JP2849591B2 - Nonwoven web with improved barrier properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a nonwoven web having a fine fiber and pore distribution, and to a method for producing such a nonwoven web. The method according to the invention uses a reactant particulate resin having a broad initial molecular weight distribution. This resin is
It is processed to narrow its molecular weight distribution and increase its melt flow. As a result, the nonwoven web according to the present invention can be manufactured with high efficiency by the melt spraying method. Such nonwoven webs are particularly useful for use as barrier layers in laminated fabrics.

[0002]

BACKGROUND OF THE INVENTION Nonwoven laminated fabrics are useful in many applications. For example, it is useful for wipes, towels, industrial clothing, medical clothing, medical drapes, and the like. Disposable laminating fabrics have particularly enabled a wide range of applications in hospital medical rooms. For example, drapes, gowns, towels,
Leg covers, sterilization wraps, etc. Such medical bonded fabrics are generally spun-bonded /
It consists of a melt-blown / spunbond (SMS) fabric and has a nonwoven outer layer of spunbond polypropylene and an inner barrier layer of meltblown polypropylene. In particular, the applicant, Kimberly
Clark has long been a registered trademark of Spunguard.
guard) "and" Evolution "
Has been used to manufacture and sell SMS medical non-woven laminated fabrics. The SMS bonded fabric has a durable outer spunbonded layer and an inner melt-blown barrier layer that is porous but does not allow liquid to pass from outside to inside of the bonded fabric. In order to properly produce such a medical bonded fabric, the melt-sprayed barrier layer must have a distribution of fiber size and pore size that ensures air permeability of the bonded fabric and does not allow liquid to pass through. is necessary.

[0003] The current pore size distribution of melt blown webs used in Kimberly-Clark's medical laminated fabric "Evolution" is primarily in the range of 10 to 15 microns, with a distribution of 10 to 15 microns. It has a peak above a micron. Such a melt-blown web has an advantage as a barrier layer, but improvement in porosity and liquid impermeability can be achieved by using the following melt-blown web. That is, a melt-blown web having a pore size distribution such that the average size of the fibers is 1 to 3 microns, the majority of the pores are in the range of 7 to 12 microns, and the peak of the pore size distribution is less than 10 microns. is there. More specifically, improvements in porosity and liquid-permeability properties can be achieved by adjusting the pore size to a Coulter Porometer.
Achieved by measuring the size distribution of the holes in the melt-blown web as measured mainly in the range of 7 to 12 microns, with fewer holes with a size of 12 to 25 microns and elimination of holes with a size of 25 microns or more. can do.

[0004]

Accordingly, it is an object of the present invention to provide a nonwoven web for use as a barrier layer in a laminated fabric, the nonwoven web comprising:
The average fiber diameter is 1 to 3 microns, the pore size distribution is mainly in the range of 7 to 12 microns, the number of pores in the size of 12 to 25 microns is reduced, and the pore size is less than 10 microns It has a distribution peak.

It is another object of the present invention to provide a nonwoven laminated fabric having a barrier layer having a distribution of fine fibers and small pores. The distribution is mainly in the range of 5-10 microns and 10
The number of pores in the range of ~ 15 microns has been reduced to 22
Substantially no micron or larger pores are present, with the peak of the pore size distribution shifting down to a maximum of 5 microns below the peak for the meltblown web alone.

[0006]

SUMMARY OF THE INVENTION These objects can be achieved by forming a melt-blown web from a resin having a broad molecular weight distribution and a high melt flow rate. The resin here has a small amount of peroxide added prior to processing so that a higher melt flow rate (lower viscosity) can be obtained. In general, the present invention comprises the step of starting with a polymer in the form of reactant particles, the polymer having a molecular weight distribution of 4.0-4.5 Mw / Mn and about 400 grams / 10 at 230 degrees Celsius. And the melting flow rate in minutes. Such a molecular weight reactant particle polymer has a polymer molecular weight distribution of 2.2 to 2.2 ppm by adding a maximum of 3000 ppm of peroxide.
Treated to reduce to 3.5 Mw / Mn. During the melt spraying process, the treated reactant particle polymer has an increased melt flow rate from 400 grams / 10 minutes to 800-5000 grams / 10 minutes at 230 degrees Celsius.

