Methods for preparing structural color films and structural color pigments

Abstract

The present invention relates to a method for producing structural color films, structural color particles, and structural color interference pigments that contain cellulose nanocrystals, such as neutralized cellulose nanocrystals. The films and particles can be used in various applications as interference pigments or colored particles, such as brighteners. The method includes depositing a nanocrystal suspension containing cellulose nanocrystals on a substrate, spreading the nanocrystal suspension over the substrate using a spreader, aging the nanocrystal suspension to partially or completely restore the cholesteric structure lost during deposition and spreading, drying the deposited nanocrystal suspension to allow the nanocrystals to self-assemble and form a structural color film, and annealing the structural color film to increase the water resistance of the film. The structural color film contains nanocrystals organized in a chiral nematic structure to provide structural color.

Description

[Technical field]

[0001] Related Applications This case claims priority to and the benefit of GB 2112102.5, filed on August 24, 2021 (2021.08.24), the contents of which are incorporated herein by reference in their entirety.

[0002] The present invention provides a method for preparing colored films from colloidal nanoparticles, such as cellulose nanocrystals. In particular, the present invention provides an industrially scalable method for producing such films. The films can be used to form interference pigments or colored particles, such as particles that can be used as brighteners. [Background technology]

[0003] Cellulose nanocrystals are rod-shaped colloidal particles that can be extracted from a variety of natural sources, e.g., cotton, wood, or wood pulp, and above a certain concentration, which can be reached, for example, when the solvent evaporates, they form stable aqueous suspensions that can exhibit cholesteric (also called chiral nematic) liquid crystalline behavior.

[0004] Evaporation of a cellulose nanocrystal suspension on a flat substrate results in the formation of a solid film with periodic chiral structure that can reflect visible light. For example, Revol et al. (J. Pulp Paper Science, 1998, 24, 146) described the preparation of a reflective film prepared from a suspension of cellulose nanocrystals deposited on a Teflon surface. The film was shown to have structural colors in the infrared (IR), ultraviolet (UV) and visible ranges.

[0005] Park et al. (Chem. Phys. Chem., 2014, 15, 1477) study the formation of films prepared from suspensions of cellulose nanocrystals deposited on large (25 mm wide) glass slides. The authors also investigate the effect of shear flow on the formation of helical structures in the film, studying film formation during drying of the suspension under orbital shaking. Here, it is said that the shear flow in the construction ensures uniformly aligned vertical cholesteric structures confined to the center of the sample. However, the shear flow does not prevent variations in the pitch of the helices.

[0006] Due to the strong need for more sustainable approaches to generate functional materials, structural color materials obtained from the self-assembly of plant-derived cellulose nanocrystals (CNCs) have attracted great interest in the scientific community and beyond. As mentioned above, colloidal cellulose nanocrystals, in suspension, can self-assemble upon evaporation of the solvent into periodic chiral nematic structures that can reflect vivid and durable photonic colors. If the characteristic size of the periodic array of cellulose nanocrystals, known as the pitch, is in the same size range as the wavelength of the interference light, the interference color can be, for example, visible, near-IR, or near-UV light according to Bragg's law (see Parker et al., Adv. Mater., 2018, 30, 1704477).

[0007] Methods for controlling the self-assembly of cellulose nanocrystals are now well understood and have a wide range of potential applications, from sensors to anticounterfeiting and decorative purposes. However, a scalable methodology for producing large-scale colored cellulose nanocrystal films remains lacking.

[0008] Roll-to-roll (R2R) printing or coating approaches have been reported to continuously deposit cellulose nanocrystal suspensions to produce transparent films. For these transparent films, the cellulose nanocrystals are typically aligned parallel to the substrate along the coating direction as a result of the high shear applied during the R2R deposition process, and this anisotropy along the coating direction remains in the final cellulose film, resulting in color defects. Structural color cellulose nanocrystal films with chiral nematic ordering have only been demonstrated over relatively small areas. Most existing methods for producing such structural color films use drop casting in shallow Petri dishes. 10~12 Drop casting is a batch process and therefore cannot be produced continuously and is not industrially scalable.

[0009] Continuous manufacturing methods, such as roll-to-roll or other similar printing or coating techniques, impose limitations on the rheological properties of the deposited material: not only does the deposited material need to be sufficiently viscous and cohesive to coat well and avoid cracking and shrinkage upon drying, but it also needs to satisfy the limited time window available for solvent evaporation during the continuous process.

[0010] It is known that to provide cellulose nanocrystal photonic materials of good optical quality (e.g., brightly colored films), long evaporation times and an initially low cellulose nanocrystal content (giving low viscosity) are required for optimal self-assembly to occur.

[0011] However, generating structural color particles of good optical quality from the self-assembly of cellulose nanocrystals remains challenging, and the use of scalable methods has not been demonstrated.

[0012] WO2018 / 033584 describes a method to generate particles with self-assembled cellulose nanocrystals through microfluidics. The radial chiral nematic ordering within the spherical particles makes the reflection less angle-dependent compared to previously reported flat-plate cast films, and the structure of the particles results in weak reflection of incident light as a result of refraction at the spherical interface of the particles upon solvent drying, preventing their use as pigments. The microfluidic process described in WO2018 / 033584 forms individual particles, and no method is described for preparing films.

[0013] WO2014 / 118466 describes a batch process for producing particles from pearlescent cellulose nanocrystal films for marking tobacco paper. However, the particles in this document are not visible to the naked eye, and the pearlescent and optical response of the particles and films are not disclosed. In addition, the particles are said to disintegrate within a few hours after immersion in water, resulting in a significant particle mass loss, preventing their use as pigments. The use of two post-treatments, one aimed at removing sulfur groups and one aimed at shielding the charge by salt, did not result in any significant improvement in the retention of the particle structure. The films produced by this method are said to have a thickness of at least 20 μm.

[0014] Zhao et al. (Adv. Funct. Mater., 2019, 29, 1804531) prepared structural color microfilms containing cellulose particles with diameters larger than 600 μm using an offset printing technique that produces sub-millimeter-scale microfilms on glass and other silanizable substrates, such as PDMS. Highly regular chiral nematic structures are obtained by keeping the printed film under oil and drying for up to two days to limit the "coffee stain" effect that occurs at the boundaries of the microdroplets. However, this drying method severely limits the scalability of the process. In addition, the microdroplets of cellulose ink were deposited on a sacrificial layer, and the microdroplets were tightly bound to the substrate. As a result, the microfilms were not recovered as individual particles, and their use as pigments has not been demonstrated. Importantly, the films are highly sensitive to ambient humidity, suggesting that the structure may collapse when immersed in water.

[0015] WO2017 / 091893 describes a method for preparing cellulosic pigments that relies on a core of cellulose aggregates that are dyed with chemicals that produce color through a light absorption mechanism.

[0016] Nan et al. (ACS Sustain. Chem. Eng., 2017, 5, 8951-8958) showed that treating cellulose nanocrystal films with strong sodium hydroxide for 3 to up to 12 hours slightly increased the penetration of water into the films in alkaline water. However, even with the longest treatment times, the films could disintegrate and the constituent nanocrystals could redisperse when immersed in water, preventing any use in such liquids. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] WO2018 / 033584 [Patent Document 2] WO2014 / 118466 [Patent Document 3] WO2017 / 091893 [Patent Document 4] CN113061274 [Patent Document 5] US Patent Application Publication No. 2010 / 151159 [Patent Document 6] KR20210062268 [Patent Document 7] WO95 / 21901 [Non-patent literature]

[0018] [Non-Patent Document 1] Revol et al. (J. Pulp Paper Science, 1998, 24, 146) [Non-Patent Document 2] Park et al. (Chem. Phys. Chem., 2014, 15, 1477) [Non-Patent Document 3] Parker et al., Adv. Mater., 2018, 30, 1704477 [Non-Patent Document 4] Zhao et al. (Adv. Funct. Mater., 2019, 29, 1804531) [Non-Patent Document 5] Nan et al. (ACS Sustain. Chem. Eng., 2017, 5, 8951~8958) [Non-Patent Document 6] Lagerwall et al. (NPG Asia Materials, 2014, 6, e80) [Non-Patent Document 7] Parker, RM et al., Adv. Mater., 30, 1704477 (2018) Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention provides an alternative to known methods for producing cellulose nanocrystal-based films and particles with structural color. Preferably, the present invention solves one or more of the problems associated with the prior art methods.

[0020] The present invention relates to methods for producing structural color films, structural color particles, and structural color interference pigments comprising cellulose nanocrystals, such as neutralized cellulose nanocrystals.

[0021] The films have structural color, which can be visible, infrared or ultraviolet. Structural color comes from the presence of chiral nematic structures, which give rise to strong reflections at specific wavelengths. [Means for solving the problem]

[0022] In general, the present invention comprises: a) depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate using a coating applicator; b) spreading the nanocrystal suspension over the substrate using a spreader; and c) drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form the structural color film. The present invention provides a method for producing a structural color film, comprising:

[0023] Preferably, the method includes an aging step between the spreading and drying steps, which allows the nanocrystal suspension to partially or completely regain any cholesteric structure lost during the deposition and spreading steps.

[0024] In this manner, structural color films with high reflectivity and chroma can be reliably and reproducibly prepared on a large scale using industrially viable methods.

[0025] In a first aspect of the present invention, a) depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate; b) spreading the nanocrystal suspension over the entire substrate using a spreader; c) aging the nanocrystal suspension so as to partially or completely restore the cholesteric structure lost during the deposition and spreading steps; d) drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form a structural color film; e) Annealing the structural color film to increase the water resistance of the film. The present invention provides a method for producing a structural color film, comprising:

[0026] The method provides structural color films with high reflectivity and color saturation, and increased solvent resistance, especially increased water resistance.

[0027] In a second aspect of the present invention, there is provided a structural color film obtainable by the method of the first aspect.

[0028] In a third aspect of the present invention, there is provided a structural color film comprising cellulose nanocrystals, preferably neutralized cellulose nanocrystals, wherein the nanocrystals are organized in a chiral nematic structure, preferably the film has a thickness such that the director of the chiral nematic structure rotates at least once within the film, and the film has a thickness of 20 μm or less.

[0029] The highly ordered chiral nematic structure in the structural color films of the second and third embodiments is highly reflective, reflecting very vivid colors with high intensity.

[0030] In a fourth aspect of the present invention, there is provided structural color particles obtained or obtainable by dividing a structural color film obtained or obtainable by the method of the first aspect.

[0031] In a fifth aspect of the present invention, there is provided a structural color particle comprising cellulose nanocrystals, preferably neutralized cellulose nanocrystals, wherein the nanocrystals are organized in a chiral nematic structure, preferably the particle having a faceted geometry corresponding to at least one chiral nematic domain.

[0032] The highly regular chiral nematic structure in the fourth and fifth structural color particles provides high reflectivity, and the structural color particles maintain their coloration for long periods of time when suspended in a solution such as water.

[0033] In a sixth aspect of the present invention there is provided the use of structural colour particles of the fourth or fifth aspect in cosmetics, in packaging or in paints.

[0034] These and other aspects and embodiments of the invention are described in further detail below.

[0035] The invention will now be described with reference to the figures listed below. [Brief description of the drawings]

