Method for improving the printability of web offset paper

Abstract: Web offset printing paper having a high level of gloss and smoothness is prepared by finishing a paper web, either by supercalender or with a synthetic roll calender at a moisture content greater than the moisture content typically used for web offset finishing, and then drying the web to a moisture content of less than about 3.5% using convection, radiation or conduction. Under the preferred post-drying conditions, bonding of the previously wet paper fibers takes place to lock in the desirable printing characteristics and to prevent heat roughening on the offset press.

Description: Background of Invention This application is a continuation-in-part of U.S. Application Serial No. 07/781,575 filed October 23, 1991 which is incorporated herein by reference. During the web offset printing process, ink is applied to both sides of the web simultaneously, followed by relatively severe drying with high temperature air impingement. High temperature drying is required to dry the applied inks. During this drying process, the printed surface becomes roughened if the moisture content of the web is greater than about 3.5%. Similar roughening is not present in the sheet fed printing process where inks are dried more slowly by chemical curing, generally without the application of external heat. The condition of the paper printed by the web offset printing process at high moisture is said to be "heat roughened". This roughening is dependent on both paper and press parameters. The most critical paper parameter is the paper moisture entering the press. However, the press drying conditions including web exit temperature, speed and oven temperature, also contribute to heat roughening. The moisture content of the web entering the press plays a significant role. Paper having a moisture content above about 3.5% tends to get rougher during printing while paper with a moisture content less than about 3.5% becomes smoother upon printing. Since press conditions are difficult to change because of the need to maintain register and press productivity, this means that the papermaker must make the adjustments necessary to overcome heat roughening on the press. Summary of the Invention Web offset paper for the high quality printing market must have high gloss and smoothness. Both smoothness and gloss may be enhanced during the papermaking process by finishing (calendering). The finishing may be accomplished by supercalendering or with a synthetic roll calender. Further, as in the case of the offset printing process, the moisture of the web plays an important role during finishing. Generally, paper webs finished at high moisture content have a smoother finish and greater gloss than paper webs finished at low moisture content. However, webs finished at high moisture must then be dried to reach a moisture content that will not produce heat roughening on the press. Unfortunately, when the finished web is dried, it is susceptible to the same type of heat roughening experienced during web offset printing. Thus, since moisture plays important roles in both the finishing and web offset printing processes, the problem becomes one of finishing the paper at high moisture to achieve the high gloss and smoothness desired by the printer, while still being able to deliver to the printer a web which retains as much of the gloss and smoothness as possible, at a low enough moisture content for good performance on the press. Therefore, merely finishing at high moisture is not enough, the paper must then be dried in such a manner as not to negate the improved finish achieved by htgh moisture finishing. This is accomplished by selecting a drying rate for the specific product and post drying method chosen to achieve the desired results. The most commonly used method for characterizing the drying of paper webs is the average water removal rate in kg/m 2 .h (lb/sq. ft/h). Unfortunately, it is not a rate constant since it is a function of both water content and web speed. However, a convenient method for the characterization of drying has been developed for an airfoil dryer which dries by convection heating. The method is based on the observation that drying of web offset coatings can be characterized as always in the falling rate period of drying, i.e., drying rate is dependent on water content. Since water content vs. residence time is linearized by a logarithmic function, a rate constant, K, may be defined which is dependent only on dryer output (i.e., dryer air temperature and velocity). This constant is not dependent on water content or web speed and may be defined by the equation: where W o is the initial moisture content entering the dryer and W F is the final moisture content, both in kg/m 2 (lb/ream), and t is the residence time in the dryer in hours. It is believed that this method would also apply to the drying of webs finished at high moisture as disclosed in the present invention. The calculation is fairly straight forward for convection