CA2166281A1

CA2166281A1 - Coated sodium percarbonate particles, method of producing them and their use

Info

Publication number: CA2166281A1
Application number: CA002166281A
Authority: CA
Inventors: Birgit Bertsch-Frank; Martin Bewersdorf; Claas-Jurgen Klasen; Thomas Lieser; Klaus Muller; Jurgen Rollmann
Original assignee: Individual
Current assignee: Evonik Operations GmbH
Priority date: 1993-07-17
Filing date: 1994-04-23
Publication date: 1995-01-26
Also published as: KR960703802A; IL110331A0; ES2113102T3; DE4324104A1; CO4180441A1; FI960208A0; EP0710215A1; EP0710215B1; JPH09500084A; TR27995A; SK4296A3; DE4324104C2; ZA945218B; CZ1896A3; PL312608A1; DE59405122D1; HU9600079D0; CN1127498A; IL110331A; ATE162497T1

Abstract

In the prior art, sodium percarbonate is coated with compounds which produce a stabilizing action in order to increase the shelf life. There is a need for an improved, boron-free, coating for sodium percarbonate. Proposed by the invention is a particle coating containing sodium carbonate plus one or more optionally partly hydrated magnesium compounds, preferably magnesium sulphate, in a single coat or in separate coats. The coating preferably contains two coats. The particles are produced by spraying aqueous solutions containing the coating compounds on to sodium percarbonate in a fluidized bed and at the same time evaporating off the water. Their long shelf life makes particles coated in this way suitable for use in washing, bleaching and cleaning agents.

Description

1 216~2~l Description The invention relates to coated sodium percarbonate particles consisting of a core of sodium percarbonate and a coating which contains sodium carbonate, which constitutes 1 to 25 wt.~ of the sodium percarbonate. The coated sodium percarbonate particles, which also contain a magnesium compound as a coating component, are characterised by high storage stability in the presence of washing powders in a moist and warm environment. The invention further relates to a process for preparing the coated sodium percarbonate particles by applying a mono or multi-layered coating in a fluidised bed. The invention also relates to the uæe of the coated sodium percarbonate particles as bleaching co,.,~ollents in detergent, cleansing agent and bleaching agent formulations.

Sodium percarbonate (2Na2C03.3H202) is used as an active oxygen component in detergents, bleaches and cleansing agents. Due to the unsatisfactory storage stability of sodium percarbonate in a warm and moist environment and in the presence of various components in detergents and cleansing agents, sodium percarbonate has to be stabilised against the loss of active oxygen (8). An essential principle for stabilisation consists of surrounding the sodium percarbonate particles with a coating of stabilising components.

GB 174 891 has already disclosed spraying active oxygen-containing compounds with a sodium waterglass solution and then drying, in order to increase the storage stability.
- Sodium percarbonate is also stabilised by applying an adequate amount of silicate to sodium percarbonate particles, obtained by crystallisation, in the process in DE-OS 26 52 776. A satisfactory stabilising effect, especially in the presence of detergents and cleansing agents, is not produced by the previously mentioned methods.

Processes for stabilising particulate sodium percarbonate are known from DE-OS 24 17 572 and DE-PS 26 22 610, wherein either a mixed compound which is formed by crystallising a sodium carbonate with other inorganic salts such as sodium bicarbonate and/or sodium sulphate, or a mixture of sodium carbonate, sodium sulphate and a sodium silicate is used as a coating substance. In these processes, an aqueous solution of the constituents for the coating material is sprayed onto sodium percarbonate particles in a fluidised bed while maintaining a fluidised bed temperature of between 30 and 80C, wherein a solid coating is built up by evaporation of the water which is introduced. Despite the greatly improved stability of sodium percarbonate particles 20 coated in this way, the active oxygen content still decreases too rapidly during long-term storage in the presence of a washing powder.

Numerous methods for the effective stabilisation of sodium 25 percarbonate by using boron compounds, such as boric acids (DE-PS 28 00 916), borates (DE-OS 33 21 082) and perborates (DE-PS 26 51 442 and DE-PS 28 10 379) are known. Despite a sometimes very good stabilising effect, the market is increasingly interested in coated sodium percarbonate which 30 does not contain boron compounds.

