Cellulose coating composition

Abstract

The present invention relates to a coating composition and an opacifier, and methods of preparing the coating composition and opacifier, as well as coatings and methods of forming coatings using the coating composition. The coating composition comprises an opacifier, the opacifier comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to 9 µm; and a carrier liquid; wherein the cellulose microparticles are present in the coating composition at an amount of from 1 to 40 wt.% and the surfactant is present in the coating composition at an amount of from 0.5 to 6 wt.% based on the total mass of the coating composition and wherein the composition comprises less than 5% by weight of a metal oxide.

Description

[0001] Cellulose Coating Composition

[0002] Field of the Invention

[0003] The present invention relates to a coating composition and an opacifier, and methods of preparing the coating composition and opacifier, as well as coatings and methods of forming coatings using the coating composition.

[0004] Background

[0005] Organic coatings are widely used as common, practical, and cost-effective methods to provide a barrier between substrates and environment, such as metallic substrate and the environment. Coatings ideally have high resistance to ionic movement and good adhesion to substrates, such as metals or other surfaces.1Polymer coating systems have found extensive uses in automotive, architectural, packaging and marine applications.2-6The use of renewable ingredients can be highly desirable to improve the sustainability and life cycle assessment (LCA) of the coating and production process.

[0006] Traditionally, to produce artificial white coating materials industry has relied on the use of high-refractive-index inorganic materials such as TiO2( ~ 2.6), ZnS (n « 2.4) and ZnO ( ~ 2.0). However, due to health concerns, the most common whitening agent, TiO2, has recently been banned as a food additive by the European Union. In 2022, the European Medicines Agency also stressed the ‘critical importance’ of finding benign replacements for TiO2in medicines (Commission Regulation (EU) 2022 / 63 of 14 January 2022 amending Annexes II and III to Regulation (EC) No 1333 / 2008 of the European Parliament and of the Council as regards the food additive titanium dioxide (E 171) (Text with EEA relevance) 1-5 (European Parliament and Council, 2022).

[0007] A typical paint composition consists of pigments (such as the above-mentioned titanium dioxide), binders (to form a cohesive film), solvents, for instance water (to adjust viscosity and facilitate application), and optionally additives (for stability, drying control, or special properties). Porosity in the paint film often develops as the solvent or liquid component evaporates during drying. This process leaves behind voids or micro-pores in the film, influenced by the packing density of the pigment particles and the binder content. When metal oxides, like titanium dioxide, are used as opacifying pigments, the pigment volume concentration is closely related to the porosity of the paint mixture. Below the critical pigment volume concentration (CPVC), whiteness is governed by the porosity of the paint. However, above the CPVC, whiteness is dominated by the scattering efficiency of the pigment.

[0008] Porosity is crucial for light scattering because it enhances the refractive index contrast between the solid components (e.g., TiO2) and the air-filled voids or pores. Mie scattering theory predicts that the relative intensity of scattered light as a function of particle size, angle of observation and wavelength and polarization of the incidnet beam. Therefore, when desigining pigments, there is an ideal particle size for efficient scattering based on the refractive index contrast. This increased contrast (i) improves the paint's opacity and hiding power, (ii) maximises the efficiency of the pigment, and (iii) reduces the amount of paint required for effective coverage.

[0009] A progressive strategy for identifying alternative whitening agents is to seek inspiration from nature. Many different biopolymers have been exploited to produce photonic structures, one such example is cellulose, an abundant and renewable material. Researchers have attempted to use cellulose nanofibrils, obtained using a homogenisation process combined with some enzymatic or chemical pre-treatment.

[0010] For example, WO 2019 / 063647 describes the use of cellulose nanofibrillar materials, or a fine web of interweaving cellulose fibrils, to produce porous particles that are fibrillar aggregates and can scatter white light. WO 2023 / 135261 describes cellulose microparticles produced using a chemical process and their light scattering ability is uniquely tied to their physical dimensions.8These cellulose microparticles are promising whitening agents.

[0011] WO02100955 discloses an aqueous dispersed latex paint comprising film forming polymeric binder with milled cellulose particles less than 100 microns measured by weight volume distribution. The milled cellulose particles are produced by milling together cellulose fibers with mineral extender filler pigment in a size reduction milling operation to produce rather uniform milled cellulose particles preferably having a particle size between 10 and 60 microns. The weight ratio of milled cellulose particles to milled mineral extender pigments (fillers) is between 5 / 95 to 80 / 20. However, milled cellulose within the particle size range described in WO 02 / 100955 A1 cannot sufficiently scatter light and fails to form a continuous film when mixed with an ionic surfactant in an aqueous suspension.

[0012] EP 2 653 508 discloses paint composition comprising microfibrillated cellulose, binder and solvent. The microfibrillated cellulose is present in an amount ranging from about 0.1 % to about 10 % by weight based on the total weight of the paint composition. The composition further comprises co-processed inorganic particulate material as primary pigment and / or extender pigment, wherein said inorganic particulate material is co-processed with a fibrous substrate comprising cellulose during the preparation of said microfibrillated cellulose. Microfibrillated cellulose helps to evenly distribute the pigments in the paint, resulting in homogeneous coverage and color intensity.

[0013] However, the formation of continuous white films using such cellulose-based whitening agents is challenging. When these cellulose particles are dried straight out of a solvent,9such as water, it is difficult to produce thin, continuous and homogeneous white films. It is thought that the cellulose particles settle over a substrate and do not homogeneously assemble to form a continuous and homogeneous white films. The film formed by drying such particles straight out of a solvent is variable and of poorer quality than films formed with inorganic materials such as TiO2. As a result, cellulose particle-containing films produced by known methods can have poorer light scattering ability, poorer whiteness and lack control over these parameters for example compared to TiO2films. Thus, these known white cellulose films may not be a desirable replacement for traditional inorganic material based coatings.

[0014] The present invention looks to address these issues.

[0015] Summary of the Invention

[0016] At its most general, the present invention provides a coating composition comprising a cellulose particle based opacifier. The opacifier comprises cellulose microparticles and surfactant. The cellulose microparticles impart opacity to the opacifier, while the surfactant assists in the distribution of the cellulose microparticles in the coating composition.

[0017] The present inventors have found that the enhanced distribution and the scaffolding effect provided by the surfactant results in the effective formation of cellulose microparticle coatings on substrates. It is proposed that this is because the cellulose microparticles do not prematurely settle out of the composition during the setting of the coating (as the solvent volume decreases), but rather remain homogenously dispersed in the solvent by the surfactants. This allows the formation of homogeneous and continuous cellulose coatings, white display uniformly excellent whiteness and opacity. Particularly, the mean particle length of the cellulose microparticles allows for the achievement of a very white composition without the need for TiO2in the formulation.

[0018] In general, there is provided a coating composition for providing a cellulose microparticle coating on a substrate, the coating composition comprising: an opacifier comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm; and a carrier liquid; and wherein the composition comprises less than 5% by weight of a metal oxide.

[0019] Specifically, cellulose white pigments contained in the composition according to the present invention have an ideal particle size and, in suitably designed paint and coating formulations, can fit the predictions for optimal scattering efficiency and refractive index contrast between the cellulose particles and pores when forming a continuous uniform film. Furthermore, said cellulose white pigments can tune whiteness above and below the critical pigment volume concentration in a colloidal suspension or mixture.

[0020] In some embodiments the surfactant is a non-ionic, cationic or zwitterionic surfactant. Preferably, the surfactant is a non-ionic surfactant. Non-ionic surfactants offer a distinct advantage due to their compatibility with charged cellulose particles; they do not disrupt the inherent charge. This property enhances their versatility, allowing for effective formulation in a wide range of products without risking destabilization or adverse interactions.

[0021] It was found that the use of surfactants results in a uniformly disperse cellulose microparticle coating composition, which can be formulated with a variety of polar and nonpolar solvents. This allows for formulation of the opacifier and the coating composition in a wide variety of commercial applications.

[0022] The coating formed by the coating composition has controllable thickness, and tuneable whiteness and opacity.

[0023] Accordingly, in a first aspect of the invention there is provided a coating composition for providing a cellulose microparticle coating on a substrate, the coating composition comprising: an opacifier, wherein the opacifier comprises cellulose microparticles having a mean average particle length of from 0.7 to 9 pm and a surfactant; and a carrier liquid; wherein the cellulose microparticles are present in the coating composition at an amount of from 1 to 40 wt.% and the surfactant is present in the coating composition at an amount of from 0.5 to 6 wt.% based on the on the total mass of the coating composition and wherein the composition comprises less than 5% by weight of a metal oxide.

[0024] In a second aspect of the invention there is provided an opacifier for a coating composition, the opacifier comprising cellulose microparticles having a mean average particle length of from 0.7 to 9 pm, and a surfactant, wherein the cellulose microparticles are present in the opacifier at an amount of from 50 to 99.6 wt.% and the surfactant is present in the opacifier at an amount of from 0.4 to 25 wt.% based on the on the total mass of the opacifier.

[0025] In some embodiments the opacifier is a powder. In some embodiments the opacifier is a powder and the opacifier has a water content of 6% or less, based on the mass of the opacifier.

[0026] In a third aspect there is provided a coating formed on a substrate, the coating comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to

[0027] 9 pm, wherein the cellulose microparticles are present in the coating at an amount of from 50 to 99.6 wt.% and the surfactant is present in the coating at an amount of from 0.4 to 25 wt.% based on the on the total mass of the coating. In some embodiments the cellulose microparticles are present in the coating at an amount of from 80 to 99 wt.% based on the on the total mass of the coating, preferably from 85 to 96 wt.%, more preferably from 86 to 90 wt.%

[0028] In some embodiments the surfactant is present in the coating at an amount of from 1 to 20 wt.% based on the on the total mass of the coating, preferably from 4 to 17 wt.%, more preferably from 12 to 15 wt.%.

[0029] In some embodiments the coating has a mean average thickness of 5 pm or more, preferably 10 pm or more, more preferably 15 pm or more, and / or a mean average thickness of 500 pm or less, preferably 300 pm or less, more preferably 200 pm or less.

[0030] In some embodiments the standard deviation of the coating thickness is 25% or less, preferably 20% or less, more preferably 15% or less.

[0031] In a preferred embodiment of the present invention, the composition is essentially free of metal oxides, particularly TiO2. The term 'essentially free' means that no metal oxides, especially TiO2, have been added. This composition provides significant advantages in terms of biocompatibility and environmental impact. Being free from metal oxides contributes to a lower environmental impact. It supports more sustainable production processes, reducing the ecological footprint associated with mining and processing metal oxides.

[0032] The composition according to the present invention can additionally comprise defoaming agent, a suitable polymer that can be latex or acrylic based, for example, and water for waterborne coatings or paints, or another suitable solvent for organic solvent based coatings and paints.

[0033] In some embodiments the coating has: a L* (4570°) of 65 or more, preferably from 70 or more, more preferably 80 or more, yet more preferably 90 or more, wherein L* (4570°) is a CEILAB colour-space coordinate as such a composition reflects a significant amount of light and appears very bright or nearly white; and / or a mean average reflectance of 38% or more, preferably from 50% or more, more preferably 68% or more, yet more preferably 80% or more, wherein average reflectance is measured across a wavelength range of from 400 to 700nm as such a composition is particularly effective at reflecting visible light, contributing to its brightness and visual appeal; and / or an opacity of 60% or more, preferably 70% or more, more preferably 80% or more, yet more preferably 85% or more, wherein opacity is measured across a wavelength range of from 400 to 700nm for a coating having a thickness of 10 pm as such a composition exhibits excellent coverage and hiding power.

[0034] In some embodiments the coating has a mean average reflectance of: 30% or more, preferably from 35% or more, more preferably 40% or more, wherein average reflectance is measured across a wavelength range of from 100 to 400nm; or 20% or more, preferably from 30% or more, more preferably 35% or more, wherein average reflectance is measured across a wavelength range of from 1000 to 2500nm.

[0035] In some embodiments of the first, second and third aspects the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm, preferably 1.3 to 7 pm, more preferably from 1.7 to 5 pm, yet more preferably from 1.9 to 2.8 pm.

[0036] In some embodiments of the first, second and third aspects the cellulose microparticles have a mean average aspect ratio of from 2 to 18, preferably 3 to 15, more preferably from 4 to 10, yet more preferably from 4 to 6.

[0037] In some embodiments of the first, second and third aspects the cellulose microparticles have a mean average width of from 0.1 to 1 pm, preferably from 0.2 to 0.8 pm, more preferably from 0.3 to 0.6 pm, yet more preferably from 0.45 to 0.55 pm.

[0038] In some embodiments of the first, second and third aspects the cellulose microparticles have a L* (4570°) of 38 or more, preferably from 38 to 42, wherein L* (4570°) is a CEILAB colourspace coordinate measured for a 1 wt.% suspension of the cellulose microparticles in water.

[0039] In some embodiments of the first, second and third aspects the cellulose microparticles have a reflectance of 50% or more, preferably 55% or more, more preferably 60% or more of incoming light at a wavelength of from 400 to 700 nm, wherein reflectance is measured for a 1 wt.% suspension of the cellulose microparticles in water.

[0040] In some embodiments of the first, second and third aspects, the surfactant is a non-ionic, cationic or zwitterionic surfactant, preferably the surfactant is non-ionic. Non-ionic surfactants are advantageous as they maintain the stability of charged cellulose particles by not interfering with their inherent charge.

[0041] In some embodiments of the first, second and third aspects, the surfactant is polyethylene glycol p-(1 ,1,3,3-tetramethylbutyl)-phenyl ether (Triton X-100), polyoxyethylene (20) sorbitan monolaurate (Tween 20), alcohol ethoxylate (Cg-C^ ethoxylated alcohol (ECOSURF SA-9), secondary alcohol ethoxylate (Cn-C^ ethoxylated alcohol (TERGITOL 15-S-9), trimethyl nonyl ether of polyethylene glycol (TERGITOL TMN-100X), hexadecyltrimethylammonium bromide (CTAB), 3-(Decyldimethylammonio)-propane-sulfonate inner salt, or a combination thereof.

[0042] In some embodiments of the first, second and third aspects, the surfactant is a non-ionic, cationic or zwitterionic surfactant, such as polyethylene glycol p-(1,1,3,3-tetramethylbutyl)- phenyl ether (Triton X-100), polyoxyethylene (20) sorbitan monolaurate (Tween 20), alcohol ethoxylate (Cg-Cn ethoxylated alcohol (ECOSURF SA-9), secondary alcohol ethoxylate (Cn- C15ethoxylated alcohol (TERGITOL 15-S-9), trimethyl nonyl ether of polyethylene glycol (TERGITOL TMN-100X), hexadecyltrimethylammonium bromide (CTAB), 3-(Decyldimethylammonio)-propane-sulfonate inner salt, or a combination thereof; or the surfactant is a non-ionic surfactant, such as polyethylene glycol p-(1 , 1 ,3,3- tetramethylbutyl)-phenyl ether (Triton X-100), polyoxyethylene (20) sorbitan monolaurate (Tween 20), alcohol ethoxylate (Cg-Cn ethoxylated alcohol (ECOSURF SA-9), secondary alcohol ethoxylate (Cn-C^ ethoxylated alcohol (TERGITOL 15-S-9), trimethyl nonyl ether of polyethylene glycol (TERGITOL TMN-100X) or a combination thereof.

[0043] In a fourth aspect there is provided a method of preparing the opacifier of the second aspect, the method comprising: adding a cellulose microparticle having a mean average particle length of 0.7 to 9 pm and a surfactant to a carrier liquid, to form a suspension; optionally dispersing the cellulose microparticle and surfactant in the carrier liquid to disperse the suspension; and drying the suspension to provide the opacifier.

[0044] In a fifth aspect of the invention there is provided a method of preparing the coating composition of the first aspect, the method comprising: adding a cellulose microparticle having a mean average particle length of from 0.7 to 9 pm and a surfactant, or the opacifier of the second aspect, to a carrier liquid; optionally dispersing the cellulose microparticle and surfactant, or the opacifier, in the carrier liquid, to provide the coating composition.

[0045] In a sixth aspect of the invention there is provided a method of forming a coating on a substrate, the method comprising: applying the coating composition of the first aspect to the substrate, and drying the coating composition to form a cellulose microparticle containing coating.

[0046] Further Aspects

[0047] In an aspect of the invention there is provided an opacifier, wherein the opacifier is obtained or obtainable by the method of the third aspect.

[0048] In an aspect of the invention there is provided a coating composition, wherein the coating composition is obtained or obtainable by the method of the fifth aspect.

[0049] In an aspect of the invention there is provided a coating applied to a substrate, wherein the coating is obtained or obtainable by the method of the sixth aspect.

[0050] In an aspect of the invention there is provided a use of the coating composition according to the first aspect, for applying to a substrate to form a coating. Summary of the Figures

[0051] Figure 1 shows a scanning electron microscopy (SEM) image of cellulose microparticles with unique light scattering capability, CMPLS, also known as cellulose microparticles. The scale bar is 30 pm.

[0052] Figure 2 shows size distributions by volume for three regimes of cellulose particles obtained from the sulfuric acid hydrolysis process, CMPZ(right) has an average particle size > 5 pm; 1 pm > CMPLS(middle) > 5 pm; and CMPX( left) < 1 pm. The particle size is measured as described in the examples section.

[0053] Figure 3 shows images of comparative composition coated onto an opacity chart. Upon drying, the film breaks up into inhomogeneous fragments, which demonstrates that white films cannot be produced from directly drying aqueous suspensions of CMP.

