FIELD OF THE INVENTION

[001] The present invention relates to a chitosan film and a method of producing said chitosan film.

BACKGROUND OF THE INVENTION

[002] As stated in US9982393, chitosan is a highly versatile biopolymer commonly derived from crustacean (i.e., crabs, shrimps, etc.) shell chitin, but is also found in the cell walls of certain fungi. It has found numerous applications in various industries due to its unique molecular characteristics as well as its ability to form fibers, films, hydrogels, and coatings, all with antimicrobial properties. Additionally, as a polymeric material of biological origin, chitosan is biodegradable, biocompatible, and has low toxicity. The molecules below show the respective structures for chitosan, chitin, and cellulose:

Cellulose

[003] As also stated in US9982393, paper is a fibrous, hydrophilic substrate; it can absorb liquid penetrants such as water, grease, and oil. Furthermore, uncoated paper allows for the easy passage of moisture. The main reason for this easy passage of moisture is the spaces between the interwoven fibers of paper that consist of countless air voids. It is these air voids and micropores within the cell walls of fibers that determine the porosity of the paper, which in turn is influenced by fiber refining. [004] Furthermore, according to US9982393, barrier coatings are applied to the surface of paper to decrease the porosity of the paper by filling the air voids between the fibers, coating the micropores in the fiber walls, and changing the surface chemistry of the fibers to make them resistant to fluid wetting and liquid absorption. Barrier coatings that provide rigidity and water resistance in corrugated board and paperboard products are widely used in food packaging and HVAC (Heating, Ventilation, and Air Conditioning) cooling pads for agricultural markets.

[005] Methods of producing a chitosan film on, for example, paper and cardboard products, are known. Typically, chitosan is first dissolved in dilute aqueous acetic acid. The resulting solution is then applied to the surface of the article on which the film is to be formed and left to dry. Subsequently, the resulting film can be exposed to aqueous sodium hydroxide, which deprotonates the chitosan (and neutralizes the acid), as shown for example in the diagram in Fig. 1, thereby rendering the chitosan film no longer soluble. This step is sometimes known as “curing”. Specifically, Fig. 1 shows acetic acid neutralization from chitosan/acetic acid material using aqueous sodium hydroxide, producing water and sodium acetate.

[006] However, with such conventional methods, there are still issues of strength and water impermeability in the resulting chitosan film. In addition, with conventional methods, typically another material (like wax) is needed to improve water impermeability.

SUMMARY OF THE INVENTION

[007] In accordance with the present invention, there is provided:

1. A chitosan film, wherein the chitosan film has at least one of the following characteristics: a reduced salt content compared to a conventional chitosan film; an improved water impermeability compared to a conventional chitosan film; does not exhibit (i.e., is free of) a fibrous network when observed by SEM at a magnification of 10 OOOx; a higher homogeneity than a conventional chitosan film; a lower porosity than a conventional chitosan film; or any combination of the above.

2. The chitosan film of item 1 , wherein the chitosan film comprises at most 1%, preferably at most 0.5%, even more preferably at most 0.3%, most preferably at most 0.1% or lower of the salts contained in a conventional chitosan film, wherein the salts are salts produced by neutralizing residual acid left over by the film making process.

3. The chitosan film of item 2, wherein the chitosan film is free of the salts contained in a conventional chitosan film.

4. The chitosan film of any one of items 1 to 3, wherein the chitosan film has a thickness between about 200 nm and about 20 pm.

5. The chitosan film of any one of items 1 to 4, wherein the chitosan film has a thickness of at least about 1 pm; at least about 2 pm; at least about 4 pm; or at least about 5 pm; and/or at most about 20 pm; at most about 15 pm; at most about 12 pm; or at most about 10 pm.

6. The chitosan film of any one of items 1 to 5, wherein the coat weight of the chitosan film is between about 1 and about 10g/m ² . The chitosan film of any one of items 1 to 6, wherein the chitosan film has a coat weight of at least about 1 g/m ² , at least about 1 .5 g/m ² ; at least about 2 g/m ² , at least about 3 g/m ² , or at least about 4 g/m ² ; and/or at most about 10 g/m ² ; at most about 8 g/m ² ; at most about 6 g/m ² ; or at most about 5 g/m ² . The chitosan film of any one of items 1 to 7, wherein the chitosan film has an average molecular weight between about 10 KDa and about 2000 KDa. The chitosan film of any one of items 1 to 8, wherein the chitosan film has an average molecular weight of at least about 10 Kda, at least about 100 KDa; at least about 200 KDa; or at least about 300 KDa; and/or at most about 2000 KDa; at most about 1000 KDa; at most about 600 KDa; or at most about 500 KDa. The chitosan film of any one of items 1 to 9, wherein the chitosan film further comprises additives, such as additives including molecules with hydrogen bonding capability, preferably glycerol and/or sorbitol. The chitosan film of any one of items 1 to 10, wherein the chitosan film is free of residual acid. A method for producing a chitosan film, comprising the steps of:

A) providing a dried uncured chitosan film;

B) curing the dried uncured chitosan film with an alcohol solvent to produce a cured chitosan film, and

