EP0002293A1

EP0002293A1 - Detergent tablet having a hydrated salt coating and process for preparing the tablet

Info

Publication number: EP0002293A1
Application number: EP78200307A
Authority: EP
Inventors: Daniel Biard; Gérard Jean Lichtfus; Hans Joachim Pracht; Wolfgang Kleeman
Original assignee: Procter and Gamble Co
Current assignee: Procter and Gamble Co
Priority date: 1977-11-29
Filing date: 1978-11-16
Publication date: 1979-06-13
Also published as: NL7815064A; IT7830328A0; FR2443502A1; IT1101296B; DE2857472A1; BE56T1

Abstract

A detergent tablet comprising an alkaline salt is provided with a coating of a hydrated salt having a melting point in the range from 30°C to 95°C. The coating is applied in the form of a melt of the hydrated salt so that a good coating is formed without the need for lengthy drying times. Preferred materials for the coating are sodium acetate trihydrate, sodium metaborate tetrahydrate and sodium orthophosphate dodecahydrate. The coating can have dispersed therein a particulate material having absorbed therein an organic liquid such as a nonionic surfactant or a silicone suds suppressor.

Description

The present invention relates to detergent tablets and, in particular, to coated detergent tablets, especially those adapted for use in automatic dishwashing machines.
Although cleaning compositions in tablet form have often been proposed, these have not (with the exception of soap bars for personal washing) gained any substantial success, despite the several advantages of products in a unit dispensing form. One of the reasons for this may be that detergent tablets require a relatively complex manufacturing process. In particular, it is often desirable to provide the tablet with a coating and this adds to the difficulties of manufacture.
While tablets without a coating are entirely effective in use, they usually lack the necessary surface hardness to withstand the abrasion that is a part of normal manufacture, packaging and handling. The result is that uncoated tablets suffer from abrasion during these processes, resulting in chipped tablets and loss of active material. Also, especially in the case of highly alkaline tablets such as are useful for use in automatic dishwashing machines, the outer surface of an uncoated tablet may be aggressive to the skin and even somewhat hazardous to handle. In such cases, tablet coating is highly desirable. Finally, coating of tablets is often desired for aesthetic reasons, to improve the outer appearance of the tablet cr to achieve some particular aesthetic effect.
Numerous methods of tablet coating have been proposed, and many of these have been suggested for detergent tablets. However, all of these methods have certain disadvantages, as will be explained below.
Polymeric coatings for detergent tablets have been proposed in, for example, U.K. Patent No. 989,683, No. 1,013,686 and No. 1,031,831. Although such coatings can provide excellent surface appearance and resistance to abrasicn, they tend to have a lower solubility than is desirable, resulting in the presence of undissolyed polyner in the wash solution. Also, polymeric coatings are difficult to apply ; frequently it is necessary to use organic solvents with the resultant difficulty and inconvenience of removing these solvents. Where the polymer can be applied from aqueous solution there is again a problem of drying, the removal of excess water requiring prolonged heating of the tablet.
Inorganic salt coatings have also been proposed in U.K. Patent No. 1,031,831. The preferred coating material is podium silicate solution. Again, an aqueous solution is applied to the tablet and this of course requires a drying step to remove the water before the coating is formed. Apart from the added complexity of a drying step, there is the further disadvantage that migration of excess water into the table may adversely affect the properties, especially solubility and storage stability, of the tablet.
It has also been proposed, for example in our copending European patent application No. 78 200014.5, to employ molten organic materials for coating detergent tablets. Materials such as fatty alcohols, fatty acids and polyethylene glycols are said to be useful. While such materials do form good coatings and avoid many of the disadvantages mentioned above, they in turn have the disadvantage of influencing the performance of compositions or of dissolving too slowly. In particular, when we are concerned with tablet for use in automatic dishwashing machines, it is important that the compositions have little or no foaming ability. Materials such as polyethylene glycol have sufficient surface activity to produce excess foaming in such compositions, while a material like a fatty acid can adversely affect detergency performance or can even deposit on the articles being washed.
It is an object of the present invention to provide a detergent tablet with a coating which is easily applied and which does not suffer from.the problems discussed above.
This and other objects are achieved by the use of certain hydrated salts as coating material for the tablets, the salts being applied in the form of a melt. Molten hydrated salts have been proposed for coating bleach- particles (see U.K. Patent No. 1,191,356 and U.S. Patent No. 4,048,351), but it has not heretofore been recognised that detergent tablets could be coated in this manner with such surprisingly advantageous results.
According to the present invention, there is provided a detergent tablet having a core comprising an alkaline builder salt and provided with a coating of a hydrated salt having a melting point in the range of from 30°C to 95° C, said coating having been applied to the tablet in the form of a melt.
In the context of the present invention, the term "detergent" does not necessarily imply the presence of a surfactant material. Tablets which exert their cleaning power solely by the presence of inorganic salts (such as phosphate and silicate) are encompassed within the present invention: The term "melt" will be defined hereinafter.
The present invention also provide a process for coating a detergent tablet comprising applying to the tablet a hydrated salt in the form of a melt, said hydrated salt having a melting point in the range from 30°C to 95°C.
In one embodiment of the present invention, a particulate, absorbent carrier material is dispersed in the coating of the tablet. The carrier material has previously been loaded with a liquid, organic material, preferred organic materials being either nonionic surfactant or silicone suds suppressor. This allows incorporation of small amounts of liquid organic materials into the tablet without adversely affecting the strength/solubility characteristics of the tablet.

