Optical filters

Abstract

The invention provides optical filter films comprising: a) a first colloidal crystal layer comprising polymer particles having a first average diameter; and b) a second colloidal crystal layer comprising polymer particles having a second average diameter wherein the first average diameter and second average diameter are different. The invention also provides coated substrates comprising a substrate coated with the optical filter films and methods of coating substrates with the optical filter films.

Description

[0001] OPTICAL FILTERS

[0002] Introduction

[0003] This present invention relates to transparent films that are able to block predetermined wavelengths of light, the films being formed from colloidal crystals of latex particles and 2D nanomaterials. The invention also relates to methods of making such films, their applications, as well as substrates onto which the films have been deposited.

[0004] Background to the Invention

[0005] In the last 20 years, there has been a significant expansion in the use of lasers for research, industry and medical use. The development of small but powerful diode lasers has enabled a wide range of devices to be innovated for many different applications. Improvements in quantum efficiencies and pumped laser technology have meant that these can be powered by fairly modest energy sources thus enabling lightweight devices which can be used for general beneficial intent.

[0006] Unfortunately, the grey market availability of importing cheap products via common trading platforms and various dark web sources around the world has resulted in proliferation of these devices into amateur and unregulated hands. For example, green laser pointers commonly used for professional presentations have been freely and cheaply (<$15) available with high outputs, up to 500mW in some instances. These are capable of inflicting serious damage if used inappropriately.

[0007] It has been long recognised that portable laser devices can blind or disrupt law enforcement agents for a relatively modest outlay. This is therefore of keen interest to those that wish to disrupt, protest and terrorise populations. Lasers can be used as weapons to cause long-term injury to pilots, law enforcement and security personnel with potentially devastating results for both military and civilian lives. They can also be used in protests and civil unrest to distract or cause flash-blindness, disrupting the security personnel's view of crowds and diverting their attention. Similar lasers can be used to temporarily flash-blind, or permanently disable cameras to try and shut down facial recognition systems used against protesters. The most obvious target which suffers long-term damage to laser radiation exposure is the eyes. The most effective means of protecting them is to wear goggles which are enabled by films of absorbing materials at certain frequencies. The increased use of laser target designators in urban unrest situations has exposed security forces’ operational staff to the risk of permanent and irreversible eye damage.

[0008] High-power, Class llla / lllb laser pointers and lasers for engraving and manufacturing are easily available and can be easily modified as short-range laser-directed energy weapons (LDEW). Recent experiences in urban environments have seen such items widely utilized with green lasers being especially popular as the eye is most sensitive at this frequency. Conventional laser protection equipment is highly effective at reducing the dosage to the eye, however these do not meet the additional requirements of personal protection equipment used by law enforcement agencies. Conventional laser protection equipment has low visible light transmittance which can lead to reduced visibility, reduced colour perception and can adversely affect hand-eye coordination. As a result, conventional laser protection equipment is not acceptable for use on personal protective equipment used by law enforcement agencies.

[0009] In addition to visible frequencies, infra-red lasers exist which may cause more harm to vision as the eye's blink response is not capable of responding in time to prevent damage.

[0010] Eye goggles, protected by films of absorbing or reflecting materials designed to absorb / reflect light at certain frequencies, have been the traditional means of protection.

[0011] US 2021 / 246330 A1 and US 2021 / 199865 A1 describe multi-colloidal structures, which comprises a plurality of types of colloidal particles of various sizes, dispersed in a resin matrix that fixes the colloidal particles in place.

[0012] Jurewicz et al., Adv. Funct. Mater. (2020), 30, 31 , 2002473 refers to the fabrication and use of opal-like photonic crystals containing graphene, the photonic crystals comprising evaporation-driven self-assembled soft polymer colloidal particles. However, there exists the need for further films that are able to block predetermined wavelengths of light that can be used as films that offer users protection against laser light.

[0013] Therefore, an object of the present invention is to provide films that are able to block predetermined wavelengths of light, but with high visible light transmittance in order to avoid the disadvantages associated with known laser protection equipment. Specifically, the films should have a high attenuation of specific harmful wavelengths but maintain a high overall visible light transmittance (VLT) at wavelengths outside the laser wavelength to ensure clear vision. Such films therefore avoid the disadvantages associated with known laser protection equipment which has low overall VLT.

[0014] Summary of the Invention

[0015] The inventors of the present application have found that films comprising monodispersed colloidal latex polymeric particles, which form a crystalline structure, and 2D nanomaterials, which occupy interstitial sites within the crystalline structure have good optical properties. The resultant films possess low transmissibility at structure-defined wavelengths but allow transmission at all other wavelengths, thereby achieving high resonant reflectivity and high off-resonance transmission simultaneously. As such they can be used as notch filters for multi-wavelength laser protection applications.

[0016] Accordingly, in a first aspect, the invention provides an optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first diameter; and b) a second colloidal crystal layer comprising polymer particles having a second diameter.

[0017] Optionally, the first diameter and second diameter may be the same or different. In embodiments where the first diameter and second diameter are the same, an interlayer is present between the first colloidal crystal layer and the second colloidal crystal layer. In embodiments where the first diameter and second diameter are different, it is preferred that an interlayer is present between the first colloidal crystal layer and the second colloidal crystal layer.

[0018] Accordingly, the invention also provides an optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first average diameter; and b) a second colloidal crystal layer comprising polymer particles having a second average diameter; wherein an interlayer is present between the first and second colloidal crystal layers, and wherein each colloidal crystal layer further comprises one or more species of 2D-layered nanomaterials.

[0019] In a second aspect, the invention provides an optical filter film comprising: a) a colloidal crystal layer comprising polymer particles; b) a first nanomaterial occupying interstitial sites in the colloidal crystal layer; and c) a second nanomaterial occupying interstitial sites in the colloidal crystal layer.

[0020] The optical filter film is generally provided onto a substrate. The substrate may be a high optical transmission polymer or glass. In some embodiments, the substrate may be, or form part of, premanufactured goggles, an outsert or an observation window. Alternatively, the optical filter film may be deposited onto a flexible substrate. The flexible substrate may be a high optical transmission flexible polymer film. After the optical filter film has been deposited onto the flexible substrate, it may be cut to shape and then adhered to a product of interest (e.g. premanufactured goggles, an outsert or an observation window, as noted above). The flexible substrate may be formed from materials including polycarbonate, polyester, silicone or cellulosic materials. The flexible films may be provided with an adhesive on the side opposite to the optical filter film to facilitate attachment to the product of interest.

[0021] The optical filter film may also be provided on a transfer plate for applying the optical filter film onto a product. The transfer plate may comprise a flexible polymer substrate which has been treated with the requisite surface energy characteristics to allow effective transfer by adherence and subsequent peeling of the transfer plate backing.

[0022] Further provided by the invention is a method of coating a substrate with an optical filter film as defined above, the method comprising: a) depositing onto the substrate an aqueous dispersion of polymer particles having a first diameter and optionally one or more nanomaterials to form a first colloidal crystal layer; b) allowing the first colloidal crystal layer to dry; c) optionally repeating steps a) and b) with an aqueous dispersion of polymer particles having a second, third or further diameter and optionally one or more nanomaterials to form a second, third or fourth colloidal crystal layer.

[0023] The method may comprise an additional step (step d)) of applying a high optical transmission protective coating over the colloidal crystal layers to encapsulate the optical film layers. This protective coating protects the colloidal crystal layers from damage from the environment.

