Tests

For the total metal ion content determination, 2.0 g of the sample was placed in a crucible and burned at 550° C. for 8 hours in a muffle furnace. The collected ashes were cooled down to room temperature and dissolved in ten 10 ml of concentrated nitric acid and diluted to a volume of 25 ml. Simultaneously, as controls, non-modified samples were assessed using the same method. Then, the solution was analyzed using an inductively coupled plasma mass spectrometry (ICP-MS) to determine the content of the modifying metal ion, as well as the potentially toxic metal (i.e., lead and arsenic) in paper samples.

To determine the leachability of the modifying metal ions, 2 g of Fe³⁺ or Zr⁴⁺ modified KPP samples were immersed in 40 ml of acidic (pH 1), neutral (pH 7), and basic (pH 12) aqueous solutions for 1-6 h. Then, the concentration of Fe³⁺ or Zr⁴⁺ in the solution was measured using ICP-MS.

Water Permeability (Cobb) and Swelling Index Tests

The water permeability of paper before and after modification was determined by the Cobb method with a self-design setup with the ring diameter of 5.5 mm. The diameter of the tested paper was 50 mm. The measuring process was conducted according to the ASTM D3285-93 (2005).

For water swelling index determination, unmodified and metal ion modified KPP samples (4×4 cm²) were immersed in 100 ml distilled water for 1 to 164 h. After a set period, the sample was taken out and the surface free water on the sample was removed using a dried tissue paper with a 500 g rod rolling on the sample surface. Then, weights of samples were recorded, and the water swelling index was calculated according to the below equation:

Swelling index (%)=(M_t−M₀)/M₀×100%, where m₀represents the mass of the sample before absorbing liquid, and m₂represents the mass of the sample after absorbing liquid.

Results
Hydrophobic Paper by Metal-Ion-Induced Surface Nano-Engineering

Paper is a thin mat of overlapping lignocellulosic fibers containing microfibrils (FIG. 1), which is hydrophilic due to the presence of polar groups (e.g., OH and COOH) on the fiber surface. In particular, outer microfibrils are fibrillated from the fibers during wood pulping and refining and become “hairy” nanofibrils with one end attached to the fiber's surface. These “hairy” nanofibrils, with abundant exposed surface polar groups (e.g., OH, C═O, etc), impart pulp and paper with low water resistance. Upon metal ion solution immersion, paper rapidly absorbs water and swells microfibrils, while metal ions are adsorbed on the microfibrils by these polar groups including OH and COOH through complexation. During the drying process, microfibrils self-assemble to form aggregates as water evaporates, while the trapped metal ions cross-link the microfibrils through the coordination to form a compact film-like structure (FIG. 1). Without being bound by theory or mechanism, metal ion coordination not only stabilizes surface polar groups of lignocellulose microfibrils that reduce their water affinity, and also locks the microfibrils by forming the compact structure that to prevent its water swelling. FIG. 1B shows the photographs of the typical M^X+-SNE process that converts conventional kraft packaging paper (KPP) to water-resistant hydrophobic paper. Unmodified KPP is a superhydrophilic material that immediately absorbing water drops after they are dropped onto the paper. In contrast, water drops remain spherical in shape and stable on the surface of the Zr⁴⁺, Al³⁺, Fe³⁺, or Fe²⁺, modified KPPs, indicating their hydrophobic surfaces. The measured water contact angles (WCA, after 1 min after dropping) of KPP and modified KPP are 0° and 110°-130°, respectively, suggesting metal-ion-modification is effective for the hydrophilic-to-hydrophobic transaction of lignocellulosic papers.

Additionally, M^X+-SNE process is also effective for pulp modification and the modified pulp can be directly molded to paper plates and cups (FIG. 2). This demonstrates that the M^X+-SNE process can be integrated with the papermaking process for the manufacturing of hydrophobic tableware.