The Detergent Tablet

The detergent tablet which is to be provided with a coating according to the present invention can be of any type and can, for example, be adapted for laundry washing, dishwashing or.any other type of cleaning operation which involves dissolution of the table in an aqueous medium.
In the body of the tablet is included an alkaline salt, preferably a water-soluble builder salt which normally provides a substantial part of the cleaning power of the tablet. The term "builder salt" is intended to mean all materials which tend to remove calcium ion from solution, either by ion exchange, complexation, sequestration or precipitation.
; preferred above all other salts for the purpose of providing alkalinity are water-soluble silicate salts.
Examples of suitable silicates are those having the general formula nSiO₂.M₂O where n is from 0.5 to 4.0 and Mis a cation imparting water-solubility to the salt, preferably an alkali metal such as sodium or potassium. Such silicates can contain up to 50 % by weight of water in the form of water of hydration. Preferred materials are sodium metasilicate and sodium sesquisilicate. Sodium orthosilicate may be used where very high alkalinity is desired. Sodium metasilicate is very highly preferred. In preferred compositions intended for use in automatic dishwashing machines, the silicate salt (inclusive of any water of hydration) makes up more than 50 % of the tablet, preferably from 60 % to 80 %. In compositions designed for laundry use, there is usually much less silicate, for example 5 % to 15 %.
Another preferred builder salt, usually employed in combination with the silicate salt (although possibly employed as the sole builder salt), is a water-soluble phosphate. Any water-soluble phosphate salt can be employed in the present invention, for example, sodium orthophosphate, pyrophosphate, tripolyphosphate or more condensed phosphates such as hexametaphosphate. Condensed phosphates are preferred, especially sodium tripolyphosphate. The phosphate salt can be in at least partially hydrated form, particular examples being pentasodium tripolyphesphate hexahydrate and tetrasodium pyrophosphate decahydrate.
In preferred compositions,.especially for dishwashing products, the phosphate salt (inclusive of any hydrated water) makes up from 10 % to 40 %, preferably from 20 % to 30 % of the tablet.
Other useful inorganic alkaline builder salts, which can be employed alone, or preferably in admixture with the silicate and phosphate salts include water-soluble carbonates, bicarbonates and borates.
Water-soluble organic builder components may also be employed. Examples of suitable organic detergency builder salts are : (1) water-soluble amino polyacetates, e.g., sodium and potassium ethylene-diamine tetra-acetates, nitrilotriacetates, and N-(2-hydroxyethyl) nitrilodiacetates; (2) water-soluble salts of phytic acid, e.g., sodium and potassium phytates; and (3) water-soluble polyphosphonates, including alkali metal salts of ethane-1-hydroxy-1,1-diphosphonic acid ; methylenediphosphonic acid, ethylene diamine tetramethylene phosphonic acid, nitrilotrimethyl- ene phosphonic acid and the like.
Additional organic builder salts useful herein include the polycarboxylate materials described in U.S. Patent No. 2,264,103, including the water-soluble alkali metal salts of mellitic acid. The water-soluble salts of polycarboxylate polymers and copolymers such as are described in U.S. Patent No. 3,308,067, incorporated herein by reference, are also suitable herein.
A further class of builder salts useful herein is the water-insoluble silicate type which functions by cation exchange to remove polyvalent mineral hardness and heavy metal ions from solution. A preferred builder of this type has the formulation Na_z(AlO₂)_z(SiO₂)_y.xH₂O.wherein z and y are integers of at least 6, the molar ratio of z to y is in the range from 1.0 to about 0.5 and x is an. integer of from about 15 to about 264. Compositions incorporating builder salts of this type form the subject of British Patent No. 1,429,143 published March 24., 1976, German Offenlegungsschrift No. 2,433,485 published February 6, 1975 and "Offenlegungsschrift No. 2,525,778 published January 2, 1976, the disclosures of which are incorporated herein by reference.
Detergent tablets for use in the present invention can contain a water-soluble surfactant selected from anionic, nonionic, zwitterionic, amphoteric and cationic surfactants. Surfactants of these types are described in U.S. Patent No. 3,929,678 the disclosure of which is incorporated herein by reference.
Laundry detergent tablets normally contain up to 15 % of an anionic surfactant such as C₈-C₁₈ alkyl benzene sulphonates, C₈-C₁₈ alcohol sulphates, C₈-C₁₈ alcohol ethoxylate sulphates and fatty acid soaps. Tablets designed for use in automatic dishwashing machines can contain no surfactant, but frequently contain up to 10 % of a nonionic surfactant.
