US2314797A - Paper manufacture - Google Patents

Description

March 23, 1943. MORRIS ET AL 2,314,797

PAPER MANUFACTURE Filed Nov. 19, 1937 INVENTORS 77g 9 ATTORN Patented Mar. 23, 1943 PAPER MANUFACTURE Donald F. Morris and Leonard R. Growdon, Chillicothe, Ohio, assignors to The Mead Corporation, Chllllcothe, Ohio, a corporation of Ohio Application November 19, 1937, Serial No. 175,366

Claims.

This invention relates to the manufacture of paper.

It is the principal object of the invention to provide a simple and elfective process for producing a smooth, non-curling sheet of paper that will produce highly satisfactory results in printins operations and the like.

It is a further object to provide such a process for producing a sheet of paper having had dissimilar treatment on its opposite Sides such as to normally cause a tendency to curl but which thereafter is so processed that it will lie substantially fiat and without objectionable curling in use.

It is a further object to provide a process for producing a sheet of paper which will give satisfactory results as regards register and the like in multi-color printing operations.

It is a further object to provide a sheet of paper of this character which has controlled moisture characteristics imparted thereto to control the tendency to curl, and to provide proper responsiveness as to printing characteristics, expansion and the like.

Other objects and advantages will be apparent from the following description, the accompanying drawing and the appended claims.

In the drawing Fig. 1 is a diagrammatic view showing appa ratus suitable for practicing the invention;

Fig. 2 is a diagrammatic view showing a modified form of such apparatus; and

Figs. 3, 4 and 5 are partial diagrammatic views showing modified constructions for effecting application of moisture in uniform manner to the web.

It has been found in the manufacture of paper coated on one side, as heretofore practised, that there is a tendency to produce paper having an unpredictable tendency to curl. This curling may manifest. itself during the course-of the paper manufacture and processing steps, or may not become evident until after complete processing and during subsequent handling in the paper or coating mill or in the printing establishment.

In the manufacture of so-called coated-oneside paper the opposite sides of the formed paper web are treated unequally.

In accordance with the present invention the sheet of paper asproduced is made to contain a predetermined content of moisture which is in predetermined excess with reference to th press room conditions so that the above described desired results in printing are secured. And it is found that by properly controlling such moisture application to the sheet, the tendency of the sheet toward curling is very much reduced if not completely eliminated, and a sheet of paper is provided which will give satisfactory results under a substantial range of press room conditions. Curling of the sheet is usually more objectionable in the case of certain hard sized uncoated papers and in the case of paper the opposite sides of which have been treatedunequally such as an uncoated sheet one side of which has been supercalendered or a sheet which contains a coating upon only one side thereof.

It has been found that this tendency may be greatly exaggerated by other ordinarily indeterminate conditions-such as relative moisture content in the paper web and in the paper mill atmosphere before and after processing. In accordance with the present invention a highly satisfactory sheet of paper coated or supercalendered on one side only, which lies fiat and is practically free of curl is produced by the controlled incorporation of moisture thereinto. In such processing moisture is added to the back or uncoated side of the sheet in predetermined quantity, which serves to obviate the inherent tendency to curl. At the same time this processing will provide a sheet having the desired characteristics of moisture content in predetermined relation to those of the press room. The fact that the coating is applied to only one side of the sheet, the one side being uncoated, is thought to produce stresses within the paper, upon drying and particularly upon calendering, which tend to cause the sheet to curl. For many purposes it is not necessary or desirable to coat both sides of the sheet, as for instance in the production of lithographing paper for use in th making of labels. In such paper a. coating is provided upon the printing surface to provide high gloss and whiteness, good printing characteristics and the like, for the reception of the printed matter of the label. The reverse side is adapted to be pasted upon the article or container, and no coating is necessary, andin fact-may be particularly undesirable because the raw stock surface of the sheet may provide much better adherence qualities for securing the sheet to the container, when not coated.

