EP2144990A1

EP2144990A1 - Process for the preparation of coated sodium percarbonate

Info

Publication number: EP2144990A1
Application number: EP08749858A
Authority: EP
Inventors: Jürgen RABE; Henk Venbrux; Gerd Hecken; Bernd Hoffmann
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2007-05-02
Filing date: 2008-04-29
Publication date: 2010-01-20
Also published as: CN101679917B; WO2008135464A1; CN101679917A

Abstract

A process for the preparation of coated sodium percarbonate containing particles, comprising (a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous so lution, (b) an at least partial drying step of the sodium percarbonate containing core particles, (c) a coating step comprising the application of a base coating on the so obtained core particles with one or more sodium percarbonate containing or generating solutions and/or suspensions, and optionally one or more additives, (d) a coating step comprising the application of a further coating on the particles from the previous step with sodium sulphate and sodium carbonate and optionally one or more additives and (e) a drying step of the coated sodium percarbonate containing particles, wherein step (c) and optionally (d) are carried out in a fluid bed reactor and the base coating of step (c) represents more than 20 % by weight of the sodium percarbonate containing core particles.

Description

Process for the preparation of coated sodium percarbonate

The present application claims the benefit of the European application no.

07107379.5 filed on May 2, 2007, herein incorporated by reference.

Introduction

The present invention relates to a process for the preparation of coated sodium percarbonate (PCS) containing particles presenting improved storage properties, the so obtained particles, as well as their use in detergent compositions.

The use of sodium percarbonate (or sodium carbonate peroxyhydrate,

2 NaCO₃ . 3 H₂O₂) as bleaching agent in detergent compositions for household fabric washing or dish washing is well known. Commonly such detergent compositions contain among other components zeolites as builder material, enzymes, bleach activators and/or perfumes.

Different processes are known to produce sodium percarbonate, among them so-called crystallisation processes comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from this aqueous solution, e.g. with salting-out agents, such as sodium chloride, etc. Other processes make use of fluid bed reactors, wherein small seed particles are grown by spraying solutions of sodium carbonate and hydrogen peroxide in the appropriate stoechiometric ratio. While crystallisation processes need less energy, they generally suffer from drawback that the resulting particles often contain salting-out agents and that due to the irregular shaped particles having a large surface to volume ratio, these particles are more likely to be prone to attrition and early percarbonate decomposition. On the other hand, fluid bed processes yield particles with a smooth surface and good attrition behaviour, however the need to introduce the reactants in solution and the subsequent energy intensive evaporation is economically detrimental.

Hence, in an effort to benefit from both techniques to reduce costs and nevertheless obtain PCS particles with good properties, both processes have been combined, e.g. by coating in the fluid bed reactor seed particles obtained by crystallisation from solution. To be of use in detergent compositions or mixtures, the percarbonate should preferably exhibit optimized long-term storage properties.

A first aspect among these long-term storage properties is the storage stability of the percarbonate. In fact, the interaction between sodium percarbonate and other formulation components leads to progressive decomposition of the percarbonate and hence to gradual loss of bleaching power during storage and transportation of the composition. This criterion is also called in-detergent stability and may be expressed as available oxygen (AvOx) recovery. Furthermore, a second relevant consideration in long-term storage properties of the mixtures relates to the differential segregation behaviour of their individual constituents. In other words, to avoid or at least reduce progressive inhomogeneity in the detergent compositions due to storage and transportation, the percarbonate particles should also demonstrate sufficient segregation resistance. Hence, although segregation is a very complex matter involving many parameters, the expression "segregation resistance" should be understood in this context as being the ability of the percarbonate to behave in a significantly similar manner than the other constituents in the composition, whereby progressive inhomogeneity of the compositions is reduced or delayed. Apparently, prior attempts failed to properly consider both properties at the same time, thereby resulting in more or less poor compromises. Object of the invention

The object of the present invention is hence to provide a process for the manufacture of new sodium percarbonate particles presenting improved long- term storage properties, combining both an better long-term storage stability and an enhanced segregation resistance, when said particles are stored and transported in a formulated product for a long period before being used, compared to known sodium percarbonate particles. General description of the invention In order to overcome the abovementioned problems, the present invention proposes a process for the preparation of coated sodium percarbonate containing particles, comprising:

(a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution, (b)an at least partial drying step of the sodium percarbonate containing core particles, (c)a coating/granulating step comprising the application of a base coating on the so obtained core particles with one or more sodium percarbonate containing or generating solutions and/or suspensions, and optionally one or more additives,

(d)a coating step comprising the application of a further coating on the particles from the previous step with sodium sulphate and sodium carbonate, and optionally one or more additives, and (e)a drying step of the coated sodium percarbonate containing particles, wherein step (c) and optionally (d) and/or (e) are carried out in a fluid bed reactor and the base coating of step (c) represents more than 20 % by weight of the sodium percarbonate containing core particles.

