US 3250666 A
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y 1966 R. E. CLARK ETAL 2 9 METHOD OF FORMING CELLULOSIC PAPER CONTAINING ROSIN AND POLYETHYLENE Original Filed May 28, 1962 INVENTORS Robert E. Clark Roberf C. Whitney 5% ROSIN I%ROSIN O%ROSIN osvaRosm LATEX SOLIDS LATEX SOLIDS "2%ROSIN ROSIN (PERCENT BY WEIGHT 0F 0.D. FIBERS) F lg. I
(PERCENT BY WEIGHT OF 0 D FIBERS) F lg. 2
A I I I II,
I I I I I 4% ROSINL 3%ROSIN 2%Rosm-- OR LESS Roam 4% ROSIN 3%ROSIN A 2%ROSIN ROSIN p ATTORNEY United States Patent METHOD OF FORMING CELLULOSIC PAPER This application is a continuation of our US. application Serial No. 198,058, filed May 28, 1962, now abandoned.
This invention relates to improved paper products and to methods of making the same. In particular, this invention relates to paper products stock-sized with polyethylene and rosin, and to methods of making the same.
Stock-sizing or internal sizing of paper products is performed to improved the resistance of the products to penetration by gases and liquids and has long been known in the papermaking art. In this process, the sizing agent usually rosinis added to an aqueous suspension of the fibers from which the paper is to be formed, and is then deposited onto the fibers prior to the formation of the fibrous web. The paper products may be hard-sized, that is, they may have a degree of sizing equivalent to that obtained by adding from about 35% of rosin based on the oven dry weight of the fibers, or may be slack-sized, that is, they may have a degree of sizing equivalent to that obtained with less than about 3% by weight of rosin, depending on the properties desired in the final product.
It has been suggested in US. Patent 2,739,058 that conventional sizing agents such as rosin be replaced by polyethylene added to the aqueous fiber suspension in the form of an aqueous dispersion of polyethylene solids. The
polyethylene sizing agent is of principal utility in improving the hue and brightness of sized paper products produced using polyethylene sizing agents, since even high grade rosin materials have color and-impart color to papers to which they are added, whereas polyethylene solids can be made uniformly white and interfere little with the optical properties of the final paper product. The degree of sizing effected by a given quantity of polyethylene sizing agent in a paper product is about the same as the degree of sizing effected by the same quantity of rosin added to the pulp suspension, particularly at high sizing levels.
It has now been found that when paper products are sized with a sizing agent comprising both rosin and polyethylene solids, a higher degree of sizing can be obtained per unit Weight of sizing agent added than is possible using either rosin alone or polyethylene alone. For example,
the addition of 1% of rosin and of 0.5% by weight of polyethylene solids in combination as a sizing agent produces the same degree of sizing as is achievable with 3 or more percent of rosin alone or with even still larger quantities of polyethylene solids alone. The effects of this combination are more than additive: the degree of sizing effected by such a combination of 1% of rosin and 0.5% of polyethylene solids far exceeds the effect expected from observations of the degree of sizing brought about by addition of 1% of rosin or addition of 0.5 of polyethylene solids separately.
Thus, the present invention permits the achievement of high size levels in paper products with amounts of rosin and/ or polyethylene considerably smaller than those heretofore used in the art. In view of the vast quantities of paper produced per year in the United States and throughout the world, the annual savings effected by the present invention in expenditures for sizing agents are of considerable economic importance. Also, although rosin is employed as a sizing agent according to the present invention, it is used in amounts far less than those which would be employed to give a corresponding degree of sizing if rosin were used alone. This permits improvement in the optical properties of paper products produced according to the invention, for example in hue and brightness, while still achieving the other effects associated with rosin sizing.
In addition, it has been discovered that the synergistic rosin-polyethylene sizing agent combinations of the present invention improve the dry tensile strength of slacksized products produced therefrom over those values observed employing either rosin or polyethylene alone as the sizing agent. Thus, the mechanical properties of certain papers produced according to the invention are superior to those observed in papers sized with rosin or polyethylene alone, the optical properties of the papers of the present invention are comparable with those'produced in the prior art, and high sizing levels are achieved using substantially smaller quantities of sizing agent.
These relationships will be better understood by referring to the drawings accompanying this application. In the drawings:
FIGURE 1 is a plot showing the degree of sizing as a function of additions of rosin alone, of polyethylene solids alone, and of rosin and polyethylene in combination; and
FIGURE 2 is a plot of dry tensile strength of sized paper as a function of the content of rosin sizing agent alone, of polyethylene solids alone, and of rosin and polyethylene in combination.