Nonionic surfactants which are advantageously employed in the composition of this invention include, but are not limited to, the following polyoxyalkylene nonionic detergents : Cg-C22 normal fatty alcohol-ethylene oxide condensates, i.e., condensation products of one mole of a fatty alcohol containing from 8 to 22 carbon atoms with from 2 to 20 moles of ethylene oxide, polyoxypropylenepolyoxyethylene condensates having the formula HO(C₂H₄O)_x(C₃H₆O)_y(C₂H₄O)_x1H where y equals at least 15 and (C₂H₄O)_x+x1, equals 20-90 % of the total weight of the compound ; and alkyl polyoxypropylenepolyoxyethylene condensates having the formula RO-(C₃H₆O)_x(C₂H₄O)_yH where R is a C₁-C₁₅ alkyl group and x and y each represent an integer of from 2 to 98.
Preferred nonionic surfactants for automatic dishwashing machine detergent tablets have very low sudsing ability. The afore-mentioned polyoxyethylene-polyoxypropylene surfactants are especially suitable in this respect. Specific surfactant materials which are preferred include Pluronic L-61, Lutensol LF-700 and the Plurafac series, especially Plurafac RA-40, RA-43 and RA-343.
Also useful in the present invention are the polyethylene glycols, for example those of molecular weight from 1,000 to 20,000, especially about 10,000. While these materials are not normally considered as surfactant materials, they do assist in improving the wetting ability of the compositions and they are also useful as processing aids in the tablet manufacture.
Detergent tablets of the present invention also preferably include a bleach component, preferably a chlorine bleach when a dishwashing composition is desired and an oxygen bleach when a laundry tablet is intended. Any of many known chlorine bleaches can be used in the present tablet. Examples of such bleach compounds are: chlorinated trisodium phosphate, dichloroisocyanuric acid, salts of chlorine substituted isocyanuric acid, 1,3-dichloro-5,5-di- methylhycantoin, N,N'-dichlorobenzoylene urea, paratoluene sulphodichloroamide, trichloromelamine, N-chloroammeline, N-chlorcsuccinimide, N, N'-dichloroazodicarbonamide, N-chloroacetyl urea,N,N'-dichlorobiuret,,chlorinated dicyandiamide, sodium hypochlorite, calcium hypochlorite, and lithium hypochlorite. The preferred bleach is an alkali-metal salt of dichlorcisocyanuric acid, e.g., potassium or sodium dichloroisocyanurate especially sodium dichloroisocyanurate dihydrate.
Useful oxygen bleaches include sodium perborate, sodium percarbonate and sodium persulphate.
Neutral fillers such as sodium sulphate and sodium chloride can be present and various other components can be included for various purposes. Examples of such additional components are enzymes, especially proteases and amylases, (which are useful in the absence of chlorine bleach), suds- suppressing agents, tarnish inhibitors such as benzotriazole, bactericidal agents, soil-suspending agents, dyes, brighteners and perfumes.
Tablets to be coated in the present invention can be prepared simply by mixing the solid ingredients together and compressing the mixture in a conventional tablet press as used, for example, in the pharmaceutical industry. Any liquid ingredients, for example the surfactant or suds suppressor, can be incorporated in a conventional manner into the solid particulate ingredients. Preferably the principal ingredients, silicate and phosphate, are used in granular form.
Alternatively, especially for laundry tablets, the ingredients such as builder salt and surfactant can be spray-dried in a conventional manner and then compacted at a suitable pressure.
A suitable process for manufacturing detergent tablets is described in U.S. Patent 3,081,267, the disclosure of which is incorporated herein by reference.
The present invention is particularly applicable to the automatic dishwashing machine tablets described in the copending European patent application No. 78. 200014.5. The tablets are highly alkaline and have a high density of at least 1.4g/cc and preferably about 1.5g/cc, for example 1.6-1.7. Where the invention is applied to such tablets, it is highly preferred that the compressed tablet itself is free of liquid organic materials such as nonionic surfactant and suds suppressor (e.g. silicone suds suppressor) and that these organic materials are incorporated into the coating as hereinafter described.
The detergent tablets can be made in any size or shape and can, if desired, be surface treated before coating according to the present invention. For example, it may be desired to water-spray the surface as described in U.S. Patent No. 3,081,267 to provide added surface integrity during subsequent handling steps.

