US2334102A

US2334102A - Coating equipment

Info

Publication number: US2334102A
Application number: US354242A
Authority: US
Inventors: Toivo A Kauppi; Roger W Kolderman; Richard S Mcclurg
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1940-08-26
Filing date: 1940-08-26
Publication date: 1943-11-09
Anticipated expiration: 1960-11-09

Description

Nov. 9, 1943. T. A. KAUPPI ETAL 2,334,102

COATING EQUIPMENT Filed Aug. 26, 1940 Ffam Supply 23 To Wilda- INVENTORS 70/70 A. KAl/PP/ 90652 W. ZOIDEIMAA/ BY BIC/I120 J, MtCU/IG M Kama! ATTORNEYS Patented Nov. 9, 1943 uurrso coa'mzo murmur oivoA.Kanppi, Roger W. Kolderman, and Bichi ard S. McGliu'I, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigan Appiication August 26, 1940, Serial No. 34,242.

-2 Claims.

Thisinvention relates to a specific improvement in coating equipment of the rotating doctor a rod type of .particular use for coating flexible sheet material. It relates especially to rotating doctor rods for use in applying hot, molten coating compositions to flexible sheet material.

As an instrument for removing excess coating composition and for distributing a coating more or less uniformly over flexiblesheet material, the

. doctor knife is well known. In the case of lac quers and similar liquid coating compositions which are capable of smoothing themselves by flowing after having been doctored, the knife is a satisfactory spreading instrument. When, however, the coating composition is of waxy or resinous character, and is applied from the molten state, the doctor blade almost invariably leaves longitudinal scratches on the surface of coated sheet material which has passed across the blade. Thus, it has been stated that the doctor knife is satisfactory for spreading base coats but not for the surface or finish coats when applying plural coatings. It often happens that only one coat is. to be applied, and that the doctor knife is not adapted to satisfactory production of the desired smoothness in the surface coating.

In addition to the doctor knife, and as a substitute therefor, doctor rollers have been proposed and employed. It has been proposed to use such rollers both driven and undriven. The driven rollers of the past have been said tobe objectionable in that they fail to deposit even coatings, and articles coated thereby have a splotchy surface. It has been found that in most instances this is true, and that idle or undriven rollers, whether employed singly or in pair to produce a steam roller action on the fresh coating, are also ineffective to produce a smooth blemish-free surface coating when operating on hot-melt coatings of the resinous type or of the wax-cellulose derivative type to be described hereinafter. The idle rollers heretofore specifically described have been from 1 to 2 inches in diameter (see '0'. S. Patent 2,196,894, Boice) while both idle and driven rollers, such as are employed in the Mayer paper waxing machines, have generally been at least inch, and usually over /2 inch in diameter. The tendency has been toward the use of larger rollers to assure less frequent recurrence of the reproduction of roller 7 surface defects on the coated sheet.

A particular problem arises when coating thermoplastic sheet material. When thermoplastic foils are to be given a thin coating, especially with molten compositions, the tension ordinarily applied to paper in a like coating operation stretches and distorts the foil. Even when using melts which can be applied at or below 100 0.,

'such thermoplastic foils must be coated while under very low tension. Under such circumstances neither the customary doctor knives nor the customary doctor rollers are satisfactory, as they all give uneven, streaked coatings. Both the doctor knife and the doctor roller, as ordinarily employed, pick up and retain in the pool of coating material on which they operate any minute particles of grit or other substance which is hard enough to abrade the freshly deposited coating,

and these retained foreign bodies augment the I already undesirable scratching effect of the spreading instruments themselves.

It is, accordingly, among the objects of the present invention to provide a coating equip.- ment whereby smooth, unblemished thin coatings may be applied to'fiexible sheet material. It is another object to provide a means as aforesaid, operable to coat uniformly a thermoplastic foil without subjecting the latter to such tension as will distort the same at the coating temperature. A further object is to provide a coatinglevelling device particularly adapted to operate on coatings of the resinous or waxy melt type to smooth and distribute the same without producing any material surface blemishes thereon. Specific objects include the provision of a particular form of doctoring device capable of accomplishmg the foregoing ends, and the provision of a preferred mode of employing such a device in a coating operation. These and other objects, and a mode whereby they may be realized, are embodied in the following description of the invention.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the means hereinafter fully described and particularly pointed out in the claims, the annexed drawing and the following description setting forth in detail'certain means for carrying out the invention, such disclosed means illustrating, however, but some of the various ways in which the principle of the invention may be used.

