Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21G—CALENDERS; ACCESSORIES FOR PAPER-MAKING MACHINES
  • D21G9/00—Other accessories for paper-making machines
  • D21G9/009—Apparatus for glaze-coating paper webs

Definitions

• This invention relates generally to the manufacture of paper and in particular to a novel method of finishing printing paper in a manner which improves its properties .
• High quality printing paper must have a number of physical properties . Two of the most important are a flat and smooth su rface to facilitate printing in a press and gloss to produce a more attractive su rface, particularly after printing . These properties can be obtained by a variety of techniques, such as coating the paper with pigments and binder and finishing it in one or more pressing operations .
• One of the most common finishing operations employed in the manufactu re of printing paper is supercalendering, in which paper is passed th rough a series of nips formed by steel rolls pressed against cotton filled rolls at very high pressu res, typically at nip loads between 175 KN/M and 437.5 KN/M (1000 and 2500 pounds per lineal inch) . This typically results in nip pressu res of 13, 780 KN/M 2 to 27, 560 KN/M 2 (2000 to 4000 p . s . i . ) .
• Supercalenders commonly consist of a large number of rolls (9 to 14) , alternating steel and resilient, in order to obtain the desired smoothness and gloss . I n order to obtain smooth ness on both sides it is necessary to run an even number of rolls and with two resilient rolls (so called “cushion rolls”) running together midway in the stack to perform the necessary reversing of the side toward the steel rolls . This action is only partly successful at providing two smooth sides since the first side finished towards the steel is later deformed by the exposu re to the resilient rolls .
• finishing is machine calendering wherein the paper web is passed between two normally unheated steel rolls pressed together at high pressures . This process produces smoothness, but little gloss because of the absence of shear in the nip .
• gloss calendering which uses heated finishing rolls to produce high gloss finishes on coated paper or board without the high pressu re of supercalendering .
• the nip pressures for commercial machines are typically between about 87.5 to 175 KN/M (500 to 1000 pounds per lineal inch) of nip loading . This typically results in nip pressures of 6,890 KN/M 2 to 13, 780 KN/M 2 (1000 to 2000 p. s . i . ) .
• the lower pressure causes less densification of the paper, and therefore, better opacity, while the higher temperature softens the coating and permits better gloss enchancement.
• the finishing effect is limited to the coating and the uppermost surface of the web .
• su rface of the sheet is not as smooth and flat as that produced in supercalendering and has generally been applied to coated board rather than high quality papers .
• gloss calendered sheets do not print as satisfactorily in a printing press as do supercalendered .sheets .
• gloss calendering molds, flattens, and glosses the surface of the coating and, in the case of uncoated paper the top surface of the fibrous substrate, but compacts the remainder of the sheet much less than supercalendering. Examples of gloss calendering are disclosed in U.S. patents Nos.
• the one parameter which has been found to be the most critical in gloss calendering and supercalendering has been the moisture content of the paper. High moisture improves the smoothing and glossing effects of both the coating and the paper substrate. Many developments in supercalendering and gloss calendering involve tech niques for increasing the moisture in the web or at least in some portions of it before finishing .
• moistu re is an undesirable control parameter. Small variations in moistu re cause large variations in the finished properties of the paper. Also, it is undesirable to have more than about 3.5% to about 4.5% moistu re in the finished sheet to avoid uneven reel building and sheet cu rl from later drying . This amount of moistu re is a stable amount, and the sheet will not dry significantly below this level under ambient conditions . To have a finished product with the desi red low moistu re content and still have the desired high moistu re content (e. g . 7% to 9%) to facilitate calendering, many heated calendering operations have increased the drum temperatu re to dry the moister webs .
• Nonuniformity of moisture in the sheet can be even a bigger problem than too much moisture.
• nonuniformity it is meant that the moisture content at one place on the sheet is higher or lower than at other locations across the width of the sheet.
• the nonuniformity can also exist in the machine di rection and the thickness of the sheet. Nonuniformity is most severe when calendering takes place immediately after coating, which is to say when the calender is in line with the coater. If coating is done in a separate operation from calendering, the moistu re content of the coated paper has time to equalize th roughout the web before calendering .