More specifically, an initial molecular weight distribution of 4.0 to 4.5 Mw / Mn and 1000 to 300 at 230 ° C.
A polypropylene resin as a reactant particle having a melt flow rate of 0 gram / 10 minutes is mixed with a small amount of peroxide of 500 ppm or less and a maximum of 5000
Gram / 10 min melt flow rate and 2.8-3.5 Mw / Mn
And an improved polypropylene having a reduced molecular weight distribution.

Alternatively, the improved melt-blown web as a barrier layer can be formed using a resin, more particularly, a polypropylene having a lower molecular weight distribution and a lower melt flow rate. The resin has been treated by adding a large amount of peroxide prior to melt spraying, so as to have a high melt flow rate. The starting reactant particle polypropylene resin is 4.0-4.5
Molecular weight distribution of Mw / Mn and 300 at 230 degrees Celsius
And a melt flow rate in the range of １０００1000 grams / 10 minutes. The polypropylene resin is treated by adding a peroxide in the range of 500-3000 ppm (the lower the initial melt flow, the more peroxide is used). The treated polypropylene resin has a maximum melt flow rate of about 3000 grams / 10 minutes at 230 degrees Celsius,
A reduced molecular weight distribution in the range of 2.2 to 2.8 Mw / Mn.

Most preferably, the initial polypropylene resin used in the melt-blown web according to the present invention is polypropylene reactant particles, said resin comprising 4.0 to 4.5 Mw /
Mn distribution of about 20 at 230 degrees Celsius
Has a melt flow rate of 00/10 minutes and is treated with about 500 ppm of peroxide, thereby having a melt flow rate of greater than 3000/10 minutes at 230 degrees Celsius;
An improved resin having a molecular weight distribution of 2.8-3.5 Mw / Mn is formed. At higher melt flow rates, wider molecular weight distributions are more effective in minimizing lint formation and polymer dripping.

Other objects and advantages of the present invention will become apparent from the following description and with reference to the drawings.

[0011]

The present invention will be described below with reference to preferred embodiments, but the present invention is not limited to these embodiments.
On the contrary, the scope of the invention as set forth in the claims includes all changes, modifications, and equivalents.

FIG. 1 schematically illustrates a forming machine 10 used to make an SMS bonded fabric 12 having a melt sprayed barrier layer 32 according to the present invention. In particular, the forming machine 10 has an endless forming belt 14 having small holes, which belt 14 is wound around rollers 16, 18 so that the belt 14 is driven in the direction of the arrow. I have. The forming machine 10 has three regions. The three are a span bonding region 20, a melt spraying region 22, and a span bonding region 24. Note that a layer having a larger unit weight can be formed using three or more formation regions. Alternatively, each layer can be formed separately, rotated, and later converted to an SMS bonded fabric offline. Further, depending on the needs of the final consumer of the laminated cloth 12, the laminated cloth 12 can be formed of three or more layers or three or less layers.

The spun-bond regions 20, 24 are well-known extruders having spinnerets to form continuous filaments of polymer and to form those filaments on a forming belt 14.
It is deposited so that it is interwoven alternately on top. The spunbond regions 20, 24 may have one or more spinneret heads depending on the processing speed and the particular polymer. It is known to form spunbonded materials, and the construction of such spunbonded regions is well within the ability of one of ordinary skill in the art. The nonwoven spunbond webs 28, 36 are produced by known methods as shown in the following patents. US Patent No. 3,6
Nos. 92,618, 3,338,992, 3,
No. 341,394, No. 3,502,538, No. 3,
No. 502,763, No. 3,909,009, No. 3,
No. 542,615, Canadian Patent No. 803,714, and U.S. Pat. No. 4,340,563. Other methods of forming nonwoven webs with continuous filaments of polymer can also be used.

[0014] Spunbond materials produced using continuous filaments have at least three common characteristics. The first feature is that the polymer is continuously extruded through a spinneret to form discontinuous filaments. For this reason, the filaments are pulled mechanically or hydraulically, without breaking, to adapt the polymer filaments molecularly and to achieve "strength". Finally, the continuous filaments are substantially randomly deposited on the conveyor belt to form and print the web. In detail, span connection area 2
0 is a spun-bonded filament 26 from fiber-forming polymer
Generate The filaments are randomly deposited on belt 14 to form a spun bonded outer layer 28. The fiber-forming polymer is described in detail below.