It has long been known that for printing and like purposes considerable diificulty is sometimes encountered from the tendency of the paper sheet to undergo physical changes as a result of varying moisture conditions.

tend to curl when exposed to the press room atmosphere, such that the sheets become diificult to Thus the sheet may handle properly. In the customary printing practice a pack of sheets is suspended in the atmosphere of the press room, or in an atmosphere similar thereto, for a period of time prior to the actual printing thereof. This time is of such duration as to permit the paper to arrive at a condition as regards moisture content suitably corresponding to the humidity of the press room, as such condition is desirable at the time the sheet is actually passed through the press. Additional changes in the moisture condition 'of the sheet may take place during the printing operation, particularly in the offset printing process, where moisture tends to be picked up by the sheet. Such moisture addition has resulted in the physical expansion of the sheet, so that upon a subsequent printing of the sheet in a multi-color process, improper register has occurred. This tendency may vary with the quantity of moisture picked up, the character of the web constituents, the coating or surfacing applied thereto, orthe calendering operation to which the formed sheet is subjected, or by combination of these or other factors, but when present in sufiicient degree is found to be quite objectionable in printing operations where proper register is desired.

In securing proper control over these characteristics of the sheet it has been determined that not only is it important to control the moisure content of the sheet with reference to the press room conditions, but also it has been found that the previous condition of the web, prior to its being brought into conformity with the press room conditions, exerts animportant influence thereon. Thus if the sheet contains less moisture than the press room, its moisture is increased as a result of exposure to the humidity conditions of the press room, and in the subsequent printing operation particularly in multi-color offset printing presses it is found that the moisture content of the sheet further increases by a material amount. And the result of such subsequent increase of moisture content during the printing operation is to cause expansion of the paper during the initial passages through the press with the result that improper register is obtained, subsequent printing being within the earlier printings.

We have found that when the sheet is previously processed to a moisture condition properly coordinated with the moisture conditions which will exist in the press room, very much improved results as regards register in the printing operation are secured. For example, when the sheets are processed to give them a moisture content of controlled limited excess above that of the printing room in which they are to be used any subsequent equalization will not adversely affect the responsiveness to proper printing. Measurements of moisture absorption under such conditions indicate that the sheet absorbs very little if any additional moisture during the printing operation,

and that as a result there is very little change by way of expansion in the sheet during the successive printing operations and very close and accurate register is thereby secured.

As an example, where the sheet was processed to contain approximately /'2% to 1% moisture above the condition of equilibrium with the air 'of press room in which it was to be used, the change in moisture content during the subsequent printing operation was reduced, with unobjectionable change in state of the sheets and with the result that very satisfactory register was secured.

Referring tothe drawing, Fig. 1 shows suitable apparatus for satisfactorily carrying out the present invention-that drawing illustrating mechanism for forming and conditioning a coated-oneside sheet. Therein a roll of paper adapted to be coated is shown at It! which is led over suitable guide rolls into contact with a coating applying means I l shown as comprising a pair of rolls the lower of which dips into a bath of the coating composition and transfers a suitable amount of the coating color over the paper as it passes through the nip of the rolls. The sheet with the coating thereon is then subjected to a smoothing and spreading action, shown as effected by reversely turning rolls illustrated at l2 running in contact with the coated surface. The sheet passes over suitable drier cylinders l3, of such number that as it leaves the final drying roll I! it is in substantially dried and finished condition and is wound onto a reel IS. The sheet as thus formed is generally coated according to the customary manner for the manufacture of a sheet for subsequent lithographing for the production of labels, a proper surface coating being applied to one side by suitable means, such as the roll coating mechanism described, or by the brush coating mechanism both generally used in the art. After such a sheet is dried it is preferably subjected to a calendering operation to produce the desired ironing or smoothing action; and this operation may be performed by passing the web completely through the usual calender stack, or if a finish requiring a lesser action is desired the sheet may be passed through only a part of the calender. Ordinarily it is found that most satisfactory results are secured where the sheet has received an effective calendering action.