[0054] Figure 4 shows white films of Composition 1b (Figure 4a) on paper charts the example coating composition comprising 12 wt.% cellulose microparticles, CMP, and 1.75 wt.% of a nonionic surfactant, Triton-x100, in aqueous solution, and white films of Composition 2 (Figure 4b) on paper, the example coating composition comprising12 wt.% CMP, 0.5 wt.% another type on non-ionic surfactant, Tween 20, in aqueous solution (right image). Both films have thickness = 20 pm.

[0055] Figure 5 shows cellulose microparticles coatings of Composition 1b on wood, the example coating composition comprising 12 wt.% cellulose microparticles and 1.75 wt.% nonionic surfactant, Triton-x100.

[0056] Figure 6 shows Remission spectra (Figure 6a) for Composition 1b coating on aluminium at various thicknesses (50 pm, 100 pm and 150 pm) measured as described in the examples section. One of the white films is pictured in Figure 6b. The whiteness of the film, as measured by L*, is shown to depend on film thickness.

[0057] Figure 7 shows remission spectra for cellulose microparticle coatings on paper at different coating thicknesses. The coatings are formed using coating composition 1b on paper, at thicknesses of 7 pm, 14 pm and 19 pm. The degree of whiteness, as indicated by L* (45 / 0) or the backscattering measurement, directly depends on the coating thickness, and is measured as described in the examples section.

[0058] Figure 8 shows a piece of bovine leather coated with Composition 4a (Figure 8a) the example coating composition comprising 12 w% CMP and 2 wt.% TERGITOL 15-S-9 (non-ionic surfactant) in an aqueous suspension. The Cl ELAB colour values for the white film on bovine leather were measured as L* = 95, a* = -0.1, b* = 2.8, and the value of whiteness = 100 - SQRT((100-L*)A2 + a*A2 + b*A2) = 94. It is noteworthy that the coating compositions and coatings of the invention offer a flexible white coating to suit a more flexible substrate. Figure 8b shows an image of a leather sample coated using the same formulation mixed with 10 wt.% acrylic paint base in water. The leather sample (Figure 8b) can be bent and still retains the white coating after flexion without cracking or chipping.

[0059] Figure 9 shows illustrative scattering properties of cellulose microparticles.

[0060] Figure 10 shows the opacity trends, of compositions based on compositions 1b and 2 at a range of CMP concentration (3 wt.%, 6 wt.%, 9 wt.%, 12 wt.%, 15 wt.%, 18 wt.% and 20 wt.%) for coatings on opacity charts. The opacity was measured using a spectrophotometer (sph870, ColorLite GmbH).

[0061] Figure 11 shows a polynomial plot illustrating the correlation between CIELAB L*(45 / 0) measurement and backscattering for a wide selection of samples coated with cellulose microparticles. The circled region indicates the ranges where light scattering, measured by either technique, represents properly measurable and detectable white coating. The coefficient of determination for this polynomial function, L*(x) = axb= 0.72 x055with a coefficient of determination, r2= 98.1%.

[0062] Figure 12 shows abrasion tests for CMP coating with a clear polyurethane film onto an aluminium substrate. Test was performed using a 600 grit sandpaper loaded with 100 g. Abrasion is concluded on the basis of water contact angles off the coatings, measured for both a clear Pll coat alone and a topcoat comprising the clear layer and CMP layer underneath.

[0063] Figure 13 show the spectral response for the coating of composition 1b (Figure 13a) and the spectra for the coating of composition 2 (Figure 13b) for the UV, visible and IR regions. The film thickness for coating in Figure 13a is 15 ± 2 pm and for film in Figure 13b is 12 ± 3 pm. This demonstrates the capability of these coatings to provide protection / shielding in the UV- Vis range.

[0064] Figures 14A to 14F show the appearance of the compositions according to the present invention and of comparative examples.

[0065] Figures 15A to 15F coated opacity charts.

[0066] Detailed Description of the Invention

[0067] In general, the present invention provides a coating composition comprising cellulose microparticles and a surfactant. The surfactant acts to disperse the cellulose microparticles, which the inventors have found to greatly improve the coating properties of the cellulose microparticle compositions. The compositions allow for the preparation of homogeneous, opaque coatings on substrates. The coatings can be applied using conventional industrial scale techniques.

[0068] The cellulose microparticles are typically produced using acid hydrolysis, followed by size sorting of the hydrolysed cellulose particles. The specific size and / or morphology of the cellulose microparticles gives a signature scattering response in the visible, ultra-violet and infra-red regions. The cellulose microparticles are able to act as opacifiers.

[0069] The surfactant is thought to form a scaffold around which the cellulose microparticles assemble. The properties of the surfactant, namely, critical micelle concentration, solid concentration and charge, determine the stability and robustness of this scaffold, which assists in the assembly of the cellulose microparticles to produce the desired opacity and whiteness at a particular film thickness.

[0070] The morphology of the microparticles can be altered to optimize their scattering efficiency for a desired application. The surfactant can also be altered to adjust how the cellulose microparticles assemble, to adjust the scattering and whiteness of the coating. In this way, the resulting cellulose microparticle coating has a refractive index which is much greater than that of standard cellulose (the average refractive index of cellulose is approximately 1.56).

[0071] Coating Composition

[0072] In a general aspect, there is provided a coating composition for providing a cellulose microparticle coating on a substrate, the coating composition comprising: an opacifier comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm; and a carrier liquid.

[0073] In a first aspect of the invention there is provided a coating composition for providing a cellulose microparticle coating on a substrate, the coating composition comprising: an opacifier, wherein the opacifier comprises cellulose microparticles having a mean average particle length of from 0.7 to 9 pm and a surfactant; and a carrier liquid; wherein the cellulose microparticles are present in the coating composition at an amount of from 1 to 40 wt.% and the surfactant is present in the coating composition at an amount of from 0.5 to 6 wt.% based on the on the total mass of the coating composition and wherein the composition comprises less than 5% by weight of a metal oxide.

[0074] A coating composition is a composition which is suitable for coating onto a substrate. The coating composition is for providing a cellulose microparticle coating on a substrate. The coating composition may be used for coating by any suitable means, such as spraying, curtain coating, knife coating, roll coating, dipping or draw down coating. The coating composition typically forms a homogeneous, uniform coating on the substrate. The coating composition comprises an opacifier. An opacifier is a component which enhances opacity of the resulting coating. Typically, an opacifier is a component which reduces the transmission of light, such as by scattering. In other words, the opacifier has good light scattering properties, and poor light transmittance, across a broad range of wavelengths. The opacifier may be a visible light opacifier, which opacifies in the visible region (e.g., 400 to 1000nm). The opacifier may be a UV opacifier, which opacifies in the UV region (e.g., 100 to 400nm). The opacifier may be an IR opacifier, which opacifies in the IR region (e.g., 1000 to 2500nm).

[0075] In the present case, the opacifier comprises cellulose microparticles and a surfactant. The cellulose microparticles and surfactant are described in detail below. The opacity is typically provided by the cellulose microparticles, which are described herein.

[0076] In some embodiments, the cellulose microparticles are present in the coating composition at an amount of from 1 to 40 wt.% and the surfactant is present in the coating composition at an amount of from 0.5 to 6 wt.% based on the on the total mass of the coating composition.

[0077] In some embodiments, the cellulose microparticles are present in the coating composition at an amount of 30 wt.% or less based on the total mass of the coating composition, preferably 20 wt.% or less, more preferably 15 wt.% or less. In some embodiments, the cellulose microparticles are present in the coating composition at an amount of 5 wt.% or more based on the total mass of the coating composition, preferably 10 wt.% or more, more preferably 12 wt.% or more.

[0078] In some embodiments, the coating composition according to the present invention comprises a particle population CMPXhaving an average particle length of less than 1 pm, or a particle population CMPLShaving an average particle length ranging from 1 pm to less than 5 pm, or a mixture thereof, preferably a a particle population CMPLShaving an average particle length ranging from 1 pm to less than 5 pm. Within the context of the present invention, the term "particle population" refers to a defined group of individual particles that share a particle length within the above defined range. The term "particle length of a particle population" pertains to the dimensions of each individual particle within that population, rather than the properties of an aggregate or bulk material formed by multiple particles. Both particle populations demonstrate good scattering efficiency; nonetheless, the scattering efficiency of the CMPLSparticle population is exceptional. This outstanding performance allows for a reduction in the concentration of particles needed while still achieving enhanced lightness in the coating composition. Therefore, while both populations offer significant benefits, CMPLSclearly excels with its remarkable effectiveness.

[0079] In some embodiments, the cellulose microparticles are present in the coating composition at an amount of from 5 to 20 wt.% based on the total mass of the coating composition, preferably from 10 to 18 wt.%, more preferably from 12 to 15 wt.%. In this way, the cellulose microparticles provide opacity to a coating produced using the coating composition. It has been observed that cellulose microparticles provide coatings having particularly excellent opacity when present in the coating composition at from 12 to 15 wt.%. The examples of the invention achieve over 80% opacity for these coating films (see TABLE 3, in the examples section).

[0080] In some embodiments, the surfactant is present in the coating composition at an amount of 0.05 wt.% or more based on the on the total mass of the coating composition, preferably 0.5 wt.% or more, more preferably 1.0 wt.% or more. In some embodiments, the surfactant is present in the coating composition at an amount of 5 wt.% or less based on the on the total mass of the coating composition, preferably 3 wt.% or less, more preferably 2.0 wt.% or less.

[0081] In some embodiments, the surfactant is present in the coating composition at an amount of from 0.5 to 3 wt.% based on the on the total mass of the coating composition, preferably from 1.0 to 2.0 wt.%.

[0082] In some embodiments, the ratio of the amount of cellulose microparticles as a wt.% to the amount of surfactant as a wt.% is from 2 to 25, wherein the amounts are based on the total mass of the coating composition. Preferably, the ratio is from 3 to 15, more preferably from 4 to 10, even more preferably from 5 to 9, yet more preferably from 6 to 8.

[0083] The coating composition comprises a carrier liquid. The carrier liquid may be any suitable liquid, which is able to disperse the opacifier. The carrier liquid may be a polar or a non-polar solvent. The carrier liquid may be a mixture of solvents.

[0084] The carrier liquid may be 2-propanol, 1 ,2-dichloroethane, 1 ,4-dioxane, 18-crown-6, 2-propanol, 2-ethoxyethanol, acetic acid, acetone, acetonitrile, ammonia, benzene, n-butanol, n-butyl acetate, chloroform, cyclohexane, dichloromethane, diethyl ether, diglyme, dimethyl formamide, Dimethyl sulfoxide, DME, ethane, ethanol, ethyl acetate, ethylene, ethylene glycol, formic acid, glycerine, heptane, hexane, hexamethylbenzene, HMDSO, HMPA, Hydrogen, Imidazole, isobutanol, isopropyl alcohol, methane, methanol, n-hexane, nitromethane n-pentane, propane, propylene, propylene carvonate, pyridine, pyrrole, pyrrolidine, silicone grease, tert -butyl alcohol, tetrahydrofuran, toluene, triethylamine, water, white spirit, xylene or a combination thereof.

[0085] In some embodiments, the carrier liquid is water, ethanol, dimethylsulfoxide (DMSO), dimethylformamide (DMF), N-methylpyrrolidone (NMP) or combination thereof.

[0086] The carrier liquid may also be a mineral oil which is miscible with water, such as glycerine, propylene glycol. Preferably the carrier liquid is predominantly water. Preferably the carrier liquid is water. ‘Predominantly’ refers to a 95 wt.% or more, based on the total mass of the carrier liquid, such as 96 wt.% or more, 97 wt.% or more, 99 wt.% or more, 99.5 wt.% or more.

[0087] The carrier liquid may also be an emulsion, such as a water-in-oil emulsion, an oil-in-water emulsion, a double emulsion, such as a water-in-oil-in-water emulsion or an oil-in-water-in-oil emulsion. A surfactant may assist in the formation of an emulsion. In compositions where the carrier liquid is an emulsion, an additional surfactant may be present to stabilise the emulsion. For example, the carrier liquid may be a paint.

[0088] Suitable oils for the emulsion include algal oil, annatto oil, argan oil, almond oil, apricot kernel oil, avocado oil, babassu oil, Brazil nut butter, butter, cashew butter, castor oil, camellia oil, cheery kernel oil, cocoa butter, coconut oil, corn oil, cottonseed oil, fish oil, grape seed oil, gardenia oil, ghee, hazelnut oil, jatropha oil, jojoba oil, kokum oil, linseed oil, macadamia oil, maize oil, mango seed oil, mango butter, mineral oil, mink oil, olive oil, palm oil, palm kernel oil, peach kernel oil, peanut butter, peanut oil, plum kernel oil, pomegranate oil, rapeseed seed oil, rice bran oil, rosehip oil, sal oil, sesame oil, shea butter, soybean oil, squalene, sunflower oil, teas seed oil, walnut oil. Oil derivatives obtained from the aforementioned oils such as esterified oils, fatty acids, fatty alcohol, hydrogenated oils and triglycerides can be used as suitable ingredient for the said formulation. Essential oils are also suitable oils.

[0089] Typically, the carrier liquid makes up the majority of the coating composition. In some embodiments, the carrier liquid is present in the composition at an amount of from 70 to 95 wt.% based on the total mass of the composition, preferably from 80 to 92 wt.%, more preferably from 83 to 90 wt.%, yet more preferably from 86 to 88 wt.%.

[0090] Opacifier

[0091] In a general aspect, there is provided an opacifier for a coating composition, the opacifier comprising cellulose microparticles having a mean average particle length of from 0.7 to 9 pm, and a surfactant.

[0092] The opacifier typically provides some degree of opacity, such as when incorporated into a coating composition or coating. The opacity may primarily be provided by the cellulose microparticle component. The opacity may be in the I R, UV or visible region of the spectrum.

[0093] In a second aspect of the invention there is provided an opacifier for a coating composition, the opacifier comprising cellulose microparticles having a mean average particle length of from 0.7 to 9 pm, and a surfactant, wherein the cellulose microparticles are present in the opacifier at an amount of from 50 to 99.6 wt.% and the surfactant is present in the opacifier at an amount of from 0.4 to 25 wt.% based on the on the total mass of the opacifier. In some embodiments the cellulose microparticles are present in the opacifier at an amount of 70 wt.% or more based on the on the total mass of the opacifier, preferably 80 wt.% or more, more preferably 85 wt.% or more. In some embodiments the cellulose microparticles are present in the opacifier at an amount of 99.6 wt.% or less based on the on the total mass of the opacifier, preferably 98 wt.% or less, more preferably 95 wt.% or less, yet more preferably 90 wt.% or less.

[0094] In some embodiments the cellulose microparticles are present in the opacifier at an amount of from 80 to 99 wt.% based on the on the total mass of the opacifier, preferably from 85 to 96 wt.%, more preferably from 86 to 90 wt.%.

[0095] In some embodiments the surfactant is present in the opacifier at an amount of 1 wt.% or more based on the on the total mass of the opacifier, preferably 5 wt.% or more, more preferably 10 wt.% or more. In some embodiments the surfactant is present in the opacifier at an amount of 20 wt.% or less based on the on the total mass of the opacifier, preferably 17 wt.% or less, more preferably 15 wt.% or less.

[0096] In some embodiments the surfactant is present in the opacifier at an amount of from 1 to 20 wt.% based on the on the total mass of the opacifier, preferably from 4 to 17 wt.%, more preferably from 12 to 15 wt.%.

[0097] The opacifier may be used in the coating composition of the first aspect.

[0098] An opacifier is a component which enhances opacity of the resulting coating. Typically, an opacifier is a component which reduces the transmission of light, such as by scattering. In other words, the opacifier has good light scattering properties, and poor light transmittance, across a broad range of wavelengths. The opacifier may be a visible light opacifier, which opacifies in the visible region (e.g., 400 to 1000nm). The opacifier may be a UV opacifier, which opacifies in the UV region (e.g., 100 to 400nm). The opacifier may be an IR opacifier, which opacifies in the IR region (e.g., 1000 to 2500nm).

[0099] The opacifier may provide a transmittance of 20% or less, preferably 18% or less, more preferably 15% or less for incoming light at a wavelength of from 400 to 800 nm, when applied as a coating on a substrate at a thickness of 20 pm.

[0100] The opacifier may provide a reflectance of 50% or more, preferably 55% or more, more preferably 60% or more of incoming light at a wavelength of from 400 to 800 nm, when applied as a coating on a substrate at a thickness of 20 pm. The opacifier may have a L* of 70 or more, such as 75 or more, such as 80 or more, wherein L* is a CEILAB colour-space coordinate, and is measured for the opacifier when applied as a coating on a substrate at a thickness of 20 pm.

[0101] The transmittance, reflectance and L* are all measured as described in the examples section.

[0102] The opacity is provided by the cellulose microparticles, which are described herein.

[0103] The opacifier may be particulate. The cellulose microparticle and the surfactant may be provided in separate particles or clusters, which are mixed together. Alternatively, cellulose microparticles and the surfactant may be combined in the same particles or clusters.

[0104] In some such embodiments, the opacifier is a powder. A powder typically includes of a plurality of particles.

[0105] Preferably, the opacifier powder is flowable powder. This flowable powder is useful for commercial applications where the powder is added to a coating composition.

[0106] The opacifier powder may have a flowability of 1s / g or less, preferably 0.5 s / g or less, more preferably 0.3 s / g or less, wherein flowability is measured using a 50g sample in a Hall flowmeter Funnel according to ISO 4490:2018.

[0107] The opacifier powder may have a Hausner ratio of 1.4 or less, preferably 1.25 or less, more preferably 1 or less, wherein Hausner ratio is calculated as the ratio between the tapped bulk density of the powder to the freely settled bulk density of the powder. The tap density and freely settled density may be measured according to ISO 3953:2011.

[0108] In some embodiments, the opacifier is substantially free of solvent. For example, the opacifier has a water content of 6% or less, based on the mass of the opacifier. This dry powder is useful for commercial applications due to its chemical and biological stability.

