US 3573965 A
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Description (OCR text may contain errors)
April 1971 MAMORU ISHIWATAAET AL 3,573,965
MULTILAYER COATING METHOD Filed Dec. 5, 1967 2 Sheets-Shet 1 FIG. I
INVENTORS MAMORU ISHIWATA YOSUKE UCHIDA YOSHIAKI NAGAI April 6, 1971 MAMQRU s w T ETAL 3,573,965
MULTILAYER COATING METHOD Filed Dec. 5, 1967 2 Sheets-Sheet 2 (mm) 0-10 Q 6 INVENTORS MAMORU ISHIWATA YOSUKE UCHIDA o YOSHIAKI NASA! u BY 6M 2. a, q I
ATTORNEYS United States Patent O US. Cl. 117-83 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to a multilayer coating method which is characterized in making the viscosity of a second layer greater than that of a first layer when two or more coating compositions pass through a doctor channel, the coating compositions maintaining the relation of superimposed layers.
BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a multilayer coating method for applying successively two or more liquid coating compositions in layers to a continuously travelling web-like support without conducting operations of setting or drying the coating layers between applications of the coating compositions.
More particularly, this invention relates to an improved so-called multiple doctor coating method. The multiple doctor coating method is a process for coating a plurality of layers on a web which comprises (1) within a short clearance from the surface of the continuously travelling web to be coated, providing a coating means having a plurality of separately closed chambers within said coating device, one for each fluid coating composition, and a plurality of discharge openings, separated from each other by partitions and each connected to a chamber such that said openings are positioned slightly apart from the surface of said web, (2) successively applying said coating compositions from the openings of the coating means without forming spaces between the coating composition discharged from said openings while filling the space between the surface of the web and the coating means with the continuously discharged coating compositions to form superimposed, multiple layers of the fluid coating composition between them, (3) withdrawing said filuid coating compositions from the coating means by the travelling support, while maintaining the superimposed and distinct layers thereof, and (4) then forming on the support the multiple layers of the coating compositions.
In particular, the method of the present invention is suitable for applying at a high speed, in one step, two or more fluid coating compositions, such as photographic, light-sensitive, gelatino silver halide emulsions, to a trave ling support, such as, a film base or a baryta paper in thin layers.
In the method described above, the thickness of each layer to be coated, that is, the volume of coating composition which passes through the clearance hereinafter referred to as a doctor channel between each of the doctor edges and the support, is determined by the clearance between the doctor edge and the support, pressure difference between both ends of the doctor channel, etc., and can be wrtten as the following equation,
3,573,965 Patented Apr. 6, 1971 ICC where i 2ti=Sun1 of coating thickness from the first layer to its layer bi=Clearance between doctor edge and support corresponding to slot for coating composition of its layer.
APi Pressure difference between the both ends of doctor channel corresponding to slot for coating composition of its layer.
ki Proportional constant.
(2) Description of the prior art In hitherto known methods, in order to make the coating thickness of each layer, especially the lowest layer, thinner, in one of the proposed methods the clearance between the doctor edges and the support have been made smaller. In another method, a so-called coating-composition pressure decreasing method, the coating composi tion on the doctor channel has a pressure gradient which increases in the direction of travel of the support.
Such known methods however, have the following difliculties. In the method which makes the clearance between the doctor edge and the support smaller, there is a tendency to produce comet-like spots due to other materials attached on the opposite surface of the support or on the surface of the coating roller. Additionally, the coating thickness did not become uniform across the whole width of the support, as the ratio of errors in the accuracy produced in the manufacturing of the doctor edges and the coating roller to the clearance between the doctor edges and the support was increased. Moreover, when a jointed part of the support member is passing, it was necessary to move the coating head and coating roller.
In the coating composition pressure decreasing method, as it is liable to admit air into the doctor channel from the environment, it was necessary to provide some kind of special device in the structure at the back and the sides of the doctor channel.
We, the inventors, found a coating method as described hereinafter to solve such difiiculties in hitherto known methods.
