U.S. patent application number 09/939631 was filed with the patent office on 2002-03-21 for electrostatic coating.
This patent application is currently assigned to Phoqus Limited. Invention is credited to Hogan, John E., Page, Trevor, Reeves, Linda, Stantiforth, John N..
Application Number | 20020034592 09/939631 |
Document ID | / |
Family ID | 26307004 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020034592 |
Kind Code |
A1 |
Hogan, John E. ; et
al. |
March 21, 2002 |
Electrostatic coating
Abstract
The invention provides apparatus for electrostatically coating a
pharmaceutical tablet core with powdered coating material. The
apparatus comprises a first rotary drum (12) on which a core is
held in electrical isolation from its surroundings but at a
potential difference to earth by an electrode which contacts the
core. The core is carried past a coating station B at which
particles of powder having an opposite potential difference to
earth are held in a tray (18). The surface of the drum is held at
the same potential difference to earth as the powder particles. The
powder is attracted to the core, and not to the drum, coating the
exposed surface of the core. The drum carries the coated core past
a fusing station C at which a heater fuses the powder to form a
continuous film coating. The core is then turned and transferred
onto a second drum (12') where the other surface is coated in the
same way.
Inventors: |
Hogan, John E.; (Faversham,
GB) ; Stantiforth, John N.; (Bath, GB) ;
Reeves, Linda; (Bath, GB) ; Page, Trevor;
(Southampton, GB) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
1100 North Glebe Rd., 8th Floor
Arlington
VA
22201-4714
US
|
Assignee: |
Phoqus Limited
|
Family ID: |
26307004 |
Appl. No.: |
09/939631 |
Filed: |
August 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09939631 |
Aug 28, 2001 |
|
|
|
09629439 |
Jul 31, 2000 |
|
|
|
Current U.S.
Class: |
427/483 |
Current CPC
Class: |
B05D 1/045 20130101;
Y02A 50/30 20180101; B05B 5/081 20130101; A61K 9/2866 20130101;
A61K 9/2846 20130101; B05B 5/087 20130101; A61J 3/005 20130101;
A61K 9/209 20130101; A61K 9/2893 20130101; B05B 5/08 20130101; B05B
5/082 20130101; A61K 9/2853 20130101 |
Class at
Publication: |
427/483 |
International
Class: |
B05B 005/025 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 1995 |
GB |
9509347.2 |
Oct 5, 1995 |
GB |
9520302.2 |
May 8, 1996 |
GB |
PCT/GB96/01102 |
Claims
1. A method for electrostatically coating an electrically poorly
conducting substrate comprising bringing the substrate to a coating
station at which it is held substantially electrically isolated
from its surroundings adjacent a source of particulate coating
material, the substrate and the coating material being held at a
potential difference to each other sufficient to coat the exposed
surface of the substrate with particles of coating material.
2. A method according to claim 1 in which the substrate is at a
potential difference to its surroundings.
3. A method according to claim 1 or 2 in which the electric field
between the coating material and the substrate is shaped so that
the substrate is in a potential well.
4. A method according to claim 1, 2 or 3 in which at the coating
station the substrate is supported by but electrically isolated
from an electrically conductive surface.
5. A method according to claim 4 in which the potential difference
to earth of the surface and of the coating material are of the same
sign.
6. A method according to claim 4 or 5 in which the surface is at
the same potential difference to earth as the coating material.
7. A method according to any preceding claim in which the substrate
is held at the coating station at a potential difference to
earth.
8. A method according to any preceding claim in which substantially
the only motive Force between the substrate and the coating
material is electrostatic.
9. A method according to any preceding claim in which the substrate
is supported, and in electrical contact with an electrode, the
substrate being otherwise electrically isolated from its
surroundings.
10. A method according to claim 9 in which a plurality of
substrates are supported on respective ones of plurality of
electrodes electrically isolated from each other and forming part
of a support surface.
11. A method according to claim 10 in which the support surface is
continuous.
12. A method according to any preceding claim in which the coating
material particles are at a potential different to earth.
13. A method according to any preceding claim in which a powdered
coating material is used.
14. A method according to claim 13 further comprising bringing the
substrate coated with powder to a fusing station where the powder
on the substrate is fused to a uniform coating.
15. A method according to claim 14 in which the fusing is by
heating.