[0109] In alternative embodiments, the opacifier is a paste or slurry. A paste or slurry is typically a plurality of particles suspended in a relatively small amount of carrier liquid. The paste or slurry typically have a higher viscosity than the coating composition. The paste or slurry typically have a higher concentration than the coating composition.

[0110] The carrier liquid may be the same as the carrier liquid described above, in the context of the coating composition.

[0111] A paste is typically viscous and thick. The paste may be useful for commercial applications, as it is more concentrated than coating compositions. The paste may be easier to disperse into a formulation than a powder. The opacifier paste or slurry may have a viscosity of 20,000 mPa s or less, preferably 5,000 mPa s or less, more preferably 1 ,000 mPa s or less. The opacifier paste or slurry may have a viscosity of 20 mPa s or more, preferably 50 mPa s or more, more preferably 100 mPa s or more. The viscosity is measured at a temperature of 20 °C according to ISO 2555:2018.

[0112] Cellulose Microparticles

[0113] The cellulose microparticles have a mean average particle length of from 0.7 to 9 pm.

[0114] The cellulose microparticles may be used in the coating composition and the opacifier, as well as other aspects of the invention.

[0115] Cellulose microparticles of the dimensions disclosed herein provide for excellent scattering of IR, UV and Visible light. Colour emanates from the interaction of light waves with the electrons in molecules comprising an object. Structural colour, on the other hand, is colour by reflection off a specified geometry, and hence is permanent for as long as the geometry of the identified material is unchanged.

[0116] White colour is obtained through appropriate light scattering off a specific particle, dependent on the refractive index, which is a material-dependent property. Titanium dioxide, TiO2, for instance, has a refractive index of 2.87 at 632.8 nm. All polymeric materials, including cellulose, have lower refractive indices than such metal oxides. Thus, to produce a white colour using cellulose, cellulose particles of a specific geometry that permits efficient light scattering are used. The cellulose microparticles used in the present invention have the required geometry and morphology to efficiently scatter IR, UV and / or visible light.

[0117] The length of the particle is generally the longest dimension of the particle. For example, if the particle is rod shaped, then the length is the length between the ends of the rod. The length of the particle is typically the longitudinal length of the particle.

[0118] The length of the particle is typically the longest lateral dimension. The lateral dimension is the dimension observable when viewing a particle in plan view. The particles in plan view appear to be two dimensional. For example, if the particle is measured from a top down (or plan) image, then length is the longest dimension measurable from the top down image of the particle.

[0119] The length of the particles may be measured using standard techniques. For example, scanning electron microscope (SEM) may be used. Suitable systems include a Mira3 FEG-SEM system (TESCAN) operated at 30 kV and a working distance of 5 mm. The length of the microparticles may then be analysed by Imaged.

[0120] Typically, the length of the particles is measured using SEM. The number of measurements taken of the length is typically from 100 to 1,000. Generally, over 100 measurements of the length are taken. The length is the mean average value of the measurements taken. The mean average is a number average. By calculating a mean average value for the length, the effect of anomalous values is reduced, and the representative length of the particle is indicated.

[0121] In some embodiments, the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm, preferably 1 pm to less than 5 pm (i.e. particle population CMPLS), more preferably 1.3 to 7 pm, yet more preferably from 1.7 to 5 pm, and especially preferred from 1.9 to 2.8 pm. This particle length provides the appropriate physical dimensions to scatter light efficiently in the visible range and, gives rise to producing white colour in coatings. The particle sizes can also be used to control the flow properties of suspensions and can therefore be used to tune the rheological properties of the coating composition and coatings.

[0122] Typically, the length of the particles gives a particle size distribution having a log-normal distribution. Generally, the particles have a monomodal particle size distribution, which has only one peak or maxima in the particle size distribution. The peak or maximum of the monomodal distribution corresponds to the median average particle diameter by number for all particles in the distribution.

[0123] In some embodiments, the length of the particles gives a multimodal particle size distribution, which has two or more peak or maxima in the particle size distribution. In other embodiments, the length of the particles gives a bimodal particle size distribution, which has exactly two peaks or maxima in the particle size distribution. In other embodiment, the length of the particles gives a trimodal particle size distribution, which has exactly three peaks or maxima in the particle size distribution.

[0124] Percentile values for the length can also be calculated, such as D90, D50 and D10. These values may be calculated based on the length particle size distribution on the basis of the number of particles.

[0125] D90 length is the particle length at which 90% of the particles have a length less than or equal to the D90 particle length.

[0126] In some embodiments the cellulose particles have a D90 length of 6,000 nm or less, preferably 5,000 nm or less, preferably 4,500 nm or less. In some embodiments the cellulose particles have a D90 length of 3,000 nm or more, preferably 3,500 nm or more, preferably 3,750 nm or more. In some embodiments the cellulose particles have a D90 length of from 3,000 to 5,000 nm, preferably from 3,500 to 4,500 nm.

[0127] In a first embodiment the D90 length is about 4,500 nm.

[0128] In a second embodiment the D90 length is about 3,500 nm. D50 length is the particle length at which 50% of the particles have a length less than or equal to the D50 particle length.

[0129] In some embodiments the cellulose particles have a D50 length of 5,000 nm or less, preferably 3,500 nm or less, preferably 3,000 nm or less, more preferably 2,800 nm or less. In some embodiments the cellulose particles have a D50 length of 1,000 nm or more, preferably 1,300 nm or more, preferably 1,600 nm or more, more preferably 1900 nm or more. In some embodiments the cellulose particles have a D50 length of from 1 ,000 to 5,000 nm, preferably from 1 ,300 to 3,500 nm, preferably from 1 ,600 to 3,000 nm, more preferably from 1,900 to 2,800 nm.

[0130] In a first embodiment the D50 length is from 1 ,000 to 5,000 nm, preferably from 1 ,900 to 3,500 nm, more preferably from 2,200 to 3,000 nm, even more preferably from 2,500 to 2,900 nm, and most preferably from 2,600 to 2,800 nm. In a first embodiment the D50 length is preferably about 2,700 nm.

[0131] In a second embodiment the D50 length is from 1 ,000 to 4,000 nm, preferably from 1 ,200 to 2,700 nm, preferably from 1,500 to 2,500 nm, preferably from 1,700 to 2,300 nm, preferably from 1,800 to 2,100 nm, more preferably from 1,900 to 2,000 nm. In a second embodiment the D50 length is preferably about 1,950 nm.

[0132] D10 length is the particle length at which 10% of the particles have a length less than or equal to the D10 particle length.

[0133] In some embodiments the cellulose particles have a D10 length of 2,000 nm or less, preferably 1,750 nm or less, preferably 1,500 nm or less. In some embodiments the cellulose particles have a D10 length of 500 nm or more, preferably 750 nm or more, preferably 1 ,000 nm or more. In some embodiments the cellulose particles have a D10 length of from 500 to 2,000 nm, preferably from 750 to 1,800 nm, preferably from 1 ,000 to 1,700 nm.

[0134] In a first embodiment the D10 length is about 1 ,700 nm.

[0135] In a second embodiment the D10 length is about 1 ,200 nm.

[0136] In some embodiments, the cellulose particles have a mean average width of from 0.1 to 1 pm, preferably from 0.2 to 0.8 pm, more preferably from 0.3 to 0.6 pm, yet more preferably from 0.45 to 0.55 pm.

[0137] The width of the particle is generally the shortest dimension of the particle. For example, if the particle is rod shaped, then the width is the diameter of the cross-section of the rod. The width of the particle is typically the lateral diameter of the particle. The width may also be referred to as the diameter. The width of the particle is typically the shortest lateral dimension of the particle. The lateral dimension is the shortest dimension observable when viewing a particle in plan view. The particles in plan view appear to be two dimensional. For example, if the particle is measured from a top down (or plan) image, then width is the shortest dimension measurable from the top down image of the particle.

[0138] The width of the particle is generally the shortest dimension of the particle which is perpendicular to a line defining the length dimension of the particle. As explained above, the length of the particle is the longest dimension of the particle, and so the line defining the length of the particle is the line between the furthest extremities of the particle. The width may be the shortest dimension of the particle which is perpendicular to the line defining the length of the particle. The shortest dimension may also be defined by a line between the closest extremities of the particle, wherein the line is perpendicular to the line defining the length of the particle. In other words, the shortest dimension may be the narrowest section of the particle which can be joined by a line perpendicular to the line defining the length of the particle.

[0139] The width of the particle may be measured analogously to the length of the particle.

[0140] Percentile values for the width from the particle size distribution can also be calculated, such as D90, D50 and D10. These values may be calculated based on the width particle size distribution on the basis of the number of particles.

[0141] D90 width is the particle width at which 90% of the particles have a width less than or equal to the D90 particle width.

[0142] In some embodiments the cellulose particles have a D90 width of 1,000 nm or less, preferably 900 nm or less, preferably 800 nm or less. In some embodiments the cellulose particles have a D90 width of 250 nm or more, preferably 300 nm or more, preferably 350 nm or more. In some embodiments the cellulose particles have a D90 width of from 300 to 1 ,000 nm, preferably from 350 to 900 nm.

[0143] In a first embodiment the D90 width is about 850 nm.

[0144] In a second embodiment the D90 width is about 350 nm.

[0145] D50 width is the particle width at which 50% of the particles have a width less than or equal to the D50 particle width.

[0146] In some embodiments the cellulose particles have a D50 width of 800 nm or less, preferably 700 nm or less, preferably 600 nm or less. In some embodiments the cellulose particles have a D50 width of 100 nm or more, preferably 150 nm or more, preferably 200 nm or more. In some embodiments the cellulose particles have a D50 width of from 200 to 800 nm, preferably from 300 to 600 nm, preferably from 450 to 550 nm.

[0147] In a first embodiment the D50 width is from 500 to 540 nm, such as about 520 nm.

[0148] In a second embodiment the D50 width is from 200 to 240 nm, such as about 220 nm.

[0149] D10 width is the particle width at which 10% of the particles have a width less than or equal to the D10 particle width.

[0150] In some embodiments the cellulose particles have a D10 width of 400 nm or less, preferably 300 nm or less, preferably 200 nm or less. In some embodiments the cellulose particles have a D10 width of 50 nm or more, preferably 100 nm or more, preferably 200 nm or more. In some embodiments the cellulose particles have a D10 width of from 50 to 400 nm, preferably from 100 to 300 nm, preferably from 150 to 200 nm.

[0151] In a first embodiment the D10 width is about 300 nm.

[0152] In a second embodiment the D10 width is about 150 nm.

[0153] The aspect ratio may also be the ratio between the length of a population of particles and the width of a population of particles. In some embodiments, the aspect ratio is a D50 aspect ratio, which is the ratio between the D50 length and the D50 width. In some embodiments, the aspect ratio is a mean average aspect ratio, which is a ratio between the mean average length and the mean average width.

[0154] The particles are typically not spherical, as the aspect ratio is greater than 1 (e.g. from 2 to 18). The particles may be described as prismatic and / or elongate. Preferably the particles are rod shaped.

[0155] In some embodiments the cellulose microparticles have a mean average aspect ratio of from 2 to 18, preferably 3 to 15, more preferably from 4 to 10, yet more preferably from 4 to 6.

[0156] In some embodiments the D50 aspect ratio may be from 2 to 18. Preferably, the D50 aspect ratio of the cellulose particle is from 2 to 16, more preferably 3 to 14, even more preferably from 4 to 10, and most preferably from 4 to 6.

[0157] The cellulose particle may have any suitable shape. In some embodiments, the cellulose particle has a rod or a rod-like shape, or a flake or flake-like shape. In some embodiments, the cellulose particle has a rod or a flake shape. The cellulose particle may have a rod or a rod-like shape. Thus, the width may be the diameter of the cross-section of the rod. The particle may be elongate with a length dimension greater than the width dimension.

[0158] The cellulose particle may also have a flake or flake-like shape. The flake or flake-like shape may be elongate, with a length dimension greater than the width dimension. The flake or flake-like shape typically has a uniform height over most of the length of the particle.

[0159] The cellulose particle may be substantially unbranched. That is, the cellulose particle is typically not divided into two or more branches. The cellulose particle is preferably not a branched cellulose particle and not a hyperbranched cellulose particle.

[0160] The cellulose particle is typically a primary particle. That is, the cellulose particle is not an agglomeration of smaller particles.

[0161] Alternatively, the cellulose particle may be a secondary particle formed from the agglomeration of multiple primary particles. Typically, the secondary cellulose particles are held together by non-covalent interactions.

[0162] Herein, the dimensions of the cellulose particles refers to the separately divisible particle. In other words, where the particles are provided as primary particles then the dimensions refer to the primary particles and when the particle are provided as secondary particles then the dimensions refer to the secondary particle.

[0163] Typically, the secondary cellulose particles includes pores or voids. The pores or voids are between the primary cellulose particles. The cluster may be said to be porous.

[0164] The secondary cellulose particles may be prepared using the methods described herein. For example, the secondary cellulose particles may be prepared by spray-drying or spray drying or spray-freeze a suspension of cellulose microparticles. The atomiser may be tuned to adjust the size of the droplets of CMP suspension. The size of the droplets may be used to control the size of the CMP cluster. Suitable spray-drying equipment is described herein.

[0165] The cellulose particles have excellent optical properties. The cellulose particles scatter incident light. In particular, the cellulose particles can provide high reflectance and low transmittance. As result, the cellulose particles provide good opacity.

[0166] The optical properties of the particles can be measured using standard techniques, such as using a light source coupled with a spectrometer and an integrating sphere. The signal can be normalized with respect to the intensity in the absence of sample. Typically, a white diffuser standard is used, such as a Labsphere SRS-99-010. The background can be recorded when no light is applied, and the background noise can be subtracted from the measurements. The optical properties are measured in the visible range (e.g., from 400 nm to 800 nm). Typically, the optical properties are measured in air. Transmittance may be measured, and reflectance calculated from the obtained transmittance values assuming that there is no absorption by the particles. Typically, five spectra were taken for each sample and averaged.

[0167] In some embodiments the total transmittance and reflectance measurements were performed with an integrating sphere (Labsphere). A light source (Ocean Optics HPX-2000) was coupled into an optical fibre (600 pmThorlabs FC-UV100-2-SR) via a collimator (Thorlabs) and the signal was collected by a spectrometer (Avantes HS2048), as shown in Figure 8 (T-, and T2). The signal was normalized with respect to the intensity when no sample was mounted. The background was recorded when no light was applied. The range of wavelengths was between 400 and 700 nm. Five spectra were taken for each sample and averaged to reduce the signal-to-noise ratio. Each spectrum was recorded using an integration time equal to 3 s.

[0168] L* (45 / 0) refers to an incidence angle of 45° to the normal of the surface and a reflectance angle at 0° to the normal of the surface. The measurement is taken for a 0.1 wt.% CMP suspension in a 1cm cuvette.

[0169] In some embodiments, the cellulose microparticles have a L* (4570°) of 38 or more, preferably from 38 to 42, wherein L* (4570°) is a CEILAB colour-space coordinate measured for a 1 wt.% suspension of the cellulose microparticles in water.

[0170] In some embodiments, the cellulose microparticles have a reflectance of 50% or more, preferably 55% or more, more preferably 60% or more of incoming light at a wavelength of from 400 to 700 nm, wherein reflectance is measured for a 1 wt.% suspension of the cellulose microparticles in water.

[0171] The surface of the cellulose particles may be modified. Typically, the hydroxyl groups on the surface of the cellulose particle are modified. The cellulose particles may be modified to be hydrophobic or partially hydrophobic.

[0172] In some embodiments, one or more hydroxyl groups on the surface of the cellulose particle are modified. In some embodiment the hydroxyl groups are transformed into a different functional group, such as an ester or an ether group.

[0173] For example, the cellulose particle may be prepared by acid hydrolysis. During acid hydrolysis, it is thought that the cellulose chain backbone of the cellulose particle is modified at the molecular level to provide colloidal stability to the cellulose particle. For example, sulfuric acid hydrolysis is thought to modify the cellulose chains with sulfate half ester groups.

[0174] Cellulose microparticles typically incorporate 20 to 100 mmol / kg, preferably 40-60 mmol / kg of half-ester sulfate groups on their surfaces as a result of the acid hydrolysis process. The mass contribution of the half-ester group (e.g., -SO3H) corresponds to 5 wt.% or less based on the total mass of the cellulose microparticles, preferably 1 wt.% or less.

[0175] The density of the half-ester sulfate groups is measured by conductometric titration, such as according to ISO 21400:2018.

[0176] The cellulose particles are preferably anionic. The half-ester group may contribute to the anionic charge on the cellulose particles. Thus, in some embodiments, the cellulose microparticles are anionic. In some embodiments, the cellulose microparticles are surface modified with anionic sulfate half ester groups.

[0177] The cellulose microparticles have an anionic charge density of from 50 mmol / kg or more, preferably 100 mmol / kg or more, more preferably 150 mmol / kg or more.

[0178] The cellulose particles may be further modified to increase the negative charge. For example, the cellulose particles may be modified by reaction with sulfamic acid as part of a reactive deep eutectic solvent. Urea can be used to increase the reactivity of sulfamic acid. The highly sulphated cellulose nanoparticles can then be mixed in the manner described in this patent application with the family of surfactants outlined herein.

[0179] We can increase the total surface charge density via a suitable post-treatment to reach 2000 mmol / kg. This can be useful for highly charged mixtures to ensure stability and maintain the same functionality.

[0180] The cellulose microparticles may have an anionic charge density of 2000 mmol / kg or less.

[0181] The charge density of the particles is measured by conductometric titration, such as according to ISO 21400:2018.

[0182] The charge may depend on the pH of the solution in which the particles are present. The charge of the particles is determined at the pH of the coating composition or opacifier in the invention. This is typically a pH of 7.