Influence of Metal Ion Modification Condition on Paper's Hydrophobicity

FIG. 3A shows the WCA of conventional KPP after 60 mM Fe³⁺ or Zr⁴⁺ solution immersion for different periods, followed by drying at 60° C. After being immersed in the Zr⁴⁺ solution for just 30 s and then dried, KPP becomes hydrophobic with a WCA >90°. The contact angle raises to 124±0.81° after 1 min immersing and remains at ˜130° afterward. The Fe³⁺ immersed KPP after drying also exhibits a hydrophobic surface, despite the WCA of Fe³⁺-KPP being 5-10° lower than that of Zr⁴⁺-KPP. The present inventors also tested the time dependence of A4 copy paper's WCA upon 60 mM Zr⁴⁺ and Fe³⁺ immersion. As shown in FIG. 3B, the longer immersion time is required for the wettability transition of A4 paper. For instance, 5- and 60-min immersion is needed for Zr⁴⁺ and Fe³⁺, respectively, to convert hydrophilic A4 paper to hydrophobic A4 paper with a WCA of >120°. The difference between KPP and A4 paper can be attributed to their different chemical composition, i.e., KPP contains lignin while A4 paper does not.

Considering both Fe³⁺ and Zr⁴⁺ solutions are acidic (i.e., pH 1-5 depending on the concentration) and contain anions (i.e., Cl⁻ for FeCl₃and ZrOCl₂), it is necessary to investigate the role of acid solution and anions on paper's wettability change. Therefore, KPP, A4 paper, and kraft tissue paper (KTP) are immersed into water and HCl solutions (10 mM, 50 mM, and 100 mM) for 4 h followed by drying, and their WCAs are determined. As shown in FIG. 4, none of these immersed papers exhibit the hydrophobic transition without the metal ions present. In contrast, both Fe³⁺ and Zr⁴⁺ solution immersed papers show good hydrophobicity with WCA >120°. Thus, metal ions such as Fe³⁺ and Zr⁴⁺ are responsible for paper's hydrophobicity. Notably, WCAs of raw A4 paper decreased from 46.2° to ˜0° after water or HCl solution immersion. This is because of the changing of paper surface characteristics. Specifically, conventional papers (especially A4) usually contain sizing agents like calcium carbonate and/or paraffin particles to increase their glossiness, smoothness, and/or hydrophobicity. These sizing agents are leached out during solution immersion, causing the decrease of WCA.

The hydrophobic transaction of lignocellulosic paper is concentration dependent. As shown in FIG. 5A, a low concentration of Fe³⁺ or Zr⁴⁺ immersion does not endow the modified paper hydrophobic property. Specifically, the KPP remains completed hydrophilic after immersing in 0.1 mM of Fe³⁺ or Zr⁴⁺ solution and drying. With the increase of Zr⁴⁺ concentration from 0.1 mM to 3 mM, the WCA of KPP increased from 0° to 127.0±2.2° and remains at 125˜140° with the Zr⁴⁺ concentration of 10 mM˜ 200 mM. However, further increasing the Zr⁴⁺ concentration causes the reduction of WCA to 109.0±3.5°. Similar trend is observed for Fe³⁺ treated KPP. The wettability transaction of lignocellulosic papers can be attributed to the interaction of metal ions (e.g., Zr⁴⁺ and Fe³⁺) with hydrophilic groups (e.g., —OH) in cellulose, hemicellulose, and lignin, the three major components of lignocellulosic fibers. When the concentration of metal salts is too low (e.g., 0.1 mM), there are not enough metal ions to form the stable interaction with those components to endow the hydrophobic transaction. On the other hand, the concentrated metal salt solution may induce the deposition of hygroscopic metal salt (e.g., Zr⁴⁺ and Fe³⁺), thereby decreasing the WCA of the modified papers. We also tested the concentration dependence of A4 paper's wettability transition, and a similar concentration-dependent trend is observed (FIG. 5B).

The hydrophobicity of metal-ion-modified papers is independent of the drying temperature. The WCAs of Zr⁴⁺ and Fe³⁺ modified KPP (FIG. 6A) and A4 paper (FIG. 6B) at a wide range of drying temperatures from 25° C. to 100° C. are all above 120° with no significant differences.

The metal ion modification is an effective and universal method for the hydrophilic to the hydrophobic transition of conventional papers, as indicated in FIG. 7 that this method has successfully converted different types of paper including A4 paper, writing paper (WP), kraft packaging paper (KPP), cardboard, KTP, and newspaper to a hydropic paper with WCA between 105 and 137°.

