US 2999786 A
Description (OCR text may contain errors)
United States Patent" O 2,999,786 MACHINE GLAZED PAPER Martin L. Downs and Ralph A. Nelson, Appleton, Wis.,
assignors to Thilmany Pulp & Paper Company, Kaukauna, Wis, a corporation of Wisconsin No Drawing. Filed Aug. 9, 1957, Ser. No. 677,209
2 Claims. (Cl. 162-129) I Prepare base sheet from pulp of freeness greater than 600 Prepare 50-400 freeness pulp Superimpose 50-400 freeness pulp (515%) on base sheet to form combined sheet Press low freeness surface of combined sheet against polished drying surface (Pressure 80 p.s.i. to 1500 psi.)
Dry combined sheet in contact with smooth drying surface to provide machine glazed sheet In the paper art, it has been known for many years Patented Sept. 12, 1961 ICC such a highly glazed paper is known as a Yankee machine or Yankee dryer. Such machine is sometimes referred to as a Flying Dutchman machine. As before indicated, the paper manufactured on such machine is commonly known in the art as machine glazed or M.G. paper. These presently available papers have a 1 gloss of about 25, as measured by a gloss meter.
Heretofore, it has not been believed possible to increase this gloss to any substantial degree because of the inherent limitations of the process for machine glazing paper. In this connection, and as has been previously pointed out, the machine glazed paper obtains its glaze characteristics by pressing of the cel-lulosic fibers, under high pressure, against a smooth polished dryer surface.
The fibers on the surface, substantially individually, conform to the smooth polished surface.
In order to obtain the degree of glaze which has been possible heretofore, the stock for the web of paper is beaten, or refined. However, such heating is necessarily limited in order to maintain a high enough freeness and to prevent the manufacture of a. slow stock. Such 5, slow stock makes a dense sheet having a low degree of sure against a flat surface and the web is allowed to dry in contact with such surface, a sheet will be provided having a side which generally conforms to the surface and which tends to reproduce the surface. It has been generally believed that the pulp which is employed to make the cellulosic web should be quite free, that is to say, the pulp should not be beaten to a high degree. Such freeness has been believed necessary in order to permit the escape of the moisture from the surface of the dryer. When the pulp is beaten to a substantial degree and is not free, the resulting web becomes quite dense. As a result, vapor forms between the dryer surface and the sheet, causing the sheet to be blown away from the flat surface, thereby resulting in non-uniform glazing of the sheet.
In commercial paper making, it has been the practice for many years to provide a highly glazed surface on one side of a paper sheet by pressing a wet web onto the surface of a highly polished cast iron cylinder or dryer and holding the sheet in contact with the dryer until the moisture is substantially removed from the web. When sufficient moisture is removed, the sheet automatically releases itself from the dryer, this release being known generally as popping, and the sheet has a glazed surface which approximates the dryer surface.
The equipment used in the paper making art to produce porosity and results in the sheet being blown or lifted from the dryer, as has been earlier pointed out. This blowing not only results in non-uniform glazing but also results in a lower glaze for the sheet.
, Since the porosity of the sheet must be maintained to prevent this blowing or lifting, the amount of glaze is I necessarily limited by the processing conditions Furthermore, since the stock cannot be beaten to a low freeness level, the cellulosic material does not as readily conform to the drying surface and, therefore, does not faithfully reproduce the profile of the drying surface. Thus, it will be seen that the presently known process inherently limits the amount of gloss which can be developed.
Furthermore, since the beating of the stock is necessarily limited, the fibers substantially retain their fibrous character, with the concomitant impossibility of pressing the sheet .with suilicient force to adhere every portion of the sheet to thedryer surface. In addition, the retention of the fibrous character of the fibers causes the sheet to be a spongy mass so that only portions of the fiber are in actual contact with the dryer surface.
With the foregoing limitations in mind, it will be readily seen why it has not been believed, heretofore, possible to make machine glazed paper with high gloss or obtain gloss values much in excess of the 25 now obtainable on commercial machines. Of course, the provision of a machine glazed sheet having such high glosses is quite desirable for various purposes. Gloss values referred to here are as measured on the Photovolt instrument, with a polished black glass standard being the reference for 100 percent gloss.
It is a principal object of this invention, therefore, to provide an improved machine glazed paper. A still further object of the invention is the provision of an improved process for making a paper having a high gloss thereon, such gloss being greater than 40 and as high as or 90. A particular object of the invention is the provision of a process for manufacturing a machine glazed paper with a substantially higher gloss than is obtainable on presently available paper making equipment.
