DE69109543T3 - Nonwoven fabric and process for its manufacture. - Google Patents

Description

This invention relates generally to a nonwoven web or fabric and a method of forming such a web or fabric.

Nonwoven laminates are suitable for a wide variety of applications. Such nonwoven laminates can be used for wipers, towels, industrial clothing, medical apparel, medical drapes, etc. Disposable fabric laminates have found widespread use, especially in hospital operating rooms, for drapes, gowns, towels, foot protectors, sterilization wraps, etc. Such fabric laminates used in surgery are usually spunbond/meltblown/spunbond (SMS) laminates, which consist of outer nonwoven layers of spunbond polypropylene and an inner barrier layer of meltblown polypropylene. In particular, Kimberly-Clark Corporation, the assignee of the present invention, has manufactured and sold surgical SMS nonwoven laminates under the brand names Spunguard® and Evolution® for several years. Such SMS fabric laminates have outer durable spunbond layers and an inner meltblown barrier layer that is porous but prevents the penetration of liquids from the outside of the fabric laminate to the inside. In order for such a surgical fabric to function properly, it is necessary that the meltblown barrier layer have a fiber size and pore size distribution that ensures the breathability of the fabric while preventing the penetration of liquids.

The material currently used in the manufacture of Kimberly-Clark’s Evolution® medical fabric laminates Meltblown webs have pore sizes distributed predominantly in the range of 10 to 15 µm, with the peak of the pore size distribution being greater than 10 µm. Such a meltblown web has advantages as a barrier layer. The article "Effects of Meltblown Web Structure on Filtration Efficiency and Porosity" (Wadsworth LC et al. Book of Papers, INDA-TEC, Philadelphia, Pennsylvania, USA, May 30 to June 2, 1989, pp. 585-600) refers to studies of meltblown polypropylene nonwovens concerning the relationship between processing conditions, structure and filtration effectiveness. 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 nonwovens. This object is achieved by the nonwoven of independent claim 1 and the method of independent claim 5 and the nonwoven laminates of independent claim 10. Further characteristics and details of the invention will become apparent from the dependent claims, the description, the examples and the drawings.

The invention provides a nonwoven fabric having fine fibers and a narrow pore size distribution, and a process for forming such a fabric. The process of the present invention uses a reactor pellet resin having an initially broad molecular weight distribution, the resin being modified to narrow its molecular weight distribution and increase its melt flow rate. Consequently, the nonwoven fabric can be formed by high throughput meltblowing. Such nonwoven fabrics are particularly suitable as barrier layers for fabric laminates. The present invention allows for a significant improvement in porosity and strikethrough prevention and provides a meltblown nonwoven fabric having average fiber sizes. of 1 to 3 µm and a pore size distribution in which the majority of the pores are in the range of 7 to 12 µm, with the peak of the pore size distribution being below 10 µm. In particular, improved performance characteristics with respect to porosity and strikethrough can be achieved when the meltblown nonwoven fabric has pore sizes predominantly distributed in the range of 7 to 12 µm, with a smaller amount of pores between 12 and 25 µm and virtually no pores above 25 µm, as measured by the Coulter Porometer.

The present invention therefore provides a nonwoven fabric to be used as a barrier layer in a fabric laminate, the nonwoven fabric having an average fiber diameter of 1 to 3 µm, and pore sizes distributed predominantly in the range of 7 to 12 µm, with a smaller amount of pores between 12 and 25 µm, practically no pores above 25 µm and with the peak of the pore size distribution below 10 µm.

The present invention also provides a nonwoven fabric laminate having a barrier layer comprising fine fibers and a narrow pore size distribution such that the resulting fabric laminate has pore sizes distributed predominantly in the range of 5 to 10 µm, with a lesser amount of pores between 10 and 15 µm, with virtually no pores above 22 µm, with the peak of the pore size distribution being shifted downward by up to 5 µm from the peak of the meltblown web alone.

The foregoing points are preferably realized by forming a meltblown web from a resin having a broad molecular weight distribution and a high melt flow rate, wherein the resin is modified prior to processing by adding a small amount of peroxide to achieve an even higher melt flow rate (lower viscosity). During the meltblowing process The modified reactor granulate polymer has an increased melt flow rate of 800 up to 5000 g/10 min at 230ºC.

