COATING TABLETS

Abstract: A method of forming a polymeric coating on a detergent tablet containing an alkaline agent comprises applying to the tablet a continuum of an aqueous solution or suspension of a polymer having free acid groups, and evaporating water to leave the coating on the tablet. The solution/suspension is most preferably applied by passing the tablets relatively through a falling cascade /curtain) of the solution/suspension.

Description: COATING TABLETS The present invention relates to tablets for use in laundering operations or dishwashing operations, particularly (but not exclusively) such operations as carried out in domestic washing and dishwashing machines. For convenience such tablets are referred to herein as detergent tablets although it will 'be appreciated that the level of surface active agent in a dishwashing tablet may be very low or even zero depending on the other components present. In recent years, there has been a trend to the use of laundry and dishwasher detergent formulations in the form of tablets. Such tablets are more convenient to use than powders or liquids since they (the tablets) do not need to be "measured-out" and can easily be introduced into a washing machine / dishwasher without spillage. Such tablets are generally produced by compression of a particulate mixture which generally comprises a builder (which may be water soluble or water insoluble), and as necessary . a surface active agent system and ancillary components such as a bleach, bleach activator, enzymes etc. as well known in the art. The builder is generally a substance (e.g. phosphate, citrate, silicate, bicarbonate, carbonate, and borate) such that, on dissolution, the tablets gives a solution with a pH of more than 7. For such tablets, it is necessary that they are able to disintegrate in water in an appropriate time scale to allow the various "active components" of the tablet to become dissolved or dispersed in the wash liquor. Thus, for example, a tablet to be introduced in the dispensing drawer of a domestic washing machine must be capable of dissolving the relatively short time during which water in introduced into that drawer. Such tablets will usually incorporate a water insoluble but water swellable disintegrant (e.g. based on cellulose) to facilitate disintegration and dissolution of the tablet. For a tablet to be introduced into the drum of a washing machine, a longer dissolution time may be acceptable and such tablets may incorporate a highly water soluble salt (e.g. sodium acetate or sodium citrate) to facilitate dissolution. A widely recognised problem of detergent tablets is the necessary trade off between tablet hardness and the rate of tablet dissolution. The harder the tablet the more resilient it is during transport and storage, yet the longer the tablet will take to disintegrate in solution. Conversely, the "weaker" the tablet the lower will be the dissolution time by the tablet will be prone to fragmentation and dust production in transport. A level of hardness has to give acceptable hardness and dissolution time. One method of increasing the strength of tablets without resorting to the use of a higher compaction pressure (and hence longer dissolution time) is to provide the tablet with a coating although, that said, coated detergent tablets are not widely available. Such coatings enable a weaker core to be used whilst avoiding problems with fragmentation of the tablet. Once the outer coating has been breached in the washing machine, the tablet core is able to dissolve. The coatings used for detergent tablets are generally of a synthetic polymer and various techniques have been used to apply polymeric coatings to detergent tablets. Generally the techniques involve either application of a melt of the polymer to the tablet (where the melting point of the polymer so permits) or application of a solution of the polymer in an organic solvent to the tablet. Thus, for example, EP-A-0 716 444 discloses application of coatings to tablets by spraying thereon either molten PEG or a solution of a co-polymer of vinyl pyrrolidone and vinyl acetate in ethanol. It will be appreciated that the use of a melt or an organic solution of the coating polymer has generally been proposed for the purposes of coating the tablet since an aqueous based coating system would be expected to cause degradation of the tablets which are specifically formulated for rapid dissolution and disintegration in water during a textile wash or dishwash procedure in an automatic machine for effecting such an operation. This is particularly so in the case of textile wash tablets which are intended to dissolve and disintegrate in the drawer of a domestic washing machine, such tablets generally incorporating a water insoluble but water swellable disintegrant for the purposes of rapid dissolution and disintegration. In practice, the commercially available coated tablets are produced by application to the tablet of a melt of polyethylene glycol. This results in a relatively thick (and uneven) coating which whilst increasing tablet strength does significantly resist the ingress of moisture and add significantly to the overall dissolution time of the tablet, (although once the coating has been breached dissolution of the tablet core is relatively rapid because of the low compaction pressure used during manufacture). Apart from the disadvantages mentioned in the previous paragraph the use of melt has additional disadvantages, e.g. energy costs associated with forming the melt and problems with fumes. The use of organic solutions of the coating polymer is not without disadvantages in that there is a cost associated with the organic solvent and once again there may be problems with fumes. US-A-3 630 920 discloses a copolymer which may be used for forming coatings on articles and which is obtained by copolyrnerisation of maleic anhydride and a vinyl compound (preferably a vinyl ether but other possibilities are mentioned, e.g. methyacrylic acid and esters thereof) followed by partial esterification with a compound having surface active properties. The ratio of maleic anhydride to vinyl compound is in the range 5:4 to 4:5 and the degree of esterification is a maximum of about 5%. The Examples of US-A-3 630 920 disclose formation of a coating on a tablet by spraying the tablet with a 5% aqueous suspension of the copolymer. Although the US-A-3 630 920 does disclose coating of articles with its copolymers by techniques such as spraying and dipping there is no specific disclosure of coating a tablet using an aqueous system otherwise than by spraying. In spite of the disclosure in the Examples of US-A-3 630 920 relating to the spraying of aqueous compositions to form coatings on tablets, there is to our knowledge no commercial use of aqueous based coating systems for such a purpose. Furthermore, the technique of spraying the polymer onto the tablet (whether the polymer be a melt, a solution/suspension in an organic solvent, or solution suspension in an aqueous medial is not entirely satisfactory. This is because of the irregular and absorbent nature of the surface of the tablet. In view of these properties porous channel in the tablet may be expected to remove the (discontinuous) sprayed liquid from the tablet surface and hence gaps in the coating will result. Such gaps reduce the coating strength and provide an avenue for the diffusion of moisture into the tablet core (although surprisingly this partial coating does not enhance dissolution). In the event that a continuous film can be built-up by spraying then this may involve some considerable lime. A further problem with spraying is that the nozzles are prone to blockage caused by loose detergent particles being recirculated. It is an object of the present invention to obviate or mitigate the above mentioned disadvantages. According to the present invention there is provided a method of forming a polymeric coating on a detergent tablet containing an alkaline agent, the method comprising applying to the tablet a continuum of an aqueous solution or suspension of a polymer having free acid groups, and evaporating water to leave the coating on the tablet. We have surprisingly found that aqueous solutions and suspensions of polymers (which term is also used to cover copolymers) containing free acid groups may be applied in the form of a continuum to a detergent tablet containing an alkaline agent (of the type typically employed in such tablets, see infra) so as to provide (after evaporation of water) a continuous coating of the polymer in