In FIGURE .1, a measure of the degree of sizing is plotted on the ordinate versus the quantity of polyethylene solids present as a sizing agent (by weight of oven dry fiber) plotted on the abscissa. The numerical values on the ordinate are plotted in seconds, and are values obtained on a Currier size tester with the testing circuit set for testing hard-sized papers. Also, points indicating the degree of sizing obtained in paper sized with rosin alone, that is containing no polyethylene solids, are plotted along the ordinate. The curves in the figure indicate the degree of sizing obtained employing polyethylene alone as a sizing agent (0% rosin), and the sizing level observed employing 0.5 1% and 2% rosin in combination with varying amounts of polyethylene solids. It will be evident from the curves that minor additions of polyethylene in combination with at least about 0.5 rosin achieve higher size levels than are obtainable using equivalent amounts of rosin alone, and that combinations of at least about 1% rosin with small amounts of polyethylene achieve a higher degree of sizing than is obtainable with rosin or polyethylene alone. Further, papers sized with combinations of at least 1% rosin and at least about 0.5% polyethylene have a degree of sizing equivalent to that obtainable with about 3 percent or more of rosin alone, and far exceed the degree of sizing obtainable using equivalent quantities of polyethylene alone. Thus, for example, using a total of 1.5% of a rosin-polyethylene sizing agent, the same degr e of sizing is obtained as is achieved using twice that quantity of rosin alone.
FIGURE 2 shows. dry tensile strength plotted on the ordinate versus percent of polyethylene solids plotted on the abscissa. As in FIGURE 1, there are a series of curves showing the effect of polyethylene in combination with 0%, 0.5%, 1% and 2% of rosin. Dry tensile strengths obtained with rosin alone are plotted as points on the ordinate. The curves show that the tensile strengths observed in stock-sized papers containing small amounts of rosin in combination with polyethylene solids exceed those observed in materials sized with equivalent quantities of rosin alone or polyethylene alone.
From the curves, it will be seen that the synergistic combinations of the invention arise when paper suspensions are treated with the addition of a mixture of rosin and polyethylene solids such that the rosin is at least about 0.5% by weight of the oven dried fibers, and the polyethylene content varies from about 0.10% to about 1 or 2%, on the same weight basis. Particularly good results are obtained when a minimum of at least about 1% of rosin is employed in combination with at least about 0.5% by weight of polyethylene solids, based on the dry fiber weight. It will further be evident from the curves that the use of amounts of rosin in excess of about 2.5% or of polyethylene in excess of 1 or 2%, although not detrimental to the paper products, causes no further significant increase in the degree of sizing or tensile strength, and would only be wasteful from an economic viewpoint.
The amount of rosin and/ or polyethylene deposited on the fibers and retained on the paper web will vary Widely in dependence on factors such as the temperature, pH and ionic strength of the suspension, the Water hardness, the precipitating agent, the nature of the fibers, machine conditions, and the like. In consequence, the degree of retention is highly dependent on specific conditions. However, under conditions which might normally be encountered in a papermaking process, between about 20% and about 80% by Weight of the rosin and polyethylene in the suspension have been retained in the final sheet. Thus, a ton of paper prepared from an aqueous suspension containing between 0.5% and 2.0% of rosin, based on oven dried fiber weight, may contain between 2 pounds and 32 pounds of retained rosin. When polyethylene solids are present in the aqueous phase in amounts between 0.1% and 1%, the ton of paper may contain between 0.4 and I6 pounds of retained polyethylene solids.
The process of the present invention can be used to size paper products, such as paper and paperboard, of any variety, including those prepared from a furnish ranging between 100% hardwood to 100% soft wood. These paper products may be prepared from various combinations of kraft, sulfite, or soda pulps, groundwood, semigroundwood, and/or semi-chemical pulp. These pnlps may be bleached or unbleached. The paper products may be filled or unfilled, of any weight or grade, and papers stock-sized according to the present invention can subsequently be surface-sized, coated, or treated by other procedures conventional in the art. The process of the invention can be used in the preparation of writing and printing paper, news and wrapping, cover, tissue, cardboard, Bristol and tag, book papers, paperboard, food packaging grades, and the like. In these products, opacity, smoothness, gloss, and porosity are all satisfactory and comparable with the properties observed in equivalent papers stock-sized with conventional sizing agents.
The rosin-polyethylene sizing agents of the present invention can be added to the pulp suspension at the beater, after refining, or at the fan pump or headbox, as known in the art. A conventional precipitating agent for the rosin, usually papermakers alum, sodium aluminate, or acids alone or in combination with aluminum salts, etc., as known in the art, may be added after the sizing agent according to the normal method. The precipitating agent can also be added first followed by the sizing agent as in the reverse sizing method. The agent is used to adjust pH to an acid value suitably between about 4.5 and about 6.5.
The rosin sizing agents employed in the present invention are materials conventional in the art for use in sizing papers and are prepared in the conventional manner, as discussed in Casey, Pulp and Paper, vol. II, Interscience Publishers, Inc., New York (1960), pp. 1043-4052.
