US3388028A - Multi-layered nonwoven fabrics - Google Patents

Description

`June 11, 1968 R. R. ALEXANDER MULTI-LAYERED NONWOVEN FABRICS Filed March 6, 1964 MW MMA 3,38%,28 MULTf-LAYERED NONWOVEN FABRICS Robert R. Alexander, Sudhury, Mass., assignor to The Iendali Company, Boston, Mass., a corporation of Massachusetts Filed Mar. 6, 1964, Ser. No. 350,027 3 Claizns. (Cl. Mil- 156) ABSTRACT OF THE DISCLOSURE A layer of low Wet-strength fibrous sheet material of non-textile length fibers, such as cellulose wadding, is plied between two unspun and unwoven fibrous webs. After saturation with a liquid binder, the assembly is squeezed under pressure, causing the inner layer of short fibers to aggregate into a sheet marked by broken, disconnected fibrous ridges, in which the ridges, having a higher concentration of fibers than the rest of the sheet, carry a higher concentration of binder. Drying the assembly results in the outer plies being intermittently bonded to the inner ply, resulting in a soft, flexible product.

This invention relates to bonded nonwoven fabrics. More particularly it relates to improvements in the art of making soft, fiexible, non-papery nonwoven vfabrics of enhanced esthetic hand and appearance.

Bonded nonwoven fabrics are Wi'dely used, among other applications, as disposable towels, pillowcases, sheets, surgical drapes, and the like. For such purposes, it is desirable that the bonded nonwoven fabric sirnulate a woven article as closely as possible: that is, it should possess adequate strength for its intended use, and should be soft and conformable, with a pleasing hand and drape, and should lbe substantially free of the stifness and crackle of paper.

Unfortunately, it is difficult to combine both strength and softness in bonded nonwoven fabrics. The type of bonded nonwoven fabric most widely met with consists of a plied assembly of from two to four or five webs of textile-length fibers, as from a set of cards or garnetts, which have been saturated with a binder as by spraying or `dipping, and then dried. Such products tend to be papery, if they are of adequate strength. Additionally, in the drying process the binder tends to migrate to the surfaces of the plied webs, so that these surfaces are rendered objectionably stiff, and so that the product is deficient in binder in the center plies and tends to delaminate. It is apparent that in the average bonded non- Woven fabric of commerce, the binder is not so distribute-d as to develop maximum strength and delamination resistance.

Attempts have been made to overcorne this disadvantage by processes in which the binder is printed onto' the assemblage of fibrous webs in the form of solid or broken lines, or in the form of discrete geometric areas which leave a substantial part of the fabric free from binder. Such an expedient, however, involves the expensive operation of engraving printing rolls to control the binder distribution pattern. Additionally, to maintan the binder in printable form 'often involves the use of thickening agents, usually of a water-Sensitive nature, since the fiuidity of many binder latices is so high that it must be re- -duced for a printing operation.

I have found that soft and fiexihle nonwoven fabrics comprising a plurality of superimposed fibrous webs can be produced by an overall saturation with binder dispersions, provided that the fibers comprising at least one of the webs have, due to their physical nature and to their state of distribution, an enhanced affinity for the fluid binder dispersion, so that in an overall saturation States Patent O ice process followed by drying, the binder in its final dried form is more concentrated in discrete areas, randornly distributed throughout the length and breadth of the fabric, than it is elsewhere in the fabric.

More specifically, I have found that if a layer of cellulose wadding is plied between two webs of textile length fibers and the assembly is saturated with binder dispersion, squeezed, and dried, the result is a multi-layered nonwoven fabric in which a substantial part of the fibers of the cellulose wadding layer are arranged in a series of randomly-distributed short ridge-like lines which contain a higher fiber concentration, and hence a higher binder concentration, than the wadding fibers throughout the rest of the inner layer. In this sense, the layers of the final nonwoven fabric may be regarded as being hingedly interconnected throughout the thickness of the fabric, with the result that the fabric is soft, conformable, and drapes readily while it maintains a substantial resistance to ply separation and delamination, as will appear more fully hereinbelow. This `diseontinuity of binder distribution is found only in depth and between plies, the fibers in the surface plies of the fabric being bonded to each other in a substantially unifor-m manner.

