CA2103439A1 - Method of preparing a nonwoven web having delayed antimicrobial activity - Google Patents

Abstract

Abstract A method of forming a nonwoven web having delayed antimicrobial activity, in that the web does not exhibit antimicrobial activity upon its formation but develops such activity within from about three hours to about 30 days thereafter without any post-formation treatment, which method involves the stepsof (1) melting a mixture consisting of a thermoplastic polyolefin, an additive, and a retardant coadditive; (2) forming fibers by extruding the resulting melt through under defined conditions of shear and throughput; (3) drawing the fibers; and (4) collecting the fibers on a moving foraminous surface as a web of entangled fibers. The additive is a defined siloxane quatenriary ammonium salt having a weight average molecular weight of from about 800 to about 2,000 and a polydispersity of up to about 3Ø The additive is present in an amount of from about 0.5 to about 2.0 percent by weight, based on the amount of thermoplastic polyolefin. The retardant coadditive is a high surface area particulate inorganic or organic material which is insoluble in the polymer at both ambient and melt-extrusion temperatures, is present in an amount of from about one-half to about two times the amount on a weight basis of the additive, has a surface area of from about 50 to about 500 m2, and is capable of being at least partially being coated by the additive.

Description

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MEl'HOD OF PREPARING A NONWOVEN WEB
HAVING DELAYE.D AN'I~CROBIAL ACTIVIlY

Cross-Reference to Related Application -~ ;
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Antimicrobial siloxane quaternary ammonium salts are described and claimed in copending and commonly assigned Application Serial No.
filed of even date in the names of Ronald Sinclair Nohr and John Gavin MacDonald.
Background oftheInvention This application claims priority from a U.S. continuation-in-part application, the parent of which corresponds to Canadian patent application No. 2,073,745, filed July 13, 1992.

The present invention relates to the forsnation of a nonwoven web by melt extrusion.
Traditional melt-extrusion processes for the formation of a nonwoven web from a thermoplastic polymer typically involve melting the thermoplastic polymer, extruding the molten polymer through a plurality of orifices to form a plurality of threadlines or filaments, attenuating the f;laments b~ entrainment in a rapidly moving first stream of gas, cooling the filaments with a second streamof gas, and randomly depositing the attenuated filaments, or fibers, on a movingforaminous surface. The most common and well known of these processes are meltblowing, coforming, and spunbonding. The nonwoven webs obtained by -~ . .
25 t'tlese processes are widely used in a variety of products, but especially in such disposable absorbent products as diapers; incontinent products; feminine care -products, such as tampons and sanitary napkins; wipes; sterilization wraps;
surgical drapes and related materials; hospital gowns, shoe covers, and the like, to name but a few.

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Meltblowing references include, by way of example, U.S. Patent Nos.

3,016,599 to R. W. Perry, Jr., 3,704,198 to J. S. Prentice, 3,755,527 to J. P.
Keller et al., 3,849,241 to R. R. Butin et al., 3,978,185 to R. R. Butin et al.,and 4,663,220 to T. J. Wisneski et al. See, also, V. A. Wente, "Superfine S Therrnoplastic Fibers", Industrial ~ Engineenng Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., "Mam~facture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing - A One-Step Web Process for New Nonwoven Productsn, Journal of the Technical Association Qf the Pulp ~ Paper Industry, Vol. 56, No.4, pp. 74-77 (1973)-Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are comingled with the meltblown fibers as they are formed) include U.S. Patent Nos. 4,10Q,324 to R. A. Anderson et al. and 4,118,531 to E. R. Hauser.
Finally, spunbonding references include, among others, U.S. Patent Nos.
3,341,394 to Kinney, 3,6S5,862 to Dorschner et al., 3,692,618 to Dorschner et al., 3,705,068 to Dobo et al., 3,802,817 to Matsuki et al., 3,853,651 to Porte, 4,064,605 to Akiyama et al., 4,091,140 to Harmon, 4,100,319 to Schwartz, 4,340,563 to Appel and Morman, 4,405,297 to Appel and Morman, 4,434,204 to Hartman et al., 4,627,811 to Greiser and Wagner, and 4,644,045 to Fowells.
IJ.S. Patent No. 4,923,914 to Nohr et al., which is incorporated herein by reference, describes a means of altering the surface characteristics of fibers prepared from a thermoplastic polymer, such as a polyolefin. Although various surface characteristics are described, the patent clearly emphasizes conver~ng normally hydrophobic surfaces to hydrophilic surfaces. The patent describes a ~i9 surface-segregatable, melt-ex~udable thermoplastic composition which compris-~' es at least one thermoplastic polymer and at least one defined additive. The most .

