US3764451A - Air formed adhesively supplemented hydrogen bonded webs - Google Patents

Abstract

LIGHTWEIGHT CELLULOSIC PRODUCTS CHARACTERIZED BY A DESIRABLE COMBINATION OF STRENGTH, ABSORBENCY AND TACTILE PROPERTIES ARE FORMED BY AIR LAYING A WEB OF CELLULOSIC FIBERS, AND, THEREAFTER, UNITING THE FIBERS INTO A COHERENT STRUCTURE BY BONDING THEM TOGETHER AT REGULARLY PATTERNED AREAS OF THE WEB WITH ADHESIVELY SUPPLEMENTED HYDROGEN BONDS. THE RESULTING PRODUCTS NEED NOT BE CREPED TO DEVELOP IMPROVED TACTILE CHARACTERISTICS AND MAY BE ADVANTAGEOUSLY USED AS FACIAL WIPES, TOWELING AND THE LIKE.

Description

Oct. 9, 1973 c. E. DUNNING 3,764,451

AIR-FORMED ADHESIVELY SUPPLEMENTED, HYDROGEN-BONDED WEBS Filed May 20, 1971 INVENTOR. KW/TA' J JVI AI Z United States Patent Offi ce 3,764,451 Patented Oct. 9, 1973 AIR-FORMED ADHESIVELY SUPPLEMENTED,

HYDROGEN-BONDED WEBS Dunning, Neenah, Wis., assignor to Kimberly- Clark Corporation, Neenah, Wis. Continuation-impart of application Ser. No. 882,257, Dec. 4, 1969, now Patent No. 3,692,622. This application May 20, 1971, Ser. No. 145,449 U Int. Cl. D21h /26; D04h 1/04 U.S. Cl. 161-124 Charles E.

5 Claims ABSTRACT OF THE DISCLOSURE Lightweight cellulosic products characterized by a desirable combination of strength, absorbency and tactile properties are formed by air laying a web of cellulosic fibers, and, thereafter, uniting the fibers into a coherent structure by bonding them together at regularly patterned areas of the web with adhesively supplemented hydrogen bonds. The resulting products need not be creped to develop improved tactile characteristics and may be advantageously used as facial wipes, toweling and the like.

RELATED APPLICATIONS BACKGROUND OF THE INVENTION This invention relates to cellulosic products and, more particularly, to products useful in, for example, facial wipes and toweling applications which are characterized by a desirable combination of strength, absorbency and .tactile properties.

Conventionally, disposable tissue and towel products have been formed ,on papermaking equipment by water laying a wood pulp fiber sheet and, thereafter, removing the water either by drying or a combination of pressing and drying. During water removal, strong capillary surface tension force is developed between fibers and a degree of overall bonding inevitably results. Because of this overall bonding phenomenon, sheets prepared by waterlaid methods inherently possess very unfavorable tactile properties (e.g.-harshness, stiffness, low bulk and poor overall softness) and absorbency. To enhance these latter properties, waterlaid sheets are conventionally creped, which artifically improves the tactile and absorbency properties by disrupting the excessive fiber bonding.

However, crepinghas several limitations, including the fact that the detrimental effects of the initial overall bonding 1 is not completely offset. Also, in a waterlaid methjod, there is typically preferential fiber orientation in the machine direction; and a result, the strength in this direction'is always higher than in the cross direction. Consequently, achieving creped products with acceptable cross direction strengths generallyrequires developing excessive machine direction strength. Because of this, the at tainment of optimum tactile properties is prevented.

Where products are to be used in contact with moisture such as toweling and facial tissues, it is customary to treat them with wet strength resins. The addition of such resins can detract from the tactile properties of the products.

' Air forming of wood pulp fiber sheets has been carried "out for many years; however, the resulting webs have only been used for applications where either little strength is required (such as for example absorbent products) or applications wherein a certain minimum strength is required but the tactile and absorbency properties are unimportant (such as various specialty papers). IUS. Pats. 2,447,161 and 2,810,940 and British Pat. 1,088,991 illustrate air forming techniques for such applications.

My copending applications herein identified disclose a method, and the resulting product, of air laying a cellulosic web to provide an aesthetically pleasing cellulosic Web characterized by a desirable combination of strength, absorbency and tactile properties. Such webs are made by air laying a continuum of substantially unbonded wood fibers, moisturizing the web and thereafter uniting the fibers into a coherent structure by bonding the fibers together at regularly patterned areas of the continuum. The bonded web is then dried to remove excess moisture. These webs possess a novel combination of attributes which makes them highly desirable as substitutes for conventional waterlaid paper products in a variety of disposable applications.

