CA1145213A

CA1145213A - Process for bonding organic fibers

Info

Publication number: CA1145213A
Application number: CA000342390A
Authority: CA
Inventors: Virgnia C. Menikheim; Bernard Silverman
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1978-12-21
Filing date: 1979-12-20
Publication date: 1983-04-26
Also published as: BR7908369A; DE2965649D1; JPS5584462A; EP0013125A1; EP0013125B1

Abstract

PROCESS FOR BONDING ORGANIC FIBERS
ABSTRACT OF THE DISCLOSURE
Nonwoven point-bonded fabrics of improved softness are prepared by simultaneously heating and compressing spaced, discrete areas of a nonwoven thermally bondable fiber web containing an attenuating liquid.

Description

a~s~3 PROCESS FOR BONDING ORGANIC FIBERS
... . . ........ _ . .
BACKGROUND OF THE INVENTION
This invention relates to processes for bonding non-woven webs of organic fibers to form nonwoven fabrics. More specifically, the invention relates to such processes wherein 5 the web is preferentially bonded in spaced, discrete areas.
Nonwoven fabrics and numerous uses thereof are well known to those skilled in the art. Such fabrics are prepared by orming a web o~ continuous ilament and/or s~aple fibers and bonding the fibers at points of fiber-to-fiber contact to 10 provide a fabric of requisite strength.
Depending on the intended use of the nonwoven web, satisfactory bonding can in some instances be accomplished mechanically, e.g., by needle punching or interlacing of the fibers or by application of adhesi~es to the fibrous web.
15 However, in a ~umber of applications nonwoven fabrics bonded by autogenous fiber-to-fiber fusion are desired. Bonding of this type is in some instances obtained by the application of heat to soften or plasticize the fibers and render them cohesive. In such thermal bonding technques the web can be 20 subjected to mechanical compression to increase contact of the thermally softened fibers and provide bonds of required streng~h. When web fibers are bonded at essentially all points of fiber-to-fiber contact, for example, by overall compression of the web in the presence of heat, the resultant nonwoven 25 fabric tends ~o be stiff and boardy and characterized by low elongItion and tear resistance. That is, such overall bonded f fa~rics are frequently more similar to paper than to conventional textile fabrics. In order to more closely 52~L3

-2- C-14-54-0440 simulate the properties o conventional textiles, nonwoven "point-bonded" fabrics have been prepared by processes tending to effect preferential bonding in spaced, discrete areas (primary bond sites). In order to provide point-bonded non-woven fabrics of adequate strengthl it is generally necessarythat bonding of the web in the primary bond sites be accom-panied by mechanical compression. This is generally accomplished by compressing the nonwoven web between mechanical compression means such as a pair of rollers or platens at least one of w~ich carries bosses sized and spaced to provide the desired pattern of primary bond sites or both of which carry land and groove designs interacting to provide the desired pattern. The compression means are generally heated sufficiently to effect thermal bonding.
By a proper selection o sizing and spacing of the bosses or lands and grooves, control of the bonding conditions (temperature and compressive force), it is possible to obtain nonwoven point-bonded fabrics having acceptable s~rength and improved tactile properties such as softness. However, even ~0 point-bonded fabrics are frequently less soft than conven-tional fabrics of comparable strength. This is probably due, at least in part, to "tack" bonding. ~hen the bonding condi-tions are controlled to provide fabrics having good strength and durability during washing, bonding is not limited to the primary bond sites produced in the areas compressed. Varying degxees of secondary or "~ack" bonding are generally observed between the primary bond sites. Such "tack" bonding probably results from the fact that techniques employed for preparing point-bonded nonwoven fabrics expose areas of the web between the areas being compressed to heat sufficient to effect some softening and tack bonding of fibers at points of contact.
The strength and number of the tack bonds formed may vary widely with the properties of the fiber utilized in the web as well as the condi~ions employed for effecting bonding in the primary bond sites. Desired fabric properties such as softness are progressively impaired as the degree of tack bonding is increased. There is, therefore, a need in the ar~
for processes capable of providing softer nonwoven fabrics.

