US3203392A - Freely supported rod for smoothing coating composition applied to webs - Google Patents

Description

Aug. 31, 1965 B. s. EDWARDS 3,203,392

FREELY SUPPORTED ROD FOR SMOOTHING COATING COMPOSITION APPLIED TO WEBS Filed June 27. 1961 \HMW 16 .77

INVENTOR H 11-; 7:: 1111-1": BEAW'WO 12/ I: I if United States Patent FREELY SUPPDRTEE) ROD FQR SMGGTEHNG CUA'HNG CQMPGSKTHUN APPLIED T0 WEBS Eernard S. Edwards, Tonawanda, N.Y., assignor to E. i.

du Pont de Nemours and 'Company, Wilmington, Deb,

a corporation oi Delaware Filed June 27, 1961, Ser. No. 119,847 4 Claims. (til. lid-HS) This invention relates to the coating and smoothing of continuous flexible webs and more particularly to an apparatus and process for smoothing a coating composition applied to a continuous regenerated cellulose film.

It has long been common practice to apply various types of coatings to web materials to impart certain desirable properties to the webs such as moisturepr-oofness, heat scalability or to improve their dimensional stability or surface properties. In general, it is desirable that a coating applied to a web be uniformly distributed and smooth so that all portions of the web will have uniform properties and good appearance. This is particularly true in the case of a coated transparent film which is to be used to wrap material such as foods and other consumer goods, since detects and non-uniformity of the coating adversely affects the optical properties of the film, hence, detract from the eye appeal of goods wrapped in the film.

Coatings are usually applied at high speeds by passing a continuous web into a solution or dispersion containing the coating composition, then through a metering device which regulates the amount of liquid coating to be left on the web. The coating is then rapidly smoothed while it is still in the fluid state and the web is finally dried to remove the coating solvent or dispersing liquid. In the coating of regenerated cellulose films, with which the present invention is chiefly concerned, it has been the general practice to met-er the coating bath onto the film by the use of a set of doctor rolls and then to smooth the coating as much as possible with one or more driven smoothing rolls which rotate in the direction opposite to the direction of travel of the film.

Although doctor rolls and driven smoothing rolls have proven to be the most convenient and economical combination of equipment for metering and smoothing coatings onto thin films of regenerated cellulose, films coated by this means usually have an appearance defect often referred to as coating striations which are minute grooves or chambers in the coating that appear as a series of essentially parallel lines in the longitudinal direction of the web.

Close examination of the coating process shows that coating striations originate at the doctor rolls as the film and roll separate and are caused, apparently, by the breaking up of the liquid coating composition which is being attracted to both the film and the doctor rolls. Some of the coating composition remains on the doctor rolls in the same striated pattern as that left on the film. The size of the striations produced by the doctor rolls is determined by the size of the doctor rolls being used; large diameter doctor rolls produce coarse striations while small doctor rolls produce fine striations. Although fine striations are less noticeable, hence less objectionable than coarse striations, it is not practical to use doctor rolls less than about 8 inches in diameter when coating at high speeds with viscous coating baths, since the hydraulic pressure developed at the nip of the doctor rolls would cause them to bend and thus allow more than the desired amount of coating to be applied to the center portion of the web being coated.

The driven smoothing rolls are used to brush down the coarse striation pattern produced by the doctor rolls, but

3,2d3392 Patented Aug. 31, 1965 ing rolls leave smaller striations that are caused by the splitting of the coating bath as the film and smoothing rolls separate. Although it would be desirable to use very small driven smoothing rolls, since they would leave a finer less objectionable striation pattern, it is necessary that driven smoothing rolls be suiiiciently rigid to avoid whipping as the roll is rotated under pressure against the film. In actual practice, it has been found necessary to use a roll of at least 2 inches in diameter to avoid this difiiculty.

Aside from the fact that driven smoothing rolls leave a striation pattern of their own, there are several other factors which further prevent them from being completely satisfactory as a means for smoothing fiuid coatings applied to film.

The coating bath which the smoothing rolls pick up from the film as the film and roll separate is left on the smoothing roll in a striated condition. This bath is printed back onto the film on the next revolution of the smoothing roll; thus, leaving the liquid coating on the film more striated than it would be otherwise.