Another type of coating component is known from US
4,325,933. According to this, stabilisation is achieved by treating the sodium percarbonate with an aqueous solution 35 of an alkaline earth metal salt, either magnesium sulphate or magnesium chloride. When treating the sQdium percarbonate particles, a layer made of an alkaline earth 66~l _ 3 carbonate is formed on the surface of the particles, which reduces the hygroscopic character of the sodium percarbonate and increases the stability. However, it has been shown that this type of stabilised product does not possess satisfactory storage stability.

EP-A 0 405 797 discloses sodium percarbonate compositions which are more reliable. According to one embodiment, the composition contains a compound from the group of inorganic magnesium compounds in addition to an alkali metal carbonate. The composition mentioned does not comprise sodium percarbonate particles with a uniform coating which adheres firmly to the sodium percarbonate core, but a mixture of substances which can also be granulated. During the finishing operations in the process given in the document mentioned, it was established that the storage stability of this type of composition, in the presence of a washing powder in a warm and moist environment, is not satisfactory; see comparison examples VB 2 to VB 4.
Accordingly, the object of the invention is to provide new, coated, sodium percarbonate particles with high storage stability in the presence of washing powders, wherein the coating should not contain boron compounds. The storage stability of the new coated sodium percarbonate particles is intended preferably to surpass that which has been obtainable using hitherto known, boron-free, coated sodium percarbonate particles. A further object of the invention relates to the provision of a suitable process for preparing particularly storage-stable, boron-free, coated, sodium percarbonate particles.

Coated sodium percarbonate particles comprising a core of sodium percarbonate and a coating which contains sodium carbonate, which constitutes 0.5 to 25 wt/~ (calculated hydrate-free) of the sodium percarbonate, which are characterised in that the coating also contains one or more 216~
_ 4 magnesium compounds from the group of salts of sulphuric acid, hydrochloric acid and carboxylic acids with 1 to 4 carbon atoms or the reaction products of the salts mentioned with sodium carbonate and/or other optionally present coating components, wherein the coating components may be partially hydrated and sodium carbonate and the one or more magnesium compounds may be located in a single layer or in separate layers of the coating, were found.

The coated sodium percarbonate particles according to the invention thus consist of a sodium percarbonate core and a mono or multi-layered coating which contains the stabilising components in a hydrate-free and/or partially hydrated form. The sodium percarbonate core is completely surrounded by the coating which adheres firmly thereto, wherein the thickness of the coating layer is approximately constant over the whole particle.

The amount of coating, calculated as hydrate-free, generally constitutes 0.5 to 25 wt.% of the sodium percarbonate. Although it is possible to prepare particles with a coating of less then 0.5 wt.~ or more than 25 wt.%, the products in the first case possess only moderate storage stability and in the second case have a reduced active oxygen content, corresponding to the increased amount of coating material.

In preferred products, the coating constitutes a total of about 1 to 15 wt.% of the sodium percarbonate, calculated as hydrate-free.

An essential feature of the invention is that the coating contains both sodium carbonate and one or more magnesium compounds, wherein the substances mentioned may be present in an anhydrous and/or in a partially hydrated form. The term ~'partially hydrated~ is to be understood as me~n;ng that the m~; ml]m possible water acceptance capacity of the 2i6~28l coating components due to hydrate formation has not been exhausted in the coated sodium percarbonate particles according to the invention. In the case of, for example, a coating consisting of MgSO4 and Na2CO3 in the form of their hydrates the water acceptance capacity is exhausted when Na2CO3 is present as the monohydrate and MgSO4 is present as the heptahydrate.

The coating contains one or more magnesium compounds from the group magnesium sulphate, magnesium chloride and magnesium salts of a carboxylic acid with 1 to 4 carbon atoms. Hydrates of the compounds mentioned are also included. From the magnesium salts of carboxylic acids, which may contain one or two carboxyl groups and optionally one or two hydroxyl groups, magneRium acetate is preferred.
When preparing mono-layered coated sodium percarbonate particles, the coating may also contain reaction products - of the previously mentioned salts with sodium carbonate and/or other optionally present coating components such as, for example, sodium silicates; in this way, for example, magnesium carbonate, basic magnesium carbonate, mixed salts of sodium carbonate and magnesium sulphate and, in the case of the presence of sodium silicates, also magnesium silicates, may also arise as constituents of the coating.
Coated sodium percarbonate particles according to the invention may have a mono or multi-layered, preferably two or three-layered, coating. The coating on preferred coated sodium percarbonate particles comprises at least one layer of essentially sodium carbonate and at least one layer of essentially one or more magnesium compounds, in particular magnesium sulphate and hydrates of the same. When building up the at least two-layered structure mentioned for the coating, reactions between the various components in the coating can be restricted to the relevant boundary surfaces of the layers. A two or three-layered coating of the previously mentioned type has an advantageous effect on the `` 216~28~
_ 6 storage stability of the coated sodium percarbonate particles.