It is an object of this invention to provide a soft, conformable nonwoven fabric comprising a plurality of fibrous webs, in which the webs are interconnected in depth more securely at some points than at others.

It is also an object of this invention to provide a nonwoven fabric of the above type in which the binder distribution in at least one of the webs is more concentrated in certain areas than in others.

Other objects of the invention will appear from the following description and from the drawings, in which:

FIGURE 1 is a top plan view of a multiylayeiedf bonded non-woven fabric made according to this invention.

FGURE 2 is a plan view of the interior layer of the product of FIGURE 1.

FIGURE 3 is a cross-sectional view of the product of FIGURE 1 along the line A-A.

FIGURE 4 is a tracing of the delamination strength of a multi-layered saturated nonwoven fabric made according to this invention, as measured on an Instron machine.

It is common practice, as set forth above, to superimpose on each other a plurality of carded webs of textilelength fibers, saturate the assembly With a liquid binder, and dry the saturated product. During the drying operation, the evaporation of water from the binder solution causes migration of binding material to the surfaces of the fabric. The surfaces are thereby enriched, and the central zone impoverished, in binder content. To compensate for the resulting tendency of the final product to delaminate or split into separate layers, it is common practice to oversaturate the assembly of webs with more binder than is actually needed to develop adequate strength. This further increases the boardy, papery hand of the outside plies of the fabric.

I have found that desirable improvements in multilayered nonwoven fabrics of this type may be eifected by the utilization, in at least one layer of the assembly, of at least one layer of a second species of fibers with properties of capillarity and retention which differ from the fibers in the surface layers. One example of such a second species of fibers is a sheet of short, non-textile length fibers, such as are found in a sheet of creped cellulose wadding. By celiulose Wadding I means a Sheet of Wood pulp fibers formed into a very thin, highly pervious and very tenuous crepe tissue material, with but little Wet strength but With high absorbent capacity. It is usually for-med by the mechanical creping of a Wood-pulp paper Sheet, and due to the creping such a wadding may be 3 stretched or or more. Such wadding generally ranges from 10 to grams per square yard. I have found that cellulose wadding Weighing |about 10 to 12 grams per square yard per single ply is well-suited to the practice of this invention.

When a ply of such a wadding is assembled between outer plies of webs of textile-length fibers from carding machines, garnetts, or the like, and the assernbled plies are saturated with a liquid binder, the characteristic nature of the wadding layer is to hold more binder than the layers of teXtile-length fibers. It is a general fact that for a fiber substance of a given chemical nature, the more finely divided the fibers, the higher their fiuid-absorbing capacity. Thus a given weight of Short wood-pulp fibers will generally hold a larger 'amount of an aqueous fiuid than an equal Weight of rayon fibers. Similarly, a given weight of fine rayon fibers of 1.5 or 3.0 denier will generally hold, at saturation, a larger amount of water than a similar weight of coarse rayon fibers of 10 or 15 denier.

In the production of nonwoven fabrics of this type, it is common practice to saturate the plied fi'orous asscrnbly With a small eXcess of saturant, to insure complete penetration, and then to squeeze the assernbly, as by means of a nip roll, to the desired pickup. In the practice of this iuvention, such a squeezing operation has an important effect on the distribution of the short fibers in the Wedding layer. Normally, in the dry state, the fibers in the wadding layer are uniformly distributed throughout the sheet, the surface of which is marked by numerous faint wave-like lines characteristc of a creped Sheet. When wet, however, as in the saturation process, the wadding sheet has little or no wet strength. Additionally, it tends to lose its crepe, and if a dry piece of such wadding is dropped onto a surface of water, it will expand in a direction perpendicular to the crepe pattern. In the practice of this invention, however, the 'wet wadding layer is not :free to expand, but is heid between the upper and lower layers of textiie fibers. The crepe that was built into the dry wadding therefore appcars as slack in the Wet wadding. Apparently the action of the squeeze rolls, acting on the assembly, is to iron out a considerable 'amount of the crepe or slack in the Sheet of wadding, and without destroying the integrity of the Sheet, to rearrange the wadding fibers into a series of separated broken ridges, the ridge pattern being of much coarser grain than the fine-grained ripple or crepe pattern of the original wadding.

