Patent Grant
9770058
Personal respirators are commonly used to protect a wearer from inhaling particles suspended in the air or from breathing unpleasant or noxious gases. Respirators generally come in one of two types—a molded cup-shaped form or a flat-folded form. The flat-folded form has advantages in that it can be carried in a wearer's pocket until needed, unfolded for use, and re-folded flat for storage. Commercially-available flat-fold respirators typically use a stiffening member (e.g., a resilient supporting framework or other supporting element, see, for example, U.S. Pat. No. 4,300,549 to Parker) or a stiffening layer (e.g., a high basis weight nonwoven web that contains large diameter, high modulus fibers such as polyester fibers, see, for example, U.S. Pat. No. 6,123,077 to Bostock et al.) to impart greater structural stability to the unfolded respirator. The stiffening member or stiffening layer can help the respirator resist deflection during breathing cycles to discourage or prevent the wearer's lips and nostrils from contacting the respirator inner surface.
Although stiffening members and stiffening layers are beneficial in that they improve the structural integrity of a respirator, the use of such components can undesirably increase overall respirator weight, bulk and cost. Because stiffening members and stiffening layers do not provide significant filtration capabilities, and limit the extent to which unused manufacturing scrap can be recycled, applicants sought to eliminate these components from a flat-fold respirator. Some patents say that a stiffening member or stiffening layer is merely optional or preferred (see e.g., the above-mentioned U.S. Pat. Nos. 6,123,077 and 4,920,960 to Hubbard et al.). It is difficult in practice to eliminate these components because their removal makes the respirator undesirably flimsy when unfolded and worn.
Applicants have now found a way to provide both stiffening and filtration capabilities in a single layer so that a flat-fold respirator can be fashioned which has one or more of reduced weight, bulk and manufacturing cost.
The invention provides in one aspect a flat-fold personal respirator that comprises at least one stiff filtration panel joined to the remainder of the respirator through at least one line of demarcation, the panel comprising a porous monocomponent monolayer nonwoven web that contains charged intermingled continuous monocomponent polymeric fibers of the same polymeric composition and that has sufficient basis weight or inter-fiber bonding so that the web exhibits a Gurley Stiffness greater than 200 mg and the respirator exhibits less than 20 mm H2O pressure drop. The respirator is capable of being folded to a substantially flat-folded configuration and unfolded to a convex open configuration.
In another aspect the invention provides a process for making a flat-fold personal respirator, which process comprises:
In yet another aspect the invention provides a process for making a flat-fold personal respirator, which process comprises:
Product complexity and waste may be reduced by eliminating a separate stiffening layer and by potentially eliminating other layers such as an outer cover web layer. Also, if the stiffening layer fibers and the fibers of any other layer (such as an inner or outer cover web layer) in the respirator all have the same polymeric composition and extraneous bonding materials are not employed, unused scrap may be recovered and fully recycled to make additional starting material.
These and other aspects of the invention will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution.
Like reference symbols in the various figures of the drawing indicate like elements. The elements in the drawing are not to scale.
As used in this document, the terms provided below will have the meaning as given:
“Attenuating the filaments into fibers” means the conversion of a segment of a filament into a segment of greater length and smaller size.
“Bimodal mass fraction/fiber size mixture” means a collection of fibers having a histogram of mass fraction vs. fiber size in μm exhibiting at least two modes. A bimodal mass fraction/fiber size mixture may include more than two modes, for example it may be a trimodal or higher-modal mass fraction/fiber size mixture.
“Bimodal fiber count/fiber size mixture” means a collection of fibers having a histogram of fiber count (frequency) vs. fiber size in μm exhibiting at least two modes whose corresponding fiber sizes differ by at least 50% of the smaller fiber size. A bimodal fiber count/fiber size mixture may include more than two modes, for example it may be a trimodal or higher-modal fiber count/fiber size mixture.
“Bonding” when used with respect to a fiber or collection of fibers means adhering together firmly; bonded fibers generally do not separate when a web is subjected to normal handling.
“Charged” when used with respect to a collection of fibers means fibers that exhibit at least a 50% loss in Quality Factor QF (discussed below) after being exposed to a 20 Gray absorbed dose of 1 mm beryllium-filtered 80 KVp X-rays when evaluated for percent dioctyl phthalate (% DOP) penetration at a face velocity of 7 cm/sec.
“Continuous” when used with respect to a fiber or collection of fibers means fibers having an essentially infinite aspect ratio (viz., a ratio of length to size of e.g., at least about 10,000 or more).
“Effective Fiber Diameter” (EFD) when used with respect to a collection of fibers means the value determined according to the method set forth in Davies, C. N., “The Separation of Airborne Dust and Particles”, Institution of Mechanical Engineers, London, Proceedings 1B, 1952 for a web of fibers of any cross-sectional shape be it circular or non-circular.
“Filtration panel” means a portion of a fold-flat respirator having filtration capabilities sufficient to remove one or more airborne small particle contaminants and having one or more discernible boundaries when the respirator is unfolded for use.
