Silica particles

Abstract

The present invention relates to coloured silica particles and to methods of manufacturing coloured silica particles.

Description

[0001] Silica Particles

[0002] Field of invention

[0003] The present invention relates to coloured silica particles and to methods of manufacturing coloured silica particles.

[0004] Background

[0005] Means of producing coloured compositions comprising silica particles for use in applications such as toothpaste, make-up and hair-care are known. These coloured compositions can be used to produce a range of highly attractive and interesting products. For example, by combining different coloured compositions, a product having discrete areas of colour can be produced, such as a speckled toothpaste. In order for this product to remain visually effective it is desirable for the colour to be maintained within the different areas of the product and there to be minimal colour bleeding of the colourant (the substance providing colour to the composition) from one area to another. It is therefore advantageous if the colourant is capable of remaining associated with the silica particles.

[0006] A number of methods for producing coloured silica particles are known. These include treating the surface of the silica particles to fix a dye in place and spray drying an organosilicate with a dye. However, these methods are not very practical on large scales.

[0007] EP 266248 A1 describes a method producing coloured silica particles by mixing a silicate and dye prior to forming the silica particles. The dye is added to a sodium silicate solution and then acid is added directly to the solution. This method of adding colour to the particles does not require significant changes to the production method, other than the introduction of the dye itself. However, the silica particles produced by this method show a high degree of colour bleeding (poor colour retention properties) and require a large quantity of dye, which can make the method inefficient.

[0008] Accordingly, it would be of benefit to the art for there to be an alternative and / or improved method of producing coloured silica particles.

[0009] 69631465-1Summary of the invention

[0010] In general, the present invention provides a new methodology for synthesizing coloured silica particles and the silica particles thereby obtained. Specifically, it has been found to be possible to produce coloured silica particles using a dried mixture of a colourant and an alkali metal silicate.

[0011] The method described in EP 266248 A1 uses a silicate composition comprising a very low dry solids content because the dye is added directly to a sodium silicate solution. The acid is then added directly to this solution. Compared to EP 266248 A1, the inventors have surprisingly found that by using a drier silicate composition a significant improvement in the coloured silica particles is obtained. This benefit may outweigh the disadvantages associated with obtaining a coloured silicate composition with the required dry solids content (e.g. having to introduce additional drying steps into the synthesis).

[0012] Accordingly, in a first aspect of the invention there is provided a method of manufacturing coloured silica particles, the method comprising contacting a coloured silicate composition with an acid to provide the coloured silica particles; wherein the coloured silicate composition comprises an alkali metal silicate and a colourant and has a dry solids content of 60% or more.

[0013] The term “colourant” is well known in the art but for the avoidance of doubt is used herein to indicate a dye, pigment, or other substance that is capable of colouring something, or a combination of one or more thereof.

[0014] The term “alkali metal silicate” refers to chemical compounds which have the formula (A2O)x(SiO2)y or A2xSiyC>2y+x, where is A corresponds to an alkali metal cation such as sodium or potassium and x and y vary based on the silica (SiC>2) to alkali metal oxide (A2O) molar ratio of the silicate. The silica to alkali metal oxide molar ratio (i.e. the y:x ratio) typically varies from 0.5 to 4.5.

[0015] The term “dry solids content” is well known in the art. However, for the avoidance of any doubt, the dry solids content of the coloured silicate composition can be determined by measuring the proportion of weight remaining after drying for 1 hour at a temperature of 800°C, as described in the general method section below.

[0016] 69631465-1The term “contacting a coloured silicate composition with an acid” is intended to refer to contacting of the two components in any order (e.g. the addition of the silicate to the acid, or addition of the acid to the silicate). Preferably, the silicate is added to the acid. Further, contacting may comprise mixing and / or reacting the components together.

[0017] The method according to the invention has been found to offer advantages over the prior art processes described above. For example, as shown in the examples section below, the coloured silica particles produced by the method of the invention show a reduced colour bleeding (i.e. improved colour retention properties) compared to silica particles produced by prior art methods. Without wishing to be bound by theory, this improvement is considered to result from an overall reduced ability of the colourant to diffuse out of the pore structure of the coloured silica particles, as compared to colourants contained in silica particles produced by prior art methods. A greater proportion of the colourant is therefore retained within the silica particles.

[0018] We have shown that the use of a coloured silicate composition that has a dry solids content of 60% or more results in coloured silica particles having significantly smaller pore diameters than particles produced according to prior art methods, for example, pore diameters of 3 nm or less. Without wishing to be bound by theory, it is believed that small pores are at least partially responsible for the reduced ability of the colourant to diffuse out of the pores.

[0019] Advantageously, the reduced colour bleeding properties of the particles may allow the particles to retain their colour over long periods of time despite successive washings. This may allow them to be used for a wider range of applications. The retention of the dye can also lead to a high colour intensity, leading to a vibrant and appealing product.

[0020] Advantageously, the method of the invention may also provide coloured silica particles which show a reduced accessible surface area at their external surface. This reduction in external surface area is shown and quantified in the examples below using low field NMR relaxation measurements. Without wishing to be bound by theory, the reduction of surface area at the external surface is believed to result in particles that may be considered to have a core-shell morphology, where the core contains a higher proportion of open pores and the shell contains a lower proportion of open pores. While referred to

[0021] 69631465-1as a core-shell morphology, it will be appreciated that the particles may not have discrete areas with the different surface areas but may exhibit a more continuous change in pore structure going from the centre of particles to their external surface. It is believed that the reduction in the accessible surface area at their external surface may reduce the ability for the colourant to diffuse out of the particles compared to prior art silicas, thereby contributing to the beneficial colour bleeding properties observed.

[0022] Advantageously, the method described herein shows a reduced loss of colourant during the manufacture of the coloured silica particles compared to the prior art, meaning less, e.g. only small quantities of colourant need be used, compared with previous methods. This may make the method more efficient. As shown in the examples, coloured silica particles can be produced using around 1 / 30thof the amount of a dye described in the prior art. It is surprising that the method can work with the small quantities of dye employed.

[0023] In a second aspect of the invention there is provided coloured silica particles obtained or obtainable in accordance with the first aspect of the invention.

[0024] In a third aspect of the invention there is provided coloured silica particles comprising silica particles comprising pores having a mean pore diameter of 3 nm or less, and a colourant contained within the pores.

[0025] As explained in relation to the first aspect of the invention, and shown in the examples below, coloured silica particles having mean pore diameters of 3 nm or less have advantageously been found to exhibit improved colour bleeding properties over particles with larger pore diameters produced according to prior art methods. The particles according to the invention are thus highly desirable for applications such as toothpaste, make-up and hair-care where colour retention is required. Coloured silica particles according the third aspect of the invention may be produced by the process of the invention as described herein.

[0026] The term “pore diameter” is well known in the art but for the avoidance of doubt can be measured using the method described in the general methods section below.

[0027] 69631465-1Further aspects and embodiments of the invention will be apparent from the detailed description of the invention that follows below.

[0028] Detailed Description

[0029] Embodiments of the various aspects of the invention are described below. For the avoidance of doubt, it will be appreciated, where appropriate, that any embodiments as described herein in relation to one aspect of the present invention will also apply to the other aspects of the present invention.

[0030] First aspect

[0031] In embodiments of the first aspect of the invention, the coloured silicate composition has a dry solids content of 75% or more, such as a dry solids content of 80% or more. In some embodiments, the coloured silicate composition has a dry solids content of 84% or more, such as 84% to 95%. As shown in the examples, using coloured silicate compositions with an increased dry solids content results in coloured silica particles that show reduced colour bleeding compared to using less dry silicate compositions. Without wishing to be bound by theory this is believed to arise, at least partially, because an increase in dry solids content of the silicate results in a decrease in the pore diameter of the final coloured silica particles.

[0032] In embodiments of the first aspect of the invention, the colourant comprises a dye. In other embodiments, the colourant comprises a pigment. In some embodiments the colourant comprises a dye or a pigment. In some embodiments the colourant comprises a dye and a pigment.

