WO2009151437A1

WO2009151437A1 - Humidification of polylactic acid for fiber formation

Info

Publication number: WO2009151437A1
Application number: PCT/US2008/066257
Authority: WO
Inventors: Ryan James Mceneany; Vasily A. Topolkaraev
Original assignee: Kimberly-Clark Worldwide, Inc.
Priority date: 2008-06-09
Filing date: 2008-06-09
Publication date: 2009-12-17

Abstract

A method and system for forming polylactic acid fibers is provided. The method, for instance, involves supplying a polylactic acid resin to a feed chamber (e.g., hopper) of an extrusion device and contacting the resin with humidified air within the feed chamber. Moisture within the humidified air initiates a hydrolysis reaction in which hydroxyl groups present in water attack the ester linkage of polylactic acid, thereby leading to chain scission or 'depolymerization' of the polylactic acid molecule into one or more shorter ester chains and form a hydrolytically degraded polylactic acid having a molecular weight lower than the starting polymer. Such lower molecular weight polymers have a higher melt flow index and lower apparent viscosity, which are useful in a wide variety of fiber forming applications, such as in the formation of nonwoven webs. The present inventors have discovered the hydrolysis reaction may be performed in a consistent and continuous manner by selectively adjusting the relative humidity and temperature of the humidified air entering the feed chamber. In this manner, the moisture absorption rate, residence time, and chamber size may also be optimized as desired.

Description

HUMIDIFICATION OF POLYLACTIC ACID FOR FIBER FORMATION Background of the Invention

Biodegradable nonwoven webs are useful in a wide range of applications, such as in the formation of disposable absorbent products (e.g., diapers, training pants, sanitary wipes, feminine pads and liners, adult incontinence pads, guards, garments, etc.). To facilitate formation of the nonwoven web, a biodegradable polymer should be selected that is melt processable, yet also has good mechanical and physical properties. Polylactic acid ("PLA") is a common biodegradable and sustainable (renewable) polymer. Although various attempts have been made to use polylactic acid in the formation of nonwoven webs, its high molecular weight and viscosity have generally restricted its use to only certain types of fiber forming processes. For example, conventional polylactic acids are not typically suitable for meltblowing processes, which require a low polymer viscosity for successful microfiber formation. As such, a need currently exists for a biodegradable polylactic acid that exhibits good mechanical and physical properties, but which may be readily formed into a nonwoven web using a variety of techniques (e.g., meltblowing).

Summary of the Invention In accordance with one embodiment of the present invention, a method for forming polylactic acid fibers is disclosed that comprises supplying a polylactic acid resin to a feed chamber; contacting the polylactic resin within the feed chamber with humidified air to form a humidified polylactic acid resin; and extruding the humidified polylactic acid resin through a die to form a fiber. The humidified air has a dry bulb temperature of from about 15°C to about 85°C and a relative humidity of about 30% or more.

In accordance with another embodiment of the present invention, a system for forming polylactic acid fibers is disclosed that comprises a humidifier that is configured to receive a supply of liquid water and convert the liquid water into a humidified air; a temperature control device that is configured to receive and heat the humidified air from the humidifier; a feed chamber that is configured to receive a polylactic acid resin and the humidified air from the temperature control device and form a humidified polylactic acid resin; and an extruder for melt processing the humidified polylactic acid resin through a die to form fibers. Other features and aspects of the present invention are discussed in greater detail below.

Brief Description of the Drawings

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended figures in which:

Fig. 1 is a schematic illustration one embodiment for supplying humidified air to a feed chamber in accordance with the present invention; Fig. 2 is a schematic illustration of a process that may be used in one embodiment of the present invention to form a nonwoven web; and

Fig. 3 is a representative example of a psychrometric chart that may be employed in the present invention.

Repeat use of references characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention.

Detailed Description of Representative Embodiments Reference now will be made in detail to various embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Definitions

As used herein, the term "biodegradable" or "biodegradable polymer" generally refers to a material that degrades from the action of naturally occurring microorganisms, such as bacteria, fungi, and algae; environmental heat; moisture; or other environmental factors. The biodegradability of a material may be determined using ASTM Test Method 5338.92. As used herein, the term "fibers" refer to elongated extrudates formed by passing a polymer through a forming orifice such as a die. Unless noted otherwise, the term "fibers" includes discontinuous fibers having a definite length and substantially continuous filaments. Substantially filaments may, for instance, have a length much greater than their diameter, such as a length to diameter ratio ("aspect ratio") greater than about 15,000 to 1 , and in some cases, greater than about 50,000 to 1.

As used herein, the term "monocomponent" refers to fibers formed from one polymer. Of course, this does not exclude fibers to which additives have been added for color, anti-static properties, lubrication, hydrophilicity, liquid repellency, etc.

As used herein, the term "multicomponent" refers to fibers formed from at least two polymers (e.g., bicomponent fibers) that are extruded from separate extruders. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the fibers. The components may be arranged in any desired configuration, such as sheath-core, side-by-side, segmented pie, island-in-the-sea, and so forth. Various methods for forming multicomponent fibers are described in U.S. Patent Nos. 4,789,592 to Taniguchi et ah and U.S. Patent No. 5,336,552 to Strack et aL 5,108,820 to Kaneko, et al., 4,795,668 to Krueqe, et al., 5,382,400 to Pike, et al., 5,336,552 to Strack, et al., and 6,200,669 to Marmon, et al., which are incorporated herein in their entirety by reference thereto for all purposes. Multicomponent fibers having various irregular shapes may also be formed, such as described in U.S. Patent Nos. 5,277,976 to Hoqle, et al., 5,162,074 to JHiHs, 5,466,410 to JHiMs, 5,069,970 to Larqman, et al., and 5,057,368 to Largman, et al., which are incorporated herein in their entirety by reference thereto for all purposes.

As used herein, the term "multiconstituent" refers to fibers formed from at least two polymers (e.g., biconstituent fibers) that are extruded as a blend. The polymers are not arranged in substantially constantly positioned distinct zones across the cross-section of the fibers. Various multiconstituent fibers are described in U.S. Patent No. 5,108,827 to Gessner, which is incorporated herein in its entirety by reference thereto for all purposes. As used herein, the term "nonwoven web" refers to a web having a structure of individual fibers that are randomly interlaid, not in an identifiable manner as in a knitted fabric. Nonwoven webs include, for example, meltblown webs, spunbond webs, carded webs, wet-laid webs, airlaid webs, coform webs, hydraulically entangled webs, etc. The basis weight of the nonwoven web may generally vary, but is typically from about 5 grams per square meter ("gsm") to 200 gsm, in some embodiments from about 10 gsm to about 150 gsm, and in some embodiments, from about 15 gsm to about 100 gsm.

As used herein, the term "meltblown" web or layer generally refers to a nonwoven web that is formed by a process in which a molten thermoplastic material is extruded through a plurality of fine, usually circular, die capillaries as molten fibers into converging high velocity gas (e.g., air) streams that attenuate the fibers of molten thermoplastic material to reduce their diameter, which may be to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly dispersed meltblown fibers. Such a process is disclosed, for example, in U.S. Patent Nos. 3,849,241 to Butin, et al.; 4,307,143 to Meitner, et al.; and 4,707,398 to Wisneski, et al., which are incorporated herein in their entirety by reference thereto for all purposes. Meltblown fibers may be substantially continuous or discontinuous, and are generally tacky when deposited onto a collecting surface.

