Hydrophobic nonwoven fabric with acrylic latex

Abstract

 A nonwoven fabric comprising hydrophobic polyester fibers bonded with a water-insoluble, hydrophobic emulsion polymer of 50 to 95 parts by weight of a C, to C₂ alkyl acrylate; 5 to 30 parts of an ethylenically unsaturated monomer selected from styrene, methyl methacrylate, and a-methyl styrene; and 0.1 to 5 parts of a monoethylenically unsaturated carboxylic acid containing 3 to 6 carbon atoms.

Description

Background of the Invention

[0001] Environmental and health problems associated with chemicals is a major concern for all. This problem takes on added significance if the chemical comes in contact with an infant or an ill person. Thus, increasing attention is being given to the materials used in sanitary and health care fabric products such as diapers, sanitary napkins, hospital drapes, disposable sheets, bed pads, etc.

[0002] Nonwoven fabrics for such applications are required to have good strength when wet by water or body fluid, such as urine. Currently, this strength in the nonwoven fabrics is achieved by binding the fibers with a latex. Most of the latexes now in use are crosslinked to improve their wet tensile strength as well as abrasion resistance. Generally, crosslinking in the latex is achieved by the use of a reactive monomer such as N-methylol acrylamide, N(iso-butoxy methyl) acrylamide, and the like. However, such reactive monomers undergo condensation reaction that evolves formaldehyde. Sometimes, crosslinking is achieved by post-addition of various amino-plasts, such as condensates of melamine and formaldehyde. Since formaldehyde is a suspected carcinogen, its use in sensitive applications, described above, should be avoided.

[0003] European patent application 0012033 discloses a nonwoven fabric comprising fibers bonded with an acrylic binder that is a polymer of 1 to 8% of a monoethylenically unsaturated carboxylic acid, 50 to 75% of a C₄ to C₈ alkyl acrylate, and 20 to 49% of methyl methacrylate, styrene, or a-methyl styrene. On page 10, it is disclosed that to be useful as a diaper coverstock, the wash durability of a nonwoven fabric bonded with the acrylic binder should be such as to withstand more than 1Q washes at 43°C or about one-half at 60°C. On page 22, latex E21 is disclosed of 70% ethyl acrylate and 30% styrene that survived not a single wash at 60°C; on page 31a, latex E43 of 91% ethyl acrylate, 5% styrene and 4% acrylic acid did not survive a single wash either at 60°C or at 43°C; on page 34, latex E43 of 91% ethyl acrylate, 5% styrene and 4% acrylic acid did not survive a single wash at 60°C; and again on page 34, latex E45 of 66% ethyl acrylate, 32.7% methyl methacrylate and 1.3% methacrylic acid survived 4 washes at 60°C.

Summary of the Invention

[0004] This invention relates to a hydrophobic nonwoven fabric bonded with a water-insoluble, hydrophobic binder selected from emulsion polymers of 50 to 95 parts by weight of a C₁ to C₂ alkyl acrylate and 5 to 30 parts by weight of styrene, methyl methacrylate or a-methyl styrene, optionally containing up to about 5 parts by weight of a monoethylenically unsaturated carboxylic acid. The glass transition temperature (Tg) of the binder is in the range of -10°C to +25°C.

Detailed Description of the Invention

[0005] The binders used to bond fibers of a nonwoven fabric described herein are latexes that are prepared by emulsion polymerization of 50 to 95 parts by weight of methyl and/or ethyl acrylate with 5 to 35 parts by weight of styrene, a-methyl styrene, or methyl methacrylate. Preferably, amount of the acrylate is 75 to 90 parts and that of styrene, or its equivalent, 15 to 30 parts. Up to about 5 parts by weight, such as 0.1 to 5 parts, preferably 0.5 to 2 parts, of a monooethylenically unsaturated carboxylic acid containing 3 to 6 carbon atoms can also be copolymerized with the other monomers. Examples of such carboxylic acids are acrylic, methacrylic, and itaconic acids. Furthermore, a small amount of a multifunctional monomer can also be included to reduce the tack of the saturated nonwoven fabric. Amount of the multifunctional monomer can vary up to about 5 parts by weight, such as 0.1 to 5 parts, preferably 0.5 to 2 parts. This permits manufacture of the diapers without encountering blocking of the nonwoven fabric due to the cross-linking taking place as a result of the presence of a multifunctional monomer. Suitable examples of the multifunctional monomers include trimethylol propane triacrylate and trimethacrylate, hexanediol dimethacrylate, pentaerylhritol diacrylate, and tetramethylene glycol diacrylate.

