Coating apparatus

Coating apparatus for producing multiple coatings on photographic bases wherein a layer of the coating material flows over each of two slideways which slope downwardly in a V form. An intermediate element is disposed in the flow gap bounded by the two slideways. The discharge edges of the slideways and of the intermediate element are disposed at substantially the same level. At this level the various layers of liquid combine to form a multilayer curtain which falls on to a base passing below to form a multiple layer thereon. The intermediate element can be a slot pourer so that the coating material issuing from its slot combines with the coating compositions supplied from the slideways.

FIELD OF THE INVENTION 
This invention relates to coating apparatus, more particularly but not 
exclusively to curtain coating apparatus. 
A very economical way of coating photographic materials, particularly those 
having a large number of different layers, is for a number of layers to be 
applied simultaneously in a single working step. 
PRIOR ART 
A method known as cascade coating uses apparatus comprising a substantially 
prismatic block which is combined from a plurality of smaller blocks and 
which has a number of distributing channels equal to the number of layers 
to be poured, the discrete liquids for forming the layers being meteredly 
and supplied separately to the channels. Disposed above each channel is a 
substantially vertical slot and the liquid is forced up from the 
distributing channel by pressure inside the slots. All of the slots 
terminate at their top end in an exposed sloping and substantially flat 
surface along which the liquid descends as a cohering uniformly 
distributed layer. Coating apparatus of this kind are known as "coating 
funnels" or "slideway coaters" or, in the special form for producing 
several layers as "cascade coaters". The planes associated with the 
various slots can be coplanar or offset in relation to one another near 
the various slots by small steps of about the same thickness as the 
coating. Liquids issuing from the exit slots nearest the end of the 
discharge plane are flowed over laminarly by liquid issuing from exit 
slots higher up the discharge plane a liquid film consisting of a number 
of clearly separate layers finally forming at the bottom end of the 
discharge plane or surface. This liquid film is transferred to a moving 
surface to be coated either in the form of a self-supporting meniscus or 
in the form of a free falling curtain. In the former case, the moving 
surface is spaced at a small distance from the discharge surface--as a 
rule, fractions of a millimeter,--whereas in the second case the distance 
is large enough to enable a free-falling curtain to form. Apparatus of 
this kind can readily enable three or four or even more layers to be 
simultaneously formed on the surface to be treated. It has been found from 
experience that, if the process is performed properly, the discrete layers 
do not mix with one another while flowing down the inclined surface or in 
the self-supporting meniscus or in the free-falling curtain or on the 
moving surface during subsequent drying. 
German Offenlegungsschrift No. 2,351,369 discloses a very advantageous form 
of curtain coater which can apply a number of layers simultaneously and in 
which two or more separate units are used to form the free-falling 
curtain, the various units being devised so as to make it possible for 
each and any of them alone to form a single layer or a multilayer curtain. 
The units are positioned relative to one another so that the liquid layers 
initially formed on them combine near the discharge edge to form a 
multilayer film, thereafter to drop as a multilayer free-falling curtain 
on to the carrier or base moving below the units. In a preferred 
construction this apparatus has two slideways whose surfaces are arranged 
in a V formation and each of which have a curved discharge lip; the 
discharge edges thereof being disposed at the same height and at a small 
distance from one another substantially corresponding to the total 
thickness of the liquid curtain. Other embodiments relate to the 
introduction of other liquid layers either on the slideway of one or of 
two of the slideways or vertically above the centre of the gap bounded by 
the two parallel discharge lips. A detailed description of the advantages 
of such systems is given in this Offenlegungsschrift. 
Also in German Auslegeschrifts Nos. 1,928,025 and 1,928,031, coating 
methods using free-falling liquid curtains call for the observance of 
certain conditions to ensure that the free-falling curtain is stable and 
free of disturbances which could cause the curtain to weave or break up. 
One such important condition is the maintenance of a minimum throughflow 
necessary to ensure that the curtain once formed does not break up 
spontaneously or separate into discrete partial flows. Disturbances 
causing the curtain to break up can be in the form of oscillations caused 
e.g. by movement of the air around the curtain; other disturbances are 
connected with particles and gas bubbles included in the liquid film and 
causing local weakening of film coherence. A free-falling liquid curtain 
can be regarded as stable when any disturbance which would cause local and 
instantaneous rupture is self-curing. 
The minimum necessary mass flow for this condition is very closely 
connected with the viscosity and surface tension of the liquid layers and 
with the geometry of the system, more particularly near the discharge lip, 
and approximately 0.5 ml/sec per cm of curtain width is regarded as the 
minimum. However, it has been found in practice that much larger specific 
throughflows are required for undisturbed operation, typical figures being 
from 2 to 3 times the amounts specified--i.e. between 1.0 and 1.5 ml. 
sec..sup.-1 cm.sup.-1. It is therefore difficult to apply thin layers to a 
base by curtain coating. One solution to the problem is e.g. to move the 
base through the coating zone very fast; unfortunately, the continuous 
drier which usually follows the coater must then have a correspondingly 
high evaporation ability. 
German Offenlegungsschrift No. 2,448,440 discloses another process for 
making a liquid curtain stable at relatively low throughputs. The method 
consists of imparting to a narrow liquid zone at opposite edges of the 
curtain a flow rate and possibly a viscosity which is higher than in the 
remainder of the curtain. The central zone of the curtain, which typically 
can be more than 98% of total curtain width, can then be operated at very 
low specific throughputs without risk to curtain stability. Despite its 
obvious advantages, this process has disadvantages. For instance, since 
the edge zones must be fed separately, the pourer is more expensive to 
construct; also, additional metering and pump system is required to supply 
the edge zones. 