In particular, a polypropylene resin in the form of reactor pellets with an initial molecular weight distribution of 4.0 to 4.5 Mw/Mn and a melt flow rate of 1000 to 3000 g/10 min at 230ºC is combined with a small amount of peroxide - less than 500 ppm - to produce a modified polypropylene with a very high melt flow rate of up to 5000 g/10 min at 230ºC and a narrower molecular weight distribution of 2.8 to 3.5 Mw/Mn.

Most preferably, the starter polypropylene resin for the meltblown web of the present invention is a polypropylene reactor pellet, the resin having a molecular weight distribution between 4.0 and 4.5 Mw/Mn and a melt flow rate of about 2000 g/10 min at 230°C, and is added with about 500 ppm peroxide to produce a modified resin having a melt flow rate of greater than 3000 g/10 min at 230°C and a molecular weight distribution of 2.8 to 3.5 Mw/Mn. The broader molecular weight distribution at the high melt flow rate helps to minimize the generation of lint and polymer droplets.

Further objects and advantages of the invention will become apparent upon reading the following detailed description with reference to the drawings.

Figure 1 is a schematic representation of a forming machine used to make the nonwoven laminate including the meltblown barrier layer of the present invention;

Figure 2 is a cross-sectional view of the nonwoven laminate of the present invention showing the layer arrangement including the inner meltblown barrier layer made in accordance with the present invention;

Figure 3 is a graph showing the pore size distribution for a meltblown web made in accordance with the present invention (Sample 1), an SMS fabric laminate incorporating such a meltblown web as a barrier layer (Sample 2), a conventional meltblown web (Sample 3), and a conventional SMS fabric laminate (Sample 4).

In Fig. 1, there is shown schematically a forming machine 10 used to make an SMS fabric laminate 12 having a meltblown barrier layer 32 in accordance with the present invention. In particular, the forming machine 10 consists of a foraminous endless forming belt 14 which is passed over rollers 16 and 18 such that the belt 14 is driven in the direction indicated by the arrows. The forming machine 10 has three stations, a spunbond station 20, a meltblown station 22 and a spunbond station 24. It should be understood that more than three forming stations may be used to build up higher basis weight layers. Alternatively, each of the laminate layers may be individually formed, rolled and later converted off-line into the SMS fabric laminate. In addition, the fabric laminate 12 could be formed of more or less than three layers, depending on the requirements for the particular intended use of the fabric laminate 12.

The spunbonding stations 20 and 24 are conventional extruders with spinnerets which form continuous polymer filaments and deposit these filaments in a randomly cross-linked manner on the forming belt 14. The spunbonding stations 20 and 24 may include one or more spinneret heads depending on the process speed and the particular polymer used. Forming spunbond material is common in the art and it is believed that one of ordinary skill in the art is sufficiently skilled to design such a spunbond forming station. The spunbond webs 28 and 36 are made in a conventional manner as illustrated by the following patents: U.S. Patent No. 3,692,618 (Dorschner et al.), U.S. Patent Nos. 3,338,992 and 3,341,394 (Kinney), U.S. Patent No. 3,502,538 (Levy), U.S. Patent Nos. 3,502,763 and 3,909,009 (Hartmann), U.S. Patent No. 3,542,615 (Dobo et al.), Canadian Patent No. 803,714 (Harmon), and U.S. Patent No. 4,340,563 (Appel et al.). Other methods of forming a nonwoven web comprised of continuous filaments of a polymer are contemplated for use in the present invention.

Spunbond materials made with continuous filaments generally have at least three common characteristics. First, the polymer is continuously extruded through a spinneret to form discrete filaments. Thereafter, the filaments are pulled either mechanically or pneumatically without breaking them to align the molecules of the polymer filaments and to impart strength. Finally, the continuous filaments are laid down in a substantially random manner on a carrier belt to form a web. In particular, the spunbond station 20 produces spunbond filaments 26 from a fiber-forming polymer. The filaments are laid down in a random manner on the belt 14 to form an outer spunbond layer 28. The fiber-forming polymer is described in more detail below.