These and other objects of the invention are accomplished by the formation of a web of paper or base sheet to which is applied, in a particular amount, a slow stock, i.e., a stock having a low freeness. I This stock is combined or superimposed upon the base sheet and dried In the practice of this invention, a base sheet is formed from a stock which does not have a freeness below about 600. (The term freeness as used herein is measured by the number of milliliters of water drained from a slurry of 2 grams of pulp in 1000 milliliters of water, using the standard Schopper Riegler instrument.) As a consequence, a base sheet is formed therefrom having a relatively high degree of porosity and a relatively low density. Such properties are necessary to provide a proper base for the highly beaten or low freeness stock which is carried on the base sheet in accordance with the practice of this invention.
If lower stock freenesses are employedfor the base sheet, vapor tends to form between the sheet'and the dryer surface so that the paper sheet has a non-uniformly glazed surface. Furthermore, the use of lower freeness stocks establishes a limitation upon the amount of low freeness stock which can be carried by thebase sheet and a satisfactory machine glazed surface may not be provided.
The moisture content of the base sheet is of some importance. The base sheet should be sufficiently high in moisture so as to insure satisfactory fiber-to-fiber bonding with the applied surface stock. It has been found that the moisture level should exceed 35 percent. When this bond is satisfactory, a Dennison wax pick test, when used on the finished surfaced sheet, should show a reading of at least 14.
The low freeness stock which is applied to the base sheet should "have a freeness which is-less than 400 and preferably less than 300. In general, the lower the freeness the better so that cellulosic materialwill conform on'the entire surface of the dryer. On the other hand, thelow freeness stock which is applied to the base sheet should not be too low in freeness or it will become too impermeable to vapor pressure and the sheet will be prematurely lifted from the surface of the dryer. Because of these conditions, it has been found that the low freeness stock should have a freeness in excess of 50.
The fiber furnish of the base sheet is relatively unimportant and can be made from any fiber that will give a sheet of adequate strength at the freeness level required. The selection of a suitable furnish is within the skill of the art. The fiber furnish of the low freeness top sheet is of great importance, since it will govern the level of gloss. While any type of wood pulp can be used with reasonable success, the pulps made from coniferous wood yielding fibers of relatively small diameter and easily beaten or hydrated give well bonded uniform surfaces yielding excellent gloss. Typical pulps used giving greatest success have been bleached Mitscherlich sulfite pulps made from spruce wood, bleached 'kraft pulp made from spruce wood, and bleached'kraft pulp made from northern Jack Pine wood.
The resulting sheet of the invention should comprise at least 5 percent of the low freeness stock. On the other hand, the low freeness stock should not comprise more than about 15 percent of the weight of the sheet (dry basis). In this connection, lower amounts of the low freeness stock do not provide adequate coverage for the base sh et, whereas the use of higher amounts of low freeness stock results in premature lifting of the sheet from the dryer because of the formation of vapor.
The addition of fillers, sizing rosin, alum, dyes and other special ingredients consistent with the usual methods of making machine glazed papers will not in any essential way interfere with the practice of this invention. This is true whether the additions are made to one or both portions of the sheet.
In order to impart the desired gloss to the sheet, a
pressure in excess of 80 p.s.i. should be applied. For
most satisfactory results, the pressure should exceed 300 p.s.i. While there is some gain by the use of still higher pressures, the pressure should not exceed about 1500 p.s.i. At higher pressures, loss of gloss is experienced in contrast to. pressures in the indicated range. The pressure should not be held for extended periods of time and,
5} in this .connection thepressure is desirably applied and rapidly released. Maintenance of the pressure for a time as long as one minute results in glosses which are lower than the mere application and release of pressure. It has been found that it is not desirable to repeatedly apply the pressure and, in this connection, best results are obtained by the single application of pressure.
Various systems may be employed for combining the high freeness stock with the low freeness stock. However, whatever system is employed, it is necessary that the low freeness stock be substantially uniformly distributed over the surfaceof the base sheet. The low freeness stock may be distributed on a transfer roll and applied'to the basesheet or, alternatively, thelow freeness stock may beapplied by means of a secondary headbox. In this connection, the base sheet may be formed on a Fourdriniermachine. with the low freeness stock applied after formation of the base sheet. Still other arrangements will be readily apparent to those familiar with the paper making art.
As an example of the practice of the invention, a bleached Mitscherlich sulfite pulp from spruce wood was beaten for three hours under the following conditions:
A stock of 4 percent consistency was prepared and introduced into a ball mill containing 29 balls, each of which had a diameter of approximately one inch. Sufficient stock was placed in the mill to provide 30 grams of even drided pulp. After beating, the stockwas re-- duced to a consistency of about .35 percent and was taken out in a measured amount andthereupon placed in a sheet mold which was 6 /2 inches by 6 /2 inches. In each of the examples, the moldwas filled with 1.7 grams (oven dry basis) of stock which provided a sheet which corresponded to a ream of paper weighing about 41.5 pounds, the ream comprising 500 sheets, 24 inches of 36 inches.
