Patent Application
20080038976
When practicing the invention in the manner illustrated in
The extrusion head 10 may be a conventional spinnerette or spin pack, generally including multiple orifices arranged in a regular pattern, e.g., straightline rows. Filaments 15 of fiber-forming liquid are extruded from the extrusion head and conveyed to a processing chamber or attenuator 16. The distance 17 the extruded filaments 15 travel before reaching the attenuator 16 can vary, as can the conditions to which they are exposed. Typically, quenching streams of air or other gas 18 are presented to the extruded filaments to reduce the temperature of the extruded filaments 15. Alternatively, the streams of air or other gas may be heated to facilitate drawing of the fibers. There may be one or more streams of air or other fluid—e.g., a first air stream 18a blown transversely to the filament stream, which may remove undesired gaseous materials or fumes released during extrusion; and a second quenching air stream 18b that achieves a major desired temperature reduction. Depending on the process being used or the form of finished product desired, the quenching air may be sufficient to solidify the extruded filaments 15 before they reach the attenuator 16. In other cases the extruded filaments are still in a softened or molten condition when they enter the attenuator. Alternatively, no quenching streams are used; in such a case ambient air or other fluid between the extrusion head 10 and the attenuator 16 may be a medium for any change in the extruded filaments before they enter the attenuator.
The filaments 15 pass through the attenuator 16, as discussed in more detail below, and then exit onto a collector 19 where they are collected as a mass of fibers 20. The collector 19 is generally porous and a gas-withdrawal device 14 can be positioned below the collector to assist deposition of fibers onto the collector. The distance 21 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 carrying out the invention, the mass 20 of fibers is carried by the collector 19 through a heating and quenching operation as illustrated in
In the illustrative heating device 100 the bottom wall 108 of the lower plenum 103 is formed with an elongated slot 109 through which an elongated or knife-like stream 110 of heated air from the lower plenum is blown onto the mass 20 traveling on the collector 19 below the heating device 100 (the mass 20 and collector 19 are shown partly broken away in
The number, size and density of openings in the plate 111 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 in the region 115 as the fibers reach the collector. Sufficient air passes through the web and collector in the region 116 to hold the web in place under the various streams of processing air. And sufficient openness is needed in the plate under the heat-treating region 117 and quenching region 118 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 20 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 as discussed above (this often can be stated, without reference to phases, simply as heating to cause melting of lower-order crystallites within the fiber), but b) not cause complete melting of another significant phase, e.g., the crystallite-characterized phase as discussed above. 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 (see, for example, the highly drawn fibers of Examples 11-14 and C14-20), 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 20, the temperature-time conditions should be controlled over the whole heated area of the mass. We have obtained best results when the temperature of the stream 110 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 101, 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. Preferably the temperature is held within one degree Centigrade of the intended temperature when measured at one second intervals.
To further control heating and to complete formation of the desired morphology of the fibers of the collected mass 20, the mass is subjected to quenching immediately after the application of the stream 110 of heated air. Such a quenching can generally be obtained by drawing ambient air over and through the mass 20 as the mass leaves the controlled hot air stream 110. Numeral 120 in
An aim of the quenching is to rapidly remove heat from the web and the fibers and thereby limit the extent and nature of crystallization or molecular ordering that will subsequently occur in the fibers. Generally a heating and quenching operation of the invention is performed while a web is moved through the operation on a conveyor, and quenching is performed before the web is wound into a storage roll at the end of the operation. The times of treatment depend on the speed at which a web is moved through an operation, but generally the total heating and quenching operation is performed in a minute or less, and preferably in less than 15 seconds. By rapid quenching from the molten/softened state to a solidified state, the amorphous-characterized phase is understood to be frozen into a more purified crystalline form, with reduced molecular material that can interfere with softening, or repeatable softening, of the fibers. Desirably the mass is cooled by a gas at a temperature at least 50° C. less than the Nominal Melting Point; also the quenching gas is desirably applied for a time on the order of at least one second. In any event the quenching gas or other fluid has sufficient heat capacity to rapidly solidify the fibers.