• the above cited patent No ' . 3, 124, 504 is primarily concerned with very moist webs (up to 35% or 50% moisture) and includes the concept of drying the web while finishing it. Very high temperatures are employed for drying, but temperatu res above the boiling point of water are said to be needed only if the web is wetter than 5% to 8% of the bone dry weight.
• the web moistu re content is also noted as being an important element in the process disclosed in above cited patent Nos . 3,442, 685 and 3,451 ,331 .
• the patents teach that it is best for the paper to have about 7% moistu re content, and moisture can be added before the supercalender to improve the finishing effects .
• the present invention is a new process which permits the manufacture of paper with supercalender smoothness and gloss without the above noted disadvantages of supercalendering.
• the invention is a process for producing gloss and smoothness on the surface of a paper web, comprising the steps of:
• A. providing a finishing apparatus comprising a smooth metal finishing drum and a resilient backing roll pressed against the drum at a force of up to 700 KN/M (4000 pounds per lineal inch) to form a nip with pressure against the paper web of at least
• step B heating the drum to a surface temperature having a value no less than 20°C below the value determined by the following formula:
• Ts [Ti x .357t _ - 479 - 234.2e " ,l31m] /[.357t ",479 -1]
• Much of the prior a rt discloses broad operating conditions in which some of the conditions of the present invention fall , but fail to teach the special requirements for low moistu re paper and are far too broad in their disclosu res for one to appreciate the present critical operating range. They all either calender at a temperatu re and/or pressure below the present invention , calender the web too wet, or teach a very broad temperatu re range which might accidentally include the present range.
• the Tg of cellulose in paper is greatly dependent upon the moisture content of the paper and is very low for papers as moist as those traditionally supercalendered .
• this very property which facilitates supercalendering also results in the undesi rable ultra sensitivity to moistu re variations and the undersirable ultra densification through the entire thickness of the web .
• some of the prior art relating to gloss calendering recognized the effects of temperature on moldability of the coating and the su rface fibers of uncoated paper, none recognized the existence of a critical strata beneath the su rface of the fibers which must be molded flat to obtain the flatness and smoothness of supercalendering .
• the invention which can be described as substrata thermal molding , is based upon molding the critical substrata of the web into a flat strata permitting the su rface of the fibrous web and any coating to be flattened, smoothed and glossed to the degree obtainable by supercalendering .
• This strata is the foundation for the su rface, and molding below this level is not critical to obtaining supercalender flatness .
• Th us the molding of the entire thickness of the sheet as in supercalendering is unnecessary, provides little advantage, and results in the previously noted disadvantages .
• the present invention does not requi re a web as moist as those generally subjected to supercalendering and gloss calendering .
• the present invention performs satisfactorily on a web having a moistu re content less than 7% of the bone dry weight of the fibers and even less than 6% or 5%.
• the invention works satisfactorily at even lower moisture contents, even as low as 3%. Consequently, finished products can be easily produced at desi rable moistu re levels without having to dry them in the finishing process .
• the ability to finish the web at lower moistu re contents permits drying down the web immediately before finishing to a low level where moistu re content is substantially uniform th roughout the web, preferably with no variation g reater than 0.5% from the average.
• the invention is particularly valuable where coating and finishing are done continuously in line with each other. It is even more valuable when coating and finishing are done continuously in line with the papermaking machine.
• the finishing apparatus includes a second resilient backing roll pressed against the drum preferably within the same pressu re range as the first to form a second nip .
• the web is advanced through the second nip after the fi rst nip within a short period of time, less than 4 seconds, to provide a great advantage, uniquely valuable to this invention and explained as follows .
• the key to the invention is to heat a critical substrata of the web to its Tg .
• this requi res a drum su rface temperature hotter than the Tg .
• the Tg increases with reduction in moisture.
• the enti re web and most importantly the critical substrata, has a temperatu re raised above its previous temperatu re, but below its Tg, when it enters the second nip .
• I n the second nip the same type of temperatu re gradient that existed in the fi rst nip is established, but with the interior temperatu re of the web higher than before.
• Th us the critical portion of the web can be brought to the critical temperatu re using a lower drum temperatu re or faster process speed than needed with only a single nip.
• the additional pressing time provided by two nips will result in su rface improvements also.
