Process for preparing a coloured granular alkali metal silicate

Description

[0001] The invention relates to coloured granules which can be used as speckles in detergent powders and to a process for manufacturing speckles.

[0002] Coloured granules in detergent compositions (detergent speckles) are well-known in the art, especially in phosphate containing detergent compositions and the detergent speckles therefore are usually mainly based on soluble inorganic salts like phosphates, especially sodium triphosphate, sodium sulphate or mixtures therefrom, which granules have been coated/impregnated with a solution or dispersion of a colourant i.e. a dye or pigment. Preferable the speckles are green or blue. Speckles give the detergent composition a more attractive consumer appeal, moreover it is possible to incorporate special purpose ingredients via the speckles in the detergent composition.

[0003] It is desirable that the amount of colourant in the granular material is so low as not to interfere with the washing process, moreover the speckles should be bright coloured and if possible the detergent speckle base material should serve a useful and practical purpose in the detergent composition. For "zero-phosphate detergent compositions" of course no phosphate-based speckles can be used. Consequently there was the problem of providing a suitable coloured detergent speckle which show a uniform, bright colour whilst comprising only a low percentage of pigment or dye, which granules remain stable as to colour, which are low in attrition and non, or low, dusting, which are based on a functional base material, which have a suitable bulk density as to avoid segregation from the detergent powder upon handling/storage and which are non-caking and dissolve fast in the washing liquor.

[0004] The present invention provides coloured detergent speckles satisfying most of the above requirements in the form of granular alkali metal silicate which has been coated with a colourant i.e. a pigment or dye.

[0005] Preferably the alkali metal silicate is sodium silicate, more preferably metasilicate and/or disilicate.

[0006] Silicates have the functions of improving the washing effect and of corrosion inhibitor in detergent composition and consequently are a functional additive. Especially preferred are silicate hydrates as the base material for the speckles according to the present invention and sodium silicate penta- and/or hexahydrate give optimal results. In view of the well-known high alkalinity of alkali metal silicates it was quite unexpected that these silicates could serve as the base material of uniformly, bright coloured of a deep hue and especially colour-stable detergent speckles.

[0007] Although it is recommended that the base material for the speckles consists substantially of sodium silicate it is, however, also possible to use as the base material for the speckles a mixture of materials in which at least 50% of the dry weight consists ot alkali metal silicate, the remainder being other inorganic materials like eg. zeolite, sodium sulfate or sodium phosphate if the latter can be tolerated in the eventual detergent composition.

[0008] The coloured silicate granules according to the present invention usually have an average particle size of 0.2 - 2mm, preferably from 0.8 to 1.6 mm, diameter. Granules with an spherical shape have advantages and then the diameter is taken to determine the size. Of course the actual size of the speckles desired depends on that of the detergent powder in which the speckles are incorporated.

[0009] The colourant, i.e. dye or pigment, which forms a coating on the alkali metal silicate granules is normally present in an amount of 0.1 to 2.0% (w.w.) of the granules. The colourant usually is a commercially available dye and compounds like Monastral Blue and Monastral Green + Unidispers Blue (ex CIBA-Geigy) are preferred. (Monastral dyes are ex ICI, England.)

[0010] The coloured detergent speckles are usually incorporated in detergent powders in a level between 0.1 - 5%, preferably between 0.2 and 2.0%, by weight of the total detergent composition.

[0011] The invention extends particularly to the use of sodium silicate having Na₂O:SiO₂ ratios from 1:1.2 to 1:3.3, preferably 1:1.5 to 1:3.0. The higher ratios are generally less preferred because of slower solution. The granules of alkali metal silicate can be prepared by a number of suitable procedures, for example metasilicate (Na₂O:Silica ratio 1:1) can be granulated in a rotary barrel as described in GB 1149859 (Baker-Holloway). Silicates, for example Sodium disilicate, can be prepared in compacted form of increased bulk density by use of the procedures described in US 3875282 (Stauffer Chemical Co= and US 3931036 (PQ).

[0012] The use of silicates in compacted form is particularly valuable for speckles intended for the higher density detergent powder compositions, that is having a bulk density above 700 g/l, preferably above 800 g/l. Sodium disilicate can be prepared with a bulk density between 700 and 900 g/l by compaction between rollers. Sodium metasilicate can be prepared with a bulk density between 880 and 1,000 g/l using granulation in a rotary barrel. At the lower density ranges the use of a binder, eg. silicate solution, may be necessary to provide integrity in the compacted mass obtained by twin rollers.

[0013] Traditionally detergent speckles are manufactured by a number of methods. One method involved the spray-drying of a special coloured slurry but the obvious disadvantage is that thorough cleaning is required before normal detergent powder can be produced again. Another method involved spraying a coloured solution onto a spray-dried detergent powder, but this requires, as the previous method, a large amount of coloured solution. Also it has been suggested to spray onto granules a solution of a colourant that also comprises a hydratable salt, preferably a warm and concentrated solution, to facilitate subsequent crystallisation. For zeolite containing detergent compositions this method was found not quite practical because a high percentage of colourant was required.

[0014] The coloured detergent speckles according to the present invention, which are based on alkali metal silicate, can be prepared by spraying under suitable conditions a solution of a colourant on granular alkali metal silicate, which is kept in constant motion so as to avoid caking. This can be done e.g. by carrying out the operation in a fluidized bed, or in a mixer, followed by drying while keeping the granules in constant motion. Suitable equipment for this technique comprises fluidized beds, nozzles for the coloured liquid, for hot and cool air, Tellschig*-type mixers consisting of rotating drums and fitted with a spray-nozzle, Loedige-type mixers consisting of fixed drums with an internal scraper fitted with a spray-nozzle, Nauta*-mixers of a type consisting of a conical vessel with a rotating scraper fitted with a spray-nozzle etc. (*stands for tradename).