[0036] [Figure 1]Schematic of R2R processing of CNC coatings into photonic microparticles. A) Schematic of the method of the invention using roll-to-roll coating, showing potential web reuse processes and pigment generation. B) Red, green, and blue R2R coated CNC films deposited on a black PET web (tip sonicated at 6.44 s / mL (21.5 kJ / g), 4.36 s / mL (14.5 kJ / g), and 2.24 s / mL (7.5 kJ / g), respectively). C) Roll-to-roll coating of 1 meter length of unwound cellulose nanocrystals produced by the method of the invention (tip sonicated at 4.36 s / mL (14.5 kJ / g)). D) Pristine (left) and heat-treated (right) photonic CNC particles (before size sorting) produced by the method of the invention including a fracture step, embedded in clear varnish. E) Heat-treated photonic CNC particles after size selection and immersed in (left to right) ethanol, 50% aqueous ethanol, and water. [Diagram 2] Coating thickness and optical properties versus coating parameters. A) Macroscopic photographs (top) and left circularly polarized optical microscope images (middle) of CNC coatings prepared at different coating gaps and rates with a constant shear rate of 2.2 s-1 (concentration equal to 6 wt.% and sonication for 56 s (2.24 s / mL or 7.5 kJ / g)) and scanning electron microscope (SEM) cross-sections (bottom) of free-standing CNC films. B) Left circularly polarized (LCP) reflectance spectra of the corresponding CNC coatings averaged over 15 locations. C) Relationship between wet coating gap and dry film thickness of the samples in A. The pink dotted line highlights the theoretical minimum thickness required for the CNC film to reach maximum reflectance, calculated using the Berreman 4 × 4 matrix method with refractive indices n0 and ne of 1.514 and 1.590, as in previous studies (see, e.g., Zhao et al., Adv. Funct. Mater., 2019, 29, 1804531). [Diagram 3]Figure 14: Effect of sonication and drying conditions on the visual appearance of CNC coatings. A) Macroscopic photographs (first and third rows) and left circularly polarized (LCP) optical microscope images (second and fourth rows) of CNC coatings prepared using the method of the present invention from a 6 wt. % CNC suspension sonicated for 56 s (2.24 s / mL or 7.5 kJ / g) on ​​the left, 109 s (4.36 s / mL or 14.5 kJ / g) in the middle, and 161 s (6.44 s / mL or 21.5 kJ / g) on ​​the right, respectively, dried at either 18 °C or 60 °C, at a speed of 1.5 mm.s-1, and using a coating gap equal to 700 μm. B) LCP reflectance spectra of the corresponding sample casts at room temperature (top) and on a hot plate (bottom), averaged over 15 positions. [Figure 4] 1 shows CNC photonic microparticles. A) Micrographs of size-selected and heat-treated CNC photonic microparticles prepared from 6 wt% R2R coating. CNC suspensions sonicated for 196 s (4.36 s / mL or 14.5 kJ / g) and coated by the method of the present invention observed in air, ethanol, a 1:1 (wt) mixture of ethanol and water, and water. The microscopy image for the largest size shows the same particles. B) Total reflectance spectra of the corresponding size-selected particles in the same four media, averaged over 10 positions. C) SEM top views illustrating the CNC photonic particle morphology after the breaking step for each size group. D) SEM cross-sections of CNC particles showing typical Bouligand arches, typically obtained after breaking of roll-to-roll printed films in (A), characteristic of CNC chiral nematic ordering. [Diagram 5]Figure 1 shows the effect of tip sonication on the visual appearance of CNC films prepared from a 6 wt% biphasic suspension. A) Macroscopic photograph. B) Corresponding micrograph through a left circular polarizing (LCP) filter. C) Corresponding reflectance spectra through LCP (left) and RCP (right) filters averaged over 10 positions. D) Corresponding CIE 1931 color space chromaticity diagram calculated from the Colour package in Python. The CIE coordinates are [0.20 ; 0.13], [0.32 ; 0.30], [0.40 ; 0.31], [0.50 ; 0.34], [0.42 ; 0.47], and [0.23 ; 0.44] for blue versus red. [Figure 6] Phase diagram of the CNC suspensions used: A) Photograph of a glass capillary filled with the starting CNC suspensions of increasing concentration and viewed between cross polarizers. B) Volume fraction of the anisotropic phase vs. CNC concentration extracted from A). [Figure 7] Figure 1 shows the effect of the annealing process. A) Effect of the temperature of the heat treatment (applied for 30 min) on the macroscopic and microscopic visual appearance of free-standing CNC samples cut from a CNC film produced by the method of the present invention using roll-to-roll coating. B) Corresponding spectra in reflection and transmission (smoothed and averaged over up to three positions). Measurements were taken using an optical fiber with a core diameter of 200 μm (Thorlabs FC-UV200-2-SR) with a 20x objective (Nikon T plan SLWD, NA = 0.3). [Figure 8] Figure 1 shows the effect of flow occurrence during drying of a film on the uniformity of a film produced using the method of the present invention. A-C) Three different sections of the same R2R film produced on a misaligned web showing the buildup of material on one side of the web (C, leading) and the effect of thickness variation on film uniformity. [Figure 9]CNC film dried on a perforated hotplate. A-E) Photographs of wet CNC film taken before and after complete drying on a perforated aluminum board placed on a hotplate set at 60° C. Films were prepared using the method of the present invention at a coating speed of 1.5 mm / s through a coating gap of 700 μm. A temperature gradient between the aluminum board and the circular holes created a spatial drying rate difference between the suspensions. B) Close-up of A), where nacreous self-assembled domains are clearly visible in the still wet CNC film. F) The resulting dried and patterned CNC coating. G) LCP micrographs of two regions dried at different speeds. H) LCP reflectance spectra corresponding to the two regions, each spectrum averaged over three locations. [Figure 10] 1 is a flow chart illustrating the method disclosed in the present invention. [Figure 11] Reversible color changes of cellulose nanocrystal photonic particles. Microscope snapshots (left to right) of a single cellulose nanocrystal photonic particle placed in a mixture of water and ethanol: particle in air, particle in a mixture of ethanol and water during evaporation, particle in water (ethanol evaporates, resulting in water condensation), and particle placed back in air (water has evaporated). [Figure 12]Figure 1. Pearlescence of dispersed photonic CNC microparticles. A) Snapshots of green photonic CNC particles (size: 150 > x > 75 μm) dispersed in glycerol taken at increasing illumination angles (particles were prepared from a CNC suspension tip-sonicated for 196 s (4.36 s / mL or 14.5 kJ / g), particle loading in the vial is approximately 5.5 mg / mL). Increasing illumination angle from left to right results in a strong apparent blue-shift, from green to blue, eventually beyond the visible spectrum. B) Schematic of the reflection obtained from the sample displayed in (a). Contributing particle orientations are illustrated in black. C) Photonic CNC particles (size: 75 > x > 25 μm) embedded in a clear epoxy resin coated glass slide and under direct illumination. Increasing illumination angle from left to right results in a reduced apparent blue-shift, from green to dark blue. D) Schematic of the reflection obtained from the sample shown in (c), highlighting the key role of the air / coating interface in reducing the blue shift according to Snell's law. E) The same sample as in (c), but under diffuse illumination. F) Schematic of the reflection obtained from the sample shown in (e). The combination of diffuse illumination and the refractive index contrast at the air / coating interface suppresses most of the blue shift, resulting in a coating with little iridescence. [Figure 13] Macroscopic visual appearance of CNC photonic particles embedded in transparent epoxy resin. Photographs under diffuse illumination (top row) show that the larger particles give the coating a mosaic-like appearance, while the smaller particles appear much more uniform. When viewed under a microscope equipped with a 5x objective (Zeiss, EC Epiplan Neofluar, NA = 0.13) in bright and dark field (BF and DF, rows 2 and 3, respectively), the larger particles tend to remain aligned on the glass substrate, while the smaller particles appear to be more randomly oriented. The loading of CNC photonic particles was approximately 15 mg per 150 mg of resin (Norland Optical Adhesive 81), approximately 3.1 mg / cm2. The mixture was spread on a glass slide and covered with a cover slip. [Figure 14] Scattering measurements of photonic CNC microparticles embedded in transparent epoxy resin (same samples as in Fig. 13). Samples were illuminated at θin = 0° (normal incidence, top row) or θin = 30° (bottom row). As the size of the particles decreased, the scattering signal became smoother and more continuous, up to the point where it could be fitted to a signal from randomly distributed diffraction domains of identical pitch (white curve). This scattering signal is less iridescent than the equivalent scattering signal from a typical CNC film. [Figure 15A] Graph showing the calculated reflectance versus film thickness assuming refractive indices n0 = 1.514 and ne = 1.590. Calculated spectra of cholesteric structures with increasing number of pitches (pitch p = 318 nm) using a numerical model (Berreman 4x4 matrix method). The plot on the right is a zoom-in of the reflectance peak, showing saturation after the number of helices N = 24, which corresponds to a film thickness of 7.6 μm. [Figure 15B] 1 shows a graph of calculated reflectance versus film thickness assuming refractive indices n0 = 1.514 and ne = 1.590. Calculated maximum reflectance peaks versus film thickness for three cholesteric structures with pitches p = 318, 370 and 429 nm (corresponding to the peak wavelengths of the blue, green and red films shown in FIG. 3) using an analytical model (De Vries equation). Reflectances > 99.5% are reached above film thicknesses of 6.9 μm, 8.1 μm and 9.3 μm for p = 318, 370 and 429 nm, respectively. [Figure 16]FIG. 1 shows in more detail the processing of the R2R cast CNC film into CNC photonic microparticles (steps performed after drying in the R2R machine, shown at the top of the figure). A) Removal of the dried CNC film lead from the PET web using a plastic blade (125 μm thick). The CNC film was carried by the moving web (still attached on one side), removed without destruction and slid onto another immobile web for collection. B) CNC photonic microparticles obtained after steps of drying, heat treatment and destruction of the R2R cast film. C) CNC particles immersed in water:ethanol mixtures with different mass ratios (from left to right: 1:0, 95:5, 85:15, 70:30, 1:1, 0:1). D) The same sample as in (C), photographed after 10 months. E) Example of one possible application of CNC photonic microparticles in a fizzy drink. F) CNC photonic microparticles in a range of solvents of varying polarity and ionic strength (from left to right: acetone, ethyl acetate, hexane, water with one drop of industrial surfactant (Premiere Products, Savona D2), water containing 1 mM NaCl). [Figure 17]Figure 1 shows benchmark testing of photonic CNC microparticles (ii, iv and vi) against commercial effect pigments (i), glitters (iii, v and vii) and photonic CNC refractive microspheres (viii). A) Photograph taken at approximately 45° from the light source under directed illumination. B) Photograph taken at approximately 110° from the light source under directed illumination highlighting the unique angle-dependent visual response of photonic CNC microparticles. Sample list: i. Merck Xirona® (Moonlight Sparks, size: 20-150 μm). ii. Photonic CNC microparticles (size: 150 > X > 75 μm). iii. Bio-glitter® Pure (Sea green, size: 40 mil ≈ 1 mm and Light gold, size: 15 mil ≈ 380 μm). iv. Photonic CNC microparticles (size: X < 25 μm). v. Bio-glitter® Sparkle (Spring green, size: 8 mils ≈ 200 μm). vi. Photonic CNC microparticles (size: X > 150 μm). vii. Bio-glitter® Pure (Sea green, size: 94 mils ≈ 2.4 mm). viii. Photonic CNC microspheres. The loading of the effect particles was approximately 10% by weight, except for sample (i), where the loading was reduced to 1% by weight. [Figure 18] Figure 1 shows the effect of temperature and duration of heat treatment on the hydrophilicity of CNC films. A) Photographs of R2R cast CNC film pieces heat treated at 150°C for increasing durations and placed in water (top row). Photographs of R2R cast CNC film pieces recovered after immersion in water and completely dried (second row). B) Photographs of R2R cast CNC film pieces heat treated at 190°C for increasing durations and then placed in water (top row). All films show structural color. Photographs of R2R cast CNC film pieces during immersion in water (second row). Scale bar: diameter of dish equals 3.5 cm. [Figure 19]Optical properties of CNC films cast by R2R and dried either statically or with continuous stepwise translation through an in-line hot air dryer. A) Image of a 4.2 m long blue R2R cast film (dried by coarse stepwise method, T = 60 °C) with insets showing the optical appearance of different points along the length (Pc indicates point from coating start), where points are defined relative to the start of deposition. B) Photographs (1st, 2nd and 4th rows) and optical micrographs (3rd and 5th rows) of an R2R cast CNC film sonicated at 2.24 s / mL (7.5 kJ / g) recorded through LCP and RCP filters. C) LCP and RCP reflectance spectra corresponding to the free-standing film reported in (b). LCP reflectance spectra are averaged over 80 points along the film. D) Drying time and thickness of films for different deposition methods and drying conditions. E) LCP optical micrographs of films prepared from CNC suspensions sonicated for 290 s (6.44 s / mL or 21.5 kJ / g) and dried at either 20 °C (left) or 60 °C (right). The top row shows a blade cast film (gc = 700 μm, vc = 1.5 mm.s-1) and the bottom row shows a R2R cast film (static T = 20 °C, rough step T = 60 °C). F) Corresponding LCP reflectance spectra averaged over 15 positions. G) Thickness measurements of the different films presented in (a). [Figure 20] Illustrative applications of the structural color films and structural color particles of the present invention: A) Particles divided into square shapes and presented on paper; B) Particles pasted onto a piece of fabric; C) Particles embedded in an edible host matrix and applied onto a piece of chocolate; D) Particles used as a coating to paint fingernails; E) Particle-containing host polymer matrix to paint a piece of wood. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The present invention relates to pigmented films prepared from colloidal nanoparticles, such as cellulose nanocrystals.

[0038] Colloidal cellulose nanoparticles self-assemble to form a chiral nematic (cholesteric) phase that contains helical structures (helicoids). Each helicoid can have a random spatial orientation defined along the long axis of the helix. A chiral nematic helicoid can be described as a pseudostack, which can be understood by virtually cutting the helix along its long axis into an infinite number of separate planes, thinly stacked parallel to each other. Each pseudolayer contains nanocrystals that, on average, point in the same direction. Defined outward along this direction, the director is a vector that rotates continuously from one pseudolayer to the other around the long axis of the helix. A full rotation of the director is completed in one helical pitch, which describes the periodicity of the helical stack. The mesogens can rotate around the helix to recover their properties after each half rotation of the helix. When the mesogens are birefringent molecular components, the chiral nematic phase exhibits unique optical properties, as the refractive index is periodically modulated to produce light reflection.

[0039] CN113061274 relates to a polymorphous multi-structural color film and a method for preparing the same, which includes drop casting a cellulose nanocrystal suspension onto a glass substrate, drying to obtain a self-assembled film, coating with an organic amorphous polymer, and then curing. However, CN113061274 does not describe spreading the nanocrystal suspension over the substrate using a spreader, or aging the nanocrystal suspension to partially or completely recover the cholesteric structure lost during deposition and spreading. CN113061274 does not describe a film having a thickness of 20 μm or less, and does not describe structural color particles.

[0040] US 2010 / 151159 relates to cellulose nanocrystal films with a non-uniform structure. The films are formed by evaporating water from a cellulose nanocrystal suspension over a period of 4-6 hours using temperature variation. US 2010 / 151159 does not describe spreading or aging the film during its preparation. US 2010 / 151159 also does not describe particles, and does not mention splitting the film to form particles.

[0041] KR20210062268 describes a colored cellulose nanocrystal film. The method of preparing the film does not include a step of spreading or aging the suspension, or a step of annealing the film. KR20210062268 also does not describe particles, and does not describe a step of dividing the film to form particles.

[0042] WO95 / 21901 describes a solid liquid crystal cellulose film prepared from a cellulose nanocrystal suspension. This document does not describe a method for preparing the film, including spreading or aging the suspension, or annealing the film. WO95 / 21901 also only refers to films or individual cellulose nanoparticles. This document does not describe particles with a chiral nematic phase, or particles formed from splitting the film.

[0043] method The present invention provides a high throughput method for producing structurally colored films of cellulose nanocrystals with high reflectance. In general, the present invention comprises: a) depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate using a coating applicator; b) spreading the nanocrystal suspension over the substrate using a spreader; and c) drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form the structural color film. The present invention provides a method for producing a structural color film, comprising:

[0044] In a first aspect of the present invention, a) depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate; b) spreading the nanocrystal suspension over the entire substrate using a spreader; c) aging the nanocrystal suspension so as to partially or completely restore the cholesteric structure lost during the deposition and spreading steps; d) drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form a structural color film; e) Annealing the structural color film to increase the water resistance of the film. The present invention provides a method for producing a structural color film, comprising:

[0045] The final color of the film can be tuned, for example, by adjusting the properties of the nanocrystal suspension, adjusting the deposition rate, adjusting the spreading (coating) conditions, and adjusting the drying conditions.

[0046] In the coating process, the nanocrystal suspension can be spread across the substrate using a coating applicator or spreader. The gap between the coating applicator / spreader and the substrate is fixed and is between 5 μm and 5 mm, preferably between 300 and 1500 μm, more preferably between 300 and 1100 μm. The term "fixed" is used here to mean that the gap does not change during coating except by user intervention. That is, the gap does not vary independently and uncontrolled.

[0047] The gap between the coating applicator / spreader and the substrate is sometimes referred to in the art as the coating gap. A larger gap results in a thicker layer of nanocrystal suspension being deposited and spread onto the substrate. The thickness of the nanocrystal suspension layer affects the drying time, and therefore the self-assembly and final color development of the dried film. Thinner nanocrystal suspension layers dry more quickly than thicker nanocrystal suspension layers. Nanocrystals in thinner layers tend to have shorter self-assembly times and less vibrant colors.

[0048] In the coating process, the coating speed is at least 0.6 mm / s. Coating speed refers to the speed at which the substrate moves relative to the spreader, at least during the spreading step.

[0049] In the coating process, the substrate is kept level during the depositing, spreading and drying steps. The term kept level is used herein to mean that substantially the entire surface of the substrate maintains a substantially horizontal plane during the depositing, spreading and drying steps.

[0050] In this way, undesired flow of the nanocrystal suspension during deposition, spreading and drying steps is prevented, which can lead to perturbations in the self-assembly process and therefore reduce the ability of the nanocrystals to form the chiral nematic order required for optimal structural color development. Figure 8 shows an example of the macroscopic appearance of a coating film when undesired flow occurs.

[0051] The substrate can be kept level by using a rigid substrate, by tensioning the substrate, by supporting the substrate, or any combination of these.

[0052] For example, the rigid substrate can be thick enough not to warp during the deposition and spreading process. Preferably, the rigid substrate is also flexible enough to be carried on the drum of a roll-to-roll printing machine.

