US 2295280 A
Description (OCR text may contain errors)
Sept. 8, 1942. c. R. FORDYCE COATING HOPPER Filed May 15, 1940 o fi CHARLES RFoRm cE 'many kinds of celiulosic solutions.
' NIT Eu STATES PATENT-OFFICE), 2.2 s.2 a o* up r V' Charles a; Fordyce, Rochester, n. Y; as umito Eastman Kodak Company, Rochester, N. Y., a
corporation of New Jersey Application May 15, 1940, se -mm. 335,335
. mas (01.18-)
This invention relates to coating apparatus for coating oellulosic film forming solutions into a continuous sheet, and more particularly to an improved type of apparatus especially adapted for use with gelation type coating solutions.
It has been thecommon practice for some time in the art of forming thinsheeting and film base to spread a thin coating of the desired material, generally a solution of a cellulose ester such as cellulose acetate in a suite e solvent, onto a heated surface and cause the solvent to evaporate therefrom. The method usually employed consists of feeding the solution or dope point.
not adhere strongly to the cooled wheel and equal to or greater than the weight of the cellucan be maintained fluid at temperatures above 40 C. and when allowed to cool to or below a critical temperature between 10 0,-40? ,C., de-
pending on the composition, they form transparent gels which remain homogeneous throughout the gelling operation. This gelation occurs within approximately Q. of the flowable solution When the gels are first formed they .do
although the gelled film contains relatively large amounts of solvent, 1. e. an amount of solvent lose ester, itis sufiiciently strong and resistant from an appropriate feeding device onto the polished metallic surfaceof a slowly rotating wheel or drum. The solvent evaporates from the film more orless progressively as the wheel turns. In less than a complete revolution suflicient solvent has been removed from the wheel and conveyed to a wind up or carried over other rolls or drums for further curing treatment.
The device usually employed for feeding the solution to the coating wheel comprises a V- shaped hopper provided with spaced apart blades mounted at an angle to one another at apex of the V to form a coating slot. Usually one or both of these blades is adjustable with respect to to deformation so that it .can be continuously stripped from the coating wheel. On leaving the coating wheel, the gel like film is usually further dried to reduce the solvent content to a mini-'.
mum. This can be done without employing high temperatures since the structure of the gels e such that theyreadily release the volatile solvent.
As'above pointed out these gel dopes change 1 from a fiuid to a gel within a few degrees of the other so as to provide a, means of controlling the thickness of the dope stream and, therefore, the sheet being formed.
Such hoppers are very satisfactory for coating when an attempt was made to coat what is herein termed a gel dope" from such a hopper onto a cold wheel, 1. e. one which is maintained below the gelation temperature of the dope, in place of a hot wheel various unforeseen difficulties arose. The present invention will perhaps be better understood if the characteristics and nature of a gel dope" are explained. The gel dope which the present hopper is adapted to coat may be formed by dissolving at'elevated or moderately elevated temperatures certaincellulose organic acid esters such as cellulose acetate propionates and cellulose acetate butyrates and-thelike in a solvent mixture, consisting of propylene chloride and ethylene chloride or propylene chloride However,
dopes of this type temperature. In coating the ordinary type dope, heretofore employed in the .art, onto a heated wheel such small temperature variations do not produce a physical change in the dope. When the regular type hopper was employed in coating the gel dopes onto a cold wheel, the proximity of the hopper slot to the cold wheel caused a sufflcient temperature drop in the hopper lips to permit partial gelation of the dope stream and formatlon of slugs on the edges of one or both of the hopper lips. These slugs project into the dope stream and give rise to unevenc'oating and the formation of streaks in the finished product. In forming photographic film base such imperfections are of course very undesirable.
The object of the invention is an improved coating hopper which is particularly adapted for coating heated solutions onto casting surfaces which are maintained at distinctly lower .tem-- the knife like edges of the blades will contact the outflowing film forming composition. The liquid is. preferably one which is miscible withthe' film forming composition being coated.- Therefore.
there will'be no tendency for the solution to' harden at the edges of the blades since this solvent will tend to redissolve or keep in solution turn screws may be adjusted to give a flow of as shown in the drawing is to provide particles of the coating solution base which otherwise might collect on the blades. The solvent also prevents the accumulation of the dope on V nuclear particles by either insulating them with a liquid film or removing them from the proximity of the coating slot. I have found that a slow seepage of the solvent overthe blades is suflicient to preventslug formation infthe dope stream. While the solvent may be spread over the total length of the blade I have found that no difiiculty is experienced if the solvent flow is restricted to a small area at the ends of-the blades and on the back or outer side of the coating blades. Q I H g t The invention will be more clearly understood Fig. 1. is a sectional view in elevation of the preferred type of coating hopper showing the devices attached to the back of the coating by reference to the following detailed description and attached drawing in which: 1
Fig. 3 is a broken away enlarged sectional view of one of the coating blades showing the chamber for containing the solvent.
Fig. 4 is a broken away enlarged sectional view of two of the coating blades having felt strips for conducting solvent to the blades instead of a hollow chamber.
Referring to Fig. 1 there is showna double walled coating hopper I having outside walls II and I2 and inside slanting walls I! and H. The a space i 5 between the double walls may be em-' ployed as a heating chamber, if desired. At the apex of slanting walls l3 and ii there is a slot it thru which the coating solution flows to the coating wheel I 1. The ends of the hopper are shown in Fig. 2and comprise similar double walls I8 and I9 and 20 and 2| forming'heating chambers 22 and 23 respectively. As will be understood the space 24 is filled with coating solution during operation of the coating hopper.
To regulate the thickness of the sheet or'film being coated, blades 25 and 26 walls l3 and M respectively. vAs shown in Fig.
