A METHOD FOR PRODUCING A MACHINE-GLAZED SHEET

Abstract: A method is provided for producing a machine-glazed, cellulosic sheet (11) which includes the steps of wet-creping a cellulosic web (1), and compressing and drying the creped, wet web (6) against the smooth, heated surface of a thermal-drying means (8). A high degree of smoothness is thereby imparted to the web. The average surface rugosity of the smooth and rugose sides of the sheet is an average surface center line deviation of not more than about 0.25 and 0.35 mil, respectively. The compression step is preferably conducted at a pressure of at least about 300 pli (53.7 kg/lineal cm).

Description: - 1 - A METHOD FOR PRODUCING A MACHINE-GLAZED SHEET TECHNICAL FIELD This invention relates to a method for producing a machine-glazed sheet from cellulosic fibers, including a wet-creping step. BACKGROUND ART "Wet-creping" is defined as creping of a cellulosic web, during an intermediate stage of paper manufacture while the web is still damp or wet, as distinguished, for example, from creping finished paper that has been rewet. Creped paper has been produced employing wet-creping techniques for many years. U. S. patent 951,954 (filed in 1907) to Loebbeke describes a pro¬ cess for the manufacture of creped paper in which a cellulosic web in the wet state is creped. Next, the web is gently pressed during the drying process to even the crepe ridges to firm same. The dried sheet is moistened and then smoothed using rollers. In another creped-papermaking process, U. S. 1,772,185 to Liebeck, a wet paper web is pressed between coacting first press rolls 4 and then is wet-creped from dryer 12. The wet-creped web is further pressed between second press rolls 15. Finally, creped paper is pro¬ duced by drying the pressed and creped web on receiv¬ ing dryer 21. Another product which has heretofore been made employing wet-creping techniques is a cellulosic sheet wherein one side of the sheet is smooth. As described in U. S. 2,940,591 to Swedish et al., a masking tape base paper is produced by creping a web of cellulose fibers while still in a wet or damp con- dition, and thereafter drying the wet web on a heated - 2 - drying cylinder. In order to increase sheet smooth¬ ness, the dried, creped paper is subsequently calen¬ dered. "Calendering" is the standard method in the industry for smoothing a web. An improvement to the above Swedish et al. patent is described in U. S. 2,941,661 to Picard et al. The pressure-sensitive adhesive masking tape base paper described in the Picard et al. patent is claimed to have an improved degree of surface smoothness due to the addition of a small amount of a chemical smooth¬ ing agent, i.e., water-dispersible deacetylated karaya gum, to the papermaking furnish prior to formation of the wet web. The method of formation of the paper is similar to that which is described in U. S. 2,940,591. DISCLOSURE OF INVENTION In contradistinction to the prior art creping methods, the method of this invention facilitates the production of a cellulosic sheet, which is machine- glazed, having a high degree of surface smoothness, preferably without requiring subsequent substantial smoothing operations, and more preferably without re¬ quiring any subsequent smoothing of the web. By em¬ ploying the method of the present invention f< a substan- tially higher degree of surface smoothness is provided than can be effectuated by calendering a comparably formed, wet-creped, cellulosic web, which has been thermally dried, as previously described in the prior art. The subject method comprises creping a cellu¬ losic web, in the wet state, the wet-creping step being conducted preferably under conditions whereby a high degree of extensibility is imparted to the web. The wet-creped web is then subjected to a compressing and drying step. More specifically, compressing and dry- WIP - 3 - ing of the creped, wet web is conducted against the smooth, heated surface of a thermal-drying -means which compacts the web and imparts a high degree of surface smoothness thereto. By compressing the web in this manner, the requisite high degree of surface smoothness can be imparted without requiring subse¬ quent smoothing of the web to produce the previously described machine-glazed, cellulosic sheet. Typically, a pressure of at least about 300 pounds per lineal '- inch (53.7 kg per lineal cm) , and preferably at least about 450 pounds per lineal inch t80,5 kg per lineal cm) f and more preferably at least about 600 pounds per lineal inch (107.4 kg per lineal cm) , is employed to impart the requisite high degree of surface s ooth- ness to the wet-creped web. The method of this invention produces machine-glazed, cellulosic sheets having an average surface rugosity, on the smooth side of the sheet, of an average surface center line deviation of not more than about 0.25 mil, and preferably not more than about 0.20 mil, and more preferably not more than about 0.15 mil. A description of how average surface rugosity is measured will be hereinafter set forth. A comparison of the average surface rugosity of sheets produced by the present invention compared to