Superhydrophobic coatings

Abstract

The present invention relates to a superhydrophobic coating for a substrate, a multi- layered superhydrophobic coating for a substrate, use of lycopodium shells or pine pollen shells in the formation of superhydrophobic coatings, a method of forming a superhydrophobic coating on a substrate, and a method of forming a multi-layered superhydrophobic coating on a substrate. The superhydrophobic coating comprises lycopodium shells or pine pollen shells, and a plant-based wax. The method of forming a superhydrophobic coating on a substrate comprises dispersing or dissolving a plant-based wax in a solvent and dispersing lycopodium shells or pine pollen shells in the solvent, thereby forming a coating dispersion; depositing the coating dispersion onto a surface of a substrate, thereby coating at least a portion of the surface with the coating dispersion; and drying the coated substrate.

Description

SUPERHYDROPHOBIC COATINGSField of the invention

[0001] The present invention relates to a superhydrophobic coating for a substrate, a multi-layered superhydrophobic coating for a substrate, use of plant spore shells in the formation of a superhydrophobic coating, a method of forming a superhydrophobic coating on a substrate, and a method of forming a multi-layered superhydrophobic coating on a substrate.

[0002] The detrimental effects of single-use plastics on the planet are now well understood and the growing concern has led to increased efforts for viable and sustainable alternatives. Food packaging typically uses plastic due to its low cost and inherent properties which act to protect food, improving both food quality and shelf life. Despite the increased interest in plastic substitutes, implementation has not seen full widespread use due to factors such as manufacturing costs and inferior performance.

[0003] Though eco-friendly packaging solutions are available in the form of compostable materials and card, often a secondary layer is needed to aid the water resistance of the material. This is a critical component of food packaging to limit transfer of moisture for both structural integrity of the packaging in addition to food quality and safety. A common method to overcome this within sustainable packaging is through thin coatings of polylactic acid (PLA). However, though classed as biodegradable bioplastic, PLA requires specific conditions for degradation with temperatures exceeding 50°C being required.

[0004] Due to their waterproof barrier properties, superhydrophobic materials offer a potential solution to this issue. Over the past few years, many have designed superhydrophobic materials and coatings with the intention of use for food packaging. This is due to their desirable properties of offering a waterproof barrier whilst also being self-cleaning and anti-fouling which can aid in the reduction of food waste and increased shelf life.

[0005] Notable work has been published in the following documents, all of which are incorporated by reference herein in their entirety:X. Jiang, Q. Li, X. Li, Y. Meng, Z. Ling, Z. Ji and F. Chen, IntJMol Sci, DOI: 10.3390 / ijms231911158.A. Lafraya, C. Prieto, M. Pardo-Figuerez, A. Chiva and J. M. Lagaron, Nanomaterials, 2021, 11, 3354.Y. Qiu, Z. Zhang, S. Liang, Y. Miao and C. Yao, Journal of Wood Chemistry and Technology, 2022, 42, 222-234.A. Milionis, R. Ruffilli and I. S. Bayer, RSC Adv, 2014, 4, 34395-34404.J. M. Morrissette, P. J. Carroll, I. S. Bayer, J. Qin, D. Waldroup and C. M. Megaridis, Green Chemistry, 2018, 20, 5169-5178.

[0006] Jiang et al. prepared paper-based packaging through a coating of PLA, cinnamaldehyde and nano-silica which displayed repellence to a variety of liquid foodstuffs as well as limiting bacterial growth by more than 99% through the inhibitory actions of cinnamaldehyde. Lafraya et al. similarly used silica nanoparticles and PLA with an electrospinning technique to produce food packaging that was self-cleaning to a variety of fatty and dense foods. Qiu et al. opted for a modified titanium dioxide nanoparticlebased coating capable of extending the shelf-life of cherries (water loss of 32.1% compared to a loss of 77.62% in uncoated paper). Though these materials show impressive barrier properties that can act to increase shelf-life, safety of the packaging to which they are applied may be compromised due to leaching of harmful materials when in contact with food. In addition, as mentioned above, PLA has unfavourable degradation characteristics despite being classed as a “biodegradable bioplastic”, and so these coatings still carry the concern of residual plastic and microplastics.

[0007] Lycopodium is a readily available plant-based biomaterial, suitable for the fabrication of superhydrophobic coatings owing to its inherent hydrophobicity and complex morphology which can introduce hierarchical structures to coatings. Previously, lycopodium has been successfully incorporated into the fabrication of superhydrophobic coatings by both Morrisette et al. and Milionis et al.. Despite this, the reported coatings use additional fillers or polymeric materials that are either non-biodegradable or synthetic, and thus compromise the safety and environmental friendliness of the reported coatings. Moreover, although the reported work achieved coatings with water contact angles exceeding 150°, this hydrophobicity is lost with time.Summary of the invention

[0008] In view of these problems, there is a need for safer and more environmentally friendly coatings suitable for packaging applications.

[0009] Accordingly, the present disclosure provides coatings suitable for packaging applications offering an eco-friendly alternative to plastics. In coatings making direct contact with food, it is desirable to use non-toxic and food-grade (FG) components for consumer safety. As such, coatings in accordance with the present disclosure exhibit superhydrophobic behaviour and may be prepared from 100% or near 100% plant-based components. In particular, coatings in accordance with the present disclosure exhibit superhydrophobic behaviour by virtue of comprising a plant-based roughening agent derived from plant spores, such as those of lycopodium or pine pollen, and a hydrophobic plant-based wax.

[0010] A first aspect of the present invention is a superhydrophobic coating for a substrate, the coating comprising plant spore shells and a plant-based wax.

[0011] The coating according to the first aspect comprises plant spore shells (roughening agent) and a hydrophobic plant-based wax. Plant spore shells are extracted by chemical modification of naturally-occurring plant spores in accordance with the techniques described herein. This chemical modification enables effective exploitation of the inherent hydrophobicity of plant spores, so to achieve reliable and high-performance superhydrophobic coatings.

[0012] In some embodiments, the coating according to the first aspect comprises lycopodium shells (roughening agent) and a hydrophobic plant-based wax. Lycopodium shells are extracted by chemical modification of naturally-occurring lycopodium spores in accordance with the techniques described herein. Owing to the inherent hydrophobicity and hierarchical surface roughness of lycopodium, superhydrophobic behaviour is achieved, while use of lycopodium shells extracted by chemical modification of naturally- occurring lycopodium spores provides a surprising improvement in hydrophobic lifetime compared with previously reported coatings. The hierarchical structure formed from the lycopodium shells leads to high water contact angles in excess of 150° in addition to contact angles exceeding 150° for various liquid foods, and reduced food adhesion.

[0013] In some embodiments, the coating according to the first aspect comprises pine pollen shells (roughening agent) and a hydrophobic plant-based wax. Pine pollen shellsare extracted by chemical modification of naturally-occurring pine pollen in accordance with the techniques described herein. The principles underlying the extraction of pine pollen shells and use thereof to impart superhydrophobic behaviour on coatings are similar to those underlying the extraction and use of lycopodium shells as described herein. The rough surface morphology of pine pollen shells leads to high water contact angles in excess of 150°.

[0014] The plant-based wax may comprise carnauba wax, candelilla wax, soybean wax, beeswax, ricebran wax, paraffin wax, or any combination thereof. Preferably, the plant-based wax comprises carnauba wax.

[0015] When the plant spore shells comprise lycopodium shells, a ratio of total mass of lycopodium shells to combined total mass of lycopodium shells and plant-based wax in the coating may be from 40% to 80%. Preferably, the ratio is from 50% to 70%, more preferably from 55% to 65%, most preferably about 60%.

[0016] When the plant spore shells comprise pine pollen shells, a ratio of total mass of pine pollen shells to combined total mass of pine pollen shells and plant-based wax in the coating may be from 50% to 80%. Preferably, the ratio is from 60% to 80%, more preferably from 70% to 80%.

[0017] The plant spore shells may comprise no more than 10% by mass genetic or cytoplasmic material. Preferably, the plant spore shells comprise no more than 5% by mass genetic or cytoplasmic material, more preferably no more than 2% by mass, more preferably no more than 1% by mass, more preferably no more than 0.1% by mass.

[0018] The superhydrophobic coating may further comprise a plant-based additive. The plant-based additive may be a binder. Preferably the plant-based additive is gum rosin. The superhydrophobic coating may comprise the plant-based additive at a mass ratio of about 40% additive to about 60% combined wax and plant spore shells.

[0019] A second aspect of the present invention is a multi-layered superhydrophobic coating for a substrate, the multi-layered superhydrophobic coating comprising: a first coating layer comprising the superhydrophobic coating of the first aspect; and at least one further coating layer comprising a plant-based wax and optionally comprising plant spore shells (e.g. lycopodium shells or pine pollen shells). The plant-based wax of the further coating layer may be of a different composition to the plant-based wax of the first coating layer.

[0020] By providing at least one further layer according to the second aspect, it is possible to ensure full coverage of the substrate and to optimise superhydrophobic properties. It is also possible to aid adhesion of the coating to the substrate and / or to aid resilience and conformity of the coating.

[0021] The superhydrophobic coating of the first aspect or the multi-layered superhydrophobic coating of the second aspect may be deposited on a paper or glass substrate.

[0022] A third aspect of the present invention is the use of plant spore shells in the formation of a superhydrophobic coating. The superhydrophobic coating may be applied to packaging. The packaging may be food packaging.