Other fluids that may be used include water sprayed onto the fibers, e.g., heated water or steam to heat the fibers, and relatively cold water to quench the fibers.
As discussed above, 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.
Although existing as two halves or sides, the attenuator functions as one unitary device and will be first discussed in its combined form. (The structure shown in
The attenuation chamber 24 may have a uniform gap width (the horizontal distance 33 on the page of
The length of the attenuation chamber 24 can be varied to achieve different effects; variation is especially useful with the portion between the air knives 32 and the exit opening 34, sometimes called herein the chute length 35. The angle between the chamber walls and the axis 26 may be wider near the exit 34 to change the distribution of fibers onto the collector; or structure such as deflector surfaces, Coanda curved surfaces, and uneven wall lengths may be used at the exit to achieve a desired spreading or other distribution of fibers. In general, the gap width, chute length, attenuation chamber shape, etc. are chosen in conjunction with the material being processed and the mode of treatment desired to achieve desired effects. For example, longer chute lengths may be useful to increase the crystallinity of prepared fibers. Conditions are chosen and can be widely varied to process the extruded filaments into a desired fiber form.
As illustrated in
In this illustrative embodiment, air cylinders 43a and 43b are connected, respectively, to the attenuator sides 16a and 16b through connecting rods 44 and apply a clamping force pressing the attenuator sides 16a and 16b toward one another. The clamping force is chosen in conjunction with the other operating parameters so as to balance the pressure existing within the attenuation chamber 24. In other words, the clamping force and the force acting internally within the attenuation chamber to press the attenuator sides apart as a result of the gaseous pressure within the attenuator are in balance or equilibrium under preferred operating conditions. Filamentary material can be extruded, passed through the attenuator and collected as finished fibers while the attenuator parts remain in their established equilibrium or steady-state position and the attenuation chamber or passage 24 remains at its established equilibrium or steady-state gap width.
During operation of the representative apparatus illustrated in
As will be seen, in the preferred embodiment of processing chamber illustrated in
Further details of the attenuator and possible variations are disclosed in Berrigan et al., U.S. Pat. Nos. 6,607,624 and 6,916,752, which are incorporated herein by reference.
Although the apparatus shown in
In addition, the invention may be practiced on webs prepared by procedures completely different from the direct-web preparation techniques illustrated in
Also, apparatus for heating and quenching as described or claimed in this patent specification (which to our knowledge is a novel apparatus) has other uses in addition to those described herein. For example, the apparatus can be used to obtain bonded webs without interest or intention to cause morphological refinement or to subject the treated web to subsequent operations making use of such refinement. One example of such a use is taught in a patent application being filed the same day as the present patent application, Attorney's Docket No. 60928US003, which is incorporated herein by reference. That patent application describes a nonwoven fibrous web comprising a matrix of continuous meltspun fibers and separately prepared microfibers dispersed among the meltspun fibers; the web can be treated with apparatus of the present patent application to cause bonding of the meltspun fibers to form a coherent or self-sustaining matrix; such a treated web may or may not be subjected to subsequent operations that take advantage of morphological refinement of the meltspun fibers.
Generally, any semicrystalline fiber-forming polymeric material may be used in preparing fibers and webs of the invention, including the polymers commonly used in commercial fiber formation such as polyethylene, polypropylene, polyethylene terephthalate, nylon, and urethanes. The specific polymers listed here are examples only, and a wide variety of other polymeric or fiber-forming materials are useful.