Interestingly, the metal-ion-induced hydrophobic transition is metal ion dependent, as displayed in FIG. 8. Zr⁴⁺ and Fe³⁺ modified A4 paper exhibit the highest WCA of ˜130°, followed by Y³⁺(123.3±2.4°), Al³⁺ (121.1±2.4°), Fe²⁺ (119.0±1.4°), Co²⁺, and Cu²⁺ treated ones. However, Ca²⁺, Mg²⁺, Na⁺, and Li treated papers remain hydrophilic with WCA of 0°. Similar trends are observed for KPP and KTP. These observations imply that metals with empty d or p orbitals (e.g., Fe, Zn, Y, Zr, etc) are required to endow the wettability transition of papers. Moreover, multivalent metal ions status (i.e., Zr⁴⁺, Fe³⁺, Y³⁺) seem to endow the modified paper with higher WCA than divalent metal ions (i.e., Fe²⁺, Zn²⁺, Cu²⁺, Co²⁺), which can be attributed to multivalent ions able to form a more stable interaction with lignocellulosic fibers.

Stability of Metal Ion Modified Papers

The metal ion modified papers exhibit stable hydrophobicity after washing with a wide pH range of solutions from 0 to 10 (FIG. 9A). Even after 1M HCl solution immersion for 5 h and drying, the FeCl₃modified A4 still exhibits a high WCA of 123.4±19.3. This implies the coordination between Fe³⁺ and lignocellulosic fibers is stable in strongly acidic solutions. However, the modified A4 paper loses its hydrophobicity after washing with strong alkaline solutions (i.e., pH≥12), which can be attributed to the swelling of lignocellulosic fibers in the strong alkaline environment that might alter the molecule structure of cellulose, hemicellulose, and lignin, thereby affect the coordination. The hydrophobic papers are resistant to the organic solvent wash too. As shown in FIG. 9B, Fe³⁺-modified A4 paper remains hydrophobic (WCA of) 120˜130° after washing using various solvents including methanol, ethanol, isopropanol, toluene, hexane. Furthermore, the hydrophobic papers are durable. As shown in FIG. 9C, after being stored at the ambient condition for 6 months, the Fe³⁺-modified A4 paper remains hydrophobic with the WCA of 133º.

Water Resistance of Hydrophobicity Paper

In comparison with conventional paper, hydrophobicity paper obtained from the metal ion modification displays good water resistance. As shown in FIG. 10, the KPP-based origami boat gets wet within 30 s upon water exposure and sinks after 60 s, while the origami boat fabricated from Fe³⁺-KPP keeps water impermeable and floating after 30 d water exposure.

To quantify the water resistance of hydrophobic papers, the Cobb values (FIG. 11A) and water retention values (FIG. 11B) of KPP, Fe³⁺-KPP, and Zr⁴⁺-KPP are measured and compared. The Cobb test evaluates the paper's resistance to the penetration of water by a one-face water exposure experiment, which measures the amount of water absorbed within few minutes. After 60 s of exposure, the Cobb₆₀of KPP has already reached 63.1 g/m², and further increasing the exposure time to 300 s results in a slightly higher Cobb₃₀₀of 69.4 g/m². In contrast, the Cobb₆₀of Fe³⁺-KPP (9.9 g/m²) and Zr⁴⁺-KPP (8.7 g/m²) are five times smaller than that of KPP. Increasing of exposure time to 300 s also slightly increased the Cobb₃₀₀of Fe³⁺-KPP (12.8 g/m²) and Zr⁴⁺-KPP (10.0 g/m²), while these values are still five times lower than that of KPP. The water retention measures the amount of water trapped in paper pores after water absorption, which can be used as an indicator for quantifying papers' long terms water resistance. Conventional KPP shows a water retention rate of 85% after 1 h water immersion, which increased to 145% after 7 d water immersion. However, Fe³⁺-KPP and Zr⁴⁺-KPP show lower water retention rates of 33% and 65% after 1 h and 7 d water immersion, respectively. This further confirmed long-term water resistance of hydrophobic papers.

Mechanical Strength of Hydrophobicity Paper

Owe to their excellent water resistance, hydrophobic paper exhibits superior wet strength. When a KPP scrip (2 cm×2 cm×10 cm) is getting wet by adding a few drops of water, it cannot bear a heavy load (300 g) as its dry status does (FIG. 12A). In contrast, both Fe³⁺-KPP and Zr⁴⁺-KPP could bear the heavy load no matter if it's dry or wet status. As shown in FIG. 12B, Fe³⁺- or Zr⁴⁺-papers (eg., A4, KPP, KTP) exhibit comparable dry tensile strength with their counterparts without statistic significant differences. While the wet tensile strengths of Fe³⁺- or Zr⁴⁺-papers are significantly (2-3 times) higher than that of unmodified papers (FIG. 12C).