As used herein, the term "spunbond" web or layer generally refers to a nonwoven web containing small diameter substantially continuous filaments. The filaments are formed by extruding a molten thermoplastic material from a plurality of fine, usually circular, capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, eductive drawing and/or other well-known spunbonding mechanisms. The production of spunbond webs is described and illustrated, for example, in U.S. Patent Nos. 4,340,563 to Appel, et al., 3,692,618 to Dorschner, et al., 3,802,817 to Matsuki, et ah, 3,338,992 to Kinnev, 3,341 ,394 to Kinney, 3,502,763 to Hartman, 3,502,538 to Levy, 3,542,615 to Dobo, et al.. and 5,382,400 to Pike, et al., which are incorporated herein in their entirety by reference thereto for all purposes. Spunbond filaments are generally not tacky when they are deposited onto a collecting surface. Spunbond filaments may sometimes have diameters less than about 40 micrometers, and are often between about 5 to about 20 micrometers.

As used herein, the term "coform web" generally refers to a composite material containing a mixture or stabilized matrix of thermoplastic fibers and a second non-thermoplastic material. As an example, coform materials may be made by a process in which at least one meltblown die head is arranged near a chute through which other materials are added to the web while it is forming. Such other materials may include, but are not limited to, fibrous organic materials such as woody or non-woody pulp such as cotton, rayon, recycled paper, pulp fluff and also superabsorbent particles, inorganic and/or organic absorbent materials, treated polymeric staple fibers and so forth. Some examples of such coform materials are disclosed in U.S. Patent Nos. 4,100,324 to Anderson, et al.; 5,284,703 to Everhart, et al.: and 5,350,624 to Georger, et al.; which are incorporated herein in their entirety by reference thereto for all purposes. Detailed Description

The present invention is directed to a method and system for forming polylactic acid fibers. The method, for instance, involves supplying a polylactic acid resin to a feed chamber (e.g., hopper) of an extrusion device and contacting the resin with humidified air within the feed chamber. Moisture within the humidified air initiates a hydrolysis reaction in which hydroxyl groups present in water attack the ester linkage of polylactic acid, thereby leading to chain scission or "depolymerization" of the polylactic acid molecule into one or more shorter ester chains and form a hydrolytically degraded polylactic acid having a molecular weight lower than the starting polymer. Such lower molecular weight polymers have a higher melt flow index and lower apparent viscosity, which are useful in a wide variety of fiber forming applications, such as in the formation of nonwoven webs. The present inventors have discovered the hydrolysis reaction may be performed in a consistent and continuous manner by selectively adjusting the relative humidity and temperature of the humidified air entering the feed chamber. In this manner, the moisture absorption rate, residence time, and chamber size may also be optimized as desired.

The polylactic acid may generally be derived from monomer units of any isomer of lactic acid, such as levorotory-lactic acid ("L-lactic acid"), dextrorotatory- lactic acid ("D-lactic acid"), meso-lactic acid, or mixtures thereof. Monomer units may also be formed from anhydrides of any isomer of lactic acid, including L- lactide, D-lactide, meso-lactide, or mixtures thereof. Cyclic dimers of such lactic acids and/or lactides may also be employed. Any known polymerization method, such as polycondensation or ring-opening polymerization, may be used to polymerize lactic acid. A small amount of a chain-extending agent (e.g., a diisocyanate compound, an epoxy compound or an acid anhydride) may also be employed. The polylactic acid may be a homopolymer or a copolymer, such as one that contains monomer units derived from L-lactic acid and monomer units derived from D-lactic acid. Although not required, the rate of content of one of the monomer unit derived from L-lactic acid and the monomer unit derived from D- lactic acid is preferably about 85 mole% or more, in some embodiments about 90 mole% or more, and in some embodiments, about 95 mole% or more. Multiple polylactic acids, each having a different ratio between the monomer unit derived from L-lactic acid and the monomer unit derived from D-lactic acid, may be blended at an arbitrary percentage. Of course, polylactic acid may also be blended with other types of polymers (e.g., polyolefins, polyesters, etc.) to provided a variety of different of benefits, such as processing, fiber formation, etc.

In one particular embodiment, the polylactic acid has the following general structure:

One specific example of a suitable polylactic acid polymer that may be used in the present invention is commercially available from Biomer, Inc. of Krailling, Germany) under the name BIOMER™ L9000. Other suitable polylactic acid polymers are commercially available from Natureworks LLC of Minnetonka,

Minnesota (NATUREWORKS®) or Mitsui Chemical (LACEA™). Still other suitable polylactic acids may be described in U.S. Patent Nos. 4,797,468; 5,470,944; 5,770,682; 5,821 ,327; 5,880,254; and 6,326,458, which are incorporated herein in their entirety by reference thereto for all purposes. The polyiactic acid typically has a number average molecular weight ("M_n") ranging from about 40,000 to about 160,000 grams per mole, in some embodiments from about 50,000 to about 140,000 grams per mole, and in some embodiments, from about 80,000 to about 120,000 grams per mole. Likewise, the polymer also typically has a weight average molecular weight ("M_w") ranging from about 80,000 to about 200,000 grams per mole, in some embodiments from about 100,000 to about 180,000 grams per mole, and in some embodiments, from about 110,000 to about 160,000 grams per mole. The ratio of the weight average molecular weight to the number average molecular weight ("IvUM_n"), i.e., the "polydispersity index", is also relatively low. For example, the polydispersity index typically ranges from about 1.0 to about 3.0, in some embodiments from about 1.1 to about 2.0, and in some embodiments, from about 1.2 to about 1.8. The weight and number average molecular weights may be determined by methods known to those skilled in the art. The polyiactic acid may also have an apparent viscosity of from about 50 to about 600 Pascal seconds (Pa-s), in some embodiments from about 100 to about 500 Pa-s, and in some embodiments, from about 200 to about 400 Pa-s, as determined at a temperature of 190⁰C and a shear rate of 1000 sec^"1. The melt flow index of the polyiactic acid (on a dry basis) may also range from about 0.1 to about 40 grams per 10 minutes, in some embodiments from about 0.5 to about 20 grams per 10 minutes, and in some embodiments, from about 5 to about 15 grams per 10 minutes. The melt flow index is the weight of a polymer (in grams) that may be forced through an extrusion rheometer orifice (0.0825-inch diameter) when subjected to a load of 2160 grams in 10 minutes at a certain temperature (e.g., 190⁰C), measured in accordance with ASTM Test Method D1238-E.

The polyiactic acid also typically has a melting point of from about 100⁰C to about 240⁰C, in some embodiments from about 120⁰C to about 220⁰C, and in some embodiments, from about 140⁰C to about 200⁰C. Such polyiactic acids are useful in that they biodegrade at a fast rate. The glass transition temperature ("T₉") of the polyiactic acid may be relatively high, such as from about 10^cC to about 80⁰C, in some embodiments from about 20⁰C to about 70⁰C, and in some embodiments, from about 25°C to about 65⁰C. As discussed in more detail below, the melting temperature and glass transition temperature may all be determined using differential scanning calorimetry ("DSC") in accordance with ASTM D-3417.

