Background All systems for manufacturing paper and paperboard products, hereinafter collectively referred to as paper, include a series of operations and processes. Typically, wood is either digested chemically or comminuted mechanically to form pulp. Pulp produced by chemical digestion must be washed, and pulp produced by both methods is often bleached. For most applications, the pulp must be refined before it is used to manufacture paper.

Refining consists of pumping an aqueous pulp slurry through a series of metal discs moving at a high speed. During refining, the cellulose fibers are cut and macerated in order to develop fibrillation. Fibrillation increases the number of inter-fiber contacts that will occur during formation of the paper and bonding during subsequent pressing and drying operations. A sheet that is formed from unrefined pulp will generally have a low density and be rather soft and weak. whereas if the same pulp is well-refined, the resultant paper will be much more dense, hard, and strong.

After refining, the aqueous pulp slurry is reduced in consistency by the addition of white water. At this stage in the process, the diluted aqueous pulp slurry is referred to as furnish. The furnish has a solids concentration, which is referred to as consistency, typically within the range of from between 0.2 and 1%. In general, the lower the consistency of the furnish, the better the formation and homogeneity of appearance of the finished paper. Functional additives, usually in a cationic form, are conventionally added to the furnish so that they can be attached to and retained by the fibers.

The furnish is usually fed to a headbox apparatus through one or more screens or other filtering devices to remove impurities. The furnish then enters a flow spreader which provides a uniform flowing stream along the width of the paper machine. The flow spreader discharges the slurry into a headbox where fiber agglomeration is prevented by controlled turbulence. Pressure is provided to cause the slurry to flow at the necessary velocity through the slice and onto a moving endless band of a woven synthetic fiber or metal fabric called a wire. On a modern machine, the wire can be moving at a speed of about 700 to 2000 meters per minute. Continuous sheet forming and drying can be accomplished using three different types of equipment: the cylinder, Fourdrinier (i. e., single wire), and twin-wire machines, all of which are well-known in the art.

In the cylinder machine, a wire-covered cylinder is mounted in a vat containing the refined fiber slurry. As the cylinder revolves, water drains inward through the screen. thus forming a paper web on the outside of the cylinder. The wet web is removed at the top of the cylinder, passes through press rolls for water removal, and is then dried over steam-heated, cylindrical drums.

The Fourdrinier machine is more complex than the cylinder machine and basically consists of a long continuous synthetic fiber wire which is supported by various means to facilitate drainage of water. The fiber slurry, which is introduced at one end of the machine through a headbox and slice, loses water as it progresses down the wire, thereby forming the web. The web is then directed to the press and dryer sections as in the cylinder machine.

The twin-wire machine is the latest development and consists essentially of two opposing wires. Twin-wire formers have replaced the Fourdrinier, particularly for lightweight sheets, e. g., tissue, towel, and newsprint. Twin-wire formers also are operated successfully on fine paper, corrugated media, and liner board grades. In twin-wire formers, the water is drained from the slurry from both the top and bottom of the sheet. The two wires, with the slurry between, are wrapped around a cylinder or set of supporting bars or foils. The tension in the outer wire results in a pressure which is transmitted through the slurry to the supporting structure. Some machines also use vacuum to produce the required pressure. The pressurized slurry drains through one or both of the wires. The web is then directed to the press and dryer sections as in the other papermaking machines.

In most paper machines, the wire is mounted over a breast roll at the intake end and at a couch roll at the discharge end. Between these two rolls, the wire is supported for the most part by table rolls, foils and suction boxes. A substantial vacuum is developed in the downstream nip between the table roll and the wire. This promotes water drainage from the slurry on the wire. As speeds increase, however, the suction can

become too violent and deflect the wire, causing the paper web to be thrown into the air.

A more controlled drainage action is accomplished by the use of foils. Foils are wing-shaped elements which support the wire and induce a vacuum at the downstream nip. Foil geometry can be varied to provide optimum conditions. After passing over the foils or table rolls, the wire and sheet pass over suction boxes, where more water is removed. Most machines also include a suction couch roll for further water removal.

In its most typical form, the formation of the paper web takes place in the first few feet on the wire. The stock issuing from the slice is a suspension of fibers in water, typically containing from 0.2 to 1.0% solids in a layer some 6-18 millimeters deep and up to several meters wide. It is deposited on and drains through the wire. At very low speeds, the force of gravity predominates in causing the drainage. At higher speeds, the action of gravity becomes negligible compared with the pumping action of the drainage elements (i. e., the table rolls or foils). A visible change occurs in the appearance of the stock as it proceeds down the wire when its concentration reaches about 2%. At this level, the surface ceases to appear mobile, loses its liquid sheen, and takes on a matte appearance. At this point in the process, the drainage elements are no longer effective for removing water because the web is formed. Next, consolidation begins, assisted by the action of the suction boxes. Some slight rearrangement of the fibers may still be achieved by the pressure of an overhead roller, called a dandy roll.

