Coating composition

Abstract

The present invention provides a coating composition comprising a homogenized dispersion of regenerated cellulose in water wherein the regenerated cellulose comprises both amorphous and crystalline cellulose.

Description

[0001] The present invention relates to a cellulose-based coating composition and a method of making the composition.

[0002] Cellulose-based coating compositions are known in the art and are used as barrier coatings, or for increasing the hydrophobicity or scratch resistance of articles. These coatings are compostable and biodegradable and can thus be used to produce eco-friendly articles.

[0003] The formation of cellulose-based coatings generally involves applying a cellulose dispersion to a surface and then drying the dispersion to produce a coating. Conventionally, the cellulose in the dispersion is microcrystalline cellulose. For example, CA668443A discloses boiling a cellulose material in hydrochloric acid to produce crystalline cellulose, which is then washed and dispersed in water to form a coating composition. Similarly, US6541627 discloses using acid hydrolysis to increase the crystallinity of a cellulose-based coating composition.

[0004] Crystalline cellulose is considered important for optical properties such as transparency. Thus, methods of producing cellulose coating materials generally involve steps such as acid hydrolysis to remove amorphous cellulose, leaving only crystalline cellulose in the coating. However, this step can be expensive and complex and is wasteful as it removes a portion of the cellulose.

[0005] Cellulose-based coatings in the art also often include non-biodegradable components or use cellulose derivatives such as nitrocellulose, thereby reducing the biodegradability of the coating itself and increasing the complexity of the manufacturing process.

[0006] Dusting is also a problem with coatings in the art and thus coatings with improved adhesion are also desired.

[0007] Thus, there is a desire to produce a cellulose-based coating composition that is simpler and cheaper to manufacture while still maintaining the necessary optical properties and adhesion, and is biodegradable.

[0008] According to a first aspect of the present invention, there is provided a coating composition comprising a homogenized dispersion of regenerated cellulose in water, wherein the regenerated cellulose comprises both amorphous and crystalline cellulose.

[0009] The regenerated cellulose in the coating composition can be predominantly amorphous cellulose. Thus, the regenerated cellulose can be more than 50% amorphous cellulose, preferably more than 75% amorphous cellulose, and even more preferably more than 90% amorphous cellulose.

[0010] Surprisingly, it has been found that the homogenized dispersion of the regenerated cellulose in water exhibits good optical properties, such as transparency, without the need to remove the amorphous cellulose regions as in the coatings of the prior art. In addition, the coating of the present invention has good adhesion to the surface and is completely biodegradable. Thus, the present invention provides a simpler and cheaper cellulose coating with good optical properties and adhesion.

[0011] Thus, the coating composition of the present invention has not been subjected to acid hydrolysis treatment.

[0012] Regenerated cellulose can be regenerated from an aqueous solution of alkali cellulose. This regeneration method is well known in the art and involves combining the aqueous solution of alkali cellulose with an acid to regenerate the cellulose. Using a cellulose material regenerated from an aqueous solution of alkali cellulose, even with amorphous regions, is considered to contribute to increasing the transparency of the coating, thus eliminating the need for an acid hydrolysis step.

[0013] The aqueous solution of alkali cellulose can be produced by dissolving a cellulose-containing material in an alkali. There are various methods known in the art for producing an aqueous solution of alkali cellulose, all of which can be used in the present invention.

[0014] The alkali can be a hydroxide, preferably an alkali metal hydroxide, and more preferably sodium hydroxide. The concentration of the alkali can be between 5% w / w and 25% w / w, or between 10% w / w and 25% w / w. The concentration of the alkali in the aqueous solution of alkali cellulose can be between 2% w / w and 15% w / w.

[0015] The dissolution of cellulose in the alkali can include homogenization to facilitate dissolution, preferably high-pressure homogenization. High-pressure homogenization as used herein refers to homogenization occurring at a pressure of 100 bar or higher. During the dissolution of cellulose in the alkali, more than one homogenization step can be used.

[0016] High-pressure homogenization can occur at a temperature of 0 °C or higher to ensure dissolution.