-3- C-14-54-0440 SUMMARY OF THE INVENTION
It is an object of this invention to provide processes for making point-bonded nonwoven fabrics character-ized by improved softness. It is a further object of the invention to provide processes for making such fabrics having improved softness ~ithout undue reduction in fabric strength.
These and other obj~cts of the invention are obtained by simultaneously heating and compressing spaced, discrete areas of a nonwoven web ~hich comprises thermally bondable, organic fibers and which contains an attenuating liquid as hereinafter defined. The temperature, com~ressive force, time of exposure of the web thereto and the quantity of attenuating liquid are correlated to effect bonding and to provide fabrics of improved softness. The practice of the invention will be understood from the following description of the preferred embodiments.
DESCRIPTION ~F THE PREFERRED EMBODIMENTS
The process of this invention can be utilized for making point-bonded fabrics from nonwoven webs of thermally Z0 bondable organic fibers. The phrase "thermally bondable organic fibers" is used herein in ~he specification and claims to denote fibers which can be bonded at points of fiber-to-fiber contact by the appl:Lcation of heat and com-pression. Thus, essentially any ther~oplastic polymer can be utilized. The fibers may be in ~he ~orm of continuous ~ila-ments or staples or mixtures thereof.
Examples of bondable fibers suitable for use in the practice o~ this invention include polyamide fibers such as nylon 6 and nylon 66; and polyester polymer fibers. Composite ~0 fibers such as fibers having a sheath of one polymer and a core of another polymer or side-by-side polycomponent fibers can be utilized. In the case of m~lticomponent fibers it is not essential that all polymer components thereof be bondable under the processin~ conditions hereinafter described. It is sufficient that such multicomponent fibers have bondable surface portions. If desired, the fibers can be crimped or textured ~o provide elasticity or other desired character-istics to the finished fabric.

5~

-4- C-14-54-0440 In accordance with the present invention, the thermally bondable fibers are processed in the form of nonwoven webs. The nonwoven webs of bondable organic fibers may be composed entirely of bondable fibers or, alternatively, may consist of bondable fibers interspersed with other fibers.
The art of preparing nonwoven webs is well understood and the manner of web formation is not critical. Generally webs are formed by deposition of fibers on a moving belt in either random or aligned orientation to provide a web having a weight of from ~ to 400 grams per square meter, preferably 10 to 150 grams per square meter. Particularly useful methods for web formation are disclosed in United States Patent No. 3,542,615.
In accordance with the present invention a selected quantity of attenuating liquid is applied to the web and the web is simultaneously heated and compressed in spaced discrete areas to effect bonding of the fibers in such areas.
The attenuating liquid can be any liquid whose presence in the web in quantities o~ 1000% or less of the web weight does not, under the bonding conditions employed, provide a fabric having higher strip tenacity (strength) than would be obtained in the absence of such liquid under other-wise equivalent conditions and which provide a fabric having at least a 20% lower bending modulus than that of a fabric obtained in the absence o~ such liquid under otherwise identical conditions.
A key element o~ the present invention is this unexpected discovery that utilization of an attenuating liquid in sufficient quantity will provide a reduction in fabric bending modulus (i.e., an increase in fabric 1'softness") as compared to that of fabrics prepared using no liquid under otherwise equivalent conditions. In accordance with the present invention a quantity is employed to reduce bending modulus by at least 20%. Th~ actual amount of attenuating liquid used may be any quantity sufficient to effect such reduction.
Generally, there is no theoretical objection to use of very large quantities of liquid.