similarly prepared sheets employing the prior art calendering techniques clearly shows that a much higher level of surface smoothness is obtained when the subject method is employed. In a further preferred embodiment of this invention, the degree of smoothness of the more rugose side of the sheet is unexpectedly increased when the method of this invention is employed. Even more un¬ expectedly, the degree of smoothness of the rugose side of a sheet produced by the method of this O invention is preferably higher than the degree of smoothness of the smooth side of a sheet produced by prior art calendering methods. BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a schematic representation of a pre erred configuration of the method of the present invention. HOPES FOR CARRYING OUT THE INVENTION This invention is directed to a method for producing a machine-glazed, cellulosic sheet having a high degree of surface smoothness. The first step in this method is the forma- tion of a wet web of cellulosic fibers from an aque¬ ous fiber furnish. These fibers typically have under- • * gone same degree of lignin modification such as being subjected to at least partial chemical treatment to produce materials such as cheraimechanical pulp, se i- chemical pulp, chemical pulp, or the like. Prefer¬ ably, the cellulosic component of the above furnish will be predominantly sof wood fibers because of their greater length, as compared to their hardwood counter¬ parts. Various additives employed in the production of machine-glazed, cellulosic sheets may be added to the aqueous furnish such as wet-strength agents, fil¬ lers, and the like. Other papermaking process steps such as pH adjustment, refining to a given freeness, total solids adjustment, and the like may also be performed prior to formation of the wet web. After completion of the preliminary furnish formation steps, as depicted in FIG. 1, the aqueous furnish is transferred to a headbox 2 of a paper machine and a wet web 1 is formed therefrom on a Fourdrinier wire 20, The exact manner of wet-web formation is a matter of choice. The method of FIG. 1 is one such formation scheme. The web, in general, has a very low con- sistency when it exits from the headbox 2 onto the Fourdrinier wire 20 (less than about 1%) . For pur¬ poses of this invention, "consistency" is defined as the percent by weight of total cellulosic fibers in the aqueous fiber furnish. Any dewatering device capable of increasing the consistency of the web, typically a mechanical dewatering device such as co- acted rolls, or pair of coacting double-felted press sections 3, can be employed to substantially increase the consistency of the web by removing water there- from. Generally, the consistency of the web is increased to a level which is dry enough to facilitate wet-creping, but not too dry to impair the tensile strength of the sheet 11. More specifically, the con¬ sistency is preferably maintained at a level of about at least 15% by weight, and more preferably at least about 30% by weight, and most preferably at a level of at least about 40% by weight, prior to subjecting the web to a wet-creping step. Although creping in the wet state can be accomplished using various known prior art creping devices, it is preferred that a creping device be employed which will impart a high degree of exten¬ sibility to the wet web. It is also preferred that the wet-creping step be conducted in conjunction with the partial drying of the wet web. For exaπple, if a receiving dryer 4 is employed in conjunction with a creping step, then the requisite, preferred extensibility and partial drying parameters will both be satisfied. It is preferred that the level of extensibility of the sheet, i.e., the percent OM - 6 - stretch of the machine-glazed, cellulosic sheet, be at least about 5%, and more preferably at least about 8%, and most preferably at least about 12%. After wet-creping, the wet-creped web, prior to said compressing and drying step, is prefer¬ ably maintained at an intermediate consistency of from about 40%, up to about 70%, and more preferably from about 50%, up to about 65%. This may be accomplished employing a thermal-drying system such as a series of thermal-drying means 7 which can fur¬ ther dry the wet-creped web. The wet-creped web is then subjected to a compressing and drying step against the smooth, heated surface 9 of a thermal-drying means 8. Preferably, a Yankee drying cylinder is employed for this purpose. The degree of compression applied to the web is main¬ tained so that a high degree of surface smoothness, as described below, is imparted to the web, prefer¬ ably without requiring further substantial subsequent smoothing operations, and more preferably without requiring subsequent smoothing of the web, to produce the subject machine-glazed, cellulose sheet. The above compressive forces are applied to the web dis¬ posed on thermal-drying means 8 by pressure applica- tion means coacting therewith such as by employing one or more coacting pressure rolls 10, or a pressure fabric, or the like. Preferably, the amount of compressive forces imparted to the web on the surface of