dryers such as the air foil type which have a finite length during which the web is exposed to heat and moisture is removed. Knowing the length and web speed, the residence time can be readily determined. The same calculations can be made for conduction drying and radiant drying by measuring the wrap around the conduction drying drum or the length of the radiant drying unit. However, in both of the above drying methods, moisture is removed beyond the time the web is in contact with the drying apparatus. Therefore, it is recognized and understood that the most accurate calculation of a drying rate constant depends to a great extent on the drying method used. However, the performance of the present invention may be best characterized by monitoring the temperature of the web exiting the post drying apparatus. Optimum performance is achieved by using a post drying procedure which produces a web exit temperature of less than about 149 °C (300°F), and preferably 93-127°C (200-260°F), although web exit temperatures of less than about 93°C (200°F) should produce similar results. The invention provides a process and a paper web as defined in the claims. US-A-3 230 867 discloses a compact calender comprising a centrally located hard roll around which there is situated at least one and preferably three soft covered rolls to provide one or more calender nips for finishing paper. The calender configuration permits ease of threading of the paper web through the calender nips and may be used to finish paper having an initial moisture content within the range of from about 6-15% moisture in contrast to a moisture content of 4-5% for conventional calenders. This document does not contain any mention, nor give any suggestion, for drying the paper web to a moisture content of 3.5% or less before printing as provided in the present invention. In accordance with the present invention, an improved web offset printing paper is produced by finishing the web, either with a supercalender (SC) or using a synthetic roll calender (SRC), under conditions where the web has a moisture content greater than the moisture content typically used for web offset finishing (i.e. 3-5%), and less than the moisture content that would cause excessive opacity loss, blackening or galvanizing of the paper at the temperature, pressure and web speed selected, and then drying the web to a final moisture content of less than about 3.5% at a moderate and substantially uniform rate as characterised by the web exit temperature of the drying method used, as for example, using convection (i.e., air), radiation (i.e., infrared) or conduction (i.e., heated rolls), or in the form of a low pressure heated nip (i.e., gloss calender). The paper produced has a high level of gloss and smoothness and the preferred post drying conditions minimize losses of surface properties obtained by high moisture finishing. This process effectively eliminates heat roughening of the printed surface during a subsequent web offset printing process. Heat roughening may be characterised by a loss in smoothness of the paper surface during printing as measured, for example, by its Bekk smoothness. Moreover, this same type of heat roughening may occur during any post drying step applied to a high moisture finished paper web. The Bekk smoothness test is an air leak method commonly used in the paper industry. In the Bekk test, the relative smoothness of the paper surface is measured by the time (in seconds) that it takes for a fixed volume of air to leak from between the surface of the paper and the smooth face of the Bekk instrument. The smoother the paper surface, the longer it takes for the fixed volume of air to escape. It is known that the smoothness of a paper web may be enhanced by finishing the web at a high temperature and pressure, and at a high moisture content. Smoothness and gloss generally increase during the finishing process as the moisture content is increased within the range of from about 4-10%. Above 10% moisture, both opacity loss and blackening generally occur. The finishing according to the present invention may be by supercalender or by a synthetic roll calender since either method may be practiced to yield about the same improvements in finish. However, finishing at high moisture to improve the smoothness of the paper web entering the press does not solve the heat roughening problem which occurs during web offset printing. In fact, there appears to be a relationship between the heat roughening effect during web offset printing and the moisture content of the web entering the press. If the moisture content of the web entering the press is greater than about 3.5%, the printed smoothness will almost always be less than the unprinted smoothness, notwithstanding the improvements achieved by high moisture finishing. Meanwhile, if the web is dried to a moisture content of less than about 3.5% before printing, the printed smoothness will almost always be greater than the unprinted smoothness. However, the smoothness gained by high