“Flat-fold respirator” means a device that can be folded flat for storage, can be unfolded to a shape that fits over at least the nose and mouth of a person and removes one or more airborne contaminants when worn by such person.
“Line of demarcation” means a fold, seam, weld, bond or other visible feature that provides a discernible boundary and optionally a hinge region for a respirator filtration panel.
“Meltblown” when used with respect to a nonwoven web means a web formed by extruding a fiber-forming material through a plurality of orifices to form filaments while contacting the filaments with air or other attenuating fluid to attenuate the filaments into fibers and thereafter collecting a layer of the attenuated fibers.
“Meltblown fibers” means fibers prepared by extruding molten fiber-forming material through orifices in a die into a high-velocity gaseous stream, where the extruded material is first attenuated and then solidifies as a mass of fibers. Meltblown fibers generally are not oriented. Although meltblown fibers have sometimes been reported to be discontinuous, the fibers generally are long and entangled sufficiently that it is usually not possible to remove one complete meltblown fiber from a mass of such fibers or to trace one meltblown fiber from beginning to end.
“Meltspun” when used with respect to a nonwoven web means a web formed by extruding a low viscosity melt through a plurality of orifices to form filaments, quenching the filaments with air or other fluid to solidify at least the surfaces of the filaments, contacting the at least partially solidified filaments with air or other fluid to attenuate the filaments into fibers and collecting a layer of the attenuated fibers.
“Meltspun fibers” means fibers issuing from a die and traveling through a processing station in which the fibers are permanently drawn and polymer molecules within the fibers are permanently oriented into alignment with the longitudinal axis of the fibers. Such fibers are essentially continuous and are entangled sufficiently that it is usually not possible to remove one complete meltspun fiber from a mass of such fibers.
“Microfibers” means fibers having a median size (as determined using microscopy) of 10 μm or less; “ultrafine microfibers” means microfibers having a median size of two μm or less; and “submicron microfibers” means microfibers having a median size one μm or less. When reference is made herein to a batch, group, array, etc. of a particular kind of microfiber, e.g., “an array of submicron microfibers,” it means the complete population of microfibers in that array, or the complete population of a single batch of microfibers, and not only that portion of the array or batch that is of submicron dimensions.
“Mode” when used with respect to a histogram of mass fraction vs. fiber size in μm or a histogram of fiber count (frequency) vs. fiber size in μm means a local peak whose height is larger than that for fiber sizes 1 and 2 μm smaller and 1 and 2 μm larger than the local peak.
“Monocomponent” when used with respect to a fiber or collection of fibers means fibers having essentially the same composition across their cross-section; monocomponent includes blends (viz., polymer alloys) or additive-containing materials, in which a continuous phase of uniform composition extends across the cross-section and over the length of the fiber.
“Monolayer” when used with respect to a nonwoven web means having (other than with respect to fiber size) a generally uniform distribution of similar fibers throughout a cross-section of the web, and having (with respect to fiber size) fibers representing each modal population present throughout a cross-section of the web. Such a monolayer web may have a generally uniform distribution of fiber sizes throughout a cross-section of the web or may, for example, have a depth gradient of fiber sizes such as a preponderance of larger size fibers proximate one major face of the web and a preponderance of smaller size fibers proximate the other major face of the web.
“Nominal Melting Point” means the peak maximum of a second-heat, total-heat-flow differential scanning calorimetry (DSC) plot in the melting region of a polymer if there is only one maximum in that region; and, if there is more than one maximum indicating more than one melting point (e.g., because of the presence of two distinct crystalline phases), as the temperature at which the highest-amplitude melting peak occurs.
“Nonwoven web” means a fibrous web characterized by entanglement or point bonding of the fibers.
“Of the same polymeric composition” means polymers that have essentially the same repeating molecular unit, but which may differ in molecular weight, melt index, method of manufacture, commercial form, etc., and which may optionally contain minor amounts (e.g., less than about 3 wt. %) of an electret charging additive.
“Oriented” when used with respect to a polymeric fiber or collection of such fibers means that at least portions of the polymeric molecules of the fibers are aligned lengthwise of the fibers as a result of passage of the fibers through equipment such as an attenuation chamber or mechanical drawing machine. The presence of orientation in fibers can be detected by various means including birefringence measurements and wide-angle x-ray diffraction.
“Porous” means air-permeable.
“Separately prepared smaller size fibers” means a stream of smaller size fibers produced from a fiber-forming apparatus (e.g., a die) positioned such that the stream is initially spatially separate (e.g., over a distance of about 1 inch (25 mm) or more from, but will merge in flight and disperse into, a stream of larger size fibers.
“Self-supporting” when used with respect to a nonwoven web or panel means that the web or panel does not include a contiguous reinforcing layer of wire, mesh, or other stiffening material having a composition different from that of the web panel and providing increased stiffness to one or more portions of the web or panel.
“Size” when used with respect to a fiber means the fiber diameter for a fiber having a circular cross section, or the length of the longest cross-sectional chord that may be constructed across a fiber having a non-circular cross-section.