[0053] For example, the substrate may have a tension of 10-250 N, more preferably 25-125 N, even more preferably 40-90 N. The tension difference between both ends of the web was such that it was greater than 20 N, more preferably 40 N, even more preferably 60 N. The film tension can be determined by a load cell sensor.

[0054] In this manner, structural color films can be reliably and reproducibly prepared on a large scale using industrially viable methods.

[0055] Coating Process The method of the present invention includes depositing and spreading a nanocrystal suspension over a substrate. Preferably, the nanocrystal suspension comprises neutralized cellulose nanocrystals.

[0056] The nanocrystal suspension may be deposited onto the substrate using a coating applicator. Any suitable coating applicator may be used. Suitable deposition methods include using a printing nozzle, spray head, or slot die through which the cellulosic suspension can be flowed in a controlled manner.

[0057] The nanocrystal suspension may be spread onto the substrate using a spreader. Any suitable spreader may be used. Suitable spreaders include a knife, a doctor blade, or a slot die.

[0058] Coating applicator and spreader may refer to the same machine. For example, if the coating applicator is a slot die used with a moving belt, both depositing and spreading are accomplished by the slot die. Slot die coating is well known, especially in roll-to-roll printing processes. Preferably, a slot die is used as the coating applicator and spreader.

[0059] During the spreading process, the nanocrystal suspension is subjected to shear.

[0060] The shear rate is calculated by the following equation: Shear rate = coating speed / coating gap

[0061] In some cases, the shear rate during the spreading process is 30.0 s -1 Below, 20.0s -1 Below, 10.0s -1 Below, 8.0s -1 Below, 4.0s -1 Below, 3.0s -1 Less than or equal to 2.8s, preferably -1 Less than or equal to 2.5 seconds is preferable. -1 The following is the result.

[0062] In some cases, the shear rate during the spreading step is 0.5 s -1 More than 1.0s, preferably 1.0s -1 More than 2.0s, preferably 2.0s -1 That's all.

[0063] The shear rate during the spreading step can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the shear rate during the spreading step can be 2.0 s -1 ~20.0s -1 , preferably 2.0s -1 ~2.5s -1 , e.g. 2.2s -1 It may be before or after.

[0064] The nanocrystal suspension may be deposited in separate batches, in which case each separate batch is spread across the substrate during the spreading step.

[0065] Alternatively, the nanocrystal suspension may be continuously deposited, in which case the nanocrystal suspension is continuously spread across the substrate.

[0066] The amount of nanocrystal suspension deposited per unit area of ​​the substrate (area application amount) was 100 μL / cm 2 Below, 90μL / cm 2 Less than or equal to 80 μL / cm 2 Less than or equal to 60 μL / cm 2 It could be the following:

[0067] The amount of applied surface area is 10 μL / cm 2 More than 20μL / cm 2 More than 30 μL / cm 2 More preferably, 40 μL / cm 2 It could be more than that.

[0068] The area coverage can be selected from a range with upper and lower limits selected from those given above. For example, the deposition can be 40 μL / cm 2 ~60μL / cm 2 , e.g. 50 μL / cm 2 It may be a ratio of before or after.

[0069] The amount of material deposited per unit time (deposition rate) can be 12,000 μL / min or less, 10,000 μL / min or less, preferably 8,000 μL / min or less, and more preferably 6,000 μL / min or less.

[0070] The deposition rate can be 800 μL / min or more, 1,200 μL / min or more, 1,600 μL / min or more, preferably 2,000 μL / min or more, and even more preferably 2,400 μL / min or more.

[0071] The deposition rate may be selected from a range with upper and lower limits selected from those given above. For example, deposition may be at a rate of 2,000 μL / min to 8,000 μL / min, such as around 6,000 μL / min.

[0072] Typically, the substrate is moved relative to the coating applicator and spreader, such that the nanocrystal suspension can be spread along the substrate by a combination of the movement of the spreader and the substrate.

[0073] Coating speed refers to the speed at which the substrate moves relative to the coating applicator or spreader, at least during the spreading step.

[0074] The coating speed may be at least 1.0 mm / s, at least 1.5 mm / s, at least 2.0 mm / s, at least 4.0 mm / s, preferably at least 1.0 mm / s, more preferably at least 1.5 mm / s.

[0075] The coating speed may be 60.0 mm / s or less, 30.0 mm / s or less, 15.0 mm / s or less, 3.0 mm / s or less, preferably 2.7 mm / s or less, and more preferably 2.4 mm / s or less.

[0076] The coating speed may be selected from a range whose upper and lower limits are selected from those given above. For example, the coating speed may be between 1.0 mm / s and 2.4 mm / s, such as around 1.5 mm / s.

[0077] Preferably, the coating process is a roll-to-roll printing process. Roll-to-roll printing is a well-known printing technique. Roll-to-roll printing involves the deposition of a substance from a stationary print head onto a moving substrate. Typically, the moving substrate is provided in the form of a roll, often referred to as a web. The term web refers to a flat, stretched (sometimes continuous) substrate that can be wound and rewound. During printing, the substrate or web is unwound from the roll, a substance is deposited on the unwound portion of the web, and the deposited substance is transported to the surface of the substrate or web for further processing, such as a drying step. The web or substrate may be rewound to form a second roll, either with or without the substance deposited on the surface of the web. Alternatively, the deposited substance may be removed, and the web or substrate is continuously reused in further processing steps (where the web or substrate is in the form of a closed loop).

[0078] In some cases, deposition may occur over one or more distinct areas of a substrate or web.

[0079] In this case, after deposition of the nanocrystal suspension onto the web or substrate, further processing steps include spreading, drying, and optionally removing (peeling) the structural color film from the surface of the web or substrate. Additional pre-treatment steps may also be performed on the web or substrate prior to deposition of the nanocrystal suspension. Additional pre-treatment and subsequent processing steps are discussed in more detail below.

[0080] Base material A substrate is any suitable surface upon which the nanocrystal suspension can be deposited, spread, and dried.

[0081] In some cases, the substrate has a thickness of 10,000 μm or less, 1,000 μm or less, preferably 800 μm or less, and even more preferably 500 μm or less.

[0082] In some cases, the substrate has a thickness of 50 μm or more, 100 μm or more, preferably 300 μm or more, and even more preferably 400 μm or more.

[0083] The thickness of the substrate can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the thickness of the substrate can be 300 to 500 μm, for example around 400 μm.

[0084] In this manner, the substrate is relatively rigid, which aids in keeping the substrate uniform and level during the deposition and drying steps.

[0085] The substrate may be or include any suitable material. Suitable substrate materials include polyvinyl alcohol, cellophane, polystyrene, acetal, ethylene-vinyl acetate, polyethylene, e.g., polyethylene terephthalate, polypropylene, fluoropolymers; polyimide, nylon, polyester, epoxy resin, acrylic resin, phenolic resin, polycarbonate, polyurethane, polyvinyl chloride, and polyester. Preferably, the web is polyethylene terephthalate (PET).

[0086] The substrate may comprise substantially any one suitable material.

[0087] Alternatively, the substrate may comprise two or more suitable materials, in which case the materials may be mixed (blended) together or combined to form separate domains of each material.

[0088] The substrate may be pre-structured, e.g., microstructured or nanostructured. In such cases, the substrate may be referred to as "patterned." This pre-existing patterning may affect the interaction of the deposited material with the substrate. For example, the substrate may exhibit regions where the deposited nanocrystal suspension may interact better with the substrate or have greater substrate wetting. Similarly, the patterning may result in regions where the deposited nanocrystal suspension may interact poorer with the substrate or have poorer substrate wetting, such that the suspension preferentially avoids such regions.

[0089] As a result of the patterning, the orientation of the cholesteric domains can change, for example the orientation can follow the topography of the substrate, resulting in reflection from the cholesteric domains over a wider range of angles, which can produce more complex visual effects than would be possible using a flat substrate without such pre-existing patterns.

[0090] After formation of the structural color nanocrystal film, the substrate may be separated from the film, a process sometimes referred to as a peeling process and discussed in more detail below.

[0091] In some cases, the substrate can be reused multiple times in a closed loop manner.

[0092] Additionally, in some cases the substrate is in the form of a movable belt, such as a metal belt, for example a heat resistant movable belt.

[0093] Nanocrystal Suspension The method of the invention involves depositing a nanocrystal suspension, for example a cellulose nanocrystal suspension, onto a substrate.

[0094] Cellulose nanocrystals (CNC) are well known in the art. Methods for preparing cellulose nanocrystals are also well known in the art. Many types of cellulose nanocrystals are known, including cellulose nanocrystals obtained from various biological sources and nanocrystals prepared from the same sources by various methods.

[0095] The cellulose nanocrystals used in this invention can be any suitable cellulose nanocrystals. The cellulose nanocrystals can be any cellulose nanocrystals that can self-assemble into a cholesteric structure.

[0096] Cellulose nanocrystals can be prepared from bacterial, vegetable and animal sources (e.g., chitin), including plant-based sources and biomass sources such as cotton and wood, and any components derived therefrom and subsequently processed, such as paper, filter paper, cotton linters and wood pulp.

[0097] The processing procedures performed on the source material to produce cellulose nanocrystals typically involve hydrolysis, separation and purification of the hydrolyzed compounds. Separation can be performed through centrifugation. Purification can be performed by dialysis and membrane ultrafiltration. A known method for producing cellulose nanocrystals is described by Lagerwall et al. (NPG Asia Materials, 2014, 6, e80), the contents of which are incorporated herein by reference.

[0098] Typically, the cellulose source is hydrolyzed in the preparation process, for example by sulfuric acid or hydrochloric acid, or other acid, or alkaline medium, or, for example, in the case of the preparation of TEMPO-oxidized cellulose nanocrystals, the cellulose source is oxidized. Preferably, the nanocrystal solution contains pH-neutralized cellulose nanocrystals. More preferably, the nanocrystal solution contains sodium form of cellulose nanocrystals.

[0099] It has been proposed that during hydrolysis, the cellulose chain backbone of cellulose nanocrystals is modified at the molecular level to provide colloidal stability to the nanocrystals. For example, sulfuric acid hydrolysis is believed to modify the cellulose chains with sulfate half ester groups. Another example of a change occurs during extraction with hydrogen peroxide, where the cellulose chains are believed to be modified with carboxyl groups to provide carboxylate cellulose nanocrystals. As a result of the cellulose chains being modified with charged groups, several counterions can be used to balance the charge. Most commonly, H+ (to provide the acidic form of cellulose nanocrystals) and Na + (Neutralize (e.g., sodium form) to give cellulose nanocrystals). Counterions (e.g., H + ) can be exchanged in the suspension, for example, by using concentrated NaOH or NaCl solutions, to give fully or partially neutralized cellulose nanocrystals. Na+ can be similarly exchanged in the suspension, for example, by using concentrated HCl or H2SO4.

[0100] Preferably, the nanocrystal suspension comprises pH neutralized cellulose nanocrystals, partially pH neutralized cellulose nanocrystals or acid form cellulose nanocrystals.More preferably, the nanocrystal suspension comprises sodium form cellulose nanocrystals.

[0101] In this case, preparing the cellulose nanocrystal suspension may include sonicating the cellulose nanocrystal suspension.

[0102] Cellulose nanocrystals are typically rod-shaped, and thus the crystals may extend in a length dimension that is much larger than their width dimension.

[0103] Cellulose nanocrystals for use in the present case may have a length of up to 200, up to 500, up to 1,000, or up to 1,500 nm.

[0104] Cellulose nanocrystals for use in the present case may have a length of at least 50, at least 70, or at least 100 nm.

[0105] Cellulose nanocrystals for use in the present case may have widths of up to 20, up to 30, up to 50 nm.

[0106] Cellulose nanocrystals for use in the present case may have a width of at least 1, at least 3, at least 5, or at least 10 nm.

[0107] The aspect ratio for the cellulose nanocrystals can be at least 5, 7, 10, 15, or 20.

[0108] The aspect ratio for the cellulose nanocrystals can be up to 40, 50, 100, 150 or 200.

[0109] The cellulose nanocrystals may also be provided, for example, as a suspension in a solvent, or as a powder, for example a spray-dried or freeze-dried powder, which is redispersed to provide a cellulose nanocrystal suspension for use in the present invention.

[0110] Any suitable solvent can be used, for example any solvent in which the cellulose nanocrystals can form a colloidally stable suspension, with or without the use of additives such as surfactants. Suitable solvents include water, acetic acid, acetone, acetonitrile, benzene, 1-butanol, 2-butanol, 2-butanone, t-butyl alcohol, carbon tetrachloride, chlorobenzene, chloroform, cyclohexane, 1,2-dichloroethane, diethylene glycol, diethyl ether, diglyme (diethylene glycol, dimethyl ether), 1,2-dimethoxyethane (glyme, DME), dimethylformamide (DMF), dimethylsulfoxide (DMSO), 1,4-dioxane, ethanol, ethyl acetate, Examples of suitable solvents include ethylene glycol, glycerin, heptane, hexamethylphosphoramide (HMPA), hexamethylphosphorus triamide (HMPT), hexane, methanol, methyl t-butyl ether (MTBE), methylene chloride, N-methyl-2-pyrrolidone (NMP), nitromethane, pentane, petroleum ether (ligroin), 1-propanol, 2-propanol, pyridine, tetrahydrofuran (THF), toluene, triethylamine, o-xylene, m-xylene, p-xylene, or ionic liquids.

[0111] Preferably, the solvent is water.

[0112] An increase in the concentration of cellulose nanocrystals in the suspension may be associated with an increase in the anisotropy of the suspension. Conversely, a decrease in the concentration of cellulose nanocrystals in the solvent may be associated with a decrease in the anisotropy of the suspension. For example, the cellulose nanocrystal suspension used in the working examples shows complete anisotropy in water at around or above 7% by weight (Figure 6). At around or below 3.5% by weight, a complete loss of anisotropy is observed.

[0113] Preferably, the concentration of cellulose nanocrystals in the suspension is selected to provide a mixture with at least some anisotropy, for example in the form of chiral nematic structures. The inventors have found that the use of nanocrystal mixtures, for example aqueous suspensions, does not provide films with high quality and optimal color development if the suspension has no anisotropy, very low anisotropy, or is in a partially liquid crystalline state (for example a two-phase state). It is proposed that such films have inhomogeneities that result in poorer observed color development properties.

[0114] Typically, the nanocrystal suspension comprises cellulose nanocrystals (e.g., neutralized cellulose nanocrystals) at a concentration of up to 12% by weight, preferably up to 11% by weight, more preferably up to 10% by weight, even more preferably up to 9% by weight, and most preferably up to 8% by weight.

[0115] Typically, the nanocrystal suspension comprises cellulose nanocrystals (e.g., neutralized cellulose nanocrystals) at a concentration of at least 1.5% by weight, preferably at least 2% by weight, more preferably at least 3% by weight, even more preferably at least 4% by weight, and most preferably at least 5% by weight.

[0116] The nanocrystal suspension may contain cellulose nanocrystals in a range with upper and lower limits selected from those given above. Typically, the nanocrystals are present in the mixture in an amount selected from 4 to 12% by weight, preferably 4 to 8% by weight, more preferably 6 to 8% by weight.