[0183] Preparation of Cellulose Microparticles

[0184] The cellulose microparticles used in the present invention may be obtained using the methods described in WO2023 / 135261, such as in Example 1 of WO2023 / 135261, the contents of which are incorporated herein by reference.

[0185] Typically, the cellulose microparticles are obtained via sulfuric acid hydrolysis of biomass using particular acid concentration, temperature, and reaction time to control the morphology and physical dimensions.

[0186] Generally, the cellulose microparticles are prepared by a method comprising: (a) adding acid to a cellulose material to hydrolyse the cellulose particles;

[0187] (b) removing the acid from the hydrolysed cellulose particles; and

[0188] (c) isolating a particular size of hydrolysed cellulose particles.

[0189] The step (a) of “Adding acid to the cellulose particles” may be referred to as the hydrolysing step. The step (b) of “removing acid from the hydrolysed cellulose particles” may be referred to as the washing step. The step (c) of “isolating a particular size of hydrolysed cellulose particles” may be referred to as the fractioning step.

[0190] The application also provides a method of preparing a cellulose particle, the method comprising:

[0191] (a) hydrolysing a cellulose material to provide hydrolysed cellulose particles;

[0192] (b) washing the hydrolysed cellulose particles; and

[0193] (c) fractioning a suspension of the hydrolysed cellulose particles.

[0194] The method is suitable for preparing the cellulose microparticles for use in the coating composition and opacifier of the invention. However, cellulose microparticles prepared by alternative methods may also be used, provided such particles have properties required by the opacifier.

[0195] The conditions used for hydrolysis step, washing step, and fractioning step can be adjusted to prepare a cellulose microparticle having the size and shape described herein.

[0196] Any suitable cellulose material may be used. Suitable cellulose material may be prepared from bacterial, vegetal or animal sources (e.g. chitin), including plant-based and biomass source such as cotton and wood along with subsequent processed products such as paper, filter-paper cotton linters, cellulose powder and wood pulp.

[0197] Preferably, the cellulose material is a microcrystalline cellulose powder. Suitable microcrystalline cellulose powder is commercially available (e.g. from SERVA Electrophoresis GmbH)

[0198] The cellulose material may be provided in the form of a suspension. Preferably a microcrystalline cellulose powder is dispersed in water to form an aqueous suspension.

[0199] A suspension is a heterogeneous mixture of a fluid that contains solid particles that are typically sufficiently large for sedimentation to occur if the suspension is left undisturbed for extended periods of time. The cellulose particles are suspended in the liquid. The suspension may be mixed, for example by sonification, to avoid settling of the cellulose particles. Any suspension medium suitable for holding cellulose material may be used. Suitable suspension media are typically aqueous solvents, such as water. Acidic and basic media may be used. Typically, acid is used, and suitable acids are set out below.

[0200] The method of preparing a cellulose particle comprises hydrolysing a cellulose material to provide hydrolysed cellulose particles. This may be known as the hydrolysis step. The hydrolysis step takes place before the washing step.

[0201] The hydrolysis is typically carried out in aqueous solvent (e.g. water).

[0202] In the hydrolysis step, the cellulose material is typically hydrolysed with an acid. Acid hydrolysis is a hydrolysis process in which a protic acid is used to catalyse the cleavage of a chemical bond via a substitution reaction with the addition of water. During acid hydrolysis, it is also proposed that the cellulose chain backbone of the cellulose particle is modified at the molecular level to provide colloidal stability to the cellulose particle. For example, sulfuric acid hydrolysis is thought to modify the cellulose chains with sulfate half ester groups.

[0203] The hydrolysis step may comprise contacting the cellulose material, such as the suspension of cellulose material, with acid.

[0204] The acid may be an organic acid or an inorganic acid. Typically, the acid is an inorganic acid (a mineral acid). Suitable inorganic acids include hydrobromic acid (HBr), hydrochloric acid (HCI), hydrofluoric acid (HF), hydroiodic acid (HI), nitric acid (HNO3), perchloric acid (HCIO4), phosphoric acid (H3PO4), sulfuric acid (H2SO4) or a combination thereof. Preferably the inorganic acid is sulfuric acid or hydrochloric acid. More preferably the inorganic acid is sulfuric acid.

[0205] Suitable organic acids include formic acid (HCOOH) and acetic acid (CH3COOH). Preferably the organic acid is formic acid.

[0206] Preferably the acid is sulfuric acid or hydrochloric acid. More preferably the acid is sulfuric acid.

[0207] The strength of an (aqueous) acid can be specified using the pH scale. Methods for determining the pH of an aqueous solution are known and include, for example, electrochemical methods (using a pH probe) and titration against an indicator compound such as universal indicator. Typically, the pH refers to the pH of the hydrolysis solution at the end of the hydrolysis step.

[0208] The acid used in the hydrolysis step is highly acidic. Typically, the acid has a pH of 1.0 or lower, preferably, 0.5 or lower, more preferably or 0.0 or lower. The strength of the acid is proportional to the concentration of the acid. The concentration of an aqueous acid can be specified using volume percentage (wt.%).

[0209] Typically, the concentration of the acid is from 40 to 60 wt.%, preferably from 45 to 55 wt.%, more preferably from 47 to 55 wt.%, yet more preferably from 49 to 52 wt.%. The wt.% is typically calculated with water as the solvent.

[0210] The amount of acid is selected to allow a desirable quantity of cellulose material to be suspended. The amount of aqueous acid can be specified by stating the volume aqueous acid in mL per gram of cellulose material used (the ratio of acid to cellulose material).

[0211] Typically, the hydrolysis step uses a mass ratio of acid to cellulose material of 100:1 preferably 80:1, more preferably 60:1. In addition or alternatively, the hydrolysis step may use a mass ratio of acid to cellulose material of 4: 1 , preferably 10: 1. For example, the hydrolysis step may use a mass ratio of acid to cellulose material of from 100:1 to 4:1, preferably from 80:1 to 10:1. A higher ratio of cellulose to acid make the process more efficient as less acid is required for the hydrolysis step and less solvent is required for the washing step.

[0212] The hydrolysis step may be performed for a sufficient time to allow a desirable quantity of cellulose material to be hydrolysed. Typically, the hydrolysis is performed for from 1 to 10 hours, preferably from 2 to 8 hours, more preferably from 3 to 7 hours, such as about 5 hours.

[0213] The hydrolysis takes place from the addition of the acid until the reaction is quenched. The reaction may be quenched by any suitable means, such as by adding water to dilute the acid, adding base to neutralise the acid, removing the acid (e.g. by washing such as by dialysis), or reducing the temperature.

[0214] The hydrolysis step may be performed at elevated temperature (above ambient temperature; approximately 25°C). Methods for providing heat during the hydrolysis step are known and include, for example, using a reaction vessel having an external heating jacket or using microwave heating.

[0215] Typically, the hydrolysis step is performed at a temperature of from 40 to 60°C, preferably from 45 to 55°C, more preferably from 48 to 52°C, such as about 50°C.

[0216] Preferably, the cellulose material is hydrolysed with 50 wt.% sulfuric acid at a temperature of about 50°C for 3 to 5 hours.

[0217] In some embodiments, in step (b) the cellulose material is hydrolysed with 50 wt.% sulfuric acid 5 hours at a temperature of 50°C. In some embodiments, in step (b) the cellulose material is hydrolysed with 55 wt.% sulfuric acid 5 hours at a temperature of 60°C. The hydrolysis may be stopped by quenching the acid hydrolysis. Typically, the hydrolysis is quenched by the addition of water, such as an excess of water. For example, if 60 mL of sulfuric acid is used to hydrolyse cellulose particles, then 300 mL of water may be added to quench the acid hydrolysis.

[0218] It is thought that using a higher concentration of acid, a higher temperature or a longer time period increases the rate of acid hydrolysis. Typically, a greater degree of hydrolysis results in a smaller cellulose particle size.

[0219] The hydrolysed cellulose particles may be collected by any suitable method. Typically, the hydrolysed cellulose particles are collected by centrifugation. Centrifugation can separate the hydrolysed cellulose particles from the acid supernatant, and optionally the water added to quench the acid hydrolysis. The supernatant can then be removed, for example using a pipette.

[0220] The method of preparing a cellulose particle comprises: washing the hydrolysed cellulose particles.

[0221] This may be known as the washing step. The washing step takes place after the hydrolysis step and before the fractionation step.

[0222] The hydrolysed cellulose particles may be washed by adding water. The water may then be removed. Thus, the washing step may comprise contacting the hydrolysed cellulose particles with water.

[0223] The washing step typically quenches the hydrolysis reaction, and so ends the hydrolysis step. The washing step may dilute the acid from the cellulose particles, thus quenching the hydrolysis reaction.

[0224] Typically, the washing step may add sufficient water to provide a concentration of 1 wt.% of cellulose particles. For example, for 1 g of hydrolysed cellulose particles, around 100 mL of water may be added per wash. This may be repeated one or more times, preferably two or more times, more preferably three or more times to remove the acid. The washing water may then be removed by centrifugation.

[0225] Preferably, the washing step may alternatively or additionally comprise dialyzing the hydrolysed cellulose particles with water, such as distilled water. The dialysis may take place after washing with water as set out above. Dialysis comprises resuspending the hydrolysed cellulose particles in distilled water and purifying them from dissolved ions (e.g. the acid used in the hydrolysis step) by means of their unequal rates of diffusion through the pores of semipermeable (dialysis) membrane. Any suitable dialysis membrane may be used. Suitable dialysis membranes have molecular weight cut off of 12 kDa or more, such as from 12 to 40 kDa. The distilled water may be replaced during the dialysis process. For example, the distilled water may be replaced every 12 hours during dialysis. The distilled water may be replaced 5 or more times, preferably 10 or more times.

[0226] In some embodiments, the hydrolysed cellulose material is dialyzed against distilled water for 7 days, replacing the distilled water every 12 hours.

[0227] In some embodiments, the hydrolysed cellulose material is dialyzed until the pH of the hydrolysed cellulose material has stabilized. The pH may be measured each time the distilled water is replaced during the dialysis process, such as every 12 hours. Typically, the pH of the hydrolysed cellulose material has stabilized once the pH is constant for at least two consecutive measurements, preferably four consecutive measurements, more preferably six consecutive measurements. Alternatively, the pH may be measured once per day, and the pH has stabilized once the pH is constant for at least two consecutive days, preferably three consecutive days, more preferably four consecutive days.

[0228] The constant pH indicates that the acid or base has been substantially removed from the hydrolysed cellulose material. Methods for determining the pH of an aqueous solution are known, as set out above.

[0229] In some embodiments, the washing step may comprise removing the acid from the cellulose particles by centrifugation, washing with water and removing the water by centrifugation, and dialyzing the hydrolysed cellulose material.

[0230] The method of preparing a cellulose particle comprises: fractioning a suspension of the hydrolysed cellulose particles.

[0231] This may be known as the fractionation step. The fractionation step takes place after the washing step.

[0232] The hydrolysed cellulose particles may be fractioned by any suitable method. Suitable methods include filtration and centrifugation. Preferably the hydrolysed cellulose particles are separated from the liquid by differential centrifugation.

[0233] The fractioning takes place on a suspension of the hydrolysed cellulose particles. For the fractioning step, the suspension of hydrolysed cellulose particles is typically a suspension of individual cellulose particles. That is, the hydrolysed cellulose particles are not significantly aggregated, or aggregated, in the suspension of hydrolysed cellulose particles. This is beneficial, since it avoids the fractioning step removing aggregations of cellulose particles having the desired size and shape. This improves the yield of cellulose particles having the desired size and shape.

[0234] The fractioning (such as centrifugation) is carried out on a suspension of the cellulose particles. Typically, a suspension of the cellulose particles in water, such as Millipore water, is used. Any suitable concentration may be used. Suitable concentrations comprise from 0.1 wt% to 5.0 wt% cellulose particles, such as 0.2 wt% to 2.0 wt%, such as 0.2 wt% to 1.0 wt%. Preferably, the concentration of hydrolysed cellulose particles is 0.5 wt%.

[0235] The suspension of the cellulose particles may be prepared by any suitable method. The hydrolysed cellulose particles may be mixed or agitated prior to separation. Typically, the hydrolysed cellulose particles are sonicated prior to separation, such as by tip sonication or ultrasonifi cation.

[0236] In some embodiments, a 30 ml suspension with 0.5 wt% particle concentration of the hydrolysed cellulose particles is ultrasonicated at an Amplitude of 30 % for 2 min in 2s on 2s off cycles. Ultrasonification may be performed using any suitable apparatus, such as a Fisherbrand Ultrasonic disintegrator, 20 kHz, tip diameter 12.7 mm.

[0237] Typically, the differential centrifugation comprises a first centrifugation and a second centrifugation. The first centrifugation is at a different speed than the second centrifugation. The first centrifugation is at a lower speed than the second centrifugation. The second centrifugation is typically carried out on the supernatant from the first centrifugation.

[0238] The first centrifugation may be at a speed of from 1,000 to 3,000 rpm, preferably from 1,500 to 2500 rpm, more preferably from 1 ,800 to 2200 rpm, such as around 2,000 rpm.

[0239] The second centrifugation may be at a speed of from 2,000 to 4,000 rpm, preferably from 2500 to 3500 rpm, more preferably from 2,800 to 3,200 rpm, such as around 3,000 rpm.

[0240] Centrifugation speed may also be quantified using relative centrifugal force (RCF). RCF is a measure of the force acting on the particles during centrifugation. RCF is generally expressed as multiples of the earth's gravitational field (g). RCF may be calculated by the following equation, where radius (cm) is the distance from the centre of the centrifuge to the extremity of the sample and rotation speed (rotation per minute) is the rotation speed of the centrifuge. The radius is typically about 15 cm.

[0241] RCF = 11.2 x radius x (rotation speed / 1000)2.

[0242] The RCF of the first centrifugation may be different to the second centrifugation. The RCF of the first centrifugation may be less than the second centrifugation.

[0243] The first centrifugation may be at a RCF of from 50 to 1 ,500, preferably 150 to 1 ,500, more preferably from 300 to 1 ,100, yet more preferably from 500 to 900, even more preferably from 600 to 800. The first centrifugation may be at a RCF of about 650. The second centrifugation may be at a RCF of from 600 to 2,600, preferably from 1 ,000 to 2,000, more preferably from 1,300 to 1,700, yet more preferably from 1 ,400 to 1 ,600. The second centrifugation may be at a RCF of about 1 ,500.

[0244] The first centrifugation may be at a RCF of from 168 to 1 ,512, preferably from 378 to 1 ,150, more preferably from 544 to 813. The first centrifugation may be at a RCF of about 672.

[0245] The second centrifugation may be at a RCF of from 671 to 2,688, preferably from 1 ,050 to 3,058, more preferably from 1,317 to 1,720. The second centrifugation may be at a RCF of about 1,512.

[0246] The first centrifugation may be carried out for a time of from 1 to 20 minutes, preferably from 2 to 15 minutes, preferably from 3 to 10 minutes, more preferably from 4 to 6 minutes, such as around 5 minutes.

[0247] The second centrifugation may be carried out for a time of from 1 to 20 minutes, preferably from 2 to 15 minutes, preferably from 3 to 10 minutes, more preferably from 4 to 6 minutes, such as around 5 minutes.

[0248] Preferably the first centrifugation is carried out at 2000 rpm for five minutes and the second centrifugation is carried out at 3000 rpm for five minutes, and the second centrifugation is carried out on the supernatant from the first centrifugation.

[0249] It is thought that a two-step differential centrifuging process results in a narrower particle size distribution for the cellulose particles. The first centrifugation at a lower speed sediments the larger cellulose particles. The supernatant from the first centrifugation thus still includes the desired size particles and smaller particles. This supernatant from the first centrifugation is then centrifuged again at a high speed so as to sediment the desired particles. The smaller particles remain in the supernatant from the second centrifugation. As a result, the sedimented particles from the second centrifugation do not include a large proportion of larger or smaller particles, so the particle size distribution is narrower.

[0250] The sedimented cellulose particles may be collected by any suitable method. Typically, the cellulose particles are collected by filtration.

[0251] The cellulose particles are typically collected as a slurry or a suspension. The slurry or suspension may be with water, ethanol or acetone. Preferably, the slurry or suspension is with water.

[0252] The sedimented larger cellulose particles from the sediment fraction of the first centrifugation may be recirculated and reprocessed by repeating the sonication and fractionation steps described above. This may improve the yield of the fraction containing the desired particle sizes. The method of preparing a cellulose particle may also comprise: drying the fractionated cellulose particles.

[0253] This may be known as the drying step. The drying step typically takes place after the fractionation step. The drying step is typically carried out on the slurry or suspension of cellulose particles prepared by the fractionation step.

[0254] Typically, the drying step comprises removing a solvent from the fractionated cellulose particles. The drying step typically comprises removing water, ethanol or acetone from the fractionated cellulose particles. Preferably, the drying step comprises removing water from the fractionated cellulose particles.

[0255] The fractionated cellulose particles may be dried by any suitable method. Suitable methods include evaporation, freeze-drying, spray-drying or spray-freeze drying. Preferably, the method is freeze-drying, spray-drying or spray-freeze drying. More preferably, the method is freeze-drying.

[0256] Any suitable freeze-drying apparatus may be used. Suitable freeze-drying apparatus include Scanvac and Coolsafe freeze driers by LaboGene A / S, or VirTis freeze dryer by SP Scientific.

[0257] Any suitable spray-drying apparatus may be used. Suitable spray-drying apparatus include a PrecisionCoat spray coater by Specialty Coating Systems (SCS).