[0023] A fourth aspect of the present invention is a method of forming a superhydrophobic coating on a substrate, the method comprising: dispersing or dissolving a plant-based wax in a solvent and dispersing plant spore shells in the solvent, thereby forming a coating dispersion; depositing the coating dispersion onto a surface of a substrate, thereby coating at least a portion of the surface with the coating dispersion; and drying the coated substrate. The plant spore shells may comprise lycopodium shells or pine pollen shells.

[0024] The plant-based wax may be dispersed or dissolved in the solvent by mixing, preferably mechanical mixing.

[0025] The plant spore shells may be dispersed in the solvent by mixing, preferably mechanical mixing.

[0026] The coating dispersion may be formed at a temperature above 50°C, preferably above 60°C, preferably above 70°C, preferably above 80°C.

[0027] The coating dispersion may be formed at a temperature above a melting temperature of the plant-based wax.

[0028] The method may comprise cooling the coating dispersion to a temperature below 30°C prior to deposition on the surface of the substrate.

[0029] The coating dispersion may alternatively be formed at a temperature from 10°C to 30°C, preferably at a temperature of about 20°C.

[0030] The depositing of the coating dispersion onto the surface of the substrate may comprise spray-coating the substrate with the coating dispersion.

[0031] The depositing of the coating dispersion onto the surface of the substrate may comprise roll-to-roll processing.

[0032] The method may further comprise annealing the dried, coated substrate.

[0033] The plant-based wax may be soluble or dispersible in the solvent.

[0034] The solvent may comprise ethanol, water, acetone, toluene, n-hexane, chloroform, petroleum ether, or any combination thereof.

[0035] The method may further comprise, after the drying of the coated substrate: depositing a further amount of the coating dispersion onto the dried coated substrate, thereby further coating at least a portion of the surface with the coating dispersion; and drying the further coated substrate. Accordingly, coverage of the substrate surface by the coating may be improved.

[0036] A fifth aspect of the present invention is a method of forming a multi-layered superhydrophobic coating on a substrate, the method comprising: forming a superhydrophobic coating on a substrate according to the method of any of claims 11 to 21, wherein the formed superhydrophobic coating constitutes a first coating layer; and forming at least one further coating layer on the substrate, the further coating layer comprising a plant-based wax and optionally plant spore shells (e.g. lycopodium shells or pine pollen shells). The plant-based wax of the further coating layer may be of a different composition to the plant-based wax of the first coating layer. The at least one further coating layer may be deposited on the substrate before formation of the first coating layer (hence positioned between the first coating layer and the substrate) or deposited on the first coating layer after formation of the first coating layer.

[0037] Brief description of drawings

[0038] The present invention will be further described below with reference to exemplary embodiments and the accompanying drawings in which:

[0039] Figure 1 depicts a SEM micrograph of a lycopodium spore.

[0040] Figure 2 depicts SEM micrographs at various stages in the modification of lycopodium spores to produce lycopodium shells.

[0041] Figure 3 depicts a superhydrophobic coating according to an embodiment.

[0042] Figure 4 depicts a multi-layered superhydrophobic coating according to an embodiment.

[0043] Figure 5 depicts a method of forming a superhydrophobic coating on a substrate, according to an embodiment.

[0044] Figure 6 depicts SEM micrographs of annealed coatings comprising lycopodium spores.

[0045] Figure 7 depicts changes of water contact angle over time of an annealed coating comprising lycopodium spores.

[0046] Figure 8 depicts changes of water contact angle over time for a coating comprising lycopodium spores and a coating comprising lycopodium shells, subject to submersion in deionized water.

[0047] Figure 9 depicts wettability characteristics of coatings comprising lycopodium shells and uncoated surfaces when contacted with various liquid foods.

[0048] Figure 10 depicts SEM images at various stages in the modification of pine pollen to produce pine pollen shells.

[0049] Figure 11 depicts a method of forming a superhydrophobic coating on a substrate, according to another embodiment.

[0050] Figure 12 depicts SEM images of annealed coatings comprising pine pollen shells.

[0051] Figure 13 depicts wettability characteristics of coatings comprising pine pollen shells when contacted with water droplets.

[0052] Figure 14 depicts WCA of coatings comprising rice bran wax and lycopodium shells and various quantities of a gum rosin additive during abrasion testing.

[0053] Figure 15 depicts the appearance of the coatings represented by Figure 14 before and after 5 abrasion cycles.

[0054] Figure 16 depicts microscope images of the coatings represented by Figure 14 before and after 5 abrasion cycles.

[0055] In the various drawings, like parts are indicated by like references.Detailed description of invention

[0056] The invention provides a novel way of coating substrates to impart thereon superhydrophobic properties. The invention has particular applicability in food packaging, where moisture resistance is of utmost importance in view of maintaining structural integrity of packaging and safety and quality of the packaged food. Products andtechniques in accordance with the present invention are capable of providing the requisite moisture resistance to such substrates over long periods of time, while remaining both environmentally friendly and safe to consumers owing to the use of plant-based, biodegradable and food-grade materials to form the superhydrophobic coatings. Additionally, surprising improvements in superhydrophobic properties and superhydrophobic lifetime of coatings have been found to result from the novel application of chemically-modified lycopodium to superhydrophobic coatings. Similar improvements have been found to result from the novel application of chemically-modified pine pollen to superhydrophobic coatings. Thus, the benefits of the superhydrophobic coatings according to embodiments herein are not limited to the application to packaging (such as food packaging). The coatings described herein may provide improved superhydrophobic properties and lifetime when applied to many different kinds of substrate.

[0057] Previously reported work has used lycopodium spores, in their natural state as obtained from lycopodium plants, to achieve coatings with water contact angles exceeding 150° (defined as superhydrophobic). However, a substantial loss in hydrophobicity over time is observed in such coatings. In particular, initially superhydrophobic coatings formed with natural lycopodium spores have been observed to exhibit decreased water contact angles corresponding to a loss of superhydrophobic character in as little as 24 hours when unsubmerged, or just 60 seconds when submerged.

[0058] Figure 1 is an SEM micrograph depicting a lycopodium spore 100. As shown, lycopodium spores possess an outer protective layer 1 (exine) having multi-directional microchannels. It is due to this complex morphology that lycopodium spores can introduce hierarchical surface roughness to coatings, thus imparting superhydrophobic properties on the coatings when applied in a suitable formulation. Inside the exine 1 of the naturally- occurring lycopodium spore 100 is a core region comprising genetic and cytoplasmic material. More specifically, inside the exine 1 of the naturally-occurring lycopodium spore 100 there is found cytoplasmic material, cellulosic content and various proteins as well as an intine layer.

[0059] In the development of the present invention, it has been found that coatings comprising naturally-occurring lycopodium spores lose their hydrophobicity over time with no observable morphological changes. Accordingly, it is theorized that the loss of hydrophobicity is due to leaching of genetic and cytoplasmic materials, which areamphiphilic materials, from the spores over time. That is, the leached amphiphilic components may counteract the hydrophobicity imparted by the roughness of the exines, to cause the noticeable loss of hydrophobicity of such coatings.

[0060] To test this theory, new coating formulations have been prepared in which the lycopodium is modified to remove genetic material, leaving just the rough outer shell or exine (lycopodium shell). This has been achieved, for instance, using a modified version of a technique that is known within medical applications (see, for instance, the following publications which are incorporated by reference herein in their entirety: S. U. Atwe, Y. Ma and H. S. Gill, Journal of Controlled Release, 2014, 194, 45-52; A. Diego-Taboada, S. T. Beckett, S. L. Atkin and G. Mackenzie, Pharmaceutics, 2014, 6, 80-96; R. C. Mundargi, M. G. Potroz, J. H. Park, J. Seo, E. L. Tan, J. H. Lee and N. J. Cho, Sci Rep, D01: 10.1038 / srepl9960; M. J. Uddin, N. Abidi, J. Warzywoda and H. S. Gill, ACS Appl Mater Interfaces, 2019, 11, 20628-20641.) The technique employed involves conducting a three-stage regime in which lycopodium is sequentially treated with organic solvents (e.g. acetone) to remove the outer lipid layer, alkali (e.g. KOH) to remove genetic material and finally acids (e.g. H3PO4) to remove the intine (polysaccharides).

[0061] Figure 2 shows scanning electron microscopy (SEM) micrographs of lycopodium spores at various stages of the exemplary modification technique utilising acetone, KOH and H3PO4 to produce lycopodium shells. In Figure 2 there is shown: A) Manually crushed as-received spores 100, where the arrow indicates the inner biomolecules (genetic / cytoplasmic material); B) Lycopodium after defatting with acetone where the micro- structure of the exine remains intact; and C) and D) spores after acidolysis, in which the exine is now substantially free of genetic material and inner layers, leaving lycopodium shells 8, wherein the presence of holes in the rough outer shell can now be seen. Figure 2 shows that after acidolysis, removal of inner layers has been successful, with empty lycopodium shells 8 remaining. Though the shells hold their shape, the complex microstructure can be slightly damaged, resulting in several holes present in the surface structure.

[0062] Accordingly, “lycopodium shells” is a term used herein to refer to lycopodium spores that have had at least a substantial amount of the genetic and cytoplasmic material that exists within the spores in their natural form removed, so that mostly or only exine material remains. For instance, lycopodium shells may have been obtained by sequentialtreatment of lycopodium spores with organic solvents, alkalis and acids to remove these components. Additionally, the term “lycopodium shells” refers not only to modified lycopodium spores whose exines have remained intact, but also refers to modified lycopodium spores that have collapsed leaving fragments of exine material. Herein, in the context of lycopodium, “genetic and cytoplasmic material” is a term used to refer to the various proteins and organic matter found within the outer shell (exine) of a naturally- occurring lycopodium spore and that are distinct from the exine itself. The genetic and cytoplasmic material removed from lycopodium spores to extract lycopodium shells may include cytoplasmic material, cellulosic content, proteins and intine layers.

