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Powder coating composition for electrostatic coating of pharmaceutical substratesRelated Patent Categories: Stock Material Or Miscellaneous Articles, Coated Or Structually Defined Flake, Particle, Cell, Strand, Strand Portion, Rod, Filament, Macroscopic Fiber Or Mass Thereof, Particulate Matter (e.g., Sphere, Flake, Etc.)Powder coating composition for electrostatic coating of pharmaceutical substrates description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060280943, Powder coating composition for electrostatic coating of pharmaceutical substrates. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This invention relates to the electrostatic coating of pharmaceutical substrates with a powder coating material and to powder coating materials for coating the pharmaceutical-substrates. In particular, but not exclusively, the invention relates to the electrostatic coating of cores of pharmaceutical tablets with a powder coating material and to powder coating materials for coating the cores of pharmaceutical tablets. While reference is made throughout the specification to pharmaceutical tablets and the invention is of particular application to pharmaceutical tablets of conventional shape, it should be understood that the term is to be interpreted in a broad sense as covering also other products to be taken orally such as pellets, capsules or spherules. [0002] Electrostatic coating of electrically conducting substrates such as metal objects is well known. For example in certain paint spraying processes, paint is electrically charged and droplets of paint are sprayed onto an earthed metal object. Such methods have been successful in obtaining a uniform coating on the substrate. [0003] Electrostatic coating of electrically non-conducting substrates, and pharmaceutical tablet cores in particular, is more difficult. There have been proposals for electrostatic coating of tablets for many years. For example, GB 1 075 404 proposes an apparatus for coating tablets in which an atomiser is used to spray finely divided particles of a coating solution onto tablets in a high potential field. The coating is dried using, for example, an infra-red heater. Such proposals have not however been practised on any substantial commercial scale and the coating of pharmaceutical tablet cores is most commonly carried-out as a batch process by applying a liquid coating in a revolving drum. The liquid coating material may of course be supplied in some cases in powder form but, if so, it is then dissolved or dispersed in a liquid prior to application. It is not, therefore, applied to the tablet core in powder form. [0004] It is in many ways easier to apply a liquid rather than a powder coating to the core of a pharmaceutical tablet. It is difficult to obtain adhesion of the powder to the tablet and in order to secure the coating to the core, the powder must -be transformed into a film without damaging the tablet core, which usually will include organic materials. Furthermore an even coating is required and it is difficult to obtain an even coating of powder on a tablet core. [0005] When a liquid coating is used, the coating must be dried. Theoretically such drying could in some circumstances be carried out at room temperature but in commercial practice it is important, for example because of the rate at which the process must be carried out, to heat the tablets and that is expensive because of the large input of energy required to vaporise the solvent used in the liquid coating. Another disadvantage of liquid coating is that it cannot be used for coating materials that are not soluble or suitably dispersible in a usable liquid, preferably water. [0006] WO92/14451 is concerned with electrostatic powder coating of pharmaceutical tablets and describes and illustrates a process in which the cores of pharmaceutical tablets are conveyed on an earthed conveyor belt and electrostatically charged powder is sprayed towards the cores to form a powder coating attached to the surfaces of the cores. The powder coating is then fused to give a fused film coating secured to the core. [0007] A disadvantage of such a method is that the majority of the powder sprayed towards the cores is not charged and is not deposited on the cores. That leads to overspray and to wastage of the powder material and makes it difficult to obtain a uniform coating. [0008] In a first aspect, the present invention seeks to provide a powder coating material suitable for use in the electrostatic powder coating of a pharmaceutical tablet core and to provide a method for the electrostatic coating of a pharmaceutical tablet core in which a special powder coating material is used to facilitate the electrostatic coating. [0009] The first aspect of the invention provides a powder coating material for use in the electrostatic powder coating of a tablet core, especially for use in a coating method as defined below, and having special properties to make it suitable for use in such a method. The special properties which the powder coating material advantageously has are further defined below and where the advantages provided by those properties are dependent upon the coating method employed, that method is also further defined. [0010] According to a first aspect of the invention there is provided a powder coating material suitable for use in the electrostatic powder coating of a pharmaceutical tablet core in which the material is pharmaceutically acceptable, is treatable to form a film coating and includes composite particles, the composite particles comprising two or more components having different physical and/or chemical properties. [0011] It is important that the powder coating material is a pharmaceutically acceptable material. That in itself imposes severe constraints on the powder coating material since at least most powder coating materials that are commercially available for use in electrostatic powder coating processes are not physiologically tolerable or pharmaceutically acceptable and materials that are commercially available for use as coating materials for pharmaceutical tablet cores are not in a form suitable for electrostatic powder application because other properties of the material are not suitable. [0012] The powder coating material of the invention includes at least two different components, each different component having different physical and/or chemical properties. It is much simpler to provide a powder material having the desired properties referred to above and elsewhere in the specification by providing a material composed of more than one component than by providing a single component material. For powder materials including two or more different components, we have found that improved coatings may be achieved where the powder includes composite