[0015] The melt-blown area 22 is provided with a die 31 used to form the microfibers 30. The unit of the amount of extrusion of the die 31 is the amount of polymer melted [lb] per inch of die width per hour.
H). As the thermoplastic polymer exits the die 31, the high pressure fluid (usually air) narrows and diffuses the polymer flow to form the microfibers 30. The microfibers 30 are randomly deposited on the surface of the spunbond layer 28 to form a melt sprayed layer 32. The structure and operation of the melt-sprayed region 22 for forming the microfibers 30 and the melt-sprayed layer 32 may be known, and their structure and operation are also within the abilities of those having ordinary skill in the art. It is. Such techniques include, A. Vent
t), E. L. Boon, C.I. D. Full hearty
NFL Report 4364, "Fabrication of Ultrafine Organic Fibers" by Fluharty. D. Lawrence, R.A.
T. Lukas, J.M. A. Young's NRL report 5265, "Improvement of Apparatus for Forming Ultrafine Thermoplastic Fibers", is described in U.S. Pat. No. 3,849,241. Other methods of forming a nonwoven web of microfibers can be used in the present invention.

[0016] The melt-blown area 22 creates microfibers 30 from the fiber-forming polymer. This will be described in detail below. The microfibers 30 are randomly deposited on the surface of the spunbond layer 28 to form a melt-blown initial layer 32. In SMS bonded fabric, for example, about from 0.35 to 0.50 oz / yd ² as meltblown barrier layer 32 is preferably a unit weight value.

After the initial layer 32 has been deposited on the spunbond outer layer 28 by the meltblown region 22, the spunbond region 24 creates a spunbond filament 34. The spun-bonded filaments 34 are deposited in random directions on the surface of the melt-sprayed layer 32 to form an outer spun-bonded layer 36. In SMS medical bonded fabric, for example, layers 28 and 36 have about 0.3 to about 1.2 ounces / yard ² as each preferred unit weight value.

The completed SMS bonded cloth web 1
2 (see FIG. 2) is then sent through bonding rolls 38,40. Embossed patterns such as spots and grids are provided on the surfaces of the coupling rolls 38 and 40.
These bonding rolls 38, 40 are heated and the web 12
Is the softening temperature of the polymer used to form this layer. As the web 12 passes between these heated bonding rolls 38, 40, the web 12 is pressed and heated by the bonding rolls 38, 40 according to the pattern on the roll surface, resulting in a pattern of discontinuous areas as shown in FIG. 41 is formed. These discontinuous regions are bonded from layer to layer, and are bonded to specific filaments and / or fibers within each layer. Such a discontinuous region or spot bonding is a known technique, and as described above, by a heating roll, or
Ultrasonic heating of the web 12 can form discontinuous regions where the filaments, fibers and layers are thermally bonded. According to the prior art described in U.S. Pat. No. 4,041,203, it is preferable for the fibers of the melt-sprayed layer in the laminated fabric to be melted inside the bonding area. On the other hand, the filaments of the spunbond layer retain their bonding properties to obtain good strength properties.

According to the present invention, the extrusion amount (PIH) of the die head 22 may be increased, and at the same time, the microfibers may be supplied using a polymer in the form of reactant particles rather than a pelletized polymer. The polymer here has a molecular weight distribution of 4.0-4.5 Mw / Mn and a melt flow rate of about 400 grams / 10 minutes at 230 degrees Celsius. This molecular weight reactant particle polymer has a maximum of 30
By adding 00 ppm of peroxide, 2.2 to
Treated to reduce the polymer molecular weight distribution in the range of 3.5 Mw / Mn. During the melt spraying process,
The treated reactant particle polymer has an increased melt flow rate, increasing the value at 230 degrees Celsius from 400 grams / 10 minutes to 800-5000 grams / 10 minutes. By treating the starting polymer, the resulting polymer has a lower stretch clay than before,
When the fiber leaves the die 31, the force for thinning the fiber can be reduced. Therefore, at the same air flow,
Higher melt flow polymers allow smaller fibers to be made at commercial extrusion rates. The amount of extrusion on a commercial basis is 3 PIH or more. However, when the extrusion amount becomes small,. The size of the fibers and holes of the sprayed layer 32 is further reduced.

The resulting melt-blown web 32 has fine fibers and a small pore size distribution and exhibits excellent barrier properties when incorporated into a bonded fabric. In particular, the unlaminated meltblown web 32 has an average fiber size of 1 to 3 microns, with pore sizes distributed primarily in the range of 7 to 12 microns, with pores in the range of 12 to 25 microns. And the pore size distribution peak is 1 with substantially no pores larger than 25 microns.
It is less than 0 microns.