C) drying the cured chitosan film to produce the chitosan film, wherein the dried uncured chitosan film has been produced using an aqueous acidic chitosan solution. The method of item 12, wherein the chitosan film is the chitosan film as defined in any one of items 1 to 11. The method of item 12 or 13, wherein step A comprises producing the dried uncured chitosan film using the aqueous acidic chitosan solution. The method of item 14, wherein the producing of the dried uncured chitosan film using the aqueous acidic chitosan solution comprises producing the acidic chitosan solution. The method of any one of items 12 to 15, wherein the acid used in the aqueous acidic chitosan solution is acetic acid, L-ascorbic acid, formic acid, L-glutamic acid, hydrochloric acid, lactic acid, maleic acid, malic acid, phosphorous acid, or succinic acid, preferably acetic acid. The method of any one of items 12 to 16, wherein the molar ratio of available acidic protons in the acid used to make the acidic chitosan solution compared to monomers of D-glucosamine in the chitosan polymer is between about 0.80 and about 3.0. The method of item 17, wherein the molar ratio is at least about 0.80; at least about 0.85; at least about 0.90; or at least about 1 .0; and/or at most about 3.0; at most about 2.5; at most about 2.0; or at most about 1.5. The method of any one of items 12 to 18, wherein step A comprises the step of preparing the acidic chitosan solution by first preparing an aqueous acetic acid solution; adding dried chitosan thereto; and then stirring until dissolution is achieved. The method of any one of items 12 to 19, wherein the chitosan concentration in % w/v in the acidic chitosan solution is between about 0.1 and about 5.0. The method of any one of items 12 to 20, wherein the chitosan concentration in % w/v in the aqueous acidic chitosan solution is at least about 0.1 ; at least about 0.5; or at least about 1.0; and/or at most about 5.0; at most about 4.0; at most about 3.0; at most about 2.5; or at most about 2.0. The method of any one of items 12 to 21, wherein step A comprises using the acidic chitosan solution to directly coat a surface, such as a sheet of paperboard, through dipping, spray coating, or spreading, followed by drying the acidic chitosan solution. The method of any one of items 12 to 22, wherein the acidic chitosan solution is dried at room temperature, preferably while pressed against a plastic surface, such as polystyrene or ABS. The method of any one of items 12 to 23, wherein, in step B, the alcohol solvent is present in a solution comprising, in v/v, at least about 70% alcohol, at least about 80%, at least about 90 %, at least about 99% or at least about 99.9% alcohol, preferably at least about 99.9% alcohol. The method of any one of items 12 to 24, wherein, in step B, the dried uncured chitosan film is submerged in the alcohol solvent or washed with alcohol solvent (once or multiple times, preferably multiple times). The method of any one of items 12 to 25, wherein step C is performed by letting the chitosan film dry at room temperature, or by heat pressing the chitosan film. The method of any one of items 12 to 26, wherein steps A, B, and C are performed multiple times, such that the chitosan film comprises multiple chitosan layers, preferably between 2 and 10. The method of any one of items 12 to 26, wherein the dried uncured chitosan film provided in step A comprises between 1 to 10 chitosan layers. A composite fiber stock material comprising paper or a fibrous substrate at least partially coated with the chitosan film as defined in any one of items 1 to 11 or the chitosan film produced according to the method defined in any one of items 12 to 28. The composite fiber stock material of item 29, wherein the composite fiber stock material has been made into a beverage cup or a food tray. A method of producing the composite fiber stock material as defined in item 29 or 30, wherein the paper or fibrous substrate is first cut and assembled to form a desired shape and then coated with the chitosan film using the method defined in any one of items 12 to 28. A method of producing the composite fiber stock material as defined in any one of items 29 to 31 , wherein the paper or fibrous substrate is first coated with the chitosan film using the method defined in any one of items 12 to 28, and then cut and assembled to form a desired shape. A chitosan film produced using the method defined in any one of items 12 to 28. BRIEF DESCRIPTION OF THE DRAWINGS

[008] Figure 1 is a diagram of what happens when conventional curing is used to produce a conventional chitosan film (specifically, it shows acetic acid neutralization from chitosan/acetic acid material using sodium hydroxide, producing water and sodium acetate);

[009] Figure 2 is a diagram of what happens when a curing step according to an embodiment of the method of the present invention is used to produce a chitosan film according to an embodiment of the present invention (specifically, it shows acetic acid dissolution from chitosan/acetic acid material using methanol, recovering acetic acid and leaving chitosan behind);

[010] Figure 3 is an SEM image of a chitosan film according to another embodiment of the present invention;

[011] Figure 4 is an SEM image of a conventional chitosan film;

[012] Figure 5 is an SEM image of a composite fiber stock material according to an embodiment of the present invention (comprising a chitosan film according to an embodiment of the present invention on a paper substrate);

[013] Figure 6 is an SEM image of a cross section of the composite fiber stock material of Figure 5;

[014] Figure 7 is an SEM image of a conventional paper substrate without a chitosan film;

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[015] The present inventors have found a relatively cheap and simple way of making chitosan films that are strong and have improved water impermeability.

[016] Turning to the invention in more detail, there is provided a chitosan film and a method of producing said chitosan film.

Chitosan film

[017] In a first aspect of the invention, a chitosan film is provided, wherein the chitosan film comprises a reduced salt content.

[018] Conventional chitosan films have generally been cured using sodium hydroxide or other bases, including hydroxides. As mentioned previously, and as shown in Fig. 1 , such a curing step deprotonates the chitosan, and results in the creation of a salt (e.g. sodium acetate, when acetic acid and sodium hydroxide are used).

[019] The inventors hypothesize that when the film is dried, some salt is likely left behind, and this salt generally reduces the chitosan film’s effectiveness at creating a water barrier, likely because the resulting salts are generally hygroscopic. Furthermore, the presence of this salt can lessen the uniformity of the resulting chitosan film, which generally reduces the chitosan film’s water impermeability. Moreover, as mentioned, if the chitosan film has not been cured, the resulting film will dissolve, or dissociate, when exposed to water.

[020] Conversely, with the chitosan film of the present invention, neutralization has not been performed; instead, the acid is simply dissolved with an alcohol solvent through selective dissolution. In addition, because no neutralization occurs, no salt is formed as there is no ionic bonding between the sodium ion and the anion of the acid (e.g., the acetate ion, when acetic acid is used). Instead, as shown in Fig. 2, the anion of the acid is present in acid form, and is therefore more likely to permeate the chitosan film (as it is smaller than the ionically bonded acid anion/sodium ion), which can then be washed away from the film by passing through the chitosan membrane. Specifically, Fig. 2 shows acetic acid dissolution from chitosan/acetic acid material using methanol, recovering acetic acid and leaving chitosan behind. This results in a chitosan film with a reduced salt content when compared to conventional chitosan films. In preferred embodiments, the chitosan film of the present invention is produced using the method defined in the section below.

[021] In embodiments, the chitosan film of the present invention contains at most 1%, preferably at most 0.5%, even more preferably at most 0.3%, most preferably at most 0.1 % or lower of the salt contained in a conventional chitosan film, wherein the salts are salts produced by neutralizing residual acid left over by the film making process. In preferred embodiments, the chitosan film of the present invention is free of such salt.

[022] For certainty, herein, a “conventional chitosan film” is defined as a chitosan film that has been cured using sodium hydroxide or another base.