Fibers also may be formed from blends of materials, including materials into which certain additives have been added, such as pigments or dyes. Bicomponent fibers, such as core-sheath or side-by-side bicomponent fibers, may be used (“bicomponent” herein includes fibers with two or more components, each occupying a separate part of the cross-section of the fiber and extending over the length of the fiber). However, the invention is most advantageous with monocomponent fibers, which have many benefits (e.g., less complexity in manufacture and composition; “monocomponent” fibers have essentially the same composition across their cross-section; monocomponent includes blends or additive-containing materials, in which a continuous phase of uniform composition extends across the cross-section and over the length of the fiber) and can be conveniently bonded and given added bondability and shapeability by the invention. Different fiber-forming materials may be extruded through different orifices of the extrusion head so as to prepare webs that comprise a mixture of fibers. In other embodiments of the invention other materials are introduced into a stream of fibers prepared according to the invention before or as the fibers are collected so as to prepare a blended web. For example, other staple fibers may be blended in the manner taught in U.S. Pat. No. 4,118,531; or particulate material may be introduced and captured within the web in the manner taught in U.S. Pat. No. 3,971,373; or microwebs as taught in U.S. Pat. No. 4,813,948 may be blended into the webs. Alternatively, fibers prepared by the present invention may be introduced into a stream of other fibers to prepare a blend of fibers.
Various processes conventionally used as adjuncts to fiber-forming processes may be used in connection with filaments as they enter or exit from the attenuator, such as spraying of finishes or other materials onto the filaments, application of an electrostatic charge to the filaments, application of water mists, etc. In addition, various materials may be added to a collected web, including bonding agents, adhesives, finishes, and other webs or films.
The fibers prepared by a method of the invention may range widely in diameter. Microfiber sizes (about 10 micrometers or less in diameter) may be obtained and offer several benefits; but fibers of larger diameter can also be prepared and are useful for certain applications; often the fibers are 20 micrometers or less in diameter. Fibers of circular cross-section are most often prepared, but other cross-sectional shapes may also be used. Depending on the operating parameters chosen, e.g., degree of solidification from the molten state before entering the attenuator, the collected fibers may be rather continuous or essentially discontinuous. The orientation of the polymer chains in the fibers can be influenced by selection of operating parameters, such as degree of solidification of filament entering the attenuator, velocity and temperature of air stream introduced into the attenuator by the air knives, and axial length, gap width and shape (because, for example, shape influences the venturi effect) of the attenuator passage.
Transmission electron micrographs through a section of fibers of the invention have revealed that in at least many cases, the amorphous-characterized phase in a fiber of the invention takes the form of a multitude of minute phases distributed throughout the cross-section of the fiber. Wherever their location however, at least portions of the amorphous-dominated phase appear to be at or near the exterior of the fibers, because of their participation in bonding of the fibers.
Immediately after the heating and quenching operation a web of the invention generally has a degree of bonding sufficient for the web to be handled, e.g., removed from the collection screen and wound into a storage roll. But as discussed above, additional bonding is possible and is often performed, e.g., to more permanently stabilize the web, or to shape it, including providing it with a nonplanar shape or smoothing its surfaces.
Any additional bonding is most typically done in a through-air-bonder, but also can be done in an oven or as part of a calendering or shaping operation. (Although there is seldom any reason to do so, bonding can also be accomplished or assisted by use of extraneous bonding materials included in the web during formation or applied after web-formation.) During thermal bonding of a web of the invention heat is generally applied in a narrow range, precisely selected to cause softening of the amorphous-characterized phase of a fiber to achieve bonding, while leaving the crystallite-characterized phase substantially unaffected. The unaffected crystallite-characterized phase thus can have a reinforcing function, e.g., it can function to retain fiber shape during the bonding operation, so that aside from bond regions the fiber retains its discrete fibrous form and the web retains its basic fibrous structure. In autogenous bonding operations the fiber can retain its fiber cross-section over its length outside bond regions, where there typically is some flow and coalescence of material from adjacent bonded fibers.
Another important advantage of the invention is the ability to shape a web of the invention. By shaping it is meant reconfiguring the web into a persistent new configuration, i.e., a self-sustaining configuration that the web will generally retain during use. In some cases shaping means smoothing one or both surfaces of the web and in some cases compacting the web. In other cases shaping involves configuring the web into a nonplanar shape such as perhaps a cup-shape for use in a face mask. Again the fibrous character of the web is retained during shaping, though the fibers may receive a somewhat different cross-section through the pressure of the shaping operation.