[0024] Independently, the method may comprise an additional step (step b-ii) of applying an interlayer between adjacent colloidal crystal layers. For example, the method may comprise depositing an interlayer between the first colloidal crystal layer and the second colloidal crystal layer. In embodiments where there are third or further colloidal crystal layers, there may be an interlayer between each pair of adjacent colloidal crystal layers.

[0025] The invention further defines the use of an optical filter film as defined above for selectively blocking predetermined wavelengths of visible and infra-red light. Detailed Description of the Invention

[0026] Colloidal Crystals

[0027] Colloidal crystals consist of highly ordered assemblies of monosized colloidal particles. One of the major limiting factors of colloidal crystals for certain applications is their opaque nature. The origin of the opacity is structural disorder causing strong incoherent scattering that generates diffuse light as well as a low refractive index contrast.

[0028] In a periodic structure, light is reflected at each interface where there is a modulation of the optical properties. The reflected light may experience a phase shift which affects whether there is constructive or destructive interference with light reflected from other interfaces in the structure. When a wave is reflected in going from a medium with a lower refractive index to a higher refractive index, the reflected wave undergoes a phase shift of IT. When a wave is reflected in going from a medium with a higher refractive index to a lower refractive index, the reflected wave does not undergo a phase shift. When the optical path length between two consecutive layers is equal to a multiple of the wavelength (and there is no phase shift), the light is reflected and does not propagate through the structure. Due to the periodic structure, the majority of light at that wavelength is light reflected from the structure is constant leading to constructive interference which is observed as a peak in reflection. The fraction of light reflected can approach 100% over a narrow band of wavelengths, depending on the number of repeated layers in the structure and their optical properties. Changing the layer thickness (and therefore the spacing of the layer transitions) through the structure, or producing several films with different properties, can allow manipulation of the bandwidth and reflectivity of the complete structure.

[0029] Colloidal crystals have a refractive index that changes periodically, i.e. they have a periodic dielectric profile, which can prevent light of certain wavelengths propagating through the colloidal crystal. Light may be prevented from propagating in one or more polarisation directions in the colloidal crystals. Disallowed wavelengths in colloidal crystals are referred to as photonic band gaps. Colloidal crystals of the present invention are selected to have one or more particular photonic band gaps such that one or more wavelengths commonly used for lasers are blocked. For example, a colloidal crystal of the invention may have one or more photonic band gaps in the range of 300nm to 800nm, preferably in the range of 400nm to 700nm, for example selected from 450nm, 532nm and / or 635nm.

[0030] The colloidal crystals of the invention comprise monosized colloidal particles. The colloidal particles define a close-packed structure, and more preferably a hexagonal close-packed (hep) structure.

[0031] Preferably, the monosized colloidal particles of the colloidal crystals are polymer particles.

[0032] Optical filters of the invention may comprise one or multiple layers of different colloidal crystals. Different colloidal crystal layers may comprise polymer particles of the same or different polymers.

[0033] If present, the different colloidal crystals of different layers of the optical filters will have different photonic band gaps and therefore will prevent propagation of different wavelengths of light. As such, using additional layers of colloidal crystals in the optical filters can increase the number of wavelengths blocked by the optical filters of the invention.

[0034] Polymers

[0035] In the colloidal crystal layers, the polymer particles are regularly arranged.

[0036] Accordingly, the polymer particles are preferably substantially monodisperse. The polymer particles may be viewed as being substantially monodisperse if they have a Polydispersity Index (PDI) from dynamic light scattering (DLS) of less than 0.4, more preferably less than 0.3 or less than 0.2, and most preferably less than 0.1 or less than 0.05. The calculation of the PDI from DLS is provided in the ISO standard document ISO 22412:2017.

[0037] Alternatively, or additionally, the polymer particles may be viewed as being substantially monodisperse if they have a percentage polydispersity of less than 30%, more preferably less than 25%, and most preferably less than 20%. The percentage polydispersity is derived from the PDI.

[0038] In order for the colloidal crystal layers to be prepared at ambient temperatures, the polymers typically have a thermal glass transition temperature (Tg) of from 10°C to 120°C, preferably from 10°C to 80°C, for example 10°C to 50°C. If the glass transition temperature is too low, then the colloidal crystal becomes difficult to form due to an increased tendency of particle surfaces to coalesce into a continuous film. Alternatively, if the glass transition temperature is too high, then the polymer is too brittle to form the colloidal crystal.

[0039] Similarly, the polymers typically have a melting temperature (Tm) of at least 10°C greater, or at least 15°C greater or at least 20°C greater, than Tg, this prevents the spontaneous agglomeration into coherent transparent polymer films with no optical diffraction properties.

[0040] Polymer particles of the invention should be formed from optical-grade polymers. Optical grade polymers are highly transparent polymers that are optically clear and have low haze.

[0041] The polymers may be homopolymers (formed from a single type of monomer) or copolymers (formed from two or more different monomers). Examples of suitable polymers include poly meth(acrylates), polycarbonates, poly cyclo-olefins, polystyrene and poly acrylic acids. Preferably, the polymer particles are formed from co-polymers of poly meth(acrylates) with various pendant chain groups, including linear and branched alkanes (methyl, butyl, propyl) and poly (styrenes). For example, the polymers may be formed of monomers of styrene-acrylic or acrylic co-polymers (e.g. methyl methacrylate, butyl acrylate, and methacrylic acid)

[0042] The polymers may also be core-shell polymers. Core-shell polymers are polymers where an outer coating polymer differs from an interior core polymer material. Suitable coreshell polymers may have an outer coating polymer having a lower glass transition temperature (Tg), (e.g. -40°C or below) for example butyl acrylate and polyethyleneadipate polymers. The core polymers may be those having a higher glass transition temperature (Tg), (e.g. 105°C or above) for example methyl methacrylate or styrene polymers.

[0043] The polymers may be produced within a stable dispersion in water as a synthetic latex. Suitable polymers include those that are formed from largely hydrophobic monomers. Example of suitable polymers include poly meth(acrylate)s, poly(cyclo-olefin)s, polystyrenes and poly(acrylic acid)s.

[0044] Preferably, the polymers may form a latex wherein the polymer particles are dispersed in water. A latex refers to a stable dispersion of polymer particles in water. It is preferred that the colloidal crystal layers are produced from a latex. In embodiments where the colloidal crystal layers are produced from a latex, the polymer particles are latex particles, wherein latex particles describe polymer particles which form a stable dispersion in water.

[0045] Colloidal crystal layers can also be formed by dispersing polymer particles in non-polar mediums such as monomer resins. The refractive index contrast between resin mediums and colloidal polymer particles is relatively small, due to the similar chemical compositions. Polymers (i.e. cured monomer resin and particles) typically have refractive indices in a limited range of 1 .30 - 1 .70, therefore there is a limited degree of achievable contrast. The limited refractive index contrast weakens Fresnel reflections at polymer particle interfaces, reducing the photonic bandgap performance of the colloidal crystal.