moisture finishing is affected by the rate of drying during any post drying step used to reduce the moisture to 3.5% or less for printing. Therefore, to achieve the best results with the present invention, the web is preferably finished at a nigh moisture content in the range of from about 4-10%, and then moderately and uniformly dried to a moisture content of 3.5% or less in such a manner that losses in the smoothness gained by high moisture finishing are minimized. The post drying step may be carried out with any number of available methods or combinations thereof, including radiation, convection, and conduction. The amount of finishing needed in the present invention is dependent upon the specifications for the grade being produced. While the mechanism of the present invention is not completely understood, heat roughening is believed to be due either to fiber debonding and swelling from a very fast water release or to stress relaxation from water imbibition into the fibers. This occurs from a combination of the fast rate of web drying and the fountain solution water used in offset printing. Thus, it is believed that after high moisture finishing, the use of moderate and substantially uniform drying prior to printing permits the previously wet paper fibers to become internally bonded to permanently lock in the desired printing characteristics and thereby reduce the heat roughening effect. Brief Description of Drawing Figure 1 is a bar chart showing the effect of web offset printing on surface roughening; Figure 2 is a graph showing the effect of air impingement drying on surface smoothness; Figure 3 is a graph showing the effect of non-impact (IR) drying on surface smoothness; Figure 4 is a bar chart showing the effect of web offset printing on surface smoothness of paper that is conventionally finished, and then finished and dried before printing according to the present invention; and, Figure 5 is a schematic illustration of the process according to the present invention. Detailed Description Heat roughening is a phenomenon that occurs when paper in web form at a moisture content of more than 3.5% is printed by an offset printing process, or when a web finished at high moisture is post dried under less than optimum conditions. It is a micro size surface phenomenon that may be detected visually, and shows up in smoothness measurements particularly as measured by the Bekk smoothness test. In web offset printing, inks are applied to both surfaces of the paper web simultaneously followed by relatively severe air impingement drying to dry the inks. The heated air impinging on the surface of the web roughens the web and substantially reduces its smoothness if its moisture content is greater than about 3.5%. In order to overcome the effects of neat roughening upon web offset printing and to provide the printer with a sheet of high gloss and smoothness, the present invention proposes a two step process whereby the web is first finished, either by supercalender or with a synthetic roll calender, at a relatively high moisture content in excess of about 4%, and preferably in the range of from about 4%-10%, and then dried by convection, radiation, or conduction, so as to minimize any loss in finish, to a moisture content of 3.5% or less. The actual moisture content used for finishing will depend upon the temperature, pressure and web speed selected for the finishing step. The initial moisture content selected is a highly important feature of the present invention for achieving a smooth surface initially with high gloss and opacity, since if the finishing step is carried out at too high of a moisture content, a condition may be reached where the web may suffer severe opacity loss, blackening or galvanizing. These conditions are a function of the temperature level and temperature profile in the Z-direction of the web which is achieved in the calendering nip. Thus, the critical moisture content of the web for finishing according to the method of this invention will vary with the type of paper, and with temperature, pressure, web speed and finishing method. In accordance with the present invention, the initial moisture content of the web entering the supercalender or synthetic roll calender is preferably greater than about 4% but is below the moisture at which blackening, galvanizing or opacity loss might occur. If the finishing apparatus is operated on-machine, it will ordinarily be a simple matter to control the amount of drying on the paper-machine to give the desired moisture content for finishing. When the finishing apparatus is operated off machine, it may, in some instances, be necessary to add moisture to the web before finishing. The maximum permissible moisture content for a given set of conditions, i.e., the moisture content at which the above mentioned detrimental effects might occur, can easily be determined by routine experimentation with the particular paper and finishing apparatus involved. When finishing with a supercalender, typical operating conditions