[0001] This invention relates to coating processes and is more particularly concerned with
curtain coating processes.
[0002] In curtain coating as described in US-A-3867901 and US-A-3508947, one or more liquid
layers are applied to a moving support as a freely-falling curtain. Under properly
controlled conditions, uniform coatings are achieved in which each layer retains its
separate identity. The curtain can be formed using a conventional multilayer slide
hopper fitted with a suitable lip. Edging rods may be used to maintain the width of
the curtain, and surface active agents are frequently added to the outer layers to
improve curtain stability and promote uniform deposition.
[0003] In the coating of photographic materials and other products in which one or more
liquid layers are applied to a moving support, uniform coatings are obtained only
if the operational variables are held within fairly precise limits. These limits define
the so-called "coating window".
[0004] One important variable which affects the "coating window" is coating speed. For economic
reasons, high coating speeds are desirable provided they can be achieved with low
waste and without loss of product quality. However, a major problem with high speed
coating is that if the speed is too high, the liquid fails to wet the support in a
complete and steady manner. When this occurs, air is entrained at the wetting line
and causes patchy or uneven deposition of the coating.
[0005] Air-entrainment is believed to occur whenever the dynamic contact angle formed between
the liquid and the support at the wetting line approaches its limiting value of 180°.
The coating speed at which this happens depends on many factors such as the method
of coating and the physical properties of the liquid. The viscosity of the liquid
being brought into contact with the support at the wetting line is of particular importance.
[0006] In curtain coating as well as in bead coating, the occurrence of air-entrainment
can be postponed to higher speeds by reducing the viscosity of the liquid. On the
other hand, for aqueous photographic melts and other water or solvent based coating
systems, the ability to coat at high viscosities has several advantages such as reduced
drying load and improved uniformity.
[0007] In multilayer bead coating, a liquid having a relatively low viscosity (typically
3 to 5mPas) is sometimes used to provide a bottom layer for higher viscosity liquids.
The bottom layer is added to the higher viscosity liquids out of the bottom slot of
the hopper (that is, the slot nearest the bead). Although increased coating speeds
are possible, a non-uniform coating is often produced, particularly when the bottom
layer is relatively thin, for example, less than about 30µm.
[0008] US-A-4569863 discloses a curtain coating process for applying a multiple coating
on to a moving support. The process enables a plurality of comparatively, high viscosity
layers (having viscosities greater than 20mPas) to be applied to the support at coating
speeds of 400mmin⁻¹ or more. This is achieved by the use of a thin bottom layer (otherwise
called an accelerating layer) of lower viscosity. The thickness of this layer is between
2 and 30µm, and its viscosity between 1 and 20mPas. Preferably, the thickness is between
2.5 and 5µm, and the viscosity between 2 and 3mPas. The use of an upper spreading
layer is also disclosed.
[0009] In US-A-4569863, the bottom or accelerating layer is applied directly to the back
of the curtain either by discharging the layer from the bottom slot of a conventional
slide hopper, or from a slot which flows on to a separate slide which joins at the
hopper lip the main hopper slide on which are flowing the other layer compositions.
Hoppers designed for this purpose are sometimes known as V-hoppers.
[0010] It should be noted that in US-A-4569863, the freely-falling curtain impinges on to
the moving support at an application angle of 0°. (The application angle may be defined
as the slope angle of the support at the point of impingement of the freely-falling
and substantially vertical curtain measured as a declination from the horizontal in
the direction of coating.) Furthermore, the height of the curtain is restricted within
the range of 10 to 100mm, preferably 15 to 50mm.
[0011] Although the method described in US-A-4569863 can yield high coating speeds, the
method suffers from several disadvantages. It is difficult to deliver a low viscosity
and relatively low flow rate layer down an inclined hopper slide as a bottom layer
without waves and other manifestations of unstable flow. In the arrangement wherein
the low viscosity bottom layer is delivered down a separate slide to join the main
hopper slide, a long main slide results. This is undesirable since waves and other
manifestations of unstable flow on the slide grow very rapidly as slide length is
increased, and undesirable restrictions on the relative flow rates and viscosities
of the layers on the main slide may result. It is therefore preferred that the total
length of the slide of a multiple slot hopper is kept to a minimum to minimize such
restrictions.
