US 3515633 A
Description (OCR text may contain errors)
June 2, 1970 J. M. FUTCH, JR 3, 1
COMPACTING OF PAPER AND SIMILAR FIBER WEBS' I Filed Feb. 27, 1967 I 2 Sheets-Sheet 1 v IO FIG.I
JuneZ, 1970 J.- M. FUTCH, JR 3,515,333
COMPACTING OF PAPER AND SIMILAR FIBER WEBS I Filed Feb. 27. 1967 2 Sheets- Sheet 2 FIG. 2
United States Patent 3,515,633 COMPACTING OF PAPER AND SIMILAR FIBER VVEBS James M. Futch, Jr., Yonges, S.C., assignor to Clupak, Inc., New York, N.Y., a corporation of Delaware Filed Feb. 27, 1967, Ser. No. 618,792 Int. Cl. D21f 3/02 U.S. Cl. 162206 5 Claims ABSTRACT OF THE DISCLOSURE Process and apparatus for compacting paper and similar cellulosic-fiber-containing webs, to enhance the stretchability and toughness thereof, in a pressure nip at moisture contents within a range. To achieve such compaction the pressure nip includes conventional resilient surface element and a special hard surface element the surface of which has a combination of characteristics such that it exerts very low drag with respect to the web at the moisture contents involved. The compaction of the web is immediately followed by machine glazing.
BACKGROUND OF THEINVENTION The field of the present invention is the compaction of fibrous webs such as paper and the like in which the fibers or a substantial percentage of the fibers of which the web is composed are natural cellulosic fibers which have been liberated to form pulp by chemical, semichemical or mechanical processes. A major reason for the compaction of such webs is to impart to them toughness and stretchability greater than that of a similar web which has not been compacted.
It is known in the prior art to compact webs of paper and similar products by passing the web through a pressure nip which simultaneously exerts on the web forces acting normal to the general web surfaces and forces acting parallel to the general web surfaces. In this regard reference is made, for example, to U.S. Pat. No. 2,624,- 245 to S. L. Cluett and U.S. Pat No. 3,122,469 to Fred H. Freuler. It is also known to compact asbestos fiber Webs in a manner such as is described in U.S. Pat. No. 3,148,- 108 to S. -L. Cluett. In these known processes the forces which act parallel to the general web surfaces in the pressure nip serve to rearrange and/or distort fibers within the web and while such action is known to produce various effects in the web a primarily important commercial effect is to impart to the web a definite degree of stretch which is recoverable in the web after drying and which is in excess, in the direction in which such forces were applied, of the stretch which a similar web would have had it not been subjected to the action of the pressure nip. An important commercial advantage of such additional recoverable stretch is that the treated paper or other web exhibits substantially greater toughness than is exhibited by an untreated web.
It has been thought that the amount of compaction that could be imparted to a fibrous paper web was controlled by the water to fiber ratio of the web at the time the same was passed through the pressure nip. The Cluett U.S. Pat. No. 2,624,245 indicates that at a ratio of about 1:1 (by weight) little or no compaction occurred on the first pass through the compacting device which, as disclosed in said patent, utilizes the pressure nip between the surface of an elastomeric material and a heated hard-surface pressure roll. However, said patent indicates that compaction will be obtained with further passes through the same or similar compacting devices as a result of the removal of water which occurs in the earlier passes. It has been found from commercial experience in the prior art practice of compaction, usually in a single 3,515,633 Patented June 2, 1970 pass, that a water to fiber ratio of from about 0.5 21.0 to not more than about 0.65: 1.0 produces the best balance of compaction and smoothness of the final product.
It is well known that paper webs are formed on the forming screen of a cylinder mould or Fourdrinier machine by removing water from a suspension of fiber in water by drainage of water through the screen both with and without promotion by suction. The surface tension of the water as the web is thus consolidated causes the fibers to be pulled into a wet mat. This wet mat, upon further removal of water as by pressing and drying by evaporation, is additionally densified and strengthened because of the formation between the fibers of chemical bonds generally thought to be hydrogen bonds between the hydroxyl groups of cellulose molecules.
Typically a web of paper leaves the forming screen with a moisture content of about by weight which corresponds with a water to fiber ratio of about 4: 1. This moisture content is reached when air has struck through the web just before it leaves the forming screen. It is apparent at such moisture content there is a substantial quantity of water on the surfaces of the fibers in addition to the quantity of water with which the fibers are saturated. In the commercial practice of compaction of paper webs as discussed above it has been regarded as essential to reduce the moisture content of the web to the level discussed above by pressing and by evaporative drying before the compaction is performed.
Although the exact function of water during the compaction process was unkown it was believed that sufficient water had to be removed not only to provide unfilled voids in the fibrous mat but also to permit substantial bonding to occur between fibers before compaction. That is, it was believed that bonding of the type which occurs at a moisture content below fiber saturation had first to be established before the fibers could be curled and compacted between the established bond sites. It was believed that at moisture contents above fiber saturation insuflicient fiber bonds had been formed so that the net effect of compaction at such moisture contents would be simply to consolidate the web creating a higher basis weight but imparting to the web little or no additional recoverable stretch.