[0063] This modification of lycopodium, not previously applied in the context of superhydrophobic coatings, has surprisingly been found to greatly increase the lifetime of superhydrophobic behaviour in coatings comprising lycopodium, as discussed further in the examples herein.

[0064] The present inventors have also found that another naturally-occurring plantbased material, pine pollen, may be used in a similar way to lycopodium to produce highly effective superhydrophobic coatings.

[0065] Pine pollen is the male spore of Pinus massoniana Lamb., Pinus tabuliformis Carr., and other plants of the same genus. Pine pollen is known for its positive effects on treating different diseases such as anaemia, arthritis, and asthma. In recent years the possibility of pollen and sporopollenin exine has been explored for medical applications, especially as microcarriers for drugs, vaccines and live cells. Figure 10A is a SEM image of pine pollen 200, in its naturally-occurring grain form. As shown, the pine pollen grain 200 has a bisaccate shape with a core-shell structure. The pine pollen grain 200 has an outer layer (exine) made of sporopollenin which has a rough surface morphology, thus providing the potential for superhydrophobic behaviour. Accordingly, the present inventors investigated the use of pine pollen sporopollenin as another potential roughening agent for combining with plant-based waxes to produce plant-based superhydrophobic coatings.

[0066] Within the sporopollenin exine of naturally-occurring pine pollen there is a central cavity that contains various genetic and cytoplasmic materials (including protein and lipid materials). If the pine pollen grain is ruptured, these protein and lipid materials are exposed to air, increasing their solubility in water, thus counteracting the inherentlyhydrophobic characteristics of the sporopollenin. The present inventors have found that the abundance of such exposed materials is detrimental to the production of superhydrophobic coatings using pine pollen. For example, it has been found that the hydrophobic characteristics of sporopollenin can be exploited particularly well when pine pollen is used in the form of pine pollen broken cell wall powder (PPBCW), but untreated PPBCW inherently contains an abundance of genetic and cytoplasmic material released from the ruptured exines. Moreover, even if whole pine pollen grains are used instead of PPBCW to produce coatings, there may still be an abundance of exposed genetic and cytoplasmic materials due to occasional ruptures of pine pollen grains before and / or during the coating process. To increase the hydrophobicity of pine pollen (e.g. PPBCW) while maintaining the pollen’s morphological structure, it is necessary to reduce the content of such genetic and cytoplasmic materials.

[0067] It is known in the medical field that genetic / cytoplasmic contents of pine pollen can be reduced by treatment with acid (see, for instance, the following publication which is incorporated by reference herein in its entirety: D. Li, L. Sun, L. Shi, L. Zhuo, L. Yang, J. Zhang, Y. Han, T. Ye and S. Wang, Chemical Engineering Journal, 2023, 473, 145184.) The present inventors have determined that similar techniques can be used to modify pine pollen for use in superhydrophobic coatings. That is, pine pollen (e.g. PPBCW) can be treated with acid (e.g. H3PO4) to remove genetic / cytoplasmic contents adequately, leaving behind the outer sporopollenin shell or exine (pine pollen shell), without affecting the rough morphology of the sporopollenin. Doing so induces the desired wettability change, making pine pollen shells a suitable roughening agent for reliable plant-based superhydrophobic coatings.

[0068] One example technique for producing pine pollen shells involves refluxing pine pollen (e.g. PPBCW) in acid (e.g. H3PO4), using vacuum filtration to separate out the PPBCW remnants, washing the PPBCW remnants until there is no residual acid, and drying the PPBCW remnants (i.e. pine pollen shells). An example of the effect of such treatment is shown in the SEM images of figures 10B and 10C. Figure 10B shows pine pollen 200, in the form of PPBCW, prior to acid treatment. Figure 10C shows remnants of the PPBCW (pine pollen shells 208) after treatment with H3PO4 at 75°C for 20 hours.

[0069] Accordingly, “pine pollen shells” is a term used herein to refer to pine pollen grains or fragments thereof (e.g. in PPBCW) that have had at least a substantial amount ofthe genetic and cytoplasmic material that exists within the grains in their natural form removed, so that mostly or only sporopollenin exine material remains. For instance, pine pollen shells may have been obtained by treatment of pine pollen grains or PPBCW with acid to remove these materials. It will be appreciated that the term “pine pollen shells” refers not only to modified pine pollen grains whose exines have remained intact, but also refers to modified pine pollen grains that have collapsed leaving fragments of exine material (e.g. when pine pollen is provided as PPBCW). Herein, in the context of pine pollen, “genetic” and “cytoplasmic” materials refer to the various proteins, lipids and other organic matter found within the outer shell (exine) of a naturally-occuring pine pollen grain and that are distinct from the exine itself. The genetic and cytoplasmic material removed from pine pollen to extract pine pollen shells may include proteins, lipids and intine layers.

[0070] It has surprisingly been found that the pine pollen shells produced by this modification of pine pollen constitute an effective roughening agent for plant-based superhydrophobic coatings. It will be appreciated that the principles underlying the use of pine pollen shells as a roughening agent are analogous to those underlying the use of lycopodium shells as described herein. In each case, a naturally-occurring material (lycopodium spore, pine pollen grain) having a rough outer shell and genetic / cytoplasmic contents is modified to remove the latter contents, leaving the material of the rough outer shell (lycopodium shell, pine pollen shell) as an effective plant-based roughening agent combinable with plant-based waxes to produce superhydrophobic coatings.

[0071] Lycopodium spores and pine pollen grains are examples of plant spores. In general, plant spores include a rough outer shell having inherent hydrophobicity, and genetic / cytoplasmic materials which can act against hydrophobicity as described above. Thus, plant spores in general possess the properties that enable the conversion to an effective roughening agent for plant-based superhydrophobic coatings, based on the same principles described above in relation to lycopodium and pine pollen.

[0072] Accordingly, “plant spore shells” is a term used herein to refer to plant spores or fragments thereof that have had at least a substantial amount of the genetic and cytoplasmic material that exists within the plant spores in their natural form removed, so that mostly or only the material of the rough outer shell remains. The removal of genetic and cytoplasmic material is generally achieved by chemical treatment of the plant spores,such as treatment with acid. It will be appreciated that the term “plant spore shells” refers not only to modified plant spores whose outer shells have remained intact, but also refers to modified plant spores that have collapsed or been crushed leaving fragments of outer shell material. Herein, in general, “genetic” and “cytoplasmic” materials refer to the various proteins, lipids, and other organic matter found within the outer shell of a naturally-occurring plant spore and that are distinct from the outer shell itself.

[0073] Figure 3 depicts, in simplified form and not to scale, a superhydrophobic coating 2 in accordance with an embodiment. The coating 2 is formed on substrate 3, which may be a food packaging material such as paper. The constitution of coating 2 includes plant spore shells and a plant-based wax. In some embodiments, the constitution of coating 2 includes lycopodium shells and a plant-based wax (i.e. the plant spore shells are lycopodium shells). In some embodiments, the constitution of coating 2 includes pine pollen shells and a plant-based wax (i.e. the plant spore shells are pine pollen shells). In some embodiments the plant spore shells are a mixture of plant spore shells from different species.

[0074] The coating 2 is superhydrophobic, meaning that a water contact angle (WCA) on the coated surface is at least 150°. As shown in Figure 3, the WCA is defined as the angle between a water surface (surface of water droplet 4) and a solid surface where they meet. The coating 2 derives its superhydrophobic properties by virtue of the combination of plant spore shells with a hydrophobic, plant-based wax.

[0075] When lycopodium shells are used as the plant spore shells, the lycopodium shells of the coating 2 are obtained by chemical modification of lycopodium spores to remove genetic / cytoplasmic materials, in accordance with techniques described herein. That is, the lycopodium shells comprise exines or fragments of exines of lycopodium spores, and are substantially free of the various proteins and genetic materials and other biomolecules found inside the exines of naturally-occurring lycopodium spores. The lycopodium shells act as a roughening agent in the coating, by virtue of the hierarchical surface roughness imparted by the morphology of the shells - in particular, the multidirectional microchannels present in the exine material.

[0076] Similarly, when pine pollen shells are used as the plant spore shells, the pine pollen shells of the coating 2 are obtained by chemical modification of pine pollen (e.g. PPBCW) to remove genetic / cytoplasmic materials, in accordance with techniquesdescribed herein. That is, the pine pollen shells comprise exines or fragments of exines of pine pollen grains, and are substantially free of the various protein and lipid materials and other biomolecules found inside the exines of naturally-occurring pine pollen grains. The pine pollen shells act as a roughening agent in the coating, by virtue of the rough surface morphology of sporopollenin. The same process can be applied to plant spores of other species to obtain plant spore shells.

[0077] Preferably, the plant spore shells of the coating 2 comprises no more than 10% by mass genetic or cytoplasmic material (e.g. the lycopodium shells comprise no more than 10% by mass genetic or cytoplasmic material or the pine pollen shells comprise no more than 10% by mass genetic or cytoplasmic material, as appropriate).

[0078] That is, when lycopodium shells are used, it is preferable for the lycopodium shells in the coating as initially formed (i.e. prior to any possible leaching) to comprise substantially entirely lycopodium exines and fragments thereof (at least 90% by mass), and thus be substantially free of amphiphilic materials that would otherwise be susceptible to leach from the shells to compromise superhydrophobic performance of the coating. The less such materials are present, the longer the potential superhydrophobic lifetime of the coating may be. The lycopodium shells of the coating 2 preferably comprise no more than 5% by mass genetic or cytoplasmic material, preferably no more than 2%, preferably no more than 1%, preferably no more than 0.1%. Optimally, the lycopodium shells (and hence the coating itself) comprise no genetic and cytoplasmic material.