16. A method according to claim 15 in which the heating is by
infra-red radiation.
17. A method according to any of claims 14, 15 and 16 further
comprising cooling the fused coating on the substrate.
18. A method according to any of claims 13 to 17 further
comprising, prior to bringing the substrate to the coating station,
bringing the substrate to a preconditioning station at which the
exposed surface of the substrate is coated with a capture-enhancing
liquid.
19. A method according to claim 18 in which the coating carried out
at the preconditioning station is electrostatic coating.
20. A method according to claims 18 to 19 in which the
capture-enhancing liquid is partially conducting.
21. A method according to any of claims 1 to 12 in which the
coating material is liquid.
22. A method according to any preceding claim in which the
substrate is carried by a support surface having a plurality of
individual locations adapted to receive a substrate and hold it
electrically isolated from the remainder of the surface and at a
predetermined potential difference to earth.
23. A method according to any preceding claim in which the
substrate is held in contact with an electrode at least while it is
at the coating station.
24. A continuous method according to any preceding claim in which
the substrate is carried by the surface of a rotating drum.
25. A method according to any preceding claim further comprising
turning the substrate after application of a coating to a first
surface of the substrate and applying a coating to a second surface
of the substrate.
26. A coated substrate produced by a method according to any of
claims 1 to 25.
27. Apparatus for electrostatically coating an electrically poorly
conducting substrate comprising a coating station at which a
substrate is substantially electrically isolated from its
surroundings adjacent means for supplying particulate coating
material; and means for holding a substrate and particulate coating
material at a potential difference to each other.
28. Apparatus according to claim 27 comprising means for holding a
substrate at a potential difference to its surroundings at the
coating station.
29. Apparatus according to claim 27 or 28 further comprising an
electric field shaping device adjacent the substrate which shapes
the field so that the substrate is in a potential well.
30. Apparatus according to claim 29 in which the electric field
shaping device surrounds the substrate.
31. Apparatus according to any of claims 27 to 30 further
comprising an electrically conductive support surface for, in use,
carrying a substrate at least at the coating station such that the
substrate is electrically isolated from the support surface.
32. Apparatus according to claim 31 in which the support surface
has a plurality of individual locations adapted to receive a
substrate and hold it electrically isolated from the remainder of
the support surface and at a predetermined potential difference to
earth.
33. Apparatus according to claim 31 or 32 in which the potential
difference of the support surface to earth and of the coating
material to earth are of the same sign.
34. Apparatus according to claim 31, 32 or 33 comprising means for
holding the support surface at the same potential difference to
earth as the coating material.
35. Apparatus according to any of claims 27 to 34 comprising means
for holding a substrate at the coating station at a potential
difference to earth.
36. Apparatus according to any of claims 27 to 34 further
comprising a fusing station downstream of the coating station for
fusing a powdered coating material on the substrate to a film.
37. Apparatus according to claim 36 in which the fusing station
comprises a heater.
38. Apparatus according to claim 37 in which the heater is a source
of infra-red radiation.
39. Apparatus according to claim 34, 37 or 38 further comprising a
cooling station downstream of the fusing station.
40. Apparatus according to claim 39 in which the cooling station
comprises an air blower.
41. Apparatus according to any of claims 27 to 40 further
comprising a preconditioning station for supplying
capture-enhancing liquid to the exposed surface of a substrate and
a conveyor for conveying the substrate between the preconditioning
station and the coating station, the preconditioning station being
upstream of the coating station.
42. Apparatus according to claim 41 in which the preconditioning
station comprises an electrostatic spray gun for supplying the
capture enhancing liquid.
43. Apparatus according to any of claims 27 to 42 comprising an
electrode disposed to contact a substrate at the coating
station.
44. Apparatus according to claims 43 in which the electrode is a
support for the substrate.
45. Apparatus according to claim 44 in which a plurality of the
said electrodes form part of a support surface for substrates.
46. Apparatus according to claim 45 in which the support surface is
continuous.
47. Apparatus according to claim 45 or 46 in which the support
surface is a conveyor disposed between the coating and fusing
stations to move the substrate from the coating station to the
fusing station.
48. Apparatus according to claim 47 in which the conveyor is also
disposed between the fusing and cooling stations to move the
substrate from the fusing station to the cooling station.