After the stock was beaten for three hours, it had a freeness of 100. In order to free'the stock from knots, it was placed in a mixer and'agitated for five minutes.
To provide a base sheet, a sulfite stock was beaten 30 minutes in the foregoing manner and had a freeness of about 860. The base sheet was formed in the sheet mold and the low freeness stock was then applied in the mold. After draining, the combination sheet was transferred to a dryer plate with the low freeness stock being pressed against the plate at a pressure of 40 p;s.i. in a Valley press.
The results of using-varying ratios of base sheet and low freeness stock under different pressing conditions will be seen from the following table:
Low I Base iree- Pressure Time Gloss Caliper sheet ness (p.s.i.) (min) stock 15 80 0 52 4. 9 325 0 54 3.0 325 l 58 3. 5 1,500 0 52 3.4 1,500 1- 42 3.4 10 80 0 48 5. 0 325 0 57 4.0 325 1 58 3. 7 1, 500 0 60 3. 5 s 1,500 1 50 3.4 80 0 30 5. 1 325 0 4248 4. 0 325 1 42-48 3. 7 1, 500 0 46 3. 6 v- 1,500 1 4855 3.4 0 S0 0 18 5. 4
In each of the foregoing cases, the'sheets were dried against a polished platen, the temperature of which was 200 By way of another'example', a stock having a freeness of 240 was made from a bleached sulfite pulpof the sametypeas in the previous example. This pulp was placed on a base sheet and the results thereof, for varying amounts of base sheet and low freeness pulp are set forth in the following table:
Base Low iree- Pressure Time Gloss Caliper sheet ness stock (p.s.i.) (min) Base Low Pressure Time sheet freeness (p.s.i.) (min) Gloss Caliper stock In another example, a pulp having a freeness of about 840 was prepared as a base sheet and a pulp made from coniferous wood treated by the kraft process was beaten to a freeness of 240. The results of combining the base sheet and low freeness pulp are set forth in the following table:
Base Low Pressure Time sheet freeness (psi) (min) Gloss Caliper stock It will be seen from the foregoing examples that high gloss sheets can be provided by combining low freeness 6 stock in particular amounts with base sheets made from higher freeness stocks. The gloss of the resulting sheet is substantially higher than commercial grade machine glazed papers. The low freeness stock can be applied with presently available paper making equipment to provide a highly desirable machine glazed paper.
The various features of this invention which are believed to be new are set forth in the following claims.
1. In the manufacture of machine glazed paper, the steps of preparing a base sheet from cellulosic pulp having a freeness in excess of 600, superimposing on said base sheet a low freeness cellulosic pulp having a freeness of less than 400 and more than 50, said low freeness pulp comprising at least 5 percent and not more than 15 percent, by weight, of the finished sheet, on a dry basis, and thereafter pressing the superimposed low freeness pulp against a polished drying surface at a pressure in excess of about psi. and less than about 1500 p.s.i., and drying said sheet while maintaining said low freeness surface in contact with said polished drying surface to provide a finished machine glazed sheet.
2. In the manufacture of machine glazed paper, the steps of preparing a base sheet from cellulosic pulp having a freeness in excess of 600, superimposing on said base sheet a low freeness cellulosic pulp having a freeness of less than 300 and more than 50, said low freeness pulp comprising at least 5 percent and not more than about 15 percent, by weight of the finished sheet, on a dry basis, and thereafter pressing, at a pressure in excess of about 300 p.s.i. and less than about 1500 p.s.i. the superimposed low freeness pulp surface of said sheet being pressed against a polished drying surface and drying said sheet to provide a finished machine glazed sheet having a gloss value in excess of 40.
References Cited in the file of this patent UNITED STATES PATENTS 367,424 Merrill Aug. 2, 1887 1,578,952 Campbell Mar. 30, 1926 1,687,599 Upson Oct. 16, 1928 1,730,849 Hinde Oct. 8, 1929 2,007,470 Harvey July 9, 1935 2,098,733 Sale Nov. 9, 1937 2,226,709 Colbert Dec. 31, 1940 2,313,497 Adrian Mar. 9, 1943 2,414,833 Osborne Jan. 28, 1947 2,680,468 Lewis June 8, 1954 2,930,106 Wrotnowski Mar. 29, 1960 FOREIGN PATENTS 227,260 Germany Sept. 14, 1906 515,011 Great Britain Nov. 23, 1939