[0258] Any suitable spray-freeze drying apparatus may be used. Suitable spray-freeze drying apparatus include a PrecisionCoat spray coater by SCS and Scanvac and Coolsafe freeze driers by LaboGene A / S.

[0259] Typically, the drying step provides a dry powder of cellulose particles. Freeze drying or spray-drying may provide a dry powder of cellulose particles. Preferably, freeze drying provides a dry powder of cellulose particles.

[0260] The drying step may provide clusters of cellulose particles. Preferably, spray drying or sprayfreeze drying provides clusters of cellulose particles.

[0261] The shape and size of the dried cellulose particles are generally the same as the cellulose particles before drying.

[0262] The dry powder of cellulose particles is useful for many applications. Several applications require the cellulose particles to be provided as a dry powder, such that the cellulose particles can be redispersed in different media.

[0263] The dry powder also minimizes storage and transportation cost due to a reduced mass of solvent. The dry powder also inhibits fungal and bacterial growth in the cellulose particles. The dry powder further allows the cellulose particles to be redispersed in a different polarity solvent to that used for the preparation of the cellulose particles. For example, if the cellulose particles are prepared in a polar solvent (e.g. water), the dry powder can be redispersed in a non-polar solvent (e.g. organic solvent). The dry powder may also undergo further chemical modifications. The dry powder may be redispersed in an organic solvent prior to further chemical modifications.

[0264] Optionally, the method of preparing cellulose particles may further comprise: modifying the surface of the cellulose particles.

[0265] This may be known as the surface modification step. Typically, the surface modification step is carried out after fractionation of the cellulose particles.

[0266] The surface modification may be carried out on any cellulose particles. Typically, the surface modification is carried out on the cellulose particles obtained in the fractionation step or the drying step described above.

[0267] The surface modification step typically comprises transforming a hydroxyl group on the surface of the cellulose particle into a different functional group. The hydroxyl group is preferably converted into an ester (esterification) or an ether (etherification). For example, the hydroxyl groups may be converted into an ether group, such as a silylether group.

[0268] In some embodiments, the surface modification step comprises contacting the cellulose particles with an esterification agent or an etherification agent. Preferably, the surface modification step comprises contacting the cellulose particles with a hydrophobic agent.

[0269] Any suitable reagent may be used to perform the surface modification. Preferably, the reagent is an esterification reagent or an etherification reagent. Preferably, the surface modification step involves treating the cellulose particle with anhydrite, acyl chloride or epoxy bearing reagents. The hydrophobic agent includes any suitable hydrophobic agent, such as trimethylchlorosilane (TCMS) and chlorotrimethoxysilane. Preferably the hydrophobic agent is TCMS.

[0270] The reagent may be liquid or a vapour (gas). Typically, the reagent agent is a vapour.

[0271] The surface modification may be carried out in a solution or in a gas phase reaction. The solution may be an aqueous solution or a non-aqueous solution. The surface modification may be carried out under an inert atmosphere, such as a nitrogen or an argon atmosphere.

[0272] The hydroxyl groups on the surface of the cellulose particle may be activated before the surface modification reaction. Any suitable activator may be used. Preferably the activator is a basic solution, preferably a sodium hydroxide solution. The surface modification may be carried out in the presence of a catalyst. Any suitable catalyst may be used. Preferably the catalyst is a basic catalyst. Preferably the catalyst is pyridine, 4-dimethylaminopyridine or trimethylamine.

[0273] The surface modification step may be carried out for any suitable length of time. Typically, the surface modification step is carried out 5 minutes or less, such as 3 minutes or less, such as 2 minutes or less, such as 1 minute or less.

[0274] The surface modification step provides surface modified cellulose particle. Without wishing to be bound by theory, it is thought that modifying the hydroxyl groups on the surface of the cellulose particles, such as with an ester or an ether alters the hydrogen bonding between the surfaces of the cellulose particles. The modification may increase or decrease the hydrogen bonding. Preferably the modification reduces the hydrogen bonding between the hydroxyl groups so the pores or voids between the cellulose particles are less prone to collapse under capillary pressure, for example, during solvent evaporation.

[0275] The surface modification with a hydrophobic agent provides hydrophobic cellulose particles. It is thought that replacing a portion of the hydroxyl groups on the surface of the cellulose particles with a hydrophobic group (such as -OTMS) reduces the hydrogen bonding between the hydroxyl groups so the pores or voids between the cellulose particles are less prone to collapse under capillary pressure, for example, during solvent evaporation. The dispersibility of the particles in solution is also able to be customised, for example to increase dispersibility in non-polar solvents.

[0276] Surfactant

[0277] A surfactant is present in the coating composition and the opacifier, as well as other aspects of the invention.

[0278] The surfactant assists in dispersion of the cellulose microparticles. Cellulose microparticles are hydrophilic and cannot be easily dispersed in nonpolar solvents. Due to their relatively large size, the cellulose particles can sediment under gravity if no external mechanical action is applied after a certain period.

[0279] Surfactants, such as nonionic surfactants, are physically mixed with the mildly charged cellulose microparticles to create a sterically stable combination that can disperse well in both polar and nonpolar solvents, and can efficiently interact in both hydrophobic and hydrophilic systems. Thus, non-ionic surfactants are preferred when the solvent is relatively non-polar (e.g., acetone)

[0280] For highly or diversely charged systems or suspensions, zwitterionic surfactants and CMPs can create an ideal combination for incorporation in polar solvents. Thus, zwitterionic surfactants are preferred when the solvent is relatively polar (e.g., water, ethanol). The surfactants can also improve stability in emulsions, such as oil and water emulsions.

[0281] This ensures a homogeneous and well dispersed colloidal suspension, which is stable over long time periods (e.g., exceeding several days).

[0282] A surfactant is typically an organic compound with a relatively hydrophilic section and a relatively hydrophobic section. A surfactant may include a ‘head’ group and a ‘tail’ group, wherein the head and tail are hydrophilic or hydrophobic.

[0283] Typically, the head group is hydrophilic. The head group may be a polar and optionally charged group. For example, the head group may be a carboxylate, sugar, phosphate, or ammonium group.

[0284] Typically, the tail group is hydrophobic. The tail group may be a hydrocarbon chain, such as branched, linear, or aromatic carbon chain. The tail group may be hydrophilic chain, such as a fluorocarbon chain, a polyether chain or a polyether modified siloxane chain. The tail group may be modified to alter the degree of hydrophobicity or hydrophilicity.

[0285] A surfactant may have one or more head groups and one or more tail groups.

[0286] Surfactants may be classified according to the charge of the head group or the hydrophilic group. Surfactants may be categorized as non-ionic, anionic, cationic, or zwitterionic.

[0287] Zwitterionic surfactants may also be known as amphoteric surfactants.

[0288] A non-ionic surfactant has a net zero charge at the head / hydrophilic group.

[0289] An anionic surfactant has a (net negative charge at the head / hydrophilic group).

[0290] A cationic surfactant has a net positive charge at the head / hydrophilic group.

[0291] A zwitterionic surfactant has a net zero charge, with a positive and negative charged centre within the head / hydrophilic group.

[0292] The charge may depend on the pH of the solution in which the surfactant is present. The charge of the head / hydrophilic group is determined at the pH of the coating composition or opacifier in the invention. This is typically a pH of about 7.

[0293] Preferably, the surfactant is a non-ionic, cationic or zwitterionic surfactant, such as at a pH of about 7. The surfactant is particularly preferably a non-ionic surfactant, such as at a pH of about 7.

[0294] Suitable non-ionic surfactants may include ethoxylate surfactants, polyethylene glycol monododecyl ether surfactants, alkylphenol ethoxylates, Nonoxynols, polyethoxylated tallow amine, cocamide monoethanolamine, cocamide diethanolamine, terminally blocked ethoxylates, poloxamers, fatty acid esters of polyhydroxy compounds, fatty acid esters of glycerol, glycerol monostearate, glycerol monolaurate, fatty acid esters of sorbitol, fatty acid esters of sucrose, alkyl polyglucosides, alkyl polyglycosides.

[0295] Suitable cationic surfactants may include alkyl ammonium surfactants, cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), dimethyldioctadecylammonium chloride and dioctadecyldimethylammonium bromide (DODAB).

[0296] Suitable zwitterionic surfactants may include a combination of a cationic part based on primary, secondary, or tertiary amines or quaternary ammonium cations and an anionic part based on a sulfonate, carboxylate, or phosphate. For example, zwitterionic surfactants may include phospholipids such as phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelins, betaines such as cocam idopropyl betaine, sultaines such as CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1 -propanesulfonate) and cocamidopropyl hydroxysultaine, lauryldimethylamine oxide and myristamine oxide.

[0297] Surfactants may be characterised by their critical micellar concentration (CMC). CMC refers to the concentration of surfactant at which micelle formation is first seen in the solution.

[0298] Above this concentration the surfactant present and any additional surfactant added will form a micelle. A micelle is a self-assembled aggregate of surfactant.

[0299] The CMC is provided for a given dispersant (solvent) at a specified temperature and pressure. Herein, CMC values are given for water as solvent, at a temperature of 20 °C and a pressure of 1 atm. Deviations from these conditions are noted, where present. Typically, the critical micellar concentration is measured according to ISO 4311:1979.

[0300] In some embodiments, the surfactant has a critical micellar concentration of 0.03 mM or more, preferably 0.04 mM or more, more preferably 0.06 mM or more. In some embodiments, the surfactant has a critical micellar concentration of 30 mM or less, preferably 25 mM or less, more preferably 20 mM or less, yet more preferably 1 mM or less, even more preferably 1.5 mM or less, most preferably 0.9 mM or less.

[0301] In some embodiments, the surfactant has a critical micellar concentration of from 0.03 to 30 mM, preferably from 0.04 to 25 mM, more preferably from 0.05 to 10 mM, yet more preferably from 0.05 to 1.5 mM, even more preferably from 0.06 to 0.9 mM.

[0302] For non-ionic surfactants, the critical micelle concentrations of the surfactant are preferably 0.06-0.9 mM (measured at 25°C and 1 atm, in water as solvent). For a cationic or zwitterionic surfactant, the critical micelle concentrations of the surfactant are typically higher, such as 0.9 to 25 mM (measured at 25°C and 1 atm, in water as solvent). The critical micelle concentration (CMC) is typically defined as the concentration of surfactants above which micelles form and all additional surfactants added to the system will form micelles. The critical micelle concentration may be measured using ISO 4311:1979.

[0303] The inventors have surprisingly found that the use of a surfactant having these specific CMC values results in particularly excellent dispersion of the cellulose microparticles, and thus provides coating compositions and coatings with excellent homogeneity. As a result, the coatings have reliable and consistent optical properties, such as whiteness (L*) and opacity.

[0304] In some embodiments, the surfactant is Triton X-100, Tween 20, ECOSURF SA-9, TERGITOL 15-S-9, TERGITOL TMN-100X, hexadecyltrimethylammonium bromide (CTAB), 3-(Decyldimethylammonio)-propane-sulfonate inner salt, or a combination thereof.

[0305] In some embodiments, the surfactant is a non-ionic surfactant selected from Triton X-100, Tween 20, ECOSURF SA-9, TERGITOL 15-S-9, TERGITOL TMN-100X, or a combination thereof.

[0306] In some embodiments the surfactant is a cationic surfactant which is hexadecyltrimethylammonium bromide (CTAB).

[0307] In some embodiments the surfactant is a zwitterionic surfactant which is 3-(Decyldimethylammonio)-propane-sulfonate inner salt.

[0308] Triton X-100 has a structure given by Formula (1). Triton X-100 is also known as polyethylene glycol p-(1 ,1,3,3-tetramethylbutyl)-phenyl ether. The average number of polyethylene glycol repeating units is 9.5 (n=9.5). The CMC is 0.22 mM.

[0309] Tween 20 has a structure given by Formula (2). Tween 20 is also known as polyoxyethylene (20) sorbitan monolaurate. The average total number of polyethylene glycol repeating units is 20 (hence the sum of w+x+y+z is 20). The CMC is 0.08 mM.

[0310] ECOSURF SA-9 has the CAS number 68937-66-6. The CMC is 22ppm (25 °C, 1atm, water solvent).

[0311] TERGITOL 15-S-9 has a structure given by Formula (3). The value of n + m is from 2 to 6. TERGITOL 15-S-9 has the CAS number 84133-50-6. The CMC is 52ppm. The molecular weight is 596 g / mol, which give a CMC is 0.037mM.

[0312] TERGITOL TMN-100X has the CAS number 60828-78-6. The CMC is 830ppm. The molecular weight is 570 g / mol, which gives a CMC of 1.46 mM.

[0313] Hexadecyltrimethylammonium bromide (CTAB) has a structure given by Formula (4). CTAB has a CMC of 0.9 mM.

[0314] 3-(Decyldimethylammonio)-propane-sulfonate inner salt has a structure given by Formula (5). The CMC of 3-(Decyldimethylammonio)-propane-sulfonate inner salt is 25-40 mM (20-25°C). Additives

[0315] The opacifier and / or the coating composition may further comprise additives. These additives are not particularly limited. The additives may be included in the opacifier component (e.g., combined with the cellulose microparticle and surfactant in a powder or paste). Alternatively, the additives may be included in the coating composition (e.g., incorporated into the liquid carrier).

[0316] The additives may include colourants, emollients, oils, polymers, waxes and gelling agents, The additives are typically suited for the intended application and use. The opacifier and / or the coating composition may comprise one or more additives, such as two or more additives, or three or more additives.

[0317] The additive may comprise a colourant. If a specific colourant is added to the initial CMP suspension, then films of modulated colour could be produced. It is apposite to note that the individual cellulose microparticles retain their ability to scatter light and form a colloidal white suspension, but these particles can only assemble into a continuous white film if aided by a scaffold.

[0318] The colourant may include any suitable dye or pigment. The colourant may be a organic or inorganic colourant, preferably an organic colourant. The colourant may absorb narrow bands of light (specific wavelengths of light), to impart colour to the coating. This differs from the opacifying function of the cellulose microparticles, which reflect light across a broad range of wavelengths.

[0319] The colourant may be a Beta-naphthol, BON arylamides, Benzimidazolone, Disazo condensation, Quinacridone, Perylene, Anthraquinone, Dibromanthrone, Pyranthrone, Diketopyrrolo-pyrrole pigments (DPP), Copper phthalocyanine, Indanthrone, Phthalocyanine green, Dioxazine violet, Perinone orange, Pyrazolone orange, Carbon Black, Graphite, Aniline Black, Anthraquinone Black, Benzimidazolone, Disazo condensation, or combinations thereof.

[0320] The cellulose microparticles coating formulation can also be mixed with any colourant to modulate shades of a particular colour. The colourant is preferably provided in the carrier liquid. For example, the carrier liquid may be a paint.

[0321] The additives may comprise emollients. Suitable emollients include ammonium lactate, petrolatum, salicylic acid, urea.

[0322] The additives may comprise oils.

[0323] Suitable oils include algal oil, annatto oil, argan oil, almond oil, apricot kernel oil, avocado oil, babassu oil, Brazil nut butter, butter, cashew butter, castor oil, camellia oil, cheery kernel oil, cocoa butter, coconut oil, corn oil, cottonseed oil, fish oil, grape seed oil, gardenia oil, ghee, hazelnut oil, jatropha oil, jojoba oil, kokum oil, linseed oil, macadamia oil, maize oil, mango seed oil, mango butter, mineral oil, mink oil, olive oil, palm oil, palm kernel oil, peach kernel oil, peanut butter, peanut oil, plum kernel oil, pomegranate oil, rapeseed seed oil, rice bran oil, rosehip oil, sal oil, sesame oil, shea butter, soybean oil, squalene, sunflower oil, teas seed oil, walnut oil. Oil derivatives obtained from the aforementioned oils such as esterified oils, fatty acids, fatty alcohol, hydrogenated oils and triglycerides can be used as suitable ingredient for the said formulation. Essential oils are also suitable oils.

[0324] The additives may comprise polymers. Suitable polymers may include stabilisers, plasticizers and flame retardants.

[0325] Polymer stabilisers refer to any polymer suitable for inhibit or retard the degradation of the opacifier, coating composition or coating. The polymer stabiliser may be a UV absorber, antioxidant and biocides.

[0326] Plasticizers refer to polymers which increase the plasticity of the coating. This may include a phthalate-based plasticisers, DMP, DEP, DIBP, DBP, DINP, Terephthalates, Trimellitates and organophosphates.

[0327] Flame retardant polymers refer to polymers which inhibit or retard combustion. This may include organohalogen or organophosphorous compounds.

[0328] Preferably, the polymer additive is a a poly(meth)acrylate polymer such as a polyacrylate. The polymer additive may be in the form of a monomer, preferably a (meth)acrylate. The monomer may be a mono-functional monomer (e.g., (meth)acrylate)) or a di-functional monomer (e.g., di(meth)acrylate). In this way, the monomer may act as a binder or crosslinker. The polymer additive may be cured, such as during drying of the coating. The polymer additive may enhance the adhesion of the coating to a substrate, increase the abrasion resistance, and improve the flexibility of the coating.

[0329] The additives may comprise waxes.

[0330] Suitable wax include beeswax, candelilla wax, carnauba wax, Japan wax, lanolin, palm wax, paraffin.

[0331] The additive may comprise gelling agents.

[0332] Suitable gelling agents includes cellulose-derivative thickener, such as hydroxyethyl cellulose, as well as acacia gum, agar, aloe gel, gelatin, guar gum, gum arabic, gum tragacanth, pectin, alginates, starches, carrageenan, and xanthan gum. Preferably the gelling agent is a cellulose derived gelling agent, such as hydroxyethyl cellulose. Where the additive is present in the coating composition, the additive may be provided as part of the carrier liquid. The carrier liquid may be a composition which includes such additives.