Besides improved bondability and shapability, fibers of the invention can provide other useful properties and features. For example, the improved morphological purity of the fibers as found in the amorphous-characterized phase may make the fibers chemically more reactive, enhancing use of the fiber for such purposes as grafting substrates. The fact that a web of the invention can be bonded without addition of an extraneous material is another important advantage, enhancing utility of the webs as membrane supports, electrochemical cell separators, filtration media, etc.
The invention is further illustrated in the following illustrative examples. Several examples are identified as comparative examples, because they do not show certain properties (such as softening, bonding, or DSC characteristics) desired for bondability, moldability, etc.; but the comparative examples may be useful for other purposes and may exhibit novel and nonobvious character.
Apparatus as shown in
Certain parts of the apparatus and operating conditions are summarized in Table 1. The clamping pressure reported in the table was sufficient that the walls of the attenuator remained generally fixed during preparation of fibers. Apparatus parameters not reported in the table are as follows. The plate 104 in
The heating face velocity reported in the table was measured at the center of the slot 109 at a point about one-half inch (1.27 centimeter) above the mass using a hot-wire anemometer; 10 measurements were taken over the width of the zone and arithmetically averaged. The cooling face velocity was measured in the same manner at the center (along the machine-direction axis) of the area 120 in
Various measurements and tests were performed on representative webs of the examples. Differential scanning calorimetry was performed using a Modulated DSC™ system (Model Q1000 supplied by TA Instruments Inc, New Castle, Del.). Test samples of about 2-4 milligrams were cut from a test web with a razor blade and tested using conditions as follows: For the set of Examples 1-3 and Comparative Examples 1-6 the sample was heated from −90 to 210° C. at a heating rate of 5° C. /minute, a perturbation amplitude of plus-or-minus 0.796° C. and a period of 60 seconds. For the set of Examples 4-6 and Comparative Examples C7-8 the sample was heated from −10 to 310° C. at a heating rate of 4° C./minute, a perturbation amplitude of plus-or-minus 0.636° C. and a period of 60 seconds. A heat-cool-heat test cycle was used for all materials.
Table 1 also presents data gathered from
The molding capabilities of the webs of Examples 4 and C8 were examined by molding representative samples into a respirator-shaped cup shape using conventional molding conditions but different mold temperatures shown in Table 2 below. Two samples of each example were molded using a five-second molding cycle. The mold height was 5.7 centimeters and formed a generally oval shape with a minor axis of 11.5 centimeters and 13 major axis. There was a 0.5-centimeter gap between mold sections. The height of the molded cup was measured by clamping it to a table top, placing a flat blade on top of the molded cup, and measuring the distance from the table top to the knife blade. A 100-gram weight was then laid on the blade and the height measured again. Table 2 reports the mold temperatures and the height measurements.
As will be noted, the webs of Example 1 replicated well the mold shape even when molded at a temperature of 155° C., less than the Nominal Melting Point of the webs. All the molded Example 1 webs except one of those molded at 155° C. and the two molded at 205° C. were essentially at mold height and the others were at least 87% or 83%, respectively, of mold height. (For purposes herein replication is regarded as attaining at least 75% of mold dimensions.) It is also noted that the molded Example 1 webs held their shape well under pressure, while the C8 molded webs essentially collapsed under pressure.
The webs of Examples 7 and 8 and C9-C11 were prepared by carding oriented crimped nylon 6-6 staple fibers on a Holingsworth random card; the fibers, supplied by Rhodia Technical Fibers, Gerliswilstrasse 19 CH-6021 Emmenbrucke, Germany, were characterized as 2-inch (about 5 centimeter) cut staple 6-denier (16.7 decitex) fiber having a crimp count of three per inch (1.2 per centimeter). Unbonded webs of 100 gsm basis weight were prepared and passed on a conveyor through a quenched flow heater as pictured in
The treated webs were studied in the described Melting Distortion test, and samples of the webs were also subjected to MDSCT testing the sample was heated from −25 to 300° C. at a heating rate of 4° C./minute, a perturbation amplitude of plus-or-minus 0.636° C. and a period of 60 seconds. Nonreversing-heat-flow plots for Examples C9 (Plot A), 9 (Plot B), and 10 (Plot C) are shown in
Although Example 10 showed some melting on the top surface, fibers deeper within the web were not melted, and these webs were thus regarded as meeting the desired performance characteristics; it is not clear to us why Example C11 did not demonstrate similar effects.