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preferred additives are polysiloxane polyethers which render the surfaces of thefibers hydrophilic.
Upon being melt-extruded, the compositions of U.S. Patent No. 4,923,914 result in fibers having a differential, increasing concentration of the additive from 5 the centers to the su~aces thereof, such that the concentration of additive toward the surface of each fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present. The additive forms an emulsion with the polymer at melt extrusion temperatures, under which 10 conditions the additive and the polymer form a metastable solution. As the temperature of the newly formed fiber drops below melt extrusion temperatures, the additive becomes significantly less compatible with the polymer. Concurrent with this marked change in compatibility, the polymer begins to solidify. Both factors contribute to the rapid migration or segregation of the additive toward the 15 surface which takes place in a controllable manner.
Web integrity sometimes is a problem with the compositions of U.S. Patent No. 4,923,914. When the additive is a siloxane-containing compound and the desired characteristic is water-wettability, the resulting nonwoven webs can lack integrity upon their formation because of the presence of additive on the surfaces 20 of the fibers. The additive sometimes interferes with the fiber-to-fiber bonding upon which web integrity relies, especially at additive levels of about 1.5 weight percent or higher. In such circumstances, the additive also has a tendency to accumulate over time on the forming wire.
This problem of poor web integrity in nonwoven webs prepared such 25 processes as meltblowing, coforming, and spunbonding can be rectified by instituting process changes. Alternatively, wettability can be delayed as described in Application Serial No. 07/566,938, entitled METHOD OF PREPARING A
NONWOVEN WEB HAVING DELAYED WETIABILITY and filed on August 13, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald. The delay -.. -.. ~;-. i ., . ~ . ~.i. ,- - .

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in wettability results from the use of a trisiloxane polyether having the general formula, R2 R4 Rs R,-Si{) Si~Si-R6 R3 CH2 }~q (CH~,n~C~O)DR8 in which: :
(a) R~~Rq are independently selected monovalent Cl-C3 allcyl groups; -(b) R8 is hydrogen or a monovalent Cl-C3 alkyl group;
(c) m represents an integer of from 0 to about 5; ~ ~:
(d) n represents an integer of from 3 to about 8;
(e) the molecular weight is from about 350 to about 700;
(f) the polydispersity is from about 1.0 to about 1.3; and ~:
(g) the trisiloxane polyether is present in an amount of from about 0.5 to about 1.75 percent by weight, based on the amount of therrnoplastic polymer, ~ :
which amount, if homogeneously distributed throughout the polyolefin, is not sufflcient to render the polyolefin wettable by water.
A method of increasing the wettabili~,r delay period of the nonwoven webs obtained in cross-referenced Application Serial No. 07l566,938 is disclosed in Application Serial No. 07/488,344, filed on March 2, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald, now U.S. Patent No. 5,114,636.
Such increase in the delay period results from including in the thermoplastic composition, in addition to the defined trisiloxane polyether, from about 0.1 toabout 6 percent by weight, based on the amount of thermoplas~ic polymer, of at least one material having the capacity to increase the delay penod for up to about -- 21~33!13~

two weeks. The preferred material for increasing the delay period is a phthalocyanine dye.
Previous attempts to apply the teachings of U.S. Patent No. 4,923,914 to the preparation of nonwoven webs having antimicrobial activity were not S successful. Moreover, the difficulties were deemed to be o f such a nature that they could not be corrected by means of the teachings of Application Serial Nos.07/566,9~8 and 07/488,344.