The Coating

However the detergent tablets are prepared and in whatever form they are, they are then coated according to the present invention with a hydrated salt having a melting point of from 30°C to 95°C.
It is an essential part of the present invention that the'coating is applied in a state wherein it essentially solidifies on the tablet rather than dries on to it. It is therefore essential that the salt be applied at a temperature in excess of its melting point. Clearly salts having a melting point below 30°C are not sufficiently solid at ambient temperatures and it has been found that salts having a melting point above about 95°C lose their water of hydration so quickly after melting that it is not practicable to use these materials. Preferably, the salts melt in the range from 35°C to 75°C, more preferably from 45°C to 70°C.
By "melting point" is meant the temperature at which the salt when heated slowly in, for example, a capillary tube becomes a clear liquid. It will be understocd that this is the point at which the salt dissolves in its own water of crystallisation. Solidification of this salt does not involve any loss of water, as the crystalline hydrate is again formed.
Any salt having a melting point in the required range and forming a hydrated species can be employed. This includes both inorganic and organic salts.
Suitable materials include sodium acetate trihydrate, sodium metaborate tetrahydrate, or octahydrate, scdium orthophosphate dodecahydrate, sodium dihydrogen phosphate dihydrate, the di-, depta-, or dodeca-hydrate of disodium hydrogen phosphate, sodium potassium tartrate tetrahydrate, potassium aluminium sulphate dodecahydrate, calcium bromide hexahydrate and calcium nitrate tetrahydrate.
Highly preferred materials are the sodium acetate and the sodium metaborate tetrahydrate. Mixtures of these two materials is proportions of metaborate:acetate by weight of from 3:1 to 1:3, preferably 2:1 to 1:2, are still more preferred.
When the above mentioned hydrated salts are heated to an appropriate temperature, they form a clear liquid phase and detergent tablets of the type described above are coated with the molten hydrated salt. The molten salt can be applied to the tablet in any convenient way, for example by spraying or dipping. Normally, when the molten salt is sprayed on to the tablets, it will solidify almost instantaneously to form a coherent coating. When tablets are dipped into the molten salt and then removed, the rapid cooling again causes almost instantaneous solidification of the coating material.
Certain of the hydrated salts form melts which tend to be too viscous for easy spraying. Also, the heating of the salt tends to drive off part of the water of hydration, thus also increasing viscosity and tending to cause premature crystallisation of the salt. This can be avoided by adding a small amount of water to the molten salt and, in the context of the present invention, the term'"melt" is intended to mean both the true molten hydrated salt and the molten salt when diluted with water. However, it should be understood that the diluted molten salt will still behave as a molten phase,i.e. it will, on cooling, solidify without the need for substantial drying or removal of excess'water. Normally, any water added to the melt should not exceed 30% by weight of the melt, preferably not more than 20%, for example from 5% to 15%. The presence of a slight excess of water can provide a further advantage in that migration of this water into the tablet results in an increase in tablet strength.
A coating of any desired thickness can be'applied according to the present invention. For most purposes, the coating forms from 1% to 10%, preferably from 1.5% to 5%, of the tablet weight.