In the said annexed drawing, wherein like numerals are employed to designate like parts:

Figs. 1 and 2 are diagrammatic representations, in side sectional elevation, of typical coating apparatus for applying surface coatings to flexible sheet material, embodying the improvement of the invention;

Figs. 3 and 4 areperspective views of two different modifications ofea coating machine accessory, embodying the doctor rod of the invention;

Fig.5 is a cross-sectional elevation taken along the line 5-5 of Fig. 3, illustrating one particular assembly embodying the doctor rod: and

Fig. 6 is a cross-sectional elevation taken along the line 66 of Fig. 4 illustrating one assembly alternative to that shown in Figs. 3 and 5.

According to the invention, one or a plurality of mechanically driven, rotating and heated cylindrical doctor rods rigidly supported throughout their length, are employed to smooth and distribute coatings on flexible bases, each such rod being of a diameter not to exceed fig inch, and preferably being 54; inch or less in diameter. The rods may be driven either with or counter to the direction of travel of the sheet being coated, and for best results, when, as in the preferred embodiment, two such rods are employed on each surface being coated, the first is driven counter to the sheet and the second in the same direction as that of the sheet. Numerous tests have shown that rods of $4 inch diameter and larger have many or all of the failings inherent in the rollers described in the prior art, while at or below the critical diameter of inch, they operate to form a smooth, flawless surface finish in fulfilment of the objects hereinb'efore stated. The suprisingly critical nature of this dimensional limitation on the rotating rod diameter makes necessary the provision of means for rigidly supporting the rod throughout its length, as without such support it has been found that the shaft tends to flex when rotated, departing from the coating plane, and failing to exert any continuous smoothing or leveling of the applied coating. Provision should be made for heating the rotating rod, especially when operating on waxy or resinous melt coatings, so that any of the composition which may already have congealed before reaching the smoothing rod may be softened and distributed in the instant of passage across the rod.

A preferred mode of carrying out coating operations utilizing the rotating doctor rods. of the present invention will be described with reference to the accompanying drawing, and with particular respect to the applicationof waxy or resinous melt coatings on flexible sheet material such as paper, Cellophane or ethyl cellulose foil. Two coating devices are illustrated diagrammatically in the drawing. That in Fig. 1 employs a roller coating operation and that in Fig. 2 coats by immersion of the sheet in a coating bath. A flexible sheet material It may be unwound over rollers II and I2, and past applicator roll l3 which is continuously acquiring a thin coating of the fluid composition l5 from roller II which dips into the bath contained in a suitably heated pan i6 having a drip catcher l1 extending under

rods

19 and 20. The sheet l acquires a thin coating from roller l3, and then passes around tension roller l8 and over a pair of rotating doctor rods l9 and 20, to be more fully described hereinafter. After passing the doctor rods, the coated sheet 23 is passed around and over chill rolls 2! and 22 to congeal the smooth coating, Alternatively, sheet ID are to be coated, the said sheet I is passed directly into and beneath the surface of the fluid composition i contained in the suitably heated pan l6 as shown in Fig. 2. The flexible sheet is withdrawn from the bath after passing around roller 24 and then both sides of the sheet are exposed to the smoothing action of a plurality of doctor rods l9, I9, 20 and 20', operating in pairs disposed on each side of the sheet. The said sheet with its smoothed double coating is passed through a cooling zone and thence over a cold roller 22 to a suitable wind up.

