DE68922024T2 - - Google Patents

Abstract

Disclosed is a process for making soft tissue paper which includes the steps of wet-laying cellulosic fibers to form a web, applying to the wet web, at a fiber consistency level of from about 10% to about 80%, a polysiloxane material, and then drying and creping the web to form the finished tissue paper. The process may further include the steps of applying an effective amount of a surfactant material to enhance softness and/or wettability control; and/or an effective amount of a binder material, such as starch, for linting control, and/or to contribute tensile strength to the tissue paper.

Description

TECHNICAL AREA

This invention relates generally to a process for manufacturing tissue paper; and more particularly to a process for producing high-bulk tissue paper having a soft, silky, flannel-like feel; and enhanced tactile bulk and physiological surface smoothness.

BACKGROUND OF THE INVENTION

Soft tissue paper is generally preferred for disposable paper towels, facial and toilet tissue. However, known methods and means for increasing the softness of tissue paper generally adversely affect tear strength. Therefore, the design of a tissue paper product is generally an act of balancing softness with tear strength.

Both mechanical and chemical means have been used in pursuit of the goal of producing soft tissue paper: tissue paper that users can sense to be soft by touch. Such tactile softness can be characterized by, but is not limited to, compliance, flexibility, and smoothness, as well as subjective descriptions such as silky or flannel-like feel. The present invention relates to a method of improving the tactile softness of tissue paper -- particularly high-bulk creped tissue paper -- by incorporating chemical additives: particularly polysiloxane materials, which impart a silky or flannel-like feel to the tissue paper without making it appear greasy or oily to the touch of users of products comprising such tissue paper. In addition, surfactant material may be added to further improve softness and/or surface smoothness and/or to at least partially offset any reduction in wettability caused by the polysiloxane; and binding material, such as starch, may be added to at least partially offset any reduction in strength and/or to compensate for any increase in linting tendency due to the polysiloxane and, if used, the surfactant additive.

Representative high bulk creped tissue papers that are quite soft by current standards and are susceptible to softness enhancement by the present invention are disclosed in the following U.S. Patents: U.S. Patent No. 3,301,746 issued January 31, 1967 to Lawrence H. Sanford and James B. Sisson; U.S. Patent No. 3,974,025 issued August 10, 1976 to Peter G. Ayers; U.S. Patent No. 3,994,771 issued November 30, 1976 to George Morgan Jr. and Thomas F. Rich; U.S. Patent No. 4,191,609 issued March 4, 1980 to Paul D. Trokhan; and US-A-4,637,859, issued January 20, 1987 to Paul D. Trokhan. Each of these papers is characterized by a pattern of dense areas: areas that are denser than their respective surroundings, such dense areas resulting from being compacted during papermaking, such as by the crossover bumps of an embossing support structure. Other high-bulk, soft tissue papers are disclosed in US-A-4,300,981, issued November 17, 1981 to Jerry E. Carstens; and in US-A-4,440,597, issued April 3, 1984 to Edward R. Wells and Thomas A. Hensler. In addition, the achievement of high bulk tissue paper by avoiding general compaction prior to final drying is disclosed in US-A-3,821,068 issued June 28, 1974 to D. L, Shaw; and the avoidance of general compaction in combination with the use of debonders and elastomeric binders in the papermaking furnish is disclosed in US-A-3,812,000 issued May 21, 1974 to J. L. Salvucci, Jr.

Chemical debonders referred to above and as contemplated by Salvucci and the theory of their operation are described in such representative U.S. patents as U.S. Patent No. 3,755,220 issued August 28, 1973 to Friemark et al.; U.S. Patent No. 3,844,880 issued October 29, 1974 to Meisel et al. and US-A-4,158,594 issued January 19, 1979 to Becker et al. Other chemical treatments which have been proposed for improving tissue paper include, for example, that disclosed in DE-A-3,420,940, Kenji Hara et al., namely: the impregnation of toilet tissue paper with a combination of a vegetable, animal or synthetic hydrocarbon oil and a silicone oil, such as dimethyl silicone oil, to make it easier to clean and wipe.

In addition, a well-known mechanical method for increasing the tensile strength of paper made from cellulose pulp is that in which the pulp is mechanically ground prior to papermaking. In general, greater refining results in greater tensile strength. However, consistent with the previous discussion of tissue paper tensile strength and softness, increased mechanical refining of cellulose pulp adversely affects the softness of tissue paper when all other aspects of the papermaking furnish and process are unchanged. However, by using the present invention, tensile strength can be increased without adversely affecting softness; or, alternatively, softness can be improved without adversely affecting tensile strength.

US-A-4,028,172 discloses a process for making paper in which a polysiloxane polymer is added to the wet pulp before it enters the press section of the paper machine. This document relates to wet picking and does not address the problem of making a tissue paper with increased softness and good tear strength.

It is an object of the present invention to provide a process for producing tissue paper having enhanced tactile softness.

It is another object of this invention to provide a process for making tissue paper that has a silky, flannel-like feel.

It is a further object of this invention to provide a process for making tissue paper having increased tactile softness at a specific level of tear strength relative to tissue paper softened by conventional processes.

These and other objects are achieved by practice of the present invention, as will be apparent from the following disclosure.

SUMMARY OF THE INVENTION

The present invention comprises a process for manufacturing soft tissue paper. This process includes the steps of wet-laying cellulosic fibers to form a web, applying a sufficient amount of a polysiloxane to the web at a fiber consistency of from about 10% to about 80% (based on the total weight of the web) such that between about 0.004% and about 0.75% of said polysiloxane, based on the dry fiber weight, is retained by the tissue paper, and then drying and creping the web. Preferably, the amount of polysiloxane retained by the tissue paper is between 0.004% and about 0.3%, based on the dry fiber weight of the tissue paper. The resulting tissue paper has a basis weight of about 10 to about 65 g/m² and a fiber density of less than about 0.6 g/cm³ and contains the polysiloxane compound evenly distributed on its outward-facing surfaces.