In the practice of the present invention, a variety of flat-fold personal respirators may be made using the stiff filtration panels described herein. One such flat-fold respirator is shown in
The resulting one-, two- or three-layer web assembly 134 may be held together by surface forces, electrostatic forces, thermal bonding, adhesive or other suitable measures that will be familiar to persons having ordinary skill in the art. Web assembly 134 can next be welded and trimmed at welding station 136 to form a partial preform 138. Preform 138 desirably is substantially flat so that the desired respirator may be formed at relatively high rates of speed and relatively low cost without requiring specialized manufacturing equipment such as mating shell molds. Partial preform 138 next passes through demarcation station 140 where at least one line of demarcation is formed in partial preform 138 to create demarked preform 142. The desired line or lines of demarcation may be formed by a variety of techniques including ultrasonic welding, application of pressure (with or without the presence of heat), stitching, application of adhesive bars, and the like. The demarked preform 142 shown in
Referring now to
Another flat-fold respirator which may be formed from the disclosed stiff filtration panel is shown in
A variety of other flat-fold respirators may be formed from the disclosed stiff filtration web. Exemplary such respirators include those shown in U.S. Pat. Nos. 2,007,867 (Le Duc), 2,265,529 (Kemp), 2,565,124 (Durborow), 2,634,724 (Burns), 2,752,916 (Haliczer), 3,664,335 (Boucher et al.), 3,736,928 (Andersson et al.), 3,971,369 (Aspelin et al.), 4,248,220 (White), 4,300,549 (Parker), 4,417,575 (Hilton et al.), 4,419,993 (Peterson), 4,419,994 (Hilton), 4,600,002 (Maryyanek et al.), 4,920,960 (Hubbard et al.), 5,322,061 (Brunson), 5,701,892 (Bledstein), 5,717,991 (Nozaki et al.), 5,724,964 (Brunson et al.), 5,735,270 (Bayer) and 6,474,336 B1 (Wolfe), and UK Patent Application No. GB 2 103 491 (American Optical Corporation).
The disclosed respirators may be pleated or non-pleated and desirably are non-pleated. The disclosed respirators may also include one or more molded portions or panels but desirably are made without requiring molding. The disclosed stiff filtration panel may represent a minority, majority or even all of the available respirator filtration area. The disclosed folds, seams, welds, bonds or other lines of demarcation may be straight, curved or curvilinear. In some embodiments containing multiple lines of demarcation, a line or lines of demarcation may intersect with another line or lines of demarcation. In other embodiments no line of demarcation will intersect with another line of demarcation. The disclosed respirators may have less than 20 mm H2O pressure drop when exposed to a 1 wt. % sodium chloride aerosol flowing at 95 liters/min. For example, they may have less than 10 mm H2O pressure drop. The disclosed respirators may also have less than 20% maximum penetration when exposed to a 1 wt. % sodium chloride aerosol flowing at 95 liters/min. For example, they may have less than 5% maximum loading penetration or less than 1% maximum loading penetration when exposed to a 0.075 μm 2% sodium chloride aerosol flowing at 85 liters/min.
A variety of polymeric fiber-forming materials may be used to prepare the disclosed stiff filtration webs. The polymer may be essentially any semicrystalline thermoplastic fiber-forming material that can be subjected to the chosen fiber and web formation process and that is capable of providing a charged nonwoven web that will maintain satisfactory electret properties or charge separation. Preferred polymeric fiber-forming materials are non-conductive semicrystalline resins having a volume resistivity of 1014 ohm-centimeters or greater at room temperature (22° C.). Preferably, the volume resistivity is about 1016 ohm-centimeters or greater. Resistivity of the polymeric fiber-forming material may be measured according to standardized test ASTM D 257-93. The polymeric fiber-forming material also preferably is substantially free from components such as antistatic agents that could significantly increase electrical conductivity or otherwise interfere with the fiber's ability to accept and hold electrostatic charges. Some examples of polymers which may be used in chargeable webs include thermoplastic polymers containing polyolefins such as polyethylene, polypropylene, polybutylene, poly(4-methyl-1-pentene) and cyclic olefin copolymers, and combinations of such polymers. Other polymers which may be used but which may be difficult to charge or which may lose charge rapidly include polycarbonates, block copolymers such as styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers, polyesters such as polyethylene terephthalate, polyamides, polyurethanes, and other polymers that will be familiar to those skilled in the art. The disclosed stiff filtration webs preferably are prepared from poly-4-methyl-1 pentene or polypropylene. Most preferably, the webs are prepared from polypropylene homopolymer because of its ability to retain electric charge, particularly in moist environments.