[0079] Similarly, when pine pollen shells are used, it is preferable for the pine pollen shells in the coating as initially formed to comprise substantially entirely sporopollenin exines and fragments thereof (at least 90% by mass), and thus be substantially free of amphiphilic materials that would otherwise compromise superhydrophobic performance of the coating. The pine pollen shells of the coating 2 preferably comprise no more than 5% by mass genetic or cytoplasmic material, preferably no more than 2%, preferably no more than 1%, preferably no more than 0.1%. Optimally, the pine pollen shells (and hence the coating itself) comprise no genetic and cytoplasmic material.

[0080] Plant spore shells are derived from plant spores and hence constitute an environmentally friendly, plant-based material. For example, lycopodium shells are derived from lycopodium spores and pine pollen shells are derived from pine pollen. Suchplant spore shells are also safe for contact with food (food grade), making them a suitable additive for food packaging.

[0081] The plant-based wax of the coating 2 is a hydrophobic wax. When deposited on the substrate 3 and dried, the hydrophobic wax undergoes a crystallization process upon drying and creates wax flakes which impart surface roughness on the coating. This complements the hierarchical roughness provided by the roughening agent (plant spore shells, e.g. lycopodium shells or pine pollen shells) to give superhydrophobic behaviour. The plant-based wax may also serve to melt around the plant spore shells (e.g. in an annealing step) to improve uniformity and durability of the coating.

[0082] Suitable plant-based waxes for inclusion in the coating 2 include carnauba wax, candelilla wax, soybean wax, beeswax, ricebran wax, paraffin wax, and combinations of these waxes. All of these waxes are plant-based, food grade, and hydrophobic. In preferred embodiments, the plant-based wax of the coating 2 comprises carnauba wax, and more preferably is carnauba wax. That is, a particularly preferred combination of wax and roughening agent is carnauba wax and lycopodium shells, and another particularly preferred combination of wax and roughening agent is carnauba wax and pine pollen shells.

[0083] The plant spore shells and plant-based wax may be present in the coating 2 in various mass ratios. This may be expressed in terms of a ratio of total mass of plant spore shells to combined total mass of plant spore shells and plant-based wax in the coating 2. When the plant spore shells are lycopodium shells, that ratio is preferably from 40% to 80%, more preferably from 50% to 70%, more preferably from 55% to 65%, most preferably about 60%. When the plant spore shells are pine pollen shells, that ratio is preferably from 50% to 80%, more preferably from 60% to 80%, most preferably from 70% to 80%. The term “total mass” may refer to a mass per unit coated area.

[0084] Figure 4 depicts, in simplified form and not to scale, a multi-layered superhydrophobic coating 5 in accordance with an embodiment. The coating 5 comprises a first coating layer 2 which is equivalent to the superhydrophobic coating 2 depicted in Figure 3. The discussion herein relating to the superhydrophobic coating 2 as depicted in Figure 3 is to be understood as being equally applicable to the first coating layer 2 of the multi-layered superhydrophobic coating 5 as depicted in Figure 4. The multi-layered superhydrophobic coating 5 comprises at least one further coating layer in addition to thefirst coating layer 2. The at least one further coating layer comprises a plant-based wax and optionally comprises plant spore shells, such as lycopodium shells or pine pollen shells. Two such further coating layers are depicted in Figure 4: under-layer 6 and overlayer 7. The multi-layered coating is not limited to this particular configuration of layers. For instance, the multi-layered coating 5 may comprise only the first coating layer 2 and under-layer 6, or may comprise only the first coating layer 2 and over-layer 7.

[0085] When under-layer 6 is present, under-layer 6 is formed directly on the substrate 3, and first coating layer 2 is formed on under-layer 6. The composition of under-layer 6 may be different to that of first coating layer 2. Under-layer 6 comprises a plant-based wax. The plant-based wax of under-layer 6 may be any one of the plant-based waxes described herein in relation to the first coating layer 2, or may be formed of combinations thereof. The plant-based wax of under-layer 6 may be of the same composition as the plant-based wax in the first coating layer 2, or may be of a different composition. In embodiments where the first coating layer 2 comprises lycopodium shells, under-layer 6 optionally comprises lycopodium shells, as in the first coating layer 2. In embodiments where the first coating layer 2 comprises pine pollen shells, under-layer 6 optionally comprises pine pollen shells, as in the first coating layer 2. The presence of under-layer 6 may serve to aid adhesion of first coating layer 2 to the substrate 3. That is, coating the substrate 3 first with a layer of wax or wax and lycopodium shells (under-layer 6) can aid adhesion for another coating of different composition (first coating layer 2) that is applied on top of the under-layer. The presence of plant spore shells such as lycopodium shells or pine pollen shells in the under-layer 6 may enhance anchorage of the coating 5 to the substrate. However, the presence of plant spore shells in the under-layer 6 is not essential, and under-layer 6 may be formed substantially entirely of plant-based wax (such as 99 wt% wax or more).

[0086] When over-layer 7 is present, over-layer 7 is formed on an outer-facing surface of first coating layer 2. The composition of over-layer 7 may be different to that of first coating layer 2. Over-layer 7 comprises a plant-based wax. The plant-based wax of overlayer 7 may be any one of the plant-based waxes described herein in relation to the first coating layer 2, or may be formed of combinations thereof. The plant-based wax of overlayer 7 may be of the same composition as the plant-based wax in the first coating layer 2, or may be of a different composition. In embodiments where the first coating layer 2comprises lycopodium shells, over-layer 7 optionally comprises lycopodium shells, as in the first coating layer 2. In embodiments where the first coating layer 2 comprises pine pollen shells, over-layer 7 optionally comprises pine pollen shells, as in the first coating layer 2. The presence of over-layer 7 may aid resilience and conformity of the first coating layer 2. The presence of plant spore shells such as lycopodium shells or pine pollen shells in the over-layer 6 may enhance roughness and thus water repelling properties of the coating 5. However, the presence of plant spore shells in the over-layer 7 is not essential, and over-layer 7 may be formed substantially entirely of plant-based wax (such as 99 wt% wax or more).

[0087] In any of the embodiments described and depicted herein, adherence of the coating 2 or multi-layered coating 5 to the substrate 3 may be aided by an adhesive layer which is distinct from under-layer 6 and not required to be formed from wax or plant-spore shells. Such an adhesive layer could be placed between the surface of the substrate 3 and the coating 2 or multi-layered coating 5.

[0088] In any of the embodiments described and depicted herein, the substrate 3 may be a food packaging material. For instance, the substrate may be a compostable material useful as an alternative to plastic, such as paper or card. The coatings described herein are particularly beneficial when applied to such materials, whose structural rigidity is otherwise prone to damage due to moisture. The substrate 3 may alternatively be glass.

[0089] However, the applications of coatings as described herein are not limited to food packaging. Such coatings may also be applied, for instance, to food items, food handling surfaces (including for food production), cosmetics, medical devices, fabrics, and general packaging (e.g. waterproof cardboard). The technology could also be expanded to any area in which it is desirable to replace plastic with paper, or make paper more water resilient. Additionally, coatings in accordance with embodiments described herein may be applied to substrate materials including, but not limited to, polymers, metals, and wood.

[0090] Coatings in accordance with the embodiments described herein (including coatings 2 and multi-layered coatings 5) are characterised by superhydrophobic behaviour, corresponding to a water contact angle of 150° or more, as measured by contact angle goniometry with 5 pL water droplets on the coated surface at ambient temperatures and pressures. In determining water contact angles of such coatings, plural measurements may be taken at various points on the surface of each coating, with the stated water contactangle corresponding to an average of the plural measurements for that coating. Preferably, the water contact angle is 151° or more, preferably 152° or more, preferably 153° or more, preferably 154° or more, preferably 155° or more.

[0091] The coatings also preferably exhibit high contact angles of liquid food droplets with different liquid surface tension to water, such as honey, orange juice, whole milk and cola, when measured by contact angle goniometry with 5 pL droplets of the liquid food on the coated surface at ambient temperatures and pressures. Preferably, coatings in accordance with the embodiments described herein (including coatings 2 and multi-layered coatings 5) exhibit a contact angle of 150° or more with at least one liquid food having a liquid surface tension less than that of water. Most preferably, such coatings exhibit a contact angle of 150° or more with all of honey, orange juice, whole milk and cola.

[0092] Coatings in accordance with the embodiments described herein (including coatings 2 and multi-layered coatings 5) preferably also exhibit low tilting angles at which water droplets begin to roll. For instance, the tilting angle at which water begins to roll may be 10° or less, or from 5° to 10°. Tilting angles are measured by a digital protractor to obtain the angle at which a 5 pL water droplet begins to roll along the coated surface.

[0093] Owing to the reliance on roughness properties of plant-derived materials (lycopodium or pine pollen) and inherent hydrophobicity of plant-based waxes, coatings described herein may be substantially entirely formed of plant-based materials. Preferably, coatings in accordance with the embodiments described herein (including coatings 2 and multi-layered coatings 5) may comprise 99.5 wt% or more plant-based material, more preferably 99.6 wt% or more, more preferably 99.7 wt% or more, more preferably 99.8 wt% or more, more preferably 99.9 wt% or more, most preferably 100%.