The polysiloxane is applied subsequent to the formation of the wet web and prior to complete drying. Surprisingly, it has been found that significant benefits in terms of softening the tissue can be achieved by much lower amounts of polysiloxane when the polysiloxane is applied to a wet web rather than to a dry web (e.g., during the conversion process). In fact, an essential feature of the process disclosed herein is that the amount of silicone is low enough to be economical. Also, tissue papers treated with low amounts of polysiloxane retain a high level of wettability, which is an essential feature for a tissue product.

Preferred polysiloxanes for use in the process of the present invention include an amino-functional polydimethylpolysiloxane in which less than about 10 mole percent of the side chains on the polymer contain an amino-functional group. Since the molecular weights of polysiloxanes are difficult to determine, the viscosity of a polysiloxane is used herein as an objectively ascertainable indication of molecular weight. Accordingly, for example, it has been found that about 2% substitution is very effective with polysiloxanes having a viscosity of about 125 x 10-6 m2s-1 (125 centistokes); and viscosities of about 5 m2s-1 (5,000,000 centistokes) or more are effective with or without substitution. In addition to such substitution with amino functional groups, effective substitution with carboxyl, hydroxyl, ether, polyether, aldehyde, ketone, amide, ester and thiol groups can be achieved. Of these effective substituent groups, the family of groups comprising amino, carboxyl and hydroxyl groups is more preferred than the others; and amino functional groups are most preferred.

Examples of commercially available polysiloxanes include DOW 8075 and DOW 200, available from DOW Corning ; and Silwet 720 and Ucarsil EPS, available from Union Carbide .

The process for making tissue paper treated with a polysiloxane in accordance with the present invention may further comprise the step of adding an effective amount of a surfactant to increase the tactile surface smoothness of the tissue paper and/or to at least partially offset any reduction in the wettability of the tissue paper that may otherwise result from the incorporation of the polysiloxane. The effective amount of surfactant is such that preferably from about 0.0 to about 2 percent, based on the dry fiber weight of the tissue paper; and more preferably from about 0.05 to about 1.0 percent, is retained by the tissue paper. Also preferably, the surfactant is noncationic; and is substantially, after the tissue paper was made, in situ non-migrating, so that after manufacture changes in the properties of the tissue paper which might otherwise be caused by the inclusion of surfactant are substantially avoided. This can be achieved, for example, by using surfactants whose melting temperatures are higher than the temperatures normally encountered during storage, shipping, sale and use of the tissue paper products of the invention: for example, melting temperatures of about 50°C or higher.

The process of the invention for making tissue paper may also further comprise the step of adding an effective amount of a binder material, such as starch, to at least partially offset any reduction in tear strength and/or increase in linting that would otherwise result from the incorporation of the polysiloxane and, if present, the surfactant. The effective amount of binder material is such that preferably from about 0.01 to about 2 percent, based on the dry fiber weight of the tissue paper, is retained by the tissue paper.

All percentages, ratios and proportions are by weight unless otherwise stated.

The present invention is described in more detail below.

DETAILED DESCRIPTION OF THE INVENTION

Briefly, the present invention provides tissue paper having a silky, flannel-like feel and increased tactile softness by adding a polysiloxane additive to a wet tissue web. Such a process may also include adding an effective amount of a surfactant material and/or a binding material, such as starch, to the wet web. Generally speaking, the surfactant may be included to increase the tactile physiological surface smoothness and/or to provide sufficient wettability for the intended uses of the tissue paper (e.g., as toilet tissue); and a binder material such as starch may be included to at least partially offset any reduction in the tear strength of the tissue paper and/or excess linting properties that would otherwise be caused by the addition of the polysiloxane and, if used, the surfactant. Surprisingly, it has been found that very low levels of polysiloxane provide a significant tissue softening effect when applied to wet tissue webs in accordance with the present invention, as compared to the application of polysiloxanes to dry tissue webs (e.g., in converting operations). To ensure high retention rates, the wet web is formed and dewatered prior to application of the polysiloxane additive to reduce loss of polysiloxane through free water runoff. Importantly, the levels of polysiloxane used to soften the tissue paper have been found to be low enough that the tissue paper retains high wettability.

The present invention is applicable to tissue paper in general, which conventionally includes, but is not limited to, felt-pressed tissue paper; pattern-densified tissue paper as exemplified by Sanford-Sisson and its product; and high-bulk, non-densified tissue paper as exemplified by Salvucci. The tissue paper may have a homogeneous or multi-layer construction; and tissue paper products made therefrom may have a single-ply or multi-ply construction. The tissue paper has a basis weight of between 10 g/m² and about 65 g/m² and a density of about 0.60 g/cm³ or less. Preferably, the basis weight will be less than about 35 g/m² or less; and the density will be about 0.30 g/cm³ or less. Most preferably, the density will be between 0.04 g/cm³ and about 0.20 g/cm³.

Conventionally pressed tissue paper and processes for producing such paper are known in the art. Such paper is usually produced by applying the papermaker's furnish to a permeable forming screen. This forming screen is often referred to in the art as a Fourdrinier wire. Once the furnish is applied to the forming wire, it is referred to as a web. The web is dewatered by pressing the web and is dried at an elevated temperature. The particular procedures and typical equipment for producing webs according to the process just described are well known to those skilled in the art. In a typical process, a low consistency pulp furnish is provided in a pressurized headbox. The headbox has an opening to deliver a thin layer of pulp furnish onto the Fourdrinier wire to form a wet web. The web is then dewatered, typically to a fiber consistency of between about 7% and about 25% (based on the total weight of the web), by vacuum dewatering and further dried by pressing operations in which the web is subjected to pressure induced by opposing mechanical members, for example, cylindrical rolls. The dewatered web is then further pressed and dried in a steam drum device known in the art as a Yankee dryer. Pressure may be applied to the Yankee dryer by mechanical means such as opposing cylindrical drums pressing against the web. Multiple Yankee drying drums may be used, providing additional pressing between the drums if necessary. The tissue paper structures that are formed are hereinafter referred to as conventional pressed tissue paper structures. Such sheets are considered compacted because the web is subjected to significant mechanical compression forces while the fibers are wet, after which they are dried in a compressed state.