[0012] The method described in US-A-4569863 may also increase the undesirable tendency of
the plane of the curtain to deviate from the vertical, and bend backwards towards
the hopper. Also, when the low viscosity layer which wets the support is delivered
using a V-hopper, it faces in a downward direction. Thus, the establishment of flow
on such a slide can be difficult in practice, and dripping of composition off the
slide surface may occur. Furthermore, with this slide orientation, there is a component
of gravity normal to the slide surface which is de-stabilizing and promotes the growth
of waves on the layer as it travels down the slide.
[0013] A low viscosity bottom layer also promotes "puddling" at the point where the freely-falling
liquid curtain impinges on the moving support. A "heel" appears at the foot of the
curtain. If the heel is sufficiently large, it may contain an eddy in which air bubbles
and debris may become trapped, thereby generating a line or streak in the coating.
A large heel can also oscillate, producing non-uniformities in the coating along and
across the direction of web motion. To prevent puddling, the low viscosity bottom
layer may have to be kept thin, even though a functional bottom layer may not be thin,
and the curtain height low, though this adversely affects curtain stability and restricts
the room beneath the hopper for other equipment, such as a start pan.
[0014] It is therefore an object of the present invention to provide a curtain coating process
in which high viscosity liquids can be deposited on to a moving support using a low
viscosity bottom layer to promote wetting of the support, but eliminating or significantly
mitigating the foregoing disadvantages.
[0015] According to one aspect of the present invention, there is provided a curtain coating
process in which liquid material comprising two or more layers is coated on to a moving
support, characterized in that the layer of liquid material adjacent the support has
a viscosity which is less than 1mPas.
[0016] Advantageously, the bottom layer of low viscosity liquid readily wets the support
at high coating speeds and is applied with the curtain as the bottom layer of the
liquid material being coated. This allows the liquid material to comprise one or more
layers of much higher viscosity, which, in the absence of the bottom layer, would
not wet the support so readily and which would, therefore, be more difficult to coat
without air-entrainment except at much lower speeds.
[0017] The bottom layer may be any low viscosity liquid or solution compatible with the
remainder of the liquid material. If the latter comprises aqueous coating compositions,
as in the manufacture of photographic products, then the preferred liquid for the
bottom layer is water to which small amounts of other substances, such as surfactants,
hardeners, dyes etc., may be added as necessary.
[0018] By coating a high viscosity curtain with a low viscosity bottom layer, the advantages
of high viscosity coating are combined with low viscosity wetting. High coating speeds
are possible, and the coating speed at which air-entrainment occurs is less seriously
affected by the total flow rate. At the same time, the viscosities of the layers comprising
the main bulk of the curtain may be chosen to suit factors other than wetting such
as uniformity, product specification and the drying capacity of the coating track.
[0019] According to the present invention, the low viscosity bottom layer may also be very
thin (between 0.5 and 10µm, preferably between 1 and 5 µm). This minimizes the tendency
towards puddling and lessens the necessity for the incorporation of non-functional
materials. A further advantage of using a thin bottom layer is that diffusion into
other layers occurs very rapidly, hence flow after coating is minimized.
[0020] In addition, application angles greater than 0° may be used, preferably angles between
20° and 60°. This permits the beneficial use of high curtains whilst again minimizing
the tendency towards puddling.
[0021] For a better understanding of the present invention, reference will now be made,
by way of example only, to the accompanying drawings in which:-
Figure 1 is a schematic cross-section of a curtain coating hopper used in accordance
with the present invention;
Figure 2 is schematic partial cross-section of a hopper illustrating an exit slot
for a low viscosity bottom layer;
Figure 3 is similar to Figure 2, but illustrates an alternative exit slot arrangement
for a low viscosity bottom layer; and
Figure 4 is a schematic cross-section of a curtain coating hopper, but illustrating
the use of exit slots as shown in Figures 2 or 3.
[0022] A conventional multilayer curtain-coating slide hopper 10 is shown in Figure 1. One
method of applying a freely-falling curtain 12 to a moving support 14 is shown, although
the support is preferably backed by a precision roller (not shown). The hopper 10
is oriented relative to the moving support 14 so that the liquid layers flow down
the slide surface 28 in a direction opposite to the coating direction. Layer 16 (which
wets the support 14), although discharged from slot 18 uppermost on the slide surface
28, becomes the bottom layer of the coating as the curtain 12 impinges on the support
14. The other layers are discharged from respective slots 20, 22, 24 and 26, and are
inverted in sequence from that of conventional curtain- or bead-coating.
[0023] It is to be noted that the support 14 is inclined to the horizontal so that the angle
of application of the curtain to the support, α, is not 0°, that is, the curtain 12
is applied at a forward application angle.