Summary of the Invelltion It therefore is an object of the present invention to provide a method of forming a nonwoven web having delayed antimicrobial activity.
This and other objects will be apparent to those having ordinary skill in the art from a consideration of the specification and claims which follow.
Accordingly, the present invention provides a method of forming a nonwoven web having delayed antimicrobial activity, in that said web does not exhibit antimicrobial activity upon its formation but develops such activity within -from about three hours to about 30 days thereafter without any post-forrnation ~ -treatment, which method comprises the steps of~
(A) melting a mixture which comprises a thermoplastic ~ -polyolefin, an additive, and a retardant coadditive; - -(B) forming fibers by extruding the resulting melt through a die at a shear rate of from about 50 to about 30,000 sec l and a throughput of no more than about 5.4 kg/cm/hour;
- 25 (C) drawing said fibers; and ; (D) collecting said fibers on a moving foraminous surface as ~' a web of entangled fibers; ~ -in which:
(1) said additive has the general formula, -~

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~1 1 1 1 1 1 Ae Rl-N~H2CHCH20(CH~)3~Si-O)"-Si~CH2)30CH2CHCH2-N-R9 eA
''' R3 R5 R7 Rto in which: :
(a) R2-R8 and Rlo are independently selected monovalent ~ ~:
Cl-C3 alkyl groups; -(b) Rl and R9 are independently seleeted monovalent C6-C25 aLkyl groups;
(c) A represents a monovalent anion;
(d) n represents an integer of from 1 to about 20;
(e) said additive has a molecular weight of from about 800 to about 2,000;
(f) said additive has a polydispersity of up to about 3.0; and (g) said additive is present in an arnount of from about 0.5 to about 2 percent by weight, based on the amount of thermoplastic polyolefin; and 20 (2) said retardant coadditive is a high surface area particulate inorganic or organic material, which retardant coadditive:
(a) is insoluble in the polymer at both ambient and melt-ex~usion temperatures;
~b) is present in an amount of from about one-half to about 25 ` two times the amount on a weight basis of said additive;
(c) has a surface area of from about 50 to about 1,000 m2;
and (d) is capable of being at least partially coated by said additive.

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In certain embodiments, the polyolefin is polypropylene. In other embodiments, the additive molecular weight is in the range of from about 800 to about 1,200, with a typical molecular weight being about 1,000.
Once the antimicrobial activity has developed, the nonwoven web is S capable of l~lling greater than 80 percent of both gram-negative and gram-positive bacteria.

Detailed Description of the Invention As used herein, the term "delayed antimicrs)bial activity" as applied to a nonwoven web means that the web does not exhibit antimicrobial activity upon its formation but develops such activity within from about three hours to about 30 days thereafter without any post-formation treatment.
The term "post-formation treatment" means any process step or treatment 15 of any l~nd after the fibers have been formed and collected as a nonwoven webon the moving foraminous surface, which process step or treatment is required in order to induce antimicrobial activity. Thus, in the absence of a post-formation treatment, antimicrobial activity develops spontaneously after a givenperiod of time.
In general, the term "ther noplastic polyolefin" is used herein to mean any therrnoplastic polyolefin which can be used for the preparation of nonwoven webs. Examples of thennoplastic polyolefins include polyethylene, polypropyl~
ene, poly(l-butene), poly(2-butene), poly(l-pentene), poly(2-pentene), poly(3-methyl-l-pentene), poly(4-methyl-1-pentene), 1,2-poly-1,3-butadiene, 1,4-poly-1 ,~-butadiene, polyisoprene, polychloroprene, polyacrylonitrile, poly(vinyl ~ acetate), poly(vinylidene chloride), polystyrene, and the like.
i~i In certain embodiments, the polyolefins are those which contain only hydrogen and carbon atoms and which are prepared by the addition polymeri~
zation of one or more unsaturated monomers. Examples of such polyolefins in-,., . :.