Additives may be added to the polymer to enhance the filtration web's performance, electret charging capability, mechanical properties, aging properties, coloration, surface properties or other characteristics of interest. Representative additives include fillers, nucleating agents (e.g., MILLAD™ 3988 dibenzylidene sorbitol, commercially available from Milliken Chemical), electret charging enhancement additives (e.g., tristearyl melamine, and various light stabilizers such as CHIMASSORB™ 119 and CHIMASSORB 944 from Ciba Specialty Chemicals), cure initiators, stiffening agents (e.g., poly(4-methyl-1-pentene)), surface active agents and surface treatments (e.g., fluorine atom treatments to improve filtration performance in an oily mist environment as described in U.S. Pat. Nos. 6,398,847 B1, 6,397,458 B1, and 6,409,806 B1 to Jones et al.). The types and amounts of such additives will be familiar to those skilled in the art. For example, electret charging enhancement additives are generally present in an amount less than about 5 wt. % and more typically less than about 2 wt. %.
The disclosed stiff filtration web may have a variety of Effective Fiber Diameter values, for example an EFD of about 5 to about 40 μm, or of about 6 to about 35 μm. The web may also have a variety of basis weights, for example a basis weight of about 100 to about 500 grams/m2 (gsm) or about 150 to about 250 gsm. The disclosed web may have a Gurley Stiffness value of at least about 200 mg, at least about 300 mg, at least about 400 mg, at least about 500 mg, at least about 1000 mg or at least about 2000 mg.
The disclosed stiff filtration web may conveniently be formed as a web containing a bimodal mass fraction/fiber size mixture of microfibers and larger size fibers, like webs described in the above-mentioned U.S. patent application Ser. Nos. 11/461,136 and 11/461,145 filed Jul. 31, 2006 and in copending U.S. patent application Ser. No. (Attorney Docket No. 62291US002) filed even date herewith and incorporated herein by reference. The manufacturing process described in the latter application is exemplary and may be summarized as follows.
The apparatus shown in
For the embodiment shown in
The remaining portions of the associated meltblowing apparatus will be familiar to those having ordinary skill in the art. For example, further details regarding meltblowing may be found in Wente, Van A. “Superfine Thermoplastic Fibers,” in Industrial Engineering Chemistry, Vol. 48, pages 1342 et seq. (1956), or in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled “Manufacture of Superfine Organic Fibers” by Wente, V. A.; Boone, C. D.; and Fluharty, E. L.; and in U.S. Pat. No. 5,993,943 (Bodaghi et al.).
The disclosed stiff filtration web may also be formed using meltblowing and an apparatus 270 like that shown in
The disclosed bimodal webs may be made in other ways including using meltspinning to form the larger size fibers and using meltblowing to form separately prepared smaller size fibers (e.g., microfibers) of the same polymeric composition. A larger size fiber stream from the meltspinning die and a smaller size fiber stream from the meltblowing die may be positioned so that the two streams merge in flight to provide a combined stream of intermingled larger fibers and smaller fibers which may then land on a suitable collector to provide a nonwoven web containing a bimodal mass fraction/fiber size mixture of the larger and smaller size fibers. Further details regarding this process and the nonwoven webs so made are shown in the above-mentioned U.S. patent application Ser. Nos. 11/457,906, 11/461,145 and 11/461,192.
The disclosed stiff filtration web may also conveniently be formed as a monocomponent monolayer nonwoven web of continuous monocomponent polymeric fibers made by meltspinning, collecting, heating and quenching the monocomponent polymeric fibers under thermal conditions sufficient to form a web of partially crystalline and partially amorphous oriented meltspun fibers of the same polymeric composition that are bonded to form a coherent and handleable web which further may be softened while retaining orientation and fiber structure, like webs described in the above-mentioned U.S. patent application Ser. Nos. 11/457,899, 11/461,128 and 11/461,201. The manufacturing process described in these applications is exemplary and may be summarized as follows. A collected web of oriented semicrystalline meltspun fibers which include an amorphous-characterized phase is subjected to a controlled heating and quenching operation that includes a) forcefully passing through the web a fluid heated to a temperature high enough to soften the amorphous-characterized phase of the fibers (which is generally greater than the onset melting temperature of the material of such fibers) for a time too short to melt the whole fibers (viz., causing such fibers to lose their discrete fibrous nature; preferably, the time of heating is too short to cause a significant distortion of the fiber cross-section), and b) immediately quenching the web by forcefully passing through the web a fluid having sufficient heat capacity to solidify the softened fibers (viz., to solidify the amorphous-characterized phase of the fibers softened during heat treatment). Preferably the fluids passed through the web are gaseous streams, and preferably they are air. In this context “forcefully” passing a fluid or gaseous stream through a web means that a force in addition to normal room pressure is applied to the fluid to propel the fluid through the web. In a preferred embodiment, the disclosed quenching step includes passing the web on a conveyor through a device (which can be termed a quenched flow heater, as discussed subsequently) that provides a focused or knife-like heated gaseous (typically air) stream issuing from the heater under pressure and engaging one side of the web, with a gas-withdrawal device on the other side of the web to assist in drawing the heated gas through the web; generally the heated stream extends across the width of the web. The heated stream is in some respects similar to the heated stream from a “through-air bonder” or “hot-air knife,” though it may be subjected to special controls that modulate the flow, causing the heated gas to be distributed uniformly and at a controlled rate through the width of the web to thoroughly, uniformly and rapidly heat and soften the meltspun fibers to a usefully high temperature. Forceful quenching immediately follows the heating to rapidly freeze the fibers in a purified morphological form (“immediately” means as part of the same operation, i.e., without an intervening time of storage as occurs when a web is wound into a roll before the next processing step). In a preferred embodiment, a gas apparatus is positioned downweb from the heated gaseous stream so as to draw a cooling gas or other fluid, e.g., ambient air, through the web promptly after it has been heated and thereby rapidly quench the fibers. The length of heating is controlled, e.g., by the length of the heating region along the path of web travel and by the speed at which the web is moved through the heating region to the cooling region, to cause the intended melting/softening of the amorphous-characterized phase without melting the whole fiber.