To effect this desired calendering treatment,

the roll is subsequently unwould, its unwinding location being illustrated diagrammatically at 15, the web passing over to a guide roll l6, and into the calender stack H. In the calenders the sheet receives a number of passes through the rolls, heated if desired, which act to exert the desired smoothing and ironing-like effect thereon, suitable means being provided for loading the calender rolls and exerting pressure upon the several nips thereof. Sometimes it has been found desirable to provide a coated and uncoated supercalendered label paper, that is, a coatedone-side sheet, in which one side of the sheet may have to be given only a light calendering treatment or no calendering while a heavier calendering is given the opposite side of the sheet. In both cases the treatment creates unequal stresses in the sheet such that unless properly controlled, the sheet tends to curl; especially when the internal stresses are subsequently disturbed by changes in the moisture content. In accordance with the present invention such sheet is treated to effect controlled incorporation of moisture thereinto controlled to meet these inherent stresses and characteristics of formation and also those resulting from subsequent introduction of disturbing influences.

As shown in the drawing the web leaves the calender stack, passes over guide roll 20, and into the wet press indicated generally at 2|. This press comprises the upper roll 22, and the lower roll 23 one or both of which are rubber covered rolls. The lower roll dips within a trough 24 adapted to contain water to a predetermined level, picking up a film on the surface thereof. It is important that the moisture be incorporated into the sheet uniformly and preferably by applying it uniformly over the entire surface. A non-uniform distribution of moisture increases the tendency of the sheet to curl and to form permanent puckers. The moisture may be readily applied, under most operating conditions by a suitable water press such as described. But other apparatus for securing the desired application and uniform incorporation may be used. For some operations a steam or water spray permits of even more accurate control and distribution. Where a press is used the bottom roll may be either metal surfaced or may be covered with a suitable felt the felt dipping within a supply of water to pick up the same and carry it properly into receptive contact with the sheet. Alternatively a suitable felt may be impregnated with moisture to the desired content, and the sheet then directed in contact and pressed against the damp felt by running the sheet and felt through a wet press.

Satisfactory results in commercial operations under the invention have been secured by effecting the application of the moisture to the web under predetermined pressure conditions as described and illustrated. In this operation applicants have found satisfactory results in incorporating the moisture uniformly and homogeneously into the web to be obtained with a press of two rolls in which both top and bottom rolls are hard surfaced rolls, the bottom roll running in a water bath and picking up a film of water on its surface and carrying the same into contact with the sheet passing through the nip. This has been found to'give proper pressure incorporation and to provide the desired distribution and uniformity of application throughout the entire body of the web. Satisfactory results have also been secured where the top roll was of rubber or rubber-covered.

In the press roll arrangement illustrated, the web is maintained in initial contact with the upper roll 22 by means of the guide roll 20, and is not brought into contact with the water film until its passage through the nip of the rolls. The sheet is subjected to controlled pressure to limit the incorporation of the water during such passage. The pressure is controlled to give the desired amount of incorporationthe weight of the roll being suflicient with some papers and desired end results, and added pressure being applied by spring means or the like as desired to secure most satisfactory operating results and the incorporation of the proper quantity of moisture into the body of the webdepending on the characteristics of the web and the conditions of use it is to meet.

As a specific example of pressures which are found to provide satisfactory results in regular commercialoperations, using a filled sheet containing considerable quantities of soda pulp, the press 2| was operated to give a pressure of from 30 to 150 pounds per linear inch, under varying characteristics of the sheet, which resulted in the incorporation of approximately 1.5% to 2.0% added moisture within the sheet, thereby increasing the moisture content of the sheet by this amount from anormal or customary content of about 3-5% to about 47%.