comprise a load of about 210-438 kN/m (1200-2500 pli) to yield nip pressures in excess of 13.8 MPa (2000 psi); a temperature of between about 38-99°C (100-210 degrees F) (steel roll surface temperature); and a web speed on the order of from about 305-914 m/min (1000-3000 fpm). Loads up to about 700 kN/m (4000 pli) may be used in the supercalender at greater speeds depending upon equipment availability. For a synthetic roll finishing device comprising one or more heated drums and one or more synthetic soft rolls in nipped relation to a heated drum, typical operating conditions comprise a steel roll surface temperature of about 121-177°C (250-350°F); web speed 305-1067 m/min (1000-3500 fpm); and operating loads of 210-525 kN/m (1200-3000 pli) to yield nip pressures in excess of 13.8 MPa (2000 psi). For a synthetic roll calender temperatures up to about 232°C (450°F) and web speeds to 1524 m/mn (5,000 fpm) may be acceptable. One or more nips of the supercalender or synthetic roll calender may be used depending upon the type of paper, the coat weight and the finish desired. Some moisture is lost during the finishing step, but in order to achieve the reduction in heat roughening according to the present invention, the web must be post dried to a moisture content of less than about 3.5% after finishing and before printing. Synthetic rolls suitable for the present invention are available from a number of suppliers, and includes rolls identified as Beloit XCC, Kleinewefers Elaplast, Stowe Woodward Plastech A, and Kusters Mat-On-Line. The post drying step is preferably conducted at a moderate and substantially uniform rate which minimizes any losses in the finish achieved by high moisture finishing. The preferred drying method for existing equipment with space limitations would be a non-impingement method, for example, with the use of IR (Infrared) heaters. However, other drying techniques including air impingement if done under appropriate conditions or the use of a low pressure heated nip (gloss calender) formed by a soft synthetic roll and a heated steel roll have also been found to give satisfactory results. Low pressure in this instance means less than about 13.8 MPa (2,000 psi) for most grades of paper coated or uncoated. The object of the post drying step is to dry the web at a drying rate that may be characterized by the web exit temperature from the drying apparatus so that the web is dried to achieve the internal fiber bonding mentioned before and to prevent fiber debonding upon offset printing. Thus, any known method for drying paper webs may be used in the practice of the present invention if properly configured. Example I In order to demonstrate the heat roughening effect of the printing surface in a web offset printing press, coated paper was commercially supercalendered with a moisture content of 4%, 6% and 8%. Calendering conditions were 610 m/min (2000 fpm), 210-280 kN/m (1200-1600 pli) and 82°C (180 °F). A first set of this supercalendered paper was printed once using a sheet fed process. Two additional sets were printed twice on different presses by the web offset process. All printing conditions were conducted on the finished paper without post drying. Smoothness of the paper including an unprinted control sample was measured by Bekk. The results are illustrated in Figure 1. According to the data in Figure 1, the unprinted smoothness of coated paper increases with increasing moisture content upon finishing as expected. Meanwhile, the printed smoothness of the sheet fed paper shows little if any change from the unprinted smoothness. That is, there is little or no heat roughening produced by the sheet fed printing process. On the other hand, the printed smoothness of the web fed paper decreases dramatically particularly as the moisture content increases. Thus it may be seen that the web offset printing process produces the heat roughening effect observed during the development of the present invention. Example II To show the effect of the drying rate on the heat roughening effect, coated paper finished on a supercalender at 8% moisture was post dried by air impingement to simulate print-drying by using the drier of a web offset press. The air temperature was varied over four different conditions (143, 171, 199 and 218°C) (290, 340, 390 and 425 degrees F.) and three different speeds (152, 229 and 305 m/min)(500, 750 and 1000 fpm) to achieve different drying rates. Paper moisture entering the drier was 6.7% since 1.3% moisture was lost during the supercalendering step. Figure 2 shows the decrease in Bekk smoothness for each condition of temperature and drying rate (speed). From these results it can be seen that as the temperature of the drier increased, the smoothness of the web decreased at a given moisture content, thus demonstrating that air impingement post drying can roughen a paper surface as might happen on a printing press, with the roughening increasing as the temperature and drying rate increases. Example III Samples of the same paper used in