the tablet. By reference to the solution or suspension being applied in the form of a continuum we mean that the solution or suspension being applied to the tablet is at any one time continuous. In contrast, a spray would be regarded as discontinuous in that the spray is made up of individual droplets which impinge in the tablet. Looked at in another way, the application of a continuum of the solution/suspension of the polymer can considered to be such that at any one time the contact region between the solution/suspension and the tablet is continuous (again as opposed to a spray where the contact is discontinuous). Example of applying the solution/suspension of the polymer in the form of a continuum are described more fully below but by v/ay of illustration include dipping of the tablet into a reservoir of the solution/suspension and passing the tablet relatively through a continuous cascade (e.g. a "curtain" of the solution/suspension. In each case, it can be considered that the tablet is drenched with the aqueous based system with the surprising result that tablet integrity is maintained and a continuous coating is formed. The use of a cascade of the solution/suspension of the polymer is an important feature of the invention in its own right and therefore in accordance with a second aspect there is provided a method of forming a polymeric coating on a detergent tablet containing an alkaline agent, the method comprising applying to the tablet an aqueous solution or suspension of a polymer having free acid groups, and evaporating water to leave the coating on the tablet wherein the aqueous solution or suspension of the polymer is applied to the tablet by passing the tablet relatively through a cascade of the solution/suspension. The invention thus represents a considerable advance in being able to provide for the use of aqueous based polymer solution/suspensions in forming continuous coatings thus avoiding the disadvantages associated with melt, solution/suspension in organic solvents, and spraying. Thus the present invention makes it possible to apply coatings simply, cheaply and without complex equipment/procedures associated with spraying melts or solvents. Whilst we do not wish to be bound by theory we believe that the success of the invention can be attributed to a reaction that occurs between the acid groups in the polymer and alkaline agent on at least the surface region of the tablet. The polymer is preferably a film forming polymer. The polymer used in accordance with the invention for forming the coating may have up to 100% (e.g. up to 95%) of its constituent monomer units incorporating acid groups. Generally from 10% to 90% by weight of the monomer units will incorporate acid groups, more preferably 20% to 80%, more preferably 30% to 70% and even more preferably 35% to 65% and ideally 35% to 60% (e.g. 35% to 45%). The acid group may be sulfonic acid groups (-SO 3 H) but more preferably are carboxylic acid groups (-COOH). Various embodiments of polymer may be used in accordance with the invention as set out below. First Embodiment In the first embodiment, the polymer may be an addition polymer derived from an ethylenically unsaturated acid monomer with free carboxylic acid groups. Examples of suitable carboxylic acids are α,β -unsaturated carboxylic acids and dicarboxylic acids. Specific example include acrylic acid, methacrylic acid, itaconic acid, iconatic acid, cinnamic acid, crotonic acid, mesaconic acid, carboxymethyl acrylic acid, maleic acid, fumaric acid and the like. Further examples ^ of classes of acids that may be used include vinyl esters of dicarboxylic acids. A still further example of acid that may be used is 1,4- vinyl benzoic acid. Generally the ,β -unsaturated carboxylic acids (particularly those containing 3 to 8 carbon atoms) are preferred. Preferably 10% to 90% by weight of the monomer units incorporate hydrophobic groups, more preferably 20% to 80%, more preferably 30% to 70%, even more preferably 35% to 65% and ideally 40% to 65% (e.g. 55% to 65%). Examples of hydrophobic groups that may be used include C1.50 (preferably Cιo)alkyl and C 6-50 . aromatic groups. The hydrophobic groups may for example be provided by the alkyl groups of C 1 - 5 (more preferably C ) alkyl esters of the abovementioned unsaturated monomers incorporating carboxylic acid groups, preferably the C1-5 (more preferably CM) alkyl esters of C 3-8 α,β- ethylenically unsaturated carboxylic acids. Alternatively the hydrophobic groups may be introduced by means of any ethylenically unsaturated monomer incorporating such groups, e.g. the amide analogus of the aforementioned esters, vinyl ethers (e.g. a C M alkyl vinyl ether), vinyl aromatic compounds etc. The hydrophobic groups may be bonded to the polymer backbone via polyalkylene oxide residues. Further comonomers may be present in the polymer. Second Embodiment In a second embodiment, the polymer is one having more than 70% by mole of monomer residues incorporating at least two -CH 2 - and/or -CH groups and at least 20% by mole of the monomer residues provided with acid groups attached directly or indirectly to the polymer backbone. It is preferred that the hydrophilic groups are attached directly to the polymer backbone. The -CH 2 - groups may be within the polymer backbone or attached directly thereto. The -CH 3 groups may be attached directly or indirectly to the polymer backbone. It is preferred that at least 75%, more preferably at least 80%, even more preferably at least 85% and most preferably at least 90% by mole of the monomer residues incorporate at least two -CH 2 - and/or -CH groups. Particularly preferred polymers for use in this second embodiment will have at least 95%, and ideally about 100% by mole of monomer residues incorporating at least two -CH 2 - and/or CH 3 groups. Preferably the polymer comprises 20% to 60%>, more preferably 30 to 60% and even more preferably 35% to 55% (e.g. 45% to 55%) by mole of monomer units with hydrophilic groups. The polymer of this second embodiment may be a co-polymer of an α,β- ethylenically unsaturated monocarboxylic acid and an ester of an α,β-ethylenically unsaturated monocarboxylic acid. Preferred examples of polymers for use in accordance with this embodiment of the invention contain 30-10% (more preferably 45% to 65%, e.g. 45% to 55%) by mole of residues derived from the monocarboxylic acid and 30% to 70%) (more preferably 35% to 55%) of residues derived from the ester. Examples of suitable α,β-unsaturated carboxylic acids include acrylic acid, methacrylic acid, itaconic acid, iconatic acid, cinnamic acid, crotonic acid, mesaconic acid, carboxymethyl acrylic acid, maleic acid, fumaric acid and the like. The ethylenically unsaturated acid preferably incorporates groups of the formula CH 2 =C(R)HCO 2 H where R is hydrogen or a C 1-8 (preferably C M ) alkyl group. Generally α,β-unsaturated carboxylic acids containing 3 to 8 carbon atoms are preferred. The esters may for example be provided by C MO (more preferablyCι- 5 , even more preferably C M ) alkyl esters of the abovementioned carboxylic acids, preferably the C 1 - 5 (more preferably CM) alkyl esters of C 3-8 α,β-ethylenically unsaturated carboxylic acids. The polymer may be obtained by copolymerisation of (meth)acrylic acid and a Q. 5 (more preferably CM) ester of (meth)acrylic acid. Most preferably the coating polymer comprises about 45% to 55%o by mole of C \ . 5 (more preferably CM) alkyl (meth)acrylate and about 45% to 55% by mole of (meth)acrylic acid. The alkyl group of the ester is preferably an ethyl, propyl or butyl group. Third Embodiment In a third embodiment, the polymer is a co-polymer of an α,β-ethylenically unsaturated monocarboxylic acid and an ester of an α,β -ethylenically unsaturated monocarboxylic acid. Preferred examples of polymers in accordance with this embodiment of the invention contain 30-70% (more preferably 45% to 65%, e.g. 45% to 55%) by mole of residues derived from the monocarboxylic acid and 30% to 70% (more preferably 45% to 65%, e.g. 45% to 55%) of residues derived from the ester. Examples of suitable α,β -unsaturated carboxylic acids include acrylic acid, methacrylic acid, itaconic acid, iconatic acid, cinnamic acid, crotonic acid, mesaconic acid, carboxymethyl acrylic acid, maleic acid, ήrmaric acid and the like. The ethylenically unsaturated acid preferably incorporates groups of the formula CH 2 =C(R)HCO 2 H where R is hydrogen or a Cι_ 8 (preferably C ) alkyl group. Generally α,β-unsaturated carboxylic acids containing 3 to 8 carbon atoms are preferred. The esters may for example be provided by Cwo (more preferably C 1 - 5 , even more preferably C M ) alkyl esters of the abovementioned carboxylic acids, preferably the Cι -5 (more preferably CM) alkyl esters of C 3-8 α,β-ethylenically unsaturated carboxylic acids. The polymer may be obtained by copolymerisation of (meth)acrylic acid and a Q. 5 (more preferably C M ) ester of (meth)acrylic acid. Most preferably the coating polymer comprises about 45% to 55% by mole of C-|. 