[0046] Conversely, the use of a polar medium (water) to disperse polymer particles enables the possibility of to achieve a higher refractive index contrast between the colloidal polymer particles and the bulk medium in which they are dispersed. Following deposition as a latex, and water is removed via evaporation from the structure. The colloidal crystal contains a mixture of polymer particles, that are partially coalesced for mechanical integrity, but also comprise trapped residual water, dopants (e.g. nanomaterials), stopband modifiers (e.g. plasticisers and / or interstitial spacers), and void space. Therefore, enabling the opportunity for higher refractive index contrast between polymer particles (1.30 - 1.70), dopants (1.2 to 5.5 at 2 eV), and voids (1.0), it has been found that this enhanced refractive index contrast observed in latex colloidal systems gives rise to more selective filtering of light and enhanced reflectance of light at select wavelengths.

[0047] The polymers may be formed in a latex using conventional techniques such as emulsion polymerisation, mini-emulsion polymerisation or graft polymerisation techniques. These techniques result in the desirable monodispersity of the polymer particles and when the polymer is a core-shell polymer, enables manipulation of the core and shell dimensions of the polymers.

[0048] The polymer particles may have an average particle size of between 50 nm and 1100 nm or between 100 nm and 600 nm, more preferably between 150 nm and 450 nm or between 200 nm and 400 nm, and most preferably is between 210 nm and 380 nm, between 220 nm and 360 nm, between 230 nm and 340 nm, between 240 nm and 320 nm or between 250 nm and 300 nm.

[0049] The polymer particles in each layer of the film typically have different diameters such that each film is able to block a different wavelength of visible light. However, generally the polymer particles have a diameter of from 100nm to 600nm, typically from 200nm to 400nm.

[0050] Alternatively for near-infra-red (NIR) blocking film functional properties, the particles may be composed of larger particles typically in the region of 500nm to 1000nm, preferably between 500 and 800 nm.

[0051] Typically, the diameter of polymer particles in adjacent colloidal crystal layers differs by at least 2%, preferably by at least 5%, more preferably by at least 10% (e.g. at least 15%). The percentage by which the diameter of the polymer particles differs between adjacent colloidal crystal layers is measured as a percentage of the smaller polymer particles, i.e. the larger of the average diameters of the polymer particles in adjacent colloidal crystal layers is X% greater than the smaller of the average diameters of the polymer particles in adjacent colloidal crystal layers, where X% may be 2%, preferably 5%, more preferably 10% (e.g. at least 15%). For example, the first colloidal crystal layer may comprise polymer particles with a first average diameter that is at least 2% greater than the second average diameter of polymer particles in the second colloidal crystal layer. In an alternative embodiment, the second average diameter is at least 2% greater than the first average diameter.

[0052] The particle size may be determined using a dynamic light scattering technique and / or by atomic force microscopy. It is noted that the diameter of polymer particles may not be consistent both within each polymer particle (i.e. because each polymer particle is unlikely to be exactly spherical) and between different polymer particles of the same polymer. Diameters of the polymer particles therefore refer to the average diameter (e.g. mean diameter) of polymer particles of a particular polymer.

[0053] In some examples, the film comprises colloidal crystal layers comprising: a) polymer particles having a diameter of from 364nm to 384nm; and / or b) polymer particles having a diameter of from 303nm to 323nm; and / or c) polymer particles having a diameter of from 255nm to 275nm; and / or d) polymer particles having a diameter of from 230nm to 250nm.

[0054] Nanomaterials

[0055] The nanomaterials may occupy the interstitial sites formed by packing the polymer particles in the colloidal crystal layers. The nanomaterials impart advantageous properties to the colloidal crystal layers, and thereby the optical films of the invention. For example, broad-band absorbers may be included to improve the optical properties of the films (e.g. by reducing haze and deleterious scatter). In addition, nanomaterials which act as index contrast modifiers (which are characterised by low optical absorption) may be present since they can be used to enhance reflectance. Different (2 or more) nanomaterials may be added to combine these index contrast modifying and absorbing effects for maximum functional benefit.

[0056] The nanomaterials may be nanoparticles, nanotubes or nanoplatelets. The nanomaterials may have a refractive index (at 2eV) of from 1.2 to 5.5 within the desired wavelength range. For applications for blocking visible light, the desired wavelength range is typically from 380nm to 700nm. For near-infrared applications, the desired wavelength range is typically from 380nm to 900nm.

[0057] The nanomaterials may be OD, 1 D or 2D nanomaterials.

[0058] OD nanomaterials are materials that do not have any dimensions outside of the nanometre range (for example, do not have any dimensions greater than 100nm). Examples of such nanomaterials are nanomaterials which are of 10Onm or less in all three dimensions. Examples of OD nanomaterials include but are not limited to quantum dots, carbon black, fullerenes, metal particles, noble metal particles, and inorganic particles.

[0059] 1 D nanomaterials are materials that have one dimension outside of the nanometre range. In other words, 1 D nanomaterials have two dimensions which are of 100nm or less and a dimension which is greater than 100nm. Examples of 1 D nanomaterials include nanotubes (such as carbon nanotubes), and metal nanowires.

[0060] 2D nanomaterials are materials that have two dimensions outside of the nanometre range. In other words, 2D nanomaterials have one dimension which is of 100nm or less and two dimensions which are greater than 100nm. Examples of 2D nanomaterials include nanoplatelets, such as carbon nanoplatelets. Such nanoplatelets have a thickness of 10Onm or less, but typically have a length and width of greater than 1 pm.

[0061] The 2D nanomaterials are typically nanoplatelets having a layered structure formed from stacked sheets, as in graphene. Suitable 2D-layered nanomaterials include graphene, graphene oxide, hexagonal boron nitride (h-BN), transition metal carbides / nitrides (also referred to as MXenes), mica a transition metal dichalcogenides. Transition metal dichalcogenides have the general formula MX2, where M is a transition metal and X is a chalcogen. The transition metal dichalcogenide may be selected from molybdenum disulphide (M0S2), tungsten disulphite (WS2), molybdenum diselenide (MoSe2), tungsten diselenide (WSe2) and molybdenum (IV) telluride (MoTe2). When the nanomaterials are nanoplatelets, the nanoplatelets may have on average 20 or fewer layers, typically 15 layers or fewer, preferably 10 or fewer layers. Layer numbers can be determined by UV-vis spectroscopy (see C. Backes et al., 'Spectroscopic metrics allow in-situ measurement of mean size and thickness of liquid-exfoliated graphene nanosheets', Nanoscale, 2016, doi: 10.1039 / C5NR08047A).

[0062] The nanoplatelets typically have an average thickness of less than 50nm or less than 30nm, for example less than 20nm. The term "thickness" as used herein refers to the dimension of the nanoplatelets along the axis of stacking of the layers within the nanoplatelets. The terms "length" and "width" refer to the longer and shorter dimensions of the nanoplatelets along perpendicular axes in the plane of the sheets of the layered materials respectively (see Figure 3). The nanoplatelets typically have an average length and / or width of 30nm or greater, preferably 50nm or greater or 100nm or greater. The nanoplatelets typically have an average length and / or width of 3.0pm or less, for example 2.0pm or less, typically 1 ,5pm or less, preferably 1 pm or less, for example 800nm or less. The dimensions of the nanoplatelets can be measured using scanning or transmission electron microscopy.

[0063] It has additionally been found that the size of the nanomaterials is critical to allow refractive index contrast enhancement without leading to deleterious scattering or affecting packing of the polymer particles. Enhancing refractive index contrast allows high resonant reflectivity to be achieved for laser-wavelength personal protective equipment.