As already suggested in the introduction, the optimum coating amount will be a constructive compromise taking into account both the storage stability which, to a certain extent, increases as the ratio surface to volume decreases, i.e. with growing particle size, and the segregation resistance which is a complex parameter depending i.a. on the percarbonate particle size and size distribution compared to those of the other constituents, as well as the relative bulk density of the particles. The percarbonate particles should e.g. have a particle size, size distribution and bulk density largely matching those of the other components of detergent compositions.

The core particles obtained by crystallisation have an irregular shape and hence a relatively large surface to volume ratio. Without wishing to be bound by any theory, it has been observed that during coating of such percarbonate particles, in a first stage, although their size increases only slowly due to preferential filling of the holes and cavities of the irregularly shaped cores, their bulk density is steadily raised. On the other hand, the size distribution of the particles remains substantially constant during this stage and hence largely remains that of the initial core particles.

In a further stage, when the irregularly shaped core particles have been covered so as to form a more or less even surface, the particles continue to grow, but their bulk density ceases to rise. On the other hand, if coating is continued, the particle size distribution, as measured by the "span" of the size distribution curve, will begin to decrease, resulting in particles having more and more the - A -

same size, which, as already mentioned, is detrimental to the segregation resistance.

In practice, the first stage of growing bulk density ends at coating values of more than 20 % by weight, whereas the second stage generally shows its undesirable effect at values above 30 to 50 % by weight with respect to the core particles depending on their mean particle size.

Fig. 1 visualises the particle size distribution (psd) and the effect of particle growth and span reduction with a higher fluid-bed granulation coating layer: the dotted line shows the initial product with a mean particle size of 385 μm and a span of 0,87, whereas the solid line represents the distribution after a fluid-bed granulation coating of 45 % by weight. The particle size increased to 438 μm and the span is reduced to 0,59. Finer granules are disappearing.

Hence, after filling the holes, the particle size is increasing and the span is reduced. It has consequently been established that the "optimum conditions" of the present process can easily be derived by measuring the evolution of the bulk density of the resulting coated particles - according to ISO 3424 -, e.g. by recording the mass of a sample in a stainless steel cylinder of internal height and diameter 86,1 mm, after running the sample out of a funnel (upper internal diameter 108 mm, lower internal diameter 40 mm, height 130 mm) placed 50 mm directly above the cylinder.

The funnel is placed on a stand and the receiver brought in its working position. The bottom opening of the funnel is covered by means of the closure plate, holding the plate lightly against the funnel. The funnel is then with the sample up to its upper lip, then the closure plate is quickly removed, thus allowing the contents of the funnel to run into and overflow the receiver. The contents of the receiver are carefully leveled off with the straight edge and the receiver is then gently tapped to compact the powder. Finally, the receiver is removed, cleaned outside with a dry cloth and reweighed. The storage stability and the segregation behaviour in detergent composition of sodium percarbonate particles may therefore be mutually optimised by using more than 20 % by weight, but generally less than 100 % by weight, preferably less than 50 % by weight and more preferably less than 35 % by weight of coating with respect to the core particles. In the present invention, the appropriate particle size distribution of the coated sodium percarbonate particles may be conveniently controlled by adapting the size distribution of the core particles in the manufacturing step (a) and/or the drying step (b), preferably in step (a), such as by adjusting the crystallisation conditions for example by changing stirrer speed, or the distribution of one reactant and/or by an additional sieving step, preferably after step (b) and before coating step (c).

