US2810966A - Process for drum-finishing coated paper - Google Patents

Description

Oct. 29, 1957 J. A. BICKNELL PROCESS FOR DRUM-FINISHING COATED PAPER Filed June 2, 1955 9 dig/g INVENTOR aim 6L QM ATTORNEY BYf l United States Patent John Alfred Bicknell, Westbrook, S. D. Warren Company, Boston, of Massachusetts Application June 2, 1955, Serial No. 512,619 2 Claims. (01.34-41) Maine, assiguor to Mass, a corporation This invention relates to a process for making drumfinished coated paper. In particular it concerns the removal of drum-finished coated paper from a finishing drum or comparable finishing surface in a condition devoid of wrinkles or other visible strains.

The production of drum-finished mineral-coated paper was described in U. S. Patent No. 1,719,166. According to this patent a layer of aqueous mineral-coating composition comprising finely divided mineral pigmentary matter and an adhesive such as solvated casein is applied to one side of a paper web and the wet coated side of the web is then pressed against a heated finishing surface, e. g. a chromium-plated drum, where the coated surface remains in non-slipping adherent contact until the coating has become dry whereupon the coated paper spontaneously releases from the finishing surface.

By another known process of drum-finishing coated paper, a web of paper having thereon a layer of dried and calendered mineral-coating which contains a substance easily swellable by water, such as casein adhesive, is wetted by application of water or aqueous solution to the calendered coated surface and the resulting Wet coated surface is immediately pressed into non-slipping adherent contact with a heated and polished chromium plated drum. In this process, likewise, the wet coated surface is dried while remaining in adherent contact with the polished finishing surface of the drum, and when it has become substantially dry it releases spontaneously from said finishing surface. In both processes it is a frequent practice to maintain on the finishing surface an imperceptible film of soap or oil to aid in the clean separation of the dry coated surface from the drum since any adhesion between the paper and the drum at the parting which must be overcome by pulling the paper away from the drum is likely to result in imperfections in the finished surface of the coating.

The paper web generally is conveyed away from the V finishing drum under only sufiicient tension for the conveyance and while this tension may be transmitted through the Web to the line of parting between the Web and the drum it is not sufficient to strip the paper from the drum until it has reached the stage of spontaneous release. The separation of the paper web from the finishing surface generally is effected by passing the web over a roller which is positioned out of contact with the drum.

In the normal course of drum-finishing coated paper as previously described it is found that the paper on the drum tends to dry more quickly toward the edges than nearer the middle of the web. As a result the side portions of the web release from the drum before the middle portion releases. Instead of the web releasing in a line substantially straight across the Web, the release line forms a rough V having its apex pointing in the direction of web travel. Such uneven drying and consequent uneven contraction of the paper across the Web sets up unequal stresses in the finished web and causes strains therein. Such strains may cause objectionable and even unmanagable curling tendencies in the finished product. Moreover,

in the case of fairly light weight paper, say paper weigh- 'ice ing in the order of grams per square meter, the strains may actually be visible as V-shaped distortion ridges or wrinkles in the surface, sometimes referred to as release wrinkles. Obviously strains which cause curl problems are very undesirable, and distortions which are visible are intolerable. A means or method to prevent or markedly decrease formation of such strains is greatly to be desired.

' Several palliative steps have at times been taken, either separately or conjointly, in efforts to decrease or prevent the undesirable efiect of uneven drying and/or uneven contraction of the paper web in the drum-finishing operation. For example, very strong tension applied to the web as it leaves the finishing surface in the direction substantially tangent to the drum at the line of release may in some cases prevent formation of visible distortion ridges. Such tension, however, is liable to cause excessive breakage of the'paper web, and at the same time is usually not effective to correct curl difficulties.

Another remedy has been to spray the end portions of the finishing drum with cold water to lower the temperature locally so that evaporation at the edge portions of the web is slowed down to a rate approximating the rate at the middle of the sheet. Alternatively the edge portions of the paper web have been heavily wetted with water to keep them from becoming dry before the middle portion of the web. The best that these two expedients can acomplish generally is to reduce to some degree unevenness of drying across the web. In practice it is not possible by these expedients to achieve absolutely uniform drying and/ or absolutely uniform release of the web from the finishing surface. Moreover, both expedients obviously are opposed to good thermal efficiency.