[0033] In embodiments of the first aspect of the invention, the colourant comprises two or more dyes (or two or more pigments). The two or more dyes (or pigments) may be homogenously mixed together or heterogeneously, e.g. randomly, mixed. Using a combination of two or more dyes (or two or more pigments) allows for coloured silica particles to be produced which have a colour that cannot be obtained using one dye (or pigment) alone or allow a different way of obtaining a particular colour.

[0034] It will be appreciated that the invention is not limited to any particular colourants.

[0035] 69631465-1In embodiments of the first aspect of the invention, the colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1, FD&C Blue No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), or a mixture of two or more thereof. These dyes have been approved safe for use in food products and therefore are particularly advantageous for application in toothpaste or similar oral compositions.

[0036] Additionally or alternatively, the colourant may comprise natural food colourants that are exempt from certification such as Annatto extract, Dehydrated beets, Butterfly pea flower extract, Calcium carbonate, Canthaxanthin, Caramel, p-Apo-8'-carotenal, p-Carotene, Cochineal extract, Carmine, Sodium copper chlorophyllin, Toasted partially defatted cooked cottonseed flour, Ferrous gluconate, Ferrous lactate, Grape color extract, Grape skin extract (enocianina), Synthetic iron oxide, Fruit juice, Vegetable juice, Carrot oil, Paprika, Paprika oleoresin, Mica-based pearlescent pigments, Riboflavin, Saffron, Soy leghemoglobin, Spirulina extract, Titanium dioxide, Tomato lycopene, Turmeric, Turmeric oleoresin, or a mixture of two or more thereof.

[0037] In embodiments of the first aspect of the invention, the acid is an inorganic acid. The inorganic acid may be selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, or a mixture of two or more thereof. In some embodiments, the acid is sulfuric acid. Inorganic acids, for example, sulfuric acid, have been shown to be particularly effective at producing coloured silica particles with desirable properties.

[0038] In alterative embodiments, the acid is an organic acid. The organic acid may be citric acid and / or formic acid.

[0039] In embodiments of the first aspect of the invention, the acid is in an aqueous solution with a concentration of 30 wt% or less, such as of from 0.5 to 30 wt%. The acid may be an aqueous solution with a concentration of from 2-20 wt%. The acid may be an aqueous solution with a concentration of from 4-10 wt%.

[0040] 69631465-1Without wishing to be bound by theory, it is believed that changing the concentration of acid may allow for rate of reaction to be controlled, which in turn could have an impact on the colour properties of the silica particles produced.

[0041] In embodiments of the first aspect of the invention, particularly when the acid is an inorganic acid such as sulphuric acid or hydrochloric acid, the molar ratio of the H+of the acid to the alkali metal oxide (A2O) of the alkali metal silicate of the coloured silicate composition is of from 1 to 3 (i.e. H+: A2O ratio of 1 to 3). The molar ratio of the H+of the acid to the A2O of the alkali metal silicate of the coloured silicate composition may be of from 1.5 to 2.5, such as 1.8 to 2.2. The molar ratio of the acid to the alkali silicate allows for the pH of the reaction to be controlled. As shown in the examples, controlling the pH has an effect on the properties of the final silica particles, including the amount of colour bleeding.

[0042] In embodiments of the first aspect of the invention, the acid is present in an amount of from 90 to 110%, preferably 95 to 105%, relative to the amount of acid that would be required for complete conversion of the alkali metal silicate to the silica particles.

[0043] In embodiments of the first aspect of the invention, contacting the coloured silicate composition with the acid comprises reacting the coloured silicate composition with the acid for a set period of time, such as from 1 minute to 6 hours.

[0044] As mentioned above, the term “alkali metal silicate” refers to chemical compounds which have the formula (A2O)x(SiO2)yor A2xSiyO2y+x, where A corresponds to an alkali metal cation such as sodium or potassium. In some embodiments, the alkali metal silicate may be sodium silicate or potassium silicate. In particular embodiments, the alkali metal silicate is sodium silicate.

[0045] The molar ratio of the silica (SiC>2) to alkali metal oxide (A2O) in the alkali metal silicate can varied to achieve different grades of alkali metal silicate.

[0046] In embodiments of the first aspect of the invention, the alkali metal silicate has a silica to alkali metal oxide molar ratio of from 2 to 4.5. In some embodiments, the alkali metal silicate has a silica to alkali metal oxide molar ratio of 3.3 or higher, such as a molar ratio of from 3.3 to 3.5. Advantageously, it has been found that these high molar ratio silicates

[0047] 69631465-1consume less acid in the process and provide coloured silica particles with a more intense colour.

[0048] In some embodiments the alkali metal silicate has a silica to alkali metal oxide molar ratio of 3.75 or higher, such as a molar ratio of from 3.7 to 4.5. In some embodiments the alkali metal silicate has a silica to alkali metal oxide molar ratio of 4 or higher, such as a molar ratio of from 4 to 4.5.

[0049] In embodiments of the first aspect of the invention, the coloured silicate composition comprises particles where the median particle diameter is of from 200 to 750 microns, such as 225 to 700 microns. In some embodiments, the weight median particle diameter may be from 300 to 650 microns, more preferably of from 350 to 550 microns.

[0050] In embodiments of the first aspect of the invention, the method further comprises a step of drying a mixture comprising an alkali metal silicate and a colourant to provide the coloured silicate composition having the solids content of 60% or more.

[0051] Put another way, the method may comprise

[0052] drying a mixture comprising an alkali metal silicate and a colourant to provide a coloured silicate composition having a dry solids content of 60% or more; and contacting the coloured silicate composition with an acid to provide the coloured silica particles.

[0053] Drying the mixture may comprise drying the mixture as a layer with a thickness of 1 cm or less, such as 0.01 cm to 1 cm. This is also known as thin layer drying. Accordingly, in a particular advantageous embodiment of the first aspect of the invention there is provided a method of manufacturing coloured silica particles, the method comprising: thin layer drying a mixture comprising an alkali metal silicate and a colourant to provide a coloured silicate composition having a dry solids content of 60% or more; and contacting the coloured silicate composition with an acid to provide the coloured silica particles. Thin layer drying may comprise spreading out a thin (e.g. 0.01 cm to 1 cm) layer of the mixture comprising an alkali metal silicate and a colourant and then allowing the mixture to dry. Drying may be at ambient temperature, or may be at an elevated temperature as described below. Drying may be under inert atmosphere or may be in air. Drying the mixture may comprise thin layer static drying, i.e. drying the mixture as a

[0054] 69631465-1layer without any forced air movement. In some embodiments, drying the mixture comprises drying the mixture as a layer with a thickness of 0.5 cm or less, such as 0.01 cm to 0.5 cm. In some embodiments, drying the mixture comprises drying the mixture as a layer with a thickness of 0.30 cm or less, such as 0.05 cm to 0.30 cm. This thin layer drying method allows for a wider range of particles sizes to be obtained than other drying methods, such as spray drying. In particular, it allows for the production of particles that are larger than those obtainable by spray drying, such as particles with a weight median particle diameter of from 300 to 650 microns. Typically these particles are obtained through milling and / or sieving the coloured silicate composition obtained from thin layer drying (which may be in the form of glass flakes). The inventors have found that colour bleeding can be inversely correlated to particle size (i.e. larger particles exhibit less colour bleeding). As a result, thin layer drying can allow for the production of larger alkali metal silicate particles resulting in larger silica particles and an improvement in the final colour bleeding properties.

[0055] Thin layer drying can also allow for the drying to occur over longer time periods compared to other drying methods, such as by spray drying. This can provide improved control over the final pore diameter of the silica particles. As explained above, it is believed that pore diameter is at least partially responsible for the reduced ability of the colourant to diffuse out of the pores. Control over the pore diameter, and in particular the production of the preferred pore diameters described in relation to the third aspect of the invention, is therefore advantageous.