SUMMARY OF THE INVENTION In contradistinction to the beliefs aforesaid the present inventor has discovered that fibrous webs can be compacted at moisture contents very substantially above fiber saturation, i.e. in the range of from about 1:1 to about 3:1 water to fiber ratio, by weight, and that such compaction yields recoverable extensibility in the finally dried web which generally corresponds with the amount of com paction just as is the case in compaction at the lower moisture contents heretofore used. The present inventor has discovered that the limiting factor in the prior commercial as well as experimental methods of compaction has been the relative drag between the wet web and the hard surface element in the compacting device instead of the factors heretofore thought to be limiting.
Efforts have been made in the past to reduce the drag of the wet web upon the polished metal surfaces used in the compaction process. These have included various treatments of the metal surface primarily consisting of forming a matte finish of carefully limited degree on a previously highly polished metal surface. Also, extensive studies have been made to establish empirical optimum temperatures for the metal surface in the manufacture of particular products to obtain the benefit of lubricating effect of a layer of steam. Also, commercial operations have quite frequently been improved by addition of lubricants such as water emulsions of silicone oils. These earlier efforts with the web at moisture contents in the range now made possible by the present invention.
The present invention makes use of hard pressure surfaces having very little tendency to adhere to wet cellulosic webs because they have low coefiicients of friction with respect to such webs. Examples of such surfaces are surfaces made wholly or in part of Teflon resin or other resins having similar characteristics or metal surfaces having small convex nodules formed thereon and closely and quite uniformly distributed over the entire working portion of the surface.
Hard surfaces of the types suitable for the present invention characteristically have quite high contact angles with water, have relatively low coeflicients of static friction with respect to wet cellulosic sheet material and exhibit very low or in some cases no detectable tendency to pick fibers from freshly formed wet cellulosic sheet material. As will be more fully explained below certain of the suitable surfaces are preferred for practical reasons such as durability under commercial production conditions.
The ability to compact fibrous webs at the moisture contents made practicable by the present invention affords many advantages some of which have been sought after from the beginning of commercial practice of compaction in accordance with the prior art. For example it has always been necessary to subject a web of paper first, to dewatering in the wet press section of a papermaking machine and second, to a substantial amount of evaporative drying, as on the first several heated dryer drums of the machine, to bring the moisture content down to even the highest level at which prior art compaction has been possible. The partial evaporative drying on dryer drums is inherently non-uniform across the width of the web whereby prior-art-compacted paper webs have very frequently exhibited varying degrees of compaction and/or surface appearance in the zones of differing moisture content. The moisture content of a paper web as it leaves a wet press or the wet press section is much more uniform than it is likely to be after the web has passed over the first several dryer drums. Therefore, compaction of the paper immediately after it leaves a wet press or a wet press section has long been regarded as desirable but impossible of achievement prior to the present invention.
Another long felt need in the paper industry has been a machine glazed paper with the enhanced stretch and toughness characteristics of the prior-art-compacted papers. This has not been possible since the paper must have a moisture content of at least 1:1 water to fiber ratio to be machine glazed whereas the prior art compaction could only be carried out at a much lower moisture content. Obviously the paper could not first be glazed and then compacted because the compaction process would adversely afiect the glam finish. With the present invention the paper may 'be compacted at a moisture content so high that it may be machine glazed after compaction. Thus, this invention provides a new and highly desired product for the paper industry.
DESCRIPTION OF DRAWINGS FIG. 1 is an apparatus constructed in accordance with and designed to carry out the process of the present invention;
FIG. 2 is an alternate arrangement of the apparatus of FIG. 1; and
FIG. 3 is a detail view of the pressure nip of a mechanical compactor for fibrous webs.
FIG. 4 is a fragmentary view of the compactor roll showing surface deformation generally.
4 DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows one embodiment of an apparatus adapted for the practice of the present invention and in this figure the compactor is illustrated in such a position that it operates upon a web of paper immediately after it emerges from a wet press or the wet press section of a papermaking machine. In this figure there is shown a head box 10 and a Fourdrinier screen 11 as well as a suction box 12 and the various rollers (table rolls 13, couch roll 14, takeup roll 15, guide roll 16 and idlers 17, 18 and 19) which comprise the basic elements of the forming section 20 of a typical papermaking machine.
A web of paper 22 is shown leaving the Fourdrinier screen 11 at the couch roll 14. The web 22 typically leaves the Fourdrinier screen at a water to fiber ratio of about 4: 1 by Weight, although this varies to some extent in either direction depending upon the grade of paper and the characteristics of a particular installation. Typically the web 22 passes from the Fourdrinier screen to the first press in a wet press section which is designated generally by the reference numeral 23. The function of the press section 23 is to remove some of the water from the web 22 and to assist in forming it into a suitable coherent structure by consolidation and smoothing. Only one wet press is shown although it Will be understood that two or three presses are usually included in a typical wet press section. Each press ordinarily includes a press felt or fabric 24 on the upper surface of which the web 22 is carried through the press. The press also includes a pair of generally vertically alined press rolls 25 which are pressed together upon the web 22 and felt 24 with force appropriate for squeezing of water from the web 22 and for smoothing and consolidation of the web as mentioned above.