Referring to
The filaments 315 pass through the attenuator 316 and then exit onto a collector 319 where they are collected as a mass of fibers 320. In the attenuator the filaments are lengthened and reduced in diameter and polymer molecules in the filaments become oriented, and at least portions of the polymer molecules within the fibers become aligned with the longitudinal axis of the fibers. In the case of semicrystalline polymers, the orientation is generally sufficient to develop strain-induced crystallinity, which greatly strengthens the resulting fibers. The collector 319 is generally porous and a gas-withdrawal device 414 can be positioned below the collector to assist deposition of fibers onto the collector. The distance 321 between the attenuator exit and the collector may be varied to obtain different effects. Also, prior to collection, extruded filaments or fibers may be subjected to a number of additional processing steps not illustrated in
In a preferred method of forming the web, the mass 320 of fibers is carried by the collector 319 through a heating and quenching operation as illustrated in
In the illustrative heating device 400 the bottom wall 408 of the lower plenum 403 is formed with an elongated slot 409 through which an elongated or knife-like stream 410 of heated air from the lower plenum is blown onto the mass 320 traveling on the collector 319 below the heating device 400 (the mass 320 and collector 319 are shown partly broken away in
The number, size and density of openings in the plate 411 may be varied in different areas to achieve desired control. Large amounts of air pass through the fiber-forming apparatus and must be disposed of as the fibers reach the collector in the region 415. Sufficient air passes through the web and collector in the region 416 to hold the web in place under the various streams of processing air. Sufficient openness is needed in the plate under the heat-treating region 417 and quenching region 418 to allow treating air to pass through the web, while sufficient resistance remains to assure that the air is more evenly distributed. The amount and temperature of heated air passed through the mass 320 is chosen to lead to an appropriate modification of the morphology of the fibers. Particularly, the amount and temperature are chosen so that the fibers are heated to a) cause melting/softening of significant molecular portions within a cross-section of the fiber, e.g., the amorphous-characterized phase of the fiber, but b) will not cause complete melting of another significant phase, e.g., the crystallite-characterized phase. We use the term “melting/softening” because amorphous polymeric material typically softens rather than melts, while crystalline material, which may be present to some degree in the amorphous-characterized phase, typically melts. This can also be stated, without reference to phases, simply as heating to cause melting of lower-order crystallites within the fiber. The fibers as a whole remain unmelted, e.g., the fibers generally retain the same fiber shape and dimensions as they had before treatment. Substantial portions of the crystallite-characterized phase are understood to retain their pre-existing crystal structure after the heat treatment. Crystal structure may have been added to the existing crystal structure, or in the case of highly ordered fibers crystal structure may have been removed to create distinguishable amorphous-characterized and crystallite-characterized phases.
To achieve the intended fiber morphology change throughout the collected mass 320, the temperature-time conditions should be controlled over the whole heated area of the mass. Desirable results have been obtained when the temperature of the stream 410 of heated air passing through the web is within a range of 5° C., and preferably within 2 or even 1° C., across the width of the mass being treated (the temperature of the heated air is often measured for convenient control of the operation at the entry point for the heated air into the housing 401, but it also can be measured adjacent the collected web with thermocouples). In addition, the heating apparatus is operated to maintain a steady temperature in the stream over time, e.g., by rapidly cycling the heater on and off to avoid over- or under-heating.
To further control heating and to complete formation of the desired morphology of the fibers of the collected mass 320, the mass is subjected to quenching immediately after the application of the stream 410 of heated air. Such a quenching can generally be obtained by drawing ambient air over and through the mass 320 as the mass leaves the controlled hot air stream 410. Numeral 420 in
Success in achieving the desired heat treatment and morphology of the amorphous-characterized phase often can be confirmed with DSC testing of representative fibers from a treated web; and treatment conditions can be adjusted according to information learned from the DSC testing, as discussed in greater detail in the above-mentioned application Ser. No. 11/457,899. Desirably the application of heated air and quenching are controlled so as to provide a web whose properties facilitate formation of an appropriate molded matrix. If inadequate heating is employed the web may be difficult to mold. If excessive heating or insufficient quenching are employed, the web may melt or become embrittled and also may not take adequate charge.