[0006] In the preparation of the latices described herein, the aqueous medium can contain suitable emulsifiers or it can be emulsifier-free. When emulsifiers are used to prepare the latices of this invention, the usual types of anionic and non-ionic emulsifiers can be employed. Suitable anionic emulsifiers include alkali metal or ammonium salts of the sulfates of alcohols containing 8 to 18 carbon atoms such as sodium lauryl sulfate, alkali metal and ammonium salts of sulfonated petroleum and paraffin oils, sodium salts of sulfonic acids, aralkyl sulfonates, alkali metal and ammonium salts of sulfonated dicarboxylic acid esters, and the like. Nonionic emulsifiers, such as octyl or nonylphenyl polyethoxyethanol, can also be used. Latices of excellent stability can be prepared with emulsifiers selected from alkali metal and ammonium salts of aromatic sulfonic acids, aralkyl sulfonates, long chain alkyl sulfonates, and poly(oxyalkylene) sulfonates.

[0007] Amount of emulsifiers can vary up to about 5 parts, such as 0.1 to 5 parts by weight per 100 parts by weight of the monomers, and exellent results can be obtained with 0.01 to 1 part of emulsifier. The latices described herein are more preferably prepared using very low to moderate level of emulsifier, such as 0.005 to 1 part by weight, most favored being in the range of 0.03 to 0.5 dry parts per 100 parts of monomer. The lower level of emulsifier in the latex is necessary to achieve the desired absorbency and dryness of the diaper coversheet when saturated with latex. The emulsifier can be added at the outset of the polymerization or it can be added incrementally throughout the run. Typically, a substantial amount of the emulsifier is added at the outset of the polymerization and the remainder is added incrementally to the reactor as the monomers are proportioned.

[0008] The polymerization can be conducted at temperatures of about 20°C to about 100°C, typically about 80°C, in the presence of a compound capable of initiating polymerization. Commonly used free radical initiators include the various peroxygen compounds such as persulfates, benzoyl peroxide, t-butyl hydroperoxide, and cumene hydroperoxide; and azo compounds such as azodiisobutyronitrile and dimethylazodiisobutyrate. Particularly useful initiators are the water-soluble peroxygen compunds such as hydrogen peroxide and the sodium, potassium and ammonium persulfates used by themselves or in an activated redox system. Typical redox systems include alkali metal persulfates in combination with a reducing substance such as polyhydroxyphenols and oxidizable sulfur compounds, a reducing sugar, dimethylaminopropionitrile, a diazomercaptan compound, and a water-soluble ferrous sulfate compound. Polymer latices with excellent stability can be obtained using alkali metal and ammonium persulfate initiators. The amount of initiator used will generally be in the range of 0.1 to 3% by weight, based on the weight of the monomers, preferably between 0.2 to 1%. The initiator can be charged at the outset of the polymerization, however, incremental addition of the initiator throughout polymerization can also be employed and is often advantageous.

[0009] Typical polymerizations for the preparation of the latices described herein are conducted by charging the reactor with appropriate amount of water and electrolyte, if any is employed, a portion of the initiator sufficient to initiate polymerization. The reactor is then evacuated, heated to the initiation temperature and charged with a portion of the monomer premix which has been previously prepared by mixing water, emulsifier, the monomers and polymerization modifiers, if any are employed. After the initial monomer charge has been allowed to react for a period of time, the proportioning of the remaining monomer premix is begun, the rate of proportioning being varied depending on the polymerization temperature, the particular initiator employed and the amount of the monomer(s) being polymerized. After all the monomer premix has been charged, the final addition of initiator is made and the reactor and-the latex heated with agitation for a length of time necessary to achieve the desired conversion. The pH of the latex is generally in the range of about 3 to 10.