[0094] Figure 5 depicts steps of a method of forming a superhydrophobic coating 2 on a substrate 3, according to an embodiment in which the coating comprises lycopodium shells 8 as the roughening agent. Shown in Figure 5 are the precursory steps of modifying lycopodium spores to remove amphiphilic materials and generate lycopodium shells, followed by the coating formation steps of forming a coating dispersion and depositing it on the substrate. A method as depicted in Figure 5 may result in the formation of coating 2 as depicted in Figure 3 or first coating layer 2 as depicted in Figure 4.

[0095] Lycopodium spores 100 are first treated with an organic solvent to remove lipids (step 51). The treatment with an organic solvent may comprise, for instance, acetone reflux.

[0096] Then, the lycopodium spores are subject to sequential alkaline lysis and acidolysis to remove proteins and genetic material to produce substantially empty lycopodium shells 8 upon drying (step 52).

[0097] Lycopodium shells 8 and plant-based wax 9 are introduced to a solvent 10 (step 53). Specifically, the lycopodium shells 8 are dispersed in the solvent 10, and plantbased wax 9 is dispersed or dissolved in the solvent 10. The solvent with dispersed lycopodium shells and dispersed / dissolved wax constitutes a coating dispersion. The dispersion of lycopodium shells 8 and dispersion / dissolution of wax 9 in the solvent 10 may take place in either order or may be simultaneous.

[0098] The dispersing / dissolving of plant-based wax 9 in the solvent 10 may be achieved by mixing, preferably by straightforward mechanical mixing. Accordingly, it is preferable for the plant-based wax 9 to be soluble or dispersible in the solvent 10. By “soluble” it is meant that the plant-based wax 9 is capable of forming a stable solution in the solvent 10 by mechanical mixing alone, at the temperature at which mixing of the wax 9 and solvent 10 takes place. By “dispersible” it is meant that the plant-based wax 9 is capable of forming a stable dispersion in the solvent 10 by mechanical mixing alone, without an emulsifier, at the temperature at which mixing of the wax 9 and solvent 10 takes place.

[0099] Similarly, the dispersing of lycopodium shells 8 in the solvent 10 may be achieved by mixing, preferably mechanical mixing.

[0100] Following addition of the lycopodium shells 8 and plant-based wax 9 to the solvent 10, the components may be mechanically mixed continuously for a time of around 30 minutes.

[0101] Various solvents may be utilized as solvent 10. For instance, solvent 10 may comprise ethanol, water, acetone, toluene, n-hexane, chloroform, petroleum ether, or any combination thereof. In view of minimising the need for extra additives in the coating formation process, it is desirable for the solvent 10 to be one in which the plant-based wax is able to disperse or dissolve without aid of additives such as emulsifiers. Accordingly, organic solvents may be preferred. Preferably, the solvent 10 is ethanol.

[0102] The coating dispersion may be formed at elevated temperatures. Preferably, the coating dispersion is formed at a temperature above 50°C, preferably above 60°C, preferably above 70°C, preferably above 80°C, preferably above 85°C. The coating dispersion is preferably formed at a temperature above a melting temperature of the plantbased wax 9. Accordingly, the preferred temperature for formation of the coating dispersion may be dependent on the particular composition of plant-based wax used. When the coating dispersion is formed at elevated temperatures, the method preferably comprises cooling the coating dispersion to a temperature below 30°C prior to deposition of the coating dispersion on the surface of the substrate 3. Preferably, the coating dispersion is cooled to room temperature prior to deposition on the substrate 3.

[0103] Alternatively, the coating dispersion may be formed at ambient temperatures, such as at room temperature. This is suitable for some wax-solvent-lycopodium formulations which do not require elevated temperatures to enhance or enable mixing to produce a coating dispersion suitable for application to a substrate to form a superhydrophobic coating. With such formulations, it is desirable to perform mixing to form the coating dispersion without heating to elevated temperatures such as the melting point of the wax, so as to save energy and cost. In such cases, preferably, the coating dispersion is formed at a temperature from 10°C to 30°C, more preferably at a temperature of about 20°C. Herein, “about 20°C” refers to temperatures close to standard room temperature. For instance, “about 20°C” may be interpreted as a temperature range of 18°C to 25°C.

[0104] The coating dispersion is deposited onto a surface of a substrate 3 (step 54). Thereby, at least a portion of the surface is coated with the coating dispersion. When the substrate 3 is a food packaging material, the surface of the food packaging material intended for contact with food is coated. Preferably, the entirety of the surface that is to be exposed to food is coated. Preferably, the depositing of the coating dispersion onto the substrate 3 comprises spray-coating the substrate 3 with the coating dispersion, as is depicted in Figure 5. Additionally, or alternatively, the depositing of the wax-solvent dispersion onto the surface of the substrate may comprise roll-to-roll processing. Other deposition methodologies usable with the present invention include, but are not limited to, dip-coating, screen printing, manual spreading, and drop-casting.

[0105] Following deposition of the coating dispersion on the surface of the substrate 3, the coated substrate is dried so as to harden the coating on the surface. The drying may comprise air drying at room temperature for a period of several hours.

[0106] After drying, a further amount of the coating dispersion is preferably deposited onto the dried coated substrate, thereby further coating at least a portion of the surface with the coating dispersion. This serves to fill in gaps and cracks in the dried initial coating, thus enabling full coverage and optimised hydrophobic properties and durability. The substrate may again be dried following the further coating.

[0107] Preferably, the method further comprises annealing the dried coated substrate (step 55). Following annealing, a final uniform superhydrophobic coating 2 may be obtained. An annealing step may serve to improve durability of coatings by causing wax to melt around lycopodium particles. Moreover, when holes have been produced in lycopodium shells in the acidolysis treatment, annealing and the resulting melting of wax may beneficially fill in these holes, counteracting weakness that could otherwise have resulted from the holes.

[0108] In embodiments, the method may begin at step 53, wherein lycopodium shells are provided having already been extracted from naturally-occurring lycopodium spores. Alternatively, the method may include precursory steps 51 and 52 of treating lycopodium spores to generate the lycopodium shells.

[0109] Figure 11 depicts steps of a method of forming a superhydrophobic coating 2 on a substrate 3, according to an embodiment in which the coating comprises pine pollen shells 208 as the roughening agent. Shown in Figure 11 are the precursory steps of modifying pine pollen to remove amphiphilic materials and generate pine pollen shells, followed by the coating formation steps of forming a coating dispersion and depositing it on the substrate. A method as depicted in Figure 11 may result in the formation of coating 2 as depicted in Figure 3 or first coating layer 2 as depicted in Figure 4.

[0110] Pine pollen 200 (e.g. in the form of PPBCW) is first treated (e.g. refluxed) with acid 201 (e.g. phosphoric acid) to remove genetic and cytoplasmic material (step 111).This produces pine pollen shells, i.e. remnants of the original pine pollen with substantially reduced content of genetic and cytoplasmic material.

[0111] The pine pollen shells 208 are filtered (e.g. by vacuum filtration) and washed (e.g. with water 202) to remove residual acid (step 112). The pine pollen shells 208 may then be dried.

[0112] To prepare the pine pollen shells 208 for the coating, the pine pollen shells 208 may be ground (e.g. by ball milling) to a fine powder (step 113).

[0113] Pine pollen shells 208 and plant-based wax 9 are introduced to a solvent 10 (step 114). Specifically, the pine pollen shells 208 are dispersed in the solvent 10, and plant-based wax 9 is dispersed or dissolved in the solvent 10. The solvent with dispersed pine pollen shells and dispersed / dissolved wax constitutes a coating dispersion. The dispersion of pine pollen shells 208 and dispersion / dissolution of wax 9 in the solvent 10 may take place in either order or may be simultaneous.

[0114] The dispersing / dissolving of plant-based wax 9 in the solvent 10 may be achieved by mixing, preferably by sonication. It is preferable for the plant-based wax 9 to be soluble or dispersible in the solvent 10, as defined above. Similarly, the dispersing of pine pollen shells 208 in the solvent 10 may be achieved by mixing, preferably by sonication. The plant-based wax 9 and pine pollen shells 208 may be sonicated in the solvent 10 simultaneously to create the coating dispersion.

[0115] Step 114 of Figure 11 is generally similar to step 53 of Figure 5, except for the different roughening agent used. As such, the discussion herein of preferable forms and characteristics of solvent 10 and the temperature at which the coating dispersion is formed when lycopodium shells are used as the roughening agent, applies mutatis mutandis when pine pollen shells are used as the roughening agent. As with lycopodium shells, a preferable solvent 10 for use with pine pollen shells is ethanol.

[0116] The coating dispersion is deposited onto a surface of a substrate 3 (step 115). Thereby, at least a portion of the surface is coated with the coating dispersion. This step is generally similar to step 54 of Figure 5, except for the different roughening agent used. As such, the discussion of herein of preferable ways of carrying out the deposition of the coating formulation and the drying of the substrate when lycopodium shells are used as the roughening agent, applies mutatis mutandis when pine pollen shells are used as the roughening agent. As discussed in relation to Figure 5, further amounts of the coating dispersion may be deposited onto the dried coated substrate to enhance coverage.

[0117] Preferably, the method further comprises annealing the dried coated substrate (step 116). As discussed herein in relation to Figure 5, annealing may provide a uniform and durable coating.

[0118] Following deposition and annealing, superhydrophobic coating 2 is obtained (step 117).

[0119] In embodiments, the method may begin at step 114, wherein pine pollen shells 208 are provided having already been extracted from naturally-occurring pine pollen and / or PPBCW. Alternatively, the method may include some or all of precursory steps 111 to 113 of generating and preparing the pine pollen shells.

