HK1078911B - Elastic nonwoven sheet - Google Patents

Description

Elastic nonwoven sheet

1. Field of the invention

The present invention relates to stretchable nonwoven sheets suitable for use in the production of personal hygiene articles. Specifically, the stretchable nonwoven sheet is formed by substantially uniformly impregnating a necked nonwoven substrate with an elastomeric polymer.

2. Description of the related Art

Elastic nonwoven materials are well known in the art. Examples of elastic nonwoven materials include "stretch bonded" and "neck bonded" laminates. Stretch bonded laminates are made by joining an elastic layer to an assembly, wherein the elastic layer is in a stretched state such that the assembly is shrinkable when all of the plies are in a relaxed state. The necked laminate is prepared by joining a necked non-elastic layer to an elastic film or elastic fibrous layer. The elastic layer typically comprises an elastic film or an elastic nonwoven web. These elastic nonwoven laminates require the prior preparation of at least two separate nonwoven or film layers.

U.S. patent No.4366814 to riedel (riedel) describes a breathable elastic material in the form of a strip comprising at least 50 wt.% of a stretchable fabric having an elongation of up to 30% or more and at least 15 wt.% of an elastomer impregnated in the fabric without filling the pores in the fabric.

U.S. Pat. No.5910224 to Morman (Morman) describes aA method of producing a stretchable composite. The process applies an elastic precursor to a neckable material, such as a nonwoven web, stretches the neckable material with a narrow neck and then treats the elastic precursor, such as by heat treatment, while the neckable material is in a necked state to form an elastic layer bonded to the necked material. Preferred elastic precursors include latex or thermoset elastomers. The amount of elastomeric precursor used in the neckable material is 5g/m²To about 50g/m²In the meantime. The permeability of the elastic layer in the web is preferably about 2 to 10 fibers thick, and the degree of penetration of the elastic precursor must be controlled to ensure that it does not penetrate to the side of the web opposite the side to which the elastic layer is applied. The resulting stretchable composite thus has a film-like hand on the side containing the elastic layer and also retains the original soft hand of the neckable material on the side opposite the elastic layer.

Published european patent application No.0472942 describes an elastomer saturated nonwoven material having compressibility and recoverability in the Z-direction of a fibrous web containing, for example, a nonwoven web of meltblown fibers, which web is impregnated with a polymeric material, such as an elastomeric acrylic latex, polyurethane latex or nitrile butadiene rubber latex.

Published japanese patent application No.47-24479 mainly describes a conveyor belt for conveyor and energy transmission, which is made by impregnating rubber or synthetic resin into a needle-punched nonwoven fabric.

It has been desired to obtain elastic sheets having the following characteristics: low manufacturing cost, flexible stretching, good holding power and fiber-like hand feeling on both surfaces.

Summary of The Invention

The present invention relates to a process for making a stretchable nonwoven sheet comprising the steps of:

providing a necked nonwoven substrate having a thickness, first and second outer surfaces, a machine direction and a cross-machine direction, the necked nonwoven substrate having an elongation in the cross-machine direction of at least 30%;

substantially uniformly impregnating a necked nonwoven substrate with a solution comprising an elastomeric polymer dissolved in a solvent; and

the solvent is removed from the impregnated nonwoven substrate using a wet coagulation process, thereby depositing the elastomeric polymer substantially uniformly throughout the thickness of the nonwoven substrate without forming a substantially continuous elastomeric polymer layer on the first or second outer surface of the nonwoven substrate.

The present invention also relates to a stretchable nonwoven sheet comprising a nonwoven substrate that has been necked in the neck direction and substantially uniformly impregnated with an elastomeric polymer. After the stretchable nonwoven sheet is stretched three times to 140% in the neck down direction, a ratio of a third unload cycle force at 100% extension (third unload cycle force) to a third load cycle force at 100% extension (third load cycle force) is at least 0.3: 1.

Detailed Description

A stretchable composite nonwoven sheet is prepared by impregnating a necked nonwoven substrate with a solution containing a solvent and an elastic polymer. The necked nonwoven substrate is impregnated under conditions such that the nonwoven substrate is substantially uniformly impregnated without forming a polymer layer on any surface thereof. After removal of the solvent, a breathable impregnated nonwoven sheet is obtained that has a high unload cycle force in the cross direction relative to the load cycle force (with good grip and flexible stretch) while having a textile-like hand, which is unexpected. Also, the sheet of the present invention is simple to make and is thin compared to conventional multilayer stretch laminates. The sheet material according to the invention typically has a thickness of between 0.25mm and 0.75mm, whereas the thickness of the stretch laminate is typically above 1.3 mm.