Pattern densified tissue paper is characterized by having a relatively high-bulk field of relatively low fiber density and an array of densified regions of relatively high fiber density. The high-bulk field is, on the other hand, characterized as an array of cushion regions. The densified regions are, on the other hand, referred to as neck-down regions. The densified regions may be discretely spaced within the high-bulk field. or they may be contiguous, either wholly or partially, within the high bulk field. Preferred methods for making pattern densified tissue webs are disclosed in US-A-3,301,746, issued January 31, 1967 to Sanford and Sisson, US-A-3,974,025, issued August 10, 1976 to Peter G. Ayres, and US-A-4,191,609, issued March 4, 1980 to Paul D. Trokhan.

In general, pattern densified webs are preferably made by depositing a papermaker's furnish on a foraminous forming wire, such as a Fourdrinier wire, to form a wet web and then applying the web against an array of supports. The web is pressed against the support assembly, thereby creating densified regions in the web at locations geographically corresponding to the points of contact between the support assembly and the wet web. The remainder of the web that is not compressed during this process is referred to as the high bulk field. Formation of the densified regions can be accomplished by applying fluid pressure, such as with a vacuum type device, or with a through-flow dryer, or by mechanically pressing the web against the support assembly. The web is dewatered and optionally predried in such a manner that compression of the high bulk field is substantially avoided. This is preferably accomplished by fluid pressure, such as with a vacuum type device or with a through-flow dryer, or by mechanically pressing the web against an array of supports, whereby the high bulk field is not compressed. The processes of dewatering, optional predrying and formation of the densified regions may be integrated or partially integrated to reduce the total number of processing steps performed. Following formation of the densified regions, dewatering and optional predrying, the web is fully dried, preferably still avoiding mechanical pressing. Preferably, about 8% to about 55% of the tissue paper surface comprises compacted constrictions with a relative density of at least 120% of the density of the high-bulk field.

The arrangement of the supports is preferably an embossed support structure with patterned distribution of bumps that act as the support arrangement, thereby facilitating the formation of the densified regions upon application of pressure. The pattern of the bumps represents the arrangement of the supports mentioned above. Embossing support structures are disclosed in US-A-3,301,746, Sanford and Sisson, issued January 31, 1967, US-A-3,821,068, Salvucci, Jr. et al., issued May 21, 1974, US-A-3,974,025, Ayers, issued August 10, 1976, US-A-3,573,164, Friedberg et al., issued March 30, 1971, US-A-3,473,576, Amneus, issued October 21, 1969, US-A-4,239,065, Trokhan, issued December 16, 1980 and US-A-4,528,239, Trokhan, issued July 9, 1985.

Preferably, the furnish is first formed into a wet web on a foraminous forming support, such as a Fourdrinier wire. The web is dewatered and transferred to an embossing structure. Alternatively, the furnish may be initially deposited on a permeable support support which also acts as an embossing structure. Once the wet web is formed, it is dewatered and preferably thermally pre-dried to a fiber consistency of between about 40% and about 80%. Dewatering is preferably accomplished with suction boxes or other vacuum devices or with through-flow dryers. The hump embossing of the embossing structure presses into the web, as discussed above, before the web is fully dried. One method of accomplishing this is by applying mechanical pressure. This may be accomplished, for example, by pressing a nip roll which holds the embossing structure against the surface of a drying drum, such as a Yankee dryer, with the web disposed between the nip roll and the drying drum. It is also preferred if the web is formed against the embossed structure prior to completion of drying by applying fluid pressure with a vacuum device such as a suction box or with a through-flow dryer. Fluid pressure can be applied to prevent densified areas from being indented during initial dewatering, in a separate subsequent process step or a combination thereof.

Non-compacted, non-pattern densified tissue paper structures are described in U.S. Patent No. 3,812,000, issued to Joseph L. Salvucci, Jr. and Peter N. Yiannos on May 21, 1974, and U.S. Patent No. 4,208,459, issued to Henry E. Becker, Albert L. McConnell and Richard Schutte on June 17, 1980. In general, non-compacted, non-pattern densified tissue paper structures are made by depositing a papermaking furnish on a permeable forming wire, such as a Fourdrinier wire, to form a wet web, draining liquid from the web and removing additional water without applying mechanical pressure until the web has a fiber consistency of at least 80%, and creping the web. The water is removed from the web by vacuum dewatering and thermal drying. The resulting structure is a soft, but weak, high-bulk sheet made of relatively uncompacted fibers. Preferably, binding material is applied to sections of the web before creping.

The papermaking fibers used for the present invention will normally comprise fibers derived from wood pulp. Other fibrous cellulosic pulp fibers such as cotton linters, bagasse, etc. may be used and are intended to be within the scope of this invention. Synthetic fibers such as rayon, polyethylene and polypropylene fibers may also be used in combination with natural cellulosic fibers. An exemplary polyethylene fiber that may be used is Pulpex® available from Hercules Inc. (Wilmington, Delaware).

Useful wood pulps include chemical pulps such as kraft, sulfite and sulfate pulps, and mechanical pulps including, for example, groundwood, thermomechanical pulps and chemically modified thermomechanical pulps. However, chemical pulps are preferred because they impart increased tactile softness to the tissue sheets made therefrom. Pulps derived from both deciduous trees (also referred to herein as "hardwood") and also from coniferous trees (also referred to herein as "softwood") may be used.

In addition to the papermaking fibers, the papermaking furnish used to make the tissue paper structures may also contain other ingredients or materials added thereto as are known in the art or will later become known. The types of desirable additives will depend on the particular end use of the tissue sheet under consideration. For example, in products such as toilet paper, paper towels, facial tissues and other similar products, high wet strength is a desirable feature. Therefore, it is often desirable to add to the papermaking furnish chemical substances known in the art as "wet strength" resins.

