US3438844A - Spot-bonded nonwoven fabrics and their preparation - Google Patents

Description

V. M. KUMIN A ril 15, 1969 SPOT-BONDED NONWOVEN FABRICS AND THEIR PREPARATION Filed June 4, 1965 United States Patent 3,438,844 SPOT-BONDED NONWOVEN FABRICS AND THEIR PREPARATION Victor M. Kumin, Newton Highlands, Mass, assignor to The Kendall Company, Boston, Mass, a corporation of Massachusetts Filed June 4, 1965, Ser. No. 461,480 Int. Cl. D04h 1/04, 5/06 US. Cl. 161--150 7 Claims ABSTRACT OF THE DISCLOSURE A fibrous Web consisting at least in part of thermoretractile textile-length polymeric fibers is spot-bonded at a set of discrete and spaced-apart areas. The spot-bonded web is then heated under restraint so that substantially no shrinkage occurs. The portions of the thermoretractile fibers lying between the spot-bonded areas are drawn into a tensioned configuration, and the web strength is enhanced.

This invention relates to spot-bonded nonwoven fabrics and their preparation. More particularly it relates to spotbonded nonwoven fabrics comprising thermoretractile fibers in which a major proportion of the length of these fibers has been retracted into a tensioned and generally rectilinear but unbonded configuration, while a minor proportion of the length of these fibers is in a bonded but generally cursive and relaxed configuration.

Nonwoven fabrics comprise an array of textile fibers assembled into sheet-like form without the use of conventional spinning and weaving or knitting operations. Such products are conveniently manufactured by bonding fibrous fleeces or webs derived from carding machines, garnetts, air-lay machines, or the like. Bonding may be accomplished by activating selected sensitive fibers included in the web, or by the addition of an auxiliary bonding substance.

It was early recognized in the art that the product resulting from overall bonding or impregnation with a bonding agent in latex or emulsion form had inherent qualities of rigidity, stiffness, and lack of fiber freedom which rendered it deficient and unsatisfactory where a soft, drapeable nonwoven fabric of maximum fiber freedom was desired. The desire to overcome this lack of softness and conformability led early to the concept of interrupted or discontinuous bonding, which historically began with the printing of straight or wavy lines of binder across the breadth of the web. Later, short discontinuous line segments, dots, rings, and varied other geometric designs of discrete areas of binder material were applied to fibrous webs in a technique that is commonly referred to as spot, zone, or island bonding, or in general terms as discontinuous bonding.

Most natural textile fibers, and man-made fibers intended to be carded, garnetted, or otherwise formed into a cohesive web or fleece suitable for processing into nonwoven fabrics, are curled or crimped so that inter-fiber friction renders the web to a certain extent self-sustaining. The crimped or curled configuration is natural in the case of wool, cotton, and other fibers of natural growth, and is man-induced in the case of synthetic fibers. Even when little or no deliberate curl or crimp has been imparted to fibers, after processing into a fibrous fleece or web they will be found to be entangled, and to follow a winding path throughout the plane of the web, due to the mixing action of the fiber-distributing apparatus. Such a winding, tortuous path followed by the fibers in a conventional textile web will be called herein a cursive configuration. It is a gross-scale phenomenon, relating to path followed by a fiber as a whole, a part from localized crimping which is a localized series of bends or folds in the 3,438,844 Patented Apr. 15, 1969 "ice fiber, repeated in regular order several times per inch of fiber length.