The sheet leaving the wet end has a consistency of 18-23%. Conventionally, additional water is removed from the sheet mechanically without adversely affecting sheet properties. This is achieved using one or more rotary presses in the press section

of the paper machine. Press rolls may be solid or perforated and often suction is also applied through the interior of the rolls. The sheet is passed through each rotary press on continuous felts, usually one and sometimes two for each press, which act as conveyors and porous receptors of water. Conventionally, the solids content of the sheet can be increased by such pressing to a consistency of about 30 to 40% without crushing.

Crushing, which refers to the direct flow of water in the sheet, occurs when too much pressure is applied to the wet sheet by the presses. Crushing can be minimized by applying pressure gradually, since less water is initially removed this way and the fibers are not so likely to be pushed apart. The sheet can stand higher and higher pressure as water is removed and the sheet becomes stronger. Graduated pressure is particularly important on heavy boards inasmuch as the danger of crushing increases for greater thicknesses of paper product. Pressing multi-cylinder boards while they are too wet may also lead to ply separation as well as crushing.

At a consistency of about 40%, it is no longer feasible to remove additional water from the sheet using conventional felted rotary presses and evaporative drying must be employed. This is a costly process and often is the production bottleneck of papermaking. The dryer section usually includes a series of steam-heated cylinders.

Alternate sides of the wet paper are exposed to the hot surface as the sheet passes from cylinder to cylinder. In most cases, except for heavy board, the sheet is held closely against the surface of the dryers by a fabric having carefully controlled permeability to steam and air. Heat is transferred from the hot cylinder to the wet sheet, and water evaporates. The water vapor is removed by way of elaborate air systems. Most dryer

sections are covered with hoods for collection and handling of the air, and heat recovery is practiced in cold climates. The final consistency of the dry sheet is usually between about 92-96 weight percent. depending upon the type of paper product being manufactured.

The efficiency of the drying sequence is dependent upon such factors as the amount of applied pressure which squeezes the wet web between the felts, the efficiency with which water condensed within the dryer cylinder is physically removed, and the ventilation of the pockets between dryers. During the drying sequence, the consistency of the product is increased from the entry level of generally about 30-40% up to that of the emerging dry paper product, i. e., 92-96%.

The energy requirements for removal of water depend upon the form of water which is present in the paper product. The majority of free water, being that which exists over and above what is required to saturate the fibers, can be removed on the wire by gravity or suction. Most of the remaining free water and some of the interstitial water can be removed from the web in the press section using a conventional felted rotary press operation. The most tenaciously held water (i. e., that within the lumen and pores of the fiber wall) must generally be accomplished in the dryer section of the paper machine using thermal drying.

During the early stages of drying, the fibers are free to slide over one another, but as the free water is driven off. the fibers are drawn closer together and bonding begins to take place. Surface tension is primarily responsible for drawing together the fibers in this stage, but later, molecular attraction brings about the final bonding between fibers. No

appreciable fiber-to-fiber bonding takes place until the consistency is raised above about 40 percent, but once this critical drying point is reached, shrinkage begins to take place and bonding begins.

It will be appreciated that the greatest energy use in the entire papermaking process occurs in the dryer section of the paper machine. Thus, any significant increase in the consistency of the web which can be accomplished in the press section of the paper machine will necessarily result in a substantial energy and cost savings.

U. S. Pat. Nos. 4,684,440 and 5,114, 539 to Penniman et al. describe a method for reducing the water content of the web in the press section of a papermaking machine using non-polar organic solvents (e. g., odorless kerosene) and water-insoluble liquid hydrocarbons (e. g., ISOPAR G, a proprietary aliphatic iso-paraffinic compound manufactured by Exxon Corporation), respectively. According to this method, non-polar organic solvents or water-insoluble liquid hydrocarbons are applied directly to the web or to the felt surfaces of the rotary press for transfer to the web. These compounds, which are more readily absorbed by the web than water, displace and replace at least a portion of the interstitial water in the web during pressing. According to this method, the consistency of the web can be increased by as much as 10 percent during pressing.

Furthermore, since the compounds added to the web are more volatile than water, they are more readily removed from the web during drying resulting in an energy consumption savings of as much as 40 percent.

One of the obvious drawbacks of the method described in the Penniman et al. patents is the use of non-polar organic hydrocarbon solvents in the papermaking process.

These compounds are more expensive than water, some can present odor and toxicity problems, and some can increase the possibility of fire and/or explosion. Moreover, use of these solvents requires modification of the paper machine itself such that the volatilized solvents can be separated and recovered from the water vapor leaving the dryer section of the paper machine. A method is needed whereby the consistency of the web can be significantly increased in the press section of a paper machine which does not involve use of volatile organic compounds and which requires no substantial modification of the paper machine.

