Nonwoven web and method of forming same

Abstract: A nonwoven web (32) suitable as a barrier layer in an SMS fabric laminate (12) is provided. The web is formed at commercially acceptable polymer melt throughputs (greater than 3 PIH) by using a reactor granule polyolefin, preferably polypropylene, that has been modified by the addition of peroxide in amounts ranging from up to 3000 ppm to reduce the molecular weight distribution from an initial molecular weight distribution of from 4.0 to 4.5 Mw/Mn to a range of from 2.2 to 3.5 Mw/Mn. Also the addition of peroxide increases the melt flow rate (lowers viscosity) to a range between 800 up to 5000 gms/10 min at 230°C. The resulting web has an average fiber size of from 1 to 3 µm and pore sizes distributed predominantly in the range from 7 to 12 µm, with a lesser amount of pores from 12 to 25 µm, with virtually no pores greater than 25 µm, and with the peak of the pore size distribution less than 10 µm.

Description: This invention relates generally to a nonwoven web and a method of forming such a web. Nonwoven fabric laminates are useful for a wide variety of applications. Such nonwoven fabric laminates are useful for wipers, towels, industrial garments, medical garments, medical drapes, and the like. Disposable fabric laminates have achieved especially widespread use in hospital operating rooms for drapes, gowns, towels, footcovers, sterile wraps, and the like. Such surgical fabric laminates are generally spun-bonded/melt-blown/spun-bonded (SMS) laminates consisting of nonwoven outer layers of spun-bonded polypropylene and an interior barrier layer of melt-blown polypropylene. Particularly, Kimberly-Clark Corporation, the assignee of the present invention, has for a number of years manufactured and sold SMS nonwoven surgical fabric laminates under the marks Spunguard® and Evolution®. Such SMS fabric laminates have outside spun-bonded layers which are durable and an internal melt-blown barrier layer which is porous but which inhibits the strikethrough of fluids from the outside of the fabric laminate to the inside. In order for such a surgical fabric to perform properly, it is necessary that the melt-blown barrier layer have a fiber size and a pore size distribution that assures breathability of the fabric while at the same time inhibiting strikethrough of fluids. The current melt-blown web used in the manufacture of the Kimberly-Clark Evolution® medical fabric laminate has pore sizes distributed predominantly in the range from 10 to 15 µm with the peak of the pore size distribution greater than 10 µm. Such a melt-blown web has advantages as a barrier layer. The article "Effects of Meltblown Web Structure on Filtration Efficiency and Porosity" (Wadsworth L.C. et al, Book of Papers, INDA-TEC, Philadelphia, Pennsylvania, USA, May 30 - June 2, 1989, pp. 585-600) relates to studies of polypropylene melt blown webs in terms of relationships among the processing conditions, structure and filtration efficiency. One of the resins used has a melt flow rate of 1400 and a polydispersity of 2.1. It is an object of the present invention to further improve such nonwoven webs. This object is solved by the nonwoven web of independent claim 1 and the method of independent claim 5 and the nonwoven fabric laminate of independent claim 10. Further features and details of the invention are evident from the dependent claims, the description, examples and drawings. The invention provides a nonwoven web having fine fibers and a small pore size distribution and a method for forming such a web. The method of the present invention uses a reactor granule resin having an initial broad molecular weight distribution which resin has been modified to narrow its molecular weight distribution and to increase its melt flow rate. Consequently the nonwoven web can be formed by melt-blowing at high throughputs. Such nonwoven webs are particularly useful as barrier layers for fabric laminates. The present invention allows a significant improvement in porosity and inhibition of strikethrough, and provides a melt-blown web having average fiber sizes of from 1 to 3 µm and having a distribution of pore sizes so that the majority of pores are in the range of 7 to 12 µm with the peak of the pore size distribution less than 10 µm. More particularly, improved performance characteristics with respect to porosity and strikethrough can be achieved when the melt-blown web has pore sizes distributed predominantly in the range from 7 to 12 µm, with a lesser amount of pores from 12 to 25 µm, and with virtually no pores greater than 25 µm as measure by the Coulter Porometer. The present invention, therefore, provides a nonwoven web for use as a barrier layer in a fabric laminate which nonwoven web has an average fiber diameter of from 1 to 3 µm and pore sizes distributed predominantly in the range from 7 to 12 µm, with a lesser amount of pores from 12 to 25 µm, with virtually no pores greater than 25 µm, and with the peak of the pore size distribution less than 10 µm. The present invention also provides a nonwoven fabric laminate having a barrier layer of fine fibers and small pore size distribution such that the resulting fabric laminate has pore sizes distributed predominantly in the range from 5 to 10 µm, with a lesser amount of pores from 10 to 15 µm, with virtually no pores greater than 22 µm, and with the peak of the pore size distribution shifted downward by up