The present invention relates to compositions of encapsulated smart materials, methods of making said compositions and articles or products comprising said compositions, for example inks or coatings to make security documents, and to detect counterfeiting. Preferred compositions using a negative photochromic coating are particularly useful as security features.

The present invention also relates to the use of said compositions in a range of new applications, articles and/or devices with desired material properties and associated methods of making the same. Particular examples of compositions of encapsulated materials and articles containing said materials includes inks, documents, anti counterfeiting materials, and systems comprising the same.

Background of the Invention There is a need for new and improved methods for incorporating or encapsulating functional or reactive molecules into materials systems while retaining a local environment necessary for molecule stability and functionality such that their unique properties may be exploited. Photochromic substances are substances which undergo a reversible change in colour or shade when exposed to light of a particular wavelength or intensity. Pigments which exhibit colour switching are useful in a number of industrial applications such as anti-counterfeiting, display screens and other optical devices. For example US5807625A describes a security document, banknotes, with reversibly photochromic printing inks. US5155607A describes an optical modulation display device and display method in which either a mixture of liquid crystal plus photochromic dispersed in polymer or a liquid crystal dispersed in a photochromic polymer. US20120309761A1 provides rapid reverse photochromies due to the ability to control the nano- and micro- environment surrounding the photochromic agent. A polymer with interconnected pores (10-100 nm) forms a continuous network. The photochromic agent may be contained in the pores in the polymer without leaching as long as some of the internal sections of the pores are narrow enough to trap the photochromic.

Negatively photochromic materials which are coloured and bleach (go colourless) in light are less well known or used in industry. A review by Aiken et al. (2018) Dyes and Pigments, 149, 92-121 discloses a range of negatively photochromic dyes and pigments.

Julia-Lopez et al. (2019) ACS. Appl. Mater. Interfaces, 11, 11884-11892, describes solid materials with tuneable reversible photochromies made using a reaction free emulsion-solvent evaporation method wherein the material of interest to is dissolved in a colour developing mixture of biphenol A, heptanoic acid, pentanoic acid and dihydoxy- terminated poly-(dimethylsiloxane -co-diphenylsiloxane (PDMS- OH).

There is a need for more new and improved products and methods for formulating a wider range of photochromic materials, particularly negatively photochromic materials and mixtures thereof which can be difficult to formulate with existing methods.

Current encapsulation technologies work for polymers with low flexural moduli and lower glass transition temperatures and other polymers which are traditionally not compatible with photochromic and other sensitive smart materials.

The present inventors have found that the afore-mentioned problems may be addressed by using the encapsulation method devised by the inventors. This two- phase encapsulation is applicable for any system where the polymer is immiscible with the material to be encapsulated. A problem with some smart materials requiring encapsulation in polymer or resin systems is that functionality is lost due to inability of the molecule to survive direct incorporation into polymers or resin systems, for example Van der Waals bonds causing some molecules (e.g. proteins) to fold in specific geometries are generally not supported in these environments.

Summary of the Invention

Photochromic coatings are disclosed which are particularly used for the manufacture of security inks and coatings. The coatings are normally strongly coloured and become colourless when irradiated by visible radiation such as by daylight. This ‘photo-bleaching’ is reversible.

Methods in the art can be used to prepare the inks or coatings to make security documents, and to detect counterfeiting.

This invention is particularly useful in the field of security coatings and security printing inks and preferably related to manufacture of document security features, which when exposed to electromagnetic radiation such as light, will give a reversible colour fade or bleaching called negative photochromism.

The present invention provides a coating composition, preferably inks, comprising a negative photochromic compound which normally have an intense colour and a high degree of photosensitivity and demonstrate a rapid and reversible photo-bleaching on exposure to daylight. The present invention provides negatively photochromic coatings and printing inks particularly suited to applications in the field of document security and prevention or detection of forgery. The present invention provides methods fortesting documents which have been marked with a print obtained from the coating and printing inks of this invention.

The present invention provides a rapid reset function to the negative photochromic response, whose colour may be reset under UV irradiation. The present invention further provides the production of negatively photochromic coatings by complex coacervation. Complex coacervation is known for the development of carbonless copy-paper, and it is now commonly used throughout the pharmaceutical industry. The technique involves the use of two oppositely charged polymeric materials, usually gelatine and acacia. Typically, the core material is dispersed in a solution of gelatine and water above 45°C (where the droplet size is determined by the rate of mixing), and subsequently diluted by a water and acacia solution. As the temperature is reduced, and hence the pH of the mixture changed, the two polymers form a complex, causing the shell material to deposit around the core particles. Finally, the mixture is cooled to below 10°C and a crosslinking agent added to harden the polymer shell.

It is a further object of the present invention to overcome at least some of the afore mentioned problems and, inter alia, to provide compositions and products which are suitable for use in a wide range of applications, in particular in document security and other materials and products with colour switching properties, particularly those able to exhibit reversible colour changes and a range of materials that are able to house biomolecules, including bioactive molecules in such a way that maintains or enhances their function or utility.