It is usually desirable to use a series of driven smoothing rolls to accomplish as high a degree of smoothing as possible; however, the number of rolls that can be used effectively is often limited because of the rapid thickening of the coating bath as a result of either evaporation or penetration of the coating bath solvent or dispersing liquid into the cellulose base sheet. There is, therefore, a definite need for some means to more effectively smooth liquid coatings q-uiclcly during a short span of travel.

To obtain the optimum degree of smoothing with driven smoothing rolls, it is necessary that they be torced against the moving film with a constant pressure sufi'icient to brush down the striations left by the doctor rolls, but not with enough pressure to squeegee oil the liquid coating from the film. Once the coating operation has begun and the position of the driven smoothing poll has been fixed, any motion of the film, such as fluttering, or any changes of tension on the film will cause uneven smoothing of the coating along the lengthwise direction of the film. It appears that heretofore no satisfactory means has been devised to overcome this problem.

To summarize the problems encountered in the prior art, it can "be stated that the use of doctor rolls to meter fiuid coatings onto fiexible film and driven smoothing rolls to smooth the coating, although the most practical known means of carrying out these operations, has not been completely satisfactory for the following reasons: (1) doctor rolls leave the liquid coating on the film as coarse striations, (2) driven smoothing rolls produce smaller striations and also reprint part of the original striation pattern left by the doctor rolls, (3) there is 'a limited amount of time and distance of film travel in which smoothing rolls can be advantageously used, and (4) changes in tension or movements of the film result in uneven pressure of the smoothing rolls against the film and thus cause uneven smoothing of the coating along the length of the web.

It is an object of this invention to provide for the coating and smoothing of continuous flexible webs.

It is a further object of this invention to provide an apparatus and process for smoothing a coating composition applied to a continuous regenerated cellulose film.

It is a still further object of this invention to overcome the above-mentioned dii'llculties and to provide an apparatus and process for effectively and rapidly smoothing liquid coatings during a short span of travel of the film Web. These and other objects will appear hereinafter.

These and other objects of this invention are accomplished by the improved process which comprises: contacting the film to which a coating composition has been applied with at least one freely supported roll, the roll supported in the space between the film and a positively aaoasaa driven, rotating roll disposed transversely to the film and free to rotate about its axis and to change its position in space as a result of unbalanced forces imparted to it by the positively driven rotating roll and the film.

The apparatu for carrying out the process of this invention comprises: means for applying a coating composition to a web, means for regulating the weight of the coating composition applied, and means for smoothing the coating on the web, the improvement in the last means comprising at least one positively driven, rotating roll disposed on each side of the web transversely to the web and at least one cylindrical body of smaller diameter than the positively driven roll supported in the space between each positively driven rotating roll and the web.

A particularly preferred apparatus comprises: a vessel adapted to contain a coating composition, a stationary guide rod disposed in the vessel below the level of the coating composition and adapted to guide the film web therearound, two rotatable rolls disposed above the guide rod and above the level of the coating composition spaced from each other in juxtaposition to provide a nip, the film web passing between the rolls and through the nip, the rolls adapted to regulate the amount of coating composition applied to the emerging film web, a positively driven rotating smoothing roll of at least two inches in diameter above the rotatable rolls disposed on each side of the film web transversely to the web and adapted to rotate against the film web in the direction opposite to the direction of travel of the film web and a rod of up to 1 /2 inches in diameter supported in the space between the film and each positively driven roll and free from any rigid connections.

It is preferred that the positively driven, rotating roll of this invention be a conventional driven smoothing roll made of a non-corrosive material having a polished surface and having a diameter of at least two inches and a length at least slightly longer than the width of the widest web to be coated. It is also preferred that the driven smoothing roll be caused to rotate against the web being coated in the direction opposite to the direction of travel of the film. Although the speed of the driven smoothing roll is not critical, it is preferred to rotate the roll at a speed 5 to 100% of the speed of the moving film. Most preferably, the driven roll should be caused to rotate at a speed 5 to 65% of the speed of the moving film.

The preferred cylindrical body of this invention is a rod made of a non-corrosive material having a polished surface, a uniform diameter from end to end of not more than 1 /2 inches, and a length slightly longer than the width of the widest Web to be coated. It is also preferred that the cylindrical body have just sufiicient weight to cause it to remain in essentially continuous contact with both the positively driven rotating roll and moving web.

Since the preferred cylindrical body is a rod without any rigid connections to any part of the coating machine, it will be referred to hereinafter as a floating rod.