5 (more preferably C M ) alkyl (meth)acrylate and about 45% to 55%by mole of (meth)acrylic acid. The alkyl group of the ester is preferably an ethyl, propyl or butyl group. Polymers employed in the invention may be within the scope of two or more of the abovedescribed embodiments. Preferred polymers for use in the invention are copolymers of (meth)acrylic acid and either ethyl acrylate or butyl acrylate. Detergent tablets having coatings as described above for the first to third embodiments of the invention are an important feature of the invention in their own right. Therefore according to the third, fourth and fifth aspects of the present invention there are provided detergent tablets provided with a coating of a polymer as defined above for the first, second and third embodiments respectively. Such coated tablets may be produced as defined for the first aspect (by applying a continuous of a solution or suspension of the polymer to the tablet) but could be produced by other methods. It will be appreciated from the foregoing description that polymers for use in the invention may be represented in the form (I) where A, B, and C are polymerised residues of an ethylenically unsaturated double bond (not necessarily implying that the double bond was of the formula CH 2 =CH-); Li represents the bonding of the carboxylic acid group to the polymer backbone, this either being direct to the polymer backbone or through a linking group; Hyd is a hydrophobic group; L 2 represents the bonding of the hydrophobic group to the polymer backbone, this either being direct to the polymer backbone or via a linking group, e.g. -O-, -C(0)O-, - C(O)NH- polyalkylene oxide (e.g. polyethylene oxide) etc; Funct represents a functional group; L3 represents the bonding of the functional group to the polymer backbone, this either being direct to the polymer backbone or via a linking group; Ti and T 2 are terminal groups; m is an integer; n is 0 or more preferably an integer p is 0 or an integer. The residues X, Y and Z may be arranged in the polymer in any order. Examples of preferred polymers for use in the invention are those for which A is a residue of a C 3-8 α,β -ethylenically unsaturated acid (e.g. methacrylic acid), B is a residue of a CM alkyl ester of a C 3-8 ,β-ethylenically unsaturated acid (e.g. ethyl acrylate) and m and n are such as to provide a percentage weight ratio of A:B of 35-45% A and 55% to 65%o B, P preferably being zero. Further preferred polymers for use as a coating in accordance with the invention are as represented by Formula (II): wherein A is a polymerised residue of a monomer selected from one or more C 3 -C 8 mono- ethylenically unsaturated carboxylic acids; B is the polymerised residue of a monomer selected from one or more C 3 -C 60 alkyl ( eth)acrylates, ethoxylated Cι- 2 alkyl (meth)acrylates and poly (alkylene glycol) (meth)acrylates, alkyl or aromatic ethers of polyalkylene glycol and the corresponding, maleate esters thereof; C is a polymerised residue of a monomer selected from one or more ethylenically unsaturated monomers which are copolymerisable with the monomers in A and B; A, B and C maybe randomly arranged in said polymer; Ti and T 2 are end groups; m is the total number of A residues and is from 1 to 500; n is the total number of B residues and is >0; p is the total number of C residues and is from 0 to 500; the sum of m and p is at least 1 ; q is from 0 to 100; and D is a polymerised residue of a monomer selected from poly(alkylene) oxide or alkylene oxide monomers E is selected from Cι- 50 alkyl groups and C 6 - 50 aromatic groups X is a functional group. The term "A, B and C residues are randomly arranged in said polymer" means that the residue adjacent the end group T \ may be an A, B or C residue. Preferably the polymer of Formula (I) is formed by co-polymerising two or more monomers A, B and C wherein: - . (i) Monomer A is selected from one or more C 3 -C 8 mono-ethylenically unsaturated carboxylic acid moieties; (ii) Monomer B is selected from one or more C 3 -C 6 o alkyl (meth)acrylates, ethoxylated - 2 alkyl (meth)acrylates, poly (alkylene glycol) (meth)acrylates, alkyl or aromatic ethers of polyalkylene glycol and the corresponding maleate mono and di -esters thereof; (iii) Monomer C is selected from one or more ethylenically unsaturated monomers which are copolymerisable with the monomers in A and B; • Ti and T 2 are end groups; m is between 0 and 500; n is >0; p is between 0 and 500; q is from 0-100; ' D is selected from poly(alkylene) oxide or alkylene oxide monomer units; and E is selected from Ci to C 5 o alkyl groups and C 6 to C5 0 aromatic groups X is a functional group. Such polymers are disclosed more fully in EP-A-0 995 791. The polymer for use in the invention may be of the type known as a hydrophobised (meth)acrylic acid polymer. Polymers for use in the invention may be cross-liked or non cross-linked. If cross-linked then the degree of cross-linking is preferably less than 2%, more preferably a maximum of 1% (e.g. 0.01% to 1%). The polymer may have a molecule weight (Mw) of at least 20,000 and the molecular weight may for example be in the range 20,000 to 3,000,000. More preferably the molecular weight is at least 50,000 and for certain embodiments at least 100,000. If cross-linked the molecular weight is preferably 500,000 to 3,000,000. If non cross-linked the molecular weight is preferably 50,000 to 2,000,000. The question as to whether the polymer is used in the form "of a solution or suspension for application to the tablet will of course depend on its solubility in water at the temperature to be employed for coating. It is preferred that the polymer is present in the aqueous solution/suspension in an amount greater than 15% by weight although usually not more than 50%, more preferably not more than 40%. Generally however the aqueous solution/suspension will contain more than 20%o by weight and more preferably greater then 25% by weight of the polymer, e.g. 25% to 35% by weight. A preferred value is about 30% by weight. ' We have in fact found, particularly surprisingly, that detergent tablets may be immersed is solutions/suspensions containing more that 20% by weight of the polymer for periods in excess of 20 minutes without tablets dissolution, although it will be appreciated in practice that the times employed for application of the coating will be significantly less (see infra). It is particularly preferred that the polymer is employed as a suspension/emulsion, most particularly one incorporating -surface active agents that stabilise the suspension since such activities will also assist in forming a uniform coating in the tablets. Most conveniently such a suspension/emulsion is obtained by a suspension/emulsion polymerisation reaction carried out with the use of surface active agents to assist in the formation of the suspension/emulsion of the polymer. The suspension may also incorporate a plasticiser (e.g. diglyme or a low molecular weight polyethylene glycol) to assist in formation of an even coating of the polymer on the tablet. Ideally also the polymer should be one which may be regarded as being soluble in itself so that as the solution/suspension applied to the tablet dries and becomes more concentrated the polymer dissolves into itself to prevent uneven residues. Most preferably the solution/suspension of the polymer will have a pH of at most 8, more preferably at most 5 and more preferably at most 3, e.g. 2-3. Polymer solutions/suspensions having the preferred pH of 2-3 may be of the type that with increase of the pH (e.g. to 5 or above) there is thickening of the polymer solution/suspension. The polymers may be produced with a low pH, e.g. 1.5 and partically neutralised for use in the invention. Preferably the solution suspension has a viscosity of less than 200cP, more preferably less than lOOcP, even more preferably less than 50cP, still more preferably less than 30cP and most preferably 0-20cP. Unless otherwise stated viscosities as referred to herein are measured using a Brookfield LVT Viscometer, spindle no. 1, 20°C, 