[0056] The skilled person will understand how to dry a mixture comprising an alkali metal silicate and a colourant to achieve the desired dry solids content (e.g. a dry solids content of 60% or more, or 75% or more). Typical conditions may comprise heating at a temperature of from 40 to 120 °C, such as 45 to 105 °C. One example is heating at a temperature of 50 to 80 °C. Another example is heating at a temperature of from 60 to 65 °C. Drying may comprise multiple drying steps or a single drying step. Where there are multiple drying steps, an optional milling or sieving step may be present between the drying steps. As shown in the examples below, using lower drying temperatures has advantageously been found to produce more uniform and more intensely coloured silica particles. It will be appreciated that the invention is not particularly limited by the heating time, but time may be used to control the dry solids content achieved. In some embodiments, heating is conducted for 12 to 48 hours, or for at least 16 hours.

[0057] 69631465-1In embodiments of the of the first aspect of the invention, the method further comprises a step of adding a colourant to an alkali metal silicate composition to provide the mixture comprising the alkali metal silicate and the colourant.

[0058] Put another way, the method may comprise

[0059] adding a colourant to an alkali metal silicate composition to provide a mixture comprising an alkali metal silicate and the colourant;

[0060] drying the mixture to provide a coloured silicate composition having a dry solids content of 60% or more; and

[0061] contacting the coloured silicate composition with an acid to provide the coloured silica particles.

[0062] The colourant may be added to the alkali metal silicate composition in an amount of from 0.001 % to 1 % based on the silica content of the alkali metal silicate on a dry weight basis. In some embodiments, the colourant is added to the alkali metal silicate composition in an amount of from 0.005% to 0.1 % based on the silica content of the alkali metal silicate on a dry weight basis. In some embodiments, the colourant is added to the alkali metal silicate composition in an amount of from 0.005% to 0.08 % based on the silica content of the alkali metal silicate on a dry weight basis. In some embodiments, the colourant is added to the alkali metal silicate composition in an amount of from 0.01 % to 0.07 % based on the silica content of the alkali metal silicate on a dry weight basis. In a particularly advantageous embodiments, the colourant is added to the alkali metal silicate composition in an amount of from 0.01% to 0.05 % based on the silica content of the alkali metal silicate on a dry weight basis. Advantageously, the method has been shown to provide sufficiently coloured silica particles using such amounts of dye. In comparison, the method described in EP 0266248 A1 requires greater than 1% dye to be incorporated. The smaller quantity of dye used compared to the prior art method may allow for the process to be more efficient.

[0063] In embodiments of the of the first aspect of the invention, the method further comprises a step of mixing an initial alkali metal silicate, a silica (typically an amorphous silica) and a liquid (typically water) to provide the alkali metal silicate composition, wherein the alkali metal silicate in the composition has an increased silica to alkali metal oxide molar ratio compared to the initial alkali metal silicate. For example, the silica to alkali metal oxide

[0064] 69631465-1molar ratio of the alkali metal silicate may be increased to 3.3 or higher, such as from 3.3 to 3.5.

[0065] Put another way, the method may comprise

[0066] mixing an initial alkali metal silicate, an amorphous silica and a liquid to provide an alkali metal silicate composition containing an (upgraded) alkali metal silicate with an increased silica to alkali metal oxide molar ratio compared to the initial alkali metal silicate;

[0067] adding a colourant to the alkali metal silicate composition to provide a mixture comprising the (upgraded) alkali metal silicate and the colourant;

[0068] drying the mixture to provide a coloured silicate composition having a dry solids content of 60% or more; and

[0069] contacting the coloured silicate composition with an acid to provide the coloured silica particles.

[0070] In embodiments of the of the first aspect of the invention, the method further comprises a step of milling the coloured silicate composition and / or sieving the coloured silicate composition prior to contacting the coloured silicate composition with the acid.

[0071] Put another way, the method may comprise

[0072] drying a mixture comprising an alkali metal silicate and a colourant to provide a coloured silicate composition having a dry solids content of 60% or more;

[0073] milling the coloured silicate composition and / or sieving the coloured silicate composition; and

[0074] contacting the milled and / or sieved coloured silicate composition with an acid to provide the coloured silica particles.

[0075] Milling and / or sieving the particles allows for tuning of the particle size of the coloured silicate. This has been shown to have an effect on the size and properties of the final coloured silica particles, such as the colour intensity and the colour consistency.

[0076] The weight median particle diameter of the coloured silicate composition after milling and / or sieving may be of from 200 to 750 microns, such as 225 to 700 microns. Preferably the weight median particle diameter of the coloured silicate composition after

[0077] 69631465-1milling and / or sieving is of from 300 to 650 microns, more preferably of from 350 to 550 microns.

[0078] In embodiments of the first aspect of the invention, the method further comprises a step of filtering, and optionally washing, the coloured silica particles. The method may also comprises drying the filtered coloured silica particles and, optionally, sieving the dried filtered coloured silica particles. These final processing steps allow for a solid product that can be easily stored, sold and introduced into various applications.

[0079] Second aspect of the invention

[0080] In embodiments of the second aspect of the invention, the silica particles comprise pores having a mean pore diameter of 3 nm or less and colorant is contained within the pores.

[0081] In embodiments of the second aspect of the invention, the coloured silica particles display a wetted surface area of 50 m2 / g or less when measured using low-field NMR relaxation.

[0082] Third aspect of the invention

[0083] In embodiments of the third aspect of the invention, the pores have a mean diameter of from 0.5 nm to 3 nm. In some embodiments, the pores have a mean diameter of 2.5 nm or less, such as of from 1 nm to 2.5 nm. In some embodiments, the pores have a mean diameter of 2.3 nm or less, such as of from 1.5 nm to 2.3 nm. In some embodiments, the pores have a mean diameter of 2.1 nm or less, such as of from 1 nm to 2.1 nm or 1.8 nm to 2.1 nm. As shown in the examples below, coloured silica particles with pore diameters below 2.1 nm showed particular advantageous colour bleeding properties. The inventors have found that the pore diameter of the coloured silica particles may be controlled by controlling the dry solids content of the alkali metal silicate used in producing the coloured silica particles according to the first aspect of the invention.

[0084] In embodiments of the third aspect of the invention, the coloured silica particles display a wetted surface area of 50 m2 / g or less when measured using low-field NMR relaxation. Preferably, the coloured silica particles display a wetted surface area of 20 m2 / g or less when measured using low-field NMR relaxation.

[0085] 69631465-1As explained in relation to the first aspect of the invention, and described in the examples below, coloured silica particles which show a reduced accessible surface area at their external surface have advantageously been demonstrated to exhibit reduced colour bleeding. Without wishing to be bound by theory, this is thought to arise due to the coreshell morphology described in relation to the first aspect of the invention, where the reduced surface area at the external surface reduces the diffusion of the colourant contributing to the beneficial colour bleeding properties.

[0086] The use of low-field NMR measurements are well known method for obtaining information on the wetted surface area of particles and thus the porosity at the surface of the particles. The measurements work by comparing relaxation of solvent molecules associated with the surface of the particle compared to the bulk liquid. An example of the technique being used on silica particles is work by T Cosgrove et al., Powder Technology, Vol 414, 15 Jan 2023, 118065. Compact and portable devices and methods for such measurements are available as described in US 2007 / 0210798 A1. It will be appreciated that an advantage of measuring the wetted surface area using a low-field NMR measurement compared to using a high-field NMR measurement is that the area can be determined using weaker magnets which are more easily accessible, smaller and use less electrical energy. Further details on how these measurements are conducted are found in the general methods section below.

[0087] A suitable frequency for the NMR measurement is 12.5 MHz. For example, the NMR data can be obtained using a MagnoMeter XRSTM NMR spectrometer, operating at 12.5 MHz. A CPMG pulse sequence can be used to measure the spin-spin relaxation time. The 1800 pulse spacing can be 1000 ps and the 900 pulse length can be 4.5 ps.