When the melt-blown web 32 is incorporated into the SMS laminating fabric 12, the peak of the pore size distribution in the resulting SMS laminating fabric shifts down to a maximum of 5 microns. In the SMS bonded cloth 12, the pore size is mainly distributed in the range of 5 to 10 microns, the number of pores in the range of 10 to 15 microns is small, and the pores of 22 microns or more are substantially absent. The peak in the size distribution shifts down to a maximum of 5 microns.

FIG. 3 shows a melt-blown web according to the present invention (sample 1) and an SM using the melt-blown web according to the present invention.
SMS such as Kimberly-Clark's SMS Medical Laminated Fabric "Evolution" formed using S-laminated fabric (Sample 2), conventional melt-blown web (Sample 3), and conventional melt-blown web. The hole size distribution for each of the bonded fabrics (Sample 4) is shown. In detail, the melt sprayed web according to the present invention and the SMS bonded fabric according to the present invention are formed according to Example 1 shown below.

The present invention can be practiced with polyolefins, copolymers, terpolymers, or mixtures thereof, including polypropylene, polyethylene, or other alpha olefins polymerized with Ziegler-Natta catalyst technology. Polypropylene is preferred.

Two methods can be used to achieve high melt polymers useful for producing nonwoven webs of microfibrils at commercial production rates. The first and preferred method comprises reacting particulate polypropylene having a molecular weight distribution in the range of 4.0-4.5 Mw / Mn and a high melt flow rate of 1000-3000 grams / 10 minutes at 230 degrees Celsius with starting material It is to be. A small amount of peroxide was added to the starting resin to give a molecular weight distribution of 2.8-3.5M.
Change to w / Mn range and increase the melt flow rate at 230 degrees Celsius to 5000/10 minutes.

A second, less preferred method for producing a nonwoven web of microfibers according to the present invention is 4.0.
The starting material is a reactant particle resin having a molecular weight distribution in the range of ~ 4.5 Mw / Mn and a lower melt flow rate than in the first method. By adding more peroxide to the starting resin than in the first method, the melt flow rate is increased and the molecular weight distribution is broadened. The starting material reactant particles polypropylene resin has a molecular weight distribution in the range of 4.0-4.5 Mw / Mn and 300-1000 at 230 degrees Celsius.
Grams / 10 minutes of melt flow rate. The polypropylene resin is treated by adding peroxide in an amount in the range of 500-3000 ppm (the lower the initial melt flow rate, the more peroxide is used). The treated polypropylene resin has a maximum of about 300 at 230 degrees.
It has a melt flow rate of 0 grams / 10 minutes and a reduced molecular weight distribution of 2.2-2.8 Mw / Mn. This second method has a narrower molecular weight distribution than the first preferred method.
2 to 2.8 Mw / Mn is formed, and therefore, lint and polymer are easily dropped.

(Experimental example 1)

To illustrate the invention described above, a melt-blown web was formed on a conventional melt-blow forming line using the treated polymer of the present invention. In addition, SMS using the melt-blown web according to the invention as the inner barrier layer
A laminated cloth was formed. The SMS bonded fabric had a spunbond layer formed of conventional polypropylene. This SMS-bonded cloth was preferably formed on-line by a multi-region forming machine as shown in FIG. Meltblown webs and meltblown barriers for SMS laminated fabrics are a reaction of polypropylene having an onset molecular weight distribution in the range of 4.0-4.5 Mw / Mn and a melt flow rate of about 2000 grams / 10 minutes at 230 degrees Celsius. Formed from body particles. The starting polypropylene resin is treated with about 500 ppm of peroxide and has a melt flow rate of more than 3000 grams / 10 minutes and a 2.8-3.5 Mw / M
a resin having a molecular weight distribution in the range of n. Widening the molecular weight distribution at high melt flow rates helps to minimize the formation of lint and polymer droplets.

[0028] The melt blown web formed in accordance with the foregoing had a basis weight of 0.50 oz / yd ^2. This is shown as sample 1 in FIG. SMS with melt sprayed inner barrier layer according to the present invention
Bonded fabric basis weight has a spunbonded layer is 0.55 oz / yd ^2, the meltblown barrier layer 0.50
It had a basis weight of ounces / yard ² . The SMS bonded fabric according to the present invention is shown in FIG.