The melt blowing station 22 consists of a nozzle 31 which is used to form microfibers 30. The throughput of the nozzle 31 is in pounds of polymer melt per inch (Inch) die width per hour (PIH)*. As the thermoplastic polymer exits the die 31, a high pressure fluid, usually air, attenuates and spreads the polymer jet to form microfibers 30. The microfibers 30 are randomly deposited on top of the spunbond layer 28 to form a meltblown layer 32. The construction and operation of the meltblowing station 22 for forming the microfibers 30 and the meltblown layer 32 are considered to be conventional and well within the skill of one of ordinary skill in the art to design and operate the same. Such capability is demonstrated by NRL Report 4364, "Manufacture of Super-Fine Organic Fibers," by VA Wendt, EL Boon and CD Fluharty, NRL Report 5265, "An Improved Device for the Formation of Super-Fine Thermoplastic Fibers," by KD Lawrence, RT Lukas and JA Young, and U.S. Patent No. 3,849,241 issued November 19, 1974 to Buntin et al. Other methods of forming a nonwoven fabric from microfibers are contemplated for use in the present invention.

The meltblowing station 22 produces fine fibers 30 from a fiber-forming polymer, as will be described in more detail below. The fibers 30 are randomly deposited on top of the spunbond layer 28 to form an inner meltblown layer 32. For an SMS fabric laminate, for example, the meltblown layer 32 has a basis weight of preferably about 0.35-0.50 oz./yd.2**.

After the inner layer 32 is deposited onto layer 28 by the melt blowing station 22, the spunbonding station 24 produces spunbond yarns 34 which are deposited in a random orientation on top of the melt blown layer 32 to produce an outer spunbond layer 36. For a medical SMS fabric laminate, the layers 28 have and 36, for example, a basis weight of preferably about 0.30 oz./yd.² to about 1.2 oz./yd.².

The resulting SMS fabric laminate web 12 (Fig. 2) is then passed through bonding rollers 38 and 40. The surfaces of the bonding rollers 38 and 40 are provided with a raised pattern such as dots or grids. The bonding rollers are heated to the softening temperature of the polymer used to form the layers of web 12. As the web 12 passes through the heated bonding rollers 38 and 40, the material is compressed and heated by the bonding rollers in accordance with the pattern on the rollers to create a pattern of discrete regions as shown at 41 in Fig. 2, bonding the regions from layer to layer and with respect to the particular filaments and/or fibers within each layer. Such bonding over discrete areas or points is well known in the art and can be accomplished using heated rollers as described or by ultrasonic heating of the web 12 to produce filaments, fibers and layers thermally bonded over discrete areas. In accordance with conventional practice described in U.S. Patent No. 4,041,203 (Brock et al.), it is preferred that the fibers of the meltblown layer in the fabric laminate melt within the bond areas while the filaments of the spunbond layers retain their integrity to achieve good strength properties.

In accordance with the present invention, we have found that the throughput (PIH) of the spinneret head 22 can be increased while providing fine fibers by using a reactor pellet form of the polymer rather than a pelletized form, wherein the polymer in reactor pellet form has a molecular weight distribution of 4.0 to 4.5 Mw/Mn and a melt flow rate of 1000 to 3000 g/10 min at 230°C. During the meltblowing process, the modified reactor pellet polymer will have an increased melt flow rate of 800 up to 5000 g/10 min at 230°C. By modifying the starter polymer, the resulting polymer will have a lower extensional viscosity, requiring less force to attenuate the fibers as they exit the spinneret 31. Therefore, the polymer with the higher melt flow rate will produce finer fibers with the same air flow at commercially acceptable throughputs. A commercially acceptable throughput is above 3 PIH. However, lower throughputs will further reduce the fiber and pore sizes of the meltblown layer 32.

