CA2073745A1 - 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 quaternary ammonium salt having a molecular weight of from about 800 to about 2,000 and a polydispersity of from 1 to about 2Ø 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. - 16 -

Description

MEI~OD OF P~PARING A NONWO~7EN WE:B
HA~ING D~ELAYED ANll~CROBIAL ACTI~ITY

Cross-Re~erences to Related Applications s The application of the principles of the present invention to nonwoven webs having delayed wettability is described and claimed in copending and commonly assigned Appli~ation Serial No. , entitled METHOD OF
IMPARTING DELAYED WEl~ABILITY TO A NONWOVEN WEB and filed of even date in the names of Ronald Sinclair Nohr and John Gavin MacDonald.
The application of the principles of the present invention to nonwoven webs prepared by hydraulic spinning and having delayed wettability is described and claimed in copending and commonly assigned Application Serial No.
, entitled FILAMENTS, TOW, AND WEBS FORMED BY HYDRAULIC
SPINNING AND HAVING DELAYED WEl~ABILITY and filed of even date in the names of Ronald Sinclair Nohr, Richard Allen Anderson, and John Gavin MacDonald.
The formation of a nonwoven web having delayed wettability is described and claimed in Application Serial No. 07/566,938, entitled METHOl:) OF
PREPARING A NONWOVEN WEB HAVING DELAYED WEITABILITY and filed on August 13, 1990 in the names of Ronald S. Nohr and J. C;avin MacDonald.
A method of increasing the delay period of the nonwoven webs obtained in Application Serial No. 07/566,938 is described and claimed in Applic~tion Serial No. 07/488,344, entitled METHOD OF INCRE~ASING THE DELAY
PERIOD OF NONWOVEN WEBS HAVING DELAYED WEl~ABILITY and filed on March 2, 1990 in the names of Ronald S. Nohr and J. Gavin Mac-Donald.

3~
Baclcgrolmd of the In~ention The present invention relates to the formation of a nonwoven web by melt extrusion. More particularly, the present invention relates to a method of S preparing by melt extrusion a nonwoven web having delayed antimicrobial activity.
Traditional melt-extrusion processes for the forrnation of a nonwoven web from a thermoplast;c polymer typically involve melting the thermoplastic polymer, extruding the molten polymer through a plurality of orifi(~es to forrn a plurality of threadlines or filaments, attenuating the filaments by 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 these processes are widely used in a variety of products, but especially in suchdisposable absorbent products as diapers, incontinent products, feminille care products, such as tampons and sanitary napl~ins, wipes, sterilization wraps, surgical drapes and related materials, hospital gowns, shoe covers, and the like, to name but a few.
Meltblowing references include, by way of example, U.S. Patent Nos.
3,016,599 to R. W. Perry, Jr., 3,704,19g to J. S. Prentice, 3,75~,527 to J. P.
Keller et al., 3,849,241 to R. R. Butin et al., 3,978,185 to R. 1?. Butin et al., and 4,663,220 to T. J. Wisneski et al. See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemis~, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., "Manufacture of Superf;ne 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 r~

Blowing - A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper.Industry, Vol. 56, No.4, pp. 74-77 (1973) Coforming re~erences (i.e., references disclosing a meltblowing process in S which fibers or particles are comingled with the meltblown fibers as they are formed) include IJ.S. Patent Nos. 4,100,324 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, 3,655,862 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 ~o Fowells.
U.S. Patent No. 4,923,914 to Nohr et al., which is incorporated herein by 15 reference, describes a means of altering the surface characteristics of fibers prepared from a thermoplastic polymer, such as a polyolefin. Although various surf~ce characteristics are described, the patent clearly emphasizes converting normally llydrophobic surfaces to hydrophilic surfaces. The patent describes a surface-segregatable, melt-extrudable thennoplastic composition which compris-20 es at least one thermoplastic polymer and at least one defined additive. The mostpreferred additives are polysiloxane polyethers which render the surfaces of the fibers 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 firom 25 the centers to the surfaces 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 su~face of the fiber at least one desired characteristic which otherwise would not be present. The additive ~r~L~