The tablet coatings of the present invention are very hard and yet still highly soluble, are more stable than many organic coatings and are neutral to tablet solubility and performance. Apart from the above-mentioned water, the coating melt can contain minor quantities of other ingredients, for example, dyestuffs, perfume, opacifier, suds suppressor, surfactant(especially nonionic surfactant), etc. In one preferred embodiment, the surfactant and/ or suds suppressor is incorporated into the coating in the following manner.
In general terms, an organic, liquid material can be incorporated into the tablet of the present invention by (a) sorbing the liquid material into a solid, particulate carrier material, (b) dispersing the loaded carrier material in the molten coating material and (c) applying the coating material in the manner described above.
This method can be used for the incorporation of any organic, liquid material but is especially preferred for incorporating either a liquid, nonionic surfactant or a liquid suds suppressor, especially an emulsifiable silicone suds suppressor as described in German Offenlegungsschrift No. 26 46 127, the disclosure of which is incorporated herein by reference. It has been found that direct incorporation of such organic materials into the tablet itself can adversely affect the strength/solubility characteristics of the tablet, especially if high-density tablets are desired.
Any suitable carrier material can be used. This includes inorganic salts such as socium carbonate and sodium phosphates, e.g. sodium tripolyphosphate; inorganic materials such as absorbent silicas and zeolites; and certain organic resins or natural organic polymers such as starch. Especially preferred materials are aluminosilicates, for example the synthetic material known as zeolite A and having the formula
In general, the aluminosilicate materials described in U.K. Patent Specification No. 1,429,143 can be employed, as can the aluminosilicate material sold by Sifrance under the trade name DG-12.
In general, it is desirable to load as much of the organic liquid on to the carrier material as possible so as to minimise the amount of carrier material in the coating. With aluminosilicate carriers, for example, it has been found that the carrier material can absorb from about 10% to 40% by weight of silicone suds suppressor or nonionic surfactant. The loaded carrier material can be suspended in the coating melt at a level of up to about 50% by weight of the mixture.
Another method of incorporating surfactant and/or suds suppressor into the inorganic melt coating is to emulsify the organic liquid into the inorganic melt with the help of a hydrotrope material such as sodium cumene sulphonate, sodium toluene sulphonate, sodium or potassium xylene sulphonate or a phosphate ester as described in British Patent No. 1,485,316.
The following Examples illustrate the present invention.

EXAMPLE 1

Granular sodium metasilicate (34,0g,), granular sodium tripolyphosphate (12.0g.), sodium aluminate (0.08g.) and sodium dichloroisocyanurate (1.1g.) were blended together in a mixing vessel to form a homogeneous particulate mixture. About 23 g. of this mixture were introduced into a mould of triangular shape with equilateral triangle sides of 3.3 cm and were compressed in this mould under a pressure of about 470Kg/sq. cm to give a tablet,of about 1.9 cm tickness and a density of about 1.7 g/cc.
Sodium metaborate tetrahydrate (60 parts) and sodium acetate trihydrate (40 parts) were mixed in a stainless steeel container and heated to 90°C with gentle stirring until molten. The molten product was a clear liquid with a viscosity of about 15 cP. The melt was then sprayed on to the tablet prepared as above to give a final coated tablet weight of 24.2g.