The resulting meltblown web 32, with its fine fibers and resulting small pore size distribution, exhibits excellent barrier properties when incorporated into a fabric laminate. In particular, the unlaminated meltblown web 32 has an average fiber size of 1 to 3 µm and pore sizes distributed predominantly in the range of 7 to 12 µm, with a smaller amount of pores between 12 and 25 µm, virtually no pores above 25 µm, with the peak of the pore size distribution being below 10 µm.

When the meltblown nonwoven 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 that are predominantly distributed in the range of 5 to 10 µm, with a smaller amount of pores between 10 and 15 µm, with virtually no pores above 22 µm, with the peak of the pore size distribution being shifted downward by up to 5 µm.

Fig. 3 shows the pore size distribution for a meltblown nonwoven fabric prepared in accordance with the present invention (Sample 1), an SMS fabric laminate made using the meltblown web of the present invention (Sample 2), a conventional meltblown web (Sample 3), and an SMS fabric laminate such as Kimberly-Clark's Evolution® medical SMS fabric laminate made using the conventional meltblown web (Sample 4). Specifically, the meltblown 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 process used to achieve a high melt flow polymer suitable for making a nonwoven fabric of fine fibers at economical production rates 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 g/10 min at 230ºC. A small amount of peroxide is added to the starter 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 g/10 min at 230ºC.

EXAMPLE 1

To illustrate the foregoing invention, a meltblown web was formed on a conventional meltblown molding belt using the modified polymer of the present invention. In addition, an SMS fabric laminate was formed using the meltblown web of the present invention as an inner barrier layer. The SMS fabric laminate had spunbond layers formed from polypropylene in a conventional manner. The SMS fabric laminate was preferably formed on-line on a multi-station molding machine as illustrated in Figure 1. The meltblown web and meltblown barrier layer for the SMS fabric laminate were formed from polypropylene reaction grains having an initial molecular weight distribution between 4.0 and 4.5 Mw/Mn and a melt flow rate of about 2000 g/10 min at 230°C. The polypropylene starter resin was added with about 500 ppm peroxide to produce a resin having a melt flow rate greater than 3000 g/10 min at 230°C and a molecular weight distribution of 2.8 to 3.5 Mw/Mn. The broader molecular weight distribution at the high melt flow rate helps minimize the generation of lint and polymer droplets.

The meltblown web prepared in accordance with the foregoing had a basis weight of 0.50 oz./yd.2 and was designated Sample 1. The SMS fabric laminate with an inner meltblown barrier layer prepared in accordance with the present invention had spunbond layers having a basis weight of 0.55 oz./yd.2 and the meltblown barrier layer had a basis weight of 0.50 oz./yd.2. The SMS fabric laminate of the present invention was designated Sample 2. In addition, as controls, a conventional meltblown web and a conventional SMS fabric laminate (Kimberly-Clark's Evolution® fabric laminate) were prepared having the same basis weights as the inventive web and SMS fabric laminate. The control meltblown web was designated Sample 3 and the control SMS fabric laminate was designated Sample 4. Samples 1 to 4 have the properties shown in Tables 1 and 2 below: TABLE 1 % Pore size distribution

The pore size distribution contained in Table 1 was measured using the Coulter Porometer. The pore size distribution contained in Table 1 is graphically shown in Fig. 3. The curves shown in Fig. 3 show the finer pore size distribution of Samples 1 and 2 compared to Sample 3 and Sample 4, respectively. The pore size distribution for the nonwoven fabric of the invention and the SMS fabric laminate of the invention is narrower than that of the conventional meltblown nonwoven fabric and the conventional SMS fabric laminate. Note that the pore size distribution for the SMS fabric laminate of the invention has shifted the peak of its curve down by as much as 5 µm from the peak of the meltblown nonwoven fabric alone prior to lamination. Apparently, the lamination process and the additional spunbond layers cause the pore structure to become denser, thereby improving the barrier properties of the resulting fabric laminate. The pore size distribution, which is predominantly between 5 and 10 µm, represents a fabric laminate (sample 2) that has a finer structure than conventional fabric laminates (Sample 4) plus the resulting improved barrier properties,

The improved barrier properties of the fabric laminate of the invention (Sample 2) compared to the conventional fabric laminate (Sample 4) are shown in Table 2 below. TABLE 2 Barrier Properties