The resulting product is iliustrated in FIGURES 1, 2, and 3. In FIGURE 1, lt) .represents a view of the upper surface of a three-layered nonwoven fabric made in accordance with the process of this invention, according to Example 1 below. Although the surface of the fabric is smooth and uniform, it is optically marked by the presence of numerous and randomized broken lines 12. FIG- URE 1 is 'a drawing, to exact Scale, of a tracing of a section of the actual fabric of Example 1, made by transmitted light.

If the outer layer of teXtilc fibers constituting the top layer of FIGURE 1 is removed, the inner or wadding layer of the fabric is exposed. This layer of fibers is still integral and intact, and comprises a fibrous Sheet 14 in which the optical lines 12 of FIGURE 1 are seen to be due to actual fiber ridges 13 of the Short, highly capillary Wood pulp fibers of the inner layer. These ridges, presumably formed by the rolling action of the Squeeze rolls taking the slack or crepe out of the wadding, are areas of high fiber concentration. That is, the short puip fibers are piled up together into ridges in which the average fiber density is greater than in the balance of the Sheet lying between the broken ridge lines. Due to this induced rearrangernent of the wadding fibers into discrete and dis continuous lines of fiber compaction, the capillarity of the fibers in these ridges is greater than it is througheut the balance of the sheet. As a consequence, during the drying operation, the tendcncy of the binder to migrate to the surfaces of the fabric is to a substantial degree overcome by these highly retentive discontinuous fiber ridges,

which hold a disproportionate share of bnder substance when compared with the rest of the inner sheet.

This may more clearly be seen with reference to FEG- URE 3, in which two layers of textile-length fibers, 16, are disposed one on each face of a layer of crepcd cellulose wadding 18 and the asscrnbly of plies is treated in accordance with the process of this invention. It is seen that at intervals the Wood pulp fibers of the inner layer have been aggregated into ridges 20, 22, and 24, reflecting in magniied form the ridges 20, 22, and 24 of line A-A of FIGURE 1. These iidges are not only areas of high fiber concentration, as explained above, but are areas of high binder concentration. Thus they serve to bond the plics of the fabric more firmly than the plies bonded in the vicinity of unperturbed fiber distribution, as at 26 in FIGURE 3. Since the locations of high binder concentration 20, 2.2, and 24 are separated by areas of lower binder concentration, the bond between the letter areas and the outer surfaces of the fabric is weaker than it is at the locations 20, 22, and 24. This variability lends a softness and drape to the fabric as a whole, and illustratcs what I mean by a hinged interconnection between the fabric plies, wherein areas 20, 22, and 24, being rongly rcsistant to delamination, interconnect the plies 16, 18, and 1d with the provision of soft and flexible areas 26 between the strongly bonded areas.

One typical product of this invention will be illustrated by the following example and by the discussion of the fabric properties set forth thereafter.

EXAMPLE 1 A layer of cellulose wadding Weighing 10 grams per square yard was continuously sandwiched between two card webs of 3 denier viscose fibers. The three-ply assembly was saturated, by means of a screen saturator, with an excess of a 2020 water dispersion of an acrylic binder known commercially as I-iA-, a product of Rohm and Haas. The wet pickup was adjusted to about 100% by means of a pair of squeeze rolls cxerting a pressure of about 12 pounds per inch of nip.

The resulting product resemblcd the illustration of FGURE 1, and was satisfact'ory for use as a disposable drape or towel, With a soft and attractive hand combined with satisfactory tensile strength and resistance to delamination.

The delamination-resistance pattern of the material was measured by means of an vlnstron machine with settings of 100 grams full Scale and a crosshead speed of 1 inch per minute. Strips of fabric were cut one-half inch wide, running in the machine direction. Dclamination was induced by working the fabric-strip by hand in the vicinity of a lightly-bonded area, after which the loosencd surface ply of viscose fibers Was clamped in one jaw of the Instron, and the center and bottom ply were clarnped in the Vother jaw. A tracing of the resulting curve is reproduced in FIGURE 4, which represents the delamination of approxirnately one inch of plicd fabric.