OBJECT OF THE INVENTION 
It is therefore an object of this invention to provide a multicoat curtain 
coater wherein the minimum specific throughput required to form a stable 
curtain is reduced. It has been found that the problem can be solved by a 
surprisingly simple modification of the coater disclosed in German 
Offenlegungsschrift No. 2,351,369. 
SUMMARY OF THE INVENTION 
The invention relates to an apparatus for the simultaneous application by 
curtain coating of at least two layers of liquid coating materials on a 
strip-like base conveyed along a predetermined path, the apparatus having 
slideways which are each fed by way of a slot with a coating liquid. The 
slideways downwards in a V formation to define a flow gap which is divided 
into two component gaps of substantially equal width by a wedge-shaped 
intermediate element which narrows downwardly. The discharge edges of the 
element and the pouring lips of the two slideways are preferably disposed 
at substantially the same level. The level of the discharge edge of the 
intermediate element can be higher or lower; as a rule however, all three 
discharge edges are disposed at the same level for optimum results. 
Preferably, the wedge-shaped intermediate element has one or more channels 
for the introduction of one or more layers between the two or more layers 
supplied by the slideways.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows a slideway coater for multicoat curtain coating comprising two 
units having slideways 1,1' arranged relatively to one another in the form 
of a V and having curved pouring lips 2,2' terminating in discharge edges 
3,3'. Disposed between the lips 2,2' is a wedge-shaped intermediate 
element 7 whose bottom edge is at the same level as the two discharge 
edges 3,3'. Consequently the flow of the two liquids 4,4' remains separate 
as far as the height of the edges 3,3' and only thereafter does it combine 
to form a free-falling combined liquid curtain 6. The same drops on to a 
strip 13 to be coated moving in the direction indicated by an arrow T and 
forms a multiple coating on the article. 
Referring to the embodiment shown in FIG. 2, the wedge-shaped intermediate 
element 7 is a slot pourer; accordingly, the element 7 is formed with a 
discharge slot 10 adapted to be supplied via a channel 11. The exit 
orifice of slot 10 is at substantially the same level as the two discharge 
edges 3,3' so as to form a combined zone from which the three liquids flow 
to form a free-falling liquid curtain 6 having 3 layers. 
The width of the gaps 12,12' between the lip 2 or 2' and the element 7 is 
such that the gaps completely fill up with liquid at least at the bottom 
region of the zone. So that this state of affairs may always be reached 
for different liquids of different viscosities, the gap widths are 
adjustable. To provide this adjustment the components which carry the 
slideways 1,1' are horizontally adjustable, as indicated by double arrows 
H, and/or the intermediate element is adjustable vertically, as indicated 
by a double arrow V. As shown in the drawings the entry to the gaps 12 and 
12' is funnel-shaped. Gap width in the narrowest part is preferably from 
approximately 0.25 to 1.00 mm per feed slot of the associated slideway 1 
or 1'. The drawings show each slideway as being formed with a single feed 
slot, but it can be formed with two or more feed slots. Similarly, the 
wedge-shaped element can be formed with two or more parallel exit slots so 
that curtains combined from four or five or more layers can be produced. 
FIG. 3 shows an embodiment in which the wedge-shaped intermediate element 7 
is formed with two slots. 
The supply tanks, feed piping, pumps and so forth which handle the coating 
liquids and which are necessary to supply the coaters are generally known 
and have not therefore been shown. 
Comparative tests show that an apparatus as shown in FIGS. 1, 2 or 3 can 
reduce the bottom limit for specific throughput considerably as compared 
with the known facilities, e.g. as disclosed by German Auslegeschrifts 
Nos. 1,928,025 and 1,928,031 and by German Offenlegungsschrift No. 
2,351,369. 
The comparative tests made were as follows: 
A first gelatin solution coloured with a green-blue azo dye was mixed with 
2% of isopropylnaphthalene sulphonic acid as wetting agent and diluted 
with water to a viscosity of 20 cP. 
A second gelatin solution was mixed with the same quantity of wetting 
agent, coloured with a purple azo dye and also adjusted to a viscosity of 
20 cP. 
The coater of FIG. 1 but without the intermediate element 7 was supplied 
with a continuous flow of both gelatin solutions so that a continuous 
free-falling two-layer liquid curtain 6 was produced which after a drop of 
7 cm was applied to a carrier 13 moving transversely of the curtain plane 
at a speed of 1.33 m/sec. A specific throughput of 0.7 
ml/sec.sup.-1.cm.sup.-1 for each of the two halves of the coater--i.e. in 
all 1.4 ml.sec.sup.-1.cm.sup.-1 for the combined curtain--was required to 
produce a stable curtain unaffected by disturbances. 
The same experiment was repeated but with the addition of the element 7: 
the liquid consumption for a stable curtain dropped to 0.525 
ml.sec.sup.-1.cm.sup.-1 for each half of the coater corresponding to 1.05 
ml.sec.sup.-1.cm.sup.-1 for the combined curtain. Finally, another 
experiment was performed for a coater of the kind shown in FIG. 2 wherein 
the left-hand and right-hand halves of the coater were each fed with a 
greenish-blue gelatin solution while the central wedge-shaped element 7 
was fed with a purple coloured gelatin solution. In this case 0.3 
ml.sec.sup.-1.cm.sup.-1 for each unit--i.e. a total of 0.9 ml.sec.sup.31 
1.cm.sup.-1 --was required to produce a stable curtain--i.e. only 64% of 
the quantity required for the apparatus of FIG. 1.