With a two-layered structure for the coating, the layer which contains sodium carbonate may also contain other stabilisers, but it preferably does not contain magnesium compounds. The supplementary stabilisers should produce a clear solution in the presence of sodium carbonate and also should not display any stability-reducing effect in the solid coating. Active components which can be found in the layer which contains sodium carbonate are alkali metal silicates, especially sodium silicate with a molar ratio of SiO2 to Na20 in the range 4 to 1 to 1 to 1, preferably in the range 2.5 to 1 to 3.5 to 1. Although in the case of a multi-layered structure for the coating, the layer which contains at least one magnesium compound may also contain other components, among which is also sodium carbonate, it is generally expedient to allocate no compounds other than the magnesium compounds which are being considered to this layer. Preferably, this layer consists of magnesium sulphate and/or some of its hydrates or of magnesium chloride or magnesium acetate and/or some of their hydrates. Magnesium sulphate with a hydrate content between

2 and less than 7 moles of H20 per mole of MgS04 is particularly preferred.

In addition to a layer which contains essentially sodium carbonate and a layer which contains essentially at least one magnesium compound, preferred coated sodium percarbonate particles may also possess one or more layers of essentially alkali metal silicates, especially sodium silicates of the previously mentioned composition.

With regard to the stabilising effect, the sequence of the 35- layered structure in the coating has only a moderate effect. The sequence of layers can have an effect, however, on the flowability of the coated particles. In order to ` 2l~6~l _ 7 obtain a product which flows particularly well, it may therefore be advantageouæ to build up the coating so that the external layer consistæ essentially of sodium carbonate. Coated particles with a layered structure in the sequence, from inside to outside, of magnesium sulphate, sodium carbonate, sodium silicates, wherein these substances may be partially hydrated, exhibit exceptional storage-stability. The storage stability of thiæ type of product exceedæ that of productæ which have only a magneæium sulphate and a sodiùm carbonate layer with no alkali metal silicate in either layer.

Preferred coated sodium percarbonate particles contain, in one or more coating layers, as stabilising components, essentially sodium carbonate and/or hydrates of the same in an amount of 0.2 to 10 wt.~, preferably 0.5 to 5 wt.~, calculated as Na2cO3, one or more magnesium compounds and/or hydrates of the same, especially magneæium sulphate, in an amount of 1 to 10 wt.~, preferably 0.5 to 5 wt.~, calculated as MgSO4, and æodium æilicates and/or hydrateæ of the æame with a molar ratio of SiO2 to Na2O of 4 to 1 to 1 to 1 in an amount of 0 to 5 wt.~, preferably 0.2 to 3 wt.~, calculated hydrate-free, wherein the amountæ given are each with reference to sodium percarbonate.
In addition to the substances mentioned and to the optional reaction products arising therefrom, other stabilising components may also be contained in one or more of the coating layers in generally smaller amounts, i.e. up to about 2 wt.~, with reference to sodium percarbonate. The additional stabilising componentæ are in particular typical active oxygen stabilisers such as aminopolycarboxylic acids which form chelate complexes, such as ethylenediamine tetraacetic acid and diethylenetriamine pentaacetic acid;
- 35 phosphonic acid compounds which form chelate complexes, su-ch as, for instance, 1-hydroxyethane-1,1-diphosphonic acid and ethylenediamine-tetra(methylphosphonic acid) and C~16~281 _ 8 diethylenetriamine-penta(methylenephosphonic acid) and their salts; water-soluble polymeræ with carboxyl and hydroxyl groups which are capable of forming complexes, such as, for instance, polymers based on alpha-hydroxyacrylic acid; stabilisers such as pyridinecarboxylicacids, such as, for instance, dipicolinic acid, may also be present.