In a further aspect there is provided compositions, coatings and articles, and methods of preparations thereof according to the appended claims.

Any specific and/or preferred features referred to herein are applicable to all of the aspects of the present invention.

In one aspect the material to be encapsulated are of a high purity grade, for example greater than about 95 wt% pure, or greater than about 96 wt% pure, or greater than about 97 wt% pure, or greater than about 98 wt% pure.

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawing(s) showing embodiments(s) of the invention.

Figure 1 shows a schematic method of preparation of the formulation. Figure 2 shows optical microscopy of capsules prepared according to the invention. Detailed Description of the Invention

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Like reference numerals in the drawings refer to like elements throughout. According to the invention a stable emulsion is formed where the sample of interest is dissolved in solvent A (solution 1) and dispersed throughout a bulk solution of polymer in solvent B (solution 2). The emulsion is then cast onto substrates, the polymer cures encapsulating liquid droplets of pigment in solvent A throughout the polymer films.

Care must to taken to ensure the emulsion is stable. Droplets of solution 1 cannot be allowed to coalesce since this will lead to poorly-dispersed material in polymer or full phase separation. Stability is affected by temperature. Some emulsification methods (e.g. sonication or other types of high energy mixing) can increase temperatures so may be alleviated by chilling of components.

Other variables include the ratio of solution 1 to solution 2 typically ratio 1:5 but can vary. Other ratios include 1:10 preferably 1:8 more preferably 1:5. According to the present invention solvent B evaporates faster than solvent A (i.e. polymer should cure before the emulsion loses stability and droplets coalesce), solution 1 and 2 must be immiscible (or very poorly miscible). Solution 1 should be concentrated (particularly relevant for photo/thermos/electro- chromic pigments or smart biomaterials e.g. green fluorescent protein (GFP)).

Material in solvent A should not have an affinity for the polymer (and vice versa).

Biomolecules are generally most stable in buffer solutions (aqueous, saline, with pH 5-8). As such, there is a need for a method for incorporating biomolecules into materials systems whilst retaining a local environment necessary for biomolecule stability so that long range order effects of the biomolecules can be realised on a system level. There is a need for a simple encapsulation method for biomaterials with retained functionality and stability. Currently known encapsulation methods use silica.

Encapsulation of fluorescent proteins such as Green Fluorescent Protein (GFP) and other proteins offers major opportunities in research and development, in particular in biotechnology and medicinal chemistry. By way of illustration, Green Fluorescent Protein has been encapsulated in polymer systems according to the present invention and its functional activity demonstrated by measuring its retained fluorescence using optical spectroscopy. Had fluorescence been lost this would indicate denaturing of the protein and that the encapsulation method is not suitable for these systems.

For purely synthetic materials, a variety of pigment systems have been encapsulated in immiscible polymer solutions and imaged under optical microscopy to detect the presence of capsules. Particular success has been achieved with polymers with high flexural moduli and high glass transition temperature (Tg), such as high impact polystyrene, engineering plastics (e.g. polycarbonate), and PMMA which have been precluded by currently available methods.

Examples

Example 1

Experiments on different solvent systems for positive or negative photochromies. The table below shows the results of experiments using different solvents with a variety of polarities.

Concentration Key

HC 0.002g/2mL

LC 0.002g/4mL

ULC 0.002g/8mL

Notes: where “?” there is a possibility that system will show a photochromic switch if diluted further - Polystyrene (PS) (MW 192000 g/mol) at 7 wt% in toluene was used for initial experiments.

Example 2 - Positive Photochromic - two phase encapsulation process Samples of encapsulated positive photochromies in toluene (and heptane and dodecane) in poly (vinyl alcohol) (PVA) have been prepared. This material was cast on glass, Tyvek, Dyneema, acrylic, and cotton/polyester mix fabric and these samples exhibited photochromism in solid form. 0.0119 g Palatinate Purple was added to 10 ml_ of toluene. 2 ml_ of this resting solution was cooled to 4°C and emulsified into 10 mL of a 7 wt% aqueous solution of

Mowiol 56-98. The emulsification took place using a sonic probe for 2 minutes, ensuring resting every 1 minute. 400 uL of the combined solution was then cast onto a microscope slide or glass substrate and allowed to dry forming a solid plastic film. Upon irradiation with ultraviolet (UV, -395 nm) light, the film exhibited a reversible photochromism.