12rpm. Specific examples of polymers that may be used in accordance with the invention include: (i) Acusol 830, which is believed to be a copolymer of methacrylic acid (40%) and ethyl acrylate (60%) (CAS No. 25212-88-8) (available from Rohm & Haas) having a molecular weight of 100k, a pH of 2-3and a viscosity of 25cP . (ii) Acusol 820, an acrylic acid copolymer with C 18 alkyl and EO 2 0C 18 (where EO is polyethylene oxide) side chains of molecular weight 500 000 (available from Rohm & Haas); (iii) Acrysol 220, an acrylic acid copolymer with an ester of methacrylic and an ethoxylated (20EO) stearic acid (available from Rohm & Haas); and (iv) Narlex TD20 which is a copolymer of methacrylic acid and butyl acrylate having a molecvular weight of 200k, a pH of 2 and a viscosity of 5cP. Further examples of polymers which may be used are acrylic acid/maleic acid maleate copolymers (e.g. having a molecular weight of about 70000) such as available from BASF under the name Sokalan. In accordance with the invention, we prefer to use as the coating formulation, Acusol 830 which as supplied is a 30% suspension of a methacrylic acid/ethylacrylate copolymer in water. Tablets may be immersed in such a suspension whilst still remaining undissolved for periods in excess of 20 minutes. Dilution of the suspension to a polymer content of about 20% still results in the tablet remaining undissolved after 20 minutes. Application of the solution/suspension as a continuum to the tablet may be effected in a number of ways. In one embodiment of the invention, the tablets may simply be immersed in a reservoir of the solution. It is however more preferred that the tablets (e.g. supported on a conveyor belt) are moved relatively through a vertically falling cascade which is continuous both in vertical and horizontal extent whereby the solution/suspension is applied to exposed surfaces of the tablet. Preferably the time taken for the tablet to pass relatively through the cascade is less 20 seconds, more preferably less than 10 seconds and most preferably less man 7 seconds. Preferably the tablet passes successively through two such cascades, e.g. over a total period of 5 to 15 (ideally about 10) seconds. Given that the tablets are supported on a conveyor then the solution/suspension falls on the upper and peripheral surfaces of the tablet. So far as the undersurfaces of the tablets are concerned, there will generally be sufficient coating composition remaining on the conveyor (from previous passes through the cascades) so that coating composition is "applied" to the undersurfaces. After passing through the final cascade the tablets are subjected to drying (ambient or higher temperature, e.g. 35°C), preferably by an air flow directed downwardly over the tablets. This ensures that excess polymer is "blown-off ' the tablets to leave an even coating on the upper and peripheral surfaces thereof. Subsequently the tablets are "flipped-over" and the (original) undersurfaces dried as described. Alternatively the removal of excess polymer solution suspension and drying may, in effect, be separate operations. Thus excess polymer solution/suspension may be "blown-off but without significant evaporation of water. Subsequently the tablets may be subjected to drying to evaporate water. A particular advantage of using a vertically falling cascade of the solution/suspension of the polymer is that the tablets become completely covered in a relatively short period of time and can be dried off quickly. Preferably the time between application of the solution/suspension to the tablet and drying is less than 20 seconds, more preferably less than 10 seconds and most preferably less than five seconds. A suitable apparatus for applying the coating composition in the form of a vertically falling cascade is disclosed in DE-A-42 03 086. Although the apparatus of DE- A-42 03 086 is intended for the application of chocolate (in the form of a vertically falling cascade) to confectionery items, it is eminently suitable for the purposes of the present invention. A suitable device for "flipping" tablets is disclosed in GB-A-2 193 073. Generally the thickness of the tablet coating will be in the range of from 5 to 600 microns, more preferably 5 to 500 microns, even more preferably 5 to 400 microns, and still more preferably 5 to 300 microns. The particularly preferred thickness range is 20 to 150 microns, even more preferably 40 to 100 microns and most preferably 50 to 80 microns. Generally the coating will completely encase the tablet. The coating may optionally include a plasticiser and/or liquid typically used in detergents, e.g. perfume, anti-foam and/or surfactants. The invention is applicable particularly to laundry wash tablets but also dish wash tablets. Coated tablets produced in accordance with the invention are particularly suitable for use with a flexible net bag as disclosed in GB-A-2 323 606 since they reduce the amount of fine material lost by handling the tablet. The tablets can be included in a flow-wrap as disclosed in WO-A-9840464 but this is not a preferred option. In this respect, an advantage of the coated tablets is in relation to so-called "tablet rework" since there is no need to separate the flow wrap from the tablets as described in WO-A- 0004126 and WO-A-0004127. The rework process would simply involve gently breaking the tablets and adding them back into untabletted powder at a low level. A further advantage of the invention relates to detergent tablets having a mould into which a second tablet or dissolving ball is glued. Coating the tablet, according to the invention, can be expected to reduce the possibility of the ball "breaking off, or potentially the need to glue the ball into the mould (see WO-A-0006684, WO-A- 0006683, WO-A-0006688, WO-A-0006689, WO-A-0006505 and WO-A-0004115. The coating of the polymer may be generally clear or translucent allowing coloured or speckled layers in or on the detergent tablet to be clearly visible through the coating. The breaking strength of the uncoated tablet may for example be in the range 15 (e.g. -20) to 1000 N more preferably 15 to 500 N. Generally the breaking strengths of laundry wash tablets will be toward the lower end of this range and may for example be • 15 to 200 N, more usually 20 to 200N, preferably 20 to 100N and more preferably 20 to 50N. The breaking strength values are measured along the strongest axis of the tablet. For a circular section (i.e. disc-shaped) tablet the breaking strength is measured by compression perpendicular to the major (circular) faces. Breaking strength may be measured using an INSTRON CT5 load tester. Breaking strength is the value at which there is a discontinuity in the stress-strain graph (increase in stress for no or only limited increase strain) and may (bot not necessarily be manifested by a visible crack in the tablet. The coating may provide a %ge increase in breaking strength of at least 10%, better at least 20% and even better at least 30%. The tablets may be such as to have a major face of circular, oval, square, rectangular or other shape. If the major face has straight edges and corners then the latter may be rounded. Detergent tablets in accordance with the invention may be of conventional composition and this incorporates niter alia the alkaline substrate referred to above. This substrate should be provided in at least the exposed surface region of the tablet but will generally be distributed uniformly throughout the tablet. Examples of suitable alkaline substances include the alkali metal (usually sodium or potassium) silicate, phosphates, carbonates and bicarbonates. Generally the tablet will contain a total of 10 to 100% by weight of the alkaline agent, more preferably 20% to 90% by weight of the alkaline agent. Generally at least a proportion of the alkaline agent will be provide by sodium triphosphate (e.g. of the type, or of a type similar to that disclosed in EP-A-0 839 906. The alkaline agent may be comprised wholly or partially of a water insoluble material, e.g. a zeolite. The tablets will generally also comprise at least one surface active agent. The surface active agent may be an anionic, cationic, non-ionic or amphoteric. The anionic surface active agent may comprise at least one alkyl sulphate, most preferably a C 8-22 alkyl sulphate. For preference, the alkyl group of the alkyl sulphate has 8-16 carbon atoms. The alkyl sulphate may be a single compound or may comprise a mixrare of alkyl sulphates of different chain lengths. For preference the alkyl groups are primary alkyl groups and