Bacterial filtration efficiency

Sample 2 95.4%

Sample 4 91.9%

Blood penetration was measured by the following procedure. A 7 in*** by 9 in. piece of each sample fabric was placed on a similarly sized piece of blotter paper. The blotter paper was supported by a water-filled bladder which itself rested 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 g sample of bovine blood was placed on the fabric sample and covered with a piece of plastic film. A solid plate was placed over the plastic film. The water bladder was then wound up until a pressure of 1 psi**** was reached under the blotter paper. Once the pressure was reached, this pressure was increased to the desired time. Once the time had passed, the pressure was released and the blotting paper was removed and weighed. The percentage penetration was determined based on the difference in the weight of the blotting paper between before and after.

The test results indicate that the SMS fabric laminate made in accordance with the present invention has excellent puncture properties, particularly for short periods of time. Short periods of time represent the situations most often encountered in medical applications where blood will generally not remain on the cloth or gown for long before it can drain away.

The filter properties were measured to determine the ability of the SMS fabric laminate to block the passage of suspended 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 effectiveness within the plus 90% range represents a significant improvement in filtration and ability to prevent the passage of suspended bacteria.

Claims (13)

1. A nonwoven web of fine fibers formed from a reactor granulate of a polypropylene resin having a molecular weight distribution Mw/Mn between 4.0 and 4.5 and a melt flow rate of 1000-3000 g/10 min at 230ºC, which has been modified by adding a small amount of peroxide to change the molecular weight distribution Mw/Mn to the range of 2.8 to 3.5 and to increase the melt flow rate up to 5000 q/10 min at 230ºC, the modified polypropylene being obtained by adding up to 500 ppm peroxide to the reactor granulate prior to forming the web. 2. The nonwoven web of claim 1, wherein the modified polypropylene has a melt flow rate of 3000-5000 g/10 min at 230°C. 3. A nonwoven web according to any one of the preceding claims, wherein the web is formed at a polymer throughput of more than 0.535 kg per cm of die width per hour (3PIH). 4. A nonwoven web according to any one of the preceding claims, wherein the web has an average fiber size of 1 to 3 µm and pore sizes are predominantly distributed in the range of 7 to 12 µm, with the peak of the pore size distribution being less than 10 µm. 5. A process for forming a nonwoven web having an average fiber size of 1 to 3 µm and pore sizes predominantly distributed in the range of 7 to 12 µm, wherein the peak of the pore size distribution smaller than 10 µm, comprising the steps of melt blowing a reactor pellet of a modified polypropylene having a molecular weight distribution Mw/Mn between 2.8 and 3.5 and a melt flow rate of 8400 g/10 min to 5000 g/10 min, wherein the polymer is passed through at a rate of more than 0.535 kg per cm die width per hour (~3PIH). 6. . A process for forming a nonwoven web according to claim 5, having fine fibers and a small pore size distribution, wherein the modified polypropylene has a melt flow rate of more than 3000 g/10 min at 230°C. 7. The process of claim 6, wherein the modified polymer is obtained by adding up to 500 ppm peroxide to the reactor granulate prior to forming the web. 8. Use of the nonwoven web according to one of claims 1 to 4, or produced according to one of claims 5 to 7, as a barrier layer in a nonwoven laminate. 9. Use according to claim 8, wherein the resulting fabric laminate has pore sizes distributed predominantly in the range of 5 to 10 µm, with a minor amount of pores of 10-15 µm and essentially no pores larger than 22 µm, the peak of the pore size distribution preferably being shifted downward by up to 5 µm from the peak of the meltblown web alone. 10. A nonwoven laminate having at least two layers, wherein one of the layers comprises a nonwoven web according to any one of claims 1 to 4 or manufactured according to any one of claims 5 to 7. 11. The nonwoven laminate of claim 10, wherein the web is a barrier layer in the nonwoven laminate. 12. The nonwoven laminate of claim 11, wherein the nonwoven laminate is an SMS nonwoven laminate with an inner barrier layer. 13. Use of the nonwoven laminate according to one of claims 10 to 12 in a sterilization drape or a surgical fabric.