Therefore, an object of the invention is to provide a coating method for forming these layers successively.
SUMMARY OF THE INVENTION This invention relates to a multilayer coating method Which is characterized in making the viscosity of a second layer greater than that of a first layer when two or more coating compositions pass through a doctor channel, the coating compositions maintaining the relation of superimposed layers.
BRIEF DESCRIPTION OF THE DRAWING The invention will now be further described with reference to the accompanying drawings in which:
FIGS. 1 and 2 are vertical cross-sectional views showing embodiments of the invention;
FIG. 3 is an enlarged vertical cross-sectional view showing the top part of the coating device shown in FIG. 1;
FIG. 4 is a model view of the distribution of fluid velocity of coating compositions at a doctor channel;
FIG. .5 illustrates the pressures of coating compositions at the outlet of a slot and at the front and back of a coating device; and
FIG. 6 illustrates the relation between coating thickness and the ratio of viscosities of coating compositions.
3 DETAILED DESCRIPTION OF THE INVENTION This invention will be described in detail with respect to the apparatus shown in FIG. 1 and FIG. 3 in the case of a two layer coating by employing a calculated example.
In FIG. 1 and FIG. 3, the reference numeral 1 designates a continuously travelling web-shaped support which is to be coated. The numeral 2 designates a roller which rotates with the support 1 by backing it up in order to keep it running constantly. The numeral 3 is a coating head which is located on the side of support 1, the coat ing device maintaining a slight clearance with it. The coating head 3 comprises liquid chambers 4 and 5 which supply coating compositions for the first layer (the lower layer) and the second layer (the upper layer), slots 8 and 9 for each of the coating compositions which are respectively connected to the liquid chamber 4 and 5 and are opened toward the support member 1 across the whole width thereof. Doctor edges 12 and 13 form spaced walls to isolate the slots from each other and have smoothed surfaces refined in high accuracy opposed to the support and form doctor channels 16 and 17 between the smoothed surfaces and the support. The liquid chambers 4 and 5 are respectively connected to liquid feeding equipment which supplies the coating compositions to complement only the quantity derived by the support. The first and the second coating compositions pass through the doctor channel 17, the coating compositions being superimposed one upon the other. The distribution of fluid velocity is shown in FIG. 4. In FIG. 4, A is the case in which the viscosity of the first coating composition is larger than that of the second coating composition, B is the case in which both are equal, and C is the case in which the viscosity of the second coating composition is larger. The quantity of coating composition passed through the doctor channel can be obtained as an integral value of fluid velocity. As understood from FIG. 4, the quantity of coating composition passed through the doctor channel in the case C is smaller than quantity of coating composition in the case A or B. Moreover, in the case where the viscosity of the second layer coating composition is larger than that of the first layer, resistance exerted when the coating composition passes through the doctor channel becomes larger, so that the static pressure of the second layer coating composition at the outlet of slot 9 becomes higher and can give a pressure gradient to the coating composition at the doctor channel 16, the pressure gradient being increased in the travelling direction of the support, or can make the pressure gradient larger, whereby the quantity of coating composition for the first layer becomes smaller.
As described above, by making the viscosity of the coating composition for the upper layer larger than that of the lower layer, the quantity of coating composition passing through the doctor channels 16 and 17 becomes smaller, so that coating of a thinner layer is possible. In order to be more clearly understood, a calculated example is shown in the case where the coating is performed under the following conditions.
COATING CONDITIONS Length of doctor edge 12 along travelling direction of support: 11 :3 mm.
Length of doctor edge 13 along travelling direction of support: (1 :4 mm.
Clearance between support and doctor edges: b=0=.3 mm.
Pressures in the front and back of coating device:
p =p =O mm. gauge Velocity of support: V=20 m./ min.
Quantity of coating composition for lower layer t '-=quantity of coating composition for upper layer t Viscosity of coating composition for lower layer: n =20 Viscosity of coating composition for upper layer: [cp.]