preferably straight chain. The alkyl sulphate is preferably an alkali metal alkyl sulphate, the preferred alkali metal being sodium. A suitable alkyl sulphate for use in the invention is available under the trade mark SULPHOPON, e.g. SULPHOPON 1218GF (a Cι 2-] 8 alkyl sulphate). As an alternative to an alkyl sulphate, the anionic surface active agent may be an alkyl ether sulphate, preferably one in which the alkyl group has 8-22 carbon atoms. It is particularly preferred that the alkyl ether sulphate is an alkyl (C 8-22 ) ethoxylated (n=l to 5, preferably 2 or 3) sulphate. The alkyl group of the ether sulphate may be as described for the alkyl sulphate. Further Examples of anionic surface active agents which may be used include alkylaryl sulphonates (e.g. alkylbenzene sulphonates, (e.g. Nansa HS90 ex Albright & Wilson) alpha olefin sulphonates and ether carboxylates. It is also possible for the anionic surface active agent to be comprised partially or wholly by a soap of any type known for use in detergent tablets. If a liquid non-ionic surface active agent is to be included in the tablet then it may for example an alcohol ethoxylate. The alcohol residue (which may be of a primary or secondary alcohol) may for example comprise 8 to 18 carbon atoms and be ethoxylated with an average of 3 to 20 moles of ethyl ene oxide per mole of alcohol. Suitable liquid non-ionic surface active agents are available from ICI under the designations SYNPERONIC A3 and SYNPERONIC A7. Mixtures of the A7 and A3 active agents may also be used. Also suitable are LUTENSOL AO3, LUTENSOL AO6 and LUTENSOL AO7 (ex BASF). Examples of solid non-ionic surface active agents which may be used in the formulation in accordance with the first aspect of the formulation include alky^Cs- 22 )polyglycosides. The preferred glycoside employed in the present invention is a glucoside (i.e. based on glucose), functionalised with a primary alcohol (e.g. Cι 2 -Cι 4 ). More preferably the glucoside is in the form of a polyglucoside, with a preferred degree of polymerisation of between 1-2, most preferably about 1.4. It is preferred that solid non-ionic surface active agent is used in the form of particles or granules containing at least 30% by weight, more preferably at least 40% by weight of solid non-ionic surface active agent. A suitable polygiycoside is available under the name Glucopon (Henkel). Preferably used as Glucopon G50 granules (50% APG, 20% silicate, 30% sulphate). The amphoteric surface active agent may for example be a betaine. Preferred betaines may be either of the fo mula (I) or (II). R 1 R* R 1 R 3 CONHCH 2 CH 2 CH 2 N + -CH 2 COO- (II) R^ In the above formula, R and R may be the same or different CM alkyl groups whereas R 3 is an alkyl group having 8-22 carbon atoms, more preferably 12 to 18 carbon atoms e.g. mixed do to Cj 4 . The preferred betaine for use in the tablet of the invention is cocoamidopropyl betaine (also known as cocodimethyl acetic acid betaine (CAS Registry No.66455-29-6). Further betaines which may be used are lauryl dimethyl betaine (CAS Registry No. 683- 10-3), cocoa dimethyl amidopropyl betaine (CAS Registry No. 61789-40-0) and the products identified as CAS Registry Nos. 70851-07-09 and 4292-10-8. An alternative amphoteric surface active agent for use . in the tablet of the invention is a glycinate of the formula R 3 NHCH 2 CO 2 H where R 3 is as defined above. A further glycinate which may be used is of the formula R 3 - N-(CH 2 ) 3 N-CH 2 C0 2 Na CH 2 CO 2 Na „ CH 2 C0 2 Na fn which R 3 is as defined above (more preferably C 12-22) and n is 1 to 3. Other suitable materials are as given in chapter 1 of "Amphoteric Surfactants", e.g. Lomax Ed, Marcel Decker, New York 1996. It is also possible for at least, a portion of the surface active agent in the tablet to be cationic surface active agent (although, if so, there would generally be no anionic surface active agent present). Cationic surface active agents that may be employed are those as known for use in detergent tablets. The cationic surface active agent may be one which provides disinfecting and/or softening properties. The invention is applicable particularly to laundry tablets containing a disintegrant. The disintegrant may be a substance having high solubility in water (e.g. hydrated salts of sodium citrate and sodium acetate). Such water soluble disintegrant are used particularly for tablets which are to be introduced into the drum of a domestic washing machine. It is also known to use dismtegrants which are insoluble in water but which swell in contact therewith. Such water insoluble materials tend to be used for tablets are to be loaded into the dispensing drawer of a washing machine. Laundry wash tablets that contain a disintegrant may disintegrate within 10-40 seconds when immersed in water at 30°C. It is therefore particularly surprising that tablets incorporating a disintegrant can be coated • using an aqueous ' polymer solution/suspension in accordance with the invention. The invention is particularly applicable to tablets containing a disintegrant of the latter type. Most preferably the disintegrant is a cellulose based material. Such cellulose based material may for example comprise both crystalline and amorphous cellulose. Examples of suitable materials are disclosed, for example, in WO-A-9855575 (Henkel), WO-A-9840462 (Herzog). The cellulose may be a cross-linked modified cellulose e.g. AC-DI-SOL and/or may comprise micro crystalline cellulose fibres (e.g. HANFLOC). The cellulose based material may be a cellulose derivative which may be cross- linked, e.g. a cross-linked carboxymethyl cellulose. A particularly suitable disintegrant for use in the invention is available under the trade mark NILYN. (ex FMC), grade LX16 which is an internally cross-linked carboxymethyl cellulose. The disintegrant may be a cellulose derivative, for example a sodium carboxymethyl cellulose. Examples include COURLOSE and NYMCEL. Further examples of disintegrants which may be used include various starches such as potato, rice, com ore maize starch. The disintegrant may be a starch derivative, e.g. carboxymethyl starch such as available under the trade mark PRIMOGEL or a sodium starch glycolate such as available under the trade mark EXPLOTAB. It is also possible for the disintegrating agent to be a clay. Such clays are generally of the "lamellar type" and may for example be a smectite such as a Laponite, Bentonite, Montmorrillonite, Hectorite or Saponite. For example, the clay may be a Sodium Montmorrillonite, a Sodium Hectorite, a Sodium Saponite, a Calcium Montmorrillonite or a Lithium Hectorite. Furthermore, it is possible for the disintegrating agent to be a synthetic polymer, for example a cross-linked polyvinyl pyrrolidone, POLYPLASDONE XL or OLLIDON XL. It would be appreciated that certain of the above described disintegrants will provide additional benefits during a laundering operation, e.g. a clay may contribute to fabric softening properties and synthetic polymers may act to prevent deposition of dyes. It is particularly, preferred that (prior to compaction of the tablet or region thereof) the. disintegrant has an average particle size of 30μm to 1500μm. For example, the disintegrant may have a particle size of 50μm to 500μm, preferably 50μm to 400μm, more preferably lOOμm to 300μm, and even more preferably 300μm to 350μm. Alternatively, the disintegrant may have a particle size of 500μm to 1200μm, more preferably 600μm to lOOOμm, and even more preferably 800μm to lOOOμm Typically the amount of the disintegrant present in the tablet (or region thereof) will be in the range 0.5%) to 9% by weight, more preferably 1% to 5% by weight. The tablet may incorporate a bleach, for example an activated bleaching system. Such a system may comprise a hydrogen peroxide precursor (e.g. sodium percarbonate, sodium perborate monohydrate or sodium perborate tetrahydrate) together with a bleach activator. The activator may be an N-acyl compound, particularly one having two or more N-acyl groups. Thus, for example, the activator may be tetraacetyl ethylene diamine (TAED) as conventionally used as a activator in detergent tablets. Alternatively, the bleach activator may be an ester of a polyhydric alcohol having at least 5 carbon atoms and at least 3 hydroxyl groups esterified with C 2 -sacyl groups, the polyhydric alcohol residue of said activator not having substituents with 6 or more carbon atoms. Such an activator may have an HLB value of at least 7, more preferably at least 9, and even more preferably at least 11. The