CA2246790A1 - Electrostatic coating of small falling objects - Google Patents

Abstract

This invention relates to a process and a device for producing an even and homogeneous coating of liquids or powders on small free-falling objects that are not earthed. The process and the device according to the invention are primarily intended for surface coating of medicinal products and foodstuffs.
According to the invention, the non-earthed objects are positively or negatively charged in a suitable manner or not at all and are then caused to fall freely and individually through a substantially cylindrically-shaped zone (2), mainly along its central axis, where the coating substance, having first, in a suitably manner, been charged with a charge of opposite polarity to that of the objects or if these are not charged, with a positive or negative charge, is sprayed onto the particles by means of at least one or preferably at least two spray nozzles (3) which are placed peripherally around the zone (2) and directed mainly towards the central axis thereof, whereby the coating substance is attracted to the object because of the potential difference between them, and an even, homogeneous surfrace coating is obtained.

Description

ELECTROSTATIC COATING OE SMALL FALLING OBJECTS

The present invention relates to a process and a device for electrostatic surface coating of small objects that are not earthed. More particulary, the invention relates to a process for forming an even and homogeneous coating of liquids or powder on small/ free falling objects. The process and the device according to the present invention are mainly intended fcr coating of medicinal products and for surface coatlng of foodstuffs.
Surface coating of medicinal produc-ts in tablet form, is made for both medical, practical and aestethical reasons. Coat~ing is made for example to facilitate administration, to hide unpleasant taste or smell, to improve mechanical properties of the medicine, to protect the active agents of the medlcine from being destroyed by e.g. saliva or gastric juices or to protect the tablet in storage. Coa~ing can also be used to br ng about a controlled release of the meciicine in the human body.
The processes which are used at present for coating of medicinal products involve ro~ating a large number of tablets in a slanting drum, known as a coating pan, or ln horizontally mounted perforated cylinders, while at the same time the surface coating material, dissolved in a suitable substance, is poureci or sprayed into the pan.
The rotation of the tablets causes a tablet to move upwards in the coating pan along the side of the pan and then fall downwards in the middle of the pan on top of the bed formed by the tablets that are moving upwards. As the solvent of the coating material evaporates, the material wilL be deposited on the tablets.
- 30 A large problem with the cu-rrent technology is that the tablets preferably shoulcl be spherical or at least be made by convex-shaped roundecl dies. The reason therefor is that the existence of edges on the tablets may cause W097~9~ PCT/SE97/00251 the coating applied thereon not to be even and homogeneous. However, tablets produced today usually have a flat, circ~lar form, that is, they have a special edge surface to a~oid problems with for instance caping of the tablet or in reasing demands on the tablet-producing machine (deep concave dies, etcetera). The coating processes used t:oday are batchwise and each batch goes through a large number of steps involving a large amount of manual work as well. The energy consumption is relatively hLgh and the noise level is considerable.
Space requirements are large, ventilation demands are stringent. These processes are time consuming as well.
Coating of confectionary, for example, the chocolate sweets known as "Non-Stop", is often performed in a similar manner.
In recent times, so cal:Led nigh speed methods for medicinal product coating have been developed, but in spite of this, the time needed t3 get finished dragées is about one to two working day,. The cost of the substances the tablets are coated with often is fa rly low, accounting for only a small amount cf the total cost of the coating.
It is llkewise known to perform the coating in a fluidizing bed. The tablets are made to move upwards through the bed propelled by a gas, preferably air, while the coating lS being sprayed on.
Processes for electrostatic coating of small objects have been known for some ti~e. A feature often common in earlier known technologies is that the objects to be coated are earthed, see for example US Patent No 4,774,102, wherein a process for electrGstatic spray coating of a not earthed object s described.
Electrostatic coating of medicinal products is also previously known; for example in W0 92/14451, such a process is described. AccordLng to the process of that publicatio~, mecicinal products l~re coated with a dry powder by us ng a procedure according tc which the W097/29~8 PCTtSE97/00251 medicinal products are placed on a ~ransportation devic on which they are coated with a powder. The potential difference between the medicina~ product and the powder or between the transportation device and the powder is used to attract the powder to the medicinal products. In this way those parts of the medicinal products that are in contact with the transportation device receive no coating. The powder coating is finally treated, for example by heating, to make a fused film on the medicinal product. The other side of the medicinal product can then be coated in the same way.
A big disadvantage with this earlier known technology, particulary in surface coatlng of tablets, is that in almost all cases the earthing is achieved by placing the objects on some kinc of support, for example a conveyor. This means that the objects are being coated on one side at a time and that there is a considerable risk that an edge will form in the border zone between the surface coatings of the two sides This in turn means that the coating will not be even and homogeneous, a feature that is especially important -n the coating of medicinal products to bring about a slow, controlled release of a medicine into the human body.
EP-B-0 085 l~9 describes a process and a device for electrostatic coating of an object that is not earthed, with a liquid or a powder. According to this process, the objects to be surface coated are made t3 fall, by means of a stationary dispersal (spreader) system, as a continous circular curtain alons the walls of a non-conductive tube, where the objects are passed through acorona zone and receive a positive charge. Centrally in this tube a centrifugal spreading devlce is placed. In the device, the material in liquid or particle form to be applied on the objects is negatively charged by passage through a negative corona zone and by means of this device the material is then sprayed on to the continuous curtain that the objects form. The process seems mainly W097~98~ PCT/SE97/00251 intended for addition of substances suc.. as vitamins, antibiotics and amines to different powders and pellets or for the moisturizing of granulates and masses in powder form bu~ not for surface coating of the type necessary for coating of medicines.
In the descriptive part of the above patent, many examples of application area, for the invention are given, but they all concern additio~ of some substance to materials such as skimmed milk, fertilizers, fungicides and the like. If the process were to be used for surface coating there would be several serious disadvantages.
Among them a large risk that the objects in the continuous curtain collide w:ith each other or with the walls of the tube, resulting in ~ newly applied surface coating being subjected to mechanical stresses, which can cause damage to the coating. It is also difficult to produce a completely even and homogeneous coating since the objects receive spray from one direction only.
Additionallyr there is no mention in the above patent that the process could be used for producing a hard surface, which is necessary ln the coatlng of medicinal products.
Electrostatic surface cc>ating in accordance with the present invention is carried out in one or a few steps and can be automated. The surface coating can be effected continuously and at the pace of production of the product. Furthermore, it is not necessary to electrically charge the objects. It is suXficient to charge the substance the objects are to be coated with, something that is especially favourable in the coating of medicinal products, as such products may be damaged by a possible charge.
The process of using electrostatic surface coating of small free-falling, not earthed objects with a substance in powder or liqui(1 form, according to this invention is characterized in that the objects are charged posit-ively or negatively in a suitable manner.