[0017] Thus, the dispersion of the regenerated cellulose is preferably subjected to at least two homogenizations, the first to facilitate the dissolution of cellulose in the alkali and the second after the regenerated cellulose is dispersed in water. This is considered to sufficiently reduce the particle size to improve the optical properties. There can be more than one homogenization step at each point of the process. There can be two or more homogenization steps to facilitate the dissolution of cellulose in the alkali and one or two homogenization steps after the cellulose has been regenerated.

[0018] Alternatively, any other known method can be used to dissolve cellulose in the alkali. Alternatively, cellulose can be dissolved in any other known cellulose solvent, such as ionic liquids, NMMO, and deep eutectic solvents.

[0019] The cellulose particles preferably have an average radius of less than 1 μm, more preferably less than 0.75 μm. The largest particles may have a radius of less than 2 μm, preferably less than 1.5 μm.

[0020] The aqueous solution of alkali cellulose can be regenerated in the liquid phase. This can be achieved by carrying out the regeneration process under agitation (such as stirring). The regeneration process can involve combining the aqueous solution of alkali cellulose with an excess of acid. The aqueous solution of alkali cellulose can be added to the acid, or the acid can be added to the aqueous solution of alkali cellulose.

[0021] It is well known to dissolve cellulose in alkali to allow further processing, such as to produce regenerated cellulose products in the form of films, fibers or shaped articles. The regeneration of alkali cellulose solutions is also well known in the art, for example by extruding the solution into acid. However, this produces cellulose films with the minimum possible thickness, which is too high for use as a coating in many applications. In contrast, using liquid phase regeneration, in which cellulose is regenerated under agitation, produces a dispersion of regenerated cellulose particles. This dispersion can then be used to produce a coating that is much thinner than the regenerated cellulose films known in the art.

[0022] Once the cellulose has been regenerated, the cellulose can be washed to obtain a dispersion of cellulose in water. The dispersion of cellulose in water can be substantially free of salts produced by the acid.

[0023] Washing can include separating the regenerated cellulose from the acid, washing the regenerated cellulose with water, and resuspending the regenerated cellulose in water to form a dispersion. The separation can be carried out by any conventional means, including filtration, centrifugation or using a vacuum. Alternatively, washing can include continuous washing to remove the acid and produce a dispersion of regenerated cellulose in water.

[0024] The dispersion of cellulose in water can then be homogenized, preferably by high-pressure homogenization. The inventors have found that homogenization of the dispersion of regenerated cellulose produces cellulose particles that are small enough and the resulting coating composition has good optical properties without requiring additional processing steps (such as acid hydrolysis).

[0025] Therefore, the homogenized dispersion of the regenerated cellulose in water can be obtained by a method comprising the following steps:

[0026] (a) dissolving a cellulose-containing material in an aqueous alkali solution to form an aqueous solution of alkali cellulose;

[0027] (b) regenerating the cellulose in the liquid phase by combining the aqueous solution of alkali cellulose with an excess of acid under agitation;

[0028] (c) Wash the resulting regenerated cellulose to obtain a cellulose dispersion in water that is substantially free of salts produced by said acid; and

[0029] (d) Homogenize the cellulose dispersion in water.

[0030] These steps, when combined, can have any of the features discussed above.

[0031] The cellulose dispersion in water can include from 1% to 10% w / w of cellulose, preferably from 2% to 7% w / w of cellulose.

[0032] The cellulose-containing material dissolved in an alkali to produce an aqueous solution of alkali cellulose can be at least partially purified to remove some non-cellulose components compared to the starting material. The at least partially purified cellulose-containing material can be produced using the following steps:

[0033] (a) Neutralize the alkaline cellulose-containing precursor material with an acid and obtain a neutralized solid cellulose-containing material;

[0034] (b) Mix the neutralized solid cellulose-containing material with a bleaching agent to produce a mixture; and

[0035] (c) Separate the solid cellulose-containing product from the mixture.

[0036] The above method can further include step (d): dissolving the solid purified cellulose-containing product in an aqueous alkali solution to produce an aqueous solution of alkali cellulose. One or more steps of the method can be carried out at a temperature between about 2 and about 90 °C and preferably between about 20 and about 60 °C.