[0064] As discussed above, the nanomaterials occupy the interstitial sites in the packed polymer particle matrix of the colloidal crystal layers. Accordingly, within each colloidal crystal layer, the nanomaterials may have one or more, for example two or more, preferably three or more dimensions which are 1 / 8th, typically 1 / 10th, for example 1 / 15thof the diameter of the polymer particles within that layer. It will be appreciated that for 1 D nanomaterials (such as carbon nanotubes), due to their flexibility, they may fold into a bundle having a diameter which allows it to fit within an interstitial site of the polymer particle matrix, despite the length (i.e. the longest dimension) of the 1 D nanomaterial being larger than the dimensions of the interstitial spaces. The combination of nanomaterials may be chosen from a set of refractive index modifying particles versus absorbing species according to Figure 8. Preferably the nanomaterials are 2D nanomaterials, for example 2D-layered nanomaterials.

[0065] Preferably the 2D-layered nanomaterials are selected from graphene, hexagonal boron nitride and transition metal dichalcogenides.

[0066] Typically, the same species of nanomaterials are present in the two or more colloidal crystal layers in the film. For example, the first, second and where present third and fourth polymeric crystal layers may all comprise graphene and boron nitride nanoplatelets.

[0067] The films described herein comprise one or more "species" of nanomaterials. For the purpose of the present application, a "species" of nanomaterial refers to nanomaterials sharing the same chemical composition, i.e. nanomaterials formed from the same chemical elements, preferably also in the same relative amounts. For example, a composition containing only graphene nanomaterials would be described as containing a single "species" of nanomaterial (even though the composition would likely contain a very large number of nanomaterials, e.g. graphene molecules). A composition which comprises both graphene nanomaterials and boron nitride nanomaterials would be described as a composition containing two "species" of nanomaterials.

[0068] Preferably each layer of the colloidal crystal comprises two or more species of nanomaterials in addition to the polymer particles. For example, each layer of the colloidal crystal comprises three or more or four or more species of nanomaterials in addition to the polymer particles.

[0069] The addition of multiple species of nanomaterials is considered to be beneficial for the properties of the optical filter films. The addition of a highly absorbing material (such as graphene or CNTs) allows the unwanted scatter to be absorbed. Furthermore, the addition of another nanomaterial (such as boron nitride) allows the overall refractive index and refractive index in the system to be raised without necessarily increasing the absorption and darkening the film. Examples of highly absorbing nanomaterials include those shown in Fig. 8 as broadband absorbers and include graphene, MoSe2, M0S2, WS2, WSe2, TisCh, TiO2 and MoTe2, as well as carbon nanotubes. Nanomaterials that can act as index contrast modifiers include those marked as such in Fig. 8, such as BN, NiOCI, Ni(OH)2, Mg3Si4O (OH)2, TiO2 and MoTe2.

[0070] Additionally, the nanomaterials may possess either physio or chemically reactive groups intrinsically within the material and / or on the surface or which serves to allow the reversible or non-reversible absorption of molecules. This change allows the modification of refractive index upon exposure to reactive gases.

[0071] The weight ratio of the polymer to the nanomaterials may be between 100:0.002 and 100:5, more preferably between 100:0.005 and 100:1 , between 100:0.01 and 100:0.5 or between 100:0.05 and 100:0.7.

[0072] Alternatively, or additionally, the volumetric ratio of the polymer to the nanomaterials may be between 100:0.001 and 100:0.01 , more preferably between 100:0.002 and 100:0.08 or between 100:0.003 and 100:0.007, and most preferably between 100:0.004 and 100:0.006.

[0073] The light filtering efficiency of optical filters can be quantified by comparing the minimum transmittance (i.e. at the wavelength of interest, i.e. the stopband wavelength) to the baseline transmittance (i.e. at wavelengths where no selective filtering is taking place). This is termed the on / off ratio. A greater on / off ratio shows a more selective filter that reflects a greater proportion of light at the stopband while transmitting a greater proportion of light outside the stopband. stopband transmittance (%)

[0074] On / Off ratio = - — - — - - - — background transmittance (%)

[0075] The addition of nanomaterials, specifically 2D-layered nanomaterials, has been found to enhance the peak reflectance of the optical filter films while maintaining a low background reflectance. This enables the optical filter films of the present invention to selectively filter out light at specific wavelengths while transmitting visible light more efficiently (i.e. have a greater on / off ratio).

[0076] Since 2D-layered nanomaterials can act as index contrast modifiers, they have the ability to enhance the refractive index contrast between components of the colloidal system.

[0077] The colloidal crystal layers may also be provided with surfactants to assist with dispersion of the nanomaterials within the (e.g. latex, water-based) polymeric particles.

[0078] Preferably, the polymeric opal comprises a surfactant.

[0079] The surfactant preferably comprises a non-ionic surfactant. In one embodiment, the nonionic surfactant may comprise a compound having the following formula: wherein n is from 1 to 20 and / or a polysorbate. The non-ionic surfactant may comprise triton X-100 and / or polysorbate 80 and is preferably comprises triton X-100.

[0080] The volumetric ratio of the polymer to the surfactant may be between 100:0.0001 and 100:2, more preferably between 100:0.001 and 100:1 or between 100:0.01 and 100:0.75, and most preferably between 100:0.4 and 100:0.6.

[0081] The volumetric ratio of the polymer to the non-ionic surfactant may be between 100:0.0001 and 100:2, more preferably between 100:0.001 and 100:1 or between 100:0.01 and 100:0.75, and most preferably between 100:0.4 and 100:0.6.

[0082] Stopband Modifiers

[0083] Colloidal crystal layers of the invention may comprise stopband modifiers that provide fine control over the stopband of a colloidal crystal layer. Stopband modifiers when present, occupy the interstitial sites of a colloidal crystal layer. Stopband modifiers may be selected from interstitial spacers and / or plasticisers.

[0084] Interstitial spacers are present in the interstitial sites formed by packing the polymer particles in the colloidal crystal layer. Interstitial spacers reside in the matrix in the interstitial sites increasing the average distance between each particle. The volume of the interstitial spacer added causes an increase in the volume of the interstitial space in that layer. For example, interstitial spacers might increase the average particle-particle distance by 2% or more greater, preferably 10% or more greater or 15% or more greater. Interstitial spacers therefore typically result in an increase in the unit cell volume. The increase in the unit cell volume is typically 25% or greater, typically 35% or greater, preferably 50% or greater.

[0085] Since interstitial spacers lead to an increase in the unit cell volume, they typically cause an increase in the absorbance maxima (i.e. a red-shift).

[0086] Suitable interstitial spacers can be polyols (such as glycerol and propylene glycol, or derivatives thereof) or polar polymers, such as polyethylene glycol, cellulose and substituted celluloses (such as carboxymethylcelluose, hydroxypropyl cellulose, ethyl methyl cellulose and methyl cellulose) and derivatives thereof.

[0087] Plasticisers are molecules that reduce the average diameter of interstitial spaces between polymer particles in a colloidal crystal layer. Without wishing to be bound by theory, plasticisers reduce the average volume of the interstitial space by diffusing into the edges of the polymer particles and attracting the polymer particles towards each other. Plasticisers may lead to a decrease in the average particle to particle distance by of 5% or greater, preferably 10% or greater, for example 15% or 20% of greater. Plasticisers therefore typically result in a decrease in the unit cell volume. The decrease in the unit cell volume is typically 15 % or greater, preferably 20 % or greater, for example 25% or greater.