[0120] Another embodiment is a method of forming a multi-layered superhydrophobic coating on a substrate. The method comprises forming a superhydrophobic coating on a substrate according to the above-described method as depicted in Figure 5 or Figure 11, wherein the formed superhydrophobic coating constitutes a first coating layer 2.Additionally, this method comprises forming at least one further coating layer on the substrate, the further coating layer comprising a plant-based wax and optionally plant spore shells (e.g. lycopodium shells or pine pollen shells). The further coating layer is equivalent to that described herein with reference to Figure 4. Thus, the further coating layer may be an under-layer 6 formed prior to formation of the first coating layer 2, or an over-layer 7 formed after formation of the first coating layer 2.

[0121] In such a method, the formation of the at least one further coating layer may be achieved in a number of ways, which are not limited in the same way as the formation of the first coating layer 2. This is because the superhydrophobic properties of the overall multi-layered coating 5 are not reliant on the presence or form of any further coating layer, but result from the formation technique and composition of the first coating layer 2. As described above, the further coating layer(s) provide additional benefits with regard to adhesion and / or resilience and conformity. Thus, the formation of any further coating layer may generally comprise dispersing or dissolving the desired wax, and optionally dispersing plant spore shells (e.g. lycopodium or pine pollen shells), in a solvent, and depositing the dispersion or solution onto the substrate or coated substrate. The process variables of which solvent is used, whether lycopodium / pine pollen shells are included, and whether the wax is dispersed or dissolved in the solvent, will depend on the intendedpurpose of the further coating layer and the constitution of the plant-based wax being used in the further coating layer.

[0122] The use of plant spore shells in the preparation of superhydrophobic coatings as described herein, particularly in combination with the use of plant-based waxes, enables the preparation of superhydrophobic coatings that are long-lasting (due to the effects of removal of amphiphilic materials as described herein), environmentally friendly and safe for contact with food. Such coatings may be formed in accordance with the techniques herein without the need for additional fillers or polymeric material or other agents such as emulsifiers which are non-biodegradable or synthetic.

[0123] Although additives (i.e. components other than the plant spore shells and plantbased wax) are not required for the coating to achieve long-lasting superhydrophobic properties, an additive or combination of additives may nonetheless be included in the coating to enhance other properties thereof, such as strength, flexibility and adherence to underlying substrates. Suitable additives include plant-based, food grade materials, so as not to detriment the above-described safety and environmental friendliness of the coating. That is, in some embodiments, the superhydrophobic coating 2 comprises a plant-based additive.

[0124] The superhydrophobic coating 2 may comprise a plant-based additive, or a combination of plant-based additives, at up to 40% by mass. For example, the coating may comprise a plant-based additive (or combination of plant-based additives) at a mass ratio of about 40% additive(s) to about 60% combined wax and plant spore shells, or about 30% additive(s) to about 70% combined wax and plant spore shells, or about 20% additive(s) to about 80% combined wax and plant spore shells, or about 10% additive(s) to about 90% combined wax and plant spore shells.

[0125] In the processes for forming a superhydrophobic coating 2 as described above, the plant-based additive (or combination of plant-based additives) may be dispersed in the solvent along with the dispersing / dissolving of the plant-based wax and the dispersing of the plant spore shells, so that the coating dispersion (prior to deposition on the substrate) comprises the plant-based wax, the plant spore shells, the plant-based additive, and the solvent. Alternatively, coating dispersion may initially be formed as described above (comprising only the plant-based wax, plant spore shells and solvent), and then the plantbased additive may be introduced to the coating dispersion and dispersed therein by asubsequent mixing step. The subsequent mixing step may be carried out at elevated temperature. The subsequent mixing step may comprise mechanical stirring and / or sonication.

[0126] The plant-based additive may act as a binder material which enhances the adhesion of the coating to the underlying substrate. A suitable plant-based additive for this purpose is gum rosin. Gum rosin is a natural resin obtained from the exudate of pine trees, primarily species from the Pinus genus. It is a solid, brittle, amber-coloured substance mainly composed of resin acids, including abietic acid. Gum rosin is known to exhibit hydrophobicity, and has previously been used in industrial applications such as adhesives, varnishes, and printing inks to improve stickiness. Gum rosin has been found to enhance adhesion of the coatings described herein to substrates, particularly paper and cardboard substrates. Moreover, it has been found that gum rosin can achieve these benefits without loss of superhydrophobic behaviour. Thus, in some embodiments, the plant-based additive of the superhydrophobic coating is gum rosin. For example, the superhydrophobic coating may comprise about 60 wt% combined plant-based wax and plant spore shells, and about 40 wt% gum rosin.

[0127] Other suitable plant-based additives include zein (a protein extract from corn production), and agar. These or other plant-based additives may be present in the superhydrophobic coating individually or in combination.

[0128] The coatings and associated uses and methods described herein show promise for the use in sustainable food packaging through application to biodegradable substrates (e.g. paper). This applicability is demonstrated in the examples herein through testing with various liquid foods where food residue and therefore food waste are greatly reduced.

[0129] The following comparative and experimental examples demonstrate some of the effects and advantages of the present invention compared with known techniques. The examples below are illustrative and are not to be construed as limiting.Comparative example: lycopodium spores

[0130] As-received lycopodium spores were initially used as a plant-based roughening agent for the preparation of food grade coatings.

[0131] Coating dispersions were prepared by the addition of roughening agent (lycopodium spores) and hydrophobic wax (Carnauba) to ethanol (20 mL). The mass ratioof roughening agent to hydrophobic wax was varied from 2:8 to 8:2, with the total mass of solids remaining at 1g. Formulations were termed according to the wt% of lycopodium relative to lycopodium plus wax: L-20 (representing a mix of 20% lycopodium and 80% wax), L-30, L-40, L-50, L-60, L-70 and L-80. In this example, references to wt% of lycopodium refer to the wt% of lycopodium relative to combined weight of lycopodium and plant-based wax in the coating. The dispersions were mechanically stirred at a temperature exceeding 85°C for 30 minutes to facilitate mixing before being left to cool at room temperature.

[0132] Surface morphology of coatings formed by the dispersions was analysed by SEM. In particular, surface morphology was analysed using a FEI Inspect F SEM operating at an acceleration voltage of 5-10 kV. Before visualisation, samples were vacuum sputter coated in a thin layer (~5 nm) of gold to improve electrical conductivity inside the microscope.

[0133] To form the coatings on substrates for analysis of hydrophobic properties, an Iwata Power Jet Pro Airbrush System was used. Coatings were deposited at a distance of about 4 cm away from substrates at an air pressure of 2 bar. Formulations were deposited onto substrates (glass slides) for a total of three coats. For some formulations, drying steps were included in between depositions. After air drying at room temperature overnight, a final coat was sprayed onto the substrate to ensure no cracks were present. Samples were then annealed for 5 minutes at 100°C.

[0134] Fourier Transform Infrared (FTIR) measurements were taken using a Bruker Tensor 27 FTIR spectrometer over a range of 400 to 4000 cm’1. Static water contact angle measurements were taken using an Ossila Contact Angle Goniometer using 5 pL droplets. Baselines were assigned manually to minimise errors and measurements were taken at ambient temperatures across 10 different areas of each coating, with the presented contact angles being an average of these. Tilting angles were measured using a digital protractor to obtain the angle at which droplets began to roll. The wettability of each coating was examined before and after annealing to observe the effects on the level of hydrophobicity.

[0135] Before annealing, coatings were extremely powdery and could easily be wiped off with a finger. After annealing, the wax melted around the lycopodium to create a uniform coating with increased durability.

[0136] Before annealing, the carnauba wax undergoes a crystallization process upon drying, leading to a surface roughness contributed to by lycopodium and carnauba wax flakes. In the unannealed coatings, as the mass of lycopodium increased so did the level of hydrophobicity, however, of the seven unannealed coatings only L-70 and L-80 exhibited superhydrophobic behaviour, with WCAs measuring 151 ± 4° and 150 ± 2° respectively. At lower mass ratios of lycopodium, coating morphology included highly rough areas of lycopodium / wax flakes but large regions of relatively flatter areas in which only carnauba wax contributed to roughness, likely leading to the measured WCAs of under 150°.

[0137] Once annealed, the wax crystals melt and form a direct coating around the lycopodium, altering the surface morphology and wettability. For coatings under 50 wt% lycopodium, the annealing resulted in a decreased WCA as large flat areas formed in the regions previously roughened by wax crystals. This is shown by Figure 6, which depicts SEM micrographs of post-annealing coatings L-30 and L-70. As is visible in Figure 6, at the lower mass of lycopodium, large flat regions are observed in the annealed coating which contribute to a lower WCA. For coatings > 50 wt% lycopodium, a small increase in the measured WCAs was observed versus the unannealed coatings, with L-50 through to L-80 being superhydrophobic after annealing. L-50 now measured the highest WCA at 153 ± 2°.

[0138] Tilting angles for the best performing coatings ranged from 5 to 10°, although higher tilt angles were also observed.

[0139] Though the coatings showed that superhydrophobicity was possible using plant-based materials (lycopodium spores and carnauba wax), upon observing the coatings over time in ambient conditions free of dust and light, a reduction in hydrophobicity was observed in as little as 24 hours. By 48 hours, superhydrophobicity was no longer present in any of the prepared coatings which now appeared “sticky” in wetting nature as droplets would no longer roll across the surface. The observed loss in hydrophobicity over time is shown in Figure 7. In Figure 7, WCAs for coatings L-20 through to L-80 are shown as measured for the original coating immediately after annealing (circular data points 11), the coating after 24 hours (square data points 12), and the coating after 48 hours (diamond data points 13). As shown, all coatings exhibited WCAs under 150° after 48 hours in the ambient conditions free of dust and light.