It is an object of the present invention to provide aesthetically pleasing cellulosic webs having desirable tactile properties in combination with superior strength characteristics. A related object is to provide webs having these characteristics and which, in addition, possess desirable absorbency properties.

A further object is to provide an economical and efiicient method of making webs having the hereinbefore defined characteristics wherein the problems associated with web handling are greatly minimized. A related object lies in the provision of a versatile method for forming webs of such type in which the desired product at- I tributes can be easily achieved.

Yet another object of the present invention is to provide a cellulosic web of the hereinbefore described type which also possesses desirable strength when wetted.

Other objects and advantages will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view illustrating apparatus that is capable of forming the fibrous webs of the present invention on a continuous or semi-continuous basis and FIG. 2 is a schematic view of a portion of a single ply Web showing its flufl'y mound-like configuration.

While the invention is susceptible of various modifica tions and alternative constructions, there is shown in the drawings and will herein be described in detail the preferred embodiments. It is to be understood, however, that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

Briefly, the present invention provides forming a cellulosic web with increased strength properties together with desirable absorbency and tactile properties by air laying cellulosic fibers to form a continuum of substantially unbonded fibers and thereafter uniting the fibers into a coherent structure by means of a discrete pattern of adhesively supplemented hydrogen bonds spaced less than an average fiber length apart. The web is characterized by loose fluify fiber regions wherein there is substantially no interfiber attachment combined with highly compacted discrete bonded regions. The mound regions which have a height several times that of the bonded regions contribute to the very desirable absorbency and tactile properties of the web while the bonded compact regions, due to their adhesively supplemented character, provide Web strength. In a preferred embodiment, the adhesive employed provides wet strength for the web.

Turning now to the drawings, FIG. 1 illustrates a manner in which products of the present invention can be prepared. A cellulosic web is formed by initially separating a pulp sheet 30 into its individual fibers 40 by unwinding the pulp sheet 30 from a roll 32 and forwarding the sheet by means of the driven rolls 34, 36 to a divellicating means such as a picker roll 38, powered by means not shown. The individual fibers 40 are conveyed through a forming duct 42 and onto a moving foraminous wire 44. Air forms a source 46 in combination with a vacuum box 48 creates a downwardly moving stream of air which assists in collecting the air-formed web 50 on the foraminous wire. In comparison with ordinary textile fibers, the wood fibers used are of significantly shorter length, the length of textile fibers being on the order of at least inch while the fibers used herein generally have a length of less than /2 inch. More particularly, the fibers in the pulps described above have a length distribution of about While customary air forming techniques can be utilized in forming the web, the forming duct 42 illustrated in FIG. 6 is particularly efficient in obtaining an especially suitable web, particularly at high speeds. The illustrated duct has a width approximately equal to the height of the picker teeth on the roll 38 and is positioned so as to tangentially receive the fibers as they leave the picker. By using a duct with such a width, fiber velocity can be maintained essentially constant throughout the length of the duct. Webs formed in this manner have exceptionally good uniformity and are substantially free of fiber floccing. Appropriate sizing of the forming duct and the spatial arrangement with respect to the picker and the wire are more completely described in copending Appel application Ser. No. 882,265, filed on Dec. 4, 1969, now abandoned, entitled Pulp Picking Apparatus With Improved Fiber Forrning Duct.

The weight of the airlaid Web formed in the above-illustrated manner is dependent upon the desired end-use of the subsequently prepared product. For most applications, however, webs having a basis weight of about -50 lbs./ 2880 ft. are suitable, 10 to 25 lbs/2880 ft. being preferred. The particular type of cellulosic fibers used in preparing the web is also not critical and the type selected will generally depend upon the desired surface texture.

For example, webs with a soft and fiufiy texture are gen-v erally obtained from cedar fibers While a slightly more wooly texture with increased body can be obtained from southern pine fibers.

Referring again to FIG. 1, after formation on the foraminous wire 44, the web 50 is then forwarded to a moisturizing station 52 where the moisture content of the web is raised to a level which is satisfactory for handling and bonding the Web as will be hereinafter described. Accordingly, the Web 50 is forwarded below a water spray which can comprise a nozzle 54 capable of emitting a uniform spray. On leaving the moisturizing station 52, the web typically has a moisture content of about -40%, based on the weight of the wetted web.