[0088] Since plasticisers lead to a decrease in the unit cell volume, they typically cause a decrease in the absorbance maxima (i.e. a blue-shift). Suitable plasticisers can be selected from esters and ethers (for example those having a molecular weight of 250g / mol or less), such as 2-butoxyethanol, Texanol (2,2,4-trimethyl- 1 ,3-pentanediol monoisobutyrate, Coasol (nylonate ester), ethylene glycol ethyl hexylether (EEH), di(propylene glycol) n-butyl ether (DPnB) and ethylene glycol ethyl ether acetate (EEA). Other suitable plasticisers include plasticising phthalates and alkane diols.

[0089] The use of stopband modifiers can modulate the stopband maxima of a colloidal crystal layer by + / - 30nm or greater, preferably + / - 50nm or greater (e.g. + / - 70nm). As such, stopband modifiers allow fine adjustment of the reflectance maxima. Here, the term stopband maxima refers to the wavelengths where the stopbands have the greatest intensity. This fine adjustment can be used to moderate natural variation in colloidal particle size due to the manufacturing process.

[0090] Preferably stopband modifiers are water soluble. Preferably stopband modifiers have a low vapour pressure. Preferably stopband modifiers have low solvent compatibility with organic polymers used in latex dispersions.

[0091] The presence of a stopband modifier in one layer is independent of the presence of a stopband modifier in another layer. In some embodiments, different stopband modifiers may be present in different colloidal crystal layers. In other embodiments, some colloidal crystal layers may comprise stopband modifiers, and other layers may not comprise stopband modifiers.

[0092] Overall Construction

[0093] Each layer of colloidal crystal within the optical filter film typically has a thickness of from 2pm to 800pm, typically from 5pm to 500pm, for example from 10pm to 100pm. It will be appreciated that the optimal thickness of each layer will depend to some extent on the sizes of the polymer particles therein. Accordingly, each colloidal crystal layer typically has a thickness at least 3 times greater, preferably at least 5 times greater, for example 10 times greater than the diameter of the polymer particles in that layer. In some examples, the optical filter film has an overall thickness of up to 100pm, for example up to 50pm.

[0094] Interlayer

[0095] In embodiments of the invention, an interlayer may be present between the first colloidal crystal layer and the second colloidal crystal layer. In embodiments where the optical filter film comprises additional colloidal crystal layers (e.g. third or fourth colloidal crystal layers), an interlayer may independently be present between each pair of adjacent colloidal crystal layers.

[0096] For example, in one embodiment of the invention there is provided an optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first average diameter; and b) a second colloidal crystal layer comprising polymer particles having a second average diameter; wherein an interlayer is present between the first and second colloidal crystal layers, and wherein each colloidal crystal layer further comprises one or more species of 2D-layered nanomaterials.

[0097] In embodiments where the first diameter and second diameter are the same, an interlayer must be present between the first colloidal crystal layer and the second colloidal crystal layer.

[0098] Therefore, in some embodiments the invention provides an optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first diameter; b) an interlayer between the first colloidal crystal layer and second colloidal crystal layer; and c) a second colloidal crystal layer comprising polymer particles having a second diameter; wherein the first diameter and second diameter are the same.

[0099] By using a number of colloidal crystal layers, each comprising polymer particles having the same diameter, a film with a single stopband can be formed. Stacking multiple colloidal crystal layers, each comprising polymer particles having the same diameter, gives rise to a film with a single stronger stopband.

[0100] Optical filter films of this structure enable selective transmission of all other wavelengths while strongly absorbing a single selected wavelength. This is preferable when a single wavelength wants to be blocked selectively.

[0101] In embodiments where the first diameter and second diameter are different, it is preferred that an interlayer is present between the first colloidal crystal layer and the second colloidal crystal layer.

[0102] Preferably the interlayer has a high transparency and low-haze to avoid interfering with the optical properties of the film. In addition, the interlayers should be non-porous and non-soluble with a low water vapour permeability. The interlayers should also conform to the curvature or shape of a substrate (such as goggles, visors, windows or screens) and should be stable up to temperatures of use (typically, up to 40-45°C, or in extreme scenarios up to 100°C).

[0103] The presence of an interlayer between colloidal crystal layers is desirable for improving adhesion between the colloidal crystal layers and improving the ease of production of the optical filter films. Depositing an interlayer between colloidal crystal layers leads to improved control for deposition of second and subsequent colloidal crystal layers, therefore easing production.

[0104] An interlayer may additionally reduce haze and improve the clarity of the optical filter film.

[0105] The interlayer may be formed from a high optical transmission material. Preferably the interlayer is formed from a polymer film or polymer resin. For example, the interlayer may be formed from UV curable epoxy, 2 part-silicones, polycarbonates, poly(urethanes), poly(esters), poly (acrylates), poly meth(acrylates) or poly (sty rene-acry lies).

[0106] Typically, an interlayer has a thickness of from 2pm to 100pm, for example from 5pm to 50pm, typically from 8pm to 15pm.

[0107] Protective Coating

[0108] In preferred embodiments, the optical filter film may comprise a protective coating over the colloidal crystal layers. This protective coating protects the colloidal crystal layers from damage from the environment (e.g. mechanical damage or damage as a result of UV light).

[0109] In addition to protecting the coating from damage, a protective coating may also reduce haze and improve the clarity of the optical filter film. Additionally, a protective coating may planarise the optical filter film (i.e. result in a smoother finish to the film).

[0110] The protective coating may be formed from a high optical transmission material. Preferably the protective coating is formed from a polymer film or polymer resin. For example, the protective coating may be formed from UV curable epoxy, 2 part-silicones, polycarbonates, poly (urethanes), poly(esters), poly (acrylates), poly meth(acrylates) or poly(styrene-acrylics).

[0111] Typically, a protective coating has a thickness of from 2pm to 100pm, for example from 5pm to 50pm, typically from 8pm to 15pm.

[0112] The protective coating may be deposited on the outermost colloidal crystal layer. The protective coating may encapsulate the colloidal crystal layers and, if present, interlayers of the optical filter film.

[0113] By using a number of colloidal crystal layers, each comprising polymer particles having different diameters, a film having several, different stopbands can be formed. Each colloidal crystal layer may comprise a stopband in the range of from 140nm to 1280nm.

[0114] In one embodiment, there is provided an optical filter film comprising two or more of the following layers: a) a first colloidal crystal layer having a stopband of from 625nm to 645nm; b) a second colloidal crystal layer having a stopband of from 522nm to 542nm; c) a third colloidal crystal layer having a stopband of from 440nm to 460nm; and d) a fourth colloidal crystal layer having a stopband of from 395nm to 415nm. wherein each colloidal crystal layer comprises polymer particles and one or more species of nanoplatelets as defined herein.

[0115] In a further embodiment, there is provided an optical filter film comprising two or more of the following layers: a) a first colloidal crystal layer comprising polymer particles having a diameter of from 364nm to 384nm and one or more species of nanoplatelets; b) a second colloidal crystal layer comprising polymer particles having a diameter of from 303nm to 323nm and one or more species of nanoplatelets; c) a third colloidal crystal layer comprising polymer particles having a diameter of from 255nm to 275nm and one or more species of nanoplatelets; d) a fourth colloidal crystal layer comprising polymer particles having a diameter of from 230nm to 250nm and one or more species of nanoplatelets; wherein the polymer particles and / or one or more species of nanoplatelets are as defined herein. In these specific embodiments, the labelling of the first to fourth colloidal crystal layers does not necessarily imply any specific order of the layers and the layers may be present in the film in any order.