A web of paper will leave a typical wet press or wet press section at a quite high moisture content, for example a water to fiber ratio of about 1:1 to about 3:1 depending upon the efliciency of the press or number of presses as well as upon the particular grade of paper being manufactured. Moisture contents in the range of those exhibited by webs leaving the typical Wet press 01 wet press section are thus very considerably in excess of those at which compaction of paper has been practiced in accordance with the prior art. Therefore it has been necessary in the past to run the web over a sufficient number of evaporative dryers, such as those generally indicated at 30 in FIG. 1 to lower the moisture content to ratios substantially below 1:1, usually within a range of about 0.511 to about 0.65:1.
However the present invention makes it possible to compact paper or other cellulosic fibrous webs at moisture contents within the range at which a web typically leaves the wet press section as Well as at lower moisture contents. Thus, as shown in FIG. 1, a compactor unit 40 is installed in a position ahead of the first heated drum 31 of the dryer section 30. This is made possible in accordance with the present invention by the utilization in the compactor 40 of a hard surface element having a combination of characteristics as will be described in greater detail hereinbelow. From the compactor unit 40 the compacted web 22 is passed to the first dryer drum 31 of the dryer section 30. The dryer section 30' includes as is conventional, a plurality of dryer drums 31, upper and lower dryer felts or fabrics 32 and the usual guiding and tensioning rolls 33, 34 and 35 for the felts. The felts 32 serve to hold the web 22 against the dryer drums. In some instances rolls 35 are heated to remove some moisture from the dryer felts 32. From the dryer section 30 the compacted and dried web may be conducted as desired to conventional equipment such as calenders, reels and the like, not shown.
In FIG. 1 the dryer section 30 has been broken away to indicate that any desired number of dryer drums may be used. Frequently the dryer section is made up of two or more groups of dryer drums, each group having its own conventional arrangement of dryer felts or fabrics and in some instances a size press or the like is positioned between groups of dryer drums. Any such arrangements may be used with the present invention. However, it should be observed that with the present invention as well as with prior art compacting equipment care should.
be exercised to keep longitudinal tension on the Web 22 in the dryer section 30 low enough that the desired portion of the enhanced stretchability imparted to the web by the compactor 40 will be retained in the finally dried; web 22.
In FIG. 2 an embodiment of the invention is shown for the production of compacted machine-glazed paper. In this figure a compactor 240 is shown positioned just ahead of a Yankee dryer section 251. The compactor 240 may be identical with the compactor 40 shown in FIG. 1 and it will be described in greater detail below. A web of paper 222 is shown passing through the compactor 240 and from there to the Yankee dryer section 251.
The Yankee dryer section 251 includes a large-diameter heated drum 252 which has a highly polished surface for contact with the web 222. The compacted web 222 is guided from the compactor 240 onto the drum 252 by a pressure roll 253 which assures tight contact of one surface of the Web with the highly polished surface of drum 252. A conventional Yankee dryer felt 254 and conventional tension and guiding rolls 255, 256 therefor serves to hold the web 222 in contact with a substantial portion of the periphery of the drum 252 as is customary in Yankee dryer sections. The compacted dried web 222 is guided by a roll 257 to a reel 258.
For machine glazing of paper on a Yankee dryer, as shown in FIG. 2, or on a stack or heated dryer rolls which press the paper between them like a calender stack as is sometimes used, it is known that the paper must have. a moisture content of at least about 50%, that is a Water to fiber ratio by weight of about 1:1, at the time it first comes into contact with the dryer drum surface on which it is to be glazed. When the moisture content is below this range the paper typically breaks prematurely away from the drum surface and dries without attaining the desired glossy finish. In the proper operating range for glazing the paper will adhere firmly to the drum surface and will remain so adhered long enough that the surface of the paper will take on the desired highly polished permanent finish which is a mirror image of the surface of the drum.
With the present invention a compactor 240 is provided for the first time which is capable of compacting paper at water to fiber ratios well is in excess of 1:1 whereby the web 222 may be introduced to the compactor 240 at a moisture content sufliciently high that, after compaction and such reduction in moisture content as may be caused by compaction, the web 222 will still have the 1:1, or greater, ratio required for machine glazing. Thus, the web 222, as shown in FIG. 2 may be conducted to the compactor 240 directly from a wet press section with a water to fiber ratio of, say 2:1 or greater if the drying capacity of the particular Yankee section is high enough. If not, the web 222 may be conducted from the wet press section to one or more preliminary dryer drums to bring the moisture content down to such a point that after compaction it will have a water to fiber ratio sufficiently high for machine glazing but not in excess of the capacity of the Yankee dryer section.