To demonstrate the potential application of metal-ion-modified papers and pulps, we also fabricated paper bags and paper cups using Zr⁴⁺-KPP and Zr⁴⁺-pulp, respectively. As shown in FIG. 13A, after water spray, the Zr⁴⁺-KPP bag still remains mechanically robust and able to hold 2.5 kg weight without breakage, however, the conventional KPP is broken even hold 0.6 kg weight. Similarly, the molded Zr⁴⁺-pulp molded cup keeps its good water impermeability without leaking (FIG. 13B) after storing 150 ml water for 1 h. In contrast, water quickly leaked out from the unmodified pulp molded cup within 2 min, and the cup collapsed under weight.

Economic Feasibility of Metal Ion Modification

To evaluate the economic feasibility of M^X+-SNE process for hydrophobic paper production, the metal content of hydrophobic Fe³⁺- and Zr⁴⁺-KPPs are determined. As shown in FIG. 14, the metal contents of hydrophobic papers are low, i.e., <0.6 wt % for Fe³⁺-KPP and <1.3 wt % for Zr⁴⁺-KPP, respectively. These values are in agreement with our mass change monitor results where neglectable mass gain (i.e., 1-2 wt %) are observed after treating KPP with 60 mM Fe³⁺ or Zr⁴⁺. Moreover, the metal content of hydrophobic paper is logarithmically increased with the increase of metal ion concentration during the treatment. For instance, one order of the Fe³⁺ concentration increment from 10 mM to 100 mM only caused the iron content of the Fe³⁺-KPP to increase from 0.28 wt % to 0.55 wt %. Furthermore, the WCA of Fe³⁺- and Zr⁴⁺-KPPs maintained at >120° even when the Fe or Zr content as low as 0.3 wt %. These values are comparable with conventional 2% AKD- and AKA-sized hydrophobic papers. Consider the cost of metal salts is lower than these organic sizing agents (i.e., FeCl₃: US$200-1,000/metric ton, AKD: US$2,400/metric ton), the M^X+-SNE process is a promising and economic feasibility method for the production of water resistance packaging paper and tableware.

Biodegradability of Metal Ion Modification

The hydrophobic paper is also biodegradable like conventional non-modified papers in the natural environment. For comparison, KPP, Zr⁴⁺ and Fe³⁺ modified hydrophobic KPPs are placed on grass and exposed to the sun, wind and rain (Starkville, MS, U.S., from February 2022 to September 2022). Their morphologies over time are monitored to determine their degradability (FIG. 15). Unmodified KPP can be easily wetted by dew because of its hydrophilicity (i.e., after 10 d). However, Fe³⁺- and Zr⁴⁺-KPP remain hydrophobic after 10 d of natural weathering, where the dew drops remain spherical shape on the surface of Fe³⁺- and Zr⁴⁺-KPP, as shown in FIG. 16 (the magnified image of FIG. 15, 10 d). After 20 d of natural weathering, the Fe³⁺- and Zr⁴⁺-KPP remain impermeable despite their surface dew drops not retaining the spherical shape. After 45 d natural weathering, while some blackspots are observed on the surface of unmodified KPP, probably due to the sunlight or microorganisms (for example, bacteria and fungi) degradation. Meanwhile, Fe³⁺- and Zr⁴⁺-KPP lose their water impermeability and become wettable. After 85 d, biodegradation-induced black spots are also observed in Fe³⁺- and Zr⁴⁺-KPP. Unmodified KPP is completely biodegraded after 165 d. In comparison, Fe³⁺- and Zr⁴⁺-KPP are more durable, which completely degraded after 180 and 200 d, respectively. This good balance between durable and biodegradability (that is, the hydrophobic paper is both stable and durable under working conditions yet easily degraded under natural soil or outdoor conditions) is appealing for designing next-generation sustainable, biodegradable and high-performance packaging materials.