This first step can also be omltted because uncharged objects can also be coated by using the concept of the present invention. After a posslble charge, the objects are made to fall freely and ind-vidually through a mainly cylindrical~y shaped zone, essentially along the central axis of the zone, where the substance in powder or liquid form, having first received, in a suitable manner, a charge of the opposite sign to that of the objects, or if these are not charged, with an optional charge, is sprayed onto the particles by means of at least one spray nozzle, preferably at least two spray nozzles, placed peripherall~ around the zone ancl directed essentially towards the central axis thereof, wnereby the substance in powder o~ liquid form is attracted to the objects because of the potential difference between the objects and the substance and an even and homogeneous surface coating is obtained.
The invention also refers to a device for electrostatic surface coating of small free-falling objects which are not earthed, with a s!lbstance in powder or liquid form, characterized in that it comprises at least an essentially cylindrica~ zone, a means for the supply of positively charged, negatively charged or uncharged objects to a central part passing through the essentially cylindrically shaped zone, and means for spraying a positively or negatively charged substance in powder or liquid form towards the central part passing through the cylindrically shaped zone.
The invention will be described in more detail in the followir.g by way of a preferred not limiling embodiment and with reference tc) the attached skeleton drawing. Th~ objects (1), preferably tablets, to be coated with a surface layer of a substance in powder or liquid form, are charged positively or negatively in a suitable prior-art manner, for instance by passage through a corona zone (not shown in the drawing) or by means of friction. This first step can also be omitted.