In FIGS. 1 and 2 respectively the compactors 40 and 240 are shown as comprising elastomeric blankets 41 and 241, hard surface rolls 42 and 242, idler rolls 43, 243, 44, 244, 45, 245 and optional auxiliary drive rolls 60 and 260. The hard surface rolls 42 and 242 respectively are driven rolls and the rolls 43 and 243. are nip rolls so mounted that their axes may be moved toward and from the hard surface rolls with which they are associated to adjust the amount of nip pressure exerted upon the elastomeric blankets 41 or 241 and upon the paper webs which pass between the blankets and the hard surface rolls for the compaction thereof. The idler rolls 45 and 245 are usually bodily adjustable to put desired tension upon the elastomeric blankets. It will be apparent that the compactors 40 and 240 as so far described are similar to one of the forms described in US Pat. No. 2,624,245 to S. L. Cluett.
In FIGS. 1 and 2 there is shown one optional feature which ordinarily is not utilized in the commercial practice of prior art compacting. Thus, in these figures auxiliary drive rolls 60 and 260 respectively have been illustrated as forming a nip with the idler rolls 44 and 244 for the purpose of driving the elastomeric blankets 41 and 241 at a desired lineal speed. Instead of the auxiliary rolls 6t and 260 the auxiliary driving effect may be achieved by driving the nip roll 43 or 243 of FIGS. 1 and 2 by suitable mean (not shown) which take into account the fact that the nip rolls also must be bodily movable to adjust nip pressure. In many instances in the practice of the present invention some such auxiliary drive is ad visable or necessary because the surfaces of the driven, hard surface rolls 42 and 242 have such low drag characteristics with respect to the moist webs 22 and 222 that slipping will occur and the elastomeric blanket and the moist paper web adhered thereto would tend to move at too low a speed for proper operation. The auxiliary drive rolls 60 and 260 or other auxiliary driving provisions as noted above will assure proper blanket and Web speed under all conditions. In prior art compactors the auxiliary drive for the blanket has not been required since the hard surface elements of such compactors at best have had frictional drag characteristics so high with respect to the paper web that movement of the web and blanket at desired lineal speed was assured.
In FIG. 3 there is shown in enlarged detail the nip portion of a compactor 340 of the type used as the compactors 40 and 240 of FIGS. 1 and 2 respectively. As is known from the prior art the compacting of a fibrous web to enhance toughness and stretchability is accomplished by applying simultaneously to the web forces which act in the plane parallel to the general web surfaces and in the plane normal to such surfaces. These forces push and crowd the fibers together to strengthen existing bonds and to form additional bonds and adhesions between fibers and fibrils of the web in its compacted condition. In the figures of the drawings the compactors act to shrink the web in a direction parallel to its travel through the machine and the finished web accordingly will exhibit greatest enhanced stretchability in this machine direction. As best seen in FIG. 3 the endless elastomeric blanket 341 which, typically, may comprise a strong, inextensible backing layer secured to a smooth-surfaced elastomeric paper engaging layer having a durometer hardness in the range from 40 to 60, wraps a substantial portion of the periphery of the nip roll 343 as it approaches the hard surface roll 342. Thus the paper engaging surface 346 of the blanket will be convexly curved and stretched as it approaches the nip. The web 322 is brought into contact with the stretched surface 346 of the blanket just ahead of the nip between rolls 342 and 343. As the blanket and web enter the nip the web is firmly pressed between the surface 346 of the blanket and the hard surface 347 of roll 342 and it will tend to adhere firmly to the strectched elastomeric surface 346. As the web and blanket pass through the nip the blanket is so guided as to wrap the surface of the hard surface roll 342. The resultant reversal of curvature of the elastomeric surface 346 causes it to relax and shorten and to correspondingly shorten the web 322 which is firmly adhered to the surface 346 during the shortening thereof. The web is still pressed against the hard surface 347 and it will be apparent that at some point within the nip the web 322 must begin to slip and then continue to slip relative to the surface 347 of hard surface roll 342 for such shortening to occur.
With specific reference to US Pat. 2,624,245, the process includes confining the web between two moving elements, one of which has a web contacting surface which is smooth and elastomeric and the other of which has a web contacting surface which is hard and offers lower frictional resistance to movement of the web than does the surface of said elastomeric element, and while the web is so confined contracting the web contacting surface of said elastomeric element in order to contract the web with it while said web is still confined against and slipping relative to said hard surface to thereby compress the web in the direction of contraction of said web contacting surface so that the fibers and fibrils of the web are brought into such close contact with each other that strong bonding and adhesions are produced therebetween.
As noted above it was thought that the moisture content of the web as it entered the nip of a compactor had to be below a certain level for various reasons including the constantly observed fact that when a web had a moisture content above a water to fiber ratio of about 0.65:1 it would stick to the surface 347 of the hard surface roll 342 and would not follow the recoil of the surface 346 of the elastomeric blanket 341. When the Webs were only slightly too moist they would not be compacted but simply would be scuffed as a result of slippage relative to the elastomeric blanket surface 346. At higher moisture contents the webs frequently were badly damaged or destroyed. In other words the webs did not have suflicient strength in shear to overcome the static friction of the hard surface relative to the webs at such higher moisture contents.