According to the present invention the coated paper is firmly held in contact with the finishing drum to a straight release line so that no portion of the web is permitted to part from the drum and shrink ahead of another portion and produce the release defects or wrinkles referred to above. In carrying out the process the wet coated web, either the freshly coated web or the rewetted calendered coated web referred to above, is brought into contact with the heated drum and is carried on the drum to the line of release which generally is or more from the line of initial contact of the coated web with the drum. The line of release of the Web from the drum is established by a pressure roller having a soft yielding surface e. g. a rubber covered roller, which bears against the web in contact with the drum with sufiicient pressure to prevent any portion of the web from slipping and shrinking and developing release wrinkles. Thus, although the web may have dried unequally and the edge portions may have reached a state of dryness sufficient for spontaneous release from the drum ahead of the middle portion, all portions are firmly held in contact with the drum and release or slippage of any portion is prevented until the web arrives at the straight line of release defined by the line of contact of the pressure roller with the finishing drum. At this line the web is parted from the drum.

As stated above the wet coated paper adheres to the drum while it is wet and releases therefrom spontaneously when it attains a certain degree of dryness. This adhesion prevents any harmful shrinkage and slipping of web while it is drying. It is only after a portion of the web e. g. the edge portions, have attained a degree of dryness at which it will release spontaneously from the drum ahead of another portion e. g. the middle portion, that release wrinkles are formed.

The pressure take-off roller which defines the parting line may be positioned at the point at which the last portion of the web is ready for release i. e. at the apex of the V referred to above. In this case the side portions of the web which have become ready to release earlier will V panying drawing in which of the drum be held in non slipping contact with the drum until they 7 tion of the pressure roller readily 'ntay be determined in each instance and will depend priniarilytiponthe character ot the coating, the rate of travel of the paper web; thesize of the finishing drum audits temperature etc.-

Apparatus for carrying out the invention includes in addition to the conventional apparatus for the drum finishing of coated paper such as disclosed in Patent No. 2,678,890; a pressure roller having a yielding surface positioned to bear against the paper web in contactwith the drum at a suitable point which generally is at least 180 around the circumference of the drum from the point at which the paper is first brought intocontact with the drum and means for disposing of the paper Web as it is removed from the drum at the pressure roller. Such an apparatus is illustrated diagrammatically in the accom- 10 is the finishing drum, ll is a conventional so-ca-lled roll coater, 12 is a roller for smoothing the coating applied by the coater 1-1 13 is the paper web coming from the roll thereof 14; 15 is a web supporting and conveying roller which cooperates with the coater 1 1, 16 is a roller positioned to press the web onto the finishing drum at the nip 17, 18 is a nozzle for feeding aqueous liquid into the nip 17 and for maintaining the pool 19 thereof in the nip 20 is a rubber covered pressure roller and 21, 21 and 21 are rollers for conveying the Web to the wind-up roller 22.-

In practice roller 20 is of course provided with conventional' means for adjusting its position with respect-to the I finishing drum 10 and consequently the pressure applied to the paper web in contact with the drum. It will be understood that my invention is not limited to the drum finishing apparatus illustrated. The known equivalents may be substituted therefor, any known Means net stews);

determine the position at which the paper web has dried 'sufliciently to separate 'sppnta neously from the drum.

Generally roller 20 is at least from the roller 16 in thsdirefioil-of moi/anteater the Web around tli d'riin'i; This is of course not an essential condition but if the finishifig'processdos not use at least half of the circum ference of a given drum then that drum is not being eificiently used and a smaller drum should be used. may-tie provided for shifting the position ofthe roller 20;aro und the drum so as to establish the take-off of parting line at the desired point, and' means (not shown) may be provided for rotating the roller 20 at the same g V In the foregoing description, I have'referred foi' can; venience to the use of a drum for providing thefinis g surface but as is'wellknownthe conventional dram-may be replaced by any travelling. surface such as that own endless belt. Consequently it is to" be understood that the appended claims embrace'e' a drum' and equivalent means for providing a finishing surface". Iclaim: I. In a process of finishing coated paper in which a moving'paper web' bearing a wet adhesive'coa'ting is dried while in non-slipping adhesive'contaet with a continuously moving finishing surface until it spontaneously releases fromsaid surface and'is then separated from-said surface; the step which consists in holding the entirewidt'h of said web in non-slipping contact with said surface toa sub-' stantially straight lineacross said surface perpendicular to the direction of travel of the web, sa'id' line being at least as far forward in the direction of travelot said? web as the forwardmost point of spontaneous release of said web from said surface." t L 2. Process as defined in claim 1 in which yielding pressure is applied to the web' cent saidstraight line;

Reterncescited iii the file or this patent UNITED STATES PATENTS 16 is determined by conditions which surface speed as that of the drum.

in contact with the drum adja-i'

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