The preferred floating rod of this invention is a rod of uniform diameter from end to end; however, a hollow tube may also be used providing its diameter and wall thickness are such that the weight of the tube is satisfactory. In some instances, it might be advantageous to use a floating rod which does not have a uniform diameter from end to end. For example, if a coating machine is to be used to apply a wide variety of coating compositions to a wide variety of Webs, it would be desirable to provide a means for readily changing the effective weight of the floating rods to give optimum smoothing for each set of coating conditions. This could be accomplished rather conveniently by using floating rods having small pivoted ends which are equipped with essentially frictionless bearings that are attached to adjustable springs which do not limit the direction of motion of the floating rod.

It is preferred to employ a floating rod on each side of the film to effectively and rapidly smooth the coating compositions applied; however, if the film when it emerges from the nip of the doctor rolls is only striated on one side, a floating rod need only be employed on that side.

The preferred floating rod has a polished surface; however, in certain instances it may be desirable to use a floating rod having a series of small, closely spaced grooves or threads around its circumference. In general, when more than one floating rod is being used, it is preferred that they all be of the same diameter; however, the floating rods may differ in size, providing each has the proper weight.

Although the process of this invention is particularly useful in smoothing coatings applied to flexible film in the form of liquid solutions or dispersions, it is generally applicable for smoothing fluids being applied to web materials such as synthetic polymeric films, paper and the like.

FIGURE 1, which shows a cut-away end view of the coating compartment of a machine used to apply liquid coating compositions to both surfaces of continuous webs of film, is the preferred arrangement of the equipment necessary for carrying out the process of this invention.

FIGURE 2 shows a cut-away side view of a coating compartment of a coating machine which is satisfactory for the smoothing of low viscosity liquid coating compositions which have been applied to both sides of a continuous web.

In the coating operation illustrated in FIGURE 1, a web of film 10 is brought into the coating compartment and passes in a downward direction into a coating bath 11 contained in tank or vessel 12. The film 10 then passes around the stationary guide rod 13, then travels upward out of the bath 11 carrying with it excess bath on both of its surfaces. The film 10 next passes through the nip of the juxtaposed rotatable large doctor rolls 14- and 15 which regulate the amount of coating bath left on the film. As the film 10 breaks contact with the doctor rolls 14 and 15, the liquid coating bath which is attracted to both the film 10 and doctor rolls 14 and 15 breaks up and some of it remains on the doctor rolls 14 and 15, however, the larger portion of the bath remains on the film surfaces. The liquid coating bath remaining on the film 10 as Well as that left on the doctor rolls 14 and 15 is heavily striated. The pattern of the striations on each surface of the film 10 corresponds to the pattern of the striation of the bath remaining on the doctor roll with which that surface of film was in contact. The doctor rolls are mounted on rotatable shafts which are adjustable in the horizontal plane for regulating the nip between the doctor rolls and providing the desired coating thickness.

The film 10 having the metered amount of liquid coating on its surfaces, in the form of coarse striations, passes upward until it contacts the first driven smoothing roll 18 which is rotating in the direction opposite to the direction of travel of the film 10. As the film passes the driven smoothim roll 18, the coarse striations left on that surface by doctor roll 14 are brushed out to a degree depending upon the pressure of the smoothing roll 18 against the film 10 and the viscosity of the coating bath; however, as the film 10 leaves the driven smoothing roll 18, a new set of striations is left on the film 10 because of the breaking up of the coating bath which is being attracted to both the smoothing roll 18 and the film 10. Since the driven smoothing roll 18 is considerably smaller in diameter than doctor roll 14, the striations left on the film It) by the smoothing roll 18 will be much finer and less noticeable than the striation left by doctor roll 14.

As smoothing roll 18 rotates in the direction opposite to the direction of travel of the film 10, it will pick up a quantity of the bath from the tops of the coarse striation ridges produced by the doctor roll 14. This coating bath is reapplied to the film 10 on the next revolution of smoothing roll 18, but only after being smoothed somewhat by the action of floating rod 16 which is rotating in contact with driven smoothing roll 18.