Such fibrous webs, in unbonded form, show considerable elastic recovery from distortion stresses of small magnitude, and this recovery is a reflection of the basic internal organization of the fibers in the web. In a so-called isotropic web, made on an air-lay machine, the fibers are said to be oriented almost equally in all directions. Garnett webs are less substantially randomized, while card webs are considered to be rather strongly oriented in the machine direction. Even in the case of card webs, however, it is proper only to speak of an average degree of orientation, since the fibers follow a twisted and tortuous path-i.e., they lie in a cursive configuration. When a tensile stress is applied to a web of this type which has been spot-bonded-that is, bonded in a set or pattern of discrete and spaced-apart areas, by heat or by the application of a binderonly a certain number of fibers are involved in the stress at any one time. In webs of this sort, the fibers bearing the stress are only casually engaged with neighboring fibers. Although the fibers may cross and recross each other, the strength of the fibrous web between the bonded areas depends on the summation of the fiber-to-fiber frictional forces developed by point contact. Furthermore, since the fibers are not organized into a proper stress-bearing configuration, only a few point-topoint frictional contact forces need to be overcome at any one time to permit the disengagement of one fiber from another. Thus those few fibers which have the minimum free paths between the spots at which they are bonded must assume all of the stress load. Continued application of stress will rupture these straightened-out fibers, and render them ineffective for load-bearing, while the majority of the neighboring fibers are still in a somewhat cursive and non-load-bearing configuration. There is no concerted action of the fibers in opposing stress, only the action of individual fibers, or small groups of fibers, which find themselves in transient opposition to the stress because in the haphazardly cursive paths followed by the fibers, this particular fiber or small group of fibers happened to have a minimum free path between bonded areas. This is one reason why prior art spot bonded nonwoven fabrics, though soft and extensible, are characterized by low tensile strengths.

It is with improvements in the art of making spotbonded nonwoven fabrics of enhanced tensile strength that the present invention is concerned. It is the chief object of this invention to provide a spot-bonded nonwoven fabric, composed at least in part of thermoretractile fibers, in which the thermoretractile fibers have been retracted into a tensioned, generally rectilinear load-bearing configuration along those segments of said fibers which lie between the discrete and spaced-apart bonded areas which unite the fabric into a unit. It is also an object of this invention to provide a process for producing such a product.

In general terms, the invention comprises the steps of:

(1) Forming a web consisting at least in part of thermoretractile fibers, as defined below.

(2) Bonding said web at discrete and spaced-apart intervals, the area of said bonded regions constituting a minor proportion of the total area of said Web.

(3) Heating said web to cause those segments of thermoretractile fibers lying between said bonded areas to retract from their normally cursive configuration into a tensioned, rectilinear load-bearing configuration, while restraining the web as a whole from undergoing any appreciable area shrinkage.

The invention will be more clearly understood with reference to the accompanying drawings, in which:

FIGURE 1 represents a section of a spot-bonded nonwoven fabric composed of thermoretractile fibers, magnified about 16 times.

FIGURE 2 represents the fabric section of FIGURE 1 after carrying out the process of this invention.

in both figures, only a few fibers are shown for the sake of clarity.

Referring to FIGURE 1, a section of a spot-bonded nonwoven fabric is shown, as if prepared from a carded web of thermoretractile fibers oriented generally in a north-south direction. By thermoretractile fibers is meant those textile fibers, composed of synthetic polymeric material, which tend to shrink or retract when heated to a temperature below their melting point, usually due to such fibers having been drawn out or stretched during the manufacturing process to impart strength and a certain degree of crystalline orderliness. Included in this category are polyolefine fibers such as polyethylene and polypropylene; polyvinyl fibers; polyamides such as the various nylons; polyacrylics and the modacrylics; and polyester fibers such as those derived from polyethylene terephthalate.

Although a comparison of FIGURES 1 and 2 will indicate that all the fibers are thermoretractile, it will be apparent that such fibers may be admixed with inert or non-retractile fibers, provided that the thermoretractile fibers are present in suflicient quantity to contribute substantially to the load-bearing capacity of the product.

The shaded diamond-shaped areas and 12 may be regarded as made up of an extraneous binder material, usually applied to the fibrous web by printing, drying and curing thereon a polymeric binder applied by means of a printing roll. Such a printing process is usually carried out without disturbing the arrangement of the fibers, so that the fibers 14, 16, 18, 22, 24 and 2 6 follow a winding and cursive path between the binder areas 10 and 1 2, as well as within said areas.