It will be noted that the tracing of delamination resistance in FIGURE 4 is highly irregular, being marked by peaks of high resistance B, C, D, E and F, where the resistance is between about grarns and grarns, on a one-half inch wide strip. The average resistance to delamination is reprcsented by the horizontal line at the 28 gram mark, and is calculated as follows. The original Instron Chart tracing is on graph paper ruled with ten lines to the inch. A representative 10 inch section of the graph, including the section illustrated, was scanned and the intercepts were measured and averaged.

The significance of the irregularly pealted delamination pattern shown in FIGURE 4 may be summarized statistically by stating that over 50% of the 100 measured i values of delamination resistance lie outside the range of the average (28 gram) value plus or minus 20%. When coinpared with saturated nonwoven fabrics of uniforrn fiber content, the deramination strength measurements of the product of this invention show a coefificient of Variation that is three times as high.

lt has been rny experience with products of this sort made from plied webs of identical fiber type, that a resistance to delamination of less than 30 grams per halfinch wide strip, measured as set forth above, is inadequate to hold the plies of a disposable towel, sheet, or drape together against the normal rigors of application and usage. in the product of this invention, represented by the tracing of FIGURE 4, less than 30% of the measured length of a fabric strip had a value of 30 grams or more, yet the product is eminently suited for use as a towel, sheet, drape, or other disposable article, due to the presence of the peaks B, C, D, E, and F, which may be regarded as resist points, halting the tendency of the fabric to separate into plies. lt should be noted that the live peaks shown in FIGURE 4 occurred during the delarnination of only one inch of nonwoven fabric. I have found that products rnade according to this invention which possess from two to ten such resist points per inch are generally resistant to delarnination when used as disposable towels, drapes, and the like.

Since over 70% of the length of the strip tested in FGURE 4 has a resistance to delarnination value of less than 30 grarns, the product as a whole refiects the properties of a discontinuously bonded nonwoven fabric, even though the saturation and all details of processing were analogous to the simple and economical processing of an overall saturated fabric. Due to the plies being only lightly bonded together over a preponderant portion of the fabric, the fabric has a pleasing softness and drape which closely approach those of woven textile fabrics used for the above-mentioned purposes.

lt Will be apparent to those skilled in the art that other impregnants may be added in the binder-saturation step, or separately, to alter the appearance or function of the final product without departing from the structure set forth in this invention. That is, the nonwoven fabric of this invention may be made water-repellant, or fiameretardant for special purposes. For surgical uses, it is frequently desirable to color the fabric green. This may be done by the use of dyes or by using pignients which are held in the fabric by the binder. In such cases, the discontnuous ridge lines in which binder is concentrated tend to be of a darker color than the balance of the fabric, thus enhancing the esthetic appeal of the article made therefrom.

Having thus described my invention, I claim:

1. A rnulti-layered nonwoven fabric comprising surface layers of textile-length fibers wherein said fibers are substantially uniforrnly bonded to each other in each individual surface layer,

together with at least one inner layer of a second species of fibers,

each layer of said fabric being in full face contact with and boncled to each adjacent layer throughout substantially its complete extent,

some at least of said second species of fibers of said inner layer being concentrated into a randomlydistributed pattern of ridge-like broken lines,

said lines containing a higher concentration of secondspecies fibers and of binder than the areas of said inner layer lying between said n'dge-like lines lying entirely within said inner layer,

whereby said inner layer of fibers is bonded to said outer layers of textile-length fibers more strongly at the points of contact of said ridges with said outer layers than throughout the areas lying between said ridges, said fabn'c having substantial resistance to delamination throughout substantially its entire extent and increased resistance to delarnination at said ridge-like lines.

2. The product according to claim 1 wherein the inner layer is a layer of short 'fibers of paper-making length.

3. The product according to clairn 1 in which there are between 2 and 10 said ridge-like broken lines per inch of nonwoven fabric length.

References Cited UNITED STATES PATENTS 80,913 8/1868 Chipman 161-148 2,077,720 4/1937 Seigle et al 161-148 2,531,931 11/1950 Arkell 161-128 2,980,570 4/1961 Cook et al. 161-152 3,079,29'0 2/1963 Marshall 161-148 MORRIS SUSSMAN, Primary Examiner.

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