Further, a conventional melt-blown web and a conventional SMS-laminated cloth having the same basis weight as the melt-blown web according to the present invention and the SMS-laminated cloth according to the present invention (laminated cloth “Evolution” by Kimberly-Clark). ") Was prepared as a comparative example. The melt blown web of the comparative example is shown as sample 3 and the SMS bonded fabric of the comparative example is shown as sample 4. Sample 1
No. 4 has the characteristics shown in Tables 1 and 2 below.

Table 1 Pore size distribution [%] 0-5 μ 5-10 μ 10-15 μ 15-20 μ Sample 1 50.7 45.8 2.9 Sample 2 1.8 55.4 40.3 1.9 Sample 3 10.5 67.7 21.4 Sample 4 1.2 20.0 61.6 11.6 20-25μ 25-30μ Maximum pore size Sample 1 0.60 Sample 2 0.40 22.0μ Sample 30 0.5 0.1 sample 4 1.2 0.9 38.2μ

The pore size distribution shown in Table 1 is based on Coulter
It was measured with a porometer (Coulter Porometer). FIG. 3 is a graph showing the pore size distribution shown in Table 1. The plot in FIG. 3 shows the micropore size distribution of Samples 1 and 2 as compared to Samples 3 and 4, respectively. Web according to the invention and SM according to the invention
The pore size distribution for S-bonded fabrics is narrower than conventional melt-blown webs and conventional SMS-bonded fabrics. It should be noted here that the SM according to the present invention is used.
The pore size distribution for the S-laminated fabric is that the peak of the curve shifts up to 5 microns below the peak of the meltblown web alone before lamination. Obviously, the lamination process and the addition of the spun tie layer cause the pore structure to be tight, thereby improving the barrier properties of the finished laminated fabric. The fact that the pore size distribution is mainly in the range of 5 to 10 microns means that the bonded fabric (Sample 2), which has a smaller structure than the conventional bonded fabric (Sample 4), results in a barrier property thereof. Indicates an improvement.

The bonded cloth according to the present invention (sample 2) in comparison with the conventional bonded cloth (sample 4)
Are shown in Table 2.

Table 2 Barrier properties (blood passage properties) t = 0 min t = 1 min p = 1 psi s p = 1 psi Sample 1 2.5% 12.4% Sample 2 10.6% 14.5% (Bacterial filtration efficiency ) Sample 1 95.4% Sample 2 91.9%

The blood passage characteristics were measured according to the following procedure. A 7 inch by 9 inch piece of each sample is placed on a piece of blotter paper of the same size. Each blotter is supported on a bag filled with water, and the bag is supported on a jack. The jack was equipped with a gauge, which measured the force acting on the gauge and calculated from this force the pressure exerted by the bag on the blotting paper. A 1.4 gram sample of bovine blood is placed on the laminated cloth sample and covered with a piece of plastic film. The fixing plate is placed above the plastic film. The water bag is then jacked up so that 1 psi of pressure acts on the bottom of the blotter. As soon as a pressure of 1 psi is applied, this pressure is maintained for the desired time. After the desired time, the pressure is released and the blotter is removed and weighed. Based on the weight difference before and after the blotting paper, the passing rate [%] is determined.

From the test results, it can be seen that the SMS-bonded cloth according to the present invention, especially in a short time,
It has excellent passing characteristics. Here, "in a short time" indicates that blood occurs most frequently for medical use because it generally does not stay in a drape or gown long enough to allow it to flow down.

To determine the ability of the SMS bonded fabric to block the penetration of airborne bacteria, the filtration characteristics were measured. Each sample was prepared as described in Mil. Spec. 36954-C4.4.
The test was performed according to 1.1.1 and 4.4.1.2.

The efficiency is 3.5% in the range of 90% or more.
The improvement indicates a significant improvement in filtration properties and ability to exclude the passage of airborne bacteria.

[Brief description of the drawings]

FIG. 1 is a schematic view of a forming machine used for making a nonwoven bonded fabric including a melt-sprayed barrier layer according to the present invention.

FIG. 2 is a cross-sectional view of the nonwoven laminated fabric according to the present invention, showing a layer structure including the inner melt-sprayed barrier layer according to the present invention.

FIG. 3 shows a melt-blown web according to the invention (sample 1),
SM containing the melt-blown web of Sample 1 as a barrier layer
5 is a graph showing the pore size distribution for an S bonded fabric (Sample 2), a conventional melt-blown web (Sample 3), and a conventional SMS bonded fabric (Sample 4).