Particularly preferred coated sodium percarbonate particles with a core of sodium percarbonate produced by crystallisation from aqueous phase, contain, in the coating, sodium carbonate and/or some of its hydrates in an amount of 2 to 6 wt.~, hydrate-free and/or some hydrate-cont~in;ng magnesium sulphate in an amount of 2 to 6 wt.~, hydrate-free and/or some hydrate-containing sodium silicates, with a molar ratio of SiO2 to Na20 between 3.5 to 1 and 2.5 to 1, in an amount of 0.5 to 3 wt.% and if required one or more of the previously mentioned other stabilising components in an amount of 0 to 2 wt.%, in particular 0 to 1 wt.~, each with reference to sodium percarbonate and calculated as hydrate-free.

If sodium percarbonate produced by spray granulation is coated, the amount of coating may be reduced as compared with the previously mentioned amounts because in this case the surface of the particleæ to be coated is smoother than in the case of particles produced by crystallisation. To coat sodium percarbonate particles with smooth surfaces, 0.5 to 5 wt.~ of Na203, 0.5 to 5 wt.% of MgS04 and 0.2 to 2 wt.~ of sodium silicates (SiO2:Na20 = 2.5 to 3.5 to 1), each with reference to sodium percarbonate, are used.

As can be seen from the examples and comparison examples, the coated sodium percarbonate particles according to the invention are distinguished by unexpectedly high storage stability in the presence of zeolite-containing detergent tower powders as compared with previously known, also 2~6~
g boron-free, coated particles. After 8 weeks storage at 30C
and 80~ relative humidity, the relative active oxygen content (Oa-retention) of the products according to the invention is approximately 70 to almost 100%, depending on the structure of the layers, the amount of coating and the method of preparation of the sodium percarbonate used.
Particularly preferred coated sodium percarbonate particles have an Oa-retention of between 93~ and about 97~ after 8 weeks a~m;xe~ with a detergent under the storage conditions mentioned.

Among the methods frequently used in the specialist sector for applying a coating layer to products being stabilised are methods using a mixer, wherein the product being stabilised, optionally after moistening the same, is treated with powdered stabilisers, and so-called fluidised bed methods, wherein the coating components, in the form of an aqueous solution, are sprayed onto the product being stabilised which is located in the fluidised bed and the water which is hereby introduced is simultaneously evaporated. It was found that coating sodium percarbonate with coating components according to the invention in a fluidised bed led to a product with an essentially higher storage stability than the previously known coating procedure using a mixer.

The process for preparing coated sodium percarbonate particles according to the invention comprises spraying an aqueous solution which contains one or more coating components onto the particles to be coated, which are located in a fluidised bed, and evaporating the water while maintaining a fluidised bed temperature of 30 to 100C, and is characterised in that at least one aqueous solution which contains sodium carbonate and at least one aqueous solution which contains a magnesium salt from the group Mg sulphate, Mg chloride and Mg carboxylate of a carboxylic acid with 1 to 4 carbon atoms, are sprayed onto the 2~ ~62~1 particles being coated, simultaneously or one after the other in any sequence, wherein the total amount of coating components applied is 1 to 25 wt.~ (calculated hydrate-free), with respect to the sodium percarbonate.
The solutions containing sodium carbonate and magnesium salt are preferably sprayed on one after the other. It is particularly expedient if a layer which contains soda is applied as the outermost layer on the particles. According to another embodiment of the process, an aqueous solution which contains essentially sodium carbonate is initially sprayed onto the particles being coated, then an aqueous solution which contains essentially magnesium sulphate and lastly a solution which again contains essentially sodium carbonate.

According to another embodiment of the process, a solution containing sodium carbonate which also contains alkali metal silicates, preferably sodium silicates, is used. As an alternative to this, one or more coating layers consisting essentially of an alkali metal silicate and/or some of its hydrates may be built up by spraying an aqueous solution of one or more alkali metal silicates, such as for example a sodium waterglass solution, onto the particles being coated, which may already possess one or more coating layers. When using a solution which contains sodium silicates, a molar ratio of SiO2 to Na20 of 4 to 1 to 1 to 1, preferably 2.5 to 1 to 3.5 to 1, and in particular about

3 to 3.5 to 1, is the basic parameter.
If the coating components sodium carbonate and one or more magnesium compounds are intended to be located in a single coating layer, the sodium carbonate solution and the magnesium salt solution, preferably magnesium sulphate solution, are sprayed onto the particles to be coated from two separate nozzles which are suitably positioned in the fluidised bed or from a three-component nozzle with 11 2l662~l external mixing. In this emboA;ment also, the solution containing sodium carbonate may also contain alkali metal silicates. In addition, the solutions may contain other stabilisers which are compatible with sodium carbonate or the magnesium compounds.