the thermal- drying means is as previously described herein. A high-smoothness, machine-glazed sheet 11 is then stripped from the thermal-drying surface 9. If desired, further drying of the sheet may be pro¬ vided. The subject machine-glazed, cellulose sheet OMP 11 has a surface smoothness which is higher than a sheet produced from a comparable cellulosic web, without subjecting same to a compressive, drying step, but employing, instead, subsequent calendering tech- niques known to the prior art. For purposes of this invention, "surface smoothness" is determined by meas¬ uring the average surface rugosity of a 24-25 pound/ 3000 ft 2 (121.2-126.3 kg/3000 m 2 ) basis weight sheet. More specifically, "surface rugosity" is measured employing an instrument such as a Tal surf Model 10 apparatus. This apparatus draws a 50-milligram loaded diamond stylus over the surface of the paper being tested and constantly records all of the "peak-and- valley" deviations of the sheet surface. By measuring the average surface center line deviation in mils over a given distance traversed by the stylus over the paper surface, the surface smoothness can be determined. Preferably, the surface rugosity of the smooth side of the sheet exhibits an average surface center line deviation, as previously described. Furthermore, an unexpected level of improve¬ ment in surface smoothness is also achieved on the rugose side of the sheet. In this latter case, the • ' surface rugosity of the more rugose side of the sheet preferably provides an average surface center line deviation of not more than about 0.35 mil, and more preferably not more than about 0.30 mil, and most pre¬ ferably not more than about 0.25 mil. It has been determined that the surface smoothness of the rugose side of the sheet 11 is higher than the surface smoothness of the smooth side of the previously described calendered sheet. This is totally unexpected since the rugose side has not been subjected to the smoothing action of the heated, thermal-drying surface 9. ( OM ■ A > WI The high-smoothness, machine-glazed, cellulosic sheet produced, as previously described, is preferably employed for masking tape base paper, for printable label stock, and the like. EXAMPLE 1 Employing the paper machine configuration set forth in FIG. 1, a bleached kraft southern pine furnish, at a total solids level of 3.2%, and a pH of 6.2, was disc-refined to a Canadian Standard Freeness (C.S.F.) of 538. This stock was mixed with 18 pounds per ton (1.86 kg/metric ton) of a wet-strength resin and diluted with water to a total solids level of 0.23%. This furnish was delivered from a headbox to a Fourdrinier wire traveling at 1,015 feet (307.6 m) per minute. The headbox pH and C.S.F. were 6.3 and 198, respectively. The wet web was picked up from the Fourdrinier and sent through two double-felted press sections at 230 pli (41.2 kg/lineal cm) and 600 pli (107.4 kg/lineal cm) , respectively. The wet web was then transported to a six-foot-diameter (1.83 ) receiving dryer and creped therefrom. The creped, wet web which was transferred to a bank of can dryers at 893 feet (270.6 m) per minute was partially dried, and the partially dried web was fed to a pressure nip between the Yankee dryer and a rubber coacting roll. The nip pressure was maintained at 600 pli (107.4 kg/ lineal cm) onto the surface of a 16-foot-diameter (4.8 ra) Yankee dryer. The compressed and dried web was then stripped from the Yankee dryer onto a pair of final can dryers, the finished, extensible, high- smoothness, machine-glazed, cellulosic sheet having a basis weight of between 24 and 25 pounds per 3000 square feet (121.2-126.3 kg/3000 m 2 ) . The above first experiment was repeated under substantially the same conditions, except that no compressiye forces were applied to the wet-creped web against the Yankee surface. In the second experi¬ ment, the sheet per se was analyzed for smoothness, and in the third experiment, the sheet was machine- calendered between a pair of coacting metal rolls. A summary of the results of these experiments is provided below: TABLE 1 Average Average Surface Rugosity Surface Rugosity Experiment No. (Smooth Side) (Rugose Side) 1 0.12 mil 0.25 mil 2 0.41 mil 0.50 mil 3 0.36 mil 0.38 mil By employing the subject process, an average surface rugosity of 0.12 mil was obtained as compared to 0.41 and 0.36 mil, respectively, for the other experimental sheets. This corresponds to an improve¬ ment in the surface smoothness of the smooth side of machine-glazed, cellulosic sheet of 342% and 300%, respectively. There is also a dramatic increase in the smoothness of the rugose side of the subject sheet. More specifically, an average surface rugosity of the rugose side of 0.25 mil is provided for the subject sheet, as opposed to 0.50 and 0.38 mil, respectively, for the sheets of Experiments 2 and 3. In this lat¬ ter case, there is an improvement of 200% and 152%, respectively. Finally, there is a further unexpected result manifested by the above experimental data. The average surface rugosity of the rugose side of the subject sheet produced in Experiment No. 1 is substan¬ tially lower, i.e., a 144% improvement, than the aver- age surface rugosity of the smooth side of the machine- ( OM "cAif . WIP calendered sheet produced in Experiment No. 3