[023] Also, the chitosan film of the present invention likely has improved water impermeability compared to a conventional chitosan film because the alcohol solvent does not cause as much swelling in the chitosan film once it is dry. When extracting the acid with an alcohol solvent, it results in fewer defects or changes in morphology in the film. This likely has to do with hydrogen bonding; the chitosan is dissolved in an acidic solution. When the resulting film is exposed to a sodium hydroxide solution, water is reintroduced into the film, which results in an acidic solution (due to the acetic acid present in the film) that can redissolve or swell some of the chitosan in the film. Furthermore, any water absorbed into the chitosan film would cause it to swell. It is worth mentioning that hydrogen bonding is more favorable with water than with alcohol solvents such as methanol.

[024] Moreover, it is worth mentioning that alcohol solvent is less likely to reduce the strength of the surface on which the film has been formed (e.g. paper), when compared to a base such as aqueous NaOH. Accordingly, when exposed to alcohol (e.g., methanol), the strength of the underlying material paper is not decreased as much as it is compared to when aqueous NaOH is used.

[025] Because of the above properties, the chitosan film of the present invention can have greater strength and improved water impermeability when compared to conventional chitosan films produced using conventional methods. As an example, conventional chitosan films are generally described as providing for a strong barrier to moisture or water vapour, but are not necessarily as water impermeable as the chitosan film of the present invention.

[026] The chitosan film of the present invention can have a variety of thicknesses, depending on the preference of the user. In general, a thicker film results in a stronger, more water impermeable, but more brittle, chitosan film. Typically, the thickness of the chitosan film of the present invention is between about 200 nm and about 20 pm. In embodiments, the chitosan film has a thickness of at least about 1 pm; at least about 2 pm; at least about 4 pm; or at least about 5 pm; and/or at most about 20 pm; at most about 15 pm; at most about 12 pm; or at most about 10 pm.

[027] The chitosan film of the present invention can have a variety of coat weights depending on the chitosan concentration and number of layers of chitosan. In general, a higher coat weight results in a thicker, stronger, more water impermeable, but more rigid and brittle, chitosan film. Typically, the coatweight of the chitosan film of the present invention is between about 1 and about 10g/m ² . In embodiments, the chitosan film has a coat weight of at least about 1 g/m ² , at least about 1 .5 g/m ² ; at least about 2 g/m ² , at least about 3 g/m ² , or at least about 4 g/m ² ; and/or at most about 10 g/m ² ; at most about 8 g/m ² ; at most about 6 g/m ² ; or at most about 5 g/m ² .

[028] The chitosan film of the present invention can apply across a range of molecular weights. In general, higher average molecular weight can increase the strength of the film. Typically, the average molecular weight of the chitosan film of the present invention is between about 10 KDa and about 2000 KDa. In embodiments, the chitosan film has an average molecular weight of at least about 10 Kda, at least about 100 KDa; at least about 200 KDa; or at least about 300 KDa; and/or at most about 2000 KDa; at most about 1000 KDa; at most about 600 KDa; or at most about 500 KDa. For clarity, the molecular weight of the chitosan film refers specifically to the average molecular weight of the chitosan contained in the film.

[029] The manner in which some of the above parameters can be controlled are better described in the method section below.

[030] In embodiments, the chitosan film of the invention does not exhibit (i.e., is free of) a fibrous network when observed by SEM at a magnification of 10 OOOx. Indeed, to illustrate the differences between the chitosan film according to an embodiment of the present invention when compared to conventional chitosan films cured with aqueous NaOH, 10 OOOx SEM images were obtained (using the procedure defined in the section “Producing the SEM Films” in the illustrative embodiments section below). Specifically, Fig. 3 is an SEM image of a chitosan film according to an embodiment of the present invention, while Fig. 4 is an SEM image of a conventional chitosan film. In the SEM image of the aqueous NaOH neutralized chitosan film at 10 OOOx magnification (Fig, 4), a fibrous network of chitosan fibers can be seen. At the same magnification, in the SEM image of the methanol cured chitosan film (Fig. 3), a fibrous network is not visible. This would indicate that the chitosan network is more homogenous and less porous when cured with methanol compared to when it is neutralized with aqueous NaOH. Therefore, in embodiments, the film of the invention is more homogenous than a conventional chitosan film. Also, in embodiments, the film of the invention is less porous than a conventional chitosan film.

[031 ] The chitosan film of the present invention can further comprise additives to improve, for example, the flexibility of the film. Preferable additives include molecules with hydrogen bonding capability. Potential additives include glycerol and sorbitol.

[032] For clarity, as used here, the term “water impermeability” refers to a chitosan film’s ability to prevent permeation by water (in vapour or liquid form).

[033] Water vapour permeability is generally understood as the ability for water vapour to pass through a membrane. Increasing the water vapour permeability allows more water vapour to pass through the membrane and decreasing it reduces the water vapour which can pass through.

[034] The term “porosity” refers to the quantified presence of pores. In general, increasing porosity allows more water or water vapour to pass through the membrane (meaning water impermeability decreases) and decreasing it reduces the amount of water or water vapour which can pass through (meaning water impermeability increases).

[035] In general, and as defined above, salt is present in conventional chitosan films (due to neutralization of the acid with a base). Ideally, in the chitosan film of the present invention, there is no salt present as salt would introduce defects in the film continuity and porosity. In general, increasing the salt content would reduce the film strength and decrease water impermeability, while decreasing the salt content would increase the film strength and increase water impermeability.

[036] The chitosan film of the present invention may also have a residual acid content. Ideally, there is no residual acid present as it would allow the potential for the film to dissolve when in contact with water and subsequently reduce film strength. Therefore, in general, increasing the residual acid content would reduce the film strength, while decreasing the residual acid content would increase the film strength.

Method for producing a chitosan film

[037] In a second aspect of the invention, a method for producing the above chitosan film is provided. As noted above, the chitosan film has improved strength and water impermeability due to the reduction in salts in the film.

[038] The method of producing the chitosan film thus comprises the steps of:

A) providing a dried uncured chitosan film;

B) curing the dried uncured chitosan film with an alcohol solvent to produce a cured chitosan film, and

C) drying the cured chitosan film to produce the chitosan film, wherein the dried uncured chitosan film has been produced using an aqueous acidic chitosan solution.