The relevant solutions are sprayed onto the particles to be coated in an amount such that the resulting coated particles possess the previously mentioned amounts of coating components in one or more layers. The concentration of coating components in the solutions being sprayed is any at all per se, but the most highly concentrated solutions possible are preferred in order to keep the amount of water to be evaporated as low as possible. Preferably, an approximately saturated sodium carbonate solution and an approximately saturated magnesium salt, in particular magnesium sulphate, solution, are used. Sodium silicates are preferably used in the form of a waterglass solution (35 to 40 Baumé), which contains SiO2 and Na20 in a molar ratio of about 3.5 to 1.

The technique of applying an aqueous solution which contains one or more coating components to the particles to be coated, that is sodium percarbonate or already partly coated sodium percarbonate, in a fluidised bed is known to a person skilled in the art, reference being made, for example, to DE-PS 26 22 610. In a conventional continuously or batchwise operated fluidised bed device, a fluidised bed is formed from the sodium percarbonate to be coated by using the drying air. The aqueous solutions containing the coating components are sprayed from nozzles, simultaneously or one after the other, wherein the water introduced with the solutions is simultaneously evaporated. The amount of drying air and its temperature are governed both by the amount of water introduced to the fluidised bed with the - solutions and by the degree of drying which is desired. The two parameters are matched to each other by the person ~l~628l skilled in the art in such a way that a temperature in the range 30 to 100C, preferably 50 to 80C, is maintained in the fluidised bed. Furthermore, it may be expedient that the solutions being sprayed already have a temperature in the range 30 to 80C. If required, the coated particles may subsequently be dried at 60 to 100C, in particular at 70 to 90C, in order to produce satisfactory dehydration of the hydrate-forming coating components.

For the continuous preparation of multi-layered coated sodium percarbonate particles, trough-shaped fluidised bed dryers with two or more spray zones and, if required, an after-drying zone, are preferably used.

The sodium percarbonate particles to be coated may have been produced by any method of preparation of sodium percarbonate at all. Those which may be mentioned ;n particular are: (i) wet methods, wherein soda and hydrogen peroxide are reacted in an aqueous phase and the resulting sodium percarbonate is separated from the mother liquor;
(ii) methods, wherein solid soda is reacted directly with hydrogen peroxide; (iii) so-called spray granulation methods, wherein a soda solution and a hydrogen peroxide solution, or a solution which contains Na2CO3 and H202, are sprayed onto sodium percarbonate seeds located in a fluidised bed in a fluidised bed dryer with or without a grading outlet, with simultaneous evaporation of the water, until the desired particle size is reached, reference being made, for example, to the method in accordance with DE-OS 27 33 935.

The advantage of the process according to the invention is regarded as being that uniformly coated sodium percarbonate particles with a mono or multi-layered coating which are distinguished by extraordinarily good storage stability, are thereby obtainable. The principle of applying a coating to sodium percarbonate particles and suitable devices for 21S~8~
_ 13 this procedure are known and proven per se in the specialist sector. Performing the process is simple and the amount of coating being applied can be controlled without any problems.
The coated sodium percarbonate particles according to the invention can be used as a bleaching comro~ent in detergents, cleansing agents and bleaching compositions.
This type of detergent, cleansing agent and bleaching composition is characterised in that the coated sodium percarbonate cont~;n~d therein has an unexpectedly high storage stability, even in the presence of zeolites, so that there is only a very slow loss of active oxygen during conventional storage times for this type of composition.
Detergent, cleansing agent and bleaching compositions which are suitable consist of 1 to 99 wt.~ of the coated sodium percarbonate according to the invention and the rem~;n;ng amount consists of up to 100 wt.~ of other conventional components for this type of composition. The following components in particular may be mentioned:

1. Surface active agents from the group of cationic, anionic, non-ionic, amphoteric or ampholytic surface active agents.
2. Inorganic and/or organic builders, whose main action comprises sequestering or complexing the metal ions which are responsible for hardness in the water, for example zeolites, sheet silicates, polyphosphates, aminopolyacetic acids and aminopolyphosphonic acids as well as polyoxycarboxylic acids.