After wetting, the web is transferred from the wire 44 to a transfer roll 56 by passing the wetted web through a nip formed between the wire 44 and the transfer roll 56. Successful transfer of the web to the roll 56 depends on the existence of a moisture gradient between the opposed surfaces of the web, with the higher moisture content being present on the surface which contacts the transfer roll. Satisfactory moisture distribution can be achieved by providing a suction box (not shown) below the wire 44 and drawing the water into the continuum with a suction of about 2 to 10 inches water. This will also leave suificient water on the surface for transfer to take place. Thus, the nip clearance should desirably be such that transfer can take place, desirably by providing only a relatively small draw (e.g. about 10% or less) between the Wire and the transfer roll. To this end, the clearance in the nip should preferably be less than the thickness of the web entering the nip. A more complete description of the transfer step can be found in the Dunning et al. application entitled High Speed Method for Forming Airlaid Web, filed on even date herewith.

After transfer of the Web to the roll 56, bonding of the web is accomplished by passing the web through a nip formed between the transfer roll 56 and the patterned steel roll 58 containing a plurality of raised points. In accordance with the present invention, bonding is carried out in such a fashion that bonds formed are adhesively supplemented. To this end, a pan roller 60 rotates in an adhesive solution 62 which is applied to the points of patterned roll 58 via transfer rolls 64 and 66. The pattern of the raised points on the roll 58 is not particularly important although this can, to some extent, influence the directional strength characteristics of the bonded web. To avoid significant compaction of the web in other than those areas to be specifically bonded, the sides of the raised points are desirably comparatively steep with heights of about 0015-0030 being particularly useful. Typically, the pressure exerted on the individual points will be at least about 2,000 psi. and be suificient to decrease the thickness of the web in the bonded areas to about 40% of the unbonded areas and desirably about 20%.

The total bonded area and the bond frequency may of course be varied by the selection of the raised point pattern on the roll 58. Total bond areas of about 1040% and bond frequencies on the order of about 10-40 per inch across both dimensions of the web are useful.

After bonding, the web is removed from the transfer roll 56 and onto a moving wire 68 by means of the suction roll 70. Thereafter, the web is dried by passage through a dryer 62. The use of a through dryer retains, and perhaps even enhances the lofty, three dimensional nature of the web. After drying, the web can be simply wound-up for future use.

Turning now specifically to that aspect of the abovedescribed procedure concerning the bonding of the cellulosic web in the roll 56, 58 nip, it should be appreciated that the bonds formed in the web have a distinct dual character. Due to the moisture content of the web and the accompanying high pressure during passage through the nip, the individual bonds will contain a significant degree of interfiber hydrogen bonding. Moreover, since the raised points on the patterned roll 58 contain a coating of adhesive which will be transferred to the web, the hydrogen bonds will be supplemented by a degree of adhesive bonding. While the construction of the individual bond areas has not been precisely established, it is believed that the applied adhesive, in addition to functioning as a bonding medium, may also serve to enhance the eifectiveness of the hydrogen bonds in providing web strength. In an application where wet strength is required, this can be readily achieved by merely forming the adhesively supplemented bonds with an adhesive capable of yielding such strength; and, in this situation, it is believed that the adhesive may also function to protect or insulate the readily water dispersible hydrogen bonds so that dispersion is significantly delayed.

The amount of adhesive applied to the raised points on the roll 58 should be carefully controlled so that adhesive is only present on the point surfaces. Furthermore, the use of excess quantities of adhesive which results in adhesive spreading beyond the specific bond points in the web should be avoided. Such adhesive spreading can adversely affect the tactile and absorbency characteristics of the web. Adhesive application of less than 2% by weight of the web will provide the desired strength levels for most applications. The amount of adhesive applied can be controlled by varying the application viscosity of the adhesive. Simple experimentation can be used to establish an appropriate adhesive application viscosity for a given web construction and bonding speed.

The type of adhesive used is not particularly critical, and the contemplated end use will generally dictate the selection. However, the adhesive should, of course, be capable of achieving the desired improvement in web strength without adversely alfecting the absorbency or tactile properties. Various well known starches provide economic adhesives where only dry strength improvement is desired. Other representative examples of suitable adhesives include water-soluble polymeric adhesives, latexes such as acrylics, solvent-based adhesives such as polyvinyl acetate, hot melts, and plastisols such as polyvinyl chloride.