A) Providing the Starting material

[039] The starting material can be any dried uncured chitosan film that has been produced with an aqueous acidic chitosan solution, as would be understood by the person of skill in the art. As mentioned, when making conventional chitosan films, chitosan is typically dissolved in an aqueous acidic solution, which results in an aqueous acidic chitosan solution. The acid used in the aqueous acidic chitosan solution can be any acid that is conventionally used to dissolve chitosan, including acetic acid, L-ascorbic acid, formic acid, L-glutamic acid, hydrochloric acid, lactic acid, maleic acid, malic acid, phosphorous acid, or succinic acid, preferably acetic acid.

[040] The concentration of acid used to dissolve the chitosan can vary, as would be understood by the person of skill in the art. In general, the acid concentration increases the solubility of the chitosan since it is understood that chitosan is protonated by the acid. A minimum amount of acid is required to protonate the chitosan polymer strands, enough to enable dissolution of the chitosan. Typically, the acidic solution has at least an equivalent molar ratio of available acidic protons compared to the moles of monomers of D-glucosamine in the chitosan polymer. In general, an insufficient number of acidic protons leads to poor solubility. In embodiments, the molar ratio of available acidic protons compared to the moles of monomers of D-glucosamine in the chitosan polymer is between about 0.80 and about 3.0. In embodiments, said molar ratio is at least about 0.80; at least about 0.85; at least about 0.90; or at least about 1.0; and/or at most about 3.0; at most about 2.5; at most about 2.0; or at most about 1 .5. [041] In embodiments, step A can comprise the step of preparing the acidic chitosan solution. This can be done using known steps used to prepare acidic chitosan solutions in the art, or using the steps defined in the section “Chitosan solution” in the illustrative embodiments section below. For example, a 0.75 % acetic acid aqueous solution can be prepared (by combining '5% aqueous acetic acid’ in water (15:85 v/v) solution). 5 g of chitosan (e.g., a fungal- sourced or a crustacean-sourced chitosan powder) can then be added to 200 mL of the aqueous acetic acid solution, resulting in a 2.5% chitosan (w/v) solution. The mixture can then be stirred at room temperature, e.g., at 350 RPM until complete dissolution is achieved.

[042] As another example, to produce a 1% chitosan solution in a 1% aqueous acetic acid solution, 1 g of dry chitosan can be added to 80 g of distilled water and stirred until dispersed. While stirring the mixture, 20 g of 5% aqueous acetic acid can be added to initiate the dissolution. The order of mixing can be changed but this order provides a fast way of achieving homogeneity.

[043] The acid from the acetic acid protonates the amine on the chitosan monomers (D-Glucosamine), which allows the polymer strands to be dissolved.

[044] It is worth noting that the acidic chitosan solution can be produced at different concentrations, where the concentration and average molecular weight of the polymer affect the viscosity of the solution. This solution can be used to coat substrates, cast films, fibers or can be transformed into many shapes and sizes. In embodiments, the chitosan concentration in % w/v in the acidic chitosan solution is between about 0.1 and about 5.0. In embodiments, the chitosan concentration in % w/v in the aqueous acidic chitosan solution is at least about 0.1; at least about 0.5; or at least about 1 .0; and/or at most about 5.0; at most about 4.0; at most about 3.0; at most about 2.5; or at most about 2.0.

[045] In embodiments, step A can further comprise the step of using the above aqueous acidic chitosan solution to produce the dried uncured chitosan film. As mentioned, this can be done using any known techniques in the art, as would be understood by the skilled person. For example, the acidic chitosan solution (e.g., the abovementioned 2.5% chitosan solution) can be used to directly coat a surface, such as a sheet of paperboard, through dipping, spray coating, spreading, or any other techniques known in the art.

[046] The coating can then be dried using any means known in the art.

[047] In embodiments, the coating can be allowed to dry at room temperature, preferably while pressed against a polystyrene surface. The polystyrene surface improves the smoothness of the coated surface (in this case, the coated paperboard). Alternatively, the coating can be oven-dried or dried through heat pressing or any known drying methods. [048] In embodiments, using the above aqueous acidic chitosan solution to produce the dried uncured chitosan film can be done using the steps defined in the illustrative embodiments section below (e.g., in the section “Coating Process”).

[049] In embodiments, drying the substrate on a plastic surface, such as polystyrene or ABS, can produce a glossy film by molding to the surface of the plastic, wherein the chitosan coating is in contact with the plastic surface which would need to be smooth/polished for this effect to occur. This would mean, for example, if the coating is drying on a paper substrate, the coated side of the paper product would be in contact with the plastic surface. In embodiments, drying the substrate on a plastic surface, such as polystyrene or ABS, can be done using the steps defined in the illustrative embodiments section below (e.g., in the section “Polystyrene or ABS drying substrate for chitosan film”).

[050] Alternatively, if this glossy film is not wanted, the coated side of the paper product can be dried without a contact surface, e.g., face up.

B) Curing step

[051] For step B, which is the curing step, the dried uncured chitosan film is cured by contacting the film with an alcohol solvent.

[052] The purpose of the alcohol solvent is that it dissolves acetic acid without dissolving chitosan. This allows any remaining acid to be easily washed away without changing the morphology of the film (see above for more detail). In theory, any alcohol solvent with such properties can be used. The alcohol solvent can comprise methanol, ethanol, propanol, butanol, or mixtures thereof, preferably methanol.

[053] As mentioned, if the dried uncured chitosan film were to be exposed to an aqueous sodium hydroxide solution (as is done conventionally), water would be reintroduced into the film, which results in swelling of the film and eventual deformation of the film. As is well known to skilled persons, alcohol solvents are generally provided as aqueous solutions and even in their more concentrated forms, typically still contain traces of water. Accordingly, in embodiments of the curing step (step B) of the present invention, the alcohol solution is present in a concentrated aqueous solution that ideally contains very little water. In general, the more water present in the solution comprising the alcohol, the more likely that some of the chitosan in the film will swell and possibly be redissolved, thereby altering the morphology of the film and reducing the water impermeability of the resulting chitosan film. In embodiments, the alcohol solvent is present in an aqueous solution comprising, in v/v, at least about 70% alcohol, at least about 80%, at least about 90 %, at least about 99% or at least about 99.9% alcohol, preferably at least about 99.9% alcohol.