to 5 µm from the peak peak of the melt-blown web alone. The foregoing aspects are preferably obtained by forming a melt-blown web from a resin having a broad molecular weight distribution and having a high melt flow rate which resin is modified by the addition of a small amount of peroxide prior to processing to achieve an even higher melt flow rate (lower viscosity). During the melt-blowing process, the modified reactor granule polymer has an increased melt flow rate between 800 up to 5000 gms/10 min at 230°C. Particularly, a polypropylene resin in the form of a reactor granule having a starting molecular weight distribution of 4.0 to 4.5 Mw/Mn and a melt flow rate of from 1000 to 3000 gms/10 min. at 230° C is combined with a small amount of peroxide, less than 500 ppm, to produce a modified polypropylene having a very high melt flow rate of up to 5000 gms/10 min. at 230° C and a narrower molecular weight distribution of 2.8 to 3.5 Mw/Mn. Most preferably, the starting polypropylene resin for the melt-blown web of the present invention is a polypropylene reactor granule which resin has a molecular weight distribution between 4.0 and 4.5 Mw/Mn, has a melt flow rate of about 2000 gms/10 min. at 230°C, and is treated with about 500 ppm of peroxide to produce a modified resin having a melt flow rate greater than 3000 gms/10 min. at 230°C and a molecular weight distribution of from 2.8 to 3.5 Mw/Mn. The broader molecular weight distribution at the high melt flow rate helps minimize production of lint and polymer droplets. Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. Figure 1 is a schematic diagram of a forming machine which is used in making the nonwoven fabric laminate including the melt-blown barrier layer of the present invention; Fig. 2 is a cross section view of the nonwoven fabric laminate of the present invention showing the layer configuration including the internal melt-blown barrier layer made in accordance with the present invention; Fig 3 is a graph showing the pore size distribution for a melt-blown web made in accordance with the present invention (Sample 1), an SMS fabric laminate incorporating such a melt-blown web as a barrier layer (Sample 2), a conventional melt-blown web (Sample 3), and a conventional SMS fabric laminate (Sample 4). Turning to Figure 1, there is shown schematically a forming machine 10 which is used to produce an SMS fabric laminate 12 having a melt-blown barrier layer 32 in accordance with the present invention. Particularly, the forming machine 10 consists of an endless foraminous forming belt 14 wrapped around rollers 16 and 18 so that the belt 14 is driven in the direction shown by the arrows. The forming machine 10 has three stations, spun-bond station 20, melt-blown station 22, and spun-bond station 24. It should be understood that more than three forming stations may be utilized to build up layers of higher basis weight. Alternatively, each of the laminate layers may be formed separately, rolled, and later converted to the SMS fabric laminate off-line. In addition the fabric laminate 12 could be formed of more than or less than three layers depending on the requirements for the particular end use for the fabric laminate 12. The spun-bond stations 20 and 24 are conventional extruders with spinnerettes which form continuous filaments of a polymer and deposit those filaments onto the forming belt 14 in a random interlaced fashion. The spun-bond stations 20 and 24 may include one or more spinnerette heads depending on the speed of the process and the particular polymer being used. Forming spun-bonded material is conventional in the art, and the design of such a spun-bonded forming station is thought to be well within the ability of those of ordinary skill in the art. The nonwoven spun-bonded webs 28 and 36 are prepared in conventional fashion such as illustrated by the following patents: Dorschner et al. United States Patent No 3,692,618; Kinney United States Patent Nos. 3,338,992 and 3,341,394; Levy United States Patent No. 3,502,538; Hartmann United States Patent Nos. 3,502,763 and 3,909,009; Dobo et al. United States Patent No. 3,542,615; Harmon Canadian Patent No. 803,714; and Appel et al. United States Patent No. 4,340,563. Other methods for forming a nonwoven web having continuous filaments of a polymer are contemplated for use with the present invention. Spun-bonded materials prepared with continuous filaments generally have at least three common features. First, the polymer is continuously extruded through a spinnerette to form discrete filaments. Thereafter, the filaments are drawn either mechanically or pneumatically without breaking in order to molecularly orient the polymer filaments and achieve tenacity. Lastly, the continuous filaments are deposited in a substantially random manner onto a carrier belt to form a web. Particularly, the spun-bond station 20 produces spun-bond filaments 26 from a fiber forming polymer. The filaments are randomly laid on the belt 14 to form a spun-bonded external layer 28. The fiber forming polymer is described in greater detail below. The melt-blown station 22 consists of a die 31 which is used to form microfibers 30. The throughput of the die 31 is specified in pounds of polymer melt per inch of die width per hour (PIH)* . As the thermoplastic polymer exits the die 31, high pressure fluid, usually air, attenuates and spreads the polymer stream to form microfibers 30. The