49. Apparatus according to claim 47 or 48 in which the conveyor is
also disposed between the preconditioning and coating stations to
move the substrate from the preconditioning station to the coating
station.
50. Apparatus according to any of claims 47 to 49 in which the
conveyor is the outer surface of a rotating drum having discrete
areas electrically isolated from the drum surface for the reception
of respective substrates.
51. Apparatus according to claim 50 in which the said areas are
depressions in the said surface of the drum.
52. Apparatus according to claim 50 or 51 in which the said areas
are each part of a respective moving electrode, each moving
electrode extending inside the drum, the drum further comprising a
first arcuate stationary electrode so disposed inside the drum that
as one of the said areas passes through the coating station the
associated electrode is in electrical contact with the first
stationary electrode.
53. Apparatus according to claim 52 in which, in use, the first
stationary electrode is, in use, at a potential difference to
earth.
54. Apparatus according to claim 52 or 53 further comprising a
second arcuate stationary electrode so disposed inside the drum
that as one of the said moving electrodes passes through the
preconditioning station it is in electrical contact with the second
stationary electrode.
55. Apparatus according to claim 54 in which the second stationary
electrode is, in use, earthed.
56. Apparatus according to any of claims 50 to 55 comprising a
vacuum device for holding the substrates on the surface of the
drum.
57. Apparatus according to any of claims 50 to 56 further
comprising a second drum and second coating and fusing stations,
the second being so disposed relative to the first drum that
substrates leaving the first drum with a coated surface are
transferred onto the second drum with an uncoated surface
exposed.
58. Apparatus according to claim 57 further comprising a second
preconditioning station adjacent the second drum.
59. A drum for apparatus according to any of claims 50 to 58.
60. A solid pharmaceutical dosage from having one coated face and
one uncoated face.
61. A coated pharmaceutical having one coating on one face and a
different coating on the ocher face.
62. A coated pharmaceutical the surface of one face of which is two
or more adjacent different coatings.
63. A coated pharmaceutical according to claim 61 or 62 in which
the coatings are of different colours.
64. A coated pharmaceutical according to claim 61, 62 or 63 in
which the coatings contain different polymers.
Description
[0001] The present invention relates to a method and apparatus for
the electrostatic coating of electrically poorly conducting
substrates. It finds particular application in the coating of solid
pharmaceutical dosage forms such as tablet cores, capsules,
granules and beads with particulate coating materials, including
powders and droplets of liquid.
[0002] The use of electrostatic techniques to coat electrically
conductive substrates, such as metal objects, is well known and
successful. The coating, such as droplets of liquid paint, is
electrically charged by applying a potential difference to it and
is attracted to the earthed substrate.
[0003] The conventional electrostatic coating technique described
above has not been successfully applied to the coating of
pharmaceutical tablet cores or other poor electrical conductors,
generally those with a resistivity of more than 10.sup.10-10.sup.15
.OMEGA.m. Proposals have been made in which tablet cores are
earthed, and a powdered coating material is directed at them
through a nozzle which imparts an electrical charge to the powder.
The powder coating is then fused to give a uniform coat. This
method has been found inefficient, since adequate earthing of the
cores has not been achieved, and the charge on the powder
accumulates on the surface of the cores, repelling further charged
powder. Even if the cores are carried on for example an earthed
conveyor belt, the poorly conducting nature of the cores allows
charge to build up.
[0004] Further, the bulk of the powder (95% in the case of corona
charging) is uncharged, and does not land or stay on the cores, and
must either be recovered or wasted. These difficulties lead to
non-uniformity in the weight and thickness of the coating applied
to the cores. This is pharmaceutically unacceptable, in particular
when the core coating plays a significant role in the timing of the
release of the pharmaceutical into the body after ingestion.
[0005] Improvements have been proposed, for example in WO 92/14451
which proposes moistening the cores with water prior to spraying
with the charged powder, to improve the earthing of the surfaces of
the cores and to encourage the powder, once on the surfaces, to
remain. Even with these improvements, coating remains inherently
inefficient; powder is wasted and the time necessary for complete
coating is too long for efficient production.
[0006] The present invention overcomes these problems by providing
in accordance with a first aspect a method for electrostatically
coating an electrically poorly conducting substrate comprising
bringing the substrate to a coating station at which it is held
electrically isolated from, and preferably at a potential
difference to, its surroundings adjacent a source of particulate
coating material, the substrate and the coating material being held
at a potential difference to each other sufficient to coat the
exposed surface of the substrate with particles of coating
material. Preferably, the substrate is held at a potential
difference to earth.
