Note: Descriptions are shown in the official language in which they were submitted.
CA 02220485 1997-11-07
WO 96/35516 PCT/GB96/01102
ELECTROSTATIC COATING
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
solidnharmaceutical dosage forms such as tablet cores, capsules,
granuies and beads with particulate coating ...ate== als, +._T?cludl.ng
powders and droplets of liquid.
The use of electrostatic techniaues to coat electrically
conductive substrates, such as metal objects, is well known and
successful. The coating, such as droplets of liquid paint, is
zlectrically charged by applying a potential difference to it and
is attracted to the earthed substrate.
The conventional electrostatic coating techniaue 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 1010 - lOls
C2m. 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 adectuate 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.
Further, the bulk of the powder (95a in the case of c.orona
charging) is uncharged, and does r_ot land or stay on the cores,
and must either be recovered or wasted. 'I'hese 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
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timing of the release of the pharmaceutical into the body after
ingestion.
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.
The present invention overcomes these problems by providing
in accordance with a first aspect a method of producing a
plurality of coated substrates, each comprising an electrically
poorly conducting substrate electrostatically coated by coating
material, the method comprising carrying the substrates on a
support surface to a coating station at which they are held
adjacent a source of particulate coating material, each substrate
being associated with an individual location provided on the
support surface which is electrically isolated from the remainder
of the surface, and holding the substrates and the coating
material at a potential difference to each other sufficient to
coat the exposed surface of each substrate.
According to a further aspect there is provided a method of
electrostatically coating a pharmaceutical 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.
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
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coating material and avoiding the problems associated with coating
material falling on the substrate surroundings.
Shaping of the field 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 different potential difference to earth to
that of the coating material. Coating material is therefore
attracted to the substrate and not to the surface.
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.
For powder coating applicatiorns, 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, for example
bi-coloured coated substrates may be produced. Preferably, the
method is carried out continuously.
It is preferred that powders used in the method according to
the invention has a resistivity greater than 108S2m, preferably
between 108 and 10150m.
There is provided in accordance with a further aspect of the
invention apparatus for producing a plurality of coated
substrates, each comprising an electrically poorly conducting
substrate electro-statically coated by coating material, the
apparatus comprising a support surface for carrying the substrates
to a coating station at which the substrates are held adjacent
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means for supplying particulate coating material, the support
surface being provided with a plurality of individual locations,
each of which is electrically isolated from the remainder of the
surface, and means for holding the substrates and the particulate
coating material at a potential difference to each other.
Preferably, the apparatus further comprises an electric
field shaping device adjacent the substrate. Particularly
preferably, the electric field shaping device surrounds the
substrate.
The apparatus advantageously includes an electrically
conductive support surface such as a drum electrically isolated
from the substrate which carries the substrate at least at the
coating station. A field shaping device can be provided by
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.
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.
In a further aspect, the invention provides a drum for the
preferred apparatus of the invention.
As described hereafter a coated pharmaceutical having one
coating on one face and a different coating, or no coating, on the
other face is produced. The coatings may be of different colours
or of different polymers or biologically active materials.
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
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tablet can carry two or more different polymer coatings, side by
side.
A coated pharmaceutical the surface of at least one face of
which is two or more adjacent different coatings is also
described. The coatings may be of different colours or of
different polymer composition.
The substrate, such as the core of a pharmaceutical tablet,
may be completely electrically isolated from its surroundings, for
example in free fall. Preferably, however, while coating takes
place the substrate is in contact with an electrode through which
it is maintainned 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 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.
The invention will be further described, by way of example,
with reference to the drawings, in which:
Figure 1 shows schematically a preferred embodiment of
apparatus according to the invention;
Figure 2 shows diagrammatically a cross-section of a drum of
the apparatus of Figure 1; and
Figure 3 shows diagrammatically means for providing droplets
of liquid coating material for an apparatus according to the
invention.