As indicated above, humidified air is supplied to the feed chamber to hydrolytically degrade the starting polylactic acid resin and reduce its molecular weight. It is believed that the hydroxyl groups of water can attack the ester linkages of the polylactic acid, thereby leading to chain scission or "depolymerization" of the polylactic acid molecule into one or more shorter ester chains. The shorter chains may include polylactic acids, as well as minor portions of lactic acid monomers or oligomers, and combinations of any of the foregoing. The amount of moisture employed relative to the polylactic acid affects the extent to which the hydrolysis reaction is able to proceed. However, if the moisture content is too great, the natural saturation level of polylactic acid (e.g., about 3000 ppm) may be substantially exceeded, which may adversely affect resin melt properties and the physical properties of the resulting fibers. Thus, in most embodiments of the present invention, the moisture content is from about 500 to about 3800 parts per million ("ppm"), in some embodiments from about 1000 to about 3500 ppm, in some embodiments from about 2000 to about 3200 ppm, and in some embodiments, from about 2200 to about 3000 ppm, based on the dry weight of the starting polylactic acid. Besides achieving a certain moisture content, the present inventors have also discovered that the rate of moisture absorption (moisture content per hour) may be enhanced through selective control over the relative humidity and dry bulb temperature of the humidified air. More specifically, at higher relative humidity levels, the polylactic acid generally absorbs moisture at a faster rate. Typically, the relative humidity of the air entering the feed chamber is about 30% or more, in some embodiments about 40% or more, in some embodiments from about 50% to about 95%, and in some embodiments, from about 75% to about 90%. Of course, as is well known in the art, the relative humidity may be determined from the temperature of the air (dry bulb temperature) and the dew point temperature using a psychrometric chart, such as shown in Fig. 3. The higher the dew point temperature, the more relative humidity the air will contain at a given air temperature. Conversely, at a given dew point temperature, the air will be hotter and less moisture will be available. To achieve the desired relative humidity, the dew point temperature is typically within a range of about -15°C to about 0⁰C, in some embodiments from about -12°C to about -2°C, and in some embodiments, from about -10°C to about -5⁰C. Likewise, the air temperature is typically maintained within a range of about 15°F to about 85°C, in some embodiments from about 20°C to about 70°C, and in some embodiments, from about 25°C to about 50⁰C.

At a given relative humidity and air temperature, the rate of moisture absorption may also allow for the determination of the dwell time needed to achieve a desired moisture content. Namely, the dwell time may be calculated by dividing the desired moisture content (parts per million) by the average moisture absorption rate (ppm/hour). In this regard, the present inventors have discovered that the average moisture absorption rate of polylactic acid is typically from about 350 ppm to about 650 ppm. Likewise, the present inventors have also discovered that polylactic acid typically possesses an initial moisture content of about 200 ppm and becomes saturated at a total moisture content of about 3000 ppm. Thus, using the equation given above, the desired dwell time may be calculated by dividing the moisture content needed to achieve saturation (i.e., 2800 ppm) by the average absorption rate, which equates to about 4 to about 8 hours. In turn, the dwell time (hours) and the desired throughput of the polymer material (kilograms per hour) may then be used to calculate the size of the feed chamber needed.

The control over the relative humidity, dew point, and temperature of the humidified air may be implemented in a variety of ways. Referring to Fig. 1 , one embodiment of a system 100 for supplying humidified air to a hopper 10 is schematically illustrated. As shown, the hopper 10 contains a housing 11 1 that defines an interior 112 is provided with one or more inwardly converging, sloped walls 113, to funnel the polylactic acid resin 114 into a discharge 140. The upper portion of the housing 111 may contain one or more walls 115 and top 116 that together form a generally cylindrical enclosure. As is well known in the art, the hopper 10 may also contain baffles, inverted cones, screens, fins, etc. for directing the resin along a circuitous path through the hopper 10 and assisting in the even distribution of the humidified air throughout the entire interior volume of the hopper 10.

The system 100 also includes a humidifier 120 that receives a flow of liquid water 122 from an external source (not shown). A filter 162 may optionally be employed to remove impurities (e.g., particles, metals, etc.) from the water supply before it enters the humidifier 120. The flow of the liquid water 122 may be controlled using a solenoid valve 184 (e.g., slow acting water solenoid valve) that is connected to a main computer, or programmable logic controller ("PLC") 182. In addition to receiving the liquid water 122, the humidifier 120 may also receive an air stream 189 that is recirculated from the hopper 10. If desired, the air stream 189 may be passed through a filter 160 and supplied to the humidifier 120 by a blower 162. The humidifier 120 may accomplish water vaporization in any of a number of ways, including for example, through the use of a resistive heating element for vaporizing water, a combustion-heated vaporizer, ultrasonic vaporization devices, and/or evaporative vaporization surfaces. Regardless of the mechanism employed, the humidifier 120 converts the liquid water 122 into humidified air 124 and supplies the humidified air 124 to the hopper 10. Although not required, the flow of the humidified air 124 though the hopper 10 may be cross-current (e.g., in a generally opposite direction) to the flow of the polylactic acid resin 114 through the hopper 10. For example, the polylactic acid resin 114 may be gravity fed through the hopper 10 from an article inlet 117 adjacent the top of the hopper 10, while the humidified air 124 is introduced through a vapor inlet 125 and exits though a vent 176 after transferring moisture to the polylactic acid resin 114. If desired, a blower, jet, fan, etc. may be provided to direct the flow of the humidified air 124 from the humidifier 120 through the hopper 10, and out the vent 176.

As shown in Fig. 1 , the system 100 may also include a temperature control device 174 (e.g., heater) for controlling the temperature of the humidified air 124 introduced into the hopper 10. Suitable temperature control devices for this purpose may include, for instance, resistive heating elements, combustion heaters, heat-transfer elements, etc. A temperature sensor 181 (e.g., thermocouple, resistance temperature detector, etc.) may also be employed outside of the hopper 10 (as shown in Fig. 1 ) and/or within the interior 112 of the hopper 10 to measure the temperature of the humidified air 124. As shown in Fig. 1 , the temperature sensor 181 sends a signal to the controller 182, which may then control the temperature control device 174 and in turn the temperature of the humidified air 124. For example, if it is determined that the temperature is below a predetermined set point, the controller 182 may send a signal that activates the temperature control device 174 and heats the humidified air 124. On the other hand, if the temperature is at or above the predetermined set point, the controller 182 may send a signal that restricts the amount of heat provided to the humidified air 124 by the temperature control device 174. For instance, as indicated above, the temperature of the humidified air 124 is desirably maintained within a range of about 15°F to about 85⁰C, in some embodiments from about 20⁰C to about 70⁰C, and in some embodiments, from about 25°C to about 50°C. The system 100 may also include a humidity sensor 180 that is capable of measuring the humidity of the humidified air 124 within the hopper 10. The humidity sensor 180 may directly measure relative humidity, or it may indirectly measure relative humidity through direct measurement of the dew point and/or absolute humidity, which may then be used to calculate relative humidity. Examples of such humidity sensors may include, for instance, capacitive relative humidity sensors, dew point hygrometers, resistive humidity sensors, thermal conductivity absolute humidity sensors, etc. As shown in Fig. 1 , the humidity sensor 180 sends a signal to the controller 182, which may then control the solenoid valve 184 and in turn the flow of liquid water 122 into the humidifier 120. For example, if it is determined that the relative humidity is below a predetermined set point, the controller 182 may send a signal that opens the valve 184 and allows an increased amount of liquid water 122 to flow into the humidifier 120. On the other hand, if the relative humidity is at or above the predetermined set point, the controller 182 may send a signal that closes the valve 184 and restricts the flow of liquid water 122 to the humidifier 120. As indicated above, the relative humidity of the humidified air 124 is desirably about 30% or more, in some embodiments about 40% or more, in some embodiments from about 50% to about 95%, and in some embodiments, from about 75% to about 90%.

As described above, the controller 182 is programmed to control the solenoid valve 184 and temperature control device 174. In this regard, the controller 182 may contain memory and an input/output component. The memory may be used, for example, to store previously measured data by the humidity sensor 180 and the temperature sensor 181 , data from a psychrometric chart, etc., and may be a separate component outside the processor itself, e.g., hard drive, flash memory, ROM, etc. The input/output component may be used, for example, to send any signals that the controller 182 generates or the received input to subsequent units involved with the humidity control. It should be noted that there may be further components connected to the controller 182. For example, a display mechanism may be used to indicate to a user that a component of the controller 182 has been activated. The display mechanism may be, for example, a light emitting diode (LED), a speaker, or a digital display.