The hard surface 347 of roll 342 always is maintained at a temperature at or above the boiling point of water in prior-art operation in order to promote slippage of the moist web relative to the hard surface. In prior-art operations the hard surface usually was very smooth and usually was of polished chromium. The lubricating effect of steam due to the temperature of the surface as well as the additional lubrication by silicones, for example, which was sometimes provided, were essential to permit effective compression of webs at the relatively low moisture contents heretofore regarded as the maximums for successful operation.
The present invention provides hard surfaces which may be heated but frequently to much lower temperatures than in the prior art, which do not require additional lubricants for operation upon webs at moisture contents as heretofore used and, most importantly, at moisture contents exceeding and in many instances greatly exceeding the highest moisture contents at which prior-art compression could be practiced. These hard surfaces have coefficients of static friction, with respect to webs at such higher moisture contents which are low enough to permit slipping to start within the nip at the proper time. Particular examples of such surfaces and the combinations of characteristics to guide those skilled in the art in selection of other surfaces suitable for use in the present invention will now be given.
A first suitable hard surface 347 may consist of a coating of Teflon fluorocarbon (TEE) resin (Du Pont) or similar resin applied to the relatively smooth surface of a metal roll 342. Such resins are available in various grades and for the present purpose some of the grades most recently developed by the supplier for durability under abrasion and elevated temperature conditions are preferred. The resin coating should have a thickness of about 0.002 inch or more when applied to a metal surface. If the metal surface is somewhat roughened, as it may be to improve adherence of the coating thereto, the coating should be sufficiently thick that the surface thereof may be polished to desirable smoothness while leaving a minimum thickness of about 0.002 inch. Thicker coatings of the presently-available Teflon resins are not necessary or desirable although the constant efforts of the supplier (Du Pont) to provide greater durability can be eX- pected to result in resins which can be used in thinner or thicker coatings for the present purposes.
A second suitable hard surface 347 for the roll 342 consists of a coating of resin, which may be a Teflon or similar resin as described above, applied over an etched or pitted metal surface of roll 342. Preferably the metal surface is a plating of chromium first polished to a smooth condition and then given a matte or pitted finish. With the matte finish the high spots are peaks. The pitted surface may be formed by blasting the polished plating with a suitable material such as glass beads to form closely spaced indentations upon the surface, with the high spots being what remains of the original polished surface. The matte or indented surface is then coated with a resin such as a Teflon fluorocarbon resin (TFE) (Du Pont) or other durable types of resin of suitable characteristics. The resin coat is thick enough to fill the valleys of the matte or indented chromium base and of course will extend over the high portions as well. When a roll with such a surface is put into use for the present invention the resin extending over the high portions will eventually wear away exposing the metal in such zones. The rate of wear thereafter will be progressively reduced as more of the metal high portions are exposed. The resin remaining in the valleys will still constitute the major portion of the area of the surface whereby the surface will continue to exhibit substantially the same characteristics as it would were it a smooth, polished surface consisting solely of the resin.
Another, third, suitable surface for use in the present invention is one consisting entirely of metal which exhibits characteristics sufficiently similar to the resin surfaces of the first and second surfaces discussed above to be entirely suitable for the present invention. Furthermore it is regarded as preferable over the first and second examples for the practical reason that it will have a much longer useful life than the resin surfaces aforesaid since it should wear longer and is less likely to be damaged. This third useful surface consists of a plating of chromium or other suitable hard metal upon the base metal of the roll 342. The plating preferably should be at least about 0.002 to 0.003 inch thick. The plating is treated or is so applied that the surface thereof is made up of small closely spaced convex nodules. One way to provide a surface of such conformation is to start with a roll 342 made of steel which is smooth or which has been polished to a very smooth condition. The steel surface is then blasted with suitable material to form closely spaced indentations with intervening high portions. The blasted steel surface is then plated with chromium which will form smoothly rounded convex nodules where it extends over the high portions of the blasted steel surface. Metal rolls of this general type with surfaces of various degrees of roughness may be obtained from Brame Textile Machine Company of Greensboro, NC
For use in the present invention the surface texture of the third, nodular metal, hard surface 347 should show an average Roughness Height R.M.S., of from about 40 to about rnicroinches determined in accordance with ASA B46.l-1962 with a Roughness Width cutoff value of 0.100 inches. To achieve such texture the roughness of the blasted steel roll before plating and the thickness of the chromium plating must be chosen in relation with each other in a manner known to those skilled in the art.