The polyethylene solids are incorporated to the pulp suspension in the form of an aqueous latex of the type described in copending US. patent application Serial No. 104,763, filed April 21, 1961, for example. As disclosed in that copending application, stable polyethylene latices of a type suitable for use in the present invention can be directly prepared by emulsion polymerization of ethylene at polymerization temperatures and pressures in a charge mixture comprising water, an anionic emulsifying agent, a pH adjuster in some cases, an optional solvent, and a polymerization initiator. In view of the detailed teachings of this copending application, it is impractical to recite all of its contents herein, and it is to be understood that the application is specifically incorporated into the present application by reference. In its essentials, however, the copending application teaches that ethylene can be co-polymerized at a minimum pressure of about 2500 pounds per square inch in an aqueous phase comprising between about 0.9 and 9% by weight of (l) the salts of saturated fatty acids, (2) salts of sulfates of fatty alcohols, or (3) salts of sulfates of ethoxylated fatty. alcohols, as emulsifying agents. Alkaline substances, such as potassium phosphates, giving a pH of at least about 8.5 in the final product, are suitably present in the aqueous phase. Tertiary butanol is optionally employed in the aqueous medium, but, if present, is present in amounts of from about 5 to about 2025 percent by weight of the aqueous medium. Potassium and sodium persulfates are employed in amounts of from about 0.06% to about 0.5% by weight of the aqueous phase as free radical initiators. At temperatures of from about 70 to about 100 C., stable polyethylene latices containing up to about 30% by weight of solids are obtained, and can be concentrated to latices containing as high as to about by weight of solids. As disclosed in said copending application, the
latices are reactor-stable, that is, they can be produced by emulsion polymerization as homogeneous products substantially free of coagulum and containing at least 20% of polyethylene solids. Further, the latices are stripsta'ble, that is, they are susceptible to concentration by removal of excess water and any solvent which may be present to produce concentrated latices containing solids of at least 40% by weight. Third, the latices are shelfstable, having a high resistance to creaming, gelling, or thickening for periods of days, 6 months, or a year or more.
In these latices, the polyethylene solids contained therein are colorless, odorless, non-oxidized, substantially oxygen-free (less than about 1%), substantially sulfur-free (less than about 0.1%), non-telomerized materials having a molecular Weight from between about 7,000 to about 30,000 or 40,000. The materials are of intermediate density, between about 0.91-0.93 grams/cm. and have a crystalline melting point of about 115C. The solids are present in the latices as particles of an average size between about 0.02 to about 0.5 micron.
Polyethylene solids of this type are relatively hard and non-waxy, and their use avoids the problems encountered with sizing with wax. These ditficulties include agglomeration of Wax in the aqueous phase during hydration and refining, the wax being directly introduced in the sizing operation and/ or by addition of wax-sized broke" to the aqueous phase as reclaimed pulp.
Particles of agglomerated wax may eventually be carried to the formation wire, where they may cause formation of holes in the paper web. Wax particles may also be carried with the paper to the press felts, where they will cause felt marks, or to the calender stack where they can result in the formation of fish eyes or translucent spots in the paper.
Also, whereas the addition of waxes to rosin size decreases the strength properties of the sized paper, the polyethylene-rosin combinations of the present invention give papers of greater tensile strength, for example, than those sized with rosin alone.
A better understanding of the invention and of its many advantages will be had by referring to the following specific examples, given by way of illustration.
Example A fiber furnish similar to that used in the manufacture of printing and writing papers was prepared by soaking 252 parts (oven dried weight) of bleached southern hardwood kraft and 108 parts by weight of bleached northern softwood kraft in water, and then beating to a Canadian standard freeness of 300. About 29 parts by weight of clay were added to the beater. Using a closed white-water system, the pH was adjusted to 4.8-5.2 with paper-makers alum.
A sizing agent was prepared by mixing a stock solution of rosin in water with an anionic polyethylene latex in amounts giving the desired quantities of rosin and polyethylene solids, based on the oven dried fiber weight. The resulting mixture was added to the pulp suspension from which paper was made. The pH was rechecked and readjusted to 4.8-5.2 with additional alum if necessary. Paper was made in the conventional manner from the resulting suspension containing the sizing agent.
A control containing alum, but no sizing additives, was made, as well as paper containing rosin alone, polyethylene solids alone, and rosin and polyethylene in combination at rosin levels of 0.5, 1, and 2% respectively in combination with increasing quantities of polyethylene solids between about 0.1% and 2% by weight. The paper produced in this matter was dried in the usual fashion using drum drying apparatus. For test purposes, sheets of the papers were then humidified in a constant humidity environment overnight in accordance with TAPPI Standard T402m-49.