The apparatus shown schematically in Figure 1 is for coating
both sides of pharmaceutical tablet cores. The apparatus comprises
an inclinded 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 (Figure 2) in its outer
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WO 96/35516 PCT/GB96/01102
surface in each of which a core can be held by vacuum, as will be
explained later.
The drum 12 is rotatable in the direction shown by the arrow
in Figure 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 poter_zia1 difference betrieei: che dropiezs
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 statier-, r_ot shown,
where cool air i s directed over or arou_nd the core to cool the
fused coating.
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 Figure 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).
A core collection chute 22 inclines away from the second
drum 12' downstream of thefusing station C', taking coated cores
to be further processed and packed.
The first drum 12 will be described in more detail with
reference to Figure 2. It comprises a rotatable shell 24, the
outer face of which carries the depressions 14. In Figure 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
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WO 96/35516 PCT/GB96/01102
drum, whether formed by one cor_tinuous shell or several contiguous
shells. The denressions 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 i s 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
apDlicatior-s, the depth of the depression should be such as to
allow aL leasL Sv o of the core L:Llickness to be exTnosed to z,:ie
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.
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 26 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 121.
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.
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WO 96/35516 PCT/GB96/01102
The drum 12 is held at a potential differer_ce to earth
having the same sign as the potential difference to earth of the
coating powder.
The second drum 121 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 co che second preconditioiiing statlon 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.
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 vacuun.z manifold_30, and so
the core is held_in the depression bv 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, itso exposed surface is
sprayed with charged droplets of a capture-enhancing liquid, for
example polyethylene glycol.
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
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WO 96135516 PCT/GB96/01102
attracted substantially only to the core and that the surface of
the drum remains substantially free of powder.
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.
As the shell 24 continues to rotate, the core leaves the
fusing station C, passes through the cooling scation (not shown),
so that 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 121, 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.
The drums themselves are preferably at least 60 mm in
diameter and not less than the minimum tablet diameter in width,
rotating at least % 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.
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
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WO 96/35516 PCT/GB96/01102
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 limiced high voltage (up to 50 kV at 30 to 100 A)
potential difference to earth as each core on a drum passes the
gun, and a mist of charged droplets is discharged from the
capillary toward 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 32 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 ML2) as each core leaves the
preconditioning station; this ensures a sharp cut off of the
droplets between tablet cores.
The pre-conditioning step may not always be required.
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
continuouslv recirculated. The trays may be of a plastics
material having an earthed metal strip under che arc swept by the
tablet cores on the respective drums or they may be metallic
trays. An alternative way to charge the particies 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 -i5 kV current limited to 5 A.
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A preferred powder coating composition is:
46.5% by weight Eudragit RS ammanio-methacrylate
co-polymer
28.0% by weight K1uce1~ hydroxy propyl cellulose
15.0% by weight titanium dioxide
5.0% by weight aluminium lake
5.0% by weiaht polyethylene glycol 6000
0.5% by weight Aerosil 200 colloidal silicon dioxide
Another preferred powder coating composition is:
39.75% bv weight Eudragit RS ammonio-methacrylate
co-poivmer
39.75% by weight Kluce: (hydroxypropyicelluiose)
15.0 s by weight Titanium dioxide
5.0% b_v weight Aluminium lake
0.5% by weight Aerosii (colloidal silicon dioxide)
The components are premixed under high shear, then wet
granulated by mixing under high shear with water C10-15 s by
weight). The granulated mixture is dried in fluid bed drier at
about 45 C for 20 to 30 minutes to reduce the moisture content to
below 3t 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 20gm, and about 100 s by
volume are of a size less than 60 m. The peak size is about 10~=_
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.
* Trademark
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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 an internal baffle which breaks the spray up into a
mist of 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.
A preferred liquid coating composition comprises :
hydroxyoropylmethylcellulose 70%
glycerol 7%
iron oxide yellow 23%
in aqueous dispersion.
At the fusing 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 bv 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.
Preferred coating apparatus according to the invention can
coat up to 300,000 tablet cores each hour._
.
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