While the three examples of useful surfaces given above are quite different in obvious respects they share certain characteristics which appear to explain why they, in contrast to the surfaces heretofore used in. the compacting of paper, make it possible to compact paper at moisture contents substantially higher than those to which prior art compacting has been limited. In a general sense these characteristics have to do with the behavior of the surfaces not only with respect to the water and to the wet fibers of which the wet fibrous web is primarily composed, but more importantly, as the present inventor has discovered, with respect to the wet fibrous sheet itself. For example it has been recognized that the hard surface should be one that is diflicult to wet with water, that is, the surface should exhibit a relatively high contact angle with water. Also, it has been recog nized that the hard surface should not tend to pick individual fibers from the web under the conditions existing in the pressure nip of the compactor. Application of these recognized principles has led to some improvement in the compaction of paper at the moisture contents already in use, such as the ability to operate at higher speeds, to achieve greater compaction and to operate upon somewhat lighter-weight webs but have not permitted operation on webs of significantly higher moisture content.
The point just discussed is exemplified by the results achieved by testing a highly polished chromium surface against one that is lightly blasted to a matte finish. A polished chromium surface having a Roughness Height Value of from 4 to 8 microinches R.M.S., typical of the surfaces heretofore used in the compacting of paper, which has a moderately high contact angle with water was found by the present inventor, in a series of tests which will be more fully described below, to pick a substantial quantity of paper fibers from a sheet of paper with a water to fiber ratio, by weight, of about 3:1. Another chromium surface known to be useful in priorart compaction of paper, which has been blasted to form an overall pattern of very small sharp peaks has a contact angle with water which was very much higher than the polished surface and it was found to exhibit no detectable tendency to pick fibers from a web of paper at the same water to fiber ratio of about 3:1. However, such blasted surface, in spite of its high contact angle with water and its freedom from fiber picking when used in a laboratory compactor was not any more capable of use for compacting paper at moisture contents of, say, a water to fiber ratio of about 3:1, than was the polished chromium surface. The laboratory tests conducted with labora tory compactor equipment which had revealed the results just stated were then supplemented by a series of tests upon various surfacing materials and surface textures to determine the characteristics which were controlling as regards the behavior of those surfaces with highly moist webs. It was in such studies that it was discovered that the static frictional characteristics of the surface with respect to a wet web were of much greater imporance than the dynamic frictional characteristics.
For the extended tests hand-sheets of paper were formed from pulp of carefully controlled uniformity and some of them were pressed to a substantially uniform moisture content with a water to fiber ratio of about 3:1. Other sheets were dried to air dry condition. In addition to fiber picking tests with the wet sheets and contact angle measurements for each of the surfaces a series of tests were made to determine frictional characteristics of both wet and dry sheets with the various surfaces.
The contact angle of each surface with water was measured by known techniques which, while not in accordance with any published standards, were carefully arranged so as to be uniform during this particular series of observations.
Surface texture was determined, as noted above, in accordance with ASA B46.1 --1962 and Root Mean Square averages are reported herein with Roughness Width Cutoff of 0.100 inch. It will be appreciated, however, that analysis of surface texture in this manner does not give any indication as to whether the surface protuberances are rounded or are sharp and accordingly Roughness Height is reported herein only in connection with those surfaces which have the rounded, nodular protuberances which the present inventor has found to be the best of the all-metal surfaces for use in the present invention. The Roughness Height of such surfaces was examined in order to establish a range within which successful compaction of paper could be carried out at moisture contents such as exist in paper coming from a Wet press or the wet press section of a paper making machine.
Fiber picking tests were made, using the wet hand sheets, but are not fully reported or described herein since picking of any significant amount was found only in connection with the polished chromium surface (RMS 4-8) of a type heretofore used for compacting of paper at commercial moisture contents of 0.65:1 water to fiber ratio or less. It should be reported however that the preferred convex nodular metal surfaces started to pick very slightly at Roughness Height values of RMS 55 or less. This occurred with paper sheets at a Water to fiber ratio of about 3:1. For this reason as well as the fact that the coefiicient of friction-wet began to approach that of polished chromium as the nodular surfaces became smoother, the lower limit on surface texture of the convex nodular metal has been determined to be about 40 RMS. While the complete range is operative with the highest moisture contents the smoother surfaces in such range are better adapted to operate with paper at the lower water to fiber ratios within the range of from about 1:1 to about 3:1.
To determine the coefiicient of static friction of the various surfaces apparatus was employed to observe the angle of repose between the surface under test and the hand sheets of paper, both wet and air-dry. The apparatus consisted of a table faced with plate glass and hinged at one end. The other end was arranged to be lifted at a controlled rate of 0.5 per second by a hydraulic-pneumatic system. The paper sample was clamped to the free end of the table and a flat plate having the surface under test was laid, test surface down, upon the paper sample. The free end of the table was then progressively elevated to incline it relative to the horizontal. the angle at which slippage of the test plate under the influence of gravity just starts is reported below as the angle of repose, which, for practical purposes is regarded as equivalent to the angle at which the onset of slipping is imminent. The tangent of the angle of repose is the coefficient of static friction.