Once the polylactic acid resin 114 is humidified with the humidified air 124, it may then be supplied via discharge 140 to an extruder 12, such as a single- screw extruder, twin-screw extruder, etc., which blends and melt processes the resin. One particularly suitable melt processing device is a co-rotating, twin-screw extruder (e.g., ZSK-30 twin-screw extruder available from Werner & Pfleiderer Corporation of Ramsey, New Jersey). Such extruders may include feeding and venting ports and provide high intensity distributive and dispersive mixing, which facilitate the hydrolysis reaction. Regardless of the particular melt processing technique chosen, the raw materials are blended under high shear/pressure and heat to ensure initiation of the hydrolysis reaction. For example, melt processing may occur at a temperature of from about 100⁰C to about 500⁰C, in some embodiments, from about 15O⁰C to about 350⁰C, and in some embodiments, from about 175°C to about 300⁰C. Likewise, the apparent shear rate during melt processing may range from about 100 seconds^"1 to about 10,000 seconds^"1, in some embodiments from about 500 seconds^"1 to about 5000 seconds^"1, and in some embodiments, from about 800 seconds^"1 to about 1200 seconds^"1. The apparent shear rate is equal to 4Q/πR³, where Q is the volumetric flow rate

("m³/s") of the polymer melt and R is the radius ("m") of the capillary (e.g., extruder die) through which the melted polymer flows. Of course, other variables, such as the residence time during melt processing, which is inversely proportional to throughput rate, may also be controlled to achieve the desired degree of hydrolysis.

Under appropriate temperature and shear conditions, as describe above, a hydrolytically degraded polymer species is formed that has a molecular weight lower than that of the starting polylactic acid. The weight average and/or number average molecular weights may, for instance, each be reduced so that the ratio of the starting polylactic acid molecular weight to the hydrolytically degraded polylactic acid molecular weight is at least about 1.1 , in some embodiments at least about 1.4, and in some embodiments, at least about 2.0. For example, the hydrolytically degraded polylactic acid may have a number average molecular weight ("M_n") ranging from about 10,000 to about 105,000 grams per mole, in some embodiments from about 20,000 to about 100,000 grams per mole, and in some embodiments, from about 30,000 to about 90,000 grams per mole. Likewise, the hydrolytically degraded polylactic acid may also have a weight average molecular weight ("M_w") of from about 20,000 to about 140,000 grams per mole, in some embodiments from about 30,000 to about 120,000 grams per mole, and in some embodiments, from about 50,000 to about 100,000 grams per mole.

In addition to possessing a lower molecular weight, the hydrolytically degraded polylactic acid may also have a lower apparent viscosity and higher melt flow index than the starting polymer. The apparent viscosity may for instance, be reduced so that the ratio of the starting polylactic acid viscosity to the hydrolytically degraded polylactic acid viscosity is at least about 1.1 , in some embodiments at least about 2, and in some embodiments, from about 15 to about 100. Likewise, the melt flow index may be increased so that the ratio of the hydrolytically degraded polylactic acid melt flow index to the starting polylactic acid melt flow index (on a dry basis) is at least about 1.5, in some embodiments at least about 5, in some embodiments at least about 10, and in some embodiments, from about 30 to about 100. In one particular embodiment, the hydrolytically degraded polylactic acid may have an apparent viscosity of from about 5 to about 250 Pascal seconds (Pa-s), in some embodiments from about 8 to about 150 Pa-s, and in some embodiments, from about 10 to about 100 Pa-s, as determined at a temperature of 190°C and a shear rate of 1000 sec^"1. The melt flow index of the hydrolytically degraded polylactic acid (dry basis) may range from about 10 to about 1000 grams per 10 minutes, in some embodiments from about 20 to about 900 grams per 10 minutes, and in some embodiments, from about 100 to about 800 grams per 10 minutes (190⁰C, 2.16 kg). Of course, the extent to which the molecular weight, apparent viscosity, and/or melt flow index are altered by the hydrolysis reaction may vary depending on the intended application. Although differing from the starting polymer in certain properties, the hydrolytically degraded polylactic acid may nevertheless retain other properties of the starting polymer. For example, the thermal characteristics (e.g., T₉, T_m, and latent heat of fusion) typically remain substantially the same as the starting polymer, such as within the ranges noted above. Further, even though the actual molecular weights may differ, the polydispersity index of the hydrolytically degraded polylactic acid may remain substantially the same as the starting polymer, such as within the range of about 1.0 to about 3.5, in some embodiments from about 1.1 to about 2.5, and in some embodiments, from about 1.2 to about 2.0.

In addition to the components noted above, it should be understood that the humidified polylactic resin may also contain other components that are combined with the polylactic acid, such as in the feed chamber or within the extruder. For example, a plasticizer may be used in certain embodiments of the present invention to improve a variety of characteristics of the resulting thermoplastic composition, including its ability to be melt processed into fibers and webs. Suitable plasticizers for polylactic acid include, for instance, phthalates; esters (e.g., citrate esters, phosphate esters, ether diesters, carboxylic esters, dicarboxylic esters, epoxidized esters, aliphatic diesters, polyesters, copolyesters, etc.); alkylene glycols (e.g., ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, poly-1 ,3-propanediol, polybutylene glycol, etc.); alkane diols (e.g., 1 ,3-propanediol, 2,2-dimethyl-1 ,3-propanediol, 1 ,3-butanediol, 1 ,4-butanediol, 1 ,5-pentanediol, 1 ,6- hexanediol, 2,2,4-trimethyl-1 ,6-hexanediol, 1 ,3-cyclohexanedimethanol, 1 ,4- cyclohexanedimethanol, 2,2,4,4-tetramethyl-1 ,3-cyclobutanediol, etc.); alkylene oxides (e.g., polyethylene oxide, polypropylene oxide, etc.); vegetable oils; polyether copolymers; and so forth. Certain plasticizers, such as alkylene glycols, alkane diols, alkylene oxides, etc., may possess one or more hydroxyl groups that can attack the ester linkages of the polylactic acid and result in chain scission, thus improving the flexibility of the polylactic acid. Polyethylene glycol ("PEG"), for instance, is an example of a plasticizer that is particularly effective in decreasing the constraints on mobility and as a result helps provide a higher crystallization rate within a broader thermal window. Suitable PEGs are commercially available from a variety of sources under designations such as PEG 600, PEG 3350, PEG 8000, etc. Examples of such PEGs include Carbowax™, which is available from Dow Chemical Co. of Midland, Michigan.

Another suitable plasticizer that may be employed in the present invention is a polyether copolymer contains a repeating unit (A) having the following formula:

C₂H₄O ^

(A) wherein, x is an integer from 1 to 250, in some embodiments from 2 to 200, and in some embodiments, from 4 to 150, and also a repeating unit (B) having the following formula:

^{Γ ~} C_nH_2n O

(B) wherein, n is an integer from 3 to 20, in some embodiments from 3 to 10, and in some embodiments, from 3 to 5; and y is an integer from 1 to 150, in some embodiments from 2 to 125, and in some embodiments, from 4 to 100. Specific examples of monomers for use in forming the repeating unit (B) may include, for instance, 1 ,2-propanediol ("propylene glycol"); 1 ,3-propanediol ("trimethylene glycol"); 1 ,4-butanediol

