The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which.
In describing the preferred embodiment of the present invention, reference will be made herein to
The present invention provides an efficient method of mass-producing nanometer-sized fiber fibrils for various applications by mechanical working of fibers. The term “fiber” means a solid that is characterized by a high aspect ratio of length to diameter. For example, an aspect ratio having a length to an average diameter ratio of from greater than about 2 to about 1000 or more may be using in the generation of nanofibers according to the instant invention. The term “fibrillated fibers” refers to fibers bearing sliver-like fibrils distributed along the length of the fiber and having a length to width ratio of about 2 to about 100 and having a diameter of less than about 1000 nanometers. Fibrillated fibers extending from the fiber, often referred to as the “core fiber,” have a diameter significantly less that the core fiber from which the fibrillated fibers extend. The fibrils extending from the core fiber preferably have diameters in the nanofiber range of less than about 1000 nanometers. As used herein, the term nanofiber means a fiber, whether extending from a core fiber or separated from a core fiber, having a diameter less than about 1000 nanometers. Nanofiber mixtures produced by the instant invention typically have diameters of about 50 nanometers up to less than about 1000 nm and lengths of about 0.1-6 millimeters. Nanofibers preferably have diameters of about 50-500 nanometers and lengths of about 0.1 to 6 millimeters.
The initial step in producing nanofibers is creating the fibrillated fibers having fiber cores and attached nanofiber fibrils. Such fibrillated fibers may be produced by shearing fibers in the manner described in the prior art, which shearing may include a degree of refining, crushing, beating, cutting, mechanical agitation and high shear blending. Alternatively, such fibrillated fibers may be produced by shearing without substantial crushing, beating and cutting in the manner described in U.S. patent application Ser. No. [atty. docket no. KXIN100007000] entitled “Process for Producing Fibrillated Fibers” by the same inventors filed on even date herewith, the disclosure of which is hereby incorporated by reference. This process preferably involves first open channel refining fibers at a first shear rate to create fibrillated fibers, and subsequently open channel refining the fibers at a second shear rate, higher than the first shear rate, to increase the degree of fibrillation of the fibers. The end result of either the prior art or alternate process is that the fibers are broken down into fiber cores and attached fibrils without cutting the fibers cores.
As used herein, the term open channel refining refers to physical processing of the fiber, primarily by shearing, without substantial crushing, beating and cutting, that results in fibrillation of the fiber with limited reduction of fiber length or generation of fines. Substantial crushing, beating and cutting of the fibers is not desirable in the production of filtration structures, for example, because such forces result in rapid disintegration of the fibers, and in the production of low quality fibrillation with many fines, short fibers and flattened fibers that provide less efficient filtration structures when such fibers are incorporated into the paper filters. Open channel refining, also referred to as shearing, is typically performed by processing an aqueous fiber suspension using one or more widely spaced rotating conical or flat blades or plates. The action of a single moving surface, sufficiently far away from other surfaces, imparts primarily shearing forces on the fibers in an independent shear field. The shear rate varies from a low value near the hub or axis of rotation to a maximum shear value at the outer periphery of the blades or plates, where maximum relative tip velocity is achieved. However, such shear is very low compared to that imparted by common surface refining methods where two surfaces in close proximity are caused to aggressively shear fibers, as in beaters, conical and high speed rotor refiners, and double disk refiners. An example of the latter employs a rotor with one or more rows of teeth that spins at high speed within or against a stator.
By contrast, the term closed channel refining refers to physical processing of the fiber by a combination of shearing, crushing, beating and cutting that results in both fibrillation of the fiber and reduction of fiber size and length, and a significant generation of fines compared to open channel refining. Closed channel refining is typically performed by processing an aqueous fiber suspension in a commercial beater or in a conical or flat plate refiner, the latter using closely spaced conical or flat blades or plates that rotate with respect to each other. This may be accomplished where one blade or plate is stationary and the other is rotating, or where two blades or plates are rotating at different angular speeds or in different directions. The action of both surfaces of the blades or plates imparts the shearing and other physical forces on the fibers, and each surface reinforces the shearing and cutting forces imparted by the other. As with open channel refining, the shear rate between the relatively rotating blades or plates varies from a low value near the hub or axis of rotation to a maximum shear value at the outer periphery of the blades or plates, where maximum relative tip velocity is achieved.
In the preferred embodiment of the present invention, the fibrillated fibers and nanofibers are produced in continuously agitated refiners from materials such as cellulose, acrylic, polyolefin, polyester, nylon, aramid and liquid crystal polymer fibers, particularly polypropylene and polyethylene fibers. In general, the fibers employed in the present invention may be organic or inorganic materials including, but not limited to, polymers, engineered resins, ceramics, cellulose, rayon, glass, metal, activated alumina, carbon or activated carbon, silica, zeolites, or combinations thereof. Combination of organic and inorganic fibers and/or whiskers are contemplated and within the scope of the invention as for example, glass, ceramic, or metal fibers and polymeric fibers may be used together.
The quality of the fibrillated fibers and nanofibers produced by the present invention is measured in one important aspect by the Canadian Standard Freeness value. Canadian Standard Freeness (CSF) means a value for the freeness or drainage rate of pulp as measured by the rate that a suspension of pulp may be drained. This methodology is well known to one having skill in the paper making arts. While the CSF value is slightly responsive to fiber length, it is strongly responsive to the degree of fiber fibrillation and fiber diameter distribution. Thus, the CSF, which is a measure of how easily water may be removed from the pulp, is a suitable means of monitoring the degree of fiber fibrillation and fiber diameter distribution. If the surface area is very high, which means generation of many nanofibers or nanofibrils on the surface of core fibers, then very little water will be drained from the pulp in a given amount of time and the CSF value will become progressively lower as the fibers fibrillate more extensively.