Example II were dried by IR (Infrared) heaters, a non-impingement drying method, at 50% and 100% output. Paper moisture entering the drier was approximately 6.7%. Figure III illustrates the effect of drying the web using a non-impingement method, and particularly the reduced degree of roughening that is achieved with a moderate drying rate, i.e., at 50% output of the IR driers as opposed to 100% output. For the paper dried at 100% output, Bekk smoothness decreased from about 2300 to 1100 seconds. Meanwhile at 50% output, Bekk only dropped from about 2300 to about 1800 seconds. Example IV A printed evaluation of coated web offset printing paper demonstrated the effectiveness of high moisture finishing and post drying according to the present invention. Several paper samples were finished at 4% and 8% moisture on a supercalender. Finishing conditions were 610 m/min (2000 fpm), 82 °C (180°F) and 210-280 kN/m (1200-1600 pli) to achieve a nip pressure in excess of 34.5 MPa (5000 psi). Some of the samples were then dried by IR at different drying rates to moisture contents ranging from about 2.8% to 3.2%. The post drying conditions and web exit temperatures are shown in Table I. Samples 3-8 had a Bekk smoothness of 2095 before drying. Sample 9 had a Bekk smoothness of 717 before drying. The post-IR web exit temperatures were measured with a non-contact IR pyrometer approximately 0.3 m (1 foot) after the exit of the drier. The temperature must be measured a sufficient distance from the drier to eliminate any drier effects which might influence the actual measurement. HIGH MOISTURE FINISHING RADIANT POST DRYING SAMPLE DRYING CONDITION MOISTURE % BEKK SMOOTHNESS POST DRIER EXIT TEMP °C (°F) W i W F UNPRINTED PRINTED 1 NONE 8 6.4 2095 841 - 2 NONE 4 4.2 717 619 - 3 2 Units 100% 183 m/min (600 fpm) 8 2.8 973 1266 149 (300) 4 1 Unit 100% 55 m/min (180 fpm) 8 2.9 1214 1396 127 (260) 5 3 Units 65% 183 m/min (600 fpm) 8 3.0 1284 1648 141 (285) 6 2 Units 45% 55 m/min (180 fpm) 8 3.0 1619 1973 132 (270) 7 3 Units 35% 55 m/min (180 fpm) 8 3.4 1969 2365 96 (205) 8 3 Units 100% 305 m/min (1000 fpm) 8 3.2 1005 1403 149 (300) 9 3 Units 100 % 610 m/min (2000 fpm) 4 3.2 619 635 121 (250) Note: W i represents moisture content into finishing device. W F represents moisture content after post drying. The data in Table 1 and Figure 4 shows that if the paper is post dried to a moisture content of less than about 3.5% the printed smoothness is almost always greater than the unprinted smoothness. However in this example, severe post drying at a web exit temperature greater than 149°C (300°F) substantially reduced the benefits obtained by high moisture finishing (see for example Sample 8 which lost about one half of its original smoothness at a web exit temperature of 149°C (300°F)). Therefore, according to the present invention, the use of web exit temperatures to characterize the performance of the present invention is a reasonable approach. Example V A study was conducted to optimize the effectiveness of various post drying techniques. Paper samples finished by both supercalendering (SC) and synthetic roll calendering (SRC) were post dried by IR, dryer can (CAN) and low pressure gloss calender nip (GC). The GC post drying process used a nip pressure of about 11 MPa (1600 psi), or less than the nip pressure normally used for finishing. The data for a 31.8 kg (701b.) coated web offset paper is shown in Table II. Similar results were obtained for other paper grades. These data indicate that optimum post drying of a web at about 6% moisture using various drying methods can be achieved with web. exit temperatures in the range 93-127°C of (200-260°F). For this example web exit temperatures for the contact post drying methods were measured with a non-contact pyrometer just after the web separated from the heated drum. HIGH MOISTURE FINISHING COMPARISON OF RADIANT AND CONDUCTION POST DRYING SAMPLE FINISHING DRYING METHOD MOISTURE % BEKK SMOOTHNESS POST DRIER EXIT TEMP °C (°F) W i W f BEFORE AFTER 1 SUPER GC 6.1 2.7 944 973 104 (220) CAN 2.9 1032 93 (200) IR 3.1 829 124 (255) 2 149 °C (300°F) GC 6.1 2.8 1314 1650 104 (220) SRC CAN 2.8 1323 96 (205) IR 2.8 1331 127 (260) 3 177 °C (350°F) GC 6.2 3.1 1750 2048 104 (220) SRC CAN 3.1 1786 99 (210) IR 3.1 1843 127 (260) 4. 204 °C (400°F) GC 6.5 3.2 1835 2128 110 (230) SRC CAN 3.2 2258 96 (205) IR 3.6 2222 124 (255) As will be apparent to one skilled in the art from the description and examples provided herein, the method of the present invention provides important advantages for the manufacture of web offset printing paper. By virtue of essentially a two step finishing process, the papermaker can provide the printer with high quality paper having exceptional performance on the web offset press. Thus, while the invention has been described in some detail with particular reference to the preferred embodiments, it will be understood that variations and modifications can be effected herein within the cope of the invention as defined in the appended claims.