[0010] In the latex, the particle size may be in the range of about 1600A° to 1700A°. A generally satisfactory particle size may be, however, from about 500 to about. 5000A. The total solids of the latices may be varied up to about 70% and may relate to the fluidity wanted in the composition. Generally, it is desired to use a latex containing 45 to 55% solids.

[0011] Latexes suitable for the use described herein must be film formers. This is easily determined by placing a latex in an oven and drying it to see whether a film or a powder resin is formed. Film forming latexes from a powder resin type latex by the above test can be made by uniformly blending with the latex about 10 to 100 parts by weight of one or more plasticizers per 100 parts by weight of the resin, although hard resins requiring plasticizers are not used and, therefore, use of plasticizers is obviated. The useful plasticizers may be described as the alkyl and alkoxyalkyl esters of dicarboxylic acids or the esters of a polyhydric alcohol and a monobasic acid. As examples of such materials, there may be named dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, di(2-ethyl hexyl) dilauryl phthalate, glyceryl stearate, and the like. The preferred plasticizers are the liquid diesters of aliphatic alcohols having from 4 to 20 carbon atoms and dibasic carboxylic acids having from 6 to 14 carbon atoms.

[0012] The latexes described herein can be compounded with, or have mixed therein, other known ingredients such as emulsifiers, curing agents, fillers, plasticizers, antioxidants or stabilizers, antifoaming agents, dyeing adjuvants, pigments, or other compounding aids. Furthermore, thickeners or bodying agents may be added to the polymer latices so as to control the viscosity of the latexes and thereby achieve the proper flow properties for the particular application desired.

[0013] A latex of the present invention can be applied to the web or mat of fibers in any suitable fashion such as by spraying, dipping, roll-transfer, or the like. Application of the latex to the fibers is preferably made at room temperature to facilitate cleaning of the associated apparatus. The solids concentration of the latex can be in the range of 5% to 60% by weight, and preferably from 5% to 25% when applied by dipping. When applied by roll-transfer, solids concentration of the latex is generally about 50% whereas with the spraying technique, it can range widely.

[0014] The proportion of the latex polymer that is applied to the web or mat is such as to provide 10 to 100%, preferably 25% to 40% by weight of the polymer, based on the total weight of the polymer and fibers. After application of the latex to the fibrous web, the impregnated or saturated web is dried either at room temperature or at elevated temperature. The web is subjected, either after completion of the drying or as the final step of the drying stage itself, to a baking or curing operation which may be effected at a temperature of about 21Q° to about 750°F for a period which may range from about one-half hour at the lower temperatures to as low as five seconds at the upper temperatures. The conditions of baking and curing are controlled so that no appreciable deterioration or degradation of the fibers or polymer occurs. Preferably, the curing is effected at a temperature of 25.0" to 325°F for a period of 2 to 10 minutes.

[0015] The fibers that are bonded with the latices described herein are in the form of nonwoven mats or webs in which they are ordered or are randomly distributed. The web can be formed by carding when the fibers are of such a character, by virtue of length and flexability, as to be amendable to the carding operation. The fibers need not be exclusively hydrophobic and may comprise natural textile fibers such as jute, sisal, ramie, hemp and cotton, as well as many of the artificial organic textile fibers including rayon, those of cellulose esters such as cellulose acetate, vinyl resin fibers such as those of polyvinyl chloride and copolymers thereof, polyacrylonitrile and copolymers thereof, polymers and copolymers of olefins such as ethylene and propylene, condensation polymers such as polyimides or nylon types, and the like. The fibers used can be those of a single composition or mixtures of fibers in a given web.

[0016] The preferred fibers are hydrophobic or a blend of fibers at least 50% by weight by which are hydropholic fibers, such as those of polyester, especially poly(ethylene terephthalate). Especially preferred are 100% polyester fibers.

[0017] The length of fibers is also important in producing fabrics of the present invention. The length should be a minimum of about 2 cm in order to produce uniform webs in the carding operation and it is preferred that the fiber length be between about 3 cm to about 4 cm although fibers 5 cm long and longer are useful particularly for wet laid webs. The denier of the fibers should be about 1 to 3, preferably about 1-1/2.