[0117] The weight percent value selected will depend on the level of anisotropy resulting from a given use of cellulose nanocrystals and can be selected accordingly.

[0118] Additionally, or alternatively, the amount of nanocrystals used can be expressed in terms of the level of anisotropy of the nanocrystal suspension.

[0119] The nanocrystals may be present in a mixture having a level of anisotropy of at least 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70%. Additionally, or alternatively, the nanocrystals may be present in a mixture having a level of anisotropy of up to 65, 70, 75, 80, 85, 90, or 95%. The nanocrystals may be present where the mixture is substantially all anisotropic (substantially 100% anisotropic).

[0120] The nanocrystals may be present in the mixture with a level of anisotropy selected from a range with upper and lower limits selected from the values ​​given above. For example, the nanocrystals may be present in the mixture with a level of anisotropy selected from 25-100%.

[0121] In cases where the sonication step is carried out before the deposition step, the anisotropy refers to the anisotropy of the cellulose nanocrystal suspension before sonication.

[0122] Sonication can alter the level of anisotropy in the suspension, and in some cases it may be preferable to discard the non-anisotropic portion of the suspension.

[0123] Other components may be present with cellulose nanocrystals or may be added to the cellulose nanocrystal suspension to modify the self-assembly properties of cellulose nanocrystals and / or the physical and chemical properties of cellulose nanocrystals, nanocrystal suspensions and / or structural color films. If necessary, additives, particularly polymers, functional molecules and fillers, may be included, which act as rheology adjusters, plasticizers, thickeners or reinforcers to provide additional functionality, such as increased flexibility or strength, to the structural color film. Examples of suitable additives are listed below.

[0124] Suitable acids include both organic acids and mineral acids and the corresponding salt forms.Suitable organic acids include carboxylic acids and corresponding acid anhydrides, such as non-phenolic organic acids, 2,5-furandicarboxylic acid, acetic acid, adipic acid, ascorbic acid, benzoic acid, boric acid, carbonic acid, citric acid, formic acid, fumaric acid, lactic acid, itaconic acid, levulinic acid, malic acid, oxalic acid, propionic acid, succinic acid, and phenolic organic acids, benzoic acid, caffeic acid, ferulic acid, gallic acid, gentisic acid, parahydroxybenzoic acid, paracoumaric acid, protocatechuic acid, vanillic acid, salicylic acid, sinapic acid, syringic acid, phenolic acid.In addition, uric acid can also be used. Suitable mineral acids include hydrochloric acid, chloroacetic acid, hydrobromic acid, bromoacetic acid, hydrochloric acid, hydrofluoric acid, hypobromous acid, hypochlorous acid, hypoiodous acid, iodic acid, iodoacetic acid, nitric acid, perchloric acid, phosphoric acid, phosphorous acid, selenic acid, sulfurous acid, sulfuric acid, telluric acid, tribromoacetic acid, trichloroacetic acid, and trifluoroacetic acid. The corresponding acid forms of the bases listed below may also be used. Mixtures of acids may also be used.

[0125] Suitable bases include amines, amides, and alkali salts, such as sodium acetate, sodium amide, 3-amino-3-methylpentane, ammonia, aniline, azetidine, bromopyridine, butyllithium, cadaverine, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, choline, cyclohexylamine, lithium diethylamide, diethylamine, diisopropylamine, dimethylamine, 2,4-dimethylimidazole, 1,2-dimethylaminoethane, 1,2-dimethylpyrrolidine, ethylamine, ethanediamine, ethanolamine, sodium ethanoate, potassium ethanoate, ferrous and ferric hydroxides, hexamethylenediamine, hexylamine, ethyl ... Examples of suitable bases include amines, hydrazine, sodium hydride, barium hydroxide, calcium hydroxide, iron hydroxide, lithium hydroxide, magnesium hydroxide, potassium hydroxide, sodium hydroxide, hydroxylamine, methylamine, 2-methyl-2-butanamine, 3-methyl-1-butanamine, methylglycine, 1-methylpiperidine, monoethanolamine, n-butylamine, nitrophenol, N-methylpyrrolidine, N-methylpyridinamine, 3-pentanamine, pentylamine, piperidine, propylamine, 1,3-propanediamine, 4-pyridinamine, pyridine, pyrrolidine, sec-butylamine and tert-butylamine, and triethylamine. The corresponding alkaline forms of the acids listed above may also be used. Mixtures of bases may also be used.

[0126] Suitable salts include neutral salts such as sodium chloride, potassium chloride, ferrous chloride and ferric chloride. Ionic liquids capable of suspending nanocrystals may also be used. Mixtures of salts may also be used.

[0127] Suitable polymers include polyethylene glycol, polyethyleneimine, polyethylene oxide, polyvinyl alcohol, quaternary polyamines, polyacrylamides, polyacrylic acid and its copolymers, polyacrylates including sodium polyacrylate, dicyandiamide resins, polyvinylpyrrolidones, sodium polystyrene sulfonate, sodium polyvinyl sulfonate, polyamidoamines, carboxypolymethylene, polyvinyl methyl ether-maleic anhydride; polyols such as polyether polyols and polyester polyols; Cellulose derivatives, such as cellulose nanofibers, microfibrillated cellulose, carboxymethylcellulose calcium, carboxymethylcellulose acetate butyrate, carboxymethylhydroxyethylcellulose, cellulose, cellulose acetate, cellulose acetate butyrate, cellulose gum, cellulose acetate propionate, cellulose acetate propionate carboxylate, cellulose succinate, cetyl hydroxyethylcellulose, ethylcellulose, hydrolyzed cellulose gum, hydroxybutyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl ethylcellulose, hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxypropyl methylcellulose acetate / succinate, hydroxypropyl methylcellulose phthalate, methylcellulose, methylethyl cellulose, methylhydroxyethyl cellulose, crystalline cellulose, potassium succinate, sodium cellulose sulfate, nitrocellulose, cellulose acetate, rayon, regenerated cellulose, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate, viscose; Other polysaccharides, glucose and polysaccharide derivatives include arabinoxylan, carrageenan, chitin, chitosan, fucoidan, galactogen, galactomannan, glucan, glycogen, inulin, lignin, mannan, pectin, starch and xylan. In addition to the polysaccharides listed above, sulfated or oxidized polysaccharides may also be used. In addition to the cellulose derivatives listed above, hemicellulose may also be used. Mixtures of polymers may also be used.

[0128] Suitable functional molecules include monosaccharides such as arabinose, deoxyribose, erythrose, fructose, galactose, glucose and sorbose; sugar alcohols and polyols such as arabitol, cyclitols such as pinitol, ethylene glycol, erythritol, galactitol, glycerol, isomalt, lactitol, maltitol, mannitol, pentaerythritol, sorbitol and xylitol; proteins such as collaged, gelatin and sericin; and amino acids. Small molecules also include dyes such as acid dyes, base dyes, direct dyes, sulfur dyes, vat dyes, reactive dyes and azo dyes. In addition, the dye may be a black dye and may provide a black appearance. Mixtures of functional molecules may be used.

[0129] Suitable fillers include water-soluble inorganic materials and nanomaterials, such as clays including hectorite, kaolin, mica, montmorillonite, laponite, cloisite; carbon materials, such as carbon nanotubes, graphite, graphene, carbon black; and water-soluble proteins, such as albumin, whey, plant-derived proteins, and zein. Mixtures of functional fillers may also be used.

[0130] In addition, typical fillers include water-soluble inorganic materials, micro- and nano-materials, such as clays, including hectorite, kaolin, mica, minerals, oxides, montmorillonite, laponite, and cloisite; carbon materials, such as carbon nanotubes, graphite, graphene, and carbon black; and water-soluble proteins, such as albumin, whey, plant-derived proteins, and zein.In addition, typical fillers include organic and inorganic pigments, such as aluminum-based, copper-based, cobalt-based, gold-based, iron-based, manganese-based, cadmium-based, chromium-based, arsenic-based, bismuth-based, chromium-based, lead-based, titanium-based, barium-based, tin-based, zinc-based, cerium-based, mercury-based, carbonaceous, and antimony-based pigments; and fluorescent pigments.

[0131] Further modifications to cellulose nanocrystals and to the compounds used to prepare them have been described in the art, typically with the goal of maintaining the ability of the cellulose nanocrystals to form chiral nematic phases.

[0132] aging The method of the present invention includes an aging step during which the nanocrystal suspension partially or completely regains any cholesteric structure lost during the deposition and spreading steps.

[0133] In this way, the optical properties of the resulting film can be optimized. Without wishing to be bound by theory, it has been proposed that high shear during the spreading process can destroy pre-existing chiral nematic order, resulting in poorer optical properties. In the event of any destruction of any anisotropy in the nanocrystal suspension resulting from excessive shear rate, any pre-existing chiral nematic order can be partially or completely restored by extending the aging time, since the energy dissipation of the alignment is a thermodynamically favorable process. In addition, the nanocrystal suspension has a low viscosity, which allows it to return to equilibrium with a short relaxation time. Thus, nanocrystal suspensions with a relaxation time of 30 minutes or less are preferred.

[0134] Typically, the deposited nanocrystal suspension is aged for 360 minutes or less. Preferably, the nanocrystal suspension is aged for 120 minutes or less, more preferably 60 minutes or less, even more preferably 45 minutes or less, and most preferably 30 minutes or less.

[0135] Typically, the deposited nanocrystal suspension is aged for 1 minute or more, 5 minutes or more. Preferably, the nanocrystal suspension is aged for 10 minutes or more, more preferably 15 minutes or more, and even more preferably 20 minutes or more.

[0136] The time for the aging step can be selected from a range with upper and lower limits selected from those given above. For example, the deposited nanocrystal suspension may be aged for 5 to 120 minutes, preferably 5 to 30 minutes.

[0137] The aging step can be performed as a break before any further processing steps. Alternatively, the aging step can be performed simultaneously with the drying step, for example by extending the drying time.

[0138] During the aging process, partial or complete restoration of chiral nematic ordering in the coated nanocrystal suspension can be promoted by an external electromagnetic field, either globally or locally throughout the deposited nanocrystal suspension.

[0139] Drying The method of the present invention includes a drying step to dry the deposited nanocrystal suspension to form a structural color film.

[0140] The drying step may be simultaneous with the aging step previously described.

[0141] The drying step may be carried out at room temperature without external heating.

[0142] Typically, however, the drying step is carried out at an elevated temperature.

[0143] The drying step may be carried out at a temperature of 250°C or less, 150°C or less, 100°C or less, preferably 80°C or less, and even more preferably 70°C or less.

[0144] The drying step may be carried out at a temperature of 10° C. or higher, 20° C. or higher, 30° C. or higher, preferably 40° C. or higher, even more preferably 50° C. or higher.

[0145] The temperature of the drying step can be selected from a range in which the upper and lower limits are selected from the values ​​given above. For example, the temperature of the drying step can be 10 to 70°C, for example, around 60°C.

[0146] The temperature of the drying step can be selected based on the solvent mixture used for the nanocrystal suspension, preferably a temperature is used at which the solvent mixture does not boil or is not close to boiling.

[0147] In this way, the resulting film can have the desired color-developing properties. It has been proposed that when the solvent mixture is boiling or near boiling, the solvent movement and bubble formation is increased, which can disrupt the chiral nematic structure and result in non-uniformity in the final dried film.

[0148] The drying step may be carried out in 720 minutes or less, 360 minutes or less, 120 minutes or less, 60 minutes or less, preferably 45 minutes or less, and even more preferably 30 minutes or less to form a dry film.

[0149] The drying step may be carried out for 10 minutes or more, preferably 15 minutes or more, and even more preferably 20 minutes or more, such that a dry film is formed.

[0150] The time for the drying step can be selected from a range in which the upper and lower limits are selected from the values ​​given above. For example, the drying step can be carried out for 10 to 60 minutes, for example around 30 minutes, so that a dry film is formed.

[0151] The combination of drying time and temperature depends on the coating gap, the coating speed and width, and the thickness of the deposited suspension. If the deposited suspension is thicker (i.e., a larger gap), longer drying times are observed than if the deposited suspension is thinner (i.e., a smaller gap) at the same temperature.

[0152] Time-constrained drying, e.g., short drying times relative to the amount of cellulose nanocrystal suspension deposited, affects the color development of the resulting film, especially the reflectance of the film. Without wishing to be bound by theory, it has been proposed that time-constrained evaporation, e.g., during heating, induces a less uniform and more disordered film. As described in the literature, the disorder can result from chiral nematic domains that become kinetically confined in random orientation and from non-optimal domain compression upon drying (see, e.g., Parker, RM et al., Adv. Mater., 30, 1704477 (2018)). It has also been proposed that heating results in a more significant flow of solvents and compounds in the cellulose nanocrystal suspension, e.g., due to temperature or concentration gradients that perturb the cellulose nanocrystal suspension. Generally, such effects mean that the reflectance peak is reduced and a red shift of the peak can be observed (see FIG. 9).

[0153] In some cases, the dry film has a thickness of at least 1.0 μm, 2.0 μm, at least 3.0 μm, at least 4.0 μm, at least 5.0 μm, at least 6.0 μm, at least 7.0 μm, at least 8.0 μm, or at least 9.0 μm.

[0154] In some cases, the dry film has a thickness of 50.0 μm or less, 30.0 or less, 20.0 or less, 17.0 or less, 15.0 or less, 12.0 μm or less, or 10.0 μm or less.

[0155] The film thickness can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the film thickness can be 1.0 to 50.0 μm, such as 6.0 μm to 12.0 μm, for example around 9.0 μm.

[0156] The drying process can be uniform across the width of the film, or alternatively, the drying conditions can be varied locally (Figure 9).

[0157] Drying is carried out by any suitable drying machine. Suitable drying machines include IR or UV radiation lamps, hot air dryers, ovens, convection ovens, furnaces, vacuum ovens and hot plates. A combination of drying steps using different machines may be used, either sequentially or simultaneously.

[0158] process In some cases, the method further includes a step of treating at least a portion of the substrate prior to the depositing step to modify physical and / or chemical properties of the substrate.

[0159] The treatment process may impart beneficial properties to the substrate, such as allowing the nanocrystal suspension to be more easily and more uniformly deposited and spread on the substrate, improving the optical properties of the structural color film, and increasing the yield of the film, which correspondingly increases the yield of structural color particles per area of ​​substrate used.

[0160] Typically, the treating step alters the surface energy of the substrate, for example by corona discharge or plasma etching. Preferably, the treating step comprises subjecting the substrate to corona discharge or plasma etching, more preferably to corona discharge.

[0161] Preferably, substrates having low surface energy can be treated to increase the surface energy, including polyvinyl alcohol, cellophane, polystyrene, acetal, ethylene-vinyl acetate, polyethylene, e.g., polyethylene terephthalate (PET), polypropylene, fluoropolymers, polyimides, nylons, polyesters, epoxy resins, acrylic resins, phenolic resins, polycarbonates, polyurethanes, polyvinyl chloride, and polyesters.