A commercial polypropylene spunbond web (BBA Spunbond Typar style 3141N, available from BBA Fiberweb Americas Industrial Division, Old Hickory, Tenn.) having a nominal basis weight of 50 gsm and comprising oriented polypropylene fibers having an average diameter of 40 micrometers was treated by passing it through a quenched flow heater apparatus as illustrated by the apparatus 100 in
The treated webs were studied in the described Melting Distortion test, and were also subjected to a Rebonding test in which two five-inch-long (12.7-centimeter-long) pieces of a treated web are overlaid on one another and heated and pressed in a calendering operation. The pieces are overlaid with their top surfaces (the top of the web as it went through the quenched flow heater) facing one another and with a 5-centimeter-long overlap. The overlaid pieces were passed through calender rolls having a surface temperature of 80 degrees C. at a rate of 3.9 meters per minute and with a nip pressure of 3.9 kilograms force per centimeter. After calendering, the opposite ends of the webs were grasped and one end was twisted 180 degrees. Bonded webs showed no sign of separation when viewed under a microscope.
Results of the Melting Distortion and Rebonding tests are reported in Table 4. MDSC™ testing (Model TA 2920 MDSC™ machine) was also conducted on the treated samples. Two-to-three-milligram samples were heated from −50 to 210° C. at a heating rate of 5° C./minute, a perturbation amplitude of plus-or-minus 0.796° C. and a period of 60 seconds. Results are reported in
From the testing and examination of webs Examples C14-C19 were regarded as lacking in a desired level of softening and bonding properties.
A nonwoven fibrous web was prepared from oriented polypropylene 4-denier, 4.76-centimeter crimped staple fibers (Kosa T196 White 060 Staple Fibers, available from Fiber Visions Inc., Covington, Ga.) using a Hergeth Random card. An unbonded web having a basis weight of 100 grams per square centimeter was prepared. Samples of the web were then treated with a quenched flow heater apparatus 100 as shown in
The Melting Distortion and Rebonding tests were performed on the treated samples, and the results are reported in Table 5. MDSC™ testing (using the Model 2920 machine) was also conducted on the treated samples. Two-to-three-milligram samples were heated from −50 to 210° C. at a heating rate of 5° C./minute, a perturbation amplitude of plus-or-minus 0.796° C. and a period of 60 seconds. First-heat nonreversing heat flow plots obtained are reported in
Unbonded nonwoven fibrous webs weighing 100 grams per square meter were prepared on a Rando Webber from oriented polyethylene terephthalate 4.7-decitex by 2-inch-long (about 5 cm) crimped staple fibers (Kosa T224 fibers from Fiber Visions Incorporated Covington, Ga.). The webs were passed under a quenched flow heater as shown in
For MDSC™ testing (using the Model Q1000 machine), two-to-three-milligram samples were heated from −10 to 310° C. at a heating rate of 4° C. /minute, a perturbation amplitude of plus-or-minus 0.636° C. and a period of 60 seconds. The resulting first-heat nonreversing heat flow plots are shown in
The webs were checked for fiber melting in the Melting Distortion test and for bonding in the Rebonding test. Results are reported in Table 6.
The molding test of Examples 1-6 was also conducted on webs of Example C25 and Example 19. The molding temperature was 172° C. and the mold dimensions and molding conditions were the same as for Examples 1-6. Results, shown in Table 7, demonstrate that the molding operation for Example 19 was successful, a remarkable effect given the fact that the 172° C. molding temperature was about 65° C. less than the Nominal Melting Point of the fibers (238.6° C.).