As used herein, the term "polymer" generally includes, but is not limited to, homopolymers, copolymers (e.g., block, graft, random and alternating copolymers), terpolymers, etc. and blends and modifications thereof. And unless otherwise specifically limited, the term "polymer" herein shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and random symmetries.

As used herein, the term "polyester" is intended to include polymers wherein at least 85% of the repeating units are condensation products of dicarboxylic acids and dihydric alcohols, having linkages formed when ester units are formed. This includes aromatic, aliphatic, saturated and unsaturated diacids and diols. As used herein, the term "polyester" also includes copolymers (e.g., block, graft, random, and alternating copolymers) and blends and modifications thereof. One of the most common examples of polyesters is polyethylene terephthalate (PET), which is a condensation product of ethylene glycol and terephthalic acid.

As used herein, the term "polyurethane" is intended to include block copolymers resulting from the condensation of difunctional polyols and diisocyanates with difunctional chain extenders, as described in more detail below.

As used herein, the term "polyolefin" refers to any predominantly saturated, open-chain polymeric hydrocarbon material consisting solely of carbon and hydrogen. Typical polyolefins include, but are not limited to, polyethylene, polypropylene, polymethylpentene, and combinations of various ethylene, propylene, and methylpentene monomers.

Herein, the term "polyethylene" includes not only homopolymers of ethylene but also copolymers wherein at least 85% of the recurring units are ethylene units.

Herein, the term "polypropylene" includes not only homopolymers of propylene but also copolymers wherein at least 85% of the recurring units are propylene units.

As used herein, the term "elastic polymer" means any polymer that, when formed into a sheet, fiber or film, is capable of being stretched under a biasing force to at least 160% or more of its relaxed length without a biasing force and capable of recovering to at least 55% or more of its stretched length after removal of the stretching biasing force. For example, a 1cm length sample of material can be stretched to at least 1.6cm and, upon application of a force, stretched to 1.6cm and then the force removed, the sample can return to less than 1.27 cm. Many elastic materials are capable of being stretched by more than 60%, such as 100% or more, of their relaxed length, and most of them are capable of returning to substantially their original relaxed length, such as within 105% of their original relaxed length, when the stretching force is removed.

As used herein, the term "nonwoven fabric" or "nonwoven web" refers to a structure of individual fibers, filaments or threads which are arranged in a random manner to form a flat material without a defined pattern, as opposed to knitted or woven fabrics.

The term "spunbond" filaments as used herein means filaments which are formed by extruding a thermoplastic polymeric material in the molten state from a plurality of capillaries of a spinneret with the diameter of the filaments exiting the capillaries of the spinneret being immediately reduced by drawing, the capillaries being generally very thin and generally circular. Other representative filament cross-sectional shapes, such as elliptical, multi-lobal, etc., may also be used. Spunbond filaments are generally continuous and have an average diameter greater than about 5 microns. Spunbond nonwoven fabrics or webs are formed by randomly placing spunbond filaments on a collecting surface such as a foraminous screen or belt. Spunbond webs are generally bonded by methods well known in the art, such as hot roll calendering or passing the web through a saturated steam chamber under high pressure. The webs can also be joined by thermal point bonding at a plurality of thermal bond points on the surface of the spunbond fabric.

Herein, the term "machine direction" (MD) refers to the direction of production of the nonwoven web. The term "transverse" (XD) generally refers to a direction perpendicular to the longitudinal direction.

As used herein, the term "necking" refers to a process that includes applying a force to a nonwoven, such as applying a force to the nonwoven parallel to the machine direction, causing the nonwoven to stretch in the direction of the applied force while reducing its width perpendicular to the direction of stretching, such as the cross direction, and this amount of reduction can be controlled to a predetermined amount. The direction perpendicular to the direction of the stretching force is referred to herein as the "necking direction". Controlled stretching and necking can be carried out at room temperature or at temperatures above or below room temperature, but is limited by the increase in overall dimension in the direction of stretching until the fabric is stretched to the point of tearing or splitting.