Fig. 2 shows the evolution of bulk density (BD), span and coating level vs. duration of the trial and illustrates the correlation between the span of the particle size distribution (psd), bulk density (BD) and coating/granulation level. The vertical black line is marking the area, where the particle size distribution is changing: the span is reduced. This effect starts at a coating/granulation level of about 30 % by weight of the core particles. In this trial the concentration of the soda ash solution was 30%.

As mentioned, the overall bulk density of the particles is advantageously increased through the amount of coating applied. In fact, the bulk density of the core particles from step (a) and after drying in step (b) is generally below about 1 g/cm³. When applying the coating in step (c) according to the invention, the bulk density steadily increases up to values of about 1,10 to 1,18 g/cm³ for coating amounts in step (c) exceeding 20 % by weight of the core particles. Hence, the coated sodium percarbonate particles of the present invention usually have a bulk density of at least 0,9 g/cm³, in particular at least 1,0 g/cm³. It is generally at most 1,2 g/cm³, especially at most 1,1 g/cm³.

Furthermore, the attrition of the coated sodium percarbonate particles of the invention, which is also an aspect in storage properties, is very low, usually at most 8 %, in particular at most 5 %, especially at most 3 % and in most cases at least 0,05 %, as measured according to the ISO standard method 5937-1980.

Contrary to prior allegations, the dissolution time (expressed as 90 % dissolution time) of the particles is not significantly impeded by the extent of coating used in the present invention, which is illustrated by the favourable experimental dissolution times from about 1.5 to 2 minutes. The coated sodium percarbonate particles of the present invention usually have a 90 % dissolution time of at least 0,5 min, in particular at least 0,7 min. Generally, the 90 % dissolution time is at most 3 min, especially at most 2,5 min. The 90 % dissolution time is the time taken for conductivity to achieve 90 % of its final value after addition of the coated sodium percarbonate particles to water at 15 +/- 1 ⁰C and 2 g/1 concentration. The method used is adapted from ISO 3123-1976 for industrial perborates, the only differences being the stirrer height of 10 mm from the beaker bottom and a 2 liter beaker (internal diameter 120 mm).

Fig. 3 shows the dissolution time versus dissolved percentage measured according to the above-described method of three percarbonate samples: a sample without fluid bed (FB) layer: 90 % dissolved after 128 s a sample with 26 % FB layer: 90 % dissolved after 113 s and a sample with 46 % FB layer: 90 % dissolved after 137 s

Hence, there is no correlation between the amount of added fluid bed (FB) granulation coating and dissolution time. The coating comprising sodium sulphate and sodium carbonate applied in step (d) preferably is the outer coating and may be applied by any known means, preferably in a fluid bed or in a mixer, using separate solutions of sodium sulphate and sodium carbonate or mixtures, wherein part of these compounds may also be incorporated as small solid particles of either or both of these compounds or applied as dry particles. In fact, in a preferred embodiment this sulphate-carbonate coating may be applied in two or more separate stages using two or more different processes, e.g. comprising the application of a first protective coating comprising a mixture of solid sodium carbonate and sodium sulphate in a mixer, followed by a further coating stage comprising the application of a second protective coating comprising a mixture of sodium carbonate and sodium sulphate in a fluid bed reactor.

It has been found that the stability of the above coated particles is significantly enhanced when using such a coating comprising sodium sulphate and sodium carbonate, compared to coatings using only one of these components alone. Furthermore, compared to coatings containing only sodium sulphate, these coatings have the advantage of incorporating substantial amounts of sodium carbonate, which is a builder useful in detergent compositions, contrary to sodium sulphate, which is only a filler and of no use for the detergent manufacturer. The coating of step (d) with sodium sulphate and sodium carbonate preferably comprises 70:30 to 30:70, preferably 65:35 to 35:65, still more preferably 60:40 to 40:60 and most preferably 57:43 to 43:57 by weight of sodium sulphate to sodium carbonate.