CA 02246790 l998-08-l8 After that, the charged or uncharged ob-ects are made to fall freely and essentially individually, preferably centrally through a cylindric:ally shaped zone (2). In this zone the objects are sprayed with the substance in powder or liquid form, said substance ir turn having first been charged positively or negatively in any known manner, by means of one or preferably two, peripherally p7aced spray nozzles. If onl~ one nozzle is used, it is preferably annular and is placed around essentially the whole zone (2). The nozz7es are placed so as to be directed mainly towards the vertlcal axis of the zone

(2). A governing and regulating device ~not shown in the drawing) and a suitable dosing apparatus (not shown in the drawing) ensure that the electrically charged powder or liquid substance that is sprayed intc the zone (2) always has the correct volume, quantit~ and position. The powder or liquid substance will, because of the potential difference from that of the freely falling objects, be attracted to the objects and form a homogeneous layer on the surface of the objects.
The surface coated objects then pass through a zone (4) where the applied substance will be made to form a hardened layer preferably by tempering, cooling, coagulation, drying or the like.
Optionally, the surface-coated objects are then passed through an electrical field (5) for the neutralizing of remaining electrical charges before the surface-coated objects come into contact with an earched surface. The purpose of this step is to prevent sparking, as might otherwise form as the surface-coated objects, still possessing some residual charge, come into contact with an earthed surface.
The thickness of the surface coating of the powder or liquid substance is regulated by means of the magnitude of the applied potential difference.
In accordance with a preferred embo~iment, only the liquid or powder substance that is to form the surface coating is charged, either positively or negatively, while the o~jects to be coated cre uncharged.
A barrier zone (6) can be arranged under the zone ~2) to prevent any superfluous surface coating material from falling down together with already ~oated objects.
The device according to the invent on can be coupled directly to the equipment used ~or the production of the tablets and coat the tablets as they are being produced.
This means that a very large part of today's cost for work and premises can be saved. This so called direct coating system also reduces dust formatlon and minimizes the risks of handling damages tc the tablets. With the process and equipment accordlng to th-s invention the noise level ls also much reduced.
This invention is primarily intended to be used for coating of tablets, but is is also equally useful for surface coating of capsules, suppositories, granulates and for surface coating of confectionar,~ and other kinds of foodstuffs. In these instances the surface coating can be of very different kinds, such as suga~, cacao powder, calcium carbonate, polyethylene glycoles, cellulose derivatives or polyacrylates.
The invention can also be used wlth n completely different areas for instance for surface coating of electronic components or for producing ~ust protection ln the mechanical industry.
The invention is not lirnited to the above mentioned examples, but can be modified in severai different ways within the scope of the attached patent claims.

Claims (11)

1. A process for electrostatic surface coating of small free-falling objects that are not earthed with a coating substance, c h a r a c t e r i z e d by causing the objects to fall freely and individually through an essentially cylindrically shaped zone (2), essentially along the central axis of the zone (2), wherein the coating substance, having first in a suitable manner been given an electrical charge that is different from the charge of the objects, is sprayed onto the particles by means of at least one and preferably at least two spray nozzles (3) which are placed peripherally around the zone (2) and directed essentially towards the central axis thereof, whereby the coating substance is attracted to the objects because of the potential difference between the coating substance and the objects so that an even and homogenous coating is obtained.

2. The process according to claim 1, wherein the objects, having passed through zone (2) wherein the coating substance is sprayed on, are caused to pass through a zone (4), wherein the surface layer is hardened, by for instance tempering, heating, drying, cooling or coagulating.

3. The process according to claim 1 or 2, wherein after the production of the surface coating, the objects are caused to pass through an electrically neutralizing zone (5) wherein any remaining charges are neutralized.

4. The process according to any one of claims 1-3, wherein the spray nozzle (3) consists of a substantially annular nozzle placed around the entire zone.

5. The process according to any one of claims 1-4, wherein the thickness of the applied surface layer is regulated by the potential difference between the objects and the coating substance.

6. A device for electrostatic surface coating of small free-falling objects that are not earthed with a coating substance, c h a r a c t e r i z e d in that it comprises at least a substantially cylindrically shaped zone (2), a device (1) for delivery of objects to a central part passing through the subsantially cylindrically shaped zone (2) and a device (3) for spraying a coating substance having an electrical charge that is different from the electrical charge of the objects, in a direction towards the central part passing through the central part of the cylindrically shaped zone (2).

7. A device according to claim 6, wherein the device also includes a zone (4) for the hardening of the surface layer applied on the objects.

8. The device according to claim 6 or 7, wherein the device also includes a zone (5) for neutralizing any remaining electrical charges.

9. The device according to any one of claims 6-8, wherein the device includes means (6) for the collection of superfluous coating substance.

10. Use of the process according to any one of claims 1-5, or the device according to any one of claims 6-9 for the coating of medicinal products.

11. Use of the process according to any one of claims 1-5, or the device according to any one of claims 6-10, for surface coating of foodstuffs, mainly confectionary products.