[0162] In the case of treatment of materials with low surface energy, it is proposed that increasing the surface energy of the substrate reduces the surface tension for deposition of the nanocrystal suspension. The treatment is such that the deposited nanocrystal suspension preferentially coats the substrate in the treated areas. For example, if the substrate is PET, treatment with corona discharge means that the treated areas of the web are more hydrophilic, and the aqueous suspension preferentially coats this hydrophilic surface.

[0163] In this way, the treatment process results in specific, localized surface activation that provides regions with different wetting properties, allowing controlled deposition of the suspension onto either treated or untreated parts of the substrate.

[0164] In some cases, the method may include a step of treating at least a portion of the substrate to reduce its surface energy prior to the depositing step. Suitable methods that can modify the surface chemistry and reduce the surface energy of the substrate include chemical and oxidative treatments, or altering the surface roughness, for example through sanding or laser ablation.

[0165] Preferably, substrates with high surface energy can be treated to lower the surface energy. High surface energy materials include metals, such as copper, aluminum, zinc, tin, stainless steel and their alloys, and glass.

[0166] In some cases, the treatment can be performed such that the surface energy of the substrate is locally altered. The local variation in surface energy can be in the form of a pattern.

[0167] Typically, the treating step is performed on the center of the substrate. The edges of the web surface may be masked prior to treating, preferably so that the treatment is applied only to the preferred areas, most preferably only to the center of the web or substrate. The masking is typically removed prior to deposition of the nanocrystal suspension. In this way, the deposited suspension preferentially covers the center of the substrate and is prevented from flowing off the web by the untreated (i.e., previously masked) edges.

[0168] In some cases, the treatment step is performed on a pristine substrate. Alternatively, the treatment step may be performed to enhance the pre-existing surface energy characteristics of the substrate, particularly in cases where the substrate is to be used in a closed loop fashion.

[0169] The treating step may be carried out in air, oxygen or argon rich atmospheres, under reduced or ambient pressure. Preferably, such treating is carried out in air and at ambient pressure.

[0170] Sonication The method of the present invention may further comprise the step of sonicating the nanocrystal suspension prior to the depositing step.

[0171] Sonication of the nanocrystal suspension prior to deposition, e.g., tip sonication, can be used to modify, e.g., red-shift, the final color of the dried nanocrystal film. It has been proposed that sonication acts to broaden the pitch of the chiral nematic phase, resulting in a red-shift, although the exact mechanism for this has not been clearly identified and may involve a reduction in the size of the nanocrystals and possibly the release of trapped ions.

[0172] The sonication energy delivered may depend on the device used, the power and amplitude delivered, as well as the volume of the cellulose nanocrystal suspension and the concentration of nanocrystals in the suspension. Suitable sonication devices are known in the art, and power, amplitude and time can be adjusted accordingly.

[0173] The length of time for which sonication is performed can vary, and longer times may be used for sonication of larger quantities of material. Regardless of the amount of material being sonicated, for purposes of comparing sonication between samples, sonication is generally measured in Joules per mass (J / g), e.g., Joules per mass of cellulose nanocrystals in a cellulose nanocrystal suspension (J / g CNC Sonication is expressed in seconds of treatment per milliliter of cellulose nanocrystal suspension (s / mL). Although both units are used, seconds may be preferred as they can be calculated directly from the entered parameters.

[0174] The sonication step may be carried out for 45 s / mL or less, such as 22.5 s / mL or less, preferably 11.2 s / mL or less, or more preferably 6.7 s / mL or less.

[0175] The sonication step may be carried out for at least 0.1 s / mL, at least 0.5 s / mL, at least 1 s / mL, at least 2.2 s / mL, at least 22.5 s / mL, preferably at least 0.2 s / mL, or more preferably at least 1 s / mL.

[0176] The sonication step may be carried out for a time per milliliter of suspension selected from a range with upper and lower limits selected from those given above. For example, the sonication step may be carried out for 0.1 to 45 s / mL, for example around 2.2 s / mL.

[0177] The ultrasonic treatment step may deliver an energy to the nanocrystal suspension of 200 kJ / g or less, 100 kJ / g or less, preferably 50 kJ / g or less, or more preferably 30 kJ / g or less.

[0178] The sonication step may deliver an energy to the nanocrystal suspension of at least 3 J / g, at least 5 kJ / g, at least 10 kJ / g, at least 100 kJ / g, preferably at least 3 kJ / g, more preferably at least 5 kJ / g.

[0179] The sonication process may provide to the nanocrystal suspension an energy selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the sonication process may provide to the nanocrystal suspension an energy that may be between 1 and 100 kJ / g, such as around 10 kJ / g.

[0180] The sonication step may be carried out for up to 2,000 seconds, up to 1,000 seconds, up to 500 seconds, up to 400 seconds, preferably up to 300 seconds, more preferably up to 200 seconds.

[0181] The sonication step may be carried out for 20 seconds or more, 30 seconds or more, preferably 40 seconds or more, more preferably 50 seconds or more.

[0182] The sonication step may be carried out for a duration in a range whose upper and lower limits are selected from those given above. For example, the sonication step may be carried out for 20 to 2,000 seconds, for example around 200 seconds.

[0183] Sonication results in a color shift in films prepared from sonicated nanocrystal suspensions. As the energy of sonication increases, the red shift of the colored films increases.

[0184] Sonication in the prior art is typically performed on isotropic suspensions of nanocrystals at around 2% by weight. In the present case, preferred nanocrystal suspensions contain at least 4% by weight nanocrystals and have some anisotropy.

[0185] Peeling The method of the present invention may further include a step of peeling the structural color film from the substrate. The peeling step is performed after the drying step. Preferably, the peeling step is performed on a dry substrate. The peeled film may be transferred to a different substrate or used as a free-standing film.

[0186] In this way, the substrate can be reused after the peeling step, for example in a closed loop fashion instead of rewinding, to allow for continuous printing.

[0187] In some cases, the substrate may be in the form of a movable belt, such as a heat resistant movable belt, made of metal, etc. The term belt herein refers to a closed loop configuration of the substrate.

[0188] In some cases, the edges of the structural color film are removed after the peeling step.

[0189] Split The method of the present invention may further include a step of dividing the structural color film to produce structural color particles. In the dividing step, the structural color film is reduced in size to provide particles that can be used as pigments or glitters. The present invention also provides structural color particles obtained or obtained by the method of the present invention.

[0190] The colored particles maintain the color from the film and have a similar appearance to glitters, such as microplastic glitters, and effect pigments, such as mica-based and titania-based effect pigments. Most glitters available on the market are obtained by roll-to-roll deposition of dye-containing synthetic polymer matrices onto metal-based reflective substrates, and are therefore not biodegradable. Thus, the present invention provides biodegradable glitters and photonic effect pigments.

[0191] The dividing step can be carried out using any suitable breaking or chopping machine, such as a device using a rotating blade, and any suitable crushing machine, such as a device using a crushing element and high-intensity impact (e.g., a mill such as a ball mill, a rusher, a pulverizer, or a freeze crusher). A die cutter or a laser cutter can also be used to reduce the size of the film to obtain particles with a defined shape.

[0192] Preferably, a disrupting machine is used, in which case the dividing step may also be referred to as a disrupting step.

[0193] Alternatively, a crushing machine is used, in which case the dividing step may also be referred to as a crushing step.

[0194] In some cases, the breaker and grinder are used sequentially, in which the breaker step provides a specific yield of large particles that can be broken down into smaller particles through further grinding to produce particles having a specific size.

[0195] The dividing step is preferably performed after the peeling step, in this way the substrate is not damaged during the dividing step and is not incorporated into the pigmented particles and therefore does not contribute to the thickness of the particles.

[0196] Without wishing to be bound by theory, it is understood that the structural color film breaks at the boundaries of the cholesteric domains because nanocrystals are expected to have less order between domains than within the cholesteric domains, however, fracture and cracking can propagate through the cholesteric domains upon high strength splitting.

[0197] After separation, the optical properties of the structural color film are retained (Figure 4).

[0198] The structural color particles may be sorted by size to obtain particles of desired diameter or surface area. Size sorting can be performed during the breaking and / or grinding step, for example, when the division is performed using an ultracentrifuge mill. Size sorting can be performed after the breaking and / or grinding step, for example, by sieving. Particles with a large size (e.g., large median average diameter or average surface area) have improved optical properties compared to particles with a small size (Figure 4). That is, particles with a large size have narrower and / or higher reflection peaks.

[0199] The structural color particles are suitable for use as effect pigments, such as interference pigments, metallic pigments and pearlescent pigments, and luster agents, due to their ability to reflect light and provide structural color, including visible, infrared and ultraviolet colors.

[0200] The structural color particles may be used as such pigments and glitters, or may be dispersed in solvents or other formulations, depending on the intended use. For example, the particles may be used in cosmetics, food, packaging, or paints. Preferably, the solvent for the selected application is water, glycerol, ethanol, or mixtures thereof. Mixtures of these components with oils may also be used, and such mixtures may also contain surfactants.

[0201] annealing The method of the present invention may further include a step of annealing the structural color film or structural color particles. Typically, the annealing step is performed on the structural color film, where the annealing step is performed after the drying step and before the dividing step, if present.

[0202] The step of annealing refers to the step of heating the structural color film. Without wishing to be bound by theory, it is proposed that annealing removes tightly bound water molecules and promotes the destabilization of the sulfate half ester groups that cover the surface of the cellulose nanocrystals, thus making them reactive. It is proposed that the removal of water and desulfation promotes the formation of new molecular bonds between adjacent nanocrystals. As a result, water molecules tend not to interact with the chains and penetrate the nanostructure, preventing the swelling and collapse of the particles in water.

[0203] The annealing step may result in oxidation, polymerization, and crosslinking between the cellulose nanocrystals and any additional compounds or additives remaining in the film after the drying step.

[0204] The annealing step is carried out by any suitable machine capable of heating, oxidizing, polymerizing, crosslinking between the cellulose nanocrystals and any additional compounds or additives remaining in the film, as well as removing tightly bound water molecules and destabilizing the sulfate half ester groups that cover the surface of the cellulose nanocrystals. Suitable drying machines include IR or UV irradiation lamps, furnaces, hot air dryers, ovens, convection ovens, vacuum ovens and hot plates. A combination of drying steps using different machines may be used, either sequentially or simultaneously.

[0205] The temperature of the annealing step may be 250° C. or less, 230° C. or less, preferably 220° C. or less, more preferably 190° C. or less.

[0206] The temperature of the annealing step may be 100° C. or higher, 110° C. or higher, preferably 140° C. or higher, more preferably 170° C. or higher.

[0207] The temperature of the annealing step can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the temperature of the annealing step can be 100 to 250°C, preferably 140 to 220°C.

[0208] The annealing step may be carried out for 120 minutes or less, 60 minutes or less, preferably 40 minutes or less, more preferably 30 minutes or less.

[0209] The annealing step may be carried out for 2 minutes or more, 5 minutes or more, 10 minutes or more, preferably 15 minutes or more, more preferably 20 minutes or more.

[0210] The annealing step may be carried out for a time period selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the annealing step may be carried out for a period of 10 to 120 minutes, preferably 20 to 40 minutes, for example, around 30 minutes.

[0211] The annealing step has the effect of making the structural color particles prepared from the annealed film "color fast" when placed in various solvents, especially aqueous solvents. The term "color fast" here refers to the ability of the particles to maintain some color when suspended in a solution or embedded in a matrix such as a polymer. Thus, the annealing step reduces and / or prevents processes such as swelling or collapse of the structural color film or structural color particles by modifying the physical and / or chemical properties of the nanocrystals.

[0212] The annealing step has also been proposed to prevent degradation of the film surface, for example through physical abrasion, during the subsequent cleavage step; FIG. 4 shows top and cross-sectional views of scanning electron microscopy (SEM) of particles produced from annealed and crushed films. These SEM images show that neither the particle surface nor the cholesteric structure is significantly damaged, suggesting that the film structure does not undergo undesirable degradation during the cleavage step. This lack of degradation is further supported by the optical performance of the particles, which is similar to the film from which they are derived. In addition, the particles have unique sharp-edged facets, and these facets suggest that the film will preferentially break along defects in the film, most likely between adjacent chiral nematic domains.

[0213] Preferably, where an annealing step is performed, the nanocrystals are cellulose nanocrystals, for example cellulose nanocrystals in the sodium form.

[0214] Thus, the annealing process provides improved optical properties to the annealed film and to any particles resulting from the film. + The counter ion is H + It is proposed that H+ counterions do not catalyze the destabilization of the sulfate half ester upon heating as much as the H+ counterion case. Excessive destabilization of the sulfate half ester is proposed to reduce the optical quality of the cellulose nanocrystal structure in the form of strong darkening of the films, as typically observed in the H+ counterion case.

[0215] In addition, as shown in Figure 7, the annealing step can reduce the transmittance of the film, and therefore increase the opacity of the film. The increased opacity can improve the color contrast of the film and the particles. Thus, by carefully controlling the temperature and duration of the heat treatment step, the degree of transmittance reduction and darkening of the particles can be further controlled.

[0216] The annealing step may reduce the transmittance of the film by at least 1%, at least 10%, at least 25%, at least 50% compared to an untreated film.

[0217] The annealing step can reduce the transmittance of the film by up to 90%, preferably up to 75%, compared to the untreated film.

[0218] The parameters of the annealing step can be adapted such that the reduction in the transmittance of the film compared to the untreated film falls within a range with upper and lower limits selected from those given above. For example, the reduction in the transmittance of the film can be 33%.

[0219] The transmittance of a film can be determined using standard techniques, for example using an optical microscope coupled with a spectrometer used in a bright field imaging configuration. The transmittance value is measured relative to the total light transmittance. Background noise is subtracted. The reflectance value is measured in the visible light range (300 nm to 800 nm). Typically, the sample is placed flat on the microscope stage so that the light beam can be considered to travel normal to the surface of the sample and the reflection at normal incidence is measured, and light is collected within the cone of the objective lens (numerical aperture) (maximum reflectance). Typically, the reflectance value is measured in air.

[0220] As a result of the annealing process, the films and particles can retain their integrity without disintegration when placed in water and other solvents for over a year (Figure 16), and only limited red shifts in color of the structural color films and structural color particles can be observed in aqueous solvents.

[0221] Cholesteric Structure and Structural Color In a further aspect, the present invention provides structural color films and structural color particles comprising nanocrystals, preferably cellulose nanocrystals. Typically, the structural color films or structural color particles are composed of neutralized cellulose nanocrystals, such as sodium form cellulose nanocrystals.

[0222] The structural color film or structural color particles of the present invention have a cholesteric structure, which is sometimes called chiral nematic ordering, and thus the film has a self-assembled structure that is not non-helical chiral nematic.

[0223] The nanocrystals in the film or particle are in a helicoidal collection, the helicoidal collection having a defined pitch in the film or particle.

[0224] The cholesteric pitch p can be at most 2.0, at most 3.0, at most 4.0, at most 5.0, at most 6.0, at most 7.0, at most 8.0, at most 9.0 or at most 10 μm.