[0116] Substrate

[0117] The optical films described herein may be applied to a substrate.

[0118] The substrate may be a transparent polymer, (such as an optical grade polymer film) e.g. a poly methyl(methacrylate), a polycarbonate or a polycyclo-olefin.

[0119] Alternatively, the substrate may be an opaque polymer, e.g. a PET, polypropylene, polymethyl(acrylate), a polycarbonate or a polycyclo-olefin.

[0120] In order to adhere the film to the substrate, an adhesion promotor may be provided between the substrate and the first colloidal crystal layer. The adhesion promotor may be an adhesive layer, for example a layer comprising a pressure-sensitive adhesive.

[0121] Once applied to a substrate, the optical filter may also be provided with a protective layer in order to protect the colloidal crystal layers from damage during use. Protective layers may be composed of a clear polymer layer (commonly termed an oversheet) with additional gas barrier coatings such as titanium or aluminium oxide to prevent excessive water vapour or oxygen ingression into the film. Oversheets may also include UV- absorbing materials to prevent degradation of the films under high UV exposure. Examples of such UV-absorbing materials include metal oxides (such as titanium dioxide or zinc oxide), polymers (such as polyurethane), UV curing resins and UV blocking glasses, such as Plexiglass (UF, MC or G).

[0122] Alternatively, sprayable, transparent and planarising polymer coatings may be used to afford the same effects as described above. These layers may exhibit different mechanical properties such as elasticity, viscoelasticity and hardness depending on the performance required. Examples of suitable sprayable transparent coatings include acrylic clear lacquer sprays, UV photocuring resins, reactive silicones, and polyurethanes.

[0123] Methods of Making the Films The optical filter films described above can be prepared by depositing the two or more colloidal crystal layers sequentially onto a substrate.

[0124] Accordingly, the invention also provides a method of coating a substrate with an optical filter film as described herein, the method comprising: a) depositing onto the substrate an aqueous dispersion of polymer particles having a first diameter and optionally one or more nanomaterials to form a first colloidal crystal layer; b) allowing the first colloidal crystal layer to dry; c) optionally repeating steps a) and b) with an aqueous dispersion of polymer particles having a second, third or further diameter and optionally one or more nanomaterials to form a second, third or fourth colloidal crystal layer.

[0125] The method may comprise an additional step (step b-ii) of applying an interlayer between adjacent colloidal crystal layers. For example, the method may comprise depositing an interlayer between the first colloidal crystal layer and the second colloidal crystal layer. In embodiments where there are third or further colloidal crystal layers, there may be an interlayer between each pair of adjacent colloidal crystal layers.

[0126] Therefore, in some embodiments the invention provides a method of coating a substrate with an optical filter film, the method comprising: a) depositing onto the substrate an aqueous dispersion of polymer particles having a first diameter and optionally one or more nanomaterials to form a first colloidal crystal layer; b) allowing the first colloidal crystal layer to dry; b-ii) depositing an interlayer onto the first colloidal crystal layer; c) repeating steps a) to b-ii) with an aqueous dispersion of polymer particles having a second, third or further diameter and optionally one or more nanomaterials to form a second, third or fourth colloidal crystal layer.

[0127] The methods may comprise an additional step (step d)) of applying a protective coating over the colloidal crystal layers to encapsulate the optical film layers.

[0128] The interlayer and / or protective coating may be deposited onto the colloidal crystal layers by flow coating, spray coating or dip coating.

[0129] Alternatively, the two or more colloidal crystal layers may be deposited onto a transfer plate and then transferred from the transfer plate to the substrate.

[0130] Accordingly, the invention provides a method of making an optical filter film as described herein, said method comprising: a) forming a film comprising two or more colloidal crystal layers comprising polymer particles and one or more species of nanomaterials on a transfer plate; b) providing the substrate; c) optionally applying an adhesive to the substrate; d) transferring the film from the transfer plate to the substrate.

[0131] This method may additionally include one or both of the steps of (i) applying an interlayer between adjacent colloidal crystal layers, and (ii) providing a protective coating over the colloidal crystal layers, as described above.

[0132] As will be appreciated when the colloidal crystal layers are transferred from the transfer plate to the substrate, their order will be reversed. In other words, the layer most proximal to the transfer plate will be the layer most distal from the substrate and vice versa. Accordingly, order for the colloidal crystal layers is applied to the transfer plate in the opposite order in which they are to be deposited onto the substrate. An adhesive, such as a pressure-sensitive adhesive may be applied to the substrate to adhere the film comprising the colloidal crystal layers to the substrate. Examples of suitable pressure-sensitive adhesives include butyl acrylate or acrylic co-polymer, polyvinyl ethers, synthetic rubbers and styrene copolymers.

[0133] In alternative embodiments, the invention may provide a method of coating a substrate with an optical filter film, the method comprising: a) depositing onto the substrate an aqueous dispersion of polymer particles having a first diameter and two or more nanomaterials to form a first colloidal crystal layer; b) allowing the first colloidal crystal layer to dry.

[0134] The films are typically deposited from an ink comprising the polymer particles, the nanomaterials, the additives described above and a solvent.

[0135] The non-miscible solvent may be an aqueous or non-aqueous solvent. However, the solvent preferably is or comprises water. Alternatively, the solvent may be a dipolar aprotic solvent. Examples of such dipolar aprotic solvents include cyclopentanone, cyclohexanone, N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylsulphoxide (DMSO), dimethylacetamide (DMAc), sulpholane, dihydrolevoglucosenone (Cyrene) and lactones, such as gamma-valerolactone.

[0136] The solvent may also include other film forming additives to improve the rheological or drying properties of a formulated ink according to the method of manufacture. For example, the addition of alcohols, amines, or glycols, with significant vapour pressures allows for the self-assembling process to occur whilst the film dries. Typically, these solvents comprise between 0.5% and 30% of the total ink volume.

[0137] The solvent may also include rheological modifiers (thickeners) and / or mobilisation agents of low vapour pressure to improve the printing characteristics. Such modifiers include polyethylene glycols (PEG), polysaccharides, polyurethanes, fumed silica or cellulose derivatives (carboxymethyl cellulose, chitosan). Mobilisation agents (plasticisers) may be utilised where films need a degree of mechanical flexibility to the film to prevent cracking under drying or thermal load exposure, examples include branched alcohols such as glycerine, alkane diols or phthalates. Typically, these agents comprise between 0.01 % and 5% of the total ink volume.

[0138] The colloidal crystal layers described herein may be deposited using a variety of techniques including flow coating, blade coating, spray-coating, dip-coating, drop coating, slot-die coating, screen printing, and flexographic printing.

[0139] The preferred deposition method may depend on the surface geometry of the substrate. When the substrate is planar, the colloidal crystal layers are preferably deposited by blade coating, slot-die coating, screen printing, or flexographic printing. When the substrate has a curved surface, the colloidal crystal layers are preferably deposited onto a transfer plate and subsequently transferred onto the substrate. When the substrate has a complex multidimensional surface geometry, the colloidal crystal layers are preferably deposited by flow coating, dip-coating or spray-coating.