3. Alkaline and inorganic electrolytes such as, for example, alkanolamines and silicates, carbonates and sulphates.

`` . . 21~6281

4. Bleach activators from the group of N-acyl compounds and O-acyl compounds, for example tetraacetyl ethylenediamine (TAED).

5. Other constituents in the agents may be stabilisers for peroxides such as in particular magnesium salts, anti-deposition agentæ, optical brighteners, foam inhibitors, enzymes, disinfectants, corrosion inhibitors, fragrances, dyes and agents for regulating the pH. With regard to individual compounds included in the classes of substances 1 to 5, reference is made, for example, to DE-OS 33 21 082, pages 14-30.

The following examples and comparison examples clearly show the much higher stabilising effect of the coating according to the invention as compared to similarly structured and similarly prepared previously known, boron-free, coatings on sodium percarbonate.

2i~28l _ 15 Examples a) General description of coating in a mixer, not according to the invention The sodium percarbonate to be coated, which has a moisture content of 5 to 10 wt.~ as a result of the method of preparation, is treated with the coating componentæ(s) under continuous mixing in a plough-share mixer. If dry sodium percarbonate is used, this is first brought up to the previously mentioned moisture content by spraying with water or an aqueous solution which contains one or more of the coating components and then mixed with further coating components. Magnesium sulphate was used as the lS heptahydrate, sodium carbonate as calcined soda; alkali metal silicate was used in the form of a sodium waterglass solution (37Bé) (SiO2 : Na20 = ca. 3.5 to 1) and sprayed onto dry sodium percarbonate. After completion of the m; ~; ng procedure, the coated product is dried at 60 to 70C
in a fluidised bed dryer until it has a residual moisture content (Karl Fischer) of less than 0.5~.

b) General description of coating in a fluidiæed bed according to the invention In a laboratory fluidised bed dryer, the aqueous solutions containing the coating components are sprayed, simultaneously or one after the other, onto the fluidised bed constructed by using the drying air (inlet temperature 100 to 110C) and the sodium percarbonate (NaPc) to be coated, wherein the temperature of the fluidised bed is maintained within the range 40 to 60C. After-drying is performed at 80 to 90C. The solutions are sprayed by using conventional two-component nozzles with air as the propellant, wherein, to prepare mono-layered coatings according to the invention, the solutions to be used are applied by means of two two-component nozzles, but 16 21 562~1 preferably by means of one three-component nozzle with external mixing.

Aqueous solutions used: MgSO4 solution (30 or 20 wt.%
S MgSO4); Na2CO3 solution (30 or 20 wt.~ Na2cO3); sodium silicate solution ca. 37 Bé (SiO2: Na2O3 about 3.5 : 1);
combined Na2CO3/sodium silicate solution (20 wt.% Na2CO3, 8 wt.% sodium silicate), prepared from a sodium waterglass solution with ca. 37 B~ and a SiO2 to Na2O molar ratio af about 3.5 to 1. The temperature of the solutions being sprayed is 30 to 40C.

c) Determination of the storage stability when mixed with a detergent Commercially available tower washing powder (Persil Supra SP), which is phosphate-free ~ut contains zeolite, TAED
activator and the sodium percarbonate to be tested were mixed in amounts such that the mixture contained 5% TAED
and the a content was about 2.35 wt.~.

400 g or 800 g (from example 5 and comparison example 10 onwards) of the mixture are stored at 30C and 80~ relative humidity in a climatic test cabinet in commercially available E1 detergent packets which were impregnated to be water-repellent and glued together. One packet was stored for each period prior to removal, 2, 4 and 8 weeks respectively. The a content is determined iodometrically in the usual way. The respective Oa-retentions are determined as a ~-age, from the initial a content and the a contents after 2, 4 and 8 weeks.

"' 216~28~

Examples E 1 to E 4 and comparison examples CE 1 to CE 4 Table 1 shows the storage stability when mixed with detergent of the non-coated sodium percarbonate (CE 1) which is used as the starting product for the coated sodium percarbonate particles listed in Table 1, examples E 1 to E 4 and comparison examples CE 2 to CE 4 and for comparison examples CE 5 to CE 9 in Table 3.