The film then passes upward a very short distance and contacts the small floating rod 16 which is being freely supported in space by the film 10 and driven smoothing roll 18. As a result of the combined effect of the motions of the film 10 and driven smoothing roll 18, the floating rod 16 is caused to rotate in the same direction as the film 10 is traveling. The weight and diameter of the floating rod 16 are such that the floating rod 16 exerts sufficient pressure against the film It) to brush down the striations in the coating left by the driven smoothing roll 18, but not with sufficient pressure to squeegee off coating bath from the film '10 or to cause the film 10 to raise the floating rod 16 out of contact with driven smoothing roll 18.

Since the floating rod 16 is being supported solely by the driven smoothing roll 18 and the film 10 against the force of gravity, it is free to shift its position in space to follow any movement of the film 10 which might result from vibrations or change in tension of the film 10, hence, tending to exert a uniform smoothing action along the length of the film 10.

As the film 1t) and floating rod 16 separate, there is again a splitting up of the bath to leave striations on the surface of the floating rod 16 and film 10; but due to the vary small diameter of the floating rod 16, the striation pattern is extremely fine. A small portion of the coating bath is carried along on the surface of the floating rod 16 in the same finely striated pattern. This bath is reprinted onto the film 10 on the next revolution of the floating rod 16, but only after being smoothed on its passage between floating rod 16 and driven smoothing roll 18.

After the film 10 leaves floating rod 16, it passes upward a short distance and the liquid coating on the other side of the film 11B is smoothed in a similar manner by driven smoothing roll 19 and floating rod 17.

Since the surface tension of the coating bath generally dissipates the very fine striations left by a floating rod, it is usually necessary to use only one floating rod for each surface of the film being coated. In some instances, however, particularly when a coating bath of rather high viscosity is being used, it might be advantageous to use more than one floating rod per film surface being coated.

Although the coating process described above and illustrated in FIGURE 1 is the preferred general procedure for carrying out the process of this invention, there are many other possible arrangements of the necessary equipment which might be used successfully under certain favorable conditions. As can be seen from FIGURE 2, no driven smoothing rolls are necessary, since the floating rods 16 and 17 are suspended in space by the web 10 and the doctor rolls 14 and 15. When this particular arrangement is used, it is preferred that the doctor rolls 14 and 15 be rubber coated to increase the friction between them and the floating rods 16 and 17, thus causing the floating rods to rotate in the direction opposite to the direction of travel of the film.

The invention will be more fully understood by referring to the following examples:

Example I A web of softened regenerated cellulose film which. had been produced by the well known viscose process was coated with 15% toluene-methyl ethyl ketone solution toluene, methyl ethyl ketone) of a copolymer of vinylidene chloride and acrylonitrile (91% vinylidene chloride, 9% acrylonitrile) using the coating apparatus and process of the present invention illustrated in FIG- URE 1. The liquid coating composition was metered onto the film by the 8 inch diameter doctor rolls 14 and 15 then smoothed by the combined action of the 2 inch diameter driven smoothing rolls 18 and 19, and the /2 inch diameter floating rods 16 and 17 while the film passed through the coating compartment at a. speed of 200 yards per minute.

After being dried in the usual manner, the coated film was inspected and found to be free of objectionable coarse striations. Although there were very fine striations, the film was judged to be saleable as first quality film.

The floating rods 16 and 17 were removed from the coating machine and another roll of softened regenerated cellulose film was coated with a coating bath identical to that described above. After being dried, this control sample of coated film Was inspected and found to be coarsely striated. The film was judged to be saleable only as second quality film, since the coarse striations were so conspicuous.

Example 11 A sample of softened regenerated cellulose film which had been coated on both surfaces with a copolymer of vinylidene chloride and acrylonitrile as described in Example I was treated with an aqueous sizing solution (.75 sodium lauryl sulfate) to improve its surface properties. The treatment was carried out using the apparatus and process of the present invention as illustrated in FIGURE 2. The film was passed through the coating compartment at a speed of 200 yards per minute, while the solution was metered onto the film by the 6 inch diameter, rubbercoated doctor rolls 14 and 15 and the sizing solution was smoothed by floating rods 16 and 17 which were of A2 and inch in diameter, respectively, and which were being driven in the direction opposite to the direction of travel of the film by the corresponding rubber-coated doctor roll. After drying, there were no visible size striations on the surface of the resulting film.

The floating rods 16 and 17 were removed from the coating machine and another roll of vinylidene chloride/ acrylonitrile copolymer coated film was sized as a control sample using a similar sizing solution. The resulting film had size striations on both surfaces which appeared as faint white lines in the longitudinal direction of the web, hence it was not saleable as first quality film.