As illustrated in FIG. 2 the web prepared as above described with respect to FIG. 1 is characterized by a continuum of randomly oriented fibers 80 interrupted in a pattern of adhesively supplemented hydrogen bonded areas 82. Fiber orientation in the basic plane of the Web prepared as described herein is quite random, and consequently the web exhibits substantially equal tensile strength in all directions. A cross-sectional view taken along a row of the bonded area shows that the bonded areas are alternately interrupted by fiufiy mounds of substantially unbonded fibers. The fact that substantially no bonding is present in the mound area is due to the absence of strong surface tension which normally develop between fibers in waterlaid processes when the water is removed.

In addition, certain product attributes of the present invention are present because of the relationship of the adhesively supplemented bonded areas to the fluffy, mound regions. More particularly, and as has been herein noted, the bonded areas are typically less than about 20% of the height of the fluffy, mound regions. The fibers merging from the bonded areas and extending toward the mounted areas thus have a substantial orientation out of the basic plane of the web, i.e.a Z-direction orientation. This aspect, together with the frequency of the bond areas, provides a structure which may be visualized as a spring structure. Thus, a considerable number of fibers may be viewed as starting in the bond area, emerging out of such area into the mound region with a substantial Z-direction orientation and then returning to the plane of the web in an adjacent bond area. Individual fibers can accordingly be visualized as being anchored at both ends in one plane and therebetween being bent considerably out of the plane to provide a stable structure which has spring or resilience when compressive force is applied in a direction perpendicular to the plane of the web. This structure is believed to contribute significantly to the desirable tactile properties of the web. Moreover, in combination with the adhesively supplemented character of the bond regions, it is also believed that the desirable absorbency characteristics of the web stem in part from this distinctive structure.

It is hypothesized that the spring" structure of the web described herein is at least partially responsible for the surprisingly high rate at which the present web is able to absorb liquids due to the existence of a unique capillary structure in and between the fluffy mound regions. Furthermore, when the bonds are adhesively supplemented with a wet strength adhesive, it is believed that such bonds resist disintegration on contact With liquid and the spring-like mound structure of the web does not collapse on initial web wetting. Accordingly, the desirable capillary characteristics of the structure remain even after partial wetting, thus serving to promote further fluid absorption.

Thus, as has been seen, the present invention provides a lightweight wood fiber airlaid web with adhesive supplemented hydrogen bonds and having characteristics making it particularly desirable for use in tissue and towelling applications. By selection of an appropriate adhesive, a product with superior dry strength, wet strength, or both, may be fashioned. Also, because of the unique method of forming the adhesively supplemented bonds, the amount of adhesive applied may be varied as desired in contradistinetion to the wet strength application for waterlaid cellulosic materials in which only limited amounts may be added. Similarly, the restriction of the adhesive to the bond areas allows the achievement of truly superior product aesthetics without sacrificing necessary strength.

I claim:

1. A soft, bulky cellulosic web having a basis weight of from about 5 to 50 lbs/2880 ft. comprising a continuum of random-laid wood fibers having a length less than about 0.5 inch, the continuum being interrupted by a pattern of highly compacted, adhesively supplemented, hydrogen-bonded areas, the bonded areas being spaced apart less than about an average fiber length apart and having a height of less than about 40% of the mounds and the web fibers in the other areas forming substantially unbounded fiuify mounds substantially free of adhesive having a thickness of at least about 2.5 times the thickness of the bonded areas.

2. The web of claim 1 wherein the adhesive is starch.

3. The web of claim 1 wherein the bonded areas cover from about 10 to about 40% of the web and are spaced at frequencies of about 10 to about 40 per inch.

4. The web of claim 1 wherein the bonded areas have a height of less than about 20% of the mounds.

5. The web of claim 1 wherein the adhesive provides wet strength properties.

References Cited UNITED STATES PATENTS 3,017,304 1/ 1962 Burgeni 161-145 2,955,641 10/1960 Burgeni 156-219 3,059,313 10/1962 Harmon 161-148 2,464,301 3/1949 Francis 161-124 2,902,395 9/1959 Hirschy et al. 16182 GEORGE F. LESMES, Primary Examiner I. I. BELL, Assistant Examiner US. Cl. X.R.

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