[0088] In embodiments of the third aspect of the invention, the colourant comprises a dye. In other embodiments, the colourant comprises a pigment. In some embodiments, the colourant comprises a dye and a pigment.

[0089] In embodiments of the third aspect of the invention, the colourant comprises two or more dyes (or two or more pigments). As explained in relation to the first aspect of the invention, using a combination of two or more dyes allows for coloured silica particles which have a colour than cannot be obtained through using one dye alone.

[0090] 69631465-1In embodiments of the third aspect of the invention, the colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1 , FD&C Blue No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), ora mixture of two or more thereof. These dyes are safe for use in food products and therefore are particularly advantageous for applications such as toothpaste.

[0091] In other embodiments of the third aspect of the invention, the colorant comprises natural food colorants except from certifications, as specified in relation to the first aspect.

[0092] In embodiments of the third aspect of the invention, the coloured silica particles have an acidity such that 5 g of the coloured particles on a dry mass basis dispersed in 95 g of deionised water at 20 °C and atmospheric pressure exhibits a pH of from 7 to 11, optionally a pH of from 8 to 10.

[0093] The pH of the coloured silica particles is relevant for their use in various applications. The examples below also show that coloured silica particles with the above-mentioned pH values show beneficial colour properties. Silica particles with the required pH may be produced by the process of the invention as described herein.

[0094] In embodiments of the third aspect of the invention, the silica particles have a BET surface area of from 50 m2 / g to 500 m2 / g, such as of from 100 m2 / g to 400 m2 / g.

[0095] In embodiments of the third aspect of the invention, the coloured silica particles have a weight median particle diameter value of greater than 200 pm. For example, the coloured silica particles have a weight median particle diameter value of greater than 250 pm. The coloured silica particles may have a weight median particle diameter value of greater than 300 pm.

[0096] In embodiments of the third aspect of the invention, the coloured silica particles have a weight median particle diameter value of from 200 pm to 600 pm, such as 250 to 550 pm. In some embodiments the weight median particle diameter value may be of from 300 pm to 500 pm. Advantageously, it has been found that compositions comprising larger particle sizes exhibit a reduced colour bleeding compared to composition with smaller particle sizes. Silica particles with these diameters may be produced by the

[0097] 69631465-1process of the invention as described herein, particularly when using the thin layer drying methodology.

[0098] Brief Description of Figures

[0099] Figure 1 is a process flow diagram showing a method of producing coloured silica particles according to the present invention.

[0100] Figure 2 shows photographs of coloured silica particles produced using a methodology presented in the prior art. The images also show the reaction filtrate and liquors from three wash cycles. 2a) shows silica particles produced using tartrazine (acid yellow 231 FD&C Yellow No. 5); 2b) shows silica particles produced using food blue No1 dye.

[0101] Figure 3 shows photographs of coloured silica particles produced according to the present invention. The images also show the reaction filtrate and liquors from three washes of the particles. 3a) shows silica particles produced using food blue No.1; 3b) shows silica particles produced using tartrazine (acid yellow 23 I FD&C Yellow No. 5); 3c) shows silica particles produced using Allura red AC (FD&C Red No. 40).

[0102] Figure 4 shows a photograph of green coloured silica particles produced according to the present invention using a 1:1 weight ratio of food blue No.1 and tartrazine (acid yellow 231 FD&C Yellow No. 5) as the colourant.

[0103] Figure 5 is a graph showing the correlation of pore diameter of coloured silica particles with their colour bleeding properties. Specifically, the graph plots the pore diameter against the visible absorbance recorded for an aqueous filtrate that had been separated from slurried, coloured particles.

[0104] Figure 6 is a graph showing the correlation of accessible surface area at the external surface of coloured silica particles with their colour bleeding properties. Specifically, the graph plots the visible absorbance recorded for an aqueous filtrate that had been separated from slurried, coloured particles against the wetted surface area measured using low-field NMR relaxation.

[0105] General Methods

[0106] 69631465-1Dry solids content

[0107] The solids content of the dried, coloured silicate particles and the coloured silica particles was determined by measuring the weight loss after drying for 1 hour at a temperature of 800°C using a Carbolite Gero oven model CWF1200.

[0108] The total solids content of the undried alkali silicate compositions (e.g. the sodium silicate liquor described in steps A and B of the example process below) was determined by measuring the percentage by weight of alkali metal oxide (e.g. Na2O) and SiC>2 in the silicate liquor using the titration method described below and then adding these two quantities.

[0109] Molar ratio of silicate

[0110] The molar ratio of the alkali metal silicate was determined by titration as is generally known in the art. The titration involved HCI, NaF and NaOH. Specifically, the silicate was dissolved in water and then titrated with 1M HCI according to the equation below described for a sodium silicate.

[0111] Na2O.SiO2+ 2 HCI > 2 NaCI + SiO2.H2O

[0112] The amount 1M HCI used in the titration was used to determine the amount and percentage of Na2<D.

[0113] NaF was then added to the obtained solution.

[0114] SiO2+ 6 NaF + 2H2O - Na2SiFe+ 4 NaOH

[0115] Excess HCI was then added. The excess HCI was then back-titrated with 1M NaOH. The amount of 1M HCI consumed minus the amount of 1 M NaOH consumed was used to determine the amount and percentage of SiO2.

[0116] It will be appreciated that the same method can be used for the measurement of other alkali metal silicates (A2O.SiO2) to calculate the amount of A2O.

[0117] Thin layer drying of coloured silicate

[0118] Thin layer drying of the coloured silicate was conducted by pouring the coloured silicate on a plastic tray, followed by drying using a Hereaus oven, T6060, for 20 hours at 60 °C. The trays were 26 by 35 cm and the depth of the silicate was approximately 0.25 cm. One 2 kg batch of silicate was divided between 6 trays. At the end of the 20 hours the dried layer of silica was turned upside down and dried for a further two hours at the same

[0119] 69631465-1temperature. The dried silicate was then broken into pieces and ground with a Retsch cross beater SK300, followed by sieving with 180 and 500 micron sieves to remove coarse and fines particles. Finally, granules are dried for approx. 2-4 hours in an oven at 60-65 °C to achieve the desired dry solids content.

[0120] Colour Measurements

[0121] A UV / Visible spectrometer, (Elrepho 3100) was set to D65 (6500k) llluminant and 10-degree Observer and was calibrated using a ‘black trap’ standard, a white and green tile calibration standard and Cie illuminant checker according to the instrument instructions. Ba(SO4) (white standard DIN 5033 from Merck art. No. 1.01748.0250) was measured as a standard at C / 2 llluminant / observer setting. Subsequently, a chrome-plated copper cup (2.2 cm3 volume) was used for sampling coloured silicas. The cup was completely filled with coloured silica to leave a smooth measuring surface. Colour of the silica was measured according to Hunter Lab and Cie Lab in a range of between 400-700 nm. Hunter L, a, b and Cie L*, a*, b* values were recorded.

[0122] 69631465-1Absorbance Measurements

[0123] Absorbance measurements on reaction filtrates and subsequent washing liquors were recorded by using a single-beam Camspec M501 UV / visible spectrometer. Reference spectra were acquired first using deionized water in 1 cm polystyrene cells. The same cell was then emptied and dried before it was used to measure the absorbance of the liquid under test at the absorbance maximum of the relevant dye. The wavelengths used for each dye were: FD&C blue no.1 - 628 nm, FD&C yellow no.5 - 427 nm, FD&C red no.40 - 504 nm.

[0124] Colour Bleed Measurements

[0125] Bleeding of the coloured silica particles was quantified using a technique adapted from US6143280A. The test comprised measuring the visible absorbance of an aqueous filtrate that had been separated from slurried, coloured particles. 1.0 gram of coloured silica was placed in a 50 ml beaker containing a magnetic stirrer bar and 29 g of deionized water. The beaker was placed on a magnetic stir plate and stirred for 10 minutes. The slurry was then filtered using a vacuum on #42 Whatman filter paper (Whatman International Ltd., England). A single-beam Camspec M501 UV / visible spectrometer was then used to measure the background spectra of deionized water in 1 cm polystyrene cells. The same cell was then emptied and dried before it was used to measure the absorbance of the filtrate at the absorbance maximum of the dye in the aqueous solution being examined. The wavelengths used for each dye were: FD&C blue no.1 - 628 nm, FD&C yellow no.5 - 427 nm, FD&C red no.40 - 504 nm.