Claims: A process for preparing a paper web for use on an offset press which will not experience heat roughening during offset printing comprising drying the web to a moisture content of 3.5% or less before printing. The process of claim 1 wherein the paper web is finished to enhance its gloss and smoothness before drying comprising, finishing the web in a finishing device at a nip pressure of at least about 13.8 MPa (2,000 psi) and a moisture content in the range of from about 4-10% by weight. The process of claim 2 wherein the finishing device comprises a supercalender apparatus consisting of a plurality of hard rolls and soft rolls wherein the web is passed through at least one nip formed by said hard and soft rolls. The process of claim 3 wherein at least one of the hard rolls in the supercalender is heated. The process of claim 2 wherein the finishing device comprises a hard finishing roll and at least one soft backing roll wherein the web is passed through at least one nip formed by said finishing roll and said backing roll. The process of claim 5 wherein the finishing device comprises a second hard finishing roll and at least one additional soft backing roll wherein the web is passed through at least one nip formed by said second hard finishing roll and said additional soft backing roll and at least one of the hard finishing rolls is heated. The process of claim 6 wherein the drying step is carried out with at least one gloss calender nip at a nip pressure of less than about 13.8 MPa (2,000 psi) and a calender roll temperature of between about 38 and 204 °C (100 and 400 °F . The process of claim 2 wherein the drying step is selected from the group consisting of convection, radiation or conduction. The process of claim 2 wherein the drying step is conducted at a moderate and substantially uniform rate so as to retain as much as possible of the finished improvement obtained by the finishing step. The process of claim 2 wherein the moisture content of the web in the finishing step is in the range of from about 6-8% by weight. The process of claim 2 wherein the drying step is conducted at a drying rate constant as determined by the formula: where W o and W f are the initial and final moisture contents of the web as measured in kg/m 2 (lb/ream) and t is the residence time of drying in hours, said drying rate constant being selected to achieve a web exit temperature that will permit the web to retain at least about 50% of the enhanced gloss and smoothness obtained by the finishing step. The process of claim 11 wherein the drying device is selected from the group consisting of convection, radiation and conduction. The process of claim 12 wherein the finishing device is selected from the group consisting of a supercalender or a synthetic roll calender. An offset printing paper web which will not heat roughen during printing, the web having a moisture content of 3.5% or less prior to printing. The paper web of claim 14 wherein a coating is applied to at least one surface of the web. The process of claim 2 wherein the finishing device comprises a first hard finishing roll and at least one soft backing roll and a second hard finishing roll and at least one additional backing roll, wherein the web is passed through at least one nip formed between said first hard finishing roll and said backing roll and at least one nip formed between said second hard finishing roll and said additional backing roll, and wherein at least one of said finishing rolls is heated to a surface temperature in excess of about 121 °C (250 °F), and the drying step is conducted at a web exit temperature of less than about 149 °C (300 °F). The process of claim 16 wherein the web exit temperature is in the range of from about 93-127 °C (200-260 °F). The process of claim 17 wherein the drying step is selected from the group consisting of convection, radiation and conduction. The process of claim 2 wherein the finishing device comprises a supercalender apparatus consisting of a plurality of hard rolls and soft rolls wherein the web is passed through at least one nip formed between said hard and soft rolls, and wherein at least one of said hard rolls is heated to a surface temperature in the range of from about 38-99 °C (100-210 °F), and the drying step is conducted at a web exit temperature of less than about 149 °C (300 °F). The process of claim 19 wherein the web exit temperature is in the range of from about 93-127 °C (200-260 °F) and the drying process is selected from the group consisting of convection, radiation and conduction.

Inventors: CASON DAVID B, HUIJING PAUL A, JOHNSON ERIC D, ORTMAN BRYAN J, PETRO S CRAIG, CASON, DAVID B., HUIJING, PAUL A., JOHNSON, ERIC D., ORTMAN, BRYAN J., PETRO, S. CRAIG
Applicants: WESTVACO CORP, WESTVACO CORPORATION
Application year: 1992
Main classification: D21G1
IPC codes: D21G1