A large number of hand-sheets was tested with each surface and the angles of repose and the coefficients of friction, reported below, are the result of averaging the observations. The hand-sheets tested were each about 12" x 12" and the test plates each were 8" x 10 (surface) x 1" thick. The test plates each weighed about 22.2 pounds whereby the pressure upon the paper sample was about 0.27 pound per square inch. The angles of repose with respect to the air-dry paper samples were all quite low and did not difier a great deal whereby the coefficients of friction-dry are not reported below. The coefficients of friction-wet are reported below because of their evident significance.
In the condensed table below test results are set forth in connection with surfaces as follows:
(A) Polished chromium, 4-8 R.M.S. (B) Nodular chromium, 55 R.M.S. (multi-directional lay tolerance 50-60 R.M.S.)
(C) Nodular chromium, 125 R.M.S. (multi-directional lay tolerance -150 R.M.S.) (D) Polished Teflon or similar resin coating on smooth metal base. (E) Teflon or similar resin on pitted metal, after wear (F) Matte chromium, sharp peaks Surfaces'A and F are prior art compacting surfaces and are not useful in the present invention while surfaces B,
C, D and E are useful in this invention as described below.
TAB LE Moisture Coefficient Contact content Angle Angle of static angle of paper, repose repose friction, Surface H2O (deg) percent dry (deg) wet (deg) wet 64. 2 76. 2 20. 7 31. 1 60 77. 3 76. 9 l2. 7 25. S 48 78. 9 76. 5 18. 8 23. 0 42 95. 1 76. 6 l7. 7 12. 3 22 95. l 75. 9 20. 7 13. 3 24 101.8 77. 4 20.0 29. 8 57 ever, the angle of repose-wet with respect to paper at a water to fiber ratio of about 3:1 for each of surfaces A and F is significantly higher than that angle for the other surfaces. The Teflon or similar resin surfaces, polished solid resin in surface D and part-resin, part-metal in surface E, have angles of repose-wet which are very low, less than half that of the polished or matte chromium surfaces A and F. It will be noted that the Teflon resin surfaces D and B have lower angles of repose-wet than their angles of repose-dry due to the peculiar characteristics of resins of this general type.
The resin surfaces D and E are substantially identical in characteristics reported in the table and either one may be used for the present invention. Surface E is preferred because of its greater durability. Surface E was also tested before being worn down to an extent such as to expose substantial portions of the high spots of the base metal and it exhibited substantially identical characteristics. The surface appearance before wear was quite similar to the polished surface of surface D. Accordingly a Teflon or similar resin coated, blasted or etched, metal base roll may be put into immediate use in the present invention and it eventually will wear into a smooth partresin, part-metal surface of good durability.
The convex nodular metal surfaces B and C, as shown in the table above both have angles of repose-wet which are higher than those of the resin or part resin surfaces but which are nevertheless significantly lower than the prior art compacting surfaces A and F. Surface C, with a finish of 125 microinches R.M.S. has a lower angle of repose-wet than the smoother surface B but both are entirely useful for compacting paper at water to fiber ratios of about 3 :1 as well as papers at any lower moisture contents including those employed in the prior art. The upper limit of surface roughness of these convex nodular metal surfaces appears to be established as a practical matter by the belief that the rougher surfaces may have a tendency to mark the paper. The upper limit of 175 microinches R.M.S. has been selected on this basis and rougher surfaces, if they olfer any advantage in special cases, may be used so long as marking does not become objectionable.
'From the table above it will be apparent that the frictional characteristics of a surface with respect to a dry Web, such as paper or plastic films, can not serve as a guide for determining whether a surface is suitable for the present invention. For example while surfaces A, C, E and F have very similar angles of repose-dry they differ very sharply in angles of repose-wet and only those with the lower angles of repose-wet are useful in the present invention. The coefiicients of static friction-wet calculated from the observed angles are believed by the present inventor to represent a way to give a numerical value to the relative frictional drag of a surface with respect to a wet web of paper. Taking the coefficient of static friction-wet of surface A which is a surface known to be unfitted for use in the present invention and assigning it the arbitrary percentage value of 100 the relative values of the remaining surfaces are approximately as follows: E, 82%; C, 70%; D, 37%; E, 40%; F, 95%. These normalized values thus afford a convenient way of expressing the relative drag of the several surfaces with respect to a wet paper web at a water to fiber ratio of about 3:1 from which it reasonably can be concluded that surfaces having a frictional drag about 85%, or less, that of the relative drag of polished chromium are useful in the present inven tion. Another way of expressing the effective relative frictional drags of these surfaces with respect to a wet web is to use the calculated coeflicients of static frictionwet. From the table it will be apparent that such coefiicient should not exceed about 0.5.