The coating machine was again modified by installing two conventional 2 inch diameter driven smoothing rolls, arranged so that during a coating operation one roll would contact one surface of the film while the other roll would contact the opposite film surface. A second control sample of film was prepared using this modified coating apparatus to apply a similar aqueous sizing solution to a roll of vinylidene chloride/acrylonitrile copolymer coated film. During the operation, the film was passed through the coating machine at 200 yards per minute while the smoothing rolls were being driven at 25% of the film speed, but in the reverse direction. The resulting film was free of size striations; however, the surface of the coating was severely scratched, apparently caused by abrasion of the driven smoothing rolls against the film. This control film sample was not saleable as first quality film due to its poor appearance.

Although the chief advantage of this invention is to provide a means for efiiciently smoothing liquid coating compositions applied to web materials rapidly during a short span of travel while the coating is still fluid, this invention also makes it possible to use solvents for coating compositions which are more volatile than those used heretofore. Another advantage of this invention is that it permits the use of higher coating speeds than those normally used, since the greater effectiveness of the fioating smoothing rods allow smoothing to be accomplished rapidly in a very short distance of travel of the web and with little increase in tension. Since a floating rod of this invention is less expensive than a conventional driven smoothing roll, yet provides at least as much smoothing as a series of driven smoothing rolls, the use of this invention has an additional advantage in that the cost of the coating equipment is reduced considerably.

What is claimed is:

1. In an apparatus for coating a continuous web having means for applying a coating composition to said web, means for regulating the weight of the coating composition applied, and means for smoothing the coating on said Web, the improvement in said last means comprising at least one positively driven, rotating roll disposed on each side of said web transversely to said web with each roll in circumferential contact with said web and at least one cylindrical body of smaller diameter than said positively driven roll freely supported by said positively driven rotating roll in the space between said positively driven rotating roll and said web.

2. The apparatus of claim 1 wherein the positively driven rotating roll has a diameter of at least two inches and the cylindrical body is a uniform diameter rod of up to 1 /2 inches in diameter.

3. An apparatus for coating a continuous web comprising: means for applying a coating composition to said web, two positively driven rotating doctor rolls disposed above said means in juxtaposition and adapted to regulate the amount of coating composition applied, and at least one cylindrical body of smaller diameter than said positively driven doctor rolls freely supported by said doctor roll in the space between said doctor roll and said web and adapted to smooth the coating composition on said web.

4. An apparatus for coating and smoothing a continuous regenerated cellulose film web comprising, in combination: a vessel adapted to contain a coating composition, a stationary guide rod disposed in said vessel below the level of the coating composition and adapted to guide said film web therearound, two rotatable doctor rolls disposed above said guide rod and above the level of the coating composition spaced from each other in juxtaposition to provide a nip, said film web passing between the rolls and through the nip, said doctor rolls adapted to regulate the amount of coating composition applied to the emerging film web, positively driven rotating smoothing rolls of at least two inches in diameter above said rotatable doctor rolls alternately disposed on each side of said film web transversely to said Web and adapted to rotate against said film web in the direction opposite to the direction of travel of said film web and a rod of up to 1 /2 inches in diameter freely supported by said smoothing rolls in the space between said film and each positively driven smoothing roll and free from any rigid connections.

References Cited by the Examiner UNITED STATES PATENTS Re. 22,605 2/45 Goff et al. 1l7-64 2,878,778 3/59 Kusters 118-117 X 2,977,243 3/61 Meier 1l7-64 RICHARD D. NEVIUS, Primary Examiner.

Claims (1)

1. IN AN APPARATUS FOR COATING A CONTINUOUS WEB HAVING MEANS FOR APPLYING A COATING COMPOSITION TO SAID WEB, MEANS FOR REGULATING THE WEIGHT OF THE COACTING COMPOSITION APPLIED, AND MEANS FOR SMOOTHING THE COATING ON SAID WEB, THE IMPROVEMENT IN SAID LAST MEANS COMPRISING AT LEAST ONE POSITIVELY DRIVEN, ROTATING ROLL DISPOSED ON EACH SIDE OF SAID WEB TRANSVERSELY TO SAID WEB WITH EACH ROLL IN CIRCUMFERENTIAL CONTACT WITH SAID WEB AND AT LEAST ONE CYLINDRICAL BODY OF SMALLER DIAM-

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