[054] Using the alcohol solvent, the dried uncured chitosan film can be cured using a variety of techniques, as would be understood by the person of skill in the art. For example, the dried uncured chitosan film can be submerged in the alcohol solvent, or it can be washed with alcohol solvent (once or multiple times, preferably multiple times). What matters is that the dried uncured chitosan film is exposed to enough alcohol solvent, and for a sufficient period of time, such that most remaining acid is removed without forming salts that will remain in the chitosan film. As mentioned above, and as shown in Fig. 2, there are a few likely reasons for why using alcohol solvent instead of, for example, aqueous NaOH, results in a more water impermeable film.

C) Drying step

[055] As mentioned, the next step (step C) is to dry the cured chitosan film to produce the chitosan film. This can be done using a variety of techniques, such as by simply letting the chitosan film dry at room temperature, or by heat pressing the chitosan film. This step is performed in order to remove any excess alcohol solvent.

[056] The number of layers in the chitosan film of the present invention can be varied from 1 to 10 depending on the desired thickness of the film and the concentration of the chitosan solution. In embodiments, the dried uncured chitosan film of step A can comprise multiple dried uncured chitosan layers, which are then washed and dried all at once in steps B and C. Alternatively, the entire method can be repeated multiple times (i.e., each of steps A, B, and C are repeated on the same surface), thereby also resulting in a chitosan film comprising multiple layers.

[057] As mentioned, the method of the present invention results in the chitosan film defined in the previous section. As also mentioned, various parameters of the chitosan film (e.g., thickness, porosity) can be controlled by altering various parameters of the method of making the chitosan film.

[058] For example, regarding the thickness of the chitosan film, it can be altered by either increasing the number of layers applied or increasing the chitosan concentration in the chitosan solution, or both.

Advantages of the Invention

[059] In addition to the advantages discussed above, in embodiments, the chitosan film of the present invention can present one or more of the following advantages:

• The film has low water permeability.

• The film is biodegradable.

• The film has appropriate flexibility for substrate coating.

[060] The method of the present invention produces the above-defined chitosan film. In addition, in embodiments, the method of the present invention can present one or more of the following advantages:

• The method is relatively simple and/or cheap and reduces the number of steps to obtain a cured product (for example, in embodiments, the method of the present invention does not require that the curing step be followed by an additional step of washing the film in distilled water; in comparison, in conventional methods where the dried uncured chitosan film is exposed to an aqueous sodium hydroxide solution, the film may need to be washed (potentially multiple times) in distilled water before drying to help remove sodium hydroxide, as well as some of the sodium acetate).

• The method is a green/ecological method since methanol and acetic acid can be reused and recycled during the process.

• The method is especially suitable for any base (including hydroxides) sensitive substrates, where a base would react with or negatively affect the properties of the substrate it comes into contact with.

Alternative methods of producing a chitosan film

[061] There is also provided another method of producing a chitosan film, that is identical to that described in the previous sections, except that the uncured chitosan film provided in step A is not dried. Specifically, a wet chitosan film could be provided and washed with an alcohol solvent which forms an alcogel, where the water and acid are leached out of the wet chitosan/acetic acid film by the alcohol solvent and replaced with the alcohol (or rather substituted by the alcohol). The resulting alcogel can then be dried leaving behind a chitosan film. This can also be done as defined in the section “Alcogel methodology and solubility” in the illustrative embodiments section below. This could potentially be a faster way to cure the chitosan from the aqueous acidic chitosan solution in order to obtain dry chitosan film. [062] There is also provided another method of producing a chitosan film, that is identical to that described in the previous sections, except that instead of performing step B as defined above, the chitosan film is washed with an alcohol NaOH solution, where the alcohol won’t cause the same kind of swelling as water would. Although neutralization would occur, the alcohol solvent would also draw out some of the acid without prior neutralization (although it would likely become neutralized since base is present). This could also be included as an extra step in the method defined in the previous section (performed after alcohol curing step B), defined above), where a lower concentration NaOH alcohol solution would be used.

[063] In embodiments, these alternative methods produce the chitosan film of the present invention (defined in a previous section); in other embodiments, these alternative methods produce a chitosan film that is different than the chitosan film of the present invention (defined in a previous section).

Use of the chitosan film of the invention

[064] In another aspect of the invention, there is provided a composite fiber stock material comprising the chitosan film of the invention coupled with a paper or fibrous substrate.

[065] Indeed, the chitosan film of the invention can be used to coat any paper or fibrous substrate to produce a composite fiber stock material. Some non-limiting examples of paper and fibrous substrates are dry paper, wet paper, pre-made dry paper, pre-made paper, dry fibrous substrates, wet fibrous substrates, fibrous cellulose-based materials, dry cellulose based materials, wet fibrous cellulose based materials, fibrous cellulose based sheets, dry cellulose based sheets, wet fibrous cellulose based sheets, fiber stock, or a combination thereof. The paper and/or fibrous substrate materials can be pre-made before the addition of the chitosan film or can be made in a process wherein the chitosan film is added as a step in the paper making process. The terms paper and fibrous substrate can be used interchangeably to represent the same scope of materials. In some embodiments, the chitosan film is added through at least one layer to a pre-made dry paper, pre-made dry fibrous substrate, or a combination thereof. In other embodiments, at least one layer of chitosan is added to a paper fibrous substrate, or a combination thereof.

[066] For clarity, the chitosan film added to said paper or fibrous substrate can comprise one or multiple layers of chitosan. The chitosan film can be added to one or both sides of the paper or fibrous substrate.

[067] The term “composite fiber stock material”, as used herein, refers to the material made when at least one layer of the chitosan film of the present invention is added or otherwise coupled to any paper or fibrous substrate.

[068] As with the chitosan film according to embodiments of the present invention, the composite fiber stock material can have high water impermeability. Furthermore, the composite fiber stock material can have a high wet stiffness. In general, wet stiffness of the composite fiber stock material increases with number of chitosan layers.

[069] Accordingly, the composite fiber stock material can be used for applications where the composite fiber stock material will be exposed to water or other liquids. For example, in embodiments, beverage cups (e.g., coffee cups) or food trays can be made using the composite fiber stock material of the present invention.