[0037] The acid can include a weak acid, which can be a carboxylic acid such as acetic acid. The concentration of the acid can be from about 1% to about 20% w / w. The polysaccharide-containing material can be placed in the acid for between about 10 minutes and about 3 hours, preferably between about 0.5 and about 1 hour. This can ensure that all solid materials have been neutralized. The pH of the resulting neutralized solid polysaccharide-containing material can be between 6 and 8, preferably about 7.

[0038] The bleaching agent can include a chlorine-containing bleaching agent. For example, the bleaching agent can include sodium hypochlorite. The bleaching agent can include a chlorine-free bleaching agent. For example, the bleaching agent can include hydrogen peroxide. The concentration of the bleaching agent can be between 0.1% and 10% w / w, preferably between 0.1% and 2% w / w.

[0039] One or more washing steps using hot water and / or cold water can also be included. The solid cellulose-containing product can be separated from the mixture by any conventional means (including filtration, vacuum or centrifugation).

[0040] An alkaline cellulose-containing precursor material can be produced by combining a cellulose-containing precursor material with an alkali solution to produce an alkali mixture. The alkali mixture can be stirred. Then, the solid alkaline cellulose-containing precursor material can be separated from the alkali mixture and used in the above steps. The alkali solution can include hydroxides. The alkali solution can be sodium hydroxide. The hydroxide can be present in the alkali solution at a concentration of about 0.1% to about 30% w / w by weight of the alkali solution.

[0041] It has been found that these pretreatment steps improve the dissolution of the cellulose-containing material in the alkali, thereby producing a stable aqueous solution of alkali cellulose, which can then be used to produce a dispersion of regenerated cellulose as discussed above.

[0042] The coating composition can further include one or more additives selected from the following: wax, biopolymer, pigment, active ingredient, reference particle, or dopant for stain absorption. The total amount of additives in the coating composition is preferably less than 40% by weight, with each additive present preferably at less than 20% by weight.

[0043] The wax is preferably a bio-based wax, such as carnauba wax and / or candelilla wax. The wax is preferably biodegradable. The wax can be included in the coating composition to improve hydrophobicity, gloss, and scratch resistance.

[0044] The biopolymer is a biodegradable natural polymer. For example, these include PHA and bioplastics. The biopolymer can be added to the coating composition to alter the properties of the resulting coating (depending on the properties of the biopolymer itself). For example, a biopolymer can be added to produce a seal.

[0045] A pigment or other colorant can be added to the coating composition to change the color of the resulting coating.

[0046] The active ingredient is a component that exhibits a chemical or biological effect on the surrounding environment. For example, the active ingredient can be zeolite, which can absorb ethylene from the environment. Alternatively, the active ingredient can be an antimicrobial component, such as a component having antibacterial or antiviral activity. Thus, these components can be used to make the resulting coating provide a chemical or biological effect on the surrounding environment.

[0047] The coating composition can also include one or more reference particles. These can be inert particles and serve as reference points, typically visual reference points for size or staining comparison. The reference particles can include PMMA or silica particles, cellulose fiber bubbles, and / or proteins. The reference particles can have a known size, orientation, and / or concentration within the cellulose membrane. The reference particles can also be randomly distributed to produce a unique pattern, which can be used to identify and / or track the coated article.

[0048] The coating composition of the present invention can also be stained, for example, using conventional histological stains, including hematoxylin and eosin (H&E), or diaminobenzidine (DAB) and horseradish peroxidase (hrp). Thus, the coating composition can include dopants that affect the staining characteristics of the coating composition, such as chitosan, gelatin, and / or keratin.

[0049] For example, the variability of staining decreases with an increase in the chitosan level in the coating. The coating composition can include 1% to 10%, preferably 3% to 10%, and most preferably 5% to 10% of chitosan. In addition, both keratin and gelatin have been found to increase stain absorption. The amount of dopant added can be such that the stain absorption of the coating matches that of the tissue sample of interest. Thus, the coating can be customized for the tissue sample of interest.

[0050] If the coating composition of the present invention is used in applications related to histological staining, it can contain both reference particles and dopants. The reference particles can also be used to visually represent the tissue itself to assist the pathologist in examining the sample.

[0051] According to a second aspect of the present invention, there is provided a coating comprising the cellulose coating composition discussed above. The coating can be formed by applying the coating composition or a composition comprising the coating composition to a surface and then drying the coating. This can be carried out in any conventional manner, such as by gravure printing, spraying, or printing coating.