forms an emulsion with the polymer at melt extrusion temperatures, under which 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 signifieantly less compatible with the polymer. Concurrent S wi~h this marked change in compatibility, the polymer begins to solidify. Bothfactors contribute to the rapid migration or segregation of the additive toward the surface which talces 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 10 desired characteris~ic is water-wettabili~y, the resulting nor~woven webs can lack integrity upon their formation because of the presence of additive on the surfaces of the fibers. The additive sometimes inter~eres with the fiber-to-fiber bondingupon which web integrity relies, especially at additive levels of about 1.5 weight percent or higher. In such circumstances, the addiSive also has a tendency to 15 accumulate over time on the forming wire.
This problem of poor web integrity in nonwoven webs prepared such processes as meltblowing, coforming, and spunbonding can be rectified by instituting process changes. Alternatively, wettability can be clelayed as described in Application Serial No. 07/566,938, entitled METHOD OF PREPARING A
20 NONWOVEN WEB HAVING DELAYED WEl'rABILITY and filed on August 13, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald. The delay in wettability results from the use of a trisiloxane polyether having the general formula, ~ ~ 7 ~ J~
p~ R4 Rs Rl-Si-O-Si-(~-Si-~6 S R3 CH2 ~7 (CH2)m O-(c2H4o)l~8 in which:
(a) R'-R7 are independently selected monovalent Cl-C3 alkyl gr~ups;
(b) R~ is hydrogen or a monovalent Cl-C3 allyl 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;
(fl the polydispersity is from about 1.0 to absut 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 thermoplastic polymer, which amount, if homogeneously distributed throughout the polyolefin, is not sufficient to render the polyolefin wettable by water.
A me~hod of increasing the wettability delay period of the nonwoven webs obtained in cross-referenced Application Serial No. 07/566,938 is disclosed in cross-referenced Application Serial No. 07/488~344. Such increase in the delay period results from including in the thennoplastic composition, in addition to the defined trisiloxane polyether, from about 0.1 to about 6 percent by weight, based on the amount of thermoplastic polymer, of at least one material having the capacity to increase the delay period ~or up to about two weel~s. 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 successful. Moreover, the difficulties were deemed to be of such a na~ure that they could not be corrected by means s)f the teachings of App~lcQho~n ~e4n~al Nos.
07/566,938 and 07/488,344.

Summary of t31e l[n~ention S
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 h~ving ordinary skill in the art from a consideration of the specification and claims which follow.
Accordingly, the present invention provides a method of ~onning 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 pogt-formation treatment, which method comprises the steps of:
(A) melting a mixture which comprises a the:rmoplastic polyolefin, an additive, and a retardant coadditive;
(B) forming fibers by extruding the resulting melt through a die at a she~r rate of from about 50 to about 30,000 sec 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 ~oraminous su~face as a web of entangled fibers;
in which:
(1) said additive has the general formula, R2 OH R4 OH 1~6 A Rl-N+-cH2cHcH2o(cH2)3-(si-o)n-(cH2)3c~z~HcH2-N ~-Rq A
I
R3 1~5 R8 2~73~
in which:
(a) R2-R6 and R8 are independently selected monovalent C,-C3 alkyl groups;
(b) Rl and R7 are independently selected monovalent C6-C25 S allyl 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 *om abou~ 800 to about 2,000;
(f) said additive has a polydispersity of from 1 to about 2.0;
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 15 (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~rusion temperatures;
(b~ is present in an amount of from about one-half to about tw0 times the amoullt 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.
In preferred embodiments, the polyolefin is polypropylene. In other preferred embodiments, the additive molecular weight is in the range of from about 800 to about 1,200, and most preferably about 1,000.
Once the antimicrobial activity has developed, the nonwoven web is capable of killing greater than 80 perccnt of both gram-negative and gram-positive bacteria.