[0007] It is particularly preferred that the electric field between
the coating material and the substrate is shaped. The field can be
shaped so that the substrate is in a potential well. That is, the
substrate is surrounded by a potential difference to earth
different to its own, there being a sharp cut-off between the two
potential differences. Thus, substantially all the coating material
is attracted to the substrate, reducing waste of the coating
material and avoiding the problems associated with coating material
falling on the substrate surroundings.
[0008] Shaping of the filed is achieved by manipulation of the
potential difference between the substrate, its surroundings and
the coating material. For example, a substrate is carried by but
insulated from a surface, the surface being held at the same
potential difference to earth as the coating material while the
substrate is held at a difference potential difference to earth to
that of the coating material. Coating material is therefore
attracted to the substrate and not to the surface.
[0009] Preferably, substantially the only motive force between the
substrate and the coating material is electrostatic. It may be
desirable to provide particulate coating material in the form of a
cloud of particles, formed for example by fluidising a bed of the
coating material. Also preferably, the substrate is supported on an
electrode while being electrically isolated from its
surroundings.
[0010] For powder coating applications, the substrate may be
brought to a pretreatment station at which the exposed surface of
the substrate is coated with a capture-enhancing liquid. After
coating with the coating material, the substrate can be brought to
a heating station where the coating material if powder is fused or
if liquid is dried to an effectively continuous uniform coating.
The reverse surfaces of the substrate can then be coated in the
same way with the same coating material as the first-coated surface
or with a different material. In this way, or example bi-coloured
coated substrates may be produced. Preferably, the method is
carried out continuously.
[0011] It is preferred that powders used in the method according co
the invention has a resistivity greater than 10.sup.8 .OMEGA.m,
preferably between 10.sup.8 and 10.sup.15 .OMEGA.m.
[0012] There is provided in accordance with a second aspect of the
invention apparatus for electrostatically coating an electrically
poorly conducting substrate comprising a coating station at which
the substrate is substantially electrically isolated from, and
preferably at a potential difference to, its surroundings adjacent
means for supplying a particulate coating material, and means for
holding a substrate and a coating material at a potential
difference to each other. Preferably, the substrate is held at a
potential difference to earth.
[0013] Preferably, the apparatus further comprises an electric
field shaping device adjacent the substrate. Particularly
preferably, the electric field shaping device surrounds the
substrate.
[0014] The apparatus advantageously includes a electrically
conductive support surface such as a drum electrically isolated
from the substrate which carries the substrate at least ac the
provision for the support surface to be held at a potential
difference to earth having the same sign as the potential
difference to earth of the coating material.
[0015] In the case of powder coatings, the apparatus can include a
pretreatment station for supplying capture-enhancing liquid to the
exposed surface of a substrate and a conveyor for conveying the
substrate between the pretreatment station and the coating station,
the pretreatment station being upstream of the coating station. The
conveyor is preferably a drum. The apparatus preferably includes a
heating station downstream of the coating station for fusing the
powder or drying the liquid coating material on the substrate to a
film.
[0016] In a third aspect, the invention provides a drum or the
preferred apparatus of the invention.
[0017] In a fourth aspect, the invention provides a coated
pharmaceutical having one coating on one face and a different
coating, or no coating, on the other face. The coatings may be of
different colours or of different polymers or biologically active
materials.
[0018] The source of particulate coating material, whether powder
or liquid, may be a multiple source comprising several sub-sources.
The sub-sources can be of different colour coating materials or of
coating materials containing different polymers. Thus, tablets
having more than one colour on a single surface can be provided.
The faces can be bicoloured or striped. Similarly, a tablet can
carry two or more different polymer coatings, side by side.
[0019] In a fifth aspect, the invention provides a coated
pharmaceutical the surface of at least one face of which is two or
more adjacent different coatings The coatings may be of different
colours or of different polymer composition.