-5- C-14-54-04~0 However, it will be observed that after a determinable quantity is added, the use of additional liquid will not provide substantial additional improvements in softness and, in some instances, may tend to reduce fabric s~rength, proba-bly ~y cooling the heating means employed. Of course,excessive amounts of liquid ~eyond that contributing to improvement of fabric properties will present unnecessary process problems with respect to liquid handling, recovery, etc. It is preferred that the amoun~ of liquid be chosen such ~hat in addition to reducing bending modulus by at least 20% a higher ratio of strip tenacity to bending modulus (as compared to that obtained using no liquid) is obtained.
That is, the maximum quantity utilized is preferably chosen so as not to reduce fabric strength disproportionately to improvements in softness obtained.
I~hether or not a particular liquid will function as an attenuating bonding liquid will depend on the nature of the nonwoven web to be bonded, the properties of the fibers constituting the web and the manner in which the web is heated and compressed. Therefore, it is not practical to exhaustively list all combinations o~ liquids, fibrous webs and conditions of temperature and compression suitable for the practice of the present invention. For example, water will effectively improve the bonding o~ a web of nylon fibers highly compressed in spaced discrete areas at temperatures below that required to cohesively soften an otherwise identi-cal dry web. Thus, under such conditions water is considered a bonding agent rather than an attenuating liquid. However, under low compressive ~orce and temperatures su~iciently high ta effect thermal bonding, water may function as an attenuating liquid. The effectiveness of a particular liquid as an atten-uating liquid under given bonding conditions can readily be determined by routine tests.
It is believed that attenuating liquids provide softening by limiting (for example by evaporative cooling, heat capacity, etc.) the temperatures attained in the web in areas not being simultaneously heat~d and compressed as hereinafter described. The heat attenuation provided by the ; liquid is believed to limit or prevent tack bonding outside

-6- C-14-54-0440 the discre~e, spaced areas which are heated and compressed, thereby providing a softer fabric. Thus in selecting liquids for testing, preference may be given to those which have relatively low boiling points as compared to fiber softening 5 points and/or those having high heat capacities. In general, any liquid which is not a bonding agent and has a boiling point below the fiber softening point will be an effective attenuating liquid. It is further contemplated that a number of liquids having boiling points higher than the 10 fiber softening point will be effective attenuating liquids, pxesumably due to heat attenuation resulting from heat capacity, vaporization, etc. preventing the web fibers from reaching bonding temperatures in ~he uncompressed areas when sufficient liquid is employed.
Under properly correlated simultaneous application of heat and compression to appropriate nonwoven webs, examples of liquids contemplated to be suitable attentuating liquids for polyamide fibers include water and hexane; e~amples of suitable attenuating liquids for polyester fibers include 20 water and carbon tetrachloride.
In accordance with this invention, the nonwoven web containing the attenuating liquid is simultaneously heated and compressed in spaced, discrete areas (points) to effect fiber bonding in such areas thereby forming the web into a 25 point-bonded fabric.
Simultaneous heating and compression of the web in spaced, discrete areas can readily be accomplished by com-pressing the webs between a pair of compressing means such as rolls or platens at least one of which compression means is 30 heated. Further, one or both of the compression means will have bosses or a land and groove design or combinations there-of such that compression of the web will be effec~ed in spaced discrete areas rather than overall. In order to provide adequate overall physical properties it is generally desirable 35 that from 2% to 80%, preferably 3% to 50%, most preferably 5%
to 30%, of the total surface area of the web be subjected to compression. Further, the nu~lber of spaced, discrete bond sites per square centimeter generally should be ~rom 1 to 250, ~i5;~3

-7- C-14-54-0~40 preferably from 16 to 64.
The compressive force, the temperature, and the time of exposure of the web to compression and heating will depend on the nature and quan~ity of the attenuating liquid utilized and the nature of the fîbers being processed. Therefore, for a particular nonwoven web and a particular at~enuating liquid, the compressive force, the temperature, and the time of exposure of the web to the compressive force and heating will be correlated to effect bonding of the web fibers in the heated, compressed areas.
Preferably, the heating and compression will be correlated to effect a degree o~ bonding sufficient to provide a wash stable fabric as hereinafter defined. In general, increases in bonding will be observed with increased tempera-ture until a temperature is attained beyond which furtherincreases will have little, if any, beneficial effect. If the operation is conducted at too high a temperature, the heat attenuation characteristics of the liquid may not be adequate to provide requisite improvements in fabric softness. The use of increasing quanti~ies of attenuating liquid may require increased compressive ~orce and/or temperature to provide wash stable fabrics. The optimum correlation of temperature and compressive force can, of course, be empirically deter-mined by routine tests.
The following examples will facilitate a better un~erstanding of the invention and the desirab~e properties of fabrics produced thereby. The tests described below are used to determine fabric properties as reported in the examples or otherwise referred to in the specification and claims:
Strip Tenacity Strip Tenacity is used as an indicator of fabric strength and is determined by dividing the breaking load of a cut fabric strip (as determined by American Society of Testing Ma~erials procedure D-16~2-64) by the fabric basis weight. Strip Tenacity is expressed as g/cm/g/m2. Values - reported are an average of tenacities in the machine and transverse di.rections of the fa~ric. (The machine direction