microfibers 30 are randomly deposited on top of the spun-bond layer 28 and form a melt-blown layer 32. The construction and operation of the melt-blown station 22 for forming microfibers 30 and melt-blown layer 32 is considered conventional, and the design and operation are well within the ability of those of ordinary skill in the art. Such skill is demonstrated by NRL Report 4364, "Manufacture of Super-Fine Organic Fibers", by V.A. Wendt, E.L. Boon, and C.D. Fluharty; NRL Report 5265, "An Improved Device for the Formation of Super-Fine Thermoplastic Fibers", by K.D. Lawrence, R.T. Lukas, and J.A. Young; and United States Patent No. 3,849,241, issued November 19, 1974, to Buntin et al. Other methods for forming a nonwoven web of microfibers are contemplated for use with the present invention. The melt-blown station 22 produces fine fibers 30 from a fiber forming polymer which will be described in greater detail below. The fibers 30 are randomly deposited on top of spun-bond layer 28 to form a melt-blown internal layer 32. For an SMS fabric laminate, for example, the melt-blown barrier layer 32 has a basis weight of preferably about 0.35-0.50 oz./yd 2 ** . After the internal layer 32 has been deposited by the melt-blown station 22 onto layer 28, spun-bond station 24 produces spun-bond filaments 34 which are deposited in random orientation on top of the melt-blown layer 32 to produce external spun-bond layer 36. For an SMS medical fabric laminate, for example, the layers 28 and 36 each have a basis weight of preferably from about 0.30 oz/yd. 2 to about 1.2 oz./yd. 2 . The resulting SMS fabric laminate web 12 (Fig. 2) is then fed through bonding rolls 38 and 40. The surface of the bonding rolls 38 and 40 are provided with a raised pattern such as spots or grids. The bonding rolls are heated to the softening temperature of the polymer used to form the layers of the web 12. As the web 12 passes between the heated bonding rolls 38 and 40, the material is compressed and heated by the bonding rolls in accordance with the pattern on the rolls to create a pattern of discrete areas, such as 41 shown in Fig. 2, which areas are bonded from layer to layer and are bonded with respect to the particular filaments and/or fibers within each layer. Such discrete area or spot bonding is well known in the art and can be carried out as described by means of heated rolls or by means of ultrasonic heating of the web 12 to produced discrete area thermally bonded filaments, fibers, and layers. In accordance with conventional practice described in Brock et al., United States Patent No. 4,041,203, it is preferable for the fibers of the melt-blown layer in the fabric laminate to fuse within the bond areas while the filaments of the spun-bonded layers retain their integrity in order to achieve good strength characteristics. In accordance with the present invention, we have found that the throughput (PIH) of the die head 22 may be increased while at the same time providing fine fibers by using a reactor granule form of the polymer rather than a pelletized form which polymer in reactor granular form has a molecular weight distribution of 4.0 to 4.5 Mw/Mn and a melt flow rate of 1000 to 3000 gms/10 min at 230 °C. During the melt-blowing process, the modified reactor granule polymer has an increased melt flow rate from 800 up to 5000 gms/10 min at 230°C. By modifying the starting polymer, the resulting polymer will have a lower extensional viscosity, thus taking less force to attenuate the fibers as they exit the die 31. Therefore, with the same air flow, the higher melt flow polymer will produce finer fibers at commercially acceptable throughputs. A commercially acceptable throughput is above 3 PIH. Lower throughputs, however, will further reduce the fiber and pore sizes of the melt-blown layer 32. The resulting melt-blown web 32 with its fine fibers and resulting small pore size distribution has superior barrier properties when incorporated into a fabric laminate. Particularly, the unlaminated melt-blown web 32 has an avbrage fiber size of from 1 to 3 µm and pore sizes distributed predominantly in the range from 7 to 12 µm, with a lesser amount of pores from 12 to 25 µm, with virtually no pores greater than 25 µm, and with the peak of the pore size distribution less than 10 µm. When the melt-blown web 32 is incorporated into the SMS fabric laminate 12, the peak of the pore size distribution in the resulting SMS fabric laminate is shifted downward by up to 5 µm. The SMS fabric laminate 12 has pore sizes distributed predominantly in the range from 5 to 10 µm, with a lesser amount of pores from 10 to 15 µm, with virtually no pores greater than 22 µm, and with the peak of the pore size distribution shifted downward by up to 5 µm. Figure 3 shows the pore size distribution for a melt-blown web made in accordance with the present invention (Sample 1), an SMS fabric laminate made using the melt-blown web of the present invention (Sample 2), a conventional melt-blown web (Sample 3), and an SMS fabric laminate such as Kimberly-Clark's Evolution® SMS medical fabric laminate made using the conventional melt-blown web (Sample 4). Particularly, the melt-blown web of the present invention and the SMS fabric laminate of the present invention were made in accordance with Example 1 below. The present invention is carried out with polypropylene. The method used to achieve the high melt flow polymer which is useful