Summary Of The Invention The present invention relates to a method for increasing the rate of water removal from a web of paper during pressing in the press section of a paper machine comprising treating the felt or felts used in the press section with one or more surfactants before the felt or felts contact the web. Virtually any water soluble or dispersible compound that reduces the surface or interfacial tension of water can be used as a surfactant in the invention, but surfactants with an HLB value of from about 4 to about 10 are currently the most preferred surfactants for use in the invention.

The surfactants are not applied to the press section felts to displace or replace or substitute for water in the web, but rather they are applied to improve the ability of press section felts to absorb and transfer water from the web by reducing interfacial surface tension of the water passing through the felt fibers. Moreover, during pressing the press section felts transfer a portion of the surfactants from the press section felts to the web thereby reducing the surface or interfacial tension of the remaining free and interstitial

water in the web. Once the surface or interfacial tension of the remaining free and interstitial water in the web is reduced, it can be removed at a greater rate during pressing.

This will have a positive effect upon the operation during subsequent drying steps.

The method of the present invention has several distinct advantages when compared to prior art methods. The surfactants used in the present invention are generally more cost effective than the compounds used in prior art methods, and substantially lesser amounts of surfactant must be added to the press felts in order to obtain comparable results. Furthermore, the surfactants used in the invention are non- toxic, water-dispersible compounds that do not present fire or explosion hazards.

Furthermore, practice of the invention does not require modification of the dryer section of the paper machine.

The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

Brief Description of the Drawings In the annexed drawings: FIG. 1 is a schematic illustration of a typical Fourdrinier paper-making machine; and, FIG. 2 is a schematic illustration showing a surfactant according to the method of the present invention being applied to one embodiment of a felted press roll assembly.

Detailed Description The present invention generally relates to the application of a water-soluble or dispersible surfactant to a press felt before the press felt contacts the web. After a sufficient amount of a surfactant has been applied to a press felt, the web is pressed against the press felt as the web and press felt pass through a press nip. During pressing, water is pressed out of the web in order to increase its consistency. The water passes from the web into or through the press felt where it will later be removed by suction.

When exiting the nip, the pressed web will try to reabsorb or be rewet by water, since all water cannot be removed by the press nip. According to the invention, however, the water-dispersible surfactant reduces the interfacial surface tension of any water it comes into contact with, both in the web and in the press felt, increasing the water removal capacity of the press felt. The economic effect of the improvement in water removal rate is significant because less energy must be used to further dry the web in the dryer section of the papermaking machine. Use of the present invention may also inhibit the deposition of undesirable organics in the felt and thus it may help to reduce the use of cleaning chemicals.

Referring to the drawings, and initially to Fig. 1, there is schematically illustrated a typical Fourdrinier paper machine 10. Machine 10 includes a flow spreader 11, a headbox 12, a Fourdrinier table 13, a press section 14, a dryer section 15, acalender stack 16 and a reel 17. As best seen in Fig. 2, the surfactant of the present invention is used in the press section 14. Press section 14 includes a pair of rolls 20 that press paper 22.

Contacting paper 22 is a continuous loop of felt 24. Felt 24 is conditioned with a

detergent shower 26 and a high pressure shower 28. Of course, care must be taken to prevent the felt washes or detergent from stripping the beneficial surfactant. Press section 14 includes a flooding shower 30 and lubricating shower 32. The vacuum box or felt suction pipe 34 is located after the lubricating shower 32. In accordance with the present invention, a surfactant is applied to the felt 24 at shower 38. Other locations of course, such as 39 or 40 may be utilized to apply the surfactant.

Any one or a combination of water-soluble or dispersible non-ionic surfactants, being agents that reduce surface interfacial tension when dissolved in or mixed with water, can be utilized in the invention. Preferably, the surfactant has an HLB number of from about 4 to about 10. The HLB number (hydrophilic-lipophilic balance) of an emulsifier is calculated according to the following formula: HLB= (100-L)/15 in which L denotes the percentage by weight of the lipophilic group relative to the weight of the entire molecule.

The use of a surfactant or combination of surfactants with an HLB value that is too high will result in undesirable foaming. The use of a surfactant or combination of surfactants with too low of an HLB value will result in plugging or ineffective bridging between the felt and the water. When selecting the proper surfactant, care must be taken to utilize a surfactant that is compatible with the type of roll covers being used in the press (i. e. the surfactant must not damage or attack the roll covers). Also, the type of felt being used in the machine should be considered when selecting the proper surfactant.

Felts generally comprise nylon or some other synthetic fiber. Different surfactants may have varied attractions to certain types of felt. Sulfonate-type surfactants are generally

not suited for use in the present invention for they tend to lead to excessive foam formation. The surfactants utilized in the present invention must be no or low foaming.