A general discussion of the types of wet strength resins used in the papermaking art can be found in TAPPI Monograph Series No. 29, Wet Strength in Paper and Paperboard, Technical Association of the Pulp and Paper Industry (New York, 1965). Most of the wet strength resins that have been useful have generally been cationic in character. Polyamide-epichlorohydrin resins are cationic wet strength resins that have been found to be particularly useful. Suitable types of such resins are described in U.S. Patent No. 3,700,623, issued October 24, 1972, and U.S. Patent No. 3,772,076, issued November 13, 1973, both issued to Keim. A usable polyamide-epichlorohydrin resin is available in Mandel from Hercules Inc. of Wilmington, Delaware, which markets this resin under the trademark Kymeme 557H.

Polyacrylamide resins have also been found to be useful wet strength resins. These resins are described in US-A-3,556,932, issued January 19, 1971 to Coscia et al., and in US-A-3,556,933, issued January 19, 1971 to Williams et al. Commercially available polyacrylamide resins are from the American Cyanamid Co. of Stanford, Connecticut, which markets such a resin under the trademark Parez 631 NC.

Still other water-soluble cationic resins that can find use in this invention are urea-formaldehyde and melamine-formaldehyde resins. The more common functional groups of these polyfunctional resins are nitrogen-containing groups such as amino groups and methylol groups bonded to nitrogen. Polyethyleneimine-type resins can also find use in the present invention. It is to be understood that the addition of chemical compounds such as the wet strength resins discussed above to the pulp furnish is optional and not necessary for the practice of the present development.

Types of polysiloxane materials suitable for use in the present invention include polymeric, oligomeric, copolymeric, and other multi-monomeric siloxane materials. As used herein, the term polysiloxane is intended to include all such polymeric, oligomeric, copolymeric, and other multi-monomeric siloxane materials. In addition, the polysiloxane may have either a straight-chain or branched-chain or cyclic structure.

Preferred polysiloxane materials include those containing monomeric siloxane units of the following structure:

wherein R₁ and R₂ for each siloxane monomer unit can independently be any alkyl, aryl, alkenyl, alkaryl, aralkyl, cycloalkyl, halogenated hydrocarbon or other radical. Each of these radicals can be substituted or unsubstituted. The R₁ and R₂ radicals of any particular monomer unit can differ from the corresponding functionalities of the next adjacent monomer unit. In addition, the radicals can have either a straight chain, a branched chain or a cyclic structure. In addition and independently, the R₁ and R₂ may represent, but are not limited to, other silicone functionalities such as siloxanes, polysiloxanes, and polysilanes. R₁ and R₂ may also contain any variety of organic functionalities, such as alcohol, carboxylic acid, and amine functionalities.

The degree of substitution and the type of substituent have been found to affect the relative degree of soft silky feel and hydrophilicity imparted to the tissue paper structure. In general, the degree of soft silky feel imparted by the polysiloxane increases as the hydrophilicity of the substituted polysiloxane decreases. Amino-functional polysiloxanes are particularly preferred for the present invention.

Preferred polysiloxanes include straight chain organopolysiloxane materials of the following general formula:

wherein each of R₁ - R₉ can independently be any unsubstituted C₁ - C₁₀ alkyl or aryl group, and R₁₀ is any substituted C₁ - C₁₀ alkyl or aryl group. Preferably, each of R₁ - R₉ is any unsubstituted C₁ - C₄ alkyl group. Those skilled in the art will recognize that there is no technical difference whether, for example, R₁ or R₁₀ is the substituted group. Preferably, the molar ratio of b to (a + b) is between 0 and about 20%, more preferably between 0 and about 10%, and most preferably between about 1% and about 5%.

In a particularly preferred embodiment, R₁ - R₉ are methyl groups and R₁₀ is a substituted or unsubstituted alkyl, aryl or alkenyl group. This material shall be described generally herein as a polydimethylsiloxane having a specific functionality as appropriate for the particular case. Examples Examples of polydimethylsiloxanes include polydimethylsiloxane, polydimethylsiloxane having an alkyl hydrocarbon radical R₁₀, and polydimethylsiloxane having one or more amino, carboxyl, hydroxyl, ether, polyether, aldehyde, ketone, amide, ester, thiol and/or other functionalities R₁₀, including alkyl and alkenyl analogues of such functionalities. For example, an amino-functional alkyl group as R₁₀ could be an amino-functional or an aminoalkyl-functional polydimethylsiloxane. The exemplary listing of these polydimethylsiloxanes is not intended to exclude other polydimethylsiloxanes not specifically mentioned.

The viscosity of the polysiloxanes useful in this invention can vary as widely as the viscosity of polysiloxanes in general varies, so long as the polysiloxane is flowable or can be made flowable for application to the tissue paper. This includes, but is not limited to, viscosities as low as about 25 x 10-6 m2.s-1 (25 centistokes) to about 20 m2.s-1 (20,000,000 centistokes) or even higher. Highly viscous polysiloxanes, which do not themselves have flowability, can be effectively applied to the tissue paper webs by such methods as, for example, emulsifying the polysiloxane in surfactant or providing the polysiloxane in solution using a solvent such as hexane, which is given only by way of example. Specific methods for applying polysiloxanes to tissue paper webs are described in more detail below.

In passing, and without wishing to be bound by any theory of operation, it is believed that the effectiveness of the tactile enhancement provided by the polysiloxane is directly related to its average molecular weight; and that viscosity is directly related to molecular weight. Accordingly, because of the relative difficulty of directly determining molecular weights of polysiloxanes as compared to determining their viscosities, viscosity is used herein as an obvious working parameter with a view to imparting enhanced tactile response to the tissue paper: i.e., softness, silkiness and flannel-like feel.

References disclosing polysiloxanes include US-A-2,826,551, issued March 11, 1958 to Geen; US-A-3,964,500, issued June 22, 1976 to Drakoff; US-A-4,364,837, issued December 21, 1982 to Pader; and GB-A-849,433, published September 28, 1960 to Woolston. Also, Silicon Compounds. pp.181-217, distributed by Petrarch Systems, Inc., 1984, contains a detailed summary and description of polysiloxanes in general.