-8- C-14-5~-0440 corresponds to the direction of feed to the heating and compressing means and the transverse direction is the planar direc~ion at a right angle thereto.) Bendin~ Modulus Bending Modulus is used as a measure of fabric softness and is determined in accordance with techniques as described in U.S. Patent 3,613,445. In accordance with such disclosure a test fabric is forced vertically downward through a slot at a constant speed. A signal is generated in propor-tional response to the load incurred in moving the fabric in-to and through the slot. A load-extension curve is generated by plotting the signal as a function of the distance. Hand, drape and bending modulus are determined by analyzing the load-extension curve. Hand is represented by the maximum point on the load-extension curve. Drape is represented by the slope of the load-de1ection curve and bendin~ modulus is determined by dividing the drape value by the cube of fabric thickness. ~ending Modulus, as determined on a 10.6 x 10.6 cm sample, is expressed in grn/cm4 and values reported are an average of fabric face up and face down machine and ~ransverse direction measurements.
With respect to both Strip Tenacity and Bending Modulus, the requirements of the present invention are defined in terms of relative (percent change; ratios) rather than absolute values. Accordingly, apparatus calibrations and choice of test techniques are not critical so long as reasonable consistency is maintained in a given series of compara~ive tests.
Since individual measurements are affected by variations in fabric uniformity and inherent limitations in the precision of various measuring techniques, it is important to conduct and average sufficient measurements to statistically assurP that the differences in values of bendin~ modulus and strip tenacities being compared fairly reflect differences in fabric properties as opposed to imprecisions in measurements of imperfect fabric uniformity.
Wash Stability Wash stability is determined as follows: Nonwoven fabric samples are mi~ed with at least 10 pieces of hemmed

-9- C-14-54-0440 cotton sheeting each measuring about 91 cm x 91 cm. The number and size of the non~oven fabric samples are subject to the following constraints:
1. Total area of the nonwoven samples is less than 6.5 m2, 2 2. Each sample is at least 465 cm in area with a minimum dimension of 15 cm.
3. No sample is larger than 01929 m2 in area or more than 0.305 m in its maximum dimension.
In addition, the total weight of the cotton sheeting plus the nonwoven samples s~ould not exceed about 1.8 kg.
(These constraints assure comparable results.~
The load is washed in a Kenmore*Model 76431100 washing machine (marke~ed by Sears Roebuck & Co.~ using the "normal" cycle (14 min.) "Hi" water level (55 Q), HOT WASH, W~M RINSE (water temperatures of 60C. + 3, 49C. ~ 3) and 90 g of American Association of Textile Colorists and Chemists Standard Detergent 124.
The wash load is then dried in a Kenmore*electric dryer, Model 6308603 (marketed by Sears, Roebuck and Co.) for at least 30 minu~es (or longer if required to dry the entire load~. The test specimens are then evaluated by visual observation to determine the number of pills formed. A pill is a visually discernible (usually roughly spherical) tangle of fiber, or fiber plus extraneous material, extending above the surface of a fabric and connected to the body of the fabric by one or more filamen~s. A abric is considered to fail the test when 5 or more pills are observed in any 929 square centimeters surface area or when more severe physical deterioration is visually discernible. Fabrics passing the above test are considered "wash-stable". In the test described, the pills are predominantly formed by fibers which were not `bonded in the process of which, in test procedure, were freed from bond sites. Thus the degree of pilling pro-vides a measure of the efficacy of the process for formingbonds and a measure of the resulting bond integrity. In instances of very poor bonding more severe fabric deviation than pilling, e.g., complete disintegration, may be observed.
As a practical matter, fabrics which do not pass the test * Trademark