Preferably, the coating of step (d) with sodium sulphate and sodium carbonate and optionally further additive(s) generally represents more than 1 % by weight, preferably more than 2 % by weight and more preferably more than 3 % by weight, but generally less than 50 % by weight, preferably less than 25 % by weight, more preferably less than 15 % by weight and still more preferably less than 10 % by weight of coating with respect to the core particles with their base coating. The coated sodium percarbonate particles of the present invention have a mean particle size of at least 300 μm, in particular at least 400 μm, and more particularly at least 500 μm. The mean particle size is at most 1600 μm, especially at most 1400 μm, values of at most 1000 μm being preferred, for instance at most 800 μm. The mean particle size of particles may be measured using a sieve set

(containing at least 6 sieves of known sieve aperture) to obtain several fractions and weighing each fraction. The mean particle size in μm (MPS) is then calculated according to the formula

MPS = 0.005∑[mAk_{ι +} k_ι+1)] ι=0 in which n is the number of sieves (not including the sieve pan), Hi₁ is the weight fraction in % on sieve i and Ic₁ is the sieve aperture in μm of sieve /. The index i increases with increasing sieve aperture. The sieve pan is indicated with the index 0 and has an aperture of ko = 0 μm and mo is the weight retained in the sieve pan after the sieving process. k_n+i equals to 1800 μm and is the maximum size considered for the MPS calculation. A typical sieve set which gives reliable results is defined as follows: n = 6; kβ = 1400 μm; £₅ = 1000 μm; L_t = 850 μm; k3 = 600 μm; fe = 425 μm; ki = 150 μm.

The span of a particle size distribution can be measured using the sieve set containing 6 sieves described above to obtain several fractions and weighing each fraction. The span was then calculated according to the formula

in which n: number of sieves (not including sieve pan), Wi₁: weight fraction in % on sieve i k,: sieve aperture in μm of sieve i index / increases with increasing sieve aperture

The sieve pan is indicated with the index 0, has an aperture of ko = 0 μm and mo is the weight retained in the sieve pan after the sieving process k_n+i'. 1800μm and is the maximum size of particles considered for the span calculation

MPS is the mean particle size as calculated according to the description above. The coated sodium percarbonate particles of the invention have an improved in-detergent stability. The in-detergent stability is expressed as the AvOx (or available oxygen content) after a storage time of 8 weeks at 32⁰C and 80% relative humidity.

Fig. 4 is showing the improvement of the in-detergent stability: 15 % PCS are mixed with 85 % of IECA* base and stored in laminated cartons over a period of 8 weeks. Every 2 weeks a carton is analysed. The resulting decrease of AvOx is shown in Fig. 4.

Sample 1 (comparative) - labelled as Crystallisation process PCS + 6 % by weight outer coating was prepared as follows: PCS from the crystallisation process is coated in a Glatt spray dryer with a coating solution made of 50 % sodium sulphate and 50 % sodium carbonate.

Sample 2 (according to the invention) - labelled as Crystallisation process PCS with 24 % by weight FB PCS coating + 6% outer coating was prepared as follows: On the starting PCS of sample 1 was applied a 24 % by weight (of the initial WP PCS core) of a fluid bed PCS coating and the outer coating was performed in the same way as described with sample 1.

The coated sodium percarbonate particles of the invention have a good storage or in-detergent stability, and especially long-term storage stability, which can be expressed in two different ways.

According to the first way, it is expressed as heat output at 40 ⁰C measured after storage of 1 g of the product during 12 weeks at 40 ⁰C in a closed ampoule of 3,5 ml. The measurement of heat output by microcalorimetry consists of using the heat flow or heat leakage principle using a LKB2277 Bio Activity Monitor. The heat flow between an ampoule containing the coated sodium percarbonate particles and a temperature controlled water bath is measured and compared to a reference material with a known heat of reaction. This long-term stability is generally less than 10 μW/g, in particular less than 8 μW/g, preferably less than 6 μW/g, and most preferably less than 4 μW/g. According to the second way, the long term stability is expressed as the

AvOx (or available oxygen content) recovery after storage of 1 g of the product for 8 weeks at 55 ⁰C in a closed ampoule of 3,5 ml. The AvOx recovery corresponds to the difference between the available oxygen content before and after the storage expressed as percentage of the initial available oxygen content. The available oxygen content is measured as explained below. This AvOx recovery is in many cases at least 60 %, especially at least 70 %, values of at least 75 % being very suitable, those of at least 80 % being preferred.

The coated sodium percarbonate particles of the invention have usually a content of available oxygen of at least 12,0 % by weight, in particular at least 13,0 % by weight, contents of at least 13,5 % by weight being particularly satisfactory. The content of available oxygen is generally at most 15,0 % by weight, in particular at most 14,0 %, for instance at most 14,2 %. The content of available oxygen is measured by titration with potassium permanganate after dissolution in sulphuric acid (see ISO standard 1917-1982).