HLB value may be as high as 14 or 15. The alcohol residue of the activator preferably has a maximum of 12 carbon atoms and a minimum of five hydroxyl groups esterified with C 2-5 acyl groups. Examples of suitable alcohols are sugar and sugar derived alcohols such as sorbitol, glucitol, mannitol, glucose and sucrose. For preference, the- acyl groups in the activator are aliphatic acyl groups. It is preferred that the acyl group has two or three carbon atoms and is most preferably the acetyl group. Specific examples of bleach activator which may be used in the tablets of the invention include hexa acetyl sorbitol, hexa acetyl mannitol, penta acetyl glucose and octa acetyl sucrose. Particularly preferred are hexa acetyl sorbitol and hexa acetyl mannitol which may be used in admixture, e.g. as disclosed in EP-A-0 525 239. Further Examples are compounds having nitrogen atoms in the basic carbohydrate skeleton,, e.g. the peracetylated forms of N-methyl gluxconamide, N-methyl glucamine and glucopyronosyl amine. Further details of the activators are disclosed in EP-A-0 869 170. The amount of bleach activator incorporated in the tablet of the invention will generally be in the range of 0.5% to 10% by weight of the total formulation, more preferably 1% to 8% and even more preferably 2% to 4% on the same basis. The preferred bleaching system for use in the invention comprises a hydrogen peroxide precursor compound and the bleach activator as defined above which is capable of reacting with the hydrogen peroxide to generate a peracid. The hydrogen peroxide precursor compound may, for example, be an inorganic persalt e.g. a perborate (in the monohydrate and/or tetrahydrate form), a percarbonate or a persulphate. The alkali metal salts of these compounds are preferred, particularly sodium and potassium salts. Alternatively the bleaching agent may be a urea-hydrogen peroxide complex. The amount of hydrogen peroxide precursor compound present in the formulation . of th invention is preferably such as to provide 0.5% to 3% by weight active oxygen, especially 1.0% to 2.5% by weight. In another embodiment of the invention the tablet may incorporate an organmεtallic bleach or a hydrophobic bleach. Examples of suitable bleaches are disclosed in WO-A.-0055294. The tablet may incorporate a fabric softening agent which may for example be a clay in conjunction with a surface active agent. The fabric softening clay preferably has a particle size of at least 500 μm. The fabric softening clay may be any such clay having fabric softening properties used in laundry detergent formulations. Such clays are generally of the "lamellar type" and are such that the layers "separate" to become deposited on the garments being washed. The clay may for example be a smectite such as a Laponite, Bentonite, Montmorrillonite, Hectorite or Saponite. For example, the clay may be a Sodium Montmorrillonite, a Sodium Hectorite, a Sodium Saponite, a Calcium Montmorrillonite or a Lithium Hectorite. Generally the amount of clay used as a fabric softener in the detergent tablets will be 5% to 20% by weight. The clay may be used in conjunction with a cationic and/or amide surfactant to help delamination of the clay and absorption thereof onto the garments being laundered. The cationic surfactant may for example be a quaternary ammonium salt having one long chain (e.g. Cs -22 ) alkyl group and three short chain (e.g. CM) alkyl groups. A suitable cationic surfactant is coco trimethyl ammonium chloride. The amide surfactant may contain at least one long chain (e.g. C 8- 22) alkyl group and may for example be stearyl stearamide. A suitable clay formulation may contain 20-30% by weight of the formulation (i.e. clay plus surfactants) of amide surfactant and 1-2% cationic surfactant. The fabric softening agent may be an organic compound. One class of organic fabric softening agents are amides of the formula C n H2n+l " C-NCjrHbm+l O where n and m are the same or different and are in the range 8 to 22, more preferably 10 to 20. If the alkyl groups are branched then they preferably include a chain of at least 8 carbon atoms. A particularly preferred amide for use in the invention is stearyl stearamide. Alternatively, or additionally, the organic fabric softening agent may be a quaternary ammonium salt having one long chain (e.g. C 8-22 ) alkyl group and three short chain (e.g. CM) alkyl groups. A suitable cationic surfactant is coco trimethyl ammonium chloride. It is particularly preferred that the quaternary ammonium salt be used in combination with the above described amides in which case the quaternary ammonium salt may suitably be employed in an amount of up to 5%, more preferably 1 to 2%, by weight of the clay. Further organic fabric softening agents which may be used include amine and/or amide functionalised silanes. The tablet may incorporate at least one enzyme. The enzyme may, for example, be a protease, amylase, lipase or cellulase (or mixtures thereof) such as commonly used in detergent formulations. Examples, of suitable enzymes are available under the names Opticlean, Savinase, Esperase; . Termamyl, Maxamayl, Lipomax, Lipolase; Celluzyme and Carezyme. The amount of enzyme incorporated in the tablet will depend on activity but will typically be 0.1 to 3%. This level is particularly suitable for Savinase 6.0T, Termamyl 60T, Celluzyme 0.7T and Lipomax. The tablets for coating in accordance with the invention may be mass-produced on a number of tabletting machines. Models that may be used include the Europharma Machinery (UK) and generally work by having a rotating circular turret with arrays of punches that compress the tablets from above and below. Tablets may be produced that are single or dual or multi- layer or of the tablet-in-tablet type and variations thereof. The cycle for producing dual layer tablets consists of filling the die with the powder that will make up one of the layers, followed by filling of the powder of second layer. Machines specially designed for dual layer operation usually have a small amount of pre -compression between filling the die with the powders of the first and second layers. This gives a sharper definition between the two layers which may be more aesthetically pleasing, particularly if the two layers are of different colours. The press should have a control to regulate the applied force used in the main compression. The applied pressure should typically be about 10 to 100 kN for a 44mm diameter tablet. The pressure applied is a crucial part of the tabletting operation as inadequate pressure will gives a tablet which dissolves too slowly. The tablet strength may be monitored by use of equipment to measure its breaking strength such as the Holland CT5 automatic compression tester (see below). It will be appreciated that the tablet may incorporate additional components as conventionally included in laundry detergent formulations. One Example of such an additional component is a soap which may be used in an amount up to 5% by weight as a processing aid. Further Examples include anti-foam agents, sequestrants (e.g. of the phosphonate type), whiteness maintenance agents (e.g. CMC, polyoxyethylene terephthalate, polyethylene terephthalate), colourants (e.g. dyestuffs), perfume, flow control agents (e.g. a sulphate) flow enhancer (e.g. a zeodite), pH regulators (e.g. a carbonate or bicarbonate), anti-corrosion agents, dye transfer inhibitors (e.g. PVP) and Optical brighteners (e.g. Tinopal CBS-X and Tinopal DMS-X). A further possibility is a perfume which can be added into the tablet formulation as a liquid or encapsulated in (for example starch or silica). These components may, for example, each be present in amounts up to 1% by weight of the formulation. Alternatively the perfume or a fraction thereof can be included in the coating polymer solution. The fabric softening agent may be an organic fabric softening agent which is nitrogen containing compound having at least a degree of positive charge on the nitrogen atom. The invention will be illustrated with reference to the following non-limiting Examples. Examples Tablets of various compositions (see below) were produced and evaluated using the following procedures. Beaker Test Tablets (45mm diameter, 18mm height, 40g) are placed on an open wire stand (40mm diameter, 5cm high) in a beaker containing water at 30°C. The time taken for the bulk of the tablet to fall from the stand was measured. A time of 30 seconds or less indicates suitability of the tablet for the testing in the dispensing drawer of a domestic washing machine. Maximum