[0052] The coating can have a thickness between 0.1 and 50 microns, preferably between 0.5 and 8 microns. Thus, the coating can be significantly thinner than the films produced from regenerated cellulose known in the art.

[0053] The coating can have a coat weight between 0.1 and 20 gsm, preferably between 1 and 10 gsm. Thus, this produces a thin coating on the surface, particularly compared to the cellulose films known in the art.

[0054] The coating can have a wide-angle haze of less than 75%, preferably less than 70%. Thus, the coating formed from the coating composition of the present invention has good optical properties.

[0055] The coating can have a water vapor permeability between 1000 and 1250 g / m 2 / 24 hours, preferably between 1050 and 1200 g / m 2 / 24 hours. The water vapor permeability can be changed outside these ranges by including additives in the coating in a conventional manner.

[0056] When dried at a temperature above room temperature, the coating can have good adhesion, where less than 10%, preferably less than 5% by area of the coating is removed in a tape test, and little or no visible coating is removed regarding a scratch test. The tape test involves adhering scotch tape 600 to the coating such that the end of the tape extends beyond the coating. One end of the tape is held at a 180° angle and quickly peeled off, and the area of the coating that is peeled off is calculated.

[0057] The coating can comprise more than 60% w / w of a cellulose coating composition, preferably more than 80% w / w. The additives mentioned above can constitute the remainder of the coating. In this embodiment, a cellulose coating composition, optionally with some additional additives, is used to produce the coating.

[0058] Alternatively, the coating can comprise less than 50% w / w of the cellulose coating composition discussed above, preferably less than 20% w / w. The coating can further comprise an additional coating composition, optionally with additional additives. In this embodiment, the cellulose coating composition is used as an additive within another coating composition. This can be used to impart advantageous properties, such as print receptivity or adhesion, to known coating compositions using the cellulose coating composition described above.

[0059] The coating can be dried at room temperature (above 20 °C) or around room temperature, for example, between 15 °C and 25 °C. It has been found that drying the coating composition at room temperature produces a coating containing a higher amount of amorphous cellulose. Such a coating is softer and swells when in contact with water.

[0060] Alternatively, the coating can be dried at a temperature above room temperature (e.g., above 25 °C), preferably above 35 °C and more preferably above 50 °C or at 60 °C or higher.

[0061] Surprisingly, it has been found that drying the coating composition at a temperature above room temperature produces a hard coating with good adhesion to the substrate. Without wishing to be bound by theory, it is believed that the heat of the drying process effectively crosslinks the cellulose particles via hydroxyl functionality, producing a crystalline structure. Thus, the particles can form hydrogen bonds with both other cellulose particles and with the substrate, resulting in a hard coating with good adhesion to the substrate (especially a glass substrate).

[0062] A coating that has been dried at an elevated temperature can have a crystallinity higher than 60%, preferably higher than 75%. This is much greater than the usual amount of crystallinity that can be around 30% seen in conventional cellulose coatings. It is also much greater than the amount of crystallinity seen in coatings that have been dried at room temperature or around room temperature (which are mainly amorphous).

[0063] The regenerated cellulose can be mainly cellulose II. The regenerated cellulose can be more than 75% cellulose II, preferably more than 85% cellulose II.

[0064] According to a third aspect of the present invention, there is provided a coated article comprising the coating discussed above. The coating provides a thin, biodegradable coating on the article, having good adhesion and optical properties, as well as good water vapor permeability.

[0065] The coating can be applied using any conventional means (including gravure printing, spraying or printing coating). The coating can have any of the characteristics discussed above.

[0066] The coated article can be coated unilaterally or bilaterally. In other words, a planar article (such as a film) can be coated on one or both sides.

[0067] The coated article can be any article, such as a film, such as a cellulose-based film.

[0068] The coated article can be a histological quality control device, such as a reference slide. As discussed above, the coating composition of the present invention can be stained using conventional histological stains. Thus, the coating composition can be used to produce a quality control device for histological staining procedures, which allows standardizing the staining of different tissue samples.