[070] To illustrate the porosity of the composite fiber stock material according to an embodiment of the present invention (in this case, paper coated with the chitosan film according to an embodiment of the present invention) when compared to that of paper on its own, 100x SEM images were obtained (using the procedure defined in the section “Producing the SEM Films” in the illustrative embodiments section below). Specifically, Fig. 5 is an SEM image of a composite fiber stock material according to an embodiment of the present invention (comprising a chitosan film according to an embodiment of the present invention on a paper substrate). Fig. 6 is an SEM image of a cross section of the composite fiber stock material of Fig. 5. Conversely, Fig. 7 is an SEM image of a conventional paper substrate without a chitosan film. The surface morphology of paper at 100x seen in Fig. 7 shows significant detail of cellulose fibers and filler materials. The same paper coated with chitosan and cured with methanol, as seen in Fig. 5, shows a reduced amount of detail of the paper itself, as only the cellulose fibers can be easily seen. The chitosan coated paper surface appears to have reduced porosity. Figure 6 shows that the chitosan coating is maintained at the surface with a minimal penetration depth of about 4pm. The surface shows reduced details and apparent reduced porosity, while on the edge of the cross-section the internal details of the paper can still be seen, indicating that the chitosan did not penetrate the paper deeply.

[071] In embodiments, the composite fiber stock material of the present invention is recyclable. In embodiments, the recycling of the composite fiber stock material of the present invention can be done as defined in the section “Recycling of coated cup” in the illustrative embodiments section below.

[072] In terms of potential other applications, the chitosan film of the present invention can be used as an adhesive for a paper product, such as a cup seam (for the purposes of the application, this will be referred to as the “Jointing Process"). Specifically, once coated with acidic chitosan solution (defined in step A of the method section), paper surfaces can be pressed together and dried. Once dried, the material can also be cured and dried using the steps mentioned above (steps B and C in the method section). This can result in a strong bond. In embodiments, the Jointing Process can also be as defined in the section “Jointing Process” in the illustrative embodiments section below.

[073] Furthermore, paper products coated with the chitosan film of the present invention can also be rewetted with dilute acetic acid or water, then pressed and dried again to form a seam.

[074] It is worth mentioning that the chitosan film of the present invention can be coated onto other materials besides paper or fibrous substrates, such as textiles, wood, plastics, minerals, metals, and glass.

Methods of production of the composite fiber stock material

[075] The composite fiber stock material (e.g., chitosan coated paper cups) can be manufactured using several methods.

[076] For example, the substrate can first be cut and assembled to form a desired shape. The shaped substrate can then be coated with the chitosan film of the present invention using the method of the present invention (e.g., coated with an aqueous acidic chitosan solution and dried (step A above), washed with alcohol (step B above), and then dried again (step C above)). Alternatively, and for example, the substrate can first be coated with the chitosan film of the present invention using the method of the present invention (e.g., coated with an aqueous acidic chitosan solution and dried (step A above), washed with alcohol (step B above), and then dried again (step C above)). The coated substrate (i.e., the composite fiber stock material) can then be cut and assembled into a desired shape (e.g., a cup). [077] In embodiments, when the composite fiber stock material is a paper cup coated with the chitosan film of the present invention, paperboard can be folded over, and the resulting seam can be glued (using, for example, the Jointing Process described in the previous section, if the paperboard is already coated with chitosan). Another method comprises rolling the paperboard into a cylinder, gluing the seam (using, for example, the Jointing Process described in the previous section, if the paperboard is already coated with chitosan), then preparing a bottom to adhere to the bottom of the cylinder. For either method, the cup can be coated post assembly or it can be coated prior to assembly. [078] On a larger scale, the method can comprise a production line where the cups are cut from materials, assembled, coated with a chitosan film using the method of the present invention (e.g., coated with an aqueous acidic chitosan solution, dried, washed with an alcohol, and dried again to be ready for use).

[079] Another option would be where paperboard is coated with acidic chitosan solution in a large roller/dispenser, where the coated paperboard is then passed through a heated system to dry (step A), then through alcohol to cure (step B), followed by a drying section to remove the leftover alcohol (step C).

DEFINITIONS

[080] The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

[081] The terms “comprising”, “having”, “including”, and “containing” are to be construed as open-ended terms (i.e. , meaning “including, but not limited to”) unless otherwise noted.

[082] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All subsets of values within the ranges are also incorporated into the specification as if they were individually recited herein.

[083] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

[084] The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. [085] No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[086] Herein, the term “about” has its ordinary meaning. In embodiments, it may mean plus or minus 10% or plus or minus 5% of the numerical value qualified.

[087] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[088] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings. DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

[089] The present invention is illustrated in further details by the following non-limiting examples.

[090] Specifically, the chitosan film of the invention, as well as the composite fiber stock material of the present invention, was produced using the method of the present invention. Various parameters of the resulting chitosan film and the composite fiber stock material were measured and compared with those of conventional chitosan films.

Materials and procedure:

Chitosan solution

[091] A 2.5 % w/v chitosan in 1% v/v aqueous acetic acid solution was produced by adding 5 g of chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd to 170 mL of distilled water; this mixture was stirred for at 200 RPM for 20 seconds, to temporarily disperse the powder. 30 mL of 5% aqueous acetic acid was added to the mixture, which initiated the dissolution of the chitosan, increasing the viscosity of the mixture. Stirring was continued for 1 hour at 350 RPM to homogenize the solution. Any undissolved particles were removed by centrifugation, 30 minutes at 2000 RPM. [092] A 1 .5 % w/v chitosan in 1% v/v aqueous acetic acid solution was produced by adding 3 g of chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd to 170 mL of distilled water. This mixture was stirred at 200 RPM for 20 seconds, to temporarily disperse the powder. 30 mL of 5% aqueous acetic acid was added to the mixture, which initiated the dissolution of the chitosan, increasing the viscosity of the mixture. Stirring was continued for 1 hour at 350 RPM to homogenize the solution. Any undissolved particles were removed by centrifugation, 30 minutes at 2000 RPM. [093] A 1 .0 % w/v chitosan in 1% v/v aqueous acetic acid solution was produced by adding 2 g of chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd to 170 mL of distilled water. This mixture was stirred at 200 RPM for 20 seconds, to temporarily disperse the powder. 30 mL of 5% aqueous acetic acid was added to the mixture, which initiated the dissolution of the chitosan, increasing the viscosity of the mixture. Stirring was continued for 1 hour at 350 RPM to homogenize the solution. Any undissolved particles were removed by centrifugation, 30 minutes at 2000 RPM. [094] Additional chitosan solutions of the concentrations listed above were also made using the exact same procedure, except fungal chitosan powder from Qingdao Chibio Biotech Co., Ltd was used instead of chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd. The experiments described below were all performed using the chitosan solutions made with chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd; however, most of those experiments were repeated with the chitosan solutions made with the fungal chitosan powder from Qingdao Chibio Biotech Co., Ltd., and the same results were observed.