("tetramethylene glycol"); 2,3-butanediol ("dimethylene glycol"); 1 ,5-pentanediol; 1 ,6-hexanediol; 1 ,9-nonanediol; 2-methyl-1 ,3-propanediol; neopentyl glycol; 2- methyl-1 ,4-butanediol; 3-methyl-1 ,5-pentanediol; 3-oxa-1 ,5-pentanediol ("diethylene glycol"); spiro-glycols, such as 3,9-bis(1 ,1-dimethyl-2-hydroxyethyl)- 2,4,8,10-tetraoxa-spiro [5,5] undecane and 3,9-diethanol-2,4,8,10-tetraoxa-spiro [5,5] undecane; and so forth. Among these polyols, propylene glycol, dimethylene glycol, trimethylene glycol, and tetramethylene glycol are particularly suitable for use in the present invention. In one particular embodiment, for example, the polyether copolymer may have the following general structure: A -f- C₂H₄O C₂H₄O B

wherein, x is an integer from 1 to 250, in some embodiments from 2 to 200, and in some embodiments, from 4 to 150; y is an integer from 1 to 150, in some embodiments from 2 to 125, and in some embodiments, from 4 to 100; z is an integer from 0 to 200, in some embodiments from 2 to 125, and in some embodiments from 4 to 100; n is an integer from 3 to 20, in some embodiments from 3 to 10, and in some embodiments, from 3 to 6;

A is hydrogen, an alkyl group, an acyl group, or an aryl group of 1 to 10 carbon atoms, and

B is hydrogen, an alkyl group, an acyl group, or an aryl group of 1 to 10 carbon atoms. When "z" is greater than 0, for example, the copolymer has an "ABA" configuration and may include, for instance, polyoxyethylene / polyoxypropylene / polyoxyethylene copolymers (EO/PO/EO) such as described in U.S. Patent Application Publication No. 2003/0204180 to Huang, et al., which is incorporated herein in its entirety by reference thereto for all purposes. Suitable EO/PO/EO polymers for use in the present invention are commercially available under the trade name PLURONIC® (e.g., F-127 L-122, L-92, L-81 , and L-61 ) from BASF Corporation, Mount Olive, New Jersey.

When employed, plasticizer(s) may be present in an amount of about 0.1 wt.% to about 20 wt.%, in some embodiments from about 0.2 wt.% to about 10 wt.%, and in some embodiments, from about 0.5 wt.% to about 5 wt.%, based on the dry weight of the starting polylactic acid. It should be understood, however, that a plasticizer is not required. In fact, in some embodiments of the present invention, the thermoplastic composition is substantially free of any plasticizers, e.g., less than about 0.5 wt.% based on the dry weight of the starting polylactic acid. Other components may of course be utilized for a variety of different reasons. Such materials that may be used include, without limitation, compatibilizers, wetting agents, melt stabilizers, processing stabilizers, heat stabilizers, light stabilizers, antioxidants, pigments, surfactants, waxes, flow promoters or melt flow index modifiers, particulates, nucleating agents, and other materials added to enhance processability. For example, a nucleating agent may be employed if desired to improve processing and to facilitate crystallization during quenching. Suitable nucleating agents for use in the present invention may include, for instance, inorganic acids, carbonates (e.g., calcium carbonate or magnesium carbonate), oxides (e.g., titanium oxide, silica, or alumina), nitrides (e.g., boron nitride), sulfates (e.g., barium sulfate), silicates (e.g., calcium silicate), stearates, benzoates, carbon black, graphite, and so forth. Still another suitable nucleating agent that may be employed is a "macrocyclic ester oligomer", which generally refers to a molecule with one or more identifiable structural repeat units having an ester functionality and a cyclic molecule of 5 or more atoms, and in some cases, 8 or more atoms covalently connected to form a ring. Specific examples of such ester oligomers may include macrocyclic poly(1 ,4-butylene terephthalate), macrocyclic poly(ethylene terephthalate), macrocyclic poly(1 ,3- propylene terephthalate), macrocyclic poly(1 ,4-butylene isophthalate), macrocyclic poly(1 ,4-cyclohexylenedimethylene terephthalate), macrocyclic poly(1 ,2-ethylene 2,6-naphthalenedicarboxylate) oligomers, co-ester oligomers comprising two or more of the above monomer repeat units, and so forth. When employed, the amount of such other ingredients may range from about 0.1 wt.% to about 25 wt.%, in some embodiments from about 0.2 wt.% to about 15 wt.%, in some embodiments from about 0.5 wt.% to about 10 wt.%, and in some embodiments, from about 1 wt.% to about 5 wt.%, based on the dry weight of the thermoplastic composition.

The hydrolytically degraded composition may then be formed into fibers, either directly after humidification or at a subsequent time. For example, in certain cases, pellets may be initially formed from the hydrolytically degraded resin, which are subsequently used in a fiber formation line. Regardless, fibers formed according to the present invention may generally have any desired configuration, including monocomponent, multicomponent (e.g., sheath-core configuration, side- by-side configuration, segmented pie configuration, island-in-the-sea configuration, and so forth), and/or multiconstituent (e.g., polymer blend). In some embodiments, the fibers may contain one or more additional polymers as a component (e.g., bicomponent) or constituent (e.g., biconstituent) to further enhance strength and other mechanical properties. For instance, the hydrolytically degraded polylactic acid may form a sheath component of a sheath/core bicomponent fiber, while an additional polymer may form the core component, or vice versa. The additional polymer may be a thermoplastic polymer that is not generally considered biodegradable, such as polyolefins, e.g., polyethylene, polypropylene, polybutylene, and so forth; polytetrafluoroethylene; polyesters, e.g., polyethylene terephthalate, and so forth; polyvinyl acetate; polyvinyl chloride acetate; polyvinyl butyral; acrylic resins, e.g., polyacrylate, polymethylacrylate, polymethylmethacrylate, and so forth; polyamides, e.g., nylon; polyvinyl chloride; polyvinylidene chloride; polystyrene; polyvinyl alcohol; and polyurethanes. More desirably, however, the additional polymer is biodegradable, such as aliphatic polyesters, such as polyesteramides, modified polyethylene terephthalate, polyglycolic acid, polyalkylene carbonates (such as polyethylene carbonate), polyhydroxyalkanoates (PHA), polyhydroxybutyrates (PHB), polyhydroxyvalerates (PHV), polyhydroxybutyrate-hydroxyvalerate copolymers (PHBV), and polycaprolactone, and succinate-based aliphatic polymers (e.g., polybutylene succinate, polybutylene succinate adipate, and polyethylene succinate); aromatic polyesters; or aliphatic-aromatic copolyesters.

Any of a variety of processes may be used to form the fibers. For example, a melt processed thermoplastic composition may be extruded through a spinneret, quenched, and drawn into the vertical passage of a fiber draw unit. The fibers may then be cut to form staple fibers having an average fiber length in the range of from about 3 to about 80 millimeters, in some embodiments from about 4 to about 65 millimeters, and in some embodiments, from about 5 to about 50 millimeters. The staple fibers may then be incorporated into a nonwoven web as is known in the art, such as bonded carded webs, through-air bonded webs, etc.

If desired, the fibers may also be deposited onto a foraminous surface to form a nonwoven web. Referring to Fig. 2, for example, one embodiment of a method for forming meltblown fibers is shown. Meltblown fibers form a structure having a small average pore size, which may be used to inhibit the passage of liquids and particles, while allowing gases (e.g., air and water vapor) to pass therethrough. To achieve the desired pore size, the meltblown fibers are typically "microfibers" in that they have an average size of 10 micrometers or less, in some embodiments about 7 micrometers or less, and in some embodiments, about 5 micrometers or less. The ability to produce such fine fibers may be facilitated in the present invention through the use of a thermoplastic composition having the desirable combination of low apparent viscosity and high melt flow index.

In Fig. 2, for instance, the raw materials (e.g., humidified polylactic acid, plasticizer, etc.) are fed into an extruder 12 from a hopper 10, such as described above and shown in Fig. 1. The extruder 12 is driven by a motor 11 and heated to a temperature sufficient to extrude the melted polymer. For example, the extruder 12 may employ one or multiple zones operating at a temperature of from about 100°C to about 500°C, in some embodiments from about 150°C to about 350⁰C, and in some embodiments, from about 175⁰C to about 300⁰C. If desired, the extruder may also possess one or more zones that remove excess moisture from the polymer, such as vacuum zones, etc. The extruder may also be vented to allow volatile gases to escape.