• the web will be passed through the nip or nips without contacting the heated drum except in the nips for the reasons stated above. However, there may be cases where it is desi rable and not too disadvantageous to have some additional drum contact. I n those cases, it will be preferable to limit the contact to less than 20% of the drum circumference.
• B RI EF DESCRI PTION OF THE DRAWI NGS Fig . 1 ill ustrates schematically an apparatus suitable for practicing the present invention ;
• Fig . 2 is a graph illustrating the gloss and smoothness values for the uncoated paper finished at various temperatures in Example 1 ;
• Fig . 3 is a graph illustrating the gloss and smoothness values for the coated paper finished at various temperatu res in Example 2
• Fig . 4 is a graph illustrating the gloss and smoothness values for the coated paper finished at various temperatu res in Example 3;
• Fig . 5 is a graph showing the dynamic Tg of cellulose fibers for various moistu re contents ;
• Fig . 6 illustrates schematically the temperatu re gradient into the thickness of the paper in a nip of the apparatus illustrated in Fig . 1 ;
• Fig . 7 is a graph showing the temperatu re gradient into the thickness of the web for various dwell times of the web in the nip;
• Fig . 8 is a graph showing the drum surface temperature required for the invention for various moisture contents and various dwell times ;
• Fig . 9 is a graph illustrating the gloss values for the coated paper finished at various temperatures and pressu res in Example 4.
• BEST MODE FOR CARRYI NG OUT TH E I NVENTION The following definitions are provided to better understand these terms in this specification and claims .
• Parker Print-Su rf - a quantitative measu rement commonly used in the papermaking field for the printing roughness and porosity of paper made by sensing the leakage of ai r at low pressu re between the su rface of the sample and the measu ring sensing head . The lower the value, the smoother the paper.
• Parker Print-Su rf can be measu red with several different pressu res of the dam against the paper being measured .
• Supercalendered coated woodfree paper will typically have a Parker Print-Su rf of less than 1 .4 and less than 1 .0 for very high quality.
• Gloss calendered coated woodfree paper will typically have a Parker Print-Su rf of between 1 .2 and 2.0.
• the present invention can be carried out on an apparatus like that illustrated in Fig . 1 .
• a paper web 1 is advanced th rough the fi rst nip formed by smooth su rface finishing drum 2 and resilient backing roll 3, around guide rolls 4, and th rough a second nip formed by drum 2 and a second resilient backing roll 5 pressed against drum 2.
• the web 1 is advanced to a second smooth surface finishing drum with a pair of nips formed by resilient backing rolls similar to the fi rst unit (not illustrated for simplicity) .
• the finished web is then wound onto reel 6. Variations in the process can be carried out by omitting or bypassing the second nip on each drum and/or finishing on one side only, in which case the second drum is bypassed or omitted .
• the web 1 supplied to the finishing apparatus can come directly from a papermaking machine 7 and/or coater 8, if the paper is to be coated . I n the alternative, the web 1 can be supplied from a roll of previously manufactured paper which may or may not have already been coated .
• the papermaking machine and coater are illustrated only as blocks since they can be provided by any conventional apparatus well known in the art.
• the finishing apparatus employed in the invention can be provided by any of the many disclosed in the previously described prior art relating to gloss calendering if they are designed or can be adapted to operate at the temperatu re, pressu re and speed conditions of the invention . Accordingly, little description of the apparatus will be given herein except to emphasize the importance of choosing a finishing drum which can be heated to the temperatures required by the invention and has a smooth metal su rface and choosing a resilient backing roll which is yieldable but will have sufficient hardness at operating temperatu res to provide a nip force between 35 and 700 KN/M (200 and 4000 pounds per lineal inch) of nip, which could requi re pressu res as high as 6000 KN/M 2 (8, 700 p .
• nip width of from 1 .27 to 2.54 cm (0.5 inch to 1 .00 inch) for the present invention .
• Nip widths shorter than 1 .27 cm and longer than 2.54 cm could be usable with the invention .
• the backing roll su rface prefferably has a P . &J . hardness of about 4 or harder at operating temperatu res to develop the desi red nip width and pressu re. To maintain this hardness may requi re internal cooling of the roll , since the typical resilient roll materials become soft very quickly at elevated termperatu res .