[0024] In Figure 2, an exit slot arrangement is shown in which the bottom layer 16 is directly
applied to the upstream side of the curtain as it leaves the hopper lip 36. The bottom
layer 16 is discharged from an additional slot 30. This slot 30 is formed by inserting
a sub-lip element 32 beneath the overhang 34 at the front of the hopper slide 28.
In this case, the slot exit is positioned exactly at the hopper lip 36 as shown. However,
as shown in Figure 3, the slot 38 can be recessed to form a short slide 40 (of 1cm
or less) on the underside of the hopper lip 36. In both these cases, the hopper slide
28 is oriented conventionally in the coating direction as shown in Figure 4 so that
the liquid leaving the slots 30, 38 becomes the bottom layer of the liquid material
being coated.
[0025] In conventional curtain coating processes, the curtain forming hopper is usually
oriented in the direction of web travel and the bottom layer is discharged from the
bottom slot (i.e. the slot nearest the lip). If this is used for a low viscosity bottom
layer such as water, then the coatings are subject to longitudinal and transverse
lines and streaks which are just as unacceptable as those seen in bead coating with
the same arrangement. But, by reversing the orientation of the hopper and the order
of the other layers, and discharging the bottom layer from the top slot (as discussed
with reference to Figure 1), this problem may be removed. In order to achieve this,
the hopper slide is inclined at an angle of less than 30° to the horizontal, and the
flow rate of the low viscosity liquid layer discharged from the top slot is restricted
to not more than one third of the sum of the flow rates of that layer and the high
viscosity layer immediately beneath it. Advantageously, the slide surface 28 promotes
levelling of any unevenness in the water layer, and the flow on the slide surface
is sufficiently stable against waves and other manifestations of slide instability.
[0026] A similar advantage is obtained if the bottom layer is applied directly to the upstream
side of the curtain (as shown in Figure 4 using the arrangements described with reference
to Figures 2 and 3).
[0027] In either of the two cases above, if water plus surfactants is used as the bottom
layer, uniform coatings can be produced at coating speeds of 600mmin⁻¹ or more.
[0028] It is possible, therefore, by using curtain processes according to the present invention,
to provide uniform multilayer coatings at higher speeds than previously attainable.
Although curtain coating is employed, there is flexibility in the choice of flow rate
and the viscosities for the layers forming the bulk of the coating. The coating method
according to the invention is both product tolerant and easy to engineer with minimum
changes in current technology.
[0029] Examples of coatings produced using a curtain coating process according to the present
invention are now described in the following examples:-
Example 1:
[0030] The high viscosity layers comprised an aqueous solution of gelatin having a viscosity
of 74mPas at 44°C. At a curtain height of 10.2cm and an application angle of 0°, the
maximum coating speed of the gelatin layers alone was typically 324mmin⁻¹ at a total
wet thickness of 100µm, and 372mmin⁻¹ at a thickness of 27µm. Higher coating speeds
produced severe air-entrainment.
[0031] On addition of a low viscosity bottom layer, 1.1µm in thickness, comprising water
containing suitable surfactants, and having a surface tension of 21mNm⁻¹ and a viscosity
of 0.61mPas at 44°C, uniform coatings were achieved at speeds up to 596mmin⁻¹ with
gelatin layers having a combined thickness of 45µm. Furthermore, it was found that
the thickness of the gelatin layers could be substantially increased by coating at
a forward application angle. For example, at an application angle of +20°, uniform
coatings of gelatin layers having a combined thickness of 9µm were made at speeds
of up to 600mmin⁻¹ with a low viscosity bottom layer of 4.3µm. In general, uniform
coatings were obtainable with low viscosity bottom layers having thicknesses in the
range 1 to 8µm, although some successful coatings were made with bottom layers of
only 0.5µm.
[0032] The main results are summarised in Table 1.
Table 1
Maximum coating speed (mmin⁻¹) |
Gelatin thickness (µm) |
Bottom layer thickness (µm) |
Curtain height (cm) |
Application angle (°) |
324 |
100 |
none |
10.2 |
0 |
372 |
27 |
none |
10.2 |
0 |
596 |
45 |
1.1 |
10.2 |
0 |
600 |
96 |
4.3 |
10.2 |
20 |
Example 2:
[0033] The high viscosity layers comprised an aqueous solution of gelatin having a viscosity
of 64mPas at 45°C. At a curtain height of 12.7cm and an application angle of +25°,
the maximum coating speed of the gelatin layers alone was typically 376mmin⁻¹ at a
total wet thickness of 44.3µm. Higher coating speeds produced severe air-entrainment.