[0126] Extinction coefficients were determined for each dye from a series of stock solutions. These coefficients were then used to determine the concentration of dye in each filtrate sample from absorbance data. The concentration results are reported below as ‘bleed dye concentration’.

[0127] Pore Diameter

[0128] Silica surface area was determined using the standard multi-point nitrogen adsorption method of Brunauer, Emmett and Teller (BET) using an ASAP 2460 apparatus supplied by Micromeritics of USA. The method is consistent with the paper by S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60, 309 (1938).

[0129] Silica pore volume was determined using the same instrument employing the method described in ASTM D4222-20. Total pore volume was measured from the desorption branch of the isotherm at a partial pressure P / P0 of 0.998.

[0130] 69631465-1Samples were outgassed under vacuum at 270° C for 4 hours before either surface area or pore volume measurements at about -196° C.

[0131] Pore Diameter is a calculated parameter based on an assumption of cylindrical pores. It is determined using from the following equation:

[0132] .. r * \ 40000 X Pore Volume ( in cm3g-1) Mean Pore Diameter (in Angstroms) =v a'

[0133] Surface Area (in m2g-1)

[0134] Wetted surface area

[0135] The wetted surface area was measured using low-field NMR relaxation.

[0136] A dispersion of the sample under test was made up in demineralised water. The NMR data were obtained using a MagnoMeterXRSTM NMR spectrometer, operating at 12.5 MHz, from Mageleka Inc., Naples, FL, USA. A CPMG pulse sequence was used to measure the spin-spin relaxation time; the 1800 pulse spacing was typically 1000 ps with a 900 pulse length of 4.5 ps and up to 20,000 echoes were recorded.

[0137] The specific surface of the particles S (surface area of the particles per unit mass of particles) can be determined using the equation 1 below, corresponding to equation 6 of T Cosgrove et al, Powder Technology, Vol 414, 15 Jan 2023, 118065. R is the average spin relaxation rate constant defined by equation 2, kAis the specific surface relaxivity constant defined by equation 3, p is the solid-liquid volume ratio (Vp / Vi) fora total volume of particles in the dispersion Vp, I is the thickness of the wetting solvent layer, Rsis the spin relaxation constant at the surface, Rb is the bulk spin relaxation rate constant, pb and psare the fraction of the solvent in the bulk and surface respectively and ppis the particle density.

[0138] R = kAS<p + Rb(Eq. 1)

[0139] R = psRs+ pbRb(Eq. 2)

[0140] kA= lpPRs- Rb) + Rb(Eq. 3)

[0141] 69631465-1pH of coloured silica particles

[0142] When pH is mentioned herein with reference to the acidity of solid particles, the pH may be determined by dispersing 5 g of the solid particles on a dry mass basis in 95 g of deionised water at 20 °C and atmospheric pressure and measuring the resulting pH of the water.

[0143] Particle diameter

[0144] The weight median particle diameter (d50) of coloured silica particles were determined by laser diffraction using a Malvern Mastersizer 2000 and a Hydro 2000 AG dispersion unit. Mie theory was used to calculate particle size distributions. The real value of the silica refractive index was assigned a value of 1.46 and the imaginary refractive index of the particle was assigned a value of 1.0, with water dispersant having a real refractive index of 1.33. Abrasive silica particles were dispersed ultrasonically using the Hydro 2000 AG dispersion unit at 50% power for 5 minutes in de-ionised water to form an aqueous suspension. Laser light was passed through a flow cell containing the particles dispersed in de-ionised water. The scattered light intensity was measured as a function of angle and the data used to calculate particle size distribution. Weight-based particle size measures were used, assuming constant density of the particles.

[0145] Example Process

[0146] An example method of manufacturing coloured silica particles according to the present invention is shown in Figure 1.

[0147] The example method comprises the following steps:

[0148] A) Preparing a sodium silicate liquor

[0149] B) Colouring the sodium silicate liquor with a dye

[0150] C) Drying the coloured sodium silicate liquor to obtain coloured sodium silicate glass flakes.

[0151] D) Milling the colored sodium silicate glass flakes to obtain colored sodium silicate particles.

[0152] E) Sieving of coloured sodium silicate particles.

[0153] F) Converting the sieved coloured sodium silicate particles into coloured silica particles by neutralization with diluted acid.

[0154] G) Filtration and washing the coloured silica particles

[0155] H) Drying the coloured silica particles

[0156] I) Sieving of coloured silica particles to obtain the desired particle size distribution.

[0157] 69631465-1The method is described in more detail below.

[0158] At step A, amorphous silica 2 (e.g. Britesorb BK75 available from PQ corporation) is dissolved in sodium silicate 1 (molar ratio 3.4, dry solids content 37%, e.g. Crystal 0079 available from PQ corporation) and dilution water at 80°C to prepare a sodium silicate liquor 3 with a molar ratio (SiO2:Na2O) of 4.0 and a dry solids content of 34.5%.

[0159] At Step B, the sodium silicate liquor 3 is coloured by adding a dye 4 (food blue No.1) in an amount of 0.03 wt.% (based on the silica content of the sodium silicate on a dry weight basis) to obtain a coloured sodium silicate liquor 5.

[0160] At step C, the coloured sodium silicate liquor 5 is dried using hot air 6 to remove moisture 7 and obtain coloured sodium silicate glass flakes 8. The drying may be thin layer static drying in an oven at a maximum temperature of 105 °C. To avoid intumescence and decolorization (e.g. from blue to pink) during drying, it is preferred to dry the liquor in an oven at 60-65 °C to a dry solid content of greater than 75%, such as greater than 80%, or greater than 84%.

[0161] At step D, the sodium silicate glass flakes 8 are milled with a cross beating mill with a milling speed of -2000 RPM to provide milled coloured sodium silicate particles 9. To ensure maximum particle size filtering during milling, sodium silicate particles are passed through a 1 mm sieve prior to being discharged.

[0162] At step E, milled coloured sodium silicate particles 9 are sieved to filter out small particles (dust) and coarse particles 11. Typical particle size D50 of sieved blue coloured sodium silicate particles is 250 - 750 pm, such as 350 - 650 micron, or 450 - 550 micron. The small and coarse particles 11 may be recycled, such as to step B.

[0163] At step F, the milled and sieved coloured sodium silicate particles 10 obtained from step E, are treated with diluted acid 12 (e.g. H2SO4 or HCI) to convert them into water insoluble coloured silica particles 13 (e.g. blue silica particles comprising food blue no.1 dye). Typically the silicate particles are dosed to the acid (rather than vice versa). The typical degree of conversion of the coloured sodium silicate particles 10 in the coloured silica particles 13 is greater than 99%. For example, by dosing coloured sodium silicate particles 10 into 8.5 wt.% H2SO4 at 40 °C it has been found feasible to convert them into 100 % coloured silica after a reaction time of about 2 hours. Shorter reaction times may also be used.

[0164] 69631465-1At step G, the coloured silica particles 13 obtained in step F are filtered and washed with water 14 (e.g. on a belt filter or vertical plate filter) to provide a filter cake 15 and waste water 16.

[0165] At step H, the filter cake 15 is dried using hot air 17 within a conventional drying system (e.g. ring dryer or fluid bed dryer) to remove moisture 19 and provide dried coloured silica particles 18.

[0166] At step I , the coloured silica particles 18 are sieved to obtain coloured silica with a desired particle size distribution 20.

[0167] Comparative examples 1-3

[0168] Comparative examples were prepared based on the method described in Example 2 of EP 266248 A1).