Load Test (Breaking Strength) This test was conducted using an INSTRON CT5 apparatus with a 50 kg load cell for textile wash tablets and a 500 kg load cell for dishwash tablets. Snap test Three point snap tests were carried out using a Holland (UK) CT5 compression tester, tablets are placed flat, on their circular face, on two bars placed 21mm apart in the load cell. A third bar then compresses the middle of the tablet from above. This is performed at ambient temperature. A 40 gram tablet should break at about 2-6 kg applied force. Example 1 This Example demonstrates the effect of various coating solutions / melts upon the integrity of a detergent tablet. Laundry wash tablets of the following compositions were prepared. Alkyl Sulphate 9% Alkyl poly glucoside 1% Bentonite Clay 17% 7EO Nonionic surfactant 1.50% Phosphate 41 % Percarbonate (Coarse) 16% TAED 3% Cellulose Disintegrant 5% Perfume 0.50% Enzymes 2% Minor components 4% The tablets weighed 40g and were of 45mm diameter and 18mm height. The tablets were immersed in a 250ml beaker filled with Acusol 830 (30% active suspension in v/ater) which was maintained at a temperature of 22°C. The tablet to be coated was gripped by external callipers and lowered into the solution being submersed for a given period of time (1 min or 5min). The tablet was removed from the solution and held 2.5 cm under a hairdryer (Babyliss 1650 Type S-6 hair dryer) supported on a 58cm clamp stand. The tablet was dried for 90 seconds using the hair dryer.. The areas of the tablet that had been covered by the clamp were then painted with the solution of the polymer and dried in the same way. Comparative solutions / melts were also tested. Details of the solutions and resistance of the tablet to the immersion are shown in Table 1. Table 1 wherein :- Solution #1 is a solution of 30wt% Acusol 830 Solution #2 is an aqueous solution of PEG (30%> by weight) Solution #3 is molten PEG Solution #4 water The tablet coated with the coating in accordance with the invention showed surprising and excellent resistance to dissolution in the aqueous solution of Accusol 830 and remained intact after 5 minutes immersion. In contrast, "ordinary" water (i.e. solution #4) caused dissolution of the tablet in 8 seconds whereas the aqueous PEG solution caused dissolution in 16 seconds (although molten PEG caused no dissolution). Example 2 This Example demonstrates the effect of coating upon the strength and dissolution ability of a detergent tablet. A detergent tablet of the type described in Example 1 was dipped for 5 seconds into an aqueous solution of Acusol 830 (30wt%) and dried to make a coated tablet. Comparative tablets were also prepared using a 30% aqueous solution of PEG to produce the coatings by dipping and spraying. The coated tablets were tested for the maximum load they could withstand. Using the INSTRON CT5 apparatus described above (50kg load cell) the tablets were also tested to establish the time that the tablet took to disperse. This was carried out using the beaker test. Control tests were conducted on non-coated tablets. The results of the test are shown in Table 2. Table 2 The solutions are as in Example 1. The tablet coated in accordance with the invention exhibit a good result in the maximum load test being significantly stronger than the uncoated tablet whilst also showing a relatively short time to disperse in the beaker. In comparison the tablet coated with molten PEG exhibited a similar result in the . strength test but performed poorly in the dissolution test taking an unacceptably long 300 seconds to disperse into solution. The tablet coated by spraying with PEG solution also gave inferior results to those obtained ttsing Acusol 830. Example 3 This Example demonstrates the effect of the thickness of the coating on the strength and dissolution ability of a detergent A detergent tablet of the type described in Example 1 was dipped into a solution of Acusol 830 (30wt%) for varying lengths of time to achieve a tablet coating of varying thicknesses. Once the tablets had been prepared they were tested for strength of the tablets (using the test as described in Example 2). The tablets were also tested using the beaker test. Details of the thickness of the coating and the effect on the strength of the tablet and the rate of dissolution are shown in Table 3. Table 3 The thicker the tablet coating the longer that the tablet took to dissolve. The strength of the tablet was also detrimentally affected by the dipping time required to obtain the thicker coatings. It is proposed that the decreased strength of the tablet is caused by ingress of water into the structure of the tablet at the longer dipping times. Example 4 This Example demonstrates the effect of coating tablets of the type described in Example 1 using a solution of polymer in accordance with the invention (30wt% aqueous Acusol 830) on the strength of a detergent tablet. 35 coated tablets were prepared as in Example 2 using a 30wt% aqueous solution of Acusol 830. The tablets were then tested using the snap test. Uncoated tablets were also tested by way of comparison. The results of the snap test are shown in Table 5. Table 5 The tablets coated with the polymeric coating in accordance with the invention were 37.5 % stronger in the snap test than the non-coated tablets. Example 5 This Example demonstrates the effect of coating with the dissolution rate of dishwash tablets. Dishwash tablets DW1 and DW2 of the following compositions were prepared. DW1 DW2 Phosphate 47.00% 47.00% Na-Disilicate 25.00% 13.00% Na-Carbonate 10.35% 10.00% Na-Sulphate - 13.70% Perborate (tetrahydrate) 12.50% - Percarbonate - 11.00% Non-Ionic Surfactant 3.00% 2.20% Enzymes 1.13% 1.70% Other Components to 100% to 100% The tablets were coated as in Example 2 (using 30wt% Acusol 830 as the coating solution). The tablets were tested by placing them in a Zanussi dishwasher set at program C (quick wash). The tablets were removed from the dishwasher every two minutes and weighed. Comparative non-coated tablets were also tested. Details of the weights of the tablets versus time are shown in Tables 5a- 5b. Table 5a - DW1 The increase in weight of the tablet over the period 0 to 2 minutes is due to hydration of the tablet. Table ' 5b - DW2 It appears that the coated tablets dissolve quicker than the non-coated counterparts. It is proposed that this results from the method of coating of the tablets wherein water ingress into the tablet structure causes the hardness of the tablet to be reduced. Example 6 - Black sheet Test This Example demonstrates the amount of residue remaining when coated tablets in accordance with the invention are used in a washing machine. Laundry wash ' tablets were coated as in Example 2 using a 30wt% coating solution of Acusol 830. The tablets were tested by washing two double and tliree single black sheets in a Bosch 2001 washing machine set at 40°C in the "woollens cycle". Two of the tablets were placed in the drawer dispensing drawer detergent for the wash. After the -wash cycle is complete the sheets and the washing machine (drum and seals) were checked for residue remaining from the tablets. Comparative non-coated tablets were also tested in the same way. Results The coated tablets performed almost identically to the non-coated tablets. The coated tablets left a very slight residue on the sheets. However, the quantity remaining was so small that it was impossible to calculate. Example 7 - Drawer Solubility Test This Example demonstrates the amount of residue remaining in the drawer of a. washing machine when coated tablets in accordance with the invention are employed in a laundering operation. Tablets of the type described in Example 1 were as in Example 2 using a 30wt% coating solution of Acusol 830. The tablets were tested by washing 2.5 kg of ballast in a Hoover washing machine set at 40°C in the "cotton cycle" with the water pressure set at 45psi to establish whether the tablets would dissolve in the drawer and the time taken to do so over three drawer fills. The test was carried out three times with the coated and the non-coated tablets by way of comparison. Results The coated tablets passed the drawer test after three fills. The coated tablets left less than 1% residue in two out of three drawer tests; one left a residue of 1.7g (2.25%>) after three fills. However, after the cycle was complete the residue had completely dissolved. Example 8 Using an apparatus as described below, detergent tablets were coated by passage through a cascade of an aqueous suspension of Narlex TD20 and subjected to a drying operation. The apparatus comprised three conveyor belts in succession. The first belt had a length of 50cm, the second a length of 56cm and the third a length of 1.5 metres. Detergent tablets placed on the first belt passed through a cascade of the polymer dispersion and then past two blower units before passing onto the second belt which was associated with a dryer and one blower. At then end of their travel on the second belt, the tablets tumbled onto the third belt which was associated with a blower and two drier units in a heated cabin. Polymer solution was recirculated using a low shear pump. This annaratus was used for the coating of (i) triple layer Machine Dishwasher (MDW) tablets; and (ii) Laundry Wash Tablets. The tablets used were of the following compositions: Using a variety of speeds and drying temperatures the following results were obtained. Example 9 The apparatus described in Example 8 was used for the application of Acusol 830 to Machine Dishwash tablets of the following composition. Machine Dishwash Tablet 20 3 x24mm The results obtained were as follows. Acusol 830