[0333] The carrier liquid may be a food composition (such as a pet food composition), cosmetic composition, personal care composition, pharmaceutical composition, ink, paint, laminate composition, washing powder composition, building materials composition. These composition include the typical components needed to carry out their function.

[0334] For example, the carrier liquid may be a paint base, comprising a binder and water. In particular, the carrier liquid may be an acrylic paint base, comprising an acrylate binder and water.

[0335] Method of Preparing Opacifier

[0336] In a fourth aspect there is provided a method of preparing the opacifier of the second aspect, the method comprising: adding a cellulose microparticle having a mean average particle length of from 0.7 to 9 pm and a surfactant to a carrier liquid, to form a suspension; optionally dispersing the cellulose microparticle and surfactant in the carrier liquid to disperse the suspension; and drying the suspension to provide the opacifier.

[0337] The method of preparing the opacifier may also comprise: providing cellulose microparticles and a surfactant.

[0338] This may be known as a “preparation step”, and refers to making a suitable cellulose microparticle and a suitable surfactant available. The preparation step typically takes place before the addition step.

[0339] Any suitable cellulose microparticle and surfactant may be used, as described herein. The cellulose microparticles and / or surfactant may be provided in a suspension or solution in a solvent. The suspension may also take the form of a slurry. Alternatively, the cellulose microparticles and / or surfactant may be provided as a dry powder or paste.

[0340] The step of adding a cellulose microparticle having a mean average particle length of from 0.7 to 9 pm and a surfactant to a carrier liquid, to form a suspension, may be referred to as the “addition step”.

[0341] The cellulose microparticles and surfactant may be added separately or together. The carrier liquid used may be the same as described above, in the context of the coating composition. Preferably the carrier liquid is water. Where the cellulose microparticles and / or surfactant are provided in a suspension or solution, the addition step simply refers to the combination of the suspension to form a suspension of both the cellulose microparticles and surfactant.

[0342] A suspension is a heterogeneous mixture of a fluid that contains solid particles that are typically sufficiently large for sedimentation to occur if the suspension is left undisturbed for extended periods of time. The cellulose microparticles are suspended in the liquid. The surfactant may be dissolved in the carrier liquid. The suspension may also take the form of a slurry.

[0343] The step of optionally dispersing the cellulose microparticle and surfactant in the carrier liquid to disperse the suspension may be referred to as the “dispersing step”.

[0344] The dispersing step may comprise sonicating a suspension. The sonication may be tip sonification or ultra-sonification. The sonication may be at an energy of from 400 to 1 ,000 J / g, preferably from 500 to 900 J / g, more preferably from 600 to 800 J / g. Preferably sonication is at an energy of about 700 J / g.

[0345] The sonication may be performed by any suitable apparatus, such as a Fisherbrand Ultrasonic disintegrator.

[0346] The step of drying the suspension to provide the opacifier may be referred to as the “drying step”. The drying step typically takes place after the addition step or, where present, the dispersing step. The drying step is typically carried out on the slurry or suspension of cellulose microparticles and surfactant.

[0347] Typically, the drying step comprises removing the carrier liquid (solvent) from the suspension. The drying step typically comprises removing water, ethanol or acetone from the suspension. Preferably, the drying step comprises removing water from the suspension.

[0348] The suspension may be dried by any suitable method. For example, the suspension may be dried by evaporation, freeze-drying, spray-drying or spray-freeze drying. The apparatus may be as described above for the preparation of the cellulose microparticles.

[0349] The drying step may remove substantially all of the carrier liquid (solvent). For example, the freeze-drying, spray-drying or spray-freeze drying may remove substantially all of the carrier liquid. This typically produces an opacifier powder, wherein the powder comprise clusters of cellulose microparticles and surfactant.

[0350] Typically, the drying step provides a dry powder of cellulose microparticles and surfactant. Freeze drying or spray-drying may provide a dry powder. Preferably, spray drying provides a dry powder of cellulose microparticles and surfactant. The opacifier powder may be as described herein.

[0351] Alternatively, the drying step may only remove a portion of the carrier liquid (solvent). In this way, the suspension is only partially dried.

[0352] For example, the evaporation, freeze-drying, spray-drying or spray-freeze drying may remove only some of the carrier liquid. This typically produces an opacifier powder, paste or slurry, which retains some solvent. The partial drying preferably produces an opacifier paste or slurry.

[0353] The opacifier paste or slurry is typically a relatively concentrated mixture of cellulose microparticles and surfactant, partially suspended in a carrier liquid. The opacifier paste of slurry is as described herein.

[0354] The method may of preparing the opacifier may also comprise: adding one or more additives to the opacifier.

[0355] The step of adding one or more additives may be carried out sequentially with the addition step. The step of adding one or more additives may be carried out after the dispersion step, where present. Preferably, the step of adding one or more additives is carried out after the addition step and before the dispersion step. Thus, the additives may also be dispersed in the dispersion step.

[0356] Where multiple additives are added, the additives may be added separately or together.

[0357] The additives are as described herein.

[0358] The opacifier produced by the method described herein may be used in a coating composition of the invention, or in the method of preparing a coating composition of the invention.

[0359] Accordingly, in an aspect of the invention there is provided an opacifier, wherein the opacifier is obtained or obtainable by the method of the third aspect.

[0360] Method of Preparing Coating Composition

[0361] In a fifth aspect of the invention there is provided a method of preparing the coating composition of the first aspect, the method comprising: adding a cellulose microparticle having a mean average particle length of from 0.7 to 9 pm and a surfactant, or the opacifier of the second aspect, to a carrier liquid; optionally dispersing the cellulose microparticle and surfactant, or the opacifier, in the carrier liquid, to provide the coating composition. The method of preparing the coating composition may also comprise: providing cellulose microparticles and a surfactant, or providing an opacifier.

[0362] This may be known as a “preparation step”, and refers to making a suitable cellulose microparticle and a suitable surfactant available, or a suitable opacifier available. The preparation step typically takes place before the addition step.

[0363] Any suitable cellulose microparticle and surfactant may be used, as described herein. The cellulose microparticles and / or surfactant may be provided in a suspension or solution in a solvent. The suspension may also take the form of a slurry. Alternatively, the cellulose microparticles and / or surfactant may be provided as a dry powder or paste.

[0364] Any suitable opacifier according to the second aspect may be used. The opacifier typically includes a microparticle and surfactant, as described herein. The opacifier may be provided in a suspension in a solvent. The suspension may also take the form of a slurry.

[0365] Alternatively, the opacifier may be provided as a dry powder or paste.

[0366] The step of adding a cellulose microparticle having a mean average particle length of from 0.2 to 20 pm and a surfactant, or the opacifier of the second aspect, to a carrier liquid; may be referred to as the “addition step”.

[0367] The components may be added separately or together. The carrier liquid used may be the same as described above, in the context of the coating composition. Preferably the carrier liquid is water.

[0368] Where the cellulose microparticles and / or surfactant are provided in a suspension or solution, the addition step simply refers to the combination of the suspension to form a suspension of both the cellulose microparticles and surfactant.

[0369] A suspension is a heterogeneous mixture of a fluid that contains solid particles that are typically sufficiently large for sedimentation to occur if the suspension is left undisturbed for extended periods of time. The cellulose microparticles are suspended in the liquid. The surfactant may be dissolved in the carrier liquid. The suspension may also take the form of a slurry.

[0370] The step of optionally dispersing the cellulose microparticle and surfactant, or the opacifier, in the carrier liquid to form a suspension may be referred to as the “dispersing step”.

[0371] The dispersion step may be required if the cellulose microparticle or the opacifier are not sufficiently dispersed as a result of the addition step. The cellulose microparticle or the opacifier are preferably homogeneously dispersed in the carrier liquid.

[0372] The dispersing step may comprise sonicating a suspension. The sonication may be tip sonification or ultra-sonification. The sonication may be at an energy of from 400 to 1 ,000 J / g, preferably from 500 to 900 J / g, more preferably from 600 to 800 J / g. Preferably sonication is at an energy of about 700 J / g. The sonication energy refers to the energy per total mass of the suspension being sonicated.

[0373] The sonication may be performed by any suitable apparatus, such as a Fisherbrand Ultrasonic disintegrator.

[0374] The method may of preparing the coating composition may also comprise: adding one or more additives to the coating composition.

[0375] The step of adding one or more additives may be carried out sequentially with the addition step. The step of adding one or more additives may be carried out after the dispersion step, where present. Preferably, the step of adding one or more additives is carried out after the addition step and before the dispersion step. Thus, the additives may also be dispersed in the dispersion step.

[0376] Where multiple additives are added, the additives may be added separately or together.

[0377] The additives are as described herein.

[0378] The coating composition produced by the method described herein may be used as the coating composition of the invention, or in the method of coating a substrate.

[0379] Accordingly, in an aspect of the invention there is provided a coating composition, wherein the coating composition is obtained or obtainable by the method of the fourth aspect.

[0380] Coating

[0381] In a third aspect of the invention there is provided a coating applied to a substrate, the coating comprising: cellulose microparticles having a mean average particle length of from 0.7 to 9 pm, and a surfactant, wherein the cellulose microparticles are present in the coating at an amount of from 50 to 99.6 wt.% and the surfactant is present in the coating at an amount of from 0.4 to 25 wt.% based on the on the total mass of the coating.

[0382] The coating composition of the invention can be used to produce homogeneous films of controllable thickness. Without wishing to be bound by theory, it is thought that the surfactant servers as a scaffold upon which the cellulose microparticles uniformly assemble. Layer-by- layer assembly of these films is also possible without disrupting the organisation of these microparticles or desired opacity. The coating may be formed using typical coating techniques as described herein. The coatings have the capability to scatter light in the visible as well as infra-red and ultraviolet wavelength ranges.

[0383] The cellulose microparticles are as described herein.

[0384] The cellulose microparticles may be present in the coating at an amount of 50 wt.% or more based on the on the total mass of the coating, preferably 80 wt.% or more, more preferably 85 wt.% or more. In some embodiments the cellulose microparticles are present in the coating at an amount of 99.6 wt.% or less based on the on the total mass of the coating, preferably 98 wt.% or less, more preferably 95 wt.% or less, yet more preferably 90 wt.% or less.

[0385] The cellulose microparticles are present in the coating at an amount of from 50 to 99.6 wt.% based on the on the total mass of the coating. In some embodiments, the cellulose microparticles are present in the coating at an amount of from 80 to 99 wt.% based on the on the total mass of the coating, preferably from 85 to 96 wt.%, more preferably from 86 to 90 wt.%.

[0386] The surfactant is as described herein.

[0387] In some embodiments the surfactant is present in the coating at an amount of 1 wt.% or more based on the on the total mass of the coating, preferably 5 wt.% or more, more preferably 10 wt.% or more. In some embodiments the surfactant is present in the coating at an amount of 20 wt.% or less based on the on the total mass of the coating, preferably 17 wt.% or less, more preferably 15 wt.% or less.

[0388] The surfactant is present in the coating at an amount of from 0.4 to 25 wt.% based on the on the total mass of the coating. In some embodiment the surfactant is present in the coating at an amount of from 1 to 20 wt.% based on the on the total mass of the coating, preferably from 4 to 17 wt.%, more preferably from 12 to 15 wt.%.

[0389] The coating is substantially free of carrier liquid (e.g., solvent). Typically, the coating comprises less than 5 wt.% carrier liquid based on the total mass of the coating, preferably less than 1 wt.%, more preferably less than 0.5 wt.%, yet more preferably less than 0.1 wt.%.

[0390] The coating typically has a thickness which is greater than the shortest dimension of the cellulose particle. For example, the coating typically has a thickness which is greater than the width of the cellulose microparticle. The width of the cellulose microparticle is as described in the cellulose microparticle section, herein.

[0391] The coating preferably has a thickness which is greater than the longest dimension of the cellulose particle. For example, the coating typically has a thickness which is greater than the length of the cellulose microparticle. The length of the cellulose microparticle is as described in the cellulose microparticle section, herein.

[0392] The coating thickness may be measured using any suitable method, such as using a micrometre screw head, ultrasonic thickness gauge, surface profilometer, SEM or AFM (atom force microscope). The coating thickness may be measured by taking multiple readings spread over the coated area, such as 10 readings, and calculating the mean average of those readings. The thickness may be measured using the method described in ISO 2808:2019.

[0393] In some embodiments the coating has a mean average thickness of 5 pm or more, preferably 10 pm or more, more preferably 15 pm or more. In some embodiments the coating has a mean average thickness of 500 pm or less, preferably 300 pm or less, more preferably 200 pm or less.

[0394] In some embodiments the coating has a mean average thickness of from 5 to 500 pm, preferably from 10 to 300 pm, more preferably from 15 pm to 200 pm.

[0395] The coating thickness may be substantially uniform across the coated area. For example, the coating thickness may have a standard deviation of 25% or less, preferably 20% or less, more preferably 15% or less, yet more preferably 10% or less. The coating thickness may have a standard deviation of 10 pm or less, preferably 8 pm or less, more preferably 5 pm or less, yet more preferably 3 pm or less.

[0396] The coating also has excellent optical properties, such as whiteness and reflectance.

[0397] In some embodiments the coating has a L* (4570°) of 65 or more, preferably from 70 or more, more preferably 80 or more, yet more preferably 90 or more, wherein L* (4570°) is a CEILAB colour-space coordinate. The L* is measured as described herein.

[0398] In some embodiments the coating has a mean average reflectance of 38% or more, preferably from 50% or more, more preferably 68% or more, yet more preferably 80% or more, wherein average reflectance is measured across a wavelength range of from 400 to 700nm.

[0399] The coating may also reflect in UV region. In some embodiments the coating has a mean average reflectance of 30% or more, preferably from 35% or more, more preferably 40% or more, wherein average reflectance is measured across a wavelength range of from 100 to 400nm.

[0400] The coating may also reflect in IR region. In some embodiments the coating has a mean average reflectance of 20% or more, preferably from 30% or more, more preferably 35% or more, wherein average reflectance is measured across a wavelength range of from 1000 to 2500nm. In some embodiments the coating has an opacity of 60% or more, preferably 70% or more, more preferably 80% or more, yet more preferably 85% or more. The opacity is typically measured across a wavelength range of from 400 to 700nm for a coating having a thickness of 10 pm. The opacity may be measured for a coating formed from a coating composition comprising 12 wt.% cellulose microparticles. Opacity is measured as described in the examples section.

[0401] The coating applied to a substrate may include two or more layers. The two or more layers may comprise two or more cellulose microparticle and surfactant containing layers of the invention. The two or more layers may be the same or different. Preferably the two or more layers are the same. In this way, multiple cellulose microparticle and surfactant containing layers of the invention can be built up to produce a coating having the desired thickness.

[0402] The two or more layers may be applied sequentially. Alternatively, the two or more layers may be interposed by a different layer, such as the additional layer described below. The use of multiple separate layers may be used to achieve a film have a more uniform thickness than a single layer of the equivalent thickness.

[0403] The coating applied to a substrate may include additional layers. The additional layers are different to the cellulose microparticle and surfactant containing layers of the invention.

[0404] The additional layers may include a primer layer. The primer layer is typically disposed between the substrate and the cellulose microparticle and surfactant containing layer(s). Preferably the primer layer improves the adhesion of the cellulose microparticle containing layer to the substrate.

[0405] Any suitable primer layer may be used. Suitable primers layer may include an oil-based primer, a latex-based primer or a shellac primer.

[0406] Adhesion can further be improved to the desired degree by applying a primary layer, or primer. The primer is particular suited to where the substrate is metal, wood, or leather, to close the structural pores within, or smoothen the roughness of the substrate surface.

[0407] The additional layers may include a top-coat layer. The top coating layer is typically disposed on the opposite side of the cellulose microparticle and surfactant containing layer(s) from the substrate. Preferably the top coating layer encapsulates the cellulose microparticle containing layer. The top-coating layer may provide improved abrasion resistance, scratch resistance, antimicrobial resistance and / or stain-resistance properties to the coating.

[0408] The coatings of the invention maintain whiteness when a primer or top coat layer is applied. The top coating layer is preferably transparent, such as transparent to visible light. Transparency typically refers to a transmission of 80% or more, such as 90% of more, or 95% or more of incident light.

[0409] Any suitable top-coat layer may be used. Suitable top-coat layers may include a polyurethane layer, an epoxide layer, or a shellac based layer.

[0410] Coating method

[0411] In a sixth aspect of the invention there is provided a method of forming a coating on a substrate, the method comprising: applying the coating composition of the first aspect to the substrate, and drying the coating composition to form a cellulose microparticle containing coating.

[0412] The cellulose microparticle containing coating may be referred to as a cellulose microparticle containing layer.

[0413] The method of forming the coating may also comprise: providing a coating composition and substrate.

[0414] This may be known as a “coating preparation step”, and refers to making a suitable coating composition and substrate available. The preparation step typically takes place before the application step.

[0415] Any suitable coating composition and substrate may be used, as described herein.

[0416] The step of applying the coating composition of the first aspect to the substrate may be referred to as the “application step”.

[0417] The coating may be applied to the substrate using any suitable means. Suitable means of applying the coating may include spraying, curtain coating, knife coating, roll coating, dipping or draw down coating. Preferably the coating is applied by spraying or draw down coating, more preferably spraying.

[0418] Draw down coating may include draw down with a stainless steel bar formed from stainless steel rod tightly wound with stainless wire. These rods can also be referred to as grooved metering rods. Drawdown coating may also be carried out using a Bird Film Applicator. Typically, an appropriate volume of the coating suspension is placed in front of the drawdown rod or bird applicator, the rod or applicator is held from the ends and moved at the same speed while applying the same pressure on top of the surface to be coated.