When a bimodal stiff filtration web is employed, the microfibers may for example have a size range of about 0.1 to about 10 μm, about 0.1 to about 5 μm or about 0.1 to about 1 μm. The larger size fibers may for example have a size range of about 10 to about 70 μm, about 10 to about 50 μm or about 15 to about 50 μm. A histogram of mass fraction vs. fiber size in μm may for example have a microfiber mode of about 0.1 to about 10 μm, about 0.5 to about 8 μm or about 1 to about 5 μm, and a larger size fiber mode of more than 10 μm, about 10 to about 50 μm, about 10 to about 40 μm or about 12 to about 30 μm. The disclosed bimodal webs may also have a bimodal fiber count/fiber size mixture whose histogram of fiber count (frequency) vs. fiber size in μm exhibits at least two modes whose corresponding fiber sizes differ by at least 50%, at least 100%, or at least 200% of the smaller fiber size. The microfibers may also for example provide at least 20% of the fibrous surface area of the web, at least 40% or at least 60%. When a web of partially crystalline and partially amorphous oriented meltspun fibers is employed, the fibers may for example have a size range of about 5 to about 70 μm, about 10 to about 50 μm or about 10 to about 30 μm as measured using optical microscopy. Larger meltspun fibers generally yield stiffer finished webs.
Depending on the process and process conditions used to make the disclosed stiff filtration web, some bonding may occur between the fibers during web formation, and thus the completed web may contain fibers bonded to one another at least some points of fiber intersection. Further bonding between fibers in the collected web may be needed to provide a web having the desired degree of stiffness. However, excessive bonding may also need to be avoided so as to limit pressure drop or other finished web or respirator properties.
After formation, the stiff filtration web is next subjected to charging and optional calendering. Although charging and calendering may be performed in either order, charging desirably is performed first so that charge will be distributed throughout the web thickness. Charge can be imparted to the disclosed nonwoven webs in a variety of ways. Charging may be carried out, for example, by contacting the web with water as disclosed in U.S. Pat. No. 5,496,507 (Angadjivand et al. '507), corona-treating as disclosed in U.S. Pat. No. 4,588,537 (Klasse et al.), hydrocharging as disclosed, for example, in U.S. Pat. No. 5,908,598 (Rousseau et al.), plasma treating as disclosed in U.S. Pat. No. 6,562,112 B2 (Jones et al.) and U.S. Patent Application Publication No. US2003/0134515 A1 (David et al.), or combinations thereof.
Calendering may be performed in a variety of ways that will be familiar to persons having ordinary skill in the art. Calendering usually is performed using heating and optional pressure (e.g., to a temperature between the applicable polymer softening point and melting point at the applicable pressure) and a point-bonding process or smooth calender rolls. Roll calendering is especially useful and may be performed in a variety of ways. For example, the web may be passed one or more times between two mating heated metal rolls to provide a calendered web having two smooth sides. The web may also be passed one or more times between a heated metal roll and mating resilient roll to provide a calendered web having one smooth side. Use of tighter roll gaps, greater nip pressures, higher temperatures or additional passes generally will increase the extent to which the web is stiffened. However calendering, if carried out to too great an extent, may undesirably increase pressure drop or compromise filtration performance in the completed respirator. Calendering typically also will cause the calendered surface to become denser and less porous. Calendaring one or both sides of the stiff filtration layer may discourage shedding sufficiently so that one or both cover webs will not be needed in the finished respirator. Accordingly, a calendered stiff filtration web provides particular advantages in that it may enable elimination of a stiffening layer and one or both covers layer in the completed respirator, thereby eliminating one to three of the layers in a conventional four layer construction.
The disclosed stiff filtration web may be formed in a variety of other ways. For example, the stiff filtration web may include a permeable skin layer or layers formed by melting fibers at and immediately adjacent one or both major surfaces of a nonwoven web, like those shown in U.S. Pat. Nos. 6,217,691 B1 and 6,358,592 B2 (both to Vair et al.).
The completed respirator optionally may include an inner cover web of lightweight construction. The inner cover web presents a smooth surface opposite the wearer's face and can increase respirator comfort. An outer cover web may also be employed if desired. As mentioned above, the inner or outer or both inner and outer cover webs preferably are rendered unnecessary through the use of a suitably calendered stiff filtration web. The inner and outer cover webs may have any suitable construction and composition. For example, the inner and outer cover webs may be spunbond webs, or smooth BMF webs made as described in U.S. Pat. No. 6,041,782 (Angadjivand et al. '782). In order to improve recyclability, the inner and outer cover webs desirably have the same polymeric composition as the stiff filtration web. The respirator may if desired include one or more additional layers other than those discussed above. For example, one or more porous layers containing sorbent particles may be employed to capture vapors of interest, such as the porous layers described in U.S. patent application Ser. No. 11/431,152 filed May 8, 2006 and entitled PARTICLE-CONTAINING FIBROUS WEB, the entire disclosure of which is incorporated herein by reference.