[0140] Flow coating is an alternative method of depositing the colloidal crystal layers onto the substrate. Flow coating may be used in addition to dip / spray coating to apply the colloidal crystal layers to a substrate with 3D curvatures. Flow coating requires no major ink formulation differences or procedural differences to dip coating.

[0141] Thermoforming can also be used to coat colloidal crystal layers onto 3D curvatures. Colloidal crystal layers can also be thermoformed on 2D flat surfaces, thus enabling films to be made with the desired thickness, making it easier to control the optical properties which are closely linked to film thickness.

[0142] Additionally in the case of complex multidimensional surface geometry, the colloidal crystal layers may also be preferably deposited onto a planar substrate, preferably deposited by flow coating, blade coating, slot-die coating, screen printing, or flexographic printing. An interlayer may then be applied to the film surface, and the films may then be thermoformed to a multidimensional curvature. An adhesive layer may then be used to apply the coating to the desired final application surface.

[0143] Additionally multi-layers can be produced by applying the coating to a planar substrate, preferably deposited by blade coating, slot-die coating, screen printing, or flexographic printing. An interlayer may also be applied to surface of the coating, on which additional coating layers can be stacked. Once the desired layer structure is achieved the multilayer stack can be collectively thermoformed to the desired multidimensional curvature, an adhesive layer may then be used to adhere the multi-layer to the final application surface.

[0144] Applications

[0145] The optical filter films of the invention can be used in a variety of applications where selective filtering of certain wavelengths of light is desirable.

[0146] As described herein, the optical filter films can be used on protective goggles to reflect laser light at given wavelengths that are harmful to the eyes. The high selectivity of the films of the present invention prevents harmful wavelengths of light from reaching the eye but allows other wavelengths of light to be transmitted through the filter such that it is generally transparent in nature and does not obscure the user’s view through the film.

[0147] Similarly, as described the optical filter films can be used on other surfaces to reflect laser light at given wavelengths that are harmful to the eyes. These surfaces may include transparent materials such as windows, more specifically automobile windows, or aircraft windows, or building windows. The optical filter films can be used on other surfaces to reflect laser light at given wavelengths that may dazzle or damage recording equipment such as cameras, CCTV, or recording equipment.

[0148] Other applications of the films applied to eyewear include for use in sports. In certain situations, it could be advantageous to block certain wavelengths of light to highlight other colours and make certain coloured objects more visible. For example, in golf, it may be desirable to “dial out” some of the green light reaching the user’s eye to accentuate the appearance of other coloured objects on a golf course. Similarly, for underwater applications, dialling out blue light may make seeing other coloured objects easier.

[0149] Although the films can be deposited onto eyewear, the films also find uses in other applications. For example, the films could be incorporated into food packaging to provide a transparent packing, which allows the user to see the content of the packaging, whilst preventing wavelengths of light which may be responsible for degradation of the food product(s) within the packaging.

[0150] Brief Description of the Drawings

[0151] Fig. 1 is a schematic drawing of an optical filter film of the invention (not to scale).

[0152] Fig. 2 is a schematic drawing of an optical filter film of the invention (not to scale).

[0153] Fig. 3 is a schematic diagram showing the respective width, length and thickness parameters of graphite nanoplatelets.

[0154] Fig. 4 is a schematic representation of a multi-wavelength optical filter film for a laser protection visor.

[0155] Fig. 5 is a UV / Vis spectrum for the optical filter film of Fig. 4.

[0156] Fig. 6 is a schematic representation of a monochromatic optical filter film for a laser protection visor.

[0157] Fig. 7 is a UV / Vis spectrum for the optical filter film of Fig. 6.

[0158] Fig. 8 is a graph showing the refractive index (at 2eV) against the electronic band gap for a range of layered nanomaterials.

[0159] Examples

[0160] Example 1 - Optical Filter Films of the Invention

[0161] Figure 1 shows a schematic of an optical filter film 9 of the invention on a polycarbonate substrate 10. The optical filter film 9 has a first colloidal crystal layer 11 comprising polymer particles of poly(methyl methacrylate) and 2D graphite nanoplatelets. The optical filter film 9 additionally features a poly(urethane) interlayer 12 adjacent both the first colloidal crystal layer 11 , and a second colloidal crystal layer 13. The second colloidal crystal layer 13 comprises polymer particles of poly (butyl acrylate) and 2D boron nitride nanoparticles.

[0162] Figure 2 shows a second embodiment of an optical filter film 19 of the invention on a polycarbonate substrate 20. The optical filter film 19 has a first colloidal crystal layer 21 , interlayer 22 and second colloidal crystal layer 24. The materials of the first colloidal crystal layer 21 , interlayer 22 and second colloidal crystal layer 24 are the same as for equivalent layers 11 , 12, 13 in Figure 1. Additionally, the optical filter film 19 has a protective coating 24 of UV curable epoxy resin.

[0163] The optical filter films 9 and 19 are formed by first depositing onto the flat substrate 10, 20 an aqueous dispersion of poly(methyl methacrylate) and 2D graphite nanoplatelets to form a first colloidal crystal layer 11 , 21 by blade coating. Once the first colloidal crystal layer 11 , 21 has dried, the poly(urethane) interlayer 12, 22 is then deposited by spray coating. Once the interlayer 12, 22 has dried, an aqueous dispersion of poly(butyl acrylate) and 2D boron nitride nanoparticles is deposited to form a first colloidal crystal layer 13, 23 by blade coating.

[0164] In the embodiment shown in Figure 2, a final protective coating 24 of UV curable resin is deposited by spray coating once the second colloidal crystal layer has dried. The UV curable resin is subsequently exposed to UV light to cure the protective coating.

[0165] Example 2 - Use of a multi-wavelength optical filter film in a laser protection visor

[0166] An optical filter film of the invention was integrated into existing visors to mitigate direct exposure to directed laser equipment. The integrated film was composed of multiple encapsulated colloidal crystal layers which were attached to the inside or outside of a high transparency, UV protective and impact-resistant substrate usually made from poly(methyl methacrylate) (also referred to as PM MA or Perspex®) or poly(carbonate) based materials. To achieve the preferred mitigation performance, combinations of different optical filter films may be used.

[0167] First, a green film layer was manufactured with a stop band maximum (fgreen) of 532nm. To manufacture the film, a polycarbonate substrate of thickness 175pm was coated with an aqueous suspension of (i) 50% (v / v) polymer latex spheres of average diameter 215nm, (ii) 0.005 wt. % Few Layer Graphene (FLG), (iii) 17.5% glycerol humectant which acts as a stop band modifier. This was doctor blade coated using an automated applicator and was allowed to dry under a controlled temperature of 23°C and relative humidity of 50% to produce a dry film with a thickness of approximately 100pm. An encapsulation layer of polycarbonate of thickness 175pm is then placed on the top.

[0168] A blue film layer was manufactured with a stop band maximum (fbiue) of 480nm. To manufacture the film, a polycarbonate substrate of thickness 175pm was coated with an aqueous suspension of (i) 50% (v / v) latex spheres of average diameter 190nm, (ii) 0.005 wt. % Few Layer Graphene (FLG), (iii) 3.5% butyl glycol cosolvent, (iv) 13.5% of glycerol humectant which acts as a stop band modifier. This was doctor blade coated using an automated applicator and allowed to dry under controlled temperature of 23°C and relative humidity of 50% to produce a dry film with a thickness of approximately 100pm. An encapsulation layer of polycarbonate of thickness 175pm was then placed on the top.