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Forming machine 12 SMS bonding cloth 14 Belt 16 Roller 18 Roller 20 Spun bonding area 22 Melt spraying area 24 Spun bonding area 26 Spun bonding filament 28 Nonwoven spun bonding web 30 Fine fiber 32 Melt blowing barrier layer 34 Spun bonding filament 36 Non Woven spun binding web 38 binding roll 40 binding roll 41 pattern

Continuing on the front page (72) Inventor Lin Sun Woon Georgia, USA 30062 Marietta Green Ridge Drive 4265 (58) Field surveyed (Int. Cl. ⁶ , DB name) D04H 1/00-18/00

Claims (19)

(57) [Claims] 1. A nonwoven web of microfibrils formed from treated particles of a treated polymer, wherein the polymer has a molecular weight distribution of 2.2-3.5 Mw / Mn and 800 grams / 10 at 230 degrees Celsius. A non-woven web having a melt flow rate greater than 1 minute. 2. The nonwoven web of claim 1, wherein said web is formed at a polymer extrusion rate of greater than 3 PIH. 3. The web of claim 1, wherein the web has an average fiber size of 1 to 3 microns and a pore size distributed primarily in the range of 7 to 12 microns, with a peak pore size distribution of less than 10 microns. The nonwoven web according to claim 1, wherein 4. A non-woven web formed from treated polymer reactant particles, said polymer comprising 2.8 to 2.8.
A nonwoven web having a molecular weight distribution of 3.5 Mw / Mn and a melt flow rate of greater than 3000 grams / 10 minutes at 230 degrees Celsius. 5. The process of claim 4, wherein the treatment of the polymer is performed by adding up to 500 ppm of peroxide to the reactant particles prior to forming the web.
A nonwoven web as described. 6. The nonwoven web according to claim 4, wherein said web is formed at a polymer extrusion rate of greater than 3 PIH. 7. The web according to claim 1, wherein the web has an average fiber size of 1 to 3 microns and a pore size distributed mainly in the range of 7 to 12 microns, with a peak of the pore size distribution being less than 10 microns. The nonwoven web according to claim 4 or 5, wherein 8. A nonwoven web formed from treated polymer reactant particles, said polymer comprising 2.2 to 2.2.
A nonwoven web having a molecular weight distribution of 2.8 Mw / Mn and a melt flow rate greater than 800 grams / 10 minutes at 230 degrees Celsius. 9. The nonwoven web of claim 8, wherein the treatment of the polymer is performed by adding 500-3000 ppm of peroxide to the reactant particles prior to forming the web. 10. The nonwoven web according to claim 8 or claim 9, wherein the web is formed at a polymer extrusion rate of greater than 3 PIH. 11. The web according to claim 1, wherein the web has an average fiber size of 1 to 3 microns and a pore size distributed mainly in the range of 7 to 12 microns, with a peak of the pore size distribution being less than 10 microns. The nonwoven web according to claim 8 or 9, wherein 12. A process comprising: melt spraying treated polymer reactant particles having a molecular weight distribution of 2.2 to 3.5 Mw / Mn and a melt flow rate of greater than 800 grams / 10 minutes at 230 degrees Celsius. A method for forming a nonwoven web having microfibers and pore size distribution. 13. The method of claim 12, wherein said polymer is formed at an extrusion volume greater than 3 PIH. 14. A process comprising melt-blowing reactant particles of a treated polymer having a molecular weight distribution of 2.8 to 3.5 Mw / Mn and a melt flow rate of greater than 3000 grams / 10 minutes at 230 degrees Celsius. A method for forming a nonwoven web having microfibers and pore size distribution. 15. The method of claim 14, wherein the treated polymer is generated by adding up to 500 ppm of peroxide to the reactant particles prior to forming the nonwoven web. 16. The method of claim 14 or claim 15, wherein the polymer is formed at an extrusion volume greater than 3 PIH. 17. A process comprising melt spraying reactant particles of a treated polymer having a molecular weight distribution of 2.2 to 2.8 Mw / Mn and a melt flow rate of greater than 800 grams / 10 minutes at 230 degrees Celsius. A method for forming a nonwoven web having microfibers and pore size distribution. 18. The method of claim 17, wherein said treated polymer is formed by adding 500-3000 ppm of peroxide to said reactant particles prior to forming said nonwoven web. 19. The method of claim 17 or claim 18, wherein the polymer is formed at an extrusion volume greater than 3 PIH.