[0225] The cholesteric pitch p can be at least 0.05, at least 0.1, at least 0.2, at least 0.4, at least 0.5, at least 0.6, at least 0.7, at least 0.8, at least 0.9, at least 1.0, or at least 2.0 μm.

[0226] The cholesteric pitch p can be in a range selected from the upper and lower limits given above. For example, the cholesteric pitch p can be in the range of 0.05 to 10 μm, such as 0.4 to 4.0 μm, for example 0.4 to 2.0 μm.

[0227] The cholesteric pitch p can be associated with films or particles that have structural colors in the visible light range.

[0228] The cholesteric pitch can be controlled by the selection of appropriate conditions during the preparation of the nanocrystal suspension. For example, the cholesteric pitch can be increased by sonicating the nanocrystal suspension prior to film formation (Figure 3). Similarly, the step of annealing the film can result in a small compression of the cholesteric pitch as a result of the removal of water molecules (Figure 7). The cholesteric pitch can also be altered by adjusting the properties of the nanocrystal suspension prior to deposition.

[0229] The cholesteric pitch can be measured, for example, from SEM images of the particles, where in dry particles the helicoidal aggregation of nanocrystals is visible in the form of Bouligand arches, and the periodicity can be measured.

[0230] The films and particles of the present invention have structural color. Thus, the cholesteric order allows Bragg reflection of incident electromagnetic radiation in the visible, infrared or ultraviolet region of the spectrum. The reflected wavelength obeys the Bragg law: λ = n x p x cosθ, where n describes the average refractive index, p is the pitch of the helicoid, and θ is the angle of the incident light with respect to the director m of the cholesteric structure.

[0231] The observed (reflected) color may be an ultraviolet color, a visible color, or an infrared color, and is preferably a visible color. Visible color refers to a color having a wavelength within the range of about 400 to about 800 nm. Infrared color refers to a color having a wavelength within the range of about 700 nm to about 1 mm, most commonly about 700 nm to about 5 μm. Ultraviolet color refers to a color having a wavelength within the range of about 100 nm to about 400 nm, generally about 200 nm to about 400 nm, for example about 300 nm to about 400 nm.

[0232] The color of a film or particle can be measured using standard techniques, for example, using an optical microscope coupled with a spectrometer used in a bright-field image configuration with or without a polarizer. Reflectance values ​​are measured relative to the reflectance of a mirror, typically a silver mirror (maximum reflectance), and used to obtain the normalized reflectance of the sample. Background noise is subtracted. Reflectance values ​​are measured in the visible light range (300 nm to 800 nm). Typically, the sample is mounted flat on the microscope stage so that the light beam travels perpendicular to the surface of the sample and the reflection at normal incidence can be considered to be measured, and light is collected within the cone of the objective lens (numerical aperture) (maximum reflectance). Typically, reflectance values ​​are measured in air.

[0233] film In a further aspect, the present invention provides a structural color film comprising or consisting of nanocrystals, preferably cellulose nanocrystals. The nanocrystals in the film are in a chiral nematic phase. Typically, the structural color film is composed of neutralized cellulose nanocrystals, such as cellulose nanocrystals in sodium form.

[0234] A structural color film may be produced by the above-mentioned method. Accordingly, the present invention provides a structural color film obtained or obtainable by the method of the present invention.

[0235] Preferably, the film has a thickness (e.g., dry thickness) large enough for the director of one chiral nematic structure in the film to make a full rotation, more preferably four full rotations, which gives the film excellent optical properties. Thus, the structural color film has a thickness of 1.0 μm to 50 μm, preferably 2.0 μm to 20 μm, more preferably 6.0 to 12.0 μm, for example, around 9.0 μm.

[0236] The thickness of the film or particles can be measured using standard techniques, for example by measuring the cross-sectional thickness of the film using an SEM.

[0237] The highly ordered chiral nematic structure in the film is highly reflective and reflects very vivid colors with high intensity.

[0238] Structural color films reflect 50% or less of incident light at a given wavelength in the visible range (300 nm to 800 nm: FIG. 3; FIG. 19). Preferably, the film reflects 30% or more of incident light, more preferably 40% or more, even more preferably 45% or more, and most preferably 49% or more.

[0239] The reflectance of a film can be measured using standard techniques, for example an optical microscope coupled with a spectrometer. The reflectance value is measured against the reflectance of a mirror, typically a silver mirror (maximum reflectance), and used to obtain the normalized reflectance of the sample. Background noise is subtracted. Reflectance values ​​are measured in the visible light range (300 nm to 800 nm). Typically, the sample is placed flat on the microscope stage, so that the light beam travels perpendicular to the surface of the sample and the reflection at normal incidence can be considered to be measured, and light is collected within the cone of the objective lens (numerical aperture) (maximum reflectance). Typically, reflectance values ​​are measured in air.

[0240] Films may also reflect different wavelengths of light with different reflectivities (bandgaps).

[0241] At 500 nm, the films typically exhibit a reflectance of 70% or greater, preferably 80% or greater, more preferably 90% or greater, and most preferably 99% or greater (FIG. 3).

[0242] At 600 nm, the films typically exhibit a reflectance of 65% or greater, preferably 75% or greater, more preferably 85% or greater, and most preferably 95% or greater (FIG. 3).

[0243] At 700 nm, the films typically exhibit a reflectance of 60% or greater, preferably 70% or greater, preferably 80% or greater, and most preferably 90% or greater (FIG. 3).

[0244] In addition to the high absolute reflectance values, the light reflected by structural color films is also very vivid. Thus, chiral nematic structures selectively reflect light in a given wavelength range. Thus, the reflectance spectrum of the material has a sharp peak.

[0245] The full width at half maximum of the reflected light is typically 150 nm or less, 100 nm or less, preferably 75 nm or less, and more preferably 50 nm or less (FIG. 3).

[0246] In the range of 400 nm to 550 nm, the films typically exhibit a full width at half maximum of 50 nm or less (Figure 3).

[0247] In the range of 450 nm to 800 nm, the films typically exhibit a full width at half maximum of 100 nm or less (Figure 3).

[0248] An example of a structural color film with high reflectance is shown in FIG. 5, where a CIE plot shows the gamut of colors from such a film.

[0249] The reflected structural color is iridescent and therefore angle dependent, and the level of iridescence can be controlled through the degree of disorder of the chiral nematic structure in the film.

[0250] The edges of structural color films that form chiral nematic structures tend to develop a "coffee stain" effect, which results in a less uniform and less optimal visual appearance at the edges than away from the edges. These portions of the film can be removed after the drying process is complete to further improve the optical properties.

[0251] In some cases, the structural color film has been treated, for example by annealing, to remove sulfate groups and tightly bound water molecules.

[0252] The sulfate half ester groups are generally grafted during extraction of cellulose nanocrystals, for example by sulfuric acid hydrolysis. The sulfate half ester groups are typically located at the C6 position of the repeating glucose units. The amount of sulfate half ester groups removed depends on the temperature and conditions used to carry out the heat treatment, as well as the content of sulfate half ester groups covering the nanocrystal surface.

[0253] Typically, the reduction in sulfate content is at least 15%, at least 30%, preferably at least 50%, more preferably at least 70%, even more preferably at least 90%, and most preferably at least 98%.

[0254] In this way, the film maintains its color without degradation even when placed in a solvent, such as water. The small reduction in sulfate content as a result of short or low temperature annealing is associated with low / short stability in water and other aqueous solutions.

[0255] particle In another aspect, the present invention provides structural color particles comprising or consisting of nanocrystals, preferably cellulose nanocrystals. The nanocrystals in the particles are in a chiral nematic phase. Typically, the structural color particles comprise cellulose nanocrystals, for example, in the neutralized or sodium form.

[0256] The structural color particles can be produced by the above-mentioned method, for example, by dividing the above-mentioned structural color film. Thus, the present invention provides structural color particles obtained or obtainable by the method of the present invention.

[0257] The process of processing the film into particles preserves the chiral nematic structure of the nanocrystals without significantly altering them. The particles produced by the above method have exceptional optical qualities, such as high intensity and brightness.

[0258] Generally, the structural color particles have a median average particle diameter of 5000 μm or less, 1000 μm or less, 500 μm or less, and preferably 300 μm or less.

[0259] Generally, the structural color particles have a median average particle diameter of 2 μm or more, 5 μm or more, preferably 15 μm or more, and more preferably 25 μm or more.

[0260] In general, the median average particle diameter of the structural color particles can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the median average particle diameter of the structural color particles can be 15 μm to 300 μm, preferably 25 μm to 300 μm.

[0261] In one embodiment, the structural color particles have a median average particle diameter of 14 μm to 127 μm, preferably 28 μm to 113 μm, and more preferably 35 μm to 106 μm.

[0262] In another embodiment, the structural color particles have a median average particle diameter of 84 μm to 254 μm, preferably 99 μm to 226 μm, more preferably 106 μm to 212 μm.

[0263] The median average particle diameter of the structural color particles can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the median average particle diameter of the structural color particles can be 170 μm to 495 μm, preferably 198 μm to 453 μm, and more preferably 212 μm to 424 μm.

[0264] The size of structural color particles can be determined using standard techniques. For example, the median average particle diameter of structural color particles can be measured by SEM or optical microscopy. Preferably, the surface area of ​​each particle is measured, and then the diameter of a spherical particle with the same surface area is calculated to obtain the diameter. This diameter is recorded as the diameter of the particle, and the median average of all measured particles is calculated from the individual particle diameters.

[0265] Structural color particles are typically 78,500,000 μm2 Preferably, the structural color particles have an average surface area of ​​3,140,000 μm 2 Less than or equal to 785,000 μm, more preferably 2 Less than 282,000 μm, more preferably 282,000 μm 2 Less than 100,000 μm, most preferably 2 It has the following average surface area:

[0266] Structural color particles are typically 80 μm 2 Preferably, the structural color particles have an average surface area of ​​700 μm or more. 2 More than 1,960μm, more preferably 2 More preferably, 4,000 μm 2 More than 6,000μm, most preferably 2 or more.

[0267] The average surface area of ​​the structural color particles can be selected from a range whose upper and lower limits are selected from the values ​​given above. For example, the average surface area of ​​the structural color particles can be 4,000 μm 2 ~100,000μm 2 , preferably 6,000 μm 2 ~40,000μm 2 It is possible.

[0268] The average surface area of ​​the structural color particles can be measured by SEM or optical microscopy. Preferably, the surface area of ​​each particle is measured and the mean average of all particles is calculated to obtain the average surface area.

[0269] In some cases, the structural color particles have a rock-like shape (Figure 4). In some cases, the structural color particles may have facets that reveal the chiral nematic phase.

[0270] Typically, the particles have at least four distinct facets that arise from the nucleation and growth of self-assembled cholesteric domains. The particles may contain one or more cholesteric domains, preferably one cholesteric domain.

[0271] The highly ordered chiral nematic structure in the particles results in high reflectivity, which is in sharp contrast to the particles described in WO2018 / 033584 and is demonstrated by a side-by-side comparison to the present work in Figure 17viii.

[0272] The structural color particles reflect 25% or more of the incident light at a given wavelength in the visible range (300 nm to 700 nm; Figures 4 and 14). Preferably, the particles reflect 30% or more of the incident light, more preferably 35% or more, and even more preferably 40% or more.

[0273] The reflectance of a particle can be measured using standard techniques, for example using an optical microscope coupled with a spectrometer used in a bright field imaging configuration with or without a polarizer. The reflectance value is measured relative to the reflectance of a mirror, typically a silver mirror (maximum reflectance), and used to obtain the normalized reflectance of the sample. Background noise is subtracted. The reflectance value is measured in the visible light range (300 nm to 800 nm). Typically, the sample is placed flat on the microscope stage, so that the light beam can be considered to travel normal to the surface of the sample and the reflection at normal incidence angles is measured, and the light is collected within the cone of the objective lens (numerical aperture) (maximum reflectance). Typically, the reflectance value is measured in air.

[0274] Particles may also reflect different wavelengths of light with different reflectances.

[0275] At 500 nm, the particles typically exhibit a reflectance of 30% or more of the incident light, more preferably 35% or more, and even more preferably 40% or more (FIG. 4).

[0276] After splitting, the optical properties of the structural color film are generally retained and observed in the particles (Figure 4). However, during the decomposition process, as the dimensions of the structural color film decrease to the point where the structural color film breaks into micron-sized objects, light scatters more at the particle interfaces, which gives the structural color particles a whiter appearance than the structural color film in air. However, because the scattering overlaps with the structural color from the particles, the optical response from the chiral nematic structure can still be measured, even for the smallest particles. In fact, as shown in Figure 13, once the structural color particles are embedded in a refractive index-matching medium, such as a polymer resin, color is obtained because scattering at the particle interfaces is suppressed.

[0277] The full width at half maximum of the reflected light is typically 150 nm or less, preferably 125 nm or less (FIG. 4).

[0278] In the range of 400 nm to 650 nm, the films typically exhibit a full width at half maximum of 150 nm or less, preferably 125 nm or less (FIG. 4).

[0279] An example of a structural color particle with high reflectance is shown in FIG.

[0280] The reflected structural color is iridescent and therefore angle dependent (Figure 12). The level of iridescence can be controlled through the degree of disorder of the chiral nematic structure in the particles.

[0281] Importantly, the structural color particles maintain their color when placed in solution or a matrix, such as a polymer matrix (Figure 12).

[0282] The color of the particles in solution or matrix can be the same as in air, which is the case, for example, when immersed in a dry solvent such as ethanol (Figure 4).

[0283] The color of structural color particles can change depending on the medium in which they are contained (Figure 11).

[0284] This variability in color change has been proposed to be due to different amounts of swelling in different media, which can be large in the case of water and proportional to the ionic strength of the medium.

[0285] For example, starting with green particles in air, upon swelling in a mixture of water and ethanol some particles will appear red and others will reflect infrared light.

[0286] In the case of a liquid solvent such as water, the swelling is reversible and the color of the structural color particles can be restored to its original color (ie, the color in air alone) upon drying.

[0287] The swelling of the particles can instantly result in a red shift in reflected light of at least 2 nm, at least 5 nm, at least 10 nm, or at least 50 nm.

[0288] The swelling of the particles can instantly result in a red shift in reflected light of up to 100 nm, up to 150 nm, up to 175 nm, or up to 200 nm.

[0289] The swelling of the particles can instantly result in a red shift of the reflected light selected from the upper and lower limits given above. For example, the wavelength shift of the particles can be in the range of 2 to 150 nm, such as 10 nm to 100 nm.

[0290] The swelling is limited and does not result in redispersion of nanocrystals over a long period of time.Typically, the particles are stable in water for 30 minutes or more.Preferably, the particles are stable in water for 1 hour or more, more preferably 2 hours or more, even more preferably 4 hours or more, and most preferably 8 hours or more.

[0291] Particle stability can also be assessed by examining the mass retention of the particles. Typically, after immersion in water for, e.g., an hour or more, the particles retain their integrity and lose less than 15% of their mass, preferably less than 10%, and even more preferably less than 5% of their mass.

[0292] Particle stability can also be assessed by examining the reflectance of the particles immersed in water: typically, the particles will have a reflectance similar to that in air, with the difference being that the reflectance peak may be red-shifted, as discussed above.