If a stress is applied to this section of spot-bonded fabric in a north-south direction, as indicated by arrows, the fibers will tend to straighten out until the fiber which has the shortest mean free path between the bonded areas (in FIGURE 1, fiber 20) has been drawn out into a tensioned, straight-line configuration. Continued application of stress will either rupture fiber 20 or will break up the spot-bonded zones if the binder is of a weak and soft nature, either consequence being undesirable. If the fiber is ruptured, the burden of the stress is next picked up by the fiber having the shortest unbroken mean path between spots 10 and 12, the cycle is repeated, and the fibers are either broken almost individually and seriatim or the binder spots are fragmented lWlth consequent fiber slippage and loss of strength.

If spot-bonded nonwoven fabrics comprising thermoretractile fibers are heated without restraint to the retraction point of the fibers, there will be an area shrinkage of up to 50% or more, a correspondingly increased, weight and a distortion of the spot-bonding pattern if the fibers have any predominant orientation.

If, however, the spot-bonded fabric is restrained against area shrinkage while being heated to the retraction point of the thermoretractile fibers, the fiber segments lying between the bonded areas 10 and 12 gradually shrink from their previous cursive configurations to take on a tensioned and generally rectilinear configuration as shown in FIGURE 2, wherein the inter-island fiber segments 15, 17, .19, 21, 23, 25 and 27 corresponding to the fibers 14, 16, 18, 20, 22, 24 and 26 are shown in retracted, straightline relationship with respect to the bonded areas 10 and 12. By the term tensioned configuration it is not implied that if such a fiber is cut, the cut ends will spring apart like a cut in an extended rubber band. Rather, it is meant that the fiber has been retracted to a taut condition, so that it responds with instantaneous resistance to the effect of an applied longitudinal stress. In this manner, all of the rectlinear segments of the fibers of FIGURE 2 have developed a component of resistance to any north-south displacement of the bonded areas 10 and 12, with subsequent enhancement of the onwoven f ric a a whole- The restriction against area shrinkage may be effected by holding out the spot-bonded nonwoven fabric by means of a pin frame or pin tenter, or by sandwiching the fabric firmly between two traveling surfaces, on of them preferably heat-conductive, and passing the assem bly through a heating zone with the heat applied to the conductive surface. In the case of fibers with relatively low retraction temperatures, such as the vinyl acetatevinyl chloride copolymers or the polyolefines, retraction may be effected by restraining the fabric between porous blankets and passing steam under the appropriate pressure therethrough. Various other expedients for fabric restriction will suggest themselves to those skilled in the art.

There are certain restrictions which should be understood as inherent in the process of this invention. First, it is not particularly effective in nonwoven fabrics which are overall bonded, whether by overall saturation of a fibrous web with a polymeric binder or by not-calendering a web of thermosensitive fibers. In the former case there are few if any cursive fiber segments which are free to undergo retraction. In the case of hot-pressed webs, where substantially all of the fibers are bonded to all the other fibers at their points of intersection, an average fiber in cursive configuration may be crossed by as many as 10 or 20 other fibers along its length. Even if the short segments of fiber length lying between the points of bonding were retracted to be made rectilinear, such a fiber would not make an appreciable contribution to the load-bearing properties of such a fabric since its principal contour would be cursive, although made up of short segments of straight lines. It is preferred, therefore, that the major segments of fiber length shall be unbonded, and that only minor segments of fiber length shall lie within bonded area. Obviously, if a fiber happens to be so oriented that only a single fiber segment lies within a bonded area, retraction will not cause that fiber to assume a rectilinear configuration. Preferably, a substantial number of the fibers will be bonded at at least two, and not more than six, fiber segments along the fiber length.

Another restriction on the process of this invention is that the temperature to which the thermoretractile fibers are acually exposed should be lower than the melting point of the fibers and lower than the melting point of the polymeric binder holding the fibers together, if an extraneous binder is used. In the latter case, it is known to compound polymeric binders which are normally heat sensitive. with additives or cross-linking agents which allow them to be cured to a substantially insensitive condition before the fabric is treated according to this invention.