The polysiloxane is applied following formation of the wet web and prior to complete drying. It has been found that addition of the polysiloxane to the wet end of the paper machine (i.e., the paper furnish) is impractical due to low retention levels of the polysiloxane. Therefore, in a typical process, the web is formed and then dewatered prior to application of the polysiloxane to reduce loss of polysiloxane through free water runoff. The polysiloxane is applied to the wet web at a fiber consistency level of between 10% and about 80% (based on the weight of the wet web), more preferably at a fiber consistency level of between about 15% and about 35%, in the manufacture of conventionally pressed tissue paper; and to a wet web having a fiber consistency between about 20% and about 35% in the manufacture of tissue paper in papermaking machines in which the freshly formed web is transferred from a fine mesh fourdriner to a relatively coarse embossing/support structure. The reason for this is that it is preferred to carry out such transfers at relatively low fiber consistencies so that the fibers have significant mobility during transfer; and it is preferred to apply the polysiloxane after their mobility has significantly decreased as water separation through the papermaking machine proceeds. Also, the addition of the polysiloxane at higher fiber consistencies ensures greater retention in and on the paper: ie, less polysiloxane is lost in the water withdrawn from the web to increase its fiber consistency. Surprisingly, in the preferred Fiber consistencies without the use of chemical retention aids retention rates of more than about 90% are expected.

The polysiloxane is preferably applied to the wet web in an aqueous solution, emulsion or suspension. The polysiloxane can also be applied in a solution containing a suitable non-aqueous solvent in which the polysiloxane dissolves or with which the polysiloxane is miscible: for example, hexane. The polysiloxane can be provided in its original form or, preferably, as an emulsion with a suitable surface-active emulsifier. Emulsified polysiloxane is preferred for ease of use, since an aqueous solution of the simple polysiloxane must be stirred to prevent separation into an aqueous and a polysiloxane phase.

The polysiloxane should be applied to the wet tissue paper web evenly so that virtually the entire sheet benefits from the tactile effect of the polysiloxane. Applying the polysiloxane to the wet tissue paper web in continuous and patterned distributions are both within the scope of the invention and meet the criteria set out above.

Methods for uniformly applying the polysiloxane to the web include spraying and gravure printing. Spraying has been found to be economical and to allow for careful control over the quantity and distribution of the polysiloxane, so it is most preferably practiced. Preferably, an aqueous mixture containing an emulsified polysiloxane is sprayed onto the wet tissue web as it passes through the papermaking machine: for example, and by no means limiting, this refers to a papermaking machine of the general type disclosed in Sanford-Sisson (identified above), and occurs either before the pre-dryer or after the pre-dryer, depending on the degree of fiber consistency desired. A less preferred method involves applying the polysiloxane to a forming wire or structure which is then brought into contact with the tissue web. An apparatus suitable for spraying polysiloxane-containing liquids onto wet webs includes an external mixer, air spray nozzles such as the 2 mm nozzles available from VIB Systems, Inc., Tucker, Georgia. Apparatus suitable for printing polysiloxane-containing liquids onto wet webs includes rotary gravure printing machines.

Surprisingly, it has been discovered that low levels of polysiloxane applied to wet tissue paper webs cause the tissue paper to have a softer, silky, flannel-like, non-greasy feel without the need for additional materials such as oils or lotions. Importantly, these benefits can be achieved in many embodiments of the present invention in combination with high wettability within the ranges desirable for toilet paper applications. Tissue paper treated with polysiloxane in accordance with the present invention contains about 0.75% or less polysiloxane. It is an unexpected advantage of this invention that tissue paper treated with about 0.75% or less polysiloxane can exhibit significant improvements in softness and silkiness from such low levels of polysiloxane. In general, tissue paper containing less than about 0.3% polysiloxane, preferably less than about 0.2%, can provide significant increases in softness, silkiness and flannel-like quality and yet be sufficiently wettable for use as toilet tissue without the need for the addition of surfactant to offset any negative effects on wettability caused by the polysiloxane.

The minimum level of polysiloxane to be retained by the tissue paper is at least an effective level to impart a noticeable difference in softness or silkiness or flannel-like quality to the paper. The minimum effective level may depend on the particular sheet type, the method of application and the particular type of polysiloxane and whether or not the polysiloxane has been supplemented with starch, surfactant or other additives or treatments. Without limiting the range of applicable polysiloxane retention by the tissue paper, at least about 0.004%, more preferably at least about 0.01% and most preferably at least about 0.02% polysiloxane is retained by the tissue paper.

A sufficient amount of polysiloxane to impart a tactile feeling of softness is applied to both surfaces of the tissue paper: i.e., to the outward facing surfaces of the fibers present in the surface area. When polysiloxane is applied to one surface of the tissue paper, some of it will generally penetrate at least partially into the interior of the tissue paper. In one embodiment, sufficient polysiloxane to cause a tactile response penetrates through the entire thickness of the tissue paper, so that both surfaces are imparted the benefits of the polysiloxane. One method that has been found to be effective in facilitating penetration of the polysiloxane to the opposite surface when the polysiloxane is applied to one surface of a wet tissue paper web is to vacuum dewater the tissue paper from the other surface of the wet tissue paper at the point of application of the polysiloxane.

In addition to treating tissue paper with polysiloxane as described above, it has been found desirable to also treat such tissue paper with surfactant material. This is in addition to any surfactant material that may be present as an emulsifying agent for the polysiloxane.

Tissue paper containing more than about 0.3% polysiloxane is preferably treated with surfactant when it is considered for uses where high wettability is desirable. Most preferably, a non-cationic surfactant is applied to the wet tissue paper web to provide an additional softening effect based on constant tear strength as described above. The amount of surfactant required to increase hydrophilicity to a desired level will depend on the type and amount of polysiloxane as well as the type of surfactant. As a general guideline, it is believed that between about 0.01% and about 2% surfactant, preferably between about 0.05% and about 1.0%, retained by the tissue paper sufficient to provide sufficiently high wettability for most applications, including toilet paper, at polysiloxane contents of about 0.75% or less.