[0293] Use and applications The films or particles of the present invention can be used in a variety of different ways to replace known colorants such as dyes, pigments and glitters (Figure 17).

[0294] The film before the decomposition step may be cut to a specific size to obtain elements with a given shape, such as strips that can be applied as is or laminated, and for use as moisture sensors for food safety applications, as textile fibers, and in safety labeling. Thus, the present invention provides a safety label for anti-counterfeiting applications or moisture sensors, etc., comprising the structural color film or structural color particles of the present invention.

[0295] The particles are particularly suitable for use as pigments due to their ability to provide structural color, including visible, infrared, and ultraviolet colors. The following examples are not intended to be exhaustive, but merely to highlight some possible ways in which the particles can be used.

[0296] The particles may be used in cosmetic powders for application to the skin, including but not limited to blushers, body powders, bronzing powders, eye shadows, face powders, lip powders, powder makeup, and other cosmetic products where the particles are formulated in, for example, a fluid, oil or wax based system, including but not limited to bronzing products, eye pencils, eyeliners, face makeup, facial foundations, hair gels, hair pastes, hair sprays, lip glosses, lipsticks, mascaras, nail varnishes (see FIG. 20D), body washes, shampoos, shower gels, skin creams, sun creams and tanning products. Thus, the present invention provides cosmetic products comprising the structural color particles of the present invention.

[0297] The particles may be used in inks, paints, coatings, packaging, acoustic devices, heating and wavelength management applications such as radiative cooling, electrophoretic displays and devices, seasonal products, decorative uses, clothing, etc. For applications in cosmetics, these applications may require the addition of the particles to a host formulation.

[0298] The particles provide additional functionality and modify the non-optical properties of the host formulation, such as the rheological, textural or mechanical properties, thermal, acoustic and energy transfer capabilities, electrochemical, electromagnetic or electrophoretic properties of the host formulation. For example, the particles may provide color effects to the host formulation in cosmetic applications, and act as fillers, thickeners or exfoliating particulates in consumer and health care products, including cleaning and cosmetic compositions.

[0299] As food and beverage additives, the particles may be colorants and the host formulation may further comprise ingestible ingredients for use in the preparation of a food or beverage. Additionally, the particles may be used in drinks (FIG. 16E) or confectioneries such as chocolate (FIG. 20C).

[0300] For decorative, packaging and safety applications, the particle and host formulation can be applied onto paper, polymer, cardboard, molded fiber, stamps and labels, veneer, fixtures and lignin-containing materials. In addition, the particle and host formulation can be applied onto wood (FIG. 20E), metal ceramic, glass, clothing, fabric (FIG. 20B). The substrate with the particle and host formulation can be applied to cover part or the entire article, such as a stamp, label or foil. For clothing, the particles can be used as sequins. In addition, the particles can be used as confetti (FIG. 20A). The particles can be sprayed, sewn or attached to the substrate for the desired application.

[0301] The host formulation is preferably clear, but may contain other colorants, such as dyes or pigments, along with the addition of the particles of the present invention to produce more complex colored effects.

[0302] The host formulation may be viscous enough to keep the particles in place, or may have low enough viscosity to allow the particles to move freely. In the case of low viscosity liquids (Figure 12-A), the glossy appearance arises from random changes in the orientation of the particles in the medium, and the sample does not need to be viewed from different viewing angles to obtain the "sparkle" effect. In the case of viscous host formulations (Figure 12-C), the particles may give the host suspension a glossy appearance when viewed at different angles. Furthermore, the apparent pearlescence depends on the lighting conditions (Figure 12-E).

[0303] The size and size distribution of the particles, as well as the amount of particles used and their orientation in the host medium, determine the overall visual appearance resulting from the incorporation of the particles in the host formulation.

[0304] However, as shown in Figure 13, particles below a certain size embedded in a viscous host formulation generally give the transparent coating in which they are contained a more angle-independent optical response than particles above a certain size. For particles below a certain size, the macroscopic visual response of the host formulation containing the particles averages over multiple randomly oriented particles, resulting in a more uniform and continuous visual appearance. Figure 14 shows that the scattering response of several small dispersed particles embedded in a transparent polymer resin is similar to that of a flat, continuous cellulose nanocrystal film.

[0305] The particles may be dispersed in a solvent or other formulation depending on the intended use. The solvent 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, dimethylformamide, dimethylsulfoxide, DME, ethane, ethanol, ethyl acetate, ethylene, ethylene glycol, formic acid, glycerin, heptane, hexane, hexamethylbenzene, HMDSO, HMPA, hydrogen, imidazole, isobutanol, isopropyl alcohol, methane, methanol, n-hexane, nitromethane, n-pentane, propane, propylene, propylene carbonate, pyridine, pyrrole, pyrrolidine, silicone grease, tert-butyl alcohol, tetrahydrofuran, toluene, triethylamine, water, white spirit and xylene, or mixtures thereof.

[0306] Preferably, the solvent is a water-based solution or formulation.

[0307] The formulation may also be an emulsion, e.g., a water-in-oil emulsion, an oil-in-water emulsion, a double emulsion, e.g., a water-in-oil-in-water emulsion or an oil-in-water-in-oil emulsion, a gel, a latex, a resin or a viscoelastic polymer matrix, or another type of advanced formulation involving several materials, e.g., emollients, oils, polymers, surfactants and waxes, as appropriate for the intended application and use.

[0308] Suitable emollients include ammonium lactate, petrolatum, salicylic acid, and urea.

[0309] Suitable polymers include acrylates / steareth-20 methacrylate copolymers, aromatic polymers (e.g., polycarbonates, polyesters, polystyrene), Carbopol®, dimethylhydantoin-formaldehyde, hydrogenated polymers, hydrogenated polydecene, keratin, para-aramid, poloxamer, polyacrylamide, polyacrylonitrile, polyamino acids, polyamides (e.g., nylon 6, nylon 6,6, nylon 12), polyethers, polyolefins (e.g., but not limited to, polyethylene, polyisoprene, polypropylene, polybutadiene, polyethylene glycol), polypeptides, polymethacrylates, polymethylmethacrylate crosspolymers, polymethylsilsesquioxane, polyquaternium, silicones, silk fibroin, silk cerusine, ulvan, vinyl acetate, vinyl acetate / crotonic acid copolymer, methyl vinyl ether and maleic semester copolymer, vinylpyrrolidone. The polymer may be a cellulose or lignin derivative, such as cellulose acetate, cellulose nitrate, cellophane, nitrocellulose and celluloid. The polymer may be a starch derivative. The polymer may be a chitin derivative, a chitosan derivative or a sericin derivative. The polymer may be an alginate derivative, a carrageenan derivative, a collagen derivative, a gelatin derivative, a hyaluronic acid derivative or a pectin derivative. Preferentially, the polymer is synthesized from natural feedstocks and / or bio-based and / or renewable monomers, the resulting monomers being preferably biodegradable, such as aliphatic polyesters, such as poly(lactic acid), poly(ε-caprolactone), and poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Suitable polymers also include those listed above as possible additives for the nanocrystal suspension.

[0310] Suitable oils include algae oil, annatto oil, argan oil, almond oil, apricot kernel oil, avocado oil, babassu oil, brazil nut butter, butter, cashew butter, castor oil, camellia oil, cherry kernel oil, cocoa butter, coconut oil, corn oil, cottonseed oil, fish oil, grape seed oil, gardenia oil, cow's milk oil, 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 oil, rice bran oil, rosehip oil, sal oil, sesame oil, shea butter, soybean oil, squalene, sunflower oil, teas seed oil, walnut oil. Oil derivatives derived from the aforementioned oils, such as esterified oils, fatty acids, fatty alcohols, hydrogenated oils and triglycerides, can be used as suitable ingredients in the formulation. Essential oils are also suitable oils.

[0311] Suitable resins include tosylamide formaldehyde resins and toluene-sulfonamide-formaldehyde resins.

[0312] Suitable waxes include beeswax, candelilla wax, carnauba wax, Japan wax, lanolin, palm wax, and paraffin.

[0313] Suitable gels include any chemical and / or physical gels resulting from the use of thickening agents such as cellulose derivative thickeners, acacia gum, agar, aloe gel, gelatin, guar gum, gum arabic, tragacanth gum, pectin, sodium alginate, starch and xanthan gum. In addition, the gel may be a mixture of cellulose fibers.

[0314] Other priorities Every compatible combination of the above embodiments is expressly disclosed herein as if every combination was individually and explicitly recited.

[0315] Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

[0316] As used herein, "and / or" should be taken as a specific disclosure of each of the two particular features or components with or without the other. For example, "A and / or B" should be taken as a specific disclosure of (i) A, (ii) B, and (iii) each of A and B, just as if each were individually set forth herein.

[0317] Unless the context dictates otherwise, the descriptions and definitions of features set forth above are not intended to be limited to any particular aspect or embodiment of the invention, but apply equally to all aspects and embodiments described.

[0318] Certain aspects and embodiments of the present invention will now be described, by way of example, with reference to the above-mentioned drawings. EXAMPLES

[0319] Experiments and Results material An aqueous CNC suspension was purchased from the University of Maine, Process Development Center (Batch No. 2015-FPL-077, (CNC) = 11.8 wt%, neutralized form, sulfur content of 1.2 wt%).

[0320] PET reels for roll-to-roll coating were obtained from Mitsubishi Polyester Film Co., Ltd. as Hostaphan RN 500 (thickness = 500 μm, width = 140 mm).

[0321] PET for the lab-scale and slot-die coating experiments was obtained from HIFI Film PMX727 for the lab-scale experiments.

[0322] device Polarized optical microscopy was performed using a Zeiss Axio.Scope optical microscope equipped with a halogen lamp (HAL100 Zeiss, nominal range: 350-1100 nm) and a 20x objective (Zeiss EC Epiplan APOCHROMAT, NA = 0.3) to image the CNC films and a 10x objective (Zeiss EC Epiplan APOCHROMAT, NA = 0.6) to image the CNC photonic particles.

[0323] The light reflected by the CNC film attached to the substrate passes through a quarter wave plate and a switchable polarizing filter, which allows only left-handed or right-handed circularly polarized light (LCP and RCP, respectively) to pass through. A beam splitter was used to direct the light to a CCD camera (Thorlabs - DCC3240C) and through an optical fiber to a spectrometer (Avantes, AvaSpec HS2048).

[0324] When measuring the CNC coatings using a 20x objective, a 600 μm core optical fiber (Thorlabs, FC-UV600-2-SR) was used, whereas when measuring the CNC photonic microparticles using a 10x objective, a 200 μm core optical fiber (Thorlabs, FC-UV200-2-SR) was used, resulting in spectra acquired over approximately 100 μm and 66 μm wide spots, respectively.

[0325] Unless otherwise specified, all spectra are normalized to the reflection of a silver mirror (Thorlabs, PF10-03-P01) in one polarization channel (left circular polarization, LPC); a perfectly aligned cholesteric sample would be 100% reflective in the LCP channel.

[0326] Photographs of the CNC film were taken from the CNC film attached to the PET substrate, using a 20MP digital camera (Huawei P10) at a fixed working distance and with top lighting on a black background. Shielding material was placed around the periphery of the laboratory-scale PET film, and the CNC film was laid flat without covering the edges. Other images were taken using a 40MP digital camera (Huawei P30 Pro).

[0327] The thickness of the deposited CNC films was measured using a SEM (Tescan MIRA3 FEG-SEM) operated in high vacuum mode at 4 kV accelerating voltage and a working distance of 3-4 mm. Samples were mounted on aluminum stubs using conductive carbon tape and sputter coated (Emitech K550) using a palladium target. The results are shown, for example, in Figure 2-A.

[0328] Angle-resolved optical spectroscopy measurements were performed using a custom-built laboratory goniometer. A lamp (Thorlabs, SLS201L / M) was used as the light source, and a spectrometer (AvaSpec-HS2048XL, Avantes) was used to analyze the scattered light signal. The sample was mounted on a central rotation stage of the goniometer, and a collimated incident beam illuminated the sample surface (light spot size Φ ≈ 6 mm) (through an optical fiber Φ = 1000 μm). The detector was mounted on an arm attached to the motorized rotation stage, and the scattered light was coupled to an optical fiber (Φ = 600 μm) connected to the spectrometer. The recorded light intensity was normalized to a white Lambertian diffuser (Labsphere, USRS-99-010), and the exposure time was adjusted using an automated high dynamic range method44. Measurements were performed at a fixed incident light angle (θ , defined from the normal to the sample interface). in = 0° or 30°) at various exit angles θ out The scattering spectrum was recorded by scanning the intensity of the scattering spectrum collected by a rotating detector at .

[0329] Sweep rheology measurements were performed using a rotational rheometer (TA Instruments, DHR-2) equipped with a 40 mm parallel Peltier iron plate geometry, with a gap of approximately 900 μm, by applying the CNC suspension (1 mL) to a temperature-controlled stage (20 °C).

[0330] Cellulose nanocrystal suspension Aqueous CNC suspensions (see Materials) were diluted to 6% by weight with ultrapure water in 25 mL batches in Corning Falcon® tubes (50 mL) in an ice bath and sonicated using an ultrasonic disintegrator (Fisherbrand 505 Sonic Dismembrator, 500 W, amplitude = 40%, tip diameter = 12.7 mm).

[0331] Lab-scale blade cast CNC film suspensions were prepared in 25 mL batches and sonicated for 56, 109, and 161 seconds to produce blue, green, and red films upon casting, respectively. Large-scale R2R deposition suspensions were prepared in 45 mL batches, and in this case the sonication times were scaled up to 101, 196, and 290 seconds, respectively. This allows the energy delivered per volume of CNC suspension by the advanced sonicator to remain constant, corresponding to treatments of 2.24, 4.36, and 6.44 s / mL (or 7.5, 14.5, and 21.5 kJ / g).

[0332] The suspension was allowed to equilibrate at room temperature for 1-3 days, after which the denser anisotropic phase was separated and collected for further use.

[0333] By increasing the duration of the tip sonication process, the color of the dried CNC films could be red-shifted (see Figures 3 and 5), producing red, green, and blue films from tip sonication of 6.44, 4.36, and 2.24 s / mL (or 21.5, 14.5, and 7.5 kJ / g).

[0334] Substrate Preparation and Treatment Polyethylene terephthalate (PET) was used as the substrate for CNC film deposition, and the surface was selectively activated to control wetting.

[0335] A PET sheet (thickness = 125 μm, length = 120 mm, width = 80 mm, with a coatable width of 60 mm) was fixed on a coating stage for a lab-scale experiment. A plasma etcher was used to activate the surface (EMITECH K1050X plasma etcher, 50 W, 5 min).

[0336] In contrast, roll-to-roll deposition involved the use of a PET reel (thickness = 500 μm, width = 140 mm). A corona discharge (Corona Supplies, power = 0.3 kW) was used to activate the continuously moving substrate (speed = 0.1 mm.s -1 = 1.7 mm.s -1 In these examples, PET reels were manually shielded with tape (see FIG. 1A), which could be stacked continuously as suggested in FIG. 1A.