[0069] In particular, the quality control device can be stained together with a tissue sample. Since the staining characteristics of the cellulose coating are known, staining variations in the tissue due to slight changes in time, component concentration, the camera or light intensity used can be identified and standardized based on the staining seen in the cellulose coating. Thus, the quality control device can be used to correct image variations during histopathological staining and imaging processes.

[0070] The quality control device of the present invention is an improvement over prior art devices, which use cellulose membranes that are prone to changing dimensions with different hydration states. No change in dimensions is seen in the cellulose coating of the present invention, and no additional adhesion means are required to attach it to a surface (such as a slide) to produce the quality control device.

[0071] The coating may not cover the entire surface of the quality control device. Then, a tissue sample can be positioned on the surface of the quality control device and stained simultaneously with the coating, thus ensuring that the tissue sample and the coating are subjected to the same staining conditions. This is an improvement over prior art arrangements because the coating of the present invention is thinner than the membranes conventionally used. This allows a coverslip to be positioned over the tissue sample and the coating, which is not possible when using cellulose membranes because the membrane is thicker than the tissue sample and thus holds the coverslip away from the tissue sample.

[0072] The coated article may include additional layers such as an adhesive layer or a barrier coating. The barrier coating may include ethylene acrylic acid, polyvinylidene chloride, acrylic or any other barrier coating material.

[0073] According to a fourth aspect of the present invention, there is provided a method of forming a coating composition, the method comprising the step of homogenizing a dispersion of regenerated cellulose in water. The method of the fourth aspect of the present invention may have any of the features of the previous aspects.

[0074] Thus, the homogenization may be high-pressure homogenization. The regenerated cellulose may be regenerated from an aqueous solution of alkali cellulose, preferably in the liquid phase. The regenerated cellulose may then be washed to remove salts produced by the acid.

[0075] The method of forming the coating composition may comprise the steps of:

[0076] (a) dissolving a cellulose-containing material in an alkali to form an aqueous solution of alkali cellulose;

[0077] (b) regenerating the cellulose in the liquid phase by combining the aqueous solution of alkali cellulose with an excess of acid under agitation;

[0078] (c) washing the resulting regenerated cellulose to obtain a dispersion of cellulose in water substantially free of salts produced by the acid; and

[0079] (d) homogenizing the dispersion of cellulose in water to produce a coating composition.

[0080] Any aspect disclosed herein may include the features of any previous aspect.

[0081] The present invention will be further discussed in the examples and figures outlined below, which are not intended to limit the scope of protection.

[0082] Figure 1 Illustrated is an X-ray diffraction (XRD) image of a coating material according to the present invention dried at temperatures above 100 °C; and

[0083] Figure 2 Illustrated is an XRD image of a coating composition according to the present invention that is only slightly dried.

[0084] Example 1

[0085] Wood pulp was dissolved in sodium hydroxide using high-pressure homogenization to form an aqueous solution of sodium hydroxide cellulose. Then, with continuous agitation, the solution was added to an excess of hydrochloric acid. This allowed the cellulose to regenerate in the liquid phase, producing a dispersion of regenerated cellulose in the acid.

[0086] The regenerated cellulose is filtered and washed with water, and then resuspended in water to produce an aqueous dispersion free of salts produced by the acid. Then, the aqueous dispersion is subjected to high-pressure homogenization to form a coating composition.

[0087] The coating composition is applied to a regenerated cellulose film (Natureflex TM NP), applied on one or both sides of the film, where the coating weight is shown in Table 1 below.

[0088] Table 1

[0089] Coating weight (gsm) Single-sided 2.14 Double-sided 3.34 / 2 = 1.67 per side

[0090] The coating is dried at a temperature above 60 °C. Then the water vapor permeability and the wide-angle haze are measured. The wide-angle haze is measured at 2.5° and according to ASTM D1003.

[0091] To measure the WVP, a sufficient amount of silica gel is placed in a cup such that the bottom is covered to a depth of approximately 0.8 cm. Then a circular film sample with a diameter of approximately 10 cm is cut. A thin layer of paraffin wax is wiped onto the cup surface. The circular sample is placed such that its edge contacts the surface and is pressed down, ensuring no creases. The coated surface of the film is positioned towards the outside of the cup. Then the cup is placed in an oven at the desired temperature and relative humidity (RH). After 24 hours, the cup is removed and immediately weighed. The cup is returned to the oven and weighed every 24 hours until a constant increase is obtained (usually after 72 hours).