[0169] A red film layer was manufactured with a stop band maximum (fred) of 675nm. To manufacture the film, a polycarbonate substrate of 175pm was coated with an aqueous suspension of (i) 50% (v / v) latex spheres of average diameter 276nm, (ii) 0.01 wt.% siloxane wetting agent, (iii) 17.5% of glycerol humectant which acts as a stop band modifier. This was doctor blade coated using an automated applicator and allowed to dry under controlled temperature of 23°C and relative humidity of 50% to produce a dry film with a thickness of approximately 120pm. An encapsulation layer of polycarbonate of thickness 175pm was then placed on the top.

[0170] Multiple individual layers were then grouped together into an optical filter film shown in Fig.4. The optical filter film has in order a mechanical support layer 100, a polycarbonate based interlayer 101 , a blue film layer 102, two polycarbonate based interlayers 101 , a green film layer 103, two polycarbonate based interlayers 101 , a red film layer 104, two polycarbonate based interlayers 101 , a red film layer 104, a polycarbonate based interlayer 101 , and a second mechanical support layer 100.

[0171] The resulting integrated optical filter film was analysed for its optical performance using a UV / Vis spectrometry. The UV / Vis spectrum for the optical filter film of Fig. 4 is shown in Fig. 5 and shows that the resultant optical filter film has stopbands of 532nm, 480nm and 675nm (i.e. within the green range, blue range and red range of the visible light spectrum respectively). Fig. 5 shows an optical density of around 2.0 for wavelengths of around 400nm and 700nm and optical densities of greater than 1.5 for wavelengths around 500nm.

[0172] Example 3 - Use of a monochromatic optical filter film

[0173] In an alternative embodiment, an individual monochromatic layer was assembled to optimise the optical density (OD) for the desired application.

[0174] A green film was manufactured with a stop band maximum (fgreen) of 532nm. To manufacture the film, a polycarbonate substrate of thickness 100pm was coated with an aqueous suspension of (i) 50% (v / v) polymer latex spheres of average diameter 215nm, (ii) 0.005 wt. % Few Layer Graphene (FLG), (iii) 17.5% glycerol humectant which acts as a stop band modifier. This was doctor blade coated using an automated applicator and allowed to dry under controlled temperature of 23°C and relative humidity of 50% to produce a dry film with a thickness of approximately 100pm. An encapsulation layer of polycarbonate of thickness 175pm was then placed on the top.

[0175] The individual layer was then stacked together into an optical filter film shown in Fig. 6. The optical filter film has, in order, a polycarbonate mechanical support layer 110, a green film layer 113, and a second polycarbonate mechanical support layer 110.

[0176] The resulting integrated optical filter film was analysed for its optical performance using UV / Vis spectrometry. The UV / Vis spectrum for the optical filter film of Fig. 6 is shown in Fig. 7 and demonstrates that the film has a stopband of 532nm (i.e. within the green range of the visible light spectrum) with an optical density of around 2.0. By comparing the background optical density with the peak optical density, an on / off transmittance ratio of around 71.3 was observed.

Claims

CLAIMS1 . An optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first average diameter; and b) a second colloidal crystal layer comprising polymer particles having a second average diameter; wherein the first average diameter and second average diameter are different, and wherein each colloidal crystal layer further comprises one or more species of 20- layered nanomaterials.

2. An optical filter film comprising: a) a first colloidal crystal layer comprising polymer particles having a first average diameter; and b) a second colloidal crystal layer comprising polymer particles having a second average diameter; wherein an interlayer is present between the first and second colloidal crystal layers, and wherein each colloidal crystal layer further comprises one or more species of 2D-layered nanomaterials.

3. An optical filter film according to claim 1 or 2 wherein the polymer particles are latex particles.

4. An optical filter film according to any one of claims 1 to 3 further comprising a third colloidal crystal layer comprising polymer particles having a third average diameter.

5. An optical filter film according to any one of claims 1 to 4 wherein the weight ratio of the polymer particles to the 2D-layered nanomaterials in each colloidal crystal layer is from 100:0.002 to 100:5.

6. An optical filter film according to any one of claims 1 to 5 wherein the volumetric ratio of the polymer particles to the 2D-layered nanomaterials in each colloidal crystal layer is from 100:0.001 to 100:0.01.

7. An optical filter film according to any one of claims 1 to 6 wherein the one or more species of 2D-layered nanomaterials are selected from graphene, boron nitride, transition metal dichalcogenides, or a combination thereof.

8. An optical filter film according to any one of claims 1 to 7 wherein the one or more species of 2D-layered nanomaterials are transition metal dichalcogenides.

9. An optical film according to any one of claims 1 to 8 wherein the one or more species of 2D-layered nanomaterials are located in the interstitial sites of the colloidal crystal layers.

10. An optical filter film according to any one of claims 1 to 9 wherein the polymer particles have a thermal glass transition temperature of from 10°C to 120°C, preferably from 10°C to 60° C.

11. An optical filter film according to any one of claims 1 to 10 wherein the polymer particles are particles of acrylic co-polymers.

12. An optical filter film according to any one of claims 1 to 11 wherein the polymer particles have an average diameter of from 100 to 600nm, preferably from 200 to 400nm.

13. An optical filter film according to any one of claims 1 to 12 wherein each colloidal crystal layer has a thickness of from 2pm to 100pm (e.g. from 2pm to 20pm).

14. An optical filter film according to any one of claims 1 to 13 wherein each colloidal crystal layer comprises a stopband in the range of from 140nm to 1280nm.

15. An optical filter film according to claim 1 or any claim dependent thereon, wherein the larger of the first average diameter and second average diameter is at least 2% greater than the smaller of the first average diameter and second average diameter, preferably at least 5% greater.

16. An optical filter film comprising two or more of the following layers: a) a first colloidal crystal layer comprising polymer particles having an average diameter of from 364nm to 384nm and one or more species of 2D-layered nanomaterials; b) a second colloidal crystal layer comprising polymer particles having an average diameter of from 303nm to 323nm and one or more species of 2D-layered nanomaterials; c) a third colloidal crystal layer comprising polymer particles having an average diameter of from 255nm to 275nm and one or more species of 2D-layered nanomaterials, and d) a fourth colloidal crystal layer comprising polymer particles having an average diameter of from 230nm to 250nm and one or more species of 2D-layered nanomaterials; wherein the polymer particles and / or one or more species of 2D-layered nanomaterials are as defined in any one of claims 1 to 15.

17. A coated substrate comprising a substrate coated with an optical filter film according to any one of claims 1 to 16.

18. A coated substrate according to claim 17 further comprising an adhesion layer (e.g. a layer of a pressure-sensitive adhesive) between the substrate and the optical filter film.

19. A method of coating a substrate with an optical filter film according to any one of claims 1 to 15, the method comprising: a) depositing onto the substrate an aqueous dispersion of polymer particles having a first average diameter and one or more species of 2D layered nanomaterials to form a first colloidal crystal layer; b) allowing the first colloidal crystal layer to dry;c) optionally depositing an interlayer onto the first colloidal crystal layer; d) repeating steps a) and b) with an aqueous dispersion of polymer particles having a second, third or further average diameter and one or more species of 2D-layered nanomaterials to form a second, third or fourth colloidal crystal layer.

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