[0140] To observe changes in hydrophobic behaviour when exposed to wet environments (e.g. in food packaging), wettability tests were also carried out on annealed coatings submerged in deionized water. In these tests, superhydrophobic behaviour was lost after just 60 seconds for all coatings. For coatings L-60, L-70 and L-80, complete wetting of the surface was observed, with the remaining coatings showing WCAs < 100 °.

[0141] Both ambient condition and submerged samples were observed with SEM where no difference in surface structure was seen. It was therefore assumed that the reduction in hydrophobicity with time was resulting from a surface chemistry change. WCA measurements were also taken on flat carnauba wax over the course of a week both in ambient and submerged conditions. Across this time, no notable difference was observed, with the WCA remaining ~ 85°, suggesting the lycopodium was the cause of decreasing hydrophobicity. Accordingly, as has been discussed herein, the loss of hydrophobicity was attributed to leaching of amphiphilic materials from the lycopodium spores.Experimental example: lycopodium shells

[0142] Food grade coatings were prepared using modified lycopodium in the form of lycopodium shells as the plant-based roughening agent.

[0143] Formulations were prepared in which the lycopodium was modified to remove genetic material, leaving just the rough outer shell. To do so, prior to forming a coating dispersion, a three-stage regime was conducted using a modified version of a process known within medical applications. Specifically, lycopodium spores were sequentially treated with organic solvents (acetone) to remove the outer lipid layer, alkali (KOH) to remove genetic material and finally acids (H3PO4) to remove the intine (polysaccharides). The lycopodium was analysed with Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) at each stage of the process to monitor both chemical and structural changes, in combination with addition of water to monitor hydrophobicity. The analysis resulted in the SEM micrographs of Figure 2, as have been discussed elsewhere herein. In Figure 2 it can be seen that after acidolysis, removal of inner layers was successful, with only empty shells remaining. Though the shells held their shape, the complex microstructure was slightly damaged, now showing several holes present in the surface structure.

[0144] Similarly to the comparative example, coating dispersions were prepared by the addition of roughening agent (lycopodium shells) and hydrophobic wax (Carnauba) to ethanol (20 mL). The mass ratio of roughening agent to hydrophobic wax was varied, with the total mass of solids remaining at 1g. Formulations were termed according to the wt% of lycopodium shells (modified lycopodium) relative to lycopodium shells plus wax: ML- 20 (representing a mix of 20% lycopodium shells and 80% wax), ML-30, ML-40, ML-50, ML-60, ML-70 and ML-80. In this example, references to wt% of lycopodium shells refer to the wt% of lycopodium shells relative to combined weight of lycopodium shells and plant-based wax in the coating. The dispersions were mechanically stirred at a temperature exceeding the melting point of the wax being used (for carnauba wax this is 85°C) for 30 minutes to facilitate mixing before being left to cool at room temperature.

[0145] Surface morphology of coatings formed by the dispersions was analysed by SEM under the same conditions as the comparative example.

[0146] Coatings were formed on substrates (glass slides) for analysis of hydrophobic properties, by spray-coating, drying and annealing under the same conditions as the comparative example. FTIR measurements, static water contact angle measurements and tilting angle measurements were taken under the same conditions as the comparative example. Additional contact angle measurements were taken using liquid food droplets (in the same way as the water contact angle measurements). Wettability of the coatings was observed before and after annealing.

[0147] Before annealing, formulations ML-60, ML-70 and ML-80 all exhibited superhydrophobic behaviour with WCAs of 151 ± 3°, 151 ± 4° and 153 ± 2° respectively. A decrease in hydrophobicity was again observed for formulations < 50 wt% after annealing as the crystalized wax melted leaving relatively large flat regions on the surface. This process, however, offered the benefit of filling in the holes produced during acidolysis, potentially counteracting weakness that could have resulted from the holes. Once annealed, ML-50 again recorded the highest WCA of 155 ± 2°.

[0148] In conditions free of dust and light, the coatings that were superhydrophobic in their initial state following formation remained superhydrophobic over a period of 9 months. The coatings are expected to remain superhydrophobic until perturbed. To assess the performance when exposed to liquid, annealed ML-50 was submerged in deionized water and the WCA was recorded every 10 minutes. Over 1 hour, the coating displayed asilver appearance resulting from the air remaining trapped within the surface structures, with the WCA remaining above 150° throughout the hour. This marked improvement in ability to maintain a high WCA when submerged is displayed in Figure 8, in which data points 14 represent measured WCAs over time for submerged coating L-50 of the comparative example, and data points 15 represent measured WCAs over time for submerged coating ML-50 of the experimental example.

[0149] To demonstrate the coating's applicability as a potential alternative to singleuse plastic, a variety of liquid foods were tested on ML-50. The foods, which have a different liquid surface tension to water (water: 72 mN / m, honey: ~55 mN / m, orange juice: ~30 mN / m, whole milk: ~48 mN / m, and cola ~57 mN / m), were found to all have contact angles exceeding 150°, with the lowest contact angle measuring 150 ± 3° for milk, which is likely due to the increased fat within this product. Qualitative observations are shown in Figure 9. Specifically, Figure 9 depicts: A) a photo showing near-spherical droplets of liquid foods (honey 16, orange juice 17, milk 18 and cola 19) on the coating 2; B) an uncoated substrate 3 (glass slide) on which honey slides leaving honey residue; C) a ML- 50 coating 2 demonstrating minimal adhesion to honey; and D) a ML-50 coating 2 demonstrating rolling of milk. The ML-50 coating showed excellent anti-adhesion to the tested foods, with milk, juice and cola all easily rolling off the surface (e.g. milk in Figure 9D). Honey also showed anti-adhesion but unlike the previous foods would slide across the surface (Figure 9C) - despite this, minimal adhesion was seen compared to the uncoated sample (Figure 9B).Experimental example: pine pollen shells

[0150] Food grade coatings were prepared using modified pine pollen in the form of pine pollen shells as the plant-based roughening agent.

[0151] 25 g of PPBCW was refluxed in H3PO4 (> 85%) at 15 times its weight for 20 hours at 75°C. Then, vacuum filtration was used to separate the treated PPBCW (i.e. pine pollen shells) and the treated PPBCW was washed thoroughly with hot deionised water until there was no residual H3PO4. Each wash was collected using vacuum filtration. After washing, the treated PPBCW was dried in an oven at 75°C for 12 hours. It was observed that the acid treatment caused the colour of PPBCW to change from yellow tobrown, and caused the PPBCW to become less wettable (i.e. more hydrophobic) in powder form.

[0152] The transition to superhydrophobic is influenced not only by surface roughness but also by surface chemistry. The presence of surface functional groups, sporopollenin, is crucial in determining the material's wettability. To understand the changes in chemical functional groups in pine pollen sporopollenin before and after H3PO4 treatment, FTIR analysis was used under the same conditions as in the comparative example, to identify the functional groups. The untreated pine pollen showed specific vibrations related to proteins (at 3300 and 1519 cm ' ) and lipids (at 1714 cm '). When compared with untreated pine pollen, the treated pine pollen showed a lack of carbohydrate-specific vibrations in the fingerprint regions (at 1176 and 1008 cm '). Both untreated and treated pine pollen showed C-H stretching at 2930 and 2850 cm The peak at 3300 drastically reduced after pine pollen was treated with H3PO4, indicating the reduction of hydrophilic groups (-OH).

[0153] Elemental analysis was also conducted to confirm a reduction in proteinaceous material. Pine pollen samples between 4 and 6 mg were analysed using Thermo Elemental Analysis Flash 1112 to estimate their carbon, nitrogen, and hydrogen content. The samples were pre-weighed, sealed in tin capsules, and positioned in a rotating autosampler at the top of the unit. The samples fell into a chamber purged of all atmospheric air. After purging, the samples dropped directly into the combustion tube, where they were combusted into gas at temperatures ranging from 950°C to 1060°C. The gaseous samples then flowed into a separation column before being quantified by a Thermal Conductivity Detector. It was taken less than 5 mg of untreated and treated pine pollen to visualize the content of nitrogen which is closely related to the protein amount. A multiplication factor of 6.25 was used to convert per cent nitrogen to per cent protein. The protein content of untreated pine pollen broken cells was approximately 13%. After being treated with H3PO4 for 20 hours there was in a protein content reduction to 3.5%. Approximately 68% of the spore mass was removed after chemical treatment (initial weight 25 g and final weight after chemical treatment 8 g).

[0154] Thermogravimetric analysis was also conducted to investigate thermal decomposition characteristics. Pine pollen treated and untreated were pyrolyzed in a nitrogen-controlled environment. The thermogravimetric analysis was measured by using a Thermogravimetric analyzer TGA 5500 (TA Instruments, Castle, MA, USA). Samplesbetween 5-10 mg were used in each analysis. Thermograms were conducted from room temperature to 600 °C with a heating rate of 10°C / min in a nitrogen flow at 25 mL / min. It was observed that the treated PPBCW exhibited less loss of mass at high temperatures than the untreated PPBCW. This is a result of the protein removal process, because the organic contents, which degrade at high temperatures, were eliminated from the structure, leaving behind the desired pollen, which is more resistant to temperature and chemical processes.

[0155] The treated PPBCW was ball-milled at 200 rpm for 1 hr in reverse mode using a Fritsch Pulverisette ball-milling machine. Treated PPBCW was placed in 80 mL hardened stainless grinding bowls with a capacity of 10-30 mL, using 22 steel balls, each with a 50x10 mm diameter, made of hardened stainless steel to create a fine powder.