As used herein, the terms "necked nonwoven" and "necked nonwoven substrate" refer to any nonwoven fabric that has been contracted in at least one direction by a process such as, for example, drawing. "neckable nonwoven" refers to a nonwoven that is capable of shrinking in at least one direction during necking. The term "percent necking" is the ratio obtained by measuring the difference between the un-necked dimension and the necked dimension (measured in the necked direction) and then removing the difference with the un-necked dimension, and multiplying the ratio by 100 to determine the percent necking. Necked nonwovens are generally stretchable in the necking direction and the amount of stretch is related (but not linearly related) to the percent necking during necking. The elongation of the necked nonwoven is herein expressed as the percentage of the amount of stretch in the necked direction to which the nonwoven is drawn to its maximum extent, which is the maximum extent to which the nonwoven may be stretched without stretching its individual fibers or without breaking the bonds between any of its fibers or without tearing itself.

As used herein, the term "wet coagulation process" describes a process wherein a nonwoven substrate impregnated with a solution containing an elastomeric polymer dissolved in a solvent is contacted with a coagulating liquid which is not capable of dissolving the elastomeric polymer but which is miscible with the solvent used to form the elastomeric polymer solution. Also, the coagulant should be selected so as not to dissolve the nonwoven substrate. The coagulant may coagulate the polymeric material and may remove the solvent from the coagulant. The condensate may then be removed from the polymer impregnated nonwoven by methods such as air drying or heating.

Neckable nonwoven fabrics suitable for use in the present invention include spunbond webs, bonded combsWire management and hydroentanglement of the wire. The neckable nonwoven is preferably necked in the cross-direction to a percent necking of between about 25% and about 75% by methods well known in the art to obtain a necked nonwoven substrate having a cross-directional stretch of between about 30% and about 300%. The neckable nonwoven fabrics useful in the present invention may be made from a wide variety of thermoplastic polymers, including non-elastomeric polyolefins such as polyethylene, polypropylene, ethylene copolymers, polyamides, polyesters, polystyrene, and poly-4-methylpentene-1. Preferred neckable nonwoven fabrics contain polypropylene, polyester, or polypropylene-polyethylene copolymers. In a preferred embodiment, the neckable nonwoven fabric is a spunbonded polypropylene fabric or a carded thermobonded polypropylene or polyester fabric. Starting neckable nonwoven substrates preferably having a basis weight of about 10g/m²To about 50g/m²In the meantime. The basis weight is relatively low, e.g. at about 10g/m²To about 20g/m²A neckable nonwoven in between is more preferred. Preferably, the nonwoven substrate is permeable to moisture vapor. The neckable nonwoven substrate may be provided with a basis weight generally greater than about 15g/m by necking²The necked nonwoven substrate of (a).

Necked nonwoven fabrics are well known in the art and their preparation typically employs stretching of a neckable nonwoven fabric in the machine direction to obtain a necked nonwoven fabric necked in the cross-machine direction. Examples of necking methods are disclosed, such as U.S. Pat. No.4443513 to Meitner et al (Meitner), U.S. Pat. Nos. 4965122, 4981747 and 5114781 to Morman. U.S. reissue patent 35206(Hassenboehler), to Hassenboehler jr, et al, discloses a preferred necking process. U.S. reissue patent 35206 is a reissue patent to U.S. patent No. 5244482. Nonwoven webs necked according to the process of Hassenboehler are also referred to herein as "reinforcement webs".

Necked nonwovens can be prepared by relatively low cost processes and are preferred over other stretchable nonwovens because of their better cross direction extensibility and the relatively low stretching (loading) forces required to stretch the nonwoven in the cross direction. In addition, necked nonwoven fabrics are generally substantially non-extensible in the machine direction, i.e., have an elongation of less than about 5% when subjected to a biasing force in the machine direction. Stretching in substantially one direction may be necessary in some end uses, as discussed below.

In a preferred embodiment, the necked nonwoven substrate is a consolidated web prepared by the process described by Hassenboehler which comprises passing a relatively low process extensibility bonded thermoplastic nonwoven web through a heating zone, such as an oven, to raise the temperature of the web between the softening temperature and the melting temperature of the polymeric web while stretching the web in the machine direction, whereby plastic deformation of the fibers occurs in the cross direction and consolidation (necking) of the web occurs in the cross direction. During stretching, the web is controlled to travel through the heating zone at a first linear speed and is withdrawn at a second linear speed that exceeds the first linear speed. A preferred ratio of the second speed to the first speed is between about 1.1: 1 to about 2: 1. The starting bonded nonwoven web is a non-elastic neckable nonwoven and should have an elongation at break during heat treatment of less than about 4.0: 1 and greater than about 1.4: 1 when measured in hot stretching at a strain rate of greater than 2500%/min and at a temperature above the softening temperature but at least 10 ° f below the melting temperature of the polymer web. The preferred elongation at break (strain) at room temperature should be between 2% and 40%, more preferably between 5% and 20%, based on test method astm d1117-77, performed on an Instron tensile tester.