Two- and three-layered coating according to the invention took place using the sequence of coating components given in the first column of Table 1. In the case of coating not according to the invention, in a mixer, MgSO4.7H2O and Na2CO3 were used simultaneously. The residual moisture content of the NaPc used in CE 2 and CE 3 was ca. 7%. In CE 4, NaPc was initially moistened with a waterglass solution.

The results show that the storage stability of coated sodium percarbonate according to the invention surpassed that of the product coated in a mixer, despite identical amounts of coating.

Table 2 shows the active oxygen content (a content) of sodium percarbonate coated in a mixer and in a fluidised bed, immediately after preparation and after 10 weeks storage (without mixing with a detergent). Coated sodium percarbonate according to the invention was characterised in that the a content decreased to an essentially smaller extent during storage.

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Examples E 5 to E 7 and compari~on example CE 10 Table 4 shows the storage stability when mixed with a detergent. Commercially available, non-coated sodium percarbonate from the applicant (CE 10), which had been prepared by reacting soda and hydrogen peroxide in the aqueous phase, crystallising the NaPc, separating it from the aqueous phase and drying, was uæed.
Coating was performed each time with magnesium sulphate (5 wt.~), sodium carbonate (5 wt.%) and sodium waterglass (2 wt.~, SiO2 : Na20 = 3.5 to 1~, wt.% with reference to the NaPc used. Mono-layered coating (E 5) was performed by simultaneous use of a MgS04 solution and an aqueous solution cont~;n;ng Na2C03 and sodium waterglass, which were sprayed onto the NaPc by means of a three-component nozzle. In the case of two-layered coating (E 6), the solutions mentioned were used one after the other by means of a two-component nozzle. In the case of three-layered coating (B 7), the MgS04 solution, a Na2C03 solution and lastly a sodium waterglass solution were sprayed on one after the other.

Two and three-layered coating produced an essentially better stabilising effect than mono-layered coating.

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216~2~1 _ 24 Ex~mples E 8 and E 9, comparison examrle CE 11 A sodium percarbonate (CE 11) was used which had been prepared by spray granulation in a.fluidised bed, in the same way as in the process in DE 27 33 935. 10~ of the particleæ had a particle diameter between 0.2 and 0.5 mm, 70~ had a diameter between 0.5 and 0.7 mm and 20~ had a diameter between 0.7 and 1.0 mm.

This was coated according to the invention in a fluidised bed, into which was sprayed first a MgSO4 solution and then a solution of Na2CO3 and sodium waterglass (SiO2 : Na2O = 3.5 to 1). The temperature of the spray solutions was 40C. The temperature of the fluidised bed during spraying was 50 to 60C. The temperature of the fluidised bed during after-drying was 80C.

The examples verify the extraordinary increase in storage stability of coated sodium percarbonate particles according to the invention.

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Claims

1. Coated sodium percarbonate particles comprising a core of sodium percarbonate and a coating which contains sodium carbonate, which constitutes 0.5 to 25 wt.%
(calculated hydrate-free) of the sodium percarbonate, characterised in that, the coating also contains one or more magnesium compounds from the group of salts of sulphuric acid, hydrochloric acid and carboxylic acids with 1 to 4 carbon atoms or the reaction products of the salts mentioned with sodium carbonate and/or other optionally present coating components, wherein the coating components may be partially hydrated and sodium carbonate and the one or more magnesium compounds may be located in a single layer or in separate layers of the coating.

2. Coated sodium percarbonate particles according to Claim 1, characterised in that the coating comprises at least one layer of essentially sodium carbonate and/or some hydrates of the same and at least one layer of essentially one or more magnesium compounds, in particular magnesium sulphate, and/or some hydrates of the same.

3. Coated sodium percarbonate particles according to Claim 1 or 2, characterised in that the coating also contains a layer of essentially one or more alkali metal silicates and/or some of their hydrates, in particular sodium silicates, or an alkali metal silicate is present in one or more layer which contains sodium carbonate and/or at least one magnesium compound.