Claims: CLAIMS 1. A method of forming a polymeric coating on a detergent tablet containing an alkaline agent, the method comprising applying to the tablet a continuum of an aqueous solution or suspension of a polymer having free acid groups, and evaporating water to leave the coating on the tablet. 2. A method as claimed in claim 1 wherein the aqueous solution or suspension of the polymer is applied to the tablet by passing the tablet relatively through a continuous cascade of the solution/suspension. 3. A method of forming a polymeric coating on a detergent tablet containing an alkaline agent, the method comprising applying to the tablet an aqueous solution or suspension of a polymer having free acid groups, and evaporating water to leave the coating on the tablet wherein the aqueous solution or suspension of the polymer is applied to the tablet by passing the tablet relatively through a cascade of the solution/suspension. 4. A method as claimed in claim 2 or 3 wherein the time taken for the tablet to pass relatively through the cascade is less than 7 seconds. 5. A method as claimed in any one of claims 2 to 4 wherein the tablet passes successively through two such cascades. 6. A method as claimed in any one of claims 2 to 5 wherein the time between application of the solution/suspension to the tablet and evaporation of the water is less than 5 seconds. 7. A method as claimed in any one of claims 2 to- 6 wherein water is evaporated by an airflow directed downwardly over the tablets. 8. A method as claimed in claim 7 wherein the tablets are "flipped-over" and the (original) undersurfaces are dried by an airflow directed downwardly over the tablets. 9. A method as claimed in any one of claims 1 to 8 wherein the polymer is present in the aqueous solution/suspension in an amount of 15% to 50% by weight. 10. A method as claimed in claim 9 wherein the polymer is present in the aqueous solution/suspension in an amount of 25% to 35% by weight. 11. A method as claimed in any one of claims 1 to 9 wherein the thickness of polymeric coating is 5 to 600 microns. 12. A method as claimed in claim 11 wherein the thickness of the polymeric coating is 40 to 100 microns. 13. A method as claimed in any one of claims 1 to 12 wherein the alkaline substance is selected from alkali metal silicates, phosphates, carbonates and bi-carbonates. 14. A method as claimed in any one of claims 1 to 13 wherein the solution/suspension of the polymer has a pH of 5 or less. 15. A method as claimed in any one of claims 1 to 13 wherein the solution/suspension has a viscosity of 30cP or less. 16. A method as claimed in any one of claims 1 to 15 wherein the polymer is used in the form of a suspension incorporating surface active agents. 17. A method as claimed in claim 16 wherein the suspension of the polymer has been prepared by emulsion polymerisation. 18. A method as claimed in any one of claims 1 to 17 wherein the polymer is a film- forming polymer whereof 10%> to 90%> by weight of the monomer units incorporate acid groups and 10% to 90%> by weight of the monomer units incorporate hydrophobic groups. 19. A method as claimed in claim 18 wherein the acid groups are derived from an ethylenically unsaturated acid monomer with free carboxylic acid groups. 20. A method as claimed in claim 19 wherein the ethylenically unsaturated acid monomer is selected from acrylic acid, methacrylic acid, itaconic acid, iconatic acid, cinnamic acid, crotonic acid, mesaconic acid, carboxymethyl acrylic acid, maleic acid, fumaric acid and the like. 21. A method as claimed in any one of claims 18 to 20 wherein the hydrophobic groups are provided by the alkyl groups Cι- alkyl esters of C -8 α,β-ethylenically unsaturated carboxylic acids. 22. A method as claimed in any one of claims 18 to 21 wherein the hydrophobic groups are bonded to the polymer backbone via polyalkylene oxide residues. 23. A method as claimed in any one of claims 1 to 17 wherein the polymer comprises more than 70% by mol of monomer residues incorporating at least two -CH 2 - and/or - CH 3 groups and at least 20% by mole of the monomer residues provided with acid groups attached directly or indirectly to the polymer backbone. • 24. A method as claimed in any one of claims 1 to 17 wherein the polymer is a copolymer of an α,β-ethylenically unsaturated monocarboxylic acid and an ester of an α,β- . ethylenically unsaturated monocarboxylic acid. 25. A method as claimed in claim 24 wherein the co-polymer comprises 45 to 65% by mol of residues derived from the mono carboxylic acid and 30% to 70% of residues derived from the ester. 26. A method as claimed in claims 24 or 25 wherein the polymer is a co-polymer of (meth)acrylic. acid and ethylacrylate or a co-polymer of (meth)acrylic acid and butyl acrylate. 27. A method as claimed in claim 26 wherein the polymer is a co-polymer of methacrylic acid and ethyl acrylate or a co-polymer of methacrylic acid and ethyl acrylate. 28. A method as claimed in any one of claims 1 to 27 wherein the polymer is cross- linked. 29. A detergent tablet having a coating of a polymer which is a film-forming polymer whereof 10% to 90% by weight of the monomer units incorporate acid groups and 10%) to 90% by weight of the monomer units incorporate hydrophobic groups. 30. A detergent tablet as claimed in claim 29 wherein the acid groups are derived from an ethylenically unsaturated acid monomer with free carboxylic acid groups. 31. A detergent tablet as claimed in claim 30 wherein the ethylenically unsaturated acid monomer is selected from acrylic acid, methacrylic acid, itaconic acid, iconatic acid, cinnamic acid, crotonic acid, mesaconic acid, carboxymethyl acrylic acid, maleic acid, fumaric acid and the like. 32. A. detergent tablet as claimed in any one of claims 29 to 31 wherein the hydrophobic groups are provided by the alkyl groups C 1- alkyl esters of C -8 α,β- ethylenically unsaturated carboxylic acids. 33. A detergent tablet as claimed in any one of claims 29 to 32 wherein the hydrophobic groups are bonded to the polymer backbone via polyalkylene oxide residues. 34. A detergent tablet having a coating of a polymer which comprises more than 70% by mol of monomer residues incorporating at least two -CH 2 - and/or -CH 3 groups and at least 20%> by mole of the monomer residues provided with acid groups attached directly or indirectly to the polymer backbone. 35. A detergent tablet having a coating of a polymer which is a co-polymer of an α,β- ethylenically unsaturated monocarboxylic acid and an ester of an α,β-ethylenically unsaturated monocarboxylic acid. 36. A detergent tablet as claimed in claim 35 wherein the co-polymer comprises 45 to 65% by mol of residues derived from the mono carboxylic acid and 30% to 70% of residues derived from the ester. 37. A detergent tablet as claimed in claims 35 or 36 wherein the polymer is a copolymer of (meth)acrylic acid and ethylacrylate or a co-polymer of (meth)acrylic acid and butyl acrylate. 38. A detergent tablet as claimed in claim 37 wherein the polymer is a co-polymer of methacrylic acid and ethyl acrylate or a co-polymer of methacrylic acid and ethyl acrylate. .

Inventors: ANDREWS GAVIN PAUL, GORDON JAMES WILLIAM, ANDREWS, GAVIN, PAUL, GORDON, JAMES, WILLIAM
Applicants: ANDREWS GAVIN PAUL, GORDON JAMES WILLIAM, MCBRIDE ROBERT LTD, ANDREWS, GAVIN, PAUL, GORDON, JAMES, WILLIAM, ROBERT MCBRIDE LTD
Application year: 2001
Main classification: C11D17
IPC codes: C11D3, C11D17