The following solvents were also used successfully; heptane and dodecane. The following positive photochromic pigments were also used successfully, Plum Red, Scafell Grey, Midnight Grey, Volcanic Grey, Cotswold Green and Trent Blue. Figure 2 - Optical Microscopy of Capsules

Image 1 shows capsules after excitation with UV light. Image 2 shows capsules after relaxation. Example 3 - Negative Photochromic

The following samples/combinations were prepared:

Toluene with spiropyran Benzyl alcohol with spiropyran Toluene with spirooxazine

Benzyl alcohol with spirooxazine Spiropyran 0.1 wt%

Spirooxazine 0.01 wt%

Emulsion with PVA in water - Mowiol 40-887 wt%

A method for preparing samples of encapsulated negative photochromies was achieved according to the following procedures (two phase encapsulation of negative photochromies). Example 3.1

0.004 g of spiropyran compound and 4 ml_ of benzyl alcohol were mixed together. The core material thus obtained was cooled to 4°C and emulsified into 20 ml_ of a 7 wt% aqueous solution of Mowiol 40-88. The emulsification took place using a sonic probe for 3 x 1 minute bursts. The mixture was cast into a shape and left to dry over night and day.

Example 3.2

0.0004 g of spirooxazine compound and 4 ml_ of benzyl alcohol were mixed together. The core material thus obtained was cooled to 4°C and emulsified into 20 ml_ of a 7 wt% aqueous solution of Mowiol 40-88. The emulsification took place using a sonic probe for 3 x 1 minute bursts. The mixture was cast into a shape and left to dry over night and day.

Example 3.3

0.004 g of spiropyran compound and 4 ml_ of toluene were mixed together. The core material thus obtained was cooled to 4°C and emulsified into 20 ml_ of a 7 wt% aqueous solution of Mowiol 40-88. The emulsification took place using a sonic probe for 3 x 1 minute bursts. The mixture was cast into a shape and left to dry over night and day.

Example 3.4

0.0004 g of spirooxazine compound and 4 ml_ of toluene were mixed together. The core material thus obtained was cooled to 4°C and emulsified into 20 ml_ of a 7 wt% aqueous solution of Mowiol 40-88. The emulsification took place using a sonic probe for 3 x 1 minute bursts. The mixture was cast into a shape and left to dry over night and day.

Toluene samples tended to bubble and wick during drying, as such benzyl alcohol appeared to be the better solvent choice.

Example 4 - Other Smart Materials and Test Optimisation Methods

Green fluorescent protein (GFP) fluoresces under UV light until denatured. GFP from two sources were used: LabGenius GFP (engineered for increased brightness and durability), and Sigma-Aldrich GFP (recombinant, “standard”). Experiments were also carried out using red food colouring in polymers (soluble in water and strongly coloured). Tests were done to determine the stability of emulsion using materials in water and polystyrene in a variety of solvents. Alternatively DNA stained with 4 wt% methylene blue could be used.

Further Solvent Selection

33 solvents capable of dissolving high molecular weight polystyrene were assessed for suitability for a two-phase encapsulation system - key criteria being boiling point and (lack of) miscibility with water. Of these, four were used to prepare solutions of polystyrene in solvent:

- Ethyl acetate (7 wt% and 21 wt%)

- Cyclohexane (7 wt% - PS only dissolved after heating)

- Dichloromethane (DCM) (7 wt%, 14 wt%, 21 wt%)

- Ethyl formate (7 wt% - PS only soluble while hot)

Red Food Colour tests indicated that dichloromethane (DCM) was suitable, and that the optimum concentration depended on volume of material being sonicated - more concentrated solutions of polymer in DCM were suited for larger volumes, since sonicating ~2 ml_ at a time cause the solvent to rapidly evaporate and the polymer to cure in the vessel. Emulsions formed with ethyl acetate (at either concentration) were not stable, leading to films which were not strongly coloured.

The importance of a stable, carefully formulated emulsion was demonstrated with red food colouring encapsulated in a cured polymer film - droplets varied greatly in size, some having coalesced to form localities of strongly concentrated colour and poor dispersion.

GFP was a useful test material due to the ease of determining whether it has denatured - once denatured, the protein would not fluoresce green. GFP was obtained from Sigma Aldrich and from LabGenius (a variant they had engineered to increase durability and strength of fluorescence). GFP samples were sonicated in 2 ml_ Eppendorf tubes - care being taken to be aware of the concentration of polymer since the higher temperatures generated by sonication will cause a small amount of solvent to evaporate rapidly and the material to cure in the tube.

To observe the GFP signal, a UV torch (-395 nm) was used to illuminate the sample and imaged through a yellow filter.

Methods

Sample 1 (using Sigma Aldrich GFP):

500uL of -7 wt% PS (192 kg/mol) in DCM + 160uL GFP from Sigma Aldrich, sonicated for 20-30 seconds, cast on slide immediately. Fluoresced under UV after curing and washing with water.

Sample 2 (using GFP from LabGenius)

400uL -7 wt% PS (192 kg/mol) in DCM + 50uL GFP - sonicated 20-30 seconds, cast immediately. Showed some fluorescence, not evenly dispersed. Sample 3

400uL -7 wt% PS (192k g/mol) in DCM + 20uL GFP, sonicated 20-30 seconds, cast immediately - post curing and washing, this showed very little fluorescence due to small quantities of GFP.