Sodium percarbonate is a chemical, an adduct of sodium carbonate and hydrogen peroxide (a perhydrate), with formula 2Na2CO3 · 3H2O2. It is a colorless, crystalline, hygroscopic and water-soluble solid. It is used in some eco-friendly cleaning products and as a laboratory source of anhydrous hydrogen peroxide. This product contains the carbonate anion, and should not be confused with sodium peroxocarbonate Na2CO4 or peroxodicarbonate Na2C2O6, which contain different anions. At room temperature, solid sodium percarbonate has the orthorhombic crystal structure, with the Cmca crystallographic space group. The structure changes to Pbca as the crystals are cooled below about -30 °C.
Production
Sodium percarbonate is produced industrially by reaction of sodium carbonate and hydrogen peroxide, followed by crystallization. Also, dry sodium carbonate may be treated directly with concentrated hydrogen peroxide solution. World production capacity of this compound was estimated at several hundred thousand tonnes for 2004. It can be obtained in the laboratory by treating the two substances in aqueous solution with proper control of the pH or concentrations.
Uses
As an oxidizing agent, sodium percarbonate is an ingredient in a number of home and laundry cleaning products, including non-chlorine bleach products such as OxyBoost, OxiClean, Tide laundry detergent, and Vanish. Dissolved in water, it yields a mixture of hydrogen peroxide (which eventually decomposes to water and oxygen) and sodium carbonate ("soda ash").
2Na2CO3.3H2O2 › 2Na2CO3 + 3H2O2
Sodium percarbonate can be used in organic synthesis as a convenient source of anhydrous H2O2, in particular in solvents that cannot dissolve the carbonate but can leach the H2O2 out of it. Coated sodium percarbonate particles, process for their preparation, their use in detergent compositions and detergent compositions containing them. Coated sodium percarbonate particles containing: a) A sodium percarbonate core, and b) A coating representing between 5 and 20 % in weight of the coated sodium particle, the coating comprising at least one coating layer comprising at least 25 % in weight of an hydrate forming inorganic salt, at least 75 % in weight of said salt being in a hydrated form. Coated sodium percarbonate particles, process for their preparation, their use in detergent compositions and detergent compositions containing them.
The use of sodium percarbonate (or sodium carbonate peroxyhydrate) as the active bleach constituent 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. The bleaching action of sodium percarbonate is due to the release of active oxygen when it decomposes during washing. However, the interaction between sodium percarbonate and other formulation components leads to progressive premature decomposition of the percarbonate and hence to loss of bleaching power during storage and transportation of the composition. This progressive decomposition is accelerated when the percarbonate is submitted to heating or stored in presence of humidity. Consequently, the storage of sodium percarbonate in moist and warm environments, as for instance when the product is marketed in southern countries, is difficult.
One of the critical features of the coated sodium carbonate particles according to the invention is the controlled quantity of sodium carbonate which is in the monohydrate form. In this respect, a particular attention has to be brought during the manufacture of the coated particles to the control of the temperature of the core particles, of the solution used to coat them and to the control of the temperature of the coated particles during the dying, in order to induce the proper crystallisation of the sodium carbonate in the coating in the monohydrate form. The precise temperatures to produce the monohydrate form can vary, depending on the drying air flow, drying air relative humidity and the residence time of the particles in the drying bed. However, in most standard conditions, core particles are generally contacted with the coating solution at a temperature comprised between 45 and 60°C and the coated particles are generally dried at temperatures between 60 and 75°C.
In consequence, the invention concerns also a process for preparing the coated sodium percarbonate particles according to the invention in which uncoated sodium percarbonate core particles are contacted at a temperature comprised between 45 and 60°C with a coating solution comprising sodium carbonate and the coated particles are dried at a temperature comprised between 60 and 75°C. The sodium percarbonate core particles can be obtained for example by crystallisation from a bulk saturated solution of sodium percarbonate in a crystalliser/classifier, the percarbonate solution being prepared out of sodium carbonate and hydrogen peroxide. They could also be obtained for example by a fluid bed process, wherein aqueous hydrogen peroxide and aqueous soda ash solutions are sprayed onto sodium percarbonate seeds in a fluidised bed, or through the reaction of solid sodium carbonate with an aqueous hydrogen peroxide solution. Preferably, the sodium percarbonate core particles are obtained by a crystallisation process using a salting-out agent.
In the process according to the invention, it is recommended that the coating solution comprises more than 23 % in weight of sodium carbonate. The core particles can be contacted with the coating solution in a mixer. It is however recommended that the core particles are contacted with the coating solution by spraying the coating solution on the core particles in a fluid bed. In another recommended embodiment, the coating is performed in two steps, the first one using a mixer, and the second one a fluid bed. The coated sodium percarbonate particles according to the invention can advantageously be used as active bleach constituent in detergent compositions. The present invention therefore concerns also the use of the above described coated sodium percarbonate particles as active bleach constituent in detergent compositions. The present invention also concerns detergent compositions containing the above described coated sodium percarbonate particles as active bleach constituent and a builder, either zeolitic or non-zeolitic. The detergent compositions can also contain other constituents such as surfactants, anti- redeposition and soil suspension agents, bleach activators, optical brightening agents, soil release agents, suds controllers, enzymes, fabric softening agents, perfumes, colours and processing aids. The incorporation of coated percarbonate particles according to the invention into detergent powders or granules with a bulk density of 0,4 to 1,0 g/cm^ and into detergent tablets which are either zeolite or phosphate built is particularly advantageous.tes the sodium percarbonate core. The water acceptance capacity is exhausted when the coating layer is in hydrated form. These known coating layers do not result in an optimum combination of properties since they do not confer simultaneously a good stability to the sodium percarbonate when present in the detergent composition, in particular when the detergent is stored in warm and moist environment, together with a rapid dissolution rate when the composition is used in washing applications.