4 Static pressure of coating composition for lower layer at exit of outflow slot: P [mm. H O gauge] CALCULATED RESULT A l0 0. 5 0. 215 .B 20 1. U 0. 191 C 5. 0 0.
1 is shown in FIG. 6.
As clearly understood from the above description and calculated example, when coating according to this invention is carried out, it is possible to make the thickness of coating composition coated on the support thinner without narrowing the clearance between the doctor edges and the support. Therefore, difficulties, such as generation of comet-like spots, accurate uniformity of coating thickness, difliculty in passing through jointed parts, etc., produced due to narrow the clearance between the doctor edges and the support, would be dissolved. Additionally, in this method it is not necessary to provide any kind of special device in order to protect against the introduction of air.
When three or more coating compositions are coated according to this invention, for example, when four layers are simultaneously coated, it may be achieved by employing an apparatus such as the one shown in FIG. 2, under conditions that the viscosity of the coating composition for the second layer is larger than that of the first layer (liquid reservoirs 6 and 7, slots 10 and 11, doctor edges 14 and 15, etc., are the same as in FIGS. 1 and 3). In this case, it is also desired that the viscosity of the coating compositions for the third and fourth layers be larger than that of the first and second layers, but, even if the former is smaller than the latter, the process would be effective to some extent.
AN EMBODIMENT OF THE INVENTION By employing the apparatus shown in FIG. 1, a photographic emulsion and gelatinous aqueous composition were coated on subbing cellulose triacetate film under the following conditions:
Length of doctor edge 12 along travelling direction of support: a =3 mm.
Length of doctor edge 13 along travelling direction of support: 11 :4 mm.
Clearance between edges and support: b=0.3 mm.
Pressures in the front and back of coating device: P
P =atmospheric pressure.
Velocity of support: V=20 m./min.
Viscosity of coating composition (photographic emulsion) for lower layer: ,u =2O cp.
Viscosity of coating composition (gelatinous aqueous solution) for upper layer: 100 cp.
Quantity of coating composition for lower layer=Quantity of coating composition for upper layer.
When the viscosity of the coating composition for the upper and lower layers is identical, 20 cp. respectively, the coating thickness was 0.165 mm. By comparison, with the larger viscosity in the second or upper layer as shown above, the coating thickness under the above conditions became 0.115 mm., that is, about 30% reduced. As a result of checking photographic film obtained via setting and drying after coating, there were no comet-like spots,
no mixing between layers, and uniform thickness. Therefore, quality was excellent.
While we have shown and described certain specific embodiments of our invention, we are aware that many modifications thereof are possible. Our invention, therefore, is not to be limited by dimensions and contour of the apparatus shown, by the kind of coating compositions, etc.
What is claimed is:
1. In a process for coating multiple, distinct layers on a web backed by a suitable surface and travelling at a high rate of speed comprising locating adjacent to said web a coating head having only a plurality of discharge openings directed toward said web, a plurality of chambers each connected to a respective discharge opening with a doctor blade forming a downstream portion of each discharge opening in the direction of travel of said web and located in close proximity to said web, supplying fluid coating compositions into each of said chambers, applying said coating compositions successively to the surface of said web from said discharge openings without forming spaces between said coatings while filling the space between the surface of the web and said doctor blades with said coating compositions, maintaining said coating compositions between said web and said doctor blades under pressure and metering said coating compositions by means of said doctor blades to form superimposed and distinct multiple layers of said fluid coating compositions on said web, the improvement comprising the steps of: providing coating compositions for said chambers having viscosities which increase in order from the lowest layer formed thereby to the uppermost layer, and varying the viscosity of respective compositions while maintaining said order to thereby control the thickness of the layers of said multiple layer coating.
References Cited UNITED STATES PATENTS 2,761,791 9/1956 Russell 117-83 2,932,855 4/1960 Bartlett 117-72 3,206,323 9/1965 Miller et a1 117-34 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R.