Once formed, the resulting thermoplastic composition may be subsequently fed to another extruder in a fiber formation line (e.g., extruder 12 of a meltblown spinning line). Alternatively, the thermoplastic composition may be directly formed into a fiber through supply to a die 14, which may be heated by a heater 16. It should be understood that other meltblown die tips may also be employed. As the polymer exits the die 14 at an orifice 19, high pressure fluid (e.g., heated air) supplied by conduits 13 attenuates and spreads the polymer stream into microfibers 18. Although not shown in Fig. 2, the die 14 may also be arranged adjacent to or near a chute through which other materials (e.g., cellulosic fibers, particles, etc.) traverse to intermix with the extruded polymer and form a "coform" web. The microfibers 18 are randomly deposited onto a foraminous surface 20 (driven by rolls 21 and 23) with the aid of an optional suction box 15 to form a meltblown web 22. The distance between the die tip and the foraminous surface 20 is generally small to improve the uniformity of the fiber laydown. For example, the distance may be from about 1 to about 35 centimeters, and in some embodiments, from about 2.5 to about 15 centimeters. In Fig. 2, the direction of the arrow 28 shows the direction in which the web is formed (i.e., "machine direction") and arrow 30 shows a direction perpendicular to the machine direction (i.e., "cross-machine direction"). Optionally, the meltblown web 22 may then be compressed by rolls 24 and 26. The desired denier of the fibers may vary depending on the desired application. Typically, the fibers are formed to have a denier per filament (i.e., the unit of linear density equal to the mass in grams per 9000 meters of fiber) of less than about 6, in some embodiments less than about 3, and in some embodiments, from about 0.5 to about 3. In addition, the fibers generally have an average diameter of from about 0.1 to about 20 micrometers, in some embodiments from about 0.5 to about 15 micrometers, and in some embodiments, from about 1 to about 10 micrometers.

Once formed, the nonwoven web may then be bonded using any conventional technique, such as with an adhesive or autogenously (e.g., fusion and/or self-adhesion of the fibers without an applied external adhesive). Autogenous bonding, for instance, may be achieved through contact of the fibers while they are semi-molten or tacky, or simply by blending a tackifying resin and/or solvent with the polylactic acid used to form the fibers. Suitable autogenous bonding techniques may include ultrasonic bonding, thermal bonding, through-air bonding, calendar bonding, and so forth. For example, the web may be further bonded or embossed with a pattern by a thermo-mechanical process in which the web is passed between a heated smooth anvil roll and a heated pattern roll. The pattern roll may have any raised pattern which provides the desired web properties or appearance. Desirably, the pattern roll defines a raised pattern which defines a plurality of bond locations which define a bond area between about 2% and 30% of the total area of the roll. Exemplary bond patterns include, for instance, those described in U.S. Patent 3,855,046 to Hansen et al., U.S. Patent No. 5,620,779 to Levy et a!.. U.S. Patent No. 5,962.112 to Havnes et al.. U.S. Patent 6,093,665 to Savovitz et al., as well as U.S. Design Patent Nos. 428,267 to Romano et al.; 390,708 to Brown; 418,305 to Zander, et al.; 384,508 to Zander, et al.; 384,819 to Zander, et al.: 358,035 to Zander, et al.; and 315,990 to Blenke, et al., all of which are incorporated herein in their entirety by reference thereto for all purposes. The pressure between the rolls may be from about 5 to about 2000 pounds per lineal inch. The pressure between the rolls and the temperature of the rolls is balanced to obtain desired web properties or appearance while maintaining cloth like properties. As is well known to those skilled in the art, the temperature and pressure required may vary depending upon many factors including but not limited to, pattern bond area, polymer properties, fiber properties and nonwoven properties. In addition to meltblown webs, a variety of other nonwoven webs may also be formed from the thermoplastic composition in accordance with the present invention, such as spunbond webs, bonded carded webs, wet-laid webs, airlaid webs, coform webs, hydraulically entangled webs, etc. For example, the polymer may be extruded through a spinnerette, quenched and drawn into substantially continuous filaments, and randomly deposited onto a forming surface.

Alternatively, the polymer may be formed into a carded web by placing bales of fibers formed from the thermoplastic composition into a picker that separates the fibers. Next, the fibers are sent through a combing or carding unit that further breaks apart and aligns the fibers in the machine direction so as to form a machine direction-oriented fibrous nonwoven web. Once formed, the nonwoven web is typically stabilized by one or more known bonding techniques.

If desired, the nonwoven web may also be a composite that contains a combination of the thermoplastic composition fibers and other types of fibers (e.g., staple fibers, filaments, etc). For example, additional synthetic fibers may be utilized, such as those formed from polyolefins, e.g., polyethylene, polypropylene, polybutylene, and so forth; polytetrafluoroethylene; polyesters, e.g., polyethylene terephthalate and so forth; polyvinyl acetate; polyvinyl chloride acetate; polyvinyl butyral; acrylic resins, e.g., polyacrylate, polymethylacrylate, polymethylmethacrylate, and so forth; polyamides, e.g., nylon; polyvinyl chloride; polyvinylidene chloride; polystyrene; polyvinyl alcohol; polyurethanes; polylactic acid; etc. If desired, biodegradable polymers, such as poly(glycolic acid) (PGA), poly(lactic acid) (PLA), poly(β-malic acid) (PMLA), poly(ε-caprolactone) (PCL), poly(p-dioxanone) (PDS), poly(butylene succinate) (PBS), and poly(3- hydroxybutyrate) (PHB), may also be employed. Some examples of known synthetic fibers include sheath-core bicomponent fibers available from KoSa Inc. of Charlotte, North Carolina under the designations T-255 and T-256, both of which use a polyolefin sheath, or T-254, which has a low melt co-polyester sheath. Still other known bicomponent fibers that may be used include those available from the Chisso Corporation of Moriyama, Japan or Fibervisions LLC of Wilmington, Delaware. Polylactic acid staple fibers may also be employed, such as those commercially available from Far Eastern Textile, Ltd. of Taiwan.

The composite may also contain pulp fibers, such as high-average fiber length pulp, low-average fiber length pulp, or mixtures thereof. One example of suitable high-average length fluff pulp fibers includes softwood kraft pulp fibers. Softwood kraft pulp fibers are derived from coniferous trees and include pulp fibers such as, but not limited to, northern, western, and southern softwood species, including redwood, red cedar, hemlock, Douglas fir, true firs, pine (e.g., southern pines), spruce (e.g., black spruce), bamboo, combinations thereof, and so forth. Northern softwood kraft pulp fibers may be used in the present invention. An example of commercially available southern softwood kraft pulp fibers suitable for use in the present invention include those available from Weyerhaeuser Company with offices in Federal Way, Washington under the trade designation of "NF-405." Another suitable pulp for use in the present invention is a bleached, sulfate wood pulp containing primarily softwood fibers that is available from Bowater Corp. with offices in Greenville, South Carolina under the trade name CoosAbsorb S pulp. Low-average length fibers may also be used in the present invention. An example of suitable low-average length pulp fibers is hardwood kraft pulp fibers. Hardwood kraft pulp fibers are derived from deciduous trees and include pulp fibers such as, but not limited to, eucalyptus, maple, birch, aspen, etc. Eucalyptus kraft pulp fibers may be particularly desired to increase softness, enhance brightness, increase opacity, and change the pore structure of the sheet to increase its wicking ability. Bamboo fibers may also be employed. Nonwoven composites may be formed using a variety of known techniques.