particles of the components. [0013] It is particularly important for the particles of the coating material to include more than one of the components where one or more of the components do not have the necessary electrical properties to become coated onto the core when, for example, the method of coating used is as described in the example below. Where the particles are not composite particles those particles of components that do not have the necessary electrical properties may simply remain at the source of powder and will not become coated onto the tablet core. The inclusion of the composite particles is thought to improve the efficiency of coating of the substrate as well as the uniformity of coating applied. The improved efficiency of coating can help to reduce the time required for coating each substrate. [0014] The term "composite particles" as used in this specification refers to particles which have been formed from two or more different components. The composite particles are not homogeneous, that is they have two or more regions each comprising different components of the particle. The composite particle may have the form of a discrete composite particle or may be in the form of agglomerates or aggregates of discrete particles of the different components, the agglomerates or aggregates behaving as discrete composite particles. [0015] Advantageously at least 50% by weight of the particles of the powder are composite particles. Ideally, substantially all of the particles are composite particles but that may not be feasible, in particular where the particle size of the composite particle is small. Where the powder includes more than two components having different physical and/or chemical properties, advantageously the composite particle also includes those other components. Ideally, substantially all of the composite particles would comprise discrete particles including each of the different components. However, in practice, satisfactory coatings can be achieved where the individual component particles are formed into the composite particle as aggregates or agglomerates. [0016] Advantageously, the two or more components have be co-processed. The co-processing may comprise granulation, spray congealing, spray drying or co-milling. Where the method of co-processing results in particles of a relatively large size, for example in the case of granulation, in some cases it would be advantageous to perform a subsequent milling step to reduce the particle size. Alternatively, a micronising step may be performed. [0017] As will be understood, blending powder components together will usually form an ordered mix of the components. However, in special cases, by careful selection of the blending conditions, for example the initial particle sizes of the components and the blending method, composite particles may be obtained by a blending method. [0018] In accordance with the first aspect of the invention, the powder coating material is one which, after it has coated the surface of the tablet core, can be treated to form a film coating secured to the core. Thus the method of coating advantageously includes the step that, after the surface of the tablet core has been coated with the powder, the powder is treated to form a film coating secured to the tablet core. The film coating is advantageously continuous, in that it is not divided into separate parts, but there may be small gaps, not visible to the naked eye, for example between particles of coating that have become secured to one another during the treatment step. Thus the coating may be sintered. For certain applications it is preferred that the film coating is free of any gaps and/or is substantially homogeneous. [0019] When the powder material is first deposited on the tablet core it is in most cases only weakly adhered to the surface of the substrate and is easily dislodged. Treatment to form a film coating is especially advantageous when coating a pharmaceutical tablet core because the core itself is likely to be of low mechanical strength and the film coating can be used to impart strength and make the coated tablets more resistant to subsequent processing such as packaging and opening of packages. The film coating, although it may impart extra strength to the tablet core, will often be very weak when isolated from the surface of the core. The tensile strength of the film coating as a free film may be, for example, 8 MNm.sup.-2 or even lower, and the tensile strength is found to decrease with the increase of the amount of TiO.sub.2 in the coating material. [0020] In the cases where the tensile strength of the free film is low, it is especially important for the film to be a coherent coating on the surface of the tablet core with good adhesion to the core. [0021] Where the powder material is transformed into a liquid phase during the formation of the film coating it is preferable that the viscosity of the powder material when in the liquid phase is less than 500 Pas, more preferably 75 Pas. [0022] Advantageously, the powder coating material is treatable at a temperature of less than 250.degree. C., more preferably less than 200.degree. C., to form a film coating. Advantageously, the powder coating material is fusible at atmospheric pressure at a temperature of less than 250.degree. C., more preferably less than 200.degree. C. It is important that the powder can be treated to form a coating around the tablet core without damaging the tablet core and that imposes a demand on the material because of the sensitivity to heat of most tablet cores, which contain organic materials. Preferably the powder coating material has a melting point in the range of 50.degree. C. to 180.degree. C., more preferably in the range of from 60.degree. C. to 100.degree. C. For a material exhibiting a glass transition, the powder coating material preferably has a softening point in the range of 30.degree. C. to 180.degree. C. [0023] The above requirements place further restrictions on the powder coating material. Many materials are not fusible and would char on the application of heat. Other materials, although they may be fusible, require a long exposure to the heat source for fusing to occur such that the risk of damage to the core is significantly increased and the time taken to form the film on the core is unacceptable for economic reasons. [0024] The desired temperature at which the powder coating material is treatable will of course depend on the material making up the tablet core and for some materials it may be possible for the treatment step to involve temperatures above 250.degree. C. In such cases, the duration of exposure to such high temperatures will be short. Continue reading about Powder coating composition for electrostatic coating of pharmaceutical substrates... 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