[0015] It is preferred to carry out the process in 4 steps to wit: pre-heating the granules, spraying on the colour, drying and cooling. The amount of dye solution or pigment suspension sprayed on the alkali metal silicate granules normally lies between 0.1 and 5.0% by weight, preferably between 0.5 - 2.0%, calculated on the alkali metal silicate.

[0016] The particle size distribution of the starting granules determines the size of the coloured speckles and most alkali metal silicate granules are available in various, often uniform or narrow particle size ranges which can be chosen dependent on the size desired in the detergent composition.

[0017] The solution which is sprayed onto the granular alkali metal silicate materiai usually contains from 0.05 to 50% of colourant preferably completely dissolved. Of course there is also the limitation of the solubility of e.g. the dye in the solvent to be observed. Water is preferred as the solvent but it is sometimes advantageous to use (aqueous) methanol to increase the solubility of the dye.

[0018] The quantity of liquid sprayed onto the granules is normally between 0.1 and 5%, preferably between 0.5 and 2%, by weight calculated on the silicate and spraying normally is completed within a few minutes to half an hour, 2 to 10 minutes is typical.

[0019] Very good results were obtained by pre-heating the alkali metal silicate granules before starting spraying with colourant solution. In any case it should be avoided to heat the alkali metal silicate granules to a temperature above their melting points, which is, in the case of sodium metasilicate pentahydrate, around 72°C. The temperature of the gas (air) blown onto the still wet granules normally varies between 25 and 70, preferably between 30 and 40°C. In order to ensure that certain silicate hydrates are substantially formed or retained it is necessary to monitor the water content of the granules during the drying operation. The percentage of water in the granule also determines its bulk density. Cooling whilst keeping the detergent speckles in constant movement also helps solidification of the surface and to prevent caking.

[0020] The detergent speckles so obtained are distinctive, of uniform colour, resistant to attrition and have virtually the same particle size distribution as the alkali metal silicate starting granules have.

[0021] The use of alkali metal silicates as a core for speckles is particularly efficient because, for one or more of the reasons,

i) silicates are an active component of detergent compositions and thus no superfluous ingredients are introduced,

ii) the silicate cores are utilised in a form in which they would, in any case, be added to a powder composition,

iii) the silicate speckles dissolve rapidly so as to provide rapid dissolution and dilution of the surface colourant,

iv) the silicate surface does not absorb the colourant, so that the latter is used efficiently, but retains the colourant so that it does not become dispersed throughout the powder,

v) the silicate speckles have a uniform and acceptable appearance, and

vi) they can be prepared with a bulk density suitable for the higher density powders.

[0022] The invention is further described and illustrated in the following examples.

EXAMPLE 1

[0023] 1.5 kg sodium metasilicate pentahydrate granules with a particle size between 0.2 and 1.2 mm was placed in a Loedige mixer of 5 litres. The mixer was switched on and the contents were pre-heated to 35 - 45°C by blowing in hot air with a temperature of 50°C. An aqueous solution containing 25% by weight of Monastral Blue was sprayed onto the granules for 5 minutes. The amount of solution sprayed onto the granules was 1% by weight calculated on the metasilicate. Consequently only 0.25% of dye was used relative to the metasilicate. Drying was also carried out at a temperature of 35-45°C for a period of 20 minutes. The water content of the metasilicate was monitored during the drying and kept at around 43% by weight. The detergent speckles were then cooled to 20°C by blowing in cool air. The detergent speckles thus obtained showed excellent brightness, a deep hue, were uniformly coloured, colour-stable and the original particle size distribution had substantially been retained. Whe mixed in a detergent powder at a level of 1% by weight they imparted an excellent consumer appeal to the product.

EXAMPLE 2

[0024] Sodium disilicate (Na₂O:SiO₂ ratio of 1:2) was spray dried to provide granules.

[0025] These were fed to a two roller press by a feed screw and compressed to a thin, ca 1mm, sheet under pressure. The sheet was then broken up by passage between intermeshing toothed wheels and sieved to provide a granular product having particle size distribution between 0.3 and 1.2 mm. The product had a bulk density of about 850 g/litre.

[0026] These granules were coated with Monastral blue using the process described in Example 1 but using a 50% aqueous suspension of the pigment to provide a coating level of 0.25% by weight of silicate. This product was then used as speckles in a detergent formulation.

Claims

1. A coloured granular material characterised in that it comprises an alkali metal silicate which granules are coated with a colourant material.

2. A coloured granular material according to claim 1 characterised in that it comprises a sodium silicate.

3. A coloured granular material according to claim 1 or 2 characterised in that the alkali metal silicate is metasilicate.

4. A coloured granular material according to any of the preceding claims characterised in that the alkali metal silicate is an alkali metal silicate hydrate.

5. A coloured granular material according to any of the claims 2 to 4 characterised in that the sodium silicate is a hydrate containing 5-6 water molecules per molecule of silicate in the crystal.

6. A coloured granular material according to any of the preceding claims characterised in that the granules have an average size of 0.2-2.0 mm.

7. A coloured granular material according to any of the preceding claims characterised in that the granules have a spherical shape.

8. A coloured granular material according to any of the preceding claims characterised in that the pigment and/or dye is chosen amongst the class consisting of Monastral Blue and Monastral Green + Unidispers Blue (ex CIBA-Geigy).

9. A process for preparing a coloured granular material characterised in that alkali metal silicate granules are sprayed with a solution of a dye or suspension of a pigment in such a way that granules are kept in motion whilst spraying, drying and cooling.

10. A detergent powder composition characterised in that this composition comprises a coloured granule as defined in any of the preceding claims.