• An example of a roll which can perform satisfactorily in the invention is disclosed in U . S . Patent No. 3, 617,445.
• Example 1 An uncoated and uncalendered bodystock of a mixtu re of Northern hardwood and softwood fibers produced in a Kraft pulping process was unwound from a roll and passed through an apparatus similar to that illustrated in Fig. 1.
• the web had been mineral filled and sized to have 10% ash content by weight, and the web weighed 93.3 g/m 2 (63 pounds per ream of 3300 ft 2 ).
• the finishing apparatus was operated with only one nip at a force of 175 KN/M (1000 pounds per lineal inch) and a nip width of .47 cm (.185 in).
• the temperature of the web was about 26.7°C (80°F) just before entering the nip.
• the moisture content of the web was measured to be 4.8% of the bone dry weight of the fibers.
• the web was passed through the finishing apparatus at 1.02 m/s
• Example 2 A bodystock like that of Example 1 was coated on one side with a conventional pigment binder coating having a weight of 14.8 g/m 2 (10 pounds per ream of 3300 ft 2 ), dried and passed through the same apparatus and same procedure as Example 1, except the finishing drum surface temperature was adjusted from 25.6°C (78°F) to 190.6°C (375°F). The coater was in line with the finishing apparatus. The moisture content of the coated web was about 3.9% of the bone dry weight of fibers. The temperature of the web was about 48.9°C
• Example 3 A bodystock like that of Examples 1 and 2 was coated on both sides with coatings of the same type and amount as in Example 2 and passed through a finishing apparatus in line with the coater and similar to that employed for Examples 1 and 2, but with two finishing drums. Each of the drums had two resilient backing rolls forming a pair of nips. One side of the paper was finished against one drum and the other side against the other drum. The nip pressure for the first drum was varied during the test from 263 KN/M (1500 pounds per lineal inch) to 333 KN/M (1900 pounds per lineal inch). The nip pressure on the second drum was held at 333 KN/M (1900 pounds per lineal inch) and its drum surface temperature at 162.8°C (325°F) throughout the test.
• the moisture content of the web was about 4.7% just prior to the first drum and about 0.5% less at the second drum. (The decrease was due to evaporation of moisture from the heated web surface between drums.)
• the web was passed through the nips at 8.89 m/s (1750 feet per minute).
• the nip widths were about 2.21 cm (.87 in), resulting in a nip dwell time of about 1.5 milliseconds.
• the temperature of the web was about 71.1°C (160°F) just before entering the first nip. Samples of the product produced were taken at the following conditions for the first side and first finishing drum.
• Example 4 An uncoated and uncalendered bodystock of a mixture of Southern hardwood and softwood fibers produced in a kraft pulping process was prepared for this example.
• the web was mineral filled and sized to have an ash content of about 10% by weight.
• the web weighed about 79.9 g/m 2 (54 pounds per ream of 3300 ft. 2 ).
• the web was coated on one side with a conventional pigment binder coating having a weight of 12 g/m 2 (8.1 pounds per ream of 3300 ft. 2 ), dried and passed through an apparatus similar to that illustrated in Figure 1. The apparatus was operated with both finishing nips.
• the coater was in line with the finishing apparatus. After coating, the web was dried and it entered the finishing apparatus at a moisture content of about 4.0% of the bone dry weight of the fibers and with a web temperature of about 60°C (140°F) just before entering the first nip.
• the web was passed through the finishing apparatus at a speed of 2.73 m/s (500 feet/min.), resulting in nip dwell times of 3.21, 3.59, 3.91, and 5.0 milliseconds for the aforementioned pressures. At each of these pressures, the temperature of the drum surface was allowed to drop from a starting surface temperature of 177°C (350°F) to a temperature of 110°C (230°F) at the finish while taking on-machine measurements for 75° Hunter gloss values.
• Fig. 2 the curve is shown in two portions, the left covering temperature ranges up to about 110°C (230°F) and the right from about 104.4°C (220°F) up. On the left, one can see that gloss and Parker Print-Surf increase at a steady rate with increasing temperature up to about 104.4°C (220°F). This is believed to be the effects from molding and coalescing the surface of the web and is what one would expect from the prior art.