4. Coated sodium percarbonate particles according to one or more of Claims 1 to 3, characterised in that the coating comprises one layer containing essentially magnesium sulphate, one layer containing essentially soda and one layer containing essentially sodium silicates, wherein some of the substances mentioned may also be present in the form of their hydrates and the sequence of layers mentioned is preferably from inside to outside.

5. Coated sodium percarbonate particles according to one or more of Claims 1 to 4, characterised in that the coating constitutes 1 to 15 wt.%, with reference to the sodium percarbonate, and the coated particles contain, in the coating layer(s), essentially sodium carbonate and/or hydrates of the same in an amount of 0.2 to 10 wt.%, preferably 0.5 to 5 wt.%, calculated as Na2CO3, one or more magnesium compounds, especially magnesium sulphate, and/or its hydrates, in an amount of 0.2 to 10 wt.%, preferably 0.5 to 5 wt.%, calculated as MgSO4, and sodium silicates or their hydrates with a molar ratio of SiO2 to Na2O of 4 to 1 to 1 to 1, in an amount of 0 to 5 wt.%, preferably 0.2 to 3 wt.%, calculated hydrate-free, each being with reference to the sodium percarbonate, as stabilising coating components.

6. A process for preparing coated sodium percarbonate particles as in Claims 1 to 5, by applying a coating of solid coating components onto sodium percarbonate particles, comprising spraying an aqueous solution containing one or more coating components onto the particles to be coated which are located in a fluidised bed and evaporating the water while maintaining a fluidised bed temperature of 30 to 100°C, characterised in that at least one aqueous solution containing sodium carbonate and at least one aqueous solution containing a magnesium salt from the group Mg sulphate, Mg chloride and a Mg carboxylate of a carboxylic acid with 1 to 4 carbon atoms is sprayed onto the particles to be coated, simultaneously or in any sequence one after the other, wherein the total amount of coating components applied is 0.5 to 25 wt.%
(calculated hydrate-free), with reference to the sodium percarbonate.

7. A process according to Claim 6, characterised in that the solution containing sodium carbonate and the solution containing a magnesium salt are sprayed on one after the other, wherein the solution containing sodium carbonate is preferably sprayed on after the solution containing a magnesium salt and magnesium sulphate is used as the preferred magnesium salt.

8. A process according to Claim 6 or 7, characterised in that a solution containing sodium carbonate is used which also contains one or more alkali metal silicates, preferably sodium silicates, or that a further coating layer is formed by spraying into the fluidised bed, using an aqueous solution which contains essentially one or more alkali metal silicates, preferably sodium silicates.

9. A process according to one of Claims 6 to 8, characterised in that sodium carbonate in an amount of 0.2 to 10 wt.%, a magnesium salt, especially magnesium sulphate, in an amount of 0.2 to 10 wt.% and sodium silicates with a molar ratio of SiO2 to Na2O of 4 to 1 to 1 to 1, preferably 2.5 to 1 to 3.5 to 1, in an amount of 0 to 5 wt.%, each with reference to the sodium percarbonate, are sprayed onto sodium percarbonate.

10. A process according to Claim 9, characterised in that sodium percarbonate which has been prepared by crystallisation from aqueous phase is coated and the amount of sodium carbonate sprayed on is 2 to 6 wt.%, the amount of magnesium sulphate sprayed on is 2 to 6 wt.% and the amount of sodium silicates with a molar ratio of SiO2 to Na2O of 2.5 to 1 to 3.5 to 1 is 0.5 to 3 wt.%, each calculated hydrate-free and with reference to sodium percarbonate.

11. A process according to Claim 9, characterised in that sodium percarbonate which has been produced by spray granulation is coated and that the amount of sodium carbonate sprayed on as coating material is 0.5 to 5 wt.%, the amount of magnesium sulphate sprayed on is 0.5 to 5 wt.% and the amount of sodium silicates with a molar ratio of SiO2 to Na2O of 2.5 to 1 to 3.5 to 1 is 0.2 to 2 wt.%, each being with reference to sodium percarbonate.

12. A process according to one of Claims 6 to 11, characterised in that a temperature in the range 50 to 80°C is maintained in the fluidised bed.

13. Use of the coated sodium percarbonate particles as in Claims 1 to 5 or obtained by a process as in Claims 6 to 12 as a bleaching component in detergents, cleansing agents and bleaches, in particular in zeolite-containing detergents and bleaches.