Surfactants preferred for use in the present invention are non-cationic; and are more preferably non-ionic. However, cationic surfactants may also be used. Non-cationic surfactants include anionic, non-ionic, amphoteric and zwitterionic surfactants. As indicated above, the surfactant is preferably a substantially non-migrating surfactant in situ after the tissue paper has been made, in order to substantially avoid post-manufacture changes in the properties of the tissue paper that would otherwise be caused by the inclusion of surfactant. This can be achieved, for example, by using surfactants having melting temperatures above the temperatures typically encountered during storage, shipping, sale and use of tissue paper product embodiments of the present invention: for example, having melting temperatures of about 50°C or higher. Also, the surfactant is preferably water soluble when applied to the wet web.

The level of noncationic surfactant applied to the wet tissue paper webs to provide the softness and tear strength benefits mentioned above is, based on a constant tear strength for the final product, in the range of the minimum effective level required to provide such benefits, up to about two (2) percent: preferably between about 0.01% and about 1% noncationic surfactant retained by the web; more preferably between about 0.05% and about 1.0%; and most preferably between about 0.05% and about 0.3%.

The surfactants preferably have alkyl chains with eight or more carbon atoms. Examples of anionic surfactants are linear alkyl sulfonates and alkylbenzene sulfonates. Examples of nonionic surfactants are alkyl glycosides, including alkyl glycoside esters such as Crodesta SL-40 available from Croda, Inc. (New York, NY); alkyl glycoside ethers as described in US-A-4,011,389 filed on March 8, 1977 to WK Langdon et al.; and alkyl polyethoxylated esters such as Pegosperse 200 ML available from Glyco Chemicals Inc. (Greenwich, CT). Particularly preferred for use in the present invention are alkyl polyglycosides.

The surfactant, in addition to any emulsifying surfactant that may be present on the polysiloxane, can be applied by the same devices and equipment used to apply the polysiloxanes. These methods include spraying and gravure printing. Preferably, an aqueous mixture containing the surfactant is sprayed onto the wet tissue web as it passes through the papermaking machine. Other methods include application to a forming screen or structure prior to contact with the web. Any surfactant other than the surface active material used to emulsify the polysiloxane is hereinafter referred to as a "surfactant" and any surfactant present as an emulsifying component of the emulsified polysiloxane is hereinafter referred to as an "emulsifier."

The surfactant can be applied to the tissue paper simultaneously with, after, or before the polysiloxane. In a typical process, the surfactant is applied following formation of the wet web and prior to final drying. Preferably, the surfactants are applied to the wet tissue webs at a fiber consistency level between about 10% and about 80%; and more preferably between about 15% and about 35%. Surprisingly, retention rates of noncationic surfactant applied to wet webs are high, even when the surfactant is applied under conditions where it is not ionically substantive to the fibers. Retention rates in excess of about 90% are expected at the preferred fiber consistencies without the use of chemical retention aids.

As indicated above, it is also desirable to treat polysiloxane-containing tissue paper with a relatively small amount of a binder to control fuzz formation and/or to increase tear strength. As used herein the term "binder" refers to the various wet and dry strength additives known in the art. The binder may be applied to the tissue paper simultaneously with, after, or before the polysiloxane and surfactant, if used. Preferably, the binders are applied to the wet tissue webs at fiber consistency levels between about 10% and about 80%, and more preferably between about 15% and about 35%.

Starch has been found to be the preferred binder for use in the present invention. Preferably, the tissue paper is treated with an aqueous starch solution and, as mentioned above, the sheet is wet at the time of application. In addition to reducing the linting of the final tissue paper product, low levels of starch also provide a modest improvement in the tear strength of the tissue paper without causing the cardboardiness (i.e., stiffness) that would result from additions of higher levels of starch.

This also provides tissue paper that has improved strength/softness ratios compared to tissue paper strengthened by conventional methods to increase tensile strength: for example, sheets with increased tensile strength by more intensive pulp refining; or by the addition of other dry strength additives. This result is particularly surprising since starch has traditionally been used to build strength at the expense of softness in applications where softness is not an important characteristic: for example, in paperboard. In addition to this and independently, starch has been used as a filler for printing and writing paper to improve the printability of the surface.

In general, a starch suitable for the practice of the present invention is characterized by water solubility and hydrophilicity. Examples of such starch materials include corn starch and potato starch, although it is not intended to limit the scope of suitable starch materials in any way; and waxy corn starch, known commercially as amioca starch, is particularly preferred. preferred. Amioka starch differs from ordinary corn starch in that it consists entirely of amylopectin, whereas ordinary corn starch contains both amylopectin and amylose. Several unique characteristics of Amioka starch are further described in "Amioka - The Starch From Waxy Corn", HH Schopmeyer, Food Industries, December 1945, pp. 106-108 (vol. pp. 1476-1478).

The starch may be in granulated or dispersed form, although granulated form is preferred. The starch is preferably cooked sufficiently to cause swelling of the granules. More preferably, the starch granules are swollen, such as by cooking, to a point just prior to dispersion of the starch granules. Starch granules swollen to such an extent are referred to as "fully cooked." The conditions of dispersion in general may vary depending on the size of the starch granules, the degree of crystallinity of the granules, and the amount of amylose present. Fully cooked amioca starch may be prepared, for example, by heating an aqueous slurry of the starch granules of about 4% consistency to about 190°F (about 88°C) for about 30 to about 40 minutes.

Other exemplary starch materials that may be used include modified cationic starches, such as those modified to contain nitrogen-containing groups such as amino groups and methylol groups attached to the nitrogen, and are available from National Starch and Chemical Company, (Bridgewater, New Jersey). Such modified starch materials have heretofore been used primarily as additives to pulp furnish to increase wet and/or dry strength. However, when applied in accordance with this invention by application to a wet tissue paper web, they may have a reduced effect on wet strength relative to the wet end addition of the same modified starch materials. Considering that such modified starch materials are more expensive than unmodified starches, the latter have generally been preferred.