[0419] Spray coating is typically carried out by spray the suspension using a suitable reservoir connected to a nozzle. For example, the suspension is placed in the reservoir and the pressure on the nozzle is adjusted to minimise dissipation outside of intended coating area. The pressure in the nozzle typically does not exceed 1 bar for volumes <50 ml. The coating application method may depend on the intended substrate and use of the coating. For example, coating of a wall or roof as the substrate may preferably be achieved by spraying. However, coating of paper or wood may preferably be achieved by draw down coating.

[0420] In some embodiments the method further comprises, before applying the coating composition, a step of applying a primer layer to the substrate, wherein the coating composition is applied onto the primer layer. The primer layer is as described above.

[0421] In some embodiments the method further comprises, after drying the coating composition, a step of applying a top coating layer to the cellulose microparticle containing layer, preferably wherein the top coating layer coat encapsulates the cellulose microparticle containing layer. The top coating layer is as described above.

[0422] Any substrate can be used which is suitable to receive the coating composition. The substrate is preferably flat, although the substrate may be curved or of complex shape (e.g., where applied by spray or drip coating).

[0423] In some embodiments the substrate is paper, wood, plastic, leather, or metal. The substrate may be any suitable type of paper, wood, plastic, leather, or metal.

[0424] Suitable paper includes copy paper, bond paper, cardstock, glossy paper, matte paper, newsprint, tissue paper, construction paper, watercolor paper, vellum, tracing paper, and parchment paper.

[0425] Suitable wood include natural woods, including seasoned and unseasoned timber, and manufactured woods, such as plywood, MDF, chipboard, hardboard and veneer.

[0426] Suitable plastics include Acrylic or Polymethyl Methacrylate (PMMA), Polycarbonate (PC), Polyethylene (PE), Polypropylene (PP), Polyethylene Terephthalate (PETE or PET), Polyvinyl Chloride (PVC) or Acrylonitrile-Butadiene-Styrene (ABS).

[0427] Suitable leathers include Bonded Leather, Bridle Leather, Deerskin Leather, Full Grain Leather and Bovine leather.

[0428] Suitable metals include aluminium, cadmium, chromium, copper, iron, lead, nickel, tin, zinc and combinations or alloys thereof.

[0429] In embodiments where the cellulose microparticles provide good IR scattering, the substrate may be a walls or roof of a building. In this way, the coating may reduce radiation via light reflection off these coated surfaces. Such cellulose white coatings can thus become a viable solution to heat management in buildings and elsewhere, and ultimately contribute to reducing global warming using renewable materials in a sustainable fashion.

[0430] In embodiments where the cellulose microparticles good UV scattering, the substrate may be the skin of a subject. In this way, the coating may provide UV protection to the subject.

[0431] The step of drying the coating composition to form a cellulose microparticle containing layer may be referred to as the “setting step”.

[0432] The setting step may be achieved by any suitable means. The setting step comprises removing the carrier liquid (solvent) from the suspension. The setting step occurs after the coating is applied to the substrate, to set the coating in position on the substrate.

[0433] The coating may be dried at ambient temperature or at elevated temperatures. The coating may be passively air dried (e.g., relying on the convection of air to remove evaporated solvent) or may be actively air dried (e.g., by passing air over the coating surface).

[0434] A conduction dryer or a convection dryer may be used in the setting step. Examples of suitable conduction dryers include paddle dryers, disc dryers or thin-film dryers. Examples of suitable convective dryers include vacuum dryers or fluidised-bed dryers.

[0435] The setting step results in a coating which is substantially free of carrier liquid (e.g., solvent). This thereby ‘sets’ the coating. Typically, the drying step produces a coating with less than 5 wt.% carrier liquid based on the total mass of the coating, preferably less than 1 wt.%, more preferably less than 0.5 wt.%, yet more preferably less than 0.1 wt.%.

[0436] During the drying step, it is thought that the cellulose microparticles assemble onto the surfactant scaffold to form continuous white films. As a result of the surfactants used, the cellulose microparticles do not prematurely settle out of the coating composition during the setting process (as the solvent volume decreases), but rather remain homogenously dispersed in the solvent by the surfactants. The cellulose microparticles then assemble into a coating layer, which has a homogenous distribution of the cellulose particles, and thus provides uniform and continuous whiteness and opacity.

[0437] The coating produced by the method described herein may be used as the coating composition of the invention.

[0438] Accordingly, in an aspect of the invention there is provided a coating, wherein the coating is obtained or obtainable by the method of the sixth aspect. Uses

[0439] In an aspect of the invention there is provided a use of the coating composition according to the first aspect, for applying to a substrate to form a coating.

[0440] The coating of the invention may be used to enhance the whiteness or provide opacity to a substrate. The coatings are particularly suitable as whiteness enhancers for substrates, as they have a high level of reflectance and reflect light at a similar level across the full spectrum of visible light.

[0441] This may find uses in food additives (such as pet food additives), cosmetics, personal care products, pharmaceuticals, inks, paints, laminates, washing powders, light harvesting devices (such as photovoltaic cells), light distribution devices (such as LEDs), homeware (such as furniture), and building materials.

[0442] Other Preferences

[0443] Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited.

[0444] Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

[0445] “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

[0446] Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

[0447] Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

[0448] Examples

[0449] The following examples are provided to further illustrate the present invention and are not intended to limit the scope of the invention.

[0450] Materials

[0451] Microcrystalline cellulose powder (MCC) was purchased from SERVA Electrophoresis. Sulfuric acid (concentration > 95%) was purchased from Fisher Chemical.

[0452] Acrylic paint base was obtained from Galaxus and includes a binder (acrylic) and water as its main components.

[0453] Triton X-100 was obtained from Sigma-Aldrich.

[0454] Tween 20 was obtained from Sigma-Aldrich.

[0455] ECOSURF SA-9 was obtained from Dow.

[0456] TERGITOL 15-S-9 was obtained from Dow.

[0457] TERGITOL TMN-100X was obtained from Dow.

[0458] Hexadimethylammonium bromide (CTAB) was obtained from Sigma-Aldrich.

[0459] 3 (Decyldimethylammonio)-propane-sulfonate inner salt was obtained from Sigma-Aldrich.

[0460] Opacity charts, also known as contrast-ratio charts, were obtained from Leneta. The opacity charts have simple combinations of white and black areas that are large enough for wide aperture reflectance measurements. The black and white areas are sealed with a clear, impervious topcoat which prevents penetration of applied coatings into the paper.

[0461] Measurement Methods

[0462] The SEM images are obtained using an FEI Nanolab 650 FIB-SEM operated at 5.0 kV and a working distance of 6.9 mm.

[0463] Whiteness can be quantified by converting the reflectance (measured as described below) into CEILAB (L*a*b*) colour-space coordinates. Here, a* represent the position between red (positive values) and green (negative values, and b* represents the position between blue (negative values), and yellow (positive values). L* represents the perceptual lightness or luminosity of the reflection. A lightness of 0 is black and a lightness of 100 is diffuse white. A lightness close to 100 is indicative of excellent whiteness of a material.

[0464] Whiteness may be measured at difference incidence angles, referring to the angle of the incident light. L* (45 / 0) refers to an incidence angle of 45° to the normal of the surface and a reflectance angle at 0° to the normal of the surface.

[0465] The total reflectance measurements were performed with an integrating sphere (Labsphere).

[0466] A light source (Ocean Optics HPX-2000) coupled into an optical fibre (600 pmThorlabs FC-UV100-2-SR) via a collimator (Thorlabs) and the signal collected by a spectrometer (Avantes HS2048). The signal may be normalized with respect to the intensity when no sample was mounted. The background is typically recorded when no light is applied. The range of wavelengths may be between 400 and 700 nm. Five spectra may be taken for each sample and averaged to reduce the signal-to-noise ratio. Each spectrum may be recorded using an integration time equal to 3 s.

[0467] For quantification of whiteness, it may be assumed that perfect white has the colour space coordinate (100, 0, 0). Whiteness may be defined and calculated as set out in WO2023 / 135261.

[0468] The thickness of the coatings were measured using a micrometre screw head. The coating thickness was measured by taking multiple readings spread over the coated area, such as 10 readings, and calculating the mean average of those readings.

[0469] When microcellulose particles are applied as paint, the solvent begins to evaporate, a process that leads to the formation of porosity in the dried film. As the solvent evaporates, the liquid phase decreases, causing the microcellulose particles to come into closer contact with one another. This results in the creation of small voids or pores within the matrix of the applied paint. The ideal pore size should be in the order of the half wavelength of light, preferably in the range of 50 to 900 nm, with an especially preferred range of 100 to 500 nm. Small pores can be more suitable for UV protection, and larger pores for IR scattering.

[0470] It is also critical to consider the uniformity of the pore size distribution. The narrower the pore size distribution, the more effective is the scattering power of the material or structure. The presence of these pores, and the uniformity of their size distribution, allow for increased light scattering, which not only improves the aesthetic qualities of the paint or coating but also results in a more uniform appearance. Therefore, the pore size and distribution can contribute to optimal light scattering, which increases the brightness and opacity of the applied white paint. Thanks to these microporous properties, the paint can not only create a high-gloss surface but also improve the breathability of the coated materials. As noted above, engineering the pore size and pore size distribution can significantly contribute to the scattering performance for coatings and paints below the critical pigment volume concentration, CPVC.

[0471] Preparation of CMPs

[0472] Three different CMPs were prepared: CMP-LS, CMP-X and CMP-Z.

[0473] For CMP-LS, cellulose microcrystalline powder (1g) was hydrolysed with sulfuric acid (50 wt.%, 60 mL) for 5 hours at 50 °C, and then quenched by adding 300 mL milli-Q water. The acid supernatant was removed by centrifugation. The hydrolysed cellulose particles were dispersed by adding 100 mL milli-Q water and then centrifuged. This process was repeated three times to remove most of the acid and the suspension of hydrolysed cellulose particles were dialyzed against milli-Q water (MWCO 12-14 kDa) for one week while changing water two times a day. The dialyzed suspension of hydrolysed cellulose particles (0.5% wt, 30 mL) was tip sonicated in an ice bath (Fisher brand ultrasonic disintegrator 500 W, 20 kHz, tip diameter 12.7 mm, amplitude 30%, 2 seconds on and 2 seconds off). The suspension was centrifuged at 2000 rpm for five minutes, and then the supernatant was collected and centrifuged at 3000 rpm for five minutes to get the cellulose nanoparticles CMP-LS.

[0474] CMP-X were obtained using the same method as for CMP-LS, except that the cellulose microcrystalline powder was replaced by cellulose filter paper (Whatman No. 1). The cellulose filter paper (Whatman No. 1) was hydrolysed with sulfuric acid (55 wt.%, 60 mL) for 0.5 hours at 50 °C.

[0475] CMP-Z were prepared from Cellulose filter paper (Whatman No. 1), which was first ground into small pieces by a coffee grinder, followed by TEMPO oxidation. 1 g of cellulose was suspended in 150 mL milli-Q water, 0.123 g TEMPO, 1.23 g NaBr and 1.23 g NaCIO was added and stirred for 4.5 h at room temperature while the pH was kept at 10 by the addition of 1 M NaOH solution. The reaction was stopped by adjusting the pH to 6 with 5 M HCI, and then the oxidized cellulose fibres were washed by filtration and dialyzed against milli-Q water to provide the Cellulose fibres CMP-Z.

[0476] Characterization of CMPs

[0477] CMP-LS was characterised by Scanning electron microscopy (SEM). Figure 1 shows an SEM image of the cellulose microparticles with unique light scattering capability. The scale bar is 30 pm.

[0478] The size distribution of CMP-LS, CMP-X and CMP-Z was measured by scanning electron microscope (SEM). A dilute suspension of CMPs (0.001wt.%) was dropped on a carbon coated copper grid (300 mesh) for 2 minutes and removed by a piece of filter paper, then a drop of uranyl acetate solution (2%) was applied as stain for 1 minute before being removed by a piece of filter paper. The length of the particles was analysed by Imaged. The length refers to the largest diameter of the particle which is visible in the SEM image. The number of measurements taken to give the size distribution is typically from 100 to 1,000. Generally, over 100 measurements of the length are taken.

[0479] The size distribution of the particle length of CMP-LS, CMP-X and CMP-Z is shown in Figure 2. The size distribution is quantified by volume of the particles.

[0480] Preparation of Compositions

[0481] Coating compositions of the invention (composition 1a to 7) were prepared by mechanically mixing a surfactant solution (water as solvent) with CMP-LS powder, followed by sonication. A comparative coating composition (comp. 1) was prepared by mechanically mixing water with CMP-LS powder, followed by sonication. No surfactant was present.

[0482] The surfactant containing solution was prepared by combining the surfactant with water at the concentration specified in Table 1. The amount of CMP added was as specified in Table 1. The mixture was sonicated at 700 J / g to produce the coating composition.

[0483] For example, composition 1a was prepared by combining 2 wt.% Triton-X-100 aqueous solution and CMP powder in 3:17 w:w ratio, which was then sonicated at 700 J / g to produce a suitable coating composition for coating a variety of surfaces.

[0484] Table 1. Cellulose microparticle (CMP) formulations involving different families of surfactants

[0485] Coating of Substrates

[0486] Drawdown coating was carried out using a stainless steel bar formed from stainless steel rod tightly wound with stainless wire. The bar can also be referred to as grooved metering rods. Drawdown coating was also carried out using a Bird Film Applicator where specified. The depth of the drawdown bar / rod is used to determine the coating thickness.

[0487] The substrate was taped on a flat surface. An appropriate volume of the coating composition was placed in front of the drawdown rod or bird applicator. The rod or applicator was held from the ends and moved at the same speed while applying the same pressure on top of the surface to be coated. Spray coating was carried out using a suitable reservoir connected to a nozzle. The composition was placed in the reservoir and the pressure on the nozzle adjusted to minimise dissipation outside of intended coating area. The pressure in the nozzle is controlled not to exceed 1 bar for volumes <50 mL.

[0488] The composition was allowed to air dry for 1-60 min depending on the volume of coating composition used.

[0489] Comparative composition 1 was coated onto an opacity chart. 5-10 mL of the comparative coating composition was placed in front of the drawdown rod, which was then pulled over the opacity chat to form a film. The film was allowed to dry for 45 minutes. Upon drying, the film breaks up into inhomogeneous fragments.

[0490] The coating formed is pictured in Figure 3. The coating was visually inspected The coating is discontinuous, with large holes formed in the coating. The coating is also inhomogeneous and inconsistent thickness across the coated area. As a result, the visual appearance is non- uniform with varying whiteness and opacity across the coated area. The coating is also uneven.

[0491] Without wishing to be bound by theory, when the solvent is evaporated from this comparative a CMP composition, it is thought that the cellulose particles sediment onto the surface, without the ability to form a continuous white film (FIGURE 3). This demonstrates that white films cannot be produced from directly drying aqueous suspensions of CMP.

[0492] Composition 1b was coated onto paper by drawdown. The coating was allowed to air dry for 45 minutes. The resulting film has a thickness of 20 pm. The resulting white film is shown in Figure 4a.

[0493] Further coatings of composition 1b were formed at thicknesses of 7 pm, 14 pm and 19 pm.

[0494] Composition 2 was coated onto paper by drawdown. The coating was allowed to air dry for 45 minutes. The resulting film has a thickness of 20 pm. The resulting white film is shown in Figure 4b.

[0495] Composition 1b was coated onto wood by drawdown. The coating was allowed to air dry for 30 minutes. The resulting film has a thickness of 20 pm. The resulting white film is shown in Figure 5.

[0496] Composition 1b was coated onto aluminium by drawdown at a coating thickness of 50 pm, 100 pm and 150 pm. The coating was allowed to air dry for 20 minutes. The resulting 10 pm white film is shown in Figure 6b. Composition 4a was coated onto bovine leather by drawdown at a coating thickness of ~20 pm. The coating was allowed to air dry for 45 minutes. The resulting white film on the leather is shown in Figure 8a.

[0497] Composition 4b (including 10 wt.% acrylic paint base in water) was coated onto bovine leather by drawdown at a coating thicknesses of 20 pm. The coating was allowed to air dry for 45 minutes. The resulting white film on the leather is shown in Figure 8b.

[0498] The leather sample shown in Figure 8b can be bent and retains the white coating after flexion without cracking or chipping. The white films thus produced maintain a good degree of dimensional stability even under deformation.

[0499] The coatings of compositions 1b, 2, 4a and 4b shown in Figures 4, 5, 6 and 8 were visually inspected. The coatings formed were continuous without voids or gaps. The coatings were also found to be homogeneous and of consistent thickness across the coated area. As a result, the visual appearance is highly uniform and flat, with excellent whiteness and opacity across the entire coated area. The thickness of the film was highly controllable by the coating process.

[0500] Without wishing to be bound by theory, it is thought that the surfactant forms a scaffold upon which the CMPs assemble to form a continuous lattice structure by maintaining uniformly periodic distances between the microparticles. This periodic structure amplifies light scattering contrast between the cellulose microparticles and adjoining spaces (occupied by air), leading to the formation of distinct continuous white films.

[0501] It is also thought that the surfactant decreases the surface tension or interfacial tension between the cellulose microparticles and solvent. However, the surfactants do not inter with the CMPs light scattering ability.

[0502] The cellulose white films were found to provide good adhesion to paper, wood, aluminium and leather substrates.

[0503] Optical Characterization of Coatings

[0504] The whiteness of various coating compositions, as measured by L*, is given in Table 2.

[0505] For the whiteness measurements provided in Table 2, a 10 pm coating was formed on an opacity chart using drawdown coating.

[0506] The example compositions provide excellent L* values, typically having an L* of 77 or more, and in several cases having an L* of 85 or more. Table 2: Whiteness of the Coatings

[0507] Remission spectra for the coating formed from composition 1b on aluminum is shown in Figure 6a. The whiteness of the film, as measured by L*, depends on film thickness.