In the above process, the sodium percarbonate containing or generating solutions and/or suspensions are preferably chosen from (1) a solution or suspension of sodium carbonate and sodium percarbonate, (2) a solution or suspension of sodium carbonate optionally comprising sodium percarbonate and a solution of hydrogen peroxide or (3) a solution or suspension of sodium carbonate, a solution or suspension of sodium percarbonate and a solution of hydrogen peroxide.

It is to be noted that at least part of the sodium carbonate or the sodium percarbonate used for the coating may also be incorporated in solid form, e.g. in fine powder or dust form, preferably after a partial drying step (b) e.g. in a centrifuge. This solid carbonate is preferably included in the process before step (c) in the form of a fine powder or dust with a dso < 0,2 mm, which adheres on the still moist core particles. This may be achieved using a mixing screw conveyor to granulate the fines onto the moist core particles. The fine material can also be recycled into the fluid bed granulator, preferably near the spray nozzles. A further advantage of such an embodiment is that fine powder or dust always occurring in such processes may be efficiently and conveniently recycled. In a further embodiment, the above process further comprises one or more additional coating steps (c'), (c"), (c'"), ... between steps (c) and (d), and/or one or more additional coating steps (d'), (d"), (d'"), ... between steps (d) and (e), one, more or all of these additional coating step(s) being optionally associated with (e.g. preceded by or concomitant to) an at least partial drying step, wherein each additional coating step comprises the coating of the particles from the previous step with one or more additives, and optionally one or more sodium percarbonate containing or generating solutions and/or suspensions, the composition of each coating being different from that of its adjacent coating(s). Preferably, one or more, more preferably all of the additional coating step(s) and if applicable one or more, more preferably all of their optionally associated drying step(s) are carried out in fluid bed reactor(s).

The additive(s) used in the additional coatings or optionally used for the base coating is/are preferably chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate, polyphosphonate or polyhydroxyacrylate salts, as such or in acid form, for example polyaminocarboxylates like EDTA or DTPA, or polyaminomethylene-phosphonates like EDTMPA, CDTMPA or DTPMPA, or hydroalkylenephosphonates like hydroxyethylidenediphosphonate), or from mixtures of the above.

It is to be understood that further to or alternatively to the already mentioned sieving step between steps (b) and (c), the process may also comprise at least one sieving step between steps (c) and (d) and/or after step (e), e.g. to collect undesirable size fractions of the particles.

As a further aspect, the invention pertains to coated sodium percarbonate particles obtained by a process as described above. Hence, such particles comprise a core of sodium percarbonate, obtained by the crystallisation of sodium percarbonate from aqueous solution and its subsequent separation from aqueous solution, and directly on said core a sodium percarbonate containing base coating optionally comprising one or more additive(s), wherein the base coating represents more than 20 % by weight of the sodium percarbonate core particle, and at least one further coating comprising sodium sulphate and sodium carbonate.

These particles may further comprise on said base coating and/or on said coating comprising sodium sulphate and sodium carbonate, preferably on said base coating, one or more additional coating(s) containing sodium percarbonate and/or one or more additive(s), the composition of each additional coating being different from that of its adjacent coating(s). As already mentioned, the coating comprising sodium sulphate and sodium carbonate preferably is the outer coating and may be applied by any known means, preferably in a fluid bed or in a mixer. In fact, in a preferred embodiment this sulphate-carbonate coating may be applied in two or more separate stages using two or more different processes, e.g. comprising the application of a first protective coating comprising a mixture of solid sodium carbonate and sodium sulphate in a mixer, followed by a further coating step comprising the application of a second protective coating comprising a mixture of sodium carbonate and sodium sulphate in a fluid bed reactor, the composition of each of these coatings being chosen such that it comprises 70:30 to 30:70, preferably 65:35 to 35:65, still more preferably 60:40 to 40:60 and most preferably 57:43 to 43:57 by weight of sodium sulphate to sodium carbonate.

Further characteristics of the coated sodium percarbonate particles are as already described above. A still further aspect of the invention pertains to the use of the coated sodium percarbonate particles as described above as bleaching agent in detergent compositions.