During formation of the disclosed stiff filtration web it typically will be helpful to monitor web properties such as basis weight, web thickness, solidity and Gurley Stiffness. It may also be helpful to monitor additional web properties such as EFD and Taber Stiffness, or completed respirator properties such as pressure drop, initial % NaCl penetration, % DOP penetration or the Quality Factor QF. When exposed to a 1 wt. % sodium chloride aerosol flowing at 95 liters/min, the completed respirator may for example have no more than 20% maximum NaCl penetration. In another embodiment the respirator, if exposed to a 0.075 μm 2% sodium chloride aerosol flowing at 85 liters/min, may have a pressure drop less than 20 mm H2O or less than 10 mm H2O, and may have a % maximum NaCl loading penetration less than about 5% or less than about 1%.
Basis weight may be determined gravimetrically using samples taken from several (e.g., 3 or more) evenly-space locations across the web widthwise direction. Similar sampling may be used to determine web thickness. Solidity may be calculated from the basis weight and web thickness measurements.
Gurley Stiffness may be determined using a Model 4171E GURLEY™ Bending Resistance Tester from Gurley Precision Instruments. Rectangular samples (3.8 cm×5.1 cm unless otherwise indicated) are die cut from the webs with the sample long side aligned with the web transverse (cross-web) direction. The samples are loaded into the Bending Resistance Tester with the sample long side in the web holding clamp. The samples are flexed in both directions, viz., with the test arm pressed against the first major sample face and then against the second major sample face, and the average of the two measurements is recorded as the stiffness in milligrams. The test is treated as a destructive test and if further measurements are needed fresh samples are employed.
EFD may be determined (unless otherwise specified) using an air flow rate of 32 L/min (corresponding to a face velocity of 5.3 cm/sec), using the method set forth in Davies, C. N., “The Separation of Airborne Dust and Particles”, Institution of Mechanical Engineers, London, Proceedings 1B, 1952.
Taber Stiffness may be determined using a Model 150-B TABER™ stiffness tester (commercially available from Taber Industries). Square 3.8 cm×3.8 cm sections are carefully vivisected from the webs using a sharp razor blade to prevent fiber fusion, and evaluated to determine their stiffness in the machine and transverse directions using 3 to 4 samples and a 15° sample deflection.
Pressure drop, percent penetration and the filtration Quality Factor QF may be determined using a challenge aerosol containing NaCl or DOP particles, delivered (unless otherwise indicated) at a flow rate of 95 or 85 liters/min, and evaluated using a TSI™ Model 8130 high-speed automated filter tester (commercially available from TSI Inc.). An MKS pressure transducer (commercially available from MKS Instruments) may be employed to measure pressure drop (ΔP, mm H2O) through the filter. For NaCl testing at 95 liters/min, the particles may generated from a 1% NaCl solution, and the Automated Filter Tester may be operated with both the heater and particle neutralizer on. For NaCl testing at 85 liters/min and using 0.075 μm diameter particles, the particles may be generated from a 2% NaCl solution to provide an aerosol containing particles at an airborne concentration of about 16-23 mg/m3, and the Automated Filter Tester may be operated with both the heater and particle neutralizer on. For DOP testing, the aerosol may contain particles with a diameter of about 0.185 μm at a concentration of about 100 mg/m3, and the Automated Filter Tester may be operated with both the heater and particle neutralizer off. The samples may be loaded to the maximum NaCl or DOP particle penetration and calibrated photometers may be employed at the filter inlet and outlet to measure the particle concentration and the % particle penetration through the filter. The equation:
may be used to calculate QF. Parameters which may be measured or calculated for the chosen challenge aerosol include initial particle penetration, initial pressure drop, initial Quality Factor QF, maximum particle penetration, pressure drop at maximum penetration, and the milligrams of particle loading at maximum penetration (the total weight challenge to the filter up to the time of maximum penetration). The initial Quality Factor QF value usually provides a reliable indicator of overall performance, with higher initial QF values indicating better filtration performance and lower initial QF values indicating reduced filtration performance.
The invention is further illustrated in the following illustrative examples, in which all parts and percentages are by weight unless otherwise indicated.
Using an apparatus like that shown in
The calendered filtration web was combined with a 17 gsm spunbond polypropylene inner cover web and a 17 gsm spunbond polypropylene outer cover web in an apparatus like that shown in
The data in Table 1B shows that the Run No. 1-1R respirator had lower initial pressure drop and lower initial NaCl penetration than the comparison 4-layer respirator.
Using the method of Example 1, a meltblown monocomponent monolayer web was formed from larger fibers and smaller size fibers of the same polymeric composition. The larger size fibers were formed using EXXON PP3155 polypropylene (a 36 melt flow rate polymer) available from Exxon Mobil Corporation to which had been added 0.8% CHIMASSORB 944 hindered amine light stabilizer as an electret charging additive and 2% POLYONE No. CC10054018WE blue pigment. The resulting blue polymer blend was fed to a Model 20 DAVIS STANDARD extruder like that used in Example 1. The smaller size fibers were formed using EXXON PP3746 polypropylene to which had been added 0.8% CHIMASSORB 944 hindered amine light stabilizer and 2% POLYONE No. CC10054018WE blue pigment. This latter polymer was fed to a KILLION extruder like that used in Example 1. By using a 13.5 in. (34.3 cm) DCD and adjusting the polymer rate from each extruder, webs with 65% larger size fibers and 35% smaller size fibers were produced. The collector speed was adjusted as needed to provide webs with about 200 to about 250 gsm basis weights, and the extrusion temperatures and heated air pressures were adjusted as needed to provide webs with about 16 to about 18 μm EFD values. The webs were hydrocharged with distilled water according to the technique taught in Angadjivand et al. '507 and allowed to dry. The resulting webs were made into flat fold respirators like the device shown in
The results in Table 2A show that each respirator should meet European FFP1 filtering facepiece requirements (see EN149:2001, Respiratory protective devices. Filtering half masks to protect against particles).