in producing a nowoven web of fine fibers at commercial production speeds is to start with a reactor granule polypropylene resin having a molecular weight distribution between 4.0 and 4.5 Mw/Mn and a high melt flow rate of 1000 to 3000 gms/10 min. at 230°C. A small amount of peroxide is added to the starting resin to modify the molecular weight distribution to a range of 2.8 to 3.5 Mw/Mn and to increase the melt flow rate up to 5000 gms/10 min at 230°C. Example 1 In order to illustrate the foregoing invention, a melt-blown web was formed on a conventional melt-blowing forming line using the modified polymer of the present invention. In addition, an SMS fabric laminate was formed using the inventive melt-blown web as an internal barrier layer. The SMS fabric laminate had spun bonded layers formed in conventional fashion of polypropylene. The SMS fabric laminate was preferably formed on-line by a multi-station forming machine as illustrated in Fig. 1. The melt-blown web and melt-blown barrier layer for the SMS fabric laminate were formed from reactor granules of polypropylene having a starting molecular weight distribution between 4.0 and 4.5 Mw/Mn and a melt flow rate of about 2000 gms/10 min. at 230°C. The starting polypropylene resin was treated with about 500 ppm of peroxide to produce a resin having a melt flow rate greater than 3000 gms/10 min. at 230°C and a molecular weight distribution of from 2.8 to 3.5 Mw/Mn. The broader molecular weight distribution at the high melt flow rate helps minimize production of lint and polymer droplets. The melt-blown web, prepared in accordance with the foregoing, had a basis weight of 0.50 oz./yd. 2 and was designated as Sample 1. The SMS fabric laminate, having a melt-blown internal barrier layer made in accordance with the present invention, had spun-bonded layers with a basis weight of 0.55 oz./yd. 2 , and the melt-blown barrier layer had a basis weight of 0.50 oz./yd. 2 . The inventive SMS fabric laminate was designated as Sample 2. In addition, a conventional melt-blown web and a conventional SMS fabric laminate (Kimberly-Clark's Evolution® fabric laminate) having the same basis weights as the inventive web and inventive SMS fabric laminate were prepared as controls. The control melt-blown web was designated Sample 3, and the control SMS fabric laminate was designated Sample 4. The Samples 1 through 4 possess the characteristics set forth in Tables 1 and 2 below: The pore size distribution set out in Table 1 was measured by the Coulter Porometer. The pore size distribution set out in Table 1 is shown graphically in Fig. 3. The plots shown in Fig. 3 show the finer pore size distribution for Samples 1 and 2 as compared to Samples 3 and 4 respectively. The pore size distribution for the inventive web and inventive SMS fabric laminate is narrower than the conventional melt-blown web and conventional SMS fabric laminate. It should be noted that the pore size distribution for the inventive SMS fabric laminate has the peak of its curve shifted downward by up to 5 µm from the peak of the melt-blown web alone before lamination. Apparently the lamination process and the additional spunbonded layers cause the pore structure to close up thereby increasing the barrier properties of the resulting fabric laminate. The distribution of the pore sizes predominantly between 5 to 10 µm represents a fabric laminate (Sample 2) that is finer in its construction than conventional fabric laminates (Sample 4) with the resulting improved barrier properties. The improved barrier properties of the inventive fabric laminate (Sample 2) as compared to the conventional fabric laminate (Sample 4) is shown in Table 2 below. The blood strike through was measured by the following procedure. A 7 in*** . by 9 in. piece of each sample fabric was laid on top of a similar sized piece of blotter paper. The blotter paper was supported on a water filled bladder which was in turn supported on a jack. The jack was equipped with a gauge to determine the force exerted from which the pressure exerted by the bladder on the blotter paper was calculated. A 1.4 gm sample of bovine blood was placed on top of the fabric sample and covered with a piece of plastic film. A stationary plate was located above the plastic film. The water bladder was then jacked up until a pressure of 1 psi**** was attained on the bottom of the blotter paper. As soon as the pressure was achieved, that pressure was held for the desired time. Once the time had elapsed, the pressure was released, and the blotter paper was removed and weighed. Based on the difference in weight of the blotter paper before and after, the percentage strike through was determined. The test results indicate that the SMS fabric laminate made in accordance with the present invention has superior strike through characteristics especially for short elapsed times. Short elapsed times represent the situations that are most often encountered in medical use where blood generally will not remain for long on the drape or gown before it can run off. The filter properties were measured to determine the ability of the SMS fabric laminate to block the penetration of air born bacteria. The samples were tested in accordance with Mil. Spec. 36954-C 4.4.1.1.1 and 4.4.1.2. The 3.5% increase in efficiency within the plus 90% range represents a significant improvement in filtration and the ability to preclude the passage of air born bacteria.