In addition to what are regarded as conventional spotbonded nonwoven fabrics, other types of intermittentlybonded nonwovens may be improved by this process. For example, certain types of so-called spunbonded products are known, prepared directly from the basic polymer in molten or solution form, in which the filaments lie in cursive configuration and are only occasionally bonded at intersection points with one another. Such products may be heated, under restraint, to the point where the free cursive fiber segments lying between the widelyseparated bonding points are drawn into rectilinear configuration. Since the filaments constituting such products are extremely long, if not continuous, the abovementioned preferred limitation of not more than six points of bonding along the fiber length does not apply,

said limitation relating to staple textile fibers of discrete length.

Similarly suitable for the process of this invention are nonwoven fabrics comprising thermoretractile fibers which have been bonded by a spray or polymeric binder which is distributed throughout the length and width of the fabric as discrete globules or islands of binder with substantial lengths of fiber segments lying in unbonded and cursive configuration between the bonded areas.

One embodiment of the invention will be illustrated by the following example.

A carded web weighing 55 grams per square yard was prepared from 1.5 inch 3 denier Dynel fibers, a modacrylic fiber made by Union Carbide. The web was spotbonded by passing it between heated rolls, one of which had a raised pattern of lands which fused the fibers together in a pattern of spots which were 0.040 inch in diameter and 0.10 inch apart, in staggered rows. Microscopic examination of the spot-bonded product showed that the fiber segments between bonded areas were in a generally relaxed and cursive configuration. The strength of a one-inch strip of the fabric was 4.5 pounds.

A sampleof the fabric was impaled on a pin frame and heated at 300 F. for one minute. There was no area shrinkage and no increase in weight. A majority of the fibers after this treatment were found to be in rectilinear configuration between the bonded areas, with no evidence of fiber fusion. The tensile strength of the heat-treated sample was 7.0 pounds per one-inch Wide strip, an increase of over 55%.

Having thus described my invention, I claim:

1. The process for making spot-bonded nonwoven fabrics of enhanced tensile strength which comprises forming a web consisting at least in part of textilelength thermoretractile synthetic polymeric fibers,

bonding the fibers of said Web in a pattern of discrete,

spaced-apart bonded areas,

said bonded areas comprising a minor portion of the total area of said web,

a major portion of the total area of said web consisting of fibers lying between said bonded areas in a relaxed and cursive configuration,

heating said bonded fibrous web while preventing said web from appreciable shrinkage,

and causing the fibers lying between said bonded areas to retract into a tensioned and generally rectilinear configuration.

2. The process according to claim 1 in which the bonded areas constitute not more than one-third of the total area of the fibrous web.

3. The process according to claim 1 in which the thermoretractile fibers are heated sufliciently to cause them to retract but not sufficiently to effect any substanial fiber fusion.

4. A spot-bonded nonwoven fabric of enhanced tensile strength which comprises a fibrous web consisting at least in part of synthetic thermotractile polymeric fibers,

said web being bonded in a pattern of discrete, spacedapart binder areas,

the segments of the polymeric fibers lying within said binder areas being in a generally relaxed and cursive configuration,

and the segments of the polymeric fibers lying between and bonded by said binder areas being in a generally tensioned and rectilinear configuration.

5. The product according to claim 4 in which the binder areas constitute not more than one-third of the total area of the nonwoven fabric.

6. The product according to claim 4 in which the rectilinear segments of polymeric fibers lying between the bonded areas are in substantially unfused condition.

7. The product according to claim 4 in which a majority of the polymeric fibers are bonded by at least two and not more than six bonded areas along the length of said fibers.

References Cited UNITED STATES PATENTS 2,774,129 12/1956 Secrist 161157 3,117,055 1/1964 Guandique et a1 161--157 3,276,944 10/1966 Levy 161-157 M. SUSSMAN, Primary Examiner.

US. Cl. X.R.

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