Using an apparatus like that shown in
The webs were next lightly calendered for one or two passes between rolls heated to 141° C. and operating at a 3.05 m/min line speed. Calendering gaps of about 1.5 to 2.2 mm were employed. The calendering gaps and web thicknesses for each sample are shown below in Table 3B:
The Gurley Stiffness values (measured using 25.4×38.1 mm samples) and pressure drop values (measured using a 32 l/min flow rate) for each sample are shown below in Table 3C:
The results in Table 3C show, inter alia, that pressure drop was not significantly adversely affected by calendering. The webs were made into flat fold respirators like the device shown in
The results in Table 3D show, inter alia, that % penetration and the Quality factor QF were not significantly adversely affected by calendering.
Set out below in Table 3E are the run number, initial pressure drop, initial % penetration, pressure drop at maximum penetration, maximum % penetration, challenge at maximum penetration and total aerosol challenge for the 3-layer respirators made from the uncalendered web samples:
The results in Table 3E show that the 3-layer respirators made using the uncalendered webs of Sample Nos. 3-1 through 3-4, 3-7 and 3-8 should pass the N95 NaCl loading test of 42 C.F.R. Part 84.
Set out below in Table 3F are the run number, initial pressure drop, initial % penetration, pressure drop at maximum penetration, maximum % penetration, challenge at maximum penetration and total aerosol challenge for the 2-layer respirators made from the web samples which had been calendered on one side:
The results in Table 3F show that the 2-layer respirators made using the single-side calendered webs of Sample Nos. 3-1 through 3-4 and 3-8 should pass the N95 NaCl loading test of 42 C.F.R. Part 84.
Set out below in Table 3G are the run number, initial pressure drop, initial % penetration, pressure drop at maximum penetration, maximum % penetration, challenge at maximum penetration and total aerosol challenge for the 1-layer respirators made from the web samples which had been calendered on both sides:
The results in Table 3G show that the 2-layer respirators made using webs of Sample Nos. 3-1 through 3-4 and 3-7 calendered on both sides should pass the N95 NaCl loading test of 42 C.F.R. Part 84.
Using an apparatus like that shown in
The web leaving the quenching area 420 was bonded with sufficient integrity to be self-supporting and handleable using normal processes and equipment; the web could be wound by normal windup into a storage roll or could be subjected to various operations such as heating and compressing the web over a hemispherical mold to form a molded respirator. The web was hydrocharged with distilled water according to the technique taught in Angadjivand et al. '507, and allowed to dry.
A second monocomponent monolayer web (web 4-2) was similarly made from FINA 3860 polypropylene to which had been added 0.5 wt. % of CHIMASSORB 944 hindered-amine light stabilizer from Ciba Specialty Chemicals. The conditions were the same as for web 4-1 except that extrusion head 10 had 512 holes arranged in a 10 cm (4 in) by 20 cm (8 in) pattern with 0.64 cm (0.25 in) hole spacing and with the long dimension of the pattern arranged across the web. The upper quench stream had an approximate face velocity of 0.32 m/sec (63 ft/min). The attenuator bottom gap width was 4.8 mm (0.19 in). The meltspun fiber stream was deposited on the collection belt 319 at a width of about 46 cm (about 18 in). Collection belt 319 moved at a rate of about 1.77 meters/min (5.8 ft/min). The bottom of the plate 408 was 4.1 cm (1.6 in) from the collected web 320 on collector 319. The collected web was hydrocharged with distilled water according to the technique taught in Rousseau et al. and allowed to dry.
The charged webs were evaluated to determine the flat web properties shown below in Table 4A:
The webs were made into flat fold respirators like the device shown in
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the invention. Accordingly, other embodiments are within the scope of the following claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 11/457,899 now abandoned and Ser. No. 11/457,906 now abandoned (both filed Jul. 17, 2006) and of U.S. patent application Ser. No. 11/461,128, now U.S. Pat. No. 7,905,973, Ser. No. 11/461,136, now U.S. Pat. No. 7,902,096, Ser. No. 11/461,145, now U.S. Pat. No. 7,858,163, Ser. No. 11/461,192 now U.S. Pat. 7,807,591 and Ser. No. 11/461,201 now U.S. Pat. No. 9,139,940, (each filed Jul. 31, 2006), the entire disclosures of each of which are incorporated herein by reference. This invention relates to flat-fold respirators that are worn by persons to protect them from inhaling airborne contaminants.
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| Number | Date | Country | |
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