The use of the low frictional drag surfaces provided by the present invention is not limited to compactors in which the elastomeric element is an endless belt such as the belts 41, 241 and 341 illustrated in the drawings. In another well known form of compactor the elastomeric element consists of a rubber, or equivalent, jacket applied to the nip roll which otherwise corresponds with the nip rolls 43, 243 and 343. Also it is known to drive the hard surface roll corresponding with the hard surface rolls 42, 242 and 342 at a surface speed somewhat in excess of the surface speed of the elastomeric element whether the latter is a belt as illustrated herein or is a jacket on the nip roll as just described. In all cases the low frictional drag of the hard surfaces provided by the present invention gives the advantage of providing improved compacting in the pressure nip at the moisture levels of the prior art while also giving the unique advantage of permitting compacting of paper webs in the pressure nip at much higher moisture contents, for example up to a water to fiber ratio by weight of about 3:1.
As can be seen from the foregoing description of preferred forms of the present invention an important aspect of the invention is that rather than being limited by the moisture content of the web the ability to compact is limited only by the ability to adjust properly the characteristics of the hard surface roll of the compactor. Al though it is not intended that this invention shall be limited to any specific theory it is believed that a substantial amount of original bonding has occurred between fibers and that sufficient voids exist between fibers for inter-fiber movement at the moisture contents at which paper webs typically leave a wet press or the wet press section of a papermaking machine. The forces exerted by the surface tension of the water upon the fibers becomes very great when air strikes through the mat before it leaves the forming screen and the water consequently changes from continuous to discontinuous phase, whereby there may be substantial bonding of the fibers at this point. In any event the wet press or presses probably form more fiber bond and certainly reduce the moisture content to create more voids.
What is claimed is:
1. The process of producing a web of adherent waterlaid cellulose fibers having substantial permanent extensibility in excess of that of the web as laid, and which has no substantial decrease in thickness when stretched, consisting of the steps of: confining the webat a point in its manufacture when its moisture content expressed as a ratio of water to fiber by Weight is the range of from about 1: 1 to about 3:1, between two moving elements, one of which has a web contacting surface which is smooth and elastomeric and the other of which has a web contacting surface which is hard and offers lower frictional resistance to movement of the web than does the surface of said elastomeric element, the web contacting surface of said other element having small nodular protuberances on the surface of said hard roll, said protuberances having smoothly rounded convex configurations and said protuberances being closely spaced, and while the web is so confined contracting the web contacting surface of said elastomeric element in order to contract the web with it while said web is still confined against and slipping relative to said hard surface to thereby compress the web in the direction of contraction of said web contacting surface so that the fibers and fibrils of the web are brought into such close contact with each other that strong bonding and adhesions are produced therebetween, and then passing the web to a Yankee drying cylinder at a water to fiber ratio of at least about 1:1 and adhering the web to the surface thereof while the web is still in the condition of its emergence from the compactor whereby the web is dried and glazed on one of its surfaces.
2. The process of producing a web of adherent waterlaid cellulose fibers having substantial permanent extensibility in excess of that of the web as laid, and which has no substantial decrease in thickness when stretched, consisting of the steps of: confining the web at a point in its manufacture when its moisture content expressed as a ratio of water to fiber by weight is in the range of from about 1:1 to about 3:1, between two moving elements, one of which has a web contacting surface which is smooth and elastomeric and the other of which has a web contacting surface which is hard and offers lower frictional resistance to movement of the web than does the surface of said elastomeric element, the web contacting surface of said other element comprising a layer of hard metal plating on the surface thereof, the layer of chromium having closely spaced indentations and a layer of resin being sufiiciently thick to fill in all the indentation and extending over the high portions as well as the indentation, and while the web is so confined contracting the web contacting surface of said elastomeric element in order to contract the web with it while said web is still confined against and slipping relative to said hard surface to thereby compress the web in the direction of contraction of said web contacting surface so that the fibers and fibrils of the web are brought into such close contact with each other that strong bonding and adhesions are produced therebetween, and then passing the web to a Yankee drying cylinder at a water to fiber ratio of at least about 1:1 and adhering the web to the surface thereof while the web is still in the condition of its emergence from the compactor whereby the web is dried and glazed on one of its surfaces.
3. A machine glazingapparatus and a Yankee drier for machining glazing a web of paper wherein the compacting apparatus includes a pressure nip in which the web of paper is supported by an elastic surface which presses the web against the surface of a hard roll while the web is sliding relative to the surface of the hard roll, said hard roll having: small nodular protuberances on the surface, said protuberances having smoothly rounded convex configurations of a size and spacing that the surface which they form has a Roughness Height R.M.S. with Roughness Width cutoff of about 0.100 inch, of from about 40 to about 175 microinches and wherein said Yankee drier immediately follows said compacting apparatus.
4. The apparatus of claim 3 wherein the nodular protuberance of said hard roll consist of a chromium layer on a pitted metal base.
5. The apparatus of claim 4 and including a layer of resin on the surface of said roll.
References Cited UNITED STATES PATENTS 2,114,072 4/1938 Cleveland 162358 XR 2,624,245 1/ 1953 Cluett 162206 XR 3,011,545 12/1961 Welsh et al 162-206 XR 3,290,209 12/1966 Ihrrnan 162361 3,362,869 1/1968 Welsh 162-206 XR S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R. 2618.6; 162361