[0034] Addition of a low viscosity bottom layer, 3.6µm in thickness, having the same composition
as that used in Example 1, allowed coatings to be achieved at speeds up to 600mmin⁻¹
with gelatin layers having a combined thickness of 96µm. Coating was stable: it was
only transiently disturbed by splices and would rapidly re-establish if the curtain
was momentarily broken by a rod. However, if the low viscosity layer was interrupted,
air-entrainment ensued and it was necessary to slow down and/or reduce flow rates
in order to resume uniform coating. Nevertheless, at lower speeds (for example at
465mmin⁻¹) uniform coating with heavy laydown (for example a combined thickness of
124µm) would automatically resume following restoration of the supply of the low viscosity
bottom layer. With thinner gelatin layers, this reversibility was available at higher
speeds.
[0035] Results for this example are summarised in Table 2.
Table 2
Maximum coating speed (mmin⁻¹) |
Gelatin thickness (µm) |
Bottom layer thickness (µm) |
Curtain height (cm) |
Application angle (°) |
376 |
44.3 |
none |
12.7 |
25 |
600 |
96 |
3.6 |
12.7 |
25 |
465 |
124 |
4.6 |
12.7 |
25 |
Example 3:
[0036] The high viscosity layers comprised aqueous solutions of gelatin having viscosities
of 67.1mPas and 63.2mPas at 42°C. At a curtain height of 25.4cm and an application
angle of +45°, the maximum practical coating speed of the gelatin layers alone was
typically 313mmin⁻¹ at a total wet thickness of 179µm. Higher coating speeds were
prone to air-entrainment.
[0037] On addition of a low viscosity bottom layer, comprising water containing suitable
surfactants, and having a surface tension of 19.3mNm⁻¹ and a viscosity of 0.63mPas
at 42°C, uniform coatings were achieved at speeds up to 738mmin⁻¹ with gelatin layers
having a combined thickness of 76µm and a bottom layer thickness of 2.6µm. Coating
speeds of up to 600mmin⁻¹ were achieved with a bottom layer thickness of 1.1µm and
gelatin layers having a total thickness of 93µm.
[0038] The results are summarised in Table 3.
Table 3
Maximum coating speed (mmin⁻¹) |
Gelatin thickness (µm) |
Bottom layer thickness (µm) |
Curtain height (cm) |
Application angle (°) |
313 |
179 |
none |
25.4 |
45 |
738 |
76 |
2.6 |
25.4 |
45 |
600 |
93 |
1.1 |
25.4 |
45 |
[0039] Although the examples cited above make use of dilute aqueous surfactant solutions
as the low viscosity bottom layer, it is envisioned that the advantages claimed herein
could be obtained with a wide range of liquids having viscosities less than 1mPas,
such as aqueous solutions of dyes, hardeners and adhesion promoting addenda, and water-miscible
and other low viscosity solvents compatible with the other coated layers.
1. A curtain coating process in which liquid material (12, 16) comprising two or more
layers is coated on to a moving support (14), characterized in that the layer (16)
of liquid material adjacent the support (14) has a viscosity which is less than 1mPas.
2. A process according to claim 1, wherein the layer (16) of liquid material adjacent
the support (14) has a wet thickness in a range between 0.5 and 10µm.
3. A process according to claim 2, wherein the wet thickness of the liquid material is
in the range between 1 and 5µm.
4. A process according to any one of claims 1 to 3, wherein the liquid material is applied
to the moving support (14) at an application angle (α) which is greater than 0°.
5. A process according to claim 4, wherein the application angle (α) lies between +20°
and +60°.
6. A process according to any one of the preceding claims, wherein the low viscosity
layer (16) is applied as the top layer on a slide surface (28) inclined at an angle
of less than 30° to the horizontal, and the flow rate of that top layer (16) comprises
not more than one third of the sum of the flow rates of that top layer and the high
viscosity layer immediately beneath it.
7. A process according to any one of claims 1 to 5, wherein the low viscosity layer (16)
is applied directly to the liquid material from an exit slot (30) positioned at the
hopper lip (36).
8. A process according to any one of claims 1 to 5, wherein the low viscosity layer (16)
is applied directly to the liquid material from an exit slot (38) positioned to form
a short slide (40) on the underside of the hopper lip (36).
9. A process according to any one of the preceding claims, wherein the layer (16) of
liquid material adjacent the support (14) comprises water including surfactants.