Additionally, the present inventors also placed KPP, Fe³⁺- and Zr⁴⁺-KPP to the outdoor soil contacted condition to test their biodegradation (FIG. 17). The conventional KPP immediately absorbs moisture and becomes wetted after contact with the fresh soil, while hydrophobic KPPs remain in a dry state (FIG. 10a). After 10 d, hydrophobic KPPs maintain in the dry state in contact with wet soil, indicating their good water impermeability (FIG. 10b). After 30 d, all KPPs show slightly color change and some blackspots are observed on their surfaces, probably due to the sunlight or microorganisms (for example, bacteria and fungi) degradation (FIG. 10c). Meanwhile, Zr⁴⁺ and Fe³⁺ modified KPPs lose their water impermeability and become wettable. After 50 d, the unmodified KPP is partly decomposed with surface broken, while the modified KPPs remain intact despite abundant blackspots are observed. (FIG. 10d). Unmodified KPP is become fractured after 75 d (FIG. 10e) and is completely biodegraded after 100 d (FIG. 10f). In comparison, Zr⁴⁺ and Fe³⁺ modified KPPs are more durable, and the biodegradation starts after 75 d (FIG. 10e) and is completely degraded after 130 d.

Recyclability of Metal Ion Modified Hydrophobic Papers

M^X+-SNE-derived hydrophobic lignocellulosic products also demonstrate good recyclability. The end-of-life hydrophobic paper and tableware can be broken back down into the pulp slurry by mechanical stirring, allowing it to be reapplied as a recycled material (FIG. 18A). Owe to the stable metal-lignocellulose coordination bonds, the recycled pulp remains hydrophobic. As shown in FIG. 18B, lignocellulosic paper produced first-round recycled pulps show a WCA of 135°. However, after two rounds of recycling, the recycled pulp loss its hydrophobicity (WCA=40°). This probably because the intense mechanical blending has caused the fibrillation of recycled pulps and the dissociation of coordinated metal ions.

Metal Ion Modification Mechanism

FTIR spectra of conventional papers before and after modification are recorded and compared (FIG. 19A). Characteristic peaks of cellulose (e.g., 1372, 1201, 1160, 1105, 1059, 1033, and 896 cm⁻¹, etc) and hemicellulose (i.e., ˜1734 and ˜1268 cm⁻¹) are observed in the FTIR spectra of all the papers, while lignin fingerprint peaks (e.g., 1593, 1511, 1450, 1266, and 814 cm⁻¹) are observed only for KPP, KTP, and cardboard. This is because the chemical composition of papers varies with their manufacturing procedures. For instance, A4 and WP are usually fabricated from bleached kraft pulps where lignin is removed, while KTP, KPP, and cardboard are typically produced from unbleached kraft pulps where lignin is retained. After Fe³⁺ and Zr⁴⁺ modification, several changes are observed in the FTIR spectra of papers. For instance, hemicellulose-related (˜1734 cm⁻¹, corresponds to C═O stretching of O═C—O) and lignin-related (1266 cm⁻¹) bands are found shift to low wavenumber regions (FIGS. 19B and 19C). Additionally, the vanish of peaks at 1420, 872, and 711 cm⁻¹indicates that CaCO₃are leached out from A4 and WP after Fe³⁺ and Zr⁴⁺ modification. However, the leaching of CaCO₃does not contribute to the hydrophilic-to-hydrophobic transition of A4 and WP. This is because CaCO₃leaching is also observed in HCl immersed A4 and WP, while these papers remain hydrophilic with WCA of 0.