EXAMPLE 2

Following the procedure of Example 1, a tablet was prepared and the melt of sodium metaborate and sodium acetate was made. To this melt (100 parts) was added 5 parts of water and the diluted melt was sprayed as before to give a coating of 5% of the tablet weight.
The above procedure was repeated by using 10 parts of water to dilute the melt. This made it easier to spray the melt and avoided a tendency for blockage of the spray nozzle.
The coated tablets of Examples 1 and 2 are especially suitable for use in automatic dishwashing machines and have an excellent, strong, water-soluble coating.

EXAMPLE 3

Similar results are obtained when the procedures of Examples 1 and 2 are followed, but using as the melt material any one of the following : - sodium acetate trihydrate, sodium metaborate tetrahydrate, sodium orthophosphate dodecahydrate, sodium potassium tartrate and potassium aluminium sulphate.

EXAMPLE 4

A spray-dried granular detergent composition has the following composition:
990 parts of this spray dried composition are uniformly mixed with 840 parts of anhydrous granular sodium tripolyphosphate and 2 parts by weight of perfume are sprayed on the mixture.
. An aliquot of the mixture is charged into a tablet cavity and formed under a pressure of 5 kg/sq.cm into a tablet.
The tablet is then coated with a mixture of sodium metaborate and sodium acetate according to the procedures of Examples 1 or 2. The coated tablet is suitable for use as a laundry detergent tablet.
Substantially similar results are obtained when the materials indicated in Example 3 are used as coating agents.

EXAMPLE 5

The procedure of Example 1 is repeated except that the tablet was dipped into the melt of sodium tetraborate and sodium acetate and then removed. The melt solidified quickly at ambient temperature and a coated tablet was obtained having a coating weight of about 5% of the total tablet weight.

EXAMPLE 6

A silicone suds suppressor (0.3 parts) was sorbed on to 1.2 parts of a zeolite A having a particle size of 5 microns and the loaded carrier material was then suspended in 3 parts of a coating melt as described in Example 1. The melt containing the suspended particulate carrier material was then sprayed on to the tablet of Example 1 to give a tablet with a 4.5% weight of coating.
Using an analogous method, a coated tablet containing 0.3% of a nonionic surfactant can be prepared.

Claims

1. A detergent tablet having a core comprising an alkaline builder salt, characterized in that the core is provided with a coating of a hydrated salt having a melting point in the range of from 30°C to 95°C, said coating having been applied to the tablet in the form of a melt.

2. A tablet according to claim 1, characterized in that the melting point of the hydrated salt is from 35°C to 75°C.

3. A tablet according to claim 2, characterized in that the hydrated salt is selected from hydrates of sodium acetate, sodium metaborate, sodium orthophosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium potassium tartrate, potassium aluminium sulphate and calcium bromide.

4. A tablet according to claim 3, characterized in that the said salt is a mixture of sodium acetate trihydrate and sodium metaborate tetrahydrate in a proportion by weight of from 3:1 to 1:3.

5. A tablet according to any one of claims 1-4, characterized in that the said alkaline salt comprises a mixture of a water-soluble silicate salt and a water-soluble phosphate salt in a weight ratio of at least 1:1.

6. A detergent tablet according to any one of claims 1-5, characterized in that wherein the coating melt has suspended therein an organic liquid sorbed on to a carrier material.

7. A tablet according to claim 6, characterized in that the organic liquid is a nonionic surfactant or a silicone suds suppressor.

8. A process for coating a detergent tablet comprising applying to the tablet a hydrated salt in the form of a melt, the said hydrated salt having a melting point in the range of from 30°C to 95°C.

9. A process according to claim 8, characterized in that the melt comprises the hydrated salt in a molten state and up to 20% by weight of the melt of the water.

10. A process according to claim 8 or claim 9, characterized in that the melt is applied to the tablet by spraying.