The fibers of the raw web may be bonded by fusion between fibers, entanglement of fibers, or thermal bonding such as point bonding. Preferably, the fibers of the neckable nonwoven fabric have a relatively small average fiber diameter, such as less than about 50 microns. The bonding of the spunbond precursor is preferably strong (e.g., high temperature point bonding) to ensure that the filament segments are locally stretched, deformed, and bent without affecting the integrity of the web. In point bonding, the bond points and bond patterns are typically selected such that the bond point area comprises between about 5% and about 25% of the web area. The bond points may be diamond shaped or many other shapes known in the art.

The hot-stretching step may plastically deform the fibers in the cross direction while further reinforcing the web so that the majority of the fibers are generally aligned in the direction of stretching (machine direction). The treated web is consolidated in the cross direction relative to the starting nonwoven because it is stretched in the machine direction and heat treated.

Necked nonwoven substrates having an elongation in the cross direction of at least 30%, preferably at least 50%, can be used to prepare the elastic nonwoven sheets of the present invention. The preferred percent necking of the nonwoven web in the consolidation process is between about 50% and about 75%, more preferably between 60% and 70%, with corresponding elongations of between 100% and 300% and 150% and 250%, respectively.

The basis weight of the necked nonwoven web can be up to 3 times or more the basis weight of the starting neckable nonwoven web. The basis weight of the necked web is preferably about 15g/m²To about 70g/m²More preferably between about 20g/m²To about 70g/m²Between, and most preferably at about 25g/m²To about 70g/m²In the meantime. The basis weight of the necked nonwoven substrate should be selected based on the end use. If used as an elastic liner, the basis weight of the necked nonwoven should preferably be about 30g/m²To 70g/m²If used as an end product in the hygiene industry, such as a diaper, waistband, etc., the basis weight is preferably about 15g/m²To 40g/m²In the meantime. The basis weight of the necked nonwoven substrate is also selected to meet the elasticity requirements of the final impregnated nonwoven. A nonwoven substrate with a higher basis weight allows more elastomeric polymer to impregnate the nonwoven, thereby increasing the unload power of the impregnated nonwoven sheet.

In the necking process described by Hassenboehler, it should be preferred to use a nonwoven having a relatively low basis weight to prepare the material made according to the present invention. These factors combine to produce a stretchable nonwoven that requires relatively low forces (load) to stretch the material, while providing relatively high resilience (unload) when relaxed, after impregnation with the elastic polymer. This feature is preferred for the envisaged end use of the material. The relationship between unload force and load force is related to elastic nonwoven hysteresis. For the preferred products of the invention, the cross direction elongation is at least 150%, and the ratio of the unload force at 100% extension to the load force at 100% extension is at least 0.3: 1, more preferably at least 0.45: 1 after three stretches of the impregnated nonwoven to 140% (relaxed between two stretches).

Elastomeric polymers useful in the present invention include polyurethanes, styrene-butadiene block copolymers, and polyether-ester block copolymers. In a preferred embodiment, the elastomeric polymer is a polyurethane.

The elastomeric polyurethanes used in the present invention can be prepared by first reacting a polymeric glycol with a diisocyanate to form a blocked glycol, dissolving the blocked glycol in a suitable solvent, and finally reacting the blocked glycol with a difunctional chain extender containing active hydrogen atoms. Such polyurethanes are referred to as "blocks" because they are composed of "rigid" urethane and urea blocks derived from diisocyanates and chain extenders, and "flexible" blocks derived primarily from polymeric glycols. Suitable solvents for preparing the polymer solution are amide solvents such as dimethylacetamide ("DMAc"), dimethylformamide ("DMF") and N-methylpyrrolidone, although other solvents such as dimethyl sulfoxide and tetramethylurea may also be used.

Polymeric glycols used in the preparation of the elastomeric polyurethanes include polyether diols, polyester diols, polycarbonate diols, and copolymers thereof. Examples of the ethylene glycol-based substance include polyethylene glycol, polybutylene glycol, butadiene-2-methyl-butylene glycol, ethylene-adipate-co-glycol, poly (2, 2-dimethyl-1, 3-dodecanoic acid propyl ester) glycol, poly (1, 5-pentyl carbonate) glycol and poly (1, 6-hexyl carbonate) glycol.