[0018] The hydropholic fibers of this invention are fibers that exhibit very little uptake of water upon water immersion or exposure to high humidity. This property can be measured by adsorption of water by a polymer film having a composition corresponding to that of the fibers or by the moisture regain of dehydrated fibers when held in an atmosphere of fixed relative humidity. Hydrophobic fibers are fibers having a moisture regain of less than 2.5%, preferably less than 1% of the fiber weight, measured at 70°F and 65 relative humidity. For purposes of comparison, moisture regain of poly(ethylene terephthalate) is 0.4%, that of nylon 6 is 2.8 to 5.0%, that of cellulose acetate is 2.5 to 6.5%, that of viscose rayon is 11 to 13%, that of acrylic is 1 to 2.5%, for polyethylene it is negligible, and for polypropylene it is 0.1%.

[0019] Among the myriad of applications that can be listed for the binders described herein, the principal group relates to sanitary products particularly table napkins, bibs, tableclothes, sanitary napkins, disposable diapers., disposable sheets, surgical dressings and compresses. These products have a desirable degree of water resistance, as indicated by their wet strength, but at the same time maintain a level of water permeability so as to permit transport of body fluids, such as prespiration and urine, through the coverstock into the underlying absorptive pad.

[0020] One of the principal uses of the fabric of this invention is as diaper coverstock. Diaper coverstock is a moisture-pervious facing layer which permits urine initially impinged thereon to pass into the internal absorbent core of the diaper. The facing layer, being in contact with the body of a baby, must be non-irritating and have an acceptable level of abrasion resistance at body temperature. Diaper coverstock must meet three principal tests, namely, tensile strength, strike through, and surface wetness. One diaper manufacturer requires a minimum of 170 g/in dry and 155 g/in wet tensile strength in across machine direction, a strike through of 7.0 seconds maximum, and surface dryness of 0.5g maximum. Strike through is a measure of the speed of a urine solution passage through a diaper coverstock disposed on an absorbent layer. This test measures how fast it takes for 5 ml of urine solution to pass through a diaper coverstock of certain area. In measuring surface dryness, i.e., rewet, additional 15 ml of urine solution is passed through the assembly that consists of a diaper coverstock on top with an absorbent layer below. A dry absorbent pad is then placed on the assembly and a weight of about 8 pounds is placed thereover. The weight of solution absorbed by the pad in a specified time period in grams is the measure of surface dryness.

[0021] It should be apparent that it is most desirable to have as low a strike through as possible in order to quickly remove urine in contact with baby's skin into the absorbent pad disposed between the inner coverstock and the outer water- impervious sheet of plastic film. However, as strike through is reduced, surface dryness increases. This condition is consonant with the wicking effect of the coverstock that allows the urine to pass through in one direction and then in the opposite direction. It should be apparent that as the passage of urine away from baby's skin is reduced, i.e., strike through is reduced, the increase in surface dryness is a direct reaction and must increase. The bonding latex is designed in order to strike a balance between these two properties. The latex described therein is of a hydrophobic nature that provides the desired balance between strike through and surface wetness properties. Since comonomers, such as acrylamide are hydrophilic, their presence in the binder copolymer can impart a hydrophilic character, depending on amount used. Presence of emulsifiars in the preparation of the copolymer binders also has a similar effect. These compounds can be used to advantage to obtain the desired characteristics in the diaper coverstock.

[0022] The following examples are presented for the purpose of illustrating the invention disclosed herein in a greater detail. The examples are not, however, to be construed to limit the invention herein in any manner, the scope of which is defined by the appended claims.

Example 1

[0023] This example illustrates preparation of a latex by polymerizing 82.5 parts by weight ethyl acrylate, 16.5 parts by weight styrene, 1.0 part by weight methacrylic acid, and 0.35 part by weight sodium lauryl sulfate emulsifier. This latex had a Tg of +5°C.

[0024] This latex was prepared by adding to a reactor, with agitation, 457.5 pounds demineralized water, 340.5 grams of a 30% sodium lauryl sulfate solution, and then heating the contents to 80°C.