Following the production of the fibrillated fibers having fiber cores and attached nanofiber fibrils, the fibrillated fibers are then subjected to processing to strip or otherwise remove the nanofibers from the core. At the end of this stage, there results a mixture of nanofibers and larger fiber cores. Preferably, the present invention produces nanofibers with very small quantities of such remaining fiber cores. This may be achieved by separating the fiber cores from the nanofibers, for example, by filtration or centrifuging, or other classification technologies. Alternatively, the fiber cores are further processed to produce additional nanofibers, preferably while still mixed with the originally stripped nanofibers, by breaking down the fiber cores by closed channel shearing. In this latter case, the nanofiber fibrils escape being further cut down to fines because shear forces employed remain insufficient to cut and destroy the small separated fibrils. The invention therefore produces high quality nanofibers without significant deterioration of the fibrils into low value shorter whiskers or fines.
Preferably, the fibrillated fibers have a CSF rating of 200 to 0, or 100 or lower, and are subjected to a two stage closed channel refining to separate nanofibers from original fiber cores. The preferred first stage of the closed channel refining is a low speed, high shear closed channel refining followed by high speed, high shear refining. The entering fibrillated fiber is an aqueous suspension having a concentration in the range of 0.1% to 25% by weight. In this first step, the nanofibers are stripped off the core fiber and the core fiber is refined further. This mixture of separated nanofibers and core fibers is then preferably fed to a second stage closed channel refining with very high shear. During this second stage closed channel refining, the fiber core is further refined to produce more nanofibers without substantially affecting already separated nanofibers. The resulting fiber mixture may then be fed back to the first stage closed channel refining and/or the second stage closed channel refining and processed again until substantially all the fiber cores are transformed into nanofibers, to yield a nanofiber slurry which has substantially reduced original fiber cores.
A preferred continuous arrangement of open and closed channel refiners is depicted in
Open channel refiner 70 includes at least one, and preferably more than one horizontally extending rotors 52 spaced-apart vertically on shaft 44. The rotors may vary in diameter, and preferably achieve a tip speed (i.e., speed at the outer diameter of rotor) of at least 7000 ft/min. (2100 m/min). The rotors may contain teeth whose number may vary, preferably from 4 to 12.
Closed channel refiners 90 and 100 follow open channel refiner 70 in process order, and the preferred embodiments of the former are shown in
The fiber suspension may then be further processed in a higher shear closed channel refiner 100, as shown in more detail in
In rotary processing equipment such as the open and closed channel refiners of
Optionally, the fiber suspension may be processed by pressurizing the suspension in a homogenizer and forcing the pressurized suspension through a small nozzle or orifice to further transform substantially all the fiber cores into nanofibers by cell disruption. This homogenization subjects the fibers to high shear forces, and may be performed after one or both of the closed channel refiner processing described above, of in place of such processing. The homogenizer may be used with (e.g., after), or in place of, the closed channel refiners shown in
As shown in
Referring back to
The open channel refiner 70 is fed continuously with fibers 38 and, after open channel refining therein for a desired time, the resulting fibrillated fiber suspension 80 preferably continuously flows to succeeding closed channel refiner 90, where it is closed channel refined at a relatively low shear rate to remove the attached nanofibers from the fiber cores. For example, the rotor speed at this first stage closed channel refining can vary from about 400 to 1800 rev./min. The partially processed fiber suspension 82 then flows from closed channel refiner 90 to closed channel refiner 100, where it is further closed channel refined at a greater shear rates in continuous mode operation. For example, the rotor speed at this second stage closed channel refining can vary from about 400 to 3600 rev./min. A mixture of fiber cores and nanofibers separated from fiber cores as produced by the closed channel refining is shown in
If desired or required, the fiber suspension may be further processed by returning the fibrillated fiber suspension 80, partially processed nanofiber suspension 86, or finally processed nanofiber suspension 88 as recycle 32 to previous refiner stages 70, 90 and/or 100 for additional open and/or closed channel refining.
The rate at which the fibers are fed into first refiner 70 is governed by the specifications of the final fibrillated fiber 84. The feed rate (in dry fibers) can typically vary from about 20-1000 lbs./hr. (9-450 kg/hr), and the average residence time in each refiner varies from about 30 min. to 2 hours. The number of sequential refiners to meet such production rates can vary from 2 up to 10. The temperature inside the refiners is usually maintained below about 175° F. (80° C.).
The processed nanofiber 84 is characterized by Canadian Standard Freeness rating of the fiber mixture, and by optical measurement techniques. Typically, entering fibrillated fibers 80 have a CSF rating of about 50 to 0. Although the final CSF rating of the processed nanofiber 84 is still about 0, optical measurement shows that the fibrils are separated from the fiber cores and the fiber cores are broken down into nanofibers as a result of the high shear forces in the closed channel refining and/or homogenization proceeds.
A slurry of fibrillated fibers with CSF 0 is fed into a closed channel low shear refiner of the type shown in
A fibrillated fiber slurry of about 0.5 wt. % solids content and CSF of 0 is fed into the inlet chamber of a homogenizer of the type shown in
Thus, the present invention provides an improved process and system for producing nanometer-sized fibers having substantially no larger fiber cores mixed therein with greater uniformity and flowability. The fiber cores have a diameter of about 500-5000 nm and a length of about 0.1-6 mm and the nanofibers have a diameter of about 50-500 nm and a length of about 0.1-6 mm. The invention also produces nanometer-sized fibers with greater energy efficient and productivity, resulting in improved volume and yield. Such nanofibers may be used for filtration and other known nanofiber applications.
While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.
Number | Date | Country | |
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60842069 | Aug 2006 | US |