US 3524758 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
8, 1970 R. c. MILLER 3,524,758
METHOD OF REDUCING LACQUER THROWING Filed Sept. 1. 1966 INVENTOR ROBERT CARLETON MILLER ATTORNEY United States Patent US. Cl. 117-102 3 Claims ABSTRACT OF THE DISCLOSURE In a method for coating webs whereina freshly coated web is passed through doctor rolls, an improvement for reduced lacquer throwing by directing-astream of air toward the exit nip of the doctor rolls.
The continuous coating of the surfaces of continuous flexible webs of organic polymeric material such as, for example, a web of regenerated cellulosg has been carried out for a number of years by passing the' film first through a dip tank containing a liquid coating' composition, and then passing the coated film through adoctoring device for regulating the amount of coating composition retained on its surfaces, and followed next by passing the coated film through a smoothing device to smooth out and improve the appearance of the coating, and finally passing the coated film through a heated drying zone.
A particularly acute problem that has been encountered with such coating devices especially as higher coating speeds have been approached is the tendency for coating liquid to be carried through the nip between the regulating doctor rolls outside the beads of the web being drawn therethrough with the result that small droplets of the coating liquid are propelled from the nip of the doctor rolls in the direction of web travel. The liquid droplets that are virtually flung from the doctor rolls become affiXed to various supporting members and machinery and often come in contact with the coated web, with the result that appearance and quality of the coated web are adversely affected. In many instances where all other elements of the coating process would permit the attainment of higher production speeds, such speeds cannot be practiced because of the occurrence of the'above described lacquer throwing. Accordingly, it is the principal object of the present invention to provide an iniprioved coating process and apparatus which is free of the above described difficulties.
According to the present invention there is provided a method of manufacture for coating webs which comprises passing a web having a liquid coating on at least one surface thereof through the nip between spaced doctoring rolls and directing a jet, blast or i's'tream of gaseous material, preferably air, into the exit nip of said doctor rolls at the marginal portions of the web extending substantially from each edge bead thereof to the journal of said rolls thereby to maintain a uniform coating over substantially the entire surface of said web and preclude excess coating material from being flung into the atmosphere above said nip of said doctor rolls and being deposited on the metered coating.
According to the present invention, there is further provided an apparatus for coating webs comprising means for continuously advancing a continuous web having a liquid coating on at least one surface thereof through the nip between spaced doctoring rolls and means associated with said doctor rolls for directing a jet, blast or stream of gaseous material into the exit nip of said doctor rolls.
The nature and advantages of the present invention will be more clearly understood from the following more detailed description thereof and the several views illustrated 3,524,758 Patented Aug. 18, 1970 in the accompanying drawings wherein like reference characters refer to the same parts throughout the several views and in which:
FIG. 1 is a vertical cross-sectional view of the general arrangement of the apparatus of the present invention; and
FIG. 2 is a vertical front elevation view of the salient features of the apparatus of the present invention.
The apparatus herein disclosed in illustration of the invention includes at least one pair of spaced doctor rolls 10 that are suitably spaced from each other to provide a nip 11 therebetween for passing therethrough a web or film 12 coated on at least one surface thereof in any suitable manner as by passing the web or film over suitable idler rolls as 13 and 14 and through a dip tank 15 adapted to contain a liquid coating composition such as 16 therein. The doctor rolls 10 are suitably journaled for rotation in an appropriate frame such as carriage frame 17, and may be positively driven by any suitable means as, for example, a conventional chain and sprocket drive assembly (not shown). The web is continuously advanced through the coating apparatus by any suitable means, not shown, as, for example, driven heated rolls located in a conventional dryer assembly downstream of the coating apparatus wherein the coating composition on the web is dried. 1
Further according to the present invention, means are provided in the apparatus for directing a stream, jet or blast of gaseous material into the exit nip of the doctor rolls. The means contemplated in the preferred embodiment of the invention comprise a pneumatic knife 18 so constructed and arranged as to direct a stream of gaseous material such as air into the exit nip of the doctor rolls 10 and over an area extending from at least 1 inch inside the edge bead 19 of the web being coated and extending essentially to the journal of the doctor rolls. As shown in greater detail in FIG. 2, the pneumatic or air knife 18 may be attached in any suitable manner to frame 17 and is positioned near each edge bead 19 of web 12. The pneumatic knife may be of any suitable configuration and is preferably of hollow rectangular configuration having rectangular sides 20 and 21 joined as at sides 22 and 23 to form a small box or plenum chamber. The pneumatic knife is connected to a supply of air by any suitable conduit means attached therebetween, and the air is directed out from slot 24 therein and into the exit nip 25 of the doctor rolls.
In general, air preferably exits from the pneumatic knife at a pressure or velocity sufficient to prevent any excess coating liquid that is pumped through the open nip spacing between the doctor rolls and the outer edge beads of the web from being thrown beyond the doctor rolls in the direction of web travel. The air pressure or velocity is adjusted so that the coating liquid beyond the bead of the web is held or confined to the surface of the doctor rolls and is carried around the rolls rather than becoming separated therefrom as in droplet form and flung in the direction of web travel.
In the preferred embodiment of the invention, the stream or blast of air from the air knife is directed over an area extending from at least 1 inch inside the bead of the film to essentially the journal of the doctor roll. The air knife itself is preferably about 6 to 12 inches long and the exit slot thereof is about 0.003 inch in width, and the air pressure supply to the knife is about 2 p.s.i.g. (minimum) and 10 p.s.i.g. (maximum). The air stream or blast is directed into the nip at an angle of about 15 from the line of travel of the web through the doctor roll.
As an illustration of the effectiveness of the above-described arrangement, it is found, in the application of a nitrocellulose coating lacquer to cellophane, that without the arrangement the maximum speed that can be used for commercial production (without the production of unacceptable film because of lacquer spots and smears being collected on the coated film) is about 310 yards per minute. In direct contrast, operating the apparatus of the invention as with the air knife arrangement above-described, highly acceptable coated cellophane film can be produced at speeds of 350 yards per minute without lacquer spotting of the coated film.
As a further illustration in the application of coating compositions comprising vinylidene chloride copolymers, the maximum production speed attainable without the use of the air knives is about 290 yards per minute. In direct contrast, operating the apparatus of the present invention as with the air knife arrangement above-described, acceptable coated film is produced at a production speed of 330 yards per minute.
What is claimed is:
1. A method of manufacture for coating webs which comprises passing a web having a liquid coating on at least one surface thereof through the nip between spaced doctor rolls and directing a stream of gaseous material to impinge on the web and doctor rolls at the exit nip of said doctor rolls at the marginal portions of a coated surface of said web from a point extending from within each edge bead thereof to about the journal of said doctor rolls thereby to maintain a uniform coating over substantially the entire surface of said web.
2. The method of claim 1 wherein the stream of gaseous material is directed into the exit nip at an angle of about 15 from the line of travel of the web through said doctoring rolls.
3. The method of claim 1 wherein the gaseous material directed into said exit nip is air.
References Cited UNITED STATES PATENTS 157,068 11/1874 Edwards. 2,229,921 1/1941 Goff et al. 2,322,533 6/1943 Mayer.
ALFRED L. LEAVITT, Primary Examiner J. A. BELL, Assistant Examiner us. 01. X.R. 118-63