[0169] Comparative example 1 (CE1)

[0170] CE1 followed the method described in EP266248A1 but was adapted to match the scale of the reactions conducted in the examples of the invention described below. Tartrazine was also used instead of Ciba Geigy 133R Irgalithe yellow dye.

[0171] Specifically, into a 50 L baffled stainless steel reaction vessel, stirred with a 6-bladed 30 degree pitched bladed unit, was added 25 L Sodium silicate with a molar ratio SiO2 / Na2O of 3.4, 127 g / L of SiO2. The solution was stirred at 250 rpm and was heated to 80 °C and maintained at this temperature.

[0172] This was followed by the progressive addition of 33.3 g Tartrazine.

[0173] Using a calibrated peristaltic pump with a flow rate of 550 ml / min, 9% H2SO4 acid solution was added over 20 minutes.

[0174] The pH which was 10.6 at the beginning of the operation was reduced to 8.7 on completion of this addition. By introduction of a further 1100 ml of 9% H2SO4 acid solution the pH was adjusted to 7.5.

[0175] The product was collected in a filter press, followed by 3 lots of sequential washing with 25 litres of water in each lot. The product was subsequently dried at 120 °C for in an oven for 15 hours.

[0176] 69631465-1The pore diameter of the final silica particles was 32.1 nm and the wetted surface area measured using low-field NMR relaxation was 347 m2 / g.

[0177] Comparative example 2 (CE2)

[0178] CE2 was prepared analogously to CE1 but the dye loading was reduced to 0.03 wt% relative to the amount of dry silica in the reaction, to better match loadings used in examples of the invention described below. The loading is roughly 35 times / ess than that described in EP266248A1. Specifically, 0.95 g of tartrazine was added to the reactor compared to the 33.3 g described above.

[0179] The pore diameter of the final silica particles was 31.3 nm and the wetted surface area measured using low-field NMR relaxation was 425 m2 / g.

[0180] Comparative example 3 (CE3)

[0181] CE3 was prepared analogously to CE2 but using FD&C blue No. 1 dye instead of tartrazine.

[0182] The pore diameter of the final silica particles was 30.2 nm and the wetted surface area measured using low-field NMR relaxation was 367 m2 / g.

[0183] Colour bleeding

[0184] Figure 2 a) shows the silica obtained from CE2, along with, from left to right, the reaction filtrate and the liquor from the each of the three washings conducted in the method described above. As can be seen, the silica of CE2 has very little colour and a lot of dye is retained in the original filtrate, or lost in the individual washing steps. The loss of dye is quantified using absorbance measurements on the filtrate and liquors in table 1 below. Figure 2 b) and table 2 shows analogous results for CE3.

[0185]

[0186] Table 1 - absorbance measurements conducted on the reaction filtrate and the liquors from the each of the three washings of CE2.

[0187] 69631465-1

[0188]

[0189] Table 2 - absorbance measurements conducted on the reaction filtrate and the liquors from the each of the three washings of CE3.

[0190] Overall, it is clear that coloured silica particles produced according to the prior art method showed poor colour retention properties. Without wishing to be bound by theory, this is believed to arise due to the large pore sizes and the high porosity of the silica.

[0191] Examples of the invention

[0192] The examples were prepared according the method described above in relation to figure 1. Unless specifically stated, the sodium silicate liquor has a molar ratio (SiO2:Na2O) of 4.0, the dye was food blue No. 1 (FD&C Blue No. 1), the dye was added in an amount 0.03% based on the silica content of the sodium silicate on a dry weight basis, the drying was thin layer static drying in an oven at 60-65 °C to achieve a solids content of 84.6%, the coloured sodium particles were sieved to provide particles in the range of 250 - 750 pm, and the silicate particles were dosed into 8.5 wt.% H2SO4 at 40 °C for 2 hours, in amount such that there would be 100 % H2SO4 consumption (i.e. a 2:1 molar ratio of H+: Na2O).

[0193] Examples 1 to 3 show the effect of controlling pH during the conversion process. Example 3 used the highest quantity of acid and showed the lowest pH. This resulted in lowest negative b-values for color (i.e. more greyness). Example 2 meanwhile used 100% H2SO4 consumption and showed good colour intensity and the least colour bleeding.

[0194] 69631465-1

[0195]

[0196] Table 3 - Properties of the coloured silica particles produced according to examples 1, 2 and 3.

[0197] Examples 4 and 5 show the effect of the drying temperature on the colour of the silica particles. Example 5 used a lower drying temperature compared to Example 4 and produced more uniform and more blue coloured silicate particles that resulted in more blue coloured silica particles and less color inconsistency. Specifically, example 5 shows the highest negative b-values for colour, whereas all other properties i.e. particle size, bulk density, BET Surface Area and pH (5%) remain similar.

[0198]

[0199] Table 4 - Properties of the coloured silica particles produced according to examples 4 and 5.

[0200] 69631465-1The drying temperature was also observed to affect the colour of the filtrates during processing. At lower drying temperatures (60 - 65°C) filtrates are less blue, indicating that more color is fixated in the silica particle. Example 5 shows the lowest negative b-values for the filtrates.

[0201]

[0202] Table 5 - Properties of the fi trate produced according to examples 4 and 5.

[0203] Examples 5, 6 and 7 show the effect of the dry solid content of the coloured sodium silicate granules on the properties of the colored silica particles, and in particular the bleeding performance and final colour of the silica. A higher dry solid content of the coloured sodium silicate granules as raw material resulted in less blueness but also less colour bleeding of the silica particles. In particular, Example 7 shows the highest dry solid content of the coloured sodium silicate granules, and shows the lowest negative b-values for both, the silica particles and bleeding liquids.

[0204]

[0205] 69631465-1

[0206]

[0207] Table 6 - Properties of the coloured silica particles produced according to examples 5, 6 and 7.

[0208] The dry solid content of the coloured sodium silicate granules also affects the colour of filtrates during processing. At higher dry solid contents of the coloured sodium silicate granules, filtrates are less blue indicating that more color is fixated in the silica particle. Specifically, Example 7 shows the lowest negative b-values for the filtrates.

[0209]

[0210] Examples 7, 8 and 9 show the feasibility of using differing dye molecules. Examples 8 and 9 use yellow and red dyes respectively, while employing similar methodology to example 7 (0.03 wt.% dye, drying temperature of 60 - 65 °C, a dry solid content of sodium silicate granules of roughly 84.6% and 100 % H2SO4 consumption). The resulting silica particles showed CIE a* and CIE b* values which represent an intense yellow color and a less intense red color.

[0211] 69631465-1

[0212] & &

[0213]

[0214] Table 8 - Properties of the coloured silica particles produced according to examples 7, 8 and 9.

[0215] Both, yellow and red colored silica samples show good bleeding performance results. Filtrates of example 8 and example 9 are low in colour indicating that more color is fixated in the silica particles.

[0216] 69631465-1

[0217] & &

[0218]

[0219] Table 9 - Properties of the filtrate produced according to examples 7, 8 and 9.

[0220] Colour bleeding of examples 7, 8 and 9

[0221] Figures 3 a), b) and c) shows the coloured silicas produced based on examples 7, 8 and 9 respectively. The solid shown on the left of each picture is the particles after synthesis. The solid shown on the right of each picture are the particles after three successive washes in water. As can be seen, the particles display a strong colour and retain their colour despite the multiple washing steps, cf. figure 2 for the comparative samples. The vials shown behind the solids represent the reaction filtrate and the liquor from each wash. As can be seen, the liquids are not highly coloured and therefore do contain much dye. This further indicates that the dye is predominantly retained in the coloured silica particles. The amount of dye present in the filtrate and in each wash liquor has been quantified in Table 10 using absorbance measurements

[0222]

[0223] Table 10 - absorbance measurements conducted on the reaction filtrate and the liquors from the each of three washings conducted on Examples 7, 8 and 9.