For example, the nonwoven composite may be a "coform material" that contains a mixture or stabilized matrix of the thermoplastic composition fibers and an absorbent material. As an example, coform materials may be made by a process in which at least one meltblown die head is arranged near a chute through which the absorbent materials are added to the web while it is forming. Such absorbent materials may include, but are not limited to, pulp fibers, superabsorbent particles, inorganic and/or organic absorbent materials, treated polymeric staple fibers, and so forth. The relative percentages of the absorbent material may vary over a wide range depending on the desired characteristics of the nonwoven composite. For example, the nonwoven composite may contain from about 1 wt.% to about 60 wt.%, in some embodiments from 5 wt.% to about 50 wt.%, and in some embodiments, from about 10 wt.% to about 40 wt.% thermoplastic composition fibers. The nonwoven composite may likewise contain from about 40 wt.% to about 99 wt.%, in some embodiments from 50 wt.% to about 95 wt.%, and in some embodiments, from about 60 wt.% to about 90 wt.% absorbent material. Some examples of such coform materials are disclosed in U.S. Patent Nos. 4,100,324 to Anderson, et al.; 5,284,703 to Everhart, et al.; and 5,350,624 to Georqer, et al.; which are incorporated herein in their entirety by reference thereto for all purposes.

Nonwoven laminates may also be formed in the present invention in which one or more layers are formed from the thermoplastic composition. For example, the nonwoven web of one layer may be a meltblown or coform web that contains the thermoplastic composition, while the nonwoven web of another layer contains thermoplastic composition, other biodegradable polymer(s), and/or any other polymer (e.g., polyolefins). In one embodiment, the nonwoven laminate contains a meltblown layer positioned between two spunbond layers to form a spunbond / meltblown / spunbond ("SMS") laminate. If desired, the meltblown layer may be formed from the thermoplastic composition. The spunbond layer may be formed from the thermoplastic composition, other biodegradable polymer(s), and/or any other polymer (e.g., polyolefins). Various techniques for forming SMS laminates are described in U.S. Patent Nos. 4,041 ,203 to Brock et al.; 5,213,881 to Timmons, et al.; 5,464,688 to Timmons, et al.; 4,374,888 to Bornslaeger; 5,169,706 to Collier, et al.; and 4,766,029 to Brock et al.. as well as U.S. Patent Application Publication No. 2004/0002273 to Fitting, et al., all of which are incorporated herein in their entirety by reference thereto for all purposes. Of course, the nonwoven laminate may have other configuration and possess any desired number of meltblown and spunbond layers, such as spunbond / meltblown / meltblown / spunbond laminates ("SMMS"), spunbond / meltblown laminates ("SM"), etc. Although the basis weight of the nonwoven laminate may be tailored to the desired application, it generally ranges from about 10 to about 300 grams per square meter ("gsm"), in some embodiments from about 25 to about 200 gsm, and in some embodiments, from about 40 to about 150 gsm. If desired, the nonwoven web or laminate may be applied with various treatments to impart desirable characteristics. For example, the web may be treated with liquid-repellency additives, antistatic agents, surfactants, colorants, antifogging agents, fluorochemical blood or alcohol repellents, lubricants, and/or antimicrobial agents. In addition, the web may be subjected to an electret treatment that imparts an electrostatic charge to improve filtration efficiency. The charge may include layers of positive or negative charges trapped at or near the surface of the polymer, or charge clouds stored in the bulk of the polymer. The charge may also include polarization charges that are frozen in alignment of the dipoles of the molecules. Techniques for subjecting a fabric to an electret treatment are well known by those skilled in the art. Examples of such techniques include, but are not limited to, thermal, liquid-contact, electron beam and corona discharge techniques. In one particular embodiment, the electret treatment is a corona discharge technique, which involves subjecting the laminate to a pair of electrical fields that have opposite polarities. Other methods for forming an electret material are described in U.S. Patent Nos. 4,215,682 to Kubik, et al.; 4,375,718 to Wadsworth; 4,592,815 to Nakao; 4,874,659 to Ando; 5,401 ,446 to Tsai, et al.; 5,883,026 to Reader, et al.; 5,908,598 to Rousseau, et al.; 6,365,088 to Knight, et al., which are incorporated herein in their entirety by reference thereto for all purposes.

The fibers and/or nonwoven web may be used in a wide variety of applications. For example, the web may be incorporated into a "medical product", such as gowns, surgical drapes, facemasks, head coverings, surgical caps, shoe coverings, sterilization wraps, warming blankets, heating pads, and so forth. Of course, the nonwoven web may also be used in various other articles. For example, the nonwoven web may be incorporated into an "absorbent article" that is capable of absorbing water or other fluids. Examples of some absorbent articles include, but are not limited to, personal care absorbent articles, such as diapers, training pants, absorbent underpants, incontinence articles, feminine hygiene products (e.g., sanitary napkins), swim wear, baby wipes, mitt wipe, and so forth; medical absorbent articles, such as garments, fenestration materials, underpads, bedpads, bandages, absorbent drapes, and medical wipes; food service wipers; clothing articles; pouches, and so forth. Materials and processes suitable for forming such articles are well known to those skilled in the art. Absorbent articles, for instance, typically include a substantially liquid-impermeable layer (e.g., outer cover), a liquid-permeable layer (e.g., bodyside liner, surge layer, etc.), and an absorbent core. In one embodiment, for example, a nonwoven web formed according to the present invention may be used to form an outer cover of an absorbent article. If desired, the nonwoven web may be laminated to a liquid- impermeable film that is either vapor-permeable or vapor-impermeable.

The present invention may be better understood with reference to the following examples. Test Methods

Apparent Viscosity:

The rheological properties of polymer samples were determined using a Gottfert Rheograph 2003 capillary rheometer with WinRHEO version 2.31 analysis software. The setup included a 2000-bar pressure transducer and a 30/1 :0/180 roundhole capillary die. Sample loading was done by alternating between sample addition and packing with a ramrod. A 2-minute melt time preceded each test to allow the polymer to completely melt at the test temperature (usually 160 to 220⁰C). The capillary rheometer determined the apparent viscosity (Pa-s) at various shear rates, such as 100, 200, 500, 1000, 2000, and 4000 s^"1. The resultant rheology curve of apparent shear rate versus apparent viscosity gave an indication of how the polymer would run at that temperature in an extrusion process.

Melt Flow Index:

The melt flow index ("MFI") is the weight of a polymer (in grams) forced through an extrusion rheometer orifice (0.0825-inch diameter) when subjected to a load of 2160 grams in 10 minutes, typically at 190°C or 230⁰C. Unless otherwise indicated, the melt flow index was measured in accordance with ASTM Test Method D1238-E. The melt flow index may be measured before or after drying. Polymers measured after drying (dry basis) generally have a water content of less than 500 parts per million. Tensile Properties:

The strip tensile strength values were determined in substantial accordance with ASTM Standard D-5034. Specifically, a nonwoven web sample was cut or otherwise provided with size dimensions that measured 25 millimeters (width) x 127 millimeters (length). A constant-rate-of-extension type of tensile tester was employed. The tensile testing system was a Sintech Tensile Tester, which is available from Sintech Corp. of Cary, North Carolina. The tensile tester was equipped with TESTWORKS 4.08B software from MTS Corporation to support the testing. An appropriate load cell was selected so that the tested value fell within the range of 10-90% of the full scale load. The sample was held between grips having a front and back face measuring 25.4 millimeters x 76 millimeters. The grip faces were rubberized, and the longer dimension of the grip was perpendicular to the direction of pull. The grip pressure was pneumatically maintained at a pressure of 40 pounds per square inch. The tensile test was run at a 300- millimeter per minute rate with a gauge length of 10.16 centimeters and a break sensitivity of 40%.