Useful diisocyanates include 1-isocyanato-4- [ (4-phenylisocyanato) methyl ] benzene, 1-isocyanato-2- [ (4-phenylisocyanato) methyl ] benzene, isophorone diisocyanate, 1, 6-hexane diisocyanate and 2, 4-tolylene diisocyanate.

The chain extender may be a diol or a diamine. Useful diols include ethylene glycol, 1, 3-propanediol, 1, 4-butanediol, and mixtures thereof. Polyurethanes can be formed using diols as chain extenders. Useful diamines include ethylenediamine, 1, 2-propylenediamine, 2-methyl-1, 5-pentylenediamine, 1, 3-pentylenediamine, 1, 4-cyclohexyldiamine, 1, 3-cyclohexyldiamine, and mixtures thereof. In this case, the resulting polymer is a polyurethane-urea (a type of polyurethane). If polyether diol and diamine chain extender are used, the resulting polymer is polyether urethane-urea; if a polyester diol is combined with a diamine chain extender, a polyester urethane-urea is formed. The addition of monofunctional amine chain terminators allows control of the molecular weight of the polymer, such as diethylamine, butylamine, cyclohexylamine and similar amines. In a preferred embodiment, the elastomeric polymer is a diamine-extended polyurethane-based elastomer.

Suitable solvents for preparing the elastomeric polymer solution include dimethylacetamide, dimethylformamide, and N-methyl-pyrrolidone. The viscosity of the elastomeric polymer solution is directly related to the concentration of the polymeric material in the solution, and thus the viscosity of the solution can affect both the degree of penetration of the polymer into the necked nonwoven fabric and the amount of polymer deposited therein. If the solution viscosity is too low, the amount of elastomer deposited on the necked nonwoven substrate will be insufficient, resulting in a reduced unload force. If the solution viscosity is too high, the amount of solution that penetrates into the nonwoven substrate may be reduced, resulting in incomplete or uneven impregnation of the polymer into the nonwoven, or formation of a polymer layer on the surface of the necked nonwoven. The viscosity of the elastomeric polymer solution used to impregnate the necked nonwoven substrate is preferably between about 1000 and 300000 centipoise ("cPS"), more preferably between 10000 and 40000 cPS. The solution may contain from about 5 wt% to about 20 wt% polymer.

It is highly desirable that the necked nonwoven substrate be capable of absorbing the polymer solution and that the polymer solution be substantially completely and uniformly impregnated into the nonwoven substrate. The necked nonwoven substrate should therefore not have any coating or other similar treatment that would prevent the necked nonwoven from absorbing the polymer solution. The elastomeric polymer solution and/or the nonwoven fabric may contain a surfactant to facilitate impregnation of the web with the polymer solution. Suitable surfactants include non-ionic wetting agents, such as polymeric surfactants.

Minor amounts of additives such as pigments, antioxidants, ultraviolet light stabilizers and lubricants may be added to the elastomeric polymer solution provided that the additives do not detract from the benefits of the invention.

The elastic polymer solution may contain dispersed, very short fine fibers, such as cellulose fibers from wood pulp, cotton pollen, or other synthetic or natural fibers, less than 0.10 inch (2.5mm) in length, preferably less than 0.5 mm. The fibers are preferably small enough to allow complete penetration into the nonwoven fabric during the impregnation step. The amount of staple fibers added to the elastomeric polymer solution should be sufficient to deposit about 3 wt% to 12 wt% (based on the total weight of the nonwoven and elastomeric polymer) of staple fibers into the impregnated nonwoven. The amount of short fiber added to the elastomeric polymer solution is preferably between about 10 wt.% and about 30 wt.%, more preferably between about 10 wt.% and about 20 wt.%, based on the total weight of short fiber, elastomeric polymer, and solvent. The elastomeric polymer solutions used to impregnate the necked nonwoven fabrics of the present invention contain powdered cellulose, and therefore the nonwoven sheets produced therefrom have a softer hand than those produced without the staple fibers in the impregnating solution. An example of a fine fiber particulate material suitable for use in the polymer solution is powdered cellulose having the trade name "Arbocel 30", available from J Rettenmaier, scout, michigan.