Hence, a still further aspect of the invention is concerned with detergent compositions containing such coated sodium percarbonate particles. Brief description of the drawings

Fig. 1 visualises the particle size distribution (psd) and the effect of particle growth and span reduction with a higher fluid-bed granulation/coating layer,

Fig. 2 shows the evolution of bulk density (BD), span and coating level vs. duration of the trial and illustrates the correlation between the span of the particle size distribution (psd), bulk density (BD) and coating/granulation level,

Fig. 3 shows the dissolution time versus dissolved percentage measured according to the above-described method of three percarbonate samples and

Fig. 4 is showing the improvement of the in-detergent stability. Examples Comparative examples V1-V6: Coating trials with Na sulphate; Na carbonate and a 1 : 1 mixture of Na sulphate and Na carbonate

Vl . PCS + 6% Sodium sulphate

V2. PCS + 9% Sodium sulphate

V3. PCS + 3% Sodium carbonate and 3% Sodium sulphate V4. PCS + 4,5% Sodium carbonate and 4,5% Sodium sulphate

V5. PCS + 6% Sodium carbonate V6. PCS + 9% Sodium carbonate Coating procedure

As raw material for each trial dry percarbonate produced with the crystallization process was used. A homogenized sample of about 20 kg with a mean particle size of ~ 700 μm was used for the coating trials.

For each trial 2000 g of the raw material was put into the Glatt dryer. The coating solutions were a 25% sodium sulphate solution and a 25% sodium carbonate solution, resp. a 1:1 mixture thereof. The coating solution was sprayed onto the particles at ~ HO⁰C air temperature and 50-55⁰C bed temperature. After spraying the calculated amount of coating solution, the product was dried until the bed temperature had reached 7O⁰C, then the material was removed from the dryer and distributed on a sheet of filter paper to cool down.

Table 1 : Analytical results

The coated samples were sieved and the mps (mean particle size) is very similar.

The above TAM values represent a microcalorimetric determination of the energy released during storage, measured by means of the TAM® Thermal Activity Monitor from Thermometric AB, Jarfalla (Sweden).

The heat-output was measured in the TAM at 4O⁰C and 5O⁰C. It is visible that the Na sulphate coated samples show the highest heat output. The coated percarbonate samples were mixed with IECA*-base (15% PCS + 85% IECA*-base) and 50 g of this formulation was put in laminated cartons. Four cartons for each grade were prepared. The cartons were stored at 32⁰C and 80% rel. humidity in a climate chamber for 8 weeks. Every 2 weeks the content of a carton was dissolved and the residual AvOx titrated.

It is surprising that the mixture of Na sulphate and Na carbonate show the highest stability compared to the samples coated with the same amount of either pure Na sulphate or pure Na carbonate.

Comparative example V7 and Example ExI: Coating trials of PCS base coated particles with Na sulphate and a 1:1 mixture of Na sulphate and Na carbonate

V7. PCS + 6% sodium sulphate

ExI. PCS + 3% sodium carbonate and 3% sodium sulphate

Coating procedure As raw material for each trial dry percarbonate produced with the crystallization process was used; this material was coated with 25% of percarbonate. A homogenized sample of about 10 kg was used for the coating trials. Particle size distribution is listed in Table 2; the mean particle size of this sample was 718 μm. For each trial 2000 g of the raw material was put into the Glatt dryer. The coating solution was sprayed onto the particles at ~ HO⁰C air temperature and 50-55⁰C bed temperature until a coating level of 6% (= 125 g solids) was reached. After spraying the calculated amount of coating solution, the product was dried until the bed temperature had reached 83⁰C, then the material was removed from the dryer and distributed on a sheet of filter paper to cool down.

Coating-solution:

As coating-solutions, two different mixtures were used:

■ 25% sodium sulphate in water (V7)

■ 25% 1:1 mixture by weight of sodium carbonate + sodium sulphate in water (ExI) Table 2: Analytical results

All samples (educts and products) show a comparable particle size distribution, proving a good stability of the PCS as no increase of fine material is visible.

When comparing the coated percarbonate samples, a clear advantage of the PCS with mixed coating is visible: the residual Avox after 6 weeks in 50 g cartons at 32⁰C and 80% rel. humidity is significantly higher with 63% versus 45%.