[0224] 69631465-1Use of multiple dyes

[0225] As mentioned above, the colourant may comprise two or more dyes. The effect of using two dyes can be seen in figure 4, which shows a photograph of coloured silicas particles produced according to the method of the present invention using a 1:1 blend of blue and yellow dye by weight (food blue No.1 and tartrazine). The silica particles appear green in colour rather than blue or yellow.

[0226] Colour bleed vs Pore diameter

[0227] Figure 5 shows the correlation of the pore diameter of coloured silica particles comprising food blue dye No.1 with their colour bleed properties. Coloured silica particles with a range of mean pore diameters were produced by varying the dry solids content of the coloured sodium silicates that were used to produce the coloured silica particles. The colour bleeding of the samples was then determined by measuring the visible absorbance of an aqueous filtrate that had been separated from slurried coloured particles. Details of this absorbance measurement can be found in the “colour bleed measurements” section within the methods described above.

[0228] It can be seen from figure 5 that the pore diameter correlates with the overall colour bleeding. Specifically, filtrates of coloured silica particles with larger pore diameters showed a larger absorbance value. A larger absorbance value indicates that the filtrate has a higher quantity of dye, which arises from an increased bleeding of the dye out of the coloured silica particles while the coloured silica particles were part of the aqueous slurry. The filtrates of silica particles with a pore diameter of less than 2.1 showed particularly low absorbance, indicating particularly advantageous colour bleeding properties for these silica particles.

[0229] Colour bleed vs NMR surface area

[0230] Figure 6 shows the correlation of the accessible surface area at the external surface of coloured silica particles comprising food blue dye No.1 to their colour bleed properties. In line with Figure 5, the colour bleeding properties are evidenced by the visible absorbance of an aqueous filtrate that had been separated from slurried coloured silica particles. The accessible surface area is shown using the wetted surface area measured using low-field NMR relaxation.

[0231] 69631465-1The samples in Figure 6 were obtained according to the method described above and then either measured as synthesised, or sieved to sub 45 micron levels. The result of the sieving was an increase in the average external surface area.

[0232] As can be seen in Figure 6, samples which showed a wetted surface area of 20 m2 / g or less using low-field NMR measurements (the unsieved samples) showed a significantly lower absorbance of the aqueous filtrate, and therefore reduced colour bleeding, compared to particles with higher external surface areas.

[0233] Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations are contemplated without departing from the principle and scope of the invention. Accordingly, the scope of the present invention defined herein and particularly the following claims should be interpreted in consideration of the appropriate equivalents. The terms "a", "an" and "the" do not preclude the presence of multiple referents, unless the context clearly dictates otherwise. Optional or optionally means that the feature or activity may or may not be present. Either is contemplated. In embodiments, the optional feature or features may be present. Alternatively, the optional feature or features may not be present. Ranges may be expressed herein as “from” one particular value, and / or “to” another particular value, which is intended to be inclusive of the end-points of the range.

[0234] The present invention may be further understood with reference to the following numbered clauses:

[0235] Clause 1. A method of manufacturing coloured silica particles, the method comprising:

[0236] contacting a coloured silicate composition with an acid to provide the coloured silica particles,

[0237] wherein the coloured silicate composition comprises an alkali metal silicate and a colourant and has a dry solids content of 60% or more.

[0238] Clause 2. The method of clause 1 , wherein the alkali metal silicate is sodium silicate or potassium silicate.

[0239] 69631465-1Clause 3. The method of clause 2, wherein the alkali metal silicate is a sodium silicate.

[0240] Clause 4. The method of any one preceding clause, wherein the coloured silicate composition has a dry solids content of 75% or more.

[0241] Clause 5. The method of clause 4, wherein the coloured silicate composition has a dry solids content of 80% or more.

[0242] Clause 6. The method of clause 5, wherein the coloured silicate composition has a dry solids content of 84% or more.

[0243] Clause 7. The method of any one preceding clause, wherein the colourant comprises a dye and / or a pigment.

[0244] Clause 8. The method of clause 7, wherein the colourant comprises a dye.

[0245] Clause 9. The method of clause 8, wherein the colourant comprises two or more dyes.

[0246] Clause 10. The method of clause 8, wherein colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1, FD&C Blue No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), or a mixture of two or more thereof.

[0247] Clause 11. The method of any one preceding clause 10, wherein the acid is an inorganic acid.

[0248] Clause 12. The method of clause 11 , wherein the inorganic acid is selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, or a mixture of two or more thereof.

[0249] Clause 13. The method of clause 12, wherein the inorganic acid is sulfuric acid.

[0250] 69631465-1Clause 14. The method of any one preceding clause, wherein the acid is in an aqueous solution with a concentration of 30 wt% or less.

[0251] Clause 15. The method of clause 14, wherein the acid has a concentration of from 0.5 to 30 wt%.

[0252] Clause 16. The method of clause 15, wherein the acid has a concentration of from 2 to 20 wt%.

[0253] Clause 17. The method of clause 16, wherein the acid has a concentration of from 4 to 10 wt%.

[0254] Clause 18. The method of any one preceding clause, wherein a molar ratio of the H+of the acid to the A2O of the alkali metal silicate of the coloured silicate composition is from 1 to 3.

[0255] Clause 19. The method of clause 18, wherein the molar ratio of the H+of the acid to the A2O of the alkali metal silicate is from 1.5 to 2.5.

[0256] Clause 20. The method of clause 19, wherein the molar ratio of the H+of the acid to the A2O of the alkali metal silicate is from 1.8 to 2.2.

[0257] Clause 21. The method of clause 20, wherein the molar ratio of the H+of the acid to the A2O of the alkali metal silicate is about 2.

[0258] Clause 22. The method of any one preceding clause, wherein contacting the coloured silicate composition with the acid comprises reacting the coloured silicate composition with the acid for a period of from 1 minute to 6 hours.

[0259] Clause 23. The method of any one preceding clause, wherein the alkali metal silicate has a silica to alkali metal oxide molar ratio of 3.3 or higher.

[0260] Clause 24. The method of clause 23, wherein the alkali metal silicate has a silica to alkali metal oxide molar ratio of 3.75 or higher.

[0261] 69631465-1Clause 25. The method of clause 25, wherein the alkali metal silicate has a silica to alkali metal oxide molar ratio of 4 or higher.

[0262] Clause 26. The method of any one preceding clause, further comprising a step of drying a mixture comprising an alkali metal silicate and a colourant to provide the coloured silicate composition.

[0263] Clause 27. The method of clause 26, wherein drying the mixture comprises drying the mixture as a layer with a thickness of 1 cm or less.

[0264] Clause 28. The method of clause 26 or 27, wherein the drying is conducted by heating at a temperature of from 40 to 120 °C.

[0265] Clause 29. The method of claim 28, wherein the drying is conducted by heating at a temperature of from 45 to 105 °C.

[0266] Clause 30. The method of clause 29, wherein the drying is conducted by heating at a temperature of from 50 to 80 °C.

[0267] Clause 31. The method of clause 30, wherein the drying is conducted by heating at a temperature of from 60 to 65 °C.

[0268] Clause 32. The method of any of clauses 26 to 31 , wherein the drying is conducted for a time period of from 12 to 48 hours.

[0269] Clause 33. The method of any of clauses 26 to 32, further comprising a step of adding a colourant to an alkali metal silicate composition to provide the mixture comprising the alkali metal silicate and the colourant.

[0270] Clause 34. The method of clause 33, wherein the colourant is added to the alkali metal silicate composition in an amount of from 0.001 % to 1 % based on the silica content of the alkali metal silicate on a dry weight basis.

[0271] 69631465-1Clause 35. The method of clause 36, wherein the colourant is added to the alkali metal silicate composition in an amount of from 0.005% to 0.1 % based on the silica content of the alkali metal silicate on a dry weight.

[0272] Clause 36. The method of clause 35, wherein the colourant is added to the alkali metal silicate composition in an amount of from 0.01% to 0.05% based on the silica content of the alkali metal silicate on a dry weight.