[0508] Remission spectra for the coating formed from composition 1b on paper at thicknesses of 7 pm, 14 pm and 19 pm is shown in Figure 7. The L* is 66 (7 pm), 77 (14 pm) and 80 (19 pm). The degree of whiteness, as indicated by L* (45 / 0) was found to directly depend on the coating thickness.

[0509] Spectra for the coating formed from composition 4a on bovine leather gives Cl ELAB colour values of: L* = 95, a* = -0.1 , b* = 2.8. The value of whiteness is 94 [calculated as 100 - SQRT((100-L*)A2 + a*A2 + b*A2)].

[0510] In this colour space, numerical differences between values roughly correspond to the amount of change humans see between colours. The whiteness of cellulose-based films, as characterised by L*(45° / 0°), show values > 85 when the cellulose microparticles are used with non-ionic or cationic surfactants and >75 when using Zwitterionic surfactants. This is indicative of excellent whiteness and excellent scattering properties of the CMPs.

[0511] Opacity Tests

[0512] The opacity dependence of the CMP concentration was tested.

[0513] A series of coating compositions were prepared based on composition 1b (1.75 wt.% of Triton X-100) and composition 2 (0.5 wt.% of Tween 20), described above. The compositions included CMPs at concentrations 3 wt.%, 6 wt.%, 9 wt.%, 12 wt.%, 15 wt.%, 18 wt.% and 20 wt.%. The coating compositions were prepared as described above.

[0514] The various coating compositions were coated onto an opacity chart (described above). The coatings were applied by drawdown coating at a thickness of 10 pm.

[0515] The opacity of the coatings was measured using the same method as for L*, described above. Opacity is the percentage ratio of the “y” value (according to the CIE Y value system) measured on black relative to white portions of the opacity chart paper. The opacity values of the coating is provided in Figure 10.

[0516] Figure 10 shows that the coating comprising CMPs with non-ionic surfactants increase with increasing CMP concentration from 3 wt.% up to 12 or 15 wt.%. The opacity increase from about 60% at 3 wt.% CMP up to over 80% at 12 wt.% CMP.

[0517] At concentrations above 12 to 15 wt.% the opacity plateaus or reduces. For some examples the capacity remains about 85% at from 12 wt.% to 20 wt.% CMP.

[0518] The exact opacity measured at 12 wt.% CMP is provided in Table 3.

[0519] Table 3. Opacity for cellulose microparticle coatings with different formulations

[0520] L* and Backscattering

[0521] The reflectance of the coatings produced in the examples described above was tested. The reflectance was meased as described above and the L*(45° / 0°) and backscattering determined for the different coatings. All the example compositions were applies to opacity charts.

[0522] Backscattering was measured using Diffusing Wave Spectroscopy (DWS), light scattering technology used to measure the Brownian motion of particles in a suspension. DWS analyses the temporal intensity-fluctuations of light scattered by the particles.

[0523] When measuring L*(45° / 0°), the measurements were calibrated using a 1 wt.% suspensions of the cellulose microparticles and 1 cm thick cuvettes. Alternatively, we can use other optical techniques that measure backscattering using, for instance, the DWS Rheolab instrument, or something similar.

[0524] The L*(45° / 0°) for the cellulose microparticles has values between 38 and 42, or backscattering response >1300 a.u. for the same 1 wt.% aqueous suspension calibrated using 1 cm-thick glass cuvettes (see FIGURE 11).

[0525] The L* and backscattering is shown in Figure 11 , which is a polynomial plot illustrating the correlation between Cl ELAB L*(45° / 0°) measurement and backscattering for a wide selection of samples coated with cellulose microparticles and how we quantify whiteness. The circled region indicates the ranges where light scattering, measured by either technique, represents properly measureable and detectable whiteness. The fitted curve to the experimental data can be described as a polynomial function, L*(x) = axb= 0.72 x055with a coefficient of determination, r2= 98.1%.

[0526] This shows a correlation between Cl ELAB L* measurement and backscattering by DWS. Acceptable levels of whiteness fall in the circled range: L* in the range 37 to 41 and backscattering between 1300 and 1400 a.u. L* higher than the circled region is also acceptable, as the whiteness is even greater. L* lower than the circled region is typically not acceptable.

[0527] This test also shows that the compositions are tuneable, to provide coatings with higher L* (or backscattering) values as needed. The whiteness and backscattering correlate well at all degrees of whiteness.

[0528] UV-Vis and IR Reflection

[0529] Composition 1b was coated onto a opacity chart by drawdown coating to produce a film having a thickness of 15 ± 2 pm.

[0530] Composition 2 was coated onto paper by drawdown coating to produce a film having a thickness of 12 ± 3 pm.

[0531] The UV and IR reflection of the films was tested using the methods described above. The UV-Vis spectra was measured at a wavelength of from 150 to 900 nm and the IR spectra was measured at a wavelength of from 1000 to 2,500 nm. The spectra for the coating of composition 1b is shown in Figure 13a and the spectra for the coating of composition 2 is shown in Figure 13b.

[0532] The spectral response of the cellulose white films in the ultra-violet (UV) and infra-red (IR) ranges is excellent, displaying high reflection (for a very thin film) in the UV-Vis range. This demonstrates the capability of these coatings to provide protection / shielding in the UV-Vis range.

[0533] Additional Coating Layers

[0534] Composition 1b was coated onto an aluminium substrate by drawdown as described above. A clear polyurethane film was applied over the white cellulose coating using drawdown coating. This resulted in an aluminium substrate with a white cellulose coating covered by a PU coating (CMP+PU).

[0535] As a control, a clear polyurethane film was applied directly to the aluminium substrate. The polyurethane film was applied using drawdown coating. This resulted in an aluminium substrate with a PU coating (PU). The PU coating thickness was about 10 pm. The Pll coating was found to have no effect on the whiteness or opacity of the CMP coating.

[0536] Abrasion Resistance

[0537] The abrasion resistance of the CMP+PLI and Pll coated aluminium was tested. The abrasion test was performed using a 600 grit sandpaper loaded with 100 g at a rate of 2 passes per cycle (back and forth). The coatings underwent 10 cycles of abrasion.

[0538] Abrasion is concluded on the basis of the water contact angles of the coatings. The water contact angle (CA, 0) was measured by using a drop shape analysis instrument (First Ten Angstroms, USA) at ambient temperature. A water droplet of 5 pL was placed on the surface of a sample, and the contact angle was a mean average of six measurements on different positions on the surface.

[0539] The results are shown in Figure 12. Prior to abrasion (cycle 0) the water contact angle for both CMP+PU and PU is about 85°. After one cycle (cycle 1) the water contact angle for the PU coating remains at about 85°, while the CMP+PU coating reduces to 65°. For cycles 2-10 the water contact angle remains consistent at 75° to 85°. The CMP+PU generally displays a lower water contact angle than the PU. This is thought to be a result of the hydrophilicity of the cellulose particles and the surfactant.

[0540] It is evident that the anti-scratch resistant of the multilayers, cellulose microparticles and polyurethane, retain an almost steady abrasion resistance as measured by the water contact angle over a wide range of cycles.

[0541] This shows that a top clear layer may be applied without affecting the whiteness of the underlying layer and simultaneously enable the white coating to have improved scratch or and abrasion resistance.

[0542] Comparative examples

[0543] Milled cellulose particles with a particle size of 40 pm was prepared using a knife mill, for instance, Retsch SM300 or a similar system, and mixed with different additives as disclosed in W002 / 100955A1.

[0544] Comparative example 1 :

[0545] A mixture comprising 12 w% of milled cellulose particles (40 pm) and 2 w% of the nonionic surfactant EcoSurf SA-9 was prepared from an aqueous suspension. This combination resulted in no film formation whatsoever; only randomly aggregated particles were observed (see CE_01 in the table below).

[0546] Comparative example 2: When milled cellulose was combined with calcium carbonate and an ionic surfactant, the outcome showed only minimal aggregation with no formation of a continuous film (refer to CE_02 in the table below).

[0547] Comparative example 3:

[0548] The combination of milled cellulose with calcium carbonate and titanium dioxide — regardless of whether a nonionic surfactant was present (CE_05 or CE_06) — yielded only faint marks during attempts to create a film, indicating inadequate film formation.

[0549] Comparative example 4:

[0550] When titanium dioxide was used independently along with a nonionic surfactant (CE_04), the resulting product was a non-homogeneous film. This demonstrates that the nonionic surfactant does not facilitate the assembly of TiO2particles into a uniform film and, in fact, disrupts the process.

[0551] Composition according to the present invention:

[0552] In contrast, a composition containing 12 w% CMP-X and 2 w% EcoSurf SA-9 from an aqueous suspension led to the successful production of a continuous and homogeneous white film (see CE03_001_01 in the table below).

[0553] Thus, milled cellulose particles, within the size range specified in WO 02 / 100955 A1 , do not exhibit the same light-scattering properties as cellulose microparticle contained in the composition according to the present invention. Moreover, when these milled cellulose particles are mixed with an ionic surfactant from an aqueous suspension, they are incapable of forming a continuous film. Instead, they tend to simply aggregate, which can further inhibit the formation of a homogeneous film with other light-scattering agents such as calcium carbonate or titanium dioxide. Further, the nonionic surfactant does not serve as a scaffold when mixed with metal oxide particles, such as titanium dioxide. The surfactant's scaffold-like function appears to be exclusive to cellulose microparticle contained in the composition according to the present invention, thus cellulose particles designed with specific physical dimensions.

[0554] References

[0555] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0556] 1. S. B. Lyon, R. Bingham and D. J. Mills, Progress in Organic Coatings, 2017, 102, 2-7.

[0557] 2. A. Javadi, A. Cobaj and M. D. Soucek, in Handbook of Waterborne Coatings, eds. P. Zarras, M. D. Soucek and A. Tiwari, Elsevier, 2020, pp. 303-344.

[0558] 3. Q. Zeng, W. Wan and L. Chen, ACS Applied Materials & Interfaces, 2019, 11, 24308- 24317.

[0559] 4. A. Balram, S. Santhanagopalan, B. Hao, Y. K. Yap and D. D. Meng, Advanced Functional Materials, 2016, 26, 2571-2579.

[0560] 5. M. Diba, D. W. H. Fam, A. R. Boccaccini and M. S. P. Shaffer, Progress in Materials Science, 2016, 82, 83-117.

[0561] 6. X.-M. Zhong, I. Wyman, H. Yang, J.-B. Wang and X. Wu, Chemical Engineering Journal, 2016, 302, 744-751.

[0562] 7. Matti, T., Olli, I., Vignolini, S., and Onelli, O. D., “Highly scattering porous material based on fibrillar, elongated, or disk-like particles,” WO2019063647.

[0563] 8. Yang, H., Jacucci, G., Schertel, L., and Vignolini, S., “Cellulose-based scattering enhancers for light management applications,” ACS Nano 2022, 16, 5, 7373-7379.

[0564] 9. Yang, H., Jacucci, G., Honorato-Rios, C., Schertel, L., and Vignolini, S., “Cellulose Particles,” WO2023 / 135261 A1.

Claims

Claims1. A coating composition for providing a cellulose microparticle coating on a substrate, the coating composition comprising: an opacifier comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm measured by scanning electron microscope; and a carrier liquid; wherein the cellulose microparticles are present in the coating composition at an amount of from 1 to 40 wt.% and the surfactant is present in the coating composition at an amount of from 0.5 to 6 wt.% based on the on the total mass of the coating composition and wherein the composition comprises less than 5% by weight of a metal oxide.

2. The coating composition, wherein the composition is essentially free of a metal oxide, preferably of TiO2.

3. The coating composition of either claim 1 or 2, wherein the cellulose microparticles are present in the coating composition at an amount of from 5 to 20 wt.% based on the total mass of the coating composition, preferably from 10 to 18 wt.%, more preferably from 12 to15 wt.%.

4. The coating composition of any preceding claim, wherein the surfactant is present in the coating composition at an amount of from 0.5 to 3 wt.% based on the on the total mass of the coating composition, preferably from 1.0 to 2.0 wt.%.

5. The coating composition of any preceding claim, wherein the ratio of the amount of cellulose microparticles to the amount of surfactant is from 2 to 25, wherein the amounts are wt.% based on the total mass of the coating composition, preferably from 5 to 9, more preferably from 6 to 8.

6. The coating composition of any preceding claim, comprising a particle population CMPXhaving an average particle length of less than 1 pm, or a particle population CMPLShaving an average particle length ranging from 1 pm to less than 5 pm, or a mixture thereof, preferably a particle population CMPLShaving an average particle length ranging from 1 pm to less than 5 pm.

7. The coating composition of any preceding claim, wherein the carrier liquid is water, ethanol, dimethylsulfoxide (DMSO), dimethylformamide (DMF), N-methylpyrrolidone (NMP) or a combination thereof, preferably wherein the carrier liquid is water.

8. An opacifier for a coating composition, the opacifier comprising: cellulose microparticles having a mean average particle length of from 0.7 to 9 pm, anda surfactant, wherein the cellulose microparticles are present in the opacifier at an amount of from 50 to 99.6 wt.% and the surfactant is present in the opacifier at an amount of from 0.4 to 25 wt.% based on the on the total mass of the opacifier; optionally wherein the opacifier is a powder.

9. A cellulose microparticle coating formed on a substrate, the coating comprising cellulose microparticles and a surfactant, wherein the cellulose microparticles have a mean average particle length of from 0.7 to 9 pm measured by scanning electron microscope, wherein the cellulose microparticles are present in the coating at an amount of from 50 to 99.6 wt.% and the surfactant is present in the coating at an amount of from 0.4 to 25 wt.% based on the on the total mass of the coating and and wherein the coating comprises less than 5% by weight of a metal oxide.

10. The cellulose microparticle coating of claim 9, wherein the coating has: a L* (4570°) of 65 or more, preferably from 70 or more, more preferably 80 or more, yet more preferably 90 or more, wherein L* (4570°) is a CEILAB colour-space coordinate measured for a coating having a thickness of 10 pm.

11. The coating composition of any of claims 1 to 7, the opacifier of claim 8, or the cellulose microparticle coating of either claim 9 or 10, wherein the cellulose microparticles have: a mean average particle length of from 1.3 to 7 pm, more preferably from 1.7 to 5 pm, yet more preferably from 1.9 to 2.8 pm, wherein the mean average particle length is measured by scanning electron microscope; and / or a mean average aspect ratio of from 2 to 18, preferably 3 to 15, more preferably from 4 to 10, yet more preferably from 4 to 6, wherein the average particle length and the average particle width for the aspect ratio are measured by scanning electron microscope; and / or a mean average width of from 0.1 to 1 pm, preferably from 0.2 to 0.8 pm, more preferably from 0.3 to 0.6 pm, yet more preferably from 0.45 to 0.55 pm, wherein the mean average particle width is measured by scanning electron microscope.

12. The coating composition of any of claims 1 to 7 or 11 , the opacifier of any of claims 8 or 11 , or the cellulose microparticle coating of any of claims 9 to 11 , wherein the cellulose microparticles are anionic, optionally wherein: the cellulose microparticles are surface modified with anionic sulfate half ester groups; and / or the cellulose microparticles have an anionic charge density of from 50 mmol / kg or more, preferably 100 mmol / kg or more, more preferably 150 mmol / kg or more, wherein the anionic charge density is determined according to ISO 21400:2018.

13. The coating composition of any of claims 1 to 7 or 11 to 12, the opacifier of any of claims 7, 8 or 11 to 12, or the cellulose microparticle coating of any of claims 9 to 12, whereinthe surfactant is a non-ionic, cationic or zwitterionic surfactant, preferably selected from the group consisting of polyethylene glycol p-(1 ,1 ,3,3-tetramethylbutyl)-phenyl ether, polyoxyethylene (20) sorbitan monolaurate, alcohol ethoxylate (Cg-CT ethoxylated alcohol), secondary alcohol ethoxylate (Cn-C^ ethoxylated alcohol, trimethyl nonyl ether of polyethylene glycol, hexadecyltrimethylammonium bromide (CTAB) and 3-(Decyldimethylammonio)-propane-sulfonate inner salt, or a combination thereof; or the surfactant is a non-ionic surfactant, preferably selected from the group consisting of polyethylene glycol p-(1 ,1 ,3,3-tetramethylbutyl)-phenyl ether, polyoxyethylene (20) sorbitan monolaurate, alcohol ethoxylate (C9-Cn ethoxylated alcohol), secondary alcohol ethoxylate (Cn-C15ethoxylated alcohol) and trimethyl nonyl ether of polyethylene glycol, or a combination thereof.

14. The coating composition of any of claims 1 to 7 or 11 to 13, the opacifier of any of claims 6 or 9 to 13, or the cellulose microparticle coating of any of claims 9 to 13, wherein the surfactant has a critical micellar concentration of from 0.03 to 30 mM, preferably from 0.04 to 25 mM, more preferably from 0.05 to 10 mM, yet more preferably from 0.05 to 1 .5 mM, wherein the critical micellar concentration is measured according to ISO 4311 :1979 at a temperature of 25°C, a pressure of 1 atm and with water as solvent.

15. A method of preparing the coating composition of any one of claims 1 to 7 or 11 to 14, the method comprising: adding a cellulose microparticles having a mean average particle length of from 0.2 to 20 pm and a surfactant, or the opacifier of any one of claims 6 or 9 to 12, to a carrier liquid; optionally dispersing the cellulose microparticle and surfactant, or the opacifier, in the carrier liquid, to provide the coating composition.

16. A method of forming a coating on a substrate, the method comprising: applying the coating composition of any one of claims 1 to 7 or 11 to 14 to the substrate, and drying the coating composition to form a cellulose microparticle coating.

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