To elaborate on metal-lignocellulose interactions, the present inventors performed the Fe³⁺ and Zr⁴⁺ modification to lignocellulose model compounds, i.e., cellulose nanofiber (CNF), xylan (the main component of hemicellulose), and kraft lignin, and monitored the changes in their FTIR spectra (FIG. 20). For CNF (FIG. 20A), band at 1028 cm⁻¹that correspond to C—O vibration of cellulose are found shifted to ˜1026 cm⁻¹after Fe³⁺ and Zr⁴⁺ modification. Moreover, intensity reductions are observed for cellulose's OH (˜1336 and 1203 cm⁻¹), C—O—C (1160 cm⁻¹), glucose ring (1112 and 897 cm⁻¹), and C—O (1051 cm⁻¹) vibration bands. The combined evidence indicates that Fe³⁺ and Zr⁴⁺ ions has chelated on cellulose's OH groups and ring oxygen. For xylan (FIG. 20B), vibrations of glycosidic C—O (896 cm⁻¹), aliphatic OH (1039 cm⁻¹), and xylan ring (987 cm⁻¹) are found shifted to the low wavenumber regions. This confirmed the coordination of metal ions with xylan's OH groups and ring oxygen. Moreover, peak at 1750 cm⁻¹(corresponds to C—O stretching of O—C—O) subject to an intensity enhancement and shifted to 1732 cm⁻¹and with the (FIG. 20B), implying the oxidation of C—O to C—O and thereafter metal coordination. For lignin (FIG. 20C), vibrations of guaiacyl C—O (1210 and 1267 cm⁻¹), phenolic hydroxyl (1363 cm⁻¹), aliphitic OH (1031 cm⁻¹), and guaiacyl C—H (1147 cm⁻¹) are subject to a low wavenumber peak shift. These shifts indicating the coordination of metal ions with lignin's ring oxygen and OH groups.

FIG. 21 shows the surface morphology and microstructure of three conventional papers, i.e., A4, KTP, and KPP, as well as their corresponding HCl and FeCl₃modified ones. Conventional papers have an isotropic net-like structure that consists of randomly oriented lignocellulosic fibers (FIG. 21A-C). The diameter of fibers ranges from 15 to 50 μm, depending on the type of paper. The surface of lignocellulosic fibers contains many microfibrils that are generated during the pulp fibrillation process for improving the mechanical strength of papers 18. After being immersed with 60 mmol HCl or FeCl₃solution and drying, the hydrophobic papers exhibit a smoother fiber surface, indicating the reorganization of micro- and macro-fibrils during paper wetting and drying. Moreover, micron-sized sizing agents (yellow circle marked regions in FIG. 21A), which were used for improving the water-resistant of A4, are found removed from the A4 surface during metal solution immersing and drying, further resulting in a smoother surface. These observations suggest that the increase of WCA after metal salt modification is not attributed to the change of paper surface roughness since the smoother surface will result in a decreased WCA. SEM-EDS mapping micrographs (FIGS. 21L to 21L) of hydrophobic papers show the homogeneous distribution of Fe element on fiber surfaces, suggesting the incorporation of metal ions with lignocellulosic fibers responsible for the increasing hydrophobicity of papers.

High-magnification views reveal the detailed morphology change of surface microfibrils of wood pulps before and after Fe³⁺ modification (FIG. 22). In both images, the aligned microfibrils are clearly visible. Unmodified pulp fiber shows a porous and loose surface characteristics with a layer of detached microfibril film (FIG. 22A). The microfibrils are mainly parallel arranged, indicating the surface is from the secondary cell wall. Moreover, nanopores range from 50 to 200 nm among microfibrils, which are resulted from the removal of lignin during the papermaking process. Despite the undulating microfibrils are observed on the surface of Fe³⁺-modified pulp, it has a denser and more compact surface characteristics than that of unmodified pulp (FIG. 22B). In particular, the surface nanopores and detached microfibrils are barely observed. These observations indicate that metal ion modification induces the self-organization of microfibrils probably via coordination interaction, which results in a more compact pulp structure with less exposed polar groups, and thereby increase the hydrophobicity of pulp fibers and papers.

CONCLUSIONS

As stated above, an embodiment of the present invention is a facile metal-ion-modification approach for preparing hydrophilic, water-resistant packaging paper and tableware. In this process, conventional papers and pulps is swelled a dilute multivalent metal ion (e.g., Fe³⁺, Zr⁴⁺) solution, followed by drying to induce the coordination interactions between metal ions and lignocellulosic fibers. The resulting hydrophobic paper and tableware exhibit a water contact angle up to 140°, good wet tensile strength of, and a low water absorptiveness of 10 g/m², which are comparable to synthetic polymer films. The hydrophobic paper is stable for long-term storage and solvent wash. This metal-ion-modification approach also can be applied in the wood pulping process for the scalable production of hydrophobic papers and tableware.

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The invention thus being described, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

PROCESSING CONVENTIONAL PAPERS AND PULPS INTO HYDROPHOBIC AND WATER-RESISTANT PACKAGING MATERIALS AND TABLEWARE

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