The starch should be applied to the tissue paper while the paper is in a wet state. The starch-based material is applied to the wet tissue paper web, preferably when the web has a fiber consistency of about 80% or less. Noncationic starch materials are retained in the web in sufficient amounts to produce an appreciable effect on softness at a particular level of strength relative to higher refining; and are preferably applied to wet tissue webs having fiber consistencies between about 10% and about 80%, more preferably between about 15% and 35%.

Starch is preferably applied to tissue paper webs in an aqueous solution. Methods of application include the same as described above with reference to the application of polysiloxane: preferably by spraying and, less preferably, by printing. The starch can be applied to the tissue paper web simultaneously with, before or after the addition of polysiloxane and/or surfactant.

Preferably, at least an effective amount of a binder, preferably starch, is applied to the sheet to provide lint control while simultaneously increasing strength upon drying relative to an unbindered but otherwise identical sheet. Preferably, between about 0.01% and about 2.0% of a binder, calculated on a fiber dry weight basis, is retained in the dried sheet; and more preferably, between about 0.1% and about 1.0% of a binder material, preferably starch-based, is retained.

Analysis of the amounts of the treating chemicals retained on the tissue paper webs herein can be performed by any method accepted in the art. For example, the amount of polysiloxane retained by the tissue paper can be determined by solvent extraction of the polysiloxane with an organic solvent followed by atomic absorption spectroscopy to determine the silicon content in the extract; the content of nonionic surfactants, such as alkyl glycosides, can be determined by extraction in an organic solvent followed by gas chromatography to determine the level of surfactant in the extract; the level of anionic surfactants, such as linear alkyl sulfonates, can be determined by water extraction followed by colorimetric analysis of the extract; the amount of starch can be determined by amylase degradation of the starch to glucose followed by colorimetric analysis to determine the glucose content. These methods are examples and are not intended to exclude other methods which may be useful for determining the levels of specific components retained by the tissue paper.

Tissue paper hydrophilicity generally refers to the tendency of the tissue paper to become wetted with water. Tissue paper hydrophilicity can be quantitatively assessed to some extent by determining the time required for dry tissue paper to become completely wetted by water. This time period is referred to as the "wetting time." To provide a consistent and repeatable test for wetting time, the following procedure can be used for wetting time determinations: first, provide a dry (greater than 90% fiber consistency grade) sample of a unit sheet, approximately 4-3/8 inch x 4-3/4 inch (about 11.1 cm x 12 cm) of the tissue paper structure; second, the sheet is folded into four (4) quarters, one above the other, and then compressed into a ball about 0.75 inch (about 1.9 cm) to about 1 inch (about 2.5 cm) in diameter; third, the balled sheet is placed on the surface of a body of distilled water at 72ºF (about 22ºC) while a stop watch is run; fourth, the stop watch is pressed and read when the balled sheet is completely wetted. Complete wetting is determined visually.

The preferred hydrophilicity of tissue paper depends on its intended final use. It is desirable for tissue paper used for many applications, e.g. toilet paper, to be hydrophilic in a relatively short period of time to prevent clogging once the water in the toilet is drained. Preferably, the wetting time is 2 minutes or less. More preferably, the wetting time is 30 seconds or less. Most preferably, the wetting time is 10 seconds or less.

Hydrophilicity characteristics of tissue paper embodiments of the present invention can, of course, be determined immediately after manufacture. However, significant increases in hydrophobicity can occur during the first two weeks after manufacture of the tissue paper: i.e., after the paper has aged two (2) weeks after manufacture. Thus, the wetting times set forth above are preferably measured after the expiration of such a two-week period. Accordingly, wetting times measured at the end of a two-week aging period at room temperature are referred to as "two-week wetting times."

Tissue paper density, as that term is used herein, is the average density calculated as the basis weight of that paper divided by the caliper, with appropriate unit conversions included therein. Tissue paper caliper, as that term is used herein, is the thickness of the paper when subjected to a compressive load of 95 g/in² (15.5 g/cm²).

EXAMPLE I

The purpose of this example is to illustrate a process that can be used to make soft tissue paper sheets treated with a polysiloxane in accordance with the present invention.

A pilot scale Fourdrinier papermaking machine is used to practice the present invention. The paper machine has a layered headbox with an upper chamber, a middle chamber and a lower chamber. Where possible, it is indicated in the following examples that the process described below is also applicable in the later examples. Briefly said, first a fibrous slurry containing primarily short papermaking fibers is pumped through the upper and lower headbox chambers and simultaneously a second fibrous slurry containing primarily long papermaking fibers is pumped through the middle headbox chamber and each is deposited in a layered manner on the Fourdrinier wire to form a three-layer embryonic web thereon. The first slurry has a fiber consistency of about 0.11% and its fiber content is Eucalyptus hardwood kraft. The second slurry has a fiber consistency of about 0.15% and its fiber content is Northern softwood kraft. Dewatering is accomplished through the Fourdrinier wire and is assisted by a deflector and vacuum boxes. The Fourdrinier wire has the configuration of a 5-ply satin weave having 35 (87) monofilaments per cm (inch) in the machine direction and 30 (76) monofilaments per cm (inch) in the cross machine direction. The wet embryonic web is transferred from the Fourdrinier wire at a fiber consistency of about 22% at the time of transfer to a support structure having a 5-ply satin weave having 14 (35) monofilaments per cm (inch) in the machine direction and 13 (33) monofilaments per cm (inch) in the cross machine direction. The side of the web not supported on the structure is sprayed with an aqueous solution containing an emulsified polysiloxane composition, which will be described below, through a 2 mm spray nozzle located directly opposite the vacuum dewatering box. The wet web has a fiber consistency of about 22% (based on the total weight of the web) when sprayed with the aqueous solution. The sprayed web is transported on the support structure beyond the vacuum dewatering box, passes through through-flow pre-dryers, whereupon the web is transferred to a Yankee dryer. The other process and machine conditions are given below. The fiber consistency is about 27% after the vacuum dewatering box and, by the action of the pre-dryers, about 65% before transfer to the Yankee dryer; creping adhesive containing a 0.25% aqueous solution of polyvinyl alcohol is applied by spraying with the aid of applicators; the fiber consistency is increased to an estimated value of 99% before the web is dry creped with a doctor blade. The doctor blade has a set angle of about 24 degrees and is positioned with respect to the Yankee dryer to provide an impact angle of about 83 degrees; the Yankee dryer is operated at about 350ºF (177ºC); the Yankee dryer operates at about 800 fpm (feet per minute) (about 244 meters per minute). The dry creped web is then passed between two calender rolls. The two calender rolls are skewed to each other in roll weight and are operated at surface speeds of 660 fpm (about 201 meters per minute).