[0020] The substrate, such as the core of a pharmaceutical tablet,
may be completely electrically isolated from its surroundings, or
example in free fall. Preferably, however, while coating takes
place the substrate is in contact with an electrode through which
it is maintained at a potential difference to earth (and to its
surroundings). If the substrate is held on a support surface, such
as the surface of a drum, it may sit in a depression in the
surface. The surface of a drum, it may sit in a depression in the
surface. The surface of the depression can be of a conductive
material and form part of the electrode. The support surface may be
surrounded by an arrangement of insulating, conducting or
semiconducting areas which act to shape the electrical field
pattern. The substrate is thus surrounded by a potential well, to
ensure that charged particles of coating material are attracted to
it, rather than to the surroundings, including the support surface,
if any, carrying the substrate.
[0021] The invention will be further described, by way of example,
with reference to the drawings, in which:
[0022] FIG. 1 shows schematically a preferred embodiment of
apparatus according to the invention;
[0023] FIG. 2 shows diagrammatically a cross-section of a drum of
the apparatus of FIG. 1; and
[0024] FIG. 3 shows diagrammatically means for providing droplets
of liquid coating material for an apparatus according to the
invention.
[0025] The apparatus shown schematically in FIG. 1 is for coating
both sides of pharmaceutical tablet cores. The apparatus comprises
an inclided tablet core feed chute 10 leading to a first rotatable
drum 12. The drum 12 is of plastic with a steel surface and has
circular depressions 14 (FIG. 2) in its outer surface in each of
which a core can be held by vacuum, as will be explained later.
[0026] The drum 12 is rotatable in the direction shown by the arrow
in FIG. 1. Adjacent the circumference of the drum 12 downstream of
the tablet feed chute 10 is a pre-conditioning station A comprising
an electrostatic spray gun 16, which produces charged droplets
which are attracted to the substrate cores on the drum by reason of
the potential difference between the droplets and the cores.
Downstream of the preconditioning station A is a coating station B
comprising a vibrating powder tray 18 for holding, fluidising and
re-circulating the powder with which the cores are to be coated.
Downstream of the coating station is a fusing station C comprising
a heater 20. After the fusing station C, the coated core passes a
cooling station, not shown, where cool air is directed over or
around the core to cool the fused coating.
[0027] A second drum 12' is adjacent the first drum 12, the nip
between the drums being downstream of the fusing station C and the
cooling station. The second drum 12' rotates in the opposite sense
to the first drum 12, as indicated by the arrow in FIG. 1. The
second drum 12' is provided with a preconditioning station A'
comprising a gun 16', a coating station B' comprising a powder tray
18', a fusing station C' comprising a heater 20' and a cooling
station (not shown).
[0028] A core collection chute 22 inclined away from the second
drum 12' downstream of the fusing station C', taking coated cores
to be further processed and packed.
[0029] The first drum 12 will be described in more detail with
reference to FIG. 2. It comprises a rotatable shell 24, the outer
face of which carries the depressions 14. In FIG. 2, only five
exemplary depressions 14 are shown; it will be appreciated that in
practice there will usually be more depressions, evenly spaced in a
circumferential row around the shell 24, and that there may be
several circumferential rows across the width of the drum, whether
formed by one continuous shell or several contiguous shells. The
depressions 14 on the drums are shaped and dimensioned to ensure
that the complete face of the core and half the depth of the side
wall is coated while the core is on one drum. In the case of a
circular tablet core, a depression diameter close to that of the
core diameter is preferred. In some applications, the depth of the
depression should be such as to allow at leas 50% of the core
thickness to be exposed to the particles of the coating material so
that exposure of first one face of the core and then the other
leads to complete coverage of the core.
[0030] The surface of each depression 14 is electrically insulated
from the surfaces of other depressions on the drum and is provided
with a pick up arm 25 extending radially inward, toward but ending
short of the centre of the drum. The pick up arms 26 are attached
to the inner surface of the shell 24 and rotate with it. The pick
up arm 26 and the depression 14 together make a moving electrode to
charge a core in a depression. Each depression 14 has means for
holding the core against forces such as gravity, for example a
passage 28 through its wall which can be in communication with a
vacuum manifold 30 which extends around a portion of the periphery
of the drum interior from immediately upstream of the core feed
chute 10 to adjacent the nip between the first drum 12 and the
second drum 12'.