[0092] The results are shown in Tables 2 and 3 below.

[0093] Table 2

[0094] Temperate water vapor permeability (g / m2 / 24hr) Single-sided 1116.25 Double-sided 1053.04

[0095] Table 3

[0096] Wide-angle haze (%) Single-sided 63.4 Double-sided 86.6

[0097] Example 2

[0098] A solution according to the present invention is prepared, having a solids content of 2%. Two glass plates measured to be 11.5 cm in length and 7.5 cm in width are placed on a drawdown bed, and the coating of the solution is applied to each plate using a standard red KBar (wet film deposition 12 μm), with a coating weight of 6.75 g / m 2 . Then the glass plates are removed from the drawdown bed and dried in an oven at 60 °C for 45 minutes. After the coating has completely dried, the plates are removed from the oven and allowed to cool to room temperature, which takes approximately 5 minutes.

[0099] Place a glass plate on a firm, flat surface and scratch the coating by continuously moving a penny along the entire length of the glass plate. No visible removal of the coating was identified.

[0100] Place another glass plate on a firm, flat surface and firmly adhere a piece of Scotch tape 600 over the entire width of the coated plate, with the ends of the tape extending beyond the width of the plate. Then hold one end of the tape at a 180° angle and quickly tear it off. Then calculate the area of the coating that was torn off and compare it to the total area of the tape that was adhered to the coated plate. Less than 5% by area of the coating was torn off, demonstrating that the coating exhibits good adhesion.

[0101] Example 3

[0102] Prepare a solution according to the present invention with a solids content of 2%. Place two glass plates measured 11.5 cm in length and 7.5 cm in width on a drawdown bed and apply the coating of the solution to each plate using a standard red KBar (wet film deposition 12 μm), with a coating weight of 6.75 g / m 2 . Then remove the glass plates from the drawdown bed and dry them at room temperature for 24 hours.

[0103] Place a glass plate on a firm, flat surface and scratch the coating by continuously moving a penny along the entire length of the glass plate. A portion of the coating was removed, exposing the underlying glass. This demonstrates that the coating of the present invention is softer when dried at room temperature compared to when dried at an elevated temperature, where the adhesion is lower.

[0104] Place another glass plate on a firm, flat surface and firmly adhere a piece of Scotch tape 600 over the entire width of the coated plate, with the ends of the tape extending beyond the width of the plate. Then hold one end of the tape at a 180° angle and quickly tear it off. Then calculate the area of the coating that was torn off and compare it to the total area of the tape that was adhered to the coated plate. More than 5% by area of the coating was torn off, demonstrating that the coating exhibits less good adhesion than the coating dried at an elevated temperature.

[0105] Example 4

[0106] Figure 1Shows the X-ray diffraction (XRD) image of the coating material according to the present invention dried at temperatures above 100 °C. In XRD, cellulose I is characterized by two peaks in the middle of the scan and broad peaks further along the scan, while cellulose II is characterized by one peak near the central beam at the start of the scan and two peaks further along the scan.

[0107] Figure 1 Shows that the coating of the present invention is almost entirely cellulose II. From Figure 1 the line scan, it is found that the coating material has a crystallinity of approximately 80%.

[0108] In contrast, Figure 2 shows the XRD image of the coating composition according to the present invention that is only slightly dried. This shows that the coating composition has little diffraction, indicating a very low level of crystallinity.

[0109] Therefore, Figure 1 and Figure 2 show that the coating composition according to the present invention is mainly amorphous before drying, but then mainly crystalline cellulose II after drying at temperatures above room temperature.

Claims

1. A coating composition, the coating composition comprising a homogenized dispersion of regenerated cellulose in water, wherein, The regenerated cellulose includes both amorphous and crystalline cellulose.

2. The coating composition according to claim 1, wherein, The regenerated cellulose is regenerated from an aqueous solution of alkali cellulose in a liquid phase.