Five samples were tested by applying the test load along the machine- direction ("MD") and five samples were tested by applying the test load along the cross direction ("CD"). In addition to tensile strength ("peak load") and peak elongation (i.e., % strain at peak load) were measured. Moisture Content:

Moisture content was determined using an Arizona Instruments Computrac Vapor Pro moisture analyzer (Model No. 3100) in substantial accordance with ASTM D 7191-05, which is incorporated herein in its entirety by reference thereto for all purposes. The test temperature (§ X2.1.2) was 130⁰C, the sample size (§ X2.1.1 ) was 2 to 4 grams, and the vial purge time (§ X2.1.4) was 30 seconds. Further, the ending criteria (§ X2.1.3) was defined as a "prediction" mode, which means that the test is ended when the built-in programmed criteria (which mathematically calculates the end point moisture content) is satisfied.

EXAMPLE 1

Polylactic acid (Biomer L9000) was conditioned using an industrial aging chamber designed to maintain 40°C/75% relative humidity. The polylactic acid ("PLA") was stored in the aging chamber until reaching an approximate saturation level of 3000 ppm. Moisture testing using an Arizona Instruments Computrac moisture analyzer confirmed that the PLA reached the desired moisture level in 3 hours. The conditioned PLA was then extruded using a co-rotating, twin-screw extruder (ZSK-30, 30 mm) manufactured by Werner and Pfleiderer Corporation of Ramsey, New Jersey. The screw length was 1328 millimeters. The extruder had 14 barrels, numbered consecutively 1-14 from the feed hopper to the die. The first barrel (#1 ) received the PLA resin via a gravimetric feeder at throughput of 30 pounds per hour. The temperature profile of the barrels was 190⁰C, 220⁰C, 220°C, 220⁰C, 220°C, 215°C, and 160°C, respectively. The screw speed was 500 revolutions per minute ("rpm"). The die used to extrude the resin had 3 die openings (6 millimeters in diameter) that were separated by 4 millimeters. Upon formation, the extruded resin was cooled on a fan-cooled conveyor belt and formed into pellets by a Conair pelletizer. Melt flow rate testing was using a conducted prior to and post compounding resulting a rate of 12 g/10 minutes (190⁰C, 2.16 kg) prior to compounding and a rate of 24 g/10 minutes (190⁰C, 2.16 kg) after compounding.

EXAMPLE 2 The process of Example 1 was repeated with the addition of 15% polyethylene glycol "PEG" (DOW Carbowax-8000) to the conditioned Biomer L9000 PLA. The PEG was added into zone 1 of the ZSK-30 via a separate gravimetric feeder at 4.5 Ib/hr and the PLA feed rate was reduced to 25.5 Ib/hr for a total of 30 Ib/hr. Under the same extrusion conditions in Example 1 , the melt flow rate increased to was 39 g/10 minutes (19O⁰C, 2.16 kg) after compounding.

EXAMPLE 3

A Kaz ultrasonic humidifier was placed inline with a Novatec MM-15 dual desiccant bed dryer. The humidifier was started and turned to 4 on the dial that ranged from 1 to a maximum setting of 7. The dryer was set to deliver air at 48.9°C. Pre-dried polylactic acid (Biomer L9000) with a moisture content of less than 200 ppm was added to the dryer hopper. The polylactic acid remained in the hopper in a static state and was allowed exposure to the humidified air. Samples were taken from the top of the dryer approximately every 30 minutes to measure the adsorption rate of the water vapor by the polylactic acid. Moisture testing was done using an Arizona Instruments Computrac moisture analyzer running at 54.4°C. The humidity and temperature in the hopper were measured using a hand-held temperature and humidity sensor mounted in the dryer hopper and visible through the hopper sight glass. After the initial introduction of polylactic acid, the relative humidity ranged from 48 to 51 % and held consistent at 47.2°C. After 240 minutes, the moisture content of the polylactic acid had increased from 133 ppm to 2851 ppm. Moisture was plotted against time and a polynomial regression was used to fit the data. The polynomial best fit suggested that the adsorption rate was approximately 1114 ppm/hr, but significantly leveled off after about 150 minutes of exposure to humidity.

Four (4) additional trials were conducted by varying the air temperature and the humidifier output dial setting. The results are shown in the table below.

While the invention has been described in detail with respect to the specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. Accordingly, the scope of the present invention should be assessed as that of the appended claims and any equivalents thereto.

Claims

WHAT IS CLAIMED iS:

1. A method for forming polylactic acid fibers, the method comprising: supplying a polylactic acid resin to a feed chamber; contacting the polylactic resin within the feed chamber with humidified air to form a humidified polylactic acid resin, wherein the humidified air has a dry bulb temperature of from about 15°C to about 85°C and a relative humidity of about 30% or more; and extruding the humidified polylactic acid resin through a die to form a fiber.

2. The method of claim 1 , wherein the extruded polylactic acid resin has a melt flow index of from about 10 to about 1000 grams per 10 minutes.

3. The method of claim 1 , wherein the extruded polylactic acid resin has a melt flow index of from about 100 to about 800 grams per 10 minutes.

4. The method of claim 1 , wherein the humidified polylactic acid resin has a moisture content of from about 500 to about 3800 parts per million.

5. The method of claim 1 , wherein the humidified polylactic acid resin has a moisture content of from about 2000 to about 3200 ppm.

6. The method of claim 1 , wherein the humidified air has a relative humidity of from about 75% to about 90%.

7. The method of claim 1 , wherein the humidified air has a dew point temperature of from about -15⁰C to about 0⁰C.

8. The method of claim 1 , wherein the humidified air has a dew point temperature of from about -1O⁰C to about -5°C.

9. The method of claim 1 , wherein the humidified air has a dry bulb temperature of from about 25°C to about 5O⁰C.

10. The method of claim 1 , further comprising supplying liquid water to a humidifier that converts the liquid water into a vapor.

11. The method of claim 11 , wherein the vapor is supplied to a temperature control device that heats the vapor to the dry bulb temperature of from about 15°C to about 85°C before entering the feed chamber.

12. The method of claim 11 , further comprising recirculating vapor that exits the feed chamber to the humidifier.

13. The method of claim 1 , wherein extrusion occurs at a temperature of from about 100⁰C to about 500°C.

14. The method of claim 1 , wherein extrusion occurs at a temperature of from about 150⁰C to about 350⁰C.

15. The method of claim 1 , wherein the humidified polylactic acid resin is initially formed into a pellet before being extruded through a die to form a fiber.

16. The method of claim 1 , wherein the humidified polylactic acid resin is extruded through a meltblowing die.

17. The method of claim 1 , further comprising randomly depositing the fiber onto a foraminous surface to form a nonwoven web.

18. A system for forming polylactic acid fibers, the method comprising: a humidifier that is configured to receive a supply of liquid water and convert the liquid water into humidified air; a temperature control device that is configured to receive and heat the humidified air from the humidifier; a feed chamber that is configured to receive a polylactic acid resin and the humidified air from the temperature control device and form a humidified polylactic acid resin; and an extruder for melt processing the humidified polylactic acid resin through a die to form fibers.

19. The system of claim 18, wherein the die is a meltblowing die.

20. The system of claim 18, further comprising a foraminous surface onto which the fibers are randomly deposited to form a nonwoven web.

21. The system of claim 18, further comprising: a temperature sensor that is configured to measure the temperature of the humidified air prior to entering the feed chamber; a humidity sensor that is configured to directly or indirectly measure the relative humidity of the humidified air within the feed chamber; and a programmable logic controller that receives a signal from the temperature sensor, the humidity sensor, or both.

22. The system of claim 21 , wherein the controller is configured to send a signal to the temperature control device for controlling the temperature of the humidified air.

23. The system of claim 21 , wherein the controller is configured to send a signal to a solenoid valve through which the liquid water flows prior to entering the humidifier for controlling the humidity of the humidified air.