1. Vorhangbeschichtungsverfahren, wobei aus zwei oder mehreren Schichten bestehendes
flüssiges Material (12, 16) auf einen bewegten Träger (14) aufgetragen wird, dadurch gekennzeichnet, daß die unmittelbar auf den Träger (14) aufzubringende flüssige Materialschicht (16)
eine Viskosität aufweist, die geringer als 1 mPas. ist.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die unmittelbar auf den Träger
(14) aufzubringende flüssige Materialschicht (16) in feuchtem Zustand eine Dicke zwischen
0,5 und 10 µm hat.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß die Dicke des flüssigen Materials
zwischen 1 und 5 µm liegt.
4. Verfahren nach einem der Ansprüche 1 - 3, dadurch gekennzeichnet, daß das flüssige
Material auf den bewegten Träger (14) unter einem Auftragswinkel (α) von über 0° aufgebracht
wird.
5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß der Auftragswinkel (α) zwischen
+20 und +60° liegt.
6. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die
Schicht geringer Viskosität (16) als oberste Schicht auf einer Gleitfläche (28), die
gegenüber der Horizontalen unter einem Winkel von weniger als 30° geneigt ist, aufgebracht
wird, wobei die Fließgeschwindigkeit dieser obersten Schicht (16) nicht mehr als ein
Drittel der Summe der Fließgeschwindigkeiten dieser obersten Schicht und der unmittelbar
darunter befindlichen Schicht hoher Viskosität beträgt.
7. Verfahren nach einem der Ansprüche 1 - 5, dadurch gekennzeichnet, daß die Schicht
geringer Viskosität (16) direkt mit dem aus einem an der Gießkante (36) befindlichen
Auslaßschlitz austretenden flüssigen Material in Kontakt gebracht wird.
8. Verfahren nach einem der Ansprüche 1 - 5, dadurch gekennzeichnet, daß die Schicht
von geringer Viskosität (16) direkt mit dem aus einem Auslaßschlitz (38) austretenden
flüssigen Material in Kontakt gebracht wird, wobei der Auslaßschlitz so angeordnet
ist, daß er an der Unterseite der Gießkante (36) eine kurze Gleitfläche (40) bildet.
9. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die
unmittelbar auf dem Träger (14) befindliche Schicht (16) aus flüssigem Material mit
oberflächenaktiven Substanzen versetztes Wasser enthält.
1. Procédé de couchage au rideau dans lequel une substance liquide (12, 16) comprenant
au moins deux couches est appliquée sur un support en mouvement (14), caractérisé
en ce que la couche (16) de substance liquide adjacente au support (14) a une viscosité
inférieure à 1mPas.
2. Procédé selon la revendication 1 dans lequel la couche (16) de substance liquide adjacente
au support (14) a une épaisseur à l'état humide compris entre 0,5 et 10 µm.
3. Procédé selon la revendication 2 dans lequel l'épaisseur à l'état humide de la substance
liquide est comprise entre 1 et 5 µm.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel la substance
liquide est appliquée sur le support en mouvement (14) avec un angle d'application
(α) supérieur à 0°.
5. Procédé selon la revendication 4 dans lequel l'angle d'application (α) est compris
entre + 20° et 60°.
6. Procédé selon l'une quelconque des revendications précédentes, dans lequel la couche
ayant une viscosité faible (16) est appliquée comme couche supérieure sur un plan
d'écoulement (28) incliné selon un angle inférieur à 30° par rapport à l'horizontale,
et le débit d'écoulement de la couche supérieure (16) n'est pas plus du tiers de la
somme des débits d'écoulement de la couche supérieure et de la couche à viscosité
élevée placée immédiatement sous cette dernière.
7. Procédé selon l'une quelconque des revendications 1 à 5 dans lequel la couche à viscosité
faible (16) est appliquée directement sur la substance liquide à partir d'une fente
de sortie (30) placée sur la lèvre du dispositif d'enduction (36).
8. Procédé selon l'une quelconque des revendications 1 à 5 dans lequel la couche à viscosité
faible (16) est appliquée directement sur la substance liquide à partir d'une fente
de sortie (38) positionnée pour former un court plan d'écoulement (40) sur le bord
inférieur de la lèvre du dispositif d'enduction.
9. Procédé selon l'une quelconque des revendications précédentes dans lequel la couche
(16) de substance liquide adjacente au support (14) comprend de l'eau et des tensioactifs.