[0025] In a separate vessel, a premix is prepared of 230 pounds of demineralized water, 7.5 pounds of a 30% sodium lauryl sulfate solution, 619 pounds of ethyl acrylate, 124 pounds of styrene, and 7.5 pounds methacrylic acid.. While heating the reactor, 70 pounds of the premix was added to the reactor with agitation.- When the temperature in the reactor reaches 80°C, 4.5 pounds of sodium persulfate predissolved in 15 pounds of water was added to commence the polymerization reaction. When the reaction slowed down, the remaining premix was metered in over 3 hours while controlling reaction temperature at 85°C following which, 136 grams of sodium persulfate predissolved in 11 pounds of water was admixed. The temperature of the reactor is maintained for addition 2 hours and then, the latex was cooled to room temperature. The latex had the following properties:

Example 2

[0026] A latex was prepared herein by polymerizing 76.0 parts by weight ethyl acrylate, 22.0 parts styrene, 1.0 part methacrylic acid and 1.0 part trimethylol propane trimethacrylate multifunctional monomer. Procedure for preparation of this latex was the same as in Example 1 except that the multifunctional monomer was added with the premix following addition of methacrylic acid.

Example 3

[0027] This example demonstrates impregnation of poly(ethylene terephthalate) webs at different pick-up rates of latex and subsequent testing for wet and dry tensile strength, strike through, and surface wetness using a standard urine solution of about 45 dynes/cm surface tension that is an aqueous solution of sodium chloride in presence of a small amount of a nonionic emulsifer.

[0028] The polyester webs used herein were carded polyester nonwoven webs weighing about 0.5 oz/yd². The webs were impregnated with the latexes of Examples 1 and 2 at solids concentration of 4, 6 and 8%. The latex was adjusted before use to a pH of 8 to 10 with ammonium hydroxide. The pick-up of the latex was varied from 20% to about 55% to test its effect on certain properties. The following results were obtained on the impregnated webs cured at 280°C for 3 minutes:

[0029] 

Claims

1. A nonwoven fabric comprising fibers at least 50% of which are hydrophobic fibers bonded together by a binder comprising a water-insoluble, hydrophobic polymer of unsaturated monomers comprising 50 to 95 parts by weight of a C₁ to C₂ alkyl acrylate and 5 to 35 parts by weight of an ethylenically unsaturated monomer selected from styrene, methyl methacrylate, a-methyl styrene, and mixtures thereof.

2. Fabric of claim 1 wherein Tg of said binder is in the range of -10°C to +25°C and amount of said binder is about 10 to 100% by weight of the dry fibers.

3. Fabric of claim 1 wherein said unsaturated monomers include 0.1 to 5 parts by weight of a multifunctional monomer to reduce tack of said binder and 0.1 to 5 parts by weight of a monoethylenically unsaturated carboxylic acid of 3 to 6 carbon atoms, on the basis of 100 parts by weight of said monomers.

4. Fabric of claim 2 wherein said fibers are 100% polyester fibers.

5. Fabric of claim 2 wherein said unsaturated monomers include about 0.1 to 5 parts by weight of a monoethylenically unsaturated carboxylic acid per 100 parts by weight of said monomers.

6. Fabric of claim 5 wherein said carboxylic acid is selected from acrylic acid, methacrylic acid, and itaionic acid, said binder being prepared in presence of 0.005 to 1 part of an emulsifier per 100 parts of said. monomers.

7. Fabric of claim 6 wherein said fibers are l00% polyester fibers and amount of said binder is 25 to 40% by weight of the dry fibers.

8. Fabric of claim 6 wherein amount of said carboxylic acid. is 0.5 to 2 parts and amount of said emulsifier is 0.03 to 0.5 part.

9. Fibers of claim 8 wherein said fibers are carded fibers about 2 to 5 cm in lenght and of about 1 to 3 denier.

1Q. Fabric of claim 8 wherein said polyester fibers are poly(ethylene terephthalate) fibers, amount of said acrylate is 75 to 90 parts and said acrylate is ethyl acrylate, and amount of said ethylenically unsaturated monomer is 15 to 30 parts.

11. A diaper comprising an outer water- impervious sheet, an inner coverstock of nonwoven fabric of claim 7, and an intermediate absorbent pad.