The aqueous solution sprayed onto the wet web through the spray nozzle contains 0.71 wt. % Dow Corning Q2-7224 (a 35% nonionic emulsion of an amino-functional polydimethylpolysiloxane marketed by Dow Corning Corp.). The volumetric flow rate of the aqueous solution through the nozzle is about 3 gal./hr.ft -cross direction- (about 37 liters/hr.meter). The retention rate of the polysiloxane applied to the web is generally about 90%.

The resulting tissue paper has a basis weight of 30 g/m², a density of 0.10 g/cm³ and contains 0.025 wt.% of the amino-functional polydimethylpolysiloxane compound.

It is important that the resulting tissue paper feels silky, flannel-like and has an increased noticeable softness.

EXAMPLE II

The purpose of this example is to illustrate a process that can be used to make soft tissue paper sheets in which the tissue paper is treated with polysiloxane, surfactant and starch.

A 3-ply paper sheet is prepared in accordance with the process previously described in Example I. The tissue web, in addition to being treated with a polysiloxane compound as described above, is also treated with Crodesta SL-40 (a nonionic alkyl glycoside polyester surfactant, marketed by Croda Inc.) and treated with a fully cooked amioca starch prepared according to the specification given. The surfactant and starch are applied simultaneously with the emulsified polysiloxane composition as part of the aqueous solution sprayed through the spray nozzle of the paper machine. The concentration of Crodesta SL-40 nonionic surfactant in the aqueous solution is adjusted so that the amount of surfactant retained is about 0.15% based on the weight of the dry fibers. Similarly, the concentration of starch in the aqueous solution is adjusted so that the amount of amioca starch retained is about 0.2% based on the weight of the dry fibers.

The resulting tissue paper has a basis weight of 30 g/m², a density of 0.10 g/cm³ and contains 0.025 wt.% polydimethylpolysiloxane Dow Q2-7224, 0.15 wt.% non-ionic surfactant Crodesta SL-40 and 0.2 wt.% cooked amioca starch. It is important that the resulting tissue paper has a silky, flannel-like feel, increased tactile softness and higher wettability and less tendency to form lint than tissue paper treated only with the polysiloxane composition.

Claims (16)

1. A process for producing soft tissue paper having a polysiloxane compound uniformly disposed on its outwardly facing surfaces, said process comprising the steps of: (a) wet-laying cellulosic fibres to form a web; b) applying said polysiloxane compound to said web; c) drying said web; wherein - the said process is intended for the manufacture of soft tissue paper with a basis weight of 10 to 65 g/m² and a density of less than 0,60 g/cm³; - in said step b) the polysiloxane compound is applied to the web at a fiber consistency of 10% to 80%, based on the total weight of the web, and the amount of polysiloxane compound applied to the web is such that 0.004% to 0.75% of said polysiloxane, based on the dry fiber weight of said tissue paper, is retained by said tissue paper; and - said step c) further comprises a step of creping the web. 2. A process according to claim 1, wherein 0.004% to 0.3% of said polysiloxane is retained by said tissue paper. 3. A process according to any one of claims 1 and 2, wherein said polysiloxane is a polydimethylpolysiloxane having a hydrogen bonding functional group selected from amino, carboxyl, hydroxyl, ether, polyether, aldehyde, ketone, amide, ester and thiol groups, said hydrogen bonding functional group being present in a molar percentage of substitution of 20% or less. 4. A process according to claim 3, wherein said polysiloxane has a molar percentage of substitution of 10% or less and a viscosity of 25 x 10-6 m2 s-1 or more. 5. A process according to any one of claims 3 and 4, wherein said polysiloxane has a molar percentage of substitution of 1.0% to 5% and a viscosity of 25 x 10⁻⁶ m²s⁻¹ to 20 m²s⁻¹. 6. A process according to claim 5, wherein said polysiloxane has a molar percent of substitution of about 2% and a viscosity of about 125 x 10-6 m2s-1. 7. A method according to any preceding claim, wherein said polysiloxane is applied to said web when said web has a fiber consistency of from 15% to 35%. 8. A method according to any preceding claim, further comprising the step of applying a water-soluble surfactant to said web at a fiber consistency of from 10% to 80% based on the total weight of the web, in an amount sufficient such that from 0.01% to 2.0% of said surfactant, based on the fiber dry weight of said tissue paper, is retained by said tissue paper. 9. A method according to claim 8, wherein the amount of surfactant applied to said web is such that from 0.05% to 1.0% surfactant, based on the dry fiber weight of said tissue paper, is retained by the tissue paper. 10. A method according to any one of claims 8 and 9, wherein the surfactant is non-cationic. 11. A method according to claim 10, wherein the non-cationic surfactant is a non-ionic surfactant. 12. A process according to any one of claims 8 to 11, wherein said surfactant has a melting point of at least 50°C. 13. A method according to any preceding claim, further comprising the step of applying a binder to said web at a fiber consistency of 10% to 80% based on the total weight of the web in an amount sufficient so that 0.01% to 2.0% of said binder, based on the dry fiber weight of said tissue paper, is retained by said tissue paper. 14. A method according to claim 13, wherein the amount of binder applied to the web is such that from 0.1% to 1.0% of said binder, based on the dry fiber weight of said tissue paper, is retained by said tissue paper. 15. A method according to claim 13, wherein said binder is starch. 16. A method according to claim 15, wherein said binder is amioca starch.