The intrinsic stability is measured by filling 1 g sample into a sealed ampoule and storing at 55⁰C in an oven. The residual AvOx after 5 weeks at 55⁰C is with 92% (ExI) appreciably higher compared to 88% (W), which is already a fair result. These results thus confirm those already described in relation with Fig. 4 above.

Claims

C L A I M S

1. A process for the preparation of coated sodium percarbonate containing particles, comprising:

(a) a manufacturing step of sodium percarbonate containing core particles, comprising the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution,

(b)an at least partial drying step of the sodium percarbonate containing core particles,

(c)a coating step comprising the application of a base coating on the so obtained core particles with one or more sodium percarbonate containing or generating solutions and/or suspensions, and optionally one or more additives,

(d)a coating step comprising the application of a further coating on the particles from the previous step with sodium sulphate and sodium carbonate and optionally one or more additives, and

(e)a drying step of the coated sodium percarbonate containing particles,

wherein step (c) and optionally (d) and/or (e) are carried out in a fluid bed reactor and the base coating of step (c) represents more than 20 % by weight of the sodium percarbonate containing core particles.

2. The process according to claim 1, wherein the sodium percarbonate containing or generating solutions and/or suspensions are chosen from (1) a solution or suspension of sodium carbonate and sodium percarbonate, (2) a solution or suspension of sodium carbonate optionally comprising sodium percarbonate and a solution of hydrogen peroxide or (3) a solution or suspension of sodium carbonate, a solution or suspension of sodium percarbonate and a solution of hydrogen peroxide.

3. The process according to claim 1 or 2, further comprising one or more additional coating steps between steps (c) and (d) and/or between steps (d) and (e), one, more or all of these additional coating step(s) being optionally associated with an at least partial drying step, wherein each additional coating step comprises the coating of the particles from the previous step with one or more additives, and optionally one or more sodium percarbonate containing or generating solutions and/or suspensions.

4. The process according to claim 3, wherein one, more or all of the additional coating step(s) and if applicable one, more or all of their optionally associated drying step(s) are carried out in fluid bed reactor(s).

5. The process according to any one of the preceding claims, wherein the coating in step (d) is the outer coating and comprises 70:30 to 30:70, preferably 60:40 to 40:60 by weight of sodium sulphate to sodium carbonate.

6. The process according to any one of the preceding claims, wherein the additive(s) is/are chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate or polyphosphonate salts, as such or in acid form, or from mixtures of the above.

7. The process according to any of the preceding claims wherein the coated particles have a mean particle size from 300 to 1600 μm.

8. The process according to any of the preceding claims, further comprising between steps (b) and (c) and/or between steps (c) and (d) and/or after step (e), at least one sieving step.

9. Coated sodium percarbonate particles obtained by a process according to claims 1 to 7.

10. The particles according to claim 9, comprising a sodium percarbonate containing core, obtained by the crystallisation of sodium percarbonate from aqueous solution and the separation from aqueous solution, and on said core a sodium percarbonate containing base coating optionally comprising one or more additive(s), wherein the base coating represents more than 20 % by weight of the sodium percarbonate core particle, and at least one further coating comprising sodium sulphate and sodium carbonate.

11. The particles according to claim 10, comprising one or more additional coating(s) containing sodium percarbonate and/or one or more additive(s).

12. The particles according to claim 10 or 11, wherein the additive(s) is/are chosen from organic or inorganic stabilizers, builders, alkaline sources, fillers, flowability enhancers and/or glass corrosion protectors, such as alkali metal or alkaline-earth metal sulphates, bicarbonates, carbonates, citrates, phosphates, borates, silicates and/or chlorides, as well as their hydrates, polycarboxylate or polyphosphonate salts, as such or in acid form, or from mixtures of the above.

13. The particles according to any one of claims 10 to 12, wherein the coating comprising sodium sulphate and sodium carbonate is the outer coating and comprises 70:30 to 30:70, preferably 60:40 to 40:60 by weight of sodium sulphate to sodium carbonate.

14. The particles according to any of claims 10 to 12, having a mean particle size from 300 to 1600 μm.

15. Use of the coated sodium percarbonate particles according to any one of claims 9 to 14 as bleaching agent in detergent compositions.

16. Detergent compositions containing the sodium percarbonate particles of any one of claims 9 to 14.