Claims: A nonwoven web of fine fibres formed from a reactor granule of a polypropylene resin having a molecular weight distribution between 4.0 and 4.5 Mw/Mn and a melt flow rate of 1000 - 3000 g/10 min at 230°C which has been modified by adding a small amount of peroxide to modify the molecular weight distribution to the range of 2.8 to 3.5 Mw/Mn and to increase the melt flow rate up to 5000 g/10 min at 230°C, wherein the modified polypropylene results from adding up to 500 ppm of peroxide to the reactor granules prior to forming the web. The nonwoven web of claim 1, wherein the modified polypropylene has a melt flow rate of 3000 - 5000 g/10 min. at 230°C. The nonwoven web of one of the preceding claims, wherein the web is formed at a polymer throughput of greater than 0.535 kg per cm of die width per hour (3PIH). The nonwoven web of one of the preceding claims, wherein the web has an average fibre size from 1 to 3 µm and pore sizes distributed predominantly in the range from 7 to 12 µ m with the peak of the pore size distribution less than 10 µm. A method of forming a nonwoven web having an average fiber size of from 1 to 3 µm and pore sizes distributed predominantly in the range from 7 to 12 µm, with the peak of the pore size distribution less than 10 µm, comprising the steps of melt-blowing a reactor granule of a modified polypropylene having a molecular weight distribution between 2.8 and 3.5 Mw/Mn and a melt flow rate of from 800 g/10 min to 5000 g/10 min, wherein the polymer is throughput at a rate greater than 0.535 kg per cm of die width per hour (3PIH). A method of forming a nonwoven web according to claim 5, having fine fibres and a small pore size distribution, wherein the modified polypropylene has a melt flow rate of greater than 3000 g/10 min. at 230°C. The method of claim 6, wherein the modified polymer results from adding up to 500 ppm of peroxide to the reactor granules prior to forming the nonwoven web. Use of the nonwoven web according to one of claims 1 to 4 or made according to one of claims 5 to 7 as a barrier layer in a nonwoven fabric laminate. The use of claim 8 wherein the resulting fabric laminate has pore sizes distributed predominantly in the range from 5 to 10 µm with a lesser amount of pores from 10-15 µm and virtually no pores greater than 22 µm, with the peak of the pore size distribution being preferably shifted downwardly by up to 5 µm from the peak of the melt-blown web alone. A nonwoven fabric laminate having at least two layers, one of said layers comprising a nonwoven web according to one of claims 1 to 4 or made according to one of claims 5 to 7. The nonwoven fabric laminate of claim 10, whereby said web is a barrier layer in the nonwoven fabric laminate. The nonwoven fabric laminate of claim 11, wherein the nonwoven fabric laminate is a nonwoven SMS fabric laminate, having an internal barrier layer. Use of the nonwoven fabric laminate according to one of claims 10 or 12 in a sterilisation wrap or a surgical fabric.

Inventors: KOBYLIVKER PETER MICHALOVICH, TIMMONS TERRY KRAMER, WOON LIN-SUN, KOBYLIVKER, PETER MICHALOVICH, TIMMONS, TERRY KRAMER, WOON, LIN-SUN
Applicants: KIMBERLY CLARK CO, KIMBERLY-CLARK WORLDWIDE, INC.
Application year: 1991
Main classification: D04H1
IPC codes: C08L101, D04H3, D04H13, A61L27, A61B19