[0156] Coating dispersions were prepared by the addition of roughening agent (pine pollen shells) and hydrophobic wax (Carnauba) to ethanol (96% v / v, 20 mL) and sonication of the mixture for 15 minutes. The mass ratio of pine pollen shells to hydrophobic wax was varied from 2:8 to 8:2, with the total mass of solids remaining at 1g. In this example, references to wt% refer to the wt% of pine pollen shells relative to the combined weight of pine pollen shells and plant-based wax in the coating.

[0157] Each coating dispersion was deposited onto a substrate (glass microscope slide) using an airbrush spray coater to deposit four layers of the coating dispersion at a pressure of 2 bars. All the sprays were carried out at ~10 cm away from the substrate, and approximately 1 ml of the solution was used for each layer. The sprayed substrates were positioned in a hot plate at 90°C until the wax was fully melted.

[0158] The treated PPBCW coatings were examined using a Scanning Electron Microscope under the same conditions as described in the comparative example, except with the thin layer of gold being approximately 8 nm thick. SEM images of the 50 wt.% carnauba wax / pine pollen coating are shown in Figure 12, at three different levels of magnification. These images show the rough surface morphology of the coating. Due to the broken nature of the PPBCW, the original pollen structure was not observable. No significant changes were noted after treatment and ball milling. The size of pine pollen ranges from 20 pm to 5 pm.

[0159] To assess the wettability of the PPBCW / CW coatings, contact angle measurements were conducted using a sessile drop of 10 pL deionised water in a Contact Angle Goniometer (Ossila). Four measurements were taken, and the averages werereported. Sliding angles were measured in the same way as in the comparative example. It was observed that as the weight percentage (wt.%) of treated PPBCW in the coating increases, the WCA also increases, reaching a range from WCA 154 to 158°. If the percentage of treated PPBCW decreases, the WCA decreases, indicating the formation of a flatter surface with less roughness. The lowest sliding angle (< 6) was achieved with 70wt% pine pollen shells. Coatings containing 20 to 40 wt% pine pollen shells were sticky and water droplets did not roll off the surface. The results are shown in Table 1 below. Figure 13 depicts water droplets on the 50wt% coating (image A) and the 70wt% coating (image B).

[0160] Table 1 : water contact angles (WCA) and sliding angles (SA) of pine pollen shells / camauba wax coatings of different wt% of pine pollen shellsExperimental example: lycopodium shells and gum rosin additive

[0161] The effect of introducing gum rosin (plant-based additive) to coatings comprising lycopodium shells was also studied. Two formulations of rice bran wax (0.3g) and lycopodium shells (0.3g) in hexane (30 mL) were prepared. 0.07g of gum rosin was added to one formulation, to create a 10% additive formulation. 0.4g of gum rosin was added to the other formulation, to create a 40% additive formulation. Each formulation was then stirred with heating (80°C) for 30 minutes to facilitate mixing before being left to cool at room temperature. To form coatings, the formulations were deposited by spray coating at a distance ~ 4 cm away from paper substrates at an air pressure of 2 bar. The formulations were deposited onto substrates for a total of three layers, waiting 10 seconds between each consecutive layer. Coatings were then heated to melt the wax layer using a heat gun (up to the wax's melting point, this is also possible on a hotplate).

[0162] The coated paper samples were placed face-down on sandpaper (grit no. 120) with a 100 g weight applied on top. Each sample was subjected to abrasion by being pulled 10 cm across the sandpaper, then rotated 90° and moved an additional 10 cm tocomplete a single cycle. Water contact angle (WCA) measurements were recorded after each cycle, with a total of five cycles performed per sample. The results are depicted in Figure 14, which also shows corresponding results for a coating formed with equal weights of rice bran wax and lycopodium shells and without any gum rosin additive. The sample without additives exhibited the highest initial WCA, likely due to the absence of higher surface energy additives. However, its WCA rapidly declined upon abrasion, and after five cycles, minimal coating remained. In contrast, samples with increasing additive content demonstrated lower initial WCA values (while still retaining superhydrophobicity), but exhibited enhanced abrasion resistance, with a reduced rate of material loss and a more gradual decline in hydrophobicity. It was also observed that, in addition to protecting the coating, the additive provided a protective effect on the paper substrate, reducing abrasion and minimising damage to the paper. Figure 15 shows the appearance of the three coatings before (left) and after (right) 5 abrasion cycles. Figure 16 shows microscope images of the three coatings ((A) 40% gum rosin, (B) 10% gum rosin, (C) 0% gum rosin) before (left) and after (right) the 5 cycles.

[0163] Numerous variations of the embodiments and examples described herein will be apparent to those skilled in the art.

Claims

Claims1. A superhydrophobic coating for a substrate, the coating comprising plant spore shells and a plant-based wax.

2. The superhydrophobic coating of claim 1, wherein the plant spore shells comprise lycopodium shells and / or pine pollen shells.

3. The superhydrophobic coating of claim 1 or claim 2, wherein the plant-based wax comprises carnauba wax, candelilla wax, soybean wax, beeswax, ricebran wax, paraffin wax, or any combination thereof.

4. The superhydrophobic coating of any preceding claim, wherein the plant-based wax comprises carnauba wax.

5. The superhydrophobic coating of any preceding claim, wherein the plant spore shells comprise lycopodium shells, and a ratio of total mass of lycopodium shells to combined total mass of lycopodium shells and plant-based wax in the coating is from 40% to 80%, preferably from 50% to 70%, preferably from 55% to 65%, preferably about 60%.

6. The superhydrophobic coating of any of claims 1-4, wherein the plant spore shells comprise pine pollen shells, and a ratio of total mass of pine pollen shells to combined total mass of pine pollen shells and plant-based wax in the coating is from 50% to 80%, preferably from 60% to 80%, preferably from 70% to 80%.

7. The superhydrophobic coating of any preceding claim, wherein the plant spore shells comprise no more than 10% by mass genetic or cytoplasmic material, preferably no more than 5% by mass, preferably no more than 2% by mass, preferably no more than 1% by mass, preferably no more than 0.1% by mass.

8. The superhydrophobic coating of any preceding claim, further comprising a plantbased additive.

9. The superhydrophobic coating of claim 8, wherein the plant-based additive is a binder, preferably wherein the plant-based additive is gum rosin.

10. The superhydrophobic coating of claim 8 or 9, wherein the superhydrophobic coating comprises the plant-based additive at a mass ratio of about 40% additive to about 60% combined wax and plant spore shells.

11. A multi-layered superhydrophobic coating for a substrate, the multi-layered superhydrophobic coating comprising: a first coating layer comprising the superhydrophobic coating of any preceding claim; and at least one further coating layer comprising a plant-based wax and optionally comprising plant spore shells.

12. The multi-layered superhydrophobic coating of claim 11, wherein the plant-based wax of the further coating layer is of a different composition to the plant-based wax of the first coating layer.

13. The superhydrophobic coating of any of claims 1-10 or the multi-layered superhydrophobic coating of claim 11 or 12, deposited on a paper or glass substrate.

14. Use of plant spore shells in the formation of a superhydrophobic coating.

15. The use of claim 14, wherein the superhydrophobic coating is applied to packaging.

16. The use of claim 15, wherein the packaging is food packaging.

17. A method of forming a superhydrophobic coating on a substrate, the method comprising: dispersing or dissolving a plant-based wax in a solvent and dispersing plant spore shells in the solvent, thereby forming a coating dispersion;depositing the coating dispersion onto a surface of a substrate, thereby coating at least a portion of the surface with the coating dispersion; and drying the coated substrate.

18. The method of claim 17, wherein the plant spore shells comprise lycopodium shells and / or pine pollen shells.

19. The method of claim 17 or claim 18, wherein the plant-based wax is dispersed or dissolved in the solvent by mixing, preferably mechanical mixing.

20. The method of any of claims 17 to 19, wherein the plant spore shells are dispersed in the solvent by mixing, preferably mechanical mixing.

21. The method of any of claims 17 to 20, wherein the coating dispersion is formed at a temperature above 50°C, preferably above 60°C, preferably above 70°C, preferably above 80°C, preferably above 85°C.

22. The method of any of claims 17-20, wherein the coating dispersion is formed at a temperature above a melting temperature of the plant-based wax.

23. The method of claim 21 or claim 22, comprising cooling the coating dispersion to a temperature below 30°C prior to deposition on the surface of the substrate.

24. The method of any of claims 17-20, wherein the coating dispersion is formed at a temperature from 10°C to 30°C, preferably at a temperature of about 20°C.

25. The method of any of claims 17-24, wherein the depositing of the coating dispersion onto the surface of the substrate comprises spray-coating the substrate with the coating dispersion.

26. The method of any of claims 17-25, wherein the depositing of the coating dispersion onto the surface of the substrate comprises roll-to-roll processing.

27. The method of any of claims 17-26, further comprising annealing the dried, coated substrate.

28. The method of any of claims 17-27, wherein the plant-based wax is soluble or dispersible in the solvent.

29. The method of any of claims 17-28, wherein the solvent comprises ethanol, water, acetone, toluene, n-hexane, chloroform, petroleum ether, or any combination thereof.

30. The method of any of claims 17-29, further comprising, after the drying of the coated substrate: depositing a further amount of the coating dispersion onto the dried coated substrate, thereby further coating at least a portion of the surface with the coating dispersion; and drying the further coated substrate.

31. A method of forming a multi-layered superhydrophobic coating on a substrate, the method comprising: forming a superhydrophobic coating on a substrate according to the method of any of claims 17 to 30, wherein the formed superhydrophobic coating constitutes a first coating layer; forming at least one further coating layer on the substrate, the further coating layer comprising a plant-based wax and optionally plant spore shells.

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