The rotating-doctor rods illustrated diagrammatically in Figs. 1 and 2 are those shown in perspective in Fig. 4 and in cross-section in Fig. 6. In said Figs. 4 and 6, the rotatable doctor rod or when both sides of the flexible l9, which is inch or less in diameter, is rigidly supported throughout its length by an extra heavy steam tube 34 having a suitably profiled channel 38 machined in the wall 33 thereof. The said wall 33 is preferably further machined to provide scraper edges 35 to remove any excess coating which may tend to adhere to rod I9, and to prevent transport of the coating into channel 38. In assembly, rod l9 having a bearing 30 at each end and a driving means, such as a belt driven pulley 39, whereby it may be rotated, is

supported in the channel 38 of steam tube 34. Steam may be admitted to the tube through valve 40 and pipe 31. Channel 38 is machined to provide a close fit between rod i9 and tube 3!, much like the fit between a shaft and a journal hearing. The spacing in Fig. 6, like that in Fig. 5, is greatly exaggerated for ease of understanding. The rod l9 and supporting tube 34 are mounted in suitable end supports 3| provided with openings for bearing 30 and for the steam coupling 31. In operation, rod I9 is rotated while steam or other heating fluid is passed through the tube 34.

Another assembly of the rotating rods of the present invention is illustrated in Figs. 3 and 5. The rod I 9 is supported longitudinally'by rigid member 21, to which are secured apair of rigid scraper blades 26 having knife edges 32 in contact with the rod l9, furnishing lateral support and rigidity thereto. The blades 26 are mounted on bar 21 suitably by means of bolts 28. In a preferred embodiment, bar 21 and blades 26 are electrical and thermal conductors, which may be heated, for example, by means of resistance bar 29 mounted on the rigid member 21, and heated by current supplied through leads 36. This type of assembly is mounted on suitable end supports 3| which are preferably of insulating material, the end of rod l9 extending through the bearing 30 in support 3| and being driven in the desired direction by the pulley 39. The support 21 and scraper blades 26 hold rod l9 rigidly in line, preventing any whi or flexing thereof during its rotation against the flat sheet being coated.

When operating a coating device embodying the small rotating rods of the present invention, especially when coating thermoplastic foils under low tension to avoid distortion and wrinkling, the first rod or group of rods oper-- ating on a coated face of the foil is preferably rotated against the direction of travel of the sheet, at a speed sufljlcient to do all the leveling and scraping required to bring the depositedcoating to the required thickness. The second rod or group of rods is preferably rotated in the direction of travel of the sheet but with a surface speed less than the linear speed of the sheet, thus smoothing the coating.

A smooth, flawless coating was applied to one face of an ethyl cellulose foil, using a set-up similar to that shown in Fig. 1, using a waxy melt as the coating composition. The melt consisted of Per cent Ethyl cellulose 10 B'eckacite 2000 (pure phenolic resin) 13 Stabellite A-1 (hydrogenated rosin) 5 Paraflin wa 28 Opal wax (hydrogenated castor oil) 44 The ethyl cellulose film was moved through the machine at 60 feet per minute. The doctor rods each were of V inch in diameter, the first rotating against the foil. at 250 revolutions per minute and the second rotating with the rbn at 60 rollers, e. g; of inch to 2 inches diameter, were substituted for the rods here employed, blotchy and streaked coatings were usually obtained reregardless of whether the rollers were idle or driven.

For best results the rod employed should be sufllciently hard and abrasion-resistant to avoid being scratched byv grit particles which may come in contact therewith during coating operations. Usually a rod of Brinell hardness over 500 is preferred, though this is not an invariably essential limitation.

We claim:

1. In apparatus for coating flexible sheet ma- 'R. P. M. The finished article had'a smooth V flawless coating 0.0001 inch thick. When larger terial, the combination of a rotatable smooth doctor rod of diameter not exceeding inch disposed transversely of the path of the'sheet material to be coated, means to support the doctor rod rigidly throughout its length, means for heating' said rod and means forrotating the rod.

2. In apparatus for coating flexible sheet ma.- terial, the combination of a plurality of rotatable smooth doctor rods of diameter not exceeding inch disposed transversely of the path of the sheet material to be coated,- means to support each of the rotatable doctor rods rigidly throughout its length, means for heating said rods and means for driving them independently of one another.

TOIVO A. KAUPPI. ROGER W. KOLDERMAN. RICHARD S. McCLURG.