The size levels in the resultant paper sheets were tested with a Currier size tester with the instrument settings on the hard-size scale. As known in the art, the tester measures the time required for Water to permeate a sized paper sufficiently to raise the electrical conductivity of the paper to a fixed standard value. Tensile strengths were determined according to TAPPI Standard T404m-50. The results of these tests have been reported in FIGURES 1 and 2 of the drawings.
The rosin used in making the papers was a commercial dry rosin size available under the trade name Accobrite.
The polyethylene latex was colorless and odorless and had a solids content of 41.65%, a latex density of 0.978, a pH of 10.2, and was stable to dilution. The average particle size of the suspended polyethylene solids was about 0.083 micron. The suspended polymer had a ring and ball softening point of 109 C. and a polymer density of 0.923.
The latex was prepared by polymerizing ethylene at a temperature of 85 C. under a pressure of 300 psi. in an aqueous medium comprising 90 parts by weight of water, parts of t-butanol, about 3 parts of myristic acid, about 0.8 part of KOH, about 0.4 part of K PO and employing about 0.1 part of potassium persulfate as an initiator.
Although specific embodiments have been shown and described, it will be understood that departures may be made therefrom within the scope of the invention.
What is claimed is:
1. -The method of internally adding additaments to paper products which consists essentially of adding from about 0.5 percent to about 2 percent of rosin and from about 0.1 percent to about 1 percent of suspended polyethylene particles to an aqueous fibrous pulp suspension and forming a fibrous web from said suspension after acidification of the suspension, said percentages being by weight of dry fibers in said suspension; wherein said polyethylene particles are non-oxidized, contain less than about 1 percent oxygen, contain less than about 0.1 percent sulfur, and are further characterized as non-telomerized material having a molecular weight from about 7,000 to about 40,000, an average particle size of about 0.02 to about 0.5 micron, a density of about 0.91 to about 0.93 gram/cm. and a melting point of about 80115 C.
2. In a method of internally adding additaments to paper products by deposition of rosin solids on suspended cellulosic fibers, the improvement consisting essentially of codepositing polyethylene particles with said rosin from an aqueous suspension of said fibers containing about 0.1 percent of polyethylene and about 0.5 percent of rosin, said percentages being by weight of dry fibers; and wherein said particles of polyethylene are nonoxidized, contain less than about 1 percent oxygen, contain less than about 0.1 percent sulfur, and are further characterized as non-telomerized material having a molecular weight from about 7,000 to about 40,000, an average particle size of about 0.02 to about 0.5 micron, a density of about 0.91 to about 0.93gram/cmfi, and a melting point of about l15 C.
3. In a method of internally adding rosin solids to paper products by deposition of rosin solids on suspended cellulosic fibers, the improvements which consists essentially of codepositing polyethylene with said rosin on said suspended cellulosic fibers by adding rosin and a latex consisting essentially of finely divided particles of polyethylene in an aqueous medium to an aqueous suspension of cellulosic fibers such that between about 0.5 to about 2 percent of rosin solids and between about 0.1 and 1 percent of polyethylene solids are present in said suspension, and then forming a fibrous web from said sus-' pension, said suspension having a pH between about 4.5 and about 6.5 at the time of web formation, said percentages being by weight of dry fibers in said suspension; and wherein said polyethylene particles are non-oxidized, contain less than about 1 percent oxygen, contain less than about 0.1 percent sulfur, and are further characterized as non-telomerized material having a molecular weight from about 7,000 to about 40,000, an average particle size of about 0.02 to about 0.5 micron, a density of about 0.91 to 0.93 gram/cmfi, and a melting point of about 80 C. 4
4. The method as in claim 3 wherein said rosin is present in amounts of from about 1 to about 2 percent, and said polyethylene is present in amounts of at least 0.5 percent to 1 percent.
5. An internally modified paper product comprising a fibrous web having incorporated therein by addition between about 2 pounds and about 32 pounds of rosin per ton of product together with between 0.4 pound and 16 pounds of polyethylene solids per ton of product; said addition of rosin and polyethylene solids being effected in an aqueous suspension of cellulosic fibers containing rosin and polyethylene particles wherein said polyethylene particles are non-oxidized, contain less than about 1 percent oxygen, contain less than about 1 percent sulfur, and are further characterized as non-telomerized material having a molecular weight from about 7,000 to about 40,000, an average particle size of about 0.02 to 0.5 micron, a density of about 0.91 to 0.93 gram/emf, and a melting point of about 801 15 C., with said product being prepared by the process of claim 1.
References Cited by the Examiner UNITED STATES PATENTS- 8/1954 Latham et al 162-169 3/1956 OFlynn et al 162168 OTHER REFERENCES Casey: Pulp and Paper, vol. 1, 1952, Interscience Publishers Inc., N.Y., page 515.