[0031] A first, earthed, stationary arcuate electrode 32 is located
inside the drum at an angular position corresponding to the
preconditioning station A. A second stationary arcuate electrode 34
at a potential difference to earth is located inside the drum at an
angular position corresponding to the coating station B. The outer
arcuate surfaces of the stationary electrodes are at the same
radial distance from the centre of the drum as the free ends of the
pick up arms 26 of the moving electrodes. As the shell 24 rotates,
the moving electrodes contact the first and second stationary
electrodes sequentially.
[0032] The drum 12 is held at a potential difference to earth
having the same sign as the potential difference to earth of the
coating powder.
[0033] The second drum 12' is constructed similarly to the first
drum, comprising a rotatable shell with depressions, pick up arms,
first and second stationary electrodes and a vacuum manifold. The
angular locations of the first and second stationary electrodes
correspond to the second preconditioning station A' and the second
coating station B', and the vacuum manifold extends from
immediately upstream of the nip between the two drums to adjacent
the core collection chute 22.
[0034] In use, cores are fed continuously to the core feed chute
10. A core passes down the core feed chute 10 into a depression 14
in the rotating shell 24 of the first drum 12. At that angular
position, the depression overlies the vacuum manifold 30, and so
the core is held in the depression by the vacuum through the
passage 28 in the shell. The shell 24 continues to rotate bringing
the core to the preconditioning station A, at which point the pick
up arm 26 attached to the depression 14 contacts the first
stationary electrode 32, earthing the moving electrode and thus the
core held in the depression. As the earthed tablet core passes the
electrostatic spray gun 16, its exposed surface is sprayed with
charged droplets of a capture-enhancing liquid, for example
polyethylene glycol.
[0035] The shell 24 continues to rotate, taking the moving
electrode 26 out of contact with the first stationary electrode 32
and bringing it into contact with the second stationary electrode
34, as the tablet core approaches the coating station B. The
exposed polyethylene glycol treated core surface is now at a
potential difference to earth, and coating powder is attracted to
it from the powder tray 18. The potential well generated by holding
the surface of the drum and the powder at the same potential
difference to earth as each other and the core at a different
potential difference to earth ensures that powder is attracted
substantially only to the core and that the surface of the drum
remains substantially free of powder.
[0036] The shell 24 continues to rotate, taking the moving
electrode 26 out of contact with the second stationary electrode 34
and bringing the core to the fusing station C, where the heater 20
fuses the powder on the coated surface of the core to form an
effectively continuous film.
[0037] As the shell 24 continues to rotate, the core leaves the
fusing station C, passes through the cooling station (not shown),
so chat the depression carrying the core no longer overlies the
vacuum manifold 30. The core drops from the first drum 12 into a
depression on the outer surface of the second drum 12', with its
uncoated surface uppermost; the depression is in communication with
the vacuum manifold of the second drum. The coating of the core is
completed as it travels past the second preconditioning A', coating
B' and fusing C' stations. The coating powder at the second coating
station may be the same as that at the first, or different. Thus,
tablets having differently coated surfaces can be produced. Such
dissimilar coatings can be used to provide functionally modified
behaviour such as altered diffusion or dissolution controlled drug
release or cosmetically different coatings such as those which
would produce a bicoloured tablet. As the coated tablet draws
adjacent the collection chute 22, the depression carrying it ceases
to overlie the vacuum manifold, and the tablet falls into the chute
and is further processed and packed.
[0038] The drums themselves are preferably at least 60 mm in
diameter and not less than the minimum tablet diameter in width,
rotating at least 1/2 r.p.m. The pressure in the vacuum manifold is
sufficiently low to hold the tablet against gravity, preferably
between 0.2 and 0.6 bar absolute.
[0039] In the electrostatic spray guns 16,16' at the
preconditioning stations A,A', a semiconducting, non-volatile
fluid, such as polyethylene glycol or an in aqueous solution
thereof is fed at a rate of 0.1 to 1 ml/min. to a steel capillary
of internal diameter 0.05 to 2 mm. The capillary is connected to a
current limited high voltage (up to 50 kV at 30 to 100 .mu.A)
potential difference to earth as each core on a drum masses the
gun, and a mist of charged droplets is discharged from the
capillary coward the core on the drum; since the cores on the drums
are earthed at the preconditioning stations, the charged droplets
are guided by the electric field between the capillary and the core
to the exposed surface of the core, where they are captured. The
cores may be held at a potential difference to earth at the
preconditioning stations, providing that they are also at a
potential difference to the capillaries. In this case, the first
stationary arcuate electrode 2 is at a potential difference to
earth. The supply of droplets from each capillary is controlled by
switching the voltage off and earthing the capillary through a
resistor (1 to 10 M.OMEGA.) as each core leaves the preconditioning
station; this ensures a sharp cut off of the droplets between
tablet cores.