Sheet Coating Process

[095] A 5 cm x 5 cm Bristol board was coated with the 1 .5% w/v chitosan in 1% v/v aqueous acetic acid solution (obtained using the above procedure) using a brush to evenly spread out the solution. 0.40 g of chitosan solution was added to the substrate, which was then air dried. The coated substrate was then placed in a beaker containing 100 mL of methanol for 6 hours. The coated substrate was then removed from the methanol and air dried, yielding a composite fiber stock material (a cured film coated substrate).

[096] In addition, a 5 cm x 5 cm Bristol board was coated with the 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution (obtained using the above procedure) using a brush to evenly spread out the solution. Three layers of chitosan was added, where for each coating step, 0.40 g of chitosan solution was added to the substrate, which was then air dried. For the three layers (each one containing 0.40 g of chitosan solution), a total of 1 .2 g of chitosan solution was added to the substrate. The coated substrate was then placed in a beaker containing 100 mL of methanol for 6 hours. The coated substrate was then removed from the methanol and air dried, yielding a composite fiber stock material (a methanol-cured chitosan film coated substrate).

Jointing Process

[097] Jointing can be used to adhere two layers of a chitosan coated substrate together using water or aqueous acetic acid or another acid solution.

[098] Two Bristol board substrates, coated with chitosan, prepared as described in the above coating process, were adhered to each other by adding one drop of water to one of the coated surfaces, followed by pressing the coated surfaces together. The water bond was present but weak; the surfaces could be separated.

[099] Two substrates coated with chitosan, prepared as described in the coating process, were adhered to each other by adding one drop of 5% aqueous acetic acid to one of the coated surfaces, followed by pressing the coated surfaces together. The acetic acid bond was strong; separation ripped the substrate. This adhesion can be achieved between two cured coated substrates (including composite fiber stock materials of the present invention, as well as conventional cured coated substrates), between two conventional uncured coated substrates, between a cured coated substrate and a conventional uncured coated substrate, between a cured coated substrate and a conventional uncoated substrate, and a conventional uncured coated substrate and a conventional uncoated substrate.

[0100] In general, increased pressure and heat improved the adhesion between the coated substrates, for example when using a heat press, where the pressure can range between 1-5 bars and the temperature can range between 10-70 °C.

Polystyrene or ABS drying substrate for chitosan film

[0101] 25 g of a 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution, prepared as described above, was poured into a polystyrene dish to cast a chitosan film. The solution was left to dry in the polystyrene dish. The film was washed in 100 ml of methanol for 6 hours. The film was removed from the methanol and air dried, yielding a cured film.

Recycling of coated sheet

[0102] A chitosan coated Bristol sheet, 0.68 g, was blended in 100 mL of water using a 900 W Ninja blender, producing a pulp. The pulp was then concentrated by centrifugation at 4000 RPM for one hour, removing excess water, and then dried at 50°C for 10 hours.

[0103] The resulting pulp sheet was uniform and similarly strong as the non-coated pulped Bristol sheet, thereby demonstrating its ability to be recycled.

Producing the SEM Films

[0104] The films used for SEM analysis (resulting in Figs. 3-7) were produced using the following methods. SEM Image of Methanol cured chitosan film - Fig. 3

[0105] 25 g of a 2.5% w/v chitosan in 1 % v/v aqueous acetic acid solution, prepared as described above, was poured into a polystyrene dish to cast a chitosan film. The solution was left to dry in the polystyrene dish. The film was washed in 100 ml of methanol for 6 hours. The film was removed from the methanol and air dried, yielding a cured film (the SEM image of which is shown in Fig. 3, discussed above).

Comparative Example - SEM Image of conventional Sodium hydroxide cured film - Fig. 4

[0106] 25 g of a 2.5% w/v chitosan in 1 % v/v aqueous acetic acid solution, prepared as described above, was poured into a polystyrene dish to cast a chitosan film. The solution was left to dry in the polystyrene dish. The film was washed in 200 ml of 1 M NaOH (aq) for 2 hours. The film was then placed in a beaker containing 200 mL of distilled water for 1 hour, removed and placed in a beaker containing 200 mL of new distilled water. The second wash water was neutral. The film was removed from the water and air dried, yielding a cured film (the SEM image of which is shown in Fig. 4, discussed above).

SEM of composite fiber stock material (cured chitosan coated paper) - Figs. 5 & 6

[0107] A 5 cm x 5 cm section of printer paper was coated with a 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution, prepared as described above, using a brush to evenly spread out the solution. 0.40 g of chitosan solution was added to the substrate, which was then air dried. The coated substrate was then placed in a beaker containing 100 mL of methanol for 6 hours. The coated substrate was then removed from the methanol and air dried, yielding a cured film coated substrate (the SEM images of which are shown in Figs. 5 and 6, discussed above).

Comparative Example - SEM Image of uncoated conventional printer paper - Fig. 7

[0108] As a comparative example, an SEM image of uncoated conventional printer paper was taken (the SEM image of which is shown in Fig. 7, discussed above).

SEM preparation - Figs. 3-7

[0109] All samples for SEM imaging were sputter coated with platinum prior to imaging. The coated paper was cut to prepare a fresh edge and placed on a 45° stub prior to sputter coating for imaging the cross section of the coated paper (shown in Fig. 6).

Alcogel methodology and solubility

[0110] 25 g of a 2.5% w/v chitosan in 1 % v/v aqueous acetic acid solution, prepared as described above, was poured into a polystyrene dish. The dish containing the chitosan solution was placed into a large beaker containing 200 mL of methanol for 12 hours. The polystyrene dish was removed from the methanol having formed an alcogel. The gel was allowed to air dry which formed a film. The resulting film did not break apart in water.

Characterization methodology

Water contact test - methanol cured chitosan coated cup vs conventional uncoated cup

[0111] To test water impermeability of coated and uncoated Bristol cups, water was added to two cups to determine the presence of leaks and onset time of leaks. Using 100 mL of distilled water for two cups, 3 drops of blue food coloring were added to aid in detecting the appearance of leaks. One cup was a conventional uncoated cup.