[0337] Deposition of photonic cellulose nanocrystal coatings Lab-scale CNC coatings were prepared using a custom-built film coater with a maximum coating length of approximately 30 cm. The coater contains a motor (Reliance Cool Motion Stage) capable of moving a flat stage along a track, on which a coating applicator (BEVS 1806 / A50) was mounted at a fixed position.

[0338] To prepare the CNC coating, the PET substrate was attached to the stage on three sides, leaving the trailing edge free so that excess CNC suspension could be removed by the coater. The blade was set to the desired height above the substrate, and the coating applicator was positioned near the leading edge of the rectangular PET sheet. The CNC suspension (3.5 mL) was deposited in front of the blade, and the stage was moved so that an area of ​​6 × 10 cm was uniformly coated (speeds of 0.65 to 2.4 mm / s and coating gaps of 300 to 1100 μm for the samples shown in the example example shown in Figure 2).

[0339] Using roll-to-roll coating, continuous deposition of the CNC suspension was achieved, demonstrating the scalability of the method. Roll-to-roll printing was achieved using a modified roll-to-roll coating system (Coatema Coating Machinery, Smartcoater 28) equipped with a custom-made slot die (coating width = 10 cm, internal reservoir 22 mL). The slot die was made of two aluminum plates separated by a 125 μm thick spacer shim and joined by screws, constituting a 100 mm slot gap aligned perpendicular to the web.

[0340] A syringe pump (New Era) was used to continuously feed the CNC suspension into the slot die, and the feed rate (approximately 6000 μL / min) was adjusted depending on the desired film thickness and coating width.

[0341] The distance between the slot opening and the substrate was controlled using a thickness feeler gauge.

[0342] The web holders were positioned such that the average distance between each holder was reduced to 30 cm. Prior to coating, the substrate was leveled using a bull's-eye level (Thorlabs LVL01) at the center of the width along the web path at multiple locations. The minimum available speed (speed = 0.1 mm.s) was used throughout the roll-to-roll system. -1 = 1.7 mm.s -1The web was translated at 1000 rpm. The shear rate was calculated from the web translation speed and the coating gap thickness.

[0343] Additionally, removal of the CNF film from the web is convenient so that the web can be reused multiple times in a closed loop fashion.

[0344] For "static drying", the minimum speed available (vc = 0.1m.min -1 The web was translated at vc = 1.67 mm.s-1), with a maximum casting length of about 3 m, corresponding to the web path limit. The web translation was then stopped and the deposited suspension was allowed to dry under ambient conditions. Alternatively, to investigate faster drying, a blast heating chamber (length ≈ 40 cm, = 20-60 °C) was placed between the R2R passes after the coating step. This allowed a stepwise deposition and drying process to be carried out in succession, where the web translation was divided into several steps interrupted by stationary rest periods. Two step sizes were demonstrated (denoted "coarse" and "fine"), and for the "coarse" process the web was moved at vc = 1.67 mm.s -1 In the "fibrillation" process, the web was translated at the same speed but in steps of 5 cm every 3.75 minutes. In both cases, this resulted in a eff ≒0.2 mm.s -1 corresponds to an effective translational velocity of (Figure 19).

[0345] Roll-to-roll printed films are shown in Figures 1B and 1C, as well as Figure 19. These show that the color development of films produced using a continuous roll-to-roll process is maintained, and in the case of Figure 1C, demonstrates the scalability of the method of the present invention.

[0346] The effect of coating gap and speed on color was studied. The coating gap and speed were varied and experiments were carried out as above. The results are shown in Figure 2. cFor a low coating gap of 300 μm, reasonable color development (peak height corresponds to 55% of the maximum LCP reflectance) can be achieved, indicating that the optical properties improve with increasing coating gap and speed. Figure 2 also shows the relationship between coating gap and resulting dry film thickness.

[0347] Drying For the plate cast films, lab-scale coatings, and large-scale R2R coatings, the CNC suspension was allowed to dry at ambient conditions without moving until film formation was complete (several hours, typically less than a day, the duration of which depends on the amount of material deposited, the length and thickness of the coated area). In addition, the lab blade coated films were also dried more quickly using a hot plate set at 60° C. The lowest speed available in the R2R machine (v c = 0.1m.min -1 ), the time required to dry the large scale CNC suspension at ambient conditions exceeded the available length of the pilot scale R2R machine. Thus, drying was either (i) performed stationary for several hours until a film formed, referred to in the discussion as "statically dried," or (ii) using the "staged continuous" translation process described above, the deposited CNC suspension was slowly translated through an in-line hot air dryer (T = 20-60 °C) to accelerate the drying process such that the film was dry before reaching the end of the web path.

[0348] The effect of drying rate on color development was studied. Heated laboratory blade-coated films were dried on a hotplate set at either 40°C or 60°C (Figure 3-A, bottom). The resulting films maintained color development, demonstrating that heat can be used to accelerate drying, which is industrially advantageous.

[0349] Additionally, the CNC film was dried on a perforated aluminum grid placed on a hot plate at 60° C. The results are shown in Figure 9. The temperature difference between the area above the aluminum board and the area above the air holes results in a differential color shift.

[0350] Breaking, annealing and size selection The conversion of the roll-to-roll film to CNC photonic particles was performed offline due to the limitations of the system used, but the conversion, including the annealing step, could be performed in-line on a heat-resistant conveyor belt as shown in Figure 1A.

[0351] The R2R CNC film was removed from the substrate by placing a thin plastic blade attached to the upper collecting web at an angle (as illustrated in Figures 1 and 16A) between the substrate and several centimeters of previously manually peeled CNC film and moving the web at a constant speed.

[0352] The CNC films were annealed (heat treated) in an oven (Nabertherm, P330) at 180° C. for 30 min.

[0353] A coffee grinder was used to cut the CNC film. The CNC polish particles were sequentially sized using sieves with decreasing mesh sizes of 150, 75, and 25 μm. The results are shown in Figure 4. The median particle size was obtained from the SEM images for each size category by highlighting the contours of individual particles and fitting them with the Feret area function in ImageJ.

[0354] The results show good color development in air and in multiple solvents for all particle sizes, with larger particles exhibiting the best optical properties.

[0355] The effect of annealing temperature was studied. The annealing temperature was varied and the experiments were carried out as above. The results are shown in Figure 7. The results show that annealing the film retains the optical properties up to 220°C. Thus, the annealing process can maintain the optical properties and provide a stable film for the destruction process. High temperatures resulted in carbonization of the film and it lost its color completely.

[0356] The decrease in peak reflectivity from the red film in Figure 3 can be attributed to the film thickness given the number of repeating chiral nematic units that reflect light. Figure 15-A clearly shows that a larger number of chiral nematic repeat units reflects more light from the chiral nematic structure. Thus, for a film to reflect larger wavelengths, such as red or infrared wavelengths, a thicker film is needed to reach the maximum reflectance from the cholesteric structure. It follows that the film thickness may be adjusted according to the wavelengths reflected from the film.

[0357] As analytical calculations show (FIG. 15-B), a film with a chiral nematic pitch centered at p = 318 nm (reflecting blue wavelengths) will reflect approximately 50% of the maximum amount of light that the chiral nematic structure is capable of if the film is 1.8 μm thick, but can reflect 95% of this maximum if the film is 4.5 μm thick, and can reflect more than 99% of the maximum if the film is more than 6.2 μm thick. A film with a chiral nematic pitch centered at 429 nm (reflecting red wavelengths) will reflect approximately 50% of the maximum amount of light that the chiral nematic structure is capable of if the film is 2.3 μm thick, but can reflect 95% of this maximum if the film is 6.1 μm thick, and can reflect more than 99% of the maximum if the film is more than 8.4 μm thick.

[0358] Additional uses for structural color films and structural color particles The films and particles prepared as described above were tested in a variety of applications, which are shown in FIG.

[0359] A) The structural color film was divided into square-shaped structural color particles. The particles were separated and presented on a piece of paper.

[0360] B) The structural color film was divided into particles and applied to a small piece of fabric using an adhesive.

[0361] C) Structural color particles were embedded in an edible host matrix and spread onto a small piece of chocolate.

[0362] D) The structural color particles were dispersed in a cosmetic nail varnish composition and used to paint fingernails.

[0363] E) Structural color particles were dispersed in a host polymer matrix and a piece of wood was painted.

[0364] References All documents mentioned within this specification are incorporated herein by reference in their entirety. Chowdhury, RA, et al. Continuous roll-to-roll fabrication of transparent cellulose nanocrystal (CNC) coatings with controlled anisotropy. Cellulose 25, 1769-1781 (2018). Frka-Petesic, B. & Vignolini, S. So much more than paper. Nat. Photonics 13, 365-367 (2019). Gicquel, E. et al. Impact of sonication on the rheological and colloidal properties of highly concentrated cellulose nanocrystal suspensions. Cellulose, 7, 7619-7634 (2019). Giese, M. et al. Responsive mesoporous photonic cellulose films by supramolecular cotemplating. Angew. Chemie - Int. Ed. 53, 8880-8884 (2014). Gray, D. G. Recent advances in chiral nematic structure and iridescent color of cellulose nanocrystal films. Nanomaterials 6, 213 (2016). Koppolu, R. et al. Continuous roll-to-roll coating of cellulose nanocrystals onto paperboard. Cellulose 25, 6055-6069 (2018). Lagerwall, J. P. F. et al. Cellulose nanocrystal-based materials: from liquid crystal self-assembly and glass formation to multifunctional thin films. NPG Asia Mater. 6, e80 (2014). Nan, F. et al. Enhanced toughness and thermal stability of cellulose nanocrystal iridescent films by alkali treatment. ACS Sustain. Chem. Eng. 5, 8951-8958 (2017). Park, J. H. et al. Macroscopic control of helix orientation in films dried from cholesteric liquid-crystalline cellulose nanocrystal suspensions. ChemPhysChem 15, 1477-1484 (2014). Parker, R. M. et al. The self-assembly of cellulose nanocrystals: hierarchical design of visual appearance. Adv. Mater. 30, 1704477 (2018). Parker, R. M. et al. Hierarchical self-assembly of cellulose nanocrystals in a confined geometry. ACS Nano 10, 8443-8449 (2016). Revol, J.-F. et al. Solid self-assembled films of cellulose with chiral nematic order and optically variable properties. J. Pulp Pap. Sci. 24, 146-149 (1998). Shafiei-Sabet, S. et al. Rheology of nanocrystalline cellulose aqueous suspensions. Langmuir 28, 17124-17133 (2012). WO 2017 / 091893 - Andrews, M. & Morse, T. Cellulose-based organic pigments. (2017). WO 2014 / 118466 - BARDET, R., BRAS, J., BELGACEM, N., Agut, P. & DUMAS, J. Procede de marquage d’un papier. (2014). WO 2018 / 033584 - VIGNOLINI, S., PARKER, R. & FRKA-PETESIC, B. Self-assembled nanocrystals. (2018). Zhang, Y. P. et al. Nanocrystalline cellulose for covert optical encryption. J. Nanophotonics 6, 063516 (2012). Zhao, T. H. et al. Coating of responsive photonic cellulose nanocrystal microfilm arrays. Adv. Funct. Mater. 29, (2019), 1804531.

Claims

1. a) depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate; b) spreading the nanocrystal suspension over the substrate using a spreader; c) aging the nanocrystal suspension to partially or completely restore the cholesteric structure lost during the deposition and spreading steps; d) drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form the structural color film; e) annealing the structural color film to increase the water resistance of the film; 10. A method for producing a structural color film, comprising:

2. 10. The method of claim 1, wherein the nanocrystal suspension comprises cellulose nanocrystals in neutralized, partially neutralized, or acidic form.

3. The method of claim 1 , wherein the nanocrystal suspension is biphasic or anisotropic.

4. The method described in claim 1, wherein the suspension has at least some anisotropy.

5. The method of claim 1, wherein the nanocrystal suspension contains cellulose nanocrystals at a concentration of 4 to 12% by weight.

6. The method described in claim 1, wherein the depositing process is a roll-to-roll printing process.

7. The method according to claim 1, wherein the drying step is carried out at a temperature of from 10 to 250°C, preferably from 10 to 70°C.

8. The method of claim 1 , further comprising the step of treating at least a portion of the substrate prior to the depositing step to modify the substrate to increase its surface energy.

9. The method of claim 8 , wherein the treatment in the treating step is plasma etching or corona discharge.

10. The method of claim 8 , wherein the treating step comprises treating a central portion of the substrate.

11. 10. The method of claim 1, further comprising sonicating the nanocrystal suspension prior to the depositing step, optionally at a rate of 0.1 to 45 s / mL, for example around 2.2 s / mL.

12. 10. The method of claim 1, wherein the cellulose nanocrystal suspension comprises at least one additive, the at least one additive being an acid or base, a filler, a polymer, a salt, or a functional molecule.

13. The method of claim 1 further comprising the step of peeling the structural color film from the substrate.

14. 2. The method according to claim 1, wherein the temperature of the annealing step is 100 to 250°C, preferably 140 to 220°C.

15. 10. The method of claim 1, wherein the annealing step is carried out for 1 minute to 120 minutes.

16. The method of claim 1 , further comprising dividing the structural color film to produce structural color particles.

17. 17. The method of claim 16, wherein the dividing step comprises breaking and / or crushing the structural color film.

18. Optionally, a structural color film obtainable by the method of any one of claims 1 to 15, wherein the film has a thickness of 1.0 to 50.0 μm.

19. A structural color film comprising cellulose nanocrystals, wherein the nanocrystals are organized in a chiral nematic structure, the film has a thickness such that the director of the chiral nematic structure rotates at least once within the film, and the film has a thickness of 20 μm or less.

20. A structural color film as described in claim 18, wherein the cellulose nanocrystals are neutralized cellulose nanocrystals.

21. 19. The structural color film of claim 18, wherein the film reflects 5% or more of incident light in the wavelength range of 200 to 1300 nm.

22. 20. The structural color film of claim 18, wherein the reflected light has a full width at half maximum of 150 nm or less.

23. Structural color particles obtained or obtainable by the method of claim 16 or 17.

24. A structural color particle comprising cellulose nanocrystals, the nanocrystals being organized in a chiral nematic structure, the particle having a faceted contour corresponding to at least one chiral nematic domain.

25. A structural color particle as described in Claim 24, wherein the cellulose nanocrystals are neutralized cellulose nanocrystals. (i) a median average particle diameter of 2 μm or more; and / or (ii) the particles reflect 5% or more of incident light at wavelengths in the range of 200 to 1300 nm; and / or (iii) the particles are stable to immersion in water for more than 1 hour; and / or (iv) The structural color particle according to claim 24, wherein the reflected light is red-shifted by 5 nm or more when immersed in water.

27. ​​The structural color particles of claim 24, wherein the structural color particles are provided in a composition, and the composition is a cosmetic, paint, coating, packaging, decoration, clothing, food, or beverage.

28. The structural color particle of claim 24, wherein the structural color particle is for use in cosmetics, paints, coatings, packaging, decorations, clothing, food, or beverages.

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