' ing surface for about are positioned on 5 1 blade 26 is fixedly attached to-wall l4 by screws hopper 24 as desired,
42, while as shown more clearly in Fig. 2, blade 5 Mountedbeneath the coating edge of the blades and attached to the blades near the ends of the coating slot are liquid distributing chambers. As
shown in Fig. 1 at 32 and 34 and in the magnifled view in Fig. -3 these liquid distributing chambers comprise small boat like members having one end'wall lower than the other. They are fastened to the'lowe portion of the knife bladesby a suitable strap 40 with the open end'of the. boat toward the coating slot. The boats will permit the solvent contained therein to flow on the outside of the coating blades and toward the coating slot 30. By regulating the flow of solvent the blades can be maintained wet with solvent during the coating operation. "Therefore, there will be no slugging on the edges of the blades since the solvent will keep the coating solution in a liquid form. The wetting solvent may be introduced into the boats in any suitable manner. One way conduits 35 in the knife blades thru which solvent can be introduced from connecting tubes 36 attached to 3 pp y.
Instead of a hollow boat as the distributing Example I A solution of parts by weight of a cellulose acetate butyrate containing 25% acetyl and 20.7% butyryl in a solvent mixture composed of 420 parts of toluene and parts of n-propyl alcohol was prepared by mixingthe ingredients with continued stirring at 70 C. The solution was then filtered to remove incompletely dissolved particles. The dope was admitted to the hopper where its temperature was maintained at about 50 C. The blades of the hopper were so adjusted as to feed a stream of the warm dope to the wheel surface of such thickness as to give an eventual film thickness of .005 inch, the wheel being maintained at a constant temperature of about 15 0. During the flow of the coating solutions thru the coating slot butyl acetate was caused to flow at a slow rate from a source not shown thru tube 36 into conduits 35 to boats 32 and 34. The solvent flows slowly out of the mouth of the boat and spreads over the outside of the coating blades adjacent the ends of the blades where slugging usually occurs and no slugging was experienced. The wheel speed that the film remained on the film formsix minutes. On coming in contact with the wheel the coating solution was transformed into a non fluid gel; and after completing somewhat more than of a'revolution on the wheel the film was stripped off the wheel and further treated by other apparatus, not shown, to remove solvent.
Example II curred. Other solvents such as ethyl and methyl acetate may be employed instead of ethylene chloride when coating this solution.
Example III A filtered solution of 100 parts by weight of a o cellulose acetate of 40.5
acetyl content in 500 parts by weight of a solvent mixture composed of 70% by weight of propylene chloride-and 30% iso-propyl alcohol was coated from my improved hopper and a lubricating solvent of methyl acetate was employed on the blades to keep-the was rotated at such a.
solution from slugging and no. slugging of the solution was experienced. I
Depending somewhat on the type of solution the following solvents may be spread on the coating blades to prevent slugging of the coating solution: ethylene chloride, propylene chloride,
methyl and ethyl acetate. In general these solvents are compatible with the solutions but are not active enough to blend rapidly with them. I have also found that more active solvents may be employed provided they are highly volatile such as, for example, methylene chloride.
It will be understood that my invention can be applied to various types of coating devices and is not restricted to the V type of coating hopper disclosed in the drawing.
My coating apparatus may be used with various coating solutions, particularly in cases where it is desirable to coat a solution onto a relatively cold sheet or film forming surface.
It will be also understood that while I have shown the solvent distributors positioned on both of the coating slot blades with some solutions it may be necessary to employ only one or the other of the distributors to achieve satisfactory sheets or films. It is, therefore, within the scope of the invention to employ a distributor on only one of the coating slot blades. Furthermore while the solvent distributors are shown in the drawings mounted on the back of the blades it will be understood that they may, within the scope of the invention, be suitably positioned atthe ends of the blades whereby they will also distribute the solvent over the portions of the blades where slugging most generally occurs.
By employing my apparatus I have successfully overcome slugging of coating solutions and am able to form. uniform sheets and films from solutions which change to solids and gels within a relatively small temperature range.
I have also found that my improved apparatus may be advantageously employed with coating solutions which coagulate to form a gel upon the wheel when subjected to moisture vapors. Such solutions are described in the copending application Serial Number 143,302 filed May 18, 1937.
The apparatus may also be advantageously employedin coating solutions which form gels by loss by evaporation of one or more of the component solvents of the coating solution.
If desired the apparatus may even be used for coating solutions which do not gel but set by simple progressive evaporation of the solvents in the solution.
In these instances the wetting of the edges of the coating slot by a suitable solvent will prevent slug formation.
1. In a coating hopper for coating film support, a pair of thin edged blades disposed to form a V and spaced apart to form an open coating slot between the blades at the apex of the V through which coating solution may flow, and means mounted on the back side of each blade near the end of each blade and near the apex of the V, for spreading another liquid on the edges of the blades at their end portions during the coating operation.
2. In a coating hopper for coating film support, a pair of thin edged blades disposed to form a V and spaced apart to form an open coating slot between the blades at the apex of the V through which coating solution may flow, and liquid distributing members mounted on the back side of each blade near the end of each blade and near the apex of the V, for spreading another liquid on the edges of the blades at their end portions during the coating operation.
3. In a coating hopper for coating film support, a pair of thin edged blades disposed to form a V and spaced apart to form an open coating slot between the blades at the apex of the v through which coating solution may flow, and permeable liquid distributing members mounted on the back side of each blade near the end of each blade and near the apex of the V, for spreading another liquid on the edges of the blades at their end portions during the coating operation.
CHARLES R. FORDYCE.