Any suitable method of coating or impregnating the elastomeric polymer solution onto the necked nonwoven substrate may be used so long as the fabric is uniformly impregnated and the coating is not concentrated on one or the other surface of the necked nonwoven substrate. It should also be noted that while a coating process may be used when treating the necked nonwoven substrate with the elastomeric polymer solution, the nature of the solution and nonwoven, as well as the conditions of the coating process, should be selected to allow the polymer solution to completely wet into or enter the necked nonwoven substrate, or to be completely absorbed by the nonwoven substrate, thereby ensuring that no polymer layer is formed on any surface of the nonwoven substrate. Generally, the amount of polymer solution used in the coating can be controlled by the coating equipment at a specified distance above the necked nonwoven fabric. The solution may also be mechanically pressed onto the necked nonwoven substrate. In the processing method of the present invention, a roll mill, a platen press, a scraper, a doctor blade and the like can be used as the coating apparatus. Spraying the solution onto the necked nonwoven substrate is also an effective method so long as the elastomeric polymer solution is substantially completely and uniformly impregnated into the nonwoven substrate. Better penetration effect can be obtained by adjusting the spraying force. The necked nonwoven substrate may also be impregnated with the elastomeric polymer solution by a "dip-press" process, as is well known in the art, wherein the web is dipped or submerged in a tank containing the elastomeric polymer solution and then pressed by a device such as a press roll to remove excess polymer solution. This method is preferred for reducing the difference between the two surfaces of the stretchable composite nonwoven sheet.

Impregnating the necked nonwoven substrate with a sufficient amount of polymer solution can achieve the desired unload/load force ratio of the final impregnated nonwoven sheet. The necked nonwoven substrate is preferably impregnated with a sufficient amount of the polymer solution to deposit therein from about 15 wt.% to about 55 wt.% of the elastic polymer, more preferably an amount of the elastic polymer between about 30 wt.% and about 50 wt.%, based on the total weight of the elastic polymer and the nonwoven substrate. If the amount of elastomer is too low, the ratio of unload force to load force will be low, and if the amount of elastomer is too high, the surface of the sheet will exhibit an undesirable sticky hand. The polymer content of the impregnated sheet can be tailored by adjusting the solution concentration and/or the amount of solution impregnated into the necked nonwoven fabric. If observed, a good balance between handle and unload/load ratio was achieved during solution impregnation by increasing the nip between the rolls, using a lower polymer concentration in the solution, while maintaining a similar elastomer content in the impregnated sheet.

Once the nonwoven substrate is impregnated with the solution containing the solvent and the elastomeric polymer, the solvent is removed. The solvent is removed by wet coagulation and the condensate is subsequently removed. The wet coagulation process produces products which are surprisingly soft and have a better cloth-like hand than the heat-dried products. Wet coagulation processes are well known in the art and are commonly used for the production of artificial leather. Water is preferred as the coagulation liquid because it is easy to handle and low cost. Other suitable condensates include methanol, ethanol, isopropanol, acetone, or methyl ethyl ketone. The addition of solvents such as dimethylformamide, dimethylacetamide or N-methyl-pyrrolidone, which dissolve the elastomeric polymer, or other additives such as surfactants to the coagulation liquid can alter the rate of coagulation. In addition, the coagulation rate can be varied by controlling the temperature of the coagulation bath. The lower the rate of coagulation, the more attractive the hand of the impregnated nonwoven after solvent removal.

In the impregnated nonwoven sheets of the present invention, the elastic polymer phase, which is uniformly distributed throughout the necked nonwoven substrate, is breathable. Preferably the impregnated nonwoven sheet is also moisture vapor permeable.

The hand of the impregnated nonwoven sheet can be improved by sanding or napping, which increases the fiber content of the surface of the impregnated sheet, making the hand softer. Napping involves passing the fabric through a rotating roll containing small metal dots to effectively brush the fabric, increasing the surface fibers. In sanding, the metal brush is replaced by a rotating roller coated with sandpaper. Preferably, both surfaces of the impregnated fabric are sanded or napped. For example, the fabric may be sanded with sandpaper having a grit size of 80-200.