3. The coating composition according to claim 2, wherein, The homogeneous dispersion of the regenerated cellulose in water is obtained by a method comprising the following steps: (a) Dissolving a cellulose-containing material in an aqueous alkali solution to form an aqueous solution of alkali cellulose; (b) Regenerating the cellulose in the liquid phase by combining the aqueous solution of alkali cellulose with an excess of acid under agitation; (c) Washing the resulting regenerated cellulose to obtain a dispersion of cellulose in water substantially free of salts produced by the acid; and (d) Homogenizing the dispersion of cellulose in water.

4. The coating composition according to claim 3, wherein, The aqueous alkali solution in step (a) is a hydroxide, preferably an alkali metal hydroxide, and more preferably sodium hydroxide.

5. The coating composition according to claim 3 or claim 4, wherein Step (a) includes homogenization to facilitate dissolution, preferably high-pressure homogenization.

6. The coating composition according to any one of claims 3 to 5, wherein, Step (b) involves adding the aqueous solution of alkali cellulose to the acid, or adding the acid to the aqueous solution of alkali cellulose.

7. The coating composition according to any one of claims 3 to 6, wherein Step (c) includes separating the regenerated cellulose from the acid, washing and resuspending it in water to form a dispersion in water, or alternatively, step (c) includes continuous washing to remove the acid and produce a dispersion in water.

8. The coating composition according to any one of claims 1 to 7, wherein, The dispersion of cellulose in water has been subjected to high-pressure homogenization.

9. The coating composition according to any one of claims 1 to 8, wherein, The dispersion of cellulose in water comprises between 1% and 10% w / w of cellulose, preferably between 2% and 7% w / w of cellulose.

10. The coating composition according to any one of claims 2 to 9, wherein, The aqueous solution of alkali cellulose is produced using the following steps: (a) Neutralizing an alkaline cellulose-containing precursor material with an acid and obtaining a neutralized solid cellulose-containing material; (b) Mixing the neutralized solid cellulose-containing material with a bleaching agent to produce a mixture; (c) Separating a solid purified cellulose-containing product from the mixture; and (d) Dissolving the solid purified cellulose-containing product in an aqueous alkali solution.

11. The coating composition according to any one of claims 1 to 10, further comprising one or more additives selected from the following: wax, biopolymer, pigment, active ingredient, reference particle or dopant for dye absorption.

12. The coating composition according to any one of claims 1 to 11, wherein, The cellulose particles have an average radius of less than 1 micron, more preferably less than 0.75 micron.

13. A coating, the coating comprising the cellulose coating composition according to any one of claims 1 to 12.

14. The coating according to claim 13, wherein, The coating has a thickness between 0.1 and 50 microns, preferably between 0.5 and 8 microns.

15. The coating according to claim 13 or claim 14, having a coating weight between 0.1 and 20 gsm, preferably between 1 and 10 gsm.

16. The coating according to any one of claims 13 to 15, having a wide-angle haze between 50% and 75%, preferably between 55% and 70%.

17. The coating according to any one of claims 13 to 16 has a water vapor permeability of 1000 to 1250 g / m 2 / 24 hours, preferably 1050 to 1200 g / m 2 / 24 hours.

18. The coating according to any one of claims 13 to 17, wherein, The coating comprises less than 50% w / w of the cellulose coating composition, or alternatively, the coating comprises more than 60% w / w of the cellulose coating composition.

19. The coating according to any one of claims 13 to 18, wherein, The coating is dried at a temperature higher than 25°C, preferably higher than 35°C and more preferably higher than 50°C.

20. The coating according to claim 19 has a crystallinity higher than 60%, preferably higher than 75%.

21. A coated article, the coated article comprising a coating according to any one of claims 13 to 20.

22. The coated article according to claim 21, wherein, The coating is applied by gravure printing, spraying or printing coating.

23. The coated article according to claim 21 or claim 22, wherein, The article is a film or a histological quality control device.

24. A method of forming a coating composition, the method comprising the steps of: (a) dissolving a cellulose-containing material in an alkali to form an aqueous solution of alkali cellulose; (b) regenerating the cellulose in the liquid phase by combining the aqueous solution of alkali cellulose with an excess of acid under agitation; (c) washing the resulting regenerated cellulose to obtain a cellulose dispersion in water substantially free of salts produced by the acid; and (d) homogenizing the cellulose dispersion in water to produce a coating composition.

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