~ ~ ~ 3 ~ ~ ~
Detailed Descriptioll of the In~rention As used herein, the term "delayed antimicrobial activity" as applied to a nonwoven web means that the web does not exhibit antimicrobial activity upon S its formation but develops such activi~y within from about three hours to about 30 days thereafter withou~ any post-formation treatment.
The term "post-formation treatment" means any process step or treatment of any Idnd after the fibers have been formed and collected as a nonwoven web on the moving foraminous surface, which process step or treatment is rcquired in order to induce antimicrobial act~ity. Thus, in the absence of a ps)st-formation treatment, antimicrobial activity develops spontaneously after a givenperiod of time.
In general, the term "thermoplastic polyolefin" is used herein to mean any thermoplastic polyolefin which can be used for the preparation of nonwoven webs. Examples of thermoplastic polyolefins include polyethylene, polypropyl-ene, poly(l-butene), poly(2-butene), poly(1-pentene3, poly(2-pentene), poly(3-methyl-1-pentene), poly(4-me~hyl-1-pentene), 1,2-poly-1,3-butadiene, 1,4-poly-1,3-butadiene, polyisoprene, polychloroprene, polyacrylonitrile, poly(vinyl acetate), poly(vinylidene chloride), polystyrene, and the like.
The preferred polyolefins are those which contain only hydrogen and carbon atoms and which are prepared by the addition polymerization of one or more unsaturated monomers. Examples of such polyolefins include, arnong others, polyethylene, polypropylene, poly(1-butene), poly(2-butene), poly(l-pentene), poly(2-pentene), poly(3-methyl-1-pentene), poly(4-methyl-1-pentene), 1,2-poly-1,3-butadiene, 1,4-poly-1,3-butadiene, polyisoprene, polystyrene7 and the like. In addition, such term is meant to include blends of two or more polyolefins and random and block copolymers prepared from two or more 2 ~ 7 ~
different unsaturated monomers. Because of their commercial importance, the most preferred 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, s R~ OH R4 OH ~6 A Rl-N+-cH2cHcH2o(~H2)3-(si-o~n-(cH2)3cH2cHcH2-N+-R7 A
R3 R5 ~8 in which:
(a) R2-R6 and R8 are independently selected monovalent C,-C3 allyl groups;
(b) Rl and R7 are independently selected monovalent C6-C25 IS allyl groups;
(c3 A represents a monovalent anion;
(d) n represents an integer of from 1 to about 20;
(e) said additive has a molecular weight of ~rom about 800 to about 2,000; and (~) said additive has a polydispersity of from 1 to about ?.0 In preferred embodiments, each of R2-R~ and R8 is a methyl group. In other preferred embodiments, Rl and R7 independently are monovalent C,2-C,8 alkyl groups. In yet other preferred embodiments, n is an integer from abou~ 6 to about 10. In still other preferred embodiments, A is a halide, with ehloride being most preferred.
While the additive molecular weight can vary from about 800 to about 2,000, it preferably will be in the range of from about 800 to about 1,200, witha molecular weight of about 1,000 being most preferred.
As noted, the polydispersity of the additive will be in the range of from 1 to about 2Ø As used herein, the term "polydispersity" refers to the ratio of ~3~
the weight-average molecular weight to ehe number-average molecular weight.
Preferably, the polydispersity of the additive will be in the range of ~rom 1.3 to about 1.8.
In general, the additive will be present in an amount of from about 0.5 S to about 2 percent by weight, based on the amount of thermoplastic polyolefin.Preferably, the amount of additive will be in the range of from about 0.8 to about 1.2 percent by weight.
The eerm "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 10 additives. Moreover, it should be appreciated by those having ordinary skill in the art that additives as defined herein typically are not available as pure compounds. Thus, the presence of impurities or related materials whic~ 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 addition to the additive, the thermoplastic polyolefin to be melt-processed to form a nonwoven web must include a retardant coadditive which is a hi~gh surface area particulate inorganic or organic material, which re~ardant 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)20 is capable of being at least partially coated by said additive.
The retardant coadditive must be present in an amount equal to ~rom 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 must be stable under 25 melt-extrusion conditions. Moreover, the retardant coadditive must be capableof being at least partially coated by the additive. Stated di~ferently, the additive must have a surface tension which is less than the sur~ace free energy of the retardant coadditive particles.

~3~ ~7~
In general, the shear rate required by the rnethod of the present invention will be in the range of from about S0 to about 30,00~ sec ~. Preferably, the shear rate will be in the range of from about lS0 to about 5900() sec-', and most preferably from about 300 to about 2,~0 sec~'.
S Throughput is ofimportance because it affects the time ~he newly fornned fiber or film is in a sufficiently molten or fluid state to allow migration or segregation of the additive toward the newly formed surfaces, even though throughput also af~ects the shear rate.
Throughput typically will be in the range of from ~bout 0.01 to about 5.4 kg/cm/hour. Preferably, throughput will be in the range from about 0.1 to about 4.0 kg/cm.hour. The throughput most preferably will be in the range of from about O.S to about 2.5 kg/cmlhour.
Without wishing to be bound by theory, it is believed ~hat the additives emulsify readily in a polyolefin such as polypropylene to ~orm micelle structures or aggregates. However, additives with molecular weights below about 1,400 form thennally unstable aggregates. lllat is, the lower the molecular weight of the additive, the more thermally unstable are the micelle structures. At fiber process conditions at temperatures above about 170C, such additives with molecular weights of around 600-700 readily "break apart" from their poorly packed aggregate structures. The additives then are able to diffuse to ~he newlyforming fiber surfaces.
However, the lower molecular weight components, in the total molecular weight distribution, not only break apart more readily from their micelle ~tructures at temperature above about 170C, but they also are capable of diffusing more rapidly than the higher molecular weight species. Thus, Ihe molecular weight distribution or polydispersity requirement is central to the present invention. That is, it is essential that the additive have a relatively lligh polydispersity in order to minimize the amounts of lower molecular wei~
components.
In other words, broad molecular weight dispersions contain molecular species that will migrate to the fiber surfaces long after the fibers have been S formed. In order to avoid spontaneous surface segrega~ion 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 d;spersions or polydispersities in concert wi~h higher additive concentrations.
While the additive still tends to migrate to the sur~aces 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 retardant coadditive. It is believed that the delay results from a temporary 15 affinity of the additive for the surfaces of the retardant coadditive pa~ticles.
Consequently, the retardant coadditive must have a relatively high surface area in order to a~fect essentially all of the additive.
Having thus described the invention, numerous changes and modifications thereof will be readily apparent to ~hose having ordinary skill in the art without 20 departing from the spirit or scope of the invention.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 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-R6 and R8 are independently selected monovalent C1-C3 allyl groups;
(b) R, and R7 are independently selected monovalent C6-C25 allyl 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 from 1 to about 2.0;
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 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-R6 and R8 is a methyl group.

6. The method of claim 1, in which R1 and R7 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 molecular weight, additive polydispersity, additive concentration, retardant coadditive, and retardant coadditive concentration are selected so as to give a predetermined delay time.