Claims: CLAIMS 1. A method for producing a creped, machine- glazed, cellulosic sheet, having a high degree of surface smoothness, which comprises: a) forming a wet web of cellulosic fibers; b) creping said web in a wet state; and c) compressing and drying said creped, wet web against a smooth, heated surface of a thermal-drying means to com¬ pact said creped, wet web and to impart, without requiring substantial subsequent smoothing of said web, a high degree of surface smoothness to said wet web, the surface rugosity of the smooth side of said creped, machine-glazed, cellulosic sheet having an average surface center line deviation of not more than about 0.25 mil. 2. The method of claim 1, wherein the average surface rugosity of the smooth side of the sheet is not more than about 0.20 mil. 3. The method of claim 1, wherein the average surface rugosity of the smooth side of the sheet is not more than about 0.15 mil. 4. The method of claim 1, wherein the average surface rugosity of the rugose side of the sheet is ot more than about 0.35 mil. 5. The method of claim 1, wherein the exten¬ sibility of said machine-glazed, cellulosic sheet is at least about 5%. 6. The method of claim 1, wherein the creped, wet web, prior to said compressing and drying step, is maintained at a consistency of from about 40%, up to about 70%, by weight. 7. A method for producing an extensible, creped, machine-glazed, cellulosic sheet having a high degree of surface smoothness, which comprises: a) forming a wet web of cellulose fibers; b) creping said web in its wet state; and c) compressing and drying said creped, wet web on the smooth, heated surface of a thermal-drying means, at a pressure of at least about 300 pounds per lineal inch (53.7 kg/lineal cm) to impart, without sub¬ stantial smoothing of said web, a high degree of surface smoothness to said wet- creped web, to produce said creped, machine-glazed, cellulosic sheet. 8. The method of claim 7, wherein said co - pressive pressure is at least about 450 pounds per lineal inch (80.5 kg/lineal cm) . 9. The method of claim 7, wherein said com- pressive pressure is at least about 600 pounds per lineal inch (107.4 kg/lineal cm) . 10. A method for producing a creped, machine- glazed, cellulosic sheet, having a high degree of surface smoothness, which comprises: a) forming a wet web of cellulosic fibers; b) .creping said web in a wet state; and c) compressing and drying said creped, wet web against a smooth, heated surface of a thermal-drying means to compact said creped, wet web and to impart, without requiring further substantial smoothing of said web, a high degree of surface smooth¬ ness to said wet web, the surface rugosity of the rugose side of said machine-glazed, cellulosic sheet having an average surface center line deviation of not more than about 0.35 mil. 11. The method of claim 10, wherein the average surface rugosity of the rugose side of the sheet is not more than about 0.30 mil. 12. The method of claim 10, wherein the average surface rugosity of the rugose side of the sheet is not more than about 0.25 mil. 13. A method for producing a creped, machine- glazed, cellulosic sheet, having a high degree of surface smoothness, which comprises: a) forming a wet web of cellulosic fibers; b) creping said web in a wet state; and c) compressing and drying said creped, wet web against a smooth, heated surface of ( OM - 14 - a thermal-drying means to compact said creped, wet web and to impart, without requiring further substantial smoothing of said web, a high degree of surface smoothness to said wet web, the surface rugosity of the rugose side of said machine-glazed, cellulosic sheet having a lower rugosity than the surface smooth¬ ness of a sheet produced without said compressing and drying step, but employ¬ ing, instead, subsequent calendering techniques. 14. A creped, machine-glazed, cellulosic sheet, having a high degree of surface smoothness, the aver¬ age rugosity of the smooth side of said sheet having an average surface center line deviation of not more than about 0.25 mil. 15. The cellulosic sheet of claim 14, wherein the average surface rugosity of the smooth side of said sheet is not more than about 0.20 mil. 16. The cellulosic sheet of claim 14, wherein the average surface rugosity of the smooth side of said sheet is not more than about 0.15 mil. 17. A creped, machine-glazed, cellulosic sheet, having a high degree of surface smoothness, the aver- age surface rugosity of the rugose side of the sheet having an average center line deviation of not more than about 0.35 mil. 18. The cellulosic sheet of claim 17, wherein the average surface rugosity of the rugose side of the sheet is not more than about 0.30 mil. 19. The cellulosic sheet of claim 17, wherein the average surface rugosity of the rugose side of the sheet is not more than about 0.25 mil.

Inventors: HANKE D, KINDLER W
Applicants: HANKE D, KINDLER W
Application year: 1981
Main classification: D21H5
IPC codes: D21F3, D21F9, D21G9