• Fig 2 On the right side of Fig 2 is illustrated the unexpected results of the invention. That is, at a specific temperature, about 110°C (230°F) in this case, there is a sudden rapid improvement in Parker Print-Surf for increasing temperatures. There is also a similar increase in gloss, and this is believed to be due to the interrelationship of flatness to gloss. This additional increment of gloss and flatness was unexpected, but once discovered is believed to be due to the portion of the web beneath the surface, or the subsurface strata, being heated to its glass transition temperature and suddenly softening and becoming moldable to allow the surface to be flattened to a greater degree than before. The advantages provided by the thermal moldability of the subsurface strata continue only up to about 148.8°C (300°F), after which there is no improvement in gloss or flatness for the next I6.7°C (30°F).
• Fig.3 displays a similar phenomenon to Fig. 2.
• the Parker Print-Surf and gloss increase at a steady rate with increasing temperatu re up to about 93.3°C (200°F) , after which there appears to be no fu rther increase with increasing temperature.
• This flattening of the cu rve is believed to be due to the behavior of coating being thermally molded and is believed to be what one would expect from the prior art.
• This may also explain why gloss calendering, which is more temperatu re controlled than supercalendering, was thought to have limited ability to improve Parker Print-Su rf values .
• On the right of Fig . 3 is illustrated the results of the invention . At about 126.7°C (260°F) there is a rapid improvement in gloss and flatness for the next 36.8°C (65°F) . This result is totally unexpected .
• the same phenomenon which facilitates flattening of the critical substrata in the present invention causes the entire thickness of the web in supercalendering to be molded at a temperature above its Tg. The reason is that the high moisture content of paper employed in supercalendering, can result in a Tg low enough to be reached throughout the web by the temperature conditions of supercalendering, even when unheated.
• the first drum temperature in a two drum apparatus will be set for the moisture content at the second nip. If there are two drums, the second drum temperature will preferably be higher than the first to accommodate the lower moisture content of the web resulting from heating at the first drum. Since satisfaction of any needed drum surface temperature for any one nip will provide some of the advantages of the invention, this invention includes a process wherein one or more of the nip conditions do not satisfy the temperature requirements.
• Fig. 5 illustrates Tg values for cellulose fibers at various moisture levels.
• the curve was derived from the experimental work of N.L. Salmen & E.L. Beck (The Influence of Water on the Glass Transition Temperature of Cellulose, TAPPI Journal, Dec. 1977. Vol. 60, No. 12) and (Glass Transitions of Wood Components Hold Implications for Molding and Pulping Processes, TAPPI Journal, July 1982, Vol. 65, No. 7, pp. 107-110). The curve was adjusted for the dynamic conditions in a finishing nip.
• the Tg values have been increased over those derived by Salmen & Beck by about 12°C, since the yieldability of any polymer-like material will become less for any given temperature if the force is applied over a shorter time span. The result is that the Tg of the material appears to be higher at dynamic conditions than for static conditions. To make this adjustment, the Williams-Landel-Ferry equation was employed. The very large increase in Tg for small reductions in moisture content in the range of the invention, 3% to 7%, should be noted.
• the web dwells in the nip very briefly, due to short nip widths and fast operating speeds. For example consider nip widths of .635 to 2.54 cm U" to 1") and machine speeds of 2.54 to 25.4 M/S (500 to 5000 feet per minute).
• the web dwell time in the nip will be from 0.3 to 12 milliseconds. At these short dwell times, the heat from the drum does not penetrate very far into the web.
• Fig. 6 illustrates the temperature gradient into a web at 1.5 milliseconds of dwell time (corresponding to a nip width of 1.32 cm and a machine speed of 8.9 M/S).
• the drum surface temperature is 138°C
• the web temperature prior to entering the nip is 71°C
• the backing roll surface temperature is 71°C.
• the temperature gradient in the web was determined by the formula:
• Fig. 7 illustrates the temperature gradient into the thickness of the web for various nip dwell times. In this illustration the drum surface temperature is 137.8°C (280°F) and the paper temperature just prior to reaching the nip is 71 °C.