Thereafter the sheet with the added moisture therein was subjected to an additional calendering action. This action seemingly rearranges the inherent stresses and causes the sheet to be finished free from curl. For this purpose the web may be wound onto a reel and passed through the same calender l1; or into a second calender stack 25, where a suitably heavy calendering acwhich has received tion is obtained. In some operations the sheet is suitably acted on and left fiat by passing through only two nips of the calender. The sheet then'passes to the final reel 26. It will be understood that any necessary or additional calendering may be effected at this point, but it is usually found that a relatively light calendering will suffice and that a heavy calendering will result in objectionable blackening of the sheet.

Satisfactory operating results have been obtained through the use of a fpress located in the vicinity of the coating calender and driven in coordinated speed relationship with the calender. As one example the paper is unwound from the reelstand on the calender and is passed through all or part of the calender rolls and then through .the Water bath; the sheet is then passed a second time through one or two nips of the calender. In most cases it has been found that the application of the water to the sheet suitably reduces the tendency of the sheet to curl, but sometimes in order to make the paper lie flat it is desirable to lightly recalender the sheet following such moisture addition. This subsequent calendering operation is preferably carried out as an operation separate from the first calendering for the reason that the sheet usually exhibits a tendency to expand and thus the second operation should either take place at a slightly greater speed or there should be some means to take up the slack and avoid the formation of puckers. Furthermore the spacing of the calenders from the point of moisture application, the speed of the paper and the amount of moisture added may require close coordination.

Fig. 2 shows a modified form of apparatus in which the roll of paper 30 after drying is passed through calender 3|, and at the bottom nip of the calender the web i drawn outwardly and passed over guide rolls 32 into and through water press 33 similar to press 2|. Thereafter the web passes over guide roll 34 and around the lowermost roll 35 of the stack onto final Wind-up roll 36. This modification is desirable where no additional calendering is sought following the water press treatment, and it is found that a final calendering operation is not necessary in all cases and where the sheet after receiving the water incorporating treatment does not contain objectionable puckers.

Figs. 3, 4 and 5 show modified apparatus to effect the application of the moisture in proper controlled quantity and with desired uniformity into and upon the sheet by using a spray which is constructed and controlled accurately to assure both quantity and distribution of the added moistuie as desired. Thus in Fig. 3 the web,

the treatment as regards coating, calendering and the like as above described, passes into the nip between upper and lower press rolls 40, 4|, and a spray nozzle 42 directs a predetermined quantity of moisture directly into the hip and in incorporating contact with the lower. and untreated, surface of the web. The web then passes through the press where pressure incorporation of the moisture is brought about as above described, following which it receives a desired calendaring treatment in calender 43. The arrangement of the nozzle or nozzles 42 is such as to effect a uniform application of spray across the entire width of the web, and is such as to produce a finely divided spray of accurately controlled volume, so that complete uniformity of moisture application results. Suitable Valve means, nozzle constructions and the like may be provided to secure the desired operation of the sprays.

In Fig. 4, the spray nozzle 45 is arranged to direct a finely divided spray directly upon the surface of the web, shortly prior to its passage to the nip of press rolls. 46, 41. Thereafter, the sheet is subjected to suitable calendering action if desired in calender l8.

In Fig. 5 the spray nozzle 50 projects a uniform film in desired quantity upon lower press roll 5|. The sheet is maintained in contact with upper press roll, 52 and the film of moisture is thus brought into incorporating contact with the untreated side of the web as it passes through the nip. As above, the web may be passed to calender 53if desired.

The sheet of paper as time produced is properly protected in shipment, storage, and the like, to avoid objectionable or material loss of moisture, and to provide for the delivery of the sheet to the user at substantially the original predetermined moisture content therein. When such sheet is then brought into the press room, the operating moisture content of which is within the predetermined range for such paper, it will not adversely react before or during printing. It has been found that when the sheet has an additional amount of moisture added thereto it does not thereafter give up such excess moisture, upon exposure to the air, so readily as it takes up moisture from the air when its moisture content is deficient with respect to the air.