[0273] Clause 37. The method of any of clauses 33 to 36, further comprising a step of mixing an initial alkali metal silicate, an amorphous silica and a liquid to provide the alkali metal silicate composition, wherein the alkali metal silicate in the composition has an increased silica to alkali metal oxide molar ratio compared to the initial alkali metal silicate.

[0274] Clause 38. The method of any one preceding clause, further comprising a step of milling the coloured silicate composition and / or sieving the coloured silicate composition prior to contacting the coloured silicate composition with the acid.

[0275] Clause 39. The method of clause 38, wherein the weight median particle diameter of the coloured silicate composition after milling and / or sieving is of from 200 to 750 microns.

[0276] Clause 40. The method of clause 39, wherein the weight median particle diameter of the coloured silicate composition after milling and / or sieving is of from 300 to 650 microns.

[0277] Clause 41. The method of clause 40, wherein the weight median particle diameter of the coloured silicate composition after milling and / or sieving is of from 350 to 550 microns.

[0278] Clause 42. The method of any one preceding clause, further comprising a step of filtering, and optionally washing, the coloured silica particles.

[0279] Clause 43. The method of clause 42, further comprising drying the filtered coloured silica particles.

[0280] Clause 44. The method of clause 43, further comprising sieving the dried filtered coloured silica particles.

[0281] 69631465-1Clause 45. Coloured silica particles obtained or obtainable by the method of any one of clauses 1 to 44.

[0282] Clause 46. The coloured silica particles of clause 45, wherein the silica particles comprise pores having a mean pore diameter of 3 nm or less and colourant is contained within the pores.

[0283] Clause 47. Coloured silica particles comprising:

[0284] silica particles comprising pores having a mean pore diameter of 3 nm or less; and

[0285] a colourant contained within the pores.

[0286] Clause 48. The coloured silica particles of clause 47, wherein the pores have an average diameter of from 1 nm to 2.5 nm.

[0287] Clause 49. The coloured silica particles of clause 48 wherein the pores have an average diameter of from 1.5 nm to 2.3 nm.

[0288] Clause 50. The coloured silica particles of clause 49 wherein the pores have an average diameter of from 1.8 nm to 2.1 nm.

[0289] Clause 51. The coloured silica particles of any of clauses 47 to 50, wherein the coloured silica particles display a wetted surface area of 50 m2 / g or less when measured using low-field NMR relaxation.

[0290] Clause 52. The coloured silica particles of clause 51, wherein the coloured silica particles display a wetted surface area of 20 m2 / g or less when measured using low-field NMR relaxation.

[0291] Clause 53. The coloured silica particles of any of clause 47 to 52, wherein the colourant comprises a dye or a pigment.

[0292] Clause 54. The coloured silica particles of clause 53 wherein the colourant comprises a dye.

[0293] Clause 55. The coloured silica particles of clause 54, wherein the colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1, FD&C Blue

[0294] 69631465-1No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), or a mixture of two or more thereof.

[0295] Clause 56. The coloured silica particles of any of clauses 47 to 56, wherein the coloured silica particles have an acidity such that 5 g of the coloured particles on a dry mass basis dispersed in 95 g of deionised water at 20 °C and atmospheric pressure exhibits a pH of from 7 to 11.

[0296] Clause 57. The coloured silica particles of clause 56, wherein the coloured silica particles have an acidity such that 5 g of the coloured particles on a dry mass basis dispersed in 95 g of deionised water at 20 °C and atmospheric pressure exhibits a pH of from 8 to 10.

[0297] Clause 58. The coloured silica particles of any of clauses 47 to 57, wherein the coloured silica particles have a weight median particle diameter value of 200 pm to 600 pm.

[0298] 69631465-1

Claims

CLAIMS:

1. A method of manufacturing coloured silica particles, the method comprising:contacting a coloured silicate composition with an acid to provide the coloured silica particles,wherein the coloured silicate composition comprises an alkali metal silicate and a colourant and has a dry solids content of 60% or more.

2. The method of claim 1, wherein the alkali metal silicate is a sodium silicate.

3. The method of any preceding claim, wherein the coloured silicate composition has a dry solids content of 75% or more.

4. The method of any preceding claim, wherein the colourant comprises a dye and / or a pigment.

5. The method of claim 4, wherein colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1, FD&C Blue No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), or a mixture of two or more thereof.

6. The method of any preceding claim, wherein the acid is an inorganic acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, or a mixture of two or more thereof.

7. The method of any preceding claim, wherein a molar ratio of the H+of the acid to the A2O of the alkali metal silicate of the coloured silicate composition is from 1 to 3.

8. The method of any preceding claim, wherein the alkali metal silicate has a silica to alkali metal oxide molar ratio of 3.3 or higher.

9. The method of any one preceding claim, further comprising a step of drying a mixture comprising an alkali metal silicate and a colourant to provide the coloured silicate composition.69631465-110. The method of claim 9, wherein drying the mixture comprises drying the mixture as a layer with a thickness of 1 cm or less.

11. The method of claims 9 or 10, further comprising a step of adding a colourant to an alkali metal silicate composition to provide the mixture comprising the alkali metal silicate and the colourant.

12. The method of claim 11 , wherein the colourant is added to the alkali metal silicate composition in an amount of from 0.001 % to 1 % based on the silica content of the alkali metal silicate on a dry weight basis.

13. The method of claim 12, wherein the colourant is added to the alkali metal silicate composition in an amount of from 0.01% to 0.05% based on the silica content of the alkali metal silicate on a dry weight.

14. The method of any one of claims 11 to 13, further comprising a step of mixing an initial alkali metal silicate, an amorphous silica, and a liquid to provide the alkali metal silicate composition, wherein the alkali metal silicate in the composition has an increased silica to alkali metal oxide molar ratio compared to the initial alkali metal silicate.

15. The method of any one preceding claim, further comprising a step of milling the coloured silicate composition and / or sieving the coloured silicate composition prior to contacting the coloured silicate composition with the acid, wherein the weight median particle diameter of the coloured silicate composition after milling and / or sieving is of from 200 to 750 microns.

16. Coloured silica particles obtained or obtainable by the method of any one of claims 1 to 15.

17. Coloured silica particles comprising:silica particles comprising pores having a mean pore diameter of 3 nm or less; anda colourant contained within the pores.

18. The coloured silica particles of claim 17 wherein the pores have an average diameter of from 1 nm to 2.5 nm.69631465-119. The coloured silica particles of claim 18 wherein the pores have an average diameter of from 1.5 nm to 2.3 nm.

20. The coloured silica particles according to any one of claims 17 to 19, wherein the coloured silica particles display a wetted surface area of 50 m2 / g or less when measured using low-field NMR relaxation.

21. The coloured silica particles of claim 20, wherein the coloured silica particles display a wetted surface area of 20 m2 / g or less when measured using low-field NMR relaxation.

22. The coloured silica particles of according to any of claims 17 to 21 , wherein the colourant comprises a dye or a pigment.

23. The coloured silica particles of claim 22, wherein the colourant comprises a dye selected from the group consisting of: FD&C Blue No. 1, FD&C Blue No. 2, FD&C Green No. 3, Orange B, Citrus Red No. 2, FD&C Red No. 3, FD&C Red No. 40, FD&C Yellow No. 5, FD&C Yellow No. 6, FD&C Red No. 2, Food Red 102 (New Coxin), Food Red 104 (Phloxine), Food Red 105 (Rose Bengal), Food Red 106 (Acid Red), ora mixture of two or more thereof.

24. The coloured silica particles according to any of claims 17 to 23, wherein the coloured silica particles have an acidity such that 5 g of the coloured particles on a dry mass basis dispersed in 95 g of deionised water at 20 °C and atmospheric pressure exhibits a pH of from 7 to 11.

25. The coloured silica particles according to any one of claims 17 to 24, wherein the coloured silica particles have a weight median particle diameter value of 200 pm to 600 pm.69631465-1

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