The stretch-impregnable nonwoven sheet of the invention preferably has a basis weight of about 40g/m²To about 100g/m²In the meantime. They are particularly suitable for use on the side of a belt or diaper and other disposable personal hygiene garments. Diapers are assembled commercially on high-speed long production lines, wherein the individual components of the diaper are preferably added in the machine direction to avoid reducing the assembly speed. This is true for elastic materials, which are often usedThe elastic material stretches before insertion. Diapers generally comprise about 20 or more individual components which must be mounted in a precise location on the diaper during high speed production. This is very easy to achieve if the direction of feed of the components (tape, sheet, fibre, etc.) is the same as the direction of movement of the diaper. To add components in the transverse direction (e.g., waistband), a material that stretches itself in the transverse direction should be preferred so that it can be taped in the longitudinal direction into the diaper manufacturing process. For example, a tape may be a 7 inch wide, but only 1 inch long, sheet cut from a sheet for adhering to a diaper or other disposable undergarment. In this process, the diaper component entering the process is preferably substantially unstretched in the machine direction to simplify the feeding process. The stretchable nonwoven sheet of the present invention has substantially no stretch in the machine direction and a high degree of resilient stretch in the cross direction, making it particularly suitable for use in the above-described process.

Test method

Basis weight

A sample of about 1.0 inch by 8.0 inches (2.54cm by 20.32cm) rectangular nonwoven sheet was carefully placed in a relaxed state so that it did not contain any wrinkles or folds. The length and width of the sample was measured accurately to millimeters and the weight of the sample was measured accurately to 0.1 milligrams. The weight is divided by the calculated area to give an accuracy of 0.1 g in g/m²。

Load and unload force analysis

The analysis was performed on an Instron model 5565 equipped with Merlin (Merlin) data acquisition software system. Both the merlin system and its device hardware are available from instron corporation (blontry, ma). A sample of nonwoven sheet material 1 + -0.05 inches (2.54 + -0.13 cm) wide by about 8 inches (20.32cm) long was secured to the jaws of an Instron machine having a sample length setting of 3.00 inches (7.62 cm). The samples were prepared so that their length direction was coincident with the cross direction of the nonwoven. The sample was stretched at a rate of 6 inches/minute (15.24cm/min) to an elongation of 140% and then returned to the original length, and this process was repeated two more times, with the third cycle recording the force applied (load force) by the material to the stretch cycle at 50%, 100%, 135% elongation, and similarly, the force applied (unload force) by the material to the third relaxation cycle at the same elongation, respectively. The results are expressed as third cycle load and unload forces in g at the corresponding percent elongation.

Percent elongation analysis

Two markings were made with a pen on a 1.0 inch (2.54cm) wide, about 8.0 inches (20.32cm) long, relaxed, nonwoven strip fabric without any crimp or fold, the two markings being spaced 4.0 inches (10.2cm) apart so that the two markings were approximately equidistant from the two ends of the fabric. The two ends of the fabric were then gripped with the thumb and forefinger of the two hands, respectively, and the sample was stretched sufficiently, but not so much as to tear or suffer any similar mechanical damage. The point at maximum elongation is clearly visible to the person performing the test because the resistance exerted by the fabric to stretching is significantly increased. The length between two mark points on the nonwoven was then measured and the percent elongation was calculated using the following formula, where the original length was 10.2 cm.

Percent elongation { (length after stretching-original length)/original length } × 100%

To measure the percent elongation in the neck-down direction, the fabric is cut to length to match the cross-machine direction (neck-down direction).

Examples

A30 inch (76.2cm) wide, 15g/m heavy piece was manufactured by Avgol corporation of Israel (Avgol Nonwovens)²The wettable spunbond polypropylene nonwoven was fed through a nip roll at 89 feet per minute (27m/min) and then passed through a 72 inch (1.83m) long, 290F (143℃) forced air oven to a take-up roll through a second nip roll at 115 feet per minute (35 m/min). In the above procedure, a 30 inch (76.2cm) wide nonwoven was uniformly and smoothly consolidated (necked) in the cross direction to a 10 inch (25.4cm) width. The application of a minimum force makes it wide in the transverse directionThe degree returned to the original 30 inches (76.2 cm). The necked nonwoven had essentially zero elongation in the machine direction and a basis weight of 32.0g/m²。

A solution of Dimethylacetamide (DMAC) containing 20 wt.% polyurethane-urea was coated on one surface of the necked nonwoven with a 0.015 inch (0.38mm) doctor blade. The polymer was made from poly (butylether) glycol with a molecular weight of 1800, 1-isocyanato-4- [ (4-isocyanatophenyl) methyl ] benzene (diisocyanate to ethylene glycol molar ratio of 1.69), a chain extender (ethylenediamine to 2-methyl-1, 5-pentanediamine molar ratio of 9: 1) and diethylamine. The following additives were also added: 0.5 wt.% of a polymer of bis (4-isocyanatocyclohexyl) methane and (3-tert-butyl-3-azido-1, 5-pentanediol) (Methacrol ® 2462B, a registered trademark of e.i. du Pont de Nemours and Company), 0.3 wt.% of titanium dioxide, 0.6 wt.% of silicone oil, 1.4 wt.% of 2, 4, 6-tris (2, 6-dimethyl-4-tert-butyl-3-hydroxybenzyl) isocyanurate (Cyanox ® 1790, a registered trademark of cytec industries), and 4 wt.% of a mixture of huntite and magnesite. (all percentages are based on the weight of the polyurethaneurea). The polyurethane-urea-DMAC solution was able to completely wet the nonwoven.