• the approximate location of the critical substrata is believed to be about .0076 mm (0.3 mils ) deep and is illustrated by the cross-hatched portion. It can be seen that the temperature of the critical substrata will depend upon dwell time and surface temperature. Whether or not the critical substrata temperature is as high as its Tg will depend in part upon its moisture content. Thus, for the conditions illustrated in Fig . 7, the critical temperature will be reached for moistu re contents from 5% to 7.5%, depending upon the dwell time chosen .
• the drum su rface temperatu re needed for a web entering the nip can be determined by the formula
• Ts [Ti .357t .479 - Tg]/[ .357t - .479
• the Tg can be determined from the curve in Fig . 5.
• a formula which very closely approximates that cu rve is the following :
• Tg glass transition temperatu re under the dynamic and moisture conditions existing in the nip in °C
• e the base of the natural logarithm
• m moisture content of the fibers in web in % of the bone dry weight of the fibers.
• Ts drum surface temperature
• Example 2 where moisture content was about 3.9%, nip dwell time was 4.5 milliseconds, and initial web temperature was about
• the Ts value is about 161.7°C (323°F). Looking at Fig. 3, this value, illustrated by the line identified as Ts, can be seen to be at the top of the temperature range where the unexpected rise in gloss and flatness occur also. The advantages of the invention actually begin about 40°C (70°F) lower. This is considered good correlation with the results for Fig. 2.
• Example 3 produced too little data to produce the full curves of the other examples, but the temperature settings in that test were chosen in accordance with the above formula with the intent to show the inflection of gloss and flatness near the unexpected rise.
• Moisture content of 4.7%, nip dwell times of 1.5 milliseconds, and initial web temperature of 71.1°C (160°F) result in a calculated Ts value of about 153.9°C (309°F).
• Fig. 4 shows by the line identified as Ts where this point is located on the gloss and flatness curves. This part of the cu rve appears to correspond to the end of the unexpected rise, this being consistent with the results from Examples 1 and 2 and the formula .
• the softening of polymeric materials is a second order transition and occurs over a range of temperature rather than sharply as in a first order transition , such as in the melting of ice.
• the breadth of the range is also a function of the molecular weight distribution with a wider distribution giving a wider range. This same softening may occur prior to reaching the temperature where the maximum effects are noted .
• None of these components need to be known precisely to develop a useful formula, because the formula need only be compared to the test results in the examples and a correction made to determine the starting and ending point of the unexpected rise in gloss and flatness . It is not known nor important to know which component or components have been estimated incorrectly, if any.
• Fig . 4 also includes in dotted lines the results of samples 3 and 7 of Example 3. They are located, as expected, slightly higher due to increased pressure effect of 2 nips, but in a nonimproving relationship to each other with increase in temperature. This is believed to be for the reason stated earlier, that two nips in rapid succession are equivalent to higher drum temperatu re. Thus, if the solid curves were extended into higher temperatu res in the manner predicted by Fig . 2, they wou ld be flat. The single point represents the higher pressure of sample 2. Fig .
• gloss only improves about 2 points when the nip pressu re is under 13, 780 KN/M 2 (2000 psi) , while it improves about 5 points when the nip pressure is over 13, 780 KN/M 2 (2000 psi) .
• the improvement at the higher pressu res is at the higher gloss range where a point of improvement is harder to obtain .
• the Ts not be exceeded by more than about 25°C, particularly for coated paper. It is also desirable to limit the depth of the web heated to its Tg to only the critical substrata . The reason is that all portions pressed which are hotter than the Tg will be excessively densified, in the manner of supercalendering, with the accompanying undesi rable loss in thickness and opacity. To obtain supercalender quality on the surface, only the critical substrata need be so densified and any additional flatness obtained by heating further into the web will be costly. The greater drum temperatu re, slower process speed, and/or greater sheet moisture needed to accomplish this reduce process efficiency, may requi re more expensive equipment and greater energy costs and can have the disadvantages of supercalendering .
• a fu rther su rprising and unexpected benefit was obtained from the invention . If one were to theorize the ideal finishing operation to produce glossy paper with the very smooth flat surfaces of supercalender quality, it would be necessary to closely evaluate the control parameters of pressure, temperature, moisture content, and dwell time in the nip.
• the one most controllable is pressure, because it can be changed precisely and instantaneously .
• the least controllable is moisture content, since it can be changed only slowly and is often difficult to maintain uniformly.