[0112] The other cup was a methanol cured chitosan coated cup. This cup was made by pouring a 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution, prepared as described above, into a conventional uncoated cup, rotating said cup until all internal surfaces were coated, pouring out the excess solution and drying upside down at STP. The coating was repeated two times, yielding an uncured chitosan film of two layers total. This cup, now coated in uncured chitosan, was cured by washing in 200 mL of Methanol for 6 hours. This cup, now coated in a Methanol-cured chitosan film, was then removed from the methanol and air dried, yielding a composite fiber stock cup. The cup looked nearly identical to its non-coated self.

[0113] 50 mL of blue water were placed in each of the coated and uncoated cups. The time of the appearance of leaks was measured for both coated and non-coated cups. The uncoated Bristol cup showed water absorption and wall softening in the first 10 mins with leaking in the first hour. The methanol cured chitosan film coated cup did not leak during the first 24 hours, after which the experiment was ended.

[0114] Bristol board, printer paper, pressed pulp and kraft paper were also tested as cup substrates, where the same results were observed for the methanol cured chitosan film coated versions of each of these cup substrates as they were for the above methanol cured chitosan film coated Bristol cup (i.e., they did not leak during at least the first 24 hours, at which point the test was ended). The most porous uncoated cup substrate, pressed pulp, completely emptied within 5 minutes, whereas the same methanol cured chitosan film coated pressed pulp cup substrate did not leak at all (in fact, all the water evaporated after several days without the appearance of leaks).

Water contact test - sodium hydroxide cured vs methanol cured chitosan coated cups

[0115] To test the water impermeability of each of a sodium hydroxide cured chitosan coated Bristol cup and a methanol cured chitosan coated Bristol cup, water was added to two cups to determine the presence of leaks and onset time of leaks. Using 100 mL of distilled water for two cups, 3 drops of blue food coloring were added to aid in detecting the appearance of leaks. Each cup was coated by pouring a 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution, prepared as described above, into each cup, rotating the cup until all internal surfaces were coated, pouring out the excess solution and drying upside down at STP.

[0116] One Bristol cup coated in uncured chitosan was cured by washing in 200 mL of Methanol for 6 hours. This cup coated in a Methanol-cured chitosan film was then removed from the methanol and air dried, yielding a composite fiber stock cup. The cup looked nearly identical to its non-coated self.

[0117] The other Bristol cup coated in uncured chitosan was cured by washing in 200 ml of 1 M NaOH(aq) for 2 hours. This cup coated in a NaOH(aq)-cured chitosan film was then placed in a beaker containing 200 mL of distilled water for 1 hour, removed, and then placed in another beaker containing 200 mL of distilled water. The pH of the wash water of the second beaker was neutral after the wash. The cup was removed from the water and air dried, yielding a conventional composite fiber stock cup. The cup was partially disintegrated and shriveled.

[0118] 50 mL of blue water were placed in each of the coated cups. The time of the appearance of leaks was measured for both cups. The NaOH(aq)-cured chitosan film cup started leaking immediately, likely due to the breakdown of the substrate and weaker film properties. Conversely, the methanol cured chitosan film coated cup did not leak during the first 24 hours, after which the experiment was ended.

Water Drop Test

[0119] One drop of water was added to each of two cured chitosan coated Bristol board sheets, where one sheet was coated in a chitosan film cured with MeOH and the other sheet was coated in a chitosan film neutralized with NaOH to compare water permeability of the coating processes.

[0120] To prepare the cured chitosan coated Bristol board sheets, two 5 cm x 5 cm Bristol board sheets were coated with a 2.5% w/v chitosan in 1% v/v aqueous acetic acid solution (obtained using the procedure described above) using a brush to evenly spread out the solution. Three layers of chitosan were added, where, for each coating step, 0.40 g of chitosan solution was added to the Bristol board sheet substrate, which was then air dried. For the three layers, a total of 1.2 g of chitosan solution was added to the substrate.

[0121] One coated Bristol board sheet was cured by placing it in a beaker containing 100 mL of methanol for 6 hours. This coated Bristol board sheet substrate was then removed from the methanol and air dried, yielding a composite fiber stock material (a methanol cured chitosan film coated Bristol board sheet).

[0122] The other Bristol board sheet was cured by washing in 100 ml of 1 M NaOH(aq) for 1 hour. This Bristol board sheet coated in a NaOH(aq)-cured chitosan film was then placed in a beaker containing 200 mL of distilled water for 1 hour, removed, and then placed in another beaker containing 200 mL of distilled water. The pH of the wash water of the second beaker was neutral after the wash. The Bristol board sheet was removed from the water and air dried, yielding an NaOH-cured chitosan coated Bristol board sheet.

[0123] As mentioned, one drop of water was added to each of these two cured chitosan coated Bristol board sheets. After 15 minutes, the excess water from the drop was removed from each chitosan coated Bristol board sheet. The NaOH-cured chitosan coated Bristol board sheet developed a dimple and began tearing when scratched with a lead pencil. The MeOH cured chitosan coated Bristol board sheet did not show a dimple and the lead pencil was still able to write over the chitosan film of the composite fiber stock without tearing.

Viscosity

[0124] Viscosity was measured with a spindle viscometer using 100 mL of various chitosan solutions prepared. For example: A 1 .5 % w/v chitosan in 1 % v/v aqueous acetic acid solution was produced by adding 3 g of chitosan powder from Shaanxi Pioneeer Biotech Co., Ltd to 170 mL of distilled water. This mixture was stirred at 200 RPM for 20 seconds, to temporarily disperse the powder. 30 mL of 5% aqueous acetic acid was added to the mixture, which initiated the dissolution of the chitosan, increasing the viscosity of the mixture. Stirring was continued for 1 hour at 350 RPM to homogenize the solution. Any undissolved particles were removed by centrifugation. For other solution concentrations, the quantity of chitosan was adjusted as required to prepare the corresponding percentage.

[0125] At a shear rate of 5.6 Hz, the viscosities of the chitosan solutions ranging from 0.1 % to 4% w/v were as follows.

[0126] From the above results, it is clear that the viscosity increases with higher polymer concentration. The viscosity also increases in a non-linear fashion where the difference in viscosity is greater at higher concentrations, such as comparing 3% and 4% (where viscosity goes from 1067 mPa«s to 8013 mPa«s) vs comparing 2% and 3 % (where viscosity goes from 167 mPa«s to 1067 mPa«s).

[0127] The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.