[0040] The pre-conditioning step may not always be required.
[0041] At coating stations B,B', powdered coating material is
supplied by vibrating feeders to the vibrating trays 18,18'. The
level of the powder in the trays is determined by a levelling blade
above each tray. The powder may be vibrofluidized and continuously
recirculated. The trays may be of a plastics material having an
earthed metal strip under the arc swept by the tablet cores on the
respective drums or they may be metallic trays. An alternative way
to charge the particles is triboelectrical charging. The trays are
preferably 50 to 150 mm long and 3 to 40 mm wide. If more than one
tray is used, to provide a bi- or multicoloured face or a face
carrying more than one polymer composition, the tray dimensions
will be appropriately different. The tablet cores are charged by a
voltage of -3 to -5 kV current limited to 5 .mu.A.
[0042] A preferred powder coating composition is:
[0043] 46.5% by weight Eudragit RS ammonio-methacrylate
co-polymer
[0044] 28.0% by weight Klucel hydroxy propyl cellulose
[0045] 15.0% by weight titanium dioxide
[0046] 5.0% by weigh; aluminium lake
[0047] 5.0% by weight polyethylene glycol 6000
[0048] 0.5% by weigh: Aerosil 200 colloidal silicon dioxide
[0049] Another preferred powder coating composition is:
[0050] 39.75% by weight Eudragit RS ammonio-methacrylate
co-polymer
[0051] 39.75% by weight Klucel (hydroxypropylcellulose)
[0052] 15.0% by weight Titanium dioxide
[0053] 5.0% by weight Aluminium lake
[0054] 0.5% by weight Aerosil (colloidal silicon dioxide)
[0055] The components are premixed under high shear, then wet
granulated by mixing under s shear with water (10-15% by weight).
The granulated mixture is dried in fluid bed drier at about
45.degree. C. for 20 to 30 minutes to reduce the moisture content
to below 3% by weight. The dried granules are milled and micronised
to a powder having a size distribution such that 50% by volume of
the particles are of a size less than 20 .mu.m, and about 100% by
volume are of a size less than 60 .mu.m. The peak size is about 10
.mu.m.
[0056] If the particulate coating material is liquid droplets, the
apparatus is of a similar construction to that for applying
powdered coating material to the cores. The vibrating trays holding
the powder are replaced by means for producing liquid droplets with
low momentum, such as that shown in FIG. 3. The apparatus may be
designed so that a source of powder coating material may be easily
replaced by a source of droplets of liquid coating material.
[0057] Droplets are produced by a spray gun 41 held at earth
potential and electrically connected to the drum (12). The gun may
be formed of metal or a polymer material. The direction of the
spray is towards a baffle 42 down which the coalesced droplets can
run into a re-circulating reservoir 43. The spray gun 41 produces a
spray of relatively high initial momentum. This impinges on a
internal baffle which breaks the spray up into a mist or droplets
of low momentum. The momentum of the droplets produced by the spray
gun is mainly in a direction normal to the substrate 44. If the
substrate is uncharged there will be effectively no droplet capture
onto the substrate surface. When the charge is applied to the
substrate surface the droplets are attracted thereto to form a
coating thereon which is later dried at a drying station similar to
the fusing station C of the powder treatment apparatus. The
pre-conditioning step A may be omitted in the case of liquid
coating material.
[0058] A preferred liquid coating composition comprises:
1 hydroxypropylmethylcellulose 70% glycerol 7% iron oxide yellow
23%
[0059] in aqueous dispersion.
[0060] At the musing or drying stations C,C', energy is imparted to
the core surfaces to fuse the powder or dry the liquid and provide
a uniform coating on the exposed surfaces of the core. The energy
is provided by focused radiation preferably in the infra-red
region; the energy. power requirement will be determined largely by
the coating material. After fusing or drying, the coating is set by
cooling, using an air blower.
[0061] Preferred coating apparatus according to the invention can
coat up to 300,000 tablet cores each hour.
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