The coated nonwoven was hung substantially vertically in air for about 1 minute to allow complete penetration of the polymer solution into the nonwoven and then immersed in a water bath containing 40% DMAC (vol%) at 70F (21℃). After one minute, the impregnated fabric was transferred continuously to a DMAC/water solution with 30%, 20% and 10% by volume respectively, transferred once per minute and finally allowed to soak in a 100% water bath for two minutes. The impregnated fabric was dried in air at room temperature.

The resulting impregnated nonwoven sheet had the same (dry, textile-like) hand and texture on both sides. Micrographs of the impregnated nonwoven sheet in the cross direction showed that the material had a uniform composite structure throughout the thickness and was substantially free of continuous polyurethane regions on any surface.

The nonwoven sheet was then lightly sanded with sandpaper having a grit size of 220. The soft feel of the resulting material was more pronounced and visual observation showed numerous individual short fibers protruding from the surface, while a completely smooth surface before sanding had no fibers protruding. Surprisingly, this treatment can make the material feel softer without significantly compromising the visual aesthetic or elastic properties of the sheet.

The resulting impregnated nonwoven sheet had a basis weight of 71.4g/m²It shows that the content of polyurethane-urea is 39.4g/m²Or about 55 wt.% of elastomeric polymer.

Hand stretching of the material in the transverse direction indicated an elongation of between about 160% and about 180%. The load and unload force analysis results are shown in the following table:

third cyclic load force

Elongation percentage%	Loading force (gram)
		50	67.3
100	211.2
		135	409.7

Third cycle unload force

Elongation percentage%	Unload power (gram)
		50	22.7
100	114.7
		135	340.7

Comparison of the data in the tables shows that the ratio of unload force to load force at 100% extension is 0.54.

Claims (10)

1. A method of making a stretchable nonwoven sheet comprising the steps of:

providing a necked nonwoven substrate having a thickness, first and second outer surfaces, a machine direction and a cross-machine direction, the necked nonwoven substrate having an elongation in the cross-machine direction of at least 30%;

substantially uniformly impregnating a necked nonwoven substrate with a solution comprising an elastomeric polymer dissolved in a solvent; and

the solvent is removed from the impregnated nonwoven substrate using a wet coagulation process, thereby depositing the elastomeric polymer substantially uniformly throughout the thickness of the nonwoven substrate without forming a substantially continuous elastomeric polymer layer on the first or second outer surface of the nonwoven substrate.

2. The method of claim 1 wherein the necked nonwoven substrate has an elongation in the machine direction of less than 5% and an elongation in the cross direction of between 100% and 300%.

3. The method of claim 1, further comprising the step of sanding or napping at least one outer surface of the nonwoven sheet after removing the solvent to increase the fibers of the surface of the sheet.

4. The method according to claim 1, wherein the amount of elastomeric polymer deposited on the substrate is between 15 wt.% and 55 wt.%, based on the total weight of the substrate and the elastomeric polymer.

5. A stretchable nonwoven sheet comprising a necked nonwoven substrate substantially uniformly impregnated with an elastomeric polymer, the stretchable nonwoven sheet having a ratio of a third unload cycle force at 100% extension to a third load cycle force at 100% extension of at least 0.3: 1 after being stretched three times to 140% in the neck direction.

6. The stretchable nonwoven sheet according to claim 5 wherein the ratio of unload force to load force is greater than 0.45: 1.

7. The stretchable nonwoven sheet according to claim 5 wherein the sheet comprises from 30 wt.% to 50 wt.% of the elastic polymer, based on the total weight of the elastic polymer and the nonwoven substrate.

8. The stretchable nonwoven sheet according to claim 5 wherein the nonwoven sheet has a basis weight of 40g/m²To 100g/m²In the meantime.

9. A personal hygiene garment comprising the stretchable nonwoven sheet of claim 5.

10. The personal hygiene garment of claim 9, wherein the garment comprises a diaper.