By adding moisture therefore the inherent characteristics of original formation of the sheet may be changed so that subsequent tendency to curl and to indeterminate expansion may be obviated. The inherent forces set up in the paper sheet or in its surface zones during its manufacture may be greatly unbalanced, and the formation may also be partly the cause of such unbalancing, with the resultant curl and expansion when moisture content and the balance of forces are changed. And as most press rooms have substantially constant temperature and moisture conditions maintained the paper may be given either a slight excess of moisture, or a moisture content so closely approximating that of the press room that subsequent changes when exposed Will be unobjectionable. The larger press rooms, using large quantities of papenare usually at a temperature of about 80 F., with a relative humidity of approximately to and by the operations as satisfactorily practised and described above, highly satisfactory results in printing are secured. The tendency of the'sheet to absorb moisture and toexpand during its passage through the several steps of the printing operation, is substantially eliminated, as well as the heretofore objectionable tendency of the sheet to curl prior to and during the actual printing operation. As a result the paper is highly satisfactory in printing and provides for securing improved and superior results therein.

While the process, apparatus, and product herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited tothis precise process, apparatus, and product, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. The process of forming a sheet of paper adapted to produce superior results in printing which comprises applying a coating to one surface of a sheet of paper, drying and calendering the coated sheet, effecting the incorporation of additional moisture into the uncoated side of the sheet by a wet pressing operation, thereafter lightly recalendering the uncoated side of the sheet, to provide a sheet of paper having a moisture content in predetermined excess above that of the press room.

2. The process in the forming of paper for use in printing, and the like, and adapted to have substantially reduced curling tendencies due to the stabilization of forces created within the sheet by unequal treatment of the two sides thereof. which comprises applying a surface coating to one side only of, the formed sheet, drying the coated sheet, said dried coated sheet having a tendency to curl toward the coated side resulting from forces set up within the sheet, calendering the sheet, subjecting the c'alendered sheet to a wet pressing operation to effect uniform incorporation of added moisture in the amount of approximately 1.5% to 2.0% into the sheet upon the uncoated side thereof, the added moisture being effective to stabilize the forces within the sheet and substantially reduce the curling tendencies caused by the .forces initially present within the sheet, and lightly recalendering the sheet with the added moisture therein.

3. The process in the forming of paper for use in printing, and the like, and adapted to have superior characteristics which comprises applying a coating to one side of a sheet of paper, drying the coated sheet of paper to substantial completion such that it contains approximately 3% to 5% moisture as a normal moisture content, calendering the dried sheet, effecting the controlled uniform incorporation of added moisture to the uncoated side of said sheet in predetermined limited small amount of the order of 1.5% to 2% and thereafter exerting a recalendering action upon the paper web whereby to effect proper stabilizing of the forces inherent therein and prevent subsequent objectionable curl and dimensional variations.

4. The method of forming a sheet of paper of superior character as regards printing and the like comprising applying a coating to one side of the sheet of paper, substantially completely drying the sheet, subjecting the sheet to at least a partial calendering operation, leading the sheet from the calender and while removed therefrom applying moisture under controlled pressure conditions to the uncoated side of the sheet, and thereafter lightly recalendering the sheet.

5. The method of forming a sheet of paper having a coating on one side thereof and having reduced curling tendency which comprises applying a surface coating to one side of the sheet, drying the coated sheet to a normal moisture content of approximately 3% to 5%, subjecting the coated side of the sheet to a calendering operation, removing the sheet from the calender, applying approximately 1.5% to 2.0% additional moisture under pressure of approximately 30 to pounds per linear inch to the uncoated side of the sheet to provide a non-curling sheet of paper, and thereafter lightly recalendering the sheet.

DONALD F. MORRIS. LEONARD R. GROWDON.

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