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clude, among others, polyethylene, polypropylene, poly(l-butene), poly(2-butene), poly(l-pentene), poly(2-pentene), poly(3-methyl-1-pentene), poly(4-methyl-l-pentene), 1,2-poly-1,3-butadiene, l~poly-1,3-butadiene,polyisoprene, polystyrene, and the like. In addition, such term is meant to include blends of 5 two or more polyolefins and random and block copolymers prepared from two or more different unsaturated monomers. Because of their commercial impor-tance, the most significant polyolefins are polyethylene and polypropylene.
The additive which is employed in the method of the present invention is a siloxane quaternary ammonium salt having the formula, R2 OH R4 R6 OH R~
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A9 Rl-N-CH2CHCH20(CH2~3~Si-O)~-Si~CH2)30CH2CHCH2-N-Rg eA
E~3 R5 R7 R~o ' .~
in which:
(a) R2-R8 and Rlo are independently selected monovalent Cl-C3 alkyl groups;
(b) R, and Rg are independently selected monovalent C6-C25 aLkyl groups;
(c) A represents a monovalent anion; ~ -(d) n represents an integer of from 1 to about 20; ~ -(e) said additive has a weight average molecular weight of `~
from about 800 to about 2,000; and (f) said additive has a polydispersity of up to about 3Ø ~
In some embodiments, each of R2-R" and Rlo is a methyl group. In other ~-embodiments, R, and Rg independently are monovalent C,2-CI8 alkyl groupsO In yet other embodiments, n is an integer from about 6 to about 10. In still other il 30 embodiments, A is a halide, with chloride being most typical.

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While the additive weight average molecular weight can vary from about 800 to about 2,000, it typically will be in the range of from about 800 to about1,200. A weight average molecular weight of about 1,000 is perhaps most exemplary of the additive.
S As noted, the polydispersity of the additive will be up to about 3Ø As used herein, the ter n "polydispersity" refers to the ratio of the weight average molecular weight to the number average molecular weight. In certain embodi-ments, the polydispersity of the additive will be in the range of from 1.3 to about 1.8.
In general, the additive will be present in an amount of from about 0.5 to about 2 percent by weight, based on the amount of thermoplastic polyolefin.
In some embodiments, the amount of additive will be in the range of from about 0.8 to about 1.2 percent by weight.
The term "additive" is used broadly herein to encompass the use of more than one additive in a given composition, i.e., a mixture of two or more additives. Moreover, it should be appreciated by those having ordinary sl~ll in the art that additives as defined herein typically are not available as pure compounds. Thus, the presence of impurities or related materials which may not come within the general formula given above for the additives does remove any given material from the spirit and scope of the present invention.
In general, the additive is either commercially available or readily prepared by those having ordinary skill in the art by known methods.
In addition to the additive, the thermoplastic polyolefin to be melt~
processed to forrn a nonwoven web includes a retardant coadditive which is a high surface area particulate inorganic or organic material. This retardant coadditive (a) is insoluble in the polymer at both ambient and melt-extrusion temperatures; (b) has a surface area of from about 50 to about 1,000 m2; and (c)is capable of being at least partially coated by the additive.

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The retardant coadditive typically is present in an amount equal to from about one-half to about two times the amount on a weight basis of additive employed. The retardant coadditive can be any inorganic or organic material having the requisite surface area. In addition, the retardant coadditive should be 5 stable under melt-extrusion conditions. Moreover, the retardant coadditive should be capable of being at least partially coated by the additive. Stated differently, the additive typically will have a surface tension which is less than the surface free energy of the retardant coadditive particles.
In general, the shear rate required by the rnethod of the present invention will be in the range of from about 50 to about 30~000 sec l. Typically, the shear rate will be in the range of from about 150 to about 5,000 sec~l, and most typically from about 300 to about 2,000 sec '.
Throughput is of importance because it affects the time the newly formed fiber or film is in a sufficiently molten or fluid state to allow migration or 15 segregation of the additive toward the newly formed surfaces, even though throughput also affects the shear rate.
Throughput typically will be in the range of from about 0.01 to about 5.4 kg/cm/hour. More typically, throughput will be in the range from about 0.1 to about 4.0 kg/cm.hour. The throughput most typically will be in the range of from about 0.5 to about 2.5 kglcm/hour.
Without wishing to be bound by theory, it is believed that the additives emulsify readily in a polyolefin such as polypropylene to form micelle structures or aggregates. However, additives with weight average molecular weights below about 1,400 form thermally unstable aggregates. That is, the lower the weight 25 average molecular weight of the additive, the more thermally unstable are themicelle structures. At fiber process conditions at temperatures above about 170C, such additives with weight average molecular weights of around 600-700 "'! readily "break apart" from their poorly packed aggregate structures. The ' additives then are able to diffuse to the newly forming fiber surfaces.
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However, the lower molecular weight components, in the total molecular weight distribution, not only break apart more readily from their micelle structures at temperature above about 170C, but they also are capable of diffusing more rapidly than the higher molecular weight species. Thus, the 5 molecular weight distribution or polydispersity requirement is central to the present invention. That is, it is essential that the additive have a relatively high polydispersity in order to minimize the amounts of lower molecular weight components.
In other words, broad molecular weight dispersions contain molecular 10 species that will migrate to the fiber surfaces lolng after the fibers have been formed. In order to avoid spontaneous surface segregation of low molecular weight species, larger concentrations of higher molecular weight species are required. Segregation control and to some extent, synthetic realities, require broad molecular weight dispersions or polydispersities in concert with higher 15 additive concentrations.
While the additive still tends to migrate to the surfaces of the fibers, the rate of migration is slower because the higher molecular weight components diffuse more slowly than the lower molecular weight components. Moreover, the diffusion or migration of all components of the additive are delayed by the 20 retardant coadditive. It is believed that the delay results from a temporary affinity of the additive for the surfaces of the retardant coadditive particles.Consequently, the retardant coadditiYe must have a relatively high surface area in order to affect essentially all of the additive.
Having thus described the invention, numerous changes and modifications 25 thereof will be readily apparent to those having ordinary skill in the art without depar~ing from the spirit or scope of the invention.