• the ideal process would be one in which large property changes result from small pressu re changes and small property changes result from large moistu re changes .
• the present invention provides control parameters which provide the ideal controls described above and also supercalender quality. These advantages cannot be obtained with supercalendering because its range for control parameters cause pressu re to be the least effective control and moistu re the most.
• the temperature effects of the invention are believed applicable for almost any pressu re applied in the nip . That is, it is expected that the effects of increasing pressu re will follow their known curve, except of cou rse, the results will be significantly better.
• the pressures will preferably be over 13, 780 KN/M 2 (2000 pounds per square inch) . It is at these pressu res that supercalender and better quality can be obtained, and it is at these pressures where the temperatu re effects of the invention are greatly increased . It should be noted that nip pressure determination can be complex .
• nip loads (unit force per unit roll length) are easy to determine by merely dividing the easily measured force applied to the total resilient press roll by the easily measu red nip length .
• the nip width is more difficult to measure.
• a widely accepted formula which is believed to provide a satisfactory approximation of nip width for many common installations is the Hertzian equation set forth by Narayan V. Deshpande, in Calculation of Nip Width, Penetration and Pressure for Contact Between Cylinders With Elastomeric Covering, TAPPI October 1978, Vol . 61 , No. 10, pp . 115-118.
• CH one-half the nip width
• F force per unit length of the nip
• the Young modulus will depend upon the hardness of the resilient roll cover. For example, roll covers having a P. &J . hardness of 4-5 at operating temperatu re will have a modulus of about 517,000 KN/M 2 (75,000 psi) . The modulus changes significantly with temperatu re changes in the roll cover.
• the principles of the invention are believed to be applicable to any type of web of papermaking fibers, whether coated or uncoated, groundwood or woodfree.
• the invention is valuable for woodfree papers (which will be defined herein as having at least 80% of its papermaking fibers provided by chemical pulp) , and groundwood papers (which will be defined herein as having at least 50% of its papermaking fiber provided by groundwood pulp) and those in between , which will comprise from 50% to 80% chemical pulp fibers and from 20% to 50% groundwood fibers .
• Coatings for woodfree sheets preferably will be in an amount of at least 7.5 g/m 2 and those for the other sheets preferably will be in an amount of at least 4.5 g/m 2 .
• the invention is believed to be applicable to all conventional basis weights, including the heavy weight board products .
• the invention is capable of producing, at least with the coated woodfree sheets, gloss higher than 50 and even 70, and Parker Print-Surfs better than 1 .4 and even better than 1 .0.
• the invention is believed to provide similar advantages to all papermaking fibers, groundwood is believed to provide an additional result because of the large amount of lignin in the web . N . L.
• Salmen has described lignin as having a static Tg at 115°C (239°F) or dynamic Tg of 127°C (260°F) for moistu re content of 2.5% and above. (See previously cited Salmen and Beck references and also Thermal Softening of the Components of Paper and its Effects on Mechanical Properties, N . L. Salmen , C . P. P . A . 65th Annual Meeting, Feb . , 1979, pp . B11 - B17. ) This value is equivalent to the Tg for Cellulose at a moistu re content of 4.7%.
• a typical groundwood web would have about 30% lignin , causing a similar but perhaps smaller rise in gloss and smoothness when its Tg was reached as with cellulose. A second and probably larger rise would occu r when the Tg of the cellulose was reached, which could be at a higher or lower temperature than the Tg of the lignin , depending upon moistu re content. Therefore, the invention is also subjecting a groundwood web (at least 50% groundwood) to a drum su rface temperature which is at least as high as that calculated by the formula using a moisture content of 4.7%.

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• 1985
  • 1985-11-06 DE DE8585905975T patent/DE3577894D1/de not_active Expired - Lifetime
  • 1985-11-06 WO PCT/US1985/002194 patent/WO1987002722A1/en not_active Application Discontinuation
  • 1985-11-06 EP EP85905975A patent/EP0245250B1/de not_active Expired - Lifetime
  • 1985-11-06 JP JP60505107A patent/JPS63500188A/ja active Granted
  • 1985-11-06 AT AT85905975T patent/ATE53085T1/de active
• 1987
  • 1987-07-03 FI FI872939A patent/FI88733B/fi not_active Application Discontinuation

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