Also, since the surfactant may come into contact with the paper fibers, FDA food contact issues may need to be taken into consideration when selecting the surfactant. Also, it is believed that cationic and anionic surfactants are generally not suitable for use in the present invention.

Presently, the most preferred surfactant is a polyethylene glycol 400, which is available as FELTMASTER 15-LF from GEO Specialty Chemicals of Charlotte, North Carolina. Examples of other general types of surfactants suitable for use in the invention include alcohol ethoxylates; alkylphenol ethoxylates; glycerol esters; polyoxyethylene esters; anhydrosorbitol esters; ethoxylated anhydrosorbitol esters; natural ethoxylated fats, oil, and waxes; glycol esters of fatty acids, diethanol amine condensates (amides); monoalkanol amine condensates (amides) and polyalkylene oxide block polymers.

It will be appreciated that water-dispersible surfactants used in the invention advantageously have little or no odor. Accordingly, they can be used in various paper making operations without need for special water treatment equipment and do not raise occupational health concerns. Moreover, because the surfactants used in the invention are water-dispersible, they present virtually no risk of fire or explosion (i. e. they have a high flash point and are substantially non-combustible).

If the surfactant is a liquid, it can be applied in a substantially pure form directly to the press felt, preferably by spraying. Of course, other techniques such as misting or showering may be employed. It will be appreciated that if the surfactant is a solid, it

must be dissolved in water before being applied to the press felt. Preferably, whether the surfactant is a liquid or a solid, it is applied to the press felt as a solution with water by misting, spraying, showering or dripping onto either side of the felt surface in a manner similar to that used for cleaning the felt. The surfactants may also be advantageously applied from the inside of a press section roll, and directed close to the nip so that the force of pressure can also be used to dispense water from the web.

The amount of surfactant utilized is not per se critical to the method of this invention. From a practical standpoint, a minimum amount of surfactant should be used in order to minimize cost. The minimum necessary amount of surfactant to be applied to the felt will depend upon the speed of the press (i. e. how fast the felt moves) and the amount of water being contacted and can be routinely determined by one skilled in the art. The felt surfaces must be uniformly and partially wetted by the surfactant for optimum results (preferably by spraying). Complete saturation of the felt by the surfactant is counterproductive because the felt can no longer absorb water from the web.

Preferably, some of the surfactant is available for transfer to the web to assist in the reducing the surface interfacial tension of the free and interstitial water in the web. With these considerations in mind, the lowest practical application limit would be an amount sufficient to at least partially coat the press felt before the press felt contacts the web.

The maximum practical amount utilized would be influenced by cost and performance considerations. At present. it is believed that application of surfactants at a rate of from about 0.25 pounds to about 1 pound per ton of finished paper is optimal. Excessive use could impact upon paper sizing operations.

The use of surfactants as described in the present invention increases the water transfer capacity of the press felt. It is believed that the surfactants reduce the surface interfacial tension of the water within the felt fibers making the water pass through the felt more easily. In addition, at least a portion of the surfactants are believed to be transferred to the web thereby reducing the surface interfacial tension of any remaining free and interstitial water in the web. Once the surface interfacial tension of the water in the web has been reduced, it can more easily be removed by pressing in the press section of the papermaking machine. Thus, there is less to be removed during subsequent drying.

When a surfactant is applied to the press felts as disclosed herein, an increase in consistency of the web exiting the press section is achieved as compared to the consistency of a web exiting from an untreated press section. The increase in consistency can range from about 0.1% to about 5%, with increases of at least about 0.5% to about 1.2% being most preferred. Over and above the consistency increase, use of surfactants as described herein will lead to less of a need to remove water in the dryer section.

Relative to paper machines that have limited drying ability, the use of the present invention facilitates higher machine speeds. Additionally, the use of the present invention can lower steam consumption since there is less water to remove from the paper.

The water-dispersible surfactants used in the invention preferably do not need to be recovered from waste water through special treatment. The preferred water-dispersible surfactants are biodegradable and can be discharged into conventional water treatment facilities without special treatment.

The scope of the invention is further described in connection with the following examples which are set forth for the sole purpose of illustrating the preferred embodiments of the invention and which are not to be construed as limiting the scope of the invention in any manner.

Example I Feltmaster 15-LF surfactant was applied to the felts of a paper machine at a rate of about 0.2-0.3 pounds per ton of finished paper. Use of the surfactant yielded a more open and drier felt. Use of the surfactant also yielded about a 4,500 pound/hour decrease in steam usage.

Example II Feltmaster 15-LF surfactant was applied to the felts of a paper machine at the rate of about 0.25-0.3 pounds per ton of finished paper. Use of the surfactant resulted in significantly drier felts in the machine. Moister levels exiting the first press fell from about 69.5% to as low as about 66% over a 5 hour period.