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Claims (12)

1. A method of forming a nonwoven web having delayed antimicrobial activity, in that said web does not exhibit antimicrobial activity upon its formation but develops such activity within from about three hours to about 30 days thereafter without any post-formation treatment, which method comprises the steps of:
(A) melting a mixture which comprises a thermoplastic polyolefin, an additive, and a retardant coadditive;
(B) forming fibers by extruding the resulting melt through a die at a shear rate of from about 50 to about 30,000 sec-1 and a throughput of no more than about 5.4 kg/cm/hour;
(C) drawing said fibers; and (D) collecting said fibers on a moving foraminous surface as a web of entangled fibers;
in which:
(1) said additive has the general formula, in which:
(a) R2-R8 and R10 are independently selected monovalent C1-C3 alkyl groups;
(b) R, and R9 are independently selected monovalent C6-C25 alkyl groups;
(c) A represents a monovalent anion;
(d) n represents an integer of from 1 to about 20;

(e) said additive has a weight average molecular weight of from about 800 to about 2,000;
(f) said additive has a polydispersity of up to about 3.0; and (g) said additive is present in an amount of from about 0.5 to about 2 percent by weight, based on the amount of thermoplastic polyolefin; and (2) said retardant coadditive is a high surface area particulate inorganic or organic material, which retardant coadditive:
(a) is insoluble in the polymer at both ambient and melt-extrusion temperatures;
(b) is present in an amount of from about one-half to about two times the amount on a weight basis of said additive;
(c) has a surface area of from about 50 to about 1,000 m2; and (d) is capable of being at least partially coated by said additive.

2. The method of claim 1, in which said polyolefin is polypropylene

3. The method of claim 1, in which said additive has a weight average molecular weight of from about 800 to about 1,200.

4. The method of claim 1, in which said additive is present in an amount of from about 0.8 to about 1.2 percent by weight, based on the amount of thermoplastic polymer.

5. The method of claim 1, in which each of R2-R8 and R10 is a methyl group.

6. The method of claim 1, in which R1 and R9 independently are monovalent C12-C18 alkyl groups.

7. The method of claim 1, in which n is an integer from about 6 to about 10.

8. The method of claim 1, in which A is a halide.

9. The method of claim 8, in which A is chloride.

10. The method of claim 1, in which the shear rate is from about 150 to about 5,000 sec-1.

11. The method of claim 1, in which the throughput is in the range of from about 0.1 to about 4.0 kg/cm/hour.

12. The method of claim 1, in which the additive, additive weight average molecular weight, additive polydispersity, additive concentration, retardant coadditive, and retardant coadditive concentration are selected so as to give a predetermined delay time.