-10- C-14-5~-0440 (even if not totally or partially disintegrated in the test) will not withstand substantial physical str~ss or repeated washings without excessive deterioration.
EXample T
Nonwoven webs composed of continuous filament, 24%
crystalline polyethyleneterephthalate fibers and having web weights of 5 gms/mete~2 and wetted with water to the add-on percentages (we- ~ t w~ter~ X 100%) shown in Table 1 below are simultaneously heated and compressed in spaced discrete areas by passage at a speed of .6 meters/minute between a pair of metal rolls. One roll is smooth while the other has 28 square boss sites/cm2 aligned in a square pattern covering about 18% of the surface area of the roll. The pressure at the roll nip is calculated as 65.0 kg/cm (assuming all pressure to be applied only to the boss sites).
Both rolls are heated to a temperature of 230C. Properties of the fabrics obtained are shown in Table 1 below.
Table 1 Strip Bending Strip TenacitY
Test Water Mo4dulus 5 Tenacity 2 Bending No. (% add-on) (gms/cm X 10 ) (gm/cm/gm/m ) Modulus l none 54 41 .76 2 35% 35 33 .94 3 65% ~7 38 .80 4 95% 36 39 1.1 The above tests in general (test 3 is anomalous and may reflect inaccurate measuring or sampling) show that the use of water as described enables the preparation of point-bonded polyester fabrics of improved softness.
Example II
Nonwoven webs composed of continuous filament crystalline polyethylene terephthalate fibers are passed (either wet with about 1000% add-on carbon tetrachloride or dry) through the nip of a pair of rolls at a speed of 6 meters/min. Each roll bears a helical land and groove pattern (508 micron land width; 1270 micron groove wid~h) with the lands and grooves disposed at 45 angles to the roll axis and cooperating to produce a pattern of diamond shaped depressions covering about 8% of the web surface. The rolls exert a nip pressure of 5~ ~ 3 ~ C~14-54-0440 130 kg/cm (calculated as in Example 1~. Processing of webs having iber crystallinities of 19%~ 24% and 36% demonstrated that the use of carbon tetrachloride pro~ided in fabrics having bending moduli substantially more ~han 20% lower and ratios of strip tenacity to bending moùdlus substantially higher than that of webs processed dry under similar conditions.
Example III
Nonwoven webs of continuous filament nylon 6,6 were processed at 225C. with and without about 700% add-on hexane using the same roll pattern and pressure as in Example II. The use of hexane provided a fabric having about a 40%
lower bending modulus and a substantially higher ratio of strip tenacity to bending modulus than that obtained without the use of hexane.
The foregoing description of the preferred embodiments and examples will enable those skilled in the art to practice these and all other embodiments of the invention within the scope o~ the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for making point-bonded nonwoven fabrics, said process being characterized by simultaneously heating and compressing spaced, discrete areas of a nonwoven web of thermally bondable organic fibers said web containing an attenuating liquid and the quantity of attenuating liquid, the temperature, the compressive force, and the time of exposure of the web thereto being correlated to effect bonding of web fibers in the heated and compressed areas and to provide a point-bonded nonwoven fabric having a bending modulus at least 20% lower than that of a fabric prepared without the use of said attenuating liquid under otherwise equivalent conditions.

2. The process of claim 1 further characterized in that the quantity of said liquid is selected to provide a nonwoven fabric having a higher ratio of strip tenacity to bending modulus than that of a fabric prepared using no liquid under otherwise equivalent conditions.

3. The process of claim 2 further characterized in that the quantity of said liquid, the temperature, the compressive force and the time of exposure of the web thereto are correlated to provide a wash-stable, point-bonded, nonwoven fabric.

4. The process of claim 3 further characterized in that simultaneous heating and compression of the web is effected by passing the web through and compressing said web in the nip of a pair of rolls at least one of which is heated and at least one of which has a pattern of raised surface portions which, in combination with the opposing surface of the other roll, effects compression of the web in spaced, discrete areas.

5. The process of claim 4 further characterized in that the surfaces of said rolls are designed to effect compression providing a point-bonded, nonwoven fabric having a pattern of from 16 to 64 discrete bond sites per square centimeter covering from 3% to 50% of the fabric surface area.

6. The process of claim 5 further characterized in that one of the rolls is provided with boss points sized and disposed to provide a fabric having said pattern.

7. The process of claim 5 further characterized in that each roll has a helical land and groove surface design interacting with the land and groove design of the opposing roll to provide a fabric having said pattern.

8. The process of claim 